JP2000201106A - Optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress effect on optical transmission signals when a channel is identified for the multiplexed transmission signals by outputting the optical signal of every channel based on the low frequency signal that is detected out of a frequency modulating component. SOLUTION: The optical signal of wavelength λ1, which has been frequency modulated at a light source 411 by the low frequency f1 sent from a low frequency signal source 412, is modulated by an optical modulator 414 of an optical transmitter 410 and outputted to an optical multiplexer 30. Then the optical signal is demultiplexed by an optical demultiplexer 50 and transmitted through an optical filter 61 of an optical receiver 510. The frequency modulating component that is superimposed on the optical signal of every channel is detected by an optical frequency discriminator 511, and a low frequency signal is extracted at a low frequency signal detection part 512 and sent to a control circuit 90. The circuit 90 controls an electric switch 80 to output the electric signals which are reproduced by the optical receivers (RX) 62 and 72 to a prescribed output destination, based on the signals received from the low frequency signal detection parts 512 and 522.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical multiplex transmission system, and more particularly to stimulated Brillouin scattering (SBS).
The present invention relates to an optical transmission system that enables channel identification of a multiplexed optical signal by using a minute frequency modulation signal applied to a light source in order to suppress the influence of ated bursting (scattering).

[0002]

2. Description of the Related Art Recently, the amount of data to be transmitted, such as image information, has increased, and the demand for an optical transmission system for multiplexing and transmitting such information has been increasing.

Hitherto, a large capacity has been realized by increasing the modulation / demodulation speed of an optical signal.
It has become difficult to increase the speed of optical devices. While the capacity of the optical transmission system is required to be 40 G to 1 Tbit / s, the above-mentioned modulation / demodulation speed is practically up to 10 to 20 Gbit / s. This problem is solved by multiplexing in the optical domain. Technology is effective, and many have been proposed at the academic level.

There are two multiplexing techniques in the optical domain, one of which is optical wavelength division multiplexing (WD) for multiplexing optical signals having different wavelengths.
M: Wavelength Division Multiplexing, and the other is optical time division multiplexing (OTDM:
Optical Time Division Multiplexing).

Recently, a transmitter having means for assigning identification information for channel identification to each optical signal, an identification information identification circuit for extracting an identification signal included in the optical signal after reception, and a transmission circuit having the same are disclosed. There has been proposed an optical transmission system including a receiver having means for outputting each optical signal to a predetermined output destination according to an identification signal. For example, such an optical transmission system is disclosed in Japanese Unexamined Patent Application Publication No. H8-3218.
There is one disclosed in Japanese Patent Publication No. 05-2005.

FIG. 8 is a diagram showing the configuration of a conventional WDM optical transmission system.

In FIG. 8, an optical transmitting apparatus 100 includes a plurality of optical transmitters 10 and 20 having different wavelengths, and optical signals from the plurality of optical transmitters 10 and 20 are wavelength-multiplexed and transmitted to a transmission line fiber 40. The optical multiplexer 30 is configured as follows.

The optical transmitter 10 includes a light source 11 having a wavelength λ 1,
An oscillator 12 for generating a low frequency of the frequency f1, a drive circuit 13 for amplitude-modulating an input data signal at a low frequency of the frequency f1, and a drive circuit 13 for transmitting an optical signal from the light source 11
And an optical modulator 14 that modulates with the output signal of. Similarly, the optical transmitter 20 includes a light source 21 having a wavelength λ2,
An oscillator 22 for generating a low frequency of the frequency f2, a driving circuit 23 for amplitude-modulating an input data signal at a low frequency of the frequency f2, and a driving circuit 23 for transmitting an optical signal from the light source 21
And an optical modulator 24 that modulates with the output signal of. Similarly, the optical transmission device 100 includes a plurality of optical channels that are amplitude-modulated at different frequencies for each wavelength.

The optical multiplexer 30 wavelength-multiplexes the optical signals from the plurality of optical transmitters 10 and 20 having different wavelengths, and sends the resultant to the transmission line optical fiber 40.

The transmission line optical fiber 40 includes a linear repeater 4
5 is installed and transmitted via the repeater 45. As the repeater 45, an erbium-doped fiber amplifier (EDFA: Erbium-Doped Fiber Amplifier) which has recently been put to practical use is used.
lifier) is used.

The optical receiving device 200 is an optical demultiplexer 5 for demultiplexing the transmitted optical signal into transmission wavelengths λ1, λ2,.
0, a plurality of optical receivers 60 and 70 for receiving each of the demultiplexed wavelength components, an electric switch 80 for electrically switching a reception signal from the plurality of optical receivers 60 and 70, and a plurality of optical receivers 60 , 70 and a control circuit 90 for controlling the electric switch 80 based on the detection signals.

The optical receiver 60 includes an optical filter 61 that transmits an optical signal having the wavelength λ1, an optical receiver (RX) 62 that receives the optical signal having the wavelength λ1 and reproduces the electrical signal, and an optical receiver (RX) 62. An optical / electrical converter (O / E) 63 for converting a part of the optical signal to optical / electrical (O / E) and receiving light;
And a low-frequency signal detector 64 for detecting only the frequency f1 component of the amplitude modulation from the three outputs. Similarly, the optical receiver 70 includes an optical filter 71 that transmits an optical signal of the wavelength λ2, an optical receiver (RX) 72 that receives the optical signal of the wavelength λ2 and reproduces the electrical signal, and an optical receiver (RX) 72. An O / E 73 for converting a part of the optical signal into light / electricity and receiving the light;
A low-frequency signal detector 74 for detecting only the frequency f2 component of the amplitude modulation from the output of the O / E 73. As described above, the optical receiving device 200 includes a plurality of channels for demultiplexing an optical signal amplitude-modulated at a different frequency for each wavelength, and detecting an optical signal of each wavelength and a frequency component of the amplitude modulation.

The control circuit 90 includes a low frequency signal detector 64,
According to the signal from 74, the electric switch 80 is controlled so that the electric signal reproduced by the optical receivers (RX) 62 and 72 is outputted to a predetermined output destination. In addition, you may make it change an output destination as needed.

The operation of the WDM optical transmission system will be described.

First, the drive circuit 13 amplitude-modulates the input data signal at a low frequency of the frequency f1, and outputs the amplitude-modulated data signal to the optical modulator 14. The optical modulator 14 modulates an optical signal of the wavelength λ1 from the light source 11 with a drive signal input via the drive circuit 13 and outputs the modulated signal to the optical multiplexer 30. Similarly, the drive circuit 23 amplitude-modulates the input data signal at a low frequency of the frequency f2, and outputs the amplitude-modulated data signal to the optical modulator 24. The optical modulator 24 modulates an optical signal of the wavelength λ2 from the light source 21 with a drive signal input via the drive circuit 23 and outputs the modulated signal to the optical multiplexer 30.

Thereafter, the optical signal amplitude-modulated at a different frequency for each wavelength is input to the optical multiplexer 30 and is transmitted to the optical multiplexer 3.
0, the optical signals of the wavelengths λ1, λ2,... Are wavelength-multiplexed and transmitted to the transmission line fiber 40. The optical signal transmitted to the transmission line optical fiber 40 is transmitted to the optical receiving side via the repeater 45.

The transmitted optical signal is first split by the optical splitter 50. In the optical filter 61, an optical signal having the wavelength λ1 is transmitted, and is reproduced by the optical receiver 62 into an electric signal. Part of the optical signal from the optical filter 61 is O / E6
The low frequency signal detector 64 detects only the frequency f1 component of the amplitude modulation from the electric signal and outputs it to the control circuit 90.

The same function as described above is provided for each optical channel of each wavelength, and an electric signal is reproduced for each wavelength and a superposed low-frequency signal is detected.

In the control circuit 90, the low frequency signal detector 6
The optical receivers (RX) 62, 7 according to the signals from
The electric switch 80 is controlled so as to output the electric signal reproduced in 2 to a predetermined output.

FIG. 9 is a diagram showing the configuration of a conventional OTDM optical transmission system, and the same components as those in FIG. 8 are denoted by the same reference numerals.

In FIG. 9, an optical transmitter 110 includes a light source 11, an oscillator 15 for generating a frequency f0 equal to the transmission speed of each channel, an optical switch 16, a plurality of optical transmitters 17, 27 having different wavelengths, And a plurality of optical transmitters 1
It comprises an optical multiplexer 31 for wavelength-multiplexing the optical signals from 7, 27 by bit multiplexing and transmitting the multiplexed optical signals to the transmission line fiber 40.

The optical switch 16 converts an optical signal from the light source 11 having the wavelength λ1 into a frequency f0 equal to the transmission speed of each channel.
And generates a pulse train having a pulse width that is half of 1 bit.

The optical transmitter 17 drives the oscillator 12 for generating a low frequency of the frequency f1, the drive circuit 13 for amplitude-modulating the input data signal at the low frequency of the frequency f1, and the optical signal from the optical switch 16. And an optical modulator 14 for modulating the output signal of the circuit 13. Similarly, the optical transmitter 27 is connected to the oscillator 2 that generates a low frequency of the frequency f2.
2, a drive circuit 23 for amplitude-modulating the input data signal at a low frequency of f2, and an optical modulator 24 for modulating the optical signal from the optical switch 16 with the output signal of the drive circuit 23.

The optical receiving device 210 includes an optical splitter 51 for splitting the transmitted optical signal into two, a clock extracting unit 52 for extracting a clock component having a frequency f0, and an extracted frequency f.
An optical switch 53 that switches an input optical signal by a clock component of 0 to perform bit separation, a plurality of optical receivers 65 and 75 that receive each bit-separated optical signal, and a plurality of optical receivers 65 and 75. An electric switch 80 for electrically switching a received signal, and a plurality of optical receivers 65 and 75
And a control circuit 90 that controls the electric switch 80 based on the detection signal from the control circuit 90.

The optical receiver 65 receives one of the bit-separated optical signals and reproduces it into an electric signal (RX).
62, an O / E 63 for optically / electrically converting a part of the optical signal from the optical switch 53 and receiving the light, and a low frequency signal detector 64 for detecting only the frequency f1 component of the amplitude modulation from the output of the O / E 63. Consists of Similarly, the optical receiver 75
Is an optical receiver (RX) 72 that receives the other bit-separated optical signal and reproduces it as an electrical signal, and an O / E 7 that optically / electrically converts a part of the optical signal from the optical switch 53 and receives it.
3 and a low-frequency signal detector 74 for detecting only the frequency f2 component of the amplitude modulation from the output of the O / E 73.

The operation of the OTDM optical transmission system will be described.

The oscillator 15 generates a frequency having the same frequency f0 as the transmission speed of each channel, and supplies it to the optical switch 16.

An optical signal from the light source 11 having the wavelength λ1 is modulated via the optical switch 16 by a sine wave or a clock signal having the same frequency f0 as the transmission speed of each channel, and is modulated by 1b.
Generate a pulse train with a pulse width that is half of it. The optical signal from the optical switch 16 is split into two and input to two optical transmitters 17 and 27. Each of the input optical signals is modulated by the optical modulators 14 and 24 which have been modulated via the drive circuits 13 and 23 as in the case of FIG. Here, the drive signals of the drive circuits 13 and 23 are amplitude-modulated at different frequencies (frequency f1 from the oscillator 12 and frequency f2 from the oscillator 22) for each wavelength.

The optical signals from the two optical transmitters 17 and 27 are bit-multiplexed by the optical multiplexer 31 and transmitted to the transmission line fiber 40 as in the case of FIG.

An optical signal from the transmission line fiber 40 is split into two by an optical splitter 51, one of which is input to an optical switch 53, and the other is input to a clock extraction unit 52.

Since the optical signal input to the clock extracting unit 52 contains a clock component of the frequency f0, first, the input optical signal is photoelectrically converted by the clock extracting unit 52, and the band-pass filter of the center frequency f0 is used. To extract the clock component of the frequency f0. Then, the optical switch 5 is used by the extracted clock component of the frequency f0.
At 3, the input optical signal is switched. This enables bit separation.

Each optical signal from the optical switch 53 is reproduced as an electrical signal by optical receivers (RX) 62 and 72, and O / $ 63 and 73 and low frequency signal detectors 64 and 74.
, A superimposed low-frequency signal is detected from a part thereof.

In the control circuit 90, the low frequency signal detector 6
The electric switch 80 is controlled so that the electric signal reproduced by the optical receivers 62 and 72 is output to a predetermined output destination in accordance with the signals from the optical receivers 4 and 74. In this case, the output destination may be changed as needed.

[0034]

However, such a conventional optical transmission system has the following problems.

That is, since a low-frequency component is superimposed on the drive signal for modulating the optical modulator as amplitude modulation, the eye opening degree, that is, the SN ratio of the optical transmission signal is degraded. Also,
Since the low-frequency component must be detected on the receiving side, a certain degree of amplitude modulation must be applied, and the deterioration of the eye opening caused by the influence cannot be ignored.

Further, as described in detail below, in order to suppress stimulated Brillouin scattering (hereinafter, referred to as SBS), which is one of the nonlinear effects in the transmission line optical fiber, frequency modulation is applied to the optical transmission signal. And the eye opening degree deteriorates more and more.

Recently, the development of an ultra-high-speed external modulator and a high-power erbium-doped optical fiber amplifier has made it possible to realize a transmission light source with less frequency chirping and an output of +20 dBm or more. When transmitting such a coherent and high-output signal light through an optical fiber, the SBS
Occurs.

The SBS occurs in the optical fiber in a direction opposite to the propagation direction of the input signal light. As a result, SBS
Occurs, the reflected light power increases, and no matter how much the input light power is increased, the power reaching the fiber output end does not increase more than a certain level. Further, the error rate is significantly deteriorated on the receiver side due to the influence of the reflected light.

This phenomenon appears more remarkably as the input optical power to the optical fiber increases, and the input optical power that begins to appear is SB
Defined as S threshold. Therefore, the signal light power is S
It must be set below the BS threshold value, which causes a problem of input optical power limit (reference literature: IEICE, Optical Communication System Study Group OCS91-49).

However, a laser diode (LD)
The SBS threshold can be improved (ie, increased) by applying a slight frequency modulation to the SBS.

For example, a conventional optical transmitter (The Institute of Electronics, Information and Communication Engineers, OCS91-49, Optical Communication System Research Group) has a configuration as shown in FIG.

FIG. 10 is a diagram for explaining a conventional SBS suppression method.

In FIG. 10, reference numeral 301 denotes an LD, 302 denotes a current source, 303 denotes a frequency modulation signal source, 304 denotes a drive circuit, 305 denotes an optical modulator, 306 denotes an optical amplifier, and 307 denotes a transmission line fiber.

LD biased by constant current source 302
Light output from 301 is input to an external modulator 305, and is modulated by a data input signal via a drive circuit 304. The output optical signal of the external modulator 305 is
The signal is amplified to a high-output optical signal by 06 and output to the transmission line optical fiber 307. Here, the frequency modulation signal source 3
From 03, minute current modulation is applied to the bias current of the LD 301. The means for giving current modulation is the same even if the bias voltage of the LD 301 is directly modulated from a voltage source without using a current source.

FIG. 11 is a diagram showing an actual measurement example of a change in the reflected light from the transmission line fiber and a change in the optical power after transmission from the transmission line fiber when the input light to the transmission line fiber is changed.
The modulation current amplitude of the frequency modulation in FIG. 10 is used as a parameter.

As shown in FIG. 11, when the frequency modulation is not applied (see ● in FIG. 11), the input power to the transmission line fiber starts to be generated at +6 dBm or more. Conversely, in order to increase the fiber input power to +17 dBm, the modulation current amplitude must be increased to 8 mApp, as shown by the symbol ▼ in FIG.

The present invention can suppress the influence on the optical transmission signal when performing channel identification of the multiplexed optical signal, and can suppress the SBS even when the input power to the transmission line optical fiber is high. It is an object of the present invention to provide an optical transmission system capable of performing the following.

Another object of the present invention is to provide an optical transmission system capable of automatically controlling an optimum modulation degree so as not to deteriorate the eye opening degree of an optical output waveform.

[0049]

SUMMARY OF THE INVENTION An optical transmission system according to the present invention includes means for applying frequency modulation or phase modulation to a light source with a low-frequency signal different for each wavelength of each light source. Optical transmitter, means for wavelength multiplexing optical signals from a plurality of optical transmitters, means for splitting a wavelength multiplexed optical signal, and detecting a phase modulation component or a frequency modulation component from the split optical signal. Means, means for detecting a low frequency signal from the phase modulation component or the frequency modulation component, and means for outputting an optical signal of each channel to a predetermined output destination based on the output of the low frequency signal detection means. It is characterized by.

The optical transmission system according to the present invention comprises an optical time division multiplexing means for time division multiplexing a plurality of optical signal channels, and an optical signal of each channel being different from each other as identification information for identifying each optical signal channel. Means for applying phase modulation by frequency, means for detecting a phase modulation component from a time-divided optical signal, means for detecting a low frequency signal from the phase modulation component, and each channel based on the output of the low frequency signal detection means. Means for outputting the optical signal to a predetermined output destination.

In the optical transmission system according to the present invention, when the optical power when the wavelength-multiplexed optical signal is input to the transmission line fiber is large enough to cause stimulated Brillouin scattering,
The frequency and the modulation level of the frequency modulation or the phase modulation may be optimized so as not to cause the stimulated Brillouin scattering.

The optical transmission system according to the present invention comprises means for monitoring the input light power to the transmission line fiber and the reflected light power from the transmission line fiber for each wavelength of each channel, and a guided Brillouin based on the output of the monitoring unit. Control means for optimizing the frequency and modulation level of phase modulation or frequency modulation so as not to cause scattering may be provided.

[0053]

Embodiments of the present invention will be described below with reference to the drawings. First Embodiment FIG. 1 is a block diagram showing a configuration of an optical transmission system according to a first embodiment of the present invention, which is an example applied to an optical transmission system based on WDM. In the description of the present embodiment, the same components as those in FIG. 8 are denoted by the same reference numerals.

In FIG. 1, an optical transmitting apparatus 400 includes a plurality of optical transmitters 410 and 420 having different wavelengths, and multiplexes optical signals from the plurality of optical transmitters 410 and 420 with each other and sends the multiplexed signal to the transmission line fiber 40. The optical multiplexer 30 is configured as follows.

The optical transmitter 410 has a light source 411 of wavelength λ1.
And a low frequency signal source 41 for generating a low frequency of the frequency f1
2, a driving circuit 413 for outputting a driving signal from the input data signal, and an optical signal from the light source 411 for the driving circuit 4.
And an optical modulator 414 that modulates the output signal with the output signal of T.13. Similarly, the optical transmitter 420 includes a light source 421 having a wavelength λ2, a low-frequency signal source 422 generating a low frequency of a frequency f2, a driving circuit 423 outputting a driving signal from an input data signal, and a light source 421. And an optical modulator 424 that modulates the optical signal of FIG.

Similarly, the optical transmitting apparatus 400 includes a plurality of optical channels that are frequency-modulated at different frequencies for each wavelength.

Each of the light sources 411 and 421 is composed of a laser diode (LD). The signal from the low frequency signal source 412 or 422 is superimposed on the bias current of the laser diode (LD) so that the light output signal is not affected sufficiently. A small current modulation. That is, the light sources 411 and 421 are frequency-modulated at different low frequencies for each wavelength of each light source. When increasing the number of wavelength multiplexing, the current modulation may be performed with a low-frequency signal having a different frequency for each channel, as in the conventional example.

Here, frequency modulation is synonymous with phase modulation. Instead of applying current modulation to the light sources 411 and 421,
A configuration may be adopted in which the outputs of the optical modulators 414 and 424 are subjected to phase modulation by an optical phase modulator. These optical phase modulators correspond to the optical modulators (intensity modulators) 14 and 24 shown in FIG.
In the same manner as described above, for example, it can be realized by a Mach-Zehnder type optical modulator in which an optical waveguide is formed on a LiNbO3 crystal.

The driving circuits 413 and 423
4,424 driver circuit, which outputs an output signal whose logic is inverted from the input electric signal to the optical modulators 414,424.
Is output to each electrode.

As described above, in this embodiment, the method of superimposing the low-frequency signal is not the conventional amplitude modulation but frequency modulation.

On the other hand, the optical receiving device 500 comprises an optical demultiplexer 5 for demultiplexing the transmitted optical signal into transmission wavelengths λ1, λ2,.
0, a plurality of optical receivers 510, 520 for receiving the respective wavelength components that have been demultiplexed, an electrical switch 80 for electrically switching the reception signals from the plurality of optical receivers 510, 520, and a plurality of optical receivers 510 , 520 and a control circuit 90 for controlling the electric switch 80 based on the detection signal.

The optical receiver 510 includes an optical filter 61 that transmits an optical signal having the wavelength λ1, an optical receiver (RX) 62 that receives the optical signal having the wavelength λ1 and reproduces the electric signal, and an optical receiver (RX) 62. An optical frequency discriminator 511 for discriminating the optical frequency of the optical signal, and a low frequency signal detector 5 for detecting only the low frequency f1 component superimposed from the output of the optical frequency discriminator 511
And 12. Similarly, the optical receiver 520 includes an optical filter 71 that transmits an optical signal of the wavelength λ2,
Optical receiver (R) that receives the optical signal of
X) 72, an optical frequency discriminator 521 for discriminating the optical frequency of the optical signal from the optical filter 71, and an optical frequency discriminator 52
A low-frequency signal detector 522 that detects only the low-frequency f2 component superimposed from one output.

The optical receiving apparatus 500 includes a plurality of channels for demultiplexing an optical signal frequency-modulated at a different frequency for each wavelength and detecting an optical signal of each wavelength and a frequency component of frequency modulation.

As described above, on the receiving side, the O /
Optical frequency discriminators 511 and 521 instead of E71 and 72
Is used.

The electric signals from the optical frequency discriminators 511 and 521 are input to the low-frequency signal detectors 512 and 522, respectively, and extract only the superposed low-frequency components. The low-frequency signal detection units 512 and 522 may be configured by a band-pass filter or the like.

The frequency discriminators 511 and 521 are composed of, for example, Mach-Zehnder interferometers and have discrimination characteristics as shown in FIG. As shown in FIG. 2, the frequency discriminators 511 and 521 have a characteristic that the output voltage varies with respect to the optical frequency, and can discriminate the optical frequency from the peak of the output voltage. Instead of using such a frequency discriminator, a local LD may be prepared for each wavelength and detected by heterodyne detection.

The control circuit 90 includes a low-frequency signal detection section 51
2, 522, according to the signal from the optical receiver (RX) 62,
The electric switch 80 is controlled so that the electric signal reproduced at 72 is output to a predetermined output destination. In addition, you may make it change an output destination as needed.

The operation of the WDM optical transmission system configured as described above will be described below.

The light source 411 is provided with an LD having good frequency modulation efficiency.
, Frequency modulation can be performed only by applying minute current modulation. In the light source 411, a low-level current modulation is applied by the low frequency f1 from the low-frequency signal source 412 so as not to affect the optical output signal, and the optical signal of the wavelength λ1 subjected to the frequency modulation is applied. Is sent to the optical modulator 414.

The drive circuit 413 amplifies the input data signal and outputs it to the optical modulator 414. The optical modulator 414 modulates the optical signal of the wavelength λ1 from the light source 411 with the drive signal input via the drive circuit 413 and outputs the modulated signal to the optical multiplexer 30.

Similarly, in the light source 421, the low-frequency signal source 4
The low frequency f2 from 22 applies a very small level of current modulation that does not affect the optical output signal,
The optical signal of the wavelength λ2 is sent to the optical modulator 424. The drive circuit 423 amplifies the input data signal and outputs the amplified data signal to the optical modulator 424. The optical modulator 424 modulates the optical signal of the wavelength λ2 from the light source 421 with the drive signal input via the drive circuit 423, and outputs the modulated signal to the optical multiplexer 30.

Thereafter, the optical signal frequency-modulated at a different low frequency for each wavelength is input to the optical multiplexer 30, which multiplexes the optical signals of the wavelengths λ1, λ2,. The signal is transmitted to the transmission line fiber 40. The optical signal transmitted to the transmission line optical fiber 40 is transmitted to the optical receiving side via the repeater 45.

The transmitted optical signal is first demultiplexed by the optical demultiplexer 50, and the optical receivers 510 and 52 of the corresponding wavelengths are demultiplexed.
Demultiplexed into 0,. Optical filter 61 of optical receiver 510
In this case, the optical signal having the wavelength λ1 is transmitted and reproduced by the optical receiver 62 into an electric signal. The frequency modulation component superimposed on the optical signal of each channel is detected by an optical frequency discriminator 511 having a discrimination characteristic as shown in FIG. 2, extracted by a low frequency signal detection unit 512, and sent to the control circuit 90. You.

Similarly, the optical filter 71 of the optical receiver 520
In this case, the optical signal having the wavelength λ2 is transmitted and reproduced by the optical receiver 72 into an electric signal. The frequency modulation component superimposed on the optical signal of each channel is output from the optical frequency discriminator 521
, And extracted by the low frequency signal detection unit 522, and then sent to the control circuit 90.

The same function as described above is provided for each optical channel of each wavelength, and an electric signal is reproduced for each wavelength and a superposed low frequency signal is detected.

In the control circuit 90, the low frequency signal detecting section 51
2, 522, according to the signal from the optical receiver (RX) 62,
The electric switch 80 is controlled so as to output the electric signal reproduced at 72 to a predetermined output. If necessary,
The output destination may be changed.

As described above, the WDM optical transmission system according to the first embodiment includes the light sources 4 of wavelengths λ1 and λ2, which are frequency-modulated with different low-frequency signals for each wavelength of each light source.
11, 421, low frequency signal sources 412 and 422 for generating low frequencies f1 and f2, drive circuits 413 and 423 for outputting drive signals from data signals, and optical modulators 414 and 424 for modulating optical signals. Optical transmitters 410 and 4
21, an optical multiplexer 30 for wavelength-multiplexing the optical signal and sending it to the transmission line fiber 40, an optical demultiplexer 50 for splitting the transmitted optical signal into transmission wavelengths λ1, λ2,. ,
optical filters 61 and 71 for transmitting an optical signal of λ2, RXs 62 and 72 for receiving the optical signal and reproducing it as an electric signal, and optical frequency discriminators 511 and 52 for discriminating the optical frequency of the optical signal.
1 and a plurality of optical receivers 510 and 520 comprising low frequency signal detectors 512 and 522 for detecting only the superposed low frequency f1 and f2 components, and an electric signal based on detection signals from the optical receivers 510 and 520. A control circuit 90 for controlling the switch 80 is provided, so that frequency modulation or phase modulation is applied to the optical transmission signal as identification information for identifying each optical signal channel, as compared with the case where intensity modulation is applied. Thus, the effect on the optical transmission signal can be suppressed. Second Embodiment FIG. 3 is a block diagram showing a configuration of an optical transmission system according to a second embodiment of the present invention, which is an example applied to an OTDM optical transmission system. In the description of the optical transmission system according to the present embodiment, the same components as those in FIGS. 1 and 9 are denoted by the same reference numerals.

In FIG. 3, an optical transmitting apparatus 600 includes a light source 11, an oscillator 15 for generating a frequency f0 equal to the transmission speed of each channel, an optical switch 16, a plurality of optical transmitters 610 and 620 having different wavelengths, respectively. And an optical multiplexer 31 for wavelength-multiplexing the optical signals from the plurality of optical transmitters 610 and 620 by bit multiplexing and transmitting the multiplexed optical signals to the transmission line fiber 40.

The optical switch 16 converts the optical signal from the light source 11 having the wavelength λ1 into the frequency f0 equal to the transmission speed of each channel.
And generates a pulse train having a pulse width that is half of 1 bit.

The optical transmitter 610 includes a low-frequency signal source 611 that generates a low frequency of the frequency f1, a drive circuit 612 that amplifies an input data signal, and an optical signal output from the optical switch 16 to the output of the drive circuit 612. The optical modulator 613 includes an optical modulator 613 that performs intensity modulation by a signal, and an optical phase modulator 614 that performs phase modulation on an output signal from the optical modulator 613 at the low frequency f1 of the low frequency signal source 611. Similarly, the optical transmitter 62
0 is a low-frequency signal source 6 that generates a low frequency of the frequency f2.
21 and a drive circuit 62 for amplifying the input data signal
2, an optical modulator 623 for intensity-modulating an optical signal from the optical switch 16 with an output signal of the drive circuit 622, and a light for phase-modulating the output signal from the optical modulator 623 with the low frequency f1 of the low frequency signal source 621. And a phase modulator 624.

As described above, the optical phase modulators 614 and 624 can be realized by, for example, an optical modulator in which an optical waveguide is formed on a LiNbO 3 crystal.

The optical multiplexer 31 includes optical phase modulators 614, 6
The optical signal from the optical fiber 24 is bit-multiplexed, and
To send to.

On the other hand, the optical receiving device 700 includes an optical splitter 51 for splitting the transmitted optical signal into two, a clock extracting unit 52 for extracting a clock component of the frequency f0, and an extracted frequency f.
An optical switch 53 that switches an input optical signal by a clock component of 0 to perform bit separation, and a plurality of optical receivers 710 and 72 that receive each wavelength-separated wavelength component.
0, an electrical switch 80 for electrically switching received signals from the plurality of optical receivers 710 and 720, and an electrical switch 8 based on detection signals from the plurality of optical receivers 710 and 720.
It is composed of a control circuit 90 for controlling 0.

The optical receiver 710 receives an optical signal of the wavelength λ1 and reproduces it into an electric signal, and an optical frequency discriminator 511 for discriminating the optical frequency of the optical signal from the optical switch 53. And a low-frequency signal detection unit 512 that detects only the low-frequency f1 component superimposed from the output of the optical frequency discriminator 511. Similarly, the optical receiver 720
Is an optical receiver (RX) 72 that receives an optical signal of wavelength λ2 and reproduces it into an electric signal, an optical frequency discriminator 521 that discriminates the optical frequency of the optical signal from the optical switch 53, and an optical frequency discriminator 521 A low-frequency signal detector 522 that detects only the low-frequency f2 component superimposed from the output.

As described above, in the present embodiment, the method of superimposing a low-frequency signal in the conventional example shown in FIG. 9 is not amplitude modulation but phase modulation.

The optical pulse signal output from the optical switch 16 is subjected to intensity modulation in optical transmitters 610 and 620 after branching. At this time, the output signals of the optical transmitters 610 and 620 are phase-modulated by the optical phase modulators 614 and 624, respectively, without superimposing a low frequency signal by amplitude modulation as in the conventional example. The optical signals from the optical phase modulators 610 and 620 are bit-multiplexed by the optical multiplexer 31 and transmitted to the transmission line 92 in the same manner as in the conventional example.

On the other hand, on the receiving side, O / E6 in FIG.
Optical frequency discriminators 511 and 521 are used instead of 3 and 73. The electric signals from the optical frequency discriminators 511 and 521 are input to the low frequency signal detectors 512 and 522, respectively, and extract only the superposed low frequency components.

Hereinafter, the operation of the OTDM optical transmission system configured as described above will be described.

The oscillator 15 generates a frequency having the same frequency f0 as the transmission speed of each channel, and supplies it to the optical switch 16.

The optical signal from the light source 11 having the wavelength λ1 is modulated via the optical switch 16 by a sine wave or clock signal having the same frequency f0 as the transmission speed of each channel, and
Generate a pulse train with a pulse width that is half of it. The optical signal from the optical switch 16 is split into two and input to two optical transmitters 610 and 620.

In the optical transmitter 610, the data signal is
The driving circuit 612 amplifies the input data signal and outputs the amplified data signal to the optical modulator 613.
The optical modulator 613 modulates the optical signal branched by the optical switch 16 with a drive signal input via the drive circuit 612 and outputs the modulated signal to the optical phase modulator 614.

Further, the low frequency signal source 611 outputs the frequency f1
Is generated and output to the optical phase modulator 614.
The optical phase modulator 614 modulates the phase of the output signal from the optical modulator 613 with the low frequency f1 of the low frequency signal source 611. As described above, the optical phase modulator 614 includes LiN
If an optical modulator having an optical waveguide formed on a bO3 crystal is used,
Phase modulation can be performed without a change in light intensity. In the optical phase modulator 614, the low-frequency f1 from the low-frequency signal source 611 performs a minute level current modulation so as not to affect the optical output signal, and the wavelength λ1 subjected to the frequency modulation is applied. Is sent to the optical multiplexer 31.

Similarly, in the optical transmitter 620, the data signal is input to the driving circuit 622,
Amplifies the input data signal and outputs it to the optical modulator 623. The optical modulator 623 modulates the optical signal split by the optical switch 16 with a drive signal input via the drive circuit 622 and outputs the modulated signal to the optical phase modulator 624.

The low-frequency signal source 621 outputs the frequency f2
Is generated and output to the optical phase modulator 624.
In the optical phase modulator 624, the output signal from the optical modulator 623 is phase-modulated by the low frequency f2 of the low frequency signal source 621. In the optical phase modulator 624, current modulation of a very small level is performed by the low frequency f2 from the low frequency signal source 621 so as not to affect the optical output signal, and the wavelength λ2 subjected to the frequency modulation is applied. Is sent to the optical multiplexer 31.

The optical signals from the two optical transmitters 610 and 620 are bit-multiplexed by the optical multiplexer 31 and transmitted to the transmission line fiber 40 as in the case of FIG.

The optical signal from the transmission line fiber 40 is split into two by an optical splitter 51, one of which is input to an optical switch 53, and the other is input to a clock extraction unit 52.

Since the optical signal input to the clock extracting unit 52 contains a clock component of the frequency f0, first, the input optical signal is photoelectrically converted by the clock extracting unit 52, and the band-pass filter of the center frequency f0 is used. To extract the clock component of the frequency f0. Then, the optical switch 5 is used by the extracted clock component of the frequency f0.
At 3, the input optical signal is switched. This enables bit separation.

Each optical signal from the optical switch 53 is output to optical receivers 710, 720,... In the optical receiver 710, the optical signal from the optical switch 53 is reproduced into an electric signal by the optical receiver (RX) 62, and the frequency modulation component superimposed on the optical signal of each channel is as shown in FIG. After being detected by the optical frequency discriminator 511 having discrimination characteristics and extracted by the low-frequency signal detection unit 512, it is transmitted to the control circuit 90. Similarly, the optical receiver 72
0, the optical signal from the optical switch 53 is transmitted to the optical receiver (R
X) The frequency modulation component reproduced into an electric signal at 72 and superimposed on the optical signal of each channel is detected by the optical frequency discriminator 521 and extracted by the low frequency signal detecting section 522, and then transmitted to the control circuit 90. Sent out.

The same function as described above is provided for each optical channel of each wavelength, and an electric signal is reproduced for each wavelength and a superposed low-frequency signal is detected.

In the control circuit 90, the low frequency signal detecting section 51
2, 522, according to the signals from the optical receivers 710, 720.
The electric switch 80 is controlled so as to output the electric signal reproduced in the above to a predetermined output destination. In this case, the output destination may be changed as needed.

As described above, the OTDM optical transmission system according to the second embodiment includes low-frequency signal sources 611 and 621 for generating low frequencies f1 and f2, drive circuits 612 and 622 for amplifying data signals, The optical modulators 613 and 623 for intensity-modulating the signals, and the output signals from the optical modulators 613 and 623 are converted to the low-frequency signals f of the low-frequency signal sources 611 and 621.
A plurality of optical transmitters 610 and 620 comprising optical phase modulators 614 and 624 for performing phase modulation by 1, f2;
An optical multiplexer 31 for wavelength-multiplexing the optical signals from the optical signals 0 and 620 by bit multiplexing and sending the resulting signals to the transmission line fiber 40, an optical splitter 51 for splitting the transmitted optical signal into two, and a frequency f0
Clock extraction unit 52 for extracting the clock component of the above, an optical switch 53 for performing bit separation, RXs 62 and 72 for receiving the optical signals of wavelengths λ1 and λ2 and reproducing them as electric signals, and a light for discriminating the optical frequency of the optical signal Frequency discriminators 511, 52
1. A plurality of optical receivers 710 and 720 comprising low frequency signal detectors 512 and 522 for detecting only the superposed low frequency f1 and f2 components, and an electric switch based on detection signals from the optical receivers 510 and 520. Control circuit 90 for controlling 80
By applying frequency modulation or phase modulation to the optical transmission signal as identification information for identifying each optical signal channel, the influence on the optical transmission signal as compared with the case of applying intensity modulation is configured. It can be suppressed. Third Embodiment FIG. 4 is a block diagram illustrating a configuration of a WDM optical transmission system according to a third embodiment of the present invention. In the description of the optical transmission system according to the present embodiment, the same components as those in FIG. 1 are denoted by the same reference numerals.

This embodiment is based on the WD according to the first embodiment.
In the M optical transmission system, the transmission power to the transmission line optical fiber 40 is increased.

In FIG. 4, an optical transmitter 800 wavelength-multiplexes optical signals from a plurality of optical transmitters 810 and 820 having different wavelengths, and a plurality of optical signals from the plurality of optical transmitters 810 and 820, through an optical booster amplifier 850. And an optical multiplexer 30 for transmitting the signal to the transmission line fiber 40.

The optical transmitter 810 has a light source 411 of wavelength λ1.
And a low-frequency signal source 8 for generating a low frequency of the frequency f1s
11, a drive circuit 413 that outputs a drive signal from the input data signal, and an optical modulator 414 that modulates an optical signal from the light source 411 with an output signal of the drive circuit 413. Similarly, the optical transmitter 820 includes a light source 421 having a wavelength λ2, a low frequency signal source 821 generating a low frequency of a frequency f2s, a driving circuit 423 outputting a driving signal from an input data signal, and a light source 421. Modulator 42 that modulates the optical signal of FIG.
And 4.

Similarly, the optical transmitting apparatus 800 includes a plurality of optical channels that are frequency-modulated at different frequencies for each wavelength.

The low-frequency signal sources 811 and 821 operate at frequencies f of low-frequency signals in which stimulated Brillouin scattering (SBS) does not occur.
1s and f2s are generated and supplied to the light sources 411 and 421.

The light sources 411 and 421 are composed of laser diodes (LD), and superimpose low-frequency signals of low frequencies f1s and f2s, which do not generate SBS from the low-frequency signal sources 811 and 821, on the bias current thereof. A minute current modulation at a level that does not sufficiently affect the optical output signal is applied. When increasing the number of wavelength multiplexing, current modulation is performed with a low-frequency signal having a different frequency for each channel.

Also, an optical booster amplifier 850 is inserted on the output side of the optical multiplexer 30 so that the optical booster amplifier 850 increases the transmission power to the transmission line optical fiber 40.

On the other hand, the optical receiving apparatus 900 comprises an optical demultiplexer 5 for demultiplexing the transmitted optical signal into transmission wavelengths λ1, λ2,.
0, a plurality of optical receivers 910, 920 that receive the respective wavelength components that have been demultiplexed, an electrical switch 80 that electrically switches reception signals from the plurality of optical receivers 910, 920, and a plurality of optical receivers 510 , 520 and a control circuit 90 for controlling the electric switch 80 based on the detection signal.

The optical receiver 910 includes an optical filter 61 that transmits an optical signal of wavelength λ1, an optical receiver (RX) 62 that receives an optical signal of wavelength λ1 and reproduces the electrical signal, and an optical receiver (RX) 62. It comprises an optical frequency discriminator 511 for discriminating the optical frequency of the optical signal, and a low frequency signal detecting section 911 for detecting only the low frequency f1s component superimposed from the output of the optical frequency discriminator 511. Similarly, the optical receiver 920
An optical filter 71 that transmits an optical signal of wavelength λ2, an optical receiver (RX) 72 that receives an optical signal of wavelength λ2 and reproduces it as an electrical signal, and an optical discriminating optical frequency of the optical signal from the optical filter 71 It comprises a frequency discriminator 521 and a low frequency signal detector 921 that detects only the low frequency f2s component superimposed from the output of the optical frequency discriminator 521.

FIG. 1 is different from FIG.
1 only detects the frequencies f1s and f2s of the low-frequency signal set so that SBS does not occur.

The operation of the WDM optical transmission system configured as described above will be described below.

In the present embodiment, an optical booster amplifier 850 is inserted into the output of the optical multiplexer 30 to increase the transmission power to the transmission line optical fiber 40 and to set the frequencies f1s and f2s of the superposed low frequency signal. By SBS
The operation of the system itself is the same as in the first embodiment.

In general, in order to extend the transmission distance, the transmission loss of an optical fiber is compensated for by using an EDFA that has recently been put to practical use. In FIG. 4, this can be easily realized by using an EDFA for the optical booster amplifier 850 and the repeater 45. Also,
In order to extend the relay distance, the optical output power of the optical booster amplifier 850 and the repeater 45 is increased, and the deterioration of the optical SN ratio is suppressed.

However, in this case, SBS is generated as described above, and the transmission quality is degraded. In the conventional example, since the optical transmission signal is transmitted while the optical spectrum line width is narrow, SBS may occur. on the other hand,
In the first and second embodiments, since frequency modulation or phase modulation is performed, an effect of suppressing SBS is expected. However, it is reported that if the modulation frequency is too low, there is no effect (1995 IEICE General Conference, B-1082).

Therefore, in the present embodiment, even if the input power to the transmission line optical fiber 40 is increased by inserting the optical booster amplifier 850, the SBS is not generated.
The modulation frequencies f1s and f2s are set. Other operations are the same as in the first embodiment.

As described above, the WDM optical transmission system according to the third embodiment generates the low-frequency signal frequencies f1s and f2s that do not generate SBS and generates the light sources 411 and 42.
1 and the light sources 411 and 421
Since the low-frequency signals of s and f2s are superimposed and a small current modulation of a level that does not sufficiently affect the optical output signal is applied, similar to the WDM optical transmission system according to the first embodiment, When the input power to the transmission line fiber 40 is high by optimizing the modulation frequency of the frequency modulation or the phase modulation in addition to the effect of suppressing the influence on the optical transmission signal as compared with the case where the intensity modulation is applied. However, SBS can be suppressed. Fourth Embodiment FIG. 5 is a block diagram illustrating a configuration of an OTDM optical transmission system according to a fourth embodiment of the present invention. In the description of the optical transmission system according to the present embodiment, the same components as those in FIG. 3 are denoted by the same reference numerals.

This embodiment relates to the OT according to the second embodiment.
In the DM optical transmission system, an optical booster amplifier is inserted into the output of the optical multiplexer 30 to increase the transmission power to the transmission line optical fiber 40. As in the third embodiment, the superposed low frequency The frequency of the signal is set to f1s and f2s so that SBS does not occur.

In FIG. 5, an optical transmitter 1000 includes a light source 11, an oscillator 15 for generating a frequency f0 equal to the transmission speed of each channel, an optical switch 16, a plurality of optical transmitters 1010 and 1020 having different wavelengths, respectively. And an optical multiplexer 31 for wavelength-multiplexing the optical signals from the plurality of optical transmitters 1010 and 1020 by bit multiplexing and transmitting the multiplexed optical signals to the transmission line fiber 40 via the optical booster amplifier 850.

The optical switch 16 converts the optical signal from the light source 11 having the wavelength λ1 into the frequency f0 equal to the transmission speed of each channel.
And generates a pulse train having a pulse width that is half of 1 bit.

The optical transmitter 1010 includes a low-frequency signal source 1011 that generates a low frequency of the frequency f1s, a drive circuit 612 that amplifies an input data signal, and an optical switch 16.
An optical modulator 613 for intensity-modulating an optical signal from the optical modulator 612 with an output signal of a drive circuit 612 and an optical phase modulator 614 for phase-modulating the output signal from the optical modulator 613 with the low frequency f1s of the low frequency signal source 611. Be composed. Similarly, the optical transmitter 1020 includes a low-frequency signal source 1021 that generates a low frequency of the frequency f2s, a drive circuit 622 that amplifies an input data signal, and an optical signal output from the optical switch 16 by the drive circuit 622. Optical modulator 6 that intensity-modulates with a signal
23 and an optical phase modulator 6 that phase-modulates the output signal from the optical modulator 623 with the low frequency f1s of the low frequency signal source 621.
24.

Similarly, the optical transmitting apparatus 1000 includes a plurality of optical channels that are frequency-modulated at different frequencies for each wavelength.

The low-frequency signal sources 1011 and 1021
The frequencies f1s and f2s of the low-frequency signal in which S does not occur are generated and supplied to the optical phase modulators 614 and 624.

As described above, the optical phase modulators 614 and 624 can be realized by, for example, an optical modulator in which an optical waveguide is formed on LiNbO3 crystal.

The optical multiplexer 31 includes optical phase modulators 614, 6
The optical signal from the optical fiber 24 is bit-multiplexed, and
To send to.

On the other hand, the optical receiving apparatus 1100 converts the input optical signal into an optical splitter 51 for splitting the transmitted optical signal into two, a clock extracting unit 52 for extracting the clock component of the frequency f0, and a clock component of the extracted frequency f0. An optical switch 53 for performing bit separation by switching, and a plurality of optical receivers 1110, 1 for receiving each wavelength-separated wavelength component
120, an electric switch 80 for electrically switching received signals from the plurality of optical receivers 1110, 1120, and a control circuit 90 for controlling the electric switch 80 based on detection signals from the plurality of optical receivers 1110, 1120. Is done.

The optical receiver 1110 receives an optical signal of the wavelength λ1 and reproduces it into an electric signal, and an optical receiver (RX) 62;
It comprises an optical frequency discriminator 511 for discriminating the optical frequency of the optical signal from the optical switch 53, and a low frequency signal detector 1111 for detecting only the low frequency f1s component superimposed from the output of the optical frequency discriminator 511. Similarly, the optical receiver 1
Reference numeral 120 denotes an optical receiver (RX) 72 for receiving an optical signal having the wavelength λ2 and reproducing the optical signal into an electric signal; an optical frequency discriminator 521 for discriminating the optical frequency of the optical signal from the optical switch 53; Low frequency f2s superimposed from 521 output
And a low-frequency signal detector 1122 for detecting only the component.

FIG. 3 is different from FIG.
The only difference is that the frequency detection unit 121 detects the frequencies f1s and f2s of the low-frequency signal set so as not to cause SBS.

Hereinafter, the operation of the OTDM optical transmission system configured as described above will be described.

In the present embodiment, an optical booster amplifier 850 is inserted into the output of the optical multiplexer 30 to increase the transmission power to the transmission line optical fiber 40 and to set the frequency f1s and f2s of the superposed low frequency signal. By SBS
The operation of the system itself is the same as in the second embodiment.

As described above, the OTDM optical transmission system according to the fourth embodiment includes a plurality of optical transmitters 101
0, 1020 are low-frequency signal sources 1011, 10 that generate low-frequency signal frequencies f 1 s, f 2 s without SBS.
21 and an optical phase modulator 61 for phase-modulating output signals from the optical modulators 613 and 623 at low frequencies f1s and f2s.
4 and 624, similarly to the OTDM optical transmission system according to the second embodiment, in addition to the effect of suppressing the influence on the optical transmission signal as compared with the case of applying the intensity modulation, By optimizing the modulation frequency of the modulation, the SBS can be suppressed even when the input power to the transmission line fiber 40 is high. Fifth Embodiment FIG. 6 is a block diagram illustrating a configuration of a WDM optical transmission system according to a fifth embodiment of the present invention. In the description of the optical transmission system according to the present embodiment, the same components as those in FIG. 5 are denoted by the same reference numerals, and the description of the overlapping portions will be omitted.

This embodiment relates to the WD according to the third embodiment.
In the M optical transmission system, the input optical power of the transmission line optical fiber 40 and the reflected light power from the transmission line fiber 40 are monitored, and the frequency modulation level is automatically controlled so that SBS does not occur. is there.

In FIG. 6, reference numeral 1200 denotes a monitor circuit (monitor means) for monitoring the input light power to the transmission line fiber 40 and the reflected light power from the transmission line fiber 40 for each wavelength of each channel. An optical coupler 1210 for splitting the optical output of the transmission line fiber 40, and an optical splitter 1211 for splitting the optical output from the optical coupler 1210.
1212, an optical filter 1221 that transmits an optical signal having a wavelength λ1 at the branch end of the optical splitter 1211;
Optical filter 1 that transmits an optical signal of wavelength λ2 at the branch end
222, an optical filter 1223 that transmits an optical signal of wavelength λ1 at the branch end of the optical splitter 1212, and an optical filter 12 that transmits an optical signal of wavelength λ2 at the branch end of the optical splitter 1212
24, and an electric signal corresponding to the ratio of the optical input, and the low-frequency signal sources 811, 82 so that the ratio of the optical input is constant.
Modulation amount control circuits 1231 and 123 for controlling the modulation degree of 1
2

The optical output from the optical booster amplifier 850 is
The optical input from the first input terminal of the optical coupler 1210, the optical output from the first output terminal of the optical coupler 1210 is output to the transmission line fiber 40, and the optical output from the second output terminal of the optical coupler 1210 is the optical output. The optical output from the input terminal of the splitter 1211 and the optical output from the second input terminal of the optical coupler 1210 are input to the input terminal of the optical splitter 1212.

The optical output from the first output terminal of the optical splitter 1211 passes through the optical filter 1221 and is input to the first input terminal of the modulation amount control circuit 1231.
After being transmitted through the optical filter 1222, the optical output from the second output terminal is input to the first input terminal of the modulation amount control circuit 1232. The optical output from the first output terminal of the optical splitter 1212 passes through the optical filter 1223 and is input to the second input terminal of the modulation amount control circuit 1231.
After being transmitted through the optical filter 1224, the optical output from the second output terminal is input to the second input terminal of the modulation amount control circuit 1232.

The modulation amount control circuit 1231 generates an electric signal corresponding to the ratio of the light input from the second input terminal to the light input from the first input terminal, and generates the electric signal corresponding to the ratio of the light input from the second input terminal. First
The modulation degree of the low-frequency signal source 811 is controlled so that the ratio of the light input from the input terminal becomes constant. Further, the modulation amount control circuit 1232 generates an electric signal corresponding to a ratio between the optical input from the second input terminal and the optical input from the first input terminal, and generates the electric signal corresponding to the ratio between the optical input from the second input terminal and the second optical input. The modulation degree of the low-frequency signal source 821 is controlled so that the ratio of the light input from the input terminal 1 is constant.

The other structure is the same as that of the third embodiment.

Hereinafter, the operation of the WDM optical transmission system configured as described above will be described.

The reflected light from the transmission line fiber 40 is split into two by an optical splitter 1212,
After extracting only the wavelength component of each channel in 24, it is input to the modulation amount control circuits 1231 and 1232. A part of the input light to the transmission line fiber 40 is split into two by the optical splitter 1211 and only the wavelength components of each channel are extracted by the optical filters 1221 and 222, and then the modulation amount control circuit 123
1, 1232.

Each of the modulation amount control circuits 1231 and 1232 calculates the ratio between the reflected light from the transmission line fiber 40 and the input light to the transmission line fiber 40, and determines the SB based on the calculation result.
Each low-frequency signal source 811, 82 is provided so as to provide an optimum amount of frequency modulation such that S does not occur and the degree of modulation is not too large to deteriorate the eye opening of the optical output waveform.
1 is automatically controlled.

The subsequent operation is the same as in the third embodiment.

As described above, the WDM optical transmission system according to the fifth embodiment monitors the input light power to the transmission line fiber 40 and the reflected light power from the transmission line fiber 40 for each channel wavelength. A monitor circuit 1200 and a modulation amount control circuit 1231 or 1232 that generate an electric signal corresponding to the optical input ratio and control the modulation degree of the low frequency signal sources 811 and 821 so that the optical input ratio is constant. With this configuration, in addition to the effect of the third embodiment, it is possible to automatically control the modulation degree to an optimum degree so as not to deteriorate the eye opening degree of the optical output waveform. Sixth Embodiment FIG. 7 is a block diagram illustrating a configuration of an OTDM optical transmission system according to a sixth embodiment of the present invention. In the description of the optical transmission system according to the present embodiment, the same components as those in FIG. 5 are denoted by the same reference numerals, and the description of the overlapping portions will be omitted.

This embodiment relates to the OT according to the fourth embodiment.
In the DM optical transmission system, as in the fifth embodiment, the input optical power of the transmission line optical fiber 40 and the reflected light power from the transmission line fiber 40 are monitored, and the frequency modulation level is adjusted so that SBS does not occur. It is controlled automatically.

In FIG. 7, reference numeral 1300 denotes a monitor circuit for monitoring the input light power to the transmission line fiber 40 and the reflected light power from the transmission line fiber 40 for each wavelength of each channel. Optical coupler 1210 for branching optical output of 40, and optical coupler 1210
The optical output from the optical input is optically input, an electric signal corresponding to the optical input ratio is generated, and the modulation amount control circuit 1310 for controlling the modulation degree of the low frequency signal sources 1011 and 1021 so that the optical input ratio is constant. Consists of

The optical output from the optical booster amplifier 850 is
The optical input from the first input terminal of the optical coupler 1210, the optical output from the first output terminal of the optical coupler 1210 is output to the transmission line fiber 40, and the optical output from the second output terminal of the optical coupler 1210 is modulated. The optical output from the first input terminal of the amount control circuit 1310 and the optical output from the second input terminal of the optical coupler 1210 are input to the second input terminal of the modulation amount control circuit 1310.

The modulation amount control circuit 1310 generates an electric signal corresponding to the ratio of the optical input from the second input terminal to the optical input from the first input terminal, and generates an electric signal corresponding to the ratio of the optical input from the second input terminal. First
Of the low-frequency signal sources 1011 and 1021 is controlled so that the ratio of the optical input from the input terminal of the low-frequency signal source becomes constant.

The other structure is the same as that of the fourth embodiment.

The operation of the OTDM optical transmission system configured as described above will be described below.

The reflected light from the transmission line fiber 40 and the input light to the transmission line fiber 40 are input to a modulation amount control circuit 1310.

The modulation amount control circuit 1310 calculates the ratio between the reflected light from the transmission line fiber 40 and the input light to the transmission line fiber 40, and based on the calculation result, does not generate SBS and the modulation degree is large. The output amplitude of each of the low-frequency signal sources 1011 and 1021 is automatically controlled so as to provide an optimal amount of frequency modulation so that the eye opening of the optical output waveform is not deteriorated due to the excessive output.

The subsequent operation is the same as in the fourth embodiment.

As described above, the OTDM optical transmission system according to the sixth embodiment monitors the input light power to the transmission line fiber 40 and the reflected light power from the transmission line fiber 40 for each wavelength of each channel. A monitor circuit 1300;
A modulation amount control circuit 1310 that generates an electric signal corresponding to the ratio of the light input and controls the modulation degree of the low frequency signal sources 1011 and 1021 so that the ratio of the light input becomes constant. In addition to the effects of the fourth embodiment, it is possible to automatically control the degree of modulation to an optimal degree so as not to further deteriorate the eye opening of the optical output waveform.

Therefore, if an optical transmission system having such excellent features is applied to, for example, an optical subscriber network system, a system margin can be increased in this device, and particularly, with an increase in communication capacity. It is suitable to be applied to an apparatus that requires channel identification of a multiplexed optical signal.

Note that the optical transmitter according to each of the above embodiments is
The present invention can be applied to the backbone transmission system and the optical subscriber network system as described above, but is not limited to this. Of course, the present invention can be applied to all devices provided with a system for transmitting an optical signal. is there.

The low frequency oscillator, optical modulator, optical repeater, and optical coupler, filter,
M, the type and number of various detection units, the connection method, the type of parameters in each device, and the control method are not limited to the above-described embodiments.

[0156]

According to the first aspect of the present invention, there is provided an optical transmission system having means for applying frequency modulation or phase modulation to a light source with a low-frequency signal different for each wavelength of each light source, and a plurality of light sources having different wavelengths. An optical transmitter, a unit for wavelength-multiplexing optical signals from a plurality of optical transmitters, a unit for splitting the wavelength-multiplexed optical signal, and a unit for detecting a phase modulation component or a frequency modulation component from the split optical signal. Means for detecting a low-frequency signal from a phase modulation component or a frequency modulation component,
Means for outputting the optical signal of each channel to a predetermined output destination based on the output of the low-frequency signal detecting means, so that the channel of the multiplexed optical signal is It is possible to realize a WDM optical transmission system capable of suppressing an influence on an optical transmission signal when performing identification.

According to the optical transmission system of the second aspect,
Time division multiplexing means for time division multiplexing a plurality of optical signal channels; means for applying phase modulation to optical signals of each channel at different frequencies from each other as identification information for identifying each optical signal channel; Means for detecting a phase modulation component from the detected optical signal, means for detecting a low frequency signal from the phase modulation component, and outputting an optical signal of each channel to a predetermined output destination based on the output of the low frequency signal detection means. Means, it is possible to realize an OTDM optical transmission system capable of suppressing an influence on an optical transmission signal when performing channel identification of a multiplexed optical signal, as compared with a case where intensity modulation is performed. .

According to the optical transmission system of the third aspect,
If the optical power when the wavelength-multiplexed optical signal is input to the transmission line fiber is large enough to cause SBS, SB
Since the frequency and the modulation level of the frequency modulation or the phase modulation are optimized so as not to cause S, the SBS can be suppressed even when the input power to the transmission line optical fiber is high.

According to the optical transmission system of the fourth aspect,
Means for monitoring the input light power to the transmission line fiber and the reflected light power from the transmission line fiber for each channel wavelength,
Control means for optimizing the frequency and modulation level of phase modulation or frequency modulation so as not to cause stimulated Brillouin scattering based on the output of the monitoring means does not deteriorate the eye opening of the optical output waveform As described above, the degree of modulation can be automatically controlled to the optimum.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration of an optical transmission system according to a first embodiment to which the present invention has been applied.

FIG. 2 is a diagram illustrating discrimination characteristics of a frequency discriminator of the optical transmission system.

FIG. 3 is a diagram illustrating a configuration of an optical transmission system according to a second embodiment to which the present invention has been applied.

FIG. 4 is a diagram illustrating a configuration of an optical transmission system according to a third embodiment to which the present invention has been applied.

FIG. 5 is a diagram illustrating a configuration of an optical transmission system according to a fourth embodiment to which the present invention has been applied.

FIG. 6 is a diagram illustrating a configuration of an optical transmission system according to a fifth embodiment to which the present invention has been applied.

FIG. 7 is a diagram illustrating a configuration of an optical transmission system according to a sixth embodiment to which the present invention has been applied.

FIG. 8 is a diagram illustrating a configuration of a conventional WDM optical transmission system.

FIG. 9 is a diagram showing a configuration of a conventional OTDM optical transmission system.

FIG. 10 is a diagram for explaining a conventional SBS suppression method.

FIG. 11 is a characteristic diagram illustrating a conventional method of suppressing SBS.

[Explanation of symbols]

11, 411, 421 light source, 16, 53 optical switch, 30, 31 optical multiplexer, 40 transmission line fiber, 5
0 optical demultiplexer, 51, 1211, 1212 optical splitter,
52 clock extraction unit, 61, 71, 1221, 122
2, 1123, 1124 optical filter, 62, 72 optical receiver (RX), 80 electric switch, 90 control circuit, 400, 600, 800, 1000 optical transmitter,
410, 420, 610, 620, 810, 820, 1
010,1020 Optical transmitter, 412,422,61
1,621,1011,1021 Low frequency signal source, 41
3,423,612,622 drive circuit, 414,42
4613,623 Optical modulator, 500,700,90
0, 1100 optical receiver, 510, 520, 710,
720, 910, 920, 1110, 1121 Optical receiver, 511, 521 Optical frequency discriminator, 512, 52
2, 911, 921 Low frequency signal detector, 850 optical booster amplifier, 1200 monitor circuit (monitor means),
1210 Optical coupler, 1231, 1232, 1310
Fluctuation control circuit

Claims (4)

[Claims] A means for frequency-modulating or phase-modulating a light source with a low-frequency signal different for each wavelength of the light source;
A plurality of optical transmitters each having a different wavelength of the light source; a unit for wavelength-multiplexing optical signals from the plurality of optical transmitters; a unit for splitting the wavelength-multiplexed optical signal; and the split optical signal Means for detecting the phase modulation component or the frequency modulation component from; means for detecting the low-frequency signal from the phase modulation component or the frequency modulation component; and for each channel based on the output of the low-frequency signal detection means. Means for outputting an optical signal to a predetermined output destination. 2. An optical time division multiplexing means for time division multiplexing a plurality of optical signal channels, and applying phase modulation at different frequencies to optical signals of the respective channels as identification information for identifying the respective optical signal channels. Means, means for detecting the phase modulation component from the time-divided optical signal, means for detecting a low frequency signal from the phase modulation component, and light of each channel based on the output of the low frequency signal detection means. Means for outputting a signal to a predetermined output destination. 3. When the wavelength-multiplexed optical signal is input to a transmission line fiber with an optical power large enough to cause stimulated Brillouin scattering, the frequency modulation or the frequency modulation is performed so as not to cause the stimulated Brillouin scattering. 2. The frequency and the modulation level of the phase modulation are optimized.
Or the optical transmission system according to any one of 2. 4. A means for monitoring the input light power to the transmission line fiber and the reflected light power from the transmission line fiber for each wavelength of each channel, and for preventing stimulated Brillouin scattering based on the output of the monitoring means. 4. The optical transmission system according to claim 1, further comprising control means for optimizing a frequency and a modulation level of the phase modulation or the frequency modulation.