JPH01179379A - Frequency stabilization of semiconductor laser and device therefor - Google Patents

Abstract

PURPOSE:To simplify the formation and handling of an optical part and allow its optical part to applied to a length measuring machine which is used at the site of production, by controlling beat frequencies of two pieces of semiconductor laser elements having different inrush current vs. oscillation frequency characteristics so that the beat frequencies are held constant. CONSTITUTION:Laser beams having different frequencies which are outgone from each element are collimated by lenses 2a and 2b and then, are divided into two parts by beam splitter 3a and 3b respectively. Then, one of beams splitted respectively is used in a length measuring machine. Other two beams are mixed by a beam mixer 11 to produce a beat frequency having a difference between frequencies. Its beam frequency is joined to a signal of a beat frequency (nua-nub) by a light receiving apparatus 6. The beat frequency is converted into a voltage signal by an F-V converter 12 and is inputted to a current control circuit 13 as positive and negative signal voltages after comparing its frequency with a constant reference value Vs by comparator 8. In this way, the control signal of an inrush current is given to an inrush current source 10 and the inrush current which makes oscillation frequencies regular is supplied to elements.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] This invention relates to a method and apparatus for stabilizing the frequency of a semiconductor laser.

[Prior Art] Semiconductor lasers, which have been rapidly progressing in recent years, are smaller and consume less power than conventional gas laser tubes, and are widely used in various fields. However, semiconductor lasers have the disadvantage that their oscillation frequency fluctuates depending on temperature and injection current.
In this state, it cannot be put to practical use as a precision measuring instrument, such as an interferometer or a length measuring instrument using this. To eliminate and stabilize such frequency fluctuations, it is necessary to detect the fluctuations and feed them back into the temperature and injection current control.
There have been various methods of detecting and feeding back frequency fluctuations, one example of which is shown in FIG. In the figure, a laser beam emitted from a semiconductor laser device 1 and collimated by a lens 2 is transmitted to a beam splitter 3.
It is divided into two parts. The straight-progressing beam forms interference fringes in the interferometer 5, a part of which is received by the light receiver 6-2, and a voltage V2 is obtained. The other beam reflected by the beam splitter passes through mirror 4 and is input to photoreceiver 6-1, where voltage v1 is output. The voltage ratio V2/V is determined by the divider 7.
1 is calculated, and a constant reference voltage V is further calculated by the comparator 8. is set as the flash value, and the portion exceeding this value is input to the calculation section 9 as an interference fringe signal.

In the calculation section, a control value for the injection current is calculated based on the change in the interference fringe signal caused by the wavelength fluctuation, based on the injection current vs. wavelength characteristic of the semiconductor laser element known in advance, and is applied to the injection current source 1o. and the oscillation frequency is held constant. Note that the voltage V1 mentioned above is for eliminating the influence of fluctuations in the intensity of the laser beam, and although not shown, stabilization is performed with respect to temperature fluctuations by a feedback method.

[Problems to be Solved by the Invention] Now, as the interferometer 5 in the above, a conventionally known Fabry-Bérot interferometer is exclusively used. However, although the Fabry-Bérot interferometer is originally extremely effective for precisely examining the fine structure of spectra, there are certain difficulties in its adjustment and handling methods, and it is also inexpensive. isn't it.

On the other hand, semiconductor lasers used in length measuring instruments used at production sites have considerably large fluctuations in one frequency, and the speed of fluctuation must constantly change, so it is difficult to respond to frequency fluctuations. Applying the Fabry-Bérot interferometer, which requires complicated adjustment, is efficient and
There isn't.

SUMMARY OF THE INVENTION An object of the present invention is to provide a simple and effective method for stabilizing the frequency of a semiconductor laser, which can be applied to a length measuring device, etc., and an apparatus therefor.

[Means for Solving the Problems] The present invention is a method and device for stabilizing the frequency of a semiconductor laser.

First, the stabilization method is to connect two semiconductor laser devices in series, which have different slope angles of the oscillation frequency characteristic curve (straight line) with respect to the injection current and are maintained at a constant temperature, and oscillate with the same injection current. The laser beams emitted from these two elements are mixed to obtain a beat frequency of the frequencies of both beams, and the value of the injection current is controlled so as to maintain the beat frequency at a constant value.

Next, the stabilizing device is constructed as follows. Two semiconductor laser elements whose inclination angles of characteristic curves (straight lines) of injection current versus oscillation frequency are different and are connected in series, a temperature stabilizer that maintains these elements at a constant temperature, and two semiconductor laser elements are connected in series. A beam splitter that splits each laser beam emitted by the element into two, a beam mixer that mixes one of the split beams, a light receiver that receives the laser beam at the beat frequency generated by the mixing, and a light receiver that It consists of a frequency-voltage converter that converts the output beat frequency signal into a signal voltage proportional to the frequency, a current control circuit and an injection current source that controls the injection current using this signal voltage and injects it into the semiconductor laser element. It is something that

[Operation] First, the principle of frequency stabilization in this invention will be explained. Generally, the oscillation frequency of a semiconductor laser varies depending on the operating temperature and injection current. Now, when two laser elements with different injection current vs. oscillation frequency characteristics are kept at a constant temperature and supplied with the same injection current, they oscillate at different frequencies, but the slope angles of their characteristic curves (straight lines) are different. When, the frequency of the difference between the two frequencies changes in response to the injected current,
For a certain injection current value, the difference frequency takes one value.

That is, the difference frequency, and therefore the two frequencies, are uniquely determined for the injected current. Therefore, the two oscillation frequencies can be stabilized by creating beat frequencies for both beams and performing control to maintain them constant. In this case, compared to the conventional method of detecting interference fringes at an extremely high optical frequency, the beat frequency is processed much lower, and there is an advantage that an interferometer and its adjustment are not required.

Based on the stabilization principle described above, in the method for stabilizing the frequency of a semiconductor laser according to the present invention, two semiconductor laser elements having different slope angles of the characteristic curve of injection current versus oscillation frequency are connected in series. The same injection current is supplied at a constant temperature and oscillations are made at different frequencies.

The two frequencies are mixed to create a beat frequency, and the injected current is controlled so that the beat frequency remains at a constant value, thereby stabilizing both frequencies.

Next, the frequency stabilizing device according to the present invention is an embodiment of the above method, in which the two semiconductor laser elements are maintained at a constant temperature by the temperature stabilizing device, and the same current is injected into the pixel element to generate different oscillate the frequency. The beat frequency signal obtained by mixing these oscillation frequencies is converted into a voltage value and input to the current control circuit, and under the control of the current control circuit, an injection current source that keeps the oscillation frequency constant is supplied to the element. It is.

[Example 1] A method for stabilizing the oscillation frequency of a semiconductor laser according to the present invention will be explained with reference to FIGS. 1(a) and 1(b).

Now, if we take two arbitrary semiconductor laser devices and look at their respective characteristic curves of injection current Ii versus oscillation frequency ν, since there are variations in the individual characteristics, the characteristic curves for the two devices are as shown in Figure 1 (a> Straight lines Da and Db with different inclination angles as
shows. Note that these straight lines change discontinuously at a certain value of the injection current Ii (Ia, Ib in the figure), resulting in Da',
Db', but such a range is excluded. Now, let the injection current Ii be the same I. Then, each oscillation frequency for this is νaO+straight line D by straight line D&
b is νbo. Further, when the injection current is 1, sial and sial are respectively b1. Here, since the slopes of both straight lines Da and Db are different, the difference between these frequencies, (
SheaQ-shibo) and (stain1-shib+) are different values. Now, let's say that the injection current is 1. If the frequency difference (a□−shibo) is held constant, the respective oscillation frequencies νaO and νbo uniquely correspond to each other and are held at a constant value. Therefore, the injection current is controlled so as to keep the frequency difference (va,)-vbo) constant. Based on this idea, the oscillation frequency of the semiconductor laser can be stabilized by mixing the laser beams of two laser elements to create a beat frequency, detecting its fluctuation, and controlling the injection current.
You can do this. In the above, if the distance between the two straight lines and the angle of inclination are not appropriate, adjust them using the following method. In other words, FIG. 1(b) is an equal circuit of the above two semiconductor laser devices, and the parallel resistances Ra and Rh of the devices Da and Db are the combined resistances of the equivalent resistance representing the loss of the device and the added external resistance. It is.

By changing the external resistance value, the spacing and inclination angle of the straight lines shown in FIG. 1(a) can be adjusted, and an appropriate difference frequency can be obtained for the injected current.

FIG. 2 is a block diagram of an embodiment of a semiconductor laser frequency stabilization method and stabilization device according to the present invention. In the figure, two semiconductor laser elements 1a and lb are attached to a heat sink 14b.

This temperature is detected by the temperature sensor 15b, and the temperature of the Bertier element 14a is controlled by the temperature stabilization circuit 15a, so that the temperature of the heat sink fixed thereto and therefore of the element is maintained constant.

The laser beams of different frequencies emitted from each element are
It is collimated by lenses 2a and 2b and divided into two by beam splitters 3a and 3b, respectively. One of the split beams is used for the tunnel length machine, and the two Ikekata beams are mixed in a beam mixer 11 to produce a beat frequency that is the difference between the two frequencies. As a concrete numerical value, the beat frequency is several GHz. This light is received by the light receiver 6, and a signal of the beat frequency (sha-b) is output.

Next, the beat signal is converted into a voltage signal by the F-V converter 12, and the beat signal is converted to a voltage signal by the comparator 8.
s and becomes a positive and negative signal voltage, which is input to the current control circuit 13. As a result, an injection current control signal is given to the injection current source 10, and an injection current that keeps the oscillation frequency constant is supplied to the element.

Note that items whose characteristics change depending on temperature, such as an F-V converter and a comparator, are housed in a constant temperature oven if necessary.

[Effects of the Invention] As is clear from the above explanation, in the semiconductor laser frequency stabilization method and device according to the present invention, the beat frequency of two semiconductor laser elements having different injection current vs. oscillation frequency characteristics can be maintained constant. By controlling the oscillation frequency of each element to be constant,
Compared to the conventional method of directly measuring and controlling frequency fluctuations using an interferometer, this method processes beat signals at a very low frequency and requires no adjustment, making the configuration and handling of the optical section simpler. Therefore, it has great effects when applied to length measuring instruments used at production sites.

[Brief explanation of the drawing]

1(a) and 1(b) are an injection current versus oscillation frequency characteristic curve diagram of a semiconductor laser device, an equivalent circuit diagram of the semiconductor laser device, and a second FIG. 3 is a block diagram of an embodiment of a semiconductor laser frequency stabilization method and device according to the present invention, and FIG. 3 is a block diagram of an example of a conventional frequency stabilization device using an interferometer. 1...-laser light source, 2...lens, 3...-
Beam splitter, 4...Mirror, 5...Fapley-Bello interferometer, 6...Receiver, 7...-divider, 8...
Comparator, 9...-Arithmetic unit, 10... Injection current source, 11... Beam mixer, 12...F
-V converter, 13... current control circuit, 1-43.
...Beltier element, 14b...Heat sink, 15
a... Temperature stabilization circuit, 15b... Temperature sensor.

Claims (2)

[Claims] (1) Characteristic curve of oscillation frequency versus injection current (straight line)
Two semiconductor laser devices with different inclination angles and maintained at a constant temperature are connected in series and oscillated with the same injection current, and the two semiconductor laser devices emit laser beams of different frequencies. A method for stabilizing the frequency of a semiconductor laser, the method comprising: controlling the value of the injection current so as to maintain a beat frequency obtained by mixing the above at a constant value. (2) Characteristic curve of oscillation frequency versus injection current (straight line)
two semiconductor laser elements having different inclination angles and connected in series; a temperature stabilizing device for maintaining the semiconductor laser elements at a constant temperature; and each laser beam emitted by the two semiconductor laser elements. a beam splitter that splits each into two, a beam mixer that mixes one of the splits, a light receiver that receives the laser beam of the beat frequency generated by the mixing, and a beat frequency signal output from the light receiver. , a frequency-voltage converter that converts the current into a signal voltage proportional to the frequency, and a current control circuit and an injection current source that control the injection current and inject it into the two semiconductor laser elements using the signal voltage. A frequency stabilizing device for a semiconductor laser, characterized in that: