JPS6364385A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To elevate an optical output level where optical damage is generated remarkably by using sections in the vicinity of both reflection as guide layers having wide band gaps to laser oscillation beams. CONSTITUTION:Double hetero-junction structure in which an active layer 14 is held by a first clad layer 13 and a second clad layer 15 consisting of a material having a band gap wider than the active layer 14 is formed onto a substrate 10. A guide layer 16 having a refractive index larger than the clad layers 13, 15 is shaped onto the clad layer 15. A first block layer 17, which has a first trench in the longitudinal direction of a resonator except sections in the vicinity of both reflecting surfaces and on the extension of a striped recessed section region while having a conductivity type reverse to the guide layer 16, is formed onto the guide layer 16. A second block layer 18, which has a second trench in width wider than the first trench in the longitudinal direction of the resonator while having a conductivity type the same as the block layer 17 and a band gap narrower than the active layer 14, is shaped at the center on the block layer 17. Accordingly, an optical output level where optical damage is generated is elevated remarkably.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor laser, and particularly to a semiconductor laser used as a light source of an optical information processing device.

[Conventional technology]

Among semiconductor lasers for optical information processing, when used as a light source for reading a video disk or an optical disk, noise characteristics, especially noise induced by return light, become a problem. Various methods have been tried to reduce the feedback-induced noise of semiconductor lasers, and among them, reducing output coherence is particularly effective. One of the methods for this is to reduce the noise of semiconductor lasers by high-frequency superposition. It has been proposed and shown to be effective in ``Laser Noise Reduction and Longitudinal Mode Characteristics''.

On the other hand, a method of generating self-l1ilj oscillation and multiplying the longitudinal modes to reduce noise is described by Meki, Matsumoto, Tamura, Watanabe, and Kurihara in Technical Report of Institute of Electronics and Communication Engineers, Photon Quantum Electronics 0QE84-57.39 p. r I SSS It was proposed and attempted in ``Laser Noise Characteristics and Mechanism of Self-Pulse Modulation''. Furthermore, with the aim of increasing the number of functions, there is now a demand for a composite semiconductor laser device for optical information processing that is not only a light source for reading optical discs, but also a light source for optically writing onto optical discs. Particularly 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, Kuwamura, Ueno, Furuse published the 1984 Autumn 4th
5th Japan Society of Applied Physics Academic Conference Lecture Preliminary mA, 190 pages, 1
5a-R-7rA&GaAs BCM laser array”
A device equipped with two independently driven lasers with separate electrodes has been proposed and prototyped.

[Problem that the invention seeks to solve]

The above-mentioned method using high frequency superimposition not only requires 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 automatic oscillations, it is expected that the characteristics of the automatic signal will depend very sensitively on the laser structure (layer thickness, groove width, etc.), and this will reduce the yield of devices exhibiting stable automatic oscillations. It has the disadvantage of being low, and furthermore, it is not possible to operate with a large optical output necessary for optical writing. Furthermore, in the case of composite semiconductor laser devices, the ones proposed so far, including the example mentioned above, are simply two lasers arranged side by side. It did not have both the high optical output oscillation characteristics of 2000 and low noise characteristics as a reading light source.

The purpose of the present invention is to provide a reading light source that eliminates the above-mentioned drawbacks, generates stable automatic vibration, and has low noise characteristics, and an optical writing light source that maintains stable fundamental transverse mode oscillation and is capable of oscillating large optical output. It is an object of the present invention to provide a semiconductor laser which has the characteristics of combining the above functions with a single light source, and has excellent controllability and reproducibility.

[Means for solving problems]

In order to solve the above-mentioned problems, the semiconductor laser of the present invention has an active layer formed on a substrate having striped recessed regions in the longitudinal direction of the resonator in the vicinity of both reflective surfaces of the resonator. It has a double heterojunction structure sandwiched between first and second cladding layers made of a material with a wide bandgap, and a guide layer having a higher refractive index than the first and second cladding layers on the second cladding layer. A first groove is provided on the guide layer in the longitudinal direction of the resonator excluding the vicinity of both reflective surfaces and on an extension of the striped recessed region, and has a conductivity type opposite to that of the guide layer. The first block layer has a second groove wider than the first groove in the longitudinal direction of the resonator, and is of the same conductivity type as the first block layer. In addition, a second block layer having a narrower band gap than the active layer is provided in the central region of the resonator excluding the vicinity of both reflective surfaces 3d, and a semiconductor layer provided by filling the first and second grooves is provided in the second block layer. It is characterized by being provided on the block layer. Note that it is desirable to determine the width of each groove so that the carrier diffusion length within the active layer is shorter than half of the length of the first and second grooves.

〔Example〕

The present invention will be explained below using the drawings. FIG. 1 is a perspective view of one embodiment of the present invention, and FIGS. 2, 3, and 4 are sectional views taken along lines AA', BB', and C-c' in FIG. 1, respectively.

First, as shown in FIG. 5, a photoresist method is applied to an n-type GaAs substrate 10 with the (100) plane as the surface, leaving a central region of the resonator with a length of 250 μIn in the longitudinal direction of the resonator, and forming a central region on both ends of the resonator. A striped window with a width of 5 μm and a length of 30 μm is opened, and etching is performed to a depth of 0.4 μm using the resist film 11 as a mask to form a striped recessed region 12 in the vicinity of both reflective surfaces. After removing the photoresist film 11, the n-type Ae 0.5aaO, 5S first cladding layer 13 is coated with a thickness of 2.0 μm, n-type^e 0-1
5ca0.85^S active layer (n-type concentration n = 1.5
:', 1018C111-3) 14 to 0.08 μm
, p-shaped person! ! +1.45GaO0'55^S 2nd
The crat layer 15 is 0.1m, p-type ^I;! 0-35
GaO16SAS guide layer 16 is 0.7 μm, n-type A/
), 5Ga□, 5As first block layer 17 1)j
) t m , an n-type GaAs second block layer 18 is continuously grown to 1.0 μm using the MOcVD method.

The liquid phase growth method conventionally used for the above growth is a method in which a melt with controlled composition is prepared for each growth layer and the substrate is moved to grow each layer. Not only is growth extremely difficult, but it is also impossible to control the thickness of each layer of each composition. On the other hand, since the MOCVD method is a vapor phase growth method using an organic metal, layers of any composition can be easily grown into any multilayer by changing the composition of the mixed gas. The growth of these structures can be easily and well controlled.

Furthermore, in the MOCVD method, fine particles of each composition grow while bonding, so there is no dependence on the plane orientation of the growth, and the growth is uniform in thickness in all directions.

Therefore, even if multiple layers are grown in the concave region 12 as in the structure of the present invention, layers with uniform thickness will grow along the shape of the four parts.

Further, as mentioned above, the n-type concentration of the active layer 14 is set to 1.5×10
If the CI is set to 11018CI, the carrier diffusion length will be 1μm.
It can be: It was also revealed that the luminous efficiency was highest at this concentration. Next, a stripe-shaped window with a width of 2 μm and a length of 230 μm is opened in the longitudinal direction of the resonator so as to coincide with the center line of the recessed region 12 in the central region of the resonator using a photoresist method.
The block layer 18 and the first block layer 17 are etched to form a first groove, and the p-type ^e 0035G10.6
The upper surface of the 5-person S guide layer 16 is exposed. After removing the photoresist I film, the photoresist method was performed again to align the center line of the first groove with a width of 4 μrn.
A stripe-shaped window with a length of 230 μm is opened in the center of the resonator, and windows with a width of 30 μm are opened in parallel to the reflective surfaces near both reflective surfaces. Using this photoresist film 19 as a mask, a second GaAs film layer is formed. By machining and sowing only 18, the A lo, 5Ga□, 5As first pro-nk layer 17 was formed.
A second groove is formed in the central region of the resonator, and the second block layer 18 near both reflective surfaces is removed (FIG. 6).

Next, after removing the photoresist film 19, the p-type
o, 4 Gao, 6 people S third cladding layer 20 1.5 μm
, a high concentration p + type GaAs cap layer 21 with a thickness of 1.0 μm
Continuous growth. In this growth, in the conventional liquid phase growth method, the Aff0135GaO-65S guide layer 16, which is an Aj'xGal-yAS layer, is used.
Although no liquid phase layer is grown on the first block layer 17, it can be easily grown using the MOCVD method. Especially this MOCV
Immediately before growing the third cladding layer 20 in the D method, HC
The reproducibility and reliability of the grown device can be further improved by etching a small amount of the back side of the growing side with a gas such as I. Thereafter, a p-type ohmic contact 225 is attached to the entire growth surface, and an n-type ohmic contact 23 is attached to the substrate side to obtain the semiconductor laser of the present invention (first
(Fig. 2, Fig. 3, Fig. 4).

In the structure of the present invention, the current injected from the entire surface electrode is transferred to the cap layer 21. Adjacent to the third cladding layer 20, an n-type GaAs second block layer 18, which spreads over the entire surface of the third cladding layer 20 and has a different electrical polarity, is further adjoined to the n-type GaAs second block layer 18. Because of the presence of the S first block layer 17, the current is blocked by the first and second block layers, and finally the p Shape A
f 0-35GaO, 65S guide layer 16 and adjacent p-type he 0.45ca0.55S second clad, through layer 15, n-type keO, 15Ga,)
45 people are injected into the active layer 14. 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 above, 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 17, and its shape is breathless. As a result, only the lower part of the striped window has a high gain, and the outside becomes a loss region. In the structure of the present invention, current is blocked by the second blocking layer even in the longitudinal direction of the resonator, and flows concentratedly into the active layer in the central region. The current that spreads to the vicinity of both reflective surfaces and flows into the active layer becomes negligible. Furthermore, since the active layer in the central region of the resonator is in contact with the second cladding layer 15 having a wide bandgap at both ends in the longitudinal direction of the resonator at the boundary with the groove portion near both reflective surfaces,
The carriers injected into the active layer in the central region are confined not only in the vertical direction but also in the longitudinal direction of the resonator, thereby promoting an increase in gain. As a result, it is possible to oscillate with a low threshold current. On the one hand, light seeps out of the active layer 14 and spreads in the vertical direction. At this time, the light seeping into the second cladding layer 15 spreads to the guide layer 16 having a high refractive index adjacent to the second cladding layer 15 and the first block layer 17 adjacent thereto. Furthermore, there is an n-type GaAs second block layer 18 adjacent to the first block layer 17, but this layer has a higher refractive index than the first block layer 17 and not only draws in light but also resists laser oscillation light. It is a light absorbing layer with a narrow band gap of 100100O0' or more. Therefore, the light is drawn into the second blocking layer 18 and suffers a large absorption loss there. As a result, there is a positive refractive index difference Δ across the window formed in the second block layer 18.
η8 occurs. In this example, the value is ΔηB=5
The inventor's calculation results revealed that the difference is x10-'. As a result of the above, in the structure of the present invention, light spreads over a wider window width than the narrow window width formed in the second block layer 18, whereas the gain distribution is about the narrow width of the window formed in the first block layer 17, and there, positive refraction occurs. This means that a rate guiding mechanism is built in. By the way, when carriers are injected into the active layer to form a gain distribution, the refractive index decreases due to the negative dependence of the refractive index on the carrier density.

However, since its value is about 3 to 4X10-3, in this example, a refractive index of 1 to 2X10-3 is built in during laser oscillation, and this positive refractive index guiding and the above-mentioned The fundamental transverse mode oscillation can be maintained due to the synergistic effect with the rapid absorption of light by the second blocking layer.

In the Ii structure of the present invention, 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 isometrically has a saturable absorber. As a result, 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. It has a promoting effect. That is, first of all, because the carrier diffusion length is short, the fluctuation of the injected carrier density distribution becomes severe, and this causes the width of the fundamental transverse mode to fluctuate greatly, causing its contraction and expansion, and as a result, the magnitude of automatic oscillation is promoted. be done.

According to the analysis results of the present inventor, as a result of calculation using carrier diffusion lengths of 1 μm and 2 μm in the structure of this example, the automatic vibration of carrier diffusion length of 1 μm is 5.5 to 2 μm.
It turned out to be 6 times as much. 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 inventor's analysis results,
As a result of calculation using a carrier diffusion length of 1 μm in the structure of this example, the ratio of the first peak intensity of automatic oscillation to the intensity at the first valley is Δηa. ``''It turned out that 5X10-3 would be 195.

As a result of all the above synergistic effects, automatic vibration easily occurs in the structure of the present invention, resulting in the axial mode becoming multimodal,
Since the coherence of the axial mode is reduced, the noise caused by the reflected light is also extremely low, and low-noise characteristics can be obtained. Therefore, the laser device of the present invention becomes a low-noise laser necessary for optical reading.

In the side path of the present invention, the active layer 14 is connected to the guide layer 1.6 of the striped recessed region 12 via the second cladding layer 15 in the vicinity of both reflective surfaces in the longitudinal direction of the resonator. Further, the guide layer 16 is continuous with a constant layer thickness over the entire length of the resonator.

As mentioned above, the light spreads across the groove width of the second block layer 18 in the horizontal direction of the active layer, and spreads from the active layer to the guide layer 16 in the vertical direction, so most of the light passes through the second cladding layer 15. It advances into the guide layer 16 of the recessed region 12. At this time, part of the light travels into the active layer in the four-part region, but since the active layer in this region is not excited, 150C11-1
It becomes an absorption area of about 100%. Since laser oscillation occurs where the gain is greatest and the loss is smallest, as described above, most of the light travels from the central region of the resonator through the guide layer 16 of the recessed region 12 and starts laser oscillation. Further, in the structure of the present invention, the guide layer 16 of the recessed region 12 is sandwiched between the second cladding layer 15 and the first block layer 17 in the vertical direction, and both ends of the guide layer 16 in the horizontal direction are partially sandwiched by the first block layer 17. are sandwiched between the two to form a striped optical waveguide. In the structure of the present invention, in the four-part region 12, there is a third cladding layer 20 adjacent to the first block layer 17, and as in the central region of the cavity, there is a second cladding layer 20 adjacent to the first block layer 17, which serves as a light absorption layer.
Since the blocking layer 18 has been removed, light travels while being confined within the optical waveguide.

In this example, the band gap of the guide layer is widened by 157 meV or more with respect to the laser oscillation light, so that the light traveling within the guide layer does not undergo absorption loss. In this way, the light that has traveled through the guide layer 16 forming a striped optical waveguide in the concave areas 12 near both reflective surfaces is partially reflected by the reflective surfaces, and is returned to the guide layer 16 of the concave areas 12 having an optical waveguide function.
6 without any loss, and the active layer 1 in the central region
4 and is re-excited, making it possible to perform laser oscillation with a low threshold and high efficiency. In particular, if the active layer of the central region, which is the active region of the central part of the resonator, is located near half the height of the guide layer of the four-part region as in this example, the light reflected by the reflective surface will be activated more effectively. You can enter the layers.

In the structure of this example, since the area near the re-reflection is a guide layer with a wide bandgap for the laser oscillation light, optical loss is reduced. In other words, in a normal semiconductor laser, the end face of the active layer, which becomes the excitation region due to carrier injection, is exposed as a reflective surface, where surface recombination occurs. It becomes a depletion layer and the bandgap shrinks.When oscillating with a large optical output, this reduced bandgap causes absorption of light, which generates heat and raises the temperature to near the melting point.
Eventually, optical damage occurs. On the other hand, in the structure of this example, not only are the areas near both reflecting surfaces a non-excitation region, but also the laser oscillation light is oscillated by passing through a layer with a wide band gap difference of 157 meV or more. Since there is no absorption of light, the optical output level at which optical damage occurs can be increased by more than one order of magnitude, and large optical output oscillation is possible.

In the structure of the present invention, the vicinity of both reflective surfaces is a guide layer with a guiding mechanism, so the laser oscillation light is controlled while traveling in this area and is adjusted to the width and thickness of the guide layer forming the optical waveguide mechanism. Since it is limited, coupling with an external optical system is easy and the efficiency can be increased.

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. Furthermore, although this embodiment has been described with respect to the AeGλAs/GaAs double heterojunction crystal material, other crystal materials, such as 1nGaP
/person E InP, InGaAsP/1nGal', 1n
GaAsP/InP, human (2GaAsSb/GaAsS
The present invention can also be applied to semiconductor lasers made of many crystal materials such as B.

〔Effect of the invention〕

As detailed above, the semiconductor laser of the present invention has (a) small astigmatism and can maintain stable fundamental transverse mode oscillation up to high optical output.

(υ) Large optical output oscillation is possible.

(C) It generates self-excited vibration and has a wide tolerance range of conditions and low noise characteristics.

(d) Since the light source can be made into a concentric circular light source and the coupling efficiency with an external optical system increases, the optical system can be made compact.

It has the following advantages.

[Brief explanation of drawings]

FIG. 1 is a perspective view of one embodiment of the present invention, and FIG. 2 is a perspective view of one embodiment of the present invention. Figures 3 and 4 are AA' and BB' of Figure 1, respectively.
, CC' sectional view, FIG. 5 is a perspective view when the substrate is formed in this example, and FIG. 6 is a striped current injection after growing a double heterojunction crystal in the process of manufacturing this example. It is a perspective view when an entrance is formed. DESCRIPTION OF SYMBOLS 10... N-type GaAs substrate, 11... Photoresist film, 12... Concave region, 13 -n-type Af O,
5GaO, 5As first cladding layer, 14-n type ^e o
, +5Gao, g5As active layer, 15-p type e 0-
450aO-55S second cladding layer, 16 ・-p-type 0135GaO, 65AS guide layer, 17--・
N-type ^l! 0.5Gao, 5As first block layer, 18
-... N-type GaAs second block layer, 19... Photoresist film, 20 ・P-type layer e 0-4GaO-6AS
As third cladding 21...p+ type GaAs cap layer, 22...p type ohmic contact, 23...n
Ohmic contact. , Pa, ゛- Agent Patent Attorney Uchihara Akira'1, Ni 1, s 3
figure

Claims (1)

[Claims] An active layer is formed on a substrate having a striped recessed region in the longitudinal direction of the resonator in the vicinity of both reflective surfaces of the resonator, and first and second cladding layers are formed of a material having a wider band gap than the active layer. A guide layer having a higher refractive index than the first and second cladding layers is provided on the second cladding layer, and a first groove is formed on both sides of the guide layer. a first block layer disposed in the longitudinal direction of the resonator excluding the vicinity of the reflective surface and on an extension of the striped recessed region and having a conductivity type opposite to that of the guide layer; A second groove having a second groove wider than the first groove in the longitudinal direction of the resonator and having the same conductivity type as the first block layer and having a narrower bandgap than the active layer. A semiconductor characterized in that a block layer is provided in the central region of the resonator excluding the vicinity of both reflective surfaces, and a semiconductor layer provided by filling the first and second grooves is provided on the second block layer. laser.