JP3210061B2 - Operating point stabilizer for optical interferometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for stabilizing an operating point of an optical interferometer. In the field of optical communication, an optical interferometer is often used to obtain intensity-modulated light. The optical interferometer is configured to split the light input to one input port into two optical paths, to merge the propagating lights in the respective optical paths, and to output the combined light from one output port. The output intensity changes according to the phase difference of light. In order to generate this phase difference, frequency-modulated or phase-modulated light having a constant amplitude input to the optical interferometer, or
Light having a constant frequency and a constant amplitude is input to the optical interferometer, and a phase difference is given to the propagation light of each optical path between the branch point and the junction.

[0002] The former optical interferometer is further classified into two types. That is, there are a case where this optical interferometer is used as a component of an optical receiver and a case where it is used as a component of an optical transmitter. An optical receiver provided with an optical interferometer is used, for example, in FDM (Frequency Division Multiplexing) transmission in which light waves having different frequencies are multiplexed and transmitted at a high density, in order to selectively detect a signal of any one channel. Used. This detection technique has an advantage that a receiving circuit can be greatly simplified as compared with heterodyne detection using local light.

On the other hand, an optical transmitter having an optical interferometer is a DP.
SH-IM (Direct Phase-shift and Self-Homodyne Int
ensity Modulation) system. DPSH
In the -IM system, a small amplitude modulation current pulse is applied to a laser diode to which a bias current higher than the threshold current is applied to phase-modulate the oscillation light, and the phase-modulated light causes the optical interferometer to operate. (Shirasaki, M., Nishimoto, H., Okiyama, T. and T
ouge, T .: Fiber transmission propertiesof optical
pulses produced through direct phase modulation of
DFB laserdiode, ELECTRONICS LETTERS 14th April 19
88 Vol. 24 No.8 pp.486-488).

In either case, the optical interferometer converts the frequency- or phase-modulated light into intensity-modulated light according to the optical frequency discrimination characteristics. Therefore, in order to perform frequency discrimination efficiently, it is desirable that the operating point of the optical interferometer is at the maximum inclination position of the optical frequency discrimination characteristic. However, since this operating point tends to fluctuate due to the temperature, aging, etc. of the laser diode or optical interferometer serving as a light source, a technique for constantly controlling the operating point to an optimum point is required.

On the other hand, an optical modulator used in an IM / DD (intensity modulation / direct detection) system is known as an optical interferometer that gives a phase difference to the propagation light in each optical path between a branch point and a junction. ing. When this type of optical modulator is used, a laser diode as a light source can be driven with a constant current, so that the effect of chromatic dispersion is minimized by suppressing wavelength chirping (dynamic fluctuation of wavelength) during modulation. , High-speed and long-distance communication becomes possible.

As a problem in putting an optical modulator into practical use,
There are variations in the initial operating point and variations in the operating point due to temperature and aging. Variations and fluctuations in the operating point cause characteristic deterioration in the eye opening and the extinction ratio of the output light, resulting in deterioration in the receiving sensitivity. Therefore, in order to avoid this, it is necessary to stabilize the operating point.

[0007]

2. Description of the Related Art Conventionally, two complementary outputs of an optical interferometer are respectively received by a light receiver, and a difference component between the average level (the DC component of the photocurrent) of the outputs of the two light receivers is used as an operating point detection signal. The operating point stabilization technology to be used is known (Toba, Oda, Nakanishi, Shibata, Nosu, Takato, Fukuda, "Study of 100-channel optical FDM information distribution transmission system configuration method" OCS8
9-64, pp15-20).

[0008]

In the prior art described above, (a) whether or not the characteristics of the two photodetectors are uniform determines the control accuracy. (A) Due to the circuit configuration, DC in the feedback loop is determined. (C) The operating point detection signal becomes smaller as the optical output from the optical interferometer approaches a rectangular wave, and in principle, the operating point detection signal in the case of an ideal rectangular wave. Becomes zero.

An object of the present invention is to provide a device for stabilizing the operating point of an optical interferometer with high accuracy.

[0010]

FIG. 1 shows the basic configuration of the present invention .
Is a diagram illustrating a. The operating point stabilizing device for an optical interferometer according to the present invention includes an optical interferometer 1, a light receiving unit 2, a peak detecting unit 3, a low frequency oscillating unit 4, a first synchronous detecting unit 5, and an operating point setting unit 6.

The optical interferometer 1 outputs an intensity-modulated interference light. The light receiving means 2 receives the interference light from the optical interferometer 1 and converts the light intensity into an electric signal.

The peak detecting means 3 receives the electric signal from the light receiving means 2 and detects the maximum amplitude of the waveform. The low frequency oscillating means 4 changes the frequency of light input to the optical interferometer 1 or the interference characteristics of the optical interferometer 1 at a low frequency.

The first synchronous detection means 5 synchronously detects the output signal of the peak detection means 3 using the oscillation output from the low frequency oscillation means 4 as a reference signal. The operating point setting means 6 feeds back the frequency of the light input to the optical interferometer 1 or the interference characteristics of the optical interferometer so that the synchronous detection output of the first synchronous detecting means 5 becomes zero or a predetermined value.

In one embodiment of the present invention, the optical interferometer 1 receives frequency-modulated or phase-modulated light and converts the light into intensity-modulated interference light according to optical frequency discrimination characteristics. In this case, when the optical interferometer is used as a component of the optical receiver, the optical receiver receives the frequency-modulated or phase-modulated light, and the optical interferometer is used as a component of the optical transmitter. When this is done, the optical transmitter outputs intensity-modulated interference light.

In another embodiment of the present invention, the optical interferometer 1 is an optical modulator that receives light having a constant intensity and converts the light into interference light that is intensity-modulated according to a modulation signal.
According to another aspect of the invention, an intensity modulated interference light is emitted.
Receiving the interference light from the optical interferometer
Light receiving means for converting the light intensity of the light into an electric signal, and the light receiving means
To detect the maximum amplitude of the waveform
And the frequency of the light input to the optical interferometer or
Low frequency that changes the interference characteristics of the optical interferometer at a low frequency
Oscillating means and an oscillation output from the low-frequency oscillating means.
Synchronously detects the output signal of the peak detection means
First synchronous detection means, and synchronous detection output of the synchronous detection means
Is input to the optical interferometer so that is equal to zero or a predetermined value.
Feedback on the frequency of light or the interference characteristics of the optical interferometer
Operating point setting means and oscillation output from the low frequency oscillation means
The electric signal from the light receiving means is synchronously detected using the
Second synchronous detection means for detecting the frequency, and an output signal of the synchronous detection means.
Discriminating means for discriminating the sign of the signal.
The output signal is fed back to the operating point setting means, and the second
The output signal of the synchronous detection means is controlled to either positive or negative.
Operating point stabilizing device for optical interferometer characterized by being controlled
Is provided.

[0016]

The interference light output from the optical interferometer 1 is intensity-modulated. The modulation amplitude is detected by the peak detecting means 3. The modulation amplitude detected by the peak detecting means 3 reflects the fluctuation of the operating point due to the optical frequency discrimination characteristic of the optical interferometer or the fluctuation of the operation characteristic when the optical interferometer is an optical modulator. Specifically, the operating point is at the optimum point when the detected amplitude is maximum. Therefore, the operating point can be maintained at the optimum point by feeding back the synchronous detection output of the output signal of the peak detecting means 3 to the interference characteristic of the optical interferometer 1 or the like.

In practicing the present invention, since only one light receiver is required, it is not necessary to use two or more light receivers having uniform characteristics. Further, since a control loop based on synchronous detection is configured, there is no need to consider DC drift.

Further, since the maximum amplitude in the intensity modulation waveform is detected, a detection signal can be obtained regardless of the optical output waveform of the optical interferometer.

[0019]

Embodiments of the present invention will be described below. FIG. 2 is a block diagram of a main part of an optical receiver including an optical interferometer and its operating point stabilizing device. The optical interferometer 11 is formed in a so-called Mach-Zehnder type by forming an optical waveguide on a waveguide substrate. Light input to the input ports 12 and 13 is, for example, directional-coupled by the optical coupling unit 14 and distributed to two optical waveguides 15 and 16 having different optical path lengths.

The light propagating through the optical waveguides 15 and 16 is, for example, directional-coupled by the optical coupling unit 17, and is output from the output port 18 and / or the output port 18.
Or 19. In this embodiment, an input port 12 and an output port 18 are used, and the input port 13 and the output port 19 are dead-ended.

The frequency-modulated or phase-modulated light transmitted from the transmitting side via an optical transmission line (not shown) is input to the input port 12 of the optical interferometer 11. This optical interferometer 11
Converts the frequency-modulated or phase-modulated light into intensity-modulated interference light, which is output from an output port 18 and input to a light receiver 20 composed of a photodiode or the like.

The output signal of the light receiver 20 is amplified by an amplifier 21 and then sent to an identification circuit (not shown). The peak detection circuit 22 including a full-wave rectifier receives the output signal of the amplifier 21 and detects the maximum amplitude of a waveform that changes according to intensity modulation.

The output signal of the peak detection circuit 22 is normalized by a low-pass filter 23 and a division circuit 24 and then input to a synchronous detection circuit 26. The oscillator 25 for synchronous detection outputs a low-frequency signal whose frequency is sufficiently lower than the frequency corresponding to the transmission speed of the transmission data. This low-frequency signal is superimposed on, for example, a bias of the optical interferometer 11, whereby the interference characteristic of the optical interferometer 11 is changed at a low frequency.

The interference characteristics are specifically described in the optical interferometer 1
1 is a propagation delay time difference between the optical waveguides 15 and 16. As a method of changing the interference characteristics of the optical interferometer,
In addition to the method using the electro-optic effect using a bias (voltage), there are a method using a photoelastic effect, a method using a mechanical external force to change, a method using the thermo-optic effect, and the like.

The synchronous detection circuit 26 performs synchronous detection on the output signal of the division circuit 24 using the low frequency signal from the oscillator 25 as a reference signal. The synchronous detection circuit 26 includes, for example, a mixer that multiplies an output signal of the division circuit 24 by a reference signal, and a low-pass filter that extracts a DC component of an output of the mixer.

The synchronous detection circuit 26 may directly perform synchronous detection on the output signal of the peak detection circuit 22 without passing through the low-pass filter 23 and the division circuit 24. The operating point setting circuit 27 feeds back the interference characteristics of the optical interferometer 11 so that the output level of the synchronous detection circuit 26 becomes zero. In this case, as a method of changing the interference characteristics of the optical interferometer 11, any of the above-described methods can be used. In this embodiment, the output signal of the operating point setting circuit 27 is fed back to the bias of the optical interferometer 11.

The operating point setting circuit 27 may be configured to perform control such that the synchronous detection output matches a predetermined offset value based on a control input such as a mark ratio correction signal. Note that the mark ratio correction will be described in an embodiment of an optical transmitter described later.

The output signal of the light receiver 20 is amplified by an amplifier 21 and amplified by an amplifier 28 and input to another synchronous detection circuit 29. The synchronous detection circuit 29 synchronously detects the received light output using the low-frequency signal from the oscillator 25 as a reference signal, and determines whether the synchronous detection output is positive or negative by the operating point determination circuit 30. The output signal of the operating point determination circuit 30 is fed back to the operating point setting circuit 27. Synchronous detection circuit 2
9 and the operation of the operating point determination circuit 30 will be described later.

FIG. 3 is a diagram for explaining the principle of stabilizing the operating point of the optical interferometer in the optical receiver of FIG. Code 4
Reference numeral 1 denotes an optical frequency discrimination characteristic of the optical interferometer 11, in which the vertical axis represents the intensity of the interference light output from the output port 18 and the horizontal axis represents the frequency or operating point of the light input to the input port 12.

When a Mach-Zehnder type optical interferometer is used, the optical frequency discrimination characteristic exhibits a characteristic that the intensity of the interference light changes sinusoidally with respect to a change in the optical frequency.
The maximum value MAX and the minimum value MIN appear alternately.

As shown by reference numeral 42, the frequency corresponding to one of the binary logic levels matches the frequency giving the minimum point MIN, and the frequency corresponding to the other matches the frequency giving the maximum point MAX. When light that has been subjected to FSK (frequency shift keying) modulation with a waveform is input, the most efficient frequency discrimination is performed, and intensity-modulated light (interference light) having a waveform represented by reference numeral 43 is obtained. In this case, assuming that the mark ratio of the transmission data is 1/2, the operating point coincides with the frequency at which the inflection point A in the optical frequency discrimination characteristic is given, and this operating point is the optimum operating point.

Here, the input waveform 42 shifts in the optical frequency axis direction due to deterioration of the light source on the transmitting side, or the curve of the optical frequency discrimination characteristic shifts in the optical frequency axis direction due to the temperature fluctuation of the optical interferometer. Assume the case. In this case, since the optical frequency corresponding to the binary logical level of the input waveform no longer matches the maximum point and the minimum point in the optical frequency discrimination characteristics, the maximum amplitude in the output waveform 43 decreases.

In the present invention, the decrease in the maximum amplitude of the output waveform is detected by a peak detection circuit, and control is performed so that the detected output is maximized, so that the operating point of the optical interferometer is stabilized at the optimum point. It is becoming.

Reference numeral 44 is a graph showing the frequency characteristics of the output of the peak detection circuit in correspondence with the optical frequency discrimination characteristics (reference numeral 41) of the optical interferometer. It can be seen that the output of the peak detection circuit is maximum when the operating point of the optical interferometer is at the optimum point. In the present embodiment, synchronous detection is performed to maintain this state.

The optical frequency characteristic of the synchronous detection signal output from the synchronous detection circuit 26 is equivalent to a waveform obtained by differentiating the optical frequency characteristic (reference numeral 44) of the output of the peak detection circuit 22.
Reference numeral 45 is a diagram showing the optical frequency characteristics of the synchronous detection signal in correspondence with the optical frequency characteristics of the output of the peak detection circuit represented by reference numeral 44.

When the operating point of the optical interferometer coincides with the optimum point, the level of the synchronous detection signal becomes zero. And
As the operating point of the optical interferometer deviates from the optimal point, the value of the synchronous detection signal changes from zero to a positive or negative direction, as indicated by reference numeral 4 in FIG.
It changes according to the characteristic represented by 5.

Based on the above operating principle, the operating point setting circuit 27 determines that the polarity of the synchronous detection signal is negative if the polarity is positive, and that the polarity is positive if the polarity of the synchronous detection signal is negative. The bias or the like of the optical interferometer 11 is adjusted in such a direction as follows. By such negative feedback control, a control operation in which the operating point of the optical interferometer matches the optimum point is realized.

In this embodiment, the output signal of the peak detection circuit 22 is not directly input to the synchronous detection circuit 26, but is input via the low-pass filter 23 and the division circuit 24. This is for keeping the gain constant and expanding the input dynamic range of the optical receiver. This will be described specifically.

The low-pass filter 23 extracts the DC component of the output signal of the peak detection circuit 22. Division circuit 24
Divides the output signal of the peak detection circuit 22 by the DC signal from the low-pass filter 23.

According to such a configuration, a low-frequency component (frequency component of the low-frequency signal from the oscillator 25) included in the output signal of the peak detection circuit 22 can be stably obtained regardless of the intensity of the received light. Because it is possible, the input dynamic range is expanded.

When such negative feedback control is performed, since the optical frequency characteristics of the synchronous detection signal of the synchronous detection circuit 26 have periodicity, there are a plurality of stable points. That is, there are a plurality of stable points where the differential coefficient of the optical frequency characteristic of the synchronous detection signal becomes negative and the synchronous detection signal becomes zero.

In the following description, only a stable point corresponding to an inflection point having a positive differential coefficient in the optical frequency discrimination characteristic (reference numeral 41) of the optical interferometer is represented by a stable point (for example, represented by reference numeral B).
And a stable point corresponding to an inflection point at which the differential coefficient in the optical frequency discrimination characteristic is negative is referred to as a pseudo stable point (for example, represented by reference symbols C ₁ and C ₂ ).

Synchronous detection circuit 29 and operating point determination circuit 30
Is for discriminating such a stable point and a pseudo stable point. When the synchronous detection circuit 29 performs synchronous detection on the output signal of the photodetector 20, the optical frequency characteristic of the synchronous detection signal becomes the optical frequency discrimination characteristic of the optical interferometer 11 (reference numeral 4).
It becomes equivalent to a waveform obtained by differentiating 1).

Reference numeral 46 is a diagram showing the optical frequency characteristics of the synchronous detection signal output from the synchronous detection circuit 29 in correspondence with the optical frequency characteristics of the synchronous detection signal of the synchronous detection circuit 26. If the operating point is stabilized in the stable point B, when the polarity of the output signal of the synchronous detection circuit 29 is positive, the operating point is stabilized in a pseudo stable point C ₁ or C ₂ is
The polarity of the output signal of the synchronous detection circuit 29 becomes negative.

Therefore, by determining the polarity by the operating point determining circuit 30, the operating point can be stabilized not at the pseudo stable point but at a desired stable point. In the present embodiment, the output signal of the operating point determination circuit 30 is
7, the stabilizing operation is automatically performed.

FIG. 4 is a block diagram of a main part of an optical transmitter including an optical interferometer and its operating point stabilizing device. DP
Light from the laser diode 51 driven according to the SH-IM operation principle is frequency-modulated or phase-modulated, and this light is input to the input port 12 of the optical interferometer 11.

The optical interferometer 11 converts the input light into intensity-modulated light by self-homodyne and outputs the light from the output port 18. Most of the intensity-modulated light output from the output port 18 is transmitted to an optical transmission line (not shown) via an optical branching circuit 52 including an optical coupler and the like, and a part of the light is converted into an electric signal by the light receiver 20. You.

The control mode for stabilizing the operating point of the optical interferometer based on the output signal of the light receiver 20 is described in the operating point setting circuit 2
7 is the same as that in the optical receiver of FIG. 2 except that the feedback destination of the output signal and the mark ratio correction are performed.

The bias / temperature control circuit 53 supplies a bias current necessary for laser oscillation to the laser diode 51 and controls the temperature of the laser diode 51. The modulation circuit 54 supplies a modulation current to the laser diode 51 based on the data input, and the modulation current
The integrated value of the oscillation frequency of the laser diode 51 fluctuated in the time slot is kπ or −kπ (k ≧
1).

Optical waveguides 15 and 1 in optical interferometer 11
The propagation delay time difference between 6 is set to a time corresponding to 1 / k of one time slot in data input. The mark rate monitor circuit 55 detects the mark rate of the input data to the modulation circuit 54, and feeds back a correction signal corresponding to the detected mark rate to the operating point setting circuit 27.

In this embodiment, the output signal of the operating point setting circuit 27 is fed back to the bias / temperature control circuit 53, and the oscillation frequency of the laser diode 51 is controlled by the bias current applied to the laser diode 51 or the temperature of the laser diode 51. Is done.

The principle of operation of the DPSH-IM will be briefly described with reference to FIG. The intensity of the light output from the optical interferometer 11 depends on the phase difference between the light waves propagated through the optical waveguide 15 and the light propagated through the optical waveguide 16 at the optical coupling section 17. If the phase difference is fixed to π (or (2n + 1) π, n is an integer), the intensity of the output light becomes zero. When the phase of the light wave input to the optical interferometer 11 changes within one time slot, the intensity of the output light changes.

The oscillation frequency of the laser diode 51 depends on the drive current (the bias current and the modulation current superimposed thereon) provided by the bias / temperature control circuit 53 and the modulation circuit 54. The waveform is reflected on the frequency change of the light wave output from the laser diode 51. FIG. 5A is a waveform chart showing this frequency shift, and an RZ type waveform is obtained. here,
The RZ type simply means that the signal level returns to zero for each modulation time slot, and the duty ratio is arbitrary.

Since the phase of the light wave is the time integral of the frequency, if the amplitude of the modulation current pulse is appropriately determined, the signal “1” is obtained.
The phase change in the time slot corresponding to
It becomes π as shown in FIG. This light interferes with the light delayed by one time slot with equal amplitude by the interferometer 11. The phase of the light delayed by one time slot is shown by a broken line in FIG. Here, the initial phase of the delayed light is zero or π
(In FIG. 5B, the initial phase is π.)

The intensity of the light output from the optical interferometer 11 is
It is determined by the relative phase difference between the two interfering lights. The relative phase difference is the difference between the solid line and the dashed line in FIG. 5 (B), which becomes as shown in FIG. 5 (C), and changes between zero and π according to the data input. As a result of the interference, the optical output becomes maximum when the phase difference is zero and becomes zero when the phase difference is π, and the intensity output waveform becomes as shown in FIG. 5D.

Although the output waveform corresponds to the data input, there is a time delay corresponding to half of one time slot between the output waveform and the data input waveform. The output waveform is of the NRZ type regardless of the duty ratio of the RZ data input, and the duty ratio of the data input determines the rise time and the fall time of the output waveform.

According to DPSH-IM, it is possible to perform small amplitude modulation under a high bias of the laser diode.
It is possible to construct a system that is hardly affected by chromatic dispersion due to chirping and that performs differential operation with a low driving voltage.

In the DPSH-IM, the intensity-modulated light output from the optical interferometer is distorted or its polarity is inverted due to a small change in the oscillation frequency of the laser diode or the delay time difference of the optical interferometer. The oscillation frequency of the laser diode and the delay time difference in the optical interferometer change according to a temperature change and aging. Therefore, when the optical transmitter to which the DPSH-IM is applied is put to practical use, it is required to stabilize the operating point of the optical interferometer so that waveform distortion and polarity inversion do not occur.

The waveform of the optical frequency in FIG. 5A corresponds to the waveform indicated by reference numeral 42 in FIG. 3, and the waveform of the interference light intensity in FIG. 5D is indicated by reference numeral 43 in FIG. Corresponding to the waveform. In the embodiment shown in FIG. 2, the output signal of the operating point setting circuit 27 is fed back to the bias of the optical interferometer 11 and the like.
The optimum operating point is obtained by displacing the optical frequency discrimination characteristic (reference numeral 41 in FIG. 3) in the optical frequency axis direction.
In this embodiment, since the optical interferometer 11 is provided in the optical transmitter, the output signal of the operating point setting circuit 27 can be fed back to the bias / temperature control circuit 53 of the laser diode. In this case, if described with reference to FIG. 3, the operating point is stabilized at the optimum point by displacing the input waveform indicated by the reference numeral 42 in the optical frequency axis direction.

When controlling the bias or temperature of the laser diode to stabilize the operating point in this way, if the delay time difference in the optical interferometer is kept constant, the automatic frequency control (AFC) of the laser diode is performed. Will be realized at the same time.

Next, the mark rate monitor circuit 55 is constituted by, for example, an integrator provided in the modulation circuit 54. The mark rate monitor circuit 55 has a logic value "1" of the input data to the modulation circuit 54. And the mark ratio is calculated, and the output is fed back to the operating point setting circuit 27.

Now, when the mark ratio is 1/2, that is, when "1" and "0" occur at a ratio of 1: 1, and when the mark ratio is, for example, 1/4, that is, "1" and "0". "0" is 1:
The average intensity of the light output from the optical interferometer 11 changes when it occurs at a rate of 3.

When the average intensity of the light output from the optical interferometer 11 changes, the output signal level of the peak detection circuit 22 also changes depending on the capability of the peak detection circuit 22. Therefore, in order to prevent such an output signal of the peak detection circuit 22 from changing depending on the mark rate,
The offset value in the operating point setting circuit 27 is increased or decreased according to the mark ratio detected by the mark ratio monitor circuit 55. As a result, an operation point stabilization operation of the optical interferometer which is not affected by the mark ratio is realized.

In this embodiment, the interference characteristic of the optical interferometer 11 is changed by the low frequency signal from the oscillator 25. However, as in this embodiment, the optical interferometer 11 is provided in the optical transmitter. In such a case, the oscillation frequency of the laser diode 51 may be changed by a low-frequency signal from the oscillator 25. In this case, the oscillator 2
5 is inputted to the bias / temperature control circuit 53.

The output signal of the operating point setting circuit 27 is
Instead of feeding back to the bias / temperature control circuit 53, the operating point of the optical interferometer may be stabilized by feeding back to the optical interferometer 11 in the same manner as in the embodiment of FIG.

FIG. 6 is a block diagram of a main part of another optical transmitter including an optical interferometer and its operating point stabilizing device. The optical interferometer in this embodiment is an optical modulator such as a Mach-Zehnder optical modulator. The light modulator 61 receives light of a constant intensity from the laser diode 62, modulates the intensity of the light, and outputs the modulated light. The intensity-modulated light output from the optical modulator 61 is split by an optical splitter 52, one of which is sent to an optical transmission line (not shown), and the other is converted into an electric signal by the light receiver 20.

The optical modulator 61 has an input port 63 to which the light from the laser diode 62 is input, and a pair of optical waveguides 64 for propagating the light input to the input port 63 in two branches.
And 65, and an output port 66 for combining and outputting the lights propagated through the optical waveguides 64 and 65, respectively.
Each optical waveguide is formed on a waveguide substrate 67 made of an electro-optic crystal such as LiNbO ₃ .

Reference numeral 68 represents a signal electrode loaded on one optical waveguide 64, and reference numeral 69 represents a ground electrode loaded on the other optical waveguide 65. In this embodiment, in order to enable a high-speed modulation operation, a traveling wave type such that an electric field generated between the signal electrode 68 and the ground electrode 69 by the modulation signal propagates from the input port 63 side to the output port 66 side. Is adopted.

The modulation circuit 70 supplies a modulation signal to the optical modulator 61 based on a data input whose logical level is binary. The modulation signal is input to the end 68A on the input port 63 side of the signal electrode 68 via the decoupling capacitor 71. End 68B of signal electrode 68 on output port 66 side
Is connected to one end of a terminating resistor 73 via a decoupling capacitor 72. The other end of the terminating resistor 73 and the ground electrode 69 are grounded.

The output signal of the operating point setting circuit 27 and the low frequency signal from the oscillator 25 are input to the end 68 B of the signal electrode 68 via the inductor 74. The mark rate monitor circuit 75 detects the mark rate of the data input to the modulation circuit 70, and feeds back the detected output to the operating point setting circuit 27.

FIG. 7 is a diagram showing the input / output characteristics of the optical modulator 61. In the figure, shows the characteristics before operating point drift occurs, and shows the characteristics when operating point drift occurs. The “operating point drift” in the case where the optical interferometer is an optical modulator as in this embodiment is a drift of the operating characteristic curve representing the relationship between the output optical power and the applied voltage in the direction of increasing or decreasing the applied voltage. .

The operating characteristic curve of the optical modulator has a periodicity with respect to the applied voltage. Therefore, efficient binary modulation can be performed by using the modulation signals (driving voltages V ₀ and V ₁ ) that can obtain the minimum value and the maximum value of the output optical power corresponding to each logical value of the data input. it can.

If the drive voltages V ₀ and V ₁ are constant when the operating point drift occurs, the above-described periodicity causes waveform distortion and extinction ratio deterioration. In this embodiment,
Since the output signal of the operating point setting circuit 27 is fed back to the bias of the optical modulator 61, when the operating point drift occurs and the drift is dV, the driving voltages V ₀ and V
₁ are changed to V ₀ + dV and V ₁ + dV, respectively. As a result, the operating point drift can be compensated to prevent waveform distortion and extinction ratio deterioration.

In this embodiment, the output signal of the operating point setting circuit 27 and the low frequency signal from the oscillator 25 are fed back to the bias of the optical modulator 61. One or both of them are fed back to the laser diode 62. The oscillation frequency of the laser diode 62 may be changed by feeding back to a bias or temperature.

According to the embodiment described above, only one light receiver is required for stabilizing the operating point of the optical interferometer, the optical interferometer is not affected by DC drift in the light receiver, etc., has a large operating point detection signal, The effect that the control is not affected by the output waveform is generated, and the operating point stabilization control according to the actual operation of the system becomes possible.

In general, an IC which is a component of the optical receiver as shown in FIG. 2 includes an automatic gain control (AGC).
In this case, the peak of the received light waveform is detected. Therefore, when the present invention is applied to an optical receiver, the present invention can be implemented only by adding a new circuit for synchronous detection, and the optical receiver can be modified in a smaller number of places.

[0077]

The effects of the present invention are evident from the description of the embodiments and the like, but the main ones will be clarified and confirmed as follows. That is, according to the present invention, the operating point of the optical interferometer can be stabilized with high accuracy.

[Brief description of the drawings]

FIG. 1 is a diagram corresponding to claims.

FIG. 2 is a block diagram of a main part of an optical receiver including the operating point stabilizing device for an optical interferometer of the present invention.

FIG. 3 is a diagram for explaining the principle of stabilizing the operating point of the optical interferometer.

FIG. 4 is a block diagram of a main part of an optical transmitter including the operating point stabilizing device for an optical interferometer according to the present invention.

FIG. 5 is an explanatory diagram of the operation principle of DPSH-IM.

FIG. 6 is a block diagram of a main part of another optical transmitter including the operating point stabilizing device of the optical interferometer of the present invention.

FIG. 7 is a diagram showing input / output characteristics of an optical modulator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 11 Optical interferometer 2 Light receiving means 3 Peak detecting means 4 Low frequency oscillation means 5 First synchronous detection means 6 Operating point setting means 61 Optical modulator (optical interferometer)

Continuation of the front page (56) References JP-A-3-160831 (JP, A) JP-A-2-96719 (JP, A) Naoki Kuwata, Hiroshi Nishimoto, Tetsuo Horimatsu, Takashi Toge, "Automatic Modulator for Mach-Zehnder Type Optical Modulator" Study of Bias Control Circuits ”, 1990 IEICE Spring National Convention Lecture Volume 4, The Institute of Electronics, Information and Communication Engineers, March 5, 1990, p4-155 (58) Int.Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 G01J 3/00-4/04 G01J 7/00-9/04

Claims (9)

(57) [Claims] An optical interferometer for outputting an intensity-modulated interference light; a light receiving unit for receiving the interference light from the optical interferometer and converting the light intensity into an electric signal; and an electric signal from the light receiving unit. And a peak detecting means for detecting the maximum amplitude of the waveform, a low frequency oscillating means for changing the frequency of light input to the optical interferometer or an interference characteristic of the optical interferometer at a low frequency, First synchronous detection means for synchronously detecting the output signal of the peak detection means using the oscillation output from the means as a reference signal; and the optical interferometer so that the synchronous detection output of the synchronous detection means becomes zero or a predetermined value. Operating point setting means for feeding back the frequency of the light input to the optical interferometer or the interference characteristics of the optical interferometer, and the oscillation output from the low frequency oscillation means as a reference signal.
A second synchronous detection for synchronously detecting the electric signal from the light receiving means;
Wave means, discriminating means for discriminating whether the output signal of the synchronous detection means is positive or negative.
The provided output signal is fed back is in the operating point setting means of the discriminating means
And the output signal of the second synchronous detection means is positive or negative.
An operating point stabilizing device for an optical interferometer, wherein the operating point is controlled to one of the two directions . 2. The optical interferometer according to claim 1, wherein the optical interferometer receives the frequency-modulated or phase-modulated light, and converts the light into an intensity-modulated interference light according to an optical frequency discrimination characteristic. Operating point stabilizer for optical interferometer. 3. The optical interference apparatus according to claim 2, wherein the operating point setting means controls an optical frequency discrimination characteristic of the optical interferometer by changing a bias or a temperature of the optical interferometer. Operating point stabilization device for vessel. 4. A low-pass filter for extracting a DC component of an output signal of the peak detecting means, and a dividing circuit for dividing an output signal of the peak detecting means by a DC component from the low-pass filter. 3. The operating point stabilizing device for an optical interferometer according to claim 2, wherein the first synchronous detection means performs synchronous detection on an output signal of the division circuit. 5. The operating point stabilizing device for an optical interferometer according to claim 2, wherein the optical interferometer is provided in an optical receiver that receives frequency-modulated or phase-modulated light. . 6. The operating point stabilizing device for an optical interferometer according to claim 2, wherein the optical interferometer is provided in an optical transmitter that outputs intensity-modulated light. 7. The automatic frequency control of the laser diode, wherein a bias or temperature of a laser diode for outputting light received by the optical interferometer receives feedback from the operating point setting means, and the laser diode is automatically frequency-controlled. 7. An operating point stabilizing device for an optical interferometer according to item 6 . 8. Frequency modulation or phase modulation of light received by the optical interferometer is performed by modulating a drive current of a laser diode based on input data, and a mark ratio of the input data is fed back to the operating point setting means. 7. The operating point stabilizing device for an optical interferometer according to claim 6 , wherein: 9. The optical interferometer receives light of a constant intensity,
The operating point stabilizing device for an optical interferometer according to claim 1, wherein the optical modulator is an optical modulator that converts the light into interference light that is intensity-modulated according to a modulation signal.