JPH05260019A - Coherent scm optical transmission method and optical transmitter, optical receiver and optical transmission system used for executing the same - Google Patents

Abstract

PURPOSE:To suppress induced Brillouin scattering and to improve efficiency in the case of using an optical amplifier by generating an optical signal by performing optical modulation according to a multiple signal, and transmitting, signal while suppressing the mate carrier component. CONSTITUTION:In an SCM optical transmitter 100, a multiple signal generating means 101 generates the multiple signal, an optical modulating means 102 generates the optical signal by modulating a coherent light source according to the multiple signal or the like, and an optical filtering means 103 suppresses or removes the main carrier component in the optical signal. A local oscillating light source 111 on the reception side outputs the local oscillating light at a frequency in prescribed relation with the frequency of the main carrier component in the optical signal transmitted from an optical transmission line 12, the local oscillating light and the transmitted optical signal are added by a photocoupler 112, the added light is converted to an electric signal by a light receiving means 113, and a signal is selected out of the electric signals, at every channel by a band pass filter 114 and demodulated by a demodulating means 115. Therefore, the total power of the optical signal can be decreased without damaging information transmission.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a so-called SCM (Subcarrier Multiplexing) optical transmission method, which is an optical transmission method mainly by optical modulation using a frequency-multiplexed microwave signal, and an optical transmitter used for implementing the method. The present invention relates to an optical receiver and an optical transmission system.

The SCM system has an excellent feature that it can simultaneously transmit a large amount of all kinds of digital / analog modulated signals. However, even in this system, the transmission power of an optical signal is limited by stimulated Brillouin scattering in a single optical fiber, and the gain is limited by its gain saturation characteristic when an optical amplifier is used. Therefore, it is required to eliminate these restrictions.

[0003]

2. Description of the Related Art A plurality of carriers, each of which is assigned a different frequency, for example, a microwave frequency, are modulated by transmission signals of different channels, and the respective signals obtained are summed to generate a multiplexed signal. A coherent SCM method is known in which a coherent light source using a laser diode or the like is modulated (for example, frequency modulation or phase modulation) by this multiplexed signal or light from the coherent light source is modulated to generate an optical signal. .

Also known are an optical fiber amplifier having an optical fiber doped with a rare earth element such as Er in the core, a laser diode type optical semiconductor amplifier using a semiconductor element as a gain medium, and other optical amplifiers. There is.

On the other hand, the optical transmission line is composed of a core having a relatively high refractive index and a clad having a relatively low refractive index provided around the core, and the main component of the core and the clad is quartz (Si).
Optical fibers with O ₂ ) are well known.

[0006]

In recent optical communication technology, high-power signal light has come to be obtained due to remarkable progress of optical amplifiers. However, when an attempt is made to transmit a signal light having a power of about 10 dBm or more by the above-mentioned silica-based optical fiber, particularly a silica-based single-mode optical fiber having a small core diameter and a high power density of transmitted light, stimulated Brillouin scattering (SBS) Occurs, and the backscattered light leads to loss, which raises a problem that it is useless to increase the power of the optical transmitter.
The occurrence of this problem is no exception in the coherent SCM method.

On the other hand, the gain saturation characteristic of the optical amplifier is determined by the total power of the signal light, and if the spectrum of the signal light contains a component unnecessary for information transmission, it is not preferable in terms of the efficiency of the optical amplifier.

As a method of suppressing SBS, (a) intentionally broadening the signal spectrum, (b) blocking Brillouin scattered light, (c) using optical fibers with different Brillouin shift amounts spliced together, etc. A method can be suggested. Specifically, a spread spectrum method, a method of installing optical isolators in multiple stages in a transmission line, etc. have been proposed and verified experimentally. The research on the phenomenon of SBS and the examination of the suppression method have been advanced for the intensity modulation / direct detection method and the ordinary coherent method, but the coherent SCM method has not been studied at present.

An object of the present invention is to provide a coherent SCM optical transmission method capable of suppressing SBS and improving efficiency when using an optical amplifier.

It is also an object of the present invention to provide an optical transmitter, an optical receiver and an optical transmission system used for implementing this method.

[0011]

The coherent S of the present invention
The CM optical transmission method includes a first step of generating multiplex signals by adding respective signals obtained by modulating a plurality of carriers, each of which is assigned a different frequency, with transmission signals of different channels. A second step of modulating a coherent light source with the multiplexed signal or modulating light from the coherent light source to generate an optical signal; and a third step of suppressing or removing a main carrier component in the optical signal.
And a fourth step of transmitting the optical signal in which the main carrier component is suppressed, and a fifth step of opto-electrically converting the transmitted optical signal together with local light to generate an electrical signal, A sixth step of demodulating a signal selected for each channel based on the electric signal.

Preferably, in the third step, either one of the double sidebands in the optical signal is further suppressed or removed. Preferably, in the third step, the main carrier component is suppressed so that its power becomes substantially equal to the power of the signal component in the optical signal after suppression.

Preferably, a step of optically amplifying the optical signal in which the main carrier component is suppressed is further provided between the third step and the fourth step. Figure 1
FIG. 1 is a block diagram showing a basic configuration of an SCM optical transmission system of the present invention.

The SCM optical transmission system of the present invention is an SCM
An optical transmitter 100, an optical transmission line 120 for transmitting an optical signal from the SCM optical transmitter 100, and an SCM optical receiver 110 for receiving the optical signal transmitted by the optical transmission line 120 are provided.

The configuration or function of each component of the SCM optical transmitter 100 is as follows. Multiplex signal generation means 101
Generates a multiplex signal by adding respective signals obtained by modulating a plurality of carriers, each of which is assigned a different frequency, with transmission signals of different channels.

The optical modulator 102 is a multiple signal generator 1
The coherent light source is modulated by the multiplexed signal from 01 or the light from the coherent light source is modulated to generate an optical signal. The optical filtering means 103 is the optical modulation means 102.
The main carrier component in the optical signal from is suppressed or removed.

The configuration or function of each component of the SCM optical receiver 110 is as follows. The local light source 111 outputs local light having a frequency having a predetermined relationship with the frequency of the main carrier component in the optical signal transmitted from the optical transmission line 120. The optical coupler 112 adds together the local light from the local light source 111 and the optical signal transmitted from the optical transmission line 120.

The light receiving means 113 converts the light output from the optical coupler 112 into an electric signal and outputs the electric signal. The band pass filter 114 selects a signal for each channel from the electric signal from the light receiving means 113.

The demodulation means 115 demodulates based on the signal from the bandpass filter 114. Band pass filter 114
The demodulation means 115 is provided in a plurality (# 1 to #n) according to the number of multiplexed channels, for example.

[0020]

In the coherent SCM system, the presence or absence of the main carrier component in the optical signal generated by modulating the coherent light source by the multiplex signal or the magnitude of the power of this main carrier component does not affect the information transmission. Further, one of the sidebands of the optical signal is redundant for information transmission, and the other sideband allows reproduction of information.

Therefore, an optical signal generated by modulating a coherent light source with a multiplex signal suppresses or removes the main carrier component in this optical signal, or suppresses either the main carrier component or one of the double sidebands. Alternatively, by removing and transmitting the signal, the total power of the optical signal can be reduced without hindering the information transmission.

As described above, according to the present invention, since the total power of the optical signal can be reduced, it is possible to suppress the SBS and improve the efficiency when using the optical amplifier.

[0023]

EXAMPLES Examples of the present invention will be described below. 2 is SC
It is a block diagram showing a 1st example of an M optical transmitter. The multiplex signal generation means 101 is the number of multiplex channels (n channels).
A plurality of modulators 201 (# 1 to #n) according to the number of the bandpass filters 2
02 (# 1 to #n) and a multiplexer 203 that synthesizes the output signal of the bandpass filter 202.

The light modulating means 102 is the laser diode 2
04, a bias circuit 205 that applies a bias current to the laser diode 204 so that the laser diode 204 oscillates, and a modulation drive circuit 206 that applies a modulation current to the laser diode 204 based on the multiplexed signal from the multiplexer 203. ..

An optical low pass filter 207 is used as the optical filtering means 103. In addition to the basic configuration of FIG. 1, an optical amplifier 208 such as an optical fiber amplifier for optically amplifying the optical signal from the optical low pass filter 207 is provided.

In the modulator 201, carriers of different frequencies f ₁ , f ₂ , ..., F _n allocated to each channel are
Each of the transmission signals (input data) D ₁ , D ₂ , ..., D _n is modulated.

Here, the modulation by the modulator 201 is, for example, amplitude modulation, frequency modulation, phase modulation, amplitude shift keying (ASK), frequency shift keying (FS).
K) and phase shift keying (PSK). Different modulation may be performed on each channel corresponding to the transmission signal.

The frequency arrangement of the transmission signal of each channel and the band of each band pass filter 202 will be described later. The signals passed through the bandpass filter 202 are combined by the multiplexer 203 to generate a frequency multiplexed signal.
As the multiplexer 203, for example, a microwave coupler or the like having a function of simply adding up an electric signal is used.

On the basis of the multiplex signal thus obtained, the modulation drive circuit 206 directly frequency-modulates the laser diode 204 in the lightwave region to generate an optical signal.
Instead of directly modulating the bias current of the laser diode 204, an external optical modulator may be used to generate the optical signal.

The characteristics of the low pass optical filter 207 will be described later. FIG. 3 is a diagram showing a spectrum example of a frequency-modulated signal. The functions of the modulator 201 and the bandpass filter 202 of FIG. 2 will be described with reference to this figure. In general, when the carrier of frequency f _C is modulated at the bit rate B, the spectrum of the modulated signal is the carrier frequency f
The components of the higher-order Bessel function are placed around the main lobe centered on _C. Therefore, it is desirable that the pass band of each band pass filter 202 be selected so that only the above-mentioned main lobe component passes.

Specifically, the pass band of the band filter is
By a range of f _c -B~f _C + B, reducing the channel spacing in a multiple signal obtained by synthesizing the signal passed through a respective bandpass filter, to a frequency corresponding to twice the bit rate B It is possible to obtain a high-density frequency-multiplexed signal.

In this example, the method of using the bandpass filter for the modulated signal is described as the filtering of the signal of each channel, but the method of modulating the original transmission signal after passing it through the lowpass filter is used. May be.

That is, the transmission signal (baseband signal) is
Since a digital signal of bit rate B has a Bessel function-like spectrum as shown in FIG. 4 (A), only the main lobe (0-B) is extracted by the low-pass filter and then If the carrier is modulated by the extracted signal, as shown in FIG. 4 (B), the same modulated signal as in the case of using the aforementioned bandpass filter can be obtained.

If these signals are added up, the result shown in FIG.
An SCM signal as shown in (C) can be obtained. If the frequencies of the transmission signals increase in the order of f ₁ , f ₂ , ..., F _n , the modulation signals do not overlap on the frequency axis, and therefore, the band pass filters (BP)
These can be extracted by F) 202.

FIG. 5 is a diagram showing an example of a spectrum of an optical signal output from the laser diode 204 of FIG. When the laser diode 204 is frequency-modulated based on the multiplexed signal obtained as described above, the laser diode 20
The spectrum of the optical signal output from 4 is the main carrier component MC due to laser oscillation when a steady bias current is applied, and the upper sideband USB and the lower side band that occur at symmetrical positions with respect to this main carrier component MC. Wave band LSB.

When the center frequency of the main carrier component is f _LD , when the multiple signal shown in FIG. 4C is used, the center frequencies of the respective components of the upper sideband USB are:
F _LD + f ₁ , f _LD + f ₂ , ..., F _LD + f _n , respectively, and the center frequencies of the respective components of the lower sideband LSB are f _LD −f ₁ , f _LD −f ₂ , ..., F, respectively. _LD- f _n .

In the coherent SCM method, since a frequency-multiplexed signal is generated in the electric stage and one laser diode can be modulated by this frequency-multiplexed signal to generate an optical signal, the optical configuration is simple. It becomes possible to provide a transmitter.

FIG. 6 is a diagram showing an example of characteristics of the optical low pass filter 207 of FIG. In the example shown in FIG. 6A, a cutoff frequency (transmittance corresponding to 1/2 of the maximum transmittance is given so that the lower sideband LSB and the main carrier component MC in the optical signal are removed. Frequency) f
_0.5 is set. The characteristic curve representing the relationship between the transmittance of the optical low-pass filter 207 and the optical frequency at this time is LP
It is represented by F ₁ .

In the example shown in FIG. 6B, the upper sideband USB in the optical signal is removed and the main carrier component MC is substantially equal to each signal component of the lower sideband LSB as represented by RMC. So that it is suppressed to the extent that it becomes power,
The slope of the falling portion of the characteristic curve LPF ₂ and the cutoff frequency are set. As described above, the residual main carrier component RMC is not completely removed without removing the main carrier component.
The reason for transmitting as will be described later.

The effect of suppressing or removing the main carrier component in the SCM optical transmitter or suppressing or removing the upper sideband in addition to this will be described in detail.
Now, when light is incident on the optical fiber with a certain power, the incident light is backscattered by acoustic phonons. This phenomenon is called Brillouin scattering, and scattered light is called Stokes light. When the power of the light incident on the optical fiber is small, only natural Brillouin scattering occurs. When the power of the light entering the optical fiber increases and exceeds a certain threshold,
The intensity of the Stokes light rapidly increases, and most of the power of the incident light is converted into the power of the Stokes light. This phenomenon is stimulated Brillouin scattering (SBS).

Although the threshold value at which SBS is generated is not always clear and varies depending on the modulation method, in a silica single mode fiber, when SBS is generated when light with a power of about 10 dBm or more is incident. Conceivable.

SBS reflects the power spectral density of signal light. The largest frequency component in the spectrum of the signal light causes SBS first. In the spectrum as shown in FIG. 5, since the modulation index is made small so as to suppress the modulation distortion, a large main carrier component remains. Also, subcarriers (upper sideband,
The frequency arrangement of carriers in each signal component of the lower sideband is usually an octave band arrangement in order to avoid second-order distortion, and the frequency difference between the main carrier and the subcarrier is at least several GHz. ..

Therefore, if the main carrier component is removed by optical filtering, the first SBS due to the main carrier component can be avoided. Even if the main carrier component is suppressed or removed, there is no information in the main carrier component, so there is no effect on information transmission. Further, if either one of the upper sideband and the lower sideband is present, the other side is redundant for information transmission, and even if it is suppressed or removed similarly, there is no problem in reproducing the information.

On the other hand, as shown in FIG. 2, when the optical amplifier 208 is provided immediately upstream of the output port of the SCM optical transmitter, by suppressing or removing the main carrier component, etc. Thus, efficient optical amplification can be performed. The reason is that, in the signal light in which the main carrier component and the like are not suppressed or removed, most of the pump power is consumed by the main carrier component, but when the main carrier component or the like is suppressed or removed, the pump power is reduced. This is because it is possible to efficiently convert the power of the signal component having information.

FIG. 7 is a block diagram showing a second embodiment of the SCM optical transmitter of the present invention. Components that are substantially the same as those in FIG. 2 are designated by the same reference numerals. Figure 7
2 is different from the first embodiment in FIG. 2 in that a reflection type Fabry-Perot interferometer 301 is used as an optical filtering means, and a transmission side automatic frequency control circuit (transmission side AFC circuit). 302 is further provided. The transmitting side AFC circuit 302 stabilizes the frequency of the main carrier component in the optical signal output from the laser diode 204 based on the intensity change of the interference light from the Fabry-Perot interferometer 301.

The optical signal output from the laser diode 204 passes through the half mirror 303 and is input to the Fabry-Perot interferometer 301. Fabry-Perot Interferometer 3
The interference light from 01 is reflected by the half mirror 303 and split into two by the other half mirror 304. One of the branched lights is amplified by the optical amplifier 208 and sent out to the optical transmission line, and the other of the branched lights is transmitted to the transmission side AFC circuit 302.
It is input to the light intensity detector 305 including a photodiode or the like.

A polarization beam splitter is used in place of the half mirror 303, and a half-wave plate is interposed between the polarization beam splitter and the Fabry-Perot interferometer 301 to avoid light loss in the half mirror 303. can do.

The output signal of the light intensity detector 305 is amplified by the amplifier 306 and input to the multiplier 307 including a mixer or the like. Reference numeral 308 is a low-frequency oscillator that outputs a low-frequency signal that does not affect information transmission. The low-frequency signal from the low-frequency oscillator 308 is the bias circuit 205.
And to the multiplier 307.

Reference numeral 309 is D of the output signal of the multiplier 307.
It is a low-pass filter that extracts the C component, and the output signal of this low-pass filter 309 is input to the feedback circuit 310. The feedback circuit 310 feeds back the DC component extracted by the low-pass filter 309 to the bias circuit 205 to stabilize the center frequency of the main carrier component of the optical signal output from the laser diode 204.

The principle of frequency stabilization will be described with reference to FIG. 8A is a graph showing a part of the optical frequency discrimination characteristics of the Fabry-Perot interferometer 301, where the vertical axis is the power of the interference light output from the Fabry-Perot interferometer 301 and the horizontal axis is the Fabry-Perot interference. It represents the frequency of light input to the container 301.

The Fabry-Perot interferometer 301 has a minimum point MIN regarding the intensity of the interference light with respect to the change of the optical frequency.
In the vicinity, it exhibits frequency discrimination characteristics in which a pulse-like waveform in which the intensity of the interference light sharply changes appears periodically.

In this embodiment, since the synchronous detection is performed using the multiplier 307, the low frequency oscillator 308 and the low pass filter 309, the synchronous detection signal (the above-mentioned DC component) obtained as a result is The change with respect to the change of the optical frequency is equivalent to a waveform obtained by differentiating the optical frequency discrimination characteristic of the Fabry-Perot optical interferometer 301. 8B is a diagram showing the relationship between the synchronous detection signal and the optical frequency in association with the optical frequency discrimination characteristic of A of FIG.

As is apparent from FIG. 8, the center frequency f _LD (see FIG. 5) of the main carrier component of the optical signal from the laser diode 204 of FIG. 7 coincides with the frequency that gives the minimum point MIN in the optical frequency discrimination characteristic. If so, the value of the synchronous detection signal becomes zero. Then, as the center frequency f _LD deviates from the frequency giving the minimum point MIN, the value of the synchronous detection signal changes from zero to the positive or negative direction according to the characteristic shown in B of FIG.

Therefore, based on such a principle, FIG.
Of the feedback circuit 310 of the laser diode bias circuit 2
By applying a negative feedback to 05, a control operation is realized such that the center frequency f _LD always matches the optical frequency that gives the minimum point MIN of the optical frequency discrimination characteristic.

The optical frequency that gives the minimum point MIN of the optical frequency discrimination characteristic can be kept constant by stabilizing the temperature of the Fabry-Perot interferometer 301, so that automatic frequency control on the transmitting side is facilitated. Can be realized.

Considering the optical frequency discrimination characteristic indicated by A in FIG. 8, it is understood that the Fabry-Perot interferometer 301 acts as an optical bandpass filter. Therefore,
When the Fabry-Perot interferometer is used as the optical filtering means, only the main carrier component of the optical signal output from the laser diode 204 of FIG. 7 is suppressed.

Although the oscillation frequency of the laser diode 204 varies depending on the low frequency signal input from the low frequency oscillator 308 to the bias circuit 205, the frequency of the low frequency signal (for example, several hundred Hz) and the modulation by the low frequency signal. If the modulation index is set appropriately, there is no influence on information transmission.

When only the main carrier component in the optical signal output from the laser diode 204 is to be suppressed or removed, a Mach-Zehnder interferometer or other optical interferometer can be used as the optical filtering means. When using a Mach-Zehnder interferometer, it is possible to improve the characteristics of the optical bandpass filter by arranging a plurality of Mach-Zehnder interferometers in series.

FIG. 9 is a block diagram showing a first embodiment of the optical receiver of the present invention. A feature of this embodiment is that the automatic frequency control on the receiving side is performed using the residual main carrier component as described with reference to FIG.

A local light source 401 using a laser diode or the like outputs local light of a frequency having a predetermined relationship with the center frequency f _LD (see FIG. 5) of the main carrier component of the received optical signal. The received optical signal is added to the local light from the local light source 401 by an optical coupler 402 including a half mirror, a fiber fusion type optical coupler and the like, and input to a light receiver 403 including a photodiode and the like.

The received optical signal and the local light are received by the light receiver 403.
When input to, an intermediate frequency signal (IF signal) having a frequency corresponding to the difference between the frequency of the received optical signal and the frequency of the local light is generated due to the square characteristic of the light receiver 403.

The IF signal is amplified by the amplifier 404 and input to the band pass filter 405 for demodulation and the band pass filter 407 for automatic frequency control. Here, the frequency arrangement of the IF signal will be described with reference to FIG. Figure 1
0 indicates the frequency arrangement of the IF signal on the intermediate frequency axis when the lower sideband and a part of the main carrier component are left by using the optical low pass filter having the characteristics shown in FIG. 6 (B). It was done. When the frequency of the local oscillation light is lower than the frequency of the optical signal, the arrangement of the lower sideband on the optical frequency axis is preserved as it is, as shown in FIG. on the other hand,
When the frequency of the local light is higher than the frequency of the optical signal, an image is obtained, and although not shown, the arrangement opposite to the arrangement of the lower sideband on the optical frequency axis is obtained on the intermediate frequency axis.

When the intermediate frequency corresponding to the center frequency f _LD of the main carrier component in the light wave region is f _IF , the carrier frequencies of the IF signals based on the respective signal components in the lower sideband are f _IF −f ₁ , F _IF- f ₂ , ..., f _IF- f
_n .

Returning to the explanation of FIG. A plurality (# 1 to #n) of band pass filters 405 are provided according to the number of transmission channels, and each pass band is set so as to selectively extract any one of the signal components of FIG. Has been done.

The demodulator 406 is also the band pass filter 4
The same number as 05 is provided. The demodulator 406 performs demodulation based on the signal component selectively extracted by the bandpass filter 405.

On the other hand, the pass band of the band pass filter 407 for automatic frequency control is set so that the residual main carrier component in the intermediate frequency signal can be extracted. The receiving side automatic frequency control circuit (receiving side AFC circuit) 408 feeds back the local light source 401 so that the frequency of the signal extracted by the band pass filter 407 becomes constant.
When the local light source 401 uses a laser diode, the control target of the reception side AFC circuit 408 is the bias current or temperature of the laser diode. Since the oscillation frequency of the laser diode changes depending on the bias current or the temperature, the center frequency of the residual main carrier component is kept constant by performing such feedback control.

As described above, when the optical signal is transmitted with the main carrier component remaining on the transmitting side, the SBS in the optical transmission line is suppressed by using the optical receiver as shown in FIG. The efficiency when using the optical amplifier can be improved and the intermediate frequency at the receiving side can be made constant. The requirement that the intermediate frequency be constant on the receiving side occurs when the oscillation frequency of the laser diode on the transmitting side and / or the laser diode of the local light source on the receiving side fluctuates undesirably.

FIG. 11 is a block diagram showing a second embodiment of the SCM optical receiver of the present invention. The substantially same components as those in FIG. 9 are designated by the same reference numerals. The IF signal amplified by the amplifier 404 is a 3 dB coupler 50.
The signals are equally distributed by 1, and the distributed signals are input to the bandpass filter 502 and the delay circuit 503, respectively.

The band pass filter 502 extracts the residual main carrier component of the IF signal. The delay circuit 503 is
A delay time equivalent to the delay time in the bandpass filter 502 is given.

The output signals of the band pass filter 502 and the delay circuit 503 are mixed by the multiplier 504,
It is input to each band pass filter 405 for demodulation. According to this configuration, the phase noise in the IF signal can be removed, and the reception sensitivity can be significantly increased.

By the way, when such phase noise is to be removed, in the present embodiment, the main carrier component is suppressed, and the transmitted main carrier component undergoes optical filtering processing. In some cases, the phase noise removal capability is not necessarily high.

Therefore, in such a case, the IF signal based on the reference signal is used instead of the residual main carrier component of the IF signal. The reference signal is, for example,
It can be obtained by outputting only the carrier from the corresponding modulator 201 without inputting any one of the transmission signals D _{1 to} D _n in the SCM optical transmitter of FIG.
When the frequency of this carrier is f _R , the spectrum of the IF signal by the reference signal is as shown in FIG. 12, and its center frequency is f _IF −f _R.

In the first embodiment shown in FIG. 9, the IF signal based on the residual main carrier component is not extracted, but
It is also possible to extract the IF signal based on the above-mentioned reference signal and perform AFC using this as a frequency reference.

In the first embodiment of the SCM optical transmitter of FIG. 2, the main carrier component and the upper sideband are suppressed or eliminated by using the optical low pass filter, but instead of the optical low pass filter. The main carrier component and the lower sideband may be suppressed or removed by using an optical high-pass filter.

[0075]

As described above, according to the present invention,
It is possible to suppress the stimulated Brillouin scattering in an optical transmission line formed of a silica-based optical fiber and to improve the efficiency when using an optical amplifier.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic configuration of an SCM optical transmission system of the present invention.

FIG. 2 is a block diagram showing a first embodiment of the optical transmitter of the present invention.

FIG. 3 is a diagram showing an example of a spectrum of a frequency-modulated signal.

FIG. 4 is a diagram for explaining channel arrangement of transmission signals.

FIG. 5 is a diagram showing a spectrum of an optical signal.

FIG. 6 is a diagram showing an example of characteristics of an optical low pass filter.

FIG. 7 is a block diagram showing a second embodiment of the SCM optical transmitter of the present invention.

FIG. 8 is an explanatory diagram of characteristics of a Fabry-Perot interferometer and a synchronous detection signal.

FIG. 9 is a block diagram showing a first embodiment of an SCM optical receiver of the present invention.

FIG. 10 is a diagram for explaining the frequency arrangement of IF signals.

FIG. 11 is a block diagram showing a second embodiment of the SCM optical receiver of the present invention.

FIG. 12 is a diagram for explaining an example of frequency arrangement of reference signals.

[Explanation of symbols]

 100 SCM optical transmitter 101 multiplex signal generation means 102 optical modulation means 103 optical filtering means 110 SCM optical receiver 111 local light source 112 optical coupler 113 light receiving means 114 band pass filter 115 demodulation means 120 optical transmission line

Claims (17)

[Claims] 1. A first method for generating (101) a multiplex signal by adding respective signals obtained by modulating a plurality of carriers, each of which is assigned a different frequency, with transmission signals of different channels. And a second step of modulating a coherent light source with the multiplexed signal or modulating light from the coherent light source to generate an optical signal (102), and suppressing or removing a main carrier component in the optical signal (1).
03) and the optical signal in which this main carrier component is suppressed is transmitted (12)
0) The fourth step, and the fifth step of optical-electrically converting the transmitted optical signal with local light to generate an electric signal (113), and sorting for each channel based on the electric signal. And a sixth step of demodulating (115) the received signal. 2. The coherent SCM optical transmission method according to claim 1, wherein in the third step, one of the both sidebands of the optical signal is further suppressed or removed. 3. In the third step, the main carrier component is suppressed so that its power becomes substantially equal to the power of the signal component in the optical signal after suppression, and the main carrier component is automatically detected based on the remaining main carrier component. The coherent SCM optical transmission method according to claim 1, wherein frequency control or phase noise removal is performed. 4. The method according to claim 1, further comprising a step of optically amplifying an optical signal in which the main carrier component is suppressed, between the third step and the fourth step. The coherent SCM optical transmission method described. 5. A multiple signal generation means for generating a multiple signal by adding respective signals obtained by modulating a plurality of carriers, each of which is assigned a different frequency, with transmission signals of different channels. 101), an optical modulator (102) that modulates a coherent light source by the multiplexed signal or modulates light from the coherent light source to generate an optical signal, and optical filtering that suppresses or removes a main carrier component in the optical signal. A coherent SCM optical transmitter comprising: a means (103). 6. The multiplex signal generating means (101) includes a plurality of modulators (201) for respectively modulating the plurality of carriers with the transmission signals of the plurality of channels, and a multiplexer (synthesizing signals from the modulators). 203), the optical modulation means (102) is a laser diode (204), a bias circuit (205) for applying a bias current to the laser diode so that the laser diode oscillates, and the multiplexed signal. 6. A coherent SCM optical transmitter according to claim 5, further comprising a modulation drive circuit (206) for applying a modulation current to the laser diode (204). 7. The optical filtering means (103) is a high-pass filter or a low-pass filter, the cut-off frequency of which is one of the both sidebands of the optical signal and the main carrier component is removed. The coherent SCM optical transmitter according to claim 6, wherein the coherent SCM optical transmitter is set as follows. 8. The optical filtering means (103) is a high-pass filter or a low-pass filter, and a slope and a cut of a rising portion or a falling portion of a characteristic curve representing the relationship between the transmittance of the filter and the optical frequency. The off frequency is set so that either one of the double sidebands in the optical signal is removed and the main carrier component in the optical signal is suppressed to a power substantially equal to the signal component. The coherent SCM optical transmitter according to claim 6. 9. The optical filtering means (103) is a Fabry-Perot interferometer (301), the interference characteristic of which is set so as to suppress the main carrier component in the optical signal. The coherent SCM optical transmitter according to claim 6. 10. A means for branching the light from the Fabry-Perot interferometer (301), and an automatic transmitter for stabilizing the frequency of the main carrier component in the optical signal based on the intensity change of the branched light. A frequency control circuit (302), wherein the transmission side automatic frequency control circuit is a low frequency oscillator (308) for superimposing a low frequency signal on the bias current.
And a light intensity detector (3
05), a multiplier (307) to which the signal from the light intensity detector and the low frequency signal are input, and a DC of the output signal of the multiplier.
The coherent SCM optical transmitter according to claim 9, further comprising a feedback circuit (310) for returning a component to the bias current. 11. The coherent SC according to claim 5, further comprising optical amplification means for optically amplifying an optical signal from the optical filtering means (103).
M optical transmitter. 12. The coherent SCM optical transmitter according to claim 5, wherein at least one of the plurality of carriers is a reference signal that provides a frequency reference. 13. A coherent SCM optical receiver for receiving an optical signal transmitted from the coherent SCM optical transmitter according to claim 5, wherein a frequency of a main carrier component in the optical signal and a predetermined value. Local light source (111) that outputs local light of frequencies related to
An optical coupler (112) for adding the local light and the transmitted optical signal, a light receiving means (113) for converting the light output from the optical coupler into an electric signal and outputting the electric signal, and the electric signal Coherent SCM light comprising a plurality of bandpass filters (114) for selecting signals for each channel and demodulation means (115) for respectively demodulating signals based on the signals from the plurality of bandpass filters. Receiving machine. 14. The coherent SCM optical receiver according to claim 13, which receives the optical signal transmitted from the coherent SCM optical transmitter according to claim 8 or 12, wherein the electric power output from the optical receiver is received. A bandpass filter (407) for extracting the residual main carrier component or reference signal component in the signal, and a receiving side automatic frequency control circuit (408) for feeding back to the local light source so that the frequency of the extracted component becomes constant. ) Is further provided, a coherent SCM optical receiver. 15. The coherent SCM optical receiver according to claim 13, which receives an optical signal transmitted from the coherent SCM optical transmitter according to claim 8 or 12, wherein the electric power output from the optical receiver is received. A branch circuit for branching the signal into two, a bandpass filter (502) for extracting a residual main carrier component or a reference signal component in one of the electric signals branched by the branch circuit, and an electric signal for branching by the branch circuit. On the other hand, the above bandpass filter
A delay circuit (503) for giving a delay time equivalent to the delay time in (502); and a multiplier (504) to which the signal from the delay circuit and the signal from the band pass filter (502) are input. A signal from the multiplier is input to the demodulator, and a coherent SCM optical receiver. 16. The coherent SCM optical transmitter according to claim 5, and the coherent SCM.
An optical transmission line for transmitting an optical signal from an optical transmitter, and an optical signal transmitted by the optical transmission line are received.
6. A coherent SCM optical transmission system comprising: the coherent SCM optical receiver according to any one of 5. 17. The coherent S according to claim 16, wherein the optical transmission line is an optical fiber, and a core and a clad of the optical fiber are mainly composed of SiO _2.
CM optical transmission system.