JPH0267777A - semiconductor laser - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field 1] The present invention relates to the structure of a semiconductor laser (hereinafter referred to as LD) capable of oscillating low noise and high optical output.

[Prior art I] When applying an LD to an optical information processing device, especially a rewritable/erasable optical disk system, stable fundamental transverse mode oscillation, which is a basic requirement for the LD, is required. In addition to low astigmatism characteristics, the following items must be satisfied.

1) A part of the light emitted from the LD is reflected by the optical disk surface or external optical system, etc., and returns to the LD resonator again, resulting in noise components of the output light from the LD (hereinafter referred to as return light noise) due to interference effects. ) increases, making it impossible to put it into practical use. Therefore, it has low noise characteristics that reduce this return optical noise.

2) When writing information on the optical disc surface or erasing information that has already been written,
, a high optical output of 30 mw or 40 mw or more is required.

Therefore, it has high light output characteristics.

In order to achieve the low noise characteristic (1) above, a method is generally known in which the LD is caused to oscillate in longitudinal multi-mode with a short coherence length. A method to achieve this includes longitudinal single mode oscillation L
A high frequency superimposition method in which longitudinal multi-mode oscillation is achieved by driving D by adding a high frequency current of several hundred MHz to the drive current, or the method described in Japanese Patent Application Laid-open No. 60-140774 and Japanese Patent Application Laid-open No. 6
Near the end face of the LD resonator, such as 0-150682,
Light waveguide is done by refractive index waveguide, and in the central part,
There have been methods such as achieving longitudinal multimode oscillation that reflects the waveguide mechanism at the center of the resonator by using gain waveguide or optical waveguide with a mixture of refractive index wave and gain waveguide.

Another method for realizing high optical output characteristics is a phased array type LD that has a plurality of light emitting stripes and creates optical coupling between each stripe. As a known example, the 32nd
It is shown on page 149 of the proceedings of a lecture on regenerative physics.

[Problems to be Solved by the Invention] However, in the prior art shown in the above-mentioned LD that has both refractive index guided and gain guided waveguides, a stable fundamental transverse mode is generated near the light output end face of the resonator. In order to satisfy the requirements such as characteristics and low astigmatism characteristics, the LD
Also in the direction parallel to the bonding of each layer such as the cladding layer and active layer,
In order to effectively confine light, an effective refractive index step (△neff) is provided between the active region and other regions, and the width of the active region is set to such an extent that the first-order transverse mode is cut off. This is a so-called refractive index waveguide mechanism. In addition, in the LD shown in the above-mentioned prior art, since there is only one region that has this refractive index waveguide mechanism, the guided light is strongly confined within the active layer, which is the active region, and the active layer The optical density within the layer reaches several MW/cm''.

On the other hand, AI, which is generally widely used in optical information processing equipment,
It is known that end face destruction occurs in GaAs LDs when the optical density reaches several MW/cm", and this sets the upper limit for increasing the output. Therefore, in the LD example above, the single L
It is not practical to install it in systems that require high output and low noise characteristics, such as optical disk systems that can be rewritten and erased using D, because the high optical output required for writing and erasing cannot be obtained. This poses a problem.

Furthermore, in an LD whose end face portion has a phased array structure, high optical output characteristics can be obtained, but since a high-order super mode oscillates, the emitted beam becomes bimodal, which causes problems in application.

The present invention is intended to solve these problems, and its purpose is to provide a phased array structure at the end face of the resonator, and a structure having a mode filter function and a gain waveguide mechanism near the center of the resonator. The object of the present invention is to provide an LD having high optical output characteristics, low noise characteristics, and stable single-peak transverse mode oscillation.

[Means for Solving the Problems J] The LD of the present invention is a semiconductor laser having a rib-shaped optical waveguide, and a side surface of the rib-shaped optical waveguide is embedded with a group II-IV compound semiconductor layer. Near one end face of the resonator, the rib-shaped optical waveguide is made thin so that the current injection width is approximately equal to the width of the rib-shaped optical waveguide to form a refractive index waveguide structure, and the rib-shaped optical waveguide is spaced at intervals that allow mutual optical coupling. A plurality of rib-shaped optical waveguides are arranged, and near the center of the resonator, the width of the rib-shaped optical waveguide is made much smaller than the current injection width. It is characterized by having an optical waveguide.

[Example J Figure 1 is a structural diagram in an example of the present invention, and the figure (a
) is a top view of the LD of the present invention, and (b) and (
c) is a cross-sectional structural view taken along A-A' and B-B', respectively. n-type GaAs buffer layer lo2 on n-type GaAs substrate 101/n-type AlGaAs first cladding layer 103
-AIGaAs active layer 104- p-type AI GaAs second cladding layer 105/p-type GaAs contact layer 10
6 double heterojunction structure (hereinafter referred to as DH tank structure)
A structure in which the side surface of a rib-shaped optical waveguide formed by etching the top surface shape 109 of the rib-shaped optical waveguide shown in FIG. It is. 108 is a current narrowing layer made of a high resistance layer.

Explanation will be added below using FIG. 2.

n-type GaAs buffer layer 202 - n-type AI GaAs first cladding layer 203 on n-type GaAs substrate 201
-AIGaAs active layer 204- P-type AlGaAs second
cladding layer 205 and p-type GaAs contact layer 206
are sequentially stacked to form a DH tank structure. DH tank structure, liquid phase epitaxial growth method, metal organic vapor phase epitaxy method (hereinafter MO)
CVD method) and molecular beam epitaxial growth (
It can be produced by a method such as (hereinafter referred to as MBE method).

After forming the DH structure, a dielectric mask 207 such as silicon dioxide is formed. (FIG. 2(a)) Next, through a normal photoetching process, the dielectric mask 207 is patterned into the shape shown by diagonal lines in FIG. 2(C), and then DH
Etching is performed halfway through the p-type GaAs contact layer 206 and the p-type AlGaAs second cladding layer of the substrate having a tank structure. Etching is performed by a wet etching process using an etchant such as sulfuric acid or a dry etching process using a gas such as chlorine. Go.

(FIG. 2(b)) Although FIG. 2(b) only shows the structure of the LD at the resonator end face of the LD of the present invention, FIG.
The shape shown by the diagonal line in c) is the shape of the mask used when forming the ribs, so the ribs are wide near the center of the LD. In addition, the spacing between the ribs shown in FIG. 2(b) is narrow enough to cause interaction of the electric field of light between adjacent ribs during LD oscillation. Group IV compound semiconductor layers are stacked. Although the Zn5e layer is used here, other II-IV group compound semiconductor layers can also be used. For the growth of Zn5e layer, MOC
Although growth can be performed by the VD method or the MBE method, here the MOCVD method using an adduct consisting of dimethylzinc-dimethylselenium as a source material was used. In this case, since the dielectric film used as a mask during rib etching remains above the rib, the crystallinity of the Zn5e layer is poor (and a polycrystalline Zn5e layer grows) above the rib.On the other hand, in other parts, the Zn5e layer has poor crystallinity. Since the lattice mismatch between A I and GaAs is small, a single-crystal Zn5e layer grows.The shaded area in FIG. Here, using an aqueous sodium hydroxide solution, polycrystalline Zn5e layer II on the upper part of the rib is
2 is removed by etching. When etching the Zn5e layer with an aqueous sodium hydroxide solution, the etching rate differs depending on the crystallinity of the Zn5e layer. In other words, a polycrystalline Zn5e layer with poor crystallinity has a faster etch rate than a single-crystalline Zn5e layer, and by appropriately determining the thickness of the Zn5e layer, the upper part of the rib can be exposed, and the thickness of the Zn5e layer on both sides of the rib can be adjusted to the height of the rib. Flattening can be achieved by leaving the surface equal to the surface.

(Fig. 2(e)) The resistivity of the Zn5e layer is lX
It is 10'ΩCm or more and is effective as a current narrowing layer.

In addition, the Zn5e layer is made of AlG with any A1 mixed crystal ratio of any refractive index.
Since it is smaller than the aAs layer, it is also effective for optical confinement, which is important for laser oscillation. After that, formation of the p-side electrode,
The LD process is completed through a wafer backside process and an n-side electrode formation process. An LD chip with a resonator length of about 250 μm is cut out from the wafer thus completed, bonded to a copper heat sink via a silicon submount with a junction down, and its characteristics are measured.

In this LD, a plurality of rib-shaped optical waveguides having a refractive index waveguide mechanism exist on the resonator end face, and the interval between each rib is set narrowly, so that the light oscillated at the rib portion is transmitted between the ribs. Optical coupling occurs and a phased array structure is formed at the end face of the resonator. Therefore, the optical density at the resonator end face can be reduced, and high optical output oscillation can be achieved. However, in a normal phased array structure, oscillation occurs easily in high-order supermodes, and the output beam becomes bimodal, which causes problems in practical applications. The reason why the high-order super mode is selected will be explained using FIG. 3. Here, Figure (a) shows the electric field distribution of the fundamental supermode, Figure (b) shows the electric field distribution of the higher-order supermode, and the arrows indicate the built-in optical waveguide, so-called rib-shaped optical waveguide in the main LD. This is the part where the refractive index is large and the gain is high. As can be seen from the figure, the electric field distribution of the fundamental supermode does not reach zero in the middle part of the arrow, that is, the part with large loss, whereas the electric field distribution of the higher-order supermode crosses the zero level. Therefore, in a normal phased array structure, the loss suffered by the higher-order supermode is smaller than that of the fundamental supermode, the threshold gain of the higher-order supermode is lower, and oscillation in the higher-order supermode is selected.

Therefore, in order to select the fundamental super mode, in the LD of the present invention, as shown by diagonal lines in FIG. Layers were provided. The shape of the current narrowing layer is such that current injection is limited to a width narrower than the total width of the rib-shaped optical waveguide at the end face of the resonator. By providing such a current narrowing layer, the gain distribution in the wide optical waveguide at the center of the resonator can be shaped as shown in Figure 3(c), similar to the electrode stripe LD which is a gain waveguide type LD. becomes. That is, the current injection part becomes a gain region, but the region shown by the shaded area in FIG. 1(a), particularly the region away from the current injection part, becomes a loss region. On the other hand, as can be understood from Fig. 3 (a) and (b), the electric field peak of the fundamental mode supermode becomes smaller in the outer optical waveguide, whereas the higher-order supermode does not depend on the position of the optical waveguide. Almost the same. Therefore, when these two supermodes propagate through the optical waveguide having the above-mentioned gain distribution of the LD of the present invention, the higher-order supermode, which has a large electric field peak in the loss region, reduces the loss more than the fundamental supermode. Therefore, the threshold gain of the higher-order supermode increases, and the basic supermode is selected.

In the above description, a four-wire phased array was explained at the resonator end face, but the same effect can be obtained by using a similar structure as long as there are two or more phased arrays.

Since the fundamental supermode is selected, the emitted beam is unimodal and has small astigmatism. On the other hand, since there is a wide rib-shaped optical waveguide near the center of the resonator, optical waveguide in this region is performed by a mechanism that uses gain waveguide or a combination of gain waveguide and refractive index waveguide. Become. Therefore, reflecting the gain waveguide mechanism, the longitudinal mode becomes multimode. Therefore, the coherent length of the laser beam is shortened, the interference effect due to returned light, etc. is reduced, and noise reduction is realized.

In the LD of the present invention having four phased arrays shown in the example, the width and center-to-center distance of the rib-shaped optical waveguide near the resonance end having a refractive index waveguide mechanism are 3 μm and 5 μm.
m, and the length of the rib-shaped optical waveguide at both output ends is 50 μm.
When j5 and 100 μm, the width of the narrowest part of the current injection region is lOμ, and the resonator length is 250 μm, the threshold current ~
120mA, astigmatism 6μm or less, relative noise intensity ~6×
A stable unimodal beam was obtained up to 10"Hz-' and an optical output exceeding 100 mW.

The LD according to the present invention can be similarly applied to laser materials other than AI GaAs-based materials, such as InGaAsP-based and InGaP-based materials. Furthermore, the structure of the LD is not limited to the one based on the three-layer waveguide shown in the above example, but also the LOG structure in which a light guide layer is provided adjacent to one side of the active layer, and the structure in which a light guide layer is provided adjacent to both sides of the active layer. The present invention can be similarly applied to an SCH structure in which a light guide layer is provided, and a GRIN-SCH structure in which the refractive index and forbidden band width of these light guide layers are distributed in the film thickness direction.

Furthermore, it is also effective for those whose active layer has a quantum well structure. Further, similar results can be obtained even in a structure in which the conductivity types are all reversed in the above embodiments (a structure in which p is replaced with n and n is replaced with p).

[Effects of the Invention] As described above, the LD of the present invention provides the following great effects.

l) Near the light output end face, the rib optical waveguide width and the current injection width are made similar and narrow to create a refractive index waveguide mechanism, and a phased array structure is created by arranging complex waveguides close to each other, so that the current injection width exceeds 100 mW. High optical power oscillation is now possible.

2) By making the optical waveguide wider near the center of the resonator, the optical waveguide in this area becomes a gain waveguide or a mixture of gain waveguide and refractive index waveguide, and the longitudinal mode is generated by the gain waveguide. Reflecting the wave mechanism, it becomes multimode. Therefore, the coherent length is shortened, the interference with the returned light is reduced, and noise reduction is achieved.

3) Since the structure has a mode filter function so that the fundamental supermode is selected near the resonance center by adding λ, a single peaked output beam can be stably obtained.

4) Usually, a high frequency superposition method is used to achieve low noise characteristics, but for this purpose it is necessary to install a high frequency oscillation circuit together with the LD. However, in this LD, the LD itself has low noise characteristics, so additional circuits are unnecessary, and the optical head can be made lighter and access time can be shortened.

5) This LD element has M in the manufacturing process, especially in the growth of each layer.
This can only be achieved using the OCVD method, and takes advantage of the characteristics of the MOCVD method, such as the ability to grow on large-area wafers, uniform film properties over a large area, and excellent controllability of film thickness. Uniformity, high reliability, and lower prices are achieved.

6) The ZnSe layer is a material with high resistance and low refractive index, and is effective in confining carriers and confining light.

In addition, the Zn5e layer has better thermal conductivity than a dielectric layer such as silicon dioxide, and is extremely effective in dissipating heat generated inside the LD.

[Brief explanation of the drawing]

FIGS. 1(a) to 1(c) are structural diagrams showing one embodiment of the LD of the present invention. FIGS. 2(a) to 2(f) are process diagrams of the LD of the present invention. FIGS. 3(a) to 3(c) are diagrams showing electric field distributions of modes. 101.201-n-type GaAs substrate 102. 103, l 04゜105゜106. 107・l 08. 2II ・ 212 ・ 207 ・ 208 ・ 209 ・ 202 ・ 203 ・ 204 ・ 205 ・ ・N-type GaAs buffer layer ・n-type AlGaAs first cladding layer - A I GaAs active layer'p-type AlGaAs second cladding layer ・p type GaAs contact layer, Zn5e buried layer, current narrowing layer, p-side electrode, n-side electrode, dielectric mask, polycrystalline Zn5e layer, single-crystalline Zn5e layer (〆) <C,) Applicant: Seiko Epson Corporation Agent People Patent Attorney Masa Homare Kamiyanagi (and 1 other person) YuCΣ

Claims (1)

[Claims] In a semiconductor laser having a rib-shaped optical waveguide, and in which the side surface of the rib-shaped optical waveguide is embedded with a group II-IV compound semiconductor layer, near at least one resonator end face,
The rib-shaped optical waveguide has a refractive index waveguide structure by making the current injection width and the width of the rib-shaped optical waveguide substantially equal and thin,
A plurality of rib-shaped optical waveguides are arranged at intervals that allow mutual optical coupling, and the width of the rib-shaped optical waveguide is made sufficiently wider than the current injection width near the center of the resonator to form a gain waveguide structure. A semiconductor laser comprising the rib-shaped optical waveguide having a resistance layer.