U.S. patent number RE39,785 [Application Number 10/997,559] was granted by the patent office on 2007-08-21 for system for optically transmitting frequency-division-multiplexed signal and transmitter therefor.
This patent grant is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Masaru Fuse.
United States Patent |
RE39,785 |
Fuse |
August 21, 2007 |
System for optically transmitting frequency-division-multiplexed
signal and transmitter therefor
Abstract
In an optical transmission system, a multiplexer
frequency-division-multiplexes a plurality of signals, and outputs
the resultant signal to an FM modulator. The FM modulator converts
the frequency-division-multiplexed signal into an FM modulated
signal through frequency modulation using the
frequency-division-multiplexed signal as an original signal. A
frequency-divider converts the FM modulated signal into a
frequency-divided FM modulated signal whose frequency is 1/2.sup.n
(n is an integer of not less than 1) the frequency of the FM
modulated signal. An optical modulator has a predetermined
input-voltage vs. output-optical-power characteristic, and is
biased at the minimum point (voltage) about the output optical
power. The optical modulator modulates an unmodulated light fed
from a light source with the applied frequency-divided FM modulated
signal to produce an optical signal whose optical carrier component
is suppressed, and sends the optical signal to an optical
transmission line. An optical receiver receives the optical signal,
and square-law detects the signal to convert into an FM modulated
signal. A FM demodulator demodulates the FM modulated signal to
reproduce the original frequency-division-multiplexed signal. This
configuration makes it possible to narrow the bandwidth of an FM
modulated signal while increasing the frequency deviation thereof,
and realize high-quality signal transmission as a result.
Inventors: |
Fuse; Masaru (Osaka,
JP) |
Assignee: |
Matsushita Electric Industrial Co.,
Ltd. (Osaka, JP)
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Family
ID: |
17772167 |
Appl.
No.: |
10/997,559 |
Filed: |
November 26, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
09417541 |
Oct 14, 1999 |
06486986 |
Nov 26, 2002 |
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Foreign Application Priority Data
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Oct 14, 1998 [JP] |
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10-291691 |
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Current U.S.
Class: |
398/79; 398/140;
398/141; 398/147; 398/158; 398/159; 398/161; 398/182; 398/183;
398/186; 398/187; 398/192; 398/193; 398/194; 398/201; 398/202;
398/208; 398/214; 398/76; 398/91 |
Current CPC
Class: |
H04B
10/25137 (20130101); H04B 10/505 (20130101); H04B
10/5165 (20130101); H04J 14/0298 (20130101) |
Current International
Class: |
H04B
10/18 (20060101); H04B 10/152 (20060101); H04B
10/155 (20060101); H04J 14/02 (20060101) |
Field of
Search: |
;398/79,76,91,140,141,147,158,159,182,183,186,187,192,193,194,202,208,214 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Optical Super Wide-Band FM Modulation Scheme and its Application
to Multi-Channel AM Video Transmission Systems" by Kikushima et
al., IOOC '95 Technical Digest, vol. 5, PD2-7, pp. 33-34. cited by
other.
|
Primary Examiner: Phan; Hanh
Attorney, Agent or Firm: Wenderoth, Lind & Ponack,
L.L.P.
Claims
What is claimed is:
.[.1. A transmission system for optically transmitting a
frequency-division-multiplexed signal, which is obtained by
frequency-division multiplexing a plurality of signals, said
transmission system comprising: a transmitting end comprising a
multiplexer operable to freqeuncy-division multiplex the plurality
of signals to produce the frequency-division multiplexed signal, a
FM modulator being operable to convert the
frequency-division-multiplexed signal into a frequency-modulated
signal through frequency modulation using the
frequency-division-multiplexed signal as an original signal to
output the frequency-modulated signal as a FM modulated signal, and
an optical transmitter operable to convert the FM modulated signal
into an optical-intensity-modulated signal whose optical carrier
component is suppressed in an optical frequency spectrum through
optical modulation using the FM modulated signal as an original
signal to send the optical-intensity-modulated signal to a
receiving end; and said receiving end comprising an optical
receiver operable to receive the optical-intensity-modulated signal
from said optical transmitter, and convert the
optical-intensity-modulated signal into an electrical signal
corresponding to the FM modulated signal through photodetection
based on a square-law detection characteristic to the output the
electrical signal as a received FM modulated signal, and a FM
demodulator operable to demodulate the received FM modulated signal
to reproduce the frequency-division-multiplexed signal..].
.[.2. The transmission system according to claim 1, wherein said
optical transmitter comprises: a light source being operable to
output an unmodulated light; and an optical modulator being
operable to modulate the unmodulated light with the FM modulated
signal to produce the optical-intensity-modulated signal, said
optical modulator having a Mach-Zehnder interferometer structure
with a predetermined input-voltage versus output-optical-power
characteristic, and being biased in the input-voltage versus
output-optical-power characteristic such that an output optical
power is at a minimum..].
.[.3. The transmission system according to claim 2, further
comprising a frequency-divider provided between said FM modulator
and said optical transmitter, said frequency-divider being operable
to convert the FM modulated signal outputted from said FM modulator
into a frequency-divided FM modulated signal whose frequency is
1/2.sup.n of a frequency of the FM modulated signal, the n being an
integer of not less than 1, wherein said optical modulator
modulates the unmodulated light with the frequency-divided FM
modulated signal to produce the optical-intensity-modulated
signal..].
.[.4. The transmission system according to claim 1, wherein said
optical transmitter comprises: a light source being operable to
output an unmodulated light; an optical branching circuit being
operable to branch the unmodulated light fed from said light source
into a first unmodulated light and a second unmodulated light; an
optical modulator being operable to modulate the first unmodulated
light with the FM modulated signal to produce the
optical-intensity-modulated signal, said optical modulator having a
Mach-Zehnder interferometer structure with a predetermined
input-voltage versus output-optical-power characteristic, and being
biased in the input-voltage versus output-optical-power
characteristic such that an output optical power is at a maximum;
and an optical combining circuit being operable to combine the
optical-intensity-modulated signal produced by said optical
modulator and the second unmodulated light to cancel an optical
carrier component to the optical-intensity-modulated signal with
the second unmodulated light and output the
optical-intensity-modulated signal whose optical carrier component
is suppressed..].
.[.5. The transmission system according to claim 4, wherein said
optical transmitter further comprises an optical delay circuit
provided between said optical branching circuit and said optical
combining circuit, said optical delay circuit being operable to
adjust a propagation delay of at least one of the first unmodulated
light, the second unmodulated light, and the
optical-intensity-modulated signal produced by said optical
modulator such that the second unmodulated light and the optical
carrier component of the optical-intensity-modulated signal
produced by said optical modulator are set in opposite phases to
each other..].
.[.6. The transmission system according to claim 4, further
comprising a frequency-divider provided between said FM modulator
and said optical transmitter, said frequency-divider being operable
to convert the FM modulated signal outputted from said FM modulator
into a frequency-divided FM modulated signal whose frequency is
1/2.sup.n of a frequency of the FM modulated signal, n being an
integer of not less than 1, wherein said optical modulator
modulates the first unmodulated light with the frequency-divided FM
modulated signal to produce the optical-intensity-modulated
signal..].
.[.7. The transmission system according to claim 1, further
comprising a frequency-divider provided between said FM modulator
and said optical transmitter, said frequency-divider being operable
to convert the FM modulated signal outputted from said FM modulator
into a frequency-divided FM modulated signal whose frequency is
1/2.sup.n of a frequency of the FM modulated signal, n being an
integer of not less than 1, wherein said optical transmitter
comprises an optical modulator being operable to produce the
optical-intensity-modulated signal through the optical modulation
using the frequency-divided FM modulated signal as an original
signal..].
.[.8. A transmitter for use in a transmission system for optically
transmitting a frequency-division-multiplexed signal, which is
obtained by frequency-division-multiplexing a plurality of signals,
said transmitter comprising: a multiplexer being operable to
frequency-division-multiplex the plurality of signals to produce
the frequency-division-multiplexed signal; a FM modulator being
operable to convert the frequency-division-multiplexed signal into
a frequency-modulated signal through frequency modulation using the
frequency-division-multiplexed signal as an original signal to
output the frequency-modulated signal as a FM modulated signal; and
an optical transmitter being operable to convert the FM modulated
signal into an optical-intensity-modulated signal whose optical
carrier component is suppressed in an optical frequency spectrum
through optical modulation using the FM modulated signal as an
original signal to send the optical-intensity-modulated signal to a
receiving end..].
.[.9. The transmitter according to claim 8, wherein said optical
transmitter comprises: a light source being operable to output an
unmodulated light; and an optical modulator being operable to
modulate the unmodulated light with the FM modulated signal to
produce the optical-intensity-modulated signal, said optical
modulator having a Mach-Zehnder interferometer structure with a
predetermined input-voltage versus output-optical-power
characteristic, and being biased in the input-voltage versus
output-optical-power characteristic such that an output optical
power is at a minimum..].
.[.10. The transmitter according to claim 9, further comprising a
frequency-divider provided between said FM modulator and said
optical transmitter, said frequency-divider being operable to
convert the FM modulated signal outputted from said FM modulator
into a frequency-divided FM modulated signal whose frequency is
1/2.sup.n of a frequency of the FM modulated signal, n being an
integer of not less than 1, wherein said optical modulator
modulates the unmodulated light with the frequency-divided FM
modulated signal to produce the optical-intensity-modulated
signal..].
.[.11. The transmitter according to claim 8, wherein said optical
transmitter comprises: a light source being operable to output an
unmodulated light; an optical branching circuit being operable to
branch the unmodulated light fed from said light source into a
first unmodulated light and a second unmodulated light; an optical
modulator being operable to modulate the first unmodulated light
with the FM modulated signal to produce the
optical-intensity-modulated signal, said optical modulator having a
Mach-Zehnder interferometer structure with a predetermined
input-voltage versus output-optical-power characteristic, and being
biased in the input-voltage versus output-optical-power
characteristic such that an output optical power is at a maximum;
and an optical combining circuit being operable to combine the
optical-intensity-modulated signal produced by said optical
modulator and the second unmodulated light to cancel an optical
carrier component of the optical-intensity-modulated signal with
the second unmodulated light, and output the
optical-intensity-modulated signal whose optical carrier component
is suppressed..].
.[.12. The transmitter according to claim 11, wherein said optical
transmitter further comprises an optical delay circuit provided
between said optical branching circuit and said optical combining
circuit, said optical delay circuit being operable to adjust a
propagation delay of at least one of the first unmodulated light,
the second unmodulated light, and the optical-intensity-modulated
signal produced by said optical modulator such that the second
unmodulated light and the optical carrier component of the
optical-intensity-modulated signal produced by said optical
modulator are set in opposite phases to each other..].
.[.13. The transmitter according to claim 11, further comprising a
frequency-divider provided between said FM modulator and said
optical transmitter, said frequency-divider being operable to
convert the FM modulated signal outputted from the FM modulator
into a frequency-divided FM modulated signal whose frequency is
1/2.sup.n of a frequency of the FM modulated signal, n being an
integer of not less than 1, wherein said optical modulator
modulates the first unmodulated light with the frequency-divided FM
modulated signal to produce the optical-intensity-modulated
signal..].
.[.14. The transmitter according to claim 8, further comprising a
frequency-divider provided between said FM modulator and said
optical transmitter, said frequency-divider being operable to
convert the FM modulated signal outputted from said FM modulator
into a frequency-divided FM modulated signal whose frequency is
1/2.sup.n of a frequency of the FM modulated signal, n being an
integer of not less than 1, wherein said optical transmitter
includes an optical modulator being operable to produce the
optical-intensity-modulated signal through the optical modulation
using the frequency-divided FM modulated signal as an original
signal..].
.[.15. An optical transmission apparatus comprising: a multiplexer
operable to multiplex a plurality of signals to produce a
multiplexed signal; an FM modulator operable to convert the
multiplexed signal into a frequency-modulated signal; an
electrical-optical modulator operable to convert the
frequency-modulated signal into an optical modulated signal whose
optical carrier component is suppressed in an optical frequency
spectrum; and an optical transmitter operable to transmit the
optical modulated signal.].
.[.16. An optical transmitting method comprising: multiplexing a
plurality of signals to produce a multiplexed signal; FM-modulating
the multiplexed signal into a frequency-modulated signal;
electrical-optical-modulating the frequency-modulated signal into
an optical modulated signal whose optical carrier component is
suppressed in an optical frequency spectrum; and transmitting the
optical modulated signal..].
.[.17. An optical transmission system comprising a transmission
apparatus and a receiving apparatus, said transmission apparatus
including: a multiplexer operable to multiplex a plurality of
signals to produce a multiplexed signal; an FM modulator operable
to convert the multiplexed signal into a frequency-modulated
signal; an electrical-optical modulator operable to convert the
frquency-modulated signal into an optical modulated signal whose
optical carrier component is suppressed in an optical frequency
spectrum; and an optical transmitter operable to transmit the
optical modulated signal, and said receiving apparatus including: a
receiver operable to receive the optical modulated signal; an
optical-electrical converter operable to convert the optical
modulated signal into the frequency-modulated signal; and an FM
demodulator operable to demodulate the frequency-modulated signal
to produce the multiplexed signal..].
.[.18. An optical transmission/reception method comprising:
multiplexing a plurality of signals to produce a multiplexed
signal; FM-modulating the multiplexed signal into a
frequency-modulated signal; electrical-optical-modulating the
frequency-modulated signal into an optical modulated signal whose
optical carrier component is suppressed in an optical frequency
spectrum; transmitting the optical modulated signal; receiving the
optical modulated signal; optical-electrical converting the optical
modulated signal into the frequency-modulated signal; and
FM-demodulating the frequency-modulated signal to produce the
multiplexed signal..].
.Iadd.19. A transmission system for optically transmitting a
frequency-division-multiplexed signal, which is obtained by
frequency-division multiplexing a plurality of signals, from a
transmitting end to a receiving end, comprising: at said
transmitting end, a multiplexer for frequency-division multiplexing
the plurality of signals to produce the
frequency-division-multiplexed signal; an FM modulator for
converting the frequency-division-multiplexed signal into a
frequency-modulated signal through frequency modulation using the
frequency-division-multiplexed signal as an original signal to
output the frequency-modulated signal as an FM modulated signal; an
optical transmitter for converting the FM modulated signal into an
optical-intensity-modulated signal whose optical carrier component
is suppressed in an optical frequency spectrum through optical
modulation using the FM modulated signal as an original signal to
send the optical-intensity-modulated signal to said receiving end;
and a frequency-divider provided between said FM modulator and said
optical transmitter for converting the FM modulated signal
outputted from said FM modulator into a frequency-divided FM
modulated signal whose frequency is 1/2.sup.n of a frequency of the
FM modulated signal, n being an integer of not less than 1, and at
said receiving end, an optical receiver for receiving the
optical-intensity-modulated signal from said optical transmitter,
and converting the optical-intensity-modulated signal into an
electrical signal corresponding to the FM modulated signal through
photodetection based on a square-law detection characteristic to
output the electrical signal as a received FM modulated signal; and
an FM demodulator for demodulating the received FM modulated signal
to reproduce the frequency-division-multiplexed signal, wherein
said optical transmitter includes: a light source for outputting an
unmodulated light; and an optical modulator for modulating the
unmodulated light with the FM modulated signal to produce the
optical-intensity-modulated signal, said optical modulator having
the Mach-Zehnder interferometer structure with a predetermined
input-voltage versus output-optical-power characteristic, and being
biased in the input-voltage versus output-optical-power
characteristic such that an output optical power is at a minimum,
and wherein said optical modulator modulates the unmodulated light
with the frequency-divided FM modulated signal to produce the
optical-intensity-modulated signal..Iaddend.
.Iadd.20. A transmission system for optically transmitting a
frequency-division-multiplexed signal, which is obtained by
frequency-division multiplexing a plurality of signals, from a
transmitting end to a receiving end, comprising: at said
transmitting end, a multiplexer for frequency-division multiplexing
the plurality of signals to produce the
frequency-division-multiplexed signal; an FM modulator for
converting the frequency-division-multiplexed signal into a
frequency-modulated signal through frequency modulation using the
frequency-division-multiplexed signal as an original signal to
output the frequency-modulated signal as an FM modulated signal;
and an optical transmitter for converting the FM modulated signal
into an optical-intensity-modulated signal whose optical carrier
component is suppressed in an optical frequency spectrum through
optical modulation using the FM modulated signal as an original
signal to send the optical-intensity-modulated signal to said
receiving end, and at said receiving end, an optical receiver for
receiving the optical-intensity-modulated signal from said optical
transmitter, and converting the optical-intensity-modulated signal
into an electrical signal corresponding to the FM modulated signal
through photodetection based on a square-law detection
characteristic to output the electrical signal as a received FM
modulated signal; and an FM demodulator for demodulating the
received FM modulated signal to reproduce the
frequency-division-multiplexed signal, wherein said optical
transmitter includes: a light source for outputting an unmodulated
light; an optical branching circuit for branching the unmodulated
light fed from said light source into first and second unmodulated
lights; an optical modulator for modulating the first unmodulated
light with the FM modulated signal to produce the
optical-intensity-modulated signal, said optical modulator having a
Mach-Zehnder interferometer structure with a predetermined
input-voltage versus output-optical-power characteristic, and being
biased in the input-voltage versus output-optical-power
characteristic such that an output optical power is at a maximum;
and an optical combining circuit for combining the
optical-intensity-modulated signal produced by said optical
modulator and the second unmodulated light to cancel an optical
carrier component of the optical-intensity-modulated signal with
the second unmodulated light and output the
optical-intensity-modulated signal whose optical carrier component
is suppressed..Iaddend.
.Iadd.21. The transmission system according to claim 20, wherein
said optical transmitter further includes an optical delay circuit,
provided between said optical branching circuit and said optical
combining circuit, for adjusting a propagation delay of at least
one of the first unmodulated light, the second unmodulated light,
and the optical-intensity-modulated signal produced by said optical
modulator such that the second unmodulated light and the optical
carrier component of the optical-intensity-modulated signal
produced by said optical modulator are set in opposite phases to
each other..Iaddend.
.Iadd.22. The transmission system according to claim 20, further
comprising, a frequency-divider provided between said FM modulator
and said optical transmitter for converting the FM modulated signal
outputted from said FM modulator into a frequency-divided FM
modulated signal whose frequency is 1/2.sup.n a frequency of the FM
modulated signal, n being an integer of not less than 1, wherein
said optical modulator modulates the first unmodulated light with
the frequency-divided FM modulated signal to produce the
optical-intensity-modulated signal..Iaddend.
.Iadd.23. A transmission system for optically transmitting a
frequency-division-multiplexed signal, which is obtained by
frequency-division multiplexing a plurality of signals, from a
transmitting end to a receiving end, comprising: at said
transmitting end, a multiplexer for frequency-division multiplexing
the plurality of signals to produce the
frequency-division-multiplexed signal; an FM modulator for
converting the frequency-division-multiplexed signal into a
frequency-modulated signal through frequency modulation using the
frequency-division-multiplexed signal as an original signal to
output the frequency-modulated signal as an FM modulated signal; an
optical transmitter for converting the FM modulated signal into an
optical-intensity-modulated signal whose optical carrier component
is suppressed in an optical frequency spectrum through optical
modulation using the FM modulated signal as an original signal to
send the optical-intensity-modulated signal to said receiving end;
and a frequency-divider provided between said FM modulator and said
optical transmitter for converting the FM modulated signal
outputted from said FM modulator into a frequency-divided FM
modulated signal whose frequency is 1/2.sup.n a frequency of the FM
modulated signal, n being an integer of not less than 1, and at
said receiving end, an optical receiver for receiving the
optical-intensity-modulated signal from said optical transmitter,
and converting the optical-intensity-modulated signal into an
electrical signal corresponding to the FM modulated signal through
photodetection based on a square-law detection characteristic to
output the electrical signal as a received FM modulated signal; and
an FM demodulator for demodulating the received FM modulated signal
to reproduce the frequency-division-multiplexed signal, wherein
said optical transmitter includes an optical modulator for
producing the optical-intensity-modulated signal through the
optical modulation using the frequency-divided FM modulated signal
as an original signal..Iaddend.
.Iadd.24. A transmitter for use in a transmission system for
optically transmitting a frequency-division-multiplexed signal,
which is obtained by frequency-division-multiplexing a plurality of
signals, from a transmitting end to a receiving end, comprising: a
multiplexer for frequency-division multiplexing the plurality of
signals to produce the frequency-division-multiplexed signal; an FM
modulator for converting the frequency-division-multiplexed signal
into a frequency-modulated signal through frequency modulation
using the frequency-division-multiplexed signal as an original
signal to output the frequency-modulated signal as an FM modulated
signal; an opitical transmitter for converting the FM modulated
signal into an optical-intensity-modulated signal whose optical
carrier component is suppressed in an optical frequency spectrum
through optical modulation using the FM modulated signal as an
original signal to send the optical-intensity-modulated signal to
said receiving end; and a frequency-divider provided between said
FM modulator and said optical transmitter for converting the FM
modulated signal outputted from said FM modulator into a
frequency-divided FM modulated signal whose frequency is 1/2.sup.n
a frequency of the FM modulated signal, n being an integer of not
less than 1, wherein said optical transmitter includes: a light
source for outputting an unmodulated light; and an optical
modulator for modulating the unmodulated light with the FM
modulated signal to produce the optical-intensity-modulated signal,
said optical modulator having a Mach-Zehnder interferometer
structure with a predetermined input-voltage versus
output-optical-power characteristic, and being biased in the
input-voltage versus output-optical-power characteristic such that
the output optical power is at a minimum, and wherein said optical
modulator modulates the unmodulated light with the
frequency-divided FM modulated signal to produce the
optical-intensity-modulated signal..Iaddend.
.Iadd.25. A transmitter for use in a transmission system for
optically transmitting a frequency-division-multiplexed signal,
which is obtained by frequency-division-multiplexing a plurality of
signals, from a transmitting end to a receiving end, comprising: a
multiplexer for frequency-division multiplexing the plurality of
signals to produce the frequency-division-multiplexed signal; an FM
modulator for converting the frequency-division-multiplexed signal
into a frequency-modulated signal through frequency modulation
using the frequency-division-multiplexed signal as an original
signal to output the frequency-modulated signal as an FM modulated
signal; and an optical transmitter for converting the FM modulated
signal into an optical-intensity-modulated signal whose optical
carrier component is suppressed in an optical frequency spectrum
through optical modulation using the FM modulated signal as an
original signal to send the optical-intensity-modulated signal to
said receiving end, and wherein said optical transmitter includes:
a light source for outputting an unmodulated light; an optical
branching circuit for branching the unmodulated light fed from said
light source into first and second unmodulated lights; an optical
modulator for modulating the first unmodulated light with the FM
modulated signal to produce the optical-intensity-modulated signal,
said optical modulator having a Mach-Zehnder interferometer
structure with a predetermined input-voltage versus
output-optical-power characteristic, and being biased in the
input-voltage versus output-optical-power characteristic such that
an output optical power is at a maximum; and an optical combining
circuit for combining the optical-intensity-modulated signal
produced by said optical modulator and the second unmodulated light
to cancel an optical carrier component of the
optical-intensity-modulated signal with the second unmodulated
light, and output the optical-intensity-modulated signal whose
optical carrier component is suppressed..Iaddend.
.Iadd.26. The transmitter according to claim 25, wherein said
optical transmitter further includes an optical delay circuit,
provided between said optical branching circuit and said optical
combining circuit, for adjusting a propagation delay of at least
one of the first unmodulated light, the second unmodulated light,
and the optical-intensity-modulated signal produced by said optical
modulator such that the second unmodulated light and the optical
carrier component of the optical-intensity-modulated signal
produced by said optical modulator are set in opposite phases to
each other..Iaddend.
.Iadd.27. The transmitter according to claim 25, further
comprising, a frequency-divider provided between said FM modulator
and said optical transmitter for converting the FM modulated signal
outputted from said FM modulator into a frequency-divided FM
modulated signal whose frequency is 1/2.sup.n a frequency of the FM
modulated signal, n being an integer of not less than 1, wherein
said optical modulator modulates the first unmodulated light with
the frequency-divided FM modulated signal to produce the
optical-intensity-modulated signal..Iaddend.
.Iadd.28. A transmitter for use in a transmission system for
optically transmitting a frequency-division-multiplexed signal,
which is obtained by frequency-division-multiplexing a plurality of
signals, from a transmitting end to a receiving end, comprising: a
multiplexer for frequency-division multiplexing the plurality of
signals to produce the frequency-division-multiplexed signal; an FM
modulator for converting the frequency-division-multiplexed signal
to a frequency-modulated signal through frequency modulation using
the frequency-division-multiplexed signal as an original signal to
output the frequency-modulated signal as an FM modulated signal;
and an optical transmitter for converting the FM modulated signal
into an optical-intensity-modulated signal whose optical carrier
component is suppressed in an optical frequency spectrum through
optical modulation using the FM modulated signal as an original
signal to send the optical-intensity-modulated signal to said
receiving end; and a frequency-divider provided between said FM
modulator and said optical transmitter for converting the FM
modulated signal outputted from said FM modulator into a
frequency-divided FM modulated signal whose frequency is 1/2.sup.n
a frequency of the FM modulated signal, n being an integer of not
less than 1, and wherein said optical transmitter includes an
optical modulator for producing the optical-intensity-modulated
signal through the optical modulation using the frequency-divided
FM modulated signal as an original signal..Iaddend.
.Iadd.29. An optical transmission apparatus comprising: a
multiplexer operable to multiplex a plurality of signals to produce
a multiplexed signal; an FM modulator operable to convert the
multiplexed signal into an FM modulated signal; a frequency-divider
operable to convert the FM modulated signal into a
frequency-divided FM modulated signal whose frequency is 1/2.sup.n
a frequency of the FM modulated signal, n being an integer of not
less than 1; an electrical-optical modulator operable to convert
the frequency-divided FM modulated signal into an optical modulated
signal whose optical carrier component is suppressed in an optical
frequency spectrum; and an optical transmitter operable to transmit
the optical modulated signal..Iaddend.
.Iadd.30. An optical transmitting method comprising: multiplexing a
plurality of signals to produce a multiplexed signal; FM-modulating
the multiplexed signal to produce an FM modulated signal;
frequency-dividing the FM modulated signal to produce a
frequency-divided FM modulated signal whose frequency is 1/2.sup.n
a frequency of the FM modulated signal, n being an integer of not
less than 1; electrical-optical-modulating the frequency-divided FM
modulated signal to produce an optical modulated signal whose
optical carrier component is suppressed in an optical frequency
spectrum; and transmitting the optical modulated
signal..Iaddend.
.Iadd.31. An optical transmission system comprising a transmission
apparatus and a receiving apparatus, said transmission apparatus
comprising: a multiplexer operable to multiplex a plurality of
signals to produce a multiplexed signal; an FM modulator operable
to convert the multiplexed signal into an FM modulated signal; a
frequency-divider operable to convert the FM modulated signal into
a frequency-divided FM modulated signal whose frequency is
1/2.sup.n a frequency of the FM modulated signal, n being an
integer of not less than 1; an electrical-optical modulator
operable to convert the frequency-divided FM modulated signal into
an optical modulated signal whose optical carrier component is
suppressed in an optical frequency spectrum; and an optical
transmitter operable to transmit the optical modulated signal, and
said receiving apparatus comprising: a receiver operable to receive
the optical modulated signal; an optical-electrical converter
operable to convert the optical modulated signal into the FM
modulated signal; and an FM demodulator operable to convert the FM
modulated signal to produce the multiplexed signal..Iaddend.
.Iadd.32. An optical transmission and reception method including a
transmitting method and a receiving method, said transmitting
method comprising: multiplexing a plurality of signals to produce a
multiplexed signal; FM-modulating the multiplexed signal to produce
an FM modulated signal; frequency-dividing the FM modulated signal
to produce a frequency-divided FM modulated signal whose frequency
is 1/2.sup.n a frequency of the FM modulated signal, n being an
integer of not less than 1; electrical-optical-modulating the
frequency-divided FM modulated signal to produce an optical
modulated signal whose optical carrier component is suppressed in
an optical frequency spectrum; and transmitting the optical
modulated signal; and said receiving method comprising: receiving
the optical modulated signal; optical-electrical converting the
optical modulated signal into the FM modulated signal; and
FM-demodulating the FM modulated signal to produce the multiplexed
signal..Iaddend.
Description
.Iadd.This application is a reissue application of U.S. Pat. No.
6,486,986, issued Nov. 26, 2002..Iaddend.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical transmission systems, and
more particularly to a system that optically transmits a
frequency-division-multiplexed signal obtained by
frequency-division-multiplexing a plurality of signals.
2. Description of the Background Art
FIG. 11 is a block diagram exemplarily showing the configuration of
a conventional optical transmission system for transmitting a
frequency-division-multiplexed signal. As will be known from FIG.
11, this optical transmission system comprises a multiplexer 1100,
an FM modulator 1101, an optical transmitter 1104, an optical
receiver 1106, and an FM demodulator 1107. In the optical
transmission system, an electrical transmission line 1102 connects
the FM modulator 1101 and the optical transmitter 1104 to each
other, and an optical transmission line 1105 connects the optical
transmitter 1104 and the optical receiver 1106 to each other.
The operation of the conventional optical transmission system in
the above configuration will be described below. The multiplexer
1100 frequency-division-multiplexes a plurality of signals, and
outputs the resultant signal to the FM modulator 1101. The FM
modulator 1101 converts the frequency-division-multiplexed signal
into a frequency-modulated signal (hereinafter, referred to as "FM
modulated signal") having a predetermined frequency deviation
through frequency modulation. After that, the FM modulator 1101
outputs the FM modulated signal to the electrical transmission line
1102. The optical transmitter 1104 receives the FM modulated signal
through the electrical transmission line 1102, then converts the
signal into an optical signal, and sends the optical signal to the
optical transmission line 1105. The optical receiver 1106 receives
the optical signal through the optical transmission line 1105, then
converts the signal into an FM modulated signal which is an
electrical signal, and outputs the FM modulated signal to the FM
demodulator 1107. The FM demodulator 1107 demodulates the FM
modulated signal to reproduce the original
frequency-division-multiplexed signal.
The optical transmission system in the above configuration is
described in detail in "Optical Super Wide-Band FM Modulation
Scheme and Its Application to Multi-Channel AM Video Transmission
Systems" by K. Kikushima et al. (IOOC'95 Technical Digest, Vol. 5
PD2-7, pp.33-34), and other documents. The optical transmission
system converts a frequency-division-multiplexed signal into an FM
modulated signal, and then optically transmits and demodulates the
FM modulated signal to reproduce the original
frequency-division-multiplexed signal. The optical transmission
system utilizes an FM gain in the FM transmission to improve the
signal-to-noise power ratio (SNR) of the demodulated signal (i.e.,
the frequency-division-multiplexed signal), thereby enabling
high-quality signal transmission.
Thus, the above-described optical transmission system can realize
high-quality multi-channel signal transmission with an optical
fiber.
However, the above-described system for optically transmitting an
FM modulated signal has the following specific problems due to the
properties of the FM modulated signal and the nonlinearity of an
optical fiber.
SUMMARY OF THE INVENTION
An FM modulation scheme increases a frequency deviation to acquire
a greater FM gain, thereby enabling signal transmission of higher
quality that other modulation schemes such as amplitude modulation.
On the other hand, the increased frequency deviation requires a
wider signal band. In addition, in the FM modulation scheme, linear
distortion tends to occur under the influence of the group delay
characteristic of a transmission line and the like (the
characteristic that a propagation delay varies depending on a
frequency). Therefore, the transmission line must be designed with
particular attention. However, as a signal band becomes wider, the
group delay variations in the band become more difficult to
sufficiently suppress.
In a general optical modulation scheme, the optical frequency
spectrum, of an optical signal is composed of a steep-shaped
optical carrier component, which has narrow spectral line-width,
and upper and lower sidebands, as shown in FIG. 12B. The upper and
lower sidebands are geometrically similar to the frequency spectrum
of a modulating signal. Therefore, if a wide-band signal like an FM
modulated signal is used as a modulating signal in optical
modulation, the optical freqeucy spectrum of the optical signal
also becomes wider. The optical signal having such wide optical
frequency spectrum becomes susceptible to the chromatic-dispersion
of an optical fiber (the characteristic that a propagation delay
varies depending on a wavelength). The affected optical signal
component interacts with the optical carrier component to induce
harmonic distortion in the FM modulated signal, resulting in
waveform deterioration of the transmitted signal.
As is known from the above, the conventional optical transmission
system has the specific problem that the quality of the transmitted
signal is degraded due to the wide-band property of an FM modulated
signal.
Therefore, an object of the present invention is to provide an
optical transmission system capable of narrowing the bandwidth of
an FM modulated signal while increasing the frequency deviation
thereof to realize high-quality signal transmission. The present
invention has the following features to attain the object
above.
A first aspect of the present invention is directed to a
transmission system for optically transmitting a
frequency-division-multiplexed signal, which is obtained by
frequency-division multiplexing a plurality of signals, from a
transmitting end to a receiving end. The transmission system
comprises at the transmitting end, a multiplexer for
frequency-division multiplexing the plurality of signals to produce
the frequency-division-multiplexed signal, an FM modulator for
converting the frequency-division-multiplexed signal into a
frequency-modulated signal through frequency modulation using the
frequency-division-multiplexed signal as an original signal to
output the frequency-modulated signal as an FM modulated signal,
and an optical transmitter for converting the FM modulated signal
into an optical-intensity-modulated signal whose optical carrier
component is suppressed in the optical frequency spectrum through
optical modulation using the FM modulated signal as an original
signal to send the optical-intensity-modulated signal to the
receiving end. The transmission system also comprises at the
receiving end, an optical receiver for receiving the
optical-intensity-modulated signal from the optical transmitter,
and converting the optical-intensity-modulated signal into an
electrical signal corresponding to the FM modulated signal through
photodetection based on a square-law detection characteristic to
output the electrical signal as a received FM modulated signal, and
an FM demodulator for demodulating the received FM modulated signal
to reproduce the frequency-division-multiplexed signal.
As described above, in the first aspect, the FM modulated signal is
obtained through frequency modulation using a
frequency-division-multiplexed signal as an original signal. The FM
modulated signal is converted into an optical-intensity-modulated
signal at the transmitting end. The optical-intensity-modulated
signal has an optical frequency spectrum in which upper and lower
sidebands distribute geometrically similarly to the frequency
spectrum of the original signal for the optical modulation and in
which an optical carrier component is suppressed. Then, the
optical-intensity-modulated signal is photodetected based on a
square-law detection characteristic at the receiving end. At the
receiving end, the optical transmission system thus obtains an FM
modulated signal, having a frequency deviation twice as large as
the one of the original FM modulated signal produced at the
transmitting end, as a received FM modulated signal. In this
manner, the optical transmission system can narrow (reduce in half)
the bandwidth of the FM modulated signal at the transmitting end
while securing the frequency deviation thereof large enough to
acquire a sufficient FM gain in FM demodulation. As a result, it is
possible to prevent the waveform of the transmitted signal from
being deteriorated due to the group delay characteristic of the
electrical transmission line and the chromatic-dispersion of the
optical transmission line, and to realize signal transmission of
good quality.
According to a second aspect, in the first aspect, the optical
transmitter includes a light source for outputting an unmodulated
light, and an optical modulator for modulating the unmodulated
light with the FM modulated signal to produce the
optical-intensity-modulated signal. The optical modulator has the
Mach-Zehnder interferometer structure with a predetermined
input-voltage vs. output-optical-power characteristic, and is
biased in the input-voltage vs. output-optical-power characteristic
such that the output optical power is at the minimum.
As stated above, in the second aspect, the optical modulator used
herein is an external optical modulator having the Mach-Zehnder
interferometer structure. A modulating signal (an FM modulated
signal) is applied to the optical modulator with respect to the
"valley" where the output optical power is at the minimum in the
input-voltage vs. output-optical-power characteristic (which is
periodic like a sine wave) of the optical modulator. The optical
modulator thus produces an optical-intensity-modulated signal whose
optical carrier component is suppressed. The suppression of the
optical carrier component prevents the waveform from being
deteriorated by the chromatic-dispersion of the optical
transmission line. In addition, the optical-intensity-modulated
signal has an optical frequency spectrum in which upper and lower
sidebands distribute geometrically similarly to the frequency
spectrum of the original signal for the optical modulation.
Therefore, after the optical-intensity-modulated signal is
square-law detected at the receiving end, the freqeucy deviation of
the FM modulated signal is doubled, thereby making it possible to
realize high-quality signal transmission.
According to a third aspect, in the second aspect, the transmission
system further comprises a frequency-divider provided between the
FM modulator and the optical transmitter for converting the FM
modulated signal outputted from the FM modulator into a
frequency-divided FM modulated signal whose frequency is 1/2.sup.n
the frequency of the FM modulated signal, the n being an integer of
not less than 1, wherein the optical modulator modulates the
unmodulated light with the frequency-divided FM modulated signal to
produce the optical-intensity-modulated signal.
As described above, in the third aspect, the optical transmission
system previously produces in the FM modulator an FM modulated
signal having a frequency deviation larger enough to acquire a
desired FM gain. The optical transmission system then converts the
FM modulated signal into a frequency-divided FM modulated signal,
and next converts the frequency-divided FM modulated signal into an
optical-intensity-modulated signal for transmission. This reduces
the phase noise in the FM modulated signal to be optically
transmitted and FM demodulated. As a result, high-quality signal
transmission can be realized.
According to a fourth aspect, in the first aspect, the optical
transmitter includes a light source for outputting an unmodulated
light, an optical branching circuit for branching the unmodulated
light fed from the light source into first and second unmodulated
lights, an optical modulator for modulating the first unmodulated
light with the FM modulated signal to produce the
optical-intensity-modulated signal, the optical modulator having
the Mach-Zehnder interferometer structure with a predetermined
input-voltage vs. output-optical-power characteristic, and being
biased in the input-voltage vs. output-optical-power characteristic
such that the output optical power is at the maximum, and an
optical combining circuit for combining the
optical-intensity-modulated signal produced by the optical
modulator and the second unmodulated light to cancel the optical
carrier component of the optical-intensity-modulated signal with
the second unmodulated light and output the
optical-intensity-modulated signal whose optical carrier component
is suppressed.
As described above, in the fifth aspect, the
optical-intensity-modulated signal produced by the optical
modulator is combined with the second unmodulated light set in an
opposite phase to the optical carrier component of the
optical-intensity-modulated signal. The optical carrier component
of the optical-intensity-modulated signal is thus canceled by the
second unmodulated light. As a result, it is possible to produce an
optical-intensity-modulated signal whose optical carrier component
is suppressed.
According to a sixth aspect, in the fourth aspect, the transmission
system further comprises a frequency-divider provided between the
FM modulator and the optical transmission for converting the FM
modulated signal outputted from the FM modulator into a
frequency-divided FM modulated signal whose frequency is 1/2.sup.n
the frequency of the FM modulated signal, the n being an integer of
not less than 1, wherein the optical modulator modulates the first
unmodulated light with the frequency-divided FM modulated signal to
produce the optical-intensity-modulated signal.
As stated above, in the sixth aspect, as in the third aspect, the
optical transmission system previously produces in the FM modulator
an FM modulated signal having a frequency deviation larger enough
to acquire a desired FM gain, then converts the FM modulated signal
into a frequency-divided FM modulated signal, and converts the
signal into an optical-intensity-modulated signal for transmission.
It is therefore possible to reduce the phase noise in the FM
modulated signal to be optically transmitted and FM
demodulated.
According to a seventh aspect, in the first aspect, the
transmission system further comprises a frequency-divider provided
between the FM modulator and the optical transmitter for converting
the FM modulated signal outputted from the FM modulator into a
frequency-divided FM modulated signal whose frequency is 11/2.sup.n
the frequency of the FM modulated signal, the n being an integer of
not less than 1, wherein the optical transmitter includes an
optical modulator for producing the optical-intensity-modulated
signal through the optical modulation using the frequency-divided
FM modulated signal as an original signal.
As described above, in the seventh aspect, the optical transmission
system previously produces in the FM modulator an FM modulated
signal having a frequency deviation larger enough to acquire a
desired FM gain, then converts the FM modulated signal into a
frequency-divided FM modulated signal, and next converts the signal
into an optical-intensity-modulated signal for transmission. It is
therefore possible to reduce the phase noise in the FM modulated
signal to be optically transmitted and FM demodulated.
An eighth aspect of the present invention is directed to an
transmitter for use in a transmission system for optically
transmitting a frequency-division-multiplexed signal, which is
obtained by frequency-division-multiplexing a plurality of signals,
from a transmitting end to a receiving end. The transmitter
comprises a multiplexer for frequency-division multiplexing the
plurality of signals to produce the frequency-division-multiplexed
signal, an FM modulator for converting the
frequency-division-multiplexed signal into a frequency-modulated
signal through frequency modulation using the
frequency-division-multiplexed signal as an original signal to
output the frequency-modulated signal as an FM modulated signal,
and an optical transmitter for converting the FM modulated signal
into an optical-intensity-modulated signal whose optical carrier
component is suppressed in the optical frequency spectrum through
optical modulation using the FM modulated signal as an original
signal to send the optical-intensity-modulated signal to the
receiving end.
According to a ninth aspect, in the eighth aspect, the optical
transmitter includes a light source for outputting an unmodulated
light, and an optical modulator for modulating the unmodulated
light with the FM modulated signal to produce the
optical-intensity-modulated signal, the optical modulator having
the Mach-Zehnder interferometer structure with a predetermined
input-voltage vs. output-optical-power characteristic, and being
biased in the input-voltage vs. output-optical-power characteristic
such that the output optical power is at the minimum.
According to a tenth aspect, in the ninth aspect, the transmitter
further comprises a frequency-divider provided between the FM
modulator and the optical transmitter for converting the FM
modulated signal outputted from the FM modulator into a
frequency-divided FM modulated signal whose frequency is 1/2.sup.n
the frequency of the FM modulated signal, the n being an integer of
not less than 1, wherein the optical modulator modulates the
unmodulated light with the frequency-divided FM modulated signal to
produce the optical-intensity-modulated signal.
According to an eleventh aspect, in the eighth aspect, the optical
transmitter includes a light source for outputting an unmodulated
light, an optical branching circuit for branching the unmodulated
light fed from the light source into first and second unmodulated
lights, an optical modulator for modulating the first unmodulated
light with the FM modulated signal to produce the optical
intensity-modulated signal, the optical modulator having the
Mach-Zehnder interferometer structure with a predetermined
input-voltage vs. output-optical-power characteristic, and being
biased in the input-voltage vs. output-optical-power characteristic
such that the output optical power is at the maximum, and an
optical combining circuit for combining the
optical-intensity-modulated signal produced by the optical
modulator and the second unmodulated light to cancel the optical
carrier component of the optical-intensity-modulated signal with
the second unmodulated light, and output the
optical-intensity-modulated signal whose optical carrier component
is suppressed.
According to a thirteenth aspect, in the eleventh aspect, the
transmitter further comprises a frequency-divider provided between
the FM modulator and the optical transmitter for converting the FM
modulated signal outputted from the FM modulator into a
frequency-divided FM modulated signal whose frequency is 1/2.sup.n
the frequency of the FM modulated signal, the n being an integer of
not less than 1, wherein the optical modulator modulates the first
unmodulated light with the frequency-divided FM modulated signal to
produce the optical-intensity-modulated signal.
According to a fourteenth aspect, in the eighth aspect, the
transmitter further comprises a frequency-divider provided between
the FM modulator and the optical transmitter for converting the FM
modulated signal outputted from the FM modulator into a
frequency-divided FM modulated signal whose frequency is 1/2.sup.n
the frequency of the FM modulated signal, the n being an integer of
not less than 1, wherein the optical transmitter includes an
optical modulator for producing the optical-intensity-modulated
signal through optical modulation using the frequency-divided FM
modulated signal as an original signal.
These and other objects, features, aspects and advantages of the
present invention will become more apparent from the following
detailed description of the present invention when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing the configuration of an optical
transmission system according to a first embodiment of the present
invention;
FIG. 2A is a schematic diagram showing the frequency spectrum of an
FM modulated signal outputted from an FM modulator in the optical
transmission system according to the first embodiment;
FIG. 2B is a schematic diagram showing the optical frequency
spectrum of an optical signal outputted from an optical modulator
in the optical transmission system according to the first
embodiment;
FIG. 2C is a schematic diagram showing the frequency spectrum of an
FM modulated signal outputted from an optical receiver in the
optical transmission system according to the first embodiment;
FIG. 3 is a schematic diagram used to explain optical modulation
performed by the optical modulator in the optical transmission
system according to the first embodiment;
FIG. 4 is a block diagram showing the configuration of an optical
transmission system according to a second embodiment of the present
invention;
FIG. 5A is a schematic diagram showing the frequency spectrum of an
FM modulated signal outputted from an FM modulator in a first
operation mode of the optical transmission system according to the
second embodiment;
FIG. 5B is a schematic diagram showing the frequency spectrum of an
FM modulated signal outputted from a frequency-divider in the first
operation mode of the optical transmission system according to the
second embodiment;
FIG. 5C is a schematic diagram showing the frequency spectrum of an
FM modulated signal outputted from an optical receiver in the first
operation mode of the optical transmission system according to the
second embodiment;
FIG. 6A is a schematic diagram showing the frequency spectrum of an
FM modulated signal outputted from the FM modulator in a second
operation mode of the optical transmission system according to the
second embodiment;
FIG. 6B is a schematic diagram showing the frequency spectrum of an
FM modulated signal outputted from the frequency-divider in the
second operation mode of the optical transmission system according
to the second embodiment;
FIG. 6C is a schematic diagram showing the frequency spectrum of
the FM modulated signal outputted from the optical receiver in the
second operation mode of the optical transmission system according
to the second embodiment;
FIG. 7 is a block diagram showing the configuration of an optical
transmission system according to a third embodiment;
FIG. 8 is a block diagram showing the configuration of an optical
transmission system according to a fourth embodiment;
FIG. 9 is a schematic diagram used to explain optical modulation
performed by an optical modulator in the optical transmission
system according to the fourth embodiment;
FIG. 10A is a schematic diagram showing the optical frequency
spectrum of an optical signal outputted from the optical modulator
in the optical transmission system according to the fourth
embodiment;
FIG. 10B is a schematic diagram showing the optical frequency
spectrum of a light outputted from an optical branching circuit in
the optical transmission system according to the fourth
embodiment;
FIG. 10C is a schematic diagram showing the optical frequency
spectrum of an optical signal outputted form an optical combining
circuit in the optical transmission system according to the fourth
embodiment;
FIG. 11 is a block diagram showing the configuration of a
convectional optical transmission system;
FIG. 12A is a schematic diagram showing the frequency spectrum of
an FM modulated signal outputted from an FM modulator in the
optical transmission system in FIG. 11;
FIG. 12B is a schematic diagram showing the optical frequency
spectrum of an optical signal outputted from an optical transmitter
in the optical transmission system in FIG. 11; and
FIG. 12C is a schematic diagram showing the frequency spectrum of
an FM modulated signal outputted from an optical receiver in the
optical transmission system in FIG. 11.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
FIG. 1 shows the configuration of a system for optically
transmitting a frequency-division-multiplexed signal according to a
first embodiment of the present invention. Referring to FIG. 1, the
optical transmission system of the present embodiment comprises a
multiplexer 100, an FM modulator 101, a light source 103, an
optical modulator 104, an optical receiver 106, and an FM
demodulator 107. In the optical transmission system, an electrical
transmission line 102 connects the FM modulator 101 and the optical
modulator 104 to each other, and an optical transmission line 105
connects the optical modulator 104 and the optical receiver 106 to
each other. The optical transmission system transmits an optical
signal, which is produced in a below described manner, from a
transmitting end to a receiving end through the optical
transmission line 105. At the transmitting end, the light source
103 and the optical modulator 104 constitute an optical transmitter
20a, and the optical transmitter 20a, the multiplexer 100, and the
FM modulator 101 constitute a transmitter 10a. In addition, the
optical receiver 106 at the receiving end is constituted by a
photodetector such as photodiode for converting an optical signal
into an electrical signal through photodetection based on a
square-law detection characteristic, and a preamplifier for
amplifying the electrical signal fed from the photodetector.
Next, referring to FIGS. 2A to 2C, the operation of the present
embodiment shown in FIG. 1 is described. FIGS. 2A to 2C
schematicaly illustrate the frequency spectrums of respective
signals in the optical transmission system in FIG. 1. FIG. 2A shows
the frequency spectrum of an output signal of the FM modulator 101.
FIG. 2B shows the optical frequency spectrum of an output signal
(optical signal) of the optical modulator 104. FIG. 2C shows the
frequency spectrum of an output signal of the optical receiver 106.
In the optical transmission system shown in FIG. 1, the multiplexer
100 frequency-division multiplexes a plurality of signals, and
outputs the resultant signal to the FM modulator 101. The FM
modulator 101 converts the frequency-division-multiplexed signal
into an FM modulated signal through frequency modulation. The FM
modulated signal has a frequency spectrum as shown in FIG. 2A in
which a carrier frequency is fc and a frequency deviation is
.DELTA.F. After that, the FM modulator outputs the FM modulated
signal to the electrical transmission line 102. The light source
103 outputs an unmodulated light. The optical modulator 104
receives the unmodulated light from the light source 103 and the FM
modulated signal through the electrical transmission line 102, then
modulates the unmodulated light with the FM modulated signal, and
outputs an optical signal whose optical carrier component is
suppressed. The optical modulator 104 has the Mach-Zehnder
interferometer structure, for example, and is biased at the
"valley" in its input-voltage vs. output-optical-power
characteristic, where the output optical power is at the minimum,
as shown in FIG. 3. The FM modulated signal is applied to the
optical modulator 104 with respect to the voltage of an operating
point 1001 which is set by the above-mentioned bias. The optical
modulator 104 thus produces an optical-intensity-modulated signal
(hereinafter, referred to as "optical signal") having the optical
frequency spectrum in which an optical carrier component is
suppressed as shown in FIG. 2B. The optical receiver 106 receives
the optical signal through the optical transmission line 105, and
square-law detects the signal to convert into an FM modulated
signal having the frequency spectrum as shown in FIG. 2C, that is,
an FM modulated signal whose carrier frequency is 2fc and whose
frequency deviation is 2* .DELTA.F. The optical receiver 106 then
outputs the FM modulated signal to the FM demodulator 107. The FM
demodulator 107 demodulates the FM modulated signal to reproduce
the original frequency-division-multiplexed signal.
An FM modulation scheme can arise an FM gain by increasing a
frequency deviation, and improve the signal-to-noise ratio (SNR) of
a demodulated signal. On the other hand, the increase in frequency
deviation extends the spectrum bandwidth of an FM modulated signal,
and also requires a wider band of a transmission line. The group
delay characteristic of the transmission line with such wide band
affects the FM modulated signal, to cause linear distortion thereof
in some cases. That is, as the bandwidth of an FM modulated signal
is wider, waveform distortion thereof increases. In short, a
frequency deviation in FM modulated signal transmission has a
trade-off relation between noise characteristic and waveform
distortion of an FM modulated signal. Hence, the wide-band FM
transmission system is difficult to optimally design.
The optical transmission system in FIG. 1 converts an FM modulated
signal into an optical signal whose optical carrier is suppressed,
and then optically transmits and square-law detects the optical
signal, thus producing an FM modulated signal having a frequency
deviation (or frequency bandwidth) twice as large as the one of the
original FM modulated signal. Therefore, the frequency deviation of
the FM modulated signal outputted from the FM modulator 101 is set
to be half of the frequency deviation essentially required to
acquire a predetermined FM gain in FM demodulation. The optical
transmission system thus reduces the bandwidth of the FM modulated
signal in half at the transmitting end, and reduces linear
distortion caused by the group delay characteristic of the
electrical transmission line 102 and the like while securing the
predetermined FM gain. Moreover, the optical transmission system
suppresses the optical carrier level of the optical signal, thereby
suppressing waveform deterioration due to the chromatic-dispersion
of the optical fiber transmission line. As is clear from the above,
the optical transmission system of the present embodiment can
realize high-quality signal transmission, and facilitate
implementation of a transmission line to reduce the cost of the
entire system.
Second Embodiment
FIG. 4 shows the configuration of an optical transmission system of
an FM modulated signal according to a second embodiment of the
present invention. Referring to FIG. 4, the optical transmission
system of the present embodiment comprises the multiplexer 100, the
FM modulator 101, the light source 103, the optical modulator 104,
the optical receiver 106, the FM demodulator 107, and a
frequency-divider 408. In the optical transmission system, the
electrical transmission line 102 connects the frequency-divider 408
and the optical modulator 104 to each other, and the optical
transmission line 105 connects the optical modulator 104 and the
optical receiver 106 to each other. The configuration of the second
embodiment is different from that of the above-described first
embodiment only in including the frequency-divider 408. Therefore,
the constituents identical to those in the first embodiment are
assigned the same reference numerals, and the description thereof
is simplified herein. The difference from the first embodiment is
mainly described below. Note that at the transmitting end of the
optical transmission system of the present embodiment, the light
source 103 and the optical modulator 104 constitute the optical
transmitter 20a, and the optical transmitter 20a, the multiplexer
100, the FM modulator 101, and the frequency-divider 408 constitute
the transmitter 10b.
In the above first embodiment, an FM modulated signal is outputted
from the FM modulator 101, and then directly applied to the optical
modulator 104. On the other hand, in the present invention, the
frequency-divider 408 divides each frequency component of the FM
modulated signal outputted from the FM modulator 101 into 2.sup.n
(n is an integer of not less than one) Namely, the
frequency-divider 408 converts the FM modulated signal into a
frequency-divided FM modulated signal whose frequency is 1/2.sup.n
the frequency of the FM modulated signal. The frequency-divider 408
then outputs the frequency-divided FM modulated signal to the
optical modulator 104.
In an FM modulation scheme, if an oscillator for generating an FM
modulated signal has frequency drift, which is called "phase
noise",the frequency drift is converted into intensity noise at the
FM demodulation to degrade the noise characteristic of the
demodulated signal.
The optical transmission system shown in FIG. 4 produces an FM
modulated signal having a frequency deviation more than the
frequency deviation essentially required to acquire a predetermined
FM gain in FM demodulation. Next, the optical transmission system
converts the FM modulated signal into a frequency-divided FM
modulated signal, and then converts the signal into an optical
signal for transmission. The phase noise in the FM modulated signal
received at the receiving end is thus reduced.
FIGS. 5A to 5C illustrate the operation and advantageous effects of
the frequency-divider 408 in a first operation mode of the optical
transmission system. FIGS. 5A, 5B, and 5C schematically show the
frequency spectrums included in output signals of the FM modulator
101, the frequency-divider 408, and the otpcial receiver 106,
respectively. In the first operation mode, as shown in FIGS. 5A to
5C, the optical transmission system previously produces an FM
modulated signal (whose phase noise is .DELTA..nu.) having a
frequency deviation .DELTA.F equal to a required amount of
frequency deviation .DELTA.F in FM demodulation (FIG. 5A). After
that, the FM modulated signal is converted into a frequency-divided
FM modulated signal whose frequency is 1/2the frequency of the FM
modulated signal (FIG. 5B). Then, the frequency-divided FM
modulated signal is converted into an optical signal whose optical
carrier is suppressed, and square-law detected. As a result, the
optical transmission system obtains an FM modulated signal having
phase noise .DELTA..nu. equal to the one of the original FM
modulated signal (FIG. 5C). On the other hand, in the first
embodiment, the phase noise of the FM modulated signal becomes
twice at the receiving end while the frequency deviation thereof is
large (FIG. 2C). Specifically, when the phase noise of the FM
modulated signal at the transmitting end is .DELTA..nu., it becomes
2*.DELTA..nu. at the receiving end.
FIGS. 6A to 6C illustrate the operation and advantageous effects of
the frequency-divider 408 in a second operation mode of the optical
transmission system in FIG. 4. FIGS. 6A, 6B, and 6C schematically
show the frequency spectrums included in output signals of the FM
modulator 101, the frequency-divider 408, and the optical receiver
106, respectively. In the second operation mode, as shown in FIGS.
6A to 6C, the optical transmission system previously produces an FM
modulated signal (whose phase noise is .DELTA..nu.) having a
frequency deviation 2* .DELTA.F twice as large as a required amount
of frequency deviation .DELTA.F in FM demodulation (FIG. 6A). After
that, the FM modulated signal is converted into a frequency-divided
FM modulated signal whose frequency is 1/4 the frequency of the FM
modulated signal (FIG. 6B). The frequency-divided FM modulated
signal is next converted into an optical signal, then transmitted,
and square-law detected. Thus, the optical transmission system can
obtain an FM modulated signal whose phase noise is reduced in half
of the one of the original FM modulated signal (i.e., the phase
noise after transmission becomes 1/2* .DELTA..nu.) (FIG. 6C)
As is clear from the above, the optical transmission system of the
present embodiment can reduce the phase noise in the FM modulated
signal after transmission, thereby realizing signal transmission of
better quality.
Third Embodiment
FIG. 7 shows the configuration of an optical transmission system of
an FM modulated signal according to a third embodiment of the
present invention. Referring to FIG. 7, the optical transmission
system of the present embodiment comprises the multiplexer 100, the
FM modulator 101, the light source 103, the optical modulator 104,
the optical receiver 106, the FM demodulator 107, and an amplitude
controller 709. In the optical transmission system, the electrical
transmission line 102 connects the amplitude controller 709 and the
optical modulator 104 to each other, and the optical transmission
line 105 connects the optical modulator 104 and the optical
receiver 106 to each other. The configuration of the third
embodiment is different from that of the first embodiment in
including the amplitude controller 709. Therefore, the constituents
identical to those of the first embodiment are assigned the same
reference numerals, and the description thereof is simplified
herein. The difference from the first embodiment is mainly
described below. Note that at the transmitting end of the optical
transmission system of the present embodiment, the light source 103
and the optical modulator 104 constitute the optical transmitter
20a, and the optical transmitter 20a, the multiplexer 100, the FM
modulator 101, and the amplitude controller 709 constitute the
optical transmitter 10c.
In the above first embodiment, an FM modulated signal is outputted
from the FM modulator 101, and directly applied to the optical
modulator 104. On the other hand, in the present embodiment, the
amplitude controller 709 removes variations in amplitude of the FM
modulated signal to adjust the amplitude to consistently remain
constant, and then outputs the FM modulated signal to the optical
modulator 104.
As described in the above, in the optical transmission system in
FIG. 7, the amplitude of an FM modulated signal to be applied to
the optical modulator 104 consistently remains constant.
Accordingly, the optical transmission system can avoid the
influence of the nonlinearity in electrical-to-optical conversion
performed by the optical modulator 104, and prevent the signal
quality from being degraded in FM demodulation performed by the FM
demodulator 107. Consequently, it is possible to realize
high-quality signal transmission.
Fourth Embodiment
FIG. 8 shows the configuration of an optical transmission system of
an FM modulated signal according to a fourth embodiment of the
present invention. Referring to FIG. 8, the optical transmission
system of the present embodiment comprises the multiplexer 100, the
FM modulator 101, the light source 103, the optical receiver 106,
the FM demodulator 107, an optical modulator 804, an optical
branching circuit 810, an optical combining circuit 811, and an
optical delay circuit 812. In the optical transmission system, the
electrical transmission line 102 connects the FM modulator 101 and
optical modulator 104 to each other, and the optical transmission
line 105 connects the optical combining circuit 811 and the optical
receiver 106 to each other. The fourth embodiment is different from
the first embodiment in including the otpcial branching circuit
810, the optical combining circuit 811, and the optical delay
circuit 812. In addition, the conditions for optical modulation
performed by the optical modulator 804 are different from those of
the optical modulator 104 in the first embodiment. Therefore, the
constituents identical to those in the first embodiment are
assigned the same reference numerals, and the description thereof
is simplified herein. The difference from the first embodiment is
mainly described below. Note that at the transmitting end of the
optical transmission system of the present embodiment, the light
source 103, the optical branching circuit 810, the optical
modulator 804, the optical combining circuit 811, and the optical
delay circuit 812 constitute the optical transmitter 20b, and the
optical transmitter 20b, the multiplexer 100, and the FM modulator
101 constitute the transmitter 10d.
In the above-described first embodiment, the optical modulator 104
is biased at the "valley" in its input-voltage vs.
output-optical-power characteristic to produce an optical signal
whose optical carrier is suppressed. However, in the present
embodiment, the optical modulator 804 is biased in its
input-voltage vs. output-optical-power characteristic at the "peak"
where the output optical power is at the maximum, as shown in FIG.
9. An FM modulated signal is applied to the optical modulator 804
with respect to the voltage of an operating point 1002 which is set
by the above-mentioned bias. The optical modulator 804 thus
produces an optical-intensity-modulated signal (hereinafter,
referred to as "optical signal") having an optical frequency
spectrum as shown in FIG. 10A. The optical branching circuit 810
receives an unmodulated light (shown in FIG. 10B) from the light
source 103, and branches the light into a first unmodulated light
La and a second unmodulated light Lb. The first unmodulated light
La is outputted to the optical modulator 804, and converted into an
optical signal through optical modulation using the FM modulated
signal outputted from the FM modulator 101 as an original signal.
The second unmodulated light Lb is outputted through the optical
delay circuit 812 to the optical combining circuit 811, and
combined therein with the optical signal outputted from the optical
modulator 804. The optical delay circuit 812, after receiving the
second unmodulated light Lb, controls a propagation delay thereof
to set the second unmodulated light Lb in a precisely opposite
phase to the optical signal outputted from the optical modulator
804. Thus, in the optical combining circuit 811, the optical
carrier component of the optical signal outputted from the optical
modulator 804 is canceled by the unmodulated light outputted from
the optical delay circuit 812. Consequently, the optical
transmission system produces an optical signal having the optical
frequency spectrum in which an optical carrier component is
suppressed as shown in FIG. 10C.
As is clear from the above, according to the optical transmission
system in FIG. 8, the optical carrier component of the optical
signal outputted from the optical modulator 804 is cancelled by the
unmodulated light Lb which is one of the lights branched from the
unmodulated light fed from the light source 103. The optical
transmission system thus produces an optical signal whose optical
carrier component is suppressed, thereby realizing high-quality
signal transmission.
The optical delay circuit 812 is interposed onto the path leading
from the optical branching circuit 810 directly to the optical
combining circuit 811 in the fourth embodiment, but may be
interposed onto the path leading from the optical branching circuit
810 to the optical combining circuit 811 via the optical modulator
804 or interposed onto the both paths, or may be omitted.
In addition, it is possible to apply the change of the conditions
for the optical modulation performed by the optical modulator 804,
and the change and addition of the configuration accompanied
therewith (the optical branching circuit 810, the optical combining
circuit 811, and the optical delay circuit 812), all of which are
described in the fourth embodiment, to the first embodiment and
other embodiments. In such case, the same advantageous effects can
be also provided.
While the invention has been described in detail, the foregoing
description is in all aspects illustrative and not restrictive. It
is understood that numerous other modifications and variations can
be devised without departing from the scope of the invention.
* * * * *