JP3317025B2 - Optical frequency division multiplex transmission equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency stabilizing method in an optical frequency division multiplex transmission apparatus.

[0002]

2. Description of the Related Art In an optical frequency division multiplex transmission apparatus, it is necessary to maintain a stable optical frequency in order to maintain signal quality.
FIG. 15 shows an example of a conventional frequency stabilization method for an optical frequency division multiplexed signal, which is shown in Hiromu Toba et a.
l. , "A Multi Channel Laser
Diode Frequency Stabiliz
erfor Narrowly Spaced Opt
ical Frequency-Division-M
multiPlexing Transmission
n ", Journal of Optical Com
municiations, 9, 1988, pp. 50-
54 is rewritten in an easy-to-understand manner.

In FIG. 15, 201a, 201b, 2
Are optical transmitters using semiconductor lasers for transmitting light having specific wavelengths. The outputs of the optical transmitters are multiplexed by an optical multiplexer (star coupler) 202. The output of the optical multiplexer 202 is the wavelength tunable optical filter 20.
3 is input. The transmission wavelength of the tunable optical filter 203 is swept by the sweep signal generation circuit 208. The output of the wavelength tunable optical filter 203 is converted into an electric signal by the light receiver 204 and processed by the CPU 205. 2
06 is a sweeper, and 207 is a digital-analog converter.

The operation of FIG. 15 will be described with reference to FIG.
Each of the optical transmitters 201a, 201b, 201c,... Transmits an optical signal having a specific wavelength.
The spectrum of the optical signal output from FIG.
As shown in FIG. 6A, spectral components corresponding to the output lights of the respective optical transmitters are observed. When the transmission wavelength of the tunable optical filter 203 is swept by a sawtooth wave shown in FIG. 16B, a pulse train on the time axis as shown in FIG. 16C is observed at the output of the tunable optical filter 203. As described above, by sweeping the transmission wavelength of the wavelength variable optical filter 203 with a sawtooth wave, the spectrum arranged on the frequency axis can be converted into a pulse train arranged on the time axis.

[0005] The pulse train received by the light receiver 204 is converted into an electric signal. When this pulse train is controlled so as to be equally spaced on the time axis, the optical transmitters 201a, 201b,
The transmission wavelengths of 201c,. The pulse interval is measured by the CPU 205, and 201a, 201b, 201c,
.. Are controlled via the digital-analog converter 207. The wavelength of the optical transmitter can be controlled by controlling the current injected into the semiconductor laser, or by controlling the temperature of the semiconductor laser.

[0006]

However, in this conventional method, it is not possible to specify which optical transmitter outputs a pulse train obtained from the output of the wavelength tunable optical filter 203, respectively. . Therefore, in this method, the optical transmitters 201a, 201b, 20
It is assumed that the order of the transmission wavelengths 1c,... Does not change, and the corresponding optical transmitter is determined according to the order in which the pulses appear. For this reason, it is difficult to perform control when the wavelength of the optical transmitter is greatly deviated and control when the power is turned on.

There is also a problem that the transmission wavelength of a generally available wavelength variable optical filter drifts due to a temperature change or an applied voltage.

[0008] Further, the proportionality constant when converting the frequency interval into the time interval is determined by the slope of the sawtooth sweeping the tunable optical filter. When the inclination of the sawtooth wave changes, there is a problem that the frequency interval changes.

As a technique related to the present application, there is a frequency interval stabilizing device disclosed in Japanese Patent Application Laid-Open No. Hei 4-245822. In this device, a pulse train obtained from an output of a Fabry-Perot type interference filter can be any semiconductor. It cannot be specified whether it corresponds to the optical signal output from the laser.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to stabilize optical frequencies of a frequency multiplexed optical signal at equal intervals and to specify the order of arrangement. The object of the present invention is to obtain a frequency stabilizing means which is hardly affected by a change in environmental temperature.

[0011]

According to the first aspect of the present invention, there are provided a plurality of optical transmitters for transmitting a plurality of optical signals having different frequencies, and a dither signal superimposing for superposing dither signals having different frequencies on the plurality of optical signals. Means, an optical multiplexer for multiplexing a plurality of optical signals from a plurality of optical transmitters, a reference cycle generating means for generating a reference cycle signal, and generating a sweep signal whose level changes with time by the reference cycle signal Signal generating means, a wavelength tunable optical filter that sweeps frequency multiplexed light from an optical multiplexer with a sweep signal and converts the signal into an optical pulse train arranged on a time axis, and a wavelength tunable optical Light receiving means for converting an optical pulse train signal from the filter into an electric signal,
Dither signal frequency from electric signal converted by light receiving means
By selectively transmitting the
The pulses of the electrical signal that
An optical frequency interval control unit for detecting an error signal by detecting a phase difference from a phase signal, and an optical frequency control unit for controlling the frequencies of a plurality of optical transmitters based on the error signal .

According to a second aspect of the present invention, a dither signal superimposing means for performing emphasis modulation with dither signals having different frequencies, and each optical transmitter corresponds to the intensity modulation component.
It is provided with frequency interval control means for extracting the pulse of the electric signal .

According to a third aspect of the present invention, there is provided a dither signal superimposing means for performing intensity modulation and frequency modulation by dither signals having different frequencies, and each optical transmitter by synchronously detecting the intensity modulation component and the frequency modulation component . Corresponding to
An optical frequency interval control means for extracting a pulse of the electric signal is provided.

According to a fourth aspect of the present invention, an optical frequency reference light source whose oscillation light frequency is stabilized and an optical signal of the optical frequency reference light source are input via a wavelength tunable optical filter and light receiving means. Pulse interval detection means for detecting a pulse interval based on the optical frequency of the reference light source, and a slope stabilization for controlling a slope of a temporal level change of the sweep signal of the sweep signal generation means by the pulse interval detected by the pulse interval detection means. Means.

According to a fifth aspect of the present invention, there is provided an optical frequency reference light source having two reference light sources having different oscillation light frequencies, and a pulse interval detecting means for detecting a pulse interval based on the oscillation frequencies of the two reference light sources. It is provided with.

According to a sixth aspect of the present invention, there is provided an optical frequency reference light source having a reference light source modulated at a predetermined frequency, and a pulse interval detecting means for detecting a pulse interval corresponding to a side band of the modulated reference light source. It is provided with.

The invention according to claim 7 is provided with an optical frequency control means having a temperature stabilizing means, wherein the control time constant of the temperature stabilizing means and the control time constant of the optical frequency interval control means are set to different values. It is.

The invention according to claim 8 is provided with bias stabilizing means for stabilizing the bias of the sweep signal of the sweep signal generating means by the output of the light receiving means based on the optical signal of the optical frequency reference light source. And the control time constant of the slope stabilizing means are set to different values.

[0019]

In the invention according to the first aspect, a plurality of optical signals on which dither signals having different frequencies are superimposed are input from the optical transmitter to the wavelength variable optical filter. The wavelength tunable optical filter sweeps a plurality of optical signals by using a sweep signal generated from the reference periodic signal, and converts a pulse of the optical signal on the frequency axis into a pulse on the time axis. These pulses converted into electric signals by the light receiving means are classified according to a plurality of optical signals by a dither signal, compared with a reference periodic signal, and an error thereof is detected. The frequency of the plurality of optical signals is controlled based on the detected error.

According to the second aspect of the present invention, intensity modulation is performed by dither signals having different frequencies, and a plurality of electric signals are classified into a plurality of optical transmitters according to the intensity modulation components.

In the invention according to claim 3, intensity modulation and frequency modulation are performed by dither signals having different frequencies,
By synchronously detecting the intensity modulation component and the frequency modulation component, a plurality of electric signals are separated into a plurality of optical transmitters.

In the invention according to claim 4, the pulse interval detecting means inputs the optical signal of the optical frequency reference light source whose oscillation light frequency is stabilized via the wavelength tunable optical filter and the light receiving means. A pulse interval based on the optical frequency of the reference light source is detected. Then, the slope stabilizing means controls the slope of the temporal level change of the sweep signal of the sweep signal generating means based on the detected pulse interval.

In the invention according to claim 5, the pulse interval detecting means detects a pulse interval based on the frequencies of two reference light sources having different oscillation light frequencies.

In the invention according to claim 6, the pulse interval detecting means detects a pulse interval corresponding to a side band of the reference light source modulated at a predetermined frequency.

In the invention according to claim 7, the control system is stabilized by setting the control time constant of the temperature stabilizing means for the optical transmitter and the control time constant for detecting an error between the reference periodic signal to different values. Make it work.

In the invention according to claim 8, the control time constant of the bias stabilizing means for stabilizing the bias of the sweep signal of the sweep signal generating means and the control time constant of the slope stabilizing means are set to different values. Operate the control system stably.

[0027]

【Example】

Embodiment 1 FIG. FIG. 1 is a block diagram showing an embodiment of an optical frequency division multiplex transmission apparatus according to the present invention. In FIG. 1, 201a, 201b, 201c, 202, 203,
Reference numerals 204 and 208 are equivalent to the conventional example of FIG. In FIG. 1, the optical transmitters 201a, 201b, 20
1c,..., And dither signal superimposing means 221a, 221b, 2
Are connected. Sweep signal generation circuit 2
08 generates a signal for sweeping the transmission wavelength of the wavelength tunable optical filter 203 from the output of the reference period generation means 209. The output of the wavelength tunable optical filter 203 is
The output of the optical receiver 204 and the output of the reference period generating means 209 are input to the optical frequency interval control means 226a.

The optical frequency interval control means 226a includes band pass filters 210a, 210b, 210c,..., Envelope detection circuits 213a, 213b, 213c,.
.., Low-pass filters 212a, 212b, 212c,.
Consists of The output of the light receiver 204 is input to the envelope detection circuits 213a, 213b, 213c,... Via the band-pass filters 210a, 210b, 210c,. Phase comparators 211a, 211b, 211
,... receive the output of the reference period generating means 209 and the outputs of the envelope detection circuits 213a, 213b, 213c,. 212a, 212b, 212c, ...
Is a low-pass filter whose output is the optical transmitter 201
a, 201b, 201c,... connected to the optical frequency control means 222a, 222b, 222c,.

As the wavelength tunable optical filter 203, for example, a Fabry-Perot filter whose gap length can be voltage-controlled by a piezo element, and a conductive filter whose voltage can control the light propagation time using a refractive index change by temperature and a refractive index change by electro-optic effect. A Mach-Zehnder filter using a wave path can be used. Optical frequency control means 222a, 222b, 22
As 2c,..., A method of controlling the temperature of a semiconductor laser as a light source of the optical transmitter, a method of controlling an injection current, and the like can be used. Also, the dither signal superimposing means 221a,
221b, 221c,... Superpose a low-frequency minute signal as a dither signal on the injection current of the semiconductor laser. In this case, the optical signal of the semiconductor laser is intensity-modulated by the dither signal.

Next, the operation of the configuration shown in FIG. 1 will be described with reference to FIG. The multiplexed light transmitted from the optical transmitters 201a, 201b, 201c,... Is input to the tunable optical filter 203. FIG. 2A shows the frequency multiplexed light spectrum. The tunable optical filter 203 is driven by the sweep signal generation circuit 208 to sweep the transmission wavelength. The output of the sweep signal generation circuit 208 is a sawtooth wave as shown in FIG. 2B, and is generated from the output of the reference period generation means 209 in FIG. FIG. 2 shows the envelope of the output of the tunable optical filter 203 whose transmission wavelength has been swept by the sawtooth wave.
(D). In this way, the optical spectrum arranged on the frequency axis in FIG. 2A can be converted into an optical pulse train arranged on the time axis in FIG. 2D. The converted light pulse train is converted into an electric signal by the light receiver 204 and input to the band-pass filters 210a, 210b, 210c,.

The band pass filters 210a, 210b,
, 210c,... Selectively transmit the dither signal frequencies generated by the dither signal superimposing means 221a, 221b, 221c,. For example, the waveform of FIG. 2E is obtained from the output of the bandpass filter 210a. When this is detected by the envelope detection circuit 213a, the waveform shown in FIG. 2F is obtained. Similarly, pulses corresponding to the outputs of the optical transmitters 201b, 201c,... Are obtained. FIG. 2 shows the output of the envelope detection circuit 213c.
(G). The phase difference between these pulses and the sine wave output from the reference period generating means 209 in FIG.
11a, 211b, 211c,..., And the low-pass filters 212a, 212b, 212c,.
To obtain an error signal. FIG. 2 shows an example of this error signal.
(H), FIG. 2 (i) and FIG. 2 (j).

When the transmission optical frequencies of the optical transmitters 201a, 201b, 201c,... Are controlled by the optical frequency control means 222a, 222b, 222c,. , The pulse shown in FIG. 2 (g) is stabilized to the zero point of the sine wave shown in FIG. 2 (c). As a result, the optical transmitters 201a, 201b,
The transmission optical frequencies 201c,... Are stabilized at equal intervals. Here, since the lock position of the phase comparator output pulse is set to the zero point of the sine wave as shown in FIG. 2C, there is a plurality of lock positions, and there is a possibility that the lock is performed at an erroneous point. This problem can be avoided by preparing a reference signal having only one zero point for each semiconductor laser, for example, a signal corresponding to a predetermined one cycle of the reference cycle signal.

Here, the phase comparators 211a, 211b, 2
.. And the output of the reference cycle generation means 209 supplied to the sweep signal generation circuit 208 need to be synchronized. In this example, the output is the same. You are using a signal. If they are not synchronized, the pulse interval cannot be easily detected using the phase comparators 211a, 211b, 211c,... As shown in the present embodiment, and unless a separate detecting means is provided. Will not be. As a method of measuring the pulse interval without using a phase comparator, there is a method of counting the number of pulses output from this oscillator with another oscillator as a time reference. However, it is generally difficult to obtain a high accuracy with a complicated configuration.

As described above, the time interval of the output pulse of the wavelength tunable optical filter 203 whose transmission wavelength is swept by the sawtooth wave is proportional to the optical frequency interval of the frequency multiplexed optical signal. Each of the optical transmitters 201a, 201b, 201 is set so as to coincide with the cycle of the generating means 209.
By controlling the output optical frequencies c,..., the optical frequencies of the frequency multiplexed optical signal can be stabilized at equal intervals.

Each of the optical transmitters 201a, 201b, 2
, The dither signal superimposing means 22
By superimposing a dither signal of a unique frequency by 1a, 221b, 221c,... And selectively transmitting the band-pass filters 210a, 210b, 210c,. It is possible to specify which optical transmitter each output pulse is transmitted from.

Embodiment 2 FIG. FIG. 3 is a block diagram showing another embodiment of the optical frequency division multiplexing transmission apparatus according to the present invention. FIG. 3 differs from FIG. 1 of the first embodiment in that the optical multiplexer 2
02 is connected to a multiplexer / demultiplexer 223 for demultiplexing the frequency multiplexed optical signal, and one of the demultiplexed frequency multiplexed optical signals passes directly through the photodetector 204b, and passes through the band pass of the optical frequency interval control means 226b. Filters 210a, 210b, 2
, And the light receiver 20
The output of 4a is connected to the synchronous detectors 214a, 214b, 214c,... Of the optical frequency interval control means 226b.

Next, the operation of the configuration shown in FIG. 3 will be described with reference to FIG. The difference from the first embodiment is that the dither signal superimposing means 221a, 221b, 221c,... Perform intensity modulation and frequency modulation by minutely modulating the injection current of the semiconductor laser. Optical transmitter 201
a, 201b, 201c, ... detect the intensity modulation component of the dither signal superimposed on the frequency multiplexed light, and synchronously detect the output of the wavelength tunable optical filter 203 using the obtained dither signal frequency. Is a point.

Each of the optical transmitters 201a, 201b,
The intensity modulation components of the dither signal superimposed on the bandpass filters 201c, 201c,.
c, ... are extracted. Tunable optical filter 20
In 3, the frequency modulation component of the dither signal superimposed on each optical transmitter is converted into an intensity modulation component. The optical transmitters 201a, 201b,
, 201c,... Are input. FIG. 4 (a) shows the frequency multiplexed optical spectrum.

The tunable optical filter 203 is driven by the sweep signal generation circuit 208 to sweep the transmission wavelength. The output of the sweep signal generation circuit 208 is a sawtooth wave as shown in FIG. 4B, and is generated from the output of the reference period generation means 209 of FIG. FIG. 4D shows the envelope of the output of the tunable optical filter 203 whose transmission wavelength has been swept by the sawtooth wave.

At this time, the FM signal component superimposed as the dither signal is converted into an AM component. Receiver 204
FIG. 4E shows a component near the dither frequency of the output a. The bandpass filter 210 which is an AM component
FIG. 4F shows the output waveform of FIG. 4 (e) and 4 (f) are used as the synchronous detectors 213a and 213.
, 213c,... Synchronous detector 213
., 213b, 213c,...
(G), the optical transmitters 201a and 201 as shown in FIG.
are obtained as waveforms obtained by differentiating the pulses corresponding to the frequency positions of b, 201c,.

The phase difference between these pulses and the sine wave output from the reference period generating means 209 in FIG.
1a, 211b, 211c,..., And an error signal is obtained through low-pass filters 212a, 212b, 212c,. FIGS. 4 (i) and 4 show examples of the error signal.
(J) shows. When the transmission optical frequencies of the optical transmitters 201a, 201b, 201c, ... are controlled by the optical frequency control means 222a, 222b, 222c, ... based on these error signals, for example, FIG. (H)
Is stabilized to the zero point of the sine wave shown in FIG. 4C, and as a result, the optical transmitters 201a, 201
The transmission light frequencies 1b, 201c,... Are stabilized at equal intervals.

As described above, a dither signal of a specific frequency is superimposed on the output light of each optical transmitter, and the FM component is synchronously detected using the AM component, so that the output pulse of the wavelength tunable optical filter can be changed. It is possible to specify from which optical transmitter the signal is transmitted, and to stabilize the optical frequency interval. Further, since synchronous detection is used in the present embodiment, the detection sensitivity of the error signal can be increased as compared with the first embodiment, and the accuracy of stabilizing the optical frequency can be further improved.

Embodiment 3 FIG. FIG. 5 is a configuration block diagram showing another embodiment of the optical frequency division multiplex transmission apparatus according to the present invention. In FIG. 5, reference numerals 225a and 225b denote optical frequency reference light sources for transmitting optical signals of specific wavelengths, respectively.
Its oscillation frequency is stabilized. The optical signals of the optical frequency reference light sources 225a and 225b are input to the multiplexer / demultiplexer 223a and multiplexed. The output of the multiplexer / demultiplexer 223a and the optical frequency multiplexed light output from the plurality of optical transmitters are coupled to the multiplexer / demultiplexer 22.
3b.

Reference numeral 203 denotes a wavelength variable optical filter, which is driven by a wavelength variable optical filter drive signal generation circuit 217. Wavelength variable optical filter drive signal generation circuit 21
7 generates a signal for sweeping the transmission wavelength of the wavelength tunable optical filter 203 based on the output of the tilt stabilizing circuit 216. The slope stabilizing circuit 216 includes a reference cycle generation unit 209.
And the pulse interval detecting means 215.

The output of the wavelength tunable optical filter 203 is connected to a light receiver 204, and the output of the light receiver 204 is input to a pulse interval detector 215 and an optical frequency interval controller 226. The optical frequency interval control means 226 is
And the output of the reference period generating means 209 as inputs, and are connected to optical frequency control means connected to each of the plurality of optical transmitters. As the optical frequency interval control means 226, the optical frequency interval control means 226a, 226b shown in the first or second embodiment can be used.

Next, the operation will be described. The operating principle of stabilizing the transmission light frequency of the optical transmitter at equal intervals by the optical frequency interval control means 226 using the wavelength variable optical filter 203 is the same as that shown in the first and second embodiments. As described above, the fact that the time interval of the output pulse of the wavelength variable optical filter whose transmission wavelength is swept by the sawtooth wave is proportional to the optical frequency interval of the frequency multiplexed optical signal is used.

Assuming that the slope of the sawtooth wave for sweeping the tunable optical filter is x [V / sec] and the proportional coefficient between the applied voltage of the tunable optical filter and the transmission center frequency is y [Hz / v], And two optical signals separated by a [Hz] correspond to two pulses separated by a / xy [sec] on the time axis. At this time, the inclination of the sawtooth wave is Δx [V / se
c], the interval between the two pulses is −aΔx / [x
(X + Δx) y] [sec].

When the control is performed so that the interval between the two pulses is constant, if the slope of the sawtooth changes by Δx [V / sec], the frequency interval of the optical signal becomes −aΔx / (x + Δx) [H
z] changes. Similarly, the proportionality coefficient between the applied voltage of the tunable optical filter and the transmission center frequency is Δy [Hz / v].
When it changes, the frequency interval of the optical signal becomes −aΔy / (y + Δ
y [Hz].

For the above reasons, in order to make the frequency interval of the optical signal constant, the product xy of the slope x of the sawtooth wave, the applied voltage of the wavelength variable optical filter, and the proportionality coefficient y of the transmission center frequency must be constant. . However, a proportional coefficient y [Hz] between the applied voltage and the transmission center frequency of a commercially available wavelength variable optical filter.
/ V] changes with the ambient temperature and the like, so the slope x of the sawtooth wave
It is necessary to control [V / sec] to stabilize the value of xy.

In this embodiment, the optical frequency reference light source 225a,
Tunable optical filter 203 corresponding to the output of 225b
Is detected by the pulse interval detecting means 215, and the slope of the sweeping sawtooth wave is controlled by the slope stabilizing circuit 216 so that the pulse interval becomes constant. The product xy of the proportional coefficient y between the applied voltage and the transmission center frequency is fixed.

FIG. 6 shows an example of the configuration of the pulse interval detecting means 215. In FIG. 6, 211a and 211b
Is a first phase comparator and a second phase comparator, respectively, 209 is a reference period generating means, 234 is a phase shifter,
212a and 212b are low-pass filters.

The operation will be described with reference to FIG. The sawtooth wave for sweeping the tunable optical filter is generated by the reference period generating means, and both are synchronized. Now, when the optical frequency reference light sources 225a and 225b pass through the wavelength variable optical filter 203 and are photoelectrically converted by the light receiver 204, as shown in FIG. . The phase shifter 234 is feedback-controlled by the optical frequency reference light source 225a so that the output voltage of the first phase comparator 211a becomes zero. That is, FIG.
The zero point of the reference periodic signal in FIG. 7B and the phase of the peak of the input pulse in FIG. By this operation, the phase of the reference periodic signal is determined by the optical frequency reference light source 225a.

The second phase comparator is an optical frequency reference light source 2
The phase relationship between the pulse according to 25b and the reference periodic signal is output. The level of this output signal represents the pulse interval itself.

FIG. 8 shows another example of the configuration of the pulse interval detecting means 215. In FIG. 8, 228 is a constant voltage source,
229a and 229b are resistors, 230 is a capacitor, 2
31 is an operational amplifier, 232 is an FET, and 233 is a peak hold circuit. The resistors 229a and 229b, the capacitor 230, the operational amplifier 231, and the FET 232 constitute an integrating circuit with a reset function.

The input pulse is used as a reset signal for an integrator circuit with a reset function. As an output, a voltage proportional to the time from when the reset signal is input can be obtained. FIG. 9 shows the wavelength tunable optical filter drive signal waveform, input pulse, output of the integrating circuit with reset, and output of the peak hold circuit. It shows how the interval of the input pulse is detected as a peak hold circuit output.

As described above, in this embodiment, even if the ambient temperature changes, the product of the voltage applied to the wavelength tunable optical filter and the proportional coefficient of the transmission center frequency can be kept constant. The spacing can be stabilized.

Embodiment 4 FIG. FIG. 10 is a block diagram showing the configuration of another embodiment of the optical frequency division multiplex transmission apparatus according to the present invention. In FIG. 10, reference numeral 225 denotes an optical frequency reference light source for transmitting an optical signal of a specific wavelength, the oscillation frequency of which is stabilized, and modulated by the RF oscillator 227 at a specific frequency. Other configurations are the same as those in FIG.

Next, the operation will be described. As described in the third embodiment, in order to make the frequency interval of the optical signal constant, the slope x [V / sec] of the sawtooth wave, the applied voltage of the wavelength variable optical filter, and the proportional coefficient y [Hz /
v], xy [Hz / sec,...] must be constant. However, since the proportional coefficient between the applied voltage and the transmission center frequency of a commercially available wavelength-variable optical filter changes depending on the ambient temperature or the like, it is necessary to stabilize the xy value by controlling the slope of the sawtooth wave.

FIG. 11 shows an output spectrum of the optical frequency reference light source modulated by the RF oscillator 227 at the frequency f1. In FIG. 11, f0 is the center frequency of the output of the optical frequency reference light source, and sidebands are observed at frequencies f2 and f3 that are apart from f0 by f1. That is, in the output of the frequency variable optical filter swept by the sawtooth wave, three pulses f0, f2, and f3 are observed. The interval between two of these three pulses is measured by the pulse interval detecting means 215.

Since the accuracy of the pulse interval is determined by the accuracy of the RF oscillator 227, it is sufficiently high. If the tilt stabilizing circuit 216 is controlled so that the measured value is constant, the optical frequency of the frequency multiplexed light can be stably stabilized. Can be.

As described above, in the present embodiment, even when the ambient temperature changes, the product of the voltage applied to the wavelength tunable optical filter and the proportionality coefficient of the transmission center frequency can be kept constant, and the transmission light frequency can be set at equal intervals. Can be stabilized.

Embodiment 5 FIG. FIG. 12 is a block diagram showing another embodiment of the optical frequency division multiplex transmission apparatus according to the present invention. In FIG. 12, 201a, 201b, 201
.., c are optical transmitters for transmitting optical signals of specific wavelengths, respectively.
19a, 219b, 219c,... And output stabilizing circuits (APC) 218a, 218b, 218c,. Temperature stabilizing circuits 219a, 219b,
The control time constant of the output stabilizing circuits 218a, 218b, 218c,... Is τt.

Reference numeral 202 denotes an optical multiplexer. Reference numeral 203 denotes a wavelength variable optical filter, which is driven by the sweep signal generation circuit 208. The sweep signal generation circuit 208 generates a signal for sweeping the transmission wavelength of the wavelength tunable optical filter 203 from the output of the reference period generation means 209. The light receiver 204 is connected to the output of the wavelength tunable optical filter 203, and the output of the light receiver 204 is input to the optical frequency interval control means 226a. The optical frequency interval control means 226a is the same as that shown in the first embodiment.

.., 211a, 211b, 211c,... Are phase comparators, and output from the reference period generating means 209 and band-pass filters 210a, 210b, 210c,.
・ Input the output of. 212a, 212b, 212
are low-pass filters, the outputs of which are connected to temperature stabilizing circuits 219a, 219b, 219c,... and optical transmitters 201a, 201b, 201c,.
・ Control the transmitted light frequency.

The control time constant of the optical frequency interval control means is τf
Which is almost the same as the time constant of the low-pass filter. Here, the control time constant τt of the temperature stabilization circuit is shorter than the control time constant τf of the optical frequency interval control means, and the control time constant τp of the output stabilization circuit is shorter than the control time constant τt of the temperature stabilization circuit. Is set.

The basic operation is the same as in the first embodiment. The main difference from the first embodiment is that the optical transmitter 20
.. Are controlled by the temperature stabilizing circuits 219a, 219b, 219c,... Second, the temperature stabilizing circuit and the output stabilizing circuit are controlled by the optical frequency interval. To work together with the means. Generally, in an optical transmitter using a semiconductor laser,
If the temperature is changed while the drive current is kept constant, the output intensity and the oscillation light frequency change, and if the output intensity is changed, the oscillation light frequency changes. Therefore, it is necessary to carefully set the respective control time constants. If the temperature stabilizing circuit and the output stabilizing circuit are operated simultaneously with the optical frequency interval control means, the control will oscillate.

Here, the control time constant τt of the temperature stabilizing circuit
Is set shorter than the control time constant τf of the optical frequency interval control means, and the control time constant τp of the output stabilization circuit is set shorter than the control time constant τt of the temperature stabilization circuit. The frequency can be controlled together. For example, τp is set to 0.02 sec and τt is set to 2 sec.
c and τf may be set to 20 seconds.

In the above embodiment, the control time constant τp of the output stabilization circuit, the control time constant τt of the temperature stabilization circuit, and the control time constant τf of the optical frequency interval control means are τp <τt <τf.
However, the setting of each control time constant need not be limited to this. For example, since .tau.t and .tau.f are the same control system, it is only necessary to give a difference in the control time constant, and .tau.p may be an arbitrary value since it is another control system.

As described above, in this embodiment, the optical frequency of the frequency multiplexed optical signal can be stabilized at equal intervals, together with the temperature and output intensity of the optical transmitter.

Embodiment 6 FIG. FIG. 13 is a block diagram showing another embodiment of the optical frequency division multiplex transmission apparatus according to the present invention. In FIG. 13, reference numeral 225 denotes an optical frequency reference light source for transmitting an optical signal of a specific wavelength, the oscillation frequency of which is stabilized, and modulated by the RF oscillator 227 at a specific frequency. The optical frequency multiplexed light output from the optical frequency reference light source 225 and the plurality of optical transmitters is
23. Reference numeral 203 denotes a wavelength variable optical filter, which is driven by the sweep signal generation circuit 217.

The sweep signal generation circuit 217 generates a signal for sweeping the transmission wavelength of the wavelength tunable optical filter 203 from the outputs of the tilt stabilization circuit 216 and the bias stabilization circuit 220. The light receiver 204 is connected to the output of the wavelength tunable optical filter 203, and the output of the light receiver 204 is input to the optical frequency interval control means 226a. The optical frequency interval control means 226a is the same as that shown in the first embodiment. 21
Numerals 1a, 211b, 211c,... Are phase comparators. The outputs of the reference period generating means 209 and the outputs of the band-pass filters 210a, 210b, 210c,. .., 212 a, 212 b, 212 c,... Are low-pass filters whose outputs control the output optical frequency of the optical transmitter.

The control time constant of the optical frequency interval control means has substantially the same value as the time constant of the low-pass filter. here,
The time constant of the tilt stabilizing circuit is τs, the control time constant of the bias stabilizing circuit is τb, and the control time constant of the optical frequency interval control means is τf. The time constant τs of the tilt stabilization circuit 216 is longer than the control time constant τf of the optical frequency interval control means, and the control time constant τf of the optical frequency interval control means is set longer than the control time constant τb of the bias stabilization circuit. I have.

The basic operation is the same as in the third embodiment. The main difference from the third embodiment is that first, a bias stabilizing circuit is added to the drive signal of the wavelength tunable optical filter 203, and second, the tilt stabilizing circuit and the bias stabilizing circuit are replaced by an optical frequency interval control means. To work with. As described in the third embodiment, the optical frequency interval control unit 226a applies a sawtooth potential to the tunable optical filter 203, thereby
To obtain a pulse train with a pulse interval proportional to the optical frequency interval of the frequency multiplexed optical signal, and adjust the output optical frequency of each optical transmitter so that the optical pulse train matches the cycle of the reference cycle generating means. The control stabilizes the optical frequency of the frequency multiplexed optical signal at equal intervals.

However, the wavelength tunable optical filter 203 may change its transmission wavelength with respect to a constant applied potential due to a change in environmental temperature or the like. Therefore, for more stable control, a bias stabilizing circuit that controls the DC component of the sawtooth potential and constantly keeps the transmission wavelength band to be swept constant is required. The bias stabilizing circuit 220 detects the phase relationship between the output pulse of the wavelength tunable optical filter corresponding to the optical frequency reference light source 225 and the sawtooth that drives the wavelength tunable optical filter 203, and forms a sawtooth wave so that this phase relationship is always constant. Controls the DC component of the potential.

However, if the tilt stabilizing circuit 216 and the bias stabilizing circuit 220 are operated simultaneously with the optical frequency interval control means 226a, the control will oscillate unless the respective control time constants are carefully set. Therefore, the time constant τs of the inclination stabilizing circuit is the control time constant τ of the optical frequency interval control means.
f, the control time constant τf of the optical frequency interval control means.
Is set longer than the control time constant τb of the bias stabilizing circuit. For example, τb is 0.2 sec and τf is 20
sec and τs may be set to 200 sec.

FIG. 14 is a configuration block diagram showing an example of the bias stabilizing circuit 220. 204 is a light receiver, 209
Is a reference period generating means, 211 is a phase comparator, and 212 is a low-pass filter. The phase comparator 211 detects the output pulse of the light receiver 204 corresponding to the optical frequency reference light source and the phase difference of the reference period generation means 209. Since the phase relationship between the output signal of the reference period generating means 209 and the driving sawtooth of the tunable optical filter 203 is constant, the output of the low-pass filter 212 is the output pulse of the light receiver 204 and the driving sawtooth of the tunable optical filter 203. Corresponds to the phase relationship.

In the above embodiment, the control time constant τb of the bias stabilization circuit, the optical frequency interval control means τf, and the control time constant τs of the tilt stabilization circuit are set as τb <τf <τs. The setting of the constant need not be limited to this. For example, since τb and τs are the same control system, it is only necessary to provide a difference in the control time constant. Since τf is another control system, τf may be an arbitrary value.

As described above, in this embodiment, the transmission wavelength band of the tunable optical filter can be kept constant even when the ambient temperature changes, and the transmission light frequency can be stabilized at equal intervals.

[0079]

As described above, according to the first to eighth aspects of the present invention, the optical frequencies of the frequency multiplexed optical signal can be stabilized at equal intervals, and the order of the arrangement can be specified.

Further, in the invention according to claim 3, the detection sensitivity of the error of the optical frequency can be increased, and the accuracy of the optical frequency stabilization can be further improved.

Further, in the invention according to claims 4 to 8,
It is possible to provide a frequency stabilizing unit that is hardly affected by a change in the environmental temperature.

[Brief description of the drawings]

FIG. 1 is a configuration block diagram showing a first embodiment according to the present invention.

FIG. 2 is a diagram illustrating the operation of the first embodiment.

FIG. 3 is a configuration block diagram showing a second embodiment according to the present invention.

FIG. 4 is a diagram illustrating the operation of the second embodiment.

FIG. 5 is a configuration block diagram showing a third embodiment according to the present invention.

FIG. 6 is a block diagram showing a configuration of a pulse interval detecting unit of FIG. 5;

FIG. 7 is a diagram illustrating the operation of FIG.

FIG. 8 is a block diagram showing another configuration of the pulse interval detecting means of FIG. 5;

FIG. 9 is a diagram illustrating the operation of FIG.

FIG. 10 is a configuration block diagram showing a fourth embodiment according to the present invention.

FIG. 11 is a diagram illustrating the operation of FIG.

FIG. 12 is a configuration block diagram showing a fifth embodiment according to the present invention.

FIG. 13 is a configuration block diagram showing a sixth embodiment according to the present invention.

FIG. 14 is a block diagram showing a configuration of the bias stabilizing circuit of FIG. 13;

FIG. 15 is a configuration block diagram of a conventional example.

FIG. 16 is a diagram illustrating the operation of FIG.

[Explanation of symbols]

201a, 201b, 201c,... Optical transmitter 202 Optical multiplexer 203 Wavelength tunable optical filter 204, 204a, 204b Optical receiver 205 CPU 206 Scanner 207 D / A converter 208 Sweep signal generation circuit 209 Reference period generation means 210a, 210b , 210c,... Bandpass filters 211a, 211b, 211c,... Phase comparators 212a, 212b, 212c,... Low-pass filters 213a, 213b, 213c,. Synchronous detector 215 Pulse interval detecting means 216 Inclination stabilization circuit 217 Wavelength tunable optical filter drive signal generation circuit 218a, 218b, 218c,... Output stabilization circuit 219a, 219b, 219c,. Conversion circuit 220 Bias stabilization circuit 221a, 221b, 221c,... Dither signal superimposing means 222a, 222b, 222c,. , 226a, 226b Optical frequency interval control means 227 RF oscillator 228 Constant voltage source 229a, 229b Resistor 230 Capacitor 231 Operational amplifier 232 FET 233 Peak hold circuit 234 Phase shifter

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-347605 (JP, A) JP-A-64-15992 (JP, A) JP-A-5-63642 (JP, A) JP-A-5-63642 132935 (JP, A) JP-A-6-13983 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08

Claims (8)

(57) [Claims] 1. A plurality of optical transmitters for transmitting a plurality of optical signals having different frequencies, dither signal superimposing means for superimposing dither signals having different frequencies on the plurality of optical signals, An optical multiplexer for multiplexing a plurality of optical signals; a reference cycle generating means for generating a reference cycle signal; a sweep signal generating means for generating a sweep signal whose level changes with time according to the reference cycle signal; A wavelength tunable optical filter that sweeps the frequency multiplexed light from the wave filter with the sweep signal and converts the multiplexed light into a signal of an optical pulse train arranged on the time axis; and a light pulse train from the wavelength tunable optical filter . light receiving means for converting the signals into electric <br/> electrical signal, the dither signal circumferential from the electric signal converted by the light receiving means
By selectively transmitting wave numbers, each optical transmitter
The corresponding pulses of the electrical signal are extracted and these pulses
And an error signal is obtained by detecting the phase difference between
That the optical frequency interval control unit, the error signal by the optical frequency division multiplex transmission apparatus characterized by comprising an optical frequency controlling means for controlling the frequency of the plurality of optical transmitters. 2. The dither signal superimposing means performs intensity modulation by the dither signals having different frequencies, and the frequency interval control means performs the intensity modulation by the intensity modulation component.
2. The optical frequency division multiplex transmission apparatus according to claim 1, wherein a pulse of the electric signal corresponding to each optical transmitter is extracted . 3. The dither signal superimposing means performs intensity modulation and frequency modulation with the dither signals having different frequencies, and the optical frequency interval control means synchronously detects the intensity modulation component and the frequency modulation component. By each optical transmission
2. The optical frequency division multiplex transmission apparatus according to claim 1, wherein a pulse of the electric signal corresponding to the optical signal is extracted . 4. An optical frequency reference light source whose oscillation light frequency is stabilized, and an optical signal of the optical frequency reference light source are input via the wavelength tunable optical filter and the light receiving means, and the optical frequency reference light source is provided. Pulse interval detecting means for detecting a pulse interval based on the optical frequency of the light, and a slope stable for controlling a slope of a temporal level change of the sweep signal of the sweep signal generating means by the pulse interval detected by the pulse interval detecting means. 2. The optical frequency division multiplex transmission apparatus according to claim 1, further comprising: 5. The optical frequency reference light source includes two reference light sources having different oscillation light frequencies, and the pulse interval detection means detects the pulse interval based on the oscillation frequencies of the two reference light sources. The optical frequency division multiplex transmission apparatus according to claim 4, wherein: 6. The optical frequency reference light source includes a reference light source modulated at a predetermined frequency, and the pulse interval detecting means detects a pulse interval corresponding to a side band of the modulated reference light source. The optical frequency multiplex transmission apparatus according to claim 4. 7. The optical frequency control means includes a temperature stabilizing means, and a control time constant of the temperature stabilizing means and a control time constant of the optical frequency interval control means are set to different values. Item 2. The optical frequency division multiplex transmission device according to item 1. 8. A bias stabilizing means for stabilizing a bias of a sweep signal of said sweep signal generating means by an output of said light receiving means based on an optical signal of said optical frequency reference light source, and controlling said bias stabilizing means. 5. The optical frequency division multiplex transmission apparatus according to claim 4, wherein a constant and a control time constant of said inclination stabilizing means are set to different values.