JPH0268975A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain one having high light output characteristics, low noise characteristics and that oscillates stable basic lateral mode by having a resonator end surface section structured in a phased array, having a mode filter function in the vicinity of the center of the resonator and using a gain waveguide mechanism. CONSTITUTION:In a semiconductor laser having a rib-like light waveguide, the side of which is buried with a II-VI compound semiconductor layer 107, a plurality of rib-like light waveguides in which a current injection width and a rib-like light waveguide width are made almost equal and narrow and a refraction index waveguides structure is made, are placed at an interval that causes light connection alternately at least in the vicinity of one resonator end surface. In the vicinity of the center of the resonator, the width of the rib-like light waveguide is made wider than the width of the rib-like light waveguide having the refraction index waveguide structure and narrower than the space between both sides of the plural rib-like light waveguide having the refraction index waveguide structure present on the resonator end surface and a gain waveguide structure is made. For, example, the side surface of a rib-like light waveguide formed in a shape as shown in the shaded portion is buried with ZnSe 107.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] 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] When applying an LD to an optical information processing device, especially a rewritable/erasable optical disk system, the basic requirements for the LD are stable fundamental transverse mode oscillation and low noise. In addition to point aberration 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 above-mentioned low noise characteristic in 1), a method is generally known in which the LD is caused to oscillate in a longitudinal multi-mode with a short coherent length. A method to achieve this includes longitudinal single mode oscillation L
A high-frequency superimposition method that generates longitudinal multi-mode oscillation by driving D by adding a high-frequency current of several hundred MHz to the drive current, or as in JP-A No. 60-140774, JP-A No. 60-150682, etc. In the vicinity of the end face of the LD resonator, light is guided by refractive index guiding, and in the central part, optical waveguiding is performed by gain guiding or a mixture of refractive index guiding and gain guiding. There have been methods such as realizing longitudinal multi-mode oscillation that reflects the waveguide mechanism of the

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 described above, in order to satisfy the requirements of stable fundamental transverse mode characteristics and low astigmatism characteristics near the light emitting end face of the resonator, the cladding of the LD is An effective refractive index step (△n*tj) is provided between the active region and other regions so that optical confinement can be effectively performed in the direction parallel to the junction of each layer, active layer, etc., and This is a so-called refractive index waveguide mechanism in which the width of the active region is narrowed to such an extent that the first-order transverse mode is cut off. 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, A, which is generally widely used in optical information processing devices,
In l2GaASLD, the optical density is several M W / cm
It is known that end face destruction occurs when the LD reaches the
Therefore, it is not practical to install a single LD in a system that requires high output and low noise characteristics, such as an optical disk system that can be written and erased, because the high optical output required for writing and erasing cannot be obtained. The problem is that it cannot be used for

Further, if the end face portion is simply formed into a phased array structure, high optical output characteristics can be obtained, but since a high-order supermode 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 provide 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 with high optical output characteristics, low noise characteristics, and stable fundamental transverse mode oscillation.

[Means for Solving the Problems 1] The LD of the present invention has a rib-shaped optical waveguide, and the side surface of the rib-shaped optical waveguide is embedded with a II-VI group compound semiconductor. (In the vicinity of one of the resonant end faces, the current injection width and the width of the rib-shaped optical waveguide are made almost equal and thin to form a refractive index waveguide structure.) A plurality of rib-shaped optical waveguides are arranged near the center of the resonator, and the width of the rib-shaped optical waveguide is wider than the width of the rib-shaped optical waveguide having a refractive index waveguide structure (and the width of the rib-shaped optical waveguide is wider than the width of the rib-shaped optical waveguide having a refractive index waveguide structure). The gain waveguide structure is characterized in that the width is narrower than the distance between both sides of the rib-shaped optical waveguide having the refractive index waveguide structure.

〔Example〕

FIG. 1 is a structural diagram of an embodiment of the present invention.
a) is a top view of the LD of the present invention, and FIGS. 1(b) and (
C) is a cross-sectional structural view taken along A-A' and B-B', respectively. n-type GaAs buffer layer 102 and n-type AffGaAs first cladding layer 1 on n-type GaAs substrate 101
03・AffGaAs active layer 104-p-type Ag, GaA
s The double heterojunction structure (hereinafter referred to as H structure) consisting of the second cladding layer 105 and the p-type GaAs contact layer 106 is etched to the middle of the second clad layer in the shape shown by the diagonal line in FIG. 1(a). This is a structure in which the side surfaces of a rib-shaped optical waveguide formed by the above process are buried with a II-VI group compound semiconductor, such as Zn5e. 108 and 1゜9 are the n-side electrodes and
This is the n-side electrode.

Next, an explanation will be added using FIG.

An n-type GaAs buffer layer 202 and an n-type AεGaAs first cladding layer 203 on an n-type GaAs substrate 201.
Af2GaAs active layer 204 ・p-type Af2GaAs second cladding JW 205 ・p-type GaAs contact layer 2
06 are sequentially stacked to form a DH structure. This DH structure can be obtained by liquid phase epitaxial growth method, metal organic vapor phase epitaxy method (
It can be formed by methods such as MOCVD method (hereinafter referred to as MOCVD method) and molecular beam epitaxial growth method (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 the p-type of the substrate having the DH structure is Etching is performed halfway through the GaAs contact layer 206 and the p-type Al2GaAs second cladding layer. Etching can be performed by a wet etching process using an etchant such as sulfuric acid or a dry etching process using a gas such as chlorine. (
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. 2(C)
The shape indicated by the diagonal lines in ) is the shape of a mask when forming the rib, so the rib is wide near the center of the LD.

In addition, the intervals between the ribs shown in FIG. 2(b) are narrow enough to cause interaction of the electric field of light between adjacent ribs during LD oscillation. V
Layering Group I compound semiconductor layers. Although the Zn5e layer is used here, other II-VI group compound semiconductor layers can also be used. For the growth of Zn5e layer, MOCVD method or M
Although it is possible to grow by the BE method,2 here, MO using an adduct consisting of dimethylzinc-dimethylselenium as a source material is used.
CVD method 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 above the rib is poor (polycrystalline Zn5
The e layer grows, while in other parts, Zn5e and A
Since the degree of lattice mismatch of βGaAs is small, single crystal Zn5e
The layers grow. The shaded area in Figure 2(d) is polycrystalline Zn5
e layer 208, and the other parts are single crystal Zn5e layer 2.
It is 09. Here, using an aqueous sodium hydroxide solution,
The polycrystalline Zn5eJil12 on the upper part of the rib 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. That is, polycrystalline Zn5 with poor crystallinity
The etch rate of the e layer is faster than that of the single crystal Zn5e layer, and the Z
By appropriately determining the thickness of the n5e layer, the upper part of the rib is exposed, and the thickness of the Zn5e layer on both sides of the rib can be left equal to the height of the rib, achieving flattening. (Figure 2(e))
In addition, the Zn5e layer has a resistivity of 1X10'Ωcm or more and is effective as a current confinement layer. Furthermore, since the Zn5e layer has a refractive index smaller than that of the AβGaAs layer of any AR mixed crystal ratio, it is effective for optical confinement, which is important for laser oscillation. Thereafter, the LD process is completed through the formation of an n-side electrode, a cross-sectional process on the back surface of the wafer, and an n-side electrode formation process. From the wafer thus completed, the resonator length is 30
In the 0-wire LD, the characteristics of which are measured by cutting out an LD chip of about 0 μm and bonding it to a copper heat sink via a silicon submount with a junction down, a rib-shaped optical waveguide with a refractive index waveguide mechanism is used as a resonator. Since there are multiple ribs on the end face and the spacing between each rib is narrow, the light oscillated at the ribs is optically coupled between the ribs, resulting in a phased array structure being formed on the resonator end face. 6. Therefore, the optical density at the end face of the resonator can be reduced, and high optical output oscillation can be achieved.

However, in a normal phased array LD structure, oscillation is likely to occur in a high-order super mode, and the output beam becomes bimodal, which causes problems in application. 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, the loss suffered by the higher-order supermode is smaller than that of the basic supermode, the threshold gain is lower in the higher-order supermode, and oscillation in the higher-order supermode is selected. Therefore, in the LD of the present invention, the constriction part (Fig. 1 (a)
) was provided. As can be understood from Figure 3, the electric field peak value of the fundamental supermode is smaller in the outer optical waveguide, whereas the electric field peak value of the higher-order supermode is almost the same regardless of the position of the optical waveguide. It is. Therefore, by providing a constriction near the center of the resonator for both of these supermodes, the constriction becomes a gain region (and the area outside the fin becomes a loss region, so the higher-order supermodes have a large peak in the loss region). mode suffers more loss than the fundamental supermode.Therefore, the higher-order supermodes have increased threshold gain than the fundamental supermode, and the fundamental supermode is selected. In the above explanation, four phased arrays were used at the resonator end face, but the same effect can be obtained by using two or more phased arrays with the same structure.

Because the fundamental supermode is selected, the emitted beam is single-peaked and has small astigmatism. On the other hand, since there is a wide rib-shaped optical waveguide near the center of the cavity, this Optical waveguide in the region is performed by a mechanism that uses gain waveguide or a combination of gain waveguide and refractive index waveguide. 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 the four double-sided array shown in the example, the width and center-to-center distance of the rib-shaped optical waveguide near the resonance end having the refractive index waveguide mechanism are set to 3um and 5μ.
m, and the length of the rib-shaped optical waveguide at both output ends is 50 μm.
and 100 μm, the width of the constriction at the center of the resonator is 10 μm, and the resonator length is 250 μm, the threshold current is 90~loomA, the astigmatism is less than 6 μm, and the relative noise intensity is ~6×10”Hz-'. The LD according to the present invention, in which a stable unimodal beam was obtained up to an optical output exceeding 100 mW, can be similarly applied to materials other than AJ2GaAs, such as InGaAsP and InGaP. The structure is not limited to the one based on the three-layer waveguide shown in the above example, but may also include an LOG structure in which a light guide layer is provided adjacent to one side of the active layer, or a structure in which a light guide layer is provided adjacent to both sides of the active layer. It can be similarly applied to the SCH structure in which a guide layer is provided and the GRIN-3CH structure in which the pressure contact ratio and forbidden band width of these optical guide layers are distributed in the film thickness direction. It is also effective for those with a well structure.Also, in the above embodiment, the structure in which all the conductivity types are reversed (p is changed to n
Similar results can be obtained with a structure in which n is replaced with p.

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

1) 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. High optical power oscillation is now possible.

2) By making the rib optical waveguide wider near the center of the resonator, the optical waveguide in this part becomes a gain guide or a mixture of gain guide and refractive index guide, and the longitudinal mode is generated by the gain guide. 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) Since the constriction is provided at the center of the resonator, by narrowing this constriction under the condition that the longitudinal mode becomes a multimode as described in 2), the rib-shaped optical waveguide near the center of the resonator can be Excessive spread of injected carriers in the contact layer/cladding layer in a wide region is suppressed, reducing reactive current and threshold current.

5) By appropriately controlling the width of the fin at the center of the resonator, it is possible to increase the gain difference between the optical waveguide section of the fin and the rest of the fin, thereby reducing the interference between supermodes. The threshold gain difference becomes large, making it easy to select the basic super mode.

6) 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, and no additional circuits are required, which has the effect of reducing the weight of the optical head and shortening the access time.

7) This LD element has M
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.

8) The ZnSe layer is a material with high resistance and low refractive index, and is effective in confining carriers and confining light. In addition, Zn5
The e-layer has better thermal conductivity than a dielectric layer such as silicon dioxide, and is extremely effective in radiating 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) and 3(b) are diagrams showing electric field modes. 101 ・ 201 ・ 102 ・ 202 ・ ・ 103 ・ 203 ・ ・ 104 ・ 204 ・ ・ 105 ・ 205 ・ ・ 106 ・ 107 ・ 108 ・ 109 ・ 207 ・ 208 ・ 209 ・ N-type GaAs substrate n-type Ga As buffer layer n-type AgGaAs First cladding layer AnGaAs active layer p-type AgGaAs second cladding layer p-type GaAs contact layer Zn5e buried layer n-side electrode n-side electrode dielectric mask polycrystalline Zn5e layer single crystal Zn5e layer (C thin Applicant Seiko Epson Co., Ltd. Agent Patent Attorney Masatoshi Kamiyanagi (and 1 other person) (屓) Me 2 diagram (C,) Ushimoku Gyurei

Claims (1)

[Claims] In a semiconductor laser that has a rib-shaped optical waveguide and the side surface of the rib-shaped optical waveguide is embedded with a II-VI group compound semiconductor layer, in the vicinity of at least one resonant end face, the current injection width and the rib-shaped A plurality of rib-shaped optical waveguides are arranged with a refractive index waveguide structure in which the widths of the optical waveguides are approximately equal and thin, and the rib-shaped optical waveguides are arranged at intervals that cause mutual optical coupling. The width of the wave path is wider than the width of the rib-shaped optical waveguide having a refractive index waveguide structure, and a plurality of waveguides are present at the end face of the resonator between both sides of the rib-shaped optical waveguide having a refractive index waveguide structure. A semiconductor laser characterized by having a gain waveguide structure with a width narrower than an interval.