JPH04144184A - Method for controlling coherence of semiconductor laser element - Google Patents

Abstract

PURPOSE:To reduce coherence of a laser to be generated and to reduce a return light noise by forming a clad layer between a current narrowing layer and a light emitting layer of a predetermined composition, and setting carrier concentration of the clad layer to a predetermined value or less. CONSTITUTION:A clad layer between a current narrowing layer and a light emitting layer is formed of Ga(1-x)AlxAs (0.3<x<0.5), and carrier concentration of the clad layer is set to 1.5X10<18>/cm<3> or below. the cap concentrations of the emitting layer 4, a second clad layer 6 and a cap layer 6 are respectively 1-10X10<17>/cm<3>, 1-10X10<17>/cm<3>, 2-4X10<18>/cm<3>, and carrier concentration of a first clad layer 3 between the narrowing layer 2 and the emitting layer 4 is variously varied in a range of 5-30X10<17>/cm<3> by regulating adding amount of an impurity such as Mg, Zn, etc. The lower the carrier concentration becomes, the smaller a gamma value of an index or coherence becomes. This trend is particularly remarkable if the cap concentration in a region having high threshold current is 1.5X10<18>/cm<3> or below.

Description

[Detailed description of the invention] [Industrial application field]

The present invention is applicable to Gal! used for optical pickup for compact discs and laser discs. The present invention relates to a method of controlling an As semiconductor laser device.

[Conventional technology]

In recent years, as this type of tip pickup has become smaller and the number of components has been reduced, the amount of disturbance light returning to the semiconductor laser element used has increased, making the emitted laser unstable and causing so-called return light. The noise started to increase. This return optical noise is a major factor that affects the performance of optical pickup, and is closely related to the coherence of the semiconductor laser device. Here, coherence refers to the fact that the emitted lasers interfere with each other in relation to the phase, so that a pattern can be observed. Using the Michelson interferometer shown in Figure 3, we can Defined quantitatively. That is, after the laser emitted from the semiconductor laser element 11 shown in FIG. The light is guided to the light receiving element 15 by creating an optical path difference. Then, the intensity of the interference light detected by the light receiving element 15 changes according to the optical path difference Δg, as shown in the interferogram of FIG. The maximum value on the envelope I7 of this interferogram 16 is defined as A, the next maximum value is defined as B, and the coherence is quantitatively defined by γ-B/A. Therefore, γ is 0 or more! Taking the following numerical values, when γ-1, the coherence is strongest and is completely coherent, and as γ approaches 0, the coherence becomes weaker. On the other hand, return optical noise quantitatively returns the laser emitted by the semiconductor laser element to the output laser intensity S (signal).
The fluctuation N (noise) occurring in the signal is measured and expressed as the ratio of S7/N. FIGS. 5 and 6 show the results of measuring the return optical noise S/N for semiconductor laser devices whose γ value, which is an index of coherence, is approximately 1.0 and 0.5, respectively. As is clear from the figure, the returned light noise S/N is worse for the element with a large γ value (γ-10) and strong coherence (FIG. 5). This is because the stronger the coherence, the more the oscillation wavelength becomes single, which causes a change in the wavelength due to the returned light or significant moat fluctuations, resulting in large fluctuations in the output laser intensity. It's called noise. From the above facts, in order to obtain a semiconductor laser device with low return optical noise, it is necessary to determine the γ value (γ
-B/'A) is essential.
Conventionally, in internal current pinching type GaA (!As) semiconductor devices, as a method to reduce the above γ value, the thickness of the light emitting layer is greater than Lf, and the cladding layer between the current pinching layer and the light emitting layer is made thicker. There are known methods to do this.

[Problem to be solved by the invention] On the other hand, the former conventional method increases the thickness of the light emitting layer, resulting in an increase in the threshold current for laser oscillation. Furthermore, in the latter conventional method, since the cladding layer is made thicker, the spread of the current becomes larger and the reactive current that does not contribute to light emission increases, which also causes an increase in the threshold current. Further, an increase in the threshold current increases the drive current and heat generation amount of the device, and has the drawback of shortening the life of the semiconductor laser device. Therefore, the purpose of the present invention is to reduce the return optical noise in the long term by lowering the coherence, that is, the γ value, of the generated laser without increasing the threshold current. It is an object of the present invention to provide a method for controlling the aJ modulus of a semiconductor laser device. [Means to solve the problem]

The inventor focused on the fact that in order to reduce the γ value without increasing the threshold current, it was only necessary to adjust the composition and carrier concentration of the cladding layer, and after conducting extensive experiments and research under various conditions, the results were as follows: The present invention has now been constructed. That is, the method for controlling the coherence of a semiconductor laser device according to the present invention is such that, in an internal current pinching type GaAQAs semiconductor laser device, the cladding layer between the current pinching layer and the light emitting layer is
a(+-x)AQx)AlxAs(0.3<x<0.5
), and the cladding layer has a carrier concentration of 1.5×10 16 /cm 3 or less. Note that the reason why the carrier concentration is limited to 1.5 x 10''/cm3 or less is to lower the γ value to about 0.5 or less while keeping the threshold current constant.

[Effect]

The cladding layer is made of Ga(+-ReAQx)AlxAs(0.3
<x<0.5), and the carrier concentration is 1.5x.
Since it is set to 10'8/cm3, the coherence of the generated laser can be lowered without increasing the threshold current of the semiconductor laser element, and the return optical noise can be reduced over a long period of time. Note that the lower the carrier concentration, the greater the decrease in the γ value, which is more effective in reducing return optical noise.

EXAMPLES The present invention will be described in detail below with reference to illustrated examples. Figure 1 shows the internal current pinching type Cal! VSIS (V channel substrate) is an example of A8 semiconductor laser device.
1 is a cross-sectional view of an inner stripe type semiconductor laser device. In the figure, ■ is a substrate made of P-type GaAs, and 2 is a substrate made by growing N-type GaAs to a thickness of about 1 μm on this substrate l, and etching the center part using a photoresist with a sulfuric acid-based etching solution. The current pinching layer 3 is a P-type Ga (+-re AQy, A
sC0,3<x<0.5) is grown to a thickness of approximately 0.2μ, and 4 is the first cladding layer 3.
P-type C; a (l-AQy) AlxAs (0.
1 < y < 0.2) to a thickness of about 01 μm, and 5 is an N-type Ga(+-x)A &
A second cladding layer formed by growing xAs (OJ<X<0.5) to a thickness of approximately 1 μm, 6 is this second cladding layer 5L
This is a cap layer made by growing N-type GaAs to a thickness of several μm. The light emitting layer 4. Second crust layer5. The carrier concentration of the cap layer 6 is 1 to 10 x 1017/cm3 and 1 to 1, respectively.
0X 10'7/cm", 2~4 X 10+11
/ Cm3, and the carrier concentration of the first cladding layer 3 between the current pinching layer 2 and the light emitting layer 4 is 5 to 30×10”/am3 by adjusting the amount of impurities such as Mg and Zn.
(See Figure 2). Note that the etched portion 2a of the current pinching layer 2 serves as a current path. In this way, the γ value (γ-B/A) of each of the vsrs type semiconductor laser devices fabricated so that the first cruciate layer 3 has various carrier # variations is determined by the method described in FIG. 3.4. The threshold current 1th was measured. FIG. 2 shows the relationship between the measured γ value and the threshold current rth for various carrier concentrations in the first cladding layer 3. As is clear from the figure, the threshold current I of the semiconductor laser device
Even if th is the same, the lower the carrier concentration, the smaller the γ value, which is the coherence index. Also, this trend is
Carrier concentration is 1.5x in the high threshold current region
This is particularly noticeable when the size is 10”7 cm3 or less, and the γ value decreases from 10 to 0.5 or less.
．． If the threshold current decreases to 5 or less, the return light noise, which has been a problem in the past, as described in FIG. As described above, in the present invention, since the carrier concentration in the cladding layer between the current pinching layer and the light emitting layer is kept below a certain value, the threshold current for laser oscillation is not increased.
By weakening the coherence of the generated laser, unwanted return optical noise can be reduced over time.

【Effect of the invention】

As is clear from the following explanation, the method for controlling the coherence of a semiconductor laser device according to the present invention uses Ga(+-x)A(! x) AlxA
s (0.3<x<0.5), and the carrier concentration is 15X 10”/cm3 or less, so
Without increasing the threshold current of the semiconductor laser element, the coherence of the generated laser can be lowered, and return optical noise can be reduced over a long period of time.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a vsrs type GaAρAs semiconductor laser device as an example of a semiconductor laser device according to the present invention, and FIG. 2 shows the relationship between the γ value and the threshold current Ith of the device in FIG. 1 for various carrier concentrations. 3 is a schematic diagram of a Michelson interferometer, FIG. 4 is a diagram showing an interferogram obtained with the above interferometer, and FIGS. 5 and 6 are γ21.0.0.5, respectively. FIG. 3 is a diagram showing the return optical noise characteristics of the element. ■...Substrate, 2...Current pinching layer, 3...First cladding layer, 4...Light emitting layer, 5...Second
Cladding layer, 6. Cap layer, 11. Semiconductor laser element, 15. Photodetector, 16. Interferogram.

Claims (1)

[Claims] (1) In an internal current pinching type GaAlAs semiconductor laser device, the cladding layer between the current pinching layer and the light emitting layer is made of Ga_(_
1_-_x) Al_xAs (0.3<x<0.5), and the carrier concentration of the cladding layer is set to 1
．． 5×10^1^6/cm^3 or less to reduce the coherence of the generated laser without increasing the threshold current of the semiconductor laser element and reduce the return optical noise. A method for controlling the coherence of a semiconductor laser device.