JPH07249818A - Frequency stabilization method of frequency-variable semiconductor laser - Google Patents

Abstract

PURPOSE:To solve a problem of a thermal drift without using a frequency stabilization circuit and to stabilize an oscillation frequency in a high-speed changeover by a method wherein the combination of frequency control voltages at which the sum of heat capacities generated by the individual frequency control voltages becomes definite is selected and set. CONSTITUTION:The frequency-stabilizaiton method is provided with high-speed memories 111 to 11n in which pieces of information on (n) pieces of frequency-control electrodes for a frequency-variable laser 10 are stored and with D/A convertes 12 to 12 in which pieces of information read out from the individual high-speed memories are converted into frequency control voltages so as to be applied to the individual frequency control electrodes. Then, the combination of the frequency control voltages at which the sum of heat capacities becomes definite with reference to individual oscillation frequencies is decided by a procedure before an operation, and it is stored in the high-speed memories corresponding to the individual control electrodes. In the operation the oscillation frequencies are designated to the individual high-speed memories, and the combination of the frequency control voltages at which the sum of the heat capacities with reference to the oscillation frequencies becomes definite is given to the individual control voltages for the frequency-variable semiconductor laser 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency stabilizing method for a frequency tunable semiconductor laser in which an oscillation frequency is changed according to a frequency control voltage applied to a plurality of frequency control electrodes. Frequency tunable semiconductor laser is used for optical communication, optical information processing,
Used as a light source for optical measurement and other systems.

[0002]

2. Description of the Related Art For example, a three-electrode DBR laser is a frequency variable semiconductor laser whose oscillation frequency is changed according to a combination of frequency control voltages applied to a plurality of frequency control electrodes. As shown in FIG. 7, the three-electrode DBR laser is divided into a light emitting region having an active layer 71 for generating light, a distributed Bragg reflection (DBR) region having a diffraction grating 72, and a phase adjusting region for adjusting the phase of light. An oscillating electrode 73, a DBR electrode 74 and a PC electrode 75 are provided respectively. To change the oscillation frequency, use the frequency control electrode (D
The frequency control voltage (current) is applied to the BR electrode 74 and the PC electrode 75 to change the carrier density of the optical waveguide and change the refractive index thereof. Since the response of the refractive index change according to the frequency control voltage is fast, the 3-electrode DB
The R laser can change the oscillation frequency at high speed in a wide band.

On the other hand, in the semiconductor laser, since the refractive index of the optical waveguide changes slowly in response to temperature, fluctuation of the oscillation frequency (heat drift) occurs due to heat. In the frequency variable semiconductor laser, the frequency control voltage applied to the frequency control electrode causes a change in carrier density and also causes heat generation. Therefore, in an application where the oscillation frequency needs to be switched accurately at high speed, a high-speed frequency stabilizing circuit is used to compensate for thermal drift. The frequency stabilization method includes feedback control and feedforward control.

FIG. 8 shows the configuration of a frequency stabilizing circuit by feedback control. In the figure, the laser light generated by the frequency variable semiconductor laser 81 is extracted as output light at the output terminal 82, and a part of the laser light is received by the photoelectric converter 83 and frequency-voltage converted. This electric signal is input to the error detector 84 that detects the difference between the current oscillation frequency and the set frequency, and a correction voltage corresponding to the error is generated and fed back to the frequency control electrode of the frequency variable semiconductor laser 81.

FIG. 9 shows a drive current waveform and an oscillation frequency waveform during feedforward control. As shown in (1),
With a rectangular wave drive current, the oscillation frequency does not respond to a rectangular wave due to thermal drift. Therefore, as shown in (2), the drive current waveform is corrected so that the oscillation frequency has a rectangular wave response.

[0006]

In the conventional frequency stabilization method, in the feedback control method, the time required for frequency stabilization is limited by the line length and the time constant of the frequency stabilization circuit in addition to the response time of the laser element. Receive. Also,
In the feedforward control method, when the frequency switching pattern is random, the drive current waveform must be controlled immediately by calculation, and a high-speed arithmetic device is required. Thus, in any frequency stabilization method,
An increase in the amount of hardware and an increase in frequency switching time cannot be avoided.

The present invention provides a frequency stabilizing method for a frequency tunable semiconductor laser which solves the thermal drift problem without using a frequency stabilizing circuit and can stabilize the oscillation frequency during high speed switching. To aim.

[0008]

According to the present invention, an oscillation frequency and a heat generation amount for a combination of frequency control voltages applied to a plurality of frequency control electrodes are measured and stored, and a frequency control corresponding to a predetermined oscillation frequency is performed. From the combinations of voltages, a combination in which the sum of the amount of heat generated by the application of each frequency control voltage is constant is selected and set.

Further, for each oscillation frequency, a combination of frequency control voltages in which the sum of heat quantities generated by the application of the frequency control voltage is constant is stored, and a corresponding combination of frequency control voltages is read every time the oscillation frequency is switched. To set.

The heat generation amount with respect to the frequency control voltage is obtained by measuring the oscillation frequency F ₀ with respect to a predetermined frequency control voltage V ₀ ,
The oscillation frequency F is measured when the frequency control voltage V is maintained for a time sufficiently longer than the thermal time constant of the frequency tunable semiconductor laser and then set to the frequency control voltage V ₀ for a time sufficiently shorter than the thermal time constant. Frequency control voltage V to V
_The amount of heat generated by the change to ₀ is measured from the difference between the oscillation frequencies F and F ₀ .

[0011]

The oscillating frequency and the heat generation amount for the combination of the frequency control voltages are measured, and the combination of the frequency control voltages for which the sum of the amount of heat generated by the application of the frequency control voltage is constant is determined for each oscillation frequency. Then, the frequency control voltage corresponding to the oscillation frequency is set based on this combination. As a result, even if the oscillation frequency is switched, it is possible to reduce the change in the amount of heat generation, and it is possible to reduce the thermal fluctuation that accompanies the switching of the oscillation frequency. Since the amount of change in the laser temperature and the amount of change in the oscillation frequency are in a substantially proportional relationship, variation in the oscillation frequency due to heat (thermal drift) can also be reduced by reducing thermal variation.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of a frequency stabilizing method for a frequency tunable semiconductor laser according to the present invention.

(1) shows a processing procedure performed before operation. The oscillation frequency is measured for a combination of frequency control voltages applied to the plurality of frequency control electrodes. The calorific value for each frequency control voltage combination is measured.
From combinations of frequency control voltages corresponding to a predetermined oscillation frequency, a combination in which the sum of heat amounts generated by application of each frequency control voltage is constant is determined.

(2) shows the basic structure of a frequency variable semiconductor laser driven based on the method of the present invention. High-speed memories 11 ₁ to 11 that store frequency control voltage information for n frequency control electrodes of the frequency tunable semiconductor laser 10, respectively.
11n and D / A converters 12 _{1 to} 12n for converting the frequency control voltage information read from each high-speed memory into a frequency control voltage (current) and applying it to each frequency control electrode. Before operation, a combination of frequency control voltages with which the sum of heat generation amounts is constant for each oscillation frequency is determined by the procedure described above, and stored in each high-speed memory corresponding to each frequency control electrode. In operation, when an oscillation frequency (channel) is designated for each high-speed memory, a combination of frequency control voltages that gives a constant sum of heat generation amounts to the oscillation frequency is given to each frequency control electrode of the frequency variable semiconductor laser 10.

In the following, when the 3-electrode DBR laser is used as the variable frequency semiconductor laser, the processes (1) to (3) performed before the operation will be specifically described. Two frequency control electrodes (DB
Frequency control voltage V _DBR applied to the R electrode and PC electrode,
The oscillation frequency with respect to V _PC is measured using an optical wavelength meter or the like.

FIG. 2 shows the relationship between the frequency control voltages V _DBR and V _PC and the oscillation frequency. It can be seen that a plurality of V _PC can be taken for each oscillation frequency f _{0 to} f ₄ . That is, the frequency control voltage V _PC has periodicity, and V _DBR and V _PC can take a plurality of combinations for a predetermined oscillation frequency. Incidentally, the frequency control of the conventional frequency tunable semiconductor laser is performed by the frequency control voltages V _DBR and V _DBR connected by the line A.
_It was done in the context of increasing _PCs at the same time.

The amount of heat generated with respect to the frequency control voltages V _DBR and V _PC is measured. However, since the temperature inside the semiconductor laser cannot be directly measured, it is indirectly obtained by utilizing the temperature dependence of the measured oscillation frequency. Ie 1
At a modulation frequency below MHZ, a semiconductor laser has approximately κ =-
The oscillation frequency is modulated by the temperature at a rate of 10 GHz / ° C. By utilizing this property, it is possible to know the temperature change of the semiconductor laser by measuring the frequency shift, and it is possible to measure the heat generation amount corresponding to the thermal drift.

FIG. 3 shows a system configuration for measuring the amount of heat generated with respect to the frequency control voltages V _DBR and V _PC . In the figure, the DBR electrode and P of the 3-electrode DBR laser 31 are shown.
The frequency control voltage V from the high speed variable voltage source 32 is applied to the C electrode.
_DBR, V _PC is applied. The output light of the three-electrode DBR laser 31 is input to the frequency discriminator 33 having high resolution with respect to time and frequency, and the frequency control voltages V _DBR , V _DBR , V
Oscillation frequency for _PC is measured. Frequency discriminator 33
The output light of the 3-electrode DBR laser 31 to an equal intensity of 2
The optical coupler 34 is branched, two Fabry-Perot filters 35a and 35b having an equal cycle (for example, 100 GHz) whose transmission band is slightly shifted, and a frequency error detector 36. The frequency error detector 36 is configured to detect a frequency error with respect to a predetermined frequency from the intensity difference between the transmitted lights of the two Fabry-Perot filters (FP) 35a and 35b, and is observed by the oscilloscope 37.

Frequency control voltage V using this system
_DBR, the calorific value of the measurement for V _PC, is described with reference to FIG. First, the predetermined frequency control voltage V _DBR0 ,
The oscillation frequency F ₀ with respect to V _PC0 is measured. Next, the output voltage of the high-speed variable voltage source 32 is kept at V _DBR and V _PC for a time T ₁ (1 ms or more) that is sufficiently longer than the thermal time constant of the three-electrode DBR laser 31, and subsequently, it is kept above the thermal time constant. V _DBR0 and V _PC0 are set for a sufficiently short time T ₂ (10 ns to 1 μs), and the oscillation frequency F at that time is measured. The difference between the oscillation frequencies F and F ₀ measured in this way (F−F ₀ )
A frequency shift due to heat _{generation, (F-F 0) /} κ , the frequency control voltage V _DBR, frequency from V _PC control voltage V _DBR0,
It becomes the amount of heat generation due to the change to V _PC0 . Such measurement is performed by changing the combination of the frequency control voltages V _DBR and V _PC .

FIG. 5 shows the relationship between the frequency control voltages V _DBR and V _PC and the amount of heat generation. By distributing the frequency control voltages V _DBR and V _PC according to the heat generation amounts h _{0 to} h ₆ , it is possible to form isothermal lines as shown in the figure.

Among the combinations of the frequency control voltages V _DBR and V _PC corresponding to a predetermined oscillation frequency, the combination in which the sum of the heat generated by the application of each frequency control voltage is constant is determined as follows. . The relationship between the frequency control voltages V _DBR and V _PC and the oscillation frequencies f _{0 to} f ₄ shown in FIG. 2 and the frequency control voltages V _DBR and V _PC and the heat generation amount h shown in FIG.
_When the relationship of _{0 to} h ₆ is displayed in an overlapping manner, it becomes as shown in FIG. Here, the frequency control voltage V for each oscillation frequency
_DBR, the V _PC, selects a combination in line as possible to isothermal line. For example, a combination of frequency control voltages V _DBR and V _PC connected by line B is selected.

Conventionally, the frequency control voltages V _DBR and V _PC along the line A, which does not consider the isothermal lines at all, were selected, so that they were affected by thermal drift. In the present invention, the fact that a plurality of frequency control voltages V _PC for a predetermined oscillation frequency can be used is utilized, and in the combination with the frequency control electrode V _DBR , the heat generation amount V becomes substantially constant for each oscillation frequency.
Determine the combination with the _PC . This is associated with the oscillation frequency, held in the high-speed memory corresponding to the DBR electrode and the PC electrode, and the corresponding frequency control voltages V _DBR and V _PC are read and set every time the oscillation frequency is switched. As a result, it is possible to reduce thermal fluctuations caused by switching the oscillation frequency.

The embodiment described above is a 3-electrode DB.
It is applied to R laser, but DBR with 4 or more electrodes
For the laser, if the amount of heat generation and the oscillation frequency for each frequency control voltage are obtained in advance, frequency stabilization can be achieved by the same method.

[0024]

As described above, in the frequency stabilizing method of the present invention, when the oscillation frequency of the frequency variable semiconductor laser is switched, a combination of frequency control voltages with a small change in the heat generation amount is selected. As a result, it is possible to suppress frequency fluctuation (thermal drift) due to heat during frequency switching, and to stabilize the oscillation frequency during high speed switching. Further, the frequency stabilizing circuit by external control is not required, and high-speed switching can be easily supported.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a frequency stabilizing method for a frequency tunable semiconductor laser according to the present invention.

FIG. 2 is a diagram showing a relationship between frequency control voltages V _DBR and V _PC and an oscillation frequency.

FIG. 3 is a block diagram showing a system configuration for measuring heat generation amounts with respect to frequency control voltages V _DBR and V _PC .

FIG. 4 is a diagram illustrating a method of measuring the amount of heat generated with respect to frequency control voltages V _DBR and V _PC .

FIG. 5 is a diagram showing a relationship between frequency control voltages V _DBR and V _PC and heat generation amount.

FIG. 6 is a diagram illustrating a process of determining frequency control voltages V _DBR and V _PC .

FIG. 7 is a diagram showing a configuration of a three-electrode DBR laser.

FIG. 8 is a diagram showing a configuration of a frequency stabilizing circuit by feedback control.

FIG. 9 is a diagram showing a drive current waveform and an oscillation frequency waveform during feedforward control.

[Explanation of symbols]

 10 frequency variable semiconductor laser 11 high speed memory 12 D / A converter 31 3-electrode DBR laser 32 high speed variable voltage source 33 frequency discriminator 34 optical coupler 35 Fabry-Perot filter 36 frequency error detector 37 oscilloscope 71 active layer 72 diffraction grating 73 oscillation Electrode 74 DBR electrode 75 PC electrode 81 Frequency variable semiconductor laser 82 Output terminal 83 Photoelectric converter 84 Error detector

Claims (3)

[Claims] 1. A frequency stabilizing method for a frequency tunable semiconductor laser, comprising a plurality of frequency control electrodes, wherein an oscillation frequency is changed in accordance with a combination of frequency control voltages applied to the respective frequency control electrodes. The oscillation frequency and the amount of heat generated for each combination of frequency control voltages applied to the electrodes are measured and stored, and the amount of heat generated by the application of each frequency control voltage is selected from the combinations of frequency control voltages corresponding to the specified oscillation frequency. A frequency stabilizing method for a frequency tunable semiconductor laser, comprising selecting and setting a combination in which the sum of the above is constant. 2. The frequency stabilization method for a frequency tunable semiconductor laser according to claim 1, wherein a combination of frequency control voltages is stored for each oscillation frequency so that a sum of heat amounts generated by application of the frequency control voltage becomes constant. Then, each time the oscillation frequency is switched, a combination of corresponding frequency control voltages is read out and set, and the frequency stabilization method of the frequency tunable semiconductor laser is characterized. 3. A frequency stabilization method of a frequency tunable semiconductor laser according to claim 1, to measure the oscillation frequency F ₀ for a given frequency control voltage V _0, sufficiently than the thermal time constant of the frequency-variable semiconductor laser The oscillation frequency F is measured when the frequency control voltage V is maintained for a long time, and then the frequency control voltage V ₀ is set for a time sufficiently shorter than the thermal time constant.
A method for stabilizing the frequency of a frequency tunable semiconductor laser, characterized in that the amount of heat generated by the change from V ₀ to V ₀ is measured from the difference between the oscillation frequencies F and F ₀ .