JPH05122261A - Optical multiplex signal demultiplexer - Google Patents

Abstract

PURPOSE:To realize the optical multiplex signal demultiplexer which stable demultiplexes a desired optical frequency from an optical signal subject to frequency multiplexing with less number of components. CONSTITUTION:A bipolar signal Si* is generated from an electric signal by receiving an optical signal through a variable tuning optical branching device 1 and the signal Si* is used to apply synchronization detection to the signal Si thereby obtaining an error signal Ve1. Moreover, a transmission frequency through the filter in the variable tuning optical branching device 1 is periodically varied and the error signal Ve1 is again subject to synchronization detection at the period to obtain an error signal Ve3. The error signal Ve3 is fed back to the filter. A difference between a frequency of the desired optical signal and a frequency at which the filter transmission rate is maximized is detected without being affected by other channel by using the signal Si* to apply synchronization detection to the signal Si. Furthermore, the direction of the frequency difference is detected without being affected by other filter as to each filter by applying 2nd synchronization detection to the error signal.

Description

Detailed Description of the Invention

[0001]

The present invention is used in optical frequency division multiplexing transmission. In particular, it relates to separation of multiplexed optical signals. More specifically, it relates to stabilization of a tunable optical demultiplexer using a Mach-Zehnder type filter.

[0002]

2. Description of the Related Art A tunable optical demultiplexer using a Mach-Zehnder type filter has been conventionally known as an element for selecting and transmitting a specific channel from a frequency-multiplexed optical signal. Here, before describing the tunable optical demultiplexer, the Mach-Zehnder type filter that is a component thereof will be described.

FIG. 13 is a diagram showing a structural example of a Mach-Zehnder type filter.

This Mach-Zehnder type filter has a substrate 13
0 Two optical input terminals 131a, 131b on top, 3 dB for directional coupling type
Coupler 132a, two optical waveguides 133a, 13 having different optical path lengths
3b, a directional coupling type 3dB coupler 132b and two optical output terminals 134a, 134b. Light incident on the optical input terminal 131a or 131b is divided into two by the directional coupling type 3dB coupler 132a, and each is an optical waveguide. After passing through 133a and 133b, they are multiplexed by a directional coupling type 3dB coupler 132b and output from at least one of the optical output terminals 134a and 134b. A heater electrode 135 is provided in the optical waveguide 133a in order to change its equivalent optical path length by heat.

FIG. 14 shows an example of transmittance characteristics of a Mach-Zehnder filter. Here, the transmittance from the light input terminal 131a to the light output terminal 134a is shown by a solid line, and the transmittance to the light output terminal 134b is shown by a broken line.

The transmittance T of the Mach-Zehnder type filter has a periodicity with respect to frequency, and the frequency interval Δ of the half period thereof.
f is the two optical waveguides 133 between the 3 dB couplers 132a and 132b.
There is a relationship of Δf = c / 2nΔL (1) with respect to the optical path length difference ΔL of a and 133b. Here, c is the speed of light in vacuum, and n is the effective refractive index of the optical waveguides 133a and 133b. The frequency dependence of the transmittance T is T (terminal 131a → 134a) = cos ² (πnΔL / c × fopt) (2) T (terminal 131a → 134b) = sin ² (πnΔL / c × fopt) It is expressed as (3). Where fopt is the optical frequency.

Therefore, the frequency interval is equal to Δf,
When a light wave including two components of frequencies f1 and f2 at which the transmittance T expressed by the equations (2) and (3) becomes maximum is input to the optical input terminal 131a, the two components are separated and the optical output A light wave of frequency f1 is obtained at the terminal 134a, and the light output terminal 13
A light wave of frequency f2 is obtained at 4b. At this time, one optical output terminal can be used as a signal port for obtaining a desired optical signal, and the other can be used as a monitor port for monitoring.

FIG. 15 shows an example of the configuration of a tunable optical demultiplexer using a Mach-Zehnder type filter. Here, a case is shown in which an optical signal of frequency f4 is extracted from a group of optical signals of frequencies f1 to f8 multiplexed at frequency intervals Δf. This tunable optical demultiplexer has three Mach-Zehnder filters F1, F2, F3 connected in series, and the half-cycle intervals of these three Mach-Zehnder filters F1, F2, F3 are set to Δf, 2Δf, and 4Δf, respectively. To be done.

FIG. 16 is a diagram for explaining the operating principle of the tunable optical demultiplexer shown in FIG. 15, where (a) is the frequency distribution of a signal group multiplexed at frequency intervals Δf, and (b) is a Mach-Zehnder type. Frequency characteristic of transmittance of the signal port of the filter F1,
(c) is the frequency distribution of the optical signal group transmitted to the signal port of the Mach-Zehnder filter F1, (d) is the frequency characteristic of the transmittance with respect to the signal port of the Mach-Zehnder filter F2, and (e) is the signal port of the Mach-Zehnder filter F2. Shows the frequency distribution of the optical signal group transmitted to the optical port, (f) shows the frequency characteristic of the transmittance with respect to the signal port of the Mach-Zehnder filter F3, and (g) shows the frequency of the optical signal transmitted to the signal port of the Mach-Zehnder filter F3. In this figure, the frequency at which the transmittance of each of the Mach-Zehnder filters F1 to F3 with respect to the signal port becomes maximum is represented by ftop.

When the optical signal group of frequencies f1 to f8 shown in FIG. 16 (a) is input to the input terminal of the Mach-Zehnder filter F1, the optical signal group of frequencies f2, f4, f6 and f8 at which the transmittance becomes "1". Is output to the input terminal of the next stage Mach-Zehnder filter F2. On the other hand, the optical signal groups of frequencies f1, f3, f5 and f7 at which the transmittance is "0" are output from the monitor port. The Mach-Zehnder filter F2 has frequencies f2, f4, f6 and f8.
The optical signal group of 1 is demultiplexed and the optical signal group of frequencies f2 and f8 is output to the signal port. Mach-Zehnder type filter F3
Separates the optical signal groups of frequencies f2 and f8,
Only the optical signal of is output to the signal port.

Thus, an optical demultiplexer can be realized by connecting the Mach-Zehnder type filters as shown in FIG. 13 in multiple stages. By changing the current flowing through the heater electrode of each Mach-Zehnder filter, the frequency ftop at which the transmittance of each filter becomes maximum can be changed, and one optical signal can be selected from the optical signal group. You can
In this case, the number of selectable optical signals is 2 ^N channels, where N is the number of filter connection stages.

FIG. 17 is a block diagram showing a conventional optical frequency division transmission device using a tunable optical demultiplexer. Here, an example of 128 multiplexing will be described.

Laser diodes LD1 to LD are provided on the transmitting side.
128 and an optical multiplexing circuit 171 are provided, and data signals S1 to S
128 is entered. Laser diode LD1 to LD128
Are modulated with respective data signals S1, S2, ..., S128, and 128 optical FSK modulated signals f1, f2 ,.
Is obtained. These optical FSK modulated signals f1, f2,
The center frequencies fc1, fc2, ..., Fc128 of f128
Is stabilized at a predetermined frequency interval Δf. The optical FSK modulated signals f1, f2, ..., F128 are frequency-multiplexed by an optical multiplexing circuit 171 and sent to an optical fiber 172.

On the receiving side, a tunable optical demultiplexer 173, an optical frequency discriminator 174, a balanced type light receiving element 175, an amplifier 176,
The tunable optical demultiplexer stabilization circuit 177 and the optical frequency discriminator stabilization circuit 178 are provided, and the 128-channel optical FSK modulated signal transmitted by the optical fiber 172 is tunable optical demultiplexer. Input to the container 173. The tunable optical demultiplexer 173 is composed of a seven-stage Mach-Zehnder type filter in this example, and selects one of the center frequencies fc1, fc2, ..., fc128 by an external setting,
Most of the optical power of the signal is supplied to the optical frequency discriminator 174. The optical frequency discriminator 174 is also composed of a Mach-Zehnder type filter and discriminates the optical frequency and supplies it to the balanced type light receiving element 175. This demodulates the data signal, which is output via the amplifier 176.

In the optical frequency discriminator 174, the center frequency fci of the optical FSK modulated signal to be received by the optical frequency discriminator stabilizing circuit 178 and the transmittance characteristic at the two output terminals of the optical frequency discriminator 174 intersect. The frequency is controlled to match. As a result, when the optical FSK modulation signal is converted into a light intensity modulation signal, the amplitude of the light intensity modulation signal is stabilized to be maximum.

The transmission frequency of the tunable optical demultiplexer 173 is set by the tunable optical demultiplexer stabilizing circuit 177. That is, when the channel information CH-i to be selected is input from the outside, the central processing unit 1771 in the tunable optical demultiplexer stabilizing circuit 177, according to the bias information previously stored in the read-only memory 1772, Initial bias currents I1 to I7 are applied to the heater electrodes of the filters in the tunable optical demultiplexer 173 via the D / A converter 1773. As a result, most of the optical power of the optical FSK modulated signal to be selected is supplied to the optical frequency discriminator 174, and the data signal Si is demodulated. The bias current value of the heater electrode is A / D converter 17
Monitored by 74 and sent to central processing unit 1771.

The tunable optical demultiplexer stabilizing circuit 177 also operates the tunable optical demultiplexer 17 in order to operate the system stably for a long period of time.
The frequency ftop at which the transmissivity of each Mach-Zehnder type filter forming 3 is maximized is controlled to match the center frequency fci of the optical FSK modulated signal to be received. Therefore, conventionally, it has been utilized that a part of the optical FSK modulation signal to be received is output from the monitor port of each filter as an intensity modulation component.

That is, the light receiving elements PD1 to PD7 are provided at the respective monitor ports of the seven Mach-Zehnder type filters to receive the light intensity modulated signal, and the light intensity modulated signal is synchronously detected by the demodulated data signal. Specifically, the demodulated data signal Si is branched into seven, and phase adjusters PS1 to PS7 are respectively provided.
To the mixers MIX1 to M, and the outputs of the phase adjusters PS1 to PS7 and the outputs of the light receiving elements PD1 to PD7, respectively.
Multiply by IX7, each low pass filter LP
Pass through F1 to LPF7. Thereby, the error signal is detected. These error signals are substantially proportional to the frequency difference fd between the center frequency fci of the optical FSK modulated signal to be received and ftop of each Mach-Zehnder filter. The sign also corresponds to the polarity to be returned.
Therefore, these error signals are amplified by the amplifiers A1 to A7 having an appropriate amplification factor and added to the bias currents I1 to I7 of each filter by the adders ADD1 to ADD7. As a result, negative feedback to the bias current applied to each heater electrode is performed, and the frequency ftop at which the transmittance characteristic of the tunable optical demultiplexer 173 becomes maximum is the center frequency fci of the optical FSK modulation signal.
Is kept equal to.

However, if feedback is applied before the data signal Si is stably demodulated, the demultiplexing characteristic of the tunable optical demultiplexer 173 becomes unstable. Therefore, the switch 1775 controls the start time of feedback.

With this configuration, the optical signal intensity of the desired channel input to the optical frequency discriminator 174 can be kept to the maximum, the crosstalk optical power from other channels can be kept to the minimum, and the receiving sensitivity can be kept to the maximum. be able to. The details of this are shown in Oda et al., March 1990, Proceedings of IEICE Proceedings, OCS89-65, page 21.

FIG. 18 is a diagram showing a part of the optical FSK modulated signal to be received is output as a light intensity modulated signal to the monitor port of the Mach-Zehnder filter. Here, the half-cycle interval of the transmittance characteristic of the Mach-Zehnder filter is Δf × 2 ^X (X = 1, 2, ..., 7), and the optical FS
It is assumed that the frequency shift of the K modulation signal is ΔF.

The center frequency f is applied to the Mach-Zehnder type filter.
When the optical FSK modulated signal of ci is input and the frequency difference fd = ftop-fci from the frequency ftop is smaller than the frequency deviation ΔF, both the mark and the space of the optical FSK modulated signal have a transmittance to the monitor port. It is small, and the amplitude of the light intensity modulation signal that appears at the monitor port is small. Completely f
When d = 0, this amplitude becomes the minimum. Further, since the transmittance deviates from zero for both the frequency of the mark and the space, the bit rate of the light intensity modulation signal is twice the bit rate of the actual optical FSK modulation signal. On the other hand, when the frequency difference fd is larger than the frequency deviation ΔF, the light intensity modulation signal is obtained at the same bit rate as that of the optical FSK modulation signal in almost proportion to the frequency difference fd. Also, f
In the case of d> 0 and the case of fd <0, the output phases are opposite to each other.

That is, if the light intensity modulation signal is synchronously detected by its own data signal, (1) the optical F at another frequency is detected.
Since the SK signal component has no phase relationship with the demodulated data signal, the signal component is removed, and only the error signal of the desired channel can be detected, and (2) the frequency difference fd is detected by the amplitude of the light intensity modulation signal. (3) The sign of the frequency difference fd can be detected by the amplitude of the synchronous detection output.

Therefore, for each of the Mach-Zehnder type filters constituting the tunable optical demultiplexer, the light intensity modulation signal at the monitor port is detected, and the value is used to obtain a negative value suitable for the bias current value flowing to the heater electrode of the filter. By applying the feedback, the frequency ftop at which the transmittance becomes maximum for each filter can be matched with the center frequency fci of the optical FSK modulated signal to be received. Optical FSK
If the mark ratio of the modulation signal is not 1/2, the error signal output may be corrected. For that purpose, an offset adjusting circuit according to the mark ratio is added.

[0025]

However, the conventional configuration has a problem in that the number of components of the light receiving element and the high frequency circuit required for stabilization increases. An object of the present invention is to solve such a problem and to provide an optical multiplex signal demultiplexer capable of stably demultiplexing a desired optical frequency with a small number of parts.

[0026]

The optical multiplex signal demultiplexing apparatus of the present invention regards an electric signal generated corresponding to an optical signal transmitted through a tunable optical demultiplexer as a binary signal, and outputs one of the two values. For generating a bipolar signal in which "1" is assigned to the other value and "-1" to the other value, a synchronous detecting means for synchronously detecting the electric signal by the bipolar signal, and a synchronous detecting means. And a feedback means for returning the output of the above to the tunable optical demultiplexer.

The feedback means is a low frequency oscillator which oscillates at a frequency sufficiently lower than the data rate of the optical signal to be transmitted by the tunable optical demultiplexer, and the tunable optical demultiplexer by the output of this low frequency oscillator. Means for periodically changing the transmission frequency of one of the one or more filters whose transmission characteristics change with frequency, and the output of the synchronous detection means for synchronous detection with the output of the low-frequency oscillator It is desirable to include means and means for controlling the transmitted light frequency of the one filter by the output of the means for synchronous detection. The filter is preferably a Mach-Zehnder type filter.

The feedback means can be common to one or more filters and can be switched to select individual filters. Further, the feedback means may be separately provided for each filter.

[0029]

The frequency ftop at which the transmittance of the tunable optical demultiplexer that periodically changes with respect to the frequency and the maximum transmittance of the filter, the frequency of the optical signal, and the case where the optical signal is FSK modulated When the center frequency fci is equal, the intensity of the optical signal transmitted through the tunable optical demultiplexer becomes maximum. However, when the frequency ftop does not match the frequency of the optical signal, the intensity of the transmitted optical signal decreases. If the optical signal transmitted through the tunable optical demultiplexer is the signal of only one channel, it is possible to adjust the frequency ftop based on the intensity of the transmitted optical signal. However, in reality, the optical signal components of other channels are included.

Therefore, a binary bipolar signal is generated from the demodulated data signal, and the data signal demodulated by the bipolar signal is synchronously detected. At this time, since the optical signal components other than the desired channel have no phase correlation with the optical signal of the channel, the time average of the output becomes zero. Therefore, the frequency fto that maximizes the transmittance of the filter is not affected by other channels due to the coherent detection output.
The difference between p and the frequency of the desired optical signal can be detected.

However, the positional relationship between ftop and the desired optical frequency is not known by this alone. Therefore, the transmission frequency of the filter is periodically changed at a low frequency. If this is synchronously detected at the same low frequency, the positional relationship between ftop and the desired optical frequency can be detected. Further, if different low frequencies are used for the individual filters, the positional relationship of the frequencies can be detected without being affected by other filters.

[0032]

1 is a block diagram showing the configuration of an optical multiplex signal demultiplexer according to a first embodiment of the present invention.

This apparatus is provided with a tunable optical demultiplexer 1 to which a frequency-multiplexed optical signal group is input and which selects an optical signal of a desired frequency from the signal group and transmits the selected optical signal. A light receiving circuit 2 and an amplifier 3 are provided as opto-electric conversion means for generating an electric signal corresponding to an optical signal transmitted through the optical multiplexer 1. The output of the amplifier 3 is used to change the transmitted optical frequency of the tunable optical demultiplexer 1. As adjusting means for adjusting, a high frequency synchronous detection circuit 4, a low frequency synchronous detection circuit 5, a switch 6 and an adder
10-1 to 10-x. Further, in order to set the transmission frequency of the tunable optical demultiplexer 1, a central processing unit 7, a read-only memory 8 and a D / A converter 9 are provided.

The feature of this embodiment is that the high-frequency synchronous detection circuit 4 regards the electric signal output from the amplifier 3 as a binary signal and sets one value to "1" and the other value to "1". On the other hand, a demodulation circuit 42 for generating a bipolar signal to which "-1" is assigned, and a phase adjustment circuit 41, a mixer 43 and a low frequency band as a synchronous detection means for synchronously detecting the electric signal from the amplifier 3 by the bipolar signal. The low-frequency synchronous detection circuit 5, the switch 6, and the adders 10-1 to 10
This is because -x constitutes feedback means for returning the output of the low-pass filter 44 to the tunable optical demultiplexer 1.

The low frequency synchronous detection circuit 5 includes a low frequency oscillator 51 which oscillates at a frequency sufficiently lower than the data rate of the optical signal to be transmitted by the tunable optical demultiplexer 1, and the low frequency oscillator 51. The output of the low-frequency oscillator 51 passes through the attenuator 52, the adder 57, and the switch 6 as a means for periodically changing the transmission frequency of the filter forming the tunable optical demultiplexer 1 by the output. The phase adjustment circuit 53, the mixer 54, the low-pass filter 55 and the amplifier 56 are provided as means for synchronously detecting the output of the high frequency synchronous detection circuit 4 with the output of the low frequency oscillator 51. The output of the amplifier 56 is connected to the filter of the tunable optical demultiplexer 1 via the adder 57 and the switch 6 as a means for controlling the transmitted light frequency of one filter by the output of the means.

When the channel information CH-i to be selected from the outside is inputted, the central processing unit 7 follows the bias information stored in the read-only memory 8 in advance, and then the D / A converter 9 and the adder 10 The initial bias values I1 to Ix are supplied to the respective heater electrodes of the Mach-Zehnder type filter constituting the tunable optical demultiplexer 1 via -1 to 10-x. As a result, most of the optical power of the optical modulation signal to be selected is supplied to the light receiving circuit 2.

The light receiving circuit 2 converts the inputted optical signal into an electric signal. This electric signal is amplified by the amplifier 3 and supplied to the high frequency synchronous detection circuit 4. This amplified electric signal is hereinafter referred to as "data signal". Further, it is represented by Si in the meaning of the data signal of the i-th channel.

Here, a case where the optical signal is an FSK modulation signal will be described as an example.

Although the data signal Si input to the high frequency synchronous detection circuit 4 can be regarded as a binary signal substantially, it has a frequency ftop at which the transmittance of the Mach-Zehnder type filter constituting the tunable optical demultiplexer 1 becomes maximum. When the center frequency fci of the optical FSK modulated signal does not match, the value is different from the predetermined value. It also includes signal components of other channels. Therefore, the data signal Si is demodulated by the demodulation circuit 42 to generate a bipolar signal Si ^*, synchronous detection of the data signal Si by using the bipolar signal Si ^*. Specifically, the bipolar signal Si ^* and the phase adjustment circuit 41
The data signal Si passed through is mixed by the mixer 43, and the output is passed to the low-pass filter 44. The synchronous detection output thus obtained has a value corresponding to the positional relationship between the frequency ftop and the center frequency fci. This is hereinafter referred to as "error signal Ve1". This error signal Ve1 is fed back to the tunable optical demultiplexer 1 via the low frequency synchronous detection circuit 5.

The low frequency synchronous detection circuit 5 is a circuit for specifying the Mach-Zehnder type filter to be adjusted. That is, the output of the low frequency oscillator 51 is output via the attenuator 52 and the adder 57. This output is supplied to any of the adders 10-1 to 10-x via the switch 6, added to any of the initial bias values I1 to Ix, and supplied to the heater electrode of one Mach-Zehnder filter. .. As a result, the transmission frequency of the filter changes, and the change is input to the low frequency synchronous detection circuit 5 through the light receiving circuit 2, the amplifier 3 and the high frequency synchronous detection circuit 4. In the low frequency synchronous detection circuit 5, the output of the high frequency synchronous detection circuit 4 is synchronously detected by the output of the low frequency oscillator 51 via the phase adjusting circuit 53 using the mixer 54 and the low pass filter 55.
This synchronous detection output is output via the amplifier 56, added by the adder 57 to the output of the low-frequency oscillator 51 via the attenuator 52, and either the switch 6 or the adders 10-1 to 10-x. Is supplied to the heater electrode of the corresponding Mach-Zehnder type filter via.

2 to 4 show a high frequency synchronous detection circuit 4
3 is a diagram for explaining the relationship between the output of FIG. 1 and the frequency ftop at which the transmittance of the Mach-Zehnder filter is maximum. In these figures, the optical modulation method is binary FSK, and the number x of Mach-Zehnder filters in the tunable optical demultiplexer 1 is x = 1.
I am assuming. Further, in these figures, Tofd: transmittance characteristic of the frequency discriminator in the light receiving circuit 2, T: transmittance characteristic of the Mach-Zehnder filter fci: center frequency of optical signal fm: optical frequency corresponding to optical signal mark fs: Optical frequency corresponding to the space of the optical signal ftop: Frequency at which the transmittance of the Mach-Zehnder filter is maximum fd: Frequency difference between ftop and fci Δτ: Time corresponding to one bit of the modulation signal represented by the reciprocal of the bit rate Is. Further, it is assumed that the code is "1" when the instantaneous frequency of light corresponds to the mark and "0" when it corresponds to the space.

FIG. 2 shows the case where fd = 0. In this case, since the values of the transmittance T of the filter for fs and fm are the same, the amplitudes of the light intensity modulation signals output from the optical frequency discriminator are the same. Letting Ia be the amplitude of the received signal current at the time of marking and Ib the received signal current at the time of space, | Ia | = | Ib | Therefore, when this received signal output and the demodulated bipolar data signal Si ^* are mixed and passed through the low-pass filter 44, the output Ve1 is a constant value P1.
Take

FIG. 3 shows that ftop has a higher ftop than fci.
The output characteristic of the high frequency synchronous detection circuit 4 in the case of = + fo is shown. The other conditions are the same as in the case of FIG. In the case of FIG. 3, since the value of the transmittance T of the filter with respect to fm is higher than the value with respect to fs, the amplitude of the light intensity modulation signal output from the optical frequency discriminator is Ia> Ib. Also, | Ia
Since the value of | + | Ib | becomes smaller than that in the case of FIG.
When synchronous detection is performed, the output Ve1 is a value P2 smaller than P1.
Becomes

FIG. 4 shows that ftop has a lower ftop than fci.
The output characteristic of the high frequency synchronous detection circuit 4 in the case of = -fo is shown. In this case, since the value of the transmittance T of the filter for fm is lower than the value for fs, the amplitude of the light intensity modulation signal output from the optical frequency discriminator is Ia <Ib. Further, the value of | Ia | + | Ib | becomes smaller than that in the case of FIG. 2, and the absolute value of fd is the same as that in the case of FIG. 3. Therefore, when synchronous detection is performed, its output Ve1 is the same as that in the case of FIG. It becomes the same value P2.

FIG. 5 shows the transmittance characteristics of the Mach-Zehnder filter ftop fluctuated on the frequency axis using the output of the low-frequency oscillator 51 that oscillates at a low frequency much lower than the transmission bit rate. .. Here, the typical value of the transmission bit rate is about several hundred Mb / s to several Gb / s, and the oscillation frequency of the low frequency oscillator 51 is about several tens to several kHz. In FIG. 5, the state represents a state of fd = 0, the state represents a state of fd = fo, and the state represents a state of fd = -fo.

6 to 8 respectively show fd, error signal Ve1 and low frequency oscillator 51 when ftop is changed.
2 shows the time change of the error signal Ve2 multiplied by the output of Note that fd also fluctuates due to ftop fluctuations. 6 shows a case where the center frequency of fd is zero, FIG. 7 shows a case where the center frequency of fd is a negative value −fofs, and FIG. 8 shows a case where the center frequency of fd is a positive value fofs, and (a) shows fd. , (B) shows the characteristic of the error signal Ve1, and (c) shows the characteristic of the error signal Ve2. Moreover, in these figures, the values in each state in FIG. 5 are shown.

Referring to FIG. 6 (b), it can be seen that the frequency of the error signal Ve1 is twice the frequency modulated by the Mach-Zehnder filter. Therefore, the error signal Ve2 obtained by multiplying the error signal Ve1 by the output of the low-frequency oscillator 51 has the same waveform on the positive side and the negative side, as shown in FIG. 6 (c). Therefore, if the error signal Ve2 and the low-pass filter 55 are passed and the integrated value is detected, it becomes zero.

When the center frequency of fd is -fofs,
As shown in FIG. 7 (b), the frequency of the error signal Ve1 is the same as the oscillation frequency of the low frequency oscillator 51, and the phase relationship is the same. Therefore, the error signal Ve2 obtained by multiplying the error signal Ve1 by the output of the low frequency oscillator 51 has a zero value in the state shown in FIG. 5 and a negative value in the state shown in FIG. 7 (c). No waveform appears on the negative side. Therefore, when the integrated value is detected by passing through the low pass filter 55, the value becomes positive.

When the center frequency of fd is shifted to the positive side by fofs, as shown in FIG. 8 (b), the error signal Ve1
The frequency is the same as the oscillation frequency of the low-frequency oscillator 51, and the phase relationship is opposite. Therefore, the error signal Ve2 obtained by multiplying this error signal Ve1 by the output of the low-frequency oscillator 51 is
As shown in FIG. 8C, the state shown in FIG. 5 has a zero value and a positive value in the state, and no waveform appears on the positive side. Therefore, when the integrated value is detected by passing through the low pass filter 55, the value becomes negative.

FIG. 9 shows the relationship between the value of fofs and the error signal Ve3 obtained by integrating the error signal Ve2. When the center frequency fofs of the frequency difference fd = ftop-fci is zero, the error signal Ve3 becomes zero. This error signal Ve3
If a suitable negative feedback is applied to the bias current that controls the transmittance characteristic of the Mach-Zehnder filter, then f
Ofs can be zero. However, the point at which the error signal Ve3 becomes zero may be affected by another Mach-Zehnder filter and may drift to some extent. Even then,
If the same negative feedback is applied to the Mach-Zehnder filter that is the source of the drift, it is possible to control so that the drift at the zero point converges to zero.

In the above description, the case where the optical signal is the FSK modulation signal has been described as an example, but the intensity modulation method IM or ASK is used.
A modulation method can be similarly implemented. In that case, the frequency discriminator is not provided in the light receiving circuit 2, and the optical signal transmitted through the tunable optical demultiplexer 1 may be directly received.

FIG. 10 is a block diagram showing an optical multiplex signal demultiplexing apparatus according to the second embodiment of the present invention. This embodiment is basically the same as the first embodiment, but in order to monitor the bias current of each Mach-Zehnder type filter that constitutes the tunable optical demultiplexer 1, the A / D converters 11-1 to 11- are used. 7 is provided, an optical frequency discriminator 21 and a balanced type light receiving element 22 are provided as a light receiving circuit, and a load resistor 12 is connected to the output of the balanced type light receiving element 22 to stabilize the tunable optical demultiplexer. circuit
13 and that the output of the central processing unit 7 is D / A for setting the center frequency of the optical frequency discriminator 21.
It is different from the first embodiment in that it is supplied to the optical frequency discriminator 21 via the converter 9-8.

The outputs of the A / D converters 11-1 to 11-7 are sent to the central processing unit 7. A Mach-Zehnder filter is used as the optical frequency discriminator 21, and the heater electrode of this filter is provided with the initial bias current from the D / A converter 9-8 and the output of the tunable optical demultiplexer stabilizing circuit 13 It is added by 14 and supplied.

To operate this device,
The read-only memory 8 stores the bias current values to be applied to the respective electrodes of the tunable optical demultiplexer 1 and the optical frequency discriminator 21 for each channel. After that, when the channel designation signal CH-i is input, the central processing unit 7 extracts the bias information from the read-only memory 8 and D
/ A converters 9-1 to 9-8 through the initial bias values I1 to
I8 is supplied to each electrode of the tunable optical demultiplexer 1 and the optical frequency discriminator 21. Further, the central processing unit 7 monitors the bias current supplied to the tunable optical demultiplexer 1 via the A / D converters 11-1 to 11-7. As a result, the tuning frequencies of the tunable optical demultiplexer 1 and the optical frequency discriminator 21 substantially match the frequencies of the desired channels, so that among the optical signals of frequencies f1 to f128 transmitted by the optical fiber. Most of the optical power of the optical signal of the desired frequency fi passes through the tunable optical demultiplexer 1, and the optical signal power of other frequencies is almost removed. At this time, the low frequency oscillator 51
Is added to the bias current of the filter selected by the switch 6, and ftop of the filter is modulated at a low frequency.

The optical signal transmitted through the tunable optical demultiplexer 1 is subjected to FSK-ASK conversion by the optical frequency discriminator 21, and the two output ports of the optical frequency discriminator generate marks and spaces for the FSK modulated signal. The corresponding light intensity modulated signal is output. By inputting this light intensity modulation signal to the balanced type light receiving element 22, the currents Ia, Ia corresponding to the light intensity are input.
Since b flows through the load resistor 12, the optical signal can be demodulated into the electrical data signal Si. The data signal Si is further converted into a binary bipolar signal Si ^* by the demodulation circuit 42 in the high frequency synchronous detection circuit 4.

The optical modulation signal input to the optical frequency discriminator 21 is not only the optical signal of the desired channel (optical signal of frequency fi). Therefore, it is necessary to remove other optical signal components. Therefore, the demodulated binary bipolar signal S
i ^* is used to synchronously detect the data signal Si, and those having no phase correlation with the data signal Si are removed. As a result, the error signal Ve1 can be obtained by the high frequency synchronous detection circuit 4 for the optical signal of the frequency fi, and the transmission frequency of the tunable optical demultiplexer 1 can be adjusted.

Further, in order to remove the influence of the transmittance characteristics of the filters other than the Mach-Zehnder type filter which is going to be fed back, ftop of the Mach-Zehnder type filter is previously modulated with the output of the low frequency oscillator 51, and The error signal Ve1 output from the high frequency synchronous detection circuit 4 is synchronously detected by the output of the low frequency oscillator 51 used for modulation. Depending on whether the value of the low frequency synchronous detection output is positive or negative, it is possible to determine whether the sign of fofs is positive or negative, that is, whether the filter setting deviates from the frequency of the optical signal to be received. When fofs = 0, the frequency of the error signal Ve2 output from the mixer 54 is the frequency f output from the low-frequency oscillator 51.
Since it is twice as large as osc, the synchronous detection output passed through the low pass filter 55 and the amplifier 56 becomes zero.

Therefore, if this synchronous detection output is negatively fed back to the bias current of each filter as an error signal, fof
s can be zero.

By performing the above-described series of operations sequentially by switching the switch 6 by the central processing unit 7 at certain fixed intervals, the frequency ftop at which the transmittance of all the filters becomes maximum is the center of the optical signal. Stabilized to frequency fci.

FIG. 11 is a block diagram showing the configuration of an optical multiplex signal demultiplexer according to the third embodiment of the present invention. This embodiment differs from the second embodiment in that a feedback circuit is provided for each Mach-Zehnder type filter that constitutes the tunable optical demultiplexer 1 and that feedback is performed in synchronization.

That is, the output of the high frequency synchronous detection circuit is 7
The system is divided into low frequency synchronous detection circuits 5-1 ~
5-7 are connected and each output is simultaneously supplied to the heater electrode of each filter. In this case, the frequencies of the low-frequency oscillators used by the low-frequency synchronous detection circuits 5-1 to 5-7 are set to different values.

FIG. 12 shows the control band for stabilizing each Mach-Zehnder filter and the arrangement of the oscillation frequencies of each low-frequency oscillator. When the lower limit of the optical signal transmission band is ftmin, the control band for each filter is B0, and the number of stages of the Mach-Zehnder filter is x, the number of stages Ns of the filter that can be stabilized is Ns ≤ ftmin / 2B0 (4) Becomes For example, if ftmin = 200Hz and B0 = 10Hz,
The number of controllable filter stages is Ns ≦ 10. This Ns
The value of corresponds to a duplexer for 1024 waves.

The second embodiment is effective when the lower limit frequency ftmin of the transmission band is relatively high. Compared with the first embodiment, (1) there is an advantage that the system takes a short time to converge to a stable state because of simultaneous control, and (2) a feedback is always applied, which is strong against disturbance. However, it is inferior to the first embodiment in that (3) the bandwidth used for stabilization is wide, (4) the number of parts is large, and the system is complicated.

In the above description, an example in which a Mach-Zehnder type filter is used as a filter that constitutes the tunable optical demultiplexer has been described, but the present invention can be similarly implemented when another type of filter is used. For example, the transmittance of a Fabry-Perot filter or a diffraction grating changes periodically with respect to frequency, and these can be used to realize a tunable optical demultiplexer similar to that using a Mach-Zehnder filter. Even in such a case, the present invention can be similarly implemented.

[0065]

As described above, in the optical multiplex signal demultiplexer of the present invention, the output of the light receiving element for reception is used as it is for feedback to the tunable optical demultiplexer. The light receiving element required for each filter that constitutes the device is not required, and only one high frequency synchronous detection circuit is required. Therefore, there is an effect that the number of parts can be reduced.

[Brief description of drawings]

FIG. 1 is a block configuration diagram showing an optical multiplex signal demultiplexer according to a first embodiment of the present invention.

FIG. 2 is a diagram for explaining the relationship between the output of the high frequency synchronous detection circuit and the frequency ftop at which the transmittance of the Mach-Zehnder filter becomes maximum, in the case where the frequency difference fd between ftop and the center frequency fci of the optical signal is zero. FIG.

FIG. 3 is a diagram illustrating a relationship between an output of a high frequency synchronous detection circuit and a frequency ftop at which a transmittance of a Mach-Zehnder filter is maximized, showing a case where a frequency difference fd is a positive value.

FIG. 4 is a diagram for explaining the relationship between the output of the high frequency synchronous detection circuit and the frequency ftop at which the transmittance of the Mach-Zehnder filter is maximum, and is a diagram showing a case where the frequency difference fd is a negative value.

FIG. 5 is a diagram showing transmittance characteristics of a Mach-Zehnder filter when the frequency ftop at which the transmittance is maximized is fluctuated on the frequency axis.

FIG. 6 is a diagram showing a time change of a frequency difference fd between the frequency ftop at which the transmittance is maximized and the center frequency fci of the optical signal, the error signal Ve1, and the error signal Ve2 when the frequency ftop at which the transmittance is maximized is changed; The figure which shows the case where the center frequency of fd is zero.

FIG. 6 is a diagram showing a time change of a frequency difference fd between the frequency ftop at which the transmittance is maximized and the center frequency fci of the optical signal, the error signal Ve1, and the error signal Ve2 when the frequency ftop at which the transmittance is maximized is changed; The figure which shows the case where the center frequency of fd is zero.

FIG. 7 is a diagram showing a time change of a frequency difference fd between the frequency ftop at which the transmittance is maximized and the center frequency fci of the optical signal, the error signal Ve1, and the error signal Ve2 when the frequency ftop at which the transmittance is maximized is changed, The figure which shows the case where the center frequency of fd is a negative value.

FIG. 8 is a diagram showing a time change of a frequency difference fd between the frequency ftop at which the transmittance is maximized and the center frequency fci of the optical signal, the error signal Ve1, and the error signal Ve2 when the frequency ftop at which the transmittance is maximized is changed; The figure which shows the case where the center frequency of fd is a positive value.

FIG. 9: Center frequency fofs of frequency difference fd and error signal V
The figure which shows the relationship with e3.

FIG. 10 is a block configuration diagram showing an optical multiplex signal demultiplexer according to a second embodiment of the present invention.

FIG. 11 is a block configuration diagram showing an optical multiplex signal demultiplexing device of a third exemplary embodiment of the present invention.

FIG. 12 is a diagram showing a control band for stabilizing a Mach-Zehnder filter and an arrangement of oscillation frequencies of low-frequency oscillators.

FIG. 13 is a diagram showing a structural example of a Mach-Zehnder filter.

FIG. 14 is a diagram showing an example of transmittance characteristics of a Mach-Zehnder filter.

FIG. 15 is a diagram showing an example of the configuration of a tunable optical demultiplexer using a Mach-Zehnder filter.

FIG. 16 is a diagram illustrating an operating principle of a tunable optical demultiplexer.

FIG. 17 is a block configuration diagram showing a conventional optical frequency division transmission device using a tunable optical demultiplexer.

FIG. 18 is a diagram showing how a part of an optical FSK modulation signal to be received is output as a light intensity modulation signal to a monitor port of a Mach-Zehnder filter.

[Explanation of symbols]

 1 tunable optical demultiplexer 2 light receiving circuit 3, 56 amplifier 4 high frequency synchronous detection circuit 5, 5-1 ~ 5-7 low frequency synchronous detection circuit 6 switch 7 central processing unit 8 read only memory 9, 9-1 ~ 9-8 D / A converter 10-1 to 10-x, 14, 57 Adder 11-1 to 11-7 A / D converter 12 Load resistance 13 Tunable optical demultiplexer stabilization circuit 21 Optical frequency discrimination 22 Balanced photo detector 41, 53 Phase adjustment circuit 42 Demodulation circuit 43, 54 Mixer 44, 55 Low pass filter 51 Low frequency oscillator 52 Attenuator

[Procedure amendment]

[Submission date] October 7, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1 is a block configuration diagram showing an optical multiplex signal demultiplexer according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a relationship between an output of a high frequency synchronous detection circuit and a frequency ftop at which a transmittance of a Mach-Zehnder filter is maximum, and ftop and a center frequency f of an optical signal.
The figure which shows the case where the frequency difference fd with ci is zero.

FIG. 3 is a diagram illustrating a relationship between an output of a high frequency synchronous detection circuit and a frequency ftop at which a Mach-Zehnder filter has a maximum transmittance, and is a diagram illustrating a case where a frequency difference fd is a positive value.

FIG. 4 is a diagram illustrating a relationship between an output of a high frequency synchronous detection circuit and a frequency ftop at which a Mach-Zehnder filter has a maximum transmittance, and is a diagram illustrating a case where a frequency difference fd is a negative value.

FIG. 5 is a diagram showing transmittance characteristics of a Mach-Zehnder filter when the frequency ftop at which the transmittance is maximized is varied on the frequency axis.

FIG. 6 shows the frequency ftop when the frequency ftop at which the transmittance is maximized is changed and the center frequency fc of the optical signal.
frequency difference fd from i, error signal Ve1, error signal Ve2
FIG. 6 is a diagram showing the change over time in FIG. 6B, showing the case where the center frequency of fd is zero.

FIG. 7 shows the frequency ftop when the frequency ftop at which the transmittance is maximized is changed and the center frequency fc of the optical signal.
frequency difference fd from i, error signal Ve1, error signal Ve2
FIG. 6 is a diagram showing the change over time in FIG. 4B, showing the case where the center frequency of fd has a negative value.

FIG. 8 shows the frequency ftop when the frequency ftop at which the transmittance is maximized is changed and the center frequency fc of the optical signal.
frequency difference fd from i, error signal Ve1, error signal Ve2
FIG. 7 is a diagram showing the change over time in FIG. 6B, showing the case where the center frequency of fd is a positive value.

FIG. 9 is a diagram showing the relationship between the center frequency fofs of the frequency difference fd and the error signal Ve3.

FIG. 10 is a block configuration diagram showing an optical multiplex signal demultiplexing device according to a second embodiment of the present invention.

FIG. 11 is a block configuration diagram showing an optical multiplex signal demultiplexing device according to a third embodiment of the present invention.

FIG. 12 is a diagram showing a control band for stabilizing a Mach-Zehnder filter and an arrangement of oscillation frequencies of low-frequency oscillators.

FIG. 13 is a diagram showing a structural example of a Mach-Zehnder filter.

FIG. 14 is a diagram showing an example of transmittance characteristics of a Mach-Zehnder filter.

FIG. 15 is a diagram showing an example of the configuration of a tunable optical demultiplexer using a Mach-Zehnder filter.

FIG. 16 is a diagram illustrating an operation principle of a tunable optical demultiplexer.

FIG. 17 is a block configuration diagram showing a conventional optical frequency division transmission device using a tunable optical demultiplexer.

FIG. 18 is a diagram showing how a part of an optical FSK modulated signal to be received is output as a light intensity modulated signal to a monitor port of a Mach-Zehnder filter.

Claims (2)

[Claims] 1. A frequency-multiplexed optical signal group is input,
A tunable optical demultiplexer that selects and transmits an optical signal of a desired frequency from the signal group, and an optoelectric conversion unit that generates an electrical signal corresponding to the optical signal that has passed through the tunable optical demultiplexer. In the optical multiplex signal demultiplexing device including an adjusting unit that adjusts the transmitted light frequency of the tunable optical demultiplexer using the output of the opto-electric converting unit, the adjusting unit is a binary signal. Means for generating a bipolar signal in which "1" is assigned to one of the values and "-1" is assigned to the other value, and synchronous detection for synchronously detecting the electric signal by the bipolar signal. An optical multiplex signal demultiplexing device comprising: means and feedback means for returning the output of the synchronous detection means to the tunable optical demultiplexer. 2. The tunable optical demultiplexer includes one or more filters whose transmission characteristics change with respect to frequency, and the one or more filters have a frequency at which the maximum of each transmittance is approximately the desired frequency. The feedback means are set in agreement, and the low-frequency oscillator oscillates at a frequency sufficiently lower than the data rate of the optical signal to be transmitted by the tunable optical demultiplexer, and the low-frequency oscillator outputs the low-frequency oscillator. By means for periodically changing the transmission frequency of one of the one or more filters, means for synchronously detecting the output of the synchronous detection means with the output of the low frequency oscillator, and output of the means for synchronous detection The optical multiplex signal demultiplexing device according to claim 1, further comprising means for controlling a transmitted light frequency of the one filter.