U.S. patent application number 10/751421 was filed with the patent office on 2004-07-15 for optical transmission system and optical transmitter and optical receiver used therefor.
Invention is credited to Fuse, Masaru, Ohya, Jun.
Application Number | 20040136730 10/751421 |
Document ID | / |
Family ID | 27297105 |
Filed Date | 2004-07-15 |
United States Patent
Application |
20040136730 |
Kind Code |
A1 |
Fuse, Masaru ; et
al. |
July 15, 2004 |
Optical transmission system and optical transmitter and optical
receiver used therefor
Abstract
An angle modulating portion 1 converts an inputted electrical
signal into a predetermined angle-modulated signal. An optical
modulating portion 2 converts the angle-modulated signal outputted
from the angle modulating portion 1 into an optical-modulated
signal and sends the optical-modulated signal to an optical
waveguide portion 3. An interference portion 6 separates the
optical-modulated signal transmitted through the optical waveguide
portion 3 into two optical signals having predetermined difference
in propagation delay and then combines the optical signals. An
optical/electrical converting portion 4 subjects the combined
optical signal to homodyne detection, to acquire a demodulated
signal of the original electrical signal and output the electrical
signal. That is, the interference portion 6 and the
optical/electrical converting portion 4 constitute a delayed
detection system of an optical signal, so that the delayed
detection system performs conversion processing of an optical
signal into an electrical signal and angle demodulation processing
simultaneously. In this way, a signal with a wide-band and a
high-frequency can be acquired by demodulation without electrical
part for wide-bands and high-frequencies.
Inventors: |
Fuse, Masaru; (Toyonaka,
JP) ; Ohya, Jun; (Osaka, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
27297105 |
Appl. No.: |
10/751421 |
Filed: |
January 6, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10751421 |
Jan 6, 2004 |
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09975188 |
Oct 12, 2001 |
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6690893 |
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09975188 |
Oct 12, 2001 |
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09136934 |
Aug 20, 1998 |
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6335814 |
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Current U.S.
Class: |
398/188 |
Current CPC
Class: |
H04B 10/69 20130101;
H04B 10/5053 20130101; H04B 10/676 20130101; H04B 10/505 20130101;
H04B 10/5051 20130101 |
Class at
Publication: |
398/188 |
International
Class: |
H04B 010/04 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 22, 1997 |
JP |
226291/1997 |
Mar 11, 1998 |
JP |
60135/1998 |
Apr 30, 1998 |
JP |
121498/1998 |
Claims
What is claimed is:
1. An optical transmission system for optically transmitting an
angle-modulated signal, comprising: an optical modulating portion
for converting said angle-modulated signal into an
optical-modulated signal; an interference portion for separating
said optical-modulated signal into a plurality of optical signals
having predetermined difference in propagation delay and then
combining the optical signals; and an optical/electrical converting
portion, having square-law-detection characteristics, for
converting the combined optical signal outputted from said
interference portion into an electrical signal, said interference
portion and said optical/electrical converting portion constituting
a delayed detection system of an optical signal, and the delayed
detection system performing conversion processing of an optical
signal into an electrical signal and angle demodulation processing
simultaneously.
2. The optical transmission system according to claim 1, wherein
said angle-modulated signal is an FM signal obtained by subjecting
an analog signal to frequency modulation.
3. The optical transmission system according to claim 1, wherein
said angle-modulated signal is a PM signal obtained by subjecting
an analog signal to phase modulation.
4. The optical transmission system according to claim 1, wherein
said angle-modulated signal is an FSK modulated signal obtained by
subjecting a digital signal to frequency modulation.
5. The optical transmission system according to claim 1, wherein
said angle-modulated signal is a PSK modulated signal obtained by
subjecting a digital signal to phase modulation.
6. The optical transmission system according to claim 1, wherein
said optical modulating portion generates an
optical-intensity-modulated signal as said optical-modulated
signal.
7. The optical transmission system according to claim 6, wherein
said optical modulating portion comprises: a light source for
outputting a light with a given optical intensity and a given
wavelength; an optical branch portion for branching the light from
said light source into two; first and second optical phase
modulating portions, provided for the two outputted lights from
said optical branch portion respectively, for subjecting the
outputted lights to optical phase modulation using said
angle-modulated signal as an original signal; and an optical
coupling portion for combining the two optical-phase-modulated
signals outputted from said first and second optical phase
modulating portions.
8. The optical transmission system according to claim 6, wherein
said interference portion comprises: an optical branch portion for
branching an inputted optical signal into a first optical signal
and a second optical signal; an optical delay portion for providing
the second optical signal outputted from said optical branch
portion with a predetermined delay; and an optical combining
portion for combining the first optical signal outputted from said
optical branch portion and the second optical signal outputted from
said optical delay portion.
9. The optical transmission system according to claim 6, wherein
said optical modulating portion comprises: a light source for
outputting a light with a given optical intensity and a given
wavelength; an optical branch portion for branching the light from
said light source into two; first and second optical phase
modulating portions, provided for the two outputted lights from
said optical branch portion respectively, for each subjecting each
of the outputted lights to optical phase modulation using said
angle-modulated signal as an original signal; and an optical
directional coupling portion for combining the two
optical-phase-modulated signals outputted from said first and
second optical phase modulating portions and then dividing the
resultant signal into first and second optical signals in which
optical-intensity modulated components are set in opposite phases
to each other, and said interference portion comprises: an optical
delay portion for providing the second optical signal outputted
from said optical directional coupling portion with a predetermined
delay; and an optical combining portion for combining the first
optical signal outputted from said optical directional coupling
portion and the second optical signal outputted from said optical
delay portion.
10. The optical transmission system according to claim 6, wherein
said interference portion comprises: an optical waveguide portion
for guiding the optical signal outputted from said optical
modulating portion; and first and second optical
transparent/reflecting portions, cascaded on said optical waveguide
portion at a prescribed interval, for respectively transmitting
parts of the inputted optical signals and reflecting the remained
parts, and propagation time in which an optical signal goes and
returns between said first and second optical
transparent/reflecting portions is said predetermined difference in
propagation delay.
11. The optical transmission system according to claim 1, wherein
said optical modulating portion generates an
optical-amplitude-modulated signal as said optical-modulated
signal.
12. The optical transmission system according to claim 11, wherein
said optical modulating portion comprises: a light source for
outputting a light with a given optical intensity and a given
wavelength; an optical branch portion for branching the light from
said light source into two; first and second optical phase
modulating portions, provided for the two outputted lights from
said optical branch portion respectively, for each subjecting each
of the outputted lights to optical phase modulation using said
angle-modulated signals as an original signal; and an optical
coupling portion for combining the two optical-phase-modulated
signals outputted from said first and second optical phase
modulating portions.
13. The optical transmission system according to claim 11, wherein
said interference portion comprises: an optical branch portion for
branching the inputted optical signal into a first optical signal
and a second optical signal; an optical delay portion for providing
the second optical signal outputted from said optical branch
portion with a predetermined delay; and an optical combining
portion for combining the first optical signal outputted from said
optical branch portion and the second optical signal outputted from
said optical delay portion.
14. The optical transmission system according to claim 11, wherein
said optical modulating portion comprises: a light source for
outputting a light with a given optical intensity and a given
wavelength; an optical branch portion for branching the light from
said light source into two; first and second optical phase
modulating portions, provided for the two outputted lights from
said optical branch portion respectively, for each subjecting each
of the outputted lights to optical phase modulation using said
angle-modulated signal as an original signal; and an optical
directional coupling portion for combining the two
optical-phase-modulated signals outputted from said first and
second optical phase modulating portions and then dividing the
resultant signal into first and second optical signals in which
optical-amplitude-modulate- d components are set in opposite phases
to each other, and said interference portion comprises: an optical
delay portion for providing the second optical signal outputted
from said optical directional coupling portion with a predetermined
delay; and an optical combining portion for combining the first
optical signal outputted from said optical directional coupling
portion and the second optical signal outputted from said optical
delay portion.
15. The optical transmission system according to claim 11, wherein
said interference portion comprises: an optical waveguide portion
for guiding the optical signal outputted from said optical
modulating portion; and first and second optical
transparent/reflecting portions, cascaded on said optical waveguide
portion at a predetermined interval, for respectively transmitting
parts of the inputted optical signals and reflecting the remained
parts, and propagation time in which an optical signal goes and
returns between said first and second optical
transparent/reflecting portions is said predetermined difference in
propagation delay.
16. The optical transmission system according to claim 12, wherein
predetermined optical phase modulation is performed in said first
and second optical phase modulating portions so that difference
between the optical phase shift by said first optical phase
modulating portion and the optical phase shift by said second
optical phase modulating portion is set in phase with said
angle-modulated signal.
17. The optical transmission system according to claim 14, wherein
predetermined optical phase modulation is performed in said first
and second optical phase modulating portions so that difference
between the optical phase shift by said first optical phase
modulating portion and the optical phase shift by said second
optical phase modulating portion is set in phase with said
angle-modulated signal.
18. The optical transmission system according to claim 12, wherein
predetermined optical phase modulation is performed in said first
and second optical phase modulating portions so that difference
between the optical phase shift by said first optical phase
modulating portion and the optical phase shift by said second
optical phase modulating portion is set in opposite phases with
said angle-modulated signal.
19. The optical transmission system according to claim 14, wherein
predetermined optical phase modulation is performed in said first
and second optical phase modulating portions so that difference
between the optical phase shift by said first optical phase
modulating portion and the optical phase shift by said second
optical phase modulating portion is set in opposite phases with
said angle-modulated signal.
20. The optical transmission system according to claim 1, wherein a
product value of a center angular frequency of said angle-modulated
signal and the predetermined difference in propagation delay in
said interference portion is set to be equal to .pi./2.
21. The optical transmission system according to claim 4, wherein
the predetermined difference in propagation delay in said
interference portion is set to be equal to one symbol length of
said digital signal.
22. The optical transmission system according to claim 5, wherein
the predetermined difference in propagation delay in said
interference portion is set to be equal to one symbol length of
said digital signal.
23. The optical transmission system according to claim 8, wherein
polarization states of the first optical signal and the second
optical signal to be combined in said optical combining portion are
set to be the same with each other.
24. The optical transmission system according to claim 9, wherein
polarization states of the first optical signal and the second
optical signal to be combined in said optical combining portion are
set to be the same with each other.
25. The optical transmission system according to claim 13, wherein
polarization states of the first optical signal and the second
optical signal to be combined in said optical combining portion are
set to be the same with each other.
26. The optical transmission system according to claim 14, wherein
polarization states of the first optical signal and the second
optical signal to be combined in said optical combining portion are
set to be the same with each other.
27. The optical transmission system according to claim 10, wherein
polarization states of the optical signal transmitting through said
first and second optical transparent/reflecting portions along said
optical waveguide portion and the optical signal transmitting
through said first optical transparent/reflecting portion,
reflected at said second optical transparent/reflecting portion,
reflected at said first optical transparent/reflecting portion and
transmitting through said second optical transparent/reflecting
portion are set to be the same with each other.
28. The optical transmission system according to claim 15, wherein
polarization states of the optical signal transmitting through said
first and second optical transparent/reflecting portions along said
optical waveguide portion and the optical signal transmitting
through said first optical transparent/reflecting portion,
reflected at said second optical transparent/reflecting portion,
reflected at said first optical transparent/reflecting portion and
transmitting through said second optical transparent/reflecting
portion are set to be the same with each other.
29. The optical transmission system according to claim 8, wherein
said optical modulating portion and said interference portion are
connected with a first optical waveguide portion, said interference
portion and said optical/electrical converting portion are
connected with a second optical waveguide portion, and said first
and/or second optical waveguide portions are composed of
single-mode optical fibers.
30. The optical transmission system according to claim 13, wherein
said optical modulating portion and said interference portion are
connected with a first optical waveguide portion, said interference
portion and said optical/electrical converting portion are
connected with a second optical waveguide portion, and said first
and/or second optical waveguide portions are composed of
single-mode optical fibers.
31. The optical transmission system according to claim 9, wherein
said interference portion and said optical/electrical converting
portion are connected with an optical waveguide portion, and said
optical waveguide portion is composed of a single-mode optical
fiber.
32. The optical transmission system according to claim 14, wherein
said interference portion and said optical/electrical converting
portion are connected with an optical waveguide portion, and said
optical waveguide portion is composed of a single-mode optical
fiber.
33. The optical transmission system according to claim 10, wherein
a whole or a part of the optical waveguide portion in said
interference portion is composed of a single-mode optical
fiber.
34. The optical transmission system according to claim 15, wherein
a whole or a part of the optical waveguide portion in said
interference portion is composed of a single-mode optical
fiber.
35. The optical transmission system according to claim 1, further
comprising an amplitude adjusting portion for adjusting an
amplitude of said angle-modulated signal and outputting said
angle-modulated signal of a constant amplitude.
36. The optical transmission system according to claim 1, further
comprising a bandwidth limiting portion for limiting a band of said
angle-modulated signal.
37. An optical transmitter for optically transmitting an
angle-modulated signal, comprising: an optical modulating portion
for converting said angle-modulated signal into an
optical-modulated signal; and an interference portion for
separating said optical-modulated signal into a plurality of
optical signals having predetermined difference in propagation
delay and then combining the optical signals, and said optical
transmitter transmitting the combined optical signal outputted from
said interference portion.
38. The optical transmitter according to claim 37, wherein said
angle-modulated signal is an FM signal obtained by subjecting an
analog signal to frequency modulation.
39. The optical transmitter according to claim 37, wherein said
angle-modulated signal is a PM signal obtained by subjecting an
analog signal to phase modulation.
40. The optical transmitter according to claim 37, wherein said
angle-modulated signal is an FSK modulated signal obtained by
subjecting a digital signal to frequency modulation.
41. The optical transmitter according to claim 37, wherein said
angle-modulated signal is a PSK modulated signal obtained by
subjecting a digital signal to phase modulation.
42. The optical transmitter according to claim 37, wherein said
optical modulating portion generates an optical-intensity-modulated
signal as said optical-modulated signal.
43. The optical transmitter according to claim 37, wherein said
optical modulating portion generates an optical-amplitude-modulated
signal as said optical-modulated signal.
44. An optical receiver for receiving an optical-modulated signal
and acquiring a demodulated signal of the optical-modulated signal,
comprising: an interference portion for separating said received
optical-modulated signal into a plurality of optical signals having
predetermined difference in propagation delay and then combining
the optical signals; and an optical/electrical converting portion,
having square-law-detection characteristics, for converting the
combined optical signal outputted from said interference portion
into an electrical signal, and said interference portion and said
optical/electrical converting portion constituting a delayed
detection system of an optical signal and the delayed detection
system performing conversion processing of an optical signal into
an electrical signal and angle demodulation processing
simultaneously.
45. The optical receiver according to claim 44, wherein said
optical-modulated signal is generated from a 2.sup.n-phase (n is an
integer of not less than two) PSK electrical-modulated signal as an
original signal, said interference portion includes: a received
light dividing portion for dividing an inputted optical signal into
2.sup.n-1 received lights; and first to 2.sup.n-1th optical
interference circuits, provided corresponding to said 2.sup.n-1
received lights respectively, for each branching each of the
received lights into a first optical signal and a second optical
signal, providing the second optical signal with a predetermined
delay and then combining the first and second optical signals, and
said optical/electrical signals are provided corresponding to said
first to 2.sup.n-1th optical interference circuits
respectively.
46. The optical receiver according to claim 45, wherein said
optical-modulated signal is generated from a quadrature PSK
electrical-modulated signal as an original signal, said
interference portion includes: a received light dividing portion
for dividing an inputted optical signal into a first received light
and a second received light; a first optical interference circuit
for branching said first received light into a first optical signal
and a second optical signal, providing the second optical signal
with a first predetermined delay and then combining the first and
second optical signals; and a second optical interference circuit
for branching said second received light into a first optical
signal and a second optical signal, providing the second optical
signal with a second predetermined delay and then combining the
first and second optical signals, and the first predetermined delay
in said first optical interference circuit and the second
predetermined delay in said second optical interference circuit are
both set to have the absolute magnitude of 1/2 symbol length of
said digital signal and be in opposite phases to each other.
47. An optical transmission system for optically transmitting an
angle-modulated signal, comprising: an optical modulating portion
for converting said angle-modulated signal into an
optical-modulated signal; an optical branch portion for branching
the optical-modulated signal outputted from said optical modulating
portion into two signals at least, a first optical-modulated signal
and a second optical-modulated signal; an interference portion for
separating the first optical-modulated signal outputted from said
optical branch portion into a plurality of optical signals having
predetermined difference in propagation delay and then combining
the optical signals; a first optical/electrical converting portion,
having square-law-detection characteristics, for converting the
combined optical signal outputted from said interference portion
into an electrical signal; and a second optical/electrical
converting portion, having square-law-detection characteristics,
for converting the second optical-modulated signal outputted from
said optical branch portion into an electrical signal.
48. The optical transmission system according to claim 47, further
comprising: a local light source for outputting a light of a
predetermined wavelength; and an optical combining portion,
inserted between said optical branch portion and said second
optical/electrical converting portion, for combining the second
optical-modulated signal outputted from said optical branch portion
and the light from said local light source, wherein said second
optical/electrical converting portion heterodyne detects the
combined optical signal outputted from said optical combining
portion and then converts the optical signal into an electrical
signal.
49. The optical transmission system according to claim 47, further
comprising: a local light source for outputting a light of a
predetermined wavelength; and an optical combining portion,
inserted between said optical modulating portion and said optical
branch portion, for combining the optical-modulated signal
outputted from said optical modulating portion and the light from
said local light source, wherein said second optical/electrical
converting portion heterodyne detects the second optical-modulated
signal outputted from said optical branch portion and converts the
optical-modulated signal into an electrical signal.
50. An optical transmission system for optically transmitting an
angle-modulated signal, comprising: an optical modulating portion
for converting said angle-modulated signal into an
optical-modulated signal; a local light source for outputting a
light of a predetermined wavelength; an optical combining portion
for combining the optical-modulated signal outputted from said
optical modulating portion and the light from said local light
source; an interference portion for separating the combined optical
signal outputted from said optical combining portion into a
plurality of optical signals having predetermined difference in
propagation delay and then combining the optical signals; an
optical/electrical converting portion, having square-law-detection
characteristics, for converting the combined optical signal
outputted from said interference portion into an electrical signal;
and a dividing portion for separating the electrical signal
outputted from said optical/electrical converting portion for each
of frequency components and outputting the electrical signals.
51. An optical transmission system for optically transmitting an
angle-modulated signal, comprising: an optical modulating portion
for converting said angle-modulated signal into an
optical-modulated signal; an optical branch portion for branching
the optical-modulated signal outputted from said optical modulating
portion into two signals at least, a first optical-modulated signal
and a second optical-modulated signal; an interference portion for
separating the first optical-modulated signal outputted from said
optical branch portion into a plurality of optical signals having
predetermined difference in propagation delay and then combining
the optical signals; a first optical/electrical converting portion,
having square-law-detection characteristics, for converting the
combined optical signal outputted from said interference portion
into an electrical signal; a local oscillation portion for
outputting an unmodulated signal of a predetermined frequency; and
a second optical/electrical converting portion, having
square-law-detection characteristics, in which its bias is
modulated with the unmodulated signal from said local oscillation
portion, for converting the second optical-modulated signal
outputted from said optical branch portion into an electrical
signal.
52. An optical transmission system for optically transmitting an
angle-modulated signal, comprising: an optical modulating portion
for converting said angle-modulated signal into an
optical-modulated signal; an optical branch portion for branching
the optical-modulated signal outputted from said optical modulating
portion into two signals at least, a first optical-modulated signal
and a second optical-modulated signal; an interference portion for
separating the first optical-modulated signal outputted from said
optical branch portion into a plurality of optical signals having
predetermined difference in propagation delay and then combining
the optical signals; a first optical/electrical converting portion,
having square-law-detection characteristics, for converting the
combined optical signal outputted from the interference portion
into an electrical signal; a second optical/electrical converting
portion, having square-law-detection characteristics, for
converting the second optical-modulated signal outputted from said
optical branch portion into an electrical signal; a local
oscillation portion for outputting an unmodulated signal of a
predetermined frequency; and a mixing portion for mixing the
electrical signal outputted from said second optical/electrical
converting portion and the unmodulated signal outputted from said
local oscillation portion and outputting the resultant signals.
53. An optical transmission system for optically transmitting two
signals at least, a first electrical signal and a second electrical
signal simultaneously, comprising: an angle modulating portion for
converting said first electrical signal into an angle-modulated
signal; a combining portion for combining said angle-modulated
signal and said second electrical signal; an optical modulating
portion for converting the combined signal outputted from said
combining portion into an optical-modulated signal; an optical
branch portion for branching the optical-modulated signal outputted
from said optical modulating portion into two signals at least, a
first optical-modulated signal and a second optical-modulated
signal; an interference portion for separating the first
optical-modulated signal outputted from said optical branch portion
into a plurality of optical signals having predetermined difference
in propagation delay and then combining the optical signals; a
first optical/electrical converting portion, having
square-law-detection characteristics, for converting the combined
optical signal outputted from said interference portion into an
electrical signal; and a second optical/electrical converting
portion, having square-law-detection characteristics, for
converting the second optical-modulated signal outputted from said
optical branch portion into an electrical signal.
54. The optical transmission system according to claim 53, wherein
an occupied frequency band of said first electrical signal, an
occupied frequency band of said second electrical signal and an
occupied frequency band of said angle-modulated signal do not
overlap with each other.
55. The optical transmission system according to claim 53, further
comprising: a first signal processing portion for limiting the
occupied frequency band of said first electrical signal; and a
second signal processing portion for limiting the occupied
frequency band of said second electrical signal.
56. The optical transmission system according to claim 55, further
comprising: a third signal processing portion for passing only a
frequency component corresponding to the occupied frequency band of
said first electrical signal as to the electrical signal outputted
from said first optical/electrical converting portion and
reproducing waveform information which was lost by the band
limitation in said first signal processing portion; and a fourth
signal processing portion for passing only a frequency component
corresponding to the occupied frequency band of said second
electrical signal as to the electrical signal outputted from said
second optical/electrical converting portion and reproducing
waveform information which was lost by the band limitation in said
second signal processing portion.
57. An optical transmission system for optically transmitting a
plurality of electrical signals, comprising: a plurality of angle
modulating portions for converting each of said plurality of
electrical signals into an angle-modulated signals; a combining
portion for combining the angle-modulated signals outputted from
said plurality of angle modulating portions; an optical modulating
portion for converting the combined signal outputted from said
combining portion into an optical-modulated signal; an optical
branch portion for branching the optical-modulated signal outputted
from said optical modulating portion into a plurality of
optical-modulated signals; and an plurality of optical signal
processing portions, provided corresponding to the plurality of
optical-modulated signals outputted from said optical branch
portion respectively, for each performing predetermined optical
signal processing and then individually reproducing said plurality
of electrical signals, and each of said optical signal processing
portions including: an interference portion for separating the
optical-modulated signal outputted from said optical branch portion
into a plurality of optical signals having difference in
propagation delay decided according to frequencies of
angle-modulated signals to be acquired by demodulation and then
combining the optical signals; and an optical/electrical converting
portion, having square-law-detection characteristics, for
converting the combined optical signal outputted from said
interference portion into an electrical signal.
58. The optical transmission system according to claim 57, wherein
occupied frequency bands of said plurality of electrical signals
and occupied frequency bands of said plurality of angle-modulated
signals do not overlap with each other.
59. The optical transmission system according to claim 57, further
comprising a plurality of signal pre-processing portions for
limiting the occupied frequency bands of said plurality of
electrical signals.
60. The optical transmission system according to claim 59, wherein
each of said plurality of optical signal processing portions
further includes a signal post-processing portion for passing a
frequency component corresponding to an occupied frequency band of
an electrical signal to be reproduced and reproducing waveform
information which was lost by the band limitation in said signal
pre-processing portion as to the electrical signal outputted from
said optical/electrical converting portion.
61. An optical transmission system for optically transmitting a
multichannel angle-modulated signal obtained by subjecting
plurality-channel electrical signals to angle modulation
respectively and frequency-division multiplexing, comprising: an
optical modulating portion for converting said multichannel
angle-modulated signal into an optical-modulated signal; an optical
branch portion for branching the optical-modulated signal outputted
from said optical modulating portion into a plurality of
optical-modulated signals; and a plurality of optical signal
processing portions, provided corresponding to the plurality of
optical-modulated signals outputted from said optical branch
portion respectively, for each performing predetermined optical
signal processing and then reproducing an electrical signal on an
individual channel, and each of said optical signal processing
portions including: an interference portion for separating the
optical-modulated signal outputted from said optical branch portion
into a plurality of optical signals having difference in
propagation delay decided according to frequencies of electrical
signals on channels to be reproduced and then combining the optical
signals; and an optical/electrical converting portion, having
square-law-detection characteristics, for converting the combined
optical signal outputted from said interference portion into an
electrical signal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to optical transmission
systems, more specifically to a system for optically transmitting
an angle-modulated signal.
[0003] 2. Description of the Background Art
[0004] FIG. 30 is a block diagram showing an example of the
configuration of a conventional optical transmission system which
transmits an angle-modulated signal. In FIG. 30, the optical
transmission system includes an angle modulating portion 1, an
optical modulating portion 2, an optical waveguide portion 3, an
optical/electrical converting portion 4, an angle demodulating
portion 5 and a filter F. Such optical transmission system is
described, for example, in a document (K. Kikushima, et al.,
"Optical Super Wide-Band FM Modulation Scheme and Its Application
to Multi-Channel AM Video Transmission Systems", IOOC'95, PD2-7,
1995, pp. 33-34.).
[0005] Next, the operation of the conventional optical transmission
system structured as above will be described. As an electrical
signal inputted to the angle modulating portion 1, assumed is an
analog signal such as an audio or video signal, or a digital signal
such as computer data and the like. The angle modulating portion 1
converts the inputted electrical signal into an angle-modulated
signal with a predetermined frequency and a predetermined angle
modulation scheme to output the angle-modulated signal. The angle
modulation scheme includes FM (frequency modulation) or PM (phase
modulation) for an analog signal and FSK (frequency-shift keying)
or PSK (phase-shift keying) for a digital signal, and is
generically referred to as angle modulation hereinafter. The
optical modulating portion 2 converts the inputted angle-modulated
signal into an optical-modulated signal to output the
optical-modulated signal. The optical/electrical converting portion
4, which includes a photodetector having square-law-detection
characteristics (a pin photo-diode, an avalanche photo-diode or the
like), re-converts the optical-modulated signal transmitted by the
optical waveguide portion 3 into an electrical signal to output an
angle-modulated signal. The angle demodulating portion 5 converts
variations in frequency (or variations in phase) of the
angle-modulated signal into variations in amplitude (or variations
in intensity) of an electrical signal, thereby re-generating a
signal correlating with the original electrical signal. The filter
F passes only a signal component corresponding to the original
electrical signal (that is a signal component of the same frequency
band as that of the original electrical signal) among signals
outputted from the angle demodulating portion 5.
[0006] In FIG. 31 is shown an example of the structure of the angle
demodulating portion 5 in FIG. 30. In FIG. 31, the angle-modulated
signal inputted from the optical/electrical converting portion 4 is
branched into two signals in a branch portion 51. One signal of the
two signals obtained by the branch is provided with a predetermined
delay T.sub.p in a delay portion 52. A mixing portion 53, which is
generally constituted by a mixer and the like, receives the other
signal outputted from the branch portion 51 and the signal
outputted from the delay portion 52 to generate a product signal of
these signals and output the product signal.
[0007] The conventional optical transmission system of the
angle-modulated signal as described above has an advantage in the
following, compared with an optical transmission system of an
amplitude-modulated (AM) signal. That is, the frequency deviation
(or the phase deviation) of the angle-modulated signal is set
larger, so that a larger gain in angle modulation can be acquired
at the optical transmission. As a result, SNR (signal-to-noise
power ratio) of a demodulated signal increases, realizing
transmission of a signal of good quality. Moreover, the frequency
deviation (or the phase deviation) of the angle-modulated signal is
increased to spread a frequency spectrum of the optical-modulated
signal and suppress a peak level of the frequency spectrum, which
leads to an advantage in that deterioration of signal quality due
to multipath reflection on an optical transmission line is
reduced.
[0008] As described above, in the conventional optical transmission
system, an electrical signal to be transmitted, after being
subjected to angle modulation, is converted into an
optical-modulated signal to be optically transmitted, subjected to
square-law-detection on a receiving side to be re-converted into an
angle-modulated signal, and further subjected to angle demodulation
to be the original electrical signal. Therefore, it is possible, in
the conventional optical transmission system, to perform optical
transmission of better quality by increasing the frequency
deviation (the phase deviation) even on an optical transmission
line of poor quality.
[0009] However, increasing in the frequency deviation (or the phase
deviation) of the angle-modulated signal makes the frequency and
band of the angle-modulated signal higher and wider. Accordingly,
the conventional optical transmission system as described above,
requires electrical parts for high frequencies and wide-bands in
order to constitute the angle modulating portion 1 and the angle
demodulating portion 5. Connection and matching among such
electrical parts for high frequencies and wide-bands are difficult
and multipath reflection among the parts readily occurs. This
causes deterioration of characteristics of the angle modulating
portion 1 and the angle demodulating portion 5, resulting in
significant deterioration of quality of modulated/demodulated
signals.
[0010] Further, in the case where an expensive electrical part for
wide-bands and high frequencies (for example, the branch portion 51
and the mixing portion 53 in FIG. 31) is used in the angle
demodulating portion 5 which is installed as a receiving terminal
of an optical transmission system, when configuring an optical
subscriber (optical multi-distribution) system such as a FTTH
(Fiber To The Home) system, a CATV network and the like, the system
cost per subscriber becomes very high to significantly degrade the
system from the view point of its economy.
[0011] As explained in the above, the conventional optical
transmission system is required, when optically transmitting an
angle-modulated signal with a wider-band and a higher frequency, to
use the electrical parts for wide-bands and high frequencies
especially as constituents of the demodulating portion. Thereby,
the conventional optical transmission system has a specific problem
in that group delay characteristics and modulation/demodulation
characteristics are easily deteriorated and economy of overall
system is significantly degraded because of increase in the cost of
the receiving terminal.
SUMMARY OF THE INVENTION
[0012] Therefore, an object of the present invention is to provide
an optical transmission system which realizes good angle
demodulation characteristics by adopting new optical signal
processing and is greatly economical by constituting a receiving
terminal at lower cost without electrical parts for wide-bands and
high-frequencies.
[0013] The present invention has features described below in order
to attain the above-mentioned object.
[0014] A first aspect of the present invention is an optical
transmission system for optically transmitting an angle-modulated
signal, comprising:
[0015] an optical modulating portion for converting the
angle-modulated signal into an optical-modulated signal;
[0016] an interference portion for separating the optical-modulated
signal into a plurality of optical signals having predetermined
difference in propagation delay and then combining the optical
signals; and
[0017] an optical/electrical converting portion, having
square-law-detection characteristics, for converting the combined
optical signal outputted from the interference portion into an
electrical signal,
[0018] the interference portion and the optical/electrical
converting portion constituting a delayed detection system of an
optical signal, and the delayed detection system performing
conversion processing of an optical signal into an electrical
signal and angle demodulation processing simultaneously.
[0019] In the case where an electrical circuit using parts for
wide-bands and high-frequencies is adopted as a demodulation device
for an angle-modulated signal, connection or matching among the
parts are difficult, causing deterioration of linearities of
demodulation characteristics or group delay characteristics easily
to degrade the quality of an demodulated signal. Moreover, the
parts for wide-bands and high-frequencies are generally expensive,
so that the cost of the demodulation device increases,
significantly deteriorating the economy of the system.
[0020] Hence, in the above first aspect, an angle-modulated signal
is converted into an optical-modulated signal and the
optical-modulated signal is homodyne detected employing
square-law-detection characteristics of a photodetector, so that
demodulation and optical transmission can be performed only by
optical signal processing without using electrical parts for
wide-bands and high-frequencies. Further, when the present aspect
is applied to an optical distribution system, the portions in the
configuration up to the interference portion are installed on a
transmitting equipment side and only the optical/electrical
converting portion is installed on a receiving terminal side,
whereby the expensive constituents are included in only the
transmitting equipment. Thus, it is possible to construct an
optical subscriber system which is greatly economical.
[0021] A second aspect is an aspect according to the first aspect,
wherein the angle-modulated signal is an FM signal obtained by
subjecting an analog signal to frequency modulation.
[0022] A third aspect is an aspect according to the first aspect,
wherein the angle-modulated signal is a PM signal obtained by
subjecting an analog signal to phase modulation.
[0023] A fourth aspect is an aspect according to the first aspect,
wherein the angle-modulated signal is an FSK modulated signal
obtained by subjecting a digital signal to frequency
modulation.
[0024] A fifth aspect is an aspect according to the first aspect,
wherein the angle-modulated signal is a PSK modulated signal
obtained by subjecting a digital signal to phase modulation.
[0025] A sixth aspect is an aspect according to claim 1, wherein
the optical modulating portion generates an
optical-intensity-modulated signal as the optical-modulated
signal.
[0026] A seventh aspect is an aspect according to the sixth aspect,
wherein
[0027] the optical modulating portion comprises:
[0028] a light source for outputting a light with a given optical
intensity and a given wavelength;
[0029] an optical branch portion for branching the light from the
light source into two;
[0030] first and second optical phase modulating portions, provided
for the two outputted lights from the optical branch portion
respectively, for subjecting the outputted lights to optical phase
modulation using the angle-modulated signal as an original signal;
and
[0031] an optical coupling portion for combining the two
optical-phase-modulated signals outputted from the first and second
optical phase modulating portions.
[0032] As described in the foregoing, in the seventh aspect, in
order to generate an optical-intensity-modulated signal, an
external modulation scheme is adopted. In place of such external
modulation scheme, a direct modulation scheme can be also
adopted.
[0033] An eighth aspect is an aspect according to the sixth aspect,
wherein the interference portion comprises:
[0034] an optical branch portion for branching an inputted optical
signal into a first optical signal and a second optical signal;
[0035] an optical delay portion for providing the second optical
signal outputted from the optical branch portion with a
predetermined delay; and
[0036] an optical combining portion for combining the first optical
signal outputted from the optical branch portion and the second
optical signal outputted from the optical delay portion.
[0037] As described in the foregoing, in the eighth aspect, the
inputted optical signal is branched into two optical signals by the
optical branch portion, the predetermined propagation delay is
provided for one of the two optical signals and then the two
optical signals are combined again by the optical combining
portion, which constitutes an interference system necessary for
delayed detection of an optical signal.
[0038] A ninth aspect is an aspect according to the sixth aspect,
wherein
[0039] the optical modulating portion comprises:
[0040] a light source for outputting a light with a given optical
intensity and a given wavelength;
[0041] an optical branch portion for branching the light from the
light source into two;
[0042] first and second optical phase modulating portions, provided
for the two outputted lights from the optical branch portion
respectively, for each subjecting each of the outputted lights to
optical phase modulation using the angle-modulated signal as an
original signal; and
[0043] an optical directional coupling portion for combining the
two optical-phase-modulated signals outputted from the first and
second optical phase modulating portions and then dividing the
resultant signal into first and second optical signals in which
optical-intensity modulated components are set in opposite phases
to each other, and
[0044] the interference portion comprises:
[0045] an optical delay portion for providing the second optical
signal outputted from the optical directional coupling portion with
a predetermined delay; and
[0046] an optical combining portion for combining the first optical
signal outputted from the optical directional coupling portion and
the second optical signal outputted from the optical delay
portion.
[0047] As described in the foregoing, in the ninth aspect, the
external modulation scheme is adopted in the optical modulating
portion and the optical directional coupling portion is provided,
to input the first and second optical signals, in which
optical-intensity-modulated components are set in opposite phases
to each other, to the interference portion. This eliminates the
need for branching the inputted optical signal in the interference
portion.
[0048] A tenth aspect is an aspect according to the sixth aspect,
wherein
[0049] the interference portion comprises:
[0050] an optical waveguide portion for guiding the optical signal
outputted from the optical modulating portion; and
[0051] first and second optical transparent/reflecting portions,
cascaded on the optical waveguide portion at a prescribed interval,
for respectively transmitting parts of the inputted optical signals
and reflecting the remained parts, and
[0052] propagation time in which an optical signal goes and returns
between the first and second optical transparent/reflecting
portions is the predetermined difference in propagation delay.
[0053] As described in the foregoing, according to the tenth
aspect, two optical transparent/reflecting portions are provided on
the optical waveguide portion, and a direct light which propagates
through both of the optical transparent/reflecting portions and an
indirect light which goes and returns between the optical
transparent/reflecting portions one time and then propagates are
generated, which constitutes an interference system necessary for
delayed detection of an optical signal without physically branching
the optical signal into two. This allows constitution of the
interference system with a simpler configuration.
[0054] An eleventh aspect is an aspect according to the first
aspect, wherein the optical modulating portion generates an
optical-amplitude-modulated signal as the optical-modulated
signal.
[0055] A twelfth aspect is an aspect according to the eleventh
aspect, wherein
[0056] the optical modulating portion comprises:
[0057] a light source for outputting a light with a given optical
intensity and a given wavelength;
[0058] an optical branch portion for branching the light from the
light source into two;
[0059] first and second optical phase modulating portions, provided
for the two outputted lights from the optical branch portion
respectively, for each subjecting each of the outputted lights to
optical phase modulation using the angle-modulated signals as an
original signal; and
[0060] an optical coupling portion for combining the two
optical-phase-modulated signals outputted from the first and second
optical phase modulating portions.
[0061] A thirteenth aspect is an aspect according to the eleventh
aspect, wherein
[0062] the interference portion comprises:
[0063] an optical branch portion for branching the inputted optical
signal into a first optical signal and a second optical signal;
[0064] an optical delay portion for providing the second optical
signal outputted from the optical branch portion with a
predetermined delay; and
[0065] an optical combining portion for combining the first optical
signal outputted from the second optical branch portion and the
second optical signal outputted from the optical delay portion.
[0066] As described in the foregoing, according to the thirteenth
aspect, the inputted optical signal is branched into two optical
signals by the optical branch portion, the predetermined
propagation delay is provided for one of the two optical signals in
the delay portion and then the two optical signals are combined
again in the optical combining portion, to constitute an
interference system necessary for delayed detection of an optical
signal.
[0067] A fourteenth aspect is an aspect according to the eleventh
aspect, wherein
[0068] the optical modulating portion comprises:
[0069] a light source for outputting a light with a given optical
intensity and a given wavelength;
[0070] an optical branch portion for branching the light from the
light source into two;
[0071] first and second optical phase modulating portions, provided
for the two outputted lights from the optical branch portion
respectively, for each subjecting each of the outputted lights to
optical phase modulation using the angle-modulated signal as an
original signal; and
[0072] an optical directional coupling portion for combining the
two optical-phase-modulated signals outputted from the first and
second optical phase modulating portions and then dividing the
resultant signal into first and second optical signals in which
optical-amplitude-modulate- d components are set in opposite phases
to each other, and
[0073] the interference portion comprises:
[0074] an optical delay portion for providing the second optical
signal outputted from the optical directional coupling portion with
a predetermined delay; and
[0075] an optical combining portion for combining the first optical
signal outputted from the optical directional coupling portion and
the second optical signal outputted from the optical delay
portion.
[0076] As described in the foregoing, in the fourteenth aspect, the
external modulation scheme is adopted in the optical modulating
portion and the optical directional coupling portion is provided,
to input the first and second optical signals, in which
optical-intensity-modulated components are set in opposite phases
to each other, to the interference portion. This eliminates the
need for branching the inputted optical signal in the interference
portion.
[0077] A fifteenth aspect is an aspect according to the eleventh
aspect, wherein
[0078] the interference portion comprises:
[0079] an optical waveguide portion for guiding the optical signal
outputted from the optical modulating portion; and
[0080] first and second optical transparent/reflecting portions,
cascaded on the optical waveguide portion at a predetermined
interval, for respectively transmitting parts of the inputted
optical signals and reflecting the remained parts, and
[0081] propagation time in which an optical signal goes and returns
between the first and second optical transparent/reflecting
portions is the predetermined difference in propagation delay.
[0082] As described in the foregoing, according to the fifteenth
aspect, two optical transparent/reflecting portions are provided on
the optical waveguide portion, and the direct light which
propagates through both of the optical transparent/reflecting
portions and the indirect light which goes and returns between the
optical transparent/reflecting portions one time and then
propagates are generated, which constitutes an interference system
necessary for delayed detection of an optical signal without
physically branching the optical signal into two. This allows
constitution of the interference system with a simpler
configuration.
[0083] A sixteenth aspect is an aspect according to the twelfth
aspect, wherein predetermined optical phase modulation is performed
in the first and second optical phase modulating portions so that
difference between the optical phase shift by the first optical
phase modulating portion and the optical phase shift by the second
optical phase modulating portion is set in phase with the
angle-modulated signal.
[0084] A seventeenth aspect is an aspect according to the
fourteenth aspect, wherein predetermined optical phase modulation
is performed in the first and second optical phase modulating
portions so that difference between the optical phase shift by the
first optical phase modulating portion and the optical phase shift
by the second optical phase modulating portion is set in phase with
the angle-modulated signal.
[0085] An eighteenth aspect is an aspect according to the twelfth
aspect, wherein predetermined optical phase modulation is performed
in the first and second optical phase modulating portions so that
difference between the optical phase shift by the first optical
phase modulating portion and the optical phase shift by the second
optical phase modulating portion is set in opposite phases with the
angle-modulated signal.
[0086] A nineteenth aspect is an aspect according to the fourteenth
aspect, wherein predetermined optical phase modulation is performed
in the first and second optical phase modulating portions so that
difference between the optical phase shift by the first optical
phase modulating portion and the optical phase shift by the second
optical phase modulating portion is set in opposite phases with the
angle-modulated signal.
[0087] In the sixteenth to nineteenth aspects, phase relation
between the angle-modulated signals inputted into the first and
second optical phase modulating portions is optimally adjusted, to
enlarge the optical-amplitude-modulated component in the optical
signal inputted into the optical coupling portion or the optical
directional coupling portion, which realizes high efficient
demodulation and optical transmission with optical signal
processing.
[0088] A twentieth aspect is an aspect according to the first
aspect, wherein a product value of a center angular frequency of
the angle-modulated signal and the predetermined difference in
propagation delay in the interference portion is set to be equal to
.pi./2.
[0089] As described in the foregoing, in the twentieth aspect, the
center angular frequency of the angle-modulated signal and the
predetermined difference in propagation delay in the interference
portion are set at optical values, to increase demodulation
efficiency.
[0090] A twenty-first aspect is an aspect according to the fourth
aspect, wherein the predetermined difference in propagation delay
in the interference portion is set to be equal to one symbol length
of the digital signal.
[0091] A twenty-second aspect is an aspect according to the fifth
aspect, wherein the predetermined difference in propagation delay
in the interference portion is set to be equal to one symbol length
of the digital signal.
[0092] As described in the foregoing, in the twenty-first and
twenty-second aspects, when the angle-modulated signal is an FSK
modulated signal or a PSK modulated signal obtained by subjecting a
digital signal to frequency modulation or phase modulation, the
symbol length of the digital signal and the predetermined
difference in propagation delay in the interference portion are set
to optimal values, thereby increasing the demodulation
efficiency.
[0093] A twenty-third aspect is an aspect according to the eighth
aspect, wherein polarization states of the first optical signal and
the second optical signal to be combined in the optical combining
portion are set to be the same with each other.
[0094] A twenty-fourth aspect is an aspect according to the ninth
aspect, wherein polarization states of the first optical signal and
the second optical signal to be combined in the optical combining
portion are set to be the same with each other.
[0095] A twenty-fifth aspect is an aspect according to the
thirteenth aspect, wherein polarization states of the first optical
signal and the second optical signal to be combined in the optical
combining portion are set to be the same with each other.
[0096] A twenty-sixth aspect is an aspect according to the
fourteenth aspect, wherein
[0097] polarization states of the first optical signal and the
second optical signal to be combined in the optical combining
portion are set to be the same with each other.
[0098] As described in the foregoing, in the twenty-third to
twenty-sixth aspects, the polarization states of the first and
second optical-signals in the optical combining portion are set to
be the same with each other, thereby increasing homodyne detection
efficiency in the optical/electrical converting portion, that is
demodulation efficiency.
[0099] A twenty-seventh aspect is an aspect according to the tenth
aspect, wherein
[0100] polarization states of the optical signal transmitting
through the first and second optical transparent/reflecting
portions along the optical waveguide portion and the optical signal
transmitting through the first optical transparent/reflecting
portion, reflected at the second optical transparent/reflecting
portion, reflected at the first optical transparent/reflecting
portion and transmitting through the second optical
transparent/reflecting portion are set to be the same with each
other.
[0101] A twenty-eighth aspect is an aspect according to the
fifteenth aspect, wherein
[0102] polarization states of the optical signal transmitting
through the first and second optical transparent/reflecting
portions along the optical waveguide portion and the optical signal
transmitting through the first optical transparent/reflecting
portion, reflected at the second optical transparent/reflecting
portion, reflected at the first optical transparent/reflecting
portion and transmitting through the second optical
transparent/reflecting portion are set to be the same with each
other.
[0103] As described in the foregoing, in the twenty-seventh and
twenty-eighth aspects, the polarization states of the direct light
and the indirect light are set to be the same, thereby increasing
the homodyne detection efficiency in the optical/electrical
converting portion, that is the demodulation efficiency.
[0104] A twenty-ninth aspect is an aspect according to the eighth
aspect, wherein
[0105] the optical modulating portion and the interference portion
are connected with a first optical waveguide portion,
[0106] the interference portion and the optical/electrical
converting portion are connected with a second optical waveguide
portion, and
[0107] the first and/or second optical waveguide portions are
composed of single-mode optical fibers.
[0108] The thirtieth aspect is an aspect according to the
thirteenth aspect, wherein
[0109] the optical modulating portion and the interference portion
are connected with a first optical waveguide portion,
[0110] the interference portion and the optical/electrical
converting portion are connected with a second optical waveguide
portion, and
[0111] the first and/or second optical waveguide portions are
composed of single-mode optical fibers.
[0112] As described in the foregoing, in the twenty-ninth and
thirtieth aspects, the first and/or second optical waveguide
portions are composed of single-mode optical fibers, making it
possible to perform optical transmission with optical fibers which
are inexpensive.
[0113] A thirty-first aspect is an aspect according to the ninth
aspect, wherein
[0114] the interference portion and the optical/electrical
converting portion are connected with an optical waveguide portion,
and
[0115] the optical waveguide portion is composed of a single-mode
optical fiber.
[0116] A thirty-second aspect is an aspect according to the
fourteenth aspect, wherein
[0117] the interference portion and the optical/electrical
converting portion are connected with an optical waveguide portion,
and
[0118] the optical waveguide portion is composed of a single-mode
optical fiber.
[0119] As described in the foregoing, in the thirty-first and
thirty-second aspects, the optical waveguide portion provided
between the interference portion and the optical/electrical
converting portion is composed of a single-mode optical fiber,
making it possible to perform optical transmission with an optical
fiber which is inexpensive.
[0120] A thirty-third aspect is an aspect according to the tenth
aspect, wherein a whole or a part of the optical waveguide portion
in the interference portion is composed of a single-mode optical
fiber.
[0121] A thirty-fourth aspect is an aspect according to the
fifteenth aspect, wherein a whole or a part of the optical
waveguide portion in the interference portion is composed of a
single-mode optical fiber.
[0122] As described in the foregoing, in the thirty-third and
thirty-fourth aspects, the whole or a part of the optical waveguide
portion in the interference portion is composed of a single-mode
optical fiber, allowing optical transmission with an optical fiber
which is inexpensive.
[0123] A thirty-fifth aspect is an aspect according to the first
aspect, further comprising an amplitude adjusting portion for
adjusting an amplitude of the angle-modulated signal and outputting
the angle-modulated signal of a constant amplitude.
[0124] In the case where delayed detection is performed employing
the square-law-detection characteristics of the optical/electrical
converting portion, as the amplitude of the angle-modulated signal
which is the original signal becomes smaller, the demodulation
efficiency decreases. Further, when the angle-modulated signal has
an amplitude fluctuation, deterioration in signal quality such as
waveform distortion and the like occurs. Hence, in the above
thirty-fifth aspect, the amplitude adjusting portion maintaining
the amplitude constant is provided for the inputted angle-modulated
signal, to suppress the above-mentioned deterioration.
[0125] A thirty-sixth aspect is an aspect according to the first
aspect, further comprising a bandwidth limiting portion for
limiting a band of the angle-modulated signal.
[0126] As described in the foregoing, in the thirty-sixth aspect,
the bands of the angle-modulated signal is previously limited, to
lessen the spectrum in width, thereby preventing deterioration in
quality of a demodulated signal caused by that the part of the
spread spectrum of the angle-modulated signal component is
superimposed on the band of the demodulated signal outputted from
the optical/electrical converting portion.
[0127] A thirty-seventh aspect is an optical transmitter for
optically transmitting an angle-modulated signal, comprising:
[0128] an optical modulating portion for converting the
angle-modulated signal into an optical-modulated signal; and
[0129] an interference portion for separating the optical-modulated
signal into a plurality of optical signals having predetermined
difference in propagation delay and then combining the optical
signals, and
[0130] the optical transmitter transmitting the combined optical
signal outputted from the interference portion.
[0131] A thirty-eighth aspect is an aspect according to the
thirty-seventh aspect, wherein the angle-modulated signal is an FM
signal obtained by subjecting an analog signal to frequency
modulation.
[0132] A thirty-ninth aspect is an aspect according to the
thirty-seventh aspect, wherein the angle-modulated signal is a PM
signal obtained by subjecting an analog signal to phase
modulation.
[0133] A fortieth aspect is an aspect according to the
thirty-seventh aspect, wherein the angle-modulated signal is an FSK
modulated signal obtained by subjecting a digital signal to
frequency modulation.
[0134] A forty-first aspect is an aspect according to the
thirty-seventh aspect, wherein the angle-modulated signal is a PSK
modulated signal obtained by subjecting a digital signal to phase
modulation.
[0135] A forty-second aspect is an aspect according to the
thirty-seventh aspect, wherein the optical modulating portion
generates an optical-intensity-modulated signal as the
optical-modulated signal.
[0136] A forty-third aspect is an aspect according to the
thirty-seventh aspect, wherein the optical modulating portion
generates an optical-amplitude-modulated signal as the
optical-modulated signal.
[0137] A forty-fourth aspect is an optical receiver for receiving
an optical-modulated signal and acquiring a demodulated signal of
the optical-modulated signal, comprising:
[0138] an interference portion for separating the received
optical-modulated signal into a plurality of optical signals having
predetermined difference in propagation delay and then combining
the optical signals; and
[0139] an optical/electrical converting portion, having
square-law-detection characteristics, for converting the combined
optical signal outputted from the interference portion into an
electrical signal, and
[0140] the interference portion and the optical/electrical
converting portion constituting a delayed detection system of an
optical signal and the delayed detection system performing
conversion processing of an optical signal into an electrical
signal and angle demodulation processing simultaneously.
[0141] A forty-fifth aspect is an aspect according to the
forty-fourth aspect, wherein
[0142] the optical-modulated signal is generated from a
2.sup.n-phase (n is an integer of not less than two) PSK
electrical-modulated signal as an original signal,
[0143] the interference-portion includes:
[0144] a received light dividing portion for dividing an inputted
optical signal into 2.sup.n-1 received lights; and
[0145] first to 2.sup.n-1th optical interference circuits, provided
corresponding to the 2.sup.n-1 received lights respectively, for
each branching each of the received lights into a first optical
signal and a second optical signal, providing the second optical
signal with a predetermined delay and then combining the first and
second optical signals, and
[0146] the optical/electrical signals are provided corresponding to
the first to 2.sup.n-1th optical interference circuits
respectively.
[0147] A forty-sixth aspect is an aspect according to the
forty-fifth aspect, wherein
[0148] the optical-modulated signal is generated from a quadrature
PSK electrical-modulated signal as an original signal,
[0149] the interference portion includes:
[0150] a received light dividing portion for dividing an inputted
optical signal into a first received light and a second received
light;
[0151] a first optical interference circuit for branching the first
received light into a first optical signal and a second optical
signal, providing the second optical signal with a first
predetermined delay and then combining the first and second optical
signals; and
[0152] a second optical interference circuit for branching the
second received light into a first optical signal and a second
optical signal, providing the second optical signal with a second
predetermined delay and then combining the first and second optical
signals, and
[0153] the first predetermined delay in the first optical
interference circuit and the second predetermined delay in the
second optical interference circuit are both set to have the
absolute magnitude of 1/2 symbol length of the digital signal and
be in opposite phases to each other.
[0154] A forty-seventh aspect is an optical transmission system for
optically transmitting an angle-modulated signal, comprising:
[0155] an optical modulating portion for converting the
angle-modulated signal into an optical-modulated signal;
[0156] an optical branch portion for branching the
optical-modulated signal outputted from the optical modulating
portion into two signals at least, a first optical-modulated signal
and a second optical-modulated signal;
[0157] an interference portion for separating the first
optical-modulated signal outputted from the optical branch portion
into a plurality of optical signals having predetermined difference
in propagation delay and then combining the optical signals;
[0158] a first optical/electrical converting portion, having
square-law-detection characteristics, for converting the combined
optical signal outputted from the interference portion into an
electrical signal; and
[0159] a second optical/electrical converting portion, having
square-law-detection characteristics, for converting the second
optical-modulated signal outputted from the optical branch portion
into an electrical signal.
[0160] As described in the foregoing, according to the
forty-seventh aspect, an angle-modulated signal is converted into
an optical signal and branched into a plurality of optical signals,
a part of the optical signals are subjected to homodyne detection
by the interference portion and the first optical/electrical
converting portion to reproduce the original electrical signal for
the angle modulation as described in the first aspect and the
remained part of the optical signals are subjected to direct
detection by the second optical/electrical converting portion to
reproduce the angle-modulated signal. Thereby, if a wired network
is constructed by using an optical fiber as its backbone and the
angle-modulated signal outputted from the second optical/electrical
converting portion is sent out in the air as a radio wave, the
optical transmission system can expand to a wireless network for
mobile terminals and the like. Especially, a high-frequency signal
such as a micro wave, a millimetre wave and the like, which is
thought as an suitable signal for a wireless network, is received
and subjected to demodulation, in a wired system, by a low cost
configuration with optical signal processing and at the same time a
radio wave is sent to the mobile terminals and the like, so that a
flexible and greatly economical system can be constructed.
[0161] A forty-eighth aspect is an aspect according to the
forty-seventh aspect, further comprising:
[0162] a local light source for outputting a light of a
predetermined wavelength; and
[0163] an optical combining portion, inserted between the optical
branch portion and the second optical/electrical converting
portion, for combining the second optical-modulated signal
outputted from the optical branch portion and the light from the
local light source,
[0164] wherein the second optical/electrical converting portion
heterodyne detects the combined optical signal outputted from the
optical combining portion and then converts the optical signal into
an electrical signal.
[0165] A forty-ninth aspect is an aspect according to the
forty-seventh aspect, further comprising:
[0166] a local light source for outputting a light of a
predetermined wavelength; and
[0167] an optical combining portion, inserted between the optical
modulating portion and the optical branch portion, for combining
the optical-modulated signal outputted from the optical modulating
portion and the light from the local light source,
[0168] wherein the second optical/electrical converting portion
heterodyne detects the second optical-modulated signal outputted
from the optical branch portion and converts the optical-modulated
signal into an electrical signal.
[0169] As described in the foregoing, according to the forty-eighth
and forty-ninth aspects, the frequency of the local light source is
varied, to freely up-convert or down-convert the frequency of the
angle-modulated signal outputted from the second optical/electrical
converting portion.
[0170] A fiftieth aspect is an optical transmission system for
optically transmitting an angle-modulated signal, comprising:
[0171] an optical modulating portion for converting the
angle-modulated signal into an optical-modulated signal;
[0172] a local light source for outputting a light of a
predetermined wavelength;
[0173] an optical combining portion for combining the
optical-modulated signal outputted from the optical modulating
portion and the light from the local light source;
[0174] an interference portion for separating the combined optical
signal outputted from the optical combining portion into a
plurality of optical signals having predetermined difference in
propagation delay and then combining the optical signals;
[0175] an optical/electrical converting portion, having
square-law-detection characteristics, for converting the combined
optical signal outputted from the interference portion into an
electrical signal; and
[0176] a dividing portion for separating the electrical signal
outputted from the optical/electrical converting portion for each
of frequency components and outputting the electrical signals.
[0177] A fifty-first aspect is an optical transmission system for
optically transmitting an angle-modulated signal, comprising:
[0178] an optical modulating portion for converting the
angle-modulated signal into an optical-modulated signal;
[0179] an optical branch portion for branching the
optical-modulated signal outputted from the optical modulating
portion into two signals at least, a first optical-modulated signal
and a second optical-modulated signal;
[0180] an interference portion for separating the first
optical-modulated signal outputted from the optical branch portion
into a plurality of optical signals having predetermined difference
in propagation delay and then combining the optical signals;
[0181] a first optical/electrical converting portion, having
square-law-detection characteristics, for converting the combined
optical signal outputted from the interference portion into an
electrical signal;
[0182] a local oscillation portion for outputting an unmodulated
signal of a predetermined frequency; and
[0183] a second optical/electrical converting portion, having
square-law-detection characteristics, in which its bias is
modulated with the unmodulated signal from the local oscillation
portion, for converting the second optical-modulated signal
outputted from the optical branch portion into an electrical
signal.
[0184] A fifty-second aspect is an optical transmission system for
optically transmitting an angle-modulated signal, comprising:
[0185] an optical modulating portion for converting the
angle-modulated signal into an optical-modulated signal;
[0186] an optical branch portion for branching the
optical-modulated signal outputted from the optical modulating
portion into two signals at least, a first optical-modulated signal
and a second optical-modulated signal;
[0187] an interference portion for separating the first
optical-modulated signal outputted from the optical branch portion
into a plurality of optical signals having predetermined difference
in propagation delay and then combining the optical signals;
[0188] a first optical/electrical converting portion, having
square-law-detection characteristics, for converting the combined
optical signal outputted from the interference portion into an
electrical signal;
[0189] a second optical/electrical converting portion, having
square-law-detection characteristics, for converting the second
optical-modulated signal outputted from the optical branch portion
into an electrical signal;
[0190] a local oscillation portion for outputting an unmodulated
signal of a predetermined frequency; and
[0191] a mixing portion for mixing the electrical signal outputted
from the second optical/electrical converting portion and the
unmodulated signal outputted from the local oscillation portion and
outputting the resultant signals.
[0192] As described in the foregoing, according to the fiftieth to
fifty-second aspects, the original electrical signal and the
angle-modulated signal for angle modulation can be reproduced only
by optical signal processing. Further, the frequency of the local
light source or the local oscillation portion is varied, to freely
up-convert or down-convert the frequency of the angle-modulated
signal to be reproduced.
[0193] A fifty-third aspect is an optical transmission system for
optically transmitting two signals at least, a first electrical
signal and a second electrical signal simultaneously,
comprising:
[0194] an angle modulating portion for converting the first
electrical signal into an angle-modulated signal;
[0195] a combining portion for combining the angle-modulated signal
and the second electrical signal;
[0196] an optical modulating portion for converting the combined
signal outputted from the combining portion into an
optical-modulated signal;
[0197] an optical branch portion for branching the
optical-modulated signal outputted from the optical modulating
portion into two signals at least, a first optical-modulated signal
and a second optical-modulated signal;
[0198] an interference portion for separating the first
optical-modulated signal outputted from the optical branch portion
into a plurality of optical signals having predetermined difference
in propagation delay and then combining the optical signals;
[0199] a first optical/electrical converting portion, having
square-law-detection characteristics, for converting the combined
optical signal outputted from the interference portion into an
electrical signal; and
[0200] a second optical/electrical converting portion, having
square-law-detection characteristics, for converting the second
optical-modulated signal outputted from the optical branch portion
into an electrical signal.
[0201] As described in the foregoing, according to the fifty-third
aspect, a digital signal and an analog signal, for example, which
are different types of electrical signals, can be optically
transmitted simultaneously and individually reproduced.
[0202] A fifty-fourth aspect is an aspect according to the
fifty-third aspect, wherein an occupied frequency band of the first
electrical signal, an occupied frequency band of the second
electrical signal and an occupied frequency band of the
angle-modulated signal do not overlap with each other.
[0203] A fifty-fifth aspect is an aspect according to the
fifty-third aspect, further comprising:
[0204] a first signal processing portion for limiting the occupied
frequency band of the first electrical signal; and
[0205] a second signal processing portion for limiting the occupied
frequency band of the second electrical signal.
[0206] A fifty-sixth aspect is an aspect according to the
fifty-fifth aspect, further comprising:
[0207] a third signal processing portion for passing only a
frequency component corresponding to the occupied frequency band of
the first electrical signal as to the electrical signal outputted
from the first optical/electrical converting portion and
reproducing waveform information which was lost by the band
limitation in the first signal processing portion; and
[0208] a fourth signal processing portion for passing only a
frequency component corresponding to the occupied frequency band of
the second electrical signal as to the electrical signal outputted
from the second optical/electrical converting portion and
reproducing waveform information which was lost by the band
limitation in the second signal processing portion.
[0209] As described in the foregoing, according to the fifty-sixth
aspect, the waveform distortion caused by the band limitation
performed on the transmitting side can be corrected on the
receiving side.
[0210] A fifty-seventh aspect is an optical transmission system for
optically transmitting a plurality of electrical signals,
comprising:
[0211] a plurality of angle modulating portions for converting each
of the plurality of electrical signals into an angle-modulated
signals;
[0212] a combining portion for combining the angle-modulated
signals outputted from the plurality of angle modulating
portions;
[0213] an optical modulating portion for converting the combined
signal outputted from the combining portion into an
optical-modulated signal;
[0214] an optical branch portion for branching the
optical-modulated signal outputted from the optical modulating
portion into a plurality of optical-modulated signals; and
[0215] an plurality of optical signal processing portions, provided
corresponding to the plurality of optical-modulated signals
outputted from the optical branch portion respectively, for each
performing predetermined optical signal processing and then
individually reproducing the plurality of electrical signals,
and
[0216] each of the optical signal processing portions
including:
[0217] an interference portion for separating the optical-modulated
signal outputted from the optical branch portion into a plurality
of optical signals having difference in propagation delay decided
according to frequencies of angle-modulated signals to be acquired
by demodulation and then combining the optical signals; and
[0218] an optical/electrical converting portion, having
square-law-detection characteristics, for converting the combined
optical signal outputted from the interference portion into an
electrical signal.
[0219] As described in the above, according to the fifty-seventh
aspect, a digital signal and an analog signal, for example, which
are different types of electrical signals, can be optically
transmitted simultaneously and individually reproduced.
[0220] A fifty-eighth aspect is an aspect according to the
fifty-seventh aspect, wherein occupied frequency bands of the
plurality of electrical signals and occupied frequency bands of the
plurality of angle-modulated signals do not overlap with each
other.
[0221] A fifty-ninth aspect is an aspect according to the
fifty-seventh aspect, further comprising a plurality of signal
pre-processing portions for limiting the occupied frequency bands
of the plurality of electrical signals.
[0222] A sixtieth aspect is an aspect according to the fifty-ninth
aspect, wherein each of the plurality of optical signal processing
portions further includes a signal post-processing portion for
passing a frequency component corresponding to an occupied
frequency band of an electrical signal to be reproduced and
reproducing waveform information which was lost by the band
limitation in the signal pre-processing portion as to the
electrical signal outputted from the optical/electrical converting
portion.
[0223] As described in the foregoing, according to the sixtieth
aspect, the waveform distortion caused by the band limitation
performed on the transmitting side can be corrected on the
receiving side.
[0224] A sixty-first aspect is an optical transmission system for
optically transmitting a multichannel angle-modulated signal
obtained by subjecting plurality-channel electrical signals to
angle modulation respectively and frequency-division multiplexing,
comprising:
[0225] an optical modulating portion for converting the
multichannel angle-modulated signal into an optical-modulated
signal;
[0226] an optical branch portion for branching the
optical-modulated signal outputted from the optical modulating
portion into a plurality of optical-modulated signals; and
[0227] a plurality of optical signal processing portions, provided
corresponding to the plurality of optical-modulated signals
outputted from the optical branch portion respectively, for each
performing predetermined optical signal processing and then
reproducing an electrical signal on an individual channel, and
[0228] each of the optical signal processing portions
including:
[0229] an interference portion for separating the optical-modulated
signal outputted from the optical branch portion into a plurality
of optical signals having difference in propagation delay decided
according to frequencies of electrical signals on channels to be
reproduced and then combining the optical signals; and
[0230] an optical/electrical converting portion, having
square-law-detection characteristics, for converting the combined
optical signal outputted from the interference portion into an
electrical signal.
[0231] As described in the foregoing, according to the sixty-first
aspect, the multichannel angle-modulated signal obtained by
frequency-division-multiplexing can be optically transmitted
simultaneously.
[0232] These and other objects, features, aspects and advantages of
the present invention will become more apparent from the following
detailed description of the present invention when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0233] FIG. 1 is a block diagram showing the configuration of an
optical transmission system according to a first embodiment of the
present invention.
[0234] FIG. 2 is a block diagram showing a first specific example
of configuration of the optical transmission system according to
the first embodiment of the present invention.
[0235] FIGS. 3a to 3c are diagrams for explaining FM demodulation
operation in the optical transmission system in FIG. 2.
[0236] FIG. 4 is a block diagram showing a first operational
example of the optical transmission system in FIG. 2.
[0237] FIG. 5 is a block diagram showing a second operational
example of the optical transmission system in FIG. 2.
[0238] FIG. 6 is a block diagram showing a second specific example
of configuration of the optical transmission system according to
the first embodiment of the present invention.
[0239] FIGS. 7a to 7c are diagrams for explaining FM demodulation
operation in the optical transmission system in FIG. 6.
[0240] FIG. 8 is a bock diagram showing a third specific example of
configuration of the optical transmission system according to the
first embodiment of the present invention.
[0241] FIG. 9 is a block diagram showing a first operational
example of the optical transmission system in FIG. 8.
[0242] FIG. 10 is a block diagram showing a second operational
example of the optical transmission system in FIG. 8.
[0243] FIG. 11 is a block diagram showing an example of the
structure of an optical receiver used in a system for optically
transmitting a QPSK modulated signal.
[0244] FIG. 12 is a block diagram showing a fourth specific example
of configuration of the optical transmission system according to
the first embodiment of the present invention.
[0245] FIG. 13 is a block diagram showing a fifth specific example
of configuration of the optical transmission system according to
the first embodiment of the present invention.
[0246] FIGS. 14a to 14d are diagrams for explaining FM demodulation
operation in the optical transmission system in FIG. 13.
[0247] FIG. 15 is a block diagram showing a sixth specific example
of configuration of the optical transmission system according to
the first embodiment of the present invention.
[0248] FIGS. 16a to 16d are diagrams for explaining FM demodulation
operation in the optical transmission system in FIG. 15.
[0249] FIG. 17 is a block diagram showing the configuration of an
optical transmission system according to a second embodiment of the
present invention.
[0250] FIG. 18 is a block diagram showing the configuration of an
optical transmission system according to a third embodiment of the
present invention.
[0251] FIG. 19 is a block diagram showing the configuration of an
optical transmission system according to a fourth embodiment of the
present invention.
[0252] FIG. 20 is a block diagram showing the configuration of an
optical transmission system according to a fifth embodiment of the
present invention.
[0253] FIG. 21 is a block diagram showing the configuration of an
optical transmission system according to a sixth embodiment of the
present invention.
[0254] FIG. 22 is a block diagram showing the configuration of an
optical transmission system according to a seventh embodiment of
the present invention.
[0255] FIG. 23 is a block diagram showing the configuration of an
optical transmission system according to an eighth embodiment of
the present invention.
[0256] FIG. 24 is a block diagram showing the configuration of an
optical transmission system according to a ninth embodiment of the
present invention.
[0257] FIG. 25 is a block diagram showing the configuration of an
optical transmission system according to a tenth embodiment of the
present invention.
[0258] FIG. 26 is a block diagram showing the configuration of an
optical transmission system according to an eleventh embodiment of
the present invention.
[0259] FIG. 27 is a block diagram showing the configuration of an
optical transmission system according to a twelfth embodiment of
the present invention.
[0260] FIG. 28 is a block diagram showing the configuration of an
optical transmission system according to a thirteenth embodiment of
the present invention.
[0261] FIG. 29 is a block diagram showing the configuration of an
optical transmission system according to a fourteenth embodiment of
the present invention.
[0262] FIG. 30 is a block diagram showing the configuration of a
conventional optical transmission system.
[0263] FIG. 31 is a block diagram showing the structure of an angle
demodulating portion in FIG. 30.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0264] (First Embodiment)
[0265] FIG. 1 is a block diagram showing the configuration of an
optical transmission system according to a first embodiment of the
present invention. FIG. 1 also shows schematic diagrams of
frequency spectrums of signals in respective portions. In FIG. 1,
the optical transmission system of the present embodiment includes
an angle modulating portion 1, an optical modulating portion 2, a
first optical waveguide portion 3, an interference portion 6, a
second optical waveguide portion 7, an optical/electrical
converting portion 4 and a filter F. Depending on the required
structure of a transmission side and a receiving side from the view
point of the whole system, the first and second optical waveguide
portions 3 and 7 are both needed in some cases, or either one of
the optical waveguide portions is needed in other cases.
[0266] It is to be noted in FIG. 1 that the angle demodulating
portion 5 shown in FIG. 30 is not provided. That is, the present
embodiment is characterized by that a demodulated signal can be
acquired with performing a new and peculiar optical signal
processing without performing electrical demodulation
processing.
[0267] Next, the operation of the embodiment shown in FIG. 1 will
be explained. The angle modulating portion 1 receives an analog
signal such as an audio signal, a video signal and the like, or a
digital signal such as computer data and the like as an electrical
signal to be transmitted and outputs an angle-modulated signal
originated from the above-mentioned signal. The optical modulating
portion 2 receives the angle-modulated signal outputted from the
angle modulating portion 1 and outputs an
optical-intensity-modulated signal with, for example, a direct
modulation scheme, or outputs an optical-intensity-modulated signal
or an optical-amplitude-modulated signal with an external
modulation scheme. The optical signal is transmitted through the
first optical waveguide portion 3. The interference portion 6
separates the inputted optical signal into two optical signals
having predetermined difference in propagation delay and then
combines the optical signals again. The combined optical signal is
transmitted through the second optical waveguide portion 7. The
optical/electrical converting portion 4, which includes a
photodetector having square-law-detection characteristics (a pin
photo-diode, an avalanche photo-diode or the like), re-converts the
inputted combined optical signal into an electrical signal and
re-generates an electrical signal (an analog signal or a digital
signal) correlating with the original electrical signal with angle
demodulation operation to output the electrical signal. The filter
F passes only a signal component corresponding to the original
electrical signal (that is, a signal component of the same
frequency band as that of the original electrical signal) among
signals outputted from the optical/electrical converting portion 4.
Described below are more specific examples of configuration of the
present embodiment.
[0268] (1) First Specific Example of Configuration in the First
Embodiment
[0269] FIG. 2 is a block diagram showing a first specific example
of configuration of the optical transmission system according to
the first embodiment of the present invention. In FIG. 2, the
optical transmission system of the present example of configuration
includes an FM portion 100 as an example of the angle modulating
portion 1. The optical modulating portion 2 includes a light source
201, a first optical branch portion 202, a first optical phase
modulating portion 203, a second optical phase modulating portion
204 and an optical coupling portion 205. The interference portion 6
includes a second optical branch portion 601, an optical delay
portion 602 and an optical combining portion 603.
[0270] Next, the operation of the specific example shown in FIG. 2
will be explained below. To the FM portion 100 is inputted an
electrical signal with a high frequency and a wide-band, for
example, a multichannel frequency-division-multiplexed signal and
the like as an original signal to be subjected to FM. The FM
portion 100 converts the inputted electrical signal into an FM
signal with a predetermined frequency. The optical modulating
portion 2 of the present configuration example has the
configuration of an external modulation scheme and the light source
201 outputs an unmodulated light. The first optical branch portion
202 branches the unmodulated light outputted from the light source
201 into two. The first and second optical phase modulating
portions 203 and 204, which are provided for each of the two lights
outputted from the first optical branch portion 202, perform
predetermined optical phase modulation with the FM signals
outputted from the FM portion 100. The optical-phase-modulated
signals outputted from the first and second optical phase
modulating portions 203 and 204 are each combined in the optical
coupling portion 205 to be converted into an
optical-amplitude-modulated signal. The optical-amplitude-modulated
signal is inputted to the interference portion 6 through the first
optical waveguide portion 3. In the interference portion 6, the
second optical branch portion 601 branches the inputted optical
signal into first and second optical signals. The optical delay
portion 602 provides the second optical signal outputted from the
optical branch portion 601 with a predetermined delay T.sub.p. The
optical combining portion 603 combines the first optical signal
outputted from the second optical branch portion 601 and the second
optical signal outputted from the optical delay portion 602 to
output the resultant signal to the second optical waveguide portion
7. The optical/electrical converting potion 4 creates a product of
the first and second optical signals transmitted through the second
optical waveguide portion 7 (such operation is commonly called as
homodyne detection).
[0271] Description will be made of operation of the first specific
example below using equations. It is assumed that as to the first
and second optical-amplitude-modulated signals outputted from the
second optical branch portion 601, an electric field component
Ea(t) of the first optical signal is expressed by the following
equation (1) and an electric field component Eb(t) of the second
optical signal passing through the optical delay portion 602 is
expressed by the following equation (2), respectively.
Ea(t)=m cos(2.pi.f.sub.tt).times.cos(2.pi.f.sub.0t) (1)
Eb(t)=-m
cos{2.pi.f.sub.t(t-T.sub.p)}.times.cos{2.pi.f.sub.0(t-T.sub.p)}
(2)
[0272] In the above-described equations (1) and (2), m represents
an amplitude in the electric field, f.sub.t represents an
(instantaneous) frequency of the FM signal, f.sub.0 represents an
optical frequency and T.sub.p represents the predetermined delay in
the optical delay portion 602. After these are combined to be
subjected to square-law-detection in the optical/electrical
converting portion 4, an optical current I.sub.0(t) after the
square-law-detection is expressed by the following equation (3). 1
I 0 ( t ) = 1 2 [ m 2 cos 2 ( 2 f t t ) .times. cos 2 ( 2 f 0 t ) +
m 2 cos 2 { 2 f t ( t - T p ) } .times. cos 2 { 2 f 0 ( t - T p ) }
- 2 m 2 cos ( 2 f t t ) .times. cos { 2 f t ( t - T p ) } .times.
cos { 2 f 0 t } .times. cos { 2 f 0 ( t - T p ) } ] ( 3 )
[0273] Considering that, in the above equation (3), signals
corresponding to the terms of periodic functions depending on the
optical frequency f.sub.0 are not outputted due to a frequency
response limit in the optical/electrical converting portion 4, a
signal component I.sub.s(t) derived from the optical/electrical
converting portion 4 is expressed by the following equation (4) by
expanding only a third term. 2 I s ( t ) = - m 2 cos ( 2 f t t )
.times. cos { 2 f t ( t - T p ) } .times. cos ( 2 f 0 t ) .times.
cos { 2 f 0 ( t - T p ) } ( 4 ) = - m 2 cos { 2 f t ( 2 t - T p ) }
+ cos ( 2 f t T p ) 2 .times. cos { 2 f 0 ( 2 t - T p ) } + cos ( 2
f 0 T p ) 2 } ( 4 a ) = - m 2 4 [ cos { 2 f t ( 2 t - T p ) }
.times. cos { 2 f 0 ( 2 t - T p ) } + cos { 2 f t ( 2 t - T p ) }
.times. cos ( 2 f 0 T p ) + cos { 2 f 0 ( 2 t - T p ) } .times. cos
( 2 f t T p ) + cos ( 2 f t T p ) .times. cos ( 2 f 0 T p ) ] } ( 4
b )
[0274] Since m, f.sub.0, T.sub.p are constant values, the magnitude
of the fourth term in the second expanded equation (4b) of the
equation (4) is changed depending on the instantaneous frequency
f.sub.t of the FM signal. That is, it is possible to derive an
optical current whose magnitude is changed according to variations
in frequency of the FM signal.
[0275] While, in the first specific example, the optical modulating
portion 2 outputs an optical-amplitude-modulated signal, the
optical modulating portion 2 may output an
optical-intensity-modulated signal. The operation of this case will
be described below using equations. As is the case with the above
description, as for the first and second
optical-intensity-modulated signals outputted from the second
optical branch portion 601, the electric field component Ea(t) of
the first optical signal is expressed by the following equation (5)
and the electric field component Eb(t) of the second optical signal
passing through the optical delay portion 602 is expressed by the
following equation (6), respectively. 3 Ea ( t ) = 1 + m cos ( 2 f
t t ) .times. cos ( 2 f 0 t ) { 1 + m 2 cos ( 2 f t t ) } .times.
cos { 2 f 0 t ) ( 5 ) Eb ( t ) = 1 - m cos { 2 f t ( t - T p ) }
.times. cos { 2 f 0 ( t - T p ) } { 1 - m 2 cos { 2 f t ( t - T p )
} .times. cos { 2 f 0 ( t - T p ) } ( 6 )
[0276] The optical current I.sub.0(t) obtained by subjecting the
combined signal to square-law-detection in the optical/electrical
converting portion 4 is expressed by the following equation (7). 4
I 0 ( t ) = 1 2 ( { 1 + m 2 cos ( 2 f t t ) } 2 .times. cos 2 ( 2 f
0 t ) + [ 1 - m 2 cos { 2 f t ( t - T p ) } ] 2 .times. cos 2 { 2 f
0 ( t - T p ) } + 2 { 1 + m 2 cos ( 2 f t t ) } .times. [ 1 - m 2
cos { 2 f t ( t - T p ) } ] .times. cos { 2 f 0 t } .times. cos { 2
f 0 ( t - T p ) } ) ( 7 )
[0277] Considering that, in the above equation (7), signals
corresponding to the terms of periodic functions depending on the
optical frequency f.sub.0 are not outputted due to the frequency
response limit of the optical/electrical converting portion 4, the
signal component I.sub.s(t) derived from the optical/electrical
converting portion 4 is expressed by the following equation (8). 5
I s ( t ) = - m 2 4 cos ( 2 f t t ) .times. cos { 2 f t ( t - T p )
} .times. cos ( 2 f 0 t ) .times. cos { 2 f 0 ( t - T p ) } ( 8 ) =
- m 2 4 cos { 2 f t ( 2 t - T p ) } + cos ( 2 f t T p ) 2 .times.
cos { 2 f 0 ( 2 t - T p ) } + cos ( 2 f 0 T p ) 2 } ( 8 a ) = - m 2
16 [ cos { 2 f t ( 2 t - T p ) } .times. cos { 2 f 0 ( 2 t - T p )
} + cos { 2 f t ( 2 t - T p ) } .times. cos ( 2 f 0 T p ) + cos { 2
f 0 ( 2 t - T p ) } .times. cos ( 2 f t T p ) + cos ( 2 f t T p )
.times. cos ( 2 f 0 T p ) ] } ( 8 b )
[0278] It is found that in a second expanded equation (8b) of the
above equation (8), the magnitude of the fourth term is changed
depending on the instantaneous frequency f.sub.t of the FM signal,
making it possible to derive an optical current whose magnitude is
changed according to variations in frequency of the FM signal as in
the case with the second expanded equation (4b) of the
above-described equation (4).
[0279] FIGS. 3a and 3b schematically show amplitude fluctuation
components (or intensity fluctuation components) of electric fields
of the first and second optical signals, respectively. FIG. 3c
shows a waveform of an optical current, which is outputted from the
optical/electrical converting portion 4, corresponding to the
amplitude fluctuation components or intensity fluctuation
component. As shown in FIG. 3c, the optical/electrical converting
portion 4 outputs a pulse-like signal comprising of negative
differential pulses. Each pulse duration of each differential pulse
is constant corresponding to the predetermined delay T.sub.p in the
optical delay portion 602, and occurrence intervals of the
differential pulses correspond to the variations in frequency of
the FM signal outputted from the FM portion 100. The filter F
receives the pulse-like signal to pass only a signal component (a
low-frequency component) of a band corresponding to that of the
electrical signal inputted to the FM portion 100. In this way, the
electrical signal can be obtained.
[0280] FIG. 4 is a diagram showing a first operational example of
the optical transmission system in FIG. 2. In FIG. 4, according to
the present operational example, the FM portion 100, the optical
modulating potion 2, the optical waveguide portion 3 and the
interference portion 6 constitute an optical transmitter PT, and
the optical/electrical converting portion 4 constitutes an optical
receiver PR. Further, an optical transmission medium such as an
optical fiber and the like is used as the second optical waveguide
portion 7 to expand a physical distance between the optical
transmitter PT and the optical receiver PR.
[0281] In the operational example in FIG. 4, the first and second
optical signals transmitted through the second optical waveguide
portion 7 (the optical fiber) are both created from the light
source 201, so that the optical wavelengths of the optical signals
are the same. Therefore, even when not a special optical fiber
having polarization maintaining features but a normal single-mode
optical fiber is employed as the second optical waveguide portion
7, the relative polarization states of the two optical signals
outputted from the optical combining portion 603 can be always
maintained constant even after the optical signals are transmitted
through the second optical waveguide portion 7. Accordingly, the
two optical signals are adjusted so that the polarization states of
the optical signals become the same, and then inputted to the
optical combining portion 603, thereby enabling the polarization
states of the two optical signals to be maintained the same even
after the optical signals are transmitted through the second
optical waveguide portion 7. As a result, homodyne efficiency in
the optical/electrical converting portion 4 reaches its maximum,
which makes it possible to realize high FM demodulation efficiency
with high stability.
[0282] As described in the above, in the operational example in
FIG. 4, a constituent required for the optical receiver PR is only
the optical/electrical converting potion 4 which is relatively
inexpensive, and expensive parts are all accommodated in the
optical transmitter PT. Accordingly, the present configuration can
provide the optical receiver PR (the receiving terminal) at low
costs and especially in the case of an optical distribution system,
the system cost is reduced to construct the greatly economical
system.
[0283] FIG. 5 is a diagram showing a second operational example of
the optical transmission system in FIG. 2. In FIG. 5, according to
the present operational example, the FM portion 100 and the optical
modulating portion 2 constitute the optical transmitter PT, and the
interference portion 6 and the optical/electrical converting
portion 4 constitute the optical receiver PR. Further, an optical
transmission medium such as an optical fiber and the like is used
as the first optical waveguide portion 3 to expand the physical
distance between the optical transmitter PT and the optical
receiver PR. The operational example in FIG. 5 has a feature that
the optical receiver PR can be constituted by relatively
inexpensive parts (since an electric demodulation circuit is not
required) and especially in the case of an optical distribution
system, the system cost is reduced to construct the greatly
economical system, although the feature is not so remarkable as
that of the operational example in FIG. 4.
[0284] (2) Second Specific Example of Configuration in the First
Embodiment
[0285] FIG. 6 is a block diagram showing a second specific example
of configuration of the optical transmission system according to
the first embodiment of the present invention. In FIG. 6, the
optical transmission system of the present specific example
includes an optical directional coupling portion 206 in place of
the optical coupling portion 205, the first optical waveguide
portion 3 and the second optical branch portion 601 in the first
operational example of the first specific example (refer to FIG.
4), and the other configuration is the same as that in FIG. 4.
Accordingly, description will be made of the operation below with
an emphasis on the difference from the first operational example of
the first specific example.
[0286] In the second specific example, the optical directional
coupling portion 206 combines the optical-phase-modulated signals
outputted from the first and second optical phase modulating
portions 203 and 204 to convert the resultant signal into an
optical-amplitude-modulated signal and then branches the
optical-amplitude-modulated signal into first and second optical
signals that have optical-modulated components being set in
opposite phases to each other. In this case, as shown in FIG. 7c, a
waveform of an optical current outputted from the
optical/electrical converting portion 4 becomes a pulse-like signal
being in opposite phase with respect to that of the first
operational example (refer to FIG. 3c), and the number of
occurrence of positive differential pluses included in the
pulse-like signal uniquely corresponds to the variations in
frequency of the FM signal. Accordingly, the pulse-like signal is
inputted to the filter F, whereby only a signal component of a band
(a low-frequency component) corresponding to that of an electrical
signal inputted to the FM portion 100 is derived and as a result,
the electrical signal can be acquired. Since equations of the
operation are the same as those of the first operational example
except that the phases of the signal waveforms are different,
description of the equations is omitted here.
[0287] In the second specific example, the FM portion 100, the
optical modulating portion 2 and the interference portion 6
constitute the optical transmitter PT, and the optical/electrical
converting portion 4 constitutes the optical receiver PR. Further,
an optical transmission medium such as an optical fiber is used as
the second optical waveguide portion 7 to expand the physical
distance between the optical transmitter PT and the optical
receiver PR.
[0288] As described above, the optical transmission system in the
second specific example requires as a constituent of the optical
receiver PR only the optical/electrical converting portion which is
relatively inexpensive, as in the case with the operational example
in FIG. 4, and expensive parts are all accommodated in the optical
transmitter PT. Accordingly, it is possible to provide the optical
receiver PR (the receiving terminal) at low costs and especially in
the case of an optical distribution system, the system cost is
reduced, to construct the greatly economical system.
[0289] (3) Third Specific Example of Configuration in the First
Embodiment
[0290] FIG. 8 is a block diagram showing a third specific example
of configuration of the optical transmission system according to
the first embodiment of the present invention. In FIG. 8, the
optical transmission system of the present specific example
includes first and second optical transparent/reflecting portions
606 and 607 and optical waveguide portion 605 in place of the first
and second optical waveguide portions 3 and 7, the second optical
branch portion 601, the optical delay portion 602 and the optical
combining portion 603 in the first specific example (refer to FIG.
2), and the other configuration is the same as that in FIG. 2.
Therefore, the operation will be explained below with an emphasis
on the difference from the first specific example.
[0291] In the third specific example, an optical signal outputted
from the optical coupling portion 205 is guided through the optical
waveguide portion 605 to the optical/electrical converting portion
4. The first and second optical transparent/reflecting portions 606
and 607 are cascaded on the optical waveguide portion 605 at a
prescribed interval. As shown in FIG. 8, a part of the optical
signal outputted from the optical coupling portion 205 is
transmitted through the first optical transparent/reflecting
portion 606 and then through the second optical
transparent/reflecting portion 607 and reaches the
optical/electrical converting portion 4 (such optical signal is
referred to as a direct light, hereinafter). Another part of the
optical signal outputted from the optical coupling portion 205 is
transmitted through the first optical transparent/reflecting
portion 606, reflected at the second optical
transmitting/reflecting portion 607, further reflected at the first
optical transparent/reflecting portion 606, transmitted through the
second optical transparent/reflecting portion 607 and reaches the
optical/electrical converting portion 4 (such optical signal is
referred; to as an indirect light, hereinafter). The
optical/electrical converting portion 4 subjects the direct light
and the indirect light to homodyne detection with the
square-law-detection characteristics and creates a product of the
two lights. Propagation time in which the indirect light goes and
returns between the first and second optical transparent/reflecting
portions 606 and 607 installed at the predetermined interval
(hereinafter referred to as round-trip propagation time)
corresponds to the delay T.sub.p in the optical delay portion 602
in the first and second specific examples.
[0292] FIG. 9 is a block diagram showing a first operational
example of the optical transmission system in FIG. 8. In FIG. 9,
according to the present operational example, the FM portion 100,
the optical modulating portion 2 and the interference portion 6
constitute the optical transmitter PT, and the optical/electrical
converting portion 4 constitutes the optical receiver PR. An
optical transmission medium such as an optical fiber is used as the
optical waveguide portion 605 to expand the physical distance
between the optical transmitter PT and the optical receiver PR.
That is, in the present operational example, the optical waveguide
portion 605 functions as the second optical waveguide portion 7 in
FIG. 1 as well.
[0293] In the operational example in FIG. 9, the direct light and
the indirect light transmitted through the optical waveguide
portion 605 (the optical fiber) are both created from the light
source 201, so that the optical wavelengths are the same.
Therefore, even when not a special optical fiber having
polarization maintaining features but a normal single-mode optical
fiber is employed as the optical waveguide portion 605, relative
polarization states of the direct light and the indirect light
outputted from the second optical transparent/reflecting portion
607 can be always maintained constant even while the lights are
transmitted through the optical waveguide portion 605. Accordingly,
the polarization states of the two lights are maintained the same,
if the polarization states in the first and second optical
transparent/reflecting portions 606 and 607 are adjusted so as to
be the same, even after the lights are transmitted through the
optical waveguide portion 605. As a result, the homodyne efficiency
in the optical/electrical converting portion 4 reaches its maximum,
thereby making it possible to realize high FM demodulation
efficiency with high stability.
[0294] As described in the above, in the operational example in
FIG. 9, a constituent required for the optical receiver PR is only
the optical/electrical converting potion 4 which is relatively
inexpensive and expensive parts are all accommodated in the optical
transmitter PT as in the case with the operational example in FIG.
4. Accordingly, the optical receiver PR (the receiving terminal)
can be provided at low costs and especially in the case of an
optical distribution system, the system cost decreases to construct
the greatly economical system.
[0295] FIG. 10 is a block diagram showing a second operational
example of the optical transmission system in FIG. 8. In FIG. 10,
according to the present operational example, the FM portion 100
and the optical modulating portion 2 constitute the optical
transmitter PT, and the interference portion 6 and the
optical/electrical converting portion 4 constitute the optical
receiver PR. Further, an optical transmission medium such as an
optical fiber and the like is used as the optical waveguide portion
605 to expand the physical distance between the optical transmitter
PT and the optical receiver PR. That is, in the present operational
example, the optical waveguide portion 605 functions as the first
optical waveguide portion 3 in FIG. 1 as well.
[0296] As described above, the operational example in FIG. 10 has a
feature that the optical receiver PR can be structured by
relatively inexpensive parts and especially in the case of an
optical distribution system, the system cost decreases to construct
the greatly economical system, although the feature is not so
remarkable as that of the operational example in FIG. 9.
[0297] The variations and operation requirements of the first to
third specific examples (FIGS. 2, 6 and 8) will be explained in
detail below.
[0298] A. About Modulation Schemes
[0299] While the first to third specific examples are configured so
that an analog signal is subjected to FM to be optically
transmitted, the present embodiment can be applied to a system in
which an analog signal is subjected to PM to be optically
transmitted, and in this case, the effect is the same as those of
the specific examples. As to the configuration in this case, it is
only necessary to replace the FM portion 100 with a known PM
portion and the other configuration of the optical transmission
system may be completely the same as those in the first to third
specific examples.
[0300] Moreover, the present embodiment can naturally subject a
digital signal, in place of an analog signal, to frequency
modulation or phase modulation for optical transmission, and in
this case, the effect is the same as those of the specific
examples. As to the configuration of this case, it is only
necessary to replace the FM portion 100 with a known FSK portion or
PSK portion and other configuration may be completely the same as
those in the first to third specific examples.
[0301] B. Definitions of Optical Amplitude Modulation
[0302] In the first to third specific examples, the phase of each
optical phase modulation operation in the first and second optical
phase modulating portions 203 and 204, that is, the phase of each
FM signal inputted to the first and second optical phase modulating
portions 203 and 204 is preferably set to a phase which enlarges an
optical-amplitude-modulated component in the optical signal
combined in the optical coupling portion 205 or the optical
directional coupling portion 206. This will be described below.
[0303] Here, it is assumed that an electric field component
E.sub.1(t) of an optical signal outputted from the first optical
phase modulating portion 203 is expressed by the following equation
(9) and an electric field component E.sub.2(t) of an optical signal
outputted from the second optical phase modulating portion 204 is
expressed by the following equation (10). 6 E 1 ( t ) = E 0 2 cos (
2 f 0 t + d 1 ) ( 9 ) E 2 ( t ) = E 0 2 cos ( 2 f 0 t + d 2 ) ( 10
)
[0304] In the above equations (9) and (10), f.sub.0 is an optical
frequency, d.sub.1 and d.sub.2 are the phase shift by the first and
second optical phase modulating portions 203 and 204, respectively.
After detecting a combined electric field obtained by combining the
electric field components E.sub.1(t) and E.sub.2(t), an optical
current I.sub.0(t) corresponding to the electric field is expressed
by the following equation (11). 7 I 0 ( t ) = I 0 2 { 1 + cos ( d 1
- d 2 ) } ( 11 )
[0305] Here, as expressed by the following equation (12), d.sub.b
and d(t) are introduced as parameters representing relative phases
between d.sub.1 and d.sub.2.
d.sub.b+d(t)=d.sub.1-d.sub.2 (12)
[0306] The above d.sub.b and d(t) correspond to a bias level (a
voltage) and a modulated signal for a Mach-Zehnder type optical
modulator constituted by the first optical branch portion 202, the
first and second optical phase modulating portions 203 and 204, and
the optical coupling portion 205 (or the optical directional
coupling portion 206), respectively.
[0307] When the bias level d.sub.b satisfies the following equation
(13), that is, when phase difference between the first optical
phase modulating portion 203 and the second optical phase
modulating portion 204 is in phase with the FM signal, the above
equation (11) is expressed by the following equation (14). As is
clear from the following equation (14), the optical current
I.sub.0(t) outputted from the optical/electrical converting portion
4 has a component proportional to a square of the modulated signal
d(t) and an optical-amplitude-modulated component is generated.
d.sub.b=0 (13) 8 I 0 ( t ) = I 0 2 [ 2 - 1 2 d ( t ) 2 ] ( 14 )
[0308] When the bias level d.sub.b satisfies the following equation
(15), that is, when the phase difference between the first optical
phase modulating portion 203 and the second optical phase
modulating portion 204 is in opposite phase with the FM signal, the
above equation (11) is expressed by the following equation (16). As
is clear from the following equation (16), the optical current
I.sub.0(t) outputted from the optical/electrical converting portion
4 is proportional to the square of the modulated signal d(t) and an
optical-amplitude-modulated component is generated.
d.sub.b=.pi. (15) 9 I 0 ( t ) = I 0 4 d ( t ) 2 ( 16 )
[0309] As explained above, the phase of each FM signal inputted to
the first and second optical phase modulating portions 203 and 204
is adjusted to an optimal state, which can enlarge an
optical-amplitude-modulated component in the optical signal
outputted from the optical coupling portion 205 or the optical
directional coupling portion 206. As a result, it is possible to
perform efficient FM demodulation.
[0310] As in the above, in the second and third specific examples,
the optical modulating portion 2 is structured so as to convert an
FM signal into an optical-amplitude-modulated signal and output the
optical-amplitude-modulated signal. The optical modulating portion
2, however, may adopt an optical intensity modulation scheme in
place of the optical amplitude modulation as described in the first
specific example, and the operation and effect are almost the same
as those in the above-described specific examples.
[0311] Moreover, while description was made as to the optical
modulating portion 2, mainly to the structure which adopts the
"external optical modulation scheme" using a Mach-Zehnder
interferometer structure in the above specific examples, in the
case where the optical modulating portion 2 uses "optical intensity
modulation" in the above first and third specific examples, it is
also possible to adopt a "direct optical modulation scheme" which
is more popular as an optical modulation scheme, that is a
structure in which an injection current to a semiconductor laser
element is direct modulated with an FM signal. In this case, the
optical transmission system can be configured more readily at lower
costs.
[0312] C. About Delay
[0313] In the above first to third specific examples, the
predetermined delay T.sub.p in the optical delay portion 602 or the
round-trip propagation time T.sub.p between the first and second
optical transparent/reflecting portions 606 and 607 installed at
the predetermined interval is preferably set so as to satisfy the
relation in the following equation (17) with respect to a center
angular frequency .omega..sub.c (=2.pi..multidot.f.sub.c) of the FM
signal. 10 c .times. T p = 4 ( 17 )
[0314] By satisfying the above-described relation, as is clear from
the second expanded equation (4b) of the equation (4) or the second
expanded equation (8b) of the equation (8) shown in the first
specific example, improved are linearity and demodulation
efficiency of the outputted optical current I.sub.0(t) from the
optical/electrical converting portion 4 relative to the
instantaneous frequency f.sub.t of the FM signal centering on the
frequency f.sub.c (=f.sub.c+.DELTA.f(t)). That is, the FM
demodulation characteristics improve to acquire a demodulated
signal with better quality.
[0315] Moreover, while an analog signal is subjected to FM to be
optically transmitted in the first to third specific examples, in
the case where a digital signal is subjected to modulation to be
optically transmitted in place of the analog signal, the
predetermined delay T.sub.p in the optical delay portion 602 or the
round-trip propagation time T.sub.p between the first and second
optical transparent/reflecting portions 606 and 607 installed at
the predetermined interval is preferably set so as to satisfy the
relation in the above equation (17) with respect to the center
frequency f.sub.c of an FSK modulated signal, or the relation in
the following equation (18) with respect to one symbol length
(symbol time) L of the digital signal.
T.sub.p=L (18)
[0316] When the above relation is satisfied, as is clear from the
first expanded equation (4a) of the equation (4) or the first
expanded equation (8a) of the equation (8) shown in the first
specific example, a delayed detection system of the FSK (or PSK)
modulated signal is structured to perform demodulation with higher
efficiency.
[0317] Also, when a digital signal is subjected to phase modulation
to optically transmit a quadrature PSK modulated signal (a QPSK
modulated signal), the interference portion 6 and the
optical/electrical converting portion 4 have preferably a double
parallel structure shown in FIG. 11. In FIG. 11, an optical
dividing portion 608 divides an optical signal outputted from the
optical modulating portion 2 into first and second received lights.
A first optical interference circuit 6a and a first
optical/electrical converting portion 4a perform homodyne detection
for the first received light, and a second optical interference
circuit 6b and a second optical/electrical converting portion 4b
perform homodyne detection for the second received light. A filter
Fa and a filter Fb derive the original digital signal component
from an outputted signal from the first optical/electrical
converting portion 4a and an outputted signal from the second
optical/electrical converting portion 4b, respectively. Further, a
predetermined delay T.sub.1 in the first optical interference
circuit 6a and a predetermined delay T.sub.2 in the second optical
interference circuit 6b are preferably set so as to satisfy the
relations in the following equations (19) and (20), respectively,
with respect to one symbol length (symbol time) L of the digital
signal. 11 T 1 = L 2 ( 19 ) T 2 = - L 2 ( 20 )
[0318] By satisfying the above relations, a delayed detection
system is configured for each of an I signal component and a Q
signal component of the QPSK modulated signal, making it possible
to favorably subject the QPSK modulated signal to demodulation.
[0319] While description was made of the case where the QPSK
modulated signal is optically transmitted in the above, more
generally speaking, when an electrical signal inputted to the angle
modulating portion 1 is a digital signal and a PSK modulated signal
with 2.sup.n-phase (n is a natural number) are outputted in place
of the FM signal, FIG. 11 is constituted by a received light
dividing portion which divides the inputted optical signal into
2.sup.n-1 received lights, first to 2.sup.n-1th interference
portions which are provided for each of the 2.sup.n-1 received
lights, branch each of the received lights into first and second
optical signals, give a predetermined delay to the second optical
signal and then combine the first and second optical signals, and
optical/electrical converting portions provided for each of the
first to 2.sup.n-1th interference portions.
[0320] D. About Polarization States
[0321] In the first to third specific examples, for example, two
optical propagation paths between the second optical branch portion
601 and optical combining portion 603 in FIG. 2, two optical
propagation paths between the optical directional coupling portion
206 and optical combining portion 603 in FIG. 6, or an optical
propagation part existing between the first and second optical
transparent/reflecting portions 606 and 607 on the optical
waveguide portion 605 in FIG. 8 are preferably constituted by an
optical transmission medium capable of maintaining polarization
such as a polarization maintaining fiber, an optical waveguide on
the substrate of crystal or glass and the like. This enables the
two optical signals (the first and second optical signals)
outputted from the second optical branch portion 601 or the optical
directional coupling portion 206 to be combined in the optical
combining portion 603 with the polarization states of the two
optical signals maintained the same. In other case, the
polarization states of the direct light and the indirect light can
be maintained the same in the optical waveguide portion 605 in FIG.
8. Thereby, the homodyne efficiency in the optical/electrical
converting portion 4 becomes always maximum to realize high FM
demodulation efficiency with high stability.
[0322] Further, in the first to third specific examples, the first
optical branch portion 202, the first and second optical phase
modulating portions 203 and 204 and the optical coupling portion
205 (or the optical directional coupling portion 206) are
preferably constructed on a same crystal substrate. Such structure
is the same as that of an optical intensity modulator of normal
Mach-Zehnder type. Adopting such structure makes construction of an
optical transmitter more readily.
[0323] Moreover, in order to downsize the apparatus, the second
optical branch portion 601, the optical delay portion 602 and the
optical combining portion 603 in the first specific example; the
optical delay portion 602 and the optical combining portion 603 in
the second specific example; and a part of the optical waveguide
portion 605 and the first and the second optical
transparent/reflecting portions 606 and 607 in the third specific
example may be constructed on the above-described crystal
substrate.
[0324] Additionally, in the first or second specific example, the
optical propagation paths between the second optical branch portion
601 or the optical directional coupling portion 206 and the optical
combining portion 603 are structured by optical waveguides on the
substrate of crystal or glass, thereby maintaining the polarization
states of the two optical signals to be inputted to the optical
combining portion 603 the same and stable to realize high FM
efficiency with stability.
[0325] Still further, in the third specific example, the optical
propagation paths consist of the optical waveguide portion 605 and
the first and second optical transparent/reflecting portions 606
and 607 are structured by optical waveguides on the substrate of
crystal or glass, whereby the polarization states of the direct
light and the indirect light are maintained the same and stable to
realize high FM efficiency with stability.
[0326] (4) Fourth Specific Example of Configuration in the First
Embodiment
[0327] FIG. 12 is a block diagram showing a fourth specific example
of configuration of the optical transmission system according to
the first embodiment of the present invention. In FIG. 12, in the
optical transmission system of the present specific example, to one
of the two optical propagation paths between the optical
directional coupling portion 206 and the optical combining portion
603 is inserted a polarization adjusting portion 609. The other
configuration of the present specific example is the same as that
of the optical transmission system of the second specific example
(refer to FIG. 6). The operation of the fourth specific example
will be described below with an emphasis on the difference from the
second specific example.
[0328] In the present specific example, a polarization state of one
of two optical signals outputted from the optical directional
coupling portion 206 is adjusted in the polarization adjusting
portion 609 to equate the polarization states of the first and
second optical signals in the optical combining portion 603. This
makes the homodyne efficiency in the optical/electrical converting
portion 4 maximum, which realizes high FM demodulation
efficiency.
[0329] While FIG. 12 shows the case where the polarization
adjusting portion 609 is inserted to the optical propagation path
on the other side of the optical propagation path provided with the
optical delay portion 602, the polarization adjusting portion 609
may be inserted to the optical propagation path provided with the
optical delay portion 602, further, to both of the two optical
propagation paths.
[0330] In addition, also in the first specific example (refer to
FIG. 2), the polarization adjusting portion 609 may be inserted to
both of the two optical propagation paths between the second
optical branch portion 601 and the optical combining portion 603 or
to either one of the optical propagation paths. In this case, the
same effect can be obtained as that of the fourth specific
example.
[0331] (5) Fifth Specific Example of Configuration in the First
Embodiment
[0332] FIG. 13 is a block diagram showing a fifth specific example
of configuration of the optical transmission system according to
the first embodiment of the present invention. In FIG. 13, in the
optical transmission system of the present specific example,
between the FM portion 100 and the first and second optical phase
modulating portions 203 and 204 is additionally inserted an
amplitude adjusting portion 8. The other configuration of the
present specific example is the same as that of the optical
transmission system in the second specific example (refer to FIG.
6). The operation of the fifth specific example will be described
below with an emphasis on the difference from the second specific
example.
[0333] In the present specific example, the amplitude adjusting
portion 8 receives an FM signal outputted from the FM portion 100
and subjects the FM signal to waveform shaping so that the
amplitude is constant to output the FM signal to the first and
second optical phase modulating portions 203 and 204. In the second
specific example, as shown in FIG. 14c, as the amplitude of the FM
signal is smaller, the amplitude of the pulse-like signal outputted
from the optical/electrical converting portion 4 decreases.
Therefore, the FM demodulation efficiency is degraded. In addition,
the amplitude of the FM signal fluctuates with time, causing the FM
demodulation efficiency to vary with time to occur distortion of a
demodulated waveform. Accordingly, by providing the amplitude
adjusting portion 8 as in the present specific example, the
amplitude of the FM signal is maintained constant, making it
possible to suppress the degradation or variation of the FM
demodulation efficiency and the distortion of the demodulated
signal.
[0334] Similarly, in the first or third specific example (refer to
FIG. 2 or FIG. 8), the amplitude adjusting portion 8 may be
additionally inserted between the FM portion 100 and the first and
second optical phase modulating portions 203 and 204. In this case,
the same effect as that in the fifth specific example can be
obtained.
[0335] (6) Sixth Specific Example of Configuration in the First
Embodiment
[0336] FIG. 15 is a block diagram showing a sixth specific example
of configuration of the optical transmission system according to
the first embodiment of the present invention. In FIG. 15, in the
optical transmission system of the present specific example, a
bandwidth limiting portion 9 is additionally inserted between the
FM portion 100 and the first and second optical phase modulating
portions 203 and 204. The other configuration of the present
specific example is the same as that of the optical transmission
system of the second specific example (refer to FIG. 6).
Description will be made of the operation of the sixth specific
example below with an emphasis on the difference from the second
specific example.
[0337] In the present specific example, the bandwidth limiting
portion 9 receives an FM signal outputted from the FM portion 100
and limits an occupied bandwidth of the FM signal to output the FM
signal to the first and second optical phase modulating portions
203 and 204. In the above second specific example, under the
influence of non-linearities in FM demodulation characteristics and
the like, a component of the FM signal passes through the
optical/electrical converting portion 4 to remain in the optical
current outputted from the optical/electrical converting portion 4.
At this time, as shown in FIGS. 16a and 16b, in the case where a
modulation index or the frequency deviation of the FM signal is
large, there is possibility that a spectrum of the FM signal
component remained in the output of the optical/electrical
converting portion 4 spreads out in the frequency band of the
demodulated signal, to interfere the demodulated signal. Therefore,
as in the present specific example, the bandwidth limiting portion
9 is provided to previously eliminate a part of a lower sideband of
the spectrum of the FM signal before the FM signal is inputted to
the first and second optical phase modulating portions 203 and 204.
Thus, it is possible to prevent the spectrum of the FM signal
component remained in the output of the optical/electrical
converting portion 4 from being superimposed on the frequency band
of the demodulated signal, resulting in improvement in the quality
of the demodulated signal.
[0338] Also in the first or third specific example (refer to FIG. 2
or FIG. 8), the bandwidth limiting portion 901 may be additionally
inserted between the FM portion 100 and the first and second
optical phase modulating portions 203 and 204. In this case, the
same effect as that of the sixth specific example can be
achieved.
[0339] (Second Embodiment)
[0340] FIG. 17 is a block diagram showing the configuration of an
optical transmission system according to a second embodiment of the
present invention. FIG. 17 also shows schematic diagrams of
frequency spectrums of signals in respective portions. In FIG. 17,
the optical transmission system of the present embodiment includes
an angle modulating portion 1, an optical modulating portion 2, an
optical waveguide portion 3, an optical branch portion 10, an
interference portion 6, a first optical/electrical converting
portion 4, a second optical/electrical converting portion 4', a
filter F and a filter F', and is different from the first
embodiment (refer to FIG. 1) in that the optical branch portion 10,
the second optical/electrical portion 4' and the filter F' are
added. Therefore, the same reference numbers are assigned to the
portions operating in the same manners as those in the first
embodiment and the detailed description thereof are omitted. The
difference from the first embodiment will be mainly described
below.
[0341] The optical branch portion 10 branches an optical signal (an
optical-intensity-modulated signal or an
optical-amplitude-modulated signal), which is outputted from the
optical modulating portion 2 and then guided by the optical
waveguide portion 3, into two. One optical signal of the two
optical signals is subjected to angle demodulation by the
interference portion 6 and the first optical/electrical converting
portion 4 and further subjected to filtering processing by the
filter F, to be re-converted into an electrical signal
corresponding to an electrical signal inputted to the angle
modulating portion 1. The other optical signal of the two optical
signals, for example, is subjected to square-law detection in the
second optical/electrical converting portion 4'. Thereby, an
optical-intensity-modulated component or an
optical-amplitude-modulated component in the other optical signal
is re-converted into an electrical signal. After that, the signal
outputted from the second optical/electrical portion 4' is
subjected to filtering processing in the filter F', so that the
same angle-modulated signal as an angle-modulated signal outputted
from the angle modulating portion 1 can be derived.
[0342] As described in the foregoing, the optical transmission
system in FIG. 17 converts an angle-modulated signal into an
optical signal to branch the optical signal into a plurality of
optical signals, reproduces an original electrical signal for the
angle modulation from each of some of these optical signals, using
the interference portion 6 and the first optical/electrical
converting portion 4, as described in the first embodiment and
subjects the other of these optical signals respectively to
square-law detection in the second optical/electrical converting
portion 4' to reproduce an angle-modulated signal. This can
construct a wired network using an optical fiber as a backbone and
can also integrate the optical transmission system, if, for
example, the angle-modulated signal outputted from the second
optical/electrical converting portion 4' is sent out in the air as
a radio wave, with a wireless network for mobile terminals and the
like. Especially, in the case of utilizing a high-frequency, a
microwave, a millimetre wave and the like, which is thought as an
effective signal for a wireless network, the angle-modulated signal
is received and subjected to demodulation to be the original
electrical signal by a low cost configuration with optical signal
processing in a wired system and at the same time it is sent to
mobile terminals and the like as a radio wave. Thereby, a flexible
and economical system can be constructed.
[0343] (Third Embodiment)
[0344] FIG. 18 is a block diagram showing the configuration of an
optical transmission system according to a third embodiment of the
present invention. In FIG. 18, the optical transmission system of
the present embodiment includes an angle modulating portion 1, an
optical modulating portion 2, an optical waveguide portion 3, an
optical branch portion 10, an interference portion 6, a first
optical/electrical converting portion 4, a local light source 11,
an optical combining portion 12, a second optical/electrical
converting portion 4', a filter F and a filter F', and is different
from the second embodiment (refer to FIG. 17) in that the local
light source 11 and the optical combining portion 12 are added.
Therefore, the same reference numbers are assigned to the portions
operating in the same manners as those in the second embodiment and
the detailed description thereof is omitted here. The difference
from the second embodiment will be mainly described below.
[0345] One optical signal of two optical signals, which are
obtained by a branch and outputted in/from the optical branch
portion 10, is subjected to angle modulation by the interference
portion 6 and the first optical/electrical converting portion 4,
and further subjected to filtering processing by the filter F, to
be re-converted into an electrical signal corresponding to an
electrical signal inputted to the angle modulating portion 1. The
other optical signal of the two optical signals, which is obtained
by the branch and outputted, is combined with a light outputted
from the local light source 11 by the optical combining portion 12,
to be inputted to the second optical/electrical converting portion
4'. The second optical/electrical converting portion 4' performs
heterodyne detection with the combined two lights. Thereby, from
the second optical/electrical converting portion 4' is outputted a
beat signal of a frequency corresponding to difference in
wavelength between the two lights. The filter F' derives only beat
signal component from the signal outputted from the second
optical/electrical converting portion 4', to output the beat signal
component.
[0346] (Fourth Embodiment)
[0347] FIG. 19 is a block diagram showing the configuration of an
optical transmission system according to a fourth embodiment of the
present invention. In FIG. 19, the optical transmission system of
the present embodiment includes an angle modulating portion 1, an
optical modulating portion 2, a local light source 13, an optical
combining portion 14, an optical waveguide portion 3, an optical
branch portion 10, an interference portion 6, a first
optical/electrical converting portion 4, a second
optical/electrical converting portion 4', a filter F and a filter
F', and is different from the second embodiment (refer to FIG. 17)
in that the local light source 13 and the optical combining portion
14 are added. Therefore, the same reference numbers are assigned to
the portions operating in the same manners as those in the second
embodiment and the detailed description thereof is omitted. The
difference from the second embodiment will be mainly described
below.
[0348] An optical signal outputted from the optical modulating
portion 2 is combined with a light outputted from the local light
source 13 by the optical combining portion 14, to be transmitted to
the optical branch portion 10 by the optical waveguide portion 3.
One optical signal of two optical signals, which are obtained by a
branch and outputted in/from the optical branch portion 10, is
subjected to angle demodulation by the interference portion 6 and
the first optical/electrical converting portion 4 and further
subjected to filtering processing by the filter F, to be
re-converted into an electrical signal corresponding to an
electrical signal inputted to the angle modulating portion 1. The
other optical signal of the two optical signals, which is obtained
by the branch and outputted, is subjected to heterodyne detection
in the second optical/electrical converting portion 4'. Thereby,
outputted from the second optical/electrical converting portion 4'
is a beat signal of a frequency corresponding to difference in
wavelength between the optical signal outputted from the optical
modulating portion 2 and the light from the local light source 13.
The filter F a derives the beat signal component from the signal
outputted from the second optical/electrical converting portion 4',
to output the beat signal component.
[0349] (Fifth Embodiment)
[0350] FIG. 20 is a block diagram showing the configuration of an
optical transmission system according to a fifth embodiment of the
present invention. In FIG. 20, the optical transmission system of
the present embodiment includes an angle modulating portion 1, an
optical modulating portion 2, a local light source 13, an optical
combining portion 14, an optical waveguide portion 3, an
interference portion 6, an optical/electrical converting portion 4
and a dividing portion 15, and is different from the first
embodiment (refer to FIG. 1) in that the local light source 13, the
optical combining portion 14 and the dividing portion 15 are added.
Therefore, the same reference numbers are assigned to the portions
operating in the same manners as those in the first embodiment and
detailed description thereof is omitted. The difference from the
first embodiment will be mainly described below.
[0351] An optical signal outputted from the optical modulating
portion 2 is combined with a light outputted from the local light
source 13 on the optical combining portion 14 and then transmitted
to the interference portion 6 by the optical waveguide portion 3.
The optical signal is subjected to angle demodulation and
heterodyne detection by the interference portion 6 and the
optical/electrical converting portion 4. At this time, the
outputted signal from the optical/electrical converting portion 4
includes an angle-demodulated signal component corresponding to an
electrical signal inputted to the angle modulating portion 1 and a
beat signal component of a frequency corresponding to difference in
wavelength between the optical signal outputted from the optical
modulating portion 2 and the light outputted from the local light
source 13. The dividing portion 15 branches the outputted signal
from the optical/electrical converting portion 4 into two and
subjects the two signal obtained by the branch to predetermined
filtering processing, respectively to separate the
angle-demodulated signal component and the beat signal component
and output the two signals.
[0352] As described in the above, the optical transmission systems
in FIG. 18, FIG. 19 and FIG. 20 can provide different kind of
networks (for example, a wired network using an optical fiber and a
wireless network) at the same time, as in the case with the optical
transmission system in FIG. 17. Moreover, regardless of a value of
the frequency of the angle-modulated signal outputted from the
angle modulating portion 1, the optical transmission systems can
suitably set the wavelength of the optical signal from the optical
modulating portion 2 and the wavelength of the light from the local
light source 11 or 13, to freely convert the frequency of the
angle-modulated signal which is a beat signal outputted from the
second optical/electrical converting portion 4', thereby making it
possible to generate an angle-modulated signal of the frequency
suitable for each network connected to the second
optical/electrical converting portion 4' and thereafter. Thus, a
more flexible system can be configured.
[0353] (Sixth Embodiment)
[0354] FIG. 21 is a block diagram showing the configuration of an
optical transmission system according to a sixth embodiment of the
present invention. In FIG. 21, the optical transmission system of
the present embodiment includes an angle modulating portion 1, an
optical modulating portion 2, an optical waveguide portion 3, an
optical branch portion 10, an interference portion 6, a first
optical/electrical converting portion 4, a second
optical/electrical converting portion 4', a local oscillation
portion 16, a filter F and a filter F', and is different from the
second embodiment (refer to FIG. 17) in that the local oscillation
portion 16 is added. Therefore, the same reference numbers are
assigned to the portions operating in the same manners as those in
the second embodiment and the detailed description thereof is
omitted. The difference from the second embodiment will be mainly
described below.
[0355] The optical branch portion 10 branches an optical signal,
which is outputted from the optical modulating portion 2 and guided
by the optical waveguide portion 3, into two. One optical signal of
the two optical signals is subjected to angle demodulation by the
interference portion 6 and the first optical/electrical converting
portion 4 and further subjected to filtering processing by the
filter F, to be re-converted into an electrical signal
corresponding to an electrical signal inputted to the angle
modulating portion 1. The other optical signal of the two optical
signals is inputted to the second optical converting portion 4'. A
bias voltage of the second optical/electrical converting portion 4'
is modulated with a local signal (an unmodulated signal) outputted
from the local oscillation portion 16. Accordingly, the second
optical/electrical converting portion 4' square-law detects the
received optical signal and thereby generates a beat signal induced
by the angle-modulated signal outputted from the angle modulating
portion 102 and the local signal, to output the beat signal. The
filter F' derives only beat signal component from the outputted
signal from the second optical/electrical converting portion
4'.
[0356] (Seventh Embodiment)
[0357] FIG. 22 is a block diagram showing the configuration of an
optical transmission system according to a seventh embodiment of
the present invention. In FIG. 22, the optical transmission system
of the present embodiment includes an angle modulating portion 1,
an optical modulating portion 2, an optical waveguide portion 3, an
optical branch portion 10, an interference portion 6, a first
optical/electrical converting portion 4, a second
optical/electrical converting portion 4', a local oscillation
portion 16 and a mixing portion 17, and is different from the
second embodiment: (refer to FIG. 17) in that the local oscillation
portion 16 and the mixing portion 17 are added. Therefore, the same
reference numbers are assigned to the portions operating in the
same manners as those in the second embodiment and the detailed
description thereof is omitted. The difference from the second
embodiment will be mainly described.
[0358] The optical branch portion 10 branches an optical signal,
which is outputted from the optical modulating portion 2 and guided
by the optical waveguide portion 3, into two. One optical signal of
the two optical signals is subjected to angle demodulation by the
interference portion 6 and the first optical/electrical converting
portion 4 and further subjected to filtering processing by the
filter F, to be re-converted into an electrical signal
corresponding to an electrical signal inputted to the angle
modulating portion 1. The other optical signal of the two optical
signals is subjected to square-law detection in the second
optical/electrical converting portion 4' and an
optical-intensity-modulat- ed component or an
optical-amplitude-modulated component of the optical signal is
re-converted into an electrical signal. Thereby, from the second
optical/electrical converting portion 4' is outputted the same
angle-modulated signal as the angle-modulated signal outputted from
the angle modulating portion 1. The mixing portion 17 mixes the
angle-modulated signal and a local signal (an unmodulated signal)
outputted from the local oscillation portion 16, to generate a beat
signal induced by the angle-modulated signal and the local signal
and output the beat signal. The filter F' derives only beat signal
component from the outputted signal of the second
optical/electrical converting portion 4' and outputs the beat
signal component.
[0359] As described in the above, the optical transmission system
in FIG. 21 and FIG. 22 can provide different kind of networks at
the same time, as in the case with the optical transmission system
in FIG. 17. Further, regardless of a value of the frequency of the
angle-modulated signal outputted from the angle modulating portion
1, the optical transmission system can suitably set the frequency
of the local signal outputted from the local oscillation portion 16
to freely convert the frequency of an angle-modulated signal which
is the beat signal induced by the angle-modulated signal and the
local signal, thereby making it possible to generate an
angle-modulated signal of a frequency suitable for each network
connected to the second optical/electrical converting portion 4'
and thereafter and send the angle-modulated signal. Thus, a more
flexible system can be configured.
[0360] (Eighth Embodiment)
[0361] FIG. 23 is a block diagram showing the configuration of an
optical transmission system according to an eighth embodiment of
the present invention. FIG. 23 also shows schematic diagrams of
frequency spectrums of signals in respective portions. In FIG. 23,
the optical transmission system of the present embodiment includes
an angle modulating portion 1, a combining portion 18, an optical
modulating portion 2, an optical waveguide portion 3, an optical
branch portion 10, an interference portion 6, a first
optical/electrical converting portion 4, a second
optical/electrical converting portion 4', a filter F and a filter
F', and is different from the first embodiment (refer to FIG. 1) in
that the combining portion 18, the optical branch portion 10, the
second optical/electrical converting portion 4' and the filter F'
are added. Therefore, the same reference numbers are assigned to
the portions operating in the same manners as those in the first
embodiment and the detailed description thereof is omitted here.
The difference from the first embodiment will be mainly described
below.
[0362] The combining portion 18 combines an angle-modulated signal
outputted from the angle modulating portion 1, of which original
signal is the first electrical signal, and the second electrical
signal, to output the resultant signal. The optical modulating
portion 2 converts the combined signal into an optical-modulated
signal, to output the optical-modulated signal. The optical branch
portion 10 branches the optical signal guided by the optical
waveguide portion 3 into two. One optical signal of the two optical
signals is subjected to angle demodulation with the interference
portion 6 and the first optical/electrical converting portion 4 and
further subjected to filtering processing by the filter F, to be
re-converted into an electrical signal corresponding to the first
electrical signal inputted to the angle modulating portion 1. The
second optical/electrical converting portion 4' receives the other
optical signal of the two optical signals and re-converts the
optical-intensity-modulated component or the
optical-amplitude-modulated component of the optical signal into an
electrical signal with square-law detection, to output an
electrical signal which corresponds to the second electrical signal
inputted to the combining portion 18. The filter F' derives the
second electrical signal component from the outputted signal of the
second optical/electrical converting portion 4' and outputs the
second electrical signal component.
[0363] In the eighth embodiment, it is preferable that occupied
frequency bands of the first electrical signal, the angle-modulated
signal and the second electrical signal do not overlap each other
as shown in FIG. 23, which allows to separate each signal with
filtering processing on the receiving side. Hence, a ninth
embodiment described below devises a method of avoiding the
overlaps of the occupied frequency bands of the above-mentioned
signals.
[0364] (Ninth Embodiment)
[0365] FIG. 24 is a block diagram showing the configuration of an
optical transmission system according to the ninth embodiment of
the present invention. The ninth embodiment is an application of
the above-mentioned eighth embodiment and therein first, second,
third and fourth signal processing portions 19, 20, 21 and 22 are
added to the configuration of the eighth embodiment. Each of the
third and fourth signal processing portions 21 and 22 also has a
function of filter and therefore the filters F and F' are not
provided in the present embodiment.
[0366] The first and second signal processing portions 19 and 20
limit the frequency bands of a first electrical signal and a second
electrical signal so that the frequency bands occupied by the first
and second electrical signals do not overlap each other, to output
the electrical signals. For example, when the occupied frequency
bands of the two electrical signals overlap each other, either one
or both of the bands are limited. Though this bandwidth limitation
causes a distortion of a reproduced waveform on the receiving side,
such waveform distortion is corrected in the third and fourth
signal processing portions 21 and 22. Furthermore, a carrier
frequency in the angle modulating portion 1 is set to a frequency
which prevents the occupied frequency band of the angle-modulated
signal from overlapping with both occupied frequency bands of the
first and second electrical signals.
[0367] The third signal processing portion 21 passes only a
frequency component corresponding to the occupied frequency band of
the first electrical signal among signals outputted from the first
optical/electrical converting portion 4. The third signal
processing portion 21 also reproduces waveform information, which
was lost in the signal processing by the first signal processing
portion 19, as required. The fourth signal processing portion 22
passes only a frequency component corresponding to the occupied
frequency band of the second electrical signal among signals
outputted from the second optical/electrical converting portion 4'.
The fourth signal processing portion 22 also reproduces waveform
information, which was lost in the signal processing by the second
signal processing portion 20, as required. Waveform information
lost in the first signal processing portion 19 or the second signal
processing portion 20 is a low-frequency component such as a DC
component and the like as an example, and in this case, the signal
waveform becomes, for example, a differential waveform of the
original signal. Accordingly, differential reproduction
(integration) processing is performed in the third signal
processing portion 21 or the fourth signal processing portion 22,
thereby making it possible to reproduce the original signal
waveform.
[0368] As described in the above, the optical transmission system
in FIG. 23 or FIG. 24 converts a first electrical signal into an
angle-modulated signal, optically transmits the angle-modulated
signal, subjects the signal to angle demodulation using the
interference portion 6 and the first optical/electrical converting
portion 4 to produce the first electrical signal, and at the same
time optically transmits a second electrical signal other than the
first electrical signal, to derive the second electrical signal by
the second optical/electrical converting portion 4'. This makes it
possible, for example, to simultaneously transmit different types
of signals such as an analog signal and a digital signal with one
optical fiber. Even in the case where the transmitted signal
includes a high-frequency signal such as a microwave, a millimetre
wave and the like, it is possible to construct a flexible and
greatly economical system which allows reception and demodulation
in a low cost configuration with optical signal processing. While
in the eighth and ninth embodiments, the case where two electrical
signals are simultaneously transmitted is described, three or more
electrical signals can, of course, be simultaneously
transmitted.
[0369] (Tenth Embodiment)
[0370] FIG. 25 is a block diagram showing the configuration of an
optical transmission system according to a tenth embodiment of the
present invention. In FIG. 25, the optical transmission system of
the present embodiment includes a first angle modulating portion 1,
a second angle modulating portion 1', a combining portion 18, an
optical modulating portion 2, an optical waveguide portion 3, an
optical branch portion 10, a first interference portion 6, a second
interference portion 6', a first optical/electrical converting
portion 4, a second optical/electrical converting portion 4', a
filter F and a filter F', and is different from the first
embodiment (refer to FIG. 1) in that the second angle modulating
portion 1', the combining portion 18, the optical branch portion
10, the second interference portion 6', the second
optical/electrical converting portion 4' and the filter F' are
added. Therefore, the same reference numbers are assigned to the
portions operating in the same manners as those in the first
embodiment and the detailed description thereof is omitted. The
difference from the first embodiment will be mainly described
below.
[0371] The combining portion 18 combines a first
angle-modulated-signal outputted from the first angle modulating
portion 1 performing angle modulation using a first electrical
signal as the original signal and a second angle-modulated signal
outputted from the second angle modulating portion 1' performing
angle modulation using a second electrical signal as the original
signal, to output the resultant signal. The optical modulating
portion 2 converts the combined signal into an optical-modulated
signal, to output the optical-modulated signal. The optical branch
portion 10 branches the optical signal guided by the optical
waveguide portion 3 into two. One optical signal of the two optical
signals is subjected to angle demodulation by the first
interference portion 6 and the first optical/electrical converting
portion 4 and further subjected to filtering processing by the
filter F, to be re-converted into an electrical signal
corresponding to the first electrical signal inputted to the first
angle modulating portion 1. The other optical signal of the two
optical signals is subjected to angle demodulation by the second
interference portion 6' and the second optical/electrical
converting portion 4' and further subjected to filtering processing
by the filter F', to be re-converted into an electrical signal
corresponding to the second electrical signal inputted to the
second angle modulating portion 1'.
[0372] In the present embodiment, it is preferable that occupied
frequency bands of the first and second electrical signals and the
first and second angle-modulated signals do not overlap each other,
which allows to separate each signal with filtering processing on
the receiving side. Hence, an eleventh embodiment described below
devises a method of avoiding overlaps of occupied frequency bands
of the above-mentioned signals.
[0373] (Eleventh Embodiment)
[0374] FIG. 26 is a block diagram showing the configuration of an
optical transmission system according to the eleventh embodiment of
the present invention. The eleventh embodiment is an application of
the tenth embodiment, and therein first, second, third and fourth
signal processing portions 23, 24, 25 and 26 are added to the
configuration of the tenth embodiment. The third and fourth signal
processing portions 25 and 26 also have functions of filter and
therefore the filters F and F' are not provided in the present
embodiment.
[0375] The first signal processing portion 23 and the second signal
processing portion 24 limit the frequency bands of a first
electrical signal and a second electrical signal so that frequency
bands occupied by the first electrical signal and the second
electrical signal do not overlap each other, to output the two
electrical signals. For example, when the occupied frequency bands
of the two electrical signals overlap each other, either one or
both of the bands of the two signals are limited. Though this
bandwidth limitation causes a distortion of a reproduced waveform
on the receiving side, such waveform distortion is corrected by the
third and fourth signal processing portions 25 and 26. Furthermore,
carrier frequencies in the first angle modulating portion 1 and the
second angle modulating portion 1' are set to frequencies which
prevent the occupied frequency bands of the first electrical
signal, the second electrical signal, the first angle-modulated
signal and the second angle-modulated signal from overlapping each
other.
[0376] The third signal processing portion 25 passes only a
frequency component corresponding to the occupied frequency band of
the first electrical signal among signals outputted from the first
optical/electrical converting portion 4. The third signal
processing portion 25 also reproduces waveform information, which
was lost in the signal processing by the first signal processing
portion 23, as required. The fourth signal processing portion 26
passes only a frequency component corresponding to the occupied
frequency band of the second electrical signal among signals
outputted from the second optical/electrical converting portion 4'.
The fourth signal processing portion 26 also reproduces waveform
information, which was lost in the signal processing by the second
signal processing portion 24, as required. Waveform information
lost in the first signal processing portion 23 or the second-signal
processing portion 24 is a low-frequency component such as a DC
component and the like, and in this case, the signal waveform
becomes, for example, a differential waveform of the original
signal. Accordingly, differential reproduction (integration)
processing is performed in the third signal processing portion 25
or the fourth signal processing portion 26, thereby making it
possible to reproduce the original signal waveform.
[0377] As described in the above, the optical transmission system
in FIG. 25 or FIG. 26 converts first and second electrical signals
into angle-modulated signals, then combines and optically transmits
the angle-modulated signals and subjects the resultant signal to
angle demodulation using the interference portion and the
optical/electrical converting portion, to reproduce the first and
second electrical signal. This makes it possible to simultaneously
transmit different types of signals such as an analog signal and a
digital signal with one optical fiber. Even in the case where the
transmitted signal includes a high-frequency signal such as a micro
wave, a millimetre wave and the like, it is possible to construct a
flexible and greatly economical system which can receive and
subject the signal to demodulation in a low cost configuration with
optical signal processing. While in the tenth and eleventh
embodiments, the case where two electrical signals are
simultaneously transmitted is described, three or more electrical
signals can, of course, be simultaneously transmitted.
[0378] (Twelfth Embodiment)
[0379] FIG. 27 is a block diagram showing the configuration of an
optical transmission system according to a twelfth embodiment of
the present invention. The optical transmission system of the
present embodiment has a configuration in which the configuration
of the first embodiment shown in FIG. 1 is extended in order to
transmit a multichannel frequency-division-multiplexed signal. In
FIG. 27, to an angle modulating portion 1 is inputted a n-channel
frequency-division-multiplexed signal which is obtained by
frequency-division-multiplexing n-channel electrical signals. The
optical branch portion 10 branches an inputted optical signal into
n optical signals. A plurality of optical signal processing
portions constituted by interference portions 6, optical/electrical
converting portions 4 and filters F are provided in parallel,
corresponding to each of the n optical signals outputted from the
optical branch portion 10. The optical signal processing portions
each subject the electrical signals on different channels to
demodulation. Therefore, the delay for an optical signal in the
interference portion 6 in an optical signal processing portion is
set to a value most suitable for a frequency of an electrical
signal on a channel to be subjected to demodulation in the optical
signal processing portion. A passband of each of the filters F is
designed so as to pass only an electrical signal on a channel to be
subjected to demodulation. Since the other configuration of the
present embodiment is the same as that in the first embodiment
shown in FIG. 1, the same reference numbers are assigned to the
corresponding portions and the description thereof is omitted here.
Such extension as shown in the present embodiment can, of course,
be made in the other above-described embodiments.
[0380] (Thirteenth Embodiment)
[0381] FIG. 28 is a block diagram showing the configuration of an
optical transmission system according to a thirteenth embodiment of
the present invention. The present embodiment has a configuration
in which the third embodiment shown in FIG. 18 is extended in order
to derive a plurality of beat signals of different frequencies. In
FIG. 28, a plurality of optical heterodyne portions constituted by
local light sources 11, optical combining portions 12, second
optical/electrical converting portions 4' and filters F' are
provided in parallel. Wavelengths of lights outputted from the
local light sources 11 are set so as to be different from each
other depending on frequencies of beat signals to be derived in the
optical heterodyne portions. A passband of each of the filters F'
is designed so as to pass only a beat signal of a frequency to be
derived. Since the other configuration of the present embodiment is
the same as that in the third embodiment shown in FIG. 18, the same
reference numbers are assigned to the corresponding portions and
the description thereof is omitted here.
[0382] (Fourteenth Embodiment)
[0383] FIG. 29 is a block diagram showing the configuration of an
optical transmission system according to a fourteenth embodiment of
the present invention. The present embodiment has a configuration
in which the fourth embodiment shown in FIG. 19 is extended in
order to derive a plurality of beat signals of different
frequencies. In FIG. 29, an optical combining portion 14 combines
an optical signal outputted from an optical modulating portion 2
and a light outputted from each of a plurality of local light
sources 13. Wavelengths of lights outputted from the local light
sources 13 are each set so as to be different from each other. A
plurality of optical detecting portions constituted by second
optical/electrical converting portions 4' and filters F' are
provided in parallel. A passband of each of the filters F' is
designed so as to pass only a beat signal of a frequency to be
derived. Since the other configuration of the present embodiment is
the same as that in the fourth embodiment shown in FIG. 19, the
same reference numbers are assigned to the corresponding portions
and the description thereof is omitted here.
[0384] Extension for the same purpose as that of the extension
performed in the thirteenth and fourteenth embodiments (that is
extension in order to derive a plurality of beat signals of
different frequencies) can be made in the third to seventh
embodiments, although those are not shown.
[0385] The more specific examples of configuration and the
operational examples of each portions described for the first
embodiment (refer to FIGS. 2 to 16) can be applied to the second to
fourteenth embodiments (FIGS. 17 to 29) as it is.
[0386] While the invention has been described in detail, the
foregoing description is in all aspects illustrative and not
restrictive. It is understood that numerous other modifications and
variations can be devised without departing from the scope of the
invention.
* * * * *