JPH0752271B2 - Optical frequency stabilizer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention is used for optical frequency division multiplexing transmission for transmitting a plurality of optical FSK modulated signals having different center frequencies. In particular,
The present invention relates to an optical frequency stabilizer that stabilizes the center frequency interval of an optical FSK modulated signal.

The present invention, in an optical frequency stabilizer, stabilizes the center frequency interval over a wide frequency range by stabilizing the optical FSK modulation signal and the center frequency interval with an optical filter whose transmission frequency changes periodically as a reference. It is a thing.

[Conventional technology]

FIG. 17 shows a block diagram of a conventional optical frequency stabilizer. This optical frequency stabilizing device can stabilize a plurality of optical signals each having a different center frequency. The details of this device are shown in Yamazaki et al., 1988 Proceedings of IEICE Autumn National Convention, B-405, B-1-189.

When the frequency of the swept laser diode 1 is swept at a constant rate with respect to time, and its optical output is input to the optical resonator 2,
Since the transmittance of the optical resonator 2 has a local maximum point periodically with respect to the frequency, an optical pulse is generated at a constant time corresponding to the resonance frequency interval of the optical resonator 2. This light pulse is input to the light receiving element 3 and serves as a reference pulse train of an electric signal.

The transmission lasers LD _{1 to} LD ₈ are respectively modulated by the signals of the information signal sources S _{1 to} S ₈ and output eight optical FSK modulation signals. The eight optical FSK modulation signals are multiplexed by the star coupler 6 and then mixed with the output light of the sweep laser diode 1 by the directional coupler 7. When the center frequency of the plurality of mixed optical FSK modulation signals matches the frequency of the sweep laser light, an electric pulse signal is generated in the light receiving element 4. The time interval of this pulse signal represents the center frequency interval of the optical FSK modulation signal group.

Therefore, the control circuit 5 measures the time difference between the reference pulse train obtained by the light receiving element 3 and the pulse train representing the center frequency interval of the optical FSK modulation signal obtained by the light receiving element 4, and the current corresponding to the time difference is measured. It returns to each of the transmission lasers LD _{1 to} LD ₈ . As a result, the interval between the center frequencies of the optical FSK modulation signal group is stabilized at the frequency interval of the optical resonator 2.

[Problems to be Solved by the Invention]

However, with conventional optical frequency stabilizers, the frequency band that can be swept with good linearity using a swept laser diode is 100 GHz or less at most, and if the center frequency interval of the optical FSK modulated signal is about 10 GHz, only about 10 channels are stabilized. There was a flaw that I could not do.

It is an object of the present invention to solve such a problem and to provide an optical frequency stabilizer that can stabilize the relative frequencies of optical FSK signals of many channels.

[Means for Solving the Problems]

The optical frequency stabilizing device of the present invention, selecting means for selecting the transmission light FSK modulation signal of any of a plurality of transmission lasers, the output light of this selection means as an input, the transmittance is periodic depending on the frequency of the incident light. And an optical filter whose period is equal to the center frequency interval of the optical FSK modulated signal.

The selection means has a periodic interval of Δf, 2 × Δf, ..., M × Δf.
M periodic filters include cascaded channel selection filters, the channel selection filters being m
It is desirable to include means for setting the transmission frequency of each of the periodic filters. However, Δf is a set value of the center frequency interval of the optical FSK modulation signal.

The optical filter includes one entrance port and two exit ports, and the transmittance from the entrance port to one exit port and the transmittance to the other exit port are periodic and complementary depending on the frequency of the incident light. It is desirable to use a periodic filter that changes to.

At this time, two light receiving elements that respectively receive the light emitted from the two output ports of the periodic filter, circuit means for obtaining the difference between the output currents of the two light receiving elements, and two incident light beams of the periodic filter are emitted. And means for setting the center frequency of the optical FSK modulated signal to a frequency that is equally distributed to the ports.

In addition, the light receiving element that receives the light emitted from the optical filter, the synchronous detection circuit that synchronously detects the output of this light receiving element with the information signal of the optical FSK modulation signal that has entered the optical filter, and the incident light of the optical filter is It is also possible to provide a means for setting the center frequency of the optical FSK modulated signal to a frequency at which the transmittance becomes maximum or minimum. Also in this case, a periodic filter including two emission ports is used as an optical filter,
The two light receiving elements can receive the respective emitted lights. At this time, the difference between the output currents of the two light receiving elements is synchronously detected.

[Work]

An optical filter whose transmittance changes periodically with the frequency of incident light can be used as an optical discriminator in which points exhibiting the same discrimination characteristic appear periodically with respect to the incident frequency. Hereinafter, the points showing the same discrimination characteristic will be referred to as "discrimination characteristic points".

For example, it is assumed that a periodic filter including two emission ports is used as the optical filter, and frequencies having the same light intensity at the two emission ports are selected as the discrimination characteristic points. At this time, when the optical FSK modulation signal is incident on the optical discriminator, the discrimination outputs of the two emission ports are complementary if the center frequency matches the discrimination characteristic point. When the center frequency deviates from the discrimination characteristic point, the two discrimination outputs change. Therefore, the frequency of the transmission source is controlled so that this is offset.

It is also possible to select a point at which the transmittance of the optical filter is minimal as a discrimination characteristic point. At this time, if the center frequency matches the discrimination characteristic point, the discrimination output becomes constant. However, when the center frequency deviates from the discrimination characteristic point, a discrimination output corresponding to the modulated information signal appears. The phase at this time differs from the original information signal depending on whether it is shifted to the low frequency side or the high frequency side. Therefore, if this discriminative output is synchronously detected with the original information signal, the frequency shift can be detected.

The same applies when a point at which the transmittance is maximized is selected as a discrimination characteristic point. Alternatively, a frequency filter including two emission ports may be used as the optical filter, and one of the two emission ports may be selected such that one has a maximum transmittance and the other has a minimum transmittance, and the difference is synchronously detected.

In this way, it is possible to match the oscillation frequency of each of the plurality of transmission lasers with the periodic discrimination characteristic point of the optical discriminator, and the optical FSK modulation signal of the optical FSK modulation signal with reference to the frequency interval of the discrimination characteristic point. The center frequency interval is stabilized.

When the channel selection filter is used as the selection means for selecting the optical FSK modulation signal (and the transmission laser that has transmitted it), the channels to be stabilized can be switched at high speed, and the number of channels that can be switched increases.
For example, when using a swept laser diode as in the conventional example, the transmission bit rate is 400 Mb / s and the channel spacing is 8
At GHz, the number of channels that can be stabilized is at most 8. On the other hand, in the case of the present invention, for example, a frequency of 100 or more channels can be stabilized at a transmission bit rate of 600 Mb / s and a channel interval of 10 GHz.

〔Example〕

FIG. 1 is a block diagram showing the basic configuration of the optical frequency stabilizing device of the present invention.

This device includes a plurality of transmission lasers LD _{1 to} LD _n that transmit optical FSK modulated signals at mutually different center frequencies, and light receiving elements Pd _a and Pd _b that detect the output light of the plurality of transmission lasers LD _{1 to} LD _n.
Then, by using the detection outputs of the light receiving elements Pd _a and Pd _b , the transmission lasers and the oscillation frequencies are set to be negative so that the center frequency intervals of the transmission FSK modulation signals of the plurality of transmission lasers LD _{1 to} LD _n become a predetermined value. As a control means for feedback control, load resistance 17,
An error signal detector 19 and a bias current group 21 are provided.
The bias current group 21 includes bias current sources b _{1 to} b _n corresponding to the transmission lasers LD _{1 to} LD _n , respectively.

The transmission lasers LD _{1 to} LD _n are composed of laser diodes whose optical frequency changes according to the injection current. Each transmitting laser LD ₁ to Ld _n, a digital electrical signal from the information signal source S ₁ to S _n, and the bias current source b ₁ ~b _n is an adder
It is supplied via 11-1 to 11-n. This gives n
Optical FSK modulated signals are obtained.

The n optical FSK modulation signals are frequency division multiplexed by the multiplexer 12. A star coupler or the like is usually used as the multiplexing device 12. The output of the multiplexer 12 is sent to the optical transmission line, a part of which is branched to the fiber 13.

The feature of this configuration is that the transmission lasers LD _{1 to} L
A channel selection filter 14 is provided as a selection unit that selects one of the transmitted light FSK modulated signals of D _n , and the output light of this channel selection filter 14 is input, and the transmittance changes periodically depending on the frequency of the incident light. The optical discriminator 16 is provided as an optical filter whose period is equal to the center frequency interval of the optical FSK modulation signal.

The multiplexed optical FSK modulation signal is input to the channel selection filter 14 by the fiber 13. The channel selection filter 14 is an arbitrary one of a plurality of optical FSK modulation signals.
Select a channel. The channel selection filter 14 is composed of, for example, a periodic filter. When the periodic interval of the periodic filter is defined as the frequency interval from the maximum value to the minimum value of the transmittance, the central frequency interval is Δ by connecting the periodic filters having the interval intervals of Δf to mΔf in multiple stages.
The optical FSK modulated signal of f can be separated for each channel. To tune the channel selection filter 14 to a desired frequency, the heater electrodes H-1 to Hm of each periodic filter are used.
To, applying a current i ₁ through i _m which corresponds to the tuning frequency. The channel selection filter 14 inputs only the FSK signal 15 of the desired channel to the optical discriminator 16.

The optical discriminator 16 is composed of a periodic filter and has a transmittance that periodically differs with respect to the frequency of the optical FSK modulated signal, and converts the selected optical FSK modulated signal into a light intensity modulated signal. The light intensity modulation signal obtained by this light discriminator 16 is received by the light receiving element Pd.
It is converted into a current Δi according to the frequency shift by _a and Pd _b . This current Δi is taken out as a voltage ΔV by the load resistor 17. Based on this ΔV, the error signal detection unit 19 detects the deviation between the discrimination characteristic point of the optical discriminator 16 and the center frequency of the optical FSK signal, and outputs the obtained error signal to the bias current group.
21 and the bias current source b ₁ based on this error signal.
By controlling ~ b _n , the center frequency of the optical FSK signal is made to coincide with the discrimination point of the optical discriminator 16.

The above operation is repeated by sequentially switching the selected channels of the channel selection filter 14. The discrimination characteristic of the optical discriminator 16 changes periodically with respect to the frequency, and the output frequencies of the transmission lasers LD _{1 to} LD _n are stabilized corresponding to this period. Therefore, assuming that the frequency interval ΔF from the maximum value to the minimum value of the transmittance of the optical discriminator 16 is, the frequency interval Δf of the optical FSK modulation signal group is p times the period interval ΔF of the optical discriminator 16 (p is 2 or more). Is a positive integer) and is relatively stabilized.

In this way, the center frequencies of the plurality of optical FSK modulation signals can be relatively stabilized at the frequency interval Δf that is an integral multiple of the cycle interval ΔF of the optical discriminator 16.

In order to guide the outputs of the transmission lasers LD _{1 to} LD _n to the optical discriminator 16,
Here, a configuration is used in which the multiplexed optical signal is split on the transmission side. However, the outputs of the transmission lasers LD _{1 to} LD _n may be branched as they are before being multiplexed and guided to the optical discriminator 16, or the optical discriminator 16 may be provided on the receiving side.

Specific examples of the present invention will be described below.

FIG. 2 is a block diagram of an optical frequency stabilizer according to the first embodiment of the present invention.

Digital electric signals from the information signal sources S _{1 to} S _n and bias currents from the bias current sources b _{1 to} b _n are respectively added to the transmission lasers LD _{1 to} LD _n by the adders 11-1 to 11-n. Will be supplied via. As a result, each of the transmission lasers LD _{1 to} LD _n outputs an optical FSK modulation signal, and an n-series optical signal is obtained.

These optical FSK modulated signals are frequency division multiplexed by the multiplexer 12 and input to the channel selection filter 14 via the fiber 13. The channel selection filter 14
Only the optical signal of the k-th channel is selected from the n optical FSK modulation signals and passed.

The channel selection filter 14 has a periodic interval of Δf, 2 × Δ
It has a structure in which m waveguide type periodic filters of f, ..., M × Δf are cascade-connected. However, Δf is a set value of the intermediate frequency interval of the optical FSK modulation signal. The tuning frequency of the channel selection filter 14 is obtained by passing a current through the heater electrodes H-1 to Hm of each periodic filter to change the optical path length of the waveguide and shift the phase of light. Here, the number m of the periodic filters and the number n of the optical FSK modulation signals have a condition that n ≦ 2 ^m , and n and m are integers of 1 or more.

For a channel selection filter having a structure in which such waveguide type periodic filters are cascaded in m stages, see Toba et al.
Electronics Laters, 1988, Vol. 24, No. 2, No.
This is explained in detail on page 78.

When the tuning frequency of the channel selection filter 14 is set to the k-th channel, the optical discriminator 16 receives only the optical FSK modulation signal of that channel.

The optical discriminator 16 includes, for example, a periodic filter equivalent to that used in the channel selection filter 14, and has two emission ports.
Lights of mutually complementary intensities are output to p _a and p _b .

This output light enters the light receiving elements Pd _a and Pd _b , respectively, and the currents of i _a and i _b flow in the respective light receiving elements Pd _a and Pd _b . Therefore, if the light receiving elements Pd _a and Pd _b are connected in series, Δi =
i _a −i _b is obtained.

FIG. 3 shows the relationship between the transmittance of the optical discriminator 16 and the current difference Δi. FIG. 3 (a) shows an optical separator 16 for the frequency of incident light.
And (b) shows the value of the current difference Δi at that time. Further, (c) shows the value of the current difference Δi with respect to the incident light intensity at a certain frequency.

As described above, the use of the current difference Δi has an advantage that the intensity noise included in the optical FSK modulation signal is canceled. In addition, it is possible to determine whether the incident light frequency of the optical discriminator 16 is shifted to the high frequency side or the low frequency side from the desired discrimination characteristic point based on the value of the current difference i. However, for the actual determination, the load resistance 17 converts the current difference Δi into a voltage signal. The converted voltage signal is used as the error signal ΔV, and this signal is negatively fed back to the transmission laser LD _k .

That is, the voltage of the load resistor 17 is set to an error signal ΔV, which is amplified by the differential amplifier 22, smoothed by the low pass filter 32, and the bias current source b _k is controlled via the 1 × n switch 26. As a result, the optical FSK transmitted by the transmission laser LD _k is transmitted.
The center frequency of the modulated signal is made to match the discrimination characteristic point of the optical discriminator 16. The 1 × n switch 26 is controlled by the control device 27 so that the tuning frequency of the channel selection filter 14 is set to the kth channel, and at the same time, the lowpass filter.
Connect 32 outputs to bias current source b _k .

In FIG. 2, an electric signal from the information signal source S _k is input to the differential amplifier 22 and a 1 × n switch 24 and a low pass filter 25 are provided.
An example of supply via If the error signal ΔV does not have the mark rate dependency, these circuit configurations are unnecessary.
When there is a mark rate dependency, the mark rate is subtracted from the error signal ΔV to remove the dependency. The mark rate is obtained by passing the original information signal through the low pass filter 25. The 1 × n switch 24 is controlled in the same manner as the 1 × n switch 26, and supplies the signal from the information signal source S _k to the low pass filter 25.

The frequency that passes through the channel selection filter 14 is sequentially switched to high speed by using the channel selection filter control circuit 28, and the above operation is repeated. The discrimination characteristic of the optical discriminator 16 changes periodically with respect to the incident light frequency, and the center frequency of the optical FSK modulation signal transmitted by the transmission lasers LD ₁ LD _n is made to coincide with each discrimination characteristic point. When the frequency interval from the maximum value to the minimum value of the transmittance of the optical discriminator 16 is ΔF, the center frequency interval Δf of the optical FSK modulation signal is set to p times ΔF. However, p is an integer of 2 or more.

4 to 12 show the principle of detecting an error signal by the optical discriminator 16. Hereinafter, the case of a binary signal having a signal level of "0" and "1" as the optical FSK modulation signal will be described.

4 to 6 show that the optical discriminator 16 detects an error signal when the frequency interval ΔF of the optical discriminator 16 and the frequency deviation f _m of the optical FSK modulation signal are ΔF = 1 / 2f _m. It is a figure explaining a principle. FIG. 4 shows a case where the center frequency f ₀ ′ of the optical FSK modulated signal is equal to the discrimination characteristic point of the optical discriminator 16, that is, the center frequency of the optical FSK modulated signal is equal to the frequency f ₀ to be stabilized. The figure shows the case of f ₀ ′ = f ₀ + δf, and FIG. 6 shows the case of f ₀ ′ = f ₀ −δf. However, δf is f ₀
Represents the offset frequency from and its value is positive.

4 to 6, (a) of each figure shows the relationship between the frequency deviation f _m and the transmittance of the optical discriminator 16,
(B) is the intensity of the light emitted from one emission port p _a ,
(C) is the detection current of the light receiving element Pd _a , (d) is the light receiving element Pd _b
, (E) shows the error signal ΔV, and (f) shows the time average <ΔV> of the error signal ΔV. Further, in these figures, τ represents the period of the optical FSK modulation signal on the time axis.

In FIG. 4 (a), FIG. 5 (a), and FIG. 6 (a),
The solid line shows the transmittance from the entrance port of the optical discriminator 16 to the exit port p _a , and the broken line shows the transmittance from the exit port p _b .

Here, the waveform of the optical FSK modulation signal is assumed to be a perfect rectangle.

As shown in FIG. 4, when ΔF = 1 / 2f _m , the two output ports p _{a of the} optical discriminator 16 regardless of whether the signal level of the optical FSK modulated signal is “0” or “1”. , P _b the light intensity is substantially constant. Further, when the center frequency f ₀ ′ of the optical FSK modulated signal matches the frequency f _{0 at the} discrimination characteristic point, the light intensities at the two emission ports p _a and p _b become substantially equal. Therefore, the detection currents i _a and i _{b of} the light receiving elements Pd _a and Pd _b have constant and substantially equal values, and the error signal ΔV becomes zero.

The center frequency f ₀ ′ of the optical FSK modulated signal is the frequency f at the discrimination characteristic point.
_When it is shifted from ₀ to δf on the high frequency side, the light intensities at the two emission ports p _a and p _b are different as shown in FIG. Therefore, the photodetectors Pd _a and Pd _b detect current
A difference occurs between i _a and i _b , and the polarity of the error signal ΔV becomes positive.

On the contrary, when the center frequency f ₀ ′ of the optical FSK modulated signal is deviated from the frequency f _{0 at the} discrimination characteristic point to the low frequency side by δf,
As shown in FIG. 6, the polarity of the error signal ΔV becomes negative.

In this way, it is possible to determine whether the center frequency f ₀ ′ of the optical FSK modulation signal is shifted to the high frequency side or the low frequency side depending on the polarity of the error signal ΔV. Therefore, this error signal ΔV can be fed back to the transmitting laser to adjust its oscillation frequency.

As concrete numerical values of the above conditions, the channel frequency interval Δf = 10 GHz, the frequency interval ΔF = 1 of the optical discriminator 16 is set.
GHz, frequency shift of optical FSK modulated signal f _m = 2 GHz,
The transmission bit rate per channel is 600 Mb / s, and frequency stabilization can be performed in optical frequency division multiplex transmission with 100 or more channels.

In the above description, the waveform of the optical FSK modulated signal is assumed to be a perfect rectangle, and therefore the light intensity at the emission ports p _a and p _b of the optical discriminator 16 is constant. However, in reality, the waveform of the optical FSK modulation signal does not become a perfect rectangle. Therefore, the light intensity fluctuates, and the error signal Δ
V fluctuates. However, if the time average <ΔV> is taken through the signal low pass filter 32, the fluctuation is removed.

In the above example, the case where there is a relation of ΔF = f _m / 2 between the frequency interval ΔF of the optical discriminator 16 and the frequency deviation f _m has been described. In this case, the optical discriminator
Although the light intensity at the 16 ports p _a and p _b cannot discriminate between “0” and “1” of the signal level, the optical frequency can be stabilized without depending on the mark ratio.

7 to 9 are similar to FIGS. 4 to 6,
FIG. 6 is a diagram illustrating the principle of detecting an error signal by the optical discriminator 16. However, in this example, the frequency interval Δ of the optical discriminator 16 is
The relationship between F and the frequency deviation f _m of the optical FSK modulation signal is ΔF = f _m
And the optical discriminator 16 determines the center frequency f ₀ ′ of the optical FSK modulated signal.
A case is shown in which the frequency is adjusted to a frequency f _{0 at} which the transmittance of becomes 1/2.

FIG. 7 shows that the center frequency f ₀ ′ of the optical FSK modulated signal is the optical discriminator 1
The case where the frequency is equal to the frequency f ₀ of the discrimination characteristic point of 6 is shown, FIG. 8 shows the case of f ₀ ′ = f ₀ + δf, and FIG. 9 shows f ₀ ′ −f ₀ −δ
The case of f is shown. However, δf represents an offset frequency from f ₀ , and its value is positive.

7 to 9, (a) of each figure shows the relationship between the frequency deviation f _m and the transmittance of the optical discriminator 16,
(B) is the intensity of the light emitted from one emission port p _a ,
(C) is the detection current of the light receiving element Pd _a , (d) is the light receiving element Pd _b
, (E) shows the error signal ΔV, and (f) shows the time average <ΔV> of the error signal ΔV. Further, in these figures, τ represents the period of the optical FSK modulation signal on the time axis.

In FIG. 7 (a), FIG. 8 (a), and FIG. 9 (a),
The solid line shows the transmittance from the entrance port of the optical discriminator 16 to the exit port p _a , and the broken line shows the transmittance from the exit port p _b .

Here, it is assumed that the waveform of the optical FSK modulation signal is not a perfect rectangle. In practice, optical FSK modulated signals that are not perfectly rectangular are thus more likely to occur.

As shown in FIG. 7, the center frequency f ₀ ′ of the optical FSK modulated signal
Is coincident with the frequency f ₀ of the discrimination characteristic point, the light intensities at the two emission ports p _a and p _b change complementarily corresponding to the signal level. Therefore, the detection currents i _a and i _{b of} the light receiving elements Pd _a and Pd _b are also complementary, and the polarity of the error signal ΔV changes according to the signal level of the optical FSK modulation signal.
However, the time average of the smoothed error signal ΔV <Δ
The value of V> becomes zero.

The center frequency f ₀ ′ of the optical FSK modulated signal is the frequency f at the discrimination characteristic point.
_When it is deviated from ₀ to δf on the high frequency side, the light intensities at the two emission ports p _a and p _b deviate from the complementary state, as shown in FIG. Therefore, the polarity of <ΔV> becomes positive.

On the contrary, when the center frequency f ₀ ′ of the optical FSK modulated signal deviates from the frequency f ₀ of the discrimination characteristic point by δf to the low frequency side, the polarity of <ΔV> becomes negative as shown in FIG.

Thus, by obtaining the time average <ΔV> of the error signal ΔV, it is possible to determine from the polarity whether the center frequency f ₀ ′ of the optical FSK modulation signal is shifted to the high frequency side or the low frequency side. Therefore, the oscillation frequency can be adjusted by feeding back this time average <ΔV> to the transmitting laser.

In the case of this example, the reference level V ₀ of the error signal ΔV varies depending on the mark rate. Therefore, the output of the low-pass filter 25 is input to the differential amplifier 22, and the value proportional to the mark ratio is subtracted from the error signal ΔV.

As concrete numerical values of the above conditions, the channel frequency interval Δf = 10 GHz, the frequency interval ΔF = 2 of the optical discriminator 16.
GHz, frequency shift of optical FSK modulated signal f _m = 2 GHz,
The transmission bit rate per channel is 600 Mb / s, and frequency stabilization can be performed in optical frequency division multiplex transmission with 100 or more channels.

10 to 12 are views for explaining the principle of detecting an error signal by the optical discriminator 16 as in FIGS. 4 to 6 and FIGS. 7 to 9. However, in this example, the frequency interval ΔF of the optical discriminator 16 and the frequency deviation f of the optical FSK modulation signal are
_{The case} where the relationship with _m is ΔF> f _m and the center frequency f ₀ ′ of the optical FSK modulated signal is matched with the frequency f _{0 at} which the transmittance of the optical discriminator 16 becomes 1/2 is shown.

FIG. 10 shows that the center frequency f ₀ ′ of the optical FSK modulated signal is the optical discriminator 1
The case where the frequency is equal to the frequency f ₀ of the discrimination characteristic point of 6 is shown, FIG. 11 shows the case of f ₀ ′ = f ₀ + δf, and FIG. 12 shows f ₀ ′ = f ₀ −δ
The case of f is shown. However, δf represents an offset frequency from f ₀ , and its value is positive.

10 to 12, (a) of each figure shows the relationship between the frequency deviation f _m and the transmittance of the optical discriminator 16,
(B) is the intensity of the light emitted from one emission port p _a ,
(C) is the detection current of the light receiving element Pd _a , (d) is the light receiving element Pd _b
, (E) shows the error signal ΔV, and (f) shows the time average <ΔV> of the error signal ΔV. Further, in these figures, τ represents the period of the optical FSK modulation signal on the time axis.

In FIG. 10 (a), FIG. 11 (a), and FIG. 12 (a),
The solid line shows the transmittance from the entrance port of the optical discriminator 16 to the exit port p _a , and the broken line shows the transmittance from the exit port p _b .

Here, the case where the waveform of the optical FSK modulation signal is a perfect rectangle is shown.

As shown in FIG. 10, the center frequency f ₀ ′ of the optical FSK modulated signal is
Is coincident with the frequency f ₀ of the discrimination characteristic point, the light intensities at the two emission ports p _a and p _b change complementarily corresponding to the signal level. Therefore, the detection currents i _a and i _{b of} the light receiving elements Pd _a and Pd _b are also complementary, and the polarity of the error signal ΔV changes according to the signal level of the optical FSK modulation signal.
However, the time average of the smoothed error signal ΔV <Δ
The value of V> becomes zero.

The center frequency f ₀ ′ of the optical FSK modulated signal is the frequency f at the discrimination characteristic point.
_When it is deviated from ₀ to δf on the high frequency side, the light intensities at the two emission ports p _a and p _b deviate from the complementary state, as shown in FIG. Therefore, the polarity of <ΔV> becomes positive.

On the contrary, when the center frequency f ₀ ′ of the optical FSK modulated signal deviates from the frequency f ₀ of the discrimination characteristic point by δf to the low frequency side, the polarity of <ΔV> becomes negative as shown in FIG.

Thus, by obtaining the time average <ΔV> of the error signal ΔV, it is possible to determine from the polarity whether the center frequency f ₀ ′ of the optical FSK modulation signal is shifted to the high frequency side or the low frequency side. Therefore, the oscillation frequency can be adjusted by feeding back this time average <ΔV> to the transmitting laser.

Also in this example, the reference level V ₀ of the error signal ΔV changes depending on the mark rate. Therefore, the output of the low-pass filter 25 is input to the differential amplifier 22, and the value proportional to the mark ratio is subtracted from the error signal ΔV.

As concrete numerical values of the above conditions, the channel frequency interval Δf = 10 GHz, the frequency interval ΔF = 5 of the optical discriminator 16.
GHz, frequency shift of optical FSK modulated signal f _m = 2 GHz,
The transmission bit rate per channel is 600 Mb / s, and frequency stabilization can be performed in optical frequency division multiplex transmission with 100 or more channels.

FIG. 13 is a block configuration diagram of an optical frequency stabilizer according to a second embodiment of the present invention.

This embodiment differs from the first embodiment in that a frequency shift is detected by determining the phase of the error signal ΔV.

That is, the k-th optical FS by the channel selection filter
At the same time as selecting the K modulation signal, use the 1xn switch 24
The second information signal and use this as the reference signal for the multiplier
Enter 31. The multiplier 31 further includes a differential amplifier 22.
The error signal ΔV is input from. The synchronous detection output <ΔV> obtained at the output of the multiplier 31 is converted to the low pass filter 32,
The bias current source b _k is supplied via the amplifier 33 and the 1 × n switch 26.

When performing synchronous detection, the reference level V ₀ of the synchronous detection output <ΔV> does not fluctuate depending on the mark rate of the optical FSK modulation signal. Therefore, a value proportional to the mark rate is used as the differential amplifier 22.
You don't have to type in.

In operation, the control device 27 controls the channel selection filter control circuit 28 to set the k-th channel to pass through the channel selection filter 14. At the same time, the 1 × n switch 24 connects the k-th information signal as a reference signal to the multiplier 31, and the 1 × n switch 26
By connecting the output of the amplifier 33 to the k th bias current source b _k. In this state, the center frequency of the optical FSK modulation signal transmitted by the _kth transmission laser LD _k is made to coincide with the frequency f ₀ of one discrimination characteristic point of the optical discriminator 16. Repeat this operation.

Since the discrimination characteristic of the optical discriminator 16 changes periodically with respect to the incident light frequency, the transmission laser is transmitted to each discrimination characteristic point.
When the center frequencies of the optical FSK modulated signals transmitted by LD _{1 to} LD _n are made to coincide with each other, the respective center frequencies are stabilized at equal frequency intervals. When the frequency interval from the maximum value to the minimum value of the transmittance of the optical discriminator 16 is ΔF, the center frequency interval Δf of the optical FSK modulation signal is set to p times ΔF. However, p is an integer of 2 or more.

14 to 16 are views for explaining the principle of error signal detection by synchronous detection.

FIG. 14 shows the case where the center frequency f ₀ ′ of the optical FSK modulated signal is equal to the frequency f ₀ of the discrimination characteristic point of the optical discriminator 16 to be stabilized, and FIG. 15 shows f ₀ ′ = f ₀ Shows the case of + δf,
FIG. 16 shows the case of f ₀ ′ = f ₀ −δf. However, δf is f ₀
Represents the offset frequency from and its value is positive.

14 to 16, (a) of each figure shows the relationship between the frequency deviation f _m and the transmittance of the optical discriminator 16,
(B) is the intensity of the light emitted from one emission port p _a ,
(C) is the detection current of the light receiving element Pd _a , (d) is the light receiving element Pd _b
, (E) shows the error signal ΔV, and (f) shows the time average <ΔV> of the error signal ΔV. Further, in these figures, τ represents the period of the optical FSK modulation signal on the time axis.

14 (a), 15 (a) and 16 (a),
The solid line shows the transmittance from the entrance port of the optical discriminator 16 to the exit port p _a , and the broken line shows the transmittance from the exit port p _b .

The following discussion is valid even when the waveform of the optical FSK modulated signal is not rectangular, but for simplicity, it is assumed that the waveform is perfectly rectangular.

Further, a case will be described in which the frequency f ₀ of the discrimination characteristic point to be stabilized is the frequency at which the transmittance to the emission port p _a becomes minimum. In addition, the frequency interval ΔF of the optical separator 16 and the optical FS
The frequency deviation fm of the K modulation signal is ΔF = fm.

As shown in FIG. 14, the center frequency f ₀ ′ of the optical FSK modulated signal is
Is coincident with the frequency f ₀ of the discrimination characteristic point, the light intensities at the two emission ports p _a and p _b are substantially constant. Therefore, the detection currents i _a and i _{b of} the light receiving elements Pd _a and Pd _b are also constant, and the error signal ΔV does not include the information signal component. Therefore, the value of the synchronous detection output <ΔV> becomes zero.

The center frequency f ₀ ′ of the optical FSK modulated signal is the frequency f at the discrimination characteristic point.
_When it is shifted from ₀ to δf on the high frequency side, as shown in FIG. 15, the light intensity at the two emission ports p _a and p _b includes an information signal component, which is also included in the error signal ΔV. It will be. At this time, the waveform of the error signal ΔV has an opposite phase to the modulation signal. Therefore, the polarity of the synchronous detection output <ΔV> obtained by multiplying the error signal ΔV with the reference signal and smoothing the signal becomes negative.

On the contrary, when the center frequency f ₀ ′ of the optical FSK modulated signal is deviated from the frequency f _{0 at the} discrimination characteristic point to the low frequency side by δf,
As shown in FIG. 16, the waveform of the error signal is in phase with the modulation signal. Therefore, the polarity of the synchronous detection output <ΔV> obtained by multiplying the error signal ΔV with the reference signal and smoothing is positive.

That is, it is possible to determine whether the center frequency of the optical FSK modulation signal is shifted to the high frequency side or the low frequency side based on the polarity of the synchronous detection output <ΔV>.

The frequency f _{0 of the} discrimination characteristic point to be stabilized is the output port p _a
In the case of the maximum point of the transmittance of the optical FSK modulation signal, the relationship between the deviation direction of the center frequency of the optical FSK modulation signal and the polarity of the synchronous detection output <ΔV> is only reversed.

As concrete numerical values of the above conditions, the channel frequency interval Δf = 10 GHz, the frequency interval ΔF = 5 of the optical discriminator 16.
GHz, frequency shift of optical FSK modulated signal f _m = 2 GHz,
The transmission bit rate per channel is 600 Mb / s, and frequency stabilization can be performed in optical frequency division multiplex transmission with 100 or more channels.

Further, in the above description, the frequency interval ΔF of the optical discriminator 16 and the frequency deviation fm of the optical FSK modulated signal are assumed to be equal. However, even when ΔF ≠ fm, the detection currents of the light receiving elements Pd _a and Pd _b
Similarly, the optical frequency can be stabilized only by changing the reference levels of i _a and i _b .

Furthermore, it is possible to perform synchronous detection using only the light intensity of one emission port as a reference.

In the above embodiments, the channel selection filter 14 is described as an example in which periodic filters are connected in multiple stages.
The present invention can be similarly implemented by using a tunable Fabry-Perot resonator or a tunable diffraction grating filter as long as the full width at half maximum B of the transmittance satisfies the condition of B> f _m . However,
In the case of using a Fabry-Perot resonator, the resonance frequency intervals Δf × (n-1) + f Nakere must greater than _m. However, Δf is the center frequency interval of the optical FSK modulation signal, and n is the number of channels.

The present invention can be similarly implemented by using a tunable Fabry-Perot resonator whose full width at half maximum of transmittance is B as the optical discriminator 16.

In the above description, the case where a binary signal is used as the optical FSK modulation signal has been described, but the present invention can be similarly implemented in the case of a multilevel FSK modulation signal.

〔The invention's effect〕

As described above, the optical frequency stabilizing device of the present invention,
It is possible to switch channels to be stabilized at a high speed by using a frequency selective filter without using a sweep laser diode, and the number of channels is relaxed as compared with the conventional one. This allows, for example, a transmission bit rate of 600M
Optical F with b / s, channel spacing of 10 GHz, and number of channels of 100 or more
For SK modulated signals, each center frequency interval can be stabilized by one device.

INDUSTRIAL APPLICABILITY The present invention is capable of frequency stabilization for a large number of channels and has an effect of increasing the number of channels of an optical FSK signal to be frequency-multiplexed.

[Brief description of drawings]

FIG. 1 is a block configuration diagram showing a basic configuration of an optical frequency stabilizing device of the present invention. FIG. 2 is a block diagram of the optical frequency stabilizing device according to the first embodiment of the present invention. FIG. 3 is a diagram showing the relationship between the transmittance of the light discriminator and the difference in the light-receiving current between the two light-receiving elements. FIG. 4 is a principle diagram of error signal detection by an optical discriminator. FIG. 5 is a principle diagram of error signal detection by an optical discriminator. FIG. 6 is a principle diagram of error signal detection by an optical discriminator. FIG. 7 is a principle diagram of error signal detection by an optical discriminator. FIG. 8 is a principle diagram of error signal detection by an optical discriminator. FIG. 9 is a principle diagram of error signal detection by an optical discriminator. Figure 10 shows the principle of error signal detection by the optical discriminator. Fig. 11 shows the principle of error signal detection by the optical discriminator. Figure 12 shows the principle of error signal detection by the optical discriminator. FIG. 13 is a block configuration diagram of an optical frequency stabilizer according to a second embodiment of the present invention. Figure 14 shows the principle of error signal detection by the optical discriminator. Figure 15 shows the principle of error signal detection by the optical discriminator. Fig. 16 shows the principle of error signal detection by the optical discriminator. FIG. 17 is a block diagram of a conventional optical frequency stabilizer. 1 ... Swept laser diode, 2 ... Optical resonator, 3,
4 ... Light receiving element, 5 ... Control circuit, 6 ... Star coupler, 7 ... Directional coupler, 11-1 to 11-n ... Adder,
12 ... Multiplexer, 13 ... Fiber, 14 ... Channel selection filter, 16 ... Optical discriminator, 17 ... Load resistance, 19 ...
… Error signal detector, 21 …… Bias current group, 22 …… Differential amplifier, 24,26 …… 1 × n switch, 25,32 …… Low pass filter, 27 …… Control device, 28 …… Channel Selective filter control circuit, 31 ... Multiplier, 33 ... Amplifier, S _{1 to} S _n
...... Information signal source, LD _{1 to} LD _n …… Transmitting laser, Pd _a , Pd _b ….
…Light receiving element.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsushi Iwashita 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corp. (56) Reference JP-A-2-39131 (JP, A)

Claims (1)

[Claims] 1. A plurality of transmitting lasers for transmitting optical FSK modulated signals at mutually different center frequencies, a light receiving element for detecting output light of the plurality of transmitting lasers, and the plurality of transmitting lasers by detection outputs of the light receiving elements. In the optical frequency stabilizer including the transmission lasers and the control means for performing negative feedback control of the oscillation frequency so that the center frequency interval of the transmission light FSK modulated signal becomes a predetermined value, In the path leading to the element, a selection means for selecting the transmission light FSK modulation signal of any of the above-mentioned transmission lasers and the output light of this selection means are input, and the transmittance changes periodically with the frequency of the incident light. An optical frequency stabilizing device, comprising: an optical filter whose period is equal to the center frequency interval.