Optical transmission system for frequency-multiplexed signal Number:6,832,047 from the United States Patent and Trademark Office (PTO) owispatent A detector outputs a detection signal indicating amplitude variation of an input frequency-multiplexed signal. An amplitude controller adjusts the amplitude of the frequency-multiplexed signal by referring to the detection signal. A modulator modulates the amplitude-adjusted frequency-multiplexed signal to produce a predetermined modulated signal. A second multiplexer multiplexes the modulated signal and the detection signal to produce a multiplexed signal. An optical transmitter converts the multiplexed signal into an optical signal, and then sends it out to an optical transmission path. An optical receiver converts the received optical signal into an electrical signal. A separator separates the modulated and detection signals from the electrical signal. A demodulator demodulates the modulated signal to output the frequency-multiplexed signal. An amplitude adjuster adjusts the amplitude of the frequency-multiplexed signal by referring to the detection signal to reproduce the original amplitude variation.<H transmission, multiplexed, Optical, transmi, 6832047.html, Patent, pto, 6832047

Title: Optical transmission system for frequency-multiplexed signal

Abstract: A detector outputs a detection signal indicating amplitude variation of an input frequency-multiplexed signal. An amplitude controller adjusts the amplitude of the frequency-multiplexed signal by referring to the detection signal. A modulator modulates the amplitude-adjusted frequency-multiplexed signal to produce a predetermined modulated signal. A second multiplexer multiplexes the modulated signal and the detection signal to produce a multiplexed signal. An optical transmitter converts the multiplexed signal into an optical signal, and then sends it out to an optical transmission path. An optical receiver converts the received optical signal into an electrical signal. A separator separates the modulated and detection signals from the electrical signal. A demodulator demodulates the modulated signal to output the frequency-multiplexed signal. An amplitude adjuster adjusts the amplitude of the frequency-multiplexed signal by referring to the detection signal to reproduce the original amplitude variation.

Patent Number: 6,832,047 Issued on 12/14/2004 to Fuse

Inventors: Fuse; Masaru (Toyonaka, JP)
Assignee: Matsushita Electric Industrial Co., Ltd. (Osaka-fu, JP)

Appl. No.: 593659

Filed: June 13, 2000

Foreign Application Priority Data


Jun 14, 1999 [JP]			11-166645

Current U.S. Class: 398/79 ; 398/140; 398/141; 398/182; 398/183; 398/187; 398/200; 398/201; 398/202; 398/208; 398/214

Current International Class: H04B 10/152 (20060101)

Field of Search: 398/187,182,183,200,201,140,141,202,208,214,79

References Cited [Referenced By]

U.S. Patent Documents


4722081	January 1988	Fujito et al.
4768186	August 1988	Bodell
4959862	September 1990	Davidov et al.
5016242	May 1991	Tang
5351148	September 1994	Maeda et al.
6486986	November 2002	Fuse

Other References

"Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems" IOOC 95, Technical Digest, vol. 5, PD. 2-7. .
"CNR Characteristics of Optical Video Transmission Systems Using Broadband FM Modulation Scheme", Fuse et al., Institute of Electronics, Information and Communication Engineers Papers, B-1, vol. J81-B-1, No. 9, Sep., 1998 (with English Translation attached)..

Primary Examiner: Phan; Hanh
Attorney, Agent or Firm: Wenderoth, Lind & Ponack, L.L.P.

Claims

What is claimed is:

1. A system for optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed, said system comprising: a detector, provided with the frequency-multiplexed signal, for outputting a detection signal corresponding to an amplitude variation of the frequency-multiplexed signal; an amplitude controller for adjusting an amplitude of the frequency-multiplexed signal by referring to the detection signal outputted from said detector, and outputting a suppressed multiplexed signal, which is the frequency-multiplexed signal with an instantaneous amplitude variation suppressed; a modulator for modulating the suppressed multiplexed signal outputted from said amplitude controller to produce a modulated signal; a multiplexer for multiplexing the modulated signal outputted from said modulator and the detection signal outputted from said detector to produce a multiplexed signal; an optical transmitter for converting the multiplexed signal outputted from said multiplexer into an optical signal; an optical receiver for converting the optical signal sent from said optical transmitter into an electrical signal; a separator for separating the modulated signal and the detection signal from the electrical signal outputted from said optical receiver; a demodulator for demodulating the modulated signal outputted from said separator to output the suppressed multiplexed signal; and an amplitude adjuster for adjusting, by referring to the detection signal outputted from said separator, an amplitude of the suppressed multiplexed signal outputted from said demodulator to output the frequency-multiplexed signal corresponding to the frequency-multiplexed signal provided to said detector.

2. The optical transmission system according to claim 1, wherein said amplitude controller adjusts an instantaneous amplitude of the frequency-multiplexed signal to be held under a predetermined value.

3. A system for optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed, said system comprising: a detector, provided with the frequency-multiplexed signal, for outputting a detection signal corresponding to an amplitude variation of the frequency-multiplexed signal; an amplitude controller for adjusting an amplitude of the frequency-multiplexed signal by referring to the detection signal outputted from said detector, and outputting a suppressed multiplexed signal, which is the frequency-multiplexed signal with an instantaneous amplitude variation suppressed; a multiplexer for multiplexing the suppressed multiplexed signal outputted from said amplitude controller and the detection signal outputted from said detector to produce a multiplexed signal; an optical transmitter for converting the multiplexed signal outputted from said multiplexer into an optical signal; an optical receiver for converting the optical signal sent from said optical transmitter into an electrical signal; a separator for separating the suppressed multiplexed signal and the detection signal from the electrical signal outputted from said optical receiver; and an amplitude adjuster for adjusting, by referring to the detection signal outputted from said separator, an amplitude of the suppressed multiplexed signal outputted from said separator to output the frequency-multiplexed signal corresponding to the frequency-multiplexed signal provided to said detector.

4. The optical transmission system according to claim 3, wherein said amplitude controller adjusts an instantaneous amplitude of the frequency-multiplexed signal to be held under a predetermined value.

5. A system for optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed, said system comprising: a detector, provided with the frequency-multiplexed signal, for outputting a detection signal corresponding to an amplitude variation of the frequency-multiplexed signal; an amplitude controller for adjusting an amplitude of the frequency-multiplexed signal by referring to the detection signal outputted from said detector, and outputting a suppressed multiplexed signal, which is the frequency-multiplexed signal with an instantaneous amplitude variation suppressed; a phase adjuster for adjusting a phase of the detection signal outputted from said detector to produce a phase-adjusted detection signal; a multiplexer for multiplexing the suppressed multiplexed signal outputted from said amplitude controller and the phase-adjusted detection signal outputted from said phase adjuster to produce a multiplexed signal; an optical transmitter for converting the multiplexed signal outputted from said multiplexer into an optical signal; an optical receiver for converting the optical signal sent from said optical transmitter into an electrical signal; a separator for separating the suppressed multiplexed signal and the phase-adjusted detection signal from the electrical signal outputted from said optical receiver; a phase reproducer for adjusting a phase of the phase-adjusted detection signal outputted from said separator to output the detection signal before phase adjustment; and an amplitude adjuster for adjusting, by referring to the detection signal outputted from said phase reproducer, an amplitude of the suppressed multiplexed signal outputted from said separator to output the frequency-multiplexed signal corresponding to the frequency-multiplexed signal provided to said detector.

6. The optical transmission system according to claim 5, wherein said amplitude controller adjusts an instantaneous amplitude of the frequency-multiplexed signal to be held under a predetermined value.

7. The optical transmission system according to claim 5, wherein said phase adjuster adjusts the phase of the detection signal so that instantaneous amplitude variations of the suppressed multiplexed signal and the detection signal multiplexed in said multiplexer are opposite in polarity.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical transmission systems and, more specifically, to a system of optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed therein.

2. Description of the Background Art

FIG. 14 is a block diagram showing an example of configuration of a conventional optical transmission system for a frequency-multiplexed signal. This optical transmission system is hereinafter referred to as a first background art. In FIG. 14, a multiplexer 1400 to which a plurality of signals having carriers with predetermined different frequencies are supplied; a modulator 1408 to which a signal outputted from the multiplexer 1400 is supplied; an optical transmitter 1404 to which a signal outputted from the modulator 1408 is supplied; an optical receiver 1405 for receiving an optical signal sent from the optical transmitter 1404; and a demodulator 1409 to which a signal outputted from the optical receiver 1405 is supplied.

The operation of the above optical transmission system in the first background art is now described. The multiplexer 1400 frequency-multiplexes the plurality of signals having the carriers with different predetermined frequencies, and outputs a multiplexed signal to the modulator 1408. The modulator 1408 modulates the frequency-multiplexed signal to produce a predetermined modulated signal, and outputs the same to the optical transmitter 1404. For such modulation, a frequency modulation (FM) scheme is used, for example. The optical transmitter 1404 converts the modulated signal into an optical signal, and sends the optical signal to an optical transmission path or the like (not shown). The optical receiver 1405 converts the optical signal received through the optical transmission path into the original electrical modulated signal, and outputs the same to the demodulator 1409. The demodulator 1409 demodulates the modulated signal to reproduce the original frequency-multiplexed signal.

The above described first background art is disclosed in detail in "Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems", IOOC' 95, Technical Digest, Vol. 5 PD2-7, which is incorporated herein by reference. In this optical transmission system, a frequency-multiplexed signal is modulated to be an FM modulated signal, and, after optical transmission, demodulated to be reproduced. This can improve SNR (signal-to-noise ratio) of the demodulated frequency-multiplexed signal by using FM gain in FM transmission. Therefore, multi-channel signals can be transmitted with high quality via a single optical fiber.

FIG. 15 is a block diagram showing another example of configuration of the conventional optical transmission system, which is hereinafter referred to as a second background art. In FIG. 15, the optical transmission system includes a multiplexer 1500 to which a plurality of signals having carriers with different frequencies are supplied; an optical transmitter 1504 to which a signal outputted from the multiplexer 1500 is supplied; and an optical receiver 1505 for receiving an optical signal from the optical transmitter 1504.

The operation of the above optical transmission system in the second background art is now described. The multiplexer 1500 frequency-multiplexes a plurality of signals having carriers with predetermined different frequencies, and outputs a frequency-multiplexed signal to the optical transmitter 1504. The optical transmitter 1504 converts the frequency-multiplexed signal into an optical signal, and sends the same to an optical transmission path or the like. The optical receiver 1505 converts the optical signal received via the optical transmission path into the original electrical frequency-multiplexed signal.

In the second background art, a frequency-multiplexed signal is directly converted into an optical modulated signal for optical transmission. Therefore, unlike the first background art, SNR improvement with FM gain cannot be achieved in this transmission system. However, multi-channel signals can be transmitted with simpler structure and low cost via a single optical fiber.

The conventional optical transmission system as described in the first background art can achieve multi-channel signal transmission using an optical fiber with high quality.

However, in the first background art, the following problems may arise due to the characteristics of the frequency-multiplexed signal. The frequency-multiplexed signal is generated by frequency-multiplexing a plurality of signals with different frequencies and phases. The instantaneous amplitude of such frequency-modulated signal is not constant and varies with time. FIG. 16 is a graph illustrating instantaneous amplitude variations on a time axis. As shown in FIG. 16, when a plurality of signals with different frequencies and phases are frequency-multiplexed, coincidences of their peaks in amplitude cause an instantaneous amplitude increase of the frequency-multiplexed signal at a certain time.

In the optical transmission system as described in the first background art, the frequency spectrum width of the modulated signal is determined in FM modulation according to the amplitude of the frequency-multiplexed signal. Therefore, as the amplitude of the frequency-multiplexed signal is instantaneously increased, the corresponding spectrum width of the modulated signal is instantaneously increased.

Furthermore, in an output part of the demodulator 1409 in the first background art, part of the modulated signal components may remain together with the demodulated signal due to circuitry configuration. Such component is herein called a residual modulated signal. It is known that part of the frequency spectrum of the residual modulated signal causes deterioration in the quality of the demodulated signal, which is disclosed, for example, in "CNR characteristics of optical video transmission system using broadband FM modulation scheme", Fuse et al., Institute of Electronics, Information and Communication Engineers Papers, B-1, Vol. J81-B-1, No. 9, August, 1998.

FIG. 17 is a graph illustrating the relation between the residual modulated signal and the demodulated signal on a frequency axis. Similar to variations in the frequency spectrum width of the frequency-multiplexed signal, variations in the frequency spectrum width of the residual modulation signal correspond to variations in the amplitude of the frequency-multiplexed signal. Therefore, an instantaneous amplitude increase of the frequency-multiplexed signal causes an instantaneous increase in the spectrum width of the residual modulated signal, further interfering with the frequency band of the demodulated signal. As a result, the quality of the demodulated signal deteriorates instantaneously.

In addition to the above, such instantaneous amplitude increase in the frequency-multiplexed signal causes the following problems. That is, such instantaneous increase also causes an instantaneous increase in the corresponding spectrum width of the modulated signal. Then, the modulated signal with its instantaneous spectrum width increased is transmitted through a transmission path such as an optical fiber.

In general, the frequency band of the signal that can be transmitted with good quality through the transmission path is predetermined by design. Therefore, if the instantaneous spectrum width of the modulated signal to be transmitted increases over the predetermined bandwidth predetermined by design, the increased part is clipped or distorted. As a result, the quality of the demodulated signal deteriorates instantaneously.

On the other hand, the optical transmission system as described in the second background art in which the frequency-multiplexed signal is directly converted into an optical modulated signal for optical transmission, multi-channel signal transmission using an optical fiber can be achieved with low cost.

However, also in the optical transmission system of the second background art, the following problems may occur due to the characteristics of the frequency-multiplexed signal, like the first background art.

The optical transmitter 1504 in the second background art generally uses a scheme called direct modulation. In the direct modulation scheme, a current injected to a light source such as a semiconductor laser is modulated with a modulating signal to be an optical intensity modulated signal.

FIG. 18 is a graph illustrating characteristics of input current to output optical intensity in the light source such as a laser device included in the optical transmitter. As shown in FIG. 18, when the input current falls down a threshold value (Ith), the output light power waveform is distorted with the part below the threshold value clipped. Therefore, if the frequency-multiplexed signal is used as the input current signal, its instantaneous increase in amplitude causes distortion in the waveform of the transmission signal, and therefore the quality thereof deteriorates instantaneously.

As described above, in the optical transmission systems as shown in the first and second background arts, an instantaneous amplitude increase, which characterizes the frequency-multiplexed signal, causes deterioration in the quality of the transmission signal.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide an optical transmission system capable of achieving signal transmission with less distortion by suppressing an instantaneous amplitude increase of a frequency-multiplexed signal and preventing an instantaneous quality deterioration of a transmission signal.

A first aspect of the present invention is directed to a system for optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed, comprising: a detector, provided with the frequency-multiplexed signal, outputting a detection signal corresponding to an amplitude variation of the frequency-multiplexed signal; an amplitude controller adjusting an amplitude of the frequency-multiplexed signal by referring to the detection signal outputted from the detector, and outputting a suppressed multiplexed signal, which is the frequency-multiplexed signal with an instantaneous amplitude variation suppressed; a modulator modulating the suppressed multiplexed signal outputted from the amplitude controller to produce a predetermined modulated signal; a multiplexer multiplexing the modulated signal outputted from the modulator and the detection signal outputted from the detector to produce a multiplexed signal; an optical transmitter converting the multiplexed signal outputted from the multiplexer into an optical signal; an optical receiver converting the optical signal sent from the optical transmitter into an electrical signal; a separator separating the modulated signal and the detection signal from the electrical signal outputted from the optical receiver; a demodulator demodulating the modulated signal outputted from the separator to output the suppressed multiplexed signal; and an amplitude adjuster adjusting, by referring to the detection signal outputted from the separator, an amplitude of the suppressed multiplexed signal outputted from the demodulator to output the frequency-multiplexed signal corresponding to the frequency-multiplexed signal provided to the detector.

In general, when the frequency-multiplexed signal is modulated to be a predetermined modulated signal for optical transmission, the frequency spectrum of the modulated signal may be instantaneously increased as the instantaneous amplitude of the frequency-multiplexed signal increases. With this instantaneous increase in the modulated signal, the frequency spectrum of a residual modulated signal even after demodulation instantaneously interferes with the frequency-multiplexed signal after demodulation, thereby causes deterioration in signal quality.

Further, in the transmission path, the frequency band width of the signal that can be transmitted with good quality is generally predetermined by design. Therefore, if the instantaneous spectrum width of the optical modulated signal to be transmitted is increased over the predetermined bandwidth, the increased part is clipped or distorted.

Therefore, in the first aspect, the instantaneous amplitude of the frequency-multiplexed signal is detected, and is controlled, by referring to the detection signal, so as to become moderate. Then, the controlled signal is modulated to be a predetermined modulated signal such as angle-modulated signal for optical transmission. On the other hand, the detection signal is multiplexed with the modulated signal for optical transmission. In the receiving side, by referring to the detection signal, the amplitude of the frequency-multiplexed signal after demodulation is adjusted, and the frequency-multiplexed signal having the same instantaneous amplitude variation as that of the original frequency-multiplexed signal is reproduced. Thus, it is possible to prevent instantaneous quality deterioration of the transmission signal with the instantaneous amplitude variation of the frequency-multiplexed signal, and achieve modulation/demodulation and optical transmission with less distortion.

According to a second aspect, in the first aspect, the amplitude controller adjusts an instantaneous amplitude of the frequency-multiplexed signal to be held under a predetermined value.

In the second aspect, the instantaneous amplitude variation of the frequency-multiplexed signal is detected, and then controlled, by referring to the detection signal, so as to be under a predetermined value. Then, the detection signal is optically transmitted with the frequency-multiplexed signal. In the receiving side, the amplitude of the frequency-multiplexed signal is adjusted by referring to the detection signal, and the frequency-multiplexed signal having the same instantaneous amplitude variation as that of the original frequency-multiplexed signal is reproduced. It is thus possible to prevent instantaneous quality deterioration of the transmission signal due to the instantaneous amplitude variation of the frequency-multiplexed signal and achieve optical transmission with extremely less distortion.

A third aspect is directed to a system for optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed, comprising: a detector, provided with the frequency-multiplexed signal, outputting a detection signal corresponding to an amplitude variation of the frequency-multiplexed signal; an amplitude controller adjusting an amplitude of the frequency-multiplexed signal by referring to the detection signal outputted from the detector, and outputting a suppressed multiplexed signal, which is the frequency-multiplexed signal with an instantaneous amplitude variation suppressed; a modulator modulating the suppressed multiplexed signal outputted from the amplitude controller to produce a predetermined modulated signal; a detection signal modulator modulating the detection signal outputted from the detector to produce a modulated detection signal; a multiplexer multiplexing the modulated signal outputted from the modulator and the modulated detection signal outputted from the detection signal modulator to produce a multiplexed signal; an optical transmitter converting the multiplexed signal outputted from the multiplexer into an optical signal; an optical receiver converting the optical signal sent from the optical transmitter into an electrical signal; a separator separating the modulated signal and the modulated detection signal from the electrical signal outputted from the optical receiver; a demodulator demodulating the modulated signal outputted from the separator to output the suppressed multiplexed signal; a detection signal demodulator demodulating the modulated detection signal outputted from the separator to output the detection signal; and an amplitude adjuster adjusting, by referring to the detection signal outputted from the detection signal demodulator, an amplitude of the suppressed multiplexed signal outputted from the demodulator to output the frequency-multiplexed signal corresponding to the frequency-multiplexed signal provided to the detector.

In the third aspect, the instantaneous amplitude variation of the frequency-multiplexed signal is detected, and then controlled, by referring to the detection signal, so as to become moderate. The controlled signal is then modulated to be a predetermined modulated signal for optical transmission. On the other hand, the detection signal is moderated to be a second modulated signal (modulated detection signal), and multiplexed with the modulated signal. In the receiving side, by referring to the detection signal obtained by demodulating the second modulated signal, the amplitude of the frequency-multiplexed signal after optical transmission and demodulation is adjusted, and the frequency-multiplexed signal having the same instantaneous amplitude variation as that of the original frequency-multiplexed signal is reproduced. It is thus possible to prevent instantaneous quality deterioration of the transmission signal due to the instantaneous amplitude variation of the frequency-multiplexed signal and achieve more flexible modulation/demodulation and optical transmission with less distortion.

According to a fourth aspect, in the third aspect, the amplitude controller adjusts an instantaneous amplitude of the frequency-multiplexed signal to be held under a predetermined value.

According to a fifth aspect, in the third aspect, an entire or part of an occupied frequency band of the modulated detection signal outputted from the detection signal modulator is different from an occupied frequency band of the modulated signal multiplexed in the multiplexer.

In the fifth aspect, one or both of the carrier frequencies of the modulated signal and the modulated detection signal are set to appropriated values so as to avoid overlap between the occupied frequency bands of the modulated signal and the modulated detection signal. It is thus possible to prevent quality deterioration of a transmission signal due to interference between these two signals and achieve high-quality optical transmission.

A sixth aspect is directed to a system for optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed, comprising: a detector, provided with the frequency-multiplexed signal, outputting a detection signal corresponding to an amplitude variation of the frequency-multiplexed signal; a modulator, provided with the frequency-multiplexed signal and the detection signal, modulating the frequency-multiplexed signal with a carrier to produce a predetermined modulated signal and changing a predetermined parameter of the modulated signal by referring to the detection signal; an optical transmitter converting the modulated signal outputted from the modulator into an optical signal; an optical receiver converting the optical signal sent from the optical transmitter into an electrical signal; a demodulator demodulating the electrical signal outputted from the optical receiver to output the frequency-multiplexed signal.

In the sixth aspect, instantaneous amplitude variation of the frequency-multiplexed signal is detected. By referring to the detection signal, a predetermined parameter in the modulated signal is controlled and changed. Thus, instantaneous spectrum interference by residual modulated signal components due to instantaneous amplitude variation of the frequency-multiplexed signal can be prevented. It is thus possible to prevent instantaneous quality deterioration of a transmission signal and achieve modulation/demodulation and optical transmission with simple structure and less distortion.

According to a seventh aspect, in the sixth aspect, the predetermined parameter changed by the modulator is a carrier frequency of the modulated signal.

In the seventh aspect, the instantaneous amplitude variation of the frequency-multiplexed signal is detected. By referring to the detection signal, the carrier frequency of the modulated signal is controlled. Thus, instantaneous spectrum interference by residual modulated signal components due to instantaneous amplitude variation of the frequency-multiplexed signal can be prevented. It is thus possible to prevent instantaneous quality deterioration of a transmission signal and achieve modulation/demodulation and optical transmission with simple structure and less distortion.

According to an eighth aspect, in the seventh aspect, the modulator increases the carrier frequency of the modulated signal as an instantaneous amplitude of the frequency-multiplexed signal increases, and the modulator decreases the carrier frequency of the modulated signal as the instantaneous amplitude of the frequency-multiplexed signal decreases.

In the eighth aspect, the carrier frequency of the modulated signal is set higher as the instantaneous amplitude of the frequency-multiplexed signal increases. It is thus possible to eliminate interference from the frequency spectrum of the residual modulated signal with the demodulated signal and achieve high-quality optical transmission.

According to a ninth aspect, in the sixth aspect, the predetermined parameter changed by the modulator is frequency modulation efficiency of the modulated signal.

In the ninth aspect, the instantaneous amplitude variation of the frequency-multiplexed signal is detected. By referring to the detection signal, the frequency modulation efficiency of the modulated signal is controlled. Thus, instantaneous spectrum interference by residual modulated signal components due to instantaneous amplitude variation of the frequency-multiplexed signal can be prevented. It is thus possible to prevent instantaneous quality deterioration of a transmission signal and achieve modulation/demodulation and optical transmission with simple structure and less distortion.

According to a tenth aspect, in the ninth aspect, the modulator decreases the frequency modulation efficiency of the modulated signal as an instantaneous amplitude of the frequency-multiplexed signal increases, and the modulator increases the frequency modulation efficiency of the modulated signal as the instantaneous amplitude of the frequency-multiplexed signal decreases.

In the tenth aspect, the frequency modulation efficiency of the modulated signal is set be decreased as the instantaneous amplitude of the frequency-multiplexed signal increases. It is thus possible to eliminate interference from the frequency spectrum of the residual modulated signal with the demodulated signal and achieve high-quality optical transmission.

An eleventh aspect is directed to a system for optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed, comprising: a detector, provided with the frequency-multiplexed signal, outputting a detection signal corresponding to an amplitude variation of the frequency-multiplexed signal; an amplitude controller adjusting an amplitude of the frequency-multiplexed signal by referring to the detection signal outputted from the detector, and outputting a suppressed multiplexed signal, which is the frequency-multiplexed signal with an instantaneous amplitude variation suppressed; a multiplexer multiplexing the suppressed multiplexed signal outputted from the amplitude controller and the detection signal outputted from the detector to produce a multiplexed signal; a modulator modulating the multiplexed signal outputted from the multiplexer to produce a predetermined modulated signal; an optical transmitter converting the modulated signal outputted from the modulator into an optical signal; an optical receiver converting the optical signal sent from the optical transmitter into an electrical signal; a demodulator demodulating the electrical signal outputted from the optical receiver to produce a demodulated signal; a separator separating the suppressed multiplexed signal and the detection signal from the demodulated signal outputted from the demodulator; and an amplitude adjuster adjusting, by referring to the detection signal outputted from the separator, an amplitude of the suppressed multiplexed signal outputted from the separator to output the frequency-multiplexed signal corresponding to the frequency-multiplexed signal provided to the detector.

In the eleventh aspect, the instantaneous amplitude variation of the frequency-multiplexed signal is detected, and then controlled, by referring to the detection signal, so as to become moderate. The detection signal is then multiplexed with the frequency-multiplexed signal, and then modulated to be a predetermined modulated signal for optical transmission. In the receiving side, a signal obtained by multiplexing the frequency-multiplexed signal and the detection signal is demodulated. By referring to the demodulated detection signal, the amplitude of the frequency-multiplexed signal is adjusted to reproduce the frequency-multiplexed signal having the same instantaneous amplitude variation as that of the original frequency-multiplexed signal. It is thus possible to prevent instantaneous quality deterioration of a transmission signal due to the instantaneous amplitude variation of the frequency-multiplexed signal and achieve modulation/demodulation and optical transmission with extremely less distortion.

According to a twelfth aspect, in the eleventh aspect, the amplitude controller adjusts an instantaneous amplitude of the frequency-multiplexed signal to be held under a predetermined value.

A thirteenth aspect is directed to a system for optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed, comprising: a detector, provided with the frequency-multiplexed signal, outputting a detection signal corresponding to an amplitude variation of the frequency-multiplexed signal; an amplitude controller adjusting an amplitude of the frequency-multiplexed signal by referring to the detection signal outputted from the detector, and outputting a suppressed multiplexed signal, which is the frequency-multiplexed signal with an instantaneous amplitude variation suppressed; a phase adjuster adjusting a phase of the detection signal outputted from the detector to produce a phase-adjusted detection signal; a multiplexer multiplexing the suppressed multiplexed signal outputted from the amplitude controller and the phase-adjusted detection signal outputted from the phase adjuster to produce a multiplexed signal; a modulator modulating the multiplexed signal outputted from the multiplexer to produce a predetermined modulated signal; an optical transmitter converting the modulated signal outputted from the modulator into an optical signal; an optical receiver converting the optical signal sent from the optical transmitter into an electrical signal; a demodulator demodulating the electrical signal outputted from the optical receiver; a separator separating the suppressed multiplexed signal and the phase-adjusted detection signal from the electrical signal outputted from the demodulator; a phase reproducer adjusting a phase of the phase-adjusted detection signal outputted the separator to output the detection signal before the phase thereof is adjusted; and an amplitude adjuster adjusting, by referring to the detection signal outputted from the phase reproducer, an amplitude of the suppressed multiplexed signal outputted from the separator to output the frequency-multiplexed signal corresponding to the frequency-multiplexed signal provided to the detector.

In the thirteenth aspect, the instantaneous amplitude variation of the frequency-multiplexed signal is detected, and then controlled, by referring to the detection signal, so as to become moderate. The phase-adjusted detection signal is then multiplexed with the frequency-multiplexed signal, and then modulated to be a predetermined modulated signal for optical transmission. In the receiving side, a signal obtained by multiplexing the frequency-multiplexed signal and the phase-adjusted detection signal is demodulated. By referring to the phase-reproduced detection signal, the amplitude of the frequency-multiplexed signal is adjusted to reproduce the frequency-multiplexed signal having the same instantaneous amplitude variation as that of the original frequency-multiplexed signal. It is thus possible to prevent instantaneous quality deterioration of a transmission signal due to the instantaneous amplitude variation of the frequency-multiplexed signal and achieve modulation/demodulation and optical transmission with less distortion.

According to a fourteenth aspect, in the thirteenth aspect, the amplitude controller adjusts an instantaneous amplitude of the frequency-multiplexed signal to be held under a predetermined value.

According to a fifteenth aspect, in the thirteenth aspect, the phase adjuster adjusts the phase of the detection signal so that instantaneous amplitude variations of the suppressed multiplexed signal and the detection signal multiplexed in the multiplexer are opposite in polarity.

In the fifteenth aspect, the phase of the detection signal is adjusted so that the instantaneous amplitude variations of the frequency-multiplexed signal and the detection signal multiplexed in the multiplexer are opposite in polarity. That is, the phase of the detection signal is adjusted so that the instantaneous amplitude of the detection signal is decreased as that of the frequency-multiplexed signal increases, and vice versa. After such adjustment, the detection signal is optically transmitted with the frequency-multiplexed signal. The instantaneous amplitude variation of the optical transmission signal is thus suppressed, and optical transmission with less distortion can be achieved.

A sixteenth aspect is directed to a system for optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed, comprising: a detector, provided with the frequency-multiplexed signal, outputting a detection signal corresponding to an amplitude variation of the frequency-multiplexed signal; an amplitude controller adjusting an amplitude of the frequency-multiplexed signal by referring to the detection signal outputted from the detector, and outputting a suppressed multiplexed signal, which is the frequency-multiplexed signal with an instantaneous amplitude variation suppressed; a detection signal modulator modulating the detection signal outputted from the detector to produce a modulated detection signal; a multiplexer multiplexing the suppressed multiplexed signal outputted from the amplitude controller and the modulated detection signal outputted from the detection signal modulator to produce a multiplexed signal; a modulator modulating the multiplexed signal outputted from the multiplexer to produce a predetermined modulated signal; an optical transmitter converting the modulated signal outputted from the modulator into an optical signal; an optical receiver converting the optical signal sent from the optical transmitter into an electrical signal; a demodulator demodulating the electrical signal outputted from the optical receiver to produce a demodulated signal; a separator separating the suppressed multiplexed signal and the modulated detection signal from the demodulated signal outputted from the demodulator; a detection signal demodulator demodulating the modulated detection signal outputted from the separator to output the detection signal; an amplitude adjuster adjusting, by referring to the detection signal outputted from the detection signal demodulator, an amplitude of the suppressed multiplexed signal outputted from the separator to output the frequency-multiplexed signal corresponding to the frequency-multiplexed signal provided to the detector.

In the sixteenth aspect, the instantaneous amplitude variation of the frequency-multiplexed signal is detected, and then controlled, by referring to the detection signal, so as to become moderate. The detection signal is then modulated to be a second modulated signal (modulated detection signal). The second modulated signal is multiplexed with the frequency-multiplexed signal, and then modulated into a predetermined modulated signal for optical transmission. In the receiving side, a signal obtained by multiplexing the frequency-multiplexed signal and the modulated detection signal is demodulated. By referring to the detection signal obtained by demodulating the modulated detection signal, the amplitude of the frequency-multiplexed signal is adjusted to reproduce the frequency-multiplexed signal having the same instantaneous amplitude variation as that of the original frequency-multiplexed signal. It is thus possible to prevent instantaneous quality deterioration of a transmission signal due to the instantaneous amplitude variation of the frequency-multiplexed signal and achieve modulation/demodulation and optical transmission with more flexibility and less distortion.

According to a seventeenth aspect, in the sixteenth aspect, the amplitude controller adjusts an instantaneous amplitude of the frequency-multiplexed signal to be held under a predetermined value.

According to an eighteenth aspect, in the sixteenth aspect, an entire or part of an occupied frequency band of the modulated detection signal outputted from the detection signal modulator is different from an occupied frequency band of the suppressed multiplexed signal obtained in the multiplexer.

In the eighteenth aspect, one or both of the carrier frequencies of the frequency-multiplexed signal and the modulated detection signal are set to appropriated values so as to avoid overlap between the occupied frequency bands of the frequency-multiplexed signal and the modulated detection signal. It is thus possible to prevent quality deterioration of a transmission signal due to interference between these two signals and achieve high-quality optical transmission.

A nineteenth aspect is directed to a system for optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed, comprising: a detector, provided with the frequency-multiplexed signal, outputting a detection signal corresponding to an amplitude variation of the frequency-multiplexed signal; an amplitude controller adjusting an amplitude of the frequency-multiplexed signal by referring to the detection signal outputted from the detector, and outputting a suppressed multiplexed signal, which is the frequency-multiplexed signal with an instantaneous amplitude variation suppressed; a phase adjuster adjusting a phase of the detection signal outputted from the detector to produce a phase-adjusted detection signal; a detection signal modulator modulating the phase-adjusted detection signal outputted from the phase adjuster to produce a modulated detection signal; a multiplexer multiplexing the suppressed multiplexed signal outputted from the amplitude controller and the modulated detection signal outputted from the detection signal modulator to produce a multiplexed signal; a modulator modulating the multiplexed signal to produce a predetermined modulated signal; an optical transmitter converting the modulated signal outputted from the modulator into an optical signal; an optical receiver converting the optical signal sent from the optical transmitter into an electrical signal; a demodulator demodulating the electrical signal outputted from the optical receiver to produce a demodulated signal; a separator separating the suppressed multiplexed signal and the modulated detection signal from the demodulated signal outputted from the demodulator; a detection signal demodulator demodulating the modulated detection signal outputted from the separator to output the phase-adjusted detection signal before modulation; a phase reproducer adjusting a phase of the phase-adjusted detection signal outputted from the detection signal demodulator to output the detection signal before phase adjustment; and an amplitude adjuster adjusting, by referring to the detection signal outputted from the phase reproducer, an amplitude of the suppressed multiplexed signal outputted from the separator to output the frequency-multiplexed signal corresponding to the frequency-multiplexed signal provided to the detector.

In the nineteenth aspect, the instantaneous amplitude variation of the frequency-multiplexed signal is detected, and then controlled, by referring to the detection signal, so as to become moderate. The phase-adjusted detection signal is then modulated to be a second modulated signal (modulated detection signal). The second modulated signal is multiplexed with the frequency-multiplexed signal, and then modulated into a predetermined modulated signal for optical transmission. In the receiving side, a signal obtained by multiplexing the frequency-multiplexed signal and the detection signal is demodulated. By referring to the detection signal obtained by demodulating the modulated detection signal and then adjusting the phase thereof, the amplitude of the frequency-multiplexed signal is adjusted to reproduce the frequency-multiplexed signal having the same instantaneous amplitude variation as that of the original frequency-multiplexed signal. It is thus possible to prevent instantaneous quality deterioration of a transmission signal due to the instantaneous amplitude variation of the frequency-multiplexed signal and achieve modulation/demodulation and optical transmission with more flexibility and less distortion.

According to a twentieth aspect, in the nineteenth aspect, the amplitude controller adjusts an instantaneous amplitude of the frequency-multiplexed signal to be held under a predetermined value.

According to a twenty-first aspect, in the nineteenth aspect, an entire or part of an occupied frequency band of the modulated detection signal outputted from the detection signal modulator is different from an occupied frequency band of the suppressed multiplexed signal obtained in the multiplexer.

According to a twenty-second aspect, in the nineteenth aspect, the phase adjuster adjusts the phase of the detection signal so that instantaneous amplitude variations of the suppressed multiplexed signal and the modulated detection signal multiplexed in the multiplexer are opposite in polarity.

A twenty-third aspect is directed to a system for optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed, comprising: a detector, provided with the frequency-multiplexed signal, outputting a detection signal corresponding to an amplitude variation of the frequency-multiplexed signal; an amplitude controller adjusting an amplitude of the frequency-multiplexed signal by referring to the detection signal outputted from the detector, and outputting a suppressed multiplexed signal, which is the frequency-multiplexed signal with an instantaneous amplitude variation suppressed; a multiplexer multiplexing the suppressed multiplexed signal outputted from the amplitude controller and the detection signal outputted from the detector to produce a multiplexed signal; an optical transmitter converting the multiplexed signal outputted from the multiplexer into an optical signal; an optical receiver converting the optical signal sent from the optical transmitter into an electrical signal; a separator separating the suppressed multiplexed signal and the detection signal from the electrical signal outputted from the optical receiver; and an amplitude adjuster adjusting, by referring to the detection signal outputted from the separator, an amplitude of the suppressed multiplexed signal outputted from the separator to output the frequency-multiplexed signal corresponding to the frequency-multiplexed signal provided to the detector.

In general, when the frequency-multiplexed signal is modulated to be a predetermined modulated signal for optical transmission, clipping occurs at a threshold in input-current-to output-optical-intensity characteristics of a laser due to the instantaneous amplitude increase of the frequency-multiplexed signal. Waveform distortion due to such clipping produces deterioration in transmission signal quality.

Therefore, in the twenty-third aspect, the instantaneous amplitude of the frequency-multiplexed signal is detected, and is controlled, by referring to the detection signal, so as to become moderate. Then, the detection signal is multiplexed with the frequency-multiplexed signal, and then modulated to be a predetermined optical modulated signal for optical transmission. In the receiving side, by referring to the detection signal, the amplitude of the frequency-multiplexed signal after optical transmission is adjusted, and the frequency-multiplexed signal having the same instantaneous amplitude variation as that of the original frequency-multiplexed signal is reproduced. Thus, it is possible to prevent instantaneous quality deterioration of the transmission signal with the instantaneous amplitude variation of the frequency-multiplexed signal, and achieve modulation/demodulation and optical transmission with less distortion.

According to a twenty-fourth aspect, in the twenty-third aspect, the amplitude controller adjusts an instantaneous amplitude of the frequency-multiplexed signal to be held under a predetermined value.

A twenty-fifth aspect is directed to a system for optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed, comprising: a detector, provided with the frequency-multiplexed signal, outputting a detection signal corresponding to an amplitude variation of the frequency-multiplexed signal; an amplitude controller adjusting an amplitude of the frequency-multiplexed signal by referring to the detection signal outputted from the detector, and outputting a suppressed multiplexed signal, which is the frequency-multiplexed signal with an instantaneous amplitude variation suppressed; a phase adjuster adjusting a phase of the detection signal outputted from the detector to produce a phase-adjusted detection signal; a multiplexer multiplexing the suppressed multiplexed signal outputted from the amplitude controller and the phase-adjusted detection signal outputted from the phase adjuster to produce a multiplexed signal; an optical transmitter converting the multiplexed signal outputted from the multiplexer into an optical signal; an optical receiver converting the optical signal sent from the optical transmitter into an electrical signal; a separator separating the suppressed multiplexed signal and the phase-adjusted detection signal from the electrical signal outputted from the optical receiver; a phase reproducer adjusting a phase of the phase-adjusted detection signal outputted from the separator to output the detection signal before phase adjustment; and an amplitude adjuster adjusting, by referring to the detection signal outputted from the phase reproducer, an amplitude of the suppressed multiplexed signal outputted from the separator to output the frequency-multiplexed signal corresponding to the frequency-multiplexed signal provided to the detector.

In the twenty-fifth aspect, the instantaneous amplitude variation of the frequency-multiplexed signal is detected, and then controlled, by referring to the detection signal, so as to be under a predetermined value. Then, the phase-adjusted detection signal is multiplexed with the frequency-multiplexed signal, and modulated to be an optical modulated signal for optical transmission. In the receiving side, the amplitude of the frequency-multiplexed signal after optical transmission is adjusted by referring to the phase-reproduced detection signal, and the frequency-multiplexed signal having the same instantaneous amplitude variation as that of the original frequency-multiplexed signal is reproduced. It is thus possible to prevent instantaneous quality deterioration of the transmission signal due to the instantaneous amplitude variation of the frequency-multiplexed signal and achieve optical transmission with less distortion.

According to a twenty-sixth aspect, in the twenty-fifth aspect, the amplitude controller adjusts an instantaneous amplitude of the frequency-multiplexed signal to be held under a predetermined value.

According to a twenty-seventh aspect, in the twenty-fifth aspect, the phase adjuster adjusts the phase of the detection signal so that instantaneous amplitude variations of the suppressed multiplexed signal and the detection signal multiplexed in the multiplexer are opposite in polarity.

In the twenty-seventh aspect, the phase of the detection signal is adjusted so that the instantaneous amplitude variations of the frequency-multiplexed signal and the detection signal multiplexed in the multiplexer are opposite in polarity. That is, the phase of the detection signal is adjusted so that the instantaneous amplitude of the detection signal is decreased as that of the frequency-multiplexed signal increases, and vice versa. After such adjustment, the detection signal is optically transmitted with the frequency-multiplexed signal. The instantaneous amplitude variation of the optical transmission signal is thus suppressed, and optical transmission with less distortion can be achieved.

A twenty-eighth aspect is directed to a system for optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed, comprising: a detector, provided with the frequency-multiplexed signal, outputting a detection signal corresponding to an amplitude variation of the frequency-multiplexed signal; an amplitude controller adjusting an amplitude of the frequency-multiplexed signal by referring to the detection signal outputted from the detector, and outputting a suppressed multiplexed signal, which is the frequency-multiplexed signal with an instantaneous amplitude variation suppressed; a detection signal modulator modulating the detection signal outputted from the detector to produce a modulated detection signal; a multiplexer multiplexing the suppressed multiplexed signal outputted from the amplitude controller and the modulated detection signal outputted from the detection signal modulator to produce a multiplexed signal; an optical transmitter converting the multiplexed signal outputted from the multiplexer into an optical signal; an optical receiver converting the optical signal sent from the optical transmitter into an electrical signal; a separator separating the suppressed multiplexed signal and the modulated detection signal from the electrical signal outputted from the optical receiver; a detection signal demodulator demodulating the modulated detection signal outputted from the separator to output the detection signal; and an amplitude adjuster adjusting, by referring to the detection signal outputted from the detection signal demodulator, an amplitude of the suppressed multiplexed signal outputted from the separator to output the frequency-multiplexed signal corresponding to the frequency-multiplexed signal provided to the detector.

In the twenty-eighth aspect, the instantaneous amplitude of the frequency-multiplexed signal is detected, and is controlled, by referring to the detection signal, so as to become moderate. Then, the detection signal is modulated to be a second modulated signal (modulated detection signal), and then multiplexed with the frequency-multiplexed signal for optical transmission. In the receiving side, by referring to the detection signal obtained by demodulating the second modulated signal, the amplitude of the frequency-multiplexed signal is adjusted, and the frequency-multiplexed signal having the same instantaneous amplitude variation as that of the original frequency-multiplexed signal is reproduced. Thus, it is possible to prevent instantaneous quality deterioration of the transmission signal with the instantaneous amplitude variation of the frequency-multiplexed signal, and achieve modulation/demodulation and optical transmission with more flexibility and less distortion.

According to a twenty-ninth aspect, in the twenty-eighth aspect, the amplitude controller adjusts an instantaneous amplitude of the frequency-multiplexed signal to be held under a predetermined value.

According to a thirtieth aspect, in the twenty-eighth aspect, an entire or part of an occupied frequency band of the modulated detection signal outputted from the detection signal modulator is different from an occupied frequency band of the suppressed multiplexed signal obtained in the multiplexer.

A thirty-first aspect is directed to a system for optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed, comprising: a detector, provided with the frequency-multiplexed signal, outputting a detection signal corresponding to an amplitude variation of the frequency-multiplexed signal; an amplitude controller adjusting an amplitude of the frequency-multiplexed signal by referring to the detection signal outputted from the detector, and outputting a suppressed multiplexed signal, which is the frequency-multiplexed signal with an instantaneous amplitude variation suppressed; a phase adjuster adjusting a phase of the detection signal outputted from the detector to produce a phase-adjusted detection signal; a detection signal modulator modulating the phase-adjusted detection signal outputted from the phase adjuster to produce a modulated detection signal; a multiplexer multiplexing the suppressed multiplexed signal outputted from the amplitude controller and the modulated detection signal outputted from the detection signal modulator to produce a multiplexed signal; an optical transmitter converting the multiplexed signal outputted from the multiplexer into an optical signal; an optical receiver converting the optical signal sent from the optical transmitter into an electrical signal; a separator separating the suppressed multiplexed signal and the modulated detection signal from the electrical signal outputted from the optical receiver; a detection signal demodulator demodulating the modulated detection signal outputted from the separator to output the phase-adjusted detection signal; a phase reproducer adjusting a phase of the phase-adjusted detection signal outputted the detection signal demodulator to output the detection signal before phase adjustment; and an amplitude adjuster adjusting, by referring to the detection signal outputted from the phase reproducer, an amplitude of the suppressed multiplexed signal outputted from the separator to output the frequency-multiplexed signal corresponding to the frequency-multiplexed signal provided to the detector.

In the thirty-first aspect, the instantaneous amplitude of the frequency-multiplexed signal is detected, and is controlled, by referring to the detection signal, so as to become moderate. Then, the phase-adjusted detection signal is modulated to be a second modulated signal (modulated detection signal), multiplexed with the frequency-multiplexed signal, and then modulated to be an optical modulated signal for optical transmission. In the receiving side, by referring to the detection signal obtained by demodulating the modulated detection signal and reproducing the phase thereof, the amplitude of the frequency-multiplexed signal is adjusted, and the frequency-multiplexed signal having the same instantaneous amplitude variation as that of the original frequency-multiplexed signal is reproduced. Thus, it is possible to prevent instantaneous quality deterioration of the transmission signal with the instantaneous amplitude variation of the frequency-multiplexed signal, and achieve modulation/demodulation and optical transmission with more flexibility and less distortion.

According to a thirty-second aspect, in the thirty-first aspect, the amplitude controller adjusts an instantaneous amplitude of the frequency-multiplexed signal to be held under a predetermined value.

According to a thirty-third aspect, in the thirty-first aspect, an entire or part of an occupied frequency band of the modulated detection signal outputted from the detection signal modulator is different from an occupied frequency band of the suppressed multiplexed signal obtained in the multiplexer.

According to a thirty-fourth aspect, in the thirty-first aspe