JPS61208281A - Semiconductor laser device - Google Patents

Abstract

PURPOSE:To inhibit the variation of wavelengths, and the to stabilize an output by constituting one reflecting mirror for a two-dimensional distributed feedback type laser by an elastic surface-wave grating and adding an output monitor circuit by a semiconductor detector. CONSTITUTION:A semiconductor laser device consists of a diffraction grating 21 by a periodic irregular shape formed in place of a cleavage-plane reflecting mirror and a comb-shaped electrode 22 for exciting an elastic surface wave as a diffraction grating, which excites the elastic surface wave and conducts a distributed feedback. When a wavelength is displaced to the short-wave length side only by DELTAlambda at that time, an incident position to one-dimensional position detector 5 is deviated by the diffraction grating 11 in leakage beams 25, the detector 5 detects the deviation, and the elastic surface wave 23 is changed into a long-wave length by a high-frequency signal controller 7 so that the deviation reaches zero, thus inhibiting the variation of the wavelength. The intensity of beams 25 is monitored by a detector 3, and fed back to an output control circuit 4, thus stabilizing an oscillation output.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a semiconductor laser device, and particularly to a semiconductor laser device with stabilized oscillation output and oscillation wavelength.

[Prior art]

Conventional semiconductor lasers use an open plane of a crystal as a reflecting mirror of a resonator, and do not have a means for selecting a longitudinal mode, and essentially perform multimode oscillation. For this reason, there is a drawback that the oscillation wavelength changes between modes during the oscillation operation, and the wavelength fluctuates irregularly. This is a so-called mode hopping phenomenon, and the amount of wavelength jump may vary by 2 to 3 when moving to an adjacent mode, and by 10λ or more when moving to a further distant mode. In order to solve these drawbacks, efforts have been made to construct a semiconductor laser device by forming a resonator so as to have strong wavelength selectivity.

One of them is a distributed feedback Bragg reflector (DFB) laser. In this method, a diffraction grating with a periodic uneven structure is arranged at both ends of the active layer, and these gratings are used as the reflecting mirrors of the resonator, instead of using the arm openings as reflecting mirrors. In this case, in order to use the diffraction grating as a reflecting mirror, the grating is formed so that the direction of propagation of light within the active layer due to the diffraction grating is opposite.

Assuming that the wavelength of light in vacuum is ng, the effective refractive index of the waveguide (active layer) is ng, and the period of the diffraction grating is 8, the conditions for specular reflection of light are 2 ng A/m, (m: integer )(1). This corresponds to the condition for Bragg diffraction when the incident angle is 90°. As is clear from the above equation, the wavelength that can exist in the active layer is determined by the effective refractive index ng and the grating period, and therefore, this resonator has wavelength selectivity with respect to the oscillation wavelength.

Also, regarding the longitudinal mode, if the length of the active layer is La, then the equation λ = 2!19 La/N, (M: integer) (2), which has the same form as equation (1), holds true.Equation (1) Only a wavelength input for which equations (2) and (2) hold simultaneously can exist without being attenuated in the resonator.Single wavelength oscillation of a semiconductor laser is realized by these means.

However, even in this method, fluctuations in wavelength caused by changes in the refractive index due to changes in the temperature of the active layer are unavoidable. Therefore, in order to suppress this variation, efforts have been made to stabilize the wavelength by adding a constant temperature device to keep the temperature of the laser section constant, and by placing the laser inside an interferometer that includes a diffraction grating and the like.

However, the wavelength stability achieved by these methods is about 10-5, and it has been difficult to achieve a fluctuation rate smaller than this. Furthermore, the oscillation wavelength depends on the manufacturing precision of the resonator, and the wavelength at which the oscillation will occur cannot be determined unless it is actually operated, and there is no way to correct any deviation from the target wavelength.

〔the purpose〕

Therefore, an object of the present invention is to eliminate the above-mentioned drawbacks, form a closed loop for detecting wavelength fluctuations, and configure one reflecting mirror of a two-dimensional distributed feedback laser with a surface acoustic wave grating. It is an object of the present invention to provide a semiconductor laser device in which the output voltage is suppressed and, at the same time, the output is stabilized by adding an output 72 output circuit using a semiconductor detector.

In order to achieve such an object, the present invention includes a semiconductor laser made of a piezoelectric semiconductor material, a first detection means for detecting the light output emitted from the sub-emitting end of the semiconductor laser, and a laser oscillation wavelength of the semiconductor laser. The oscillation output and oscillation wavelength are stabilized by feedback controlling the oscillation output and oscillation wavelength of the semiconductor laser using the outputs of the first and second detection means. It is characterized by being made to

〔Example〕

The present invention will be described in detail below with reference to the drawings.

FIG. 1 is an overall view showing the basic configuration of the semiconductor laser device of the present invention, where l is a substrate on which a laser and peripheral components are mounted. 2 is made of four piezoelectric semiconductor materials, and a diffraction grating 21 with a periodic uneven shape is provided instead of the arm opening surface reflector, and surface acoustic waves are excited in a direction perpendicular to the arm opening surface to generate distributed feedback. This is a two-dimensional distributed feedback semiconductor laser in which a comb-shaped electrode 22 for surface acoustic wave excitation is formed as a diffraction grating. 3 is an optical power detector for detecting the amount of light 14 leaking from the reflecting mirror (diffraction grating in this example) 11; 4 is an output from this photodetector 3 to control the driving current of the laser. It is a control device and a power supply for the laser beam, and forms a feedback loop that suppresses fluctuations in the laser beam output 24 while monitoring the laser beam with the photodetector 3.

Reference numeral 5 denotes a one-dimensional position detector, in which leakage light 25 from the semiconductor laser 2 is separated by a diffraction grating 11, and -order diffraction light 1 is separated.
5 is incident on the detector 5, the movement of the wavelength is detected. Reference numeral 6 denotes a position signal conversion device that forms a signal representing a change in wavelength based on the position detection signal from the position detector 5, and the signal representing the change in wavelength is converted into a frequency change of the surface acoustic wave 23 on the laser chip z. handle. Therefore, this signal is supplied to the high frequency signal control device 7 to generate a control signal for suppressing the frequency fluctuation of the laser light output.

Reference numeral 8 denotes a high frequency power source that drives the comb-shaped electrode 22 for excitation of surface acoustic waves, and is controlled by a control signal from the control device 7 so as to minimize the frequency change of the surface acoustic wave 23 on the laser chip 2.

The laser 2 forms a two-dimensional distributed feedback Bragg reflection type laser, and if the direction of the arm opening plane is usually the two directions, and the axis perpendicular to this and on the same plane is the y-axis direction, then A fixed Bragg reflector is formed by a pair of diffraction gratings 21, and the surface acoustic wave 23 propagates in the y direction on the plane between them, so that the light hits the grating due to the zero surface acoustic wave 23 that satisfies the Bragg condition. When the light is incident at an angle OB, it is efficiently reflected. Then 2Δasin θ5y==in/ng
(1) The following relationship holds true. Here, Δa is the wavelength of the surface acoustic wave, Δa is the wavelength of light in vacuum, and Δa is the effective refractive index.

If this reflected light also enters the Bragg reflector 21 so as to satisfy the Bragg condition, the light will be effectively reflected. At that time, as before, 2Azgfn θez=
The following relationship holds true: Nz input/nq (2). Here, Δ2 is the grating period of the Bragg reflector 21, θ8z is the incident angle to the grating,
Nz is an integer.

Considering equations (1) and (2) and the condition that the reflected light from each grating forms a closed ring as a whole, the oscillation wavelength λ2 of the two-dimensional distributed feedback mode is given by the following equation. Here, input 1 is the oscillation wavelength of a normal one-dimensional mode emitted in two directions. When the surface acoustic wave is not excited, equation (3) corresponds to the normal one-dimensional case. The wavelength input 1 at this time is taken as a reference for the oscillation wavelength, and is also used as a reference for the position of the one-dimensional position detector 5.

Now, a high frequency voltage is applied from the surface acoustic wave excitation voltage Igg to the comb-shaped electrode 22, and a driving current is supplied from the laser power source 4 to the laser chip 2 in which one surface acoustic wave 23 exists as a traveling wave, so that the radar oscillates. The oscillation output is stabilized by monitoring the intensity of the leaked emitted light 25 with the detector 3 and feeding it back to the output control circuit 4. on the other hand,
As for the oscillation wavelength input 2, the wavelength Δ of the surface acoustic wave in equation (3) is appropriately selected so as to give a desired wavelength.

Now, if for some reason the wavelength tries to shift by Δλ to the shorter wavelength side, the diffraction grating of leaked light 25! The diffraction angle of the one-dimensional diffracted light 15 by L increases, and the incident position on the one-dimensional position detector 5 moves in a direction further away from the reference position. Therefore, the detector 5 detects the amount of positional deviation, and a control signal generated by the control device 7 that lowers the frequency of the surface acoustic wave 23 so as to reduce the deviation to zero is supplied to the high-frequency power source 8. The surface acoustic wave 23 is made to have a longer wavelength, and the laser oscillation wavelength is shifted to the longer side, thereby operating so as to suppress wavelength fluctuations.

Regardless of whether the surface acoustic wave is in a traveling wave mode or a standing wave mode, the surface acoustic wave 23 is applied to the entire area of the laser chip 2 after a high frequency voltage is applied from the high frequency electric drift 8 to the comb-shaped electrode 22. There is a time delay until the laser is excited due to the relatively slow propagation speed of the surface waves, but this can be achieved by making the width of the laser active region sufficiently small and placing the electrodes very close to the active region. In practice, it can be ignored.

Specific examples of the present invention are shown below, but the present invention is not limited only to these examples.

Embodiment 1 FIG. 2 is a diagram showing the arrangement of various parts in Embodiment 1 of the present invention. Here, the size is 25mm x 5G, thickness 2
A distributed feedback laser (GaAs
) Chip 2 was fixed. A diffraction grating 21 with concave and convex pitches of 0.3471 Lm is formed in the direction in which the light is extracted as a reflector of the laser resonator, and a comb-shaped electrode 22 is provided in the direction perpendicular to this to form a surface acoustic wave with a wavelength of 2.5 pm. was excited as a traveling wave by a comb-shaped electrode.

When the other end face opposite to the main light output end face 2A of the laser is used as the light output surface 2B, a pitch of 0.57 is used as a grating for separating the light emitted from the output face.
A diffraction grating 11 of zm was provided. In order to monitor the laser output, it is necessary to detect the amount of light that has passed through one diffraction grating 11, and on the other hand, in order to detect a minute wavelength shift of the laser beam, it is necessary to detect one of the higher-order diffracted lights. For this purpose, it is best to obtain the same intensity of the transmitted light and one of the higher-order diffracted lights, and in this case, the diffracted light of other orders is unnecessary, so a blazed grating is more suitable. The light that passed through the diffraction grating 11 was detected by the silicon photodetector chip 3. A one-dimensional can array was arranged as a one-dimensional position detector 5 for detecting one piece of high-order diffracted light.

When the semiconductor device with the above configuration was caused to oscillate as a laser without exciting surface acoustic waves, the result was a one-dimensional mode and a third-order mode (
Nz=3) is excited, and its wavelength is 0.83057Al
It was l. Furthermore, high frequency power source 8 to 1.130GH
A high frequency voltage of z is applied to the comb-shaped electrode 22 at a wavelength of 2.5.
When the surface acoustic wave in the final layer is excited, a two-dimensional mode is excited, and the oscillation wavelength of the laser beam is 0.8291111Lm.
It became. At that time, 0.8305g in one-dimensional mode
The intestinal oscillation spectrum had disappeared.

Once the feedback loop including the surface acoustic wave control device was opened (however, the surface acoustic wave with a wavelength of 2.51 L was continued to be applied) and the temperature of the laser was raised by 5°C, the wavelength was 0.8310 L. It became. When the feedback loop was reconnected, the wavelength shifted to the shorter wavelength side and returned to 0.8298. The wavelength of the surface acoustic wave at that time was 1.513g. In this example, a hybrid configuration was used in which the laser chip, semiconductor detector chip diffraction grating, and position detector were mounted as individual components on a single board. However, it is of course possible to integrate these components on one semiconductor substrate in the form of an integrated circuit.

In Example 1, the surface acoustic waves were directly excited by the piezoelectricity of the laser material, but in order to excite the surface acoustic waves even more efficiently, a piezoelectric thin film, such as a sputtered film of zinc oxide (ZnO), was laminated on the laser chip. You may.

Example 2 For example, as shown in FIG. 3, gallium arsenide (GaAs)
A laser section 2 and an output monitor detector 3 are simultaneously formed on a substrate 1 with an element isolation region 30 in between. Element isolation area 30
suitable waveguide materials, such as arsenic trisulfide (As2
A waveguide is formed using a method such as S3), and a waveguide type diffraction grating 31 is formed on the waveguide by an electron beam exposure method or the like. Further, by arranging a micro diode array as a linear position detector 5 on the side of the diffraction grating 31 where light is diffracted, the semiconductor laser device of the present invention is monolithically formed on a single GaAs substrate.

〔effect〕

As described above, according to the present invention, the circuit is to control wave fluctuations and output fluctuations of a semiconductor laser in real time during laser operation. Furthermore, according to the present invention, the oscillation wavelength is selected by appropriately selecting the wavelength of the surface acoustic wave to select a desired wavelength and operate stably at that point.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the basic configuration of the laser of the present invention. FIG. 2 is a perspective view showing Embodiment 1 of the present invention. FIG. 3 is an L side view showing a second embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Board, 2... Laser light output 3... Optical power detector, 4... Laser current control device, 5... One-dimensional position detector. 6...Position signal conversion device, 7...High frequency signal control device, 8...High frequency power source for driving interdigitated electrodes, 11...Diffraction grating, 14...Leakage light, 15...-Next time Folded light. 21... Diffraction grating (Bragg reflector), 22... Comb-shaped electrode, 23... Surface acoustic wave, 24... Laser light output, 25... Leakage light.

Claims (1)

[Scope of Claims] 1) A semiconductor laser made of a piezoelectric semiconductor material, a first detection means for detecting a light output emitted from a sub-emitting end of the semiconductor laser, and a detection means for detecting a laser oscillation wavelength of the semiconductor laser. and a second detection means arranged on a single substrate,
A semiconductor laser device characterized in that the oscillation output and oscillation wavelength of the semiconductor laser are stabilized by feedback-controlling the oscillation output and oscillation wavelength of the semiconductor laser using the outputs of the first and second detection means. 2) The second detection means is composed of a diffraction grating and a one-dimensional position detector that detects the one-dimensional position of the output light from the diffraction grating, and the one-dimensional position detector detects a change in the diffraction angle due to a change in wavelength. The semiconductor laser device according to claim 1, characterized in that the semiconductor laser device detects. 3) The semiconductor laser is a two-dimensional distributed feedback Bragg reflection type laser having a pair of Bragg reflection gratings, the pair of Bragg reflection gratings are formed by traveling wave type surface acoustic waves, and the oscillation detected by the second means 3. The semiconductor laser device according to claim 1, wherein the wavelength of the surface acoustic wave is controlled by a wavelength detection output. 4) Claim 3, characterized in that the oscillator that excites the surface acoustic wave is constituted by a voltage-controlled oscillator.
The semiconductor laser device described in .