U.S. patent application number 10/190682 was filed with the patent office on 2003-02-13 for optical transmitter-receiver.
Invention is credited to Maeda, Kazuki, Sasai, Hiroyuki, Utsumi, Kuniaki.
Application Number | 20030030868 10/190682 |
Document ID | / |
Family ID | 26432113 |
Filed Date | 2003-02-13 |
United States Patent
Application |
20030030868 |
Kind Code |
A1 |
Sasai, Hiroyuki ; et
al. |
February 13, 2003 |
Optical transmitter-receiver
Abstract
A modulated electrical signal Smod produced upon
amplitude-modulating a subcarrier having a high frequency (for
example, a millimeter-wave band) by a baseband signal SBB to be
transmitted and a main carrier MC outputted from a light source 110
are inputted to an external optical modulating portion 120 in an
optical transmitter 101. The external optical modulating portion
120 amplitude-modulates the main carrier MC by the modulated
electrical signal Smod, to output a double-modulated optical signal
OSdmod to an optical filter portion 130. The optical filter portion
130 passes only a component of one of sidebands included in the
double-modulated optical signal OSdmod, and outputs the component
to an optical fiber 140 as an optical signal OS. An
optical/electrical converting portion 150 in an optical receiver
102 optical/electrical-conv- erts the optical signal OS transmitted
through the optical fiber 140, to directly obtain a baseband signal
SBB. Consequently, the optical receiver 102 is constructed simply
and at low cost without requiring a wideband optical/electrical
converting element for optical/electrical-converting a
high-frequency electrical signal and a high-frequency electrical
component (a frequency converter, a demodulator, a semirigid cable
or a waveguide) which is very high in cost or is difficult to
process.
Inventors: |
Sasai, Hiroyuki; (Katano,
JP) ; Maeda, Kazuki; (Neyagawa, JP) ; Utsumi,
Kuniaki; (Sanda, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
26432113 |
Appl. No.: |
10/190682 |
Filed: |
July 9, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10190682 |
Jul 9, 2002 |
|
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09056611 |
Apr 8, 1998 |
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6459519 |
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Current U.S.
Class: |
398/140 |
Current CPC
Class: |
H04B 10/5051 20130101;
H04B 10/505 20130101; H04B 10/572 20130101 |
Class at
Publication: |
359/154 ;
359/173; 359/181 |
International
Class: |
H04B 010/00; H04B
010/12; H04B 010/04 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 9, 1997 |
JP |
090658/1997 |
Jul 31, 1997 |
JP |
206785/1997 |
Claims
What is claimed is:
1. An optical transmitter-receiver in which an optical transmitter
and an optical receiver are interconnected such that optical
transmission is possible, comprising: a double-modulating portion,
to which a subcarrier modulated by an electrical signal to be
transmitted is inputted from outside, for double-modulating a main
carrier, which is unmodulated light having a predetermined optical
frequency, by the inputted subcarrier, to produce and output a
double-modulated optical signal, an optical spectrum of said
double-modulated optical signal inputted from said
double-modulating portion including a component of said main
carrier at said predetermined optical frequency and further
including components of an upper sideband and a lower sideband at
frequencies spaced by the frequency of said subcarrier apart from
the predetermined optical frequency; an optical filter portion for
passing an optical signal including the component of either one of
said upper sideband and said lower sideband in said
double-modulated optical signal inputted from said
double-modulating portion; and an optical/electrical converting
portion for optical/electrical-converting said optical signal
inputted from said optical filter portion, to obtain said
electrical signal to be transmitted, said optical transmitter
comprising at least said double-modulating portion, said optical
receiver comprising at least said optical/electrical converting
portion, and said optical filter portion being included in either
one of the optical transmitter and the optical receiver.
2. The optical transmitter-receiver according to claim 1, wherein
said double-modulating portion comprises a semiconductor laser
diode for outputting said main carrier, and at least one external
optical modulating portion for amplitude-modulating said main
carrier inputted from said semiconductor laser diode by a
subcarrier amplitude-modulated by an electrical signal to be
transmitted which is inputted from outside using an external
optical modulation.
3. The optical transmitter-receiver according to claim 2, wherein
the subcarrier amplitude-modulated by said electrical signal to be
transmitted is a signal transmitted from outside, further
comprising an antenna portion receiving said signal transmitted for
supplying the signal to said double-modulating portion.
4. The optical transmitter-receiver according to claim 3, wherein
said electrical signal to be transmitted is a multichannel signal
frequency-multiplexed, and said subcarrier modulated by said
multichannel signal is inputted into said double-modulating portion
from outside.
5. The optical transmitter-receiver according to claim 3, wherein
said electrical signal to be transmitted is digital information,
and said subcarrier OOK (on-off keying)-modulated by said digital
information is inputted into said double-modulating portion from
outside.
6. An optical transmitter-receiver in which an optical transmitter
and an optical receiver are interconnected such that optical
transmission is possible, comprising: a double-modulating portion,
to which a subcarrier modulated by an electrical signal to be
transmitted is inputted from outside, for double-modulating a main
carrier, which is unmodulated light having a predetermined optical
frequency, by the inputted subcarrier, to produce and output a
double-modulated optical signal, an optical spectrum of said
double-modulated optical signal inputted from said
double-modulating portion including a component of said main
carrier at said predetermined optical frequency and further
including components of an upper sideband and a lower sideband at
frequencies spaced by the frequency of said subcarrier apart from
the predetermined optical frequency; an optical filter portion for
passing an optical signal including the component of either one of
said upper sideband and said lower sideband in said
double-modulated optical signal inputted from said
double-modulating portion; and an optical branching portion for
branching the optical signal inputted from said optical filter
portion into a first optical signal and a second optical signal,
and outputting the first and second optical signals; a first
optical/electrical converting portion for
optical/electrical-converting said first optical signal inputted
from said optical branching portion, to obtain said electrical
signal to be transmitted; a second optical/electrical converting
portion for outputting as a detecting signal an electrical signal
obtained by optical/electrical-converting said second optical
signal inputted from said optical branching portion; and a
wavelength control portion for detecting the average values of
detected signals inputted from said second optical/electrical
converting portion at predetermined time intervals, and controlling
the wavelength of the double-modulated optical signal outputted
from said double-modulating portion on the basis of the maximum
value of the detected average values, said optical transmitter
comprising at least said double-modulating portion, said optical
receiver comprising at least said first optical/electrical
converting portion, and said optical filter portion being included
in either one of the optical transmitter and the optical
receiver.
7. An optical transmitter-receiver in which an optical transmitter
and first and second optical receivers are interconnected such that
optical transmission is possible, wherein said optical transmitter
comprises a local oscillating portion for outputting a subcarrier
having a predetermine frequency, a double-modulating portion for
double-modulating a main carrier which is unmodulated light having
a predetermined optical frequency by an electrical signal to be
transmitted which is inputted from outside and said subcarrier
inputted from said local oscillating portion, to produce and output
a double-modulated optical signal, a spectrum of said
double-modulated optical signal outputted from said
double-modulating portion including a component of said main
carrier at said predetermined optical frequency and further
including components of an upper sideband and a lower sideband at
frequencies spaced by the frequency of said subcarrier apart from
the predetermined optical frequency, and an optical filter portion
for dividing said double-modulated optical signal inputted from
said double-modulating portion into a first optical signal
including the component of either one of said upper sideband and
said lower sideband and a second optical signal including the
component of said main carrier and the component of the other one
of the upper sideband and the lower sideband, to output the first
optical signal and the second optical signal, said first optical
receiver optical/electrical-converts said first optical signal
transmitted from said optical transmitter, to obtain said
electrical signal to be transmitted, and said second optical
receiver optical/electrical-converts said second optical signal
transmitted from said optical transmitter, to obtain said
subcarrier that is modulated by said electrical signal to be
transmitted.
8. The optical transmitter-receiver according to claim 7, wherein
said double-modulating portion comprises an electrical modulating
portion for amplitude-modulating said subcarrier inputted from said
local oscillating portion by said electrical signal to be
transmitted which is inputted from outside, to produce and output a
modulated electrical signal, a light source for outputting said
main carrier which is unmodulated light having a predetermined
optical frequency, and an external optical modulating portion for
amplitude-modulating said main carrier inputted from said light
source by said modulated electrical signal inputted from said
electrical modulating portion, to produce said double-modulated
optical signal.
9. The optical transmitter-receiver according to claim 8, wherein
said electrical signal to be transmitted is digital information,
and said electrical modulating portion OOK (on-off
keying)-modulates said subcarrier by said digital information.
10. The optical transmitter-receiver according to claim 7, wherein
said double-modulating portion comprises a light source for
outputting said main carrier which is unmodulated light having a
predetermined optical frequency, a first external optical
modulating portion for amplitude-modulating said main carrier
inputted from said light source by said subcarrier inputted from
said local oscillating portion, to produce and output a modulated
optical signal, and a second external optical modulating portion
for amplitude-modulating said modulated optical signal inputted
from said first external optical modulating portion by said
electrical signal to be transmitted which is inputted from outside,
to produce said double-modulated optical signal.
11. The optical transmitter-receiver according to claim 7, wherein
said double-modulating portion comprises a light source for
outputting said main carrier which is unmodulated light having a
predetermined optical frequency, a first external optical
modulating portion for amplitude-modulating said main carrier
inputted from said light source by said electrical signal to be
transmitted which is inputted from outside, to produce and output a
modulated optical signal, and a second external optical modulating
portion for amplitude-modulating said modulated optical signal
inputted from said first external optical modulating portion by
said subcarrier inputted from said local oscillating portion, to
produce said double-modulated optical signal.
12. The optical transmitter-receiver according to claim 7, wherein
said optical filter portion comprises an optical circulator portion
for outputting said double-modulated optical signal inputted from
said double-modulating portion as it is, and an optical fiber
grating portion for reflecting the component of either one of said
upper sideband and said lower sideband in said double-modulated
optical signal inputted from said optical circulator portion, to
produce said first optical signal and output the produced first
optical signal to said optical circulator portion, and passing the
component of said main carrier and the component of the other one
of the upper sideband and the lower sideband, to produce and output
said second optical signal to said second optical receiver, said
optical circulator portion further outputting said first optical
signal inputted from said optical fiber grating portion as it is to
said first optical receiver.
13. The optical transmitter-receiver according to claim 7, wherein
said second optical receiver comprises an antenna portion for
radiating to a space the subcarrier that is modulated by said
electrical signal to be transmitted which is obtained by the
optical/electrical conversion.
14. The optical transmitter-receiver according to claim 7, wherein
said electrical signal to be transmitted is an electrical signal to
be transmitted which is converted analog information into digital
information.
15. The optical transmitter-receiver according to claim 7, wherein
said electrical signal to be transmitted is a carrier modulated by
an analog information and digital information, the frequency of
said carrier is an intermediate frequency lower than that of the
subcarrier outputted from said local oscillating portion.
16. The optical transmitter-receiver according to claim 7, wherein
said electrical signal to be transmitted is obtained by
multiplexing a plurality of electrical signals that have the
intermediate frequency and are modulated by analog information or
digital information using a predetermined multiplexing technique,
respectively.
17. The optical transmitter-receiver according to claim 16, wherein
said predetermined multiplexing technique is a frequency division
multiplexing access, a time division multiplexing access or a code
division multiplexing access.
18. An optical transmitter-receiver in which an optical transmitter
and first and second optical receivers are interconnected such that
optical transmission is possible, wherein said optical transmitter
comprises a local oscillating portion for outputting a subcarrier
having a predetermine frequency, a double-modulating portion for
double-modulating a main carrier which is unmodulated light having
a predetermined optical frequency by an electrical signal to be
transmitted which is inputted from outside and by said subcarrier
inputted from said local oscillating portion, to produce and output
a double-modulated optical signal, and an optical branching portion
for branching said double-modulated optical signal inputted from
said double-modulating portion, and outputting double-modulated
optical signals obtained by the branching, said first optical
receiver comprises a low-pass filter portion for passing a
component included in a low frequency band of an electrical signal
obtained by optical/electrical-converting said double-modulated
optical signal transmitted from said optical transmitter, to output
said electrical signal to be transmitted, and said second optical
receiver comprises a high-pass filter portion for passing a
component included in a high frequency band of an electrical signal
obtained by optical/electrical-converting said double-modulated
optical signal transmitted from said optical transmitter, to output
said subcarrier that is modulated by said electrical signal to be
transmitted.
19. The optical transmitter-receiver according to claim 18, wherein
said double-modulating portion comprises an electrical modulating
portion for amplitude-modulating said subcarrier inputted from said
local oscillating portion by said electrical signal to be
transmitted which is inputted from outside, to produce and output a
modulated electrical signal, a light source for outputting said
main carrier which is unmodulated light having a predetermined
optical frequency, and an external optical modulating portion for
amplitude-modulating said main carrier outputted from said light
source by said modulated electrical signal inputted from said
electrical modulating portion, to produce said double-modulated
optical signal.
20. The optical transmitter-receiver according to claim 19, wherein
said electrical signal to be transmitted is digital information,
and said electrical modulating portion OOK (on-off
keying)-modulates said subcarrier by said digital information.
21. The optical transmitter-receiver according to claim 18, wherein
said double-modulating portion comprises a light source for
outputting said main carrier which is unmodulated light having a
predetermined optical frequency, a first external optical
modulating portion for amplitude-modulating said main carrier
inputted from said light source by said subcarrier inputted from
said local oscillating portion, to produce and output a modulated
optical signal, and a second external optical modulating portion
for amplitude-modulating said modulated optical signal inputted
from said first external optical modulating portion by said
electrical signal to be transmitted which is inputted from outside,
to produce said double-modulated optical signal.
22. The optical transmitter-receiver according to claim 18, wherein
said double-modulating portion comprises a light source for
outputting said main carrier which is unmodulated light having a
predetermined optical frequency, a first external optical
modulating portion for amplitude-modulating said main carrier
inputted from said light source by said electrical signal to be
transmitted which is inputted from outside, to produce and output a
modulated optical signal, and a second external optical modulating
portion for amplitude-modulating said modulated optical signal
inputted from said first external optical modulating portion by
said subcarrier inputted from said local oscillating portion, to
produce said double-modulated optical signal.
23. The optical transmitter-receiver according to claim 18, wherein
an antenna portion for radiating to a space the subcarrier that is
modulated by said electrical signal to be transmitted which is
outputted from said high-pass filter portion is set in a back end
against the high-pass filter portion.
24. The optical transmitter-receiver according to claim 18, wherein
said electrical signal to be transmitted is a carrier modulated by
analog information or digital information, the frequency of said
carrier is an intermediate frequency lower than that of the
subcarrier outputted from said local oscillating portion.
25. The optical transmitter-receiver according to claim 18, wherein
said double-modulating portion modulates said main carrier by said
subcarrier inputted from said local oscillating portion using a
single sideband amplitude modulation technique.
26. An optical transmitter-receiver in which an optical transmitter
and an optical receiver are interconnected such that optical
transmission is possible, wherein said optical transmitter
comprises a local oscillating portion for outputting a subcarrier
having a predetermine frequency, and a double-modulating portion
for double-modulating a main carrier which is unmodulated light
having a predetermined optical frequency by an electrical signal to
be transmitted which is inputted from outside and by said
subcarrier inputted from said local oscillating portion, to produce
and output a double-modulated optical signal, said optical receiver
comprises an optical/electrical converting portion for
optical/electrical-converting said double-modulated optical signal
transmitted from said optical transmitter, to output an electrical
signal, a distributing portion for distributing the electrical
signal inputted from said optical/electrical converting portion
into at least two electrical signals, a low-pass filter portion for
passing a component included in a low frequency band of the
electrical signal obtained by the distribution, to output said
electrical signal to be transmitted, and a high-pass filter portion
for passing a component included in a high frequency band of the
electrical signal obtained by the distribution, to output said
subcarrier that is modulated by said electrical signal to be
transmitted.
27. The optical transmitter-receiver according to claim 26, wherein
said double-modulating portion comprises an electrical modulating
portion for amplitude-modulating said subcarrier inputted from said
local oscillating portion by said electrical signal to be
transmitted which is inputted from outside, to produce and output a
modulated electrical signal, a light source for outputting said
main carrier which is unmodulated light having a predetermined
optical frequency, and an external optical modulating portion for
amplitude-modulating said main carrier inputted from said light
source by said modulated electrical signal inputted from said
electrical modulating portion, to produce said double-modulated
optical signal.
28. The optical transmitter-receiver according to claim 27, wherein
said electrical signal to be transmitted is digital information,
and said electrical modulating portion OOK (on-off
keying)-modulates by said digital information.
29. The optical transmitter-receiver according to claim 26, wherein
said double-modulating portion comprises a light source for
outputting said main carrier which is unmodulated light having a
predetermined optical frequency, a first external optical
modulating portion for amplitude-modulating said main carrier
inputted from said light source by said subcarrier inputted from
said local oscillating portion, to produce and output a modulated
optical signal, and a second external optical modulating portion
for amplitude-modulating said modulated optical signal inputted
from said first external optical modulating portion by said
electrical signal to be transmitted which is inputted from outside,
to produce said double-modulated optical signal.
30. The optical transmitter-receiver according to claim 26, wherein
said double-modulating portion comprises a light source for
outputting said main carrier which is unmodulated light having a
predetermined optical frequency, a first external optical
modulating portion for amplitude-modulating said main carrier
inputted from said light source by said electrical signal to be
transmitted which is inputted from outside, to produce and output a
modulated optical signal, and a second external optical modulating
portion for amplitude-modulating said modulated optical signal
inputted from said first external optical modulating portion by
said subcarrier inputted from said local oscillating portion, to
produce said double-modulated optical signal.
31. The optical transmitter-receiver according to claim 26, wherein
an antenna portion for radiating to a space the subcarrier that is
modulated by said electrical signal to be transmitted which is
outputted from said high-pass filter portion is set in a back end
against the high-pass filter portion.
32. The optical transmitter-receiver according to claim 26, wherein
said electrical signal to be transmitted is a carrier modulated by
analog information or digital information, the frequency of said
carrier is an intermediate frequency lower than that of the
subcarrier outputted from said local oscillating portion.
33. The optical transmitter-receiver according to claim 26, wherein
said double-modulating portion modulates said main carrier by said
subcarrier inputted from said local oscillating portion using a
single sideband amplitude modulation technique.
34. An optical transmitter-receiver in which an optical transmitter
and first and second optical receivers are interconnected such that
optical transmission is possible, wherein said optical transmitter
comprises a local oscillating portion for outputting a subcarrier
having a predetermine frequency, a mode locked light source which
is mode-locked on the basis of the subcarrier inputted from said
local oscillating portion, and oscillating with spacing between
optical frequencies related to the subcarrier, to produce and
output a mode-locked optical signal, an external optical modulating
portion for amplitude-modulating said mode-locked optical signal
inputted from said mode locked light source by the electrical
signal to be transmitted which is inputted from outside, to produce
and output a double-modulated optical signal, and an optical
branching portion for branching said double-modulated optical
signal inputted from said external optical modulating portion and
outputting double-modulated optical signals obtained by the
branching, said first optical receiver comprises a low-pass filter
portion for passing a component included in a low frequency band of
an electrical signal obtained by optical/electrical-converting said
double-modulated optical signal transmitted from said optical
transmitter, to output said electrical signal to be transmitted,
and said second optical receiver comprises a high-pass filter
portion for passing a component included in a high frequency band
of an electrical signal obtained by optical/electrical-converting
said double-modulated optical signal transmitted from said optical
transmitter, to output said subcarrier that is modulated by said
electrical signal to be transmitted.
35. The optical transmitter-receiver according to claim 34, wherein
an antenna portion for radiating to a space the subcarrier that is
modulated by said electrical signal to be transmitted which is
outputted from said high-pass filter portion is set in a back end
against said high-pass filter portion.
36. The optical transmitter-receiver according to claim 34, wherein
said electrical signal to be transmitted is a carrier modulated by
analog information or digital information, the frequency of said
carrier is an intermediate frequency lower than that of the
subcarrier outputted from said local oscillating portion.
37. An optical transmitter-receiver in which an optical transmitter
and an optical receiver are interconnected such that optical
transmission is possible, wherein said optical transmitter
comprises a local oscillating portion for outputting a subcarrier
having a predetermine frequency, a mode locked light source which
is mode-locked on the basis of a subcarrier inputted from said
local oscillating portion and oscillating with spacing between
optical frequencies related to the subcarrier, to produce and
output a mode-locked optical signal, and an external optical
modulating portion for amplitude-modulating said mode-locked
optical signal inputted from said mode locked light source by an
electrical signal to be transmitted which is inputted from outside,
to produce and output a double-modulated optical signal, and said
optical receiver comprises an optical/electrical converting portion
for optical/electrical-converting said double-modulated optical
signal transmitted from said optical transmitter, to output an
electrical signal, a distributing portion for distributing the
electrical signal inputted from said optical/electrical converting
portion into at least two electrical signals, a low-pass filter
portion for passing a component included in a low frequency band of
the electrical signal obtained by the distribution, to output said
electrical signal to be transmitted, and a high-pass filter portion
for passing a component included in a high frequency band of the
electrical signal obtained by the distribution, to output said
subcarrier that is modulated by said electrical signal to be
transmitted.
38. The optical transmitter-receiver according to claim 37, wherein
an antenna portion for radiating to a space the subcarrier that is
modulated by said electrical signal to be transmitted which is
outputted from said high-pass filter portion is set in a back end
against said high-pass filter portion.
39. The optical transmitter-receiver according to claim 37, wherein
said electrical signal to be transmitted is a carrier modulated by
analog information or digital information, the frequency of said
carrier is an intermediate frequency lower than that of the
subcarrier outputted from said local oscillating portion.
40. An optical transmitter-receiver in which an optical transmitter
and first and second optical receivers are interconnected such that
optical transmission is possible, wherein said optical transmitter
comprises a first light source for outputting first unmodulated
light having a first optical frequency, an external optical
modulating portion for amplitude-modulating the first unmodulated
light inputted from said first light source by an electrical signal
to be transmitted which is inputted from outside, to produce and
output a modulated optical signal, a second light source for
outputting second unmodulated light having a second optical
frequency which differs from said first optical frequency by a
predetermined optical frequency, an optical multiplexing portion
for multiplexing the modulated optical signal inputted from said
external optical modulating portion and the second unmodulated
light inputted from said second light source such that polarization
of the modulated optical signal and the second unmodulated light
coincide with each other, to produce and output an optical signal,
and an optical branching portion for branching the optical signal
inputted from said optical multiplexing portion and outputting
optical signals obtained by the branching, said first optical
receiver comprises a low-pass filter portion for passing a
component included in a low frequency band of an electrical signal
obtained by optical/electrical-converting said optical signal
transmitted from said optical transmitter, to output said
electrical signal to be transmitted, and said second optical
receiver comprises a high-pass filter portion for passing a
component included in a high frequency band of an electrical signal
obtained by optical/electrical-converting said optical signal
transmitted from said optical transmitter, to output said
subcarrier that is modulated by said electrical signal to be
transmitted.
41. The optical receiver according to claim 40, wherein an antenna
portion for radiating to a space the subcarrier that is modulated
by said electrical signal to be transmitted which is outputted from
said high-pass filter portion is set in a back end against said
high-pass filter portion.
42. The optical transmitter-receiver according to claim 40, wherein
said electrical signal to be transmitted is a carrier modulated by
analog information or digital information, the frequency of said
carrier is an intermediate frequency lower than that of the
subcarrier outputted from said local oscillating portion.
43. An optical transmitter-receiver in which an optical transmitter
and an optical receiver are interconnected such that optical
transmission is possible, wherein said optical transmitter
comprises a first light source for outputting first unmodulated
light having a first optical frequency, an external optical
modulating portion for amplitude-modulating the first unmodulated
light inputted from said first light source by an electrical signal
to be transmitted which is inputted from outside, to produce and
output a modulated optical signal, a second light source for
outputting second unmodulated light having a second optical
frequency which differs from said first optical frequency by a
predetermined optical frequency, an optical multiplexing portion
for multiplexing the modulated optical signal inputted from said
external optical modulating portion and the second unmodulated
light inputted from said second light source such that polarization
of the modulated optical signal and the second unmodulated light
coincide with each other, to produce and output an optical signal,
and an optical branching portion for branching the optical signal
inputted from said optical multiplexing portion and outputting
optical signals obtained by the branching, and said optical
receiver comprises an optical/electrical converting portion for
optical/electrical-converting said optical signal transmitted from
said optical transmitter, to output an electrical signal, a
distributing portion for distributing the electrical signal
inputted from said optical/electrical converting portion into at
least two electrical signals, a low-pass filter portion for passing
a component included in a low frequency band of the electrical
signal obtained by the distribution, to output an electrical signal
to be transmitted, and a high-pass filter portion for passing a
component included in a high frequency band of the electrical
signal obtained by the distribution, to output said subcarrier that
is modulated by said electrical signal to be transmitted.
44. The optical transmitter-receiver according to claim 43, wherein
an antenna portion for radiating to a space the subcarrier that is
modulated by said electrical signal to be transmitted which is
outputted from said high-pass filter portion is set in a back end
against said high-pass filter portion.
45. The optical transmitter-receiver according to claim 43, wherein
said electrical signal to be transmitted is a carrier modulated by
analog information or digital information, the frequency of said
carrier is an intermediate frequency lower than that of the
subcarrier outputted from said local oscillating portion.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to an optical
transmitter-receiver, and more particularly, to an optical
transmitter-receiver in which an optical transmitter and an optical
receiver are interconnected such that optical transmission is
possible.
[0003] 2. Description of the Background Art
[0004] Optical transmission for transmitting information with light
modulated by the information has been expected to be widely used
for a future high speed communication network due to low-loss and
wideband properties. For example, an optical transmitter-receiver
for optically transmitting an electrical signal having a
high-frequency (hereinafter referred to as a first optical
transmitter-receiver), and an optical transmitter-receiver for
optically transmitting a baseband signal (hereinafter referred to
as a second optical transmitter-receiver), have been proposed. The
two optical transmitter-receivers will be specifically described
referring to the drawings.
[0005] Description is now made of the first optical
transmitter-receiver. In recent years, a wireless service such as a
portable telephone or a PHS (Personal Handyphone System) has been
rapidly enlarged. Therefore, utilization of still higher
frequencies has been examined. A micro-cell system or a pico-cell
system utilizing a millimeter-wave band of approximately 30 GHz to
300 GHz has been examined. In such a cell system, a signal having a
high frequency such as a millimeter-wave band is radiated from a
lot of base stations connected to a control station, so that a
wireless service is provided. The cell system has various
advantages. First, the signal having the millimeter-wave band does
not easily adversely affect next cells due to a propagation loss in
a space. Second, the signal having the millimeter-wave band has a
short wavelength, so that an antenna or the like set in the control
station or the like is miniaturized. Third, the signal having the
millimeter-wave band has a high frequency, so that the transmission
capacity can be increased. Consequently, it may be possible to
provide a high speed transmission service which is difficult to
realize in a conventional wireless service.
[0006] In a wireless communication system to which such a cell
system is applied, however, a lot of base stations are set
throughout a town. Therefore, the base station must be small in
size and low in cost. A first optical transmitter-receiver
employing a so-called subcarrier optical transmission system which
has been tremendously researched and developed in recent years may,
in some cases, be applied to the wireless communication system. The
subcarrier optical transmission system is described in detail in
"Microwave and millimeter-wave fiber optic technologies for
subcarrier transmission systems" (Hiroyo Ogawa, IEICE Transactions
on Communications, Vol. E76-B, No. 9, pp.1078-1090, September,
1993), for example.
[0007] In the subcarrier transmission system, the intensity of a
main carrier which is typically unmodulated light is modulated by a
modulated signal that a subcarrier is modulated by information
which is a voice signal and/or an image signal, so that an optical
signal is obtained. The change in the intensity of the optical
signal uniquely corresponds to the change in the amplitude, the
change in the frequency or the change in the phase of the modulated
signal. In the subcarrier optical transmission system, an optical
fiber which is very low in loss is used. When the modulated signal
has a millimeter-wave band, therefore, the modulated signal can be
transmitted to a remote location as it is.
[0008] FIG. 17 is a block diagram showing the structure of a
typical first optical transmitter-receiver. In FIG. 17, the first
optical transmitter-receiver comprises a light source 110, an
external optical modulating portion 120, an optical fiber 140, an
optical/electrical converting portion 150, a frequency converting
portion 1510, and a demodulating portion 1720. The light source 110
and the external optical modulating portion 120 constitute an
optical transmitter 101, and are set in a base station, while the
optical/electrical converting portion 150, the frequency converting
portion 1510, and the demodulating portion 1720 constitute an
optical receiver 102, and are set in a control station. FIG. 17
shows only signal path in the one direction, that is, the signal
path transmitted from the base station to the control station.
[0009] In the first optical transmitter-receiver, an electrical
signal to be transmitted from the base station to the control
station is typically a modulated electrical signal Smod having a
millimeter-wave band that a subcarrier is modulated by a baseband
signal such as a voice signal and/or an image signal. The modulated
electrical signal Smod is inputted to the external optical
modulating portion 120 in the light transmitter 101 through an
antenna or an amplifier (not shown) from a portable telephone, a
PHS terminal, or the like which is moved outside the base station.
The light source 110 oscillates using unmodulated light as a main
carrier MC. The main carrier MC is also inputted to the external
optical modulating portion 120. The external optical modulating
portion 120 performs external light-intensity modulation, to
modulate the intensity of the inputted main carrier MC on the basis
of the change in the amplitude of the inputted modulated electrical
signal Smod, thereby obtaining an optical signal OSmod. The optical
signal OSmod itself outputted from the external optical modulating
portion 120 to the optical fiber 140 is changed into a carrier, and
is incident on the optical/electrical converting portion 150 in the
optical receiver 102 while the modulated electrical signal Smod is
being conveyed through the optical fiber 140 as it is. The
optical/electrical converting portion 150 performs
optical/electrical conversion, to convert the incident optical
signal OSmod into an electrical signal including its intensity
modulation component. The frequency converting portion 1510
down-coverts the electrical signal inputted from the
optical/electrical converting portion 150 into an electrical signal
having an intermediate frequency band. The demodulating portion
1720 demodulates the information of the baseband signal such as the
voice signal and/or the image signal on the basis of the electrical
signal having the intermediate frequency band inputted from the
frequency converting portion 1510.
[0010] Description is now made of the second optical
transmitter-receiver for merely optically transmitting a baseband
signal. FIG. 18 is a block diagram showing the structure of a
typical second optical transmitter-receiver. In FIG. 18, the second
optical transmitter-receiver comprises a light source driving
portion 1610, a light source 110, an optical fiber 140, and an
optical/electrical converting portion 150. The light source driving
portion 1610 and the light source 110 constitute an optical
transmitter 101, while the optical/electrical converting portion
150 constitutes an optical receiver 102. In the second optical
transmitter-receiver, it is assumed that a baseband signal SBB to
be transmitted from the optical transmitter 101 to the optical
receiver 102 is digital information which is a voice signal and/or
an image signal, for example. The baseband signal SBB is inputted
to the light source driving portion 1610. The light source driving
portion 1610 drives the light source 110, and modulates the
intensity of an optical signal outputted from the light source 110
on the basis of the inputted baseband signal SBB (a direct optical
modulation system). The optical signal is transmitted through the
optical fiber 140, and is then optical/electrical-converted in the
optical/electrical converting portion 150, so that the original
baseband signal SBB is obtained. Such a light transmission
technique is general, and is described in Chapter 2 "Practice of
Optical Communication System" of "Hikari Tsushin Gijyutsu Dokuhon
(Optical Transmission Technical Book)" (edited by Shimada, Ohm
Publishing Co., Ltd.) issued in 1980, for example.
[0011] However, the optical/electrical converting portion 150 and
the frequency converting portion 1510 shown in FIG. 17 must
accurately perform optical/electrical conversion and frequency
conversion of a signal having a high frequency such as a
millimeter-wave band, so that wideband characteristics are
required. Otherwise the demodulating portion 1720 would not perform
accurate demodulation processing. In the first optical
transmitter-receiver, therefore, electrical components
corresponding to a high frequency band are interconnected. For this
connection, a dedicated connector, waveguide or semirigid cable is
used. The waveguide or the semirigid cable is difficult to freely
work, so that the first optical transmitter-receiver is difficult
to manufacture. It is necessary to use a wave guide, in the case of
an attempt to transmit an electrical signal having a high-frequency
such as milliwave band with low loss. However, the size of the
first transmitter-receiver becomes large, because the size of the
waveguide is larger than the one of coaxial cable.
[0012] As described in the foregoing, the second optical
transmitter-receiver (see FIG. 18) is frequently used for online
transmitting the baseband signal SBB which is digital information
by wire. On the other hand, it is examined whether or not the first
optical transmitter-receiver (see FIG. 17) is applied to a wireless
communication system. The first and second optical
transmitter-receivers are thus examined as separate systems because
they differ in their applications. The optical transmitter-receiver
for simultaneously optically transmitting both a baseband signal
and a high-frequency electrical signal has not so examined. If a
wavelength division multiplexing technique is used, however, such
an optical transmitter-receiver can be constructed. That is, the
optical signal outputted from the light source 110 shown in FIG. 18
and the optical signal outputted from the external optical
modulating portion 120 shown in FIG. 17 are
wavelength-division-multiplexed on the transmission side. A signal
obtained by the wavelength division multiplexing (WDM) is
transmitted through the optical fiber 140, and is separated on the
side of optical receiving. Thereafter, signals obtained by the
separation are then respectively optical/electrical-converted.
Consequently, both the signals are simultaneously obtained on the
receiving side. However, the optical transmitter-receiver to which
a wavelength division multiplexing technique is applied must
separate an optical signal obtained by accurate wavelength division
multiplexing on the side of optical receiving. Therefore, a
plurality of light sources 110 which differ in oscillation
wavelength are required, so that significant cost is required to
construct the optical transmitter-receiver.
[0013] U.S. Pat. No. 5,596,436 discloses an optical
transmitter-receiver to which a subcarrier multiplex optical
transmission system is applied, which has apparently similar
portions to those in some of optical transmitter-receivers
disclosed in the present application. In the optical
transmitter-receiver according to the U.S. patent, however,
modulated electrical signals are respectively first produced by
modulating subcarriers by baseband signals using mixers. A
multiplexed signal is produced by multiplexing the modulated
electrical signals by a combiner 40. An external optical modulator
46 modulates unmodulated light from a laser 44 by the multiplexed
signal. The optical transmitter according to the U.S. patent
differs in structure from the optical transmitter 101 according to
the present invention. That is, a single subcarrier is used in the
optical transmitter 101 according to the present invention, while a
plurality of subcarriers are used in the optical transmitter
according to the U.S. patent. Consequently, spectrums of optical
signals outputted from both the optical transmitters differ from
each other. In the optical signal according to the U.S. patent, a
component of a main carrier and a component of each of subcarriers
are in close proximity to each other on an optical frequency axis.
On the other hand, in an optical signal OS according to the present
invention (as described later), a component of a main carrier and
components of both sidebands are not in close proximity to each
other. Consequently, the optical receiver according to the present
invention produces such a significant technical effect that a
component of a baseband signal SBB can be taken out more simply and
accurately, as compared with that in the U.S. patent.
SUMMARY OF THE INVENTION
[0014] Therefore, an object of the present invention is to provide
an optical transmitter-receiver capable of optically transmitting a
high-frequency electrical signal and being simple in manufacture
and small in size.
[0015] Another object of the present invention is to provide an
optical transmitter-receiver capable of simultaneously optically
transmitting both a baseband signal and a high-frequency signal
using the same light source.
[0016] The first aspect is directed to an optical
transmitter-receiver in which an optical transmitter and an optical
receiver are interconnected such that an optical transmission is
possible, characterized by comprising a double-modulating portion,
to which a subcarrier modulated by an electrical signal to be
transmitted is inputted from outside, for double-modulating a main
carrier which is unmodulated light having a predetermined optical
frequency by the inputted subcarrier, to produce and output a
double-modulated optical signal, an optical spectrum of the
double-modulated optical signal inputted from the double-modulating
portion including a component of the main carrier at the
predetermined optical frequency and further including components of
an upper sideband and a lower sideband at frequencies spaced by the
frequency of the subcarrier apart from the predetermined optical
frequency, an optical filter portion for selectively passing an
optical signal including the component of either one of the upper
sideband and the lower sideband in the double-modulated optical
signal inputted from the double-modulating portion, and an
optical/electrical converting portion for
optical/electrical-converting the optical signal inputted from the
optical filter portion, to obtain the electrical signal to be
transmitted, the optical transmitter comprising at least the local
oscillating portion and the double-modulating portion, and the
optical receiver comprising at least the optical/electrical
converting portion, the optical filter portion being included in
either one of the optical transmitter and the optical receiver.
[0017] According to the first aspect, the optical/electrical
converting portion can directly obtain from the optical signal the
electrical signal having a relatively low frequency to be
transmitted, thereby eliminating the necessity of an electrical
component, which is high in cost and is difficult to process,
corresponding to a subcarrier band which is a relatively high
frequency as in a conventional optical transmission of a
subcarrier. Correspondingly, the optical receiver can be
constructed simply and at low cost.
[0018] A second aspect is characterized in that in the first
aspect, the double-modulating portion comprises a semiconductor
laser diode for outputting the main carrier, and at least one
external optical modulating portion for amplitude-modulating the
main carrier inputted from the semiconductor laser diode by a
subcarrier amplitude-modulated by an electrical signal to be
transmitted which is inputted from outside using an external
optical modulation.
[0019] According to the second aspect, the double-modulating
portion is constituted by an existing semiconductor laser diode and
an existing external optical modulating portion, so that the
optical transmitter-receiver is constructed at low cost.
[0020] A third aspect is characterized in that in the second
aspect, wherein the subcarrier amplitude-modulated by said
electrical signal to be transmitted is a signal transmitted from
outside, further comprising an antenna portion receiving the signal
transmitted for supplying the signal to said double-modulating
portion.
[0021] According to the third aspect, the optical
trnasmitter-receiver can be easily connected to the wireless
transmission system by comprising the antenna portion receiving the
signal transmitted from outside.
[0022] A fourth aspect is characterized in that in the third
aspect, the electrical signal to be transmitted is a multichannel
signal frequency-multiplexed, and the electrical modulating portion
amplitude-modulates the inputted subcarrier by the multichannel
signal, to produce and output the modulated electrical signal.
[0023] According to the fourth aspect, the optical
transmitter-receiver can optically transmit a lot of
information.
[0024] A fifth aspect is characterized in that in the third aspect,
the electrical signal to be transmitted is digital information, and
the electrical modulating portion OOK (on-off keying)-modulates the
subcarrier by the digital information.
[0025] According to the fifth aspect, the optical
transmitter-receiver can transmit information high in quality.
[0026] A sixth aspect is directed to an optical
transmitter-receiver in which an optical transmitter and an optical
receiver are interconnected such that optical transmission is
possible, comprising a double-modulating portion, to which a
subcarrier modulated by an electrical signal to be transmitted is
inputted from outside, for double-modulating a main carrier which
is unmodulated light having a predetermined optical frequency by
the inputted subcarrier, to produce and output a double-modulated
optical signal, an optical spectrum of said double-modulated
optical signal inputted from the double-modulating portion
including a component of said main carrier at said predetermined
optical frequency and further including components of an upper
sideband and a lower sideband at frequencies spaced by the
frequency of the subcarrier apart from the predetermined optical
frequency, an optical filter portion for selectively passing an
optical signal including the component of either one of said upper
sideband and the lower sideband in the double-modulated optical
signal inputted from the double-modulating portion, and and an
optical branching portion for branching the optical signal inputted
from the optical filter portion into a first optical signal and a
second optical signal and outputting the first and second optical
signals;
[0027] a first optical/electrical converting portion for
optical/electrical-converting the first optical signal inputted
from the optical branching portion, to obtain the electrical signal
to be transmitted; a second optical/electrical converting portion
for outputting as a detecting signal an electrical signal obtained
by optical/electrical-converting the second optical signal inputted
from the optical branching portion; and a wavelength control
portion for detecting the average values of detected signals
inputted from the second optical/electrical converting portion at
predetermined time intervals, and controlling the wavelength of the
double-modulated optical signal outputted from the
double-modulating portion on the basis of the maximum value of the
detected average values, the optical transmitter comprising at
least the local oscillating portion and the double-modulating
portion, the optical receiver comprising at least the first
optical/electrical converting portion, and the optical filter
portion being included in either one of the optical transmitter and
the optical receiver.
[0028] According to the sixth aspect, as same as the first aspect,
the optical transmitter-receiver can be constructed simply and at
low cost, eliminating the necessity of an electrical component,
which is high in cost and is difficult to process, corresponding to
a subcarrier band which is a relatively high frequency. Further,
the optical filter portion can output an optical signal possible a
constantly precise demodulation for controlling a wavelength of the
double-modulated optical.
[0029] An seventh aspect is directed to an optical
transmitter-receiver in which an optical transmitter and first and
second optical receivers are interconnected such that subcarrier
optical transmission is possible, characterized in that the optical
transmitter comprises a local oscillating portion for outputting a
subcarrier having a predetermine frequency, a double-modulating
portion for double-modulating a main carrier which is unmodulated
light having a predetermined optical frequency by an electrical
signal to be transmitted which is inputted from outside and the
subcarrier inputted from the local oscillating portion, to produce
and output a double-modulated optical signal, a spectrum of the
double-modulated optical signal outputted from the
double-modulating portion including a component of the main carrier
at the predetermined optical frequency and further including
components of an upper sideband and a lower sideband at frequencies
spaced by the frequency of the subcarrier apart from the
predetermined optical frequency, and an optical filter portion for
dividing the double-modulated optical signal inputted from the
double-modulating portion into a first optical signal including the
component of either one of the upper sideband and the lower
sideband and a second optical signal including the component of the
main carrier and the component of the other one of the upper
sideband and the lower sideband, to output the first optical signal
and the second optical signal, the first optical receiver
optical/electrical-converts the first optical signal transmitted
from the optical transmitter, to obtain the electrical signal to be
transmitted, and the second optical receiver optical/electrical
converts the second optical signal transmitted from the optical
transmitter, to output the subcarrier that is modulated by the
electrical signal to be transmitted.
[0030] The first optical signal includes the component of one of
the sidebands included in the double-modulated optical signal
obtained by the double modulation, and is
optical/electrical-converted by the first optical/electrical
converting portion, to be converted into an electrical signal to be
transmitted. Further, the second optical signal includes the
components of the other sideband and the main carrier in the
double-modulated optical signal, and is
optical/electrical-converted by the second optical/electrical
converting portion, to be converted into a signal that the
subcarrier is modulated by the electrical signal to be transmitted.
According to the seventh aspect, both the electrical signal to be
transmitted and the signal that the subcarrier is modulated by the
electrical signal to be transmitted can be simultaneously obtained
on the receiving side. Further, as apparent by referring to the
foregoing, both the signals can be transmitted by a single wave of
unmodulated light, so that the optical transmitter-receiver can be
constructed at low cast according to the seventh aspect without
requiring a plurality of light sources as in a wavelength division
multiplexing technique.
[0031] A eighth aspect is characterized in that in the seventh
aspect, the double-modulating portion comprises an electrical
modulating portion for amplitude-modulating the subcarrier inputted
from the local oscillating portion by the electrical signal to be
transmitted which is inputted from outside, to produce and output a
modulated electrical signal, a light source for outputting the main
carrier which is unmodulated light having a predetermined optical
frequency, and an external optical modulating portion for
amplitude-modulating the main carrier inputted from the light
source by the modulated electrical signal inputted from the
electrical modulating portion, to produce the double-modulated
optical signal.
[0032] According to the eighth aspect, the optical transmitter uses
the same light source to simultaneously transmit the electrical
signal to be transmitted and the signal that the subcarrier is
modulated by the electrical signal to be transmitted toward the
receiving side. Consequently, the optical transmitter-receiver is
constructed at low cost.
[0033] A ninth aspect is characterized in that in the eighth
aspect, the electrical signal to be transmitted is digital
information, and the electrical modulating portion OOK (on-off
keying)-modulates the subcarrier by the digital information.
[0034] According to the nineth aspect, the optical
transmitter-receiver can transmit information high in quality.
[0035] A tenth aspect is characterized in that in the seventh
aspect, the double-modulating portion comprises a light source for
outputting the main carrier which is unmodulated light having a
predetermined optical frequency, a first external optical
modulating portion for amplitude-modulating the main carrier
inputted from the light source by the subcarrier inputted from the
local oscillating portion, to produce and output a modulated
optical signal, and a second external optical modulating portion
for amplitude-modulating the modulated optical signal inputted from
the first external optical modulating portion by the electrical
signal to be transmitted which is inputted from outside, to produce
the double-modulated optical signal.
[0036] According to the tenth aspect, the optical transmitter uses
the same light source to simultaneously transmit the electrical
signal to be transmitted and the signal that the subcarrier is
modulated by the electrical signal to be transmitted toward the
receiving side. Consequently, the optical transmitter-receiver is
constructed at low cost.
[0037] An eleventh aspect is characterized in that in the seventh
aspect, the double-modulating portion comprises a light source for
outputting the main carrier which is unmodulated light having a
predetermined optical frequency, a first external optical
modulating portion for amplitude-modulating the main carrier
inputted from the light source by the electrical signal to be
transmitted which is inputted from outside, to produce and output a
modulated optical signal, and a second external optical modulating
portion for amplitude-modulating the modulated optical signal
inputted from the first external optical modulating portion by the
subcarrier inputted from the local oscillating portion, to produce
the double-modulated optical signal.
[0038] According to the eleventh aspect, the optical transmitter
uses the same light source to simultaneously transmit the
electrical signal to be transmitted and the signal that the
subcarrier is modulated by the electrical signal to be transmitted
toward the receiving side. Consequently, the optical
transmitter-receiver is constructed at low cost.
[0039] A twelfth aspect is characterized in that in the seventh
aspect, the optical filter portion comprises an optical circulator
portion for outputting the double-modulated optical signal inputted
from the double-modulating portion as it is, and an optical fiber
grating portion for reflecting the component of either one of the
upper sideband and the lower sideband in the double-modulated
optical signal inputted from the optical circulator portion, to
produce the first optical signal and output the produced first
optical signal to the optical circulator portion, and passing the
component of the main carrier and the component of the other one of
the upper sideband and the lower sideband, to produce and output
the second optical signal to the second optical receiver, the
optical circulator portion further outputting the first optical
signal inputted from the optical fiber grating portion as it is to
the first optical receiver.
[0040] In the twelfth aspect, the optical filter portion is
constituted by the optical circulator and the optical fiber grating
which are optical components, so that the optical
transmitter-receiver is constructed simply and at low cost.
[0041] An thirteenth aspect is characterized in that in the seventh
aspect, the second optical receiver comprises an antenna portion
for radiating to a space the subcarrier that is modulated by the
electrical signal to be transmitted which is obtained by the
optical/electrical conversion.
[0042] The subcarrier modulated by the electrical signal to be
transmitted is a signal suitable for wireless transmission.
According to the thirteenth aspect, the second optical receiver
comprises the antenna portion for radiating the subcarrier to a
space, so that the optical transmitter-receiver is easily connected
to a wireless transmission system.
[0043] A fourteenth aspect is characterized in that in the seventh
aspect, the electrical signal to be transmitted is an electrical
signal to be transmitted which is converted analog information into
digital information.
[0044] According to the fourteenth aspect, the optical
transmitter-receiver can transmit information high in quality.
[0045] A fifteenth aspect is characterized in that in the seventh
aspect, the electrical signal to be transmitted is a carrier
modulated by analog information and digital information, the
frequency of the carrier is an intermediate frequency lower than
that of the subcarrier outputted from the local oscillating
portion.
[0046] When the electrical signal to be transmitted is the
above-mentioned electrical signal, the carrier having the
intermediate frequency modulated by the analog information or the
like and the signal that the subcarrier is modulated by the carrier
having the intermediate frequency are obtained on the receiving
side of the optical transmitter-receiver according to the fifteenth
aspect. Consequently, the optical transmitter-receiver can perform
optical transmission which does not depend on a modulation
form.
[0047] A sixteenth aspect is characterized in that in the seventh
aspect, the electrical signal to be transmitted is obtained by
multiplexing a plurality of electrical signals that have the
intermediate frequency and are modulated by analog information or
digital information using a predetermined multiplexing technique,
respectively.
[0048] A seventeenth aspect is characterized in that in the
sixteenth aspect, the predetermined multiplexing technique is a
frequency division multiplexing access, a time division
multiplexing access or a code division multiplexing access.
[0049] According to the sixteenth and seventeenth aspects, the
optical transmitter-receiver can multiplex a lot of information and
optically transmit information obtained by the multiplexing.
[0050] A eighteenth aspect is directed to an optical
transmitter-receiver in which an optical transmitter and first and
second optical receivers are interconnected such that subcarrier
optical transmission is possible, characterized in that the optical
transmitter comprises a local oscillating portion for outputting a
subcarrier having a predetermine frequency, a double-modulating
portion for double-modulating a main carrier which is unmodulated
light having a predetermined optical frequency by an electrical
signal to be transmitted which is inputted from outside and by the
subcarrier inputted from the local oscillating portion, to produce
and output a double-modulated optical signal, and an optical
branching portion for branching the double-modulated optical signal
inputted from the double-modulating portion and outputting
double-modulated optical signals obtained by the branching, the
first optical receiver comprises a low-pass filter portion for
passing a component included in a low frequency band of an
electrical signal obtained by optical/electrical-converting the
double-modulated optical signal transmitted from the optical
transmitter, to output the electrical signal to be transmitted, and
the second optical receiver comprises a high-pass filter portion
for passing a component included in a high frequency band of an
electrical signal obtained by optical/electrical-converting the
double-modulated optical signal transmitted from the optical
transmitter, to output the subcarrier that is modulated by the
electrical signal to be transmitted.
[0051] On the receiving side in the eighteenth aspect, as same as
the seventh aspect, the low-pass filter portion and the high-pass
filter portion respectively pass a low frequency band part and a
high frequency band part of the electrical signal obtained by
optical/electrical-convert- ing the double-modulated optical
signal. Therefore, signals obtained upon respectively modulating
the subcarrier by an electrical signal to be transmitted which is
included in the relatively low frequency band and an electrical
signal to be transmitted which is included in the relatively high
frequency band can be simultaneously obtained. Further, the optical
transmitter-receiver can be constructed at low cost.
[0052] A nineteenth aspect is characterized in that in the
eighteenth aspect, the double-modulating portion comprises an
electrical modulating portion for amplitude-modulating the
subcarrier outputted from the local oscillating portion by the
electrical signal to be transmitted which is inputted from outside,
to produce and output a modulated electrical signal, a light source
for outputting the main carrier which is unmodulated light having a
predetermined optical frequency, and an external optical modulating
portion for amplitude-modulating the main carrier outputted from
the light source by the modulated electrical signal inputted from
the electrical modulating portion, to produce the double-modulated
optical signal.
[0053] According to the nineteenth aspect, the optical transmitter
uses the same light source to simultaneously transmit the
electrical signal to be transmitted and the signal that the
subcarrier is modulated by the electrical signal to be transmitted
toward the receiving side. Consequently, the optical
transmitter-receiver is constructed at low cost.
[0054] A twentieth aspect is characterized in that in the
nineteenth aspect, the electrical signal to be transmitted is
digital information, and the electrical modulating portion OOK
(on-off keying)-modulates the subcarrier by the digital
information.
[0055] According to the twentieth aspect, the optical
transmitter-receiver can transmit information high in quality.
[0056] A twenty-first aspect is characterized in that in the
eighteenth aspect, the double-modulating portion comprises a light
source for outputting the main carrier which is unmodulated light
having a predetermined optical frequency, a first external optical
modulating portion for amplitude-modulating the main carrier
inputted from the light source by the subcarrier inputted from the
local oscillating portion, to produce and output a modulated
optical signal, and a second external optical modulating portion
for amplitude-modulating the modulated optical signal inputted from
the first external optical modulating portion by the electrical
signal to be transmitted which is inputted from outside, to produce
the double-modulated optical signal.
[0057] According to the twenty-first aspect, the optical
transmitter uses the same light source to simultaneously transmit
the electrical signal to be transmitted and the signal that the
subcarrier is modulated by the electrical signal to be transmitted
toward the receiving side. Consequently, the optical
transmitter-receiver is constructed at low cost.
[0058] A twenty-second aspect is characterized in that in the
eighteenth aspect, the double-modulating portion comprises a light
source for outputting the main carrier which is unmodulated light
having a predetermined optical frequency, a first external optical
modulating portion for amplitude-modulating the main carrier
inputted from the light source by the electrical signal to be
transmitted which is inputted from outside, to produce and output a
modulated optical signal, and a second external optical modulating
portion for amplitude-modulating the modulated optical signal
inputted from the first external optical modulating portion by the
subcarrier inputted from the local oscillating portion, to produce
the double-modulated optical signal.
[0059] According to the twenty-second aspect, the optical
transmitter uses the same light source to simultaneously transmit
the electrical signal to be transmitted and the signal that the
subcarrier is modulated by the electrical signal to be transmitted
toward the receiving side. Consequently, the optical
transmitter-receiver is constructed at low cost.
[0060] A twenty-third aspect is characterized in that in the
eighteenth aspect, an antenna portion for radiating to a space the
subcarrier that is modulated by the electrical signal to be
transmitted which is outputted from the high-pass filter portion is
set in a back end against the high-pass filter portion.
[0061] According to the twenty-third aspect, the optical
transmitter-receiver is simply connected to a wireless transmission
system, as in the thirteenth aspect.
[0062] A twenty-fourth aspect is characterized in that in the
eighteenth aspect, the electrical signal to be transmitted is a
carrier modulated by analog information or digital information, the
frequency of the carrier is an intermediate frequency lower than
that of the subcarrier outputted from the local oscillating
portion.
[0063] According to the twenty-fourth aspect, when the electrical
signal to be transmitted is the above-mentioned electrical signal,
the carrier having the intermediate frequency modulated by the
analog information or the like and the signal that the subcarrier
is modulated by the carrier having the intermediate frequency are
obtained on the side of optical receiving. Consequently, the
optical transmitter-receiver can perform optical transmission which
does not depend on a modulation form.
[0064] A twenty-fifth aspect is characterized in that in the
eighteenth aspect, the double-modulating portion modulates the main
carrier by the subcarrier inputted from the local oscillating
portion using a single sideband amplitude modulation system.
[0065] In the twenty-fifth aspect, the double-modulated optical
signal is not easily affected by wavelength dispersion in an
optical fiber serving as an optical transmission line by applying a
single sideband amplitude modulation system, so that the
transmission distance increases.
[0066] A twenty-sixth aspect is directed to an optical
transmitter-receiver in which an optical transmitter and an optical
receiver are interconnected such that subcarrier optical
transmission is possible, characterized in that the optical
transmitter comprises a local oscillating portion for outputting a
subcarrier having a predetermine frequency, and a double-modulating
portion for double-modulating a main carrier which is unmodulated
light having a predetermined optical frequency by an electrical
signal to be transmitted which is inputted from outside and by the
subcarrier inputted from the local oscillating portion, to produce
and output a double-modulated optical signal, and the optical
receiver comprises an optical/electrical converting portion for
optical/electrical-converting the double-modulated optical signal
transmitted from the optical transmitter, to output an electrical
signal, a distributing portion for distributing the electrical
signal inputted from the optical/electrical converting portion into
at least two electrical signals, a low-pass filter portion for
passing a component included in a low frequency band of the
electrical signal obtained by the distribution, to output the
electrical signal to be transmitted, and a high-pass filter portion
for passing a component included in a high frequency band of the
electrical signal obtained by the distribution, to output the
subcarrier that is modulated by the electrical signal to be
transmitted.
[0067] On the receiving side in the twenty-sixth aspect, the
low-pass filter portion and the high-pass filter portion
respectively pass a low frequency band part and a high frequency
band part of the electrical signal obtained by
optical/electrical-converting the double-modulated optical signal,
as in the seventh aspect. Therefore, signals that a subcarrier is
modulated by an electrical signal to be transmitted which is
included in the relatively low frequency band and an electrical
signal to be transmitted which is included in the relatively high
frequency band can be simultaneously obtained. Further, the optical
transmitter-receiver can be constructed at low cost.
[0068] A twenty-seventh aspect is characterized in that in the
twenty-sixth aspect, the double-modulating portion comprises an
electrical modulating portion for amplitude-modulating the
subcarrier inputted from the local oscillating portion by the
electrical signal to be transmitted which is inputted from outside,
to produce and output a modulated electrical signal, a light source
for outputting the main carrier which is unmodulated light having a
predetermined optical frequency, and an external optical modulating
portion for amplitude-modulating the main carrier inputted from the
light source by the modulated electrical signal inputted from the
electrical modulating portion, to produce the double-modulated
optical signal.
[0069] According to the twenty-seventh aspect, the optical
transmitter uses the same light source to simultaneously transmit
the electrical signal to be transmitted and the signal that the
subcarrier is modulated by the electrical signal to be transmitted
toward the receiving side. Consequently, the optical
transmitter-receiver is constructed at low cost.
[0070] A twenty-eighth aspect is characterized in that in the
twenty-seventh aspect, the electrical signal to be transmitted is
digital information, and the electrical modulating portion OOK
(on-off keying)-modulates the subcarrier by the digital
information.
[0071] According to the twenty-eighth aspect, the optical
transmitter-receiver can transmit information high in quality.
[0072] A twenty-ninth aspect is characterized in that in the
twenty-sixth aspect, the double-modulating portion comprises a
light source for outputting the main carrier which is unmodulated
light having a predetermined optical frequency, a first external
optical modulating portion for amplitude-modulating the main
carrier inputted from the light source by the subcarrier inputted
from the local oscillating portion, to produce and output a
modulated optical signal, and a second external optical modulating
portion for amplitude-modulating the modulated optical signal
inputted from the first external optical modulating portion by the
electrical signal to be transmitted which is inputted from outside,
to produce the double-modulated optical signal.
[0073] According to the twenty-ninth aspect, the optical
transmitter uses the same light source to simultaneously transmit
the electrical signal to be transmitted and the signal that the
subcarrier is modulated by the electrical signal to be transmitted
toward the receiving side. Consequently, the optical
transmitter-receiver is constructed at low cost.
[0074] A thirtieth aspect is characterized in that in the
twenty-sixth aspect, the double-modulating portion comprises a
light source for outputting the main carrier which is unmodulated
light having a predetermined optical frequency, a first external
optical modulating portion for amplitude-modulating the main
carrier inputted from the light source by the electrical signal to
be transmitted which is inputted from outside, to produce and
output a modulated optical signal, and a second external optical
modulating portion for amplitude-modulating the modulated optical
signal inputted from the first external optical modulating portion
by the subcarrier inputted from the local oscillating portion, to
produce the double-modulated optical signal.
[0075] According to the thirtieth aspect, the optical transmitter
uses the same light source to simultaneously transmit the
electrical signal to be transmitted and the signal that the
subcarrier is modulated by the electrical signal to be transmitted
toward the receiving side. Consequently, the optical
transmitter-receiver is constructed at low cost.
[0076] A thirty-first aspect is characterized in that in the
twenty-sixth aspect, an antenna portion for radiating to a space
the subcarrier that is modulated by the electrical signal to be
transmitted which is outputted from the high-pass filter portion is
set in a back end against the high-pass filter portion.
[0077] According to the thirty-first aspect, the optical
transmitter-receiver is simply connected to a wireless transmission
system, as in the thirteenth aspect.
[0078] A thirty-second aspect is characterized in that in the
twenty-sixth aspect, the electrical signal to be transmitted is a
carrier modulated by analog information or digital information, the
frequency of the carrier is an intermediate frequency lower than
that of the subcarrier outputted from the local oscillating
portion.
[0079] According to the thirty-second aspect, the optical
transmitter-receiver can perform optical transmission which does
not depend on a modulation form, as in the fifteenth aspect.
[0080] A thirty-third aspect is characterized in that in the
twenty-sixth aspect, the double-modulating portion modulates the
main carrier by the subcarrier inputted from the local oscillating
portion using a single sideband amplitude modulation system.
[0081] In the thirty-third aspect, the double-modulated optical
signal is not easily affected by wavelength dispersion in an
optical fiber serving as an optical transmission line, so that the
transmission distance increases, as in the twenty-fifth aspect.
[0082] A thirty-fourth aspect is directed to an optical
transmitter-receiver in which an optical transmitter and first and
second optical receivers are interconnected such that subcarrier
optical transmission is possible, characterized in that the optical
transmitter comprises a local oscillating portion for outputting a
subcarrier having a predetermined frequency, a mode locked light
source which is mode-locked on the basis of the subcarrier inputted
from the local oscillating portion and oscillating with spacing
between optical frequencies related to the subcarrier, to produce
and output a mode-locked optical signal, an external optical
modulating portion for amplitude-modulating the mode-locked optical
signal inputted from the mode locked light source by an electrical
signal to be transmitted which is inputted from outside, to produce
and output a double-modulated optical signal, and an optical
branching portion for branching the double-modulated optical signal
inputted from the external optical modulating portion and
outputting double-modulated optical signals obtained by the
branching, the first optical receiver comprises a low-pass filter
portion for passing a component included in a low frequency band of
an electrical signal obtained by optical/electrical-converting the
double-modulated optical signal transmitted from the optical
transmitter, to output the electrical signal to be transmitted, and
the second optical receiver comprises a high-pass filter portion
for passing a component included in a high frequency band of an
electrical signal obtained by optical/electrical-converting the
double-modulated optical signal transmitted from the optical
transmitter, to output the subcarrier that is modulated by the
electrical signal to be transmitted.
[0083] On the receiving side in the thirty-fourth aspect, the
low-pass filter portion and the high-pass filter portion
respectively pass a low frequency band part and a high frequency
band part of the electrical signal obtained by
optical/electrical-converting the double-modulated optical signal,
as in the seventh aspect. Therefore, signals that the subcarrier is
modulated by an electrical signal to be transmitted which is
included in the relatively low frequency band and an electrical
signal to be transmitted which is included in the relatively high
frequency band can be simultaneously obtained. Further, the optical
transmitter-receiver can be constructed at low cost.
[0084] A thirty-fifth aspect is characterized in that in the
thirty-fourth aspect, an antenna portion for radiating to a space
the subcarrier that is modulated by the electrical signal to be
transmitted which is outputted from the high-pass filter portion is
set in a back end against the high-pass filter portion.
[0085] According to the thirty-fifth aspect, the optical
transmitter-receiver is simply connected to a wireless transmission
system, as in the thirteenth aspect.
[0086] A thirty-sixth aspect is characterized in that in the
thirty-fourth aspect, the electrical signal to be transmitted is a
carrier modulated by analog information or digital information, the
frequency of the carrier is an intermediate frequency lower than
that of the subcarrier outputted from the local oscillating
portion.
[0087] According to the thirty-sixth aspect, the optical
transmitter-receiver can perform optical transmission which does
not depend on a modulation form, as in the fifteenth aspect.
[0088] A thirty-seventh aspect is directed to an optical
transmitter-receiver in which an optical transmitter and an optical
receiver are interconnected such that subcarrier optical
transmission is possible, characterized in that the optical
transmitter comprises a local oscillating portion for outputting a
subcarrier having a predetermined frequency, a mode locked light
source which is mode-locked on the basis of a subcarrier inputted
from the local oscillating portion and oscillating with spacing
between optical frequencies related to the subcarrier, to produce
and output a mode-locked optical signal, and an external optical
modulating portion for amplitude-modulating the mode-locked optical
signal inputted from the mode locked light source by the electrical
signal to be transmitted which is inputted from outside, to produce
and output a double-modulated optical signal, and the optical
receiver comprises an optical/electrical converting portion for
optical/electrical-converting the double-modulated optical signal
transmitted from the optical transmitter, to output an electrical
signal, a distributing portion for distributing the electrical
signal inputted from the optical/electrical converting portion into
at least two electrical signals, a low-pass filter portion for
passing a component included in a low frequency band of the
electrical signal obtained by the distribution, to output the
electrical signal to be transmitted, and a high-pass filter portion
for passing a component included in a high frequency band of the
electrical signal obtained by the distribution, to output the
subcarrier that is modulated by the electrical signal to be
transmitted.
[0089] On the receiving side in the thirty-seventh aspect, the
low-pass filter portion and the high-pass filter portion
respectively pass a low frequency band part and a high frequency
band part of the electrical signal obtained by
optical/electrical-converting the double-modulated optical signal,
as in the seventh aspect. Therefore, signals obtained by modulating
the subcarrier by an electrical signal to be transmitted which is
included in the relatively low frequency band and an electrical
signal to be transmitted which is included in the relatively high
frequency band can be simultaneously obtained. Further, the optical
transmitter-receiver can be constructed at low cost.
[0090] A thirty-eighth aspect is characterized in that in the
thirty-seventh aspect, an antenna portion for radiating to a space
the subcarrier that is modulated by the electrical signal to be
transmitted which is outputted from the high-pass filter portion is
set in a back end against the high-pass filter portion.
[0091] According to the thirty-eighth aspect, the optical
transmitter-receiver is simply connected to a wireless transmission
system, as in the thirteenth aspect.
[0092] A thirty-ninth aspect is characterized in that in the
thirty-seventh aspect, the electrical signal to be transmitted is a
carrier modulated by analog information or digital information, the
frequency of the carrier is an intermediate frequency lower than
that of the subcarrier outputted from the local oscillating
portion.
[0093] According to the thirty-ninth aspect, the optical
transmitter-receiver can perform optical transmission which does
not depend on a modulation form, as in the fifteenth aspect.
[0094] A fortieth aspect is directed to an optical
transmitter-receiver in which an optical transmitter and first and
second optical receivers are interconnected such that optical
transmission is possible, wherein the optical transmitter comprises
a first light source for outputting first unmodulated light having
a first optical frequency, an external optical modulating portion
for amplitude-modulating the first unmodulated light inputted from
the first light source by an electrical signal to be transmitted
which is inputted from outside, to produce and output a modulated
optical signal, a second light source for outputting second
unmodulated light having a second optical frequency which differs
from the first optical frequency by a predetermined optical
frequency, an optical multiplexing portion for multiplexing the
modulated optical signal inputted from the external optical
modulating portion and the second unmodulated light inputted from
the second light source such that polarization of the modulated
optical signal and the second unmodulated light coincide with each
other, to produce and output an optical signal, and an optical
branching portion for branching the optical signal inputted from
the optical multiplexing portion and outputting optical signals
obtained by the branching, the first optical receiver comprises a
low-pass filter portion for passing a component included in a low
frequency band of an electrical signal obtained by
optical/electrical-converting the optical signal transmitted from
the optical transmitter, to output the electrical signal to be
transmitted, and the second optical receiver comprises a high-pass
filter portion for passing a component included in a high frequency
band of an electrical signal obtained by optical/electrical
converting the optical signal transmitted from the optical
transmitter, to output the subcarrier that is modulated by the
electrical signal to be transmitted.
[0095] According to the fortieth aspect, the first unmodulated
light is amplitude-modulated by the electrical signal to be
transmitted, to produce the modulated optical signal. The modulated
optical signal and the second unmodulated light are multiplexed, to
produce the optical signal. Although optical/electrical conversion
must be made twice in the seventh aspect, for example, the optical
transmitter in the fortieth aspect performs optical/electrical
conversion only once. By thus reducing the number of times of
optical/electrical conversion, low-loss optical transmission can be
realized. Further, in the optical transmitter in the fortieth
aspect, no electrical component for amplitude-modulating the
subcarrier by the electrical signal to be transmitted is required.
That is, according to the fortieth aspect, the necessity of an
electrical component, which is high in cost and is difficult to
process, corresponding to a subcarrier band which is a relatively
high frequency is eliminated. Correspondingly, the optical receiver
can be constructed simply and at low cost.
[0096] A forty-first aspect is characterized in that in the
fortieth aspect, an antenna portion for radiating to a space the
subcarrier that is modulated by the electrical signal to be
transmitted which is outputted from the high-pass filter portion is
set in a back end against the high-pass filter portion.
[0097] According to the forty-first aspect, the optical
transmitter-receiver is simply connected to a wireless transmission
system, as in the thirteenth aspect.
[0098] A forty-second aspect is characterized in that in the
fortieth aspect, the electrical signal to be transmitted is a
carrier modulated by analog information or digital information, the
frequency of the carrier is an intermediate frequency lower than
that of the subcarrier outputted from the local oscillating
portion.
[0099] According to the forty-second aspect, the optical
transmitter-receiver can perform optical transmission which does
not depend on a modulation form, as in the fifteenth aspect.
[0100] A forty-third aspect is directed to an optical
transmitter-receiver in which an optical transmitter and an optical
receiver are interconnected such that optical transmission is
possible, wherein the optical transmitter comprises a first light
source for outputting first unmodulated light having a first
optical frequency, an external optical modulating portion for
amplitude-modulating the first unmodulated light inputted from the
first light source by an electrical signal to be transmitted which
is inputted from outside, to produce and output a modulated optical
signal, a second light source for outputting second unmodulated
light having a second optical frequency which differs from the
first optical frequency by a predetermined optical frequency, an
optical multiplexing portion for multiplexing the modulated optical
signal inputted from the external optical modulating portion and
the second unmodulated light inputted from the second light source
such that polarization of the modulated optical signal and the
second unmodulated light coincide with each other, to produce and
output an optical signal, and an optical branching portion for
branching the optical signal inputted from the optical multiplexing
portion and outputting optical signals branched by the branching
portion, and the optical receiver comprises an optical/electrical
converting portion for optical/electrical-converting the optical
signal transmitted from the optical transmitter, to output an
electrical signal, a distributing portion for distributing the
electrical signal inputted from the optical/electrical converting
portion into at least two electrical signals, a low-pass filter
portion for passing a component included in a low frequency band of
the electrical signal obtained by the distribution, to output the
electrical signal to be transmitted, and a high-pass filter portion
for passing a component included in a high frequency band of the
electrical signal obtained by the distribution, to output the
subcarrier that is modulated by the electrical signal to be
transmitted.
[0101] According to the forty-third aspect, it is possible to
realize low-loss optical transmission as well as to construct the
optical transmitter-receiver simply and at low cost.
[0102] A forty-fourth aspect is characterized in that in the
forty-third aspect, an antenna portion for radiating to a space the
subcarrier that is modulated by the electrical signal to be
transmitted which is outputted from the high-pass filter portion is
set in a back end against the high-pass filter portion.
[0103] According to the forty-fourth aspect, the optical
transmitter-receiver is simply connected to a wireless transmission
system, as in the thirteenth aspect.
[0104] A forty-fifth aspect is characterized in that in the
forty-third aspect, the electrical signal to be transmitted is a
carrier modulated by analog information or digital information, the
frequency of the carrier is an intermediate frequency lower than
that of the subcarrier outputted from the local oscillating
portion.
[0105] According to the forty-fifth aspect, the optical
transmitter-receiver can perform optical transmission which does
not depend on a modulation form, as in the fifteenth aspect.
[0106] The foregoing 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
[0107] FIG. 1 is a block diagram showing the structure of an
optical transmitter-receiver according to a first embodiment of the
present invention;
[0108] FIGS. 2(a-1) to (d-1) schematically illustrate spectrums of
signals in principal parts (a-1) to (d-1) of the optical
transmitter-receiver shown in FIG. 1;
[0109] FIG. 3 is a block diagram showing the structure of an
optical transmitter-receiver according to a second embodiment of
the present invention;
[0110] FIGS. 4(a-3) to (b-3) schematically illustrate spectrums of
signals in principal parts (a-3) to (b-3) of the optical
transmitter-receiver shown in FIG. 3;
[0111] FIG. 5 is a block diagram showing the structure of only an
optical transmitter with respect to an optical transmitter-receiver
according to a third embodiment of the present invention;
[0112] FIG. 6 is a block diagram showing the structure of an
optical transmitter-receiver according to a fourth embodiment of
the present invention;
[0113] FIG. 7 is a block diagram showing the structure of an
optical transmitter-receiver according to a fifth embodiment of the
present invention;
[0114] FIGS. 8(a-7) to (f-7) schematically illustrate spectrums of
signals in principal parts (a-7) to (f-7) of the optical
transmitter-receiver shown in FIG. 7;
[0115] FIG. 9 is a block diagram showing the detailed structure of
optical filter portions 710;
[0116] FIG. 10 is a block diagram showing the structure of an
optical transmitter-receiver according to a sixth embodiment of the
present invention;
[0117] FIGS. 11(a-10) to (f-10) schematically illustrate spectrums
of signals in principal parts (a-10) to (f-10) of the optical
transmitter-receiver shown in FIG. 10;
[0118] FIG. 12 is a block diagram showing the structure of an
optical transmitter-receiver according to a seventh embodiment of
the present invention;
[0119] FIG. 13 is a block diagram showing the structure of an
optical transmitter-receiver according to an eighth embodiment of
the present invention;
[0120] FIG. 14 is a block diagram showing the structure of only an
optical transmitter with respect to an optical transmitter-receiver
according to a ninth embodiment of the present invention;
[0121] FIG. 15 is a block diagram showing the structure of only an
optical transmitter with respect to an optical transmitter-receiver
according to a tenth embodiment of the present invention;
[0122] FIGS. 16(a-15) to (d-15) schematically illustrate spectrums
of signals in principal parts (a-10) to (d-15) of the optical
transmitter shown in FIG. 15;
[0123] FIG. 17 is a block diagram showing the structure of a
conventional first optical transmitter-receiver; and
[0124] FIG. 18 is a block diagram showing the structure of a
conventional second optical transmitter-receiver.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0125] (First Embodiment)
[0126] FIG. 1 is a block diagram showing the structure of an
optical transmitter-receiver according to a first embodiment of the
present invention. In the optical transmitter-receiver shown in
FIG. 1, an optical transmitter 101 and an optical receiver 102 are
interconnected through an optical fiber 140 such that optical
transmission is possible. The optical transmitter 101 comprises a
light source 110, an external optical modulating portion 120, an
optical filter portion 130, and an antenna portion 190, and the
optical receiver 102 comprises an optical/electrical converting
portion 150.
[0127] FIGS. 2(a-1) to (d-1) schematically illustrate spectrums of
signals in principal parts (a-1) to (d-1) of the optical
transmitter-receiver shown in FIG. 1.
[0128] The operations of the optical transmitter-receiver shown in
FIG. 1 will be described on the basis of FIGS. 1 and 2. In the
optical transmitter 101, a signal amplitude-modulated an electrical
subcarrier SC having a predetermined frequency f0 by a baseband
signal SBB to be transmitted having the frequency f1 (hereinafter
referred to as a modulated electrical signal Smod) is transmitted
from the outside. The antenna portion 190 receives the modulated
electrical signal Smod and outputs it to the external optical
modulating portion 120. It is assumed that a current waveform of
the baseband signal SBB is I(t) and the amplitude modulation is
performed at the degree of modulation md. The voltage waveform
Vd(t) of the modulated electrical signal Smod is expressed by the
following equation (1):
Vd(t)=(1+mdI(t))cos(.omega.0t) (1)
[0129] where .omega.0=2.pi.f0. When (1+mdI(t)) is replaced with
D(t), the foregoing equation (1) is expressed by the following
equation (2):
Vd(t)=D(t)cos(.omega.0t) (2)
[0130] The light source 110 is typically constituted by a
semiconductor laser diode, which oscillates unmodulated light
having a predetermined optical frequency .nu. as shown in FIG.
2(a-1), and outputs the unmodulated light as a main carrier MC. The
external optical modulating portion 120 has a Mach-Zehnder type
structure, for example, and amplitude-modulates the light intensity
of the main carrier MC inputted from the light source 110 by the
modulated electrical signal Smod inputted from the antenna portion
190, to produce an optical signal double-modulated (hereinafter
referred to as a double-modulated optical signal OSdmod). More
specifically, the Mach-Zehnder type external optical modulating
portion 120 first branches the inputted main carrier MC into two
main carriers. One of the branched main carriers MC is
optical-phase-modulatd by the inputted modulated electrical signal
Smod. The modulated main carrier MC is multiplexed with the other
branched main carrier MC, thereby producing the above-mentioned
double-modulated optical signal OSdmod. The change in the amplitude
of the double-modulated optical signal OSdmod uniquely corresponds
to the change in the amplitude of the modulated electrical signal
Smod. The optical spectrum of the double-modulated optical signal
OSdmod has a component of the main carrier MC at a center optical
frequency .nu., and further has components of sidebands (an upper
sideband and a lower sideband) at frequencies spaced an integral
multiple of the optical frequency f0 apart from the optical
frequency .nu. (only .+-.f0 are illustrated). The occupied
frequency bands of components of both the sidebands depend on the
above-mentioned frequency f1.
[0131] The field strength waveform E(t) of the double-modulated
optical signal OSdmod is then expressed by an equation. It is
assumed that the minimum value of a difference between input
voltages in cases where the amplitude of the double-modulated
optical signal OSdmod outputted from the external optical
modulating portion 120 reaches zero and the maximum is V.pi..
Further, it is assumed that a phase difference between the main
carrier MC to be multiplexed in the external optical modulating
portion 120 and the phase-modulated main carrier MC is set to
.pi./2. By following the assumption, the double-modulated optical
signal OSdmod is expressed by the following equation (3): 1 E ( t )
= E 2 { cos ( 2 v t ) + cos ( 2 v t ) cos ( 1 ) - sin ( 2 v t ) sin
( 1 ) } = E 2 { cos ( 2 v t ) - cos ( k D ( t ) cos ( 0 t ) sin ( 2
v t ) - sin ( k D ( t ) cos ( 0 t ) ) cos ( 2 v t ) } ( 3 )
[0132] where k=.pi./2V.pi., and .delta.1 is expressed by the
following equation (4): 2 1 = 2 D ( t ) cos ( 0 t ) V + 2 ( 4 )
[0133] For example, when it is assumed that the baseband signal SBB
is a sine wave, and its current waveform is expressed by I(t)=cos
(.omega.1t) (.omega.1=2.pi.f1), .delta.1 is expressed by the
following equation (5), and the foregoing equation (3) can be
expanded as in the following equation (6) if the following equation
(5) is used: 3 1 = k ( 1 + m d cos ( 1 t ) ) cos ( 0 t ) + 2 ( 5 )
E ( t ) = E 2 cos ( 2 v t ) - E 2 cos ( k ( 1 + m cos ( 1 t ) ) cos
( 0 t ) ) cos ( 2 v t ) - E 2 sin ( k ( 1 + m cos ( 1 t ) ) cos ( 0
t ) ) cos ( 2 v t ) ( 6 )
[0134] In the foregoing equation (6), the field strength waveform
E(t) of the double-modulated optical signal OSdmod is finally
expressed by the following equation (7), considering the optical
frequency .nu. and linear terms of .nu., f1, and f0: 4 E ( t ) = E
2 cos ( 2 v t ) { 1 + 2 J 1 ( k ) J 0 2 ( k m d 2 ) cos ( 0 t ) - 2
J 0 ( k ) J 0 ( k m d 2 ) J 1 ( k m d 2 ) ( cos ( 0 + 1 ) t + cos (
0 - 1 ) t ) } ( 7 )
[0135] where J0 is a zero-order Bessel function, and J1 is a linear
Bessel function.
[0136] The double-modulated optical signal OSdmod as described
above is inputted to the optical filter portion 130. The passband
of the optical filter portion 130 is so set that only the component
of the upper sideband or the lower sideband out of the components
of the double-modulated optical signal OSdmod shown in FIG. 2(b-1)
can be extracted. For example, when the passband of the optical
filter portion 130 is set to the vicinity of an optical frequency
.nu.+f0 (see a portion enclosed by a dotted line in FIG. 2(b-1)),
only the component of the upper sideband passes through the optical
filter portion 130 as an optical signal OS. The optical spectrum of
the optical signal OS has only the same component as that of the
upper sideband, and is included in an optical frequency band in the
vicinity of the optical frequency .nu.+f0, as shown in FIG.
2(c-1).
[0137] The field strength waveform Ef(t) of the optical signal OS
is then expressed by the following equation (8). When the following
equation (8) is arranged, the following equation (9) is obtained: 5
E f ( t ) = E 2 J 0 ( k m d 2 ) { J i ( k ) J 0 ( k m d 2 ) cos ( +
0 ) t - J 0 ( k ) J 1 ( k m d 2 ) ( cos ( + 0 + 1 ) t + cos ( + 0 -
1 ) t ) } ( 8 ) E f ( t ) = K cos ( + 0 ) t ( 1 - m ' cos 1 t ) ( 9
)
[0138] In the foregoing equations (8) and (9), .omega.=2.pi..nu..
In the foregoing equation (9), m' is expressed by the following
equation (10), and K is expressed by the following equation (11): 6
m ' = J 0 ( k ) J 1 ( k m d 2 ) J 1 ( k ) J 0 ( k m d 2 ) ( 10 ) K
= E 2 J 0 ( k m d 2 ) J 1 ( k ) J 0 ( k m d 2 ) ( 11 )
[0139] The optical signal OS described with reference to the
foregoing equations and FIG. 2(c-1) is outputted to the optical
fiber 140 from the optical filter portion 130, is transmitted
through the optical fiber 140, and is incident on the
optical/electrical converting portion 150 in the optical receiver
102. Consequently, the optical signal OS is transmitted to a remote
location.
[0140] The optical/electrical converting portion 150
optical/electrical converts the incident optical signal OS and
outputs an electrical signal. It is found by referring to FIG.
2(c-1) that the optical signal OS is equivalent to one obtained
upon amplitude-modulating a carrier having the optical frequency
.nu.+f0 by the baseband signal SBB (=cos 2.pi.f1t) which is
information to be transmitted. Consequently, the current waveform
Ipd(t) of the electrical signal outputted from the
optical/electrical converting portion 150 is expressed by the
following equation (12): 7 I p d ( t ) = 2 K 2 ( 1 - m ' cos 1 t )
2 = I p d ( 1 - 2 m ' cos 1 t + m '2 cos 2 1 t ) ( 12 )
[0141] where .eta. is the conversion efficiency of the
optical/electrical converting portion 150, and Ipd is a DC (direct
current) component. As can be seen by referring to the foregoing
equation (12), if only a component of .omega.1 (a component having
a frequency f1) is extracted from the electrical signal outputted
from the optical/electrical converting portion 150, an amplitude
modulation component of the optical signal OS, that is, the current
waveform I(t) of the baseband signal SBB is directly obtained, as
shown in FIG. 2(d-1). It is more easily feasible to extract only
the component of .omega.1 by connecting a band-pass filter to a
back end against the optical/electrical converting portion 150. The
optical/electrical converting portion 150 may only have frequency
characteristics for a band of the frequency f1, and does not
require wideband characteristics as in conventional subcarrier
optical transmission.
[0142] As described in the foregoing, it is assumed from the
viewpoint of simplification of illustration that the baseband
signal SBB is expressed by I(t)=cos (.omega.1t), that is, it is a
one-channel signal. Even if the baseband signal SBB is a
multichannel signal, that is, it is expressed by I(t)=cos
(.omega.1t)+cos (.omega.2t)+. . . , it can be demodulated in the
optical transmitter-receiver, similarly to the one-channel
signal.
[0143] When the baseband signal SBB is particularly digital
information, the component of the subcarrier SC of the modulated
electrical signal Smod is performed digital amplitude modulation
called ASK (Amplitude Shift Keying) or on-off keying. Consequently,
the optical transmitter-receiver can optically transmit information
high in quality.
[0144] When the subcarrier SC is double sideband modulated by the
baseband signal SBB (=I(t)) which is digital information, the
voltage waveform Vd(t) of the modulated electrical signal Smod is
expressed by the following equation (13):
Vd(t)=D(t)cos(.omega.0t)=mdI(t)cos(.omega.0t) (13)
[0145] At this time, the field strength waveform E(t) of the
double-modulated optical signal OSdmod outputted from the external
optical modulating portion 120 is found by the following equation
(14): 8 E ( t ) = E 2 ( cos ( t ) - J 0 ( k ) sin ( t ) ) + E 2 J 1
( k ) cos ( + 0 ) t + E 2 J 1 ( k ) cos ( - 0 ) t ( 14 )
[0146] The double-modulated optical signal OSdmod expressed by the
foregoing equation (14) is optically transmitted through the
optical fiber 140 as an optical signal OS after passing through the
optical filter portion 130, and is then incident on the
optical/electrical converting portion 150. The optical/electrical
converting portion 150 optical/electrical converts the incident
optical signal OS and outputs an electrical signal. The current
waveform Ipd(t) of the electrical signal is expressed by the
following equation (15): 9 I p d ( t ) = 2 ( E 2 J 1 ( k m d I ( t
) ) ) 2 2 ( E 2 k m d I ( t ) 2 ) 2 ( 15 )
[0147] In the foregoing equation (15), kmdI(t)<<1.
[0148] When the double sideband modulation is thus performed, the
output current waveform of the optical/electrical converting
portion 150 is obtained as it is as a demodulated signal, as also
apparent from the foregoing equation (15). It is found from the
foregoing equation (15) that I(t) varies linearly, while Ipd(t)
varies in the second order. If M-ASK (a multivalued ASK modulation
system) is employed, therefore, spacing between thresholds of
Ipd(t) is twice in decibel spacing between thresholds of I(t).
Therefore, it is found that the optical signal OS is resistant to
noise which may occur on an optical transmission line (an optical
fiber).
[0149] In the forgoing, it is assumed that a phase difference
between the main carrier MC to be multiplexed in the external
optical modulating portion 120 and the phase-modulated main carrier
MC is set to .pi./2, the same effect can be also obtained in the
case of setting the phase difference to .pi./2. Furthermore, the
same effect can be also obtained in the case of using other
external optical modulators such as an electroabsorption modulator
in place of Mach-Zender type external optical modulator.
[0150] As described in the foregoing, in the optical
transmitter-receiver, the electrical signal having a high frequency
such as a millimeter-wave band is optically transmitted by optical
signal processing, and the optical signal is further
optical-signal-processed, thereby eliminating the necessity of a
high-frequency electrical component (a millimeter-wave band
down-converter or demodulator) which was required in a conventional
optical transmitter-receiver as well as completely eliminating the
necessity of a high-frequency component which is difficult to work,
for example, a waveguide or a semirigid cable. Consequently, it is
possible to significantly reduce the scale of the optical
transmitter-receiver.
[0151] Since the main carrier is external-optical-modulated by the
electrical signal having a high frequency such as a millimeter-wave
band, spacing between optical frequencies of the component of the
main carrier and the component of the sideband is wide
(corresponding to the millimeter-wave band) in the optical spectrum
shown in FIG. 2(b-1). Consequently, the optical filter 130 can
accurately extract only the component of the sideband by the
current technique.
[0152] In the first embodiment, the external optical modulating
portion 120 optically modulates the main carrier MC by the
modulated electrical signal Smod having the millimeter-wave band in
order to produce a significant technical effect. Even if the
external optical modulating portion 120 performs optical modulation
by the modulated electrical signal Smod having the other frequency
band, however, the optical receiver 102 can demodulate the baseband
signal SBB without requiring an electrical component (a
down-converter or a demodulator). That is, the optical
transmitter-receiver according to the first embodiment is not
limited to the millimeter-wave band. For example, it is applicable
to a wider frequency band.
[0153] The optical transmitter-receiver according to the first
embodiment does not easily perform direct optical modulation by the
modulated electrical signal Smod having a millimeter-wave band if
consideration is given to the frequency response characteristics of
the light source 110. Therefore, an external optical modulation
system has been employed. If the modulated electrical signal Smod
is approximately not more than a microwave band, it is also
possible to directly drive the light source 110 by the modulated
electrical signal Smod, and directly modulate the intensity of
output light of the light source 110 irrespective of the frequency
response characteristics. That is, the optical transmitter-receiver
can also employ a direct optical modulation system.
[0154] In the optical transmitter-receiver according to the first
embodiment, the optical filter portion 130 in the optical
transmitter 101 extracts only the optical signal OS from the
double-modulated optical signal OSdmod and outputs the optical
signal OS to the optical fiber 140. However, the optical filter
portion 130 may be provided in the optical receiver 102. In this
case, the optical transmitter 101 directly outputs to the optical
fiber 140 the double-modulated optical signal OSdmod produced by
the external optical modulating portion 120. The optical receiver
102 extracts only the optical signal OS from the double-modulated
optical signal OSdmod incident from the optical fiber 140 using the
optical filter portion 130 set at a front end, and then
optical/electrical converts the extracted optical signal OS using
the optical/electrical converting portion 150 set at a back
end.
[0155] (Second Embodiment)
[0156] FIG. 3 is a block diagram showing the structure of an
optical transmitter-receiver according to a second embodiment of
the present invention. In the optical transmitter-receiver shown in
FIG. 3, an optical transmitter 101 and an optical receiver 102 are
interconnected through an optical fiber 140 such that optical
transmission is possible. The optical transmitter 101 comprises a
light source 110, first and second external optical modulating
portions 120-1 and 120-2, an optical filter portion 130, and a
local oscillating portion 170, and the optical receiver 102
comprises an optical/electrical converting portion 150.
[0157] FIGS. 4(a-3) to (b-3) schematically illustrate spectrums of
signals in principal parts (a-3) to (b-3) of the optical
transmitter-receiver shown in FIG. 3.
[0158] The operations of the optical transmitter-receiver shown in
FIG. 3 will be described on the basis of FIGS. 3, 4 and so on. In
the optical transmitter 101, the light source 110 is typically
constructed by a semiconductor laser diode, which oscillates
unmodulated light having a predetermined optical frequency .nu. as
shown in FIG. 2(a-1), and outputs the unmodulated light as a main
carrier MC to the first external optical modulating portion 120-1.
The local oscillating portion 170 outputs an electrical subcarrier
SC having a predetermined frequency f0 which is a millimeter-wave
band to the first external optical modulating portion 120-1. The
first external optical modulating portion 120-1 has a Mach-Zehnder
type structure, for example (see the first embodiment), and
amplitude-modulates the inputted main carrier MC (see FIG. 2(a-1))
by the inputted subcarrier SC. Consequently, a modulated optical
signal OSmod is produced, and is outputted to the second external
optical modulating portion 120-2. The optical spectrum of the
modulated optical signal OSmod has a component of the main carrier
MC at a center optical frequency .nu., and further has components
of sidebands (an upper sideband and a lower sideband) at
frequencies spaced an integral multiple of the optical frequency f0
apart from the optical frequency .nu. (only .+-.f0 are
illustrated).
[0159] A baseband signal SBB having a frequency f1 to be
transmitted is inputted to the second external optical modulating
portion 120-2 from the outside of the optical transmitter 101. The
second external optical modulating portion 120-2 also has a
Mach-Zehnder type structure, for example (see the first
embodiment), and amplitude-modulates the inputted modulated optical
signal OSmod (see FIG. 4(a-3)) by the inputted baseband signal SBB.
Consequently, a double-modulated optical signal OSdmod is produced.
The optical spectrum of the double-modulated optical signal OSdmod
has a component of the main carrier MC at a center optical
frequency .nu., and further has components of sidebands (an upper
sideband and a lower sideband) at frequencies spaced an integral
multiple of the optical frequency f0 apart from the optical
frequency .nu. (only .+-.f0 are illustrated). The occupied
frequency bands of the components of both the sidebands depend on
the frequency f1. A component of the baseband signal SBB in FIG.
4(b-3) differs from that in FIG. 2(b-1) in that it also occurs with
respect to the main carrier MC.
[0160] The double-modulated optical signal OSdmod as described
above is inputted to the optical filter portion 130. In the optical
transmitter-receiver shown in FIG. 3, the optical filter portion
130 and the subsequent components perform the same operations as
those of the corresponding components in the optical
transmitter-receiver shown in FIG. 1. In the second embodiment,
therefore, description of the corresponding components is not
repeated. However, a modulating method according to the second
embodiment differs from that in the first embodiment and hence, it
will be noted that almost all of the equations used in the first
embodiment are not applied in the second embodiment.
[0161] In the optical transmitter-receiver shown in FIG. 3, the
first external optical modulating portion 120-1 performs modulation
using the subcarrier SC, and the second external optical modulating
portion 120-2 performs modulation using the baseband signal SBB.
However, the first external optical modulating portion 120-1 may
perform amplitude modulation using the baseband signal SBB, and the
second external optical modulating portion 120-2 may perform
amplitude modulation using the subcarrier SC.
[0162] Also in the optical transmitter-receiver according to the
second embodiment, the optical filter portion 130 in the optical
transmitter 101 extracts only an optical signal OS from the
double-modulated optical signal OSdmod, and outputs the optical
signal OS to the optical fiber 140. However, the optical filter
portion 130 may be provided in the optical receiver 102. In this
case, the optical transmitter 101 directly outputs the
double-modulated optical signal OSdmod produced by the second
external optical modulating portion 120-2 to the optical fiber 140.
The optical receiver 102 extracts only the optical signal OS from
the double-modulated optical signal OSdmod incident from the
optical fiber 140 using the optical filter portion 130 set at a
front end, and then optical/electrical converts the extracted
optical signal OS using the optical/electrical converting portion
150 set at a back end.
[0163] (Third Embodiment)
[0164] FIG. 5 is a block diagram showing the structure of only an
optical transmitter with respect to an optical transmitter-receiver
according to a third embodiment of the present invention. Although
an optical receiver is not illustrated in FIG. 5, the optical
receiver 102 shown in FIG. 1 or 3 can be connected.
[0165] The optical transmitter 101 shown in FIG. 5 comprises a
local oscillating portion 170, a mode locked light source 510, an
external optical modulating portion 120, and an optical filter
portion 130.
[0166] The optical transmitter 101 shown in FIG. 5 will be
described with reference to FIGS. 2, 4 and 5.
[0167] The local oscillating portion 170 outputs the same
subcarrier SC as described above. The mode locked light source 510
is mode-locked by an inputted subcarrier SC, to oscillate in a
multimode. Although there are two types of methods of mode locking:
a method by electrical driving and a method by optical injection,
either one of the methods may be used. If spacing between mode
locking frequencies is so set as to be equal to the frequency of
the subcarrier SC, the same optical signal as the modulated optical
signal OSmod shown in FIG. 4(a-3) (this optical signal is also
referred to as a modulated optical signal OSmod for convenience,
although exactly speaking, it oscillates in a multimode in a wider
optical frequency band) is outputted to the external optical
modulating portion 120 from the mode locked light source 510.
[0168] The same baseband signal SBB as described above is inputted
to the external optical modulating portion 120 from the outside of
the optical transmitter 101. The external optical modulating
portion 120 amplitude-modulates the inputted modulated optical
signal OSmod by the inputted baseband signal SBB, to produce and
output a double-modulated optical signal OSdmod shown in FIG.
4(b-5).
[0169] Although the double-modulated optical signal OSdmod as
described above is inputted to the optical filter portion 130, the
optical filter portion 130 and the subsequent components are the
same as the corresponding components in FIG. 1 or 3 and hence, the
description thereof is not repeated.
[0170] (Fourth Embodiment)
[0171] FIG. 6 is a block diagram showing the structure of an
optical transmitter-receiver according to a fourth embodiment of
the present invention. Since the optical transmitter-receiver shown
in FIG. 6 is the same as the optical transmitter-receiver shown in
FIG. 1 except that it further comprises an optical branching
portion 310, a second optical/electrical converting portion 320,
and a wavelength control portion 330, the same reference numerals
are assigned to the corresponding components and hence, the
description thereof is not repeated. It will be noted that the
optical/electrical converting portion 150 and the optical signal OS
transmitted through the optical fiber 140 shown in FIG. 1 are
respectively referred to as a first optical/electrical converting
portion 150 and a first optical signal OS1 as can be seen by
referring to FIG. 6 in the fourth embodiment for convenience of
illustration (see FIG. 6).
[0172] The operations of the optical transmitter-receiver according
to the fourth embodiment will be described centered with respect to
a difference from the optical transmitter-receiver shown in FIG. 1
on the basis of FIG. 6.
[0173] In FIG. 6, an optical signal OS is outputted from an optical
filter portion 130, and is inputted to the optical branching
portion 610, as described in the first embodiment. The optical
branching portion 610 branches the inputted optical signal OS into
a first optical signal OS1 and a second optical signal OS2, and
outputs the first optical signal OS1 to an optical fiber 140, while
outputting the second optical signal OS2 to the second
optical/electrical converting portion 620. The first optical signal
OS1 is transmitted through the optical fiber 140, and is then
processed by the first optical/electrical converting portion 150 in
the same manner as that in the first embodiment.
[0174] The second optical/electrical converting portion 620 also
optical/electrical converts the inputted second optical signal OS2
and outputs an electrical signal. The electrical signal is referred
to as a detecting signal Sdet.
[0175] The wavelength control portion 330 detects the average
values of detecting signals Sdet inputted at predetermined time
intervals. The wavelength control portion 330 selects the maximum
value Vmax out of the detected average values, and controls the
temperature of a light source 110 or a bias current such that the
maximum value Vmax is always detected, to control the wavelength
(the optical frequency) of a main carrier MC.
[0176] In the optical transmitter-receiver, the oscillation
wavelength of the light source 110 and/or the passband of the
optical filter portion 130 may, in some cases, be shifted from a
predetermined oscillation wavelength and/or a predetermined
passband due to changes with time and changes in ambient
temperature. When such a shift occurs, the optical filter portion
130 cannot accurately extract only a component of an upper sideband
or a component of a lower sideband out of components included in a
double-modulated optical signal OSdmod (a component of the main
carrier and components of both sidebands). Although in the optical
transmitter-receiver according to the fourth embodiment, the
wavelength control portion 330 monitors the optical signal OS, to
carry out feed back control of the oscillation wavelength of the
light source 110, however, it is possible to correct, even if the
above-mentioned shift occurs, the shift, so that the optical filter
portion 130 can always accurately extract only one of the
sidebands.
[0177] (Fifth Embodiment)
[0178] FIG. 7 is a block diagram showing the structure of an
optical transmitter-receiver according to a fifth embodiment of the
present invention. Since the optical transmitter-receiver shown in
FIG. 7 is roughly the same as the optical transmitter-receiver
shown in FIG. 1 except that it further comprises a second optical
receiver 102-2 connected to an optical transmitter 101 through a
second optical fiber 140-2 such that optical transmission is
possible, the same reference numerals are assigned to the
corresponding components and hence, the description thereof is
simplified. It will be noted that the optical fiber 140, the
optical receiver 102, and the optical/electrical converting portion
150 shown in FIG. 1 are respectively referred to as a first optical
fiber 140-1, a first optical receiver 102-1, and a first
optical/electrical converting portion 150-1 in the fifth embodiment
for convenience of illustration, and the optical signal OS shown in
FIG. 1 is referred to as a first optical signal OS1.
[0179] The optical transmitter 101 shown in FIG. 7 differs from the
optical transmitter 101 shown in FIG. 1 in that the optical filter
portion 130 is replaced with an optical filter portion 710.
Further, the second optical receiver 102-2 comprises a second
optical/electrical converting portion 150-2.
[0180] FIGS. 8(a-7) to (f-7) schematically illustrate spectrums of
signals in principal parts (a-7) to (f-7) of the optical
transmitter-receiver shown in FIG. 7.
[0181] The operations of the optical transmitter-receiver according
to the fifth embodiment will be described centered with respect to
a difference from the optical transmitter-receiver shown in FIG. 1
on the basis of FIGS. 7 and 8.
[0182] In the optical transmitter 101, a baseband modulating
portion 180 amplitude-modulates a subcarrier SC inputted from a
local oscillating portion 170 by a baseband signal SBB inputted
from the outside of the optical transmitter 101 at a degree of
modulation md, to produce a modulated electrical signal Smod, as
described in the first embodiment. Let I(t) be the current waveform
of the baseband signal SBB. The voltage waveform Vd(t) of the
modulated electrical signal Smod is expressed by the foregoing
equation (2), and is outputted to an external optical modulating
portion 120.
[0183] A light source 110 outputs a main carrier MC having the
optical spectrum as shown in FIG. 8(a-7). FIG. 8(a-7) is the same
as FIG. 2(a-1). The external optical modulating portion 120
amplitude-modulates the main carrier MC inputted from the light
source 110 by the modulated electrical signal Smod inputted from
the baseband modulating portion 180, as described in the first
embodiment, to produce and output a double-modulated optical signal
OSdmod having the optical spectrum as shown in FIG. 8(b-1). The
optical spectrum shown in FIG. 8(b-7) is the same as that shown in
FIG. 2(b-1). The field strength waveform E(t) of the
double-modulated optical signal OSdmod is finally expressed by the
foregoing equation (7), as described in the first embodiment.
[0184] As described in the foregoing, the double-modulated optical
signal OSdmod is inputted to the optical filter portion 710. The
optical frequency passband of the optical filter portion 710 is so
set that the inputted double-modulated optical signal OSdmod is
divided into a component of a lower sideband included in a band B1
and components of an upper sideband and the main carrier which are
included in a band B2 as shown in FIG. 8(b-7). The optical filter
portion 710 outputs the component of the lower sideband divided to
the first optical fiber 140-1 as a first optical signal OS1, while
outputting the components of the upper sideband and the main
carrier divided to the second optical fiber 140-2 as a second
optical signal OS2.
[0185] The detailed structure and the operations of the optical
filter portion 710 will be described on the basis of FIGS. 8 and 9.
In FIG. 9, the optical filter portion 710 comprises an optical
circulator portion 910 having terminals 1, 2 and 3 and an optical
fiber grating portion 920. The terminals 1, 2 and 3 are connected
to the external optical modulating portion 120, the optical fiber
grating portion 920, and the optical fiber 140-1.
[0186] The double-modulated optical signal OSdmod inputted to the
terminal 1 of the optical circulator portion 910 from the external
optical modulating portion 120 is outputted as it is only to the
optical fiber grating portion 920 connected to the terminal 2. The
optical fiber grating portion 920 is a narrow-band optical notch
filter, and is so set as to reflect only the component included in
the band B1 shown in FIG. 8(b-7) in the inputted double-modulated
optical signal OSdmod. Consequently, the first optical signal OS1
is reflected. As a result, the first optical signal OS1 is incident
on the optical circulator portion 910 from the terminal 2 again,
and is outputted as it is only to the first optical fiber 140-1
connected to the terminal 3.
[0187] Since the optical fiber grating portion 920 passes
components of bands other than a reflected band (outside the band
B1) in the inputted double-modulated optical signal OSmod, the
second optical signal OS2 is outputted to the second optical fiber
140-2.
[0188] As described in the foregoing, the optical filter portion
710 can realize optical filtering processing for a narrow band in
simple construction by combining an optical circulator and an
optical fiber grating which are existing optical components.
[0189] The optical spectrum of the first optical signal OS1 is
included in an optical frequency band in the vicinity of an optical
frequency .nu.-f0, as shown in FIG. 8(c-7). The field strength
waveform EOS1(t) of the first optical signal OS1 is expressed by
the following equation (16). When the following equation (16) is
arranged, the following equation (17) is obtained: 10 E O S 1 ( t )
= E 2 J 0 ( k m d 2 ) { J 1 ( k ) J 0 ( k m d 2 ) cos ( - 0 ) t - J
0 ( k ) J 1 ( k m d 2 ) ( cos ( - 0 + 1 ) t + cos ( - 0 - 1 ) t ) }
( 16 ) E O S 1 ( t ) = K cos ( - 0 ) t ( 1 - m ' cos 1 t ) ( 17
)
[0190] In the foregoing equation (16), m' and K are respectively
expressed by the foregoing equations (10) and (11).
[0191] The optical spectrum of the second optical signal OS2 is
included in an optical frequency band from the vicinity of an
optical frequency .nu. to the vicinity of an optical frequency
.nu.+f0, as shown in FIG. 8(d-7). The waveform EOS2(t) of the
second optical signal OS2 is expressed by the following equation
(18): 11 E O S 2 ( t ) = E 2 cos ( 2 v t ) + E 2 J 1 ( k ) J 0 2 (
k m d 2 ) cos ( + 0 ) t - E 2 J 0 ( k ) J 0 ( k m d 2 ) J 1 ( k m d
2 ) 2 cos 1 t cos ( + 0 ) t ( 18 )
[0192] When the foregoing equation (18) is arranged using m' and K,
the following equation (19) is obtained: 12 E O S 2 ( t ) = E 2 cos
( 2 v t ) + K cos ( + 0 ) t ( 1 - m ' cos 1 t ) ( 19 )
[0193] The first optical signal OS1 and the second optical signal
OS2 as described on the basis of the equations and FIG. 8, for
example, are respectively transmitted through the first optical
fiber 140-1 and the second optical fiber 140-2, and are incident on
the first optical receiver 102-1 and the second optical receiver
102-2. Consequently, both the optical signals OS1 and OS2 are
transmitted to a remote location.
[0194] In the first optical receiver 102-1, the first
optical/electrical converting portion 150-1 optical/electrical
converts the incident first optical signal OS1 and outputs an
electrical signal. It is found by referring to FIG. 8(c-7) that the
first optical signal OS1 is equivalent to one amplitude-modulated
the carrier having the optical frequency .nu.-f0 by the baseband
signal SBB (=cos 2.pi.f1t). Consequently, the current waveform
Ipd1(t) of an electrical signal outputted by the first
optical/electrical converting portion 150-1 is expressed by the
following equation (20): 13 I p d 1 ( t ) = 1 2 K 2 ( 1 - m ' cos 1
t ) 2 = I p d 1 ( 1 - 2 m ' cos 1 t + m '2 cos 2 1 t ) ( 20 )
[0195] where .eta.1 is the conversion efficiency of the first
optical/electrical converting portion 150-1, and Ipd1 is a DC
component. As can be understood by referring to the foregoing
equation (20), if only a component having a frequency f1 is
extracted using a band-pass filter or the like from the electrical
signal outputted from the first optical/electrical converting
portion 150-1, an amplitude modulation component of the first
optical signal OS1, that is, the current waveform I(t) of the
baseband signal SBB is directly obtained, as shown in FIG. 8 (e-7).
It is easily feasible to extract only the component having the
frequency f1 by connecting the band-pass filter to a back end
against the optical/electrical converting portion 150. In the first
optical/electrical converting portion 150-1, there may be a
frequency band enough to obtain the baseband signal SBB.
[0196] In the second optical receiver 102-2, the second
optical/electrical converting portion 150-2 optical/electrical
converts the incident second optical signal OS2 and outputs an
electrical signal. It is found by referring to FIG. 8(f-7) that the
second optical signal OS2 is equivalent to one that the main
carrier MC is single-sideband modulated by the above-mentioned
modulated electrical signal Smod (a signal that the subcarrier SC
is amplitude-modulated by the baseband signal SBB). Consequently,
the current waveform Ipd2(t) of an electrical signal outputted by
the second optical/electrical converting portion 150-2 is expressed
by the following equation (21): 14 I p d 2 ( t ) = ( E 2 ) 2 K 2
cos 0 t ( 1 - m ' cos 1 t ) ( 21 )
[0197] where .eta.2 is the conversion efficiency of the second
optical/electrical converting portion 150-2, and Ipd2 is a DC
component. As can be understood by referring to the foregoing
equation (21), if only a component having a frequency f0 is
extracted using a band-pass filter or the like from the electrical
signal outputted from the second optical/electrical converting
portion 150-2, an amplitude modulation component of the second
optical signal OS2, that is, the modulated electrical signal Smod
having a band of the frequency f0 is directly and reasonably
obtained, as shown in FIG. 8(f-7). It is easily feasible to extract
only the component having the frequency f0 by connecting the
band-pass filter to a back end against the optical/electrical
converting portion 150. In the second optical/electrical converting
portion 150-2, there may be a frequency band enough to obtain the
modulated electrical signal Smod.
[0198] As described in the foregoing, the optical transmitter 101
shown in FIG. 7 divides the double-modulated optical signal OSdmod
obtained by double-modulating the main carrier MC into the
component of one of the sidebands and the components of the main
carrier and the other sideband by optical filtering, and optically
transmits the components. The first and second optical receivers
102-1 and 102-2 can respectively obtain the baseband signal SBB and
the modulated electrical signal Smod by separately
optical/electrical converting the components. The optical
transmitter-receiver can thus simultaneously optically transmit the
baseband signal SBB and the modulated electrical signal Smod that
the subcarrier SC is amplitude-modulated by the baseband signal
using the same light source 110.
[0199] Although in the fifth embodiment, the optical filter portion
710 band-divides the double-modulated optical signal OSdmod into
the component of the lower sideband and the components of the upper
sideband and the main carrier, it may band-divide the
double-modulated optical signal OSdmod into the component of the
upper sideband and the components of the lower sideband and the
main carrier.
[0200] The modulated electrical signal Smod shown in FIG. 8(f-7) is
suitable for wireless transmission when f0 is a microwave band or a
millimeter-wave band. Therefore, an antenna (not shown) capable of
radiating the modulated electrical signal Smod to a space is
provided in a back end against the second optical/electrical
converting portion 150-2, and the modulated electrical signal Smod
is introduced into the antenna, so that the optical
transmitter-receiver and a wireless transmission system can be
easily interconnected.
[0201] In the fifth embodiment, when the modulated electrical
signal Smod outputted from the baseband modulating portion 180 is a
microwave band or a millimeter-wave band, it is difficult, if
consideration is given to frequency response characteristics, to
direct-optical-modulate the light source 110 by the modulated
electrical signal Smod having such a high frequency. Therefore, the
optical transmitter 101 has employed an external optical modulation
system. If the modulated electrical signal Smod outputted from the
baseband modulating portion 180 is not more than approximately a
microwave band, it is also possible to directly drive the light
source 110 by the modulated electrical signal Smod irrespective of
the frequency response characteristics, and directly modulate the
intensity of output light of the light source 110. That is, the
optical transmitter-receiver can also employ a direct optical
modulation system.
[0202] (Sixth Embodiment)
[0203] FIG. 10 is a block diagram showing the structure of an
optical transmitter-receiver according to a sixth embodiment of the
present invention. Since the optical transmitter-receiver shown in
FIG. 10 is roughly the same as the optical transmitter-receiver
shown in FIG. 3 except that it further comprises a second optical
receiver 102-2 connected to an optical transmitter 101 through a
second optical fiber 140-2 such that optical transmission is
possible, the same reference numerals are assigned to the
corresponding components and hence, the description thereof is
simplified. It will be noted that the optical fiber 140, the
optical receiver 102, and the optical/electrical converting portion
150 shown in FIG. 3 are respectively referred to as a first optical
fiber 140-1, a first optical receiver 102-1, and a first
optical/electrical converting portion 150-1 in the sixth embodiment
for convenience of illustration, and the optical signal OS shown in
FIG. 3 is also referred to as a first optical signal OS1.
[0204] The optical transmitter 101 shown in FIG. 10 differs from
the optical transmitter 101 shown in FIG. 3 in that the optical
filter portion 130 is replaced with an optical filter portion 710.
Further, the second optical receiver 102-2 comprises a second
optical/electrical converting portion 150-2.
[0205] FIGS. 11(a-10) to (f-10) schematically illustrate spectrums
of signals in principal parts (a-10) to (a-10) of the optical
transmitter-receiver shown in FIG. 10.
[0206] The operations of the optical transmitter-receiver according
to the sixth embodiment will be described centered with respect to
a difference from the optical transmitter-receiver shown in FIG. 3
on the basis of FIGS. 10 and 11, for example. In the optical
transmitter 101, a light source 110 outputs a main carrier MC
having the optical spectrum as shown in FIG. 8(a-7) to a first
external optical modulating portion 120-1. A local oscillating
portion 170 outputs the same subcarrier SC as described above to
the first external optical modulating portion 120-1. The first
external optical modulating portion 120-1 amplitude-modulates the
inputted main carrier MC by the inputted subcarrier SC, to produce
a modulated optical signal OSmod and output the produced modulated
optical signal to the second external optical modulating portion
120-2. The optical spectrum of the modulated optical signal OSmod
is the same as the optical spectrum shown in FIG. 4(a-3), as shown
in FIG. 11(a-10) and hence, the details thereof are omitted.
[0207] As described in the second embodiment, a baseband signal SBB
is inputted to the second external optical modulating portion 120-2
from the outside of the optical transmitter 101. The second
external optical modulating portion 120-2 also amplitude-modulates
the inputted modulated optical signal OSmod by the inputted
baseband signal SBB, to produce a double-modulated optical signal
OSmod, as described in the second embodiment. The optical spectrum
of the double-modulated optical signal OSdmod is the same as the
optical spectrum shown in FIG. 4(b-3), as shown in FIG. 11(b-10)
and hence, the details thereof are omitted.
[0208] The double-modulated optical signal OSdmod as described
above is inputted to the optical filter portion 710. The optical
frequency passband of the optical filter portion 710 is so set that
the inputted double-modulated optical signal OSdmod is divided into
a component of a lower sideband included in a band B1 and
components of an upper sideband and the main carrier which are
included in a band B2, as shown in FIG. 11(b-10). The optical
filter portion 710 outputs the component of the lower sideband
obtained by the division to the first optical fiber 140-1 as a
first optical signal OS1 while outputting the components of the
upper sideband and the main carrier which are obtained by the
division to the second optical fiber 140-2 as a second optical
signal OS2. The optical spectrum of the first optical signal OS1 is
included in an optical frequency band in the vicinity of an optical
frequency .nu.-f0, as shown in FIG. 11(c-10). Further, the optical
spectrum of the second optical signal OS2 is included in an optical
frequency band from the vicinity of .nu. to the vicinity of
.nu.+f0, as shown in FIG. 11(d-10).
[0209] The first optical signal OS1 and the second optical signal
OS2 as described above are respectively incident on the first
optical receiver 102-1 and the second optical receiver 102-2, as
described in the fifth embodiment. Consequently, both the optical
signals OS1 and OS2 are transmitted to a remote location.
[0210] The first optical receiver 102-1 and the second optical
receiver 102-2 are operated similarly to those in the fifth
embodiment, to output a baseband signal SBB having the spectrum as
shown in FIG. 11(e-10) and a modulated electrical signal (a signal
that the subcarrier is amplitude-modulated by the baseband signal)
Smod having the spectrum as shown in FIG. 11(f-10). As shown in
FIG. 11(f-10), the modulated electrical signal Smod is
approximately the same as the modulated electrical signal Smod
shown in FIG. 8(f-7). Exactly speaking, however, the modulated
electrical signal Smod shown in FIG. 11(f-10) induces slightly
higher strain than that in the modulated electrical signal Smod
shown in FIG. 8(f-7) due to the effect of the component of the main
carrier MC (see a hatched portion). A modulating method according
to the sixth embodiment differs from that in the fifth embodiment.
Therefore, it will be noted that almost all of the equations used
in the fifth embodiment are not applied in the sixth
embodiment.
[0211] As described in the foregoing, according to the optical
transmitter shown in FIG. 10, the optical spectrum of the
double-modulated optical signal OSdmod obtained by
double-modulating the main carrier MC (further amplitude-modulating
by the baseband signal SBB the modulated optical signal OSmod that
the main carrier is amplitude-modulated by the subcarrier) (see
FIG. 11(b-10)) is divided into the component of one of the
sidebands and the components of the main carrier and the other
sideband by optical filtering, and the components are optically
transmitted. The first optical receiver and the second optical
receiver can respectively obtain the baseband signal SBB (see FIG.
11(e-10)) and the modulated electrical signal Smod (see FIG.
11(f-10)) by separately optical/electrical converting the
components. The optical transmitter-receiver thus simultaneously
optically transmits the baseband signal and the signal obtained
upon amplitude-modulating the subcarrier by the baseband signal
using the same light source 110.
[0212] Also in the optical transmitter-receiver shown in FIG. 10,
the optical filter portion 710 may band-divide the double-modulated
optical signal OSdmod into the component of the upper sideband and
the components of the lower sideband and the main carrier.
[0213] Also in the optical transmitter-receiver shown in FIG. 10,
an antenna (as described above) is provided in a back end against
the second optical/electrical converting portion 150-2, and the
modulated electrical signal Smod is introduced into the antenna, so
that the optical transmitter-receiver can be easily connected to a
wireless transmission system, similarly to the optical
transmitter-receiver shown in FIG. 7.
[0214] Furthermore, in the optical transmitter-receiver shown in
FIG. 10, the first external optical modulating portion 120-1
performs modulation using the subcarrier, and the second external
optical modulating portion 120-2 performs modulation using the
baseband signal. However, the first external optical modulating
portion 120-1 may perform amplitude modulation using the baseband
signal, and the second external optical modulating portion 120-2
may perform amplitude modulation using the subcarrier.
[0215] (Seventh Embodiment)
[0216] FIG. 12 is a block diagram showing the structure of an
optical transmitter-receiver according to a seventh embodiment of
the present invention. Since the optical transmitter-receiver shown
in FIG. 12 is the same as the optical transmitter-receiver shown in
FIG. 10 except that an optical transmitter 101 comprises an optical
branching portion 1210 in place of the optical filter portion 710,
a first optical receiver 102-1 further comprises a low-pass filter
portion 1220, and a second optical receiver 102-2 further comprises
a high-pass filter portion 1230, the same reference numerals are
assigned to the corresponding components and hence, the description
thereof is not repeated.
[0217] The operations of the optical transmitter-receiver shown in
FIG. 12 will be described on the basis of FIGS. 11 and 12.
[0218] A second external optical modulating portion 120-2 produces
the same double-modulated optical signal OSdmod as that in the
sixth embodiment (see FIG. 11(b-10)), and outputs the produced
double-modulated optical signal to the optical branching portion
1210. The optical branching portion 1210 branches the inputted
double-modulated optical signal OSdmod into a lot of
double-modulated optical signals (two double-modulated optical
signals in the description), and outputs the double-modulated
optical signals to optical fibers 140-1 and 140-2.
[0219] One of the branched double-modulated optical signals OSdmod
and the other double-modulated optical signal are thereafter
transmitted through optical fibers 140-1 and 140-2, and are
incident on a first optical/electrical converting portion 150-1 and
a second optical/electrical converting portion 150-2, respectively.
The first optical/electrical converting portion 150-1 and the
second optical/electrical converting portion 150-2 respectively
optical/electrical converts the double-modulated optical signals
OSdmod and outputs electric signals. Received optical currents of
the first optical/electrical converting portion 150-1 and the
second optical/electrical converting portion 150-2 reasonably
respectively include a component of a baseband signal SBB (see FIG.
11(e-10)) and a component of a modulated electrical signal Smod
(see FIG. 11(f-10)).
[0220] The electrical signal outputted from the first
optical/electrical converting portion 150-1 is inputted to the
low-pass filter portion 1220, so that only a part included in a low
frequency band of the electrical signal is outputted after passing
through the low-pass filter portion 1220. Consequently, it is
possible to obtain the baseband signal SBB (see FIG. 11(e-10)), as
in the second embodiment.
[0221] On the other hand, an electrical signal outputted from the
second optical/electrical converting portion 150-2 is inputted to
the high-pass filter portion 1230, so that only a part included in
a high frequency band of the electrical signal is outputted after
passing through the high-pass filter portion 1230. Consequently, it
is possible to obtain the modulated electrical signal Smod (see
FIG. 11(f-10)), as in the second embodiment.
[0222] As described in the foregoing, according to the optical
transmitter-receiver shown in FIG. 12, the double-modulated optical
signal OSdmod obtained by double-modulating the same main carrier
MC as that in the optical transmitter-receiver shown in FIG. 10 is
branched into a lot of double-modulated optical signals, and the
double-modulated optical signals are respectively optically
transmitted. The first optical receiver and the second optical
receiver can respectively obtain the baseband signal SBB and the
modulated electrical signal Smod by separately
optical/electrical-converting the double-modulated optical signals,
followed by low-pass filtering and high-pass filtering. The optical
transmitter-receiver can thus simultaneously optically transmit the
baseband signal and the modulated electrical signal using the same
light source 110.
[0223] The above-mentioned first and second optical receivers 102-1
and 102-2 respectively comprise the first and second
optical/electrical converting portions 150-1 and 150-2 which differ
in frequency bands which can be optical/electrical-converted.
However, the first and second optical transmitter-receivers 102-1
and 102-2 may be the same, and may respectively comprise
optical/electrical converting portions having sufficiently wide
frequency bands to collectively optical/electrical-conv- ert the
double-modulated optical signal OSdmod. In such a case, the first
and second optical transmitter-receivers 102-1 and 102-2 can
respectively obtain the baseband signal SBB and the modulated
electrical signal Smod by low-pass filtering and high-pass
filtering.
[0224] A transmitter other than that described in the sixth
embodiment may be applied for the optical transmitter 101 with
respect to the present invention.
[0225] (Eighth Embodiment)
[0226] In the optical transmitter-receiver shown in FIG. 12,
two-types of optical receivers which differ in structure are
connected. If the optical receiver is constructed as described
below, however, both a baseband signal SBB and a modulated
electrical signal Smod can be obtained even if only one type of
optical receiver is connected to an optical transmitter. The
structure of the optical receiver will be described on the basis of
FIG. 13.
[0227] In the optical transmitter-receiver shown in FIG. 13, an
optical transmitter 101 and one or more optical receivers 102 are
interconnected through optical fibers 140 such that optical
transmission is possible. The optical transmitter 101 shown in FIG.
13 is the same in structure as the optical transmitter 101 shown in
FIG. 12 and hence, the description thereof is not repeated. The
optical receiver 102 shown in FIG. 13 has a different structure
from the optical receiver 102-1 or 102-2 shown in FIG. 12, and
comprises an optical/electrical converting portion 150, a
distributor 1310, a low-pass filter portion 1320, and a high-pass
filter portion 1330.
[0228] As can be also seen from the foregoing, a double-modulated
optical signal OSdmod outputted from the optical transmitter 101 is
transmitted through each of the optical fibers 140, and is incident
on the optical/electrical converting portion 150 in the optical
receiver 102. The optical/electrical converting portion 150 has
wideband characteristics in which a frequency band from a low
frequency band to a high frequency band can be
optical/electrical-converted, and collectively
optical/electrical-converts the double-modulated optical signal
OSdmod and outputs an electrical signal obtained by the conversion
to the distributor 1310. The electrical signal is distributed into
a lot of electrical signals (two electrical signals in this
description) by the distributor 1310. Only one of the electrical
signals obtained by the distribution is inputted to the low-pass
filter portion 1320, so that only a part included in a low
frequency band of the electrical signal is outputted after passing
through the low-pass filter portion 1320. Consequently, a baseband
signal SBB (see FIG. 11(e-10)) is obtained.
[0229] Furthermore, the other electrical signal obtained by the
distribution is inputted to the high-pass filter portion 1330, so
that only a part included in a high frequency band of the
electrical signal is outputted after passing through the high-pass
filter portion 1330. Consequently, a modulated electrical signal
Smod (see FIG. 11(f-10)) is obtained.
[0230] As described in the foregoing, if the optical
transmitter-receiver comprises one optical receiver shown in FIG.
13, it can obtain both the baseband signal SBB and the modulated
electrical signal Smod, similarly to the optical
transmitter-receiver shown in FIG. 12.
[0231] In FIG. 13, a plurality of (two in the drawing) optical
receivers 102 are connected. The number of optical receivers 102
may be one depending on the construction conditions of the optical
transmitter-receiver. In such a case, the necessity of an optical
branching portion 1210 is eliminated, and a second external optical
modulating portion 120-2 outputs the double-modulated optical
signal OSdmod to the optical fiber 140.
[0232] In the optical transmitter-receivers according to the
seventh and eighth embodiments, the first external optical
modulating portion 120-1 and the second external optical modulating
portion 120-2 perform amplitude modulation using a double sideband
amplitude modulation system, as apparent by referring to FIG. 11.
However, the first external optical modulating portion 120-1 and
the second external optical modulating portion 120-2 may perform
amplitude modulation using a single sideband amplitude modulation
system. According to the single sideband amplitude modulation
system, the double-modulated optical signal OSdmod first has a
component of a main carrier MC at a center optical frequency .nu..
The double-modulated optical signal OSdmod further has a component
of an upper sideband or a lower sideband on the side of higher
frequencies or lower frequencies than the optical frequency .nu.
and at a frequency spaced an integral multiple of the optical
frequency f0. Although the double-modulated optical signal OSdmod
is transmitted through each of the optical fibers, it can be
optically transmitted over a longer distance because it is not
easily affected by wavelength dispersion by the optical fiber 140,
as compared with that in the case of double sideband amplitude
modulation.
[0233] (Ninth Embodiment)
[0234] FIG. 14 is a block diagram showing the structure of only an
optical transmitter with respect to an optical transmitter-receiver
according to a ninth embodiment of the present invention. Although
an optical receiver is not illustrated in FIG. 14, the optical
receivers 102-1 and 102-2 shown in FIG. 12 or the optical receiver
102 shown in FIG. 13 can be connected. Since an optical transmitter
101 shown in FIG. 14 is the same as the optical transmitter 101
shown in FIG. 5 except that the optical filter portion 130 is
replaced with an optical branching portion 1210, the same reference
numerals are assigned to the corresponding components. The optical
branching portion 1210 was described referring to FIG. 12, for
example. Therefore, the operations of the optical transmitter 101
shown in FIG. 14 are apparent from the descriptions and hence, the
description thereof is simplified.
[0235] A mode locked light source 510 is mode-locked by a
subcarrier SC inputted form a local oscillating portion 170, to
oscillate in a multimode. If spacing between mode locking
frequencies is so set as to be equal to the frequency of the
subcarrier SC, a modulated optical signal OSmod (see FIG. 11(a-10))
is outputted to an external optical modulating portion 120 from the
mode locked light source 510. The external optical modulating
portion 120 produces a double-modulated optical signal OSdmod (see
FIG. 11(b-10)) on the basis of the inputted modulated optical
signal OSmod and a baseband signal SBB inputted from outside, and
outputs the produced double-modulated optical signal OSmod to the
optical branching portion 1210. The optical branching portion 1210
branches the inputted double-modulate optical signal OSdmod into a
lot of double-modulated optical signals, and then outputs the
obtained double-modulated optical signals to each of optical fibers
140.
[0236] Also in the optical transmitter-receiver shown in FIG. 14,
at least one optical receiver 102 may be connected. When the number
of optical receivers is one, the necessity of the optical branching
portion 1210 is eliminated, and the external optical modulating
portion 120 outputs the double-modulated optical signal OSdmod to
the optical fiber 140.
[0237] (Tenth Embodiment)
[0238] FIG. 15 is a block diagram showing the structure of only an
optical transmitter with respect to an optical transmitter-receiver
according to a tenth embodiment of the present invention. Although
an optical receiver is not illustrated in FIG. 15, the optical
receivers 102-1 and 102-2 shown in FIG. 12 or the optical receiver
102 shown in FIG. 13 can be connected. Since an optical transmitter
101 shown in FIG. 15 comprises a first light source 1510-1, a
second light source 1510-2, an external optical modulating portion
120, an optical multiplexing portion 1720, and an optical branching
portion 1210. In the optical transmitter 101 shown in FIG. 15, the
same reference numerals are assigned to components corresponding to
the components of the optical transmitter shown in FIG. 14 and
hence, the description thereof is simplified. FIGS. 16(a-15) to
(d-15) schematically illustrate spectrums of signals in principal
parts (a-15) to (d-15) of the optical transmitter-receiver shown in
FIG. 15.
[0239] The operations of the optical transmitter-receiver according
to the tenth embodiment will be described on the basis of FIGS. 15
and 16. In the optical transmitter 101, the first light source
1510-1 outputs first unmodulated light UML1 having an optical
frequency .nu. to the external optical modulating portion 120. The
first unmodulated light UML1 has an optical spectrum as shown in
FIG. 16(a-15). Further, a baseband signal SBB having a frequency f1
is inputted to the external optical modulating portion 120 from the
outside of the optical transmitter 101. The external optical
modulating portion 120 amplitude-modulates the inputted first
unmodulated light UML1 by the inputted baseband signal SBB, to
produce a modulated optical signal OSmod, as described in the
second embodiment. The optical spectrum of the modulated optical
signal OSmod has a component of the first unmodulated light UML1 at
a center optical frequency .nu., and further has components of
sidebands at frequencies spaced an integral multiple of the optical
frequency f1 apart from the optical frequency .nu. (only .+-.f0 are
illustrated), as shown in FIG. 16(b-15). The modulated optical
signal OSmod is inputted to the optical multiplexing portion
1720.
[0240] The second light source 1510-2 outputs second unmodulated
light UML2 spaced a predetermined optical frequency apart from the
optical frequency .nu. to the optical multiplexing portion 1720.
The predetermined optical frequency shall be an optical frequency
corresponding to the frequency f0 of the above-mentioned subcarrier
SC. The second unmodulated light UML2 has the optical spectrum as
shown in FIG. 16(c-15). The optical multiplexing portion 1720
multiplexes the inputted modulated optical signal OSmod and the
second unmodulated light UML2 such that their polarized waves
coincide with each other, and outputs a signal obtained by the
multiplexing to the optical branching portion 1210 as an optical
signal OS. The optical signal OS has the optical spectrum as shown
in FIG. 16(d-15) because it is obtained by merely multiplexing the
modulated optical signal OSmod and the second unmodulated light
UML2. It is found by referring to FIG. 16(d-15) that the optical
spectrum of the optical signal OS is the same as that in the case
where the single sideband amplitude modulation system described in
the eighth embodiment is applied. Consequently, the optical
transmitter 101 constructed as shown in FIG. 15 also produces the
same effect as that in the case where the single sideband amplitude
modulation system described in the eighth embodiment is applied.
Further, in the present embodiment, it is possible to optically
transmit the baseband signal SBB and a modulated electrical signal
Smod using not the local oscillating portion 170 as in the eighth
embodiment but only the two light sources (the first light source
1510-1 and the second light source 1510-2). Consequently, the
optical/electrical conversion must be made twice in the first
external optical modulating portion 120-1 and the second external
optical modulating portion 120-2 in the eighth embodiment, for
example, while the optical/electrical conversion is made only once
by the external optical modulating portion 120 in the optical
transmitter 101 according to the present embodiment. It is possible
to realize low-loss optical transmission by thus reducing the
number of times of optical/electrical conversion. Further, the
optical transmitter 101 according to the present embodiment
requires no electrical component for amplitude-modulating the
subcarrier by the electrical signal to be transmitted. That is,
according to the present embodiment, the necessity of an electrical
component, which is high in cost and is difficult to process,
corresponding to a subcarrier band which is a relatively high
frequency is eliminated. Correspondingly, the optical
transmitter-receiver can be constructed simply and at low cost.
Further, the oscillation optical frequencies of the two light
sources can be easily changed by changing their bias currents and
ambient temperatures. Therefore, the frequency band of the
modulated electrical signal Smod obtained on the side of the
optical receiver can be easily changed.
[0241] In the tenth embodiment, description was made, letting .nu.
be the oscillation optical frequency of the first light source
1510-1 and .nu.+f0 be the oscillation optical frequency of the
second light source 1510-2, as can be seen by referring to FIG. 16.
However, the oscillation optical frequency of the second light
source 1510-2 may be .nu.-f0.
[0242] In each of the above-mentioned embodiments, in a case where
the baseband signal is analog information, when the analog
information is optically transmitted with the subcarrier SC
modulated by the analog information, the optical/electrical
converting portions 150, 150-1 and 150-2 typically square-law
detect the optical signal, so that accurate square-law detection
may, in some cases, be interfered with by secondary higher
harmonics. On the side of the optical transmitter 101, the baseband
signal which is the analog information is analog-to-digital
converted, and the baseband signal which is digital information
obtained by the conversion is optically transmitted with the
subcarrier modulated by the baseband signal. The optical receiver
102 or the like digital-to-analog converts the optical signal after
the optical/electrical conversion. Consequently, the optical
transmitter-receiver can transmit information high in quality which
is not interfered with by higher harmonics.
[0243] In the optical transmitter-receiver according to each of the
embodiments, the baseband signal is inputted from outside. If a
carrier having an intermediate frequency is previously modulated by
the baseband signal using a predetermined modulation system
(amplitude modulation, frequency modulation or phase modulation),
and a signal obtained by modulating the carrier having the
intermediate frequency is optically transmitted after the
subcarrier SC outputted from the local oscillating portion 170 is
modulated by the signal, however, the carrier having the
intermediate frequency and a signal that the subcarrier SC is
modulated by the carrier having the intermediate frequency can be
obtained in the optical receiver according to each of the
embodiments, so that optical transmission which does not depend on
a modulation form is possible. The intermediate frequency is
limited to a lower frequency than the frequency f0 of the
subcarrier SC. The reason for this is that if a component of the
carrier having the intermediate frequency is not included between
.nu..+-.f0, it is difficult to accurately perform
optical/electrical conversion and filtering.
[0244] In the optical transmitter-receiver according to each of the
embodiments, a plurality of carriers respectively having different
intermediate frequencies are previously prepared, the carriers
having different intermediate frequencies are respectively
modulated by different baseband signals, and a frequency division
multiplexing access is further employed, so that it is possible to
collectively optically transmit the carriers.
[0245] A time division multiplexing access or a code division
multiplexing access is employed for the optical
transmitter-receiver according to each of the embodiments, so that
different baseband signals can be transmitted upon being
multiplexed on a single carrier having an intermediate frequency.
Further, more information can be transmitted upon being multiplexed
by simultaneously using the frequency division multiplexing access
and the time division multiplexing access or the code division
multiplexing access.
[0246] As described in the foregoing, an optical spectrum of a
double-amplitude-modulated optical signal is divided into a
component of a sideband and components of a main carrier and the
other sideband by optical filtering, and the components are
respectively optically transmitted, so that a baseband signal to be
transmitted and a modulated electrical signal taht a subcarrier is
modulated by the baseband signal to be transmitted can be
simultaneously obtained after the optical/electrical conversion.
The modulated electrical signal is suitable for wireless
transmission if it has a microwave band or a millimeter-wave band.
According to the optical transmitter-receiver, therefore, it is
possible to construct a system in which a wire communication
network by an optical fiber and a wireless transmission system
utilizing a modulated electrical signal (a signal having a high
frequency such as a microwave band or a millimeter-wave band).
Moreover, in the optical transmitter, only one light source is
used, which is advantages in terms of construction of the optical
transmitter-receiver, the maintenance cost, and the like.
[0247] When an optical signal having a band of 1.5 .mu.m whose
transmission loss is the minimum is transmitted through a single
1.3 .mu.m band single mode fiber generally used, a modulation
component disappears by several kilometers due to dispersion in a
normal optical signal generally amplitude-modulated by a signal
having a high frequency such as a millimeter-wave band. However,
the optical transmitter-receiver is not affected by dispersion
because it receives an amplitude-modulated optical signal having
only a component of one sideband.
[0248] Furthermore, an erbium doped fiber amplifier (EDFA) can be
also used by using an optical signal having a band of 1.5 .mu.m, so
that it is also possible to improve the receiving sensitivity.
[0249] Although the present invention has been described and
illustrated in detail, it is clearly understood that the same is by
way of illustration and example only and is not to be taken by way
of limitation, the spirit and scope of the present invention being
limited only by the terms of the appended claims.
* * * * *