JPS62162381A - Highly stabilized semiconductor laser light source - Google Patents

Abstract

PURPOSE:To control and stabilize laser electric current, by feeding back output lights outputted from light-emitting faces of a laser to light-emitting faces each other, and controlling the phase change of the light passing on optical paths such that a mean power of AC components of the light is minimized and a DC output is maximized or minimized. CONSTITUTION:A phase modulator 27 and a phase changer 26 are connected in a feed back path of a laser 21. The modulator 27 modulates the phase of light passing there, in accordance with an AC output signal from a signal source 28. A detector 29 photoelectrically converts light transmitted by a half mirror 24 and a phase detector 31 comparates its phase with that of the output from the signal source 28 so that a signal is generated in accordance with a difference in phase. A servo signal is generated by a servo circuit 32 in accordance with this signal and driving current is outputted by a drive circuit 33 to drive the laser 21. On the other hand, an AC component of the detector 29 is taken out by means of a low- pass filter 34 and a servo signal is generated by a servo circuit 35, so that a phase changer 26 performs DC phase change in the light passing there. A temperature controller 36 is caused to drive a Peltier element 37 by a servo signal from the servo circuit 35 and the laser 21 oscillates as determined by the driving current from the drive circuit 33. An operating temperature during the oscillation is determined by the operation of the Peltier element 37, and an optical frequency can be stabilized when there is no phase shift between the lights in both directions that are coupled within the laser.

Description

[Detailed description of the invention] [Summary] Output light from both light emitting surfaces of a semiconductor laser is returned to the light emitting surfaces on opposite sides through a closed optical path system, and a phase changer is inserted in the optical path system. A phase change is given to the passing light in accordance with the control signal, and a detection means is provided to detect the alternating current component and the direct current component of the frequency change in the light in the optical path system to generate an output, and the first control means The drive current supplied to the semiconductor laser is controlled so that the average power of the alternating current component of the output of the detection means is minimized, and the second control means is provided to control the drive current supplied to the semiconductor laser so that the average power of the alternating current component of the output of the detection means is maximized or minimized. By controlling the phase change in the phase changer so as to improve the frequency stability of the light generated by the semiconductor laser and realize a narrow spectrum band.

[Industrial application field]

The present invention relates to a highly stabilized semiconductor laser, and in particular to a highly stabilized semiconductor laser light source that has high frequency stability and a narrow spectrum band and is suitable as a light source for coherent optical communication. .

Conventionally, in optical communications via optical fibers, only amplitude information of light from a semiconductor laser, which is a light source, is used for the purpose of communication. For such purposes, stability of the light level generated by the semiconductor laser light source is required.
Frequency stability was not much of an issue.

However, as a new optical communication method, coherent optical communication that utilizes frequency information or phase information in light is being considered. A light source used for such optical communications must have sufficiently high frequency stability and an extremely narrow spectrum band, and there is a desire to realize such a highly stabilized semiconductor laser light source.

[Conventional technology]

As a highly stable semiconductor laser light source, a highly stabilized semiconductor laser light source is conventionally known in which the frequency of generated light is controlled using an etalon, which is a narrow band optical bandpass filter, as a wavelength reference.

FIG. 4 shows a conventional highly stabilized semiconductor laser light source, in which 1 indicates a semiconductor laser, and the generated light is branched through a half mirror 2, and a portion is converted into a plane wave through a lens system 3. and enters etalon 4. The etalon 4 is made of a plane-parallel glass plate with half mirrors on both sides, and a number of standing waves of a specific wavelength determined by its thickness stand between the two sides, thereby selectively passing light of that wavelength. The light that has passed through the etalon 4 is converged through a lens system 5, enters a detector 6, and is converted into an electrical signal. The other light branched by the half mirror 2 passes through an attenuator 7, receives a required attenuation, and then enters a detector 8, where it is converted into an electrical signal. A differential amplifier 9 detects the difference between the output signal of the detector 6 and the output signal of the detector 7 and generates an output. The servo circuit 10 performs control according to the error output of the differential amplifier 9, and controls the semiconductor laser 1.
Supplies drive current to.

FIG. 5 shows an example of the characteristics of the etalon, which exhibits a steep passing light intensity characteristic centered around the passing frequency fO. Now, by controlling the frequency of light generated by the semiconductor laser light source 1, indicated by fs in the figure, to be centered at the shoulder of the passing light characteristic of the etalon, the characteristic frequency fO of the etalon can be adjusted.
Control is performed to stabilize the frequency of the light generated by the semiconductor laser 1 based on .

[Problem that the invention seeks to solve]

In the highly stabilized semiconductor laser light source shown in FIG. 4, control is performed to stabilize the frequency of generated light using the characteristic frequency of the etalon as a reference, as described above.

However, due to its structure, the etalon requires a high degree of flatness for input and output light. Therefore, in order to convert the input and output light into plane waves, a high-grade optical system is required and the configuration becomes complicated.

In addition, in order to obtain the optical frequency stability required for optical coherent communication in an etalon, the distance between both end faces must be maintained with extremely high precision at all times, which requires a high degree of surface area. In order to prevent changes in the shape and two dimensions, sophisticated temperature control is required, which inevitably complicates the structure and increases the price.

[Means for solving problems]

The present invention attempts to solve the problems of the prior art, and its purpose is to provide a highly stabilized semiconductor laser light source with a simple configuration and low cost. As shown in the figure, a semiconductor laser 10
1 has the following components.

Reference numeral 102 denotes an optical path system that returns the output light from both light emitting surfaces of the semiconductor laser 101 to the light emitting surfaces on opposite sides.

Reference numeral 103 denotes a phase changer, which is inserted into the optical path system 102 and applies a phase change to the passing light in accordance with a control signal.

Reference numeral 104 is a detection means that detects the alternating current and direct current components of the frequency change in the light of the optical path system 102 and generates an output.

105 is a control means that controls the semiconductor laser 101 so that the average power of the AC component of the output of the detection means 104 is minimized.
Controls the drive current supplied to the

106 is a control means that controls the phase change in the phase changer 103 so that the DC component of the output of the detection means 104 is maximized or minimized.

[For production]

The output light from both light emitting surfaces of the semiconductor laser is configured to return to the light emitting surfaces on opposite sides through a closed optical path system, and a phase changer is inserted in the optical path system to change the output light to the passing light according to a control signal. In addition to providing a phase change, a detection means is provided to detect the alternating current and direct current components of the frequency change in the light in the optical path system to generate an output, and the first control means detects the alternating current and direct current components of the frequency change in the light in the optical path system. The drive current supplied to the semiconductor laser is controlled so that the average power of Since the changes are controlled, both direct current fluctuations and alternating current fluctuations in the frequency of light generated by the semiconductor laser are reduced.

〔Example〕

FIG. 2 shows an embodiment of the present invention, 21
is a semiconductor laser, 22, 23, 24 is a half mirror 12
5 is a mirror, 26 is an optical phase changer, 27 is an optical phase modulator, 28 is a phase modulation signal source, 29.30 is a detector, 31
32 is a phase detector, 32 is a servo circuit, 33 is a drive circuit, 34 is a low-pass filter, 35 is a servo circuit, 36 is a temperature controller, and 37 is a Vertier element.

In FIG. 2, light from one light emitting surface of a semiconductor laser 21 passes through a half mirror 22 to generate output light, and part of it is reflected by the half mirror 22 and further reflected by the half mirror 23, resulting in an optical phase shift. The light passes through the changer 2O2 optical phase modulator 27, enters the half mirror 24, is reflected, is reflected again by the mirror 5, and enters the semiconductor laser 21 from the other light emitting surface of the semiconductor laser 21. Also, the semiconductor laser 2
The light from the other light emitting surface of 1 follows the opposite path to the above case,
Half mirror 25° half mirror 24, optical phase modulator 2
7. The light enters the semiconductor laser 21 from one light emitting surface of the semiconductor laser 21 via the optical phase changer 26, the half mirror 23, and the half mirror 22.

At this time, the optical phase modulator 27 applies phase modulation to the passing light in accordance with the AC signal from the phase modulation signal source 28.

Any known type of optical phase modulator can be used as such an optical phase modulator. The detector 29 receives the light transmitted through the half mirror 24, converts it into an electrical signal, and generates an output. The phase detector 31 compares the phases of the output of the detector 29 and the signal of the phase modulation signal source 28, and generates a signal corresponding to the phase difference.

The servo circuit 32 performs servo control in response to this signal and generates an output signal, and the drive circuit 33 generates a drive current based on this output signal and supplies it to the semiconductor laser 21 .

Further, the DC component in the output signal of the detector 29 is extracted through a low-pass filter 34, and a servo circuit 35 performs servo control in accordance with this signal to generate an output signal.

The optical phase changer 26 uses this output signal to apply a direct current phase change to the destination passing through it. Examples of such an optical phase changer include, for example, an electro-optic crystal to which a DC voltage is applied, or a glass slope that is mechanically coupled to an electrostrictive element and changes the pressure in the light transmission direction depending on electrical input. elements can be used.

Furthermore, the temperature controller 36 generates a drive current based on the output signal of the servo circuit 35 and supplies it to the Bertier element 37 . The semiconductor laser 21 performs an oscillation operation determined by the drive current from the drive circuit 33, and the operating temperature conditions of the semiconductor laser 21 at this time are determined by the heating or cooling operation of the Vertier element 37.

In FIG. 2, the lights from both light emitting surfaces of the semiconductor laser 2I go around in a loop limited by each mirror in opposite directions and are combined in the semiconductor laser 21. When the optical path length is adjusted so that the light in both directions coupled within the loop does not have a phase shift, the entire loop acts like an etalon and the optical frequency is stabilized.

FIG. 3 shows the light waves in the loop. (As shown in al, when the light in both directions coupled within the semiconductor laser 21 does not cause a phase shift, a stabilized optical frequency is obtained.)
bl inserts a mode filter or a spatial filter 41 in the loop, selects a mode determined by itself in the case of a moat filter, and limits the motes in the entire loop in the case of a spatial filter. This shows the case where only the mode required for the laser beam is transmitted, and the noise component of the laser beam is removed. This allows for more (h) fine adjustment of the coherence.

Now, the variable jJIil signal frequency of the phase modulation signal source 28 is f
Since the signal extracted by the detector 29 is phase modulated, the signal is shifted upward or downward by the frequency fm from the optical signal frequency generated by the semiconductor laser 21, but this has a DC component. They are superimposed.

Here, the DC component indicates the DC component of the optical path length deviation, and this signal is used to correct the phase change in the optical phase changer 26 by servo control so that the DC component is minimized. Make it. Note that depending on the structure of the semiconductor laser 21, the desired compensation for the optical path length deviation may be performed by controlling the DC component to be maximum, so it is necessary to select one of them. Furthermore, since a change in the optical path length is also caused by a change in the temperature of the semiconductor laser 21, the same signal is used to heat or cool the Vertier element 37 to compensate for the variation due to the temperature change.

On the other hand, due to optical frequency fluctuations generated as noise in the semiconductor laser 2I, a phase change Δφ is generated in the phase modulated signal, and this phase change Δφ is detected by the phase detector 31.
is detected as an AC signal.

The servo circuit 32 takes the average value of this AC signal output, and controls the drive current supplied to the semiconductor laser 21 so that this average value becomes the minimum value. The reason for taking the average value of the AC component here is to take into account the response time of the servo loop, and
Therefore, it is desirable to make the averaging time as short as possible to the extent that the servo loop can respond.

By performing negative feedback control on the AC and DC components of the semiconductor laser 21 in this way, both DC displacement and AC fluctuation can be compensated for using the wavelength determined by the loop shown in FIG. 2 as a reference. Compensation is performed to stabilize the output optical frequency of the semiconductor laser 21.

In addition, in FIG. 2, a detector 30 is provided in addition to the detector 29, and the outputs of both detectors are coupled in a push-pull manner.
A required control output may be extracted.

Also, the detector 29 or phase detector 3 shown in FIG.
1, the above-mentioned semiconductor laser 2
It is also possible to perform alternating current fluctuation compensation using a drive current for the optical phase changer 26 and direct current compensation based on direct current driving of the optical phase changer 26. That is, the detector 29 can also detect alternating current fluctuations due to fluctuations in the optical frequency of the semiconductor laser 21, and therefore the detector 29 alone can compensate for both. At this time, an optical phase modulator 27 is provided in the optical path system to perform phase modulation from the phase modulated signal tA28, and the detector 29 detects the amplitude modulation component of the phase modulated optical signal, thereby detecting the alternating current fluctuation component. It may also be detected.

On the other hand, it is clear that DC fluctuations in the optical frequency of the semiconductor laser 21 can also be detected from the phase detector 31. Therefore, the phase detector 31 and the phase modulation signal source 28 can perform both AC compensation and DC compensation. It is also possible to do so.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to reduce both DC fluctuations and AC fluctuations in the frequency of light generated by a semiconductor laser, thereby improving frequency stability and widening the spectrum band. Since it can be made narrower, a light source suitable for coherent optical communication can be obtained.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the basic configuration of the present invention, Fig. 2 is a diagram showing an embodiment of the present invention, Fig. 3 is a diagram showing light waves in a loop, and Fig. 4 is a diagram showing a conventional highly stabilized semiconductor. FIG. 5 is a diagram showing the laser light source, and FIG. 5 is a diagram showing the characteristics of the etalon. 21-semiconductor laser, 22, 23.24-m-half mirror, 25--mirror, 26--optical phase changer, 27--optical phase modulator, 28--phase modulation signal source, 29. 30--detector, 31-phase detector, 32-servo circuit, 33-drive circuit, 34--low pass filter, 35-servo circuit, 36-temperature controller, 37--Peltier element 41--mode Filter or Spatial Filter Figure 1 shows the basic structure of the present invention Figure 2 Figure 5 shows an example of the characteristics of an etalon

Claims (3)

[Claims] (1) A semiconductor laser (101), an optical path system (102) that returns output light from both light emitting surfaces of the semiconductor laser (101) to mutually opposite light emitting surfaces, and an optical path system (102) inserted into the optical path system (102). a phase changer (103) that changes the phase of the passing light according to a control signal; and a detection means that detects an AC component and a DC component of a frequency change in the light of the optical path system (102) and generates an output. (104)
and a control means (105) for controlling the drive current supplied to the semiconductor laser (101) so that the average power of the alternating current component of the output of the detection means (104) is minimized;
104) A highly stabilized semiconductor laser light source characterized by comprising: control means (106) for controlling a phase change in the phase changer (103) so that the DC component of the output of (104) is maximized or minimized. (2) The detection means (104) includes the optical path system (102).
2. The highly stabilized semiconductor laser light source according to claim 1, wherein the highly stabilized semiconductor laser light source is a detector for detecting the light. (3) The detection means (104) includes a phase modulator that is inserted into the optical path system (102) and phase modulates the passing light according to a phase modulation signal, and a detector that detects the light of the optical path system (102). The highly stabilized semiconductor laser light source according to claim 1, further comprising a phase detector that generates an output by comparing the phases of the output signal of the detector and the phase modulation signal.