JPS5939915B2 - Wavelength stabilization method for semiconductor laser output light - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for stabilizing the wavelength of output light from a semiconductor laser.

Among semiconductor lasers, one in which the wavelength of output light changes considerably depending on the temperature of the element has been developed in recent years.

Such a laser is a wavelength tunable laser.
It is called ``aser''. These types of lasers use a multi-component semiconductor material consisting of three types of elements as the constituent material of the device, and emit light in the infrared region. When using the above wavelength tunable laser, it is often desired to extract light of a desired specific wavelength within the wavelength tunable range.

For example, when using the above laser as a light source for measuring absorption spectra in infrared spectroscopy, several types of infrared rays of different wavelengths are made incident on the sample, but the light source is not used until the absorption at a particular wavelength is measured. Needless to say, it is desirable that the emission wavelength does not change. However, due to its variable characteristics, a wavelength tunable laser, on the other hand, causes fluctuations in the emission wavelength due to temperature fluctuations. In particular, in order to generate laser light, that is, to cause oscillation, it is necessary to flow a considerably large amount of forward current, and the temperature of the laser element rises due to the heat generated by this current. Therefore, in order to stabilize the wavelength of the laser beam, a powerful cooling method is required, but generally a coolant such as liquid nitrogen is used as a powerful cooling method.
Another problem is that it requires a fairly large container for the refrigerant. The present invention solves the above-mentioned problems, and utilizes the phenomenon that the sensitivity characteristics of a semiconductor photodetector to incident light rapidly change with a negative gradient on the longer wavelength side than the maximum point of sensitivity. The present invention aims to provide a novel method for stabilizing the emission wavelength of a semiconductor laser, which stabilizes the wavelength of the output light of the laser by detecting the wavelength of the output light of the laser and returning the detected output to the cooling means of the laser. .

An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a simplified system diagram showing an example of a system for implementing the method of the present invention. In this diagram, a wavelength tunable laser 1 made of a semiconductor is supplied with a pulse current by a driving pulse generator 2. and emit light.

The operating principle of a semiconductor laser is already well known; when a sufficiently large forward current is passed through a junction diode provided with a predetermined light reflecting surface, laser oscillation occurs and coherent light is emitted from the junction. The wavelength of the coherent light, that is, the laser light, is determined by the width of the energy gap in which the carrier transition occurs, and since this width is temperature dependent, the wavelength of the laser light changes depending on the temperature of the element. As an example, a semiconductor laser made of lead-tin-tellurium (Pbl-XSnxTe) emits laser light with a wavelength of about 8.3 μm at 77K, and when the temperature rises by 1°C, it emits a laser beam of about 0.02 to 0.0 μm.
The emission wavelength becomes shorter by about 0.3 μm. Now, apart from the laser light 3 emitted by the semiconductor laser 1, a part 4 of the emitted light is input into a photoelectric conversion device 5 and converted into an electrical signal. The level of the electrical signal output by the photoelectric conversion device 5 depends on the wavelength of the incident light, and the characteristics of the photoelectric conversion device 5 will be described in detail later. The electrical signal is input to an amplifier 6, amplified, and then fed back to a cooling device 7 for cooling the semiconductor laser 1, thereby closing a feedback loop. The cooling device 7 is configured so that its cooling temperature is controlled by an electric signal. In this embodiment, the cooling device 7 is mainly composed of an electronic cooling element that utilizes the Peltier effect, and the cooling temperature can be variably controlled by controlling the current flowing through the electronic cooling element. Generally, in a semiconductor laser, the wavelength of the laser beam decreases as the element temperature increases, and in the opposite direction as the temperature decreases. Therefore, the semiconductor laser 1 mentioned above
If the polarity of the electric signal is set so that the feedback loop consisting of the photoelectric conversion device 5, the amplifier 6, and the cooling device 7 performs a negative feedback function, the element temperature of the semiconductor laser 1 is stabilized, and the laser The wavelength of light also stops changing. Next, the characteristics of the photoelectric conversion device 5 will be explained.

FIG. 2 shows the incident light wavelength vs. sensitivity characteristic of the semiconductor photodetector in the converter 5. A curve 8 representing the characteristic has a maximum point P, and the slopes of the curve are opposite on both sides of the P point. Then,
The absolute value of the slope is steep on the right side of point P, and is also gentle on the left side. Incidentally, semiconductor light-receiving elements generally have the above-mentioned tendency even if their constituent materials are different. Therefore, in this example, the same material as the semiconductor laser element is used as the material of the light receiving element, and this is kept at approximately the same temperature as the laser element, so that the wavelength of the laser light is
For example, point Q is located on the steep slope section to the right of point P in the figure. The wavelength corresponding to point Q is λ. Then,
The differential coefficient of the curve 8 at point Q corresponds to the conversion gain of the light receiving element. Therefore, by selecting the operating point as described above, a large conversion gain can be obtained. As is clear from FIG. 2, when the wavelength of the laser beam becomes longer than λo, the output of the photoelectric conversion device 5 becomes negative in polarity.

So, returning to FIG. 1 again, as mentioned above, the temperature of the semiconductor laser element and the wavelength of the laser light change in opposite directions, so when the output of the photoelectric conversion device 5 swings in the negative direction, the cooling efficiency of the cooling device is If the output polarity of the amplifier 6 is determined so that the temperature of the semiconductor laser element is increased by decreasing the temperature of the laser element, and vice versa, the direction of feedback becomes negative feedback. The wavelength of the laser beam is stabilized according to the temperature of the semiconductor laser 1. Note that even if the cooling device is not of a type that utilizes the Peltier effect, the same effect as that of the embodiment can be obtained as long as it is a type that can control the cooling temperature by the amount of electricity.

Furthermore, since the heat generated by a laser element naturally depends on the magnitude of the current flowing to emit light, even if a feedback loop is formed to control the current and adjust the amount of heat generated by the element, it will theoretically result in negative feedback. . However, this method is somewhat difficult to apply. According to the method for stabilizing the wavelength of the output light of a semiconductor laser according to the present invention described above, stabilization can be achieved without using a liquid coolant, etc. It becomes possible to make everything up to and including semiconductors, making the whole thing smaller and easier to handle.

[Brief explanation of drawings]

FIG. 1 is a system diagram showing the configuration of an apparatus in an embodiment of the method of the present invention, and FIG. 2 is a graph showing the relationship between the sensitivity of a semiconductor light receiving element and the wavelength of incident light. 1: Semiconductor laser, 2: 1 driving pulse generator, 5: Photoelectric conversion device, 6: Amplifier, 7: Cooling device, 8: Curve showing the spectral sensitivity characteristics of the semiconductor photodetector.

Claims (1)

[Claims] 1. The temperature of a semiconductor laser element whose emission wavelength changes depending on the temperature is controlled by an electrically controllable temperature control device, and a part of the output light of the laser element is controlled in the wavelength band of the output light relative to the incident light. An electrical signal whose output level changes depending on the wavelength change of the output light is obtained by making the light incident on a single semiconductor light-receiving element whose sensitivity characteristic changes rapidly with a negative gradient, and this electrical signal is transmitted to the above-mentioned temperature. A method for stabilizing the wavelength of output light from a semiconductor laser, characterized in that the wavelength of the output light is stabilized by negative feedback to an adjustment device.