JPH0728267B2 - Semiconductor laser modulation method and device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a semiconductor laser device used in various optical communication systems, and also to a method for modulating the semiconductor laser.

BACKGROUND ART In optical communications, semiconductor lasers are often used as light sources. Modulation methods can be broadly divided into direct modulation, in which the drive current of the semiconductor laser is modulated by transmission data, and external modulation, in which the output light of the semiconductor laser is modulated by an external modulator.

In conventional direct modulation methods, the threshold current I _T of a semiconductor laser is generally supplied as a bias current, and a modulation current according to transmission data is superimposed on the bias current to drive the semiconductor laser. However, in such modulation methods, the semiconductor laser needs to be driven with a pulse current of relatively large amplitude, which causes chirping (dynamic wavelength shift) of the oscillation wavelength. This chirping distorts the waveform of the optical pulse propagated through the optical fiber. Therefore, several
Direct modulation using high-speed data of over Gb/s is difficult, and the large modulation amplitude places a heavy burden on both the semiconductor laser and its driver circuit.

To reduce the chirping, a constant bias current _{I0 sufficiently larger than the laser threshold is added to a modulation current Im of} , for example, several tens of milliamps, so that modulation is performed only in the region where the current is larger than the laser threshold current _I1 , as shown in Figure 1. This method causes light emission even when the transmitted data is "0," which causes a serious problem in that the extinction ratio of the output light deteriorates.

Furthermore, conventional external modulation methods use various types of modulators, such as external modulators using electro-optical effect materials or acousto-optical effect materials, waveguide-type external modulators, optical deflection-type external modulators, etc. Therefore, there are problems in that these external modulators generate large connection losses and require complex structures for high-speed modulation.

Next, it is used as an external modulator for a semiconductor laser, etc.
The configuration of an optical interferometer consisting of a general Mach-Zehnder type electro-optic modulator is shown in Fig. 2. In the optical interferometer 10 shown in the figure, steady light from a semiconductor laser or the like is input to one optical waveguide 1 on the input side, and this is once branched into two optical waveguides 2 and 3.
After passing through the optical waveguides 2 and 3, these lights are combined and made incident on a single optical waveguide 4. At this time, electrodes 5 and 6 are provided on the two optical waveguides 2 and 3, and a voltage is applied to these electrodes to change the difference in optical path length, thereby giving a phase difference to the two lights being combined. As a result, the two lights interfere with each other when combined, and output light is obtained that is intensity-modulated according to the phase difference.

However, in such an optical interferometer 10, the actual length of the optical path may fluctuate with temperature changes, or the wavelength of the input light may fluctuate, resulting in the two light beams to be combined being distorted.
The phase difference between the two lights fluctuates over time, which causes the problem that the reference point (phase bias) of the operation during intensity modulation is not stable over time. For example, the ideal relationship between the phase difference and the output light intensity during intensity modulation is shown in Figure 3A. That is, if the phase difference is φ ₁ (= 2nπ) and φ ₂ (= 2n +
When the phase difference φ ₁ is changed between φ 1 and φ 2, the output light intensity changes to "1" and "0", respectively.
If _φ2 fluctuates to become _φ1 ' or _φ2 ', the phase bias will shift as shown in Figure 3. As a result, the maximum output light intensity will be less than the above "1" and the minimum output light intensity will be greater than the above "0", resulting in a decrease in the extinction ratio of the output light.

To solve this problem, it is conceivable to detect the phase difference deviation from the output light and correct the phase difference so that this deviation is eliminated. However, because the output light switches between "1" and "0" at high speed in response to changes in the phase difference, it is extremely difficult to directly detect the phase state from such output light, and therefore it has not been possible to sufficiently suppress the fluctuations in the phase difference until now.

DISCLOSURE OF THE INVENTION In view of the above-mentioned conventional problems, an object of the present invention is to provide a semiconductor laser modulation method that eliminates the adverse effects of chirping, reduces the burden on the semiconductor laser and its drive circuit, and enables high-speed modulation by direct modulation.

Another object of the present invention is to provide an optical interferometer that can eliminate phase shifts at the output side and obtain a stable output, in consideration of the problems of the conventional optical interferometers described above. Still another object of the present invention is to provide an optical modulation device that can stably and quickly modulate the optical intensity of a semiconductor laser using such an optical interferometer.

In general, it can be considered that the oscillation frequency ω of a semiconductor laser changes in proportion to the change ΔI in the current I in the vicinity of the average driving current. This relationship is shown in FIG. 6. That is,
When I(t)=I ₀ +ΔI(t), ω(t)=ω ₀ +Δω
(t), it can be expressed as Δω(t)=kΔI(t), where k is the chirp factor (constant).

And, the optical field is When the oscillation frequency ω varies over time as shown above, This results in Therefore, it is understood that the phase φ can be controlled by the change ΔI in the drive current.

Therefore, in the semiconductor laser modulation method of the present invention, the change ΔI(t) in the drive current is used as a modulation current pulse,
The time integral of this one time slot T and the chirping coefficient (k) are set to be π or -π. In this way, the phase φ is inverted by π or −π during one time slot T. Therefore, the modulation current pulse ΔI is inverted at a timing according to the transmission data.
By generating (t), it is possible to apply phase modulation between 0 and π to the output light of the semiconductor laser.
The shape of ΔI(t) that satisfies the above formula (1) is as follows:
A delta function (for example, ΔI(t) = (π/k)δ(t - _t0 )) can be considered, but any other function that is in a narrow time region and whose integral is π/k or -π/k is also acceptable.

This method eliminates the negative effects of chirping, which occurs in conventional direct modulation, because the oscillation wavelength broadens only due to sidebands caused by modulation. Furthermore, because the semiconductor laser is modulated at a small amplitude, the burden on both the semiconductor laser and its driver circuit can be significantly reduced. These factors enable high-speed modulation that exceeds the frequency limit of conventional direct modulation.

In the semiconductor laser modulation method of the present invention, the output light of the semiconductor laser that has been phase-modulated between 0 and π as described above is passed through a self-homodyne optical interferometer.
This causes intensity modulation according to each phase difference due to self-homodyne, i.e., the light is converted into optical pulses of "0" and "1." This enables high-speed optical intensity modulation.

Next, in an optical interferometer in which a high-speed phase-modulated optical signal is input as described above, or an optical interferometer in which stationary light is input as shown in FIG. 2, if the phase difference is moved between _φ1 and _φ2 as shown in FIG. 3A, for example,
The output light is intensity-modulated to "1" and "0" respectively. Since the probability of these two values is generally 50/50, if we consider that the light intensity moves instantaneously between "1" and "0", the average intensity of these two lights is "1/2".
On the other hand, even if the phase difference is changed between _φ1 ' and _φ2 ' as shown in Figure 3B, the extinction ratio decreases, but if we consider that the light intensity similarly moves instantly between "1" and "0," the average output of the two lights will also be "1/2."
However, when considering the actual average intensity when the light intensity switches from "1" to "0" or from "0" to "1," there is a finite transition time between the two values, so different average intensities are obtained in Figures 3A and 3B. That is, the actual average intensity between _φ1 and _φ2 is exactly "1/2."
whereas the actual average intensity between φ ₁ ' and φ ₂ ' is smaller than "1/2".

Therefore, by detecting such a change in the average intensity, the phase bias can be easily determined. Therefore, in the stabilization method of the optical interferometer of the present invention, the average intensity is obtained by integrating the intensity of the output light of the optical interferometer over a time slower than the modulation speed (for example, about 1/100 seconds for 1 Gb/s phase modulation). By integrating over a time slower than the modulation speed in this way and obtaining the average intensity, the "1" and "0" of the transmitted data can be easily determined.
Then, using the average intensity, feedback is applied to the optical wavelength of the input light or the optical path length difference between the two optical paths, thereby correcting the phase bias during intensity modulation.

This makes it possible to easily determine the deviation in the phase bias from the change in the average intensity, and to apply feedback to eliminate this deviation, thereby significantly stabilizing the output of the optical interferometer.

The optical modulation device of the present invention is configured to include a code conversion section, a differentiation circuit, a semiconductor laser, and an optical interferometer. The code conversion section converts the code so that, for example, only when the transmission data is "1" the code is inverted and output, and when the transmission data is "0" the previous code is output (or vice versa).
The converted output signal is differentiated by a differentiating circuit. A bias current selected so that the semiconductor laser oscillates and produces output light of the desired optical intensity is supplied to the semiconductor laser from a bias current source, and the differentiated output signal of the differentiating circuit is superimposed on this bias current as a modulation current pulse. This modulation current pulse changes the phase of the output light from the semiconductor laser. The modulation current pulse is selected so that this phase change is π or -π. The optical interferometer is configured to enable self-homodyning and has an optical delay circuit that provides a relative delay time of one time slot and a relative phase difference of 0 or π. Therefore, when the phase of the output light from the semiconductor laser is changed to π or -π, the output light from the optical interferometer becomes a relative value of "0" or "1."

This allows the semiconductor laser to be in a continuous oscillation state, and the modulation current pulse is small compared to the constant bias current, so the chirping problem is eliminated.
The optical interferometer obtains intensity-modulated output light in response to changes in the phase of the output light from the semiconductor laser, and has little connection loss and can operate at extremely high speed.
Stable intensity modulation is possible even for high-speed data of Gb/s or more.

4A and 4B are diagrams showing the overall configuration when the present invention is applied to an optical communication system. As shown in FIG. 4A, the optical interferometer 12 according to the present invention is located after the semiconductor laser 11 on the transmitting side and is connected to the transmission optical fiber 13.
Alternatively, as shown in Fig. 4B, it may be placed after the transmission optical fiber 13 and before the receiver 14 on the receiving side. When the optical interferometer 12 is placed on the transmitting side as shown in Fig. 4A, it is possible to stabilize it by applying feedback to the oscillation wavelength of the semiconductor laser 11 or by applying feedback to the optical path length difference of the optical interferometer 12 itself. When the optical interferometer 12 is placed on the receiving side as shown in Fig. 4B, it is possible to stabilize it by applying feedback to the optical path length difference of the optical interferometer 12 itself. In the latter case, even if the light is attenuated by the long transmission optical fiber 13, the light is still in a phase-modulated stage and its intensity is always constant, so that it is easy to amplify the light as it is, i.e., so-called direct optical amplification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram for explaining the operation of conventional direct modulation, FIG. 2 is a diagram showing the configuration of a general Mach-Zehnder electro-optic modulator, FIGS. 3A and 3B are diagrams showing the ideal and shifted relationship between the phase difference and the output light intensity in intensity modulation in an optical interferometer, FIGS. 4A and 4B are diagrams showing the overall configuration when the present invention is applied to an optical communication system, FIG. 5 is a block diagram showing a laser drive circuit according to a first embodiment of the semiconductor laser modulation method of the present invention, and FIG. 6 is a diagram showing the change Δ of the drive current in a general semiconductor laser.
8 is a waveform diagram of the output optical phase φ obtained in response to the drive current I shown in FIG. 7; FIG. 9 is a diagram showing the configuration of an optical interferometer in accordance with a second embodiment of the method for modulating a semiconductor laser of the present invention; FIG. 10 is a waveform diagram of the optical output obtained in response to the output optical phase φ of FIG. 8 from the optical interferometer shown in FIG. 9; FIG. 11 is a waveform diagram of the drive current I in accordance with a third embodiment of the method for modulating a semiconductor laser of the present invention; FIGS. 12A and 12B are diagrams showing other examples of self-homodyne optical interferometers; FIG. 13 is a diagram showing a configuration for realizing a first embodiment of the method for stabilizing an optical interferometer of the present invention; Figure 17 is a diagram showing a configuration for realizing a fifth embodiment of the method for stabilizing an optical interferometer of the present invention; Figure 18 is a block diagram showing the basic configuration of an optical modulation device of the present invention; Figure 19 is a diagram showing the configuration of a first embodiment of an optical modulation device of the present invention; Figures 20(a) to (g) are diagrams for explaining the operation of the optical modulation device shown in Figure 19; Figure 21 is a circuit diagram showing a differential circuit related to a second embodiment of an optical modulation device of the present invention; and Figure 22 is a diagram showing the configuration of an optical modulator stabilization means related to a third embodiment of an optical modulation device of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION In order to explain the present invention in more detail, the present invention will be described with reference to the accompanying drawings.

FIG. 5 is a block diagram showing a laser driving circuit according to a first embodiment of the semiconductor laser modulation method of the present invention.

In the figure, a bias current _I0 of a fixed value (for example, 100 mA) is generated by a bias current generating circuit 21.
The current pulse generating circuit 22 generates a modulated current ΔI of, for example, several mA at a timing according to the transmission data. The shape of this ΔI is set so as to satisfy the above-mentioned formula (1), that is, For example, ΔI(t)=
The shape is set to be (π/k)δ(t−t ₀ ) or a shape close to this.

The modulated current ΔI obtained by the current pulse generating circuit 22
, the bias current I ₀ obtained by the bias current generating circuit 21.
This is superimposed on the driving current I and given to the semiconductor laser 23 as the driving current I (=I ₀ +ΔI). An example of the waveform of this driving current I is shown in Figure 7. When the driving current I momentarily changes in a pulse-like manner in places like this, the oscillation frequency ω at that point fluctuates in accordance with the current change ΔI (see Figure 6), and a phase difference occurs that is equal to the time integration of this fluctuation Δω. In this embodiment, as described above, the time integration of Δω Since ΔI is set so that it is possible to generate a relative phase difference of π before and after the addition of ΔI. For example, Fig. 8 shows the change in the phase φ of the output light according to the drive current waveform in Fig. 7 at the same time position on the horizontal axis. That is, it can be seen that the phase φ of the output light is inverted by π at the part where there is a current change ΔI, and phase modulation is applied between 0 and π.

According to this embodiment, a phase difference of π can be imparted to the output light simply by adding a slight current change ΔI to the constant bias current _I0 . Since the amplitude of the modulation current can be made very small in this way, the burden on both the semiconductor laser and its drive circuit is extremely small. Moreover, the spread of the oscillation wavelength is limited to the sidebands due to modulation and is kept to the same level as when an external modulator is used, so there is no adverse effect due to chirping. From these reasons,
This solves the major problem that has limited the speed of conventional direct modulation, making it possible to achieve high-speed modulation (for example, several Gb/s or more) that far exceeds the frequency limit of conventional direct modulation.

Next, a second embodiment of the semiconductor laser modulation method of the present invention will be described.

In this embodiment, the output light (phase-modulated light) of the semiconductor laser 23 obtained in the above embodiment is input to a self-homodyne optical interferometer 24 shown in FIG.
The light is split and combined by two half mirrors 24a and 24b, respectively, and the light that has passed through one optical path _l1 is then reflected by mirror 24
The time delay is generated in the light that passes through the other optical path _l2 while being reflected by 24c and 24d. The time delay generated here is 1/2 of the driving current I shown in FIG.
The time slot (one time slot) is set equal to T.

Then, the output light of the optical interferometer 24 is a combination of light from the semiconductor laser 23 in a certain time period and light from the time period immediately before (or after) that time period, and thus a light pulse of "0" or "1" is obtained according to the phase difference of π or 0 between the two combined lights.
The output light (phase modulated light) of the semiconductor laser 23 is input to an optical interferometer 24
For example, the optical interferometer obtained according to the phase change shown in FIG.
The change in the intensity of the output light of 24 is shown in Figure 10. As is clear from the figure, when there is a phase difference of 0 between the two lights being combined, the two lights are added, but when there is a phase difference of π, the two lights cancel each other out, resulting in an optical output of "1" or "0" depending on the presence or absence of the modulation current ΔI shown in Figure 7.

According to this embodiment, just by applying a slight current change ΔI to the drive current, it is possible to apply intensity modulation of "0" and "1" to the output light of the semiconductor laser, just like in the first embodiment. Therefore, just like in the first embodiment, it is possible to eliminate the adverse effects of chirping, reduce the burden on the semiconductor laser and the drive circuit, and achieve high speeds through direct modulation.

The modulation current ΔI shown in Fig. 7 is designed to produce a phase difference of π, but it may also have a phase difference of -π, so it may be modulated so that ΔI is subtracted from _I0 as shown in Fig. 11. In this case, too, the optical interferometer shown in Fig. 9
24, an intensity modulation similar to that shown in FIG. 10 can be obtained.

Furthermore, the shape of the modulation current ΔI does not necessarily have to be a δ function or a shape close to it, but may be any shape whose integral is π/k in a narrow temporal region.

As for the optical interferometer 24, in addition to the Mach-Zehnder type optical interferometer described above, various types of optical interferometers, such as the Michelson type shown in FIG. 12A and the Fabry-Perot type shown in FIG. 12B, can be used as long as they are capable of self-homodyne interference.
f and 24h are mirrors, and 24g and 24i are half mirrors.

Next, FIG. 13 shows the first method for stabilizing an optical interferometer according to the present invention.
10 is a diagram showing a configuration for realizing the embodiment of FIG. 10, in which this embodiment is applied to the optical interferometer 24 shown in FIG.

In FIG. 13, first, as described in FIG. 9, the half mirror 24
The output light obtained by combining the beams at b is detected by a photodetector 32 via a half mirror 31.
The other light, which is output in a direction different from the output light at b and has an inverse relationship in intensity with the output light at b, is
The light is detected by another photodetector 34 via a half mirror 33. These two photodetectors 32 and 34 have a time constant that is larger than the time interval of the phase modulation by the semiconductor laser 23.
For example, this configuration is similar to connecting a large capacitance in parallel to a normal photodiode. By detecting the output light with such photodetectors 32 and 34, the average intensity can be obtained by integrating the intensity of the output light over a time slower than the phase modulation (e.g., about 1/100 seconds for 1 Gb/s phase modulation). As mentioned above, this change in average intensity corresponds to the phase bias shift during intensity modulation.

Next, since the intensities of the lights detected by the photodetectors 32 and 34 are in an inverse relationship to each other, the difference between the two photodetectors 32 and 34 is calculated by the differential amplifier 35, and the deviation of the average intensity from "1/2" is calculated. Here, if the output of the differential amplifier 35 is zero, the average intensity is "1/2".
2", which indicates that there is no phase bias deviation (see Figure 3A). On the other hand, if the output of the differential amplifier 35 is biased to either the positive or negative side, the average intensity is not "1/2", which indicates that there is a phase bias deviation (see Figure 3B). Therefore, the output value of the differential amplifier 35 corresponding to such a phase bias deviation is used to apply feedback to the oscillation wavelength of the semiconductor laser 23. That is, the oscillation wavelength of the semiconductor laser 23 is changed by superimposing the output of the differential amplifier 35 on the bias current I ₀ of the semiconductor laser 23 (see Figure 5). If the oscillation wavelength of the semiconductor laser 23 changes in this way, the phase difference between the two lights combined by the half mirror 24b changes. This changes the phase bias during intensity modulation, and therefore the average intensity of the output light. Therefore, if feedback is constantly applied so that the output of the differential amplifier 35 is zero, the phase bias can be maintained at "1/2", and the output light can be stabilized. Moreover, in this case, since the intensities of the output lights detected by the photodetectors 32 and 34 are in an inverted relationship with each other as described above, the difference between them is a doubled phase bias deviation that appears large, making it possible to detect the deviation extremely accurately.

FIG. 14 is a diagram showing a configuration for realizing a second embodiment of the method for stabilizing an optical interferometer according to the present invention.

In this embodiment, instead of applying feedback to the oscillation wavelength of the semiconductor laser 23, feedback is applied to the optical path length difference to achieve stabilization.
The mirror 24c of the mirror 24 is fixed to a piezo element 36, and the piezo element 36 is driven according to the output of a differential amplifier 35.
By moving the half mirror 24a in the direction of the arrow,
through the mirror 24c and the mirror 24d to the half mirror 24b
The actual length of the optical path from the light source to the target is changed.

Even if the optical path length difference is changed in this way, the half mirror 24b
Therefore, by constantly applying feedback so that the output of the differential amplifier 35 becomes zero, the phase bias can be maintained in an ideal state, as in the above embodiment, and the output can be stabilized.

FIG. 15 is a diagram showing a configuration for realizing a third embodiment of the method for stabilizing an optical interferometer according to the present invention.

In this embodiment, in order to change the optical path length difference as in the second embodiment, a heater is used instead of providing a piezoelectric element 36.
The heater 37 is disposed on the optical path between the mirrors 24c and 24d.
is driven in response to the output of the differential amplifier 35. If the temperature changes due to the heater 37,
The refractive index of that portion also changes, and the optical path length changes accordingly. In this way, even if feedback is applied to the optical path length difference, the output of the optical interferometer 24 can be stabilized in the same way as in the above embodiment.

FIG. 16 is a diagram showing a configuration for realizing a fourth embodiment of the method for stabilizing an optical interferometer according to the present invention, in which this embodiment is applied to the optical interferometer 10 shown in FIG. 2.

In this embodiment, similar to the embodiment shown in Fig. 13, first, two output lights whose light intensities are inverse to each other are detected by photodetectors 32 and 34 via half mirrors 31 and 33, respectively. Then, the difference between them is calculated by a differential amplifier 35, and based on this, feedback is applied to the oscillation wavelength of a semiconductor laser 38 with a steady optical output. Then, since the optical path lengths of the optical waveguides 2 and 3 are different from each other, if the oscillation wavelength changes, the optical waveguides 2 and 3 will
The phase difference of the light passing through 3 also changes, and this causes the optical interferometer
The average intensity of the output light of 10 changes.
If feedback is constantly applied so that the output of 35 becomes zero, the output of the optical interferometer 10 can be stabilized in the same way as in the above embodiment.

FIG. 17 shows a configuration for realizing a fifth embodiment of the method for stabilizing an optical interferometer according to the present invention. This is an example of application to an optical interferometer 10' in which the lengths of the two optical waveguides 2 and 3 in the optical interferometer 10 shown in FIG. 2 are made equal to each other.

In this embodiment, the phase difference is changed by applying feedback to the optical path length difference between the optical waveguides 2 and 3. That is, in FIG.
The applied voltage is changed, and the refractive index changes accordingly, thereby changing the optical path length of the optical waveguide 3.
In this embodiment, as in the previous embodiments, the phase bias can be maintained in an ideal state, and the output of the optical interferometer 10' can be stabilized.

In the above embodiments, the photodetectors 32, 34 and the differential amplifier 35 are used as the means for detecting the average intensity of the output light from the optical interferometers 24, 10, 10'. However, the present invention is not limited to these and any configuration may be used as long as it can detect the average intensity obtained by integrating the intensity of the output light from the optical interferometer over a time slower than the phase modulation.

The stabilization method of the optical interferometer of the present invention is also the same as that shown in FIGS.
In addition to the Mach-Zehnder type optical interferometer shown in the figure, it is of course possible to apply the present invention to various optical interferometers, such as Michelson type and Fabry-Perot type.

Furthermore, when the present invention is applied to an optical communication system, two types of arrangements can be considered as shown in Figures 4A and 4B, as mentioned above. For example, the configuration in which feedback is applied to the optical path length difference as shown in Figures 14, 15 and 17 is
4A and 4B, but is suitable for both arrangements.
The configuration in which feedback is applied to the oscillation wavelength as shown in Fig. 15 and Fig. 16 is suitable for the arrangement of Fig. 4A. When feedback is applied to the oscillation wavelength in the arrangement of Fig. 4A, there is an advantage in that the response is fast, while when feedback is applied to the optical path length difference in the arrangement of Fig. 4B, there is an advantage in that direct optical amplification can be easily performed even if the light is attenuated during transmission.

Next, FIG. 18 is a block diagram showing the basic configuration of an optical modulation device according to the present invention, and FIG. 19 is a diagram showing the configuration of a first embodiment thereof.

As shown in FIG. 19, the optical modulator of this embodiment comprises an AND circuit 50, a flip-flop 51, a capacitor 52, a semiconductor laser 53, an optical isolator 54, a bias current source 55, an inductance 5, a resistor 57, an optical interferometer 58 (half mirrors 58a, 58b) and a resistor 59.
and mirrors 58c, 58d), half mirrors 59, 60, 61, photodetectors 62, 63, a subtraction processing circuit 64, and a mirror driver 65. That is, when the configuration of this embodiment is compared with the configuration of FIG. 18, the AND circuit 50 and flip-flop 51
The circuit composed of these corresponds to the code conversion unit 40, and the capacitor 52
and resistor 57 corresponds to the differential circuit 41. Also, the semiconductor laser 53, the optical interferometer 58, and the bias current source 55
correspond to the semiconductor laser 42, the optical interferometer 43, and the bias current source 44, respectively.

In the optical modulation device configured as described above, transmission data consisting of "1" and "0" is applied to the clock terminal CK of the flip-flop 51, and a "1" in the transmission data causes the "1" and "0" at the output terminal Q to be inverted, while a "0" in the transmission data leaves the output terminal Q unchanged. Therefore, code conversion is performed in which the sign is inverted only when the transmission data is "1", and the signal is output from the AND circuit 50 in accordance with the transmission clock signal. This AND circuit 50 is used to convert the converted output signal into an RZ signal.

This converted output signal is supplied as a modulated current pulse ΔI to a semiconductor laser 53 via a capacitor 52 and is superimposed on a constant bias current _I0 from a bias current source 55. This constant bias current _I0 is selected to have a value that allows the semiconductor laser 53 to constantly oscillate and obtain output light of a desired intensity, as shown in Fig. 1, for example.

The output light of the semiconductor laser 53 is input to an optical interferometer 58 via an optical isolator 54. The basic configuration of this optical interferometer 58 is the same as that of the optical interferometer 24 shown in FIG. 9 etc. One optical signal branched by a half mirror 58a is input to the half mirror 58b.
The other optical signal is propagated along the path of mirrors 58c and 58d and half mirrors 60 and 58b (optical delay circuit), respectively, and the relative delay time at half mirror 58b is set to one time slot and the relative phase difference is set to 0 or π, causing optical interference by self-homodyne.
It should be noted that the optical delay circuit may use optical fibers instead of the mirrors 58c and 58d.

Also, half mirrors 59, 60, and 61, photodetectors 62 and 63,
The subtraction processing circuit 64 and mirror driver 65 are an example of a configuration for stabilizing the output of the optical interferometer 58, similar to the configuration shown in Fig. 14. Here, the subtraction processing circuit 64 can be, for example, a differential amplifier, and the mirror driver 65 can be, for example, a piezoelectric element.

In this embodiment, similarly to the semiconductor laser modulation method shown in FIG. 5, the modulation current pulse ΔI is applied to the semiconductor laser 53 so as to satisfy the above-mentioned formula (1).
The product of the time integral of one time slot of (t) and the chirping coefficient k is selected to be π or -π. To this end, a differentiation circuit consisting of a capacitor 52, a semiconductor laser 53, and a resistor 57 is connected to a modulation current pulse ΔI that satisfies the above relationship.
(t) is obtained, and the phase φ of the output light from the semiconductor laser 53 is modulated to be π or −π.

In the optical interferometer 58, as described above, the relative delay time between one branched optical signal and the other branched optical signal is one time slot, and the relative phase difference is set to 0 or π. Therefore, if the relative phase difference is set to π, for example, when the phase φ of the output light from the semiconductor laser 53 is continuously the same, the phase difference of the optical signal input to the half mirror 58b becomes π, and the resulting modulated output light intensity becomes "0". On the other hand, when the phase φ of the output light from the semiconductor laser 53
is changed to π, the phase difference of the optical signal input to the half mirror 58b becomes 2π during one time slot, and therefore the modulated output intensity becomes "1" in relative value.
The semiconductor laser 53 is driven by superimposing a modulation current pulse ΔI(t) on a constant bias current _I0 , and the phase φ of the output light from the semiconductor laser 53 is changed to π or −π. The output is then applied to the optical interferometer 58, thereby achieving optical intensity modulation.

Furthermore, if the relative phase difference in the optical interferometer 58 is set to 0, when the phase φ of the output light from the semiconductor laser 53 is changed to π, the phase difference becomes π during one time slot, and the modulated output light intensity becomes "0", while the modulated output light intensity during the other time slots becomes "1" in relative value.

Next, the operation of this embodiment will be specifically explained with reference to FIGS. 20(a) to 20(g).

For example, transmission data consisting of "1" and "0" as shown in Fig. 20(a) is converted into a code by the flip-flop 51 as shown in Fig. 20(b). That is, the sign of the converted output signal is inverted for each "1" in the transmission data, and transmission data of "01101001" becomes a converted output signal of "01001110". This converted output signal is then applied to the semiconductor laser 53.
When the modulated current pulse ΔI is applied to the semiconductor laser 53, the waveform shown in Fig. 20(c) is obtained. However, when the "1" is an isolated pulse waveform, the oscillation frequency ω of the semiconductor laser 53 can be changed so that the phase φ is π as described above.
In the case of a pulse waveform in which "1" continues, this corresponds to a case in which the average drive current increases, and it becomes impossible to give the desired change in phase φ.
The converted output signal is differentiated using a differentiating circuit 41 (capacitor 52, resistor 57) and converted into a modulated current pulse as shown in FIG. 20(d).

As a result, a change in the oscillation frequency ω corresponding to the modulation current pulse occurs in the semiconductor laser 53. At this time, the output optical phase φ of the semiconductor laser 53 corresponds to the time integral of the oscillation frequency ω as described above, and is therefore as shown in Figure 20(e). In other words, even if the converted output signal is a series of "1", the semiconductor laser 53 can be driven so that a change of π occurs in its output optical phase φ.

Furthermore, when an optical signal having such a phase passes through the optical interferometer 58, an optical signal having a phase shown by the solid line in Figure 20(e) and a delayed optical signal having a phase shown by the broken line, which is delayed from this optical signal by one time slot time T, are incident on the half mirror 58b. Therefore,
The phase difference of the optical signal incident on the half mirror 58b is
As shown in Figure 0(f), when the converted output signal is "0", it is π, and when it is "1", it is 2π or 0.
Alternatively, optical signals with a phase difference of 0 interfere with each other at the half mirror 58b, thereby obtaining output light whose intensity is modulated in accordance with the transmission data, as shown in FIG. 20(g).

According to this embodiment, the bias current is reduced by 55 to the semiconductor laser.
The bias current _I0 supplied to the semiconductor laser 53 is selected according to the characteristics of the semiconductor laser 53, and is set to, for example, 60 mA. On the other hand, the modulated current pulse ΔI
can be set to, for example, 11 mA. Therefore, since the modulation current pulse is small compared to the bias current, the chirping problem seen in conventional systems does not occur. Furthermore, the optical interferometer 58 provides an optical delay of one time slot in advance and sets the relative phase difference to 0 or π, and since the connection loss is small, very efficient intensity modulation can be achieved.

In addition, the half mirrors 59, 60, 61, the photodetector 62,
63, a subtraction processing circuit 64, and a mirror driver 65 are used to move the mirror 58c by the mirror driver 65 so that the output of the subtraction processing circuit 64 becomes zero, thereby applying feedback to the optical path length difference of the optical interferometer 58, thereby stabilizing the output of the optical interferometer 58 in the same way as in FIG. 14.

Next, FIG. 21 is a circuit diagram showing a differentiating circuit according to a second embodiment of the optical modulation device of the present invention.

The circuit shown in the figure is composed of a code converter 71, a capacitor 72, a semiconductor laser 73, a stub 74, a resistor 75, and an inductor 76. The code converter 71 converts the code of the transmitted data in the same manner as in the previous embodiment and generates a converted output signal with a pulse width of 1/2 a time slot. The length l of the stub 74 is selected so that the polarity of the converted output signal is inverted and reflected, and input to the capacitor 72 after 1/2 a time slot. Therefore, the modulation current pulse applied to the semiconductor laser 73 via the capacitor 72 has a waveform similar to that shown in Figure 20(d). This modulation current pulse is superimposed on a constant bias current from a bias current source and applied to the semiconductor laser 73. As a result, the semiconductor laser 73 produces output light with a phase φ as shown in Figure 20(e).

22 is a diagram showing the configuration of an optical modulator stabilization means according to a third embodiment of the optical modulation device of the present invention.
As with the stabilization method shown in Figure 13, the output signal of the subtraction processing circuit 64 is fed back to the oscillation wavelength of the semiconductor laser 53 by adding the output signal of the subtraction processing circuit 64 to the bias current source 55, so that the output signal of the subtraction processing circuit 64 is always zero.
The phase bias during intensity modulation of the optical interferometer 58 is set to the ideal state (the
This allows the output to be stabilized by maintaining the output at a constant value (see Figure 3A).

Although the optical interferometer 58 described above is of the Mach-Zehnder type, it may be configured with an optical waveguide, or alternatively, a Michelson or Fabry-Perot type optical interferometer may be used.

In addition, in the circuit for stabilizing the optical interference detector shown in FIG. 22, the occurrence probability of "1" and "0" in the transmission data is set to 1/2 in the above-mentioned FIG. 19. However, if other occurrence probabilities are set, the subtraction processing circuit 64 processes the data in accordance with the occurrence probability, and the photodetectors 62, 63
Alternatively, feedback may be applied so that the difference between the detection signals is a predetermined value.

INDUSTRIAL APPLICABILITY As described above, the semiconductor laser modulation method, optical interferometer stabilization method, and optical modulation device of the present invention are particularly useful in the following applications:
This is useful for optical communication systems such as those shown in Figures 4A and 4B, and of course it can also be applied to various other optical devices.

Claims (13)

[Claims] [Claim 1] A semiconductor laser device that drives a semiconductor laser with a modulated current pulse superimposed on a bias current of a constant value, comprising: a code conversion unit that inverts the sign only when transmission data is either 1 or 0; a differentiation circuit that differentiates the converted output signal of the code conversion unit; and a differential output signal of the differentiation circuit that is inverted by 1 of the modulated current pulse.
A semiconductor laser device characterized by comprising a semiconductor laser driving means for driving the semiconductor laser by superimposing a modulated pulse current on a constant bias current such that the product of the time integral of the time slot and the chirping coefficient has a phase amount of π or -π. [Claim 2] The semiconductor laser device according to claim 1, further comprising an optical interferometer which receives as input a phase-modulated optical signal which is the output light of the semiconductor laser, branches the input light, passes the branched light through an optical delay circuit which sets the relative delay phase difference between one side and the other to 0 or π and sets the relative delay time to 1 time slot, and then combines the branched light, and obtains intensity-modulated output light based on the phase difference between the two combined lights. 3. The semiconductor laser device according to claim 2, wherein the average intensity obtained by integrating the intensity of the output light of the optical interferometer at a speed slower than the phase modulation is used to apply feedback to the optical wavelength of the input light, thereby correcting the phase bias during the intensity modulation. [Claim 4] A semiconductor laser device according to claim 2, characterized in that the average intensity obtained by integrating the intensity of the output light of the optical interferometer at a speed slower than that of the phase modulation is used to apply feedback to the optical path length difference between the two optical paths, thereby correcting the phase bias during the intensity modulation. 5. The semiconductor laser device according to claim 2, wherein a Mach-Zehnder type interferometer is used as the optical interferometer. 6. The semiconductor laser device according to claim 2, wherein a Michelson type interferometer is used as the optical interferometer. 7. The semiconductor laser device according to claim 2, wherein a Fabry-Perot interferometer is used as the optical interferometer. 8. The semiconductor laser device according to claim 2, wherein the optical interferometer is provided at an output section of the semiconductor laser. 9. The semiconductor laser device according to claim 2, wherein the output light of the semiconductor laser is input to a transmission optical fiber and converted into an optical pulse of 0 or 1 by the optical interferometer at the output end of the transmission optical fiber. 10. The semiconductor laser device according to claim 1, wherein said differentiating circuit is provided with a stub. 11. The semiconductor laser device according to claim 2, wherein the differentiating circuit is provided with a stub. [Claim 12] A modulation method for a semiconductor laser device in which a semiconductor laser is driven by a modulation current pulse superimposed on a constant bias current, comprising: inverting the sign only when the transmitted data is either 1 or 0; differentiating the code-converted converted output signal; and converting the differentiated output signal into a modulation pulse current such that the product of the time integral of one time slot of the modulation current pulse and the chirp coefficient has a phase amount of π or -π, and superimposing the modulated pulse current on the constant bias current. [Claim 13] A modulation method for a semiconductor laser device according to claim 12, characterized in that a phase-modulated optical signal, which is output light from the semiconductor laser, is branched and passed through an optical delay circuit in which the relative delay phase difference between one side and the other side is set to 0 or π and the relative delay time is set to 1 time slot, and then the two beams are combined to produce output light that is intensity-modulated based on the phase difference between the two combined beams.