JPH0726856Y2 - Semiconductor laser device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a semiconductor laser device particularly suitable for an optical reading device.

(B) Conventional technology Conventionally, a semiconductor laser element is used in an optical reading device for reading information recorded on a disk, and the laser light is reflected on the disk and used. In this case, when the reflected laser light reaches the semiconductor laser element instead of the light receiving element, so-called return light noise such as interference or laser oscillation inhibition occurs, which is inconvenient. For this reason, in early optical reading devices, a λ / 4 plate was used for the optical system, but in recent years, due to downsizing and simplification, measures against returning light by the optical system are no longer taken.

On the other hand, in the semiconductor laser device, as shown in U.S. Pat. No. 4,178,564, a coating made of a derivative is provided on the opposing cleavage faces which are the light emitting faces. This was done by paying attention to the fact that when forming a Fabry-Perot resonator with a cleavage plane, increasing the edge reflectivity lowers the threshold value, which facilitates laser oscillation and facilitates control. , It was common to provide a coating with equal characteristics with a reflectance of 30% or more on both cleaved surfaces.

C) Problems to be Solved by the Invention The present invention has been made in consideration of the above points, and provides a semiconductor laser device in which the return light noise is reduced by focusing on the film.

D) Means for solving the problems In the present invention, the reflectances of the coatings are made different from each other on the opposing cleavage planes, one of which is 25 to 32% and the other of which is 15 to 25%.

(E) Action As a result, the influence of return light noise can be reduced without degrading characteristics such as coherency.

F) Embodiment FIG. 1 is a perspective view of a semiconductor laser device according to an embodiment of the present invention.
Active layer (2) sandwiched by cladding layers on a GaAs base (1)
Are epitaxially grown and electrodes (3) (3) are formed on the front and back surfaces.
Is provided. The active layer (2) is formed in a narrowed shape so that the laser light can be narrowed down, and light is emitted from the front and back of the device. The light emitting surface is the cleavage planes (4) (4).
And constitutes a Fabry-Perot resonator. (5)
(5) 'is a film made of a dielectric material provided on the cleavage planes (4) and (4), and the respective reflectances are shown by n1 and n2. FIG. 2 is a diagram showing the characteristics of the return light noise, which is indicated by the broken line (a).
Is due to the conventional laser element, and both coatings have n1 = n2 =
It is at 32%. The characteristic indicated by the alternate long and short dash line (b) is one in which the reflectance of one coating is lowered in the present invention research, and exemplifies the one in which n1 = 13% and n2 = 30%.

Both of which actually use the semiconductor laser device 1.2 <Iop / I
Return light noise is large in the current range of th <1.45. On the other hand, the characteristics shown by the solid line (C) are those of the embodiment of the present invention,
As a typical example, n1 = 20% and n2 = 30% are shown.
The return light noise could be adjusted to -70 dB or less in the above current range, and -65 dB or less even if the variation of the lot was included.

Although the above example will be described in more detail, what is shown on the horizontal axis is the drive current Iop standardized by the threshold current Ith, which is generally called an operating current. Now, the reflectivity is not actually measured by each element, but Apfel
It was derived by the circular figure method of and confirmed by the sample device. According to this method, the productivity can be improved because the conditions for forming the coating can be met. Specifically, the wavelength of laser light λ = 800 nm
When Al ² O ³ is used as the coating, the thickness of the coating is 2
The reflectivity is 32% at 400Å, and the reflectivity is 3% when the film thickness is 1200Å or 3600Å, and the characteristic changes sinusoidally during that time. And what is interesting here is that the coloration appears in the coating film according to the reflectance, and as the reflectance changes from 32% to 3%, it changes from pink → gold → yellow → green → light blue → light color.

From the above, the conditions for reducing the above-mentioned return light noise are shown. One film (5) has a reflectance n1 of 15% or more and 25% or less, and the other film (5) 'has a reflectance n2 of 25% or more 32%
In the following cases, the characteristic curve becomes almost parallel to the horizontal axis so that it becomes -60 dB or less. At this time, the color of the coatings (5) and (5) 'is yellow green to yellow to gold in the coating (5) having a lower reflectance, which is preferably yellowish and has a higher reflectance. Coating 5 '
It should have a golden to pale pink to pink purple and pink taste.

These results can be considered as follows. When the coatings (5) and (5) 'have the same reflectance and when both have high reflectance, the laser light has extremely high coherency (coherence), so that the emitted light and the returned light interfere with each other, and as a result, so-called It is considered to be a return light noise. Therefore, the higher the output of the laser light, the greater the return light noise. On the other hand, if either of the coatings (5) and (5) 'has a reflectance that is too low, it is considered that the optical confinement of the Fabry-Perot resonator is weakened and the oscillation is weakened, and the return light easily enters the resonator. Therefore, the return light noise does not decrease even after the threshold value is exceeded for a while. One of the reasons that the coherence and the oscillation conditions are considered to be the causes is the relationship between the change in the reflectance and the pattern of the characteristic curve of the return light noise. That is, as the reflectance increases, the curve in FIG. Although the mountain portion of the pattern B is gradually reduced, the coating film (5) changes from green to yellow-green (comparison of reflectance of 14% and 15%), and suddenly becomes a pattern of C. Also, the coating (5) changes from gold to pale pink (comparison of reflectance of 25% and 27%)
Also, the return light noise in the high operating current region suddenly increases. If the reflectance of both coatings (5) and (5) 'falls below 25%, laser oscillation becomes unstable.

G) Effect of the invention As described above, the reflectance of the coating provided on the cleaved surface is made different between before and after the Fabry-Perot resonator, and one is set to 15 to 25% and the other is set to 25 to 32%. It was possible to greatly reduce the return light noise without any measures against the return light.

[Brief description of drawings]

FIG. 1 is a perspective view of a semiconductor laser device according to an embodiment of the present invention, and FIG. 2 is a characteristic diagram of the semiconductor laser device. (2) ... Active layer, (4) (4) ... Cleaved surface, (5)
(5) '... coating

Claims (1)

[Scope of utility model registration request] 1. A semiconductor laser device having a cleaved surface facing each other forming an active layer and a Fabry-Perot resonator, wherein the cleaved surfaces have reflectances n ₁ and n ₂ of 15% ≦ n ₁ ≦ 25%; 25, respectively. %
A semiconductor laser device characterized by being covered with a film satisfying ≤n ₂ ≤32% and n ₁ ≠ n ₂ .