JPS62165388A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a semiconductor laser with excellent controllability and reproducibility by a method wherein the cross section of a guide layer perpendic ular to the longitudinal axis of a resonator has a protruded shape near a reflecting end surface and the carrier diffusion length is made shorter than a half of the difference between the width of 2nd trench and the width of 1st trench. CONSTITUTION:A current applied from a whole electrode is blocked by 1st blocking layer 14 and 2nd blocking layer 15 and finally injected into an n-type Al0.15Ga0.35As active layer 12 through a stripe-shape window formed in the N-type Al0.5Ga0.5As 1st blocking layer 14 and p-type Al0.4Ga0.6As 2nd cladding layer 13. The carriers injected into the active layer 12 are diffused to the hori zontal direction of the active layer to form a gain distribution and induce a laser oscillation. In this case, as the carrier diffusion length is shorter than a half of the difference between the width of a refractive index distribution and the width of a carrier injecting region which determines the width of the gain distribution and the refractive index at the time of laser oscillation is relatively small, self-exciting oscillation is promoted.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a semiconductor laser suitable for optical information processing.

(Prior Art) Among optical information processing semiconductor lasers, it is used as a light source for reading video discs and optical discs.
Noise characteristics, especially characteristics of noise induced by returned light, are a problem. In order to reduce the return light induced noise of semiconductor lasers,
Although various methods have been tried, reducing output coherence is particularly effective.

One of these methods was to reduce the noise of semiconductor lasers by high-frequency superposition by Daiyuki, Kayane, Nakamura, and Ojima at the 1983 Autumn Conference on Applied Physics, p. 26a-102.
It has been proposed and shown to be effective in reducing noise and longitudinal mode characteristics of semiconductor lasers by P-6' high frequency superposition. On the other hand, there is a method that generates self-excited vibration and multiplies the longitudinal modes to reduce noise.
This was proposed and attempted by Kurihara in Dynamic Electronics and Communication Engineers Technical Report, Photon Quantum Electronics 0QE84-57.39 page #18B8 Laser Noise Characteristics and Mechanism of Self-Pulse Modulation I.

Furthermore, with the aim of increasing the number of functions, there is now a demand for a composite semiconductor laser device for optical information that is not only a light source for reading optical discs, etc., but also a light source for optically writing to optical discs, etc. In particular, when used as a light source for optical writing onto an optical disk or the like, it is necessary to have stable fundamental transverse mode oscillation and withstand large optical output oscillation. As a composite semiconductor laser device, for example, Yono, Endo, Ito, Yozai, Ueno, Furuse, Proceedings of the 45th Japan Society of Applied Physics Academic Conference, Autumn 1984, 190 pages 15a-R-71AIGaAsBC
As announced in M Laser Array I, an element equipped with two independently driven lasers with separated electrodes has been proposed and prototyped.

(Problems to be Solved by the Invention) The above-mentioned method using high-frequency superposition not only requires the addition of a high-frequency drive circuit, but also has disadvantages such as high-frequency leakage to an external mechanism. On the other hand, in methods that generate self-excited vibration, the characteristics of self-excited vibration depend extremely sensitively on the laser structure (layer thickness, groove width, etc.), so the yield of devices that exhibit stable self-excited vibration is low. It had drawbacks. Furthermore, in this case, large optical output oscillation is impossible. In addition, the composite semiconductor laser devices that have been proposed so far, including the example mentioned above, are simply two lasers arranged side by side. It does not have both an oscillation light source and a reading light source with low noise characteristics.

Therefore, the purpose of the present invention is to eliminate these drawbacks, to provide a reading light source that generates stable self-excited vibration and has low noise characteristics, and to provide a light source that can maintain stable fundamental transverse mode oscillation and oscillate a large optical output. The object of the present invention is to provide a semiconductor laser that has the functions of a writing light source in a single light source and has excellent controllability and reproducibility.

(Means for Solving the Problems) The present invention provides means for solving the above-mentioned problems by forming the active layer with first and second cladding layers made of a material having a wider band gap than the active layer. a first 1172 layer having a sandwiched double heterojunction structure and having a first groove on the second cladding layer, the first 1172 layer having a width wider than the first groove on the first blocking layer; A semiconductor multilayer structure including a second block layer having a second groove, the first and second grooves being filled with a guide layer having a lower refractive index than the first and second cladding layers is resonant. The active layer, the second cladding layer, and the first and second blocking layers do not reach both reflective end faces of the resonator, and the guide layer reaches directly to both reflective end faces of the resonator. In the structure, the guide layer has a convex shape in a cross section perpendicular to the longitudinal axis of the resonator near both reflective end faces, and the carrier diffusion length in the active layer is equal to the second groove width and the first groove width. This semiconductor laser is characterized by being shorter than half the difference from the groove width.

(Example) An example of the present invention will be described below with reference to the drawings. 1st
The figure is a perspective view of this embodiment, and FIGS. 2, 3, and 4.
The figures are A A'# B-B' and c of Fig. 1, respectively.
-c' arrow sectional view.

To manufacture this example, first, as shown in FIG.
n-type AA on a GaAs substrate to! o,a Ga(1,
The IAs first cladding layer 11 is made of 2.5 μm ln type Ale.
, ts()a@0. l As active layer (n-type concentration n = 1
, 5 X l O"cm") 12 to 0.08'μmsp
type kl N4 Ga,,, As second cladding layer 13 is 0.3 μm type kl(,,@Ga1l,l As first block layer 14 is 0.3 μm sn type GaAs second block layer 15 is 1.0 μm e M OCV Continuous growth is performed using the D method.The MO14p method allows thin film growth and has precise film thickness controllability, making it possible to grow the layer structure as described above with good control.Also, as described above, the active layer 12 By setting the n-type concentration to 1.5×10′″α, the carrier diffusion length can be reduced to 1 μm or less.

It has become clear that the luminous efficiency is highest at this concentration.

Next, a photoresist method is performed, and a striped window with a width of 4 μm is created in the resist film at the center of the cavity in the longitudinal direction, and the resist film near both reflective end faces is removed. Using the resist film as a mask, the GaAs block is The layer 15 is etched to form a groove in the central part of both 4aW, and the GaAs block layer 15 in the vicinity of both reflective end faces is removed.
Gas,s As d l block J@! 14 is exposed. Furthermore, a photoresist method was performed to create a striped window with a width of 2 μm in the resist film 16 at the center of the resonator so as to match the center line of the groove described above, and also remove the resist film 16 near both reflective end faces. Using the film as a mask, the first block layer 14 is etched to form a groove in the center of the resonator, and the first block layer 14 in the vicinity of both reflective end faces is removed. .4 Gao, @ As Expose the surface of the second cladding layer 13 (Figure 6).

Next, the resist film 16 is removed, the central part of the cavity is masked using a photoresist method, and the vicinity of both reflective end faces is etched.
Shape AA? o,s C)aO,! 1 Expose the surface of the AI first cladding layer 11. After removing the resist film used as a mask, the central part of the resonator and the G
a Striped portions with the same width of 4 μm as the grooves are covered with a resist film 17 in the vicinity of both reflective end faces on the longitudinal extension of the resonator in the groove formed in the As block layer 15.
O0l Gao, a As first and second layer 11 to depth 1
．． Etch 5 μm (Figure 7).

Next, after removing the resist film 17, the p-type/M! o, sw(
) aLTI As guide layer 18 with a thickness of 1.0 μm, n
:MAA! o, 5Ga6. @ As The third cladding layer 19 is 1.5 μm thick. The p-type GaA3 cap layer 20 is 1.5 μm thick.
Continuous growth of 0μm.

In the liquid phase growth method conventionally used for this growth, A16 which is a kl x Ga 1-z As layer is used.
．． lGa6) As first cladding layer 11, AJ6,4
Ga6,11 A8 second cladding layer 13 and kl @, @
Although no liquid phase layer is grown on the Ga 6J As first block layer 14, it can be easily grown using the MOCVD method.

In particular, in this MOCVD method, immediately before growing the third cladding layer 18, if the surface of the growth surface is slightly etched with a gas such as HCI, the reproducibility and reliability of the grown device can be further improved. Then 81 on the entire growth surface
0. After forming the film, a window is opened in the groove-formed region at the center of the resonator using a photoresist method, and Zn is diffused into the guide layer 18 (Zn diffusion region 21). S10. After removing the film, a p-type ohmic contact 22 is formed on the entire growth surface.
, the semiconductor laser of the present invention can be obtained by attaching an n-type ohmic contact 23 to the substrate side (Figs. 1, 2, 3,
Figure 4).

(Operations and Effects of Embodiment) In the structure of the embodiment of FIG. 1 described above, the current injected from the entire surface electrode spreads over the entire surface of the Zn diffusion region in the cap layer 20, the third cladding layer 19 and the guide layer 18, but N-type Ga A with different electrical polarity adjacent to the guide layer 1B
s Second block layer 15 and adjacent to this are n-type AA!
o, s Gao, i As As there is the first blocking layer 14, the current is blocked by the first blocking layer 14 and the second blocking layer 15, and finally the n-type AI6.10a11.
The p-type Al.4Ga64As second cladding layer 13 is formed through the striped window formed in the lAs first block layer 14.
n-type Ale, t*Gao, and asAB are implanted into the active layer 12 through the . The carriers injected into the active layer diffuse in the horizontal and lateral directions of the active layer, form a gain distribution, and start laser oscillation. At this time, as mentioned earlier, since the carrier diffusion length in the active layer is short, the gain distribution is mainly formed in the part of the active layer under the striped window formed in the first block layer 14. becomes steep, and as a result, the gain is high only in the lower part of the striped window, and the outside becomes a loss region.

On the other hand, light seeps out of the active layer and spreads in the vertical direction.

At this time, the light seeping into the second cladding layer 13 is transmitted to the p-type guide layer 18, the n-type kl@,
, Gao, @As There is a first blocking layer 14, and the light spreads to this layer. Furthermore, there is an n-type GaAs second block layer 15 adjacent to the first block layer 14, but this layer not only has a higher refractive index than the first block layer and draws in light, but also has a band gap with respect to the laser oscillation light. is narrow~
It is a layer that absorbs light of 10,000 cIIL or more.

Therefore, the light is drawn into the second blocking layer 15 and suffers a large absorption loss there. As a result, a positive refractive index difference Δη8 occurs across the window formed in the second block layer 15.

The inventor's calculation results revealed that the value is ΔηB=5×10 in this example.

As a result of the above, in the structure of this embodiment, the gain distribution is similar to the narrow window width formed in the first block layer 14, but the light is spread over a wider window width formed in the second block layer 15, and there is a difference in the gain distribution. This means that a refractive index guiding mechanism is built in. However, when carriers are injected into the active layer and a gain distribution is formed, the refractive index decreases due to the negative dependence of the refractive index on the carrier density. But its value is 3
Since the refractive index is approximately 4×10 2 , in this example, a refractive index of 1 to 2×10 2 is built in during laser oscillation, and this positive refractive index guiding and the sudden change of light due to the second blocking layer described earlier The fundamental transverse mode oscillation can be maintained due to the synergistic effect with absorption.

In the structure of this example, the width of the spread of light is wider than the width of the gain distribution, so the light spreads from the gain region to the loss region outside it, which is isometrically equivalent to having a saturable absorber. Therefore, self-excited vibration is likely to occur. Furthermore, in the structure of the present invention, the carrier diffusion length is less than half the width of the carrier injection region that determines the width of the refractive index distribution and the gain distribution width, and the refractive index during laser oscillation is relatively small, which promotes self-oscillation. The effect of

In other words, first of all, because the carrier diffusion length is short, the fluctuation of the injected carrier density distribution becomes severe, and along with this, the width of the fundamental transverse mode fluctuates greatly, causing its contraction and expansion, and as a result, a large amount of self-excited vibration occurs. encouraged. According to the analysis results of the present inventor, the carrier diffusion length is 1μ in the structure of this example.
The carrier diffusion length is 1μ as a result of calculation using m and 2μm.
It was revealed that the self-excited vibration of m is 5.5 to 6 times that of 2 μm.

Furthermore, the relatively small magnitude of the refractive index during laser oscillation also promotes fluctuations in the width of the fundamental transverse mode.

According to the analysis results of the present inventor, the ratio of the first peak intensity of self-excited vibration to the intensity at the first valley is ηB = calculated using a carrier diffusion length of 1 μm in the structure of this example.
t, ox t o, ηB=5X1 for 160
I found out that 0-” would be 195.

As a result of all the synergistic effects described above, self-excited vibration easily occurs in the structure of this example, resulting in the axial mode becoming multimodal and the coherence of the axial mode being reduced, resulting in extremely low noise for reflected light and low noise characteristics. Therefore, the laser element of this embodiment becomes a low-noise laser necessary for optical reading.

In the structure of the embodiment shown in FIG. 1, an active layer 12, a p-type kl 6.4 Ga @, @As second cladding layer 13, an n-type kl 16 . @ ()aLI A5 1st
The block layer 14 is adjacent to the guide layer 18 in the longitudinal direction of the resonator, and the guide layer 1B is connected to the guide layer 18 with a constant layer thickness over the entire length of the resonator. Therefore, the light spread over the groove area in the center of the resonator does not leak and travels into the adjacent guide layer 18 in the vicinity of both reflective end faces. In the vertical direction, the guide layer 1B near the reflective end face is sandwiched between the first cladding layer 11 and the third cladding layer 19, which have a smaller refractive index than the guide layer 18, and in the horizontal and lateral directions, most of the guide layer 1B is sandwiched between the first cladding layer 11 and the third cladding layer 19, which have a lower refractive index than the guide layer 18. 19, forming an optical waveguide. Therefore, the light does not spread and the guide layer 1
The light condenses within the convex portion of No. 8 and travels. In this embodiment, the band gap of the guide layer 18 is 15 for the laser oscillation light.
Since it spreads over 7 meV, the light traveling through the guide layer 18 will not suffer any absorption loss. In this way, guide layer 1
The light that has traveled through B is reflected by the reflective end face, returns to the guide layer 18 with an optical waveguide function without any loss, enters the active layer 12, and is reexcited, resulting in laser oscillation with a low threshold and high efficiency. I can do things.

In the structure of this embodiment, since the vicinity of both reflective end faces serves as a guide layer with a wide bandgap for laser oscillation light, the optical output level at which optical damage (COD) occurs can be significantly increased. That is, in a normal semiconductor laser, the end face of the active layer, which becomes an excitation region due to carrier injection, is exposed as a reflective end face9, where surface recombination occurs and becomes a depletion layer, reducing the band gap. When a large optical output is produced, this narrowed bandgap causes absorption of light, which generates heat and raises the temperature to near the melting point, eventually causing optical damage. On the other hand, in the structure of this example, not only the vicinity of both reflective end faces is a non-excitation region, but also the band gap difference of the laser oscillation light is 157 m.
Since it oscillates by passing through a layer wider than eV, there is no light absorption near the reflective end face, and the optical output level at which optical damage occurs can be increased by more than one order of magnitude, making it possible to discover a large optical output.

Note that although an n-type GaAs substrate was used in the above embodiment, a p-type GaAs substrate was used.
The present invention can be realized even if n is inverted. Also, the example is k
Although the lGaAs/GaAs double heterojunction crystal material has been described, other crystalline materials such as lmGa
P/AIJnP# InGaA8P/ InGaP r
InGaAB P/ In P * kl GB As
The present invention can also be applied to semiconductor lasers made of many crystal materials such as Sb/C)a AB sb.

(Effects of the Invention) As explained above, the semiconductor laser of the present invention can: ■ Stably maintain the fundamental transverse mode oscillation; ■ Generate self-excited vibration, and have a wide tolerance range for reproducibility. It has the following advantages: 1) the same light source can produce the large optical output required for optical writing; and 2) the same light source can perform both writing and reading, making it possible to make the optical system more compact.

In summary, according to the present invention, the function of a reading light source that generates stable self-excited vibration and has low noise characteristics, and the function of an optical writing light source that maintains stable fundamental transverse mode oscillation and can oscillate a large optical output are achieved. It is possible to provide a semiconductor laser that has both functions in a single light source and has excellent controllability and reproducibility.

[Brief explanation of drawings]

FIG. 1 is a perspective view of one embodiment of the present invention, and FIGS. 2, 3, and 4 are views taken along arrows A-A', B-B', and C-C' in FIG. 1, respectively. , FIG. 5 is a vertical cross-sectional view showing a semiconductor stacked structure obtained by forming a double heterojunction structure on a substrate in the manufacturing process of the embodiment shown in FIG. 1, and FIG. 7th cross-sectional view of the central portion of the semiconductor stacked structure obtained by forming a groove in the central portion of the resonator;
This figure is a perspective view showing a semiconductor laminated structure obtained by providing convex regions in the vicinity of both reflective end faces in the manufacturing process of the embodiment of FIG. 1. 10...n-type Ga As substrate, 11"/n-type A16
．． II Gao, sA8 first cladding layer, 12”・n type Alo, ts Gao, ss As active layer, 13・p
Type Alo, 4 Ga@, 6 As second cladding layer, 14
... n-type kl @JGa O,I As first block layer, 15 ... n-type Ga ks second block layer, 16
...1/ cyst film, 17-... resist film, 18-p
Shape AA! ,,B,I Ga0. a B As guide layer, l 9 ... n-type kl 6. @ Ga 6.5
As third layer 20... p-type Ga As cap layer,
21...Zn diffusion region, 22...p type ohmic contact, 23...n type ohmic contact.

Claims (1)

[Claims] It has a double heterojunction structure in which an active layer is sandwiched between first and second cladding layers made of a material with a wider band gap than the active layer, and a first cladding layer is further disposed on the second cladding layer.
a first block layer having a groove, a second block layer having a second groove wider than the first groove on the first block layer; A semiconductor multilayer structure is provided in the central part of the resonator, in which a groove is filled with a guide layer having a refractive index lower than that of the first and second cladding layers, and the active layer, the second cladding layer, and the first The second blocking layer does not reach both reflective end faces of the resonator, and the guide layer reaches both reflective end faces, and in the vicinity of both reflective end faces, the cross section perpendicular to the longitudinal axis of the resonator A semiconductor laser characterized in that the guide layer has a convex shape, and the carrier diffusion length in the active layer is shorter than half the difference between the second groove width and the first groove width.