JPH09199779A - Frequency stabilized light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain output beams whose frequency is stabilized in the long term by adjusting injection current to a phase adjusting area so as to accord the wavelength of the output beams of the laser with the Bragg wavelength of a diffraction lattice reflector and simultaneously adjusting the injection current to the diffraction lattice reflector so as to fix the Bragg wavelength. SOLUTION: The output beams from a semiconductor laser 1 are projected by optical fiber. The beams, however, are partially branched in the middle by a fiber coupler 9, and are inputted to a phase synchronizing detector 13 through a photodetector 11 which performs photoelectric transfer. A part of the beam output is detected by phase synchronization by a fine modulation signal, the deviation between the Bragg frequency and a vertical mode frequency is detected, and since a signal based on such deviation is fed back to a PC area 3 as an error signal, the vertical mode frequency is stabilized at the Bragg frequency. Therefore, output beams of a stabilized frequency are obtained in the long term by separately stabilizing the Bragg wavelength and the vertical mode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency-stabilized light source for fixing the wavelength of output light of a semiconductor laser to a constant value and obtaining laser light with a stable wavelength of this semiconductor laser.

[0002]

2. Description of the Related Art At present, research on an optical frequency (wavelength) multiplex communication system has been actively conducted in order to increase the amount of communication information. In this case, a semiconductor laser is mainly used as a light source for transmission, and a semiconductor laser having a so-called single-mode oscillation characteristic having good light coherency and a narrow spectrum is required for frequency multiplexing. Furthermore, it is necessary that the optical frequency be fixed at a constant value and stable over a long period of time.

The conventional distributed feedback (DFB) semiconductor laser exhibits excellent single-mode oscillation characteristics.
It is a mainstream light source for current long-distance optical communication systems, and is expected as a light source for future optical frequency (wavelength) multiplex communication systems, but a laser with an absolute value of oscillation frequency that is a desired constant value is manufactured. It is extremely difficult to do so, and the manufacturing error from the target frequency (wavelength) is usually several hundred GHz.
(Wavelength of several nm). Further, although this frequency adjustment is possible, the adjustment range is extremely narrow. On the other hand, 3
In the electrode distributed reflection (DBR) semiconductor laser, the oscillation frequency is 1 THz (10 at wavelength) by injecting current into the distributed reflection (DBR) region and the phase adjustment (PC) region.
It is possible to change the size of the DFB laser by about 0.1 nm, and therefore, strictness in manufacturing is not required as compared with the DFB laser, and the manufacturing tolerance is relaxed. Therefore, the DBR semiconductor laser is
Since it is easier to manufacture than the B semiconductor laser and the desired wavelength can be easily fixed, it is regarded as a promising light source for multiplexing.

Since the oscillation frequency of the semiconductor laser greatly varies depending on the temperature, it is necessary to control the temperature for that purpose. However, even if the temperature control is properly performed, the active region is deteriorated in the long-term use. Since the oscillation frequency also fluctuates due to the change in the element characteristics and the change in the laser characteristics, a control circuit for stabilizing some frequency (wavelength) is required.

FIG. 5 shows an example of a frequency stabilized light source using a conventional DBR semiconductor laser. The DBR semiconductor laser 1 has a distributed reflection (DBR) region 2, a phase adjustment (PC) region 3, and a gain region 4, each of which is provided with an electrode, and electrodes of the DBR and PC regions are different. It is connected to a bias current source 7 via resistors 5 and 6, and the electrode of the gain region 4 is connected to another bias current source 8. Part of the light output from the DBR semiconductor laser 1 is input to the interference filter 10 by the fiber coupler 9, then converted into an electric signal by the photodetector 11, and added to the DBR region 2 and the PC region 3. Phase of modulated signal source 12 and synchronous detector 1 from which a frequency modulated signal can be obtained
3 is compared, and the shift amount between the resonance frequency of the interference filter 10 and the optical frequency is obtained. Then, the laser oscillation frequency is feedback-controlled based on this shift amount, and the DBR region 2 and the PC are based on the ratio of the resistance values of the resistors 5 and 6.
A current is injected into the region 3 and the oscillation frequency is controlled by changing the current. In this way, the oscillation frequency is once stabilized by the feedback control to the resonance frequency of the filter.

[0006]

However, the DBR semiconductor laser shown in FIG. 5 has a problem that it lacks long-term stability, although frequency stabilization can be achieved to some extent. That is, in the DBR semiconductor laser, only a single longitudinal mode in the vicinity of the Bragg frequency (wavelength) based on the spatial frequency of the diffraction grating formed in the distributed reflection region is selected from a large number of resonance longitudinal modes determined by the laser resonator. , Oscillates in single mode. In this case, in the DBR semiconductor laser, the Bragg frequency and the oscillation longitudinal mode frequency do not always match. For example, distributed reflection (DB
When current injection is performed only in the R region, the Bragg frequency (wavelength) can be changed, but the longitudinal mode frequency (wavelength) does not follow the change, so the oscillation frequency changes with mode jump. . In the configuration shown in FIG. 5, the current values in the DBR region and the PC region are simultaneously changed by using the two resistors 5 and 6 as appropriate values, and the longitudinal mode frequency related to the PC region and the Bragg frequency related to the DBR region are set. If they are matched, the oscillation frequency can be continuously changed.

However, even in the configuration shown in FIG. 5, there is a difference in the manner of changing the Bragg frequency and the manner of changing the longitudinal mode frequency, and therefore it is difficult to completely match these two frequencies. Further, even if these two frequencies are made to coincide with each other, if long-term element variation such as deterioration of the active region portion occurs due to long-term use of the laser, for example, if optical refraction varies, a change in longitudinal mode frequency occurs, Furthermore, there is a risk that the oscillation longitudinal mode jumps to the adjacent mode.
It is impossible to tune to zero. Thus, it has been difficult in the past to fix the Bragg frequency to a constant value in combination with matching the Bragg frequency with the longitudinal mode frequency over a long period of time.

In view of the prior art, it is an object of the present invention to provide a frequency-stabilized light source using a distributed reflection (DBR) type semiconductor laser whose oscillation frequency is always stable even if the element characteristics fluctuate over a long period of time.

[0009]

The present invention for achieving the above object has the following specific features. (1) A distributed Bragg reflector semiconductor laser having a diffraction grating reflection region, a phase adjustment region, and a gain region is provided, and the phase adjustment is performed so that the wavelength of the output light of this laser matches the Bragg wavelength of the diffraction grating reflection region. It has a function of adjusting the refractive index of the region and a function of fixing the Bragg wavelength to one wavelength. (2) A distributed Bragg reflector semiconductor laser having a diffraction grating reflection region having a first electrode and a phase adjusting region having a second electrode is provided, and a direct current and intensity modulation are applied to the first electrode. And a function of setting the current to be injected into the second electrode so that the signal obtained by phase-coherently detecting the output light of the laser by injecting a current and the wavelength of the output light of the laser are the frequency reference. The above-mentioned first to match the reference wavelength of the instrument
And the function of setting the direct current to be injected into the electrode. (3) In the above (2), the distributed reflection type semiconductor laser has a third electrode in the gain region, and a function of setting a current injected into the third electrode so that the optical output intensity of the laser is constant. Have.

First, a minute modulation signal is applied to the DBR region and the output light is phase-coherently detected to detect the amount of deviation between the Bragg frequency and the longitudinal mode frequency, and the injection current in the PC region is controlled based on this amount of deviation. The Bragg frequency and the longitudinal mode frequency are matched by. That is, as shown in FIG. 2, a change in light intensity with respect to the injection current in the DBR region (b), an error signal (c) after phase-coherent detection of the optical output by modulating the DBR current, and a change in wavelength. It has the characteristics of (a), and the point at which the light intensity becomes maximum in FIG. 2 (b) shows the state where the longitudinal mode and the Bragg wavelength match. This is because the reflectance of the DBR reflector becomes maximum at the Bragg wavelength. The point where the light intensity is maximum corresponds to the point where the error signal becomes zero. As a result, the longitudinal signal frequency can be matched with the Bragg frequency by appropriately amplifying the error signal B and feeding it back to the PC region.

[0011]

DETAILED DESCRIPTION OF THE INVENTION An embodiment of the present invention will now be described with reference to FIGS. In FIG.
The same parts as those in FIG. 3-electrode distributed reflection type (DB
R) Diffraction grating reflection (DBR) region 2 of the semiconductor laser 1,
Bias current sources 14, 15 and 8 are connected to the phase adjustment (PC) region 3 and the active region 4, respectively, so that current can be injected. Further, a minute modulation signal is applied to the DBR region 4 from the modulation signal source 12. The output light of the semiconductor laser 1 is led out by an optical fiber, but partly branched by a fiber coupler 9 on the way, and a phase-locked detector 13 is passed through a photodetector 11 which performs photoelectric conversion.
Is input to In this synchronous detector 13, a part of the optical output is phase synchronously detected by the minute modulation signal, and the amount of deviation between the Bragg frequency and the longitudinal mode frequency is detected. A signal based on this shift amount is fed back to the PC area 3 as an error signal. Therefore, the longitudinal mode frequency is stabilized at the Bragg frequency.

The optical output partially extracted by the fiber coupler 9 is further partially extracted by the fiber coupler 16, and a cross discrimination type filter 17 (for example, a volume holographic) that uses the optical frequency as a reference. , And only specific wavelengths are output. Two photodetectors 18 and 19 are arranged at the next stage of the filter 17 for photoelectric conversion, and the respective outputs of the photodetectors 18 and 19 are differential amplifier 20.
2 inputs, and each of these inputs is output after the difference is taken. The filter 17 is a filter for discriminating the reference frequency, and the photodetectors 18 and 19 are shown in FIG.
An electric signal having a bimodal transmission frequency as shown in (a) and (b) is obtained. Therefore, the output of the differential amplifier 20 is as shown in FIG. 3 (c), and becomes zero with the same electric signal input. Therefore, if the output of the differential amplifier 20 is fed back as the injection current of the DBR region 2, the oscillation frequency will be stabilized at the optical reference filter frequency.

Further, in FIG. 1, in order to make the output intensity of light constant, a part of the output of the photodetector 11 is input to the differential amplifier 21 to take the difference from the reference voltage, and this signal is sent to the active region 4
Is fed back as the injection current C of.

FIG. 4 shows the result of an experiment to confirm how the oscillation wavelength changes when the temperature of the laser is changed, instead of the fact that the element characteristics change due to long-term use. In FIG. 4, the symbol Δ indicates the case according to FIG. 5 in which the error signal with respect to the reference optical filter wavelength is fed back to the phase adjustment region without separately stabilizing the Bragg wavelength and the longitudinal mode, and the symbol ◯ according to the present embodiment. The case is shown. This figure 4
As apparent from the above, in the conventional method, when the temperature changes by about 3 ° C., the longitudinal mode wavelength jumps to the adjacent mode and the oscillation wavelength deviates from the stabilization range immediately. It is stabilized. This is because the Bragg wavelength and the longitudinal mode wavelength are always controlled to match. Therefore,
The stabilization of the present invention indicates that the device is resistant to long-term changes in device characteristics.

In FIG. 1, the cross discrimination type filter 17 is used as the optical reference frequency filter, but it is also applicable to the case where an interference type filter such as a Fabry-Perot etalon or a gas absorption cell is used. The temperature control of the DBR laser is controlled to be constant using a Peltier element or the like.

[0016]

As described above, according to the present invention, by stabilizing the Bragg wavelength and the longitudinal mode separately, it is possible to obtain output light whose frequency (wavelength) is stabilized for a long period of time.

[Brief description of drawings]

FIG. 1 is a configuration diagram illustrating an embodiment of the present invention.

FIG. 2 is a characteristic diagram of an oscillation wavelength, a light intensity, and an error signal after phase-coherent detection with respect to a DBR current.

FIG. 3 is a diagram showing the output characteristics of a photodetector and the output of a differential amplifier with respect to the wavelength of input light.

FIG. 4 is a result comparison diagram of a conventional method and the present invention.

FIG. 5 is a configuration diagram of a conventional example.

[Explanation of symbols]

1 1.55 μm 3 Electrode Distributed Reflection (DBR) Laser 2 Distributed Reflection (DBR) Region 3 Phase Adjustment (PC) Region 4 Active Region 5,6 Resistor 7,14 Bias Current Source to DBR Region 7,15 To PC Region Bias current source 8 Bias current source to active region 9,16 Fiber coupler 10 Optical frequency reference filter (Fabry-Perot etalon) 11, 18, 19 Photodetector 12 Modulation signal source 13 Synchronous detector 17 Optical frequency reference filter ( Volume holography) 20,21 Differential amplifier

Claims (3)

[Claims] 1. A distributed Bragg reflector semiconductor laser having a diffraction grating reflection region, a phase adjustment region, and a gain region, wherein the wavelength of the output light of the laser matches the Bragg wavelength of the diffraction grating reflection region. A frequency-stabilized light source having a function of adjusting the refractive index of the phase adjustment region and a function of fixing the Bragg wavelength to one wavelength. 2. A distributed reflection type semiconductor laser having a diffraction grating reflection region having a first electrode and a phase adjustment region having a second electrode, wherein a direct current and intensity modulation are applied to the first electrode. The function of setting the current to be injected into the second electrode so that the signal obtained by phase-coherently detecting the output light of the laser by injecting the generated current and the wavelength of the output light of the laser are A frequency-stabilized light source having a function of setting the DC current to be injected into the first electrode so as to match the reference wavelength of the frequency reference device. 3. A distributed reflection type semiconductor laser having a third electrode in a gain region, and having a function of setting a current to be injected into the third electrode so that the optical output intensity of the laser becomes constant. 2. The frequency-stabilized light source described in 2.