JPH10209555A - Surface emission type semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface emission type semiconductor laser which makes it possible to fetch a circular focused beam and to efficiently couple it to fiber, and utilizes a diffractive grating which makes it possible to judge a defective or non-defective judgement without cleaving. SOLUTION: A DFB type semiconductor laser is provided by forming an n-type lower clad layer 2, a waveguide laser, containing an i-type active layer 3, and a p-type upper clad layer 4 successively on an n-type semiconductor substrate 1, and a diffracive grating 5 is formed along the waveguide layer for distributed feedback of light. The diffractive grating 5 constitutes a laser resonator in 360 deg. direction on the plane in parallel with substrate surface, as the secondary diffractive grating describing a concentric circular pattern, centering on the center of the concentric circular pattern. The obtained laser beam is fetched in the direction vertical to substrate surface by diffractive effect, and the circular output light beam, which is fetched by forming a grating lens 10 of a concentric circular pattern on the substrate surface of the region where the diffractive grating is formed, is used as a focussed light beam 11.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a surface-emitting type semiconductor laser using a diffraction grating.

[0002]

2. Description of the Related Art Conventionally, DFB (Distributed Feed-Back) lasers and D
A BR (Distributed Bragg Reflector) laser is known. DFB lasers provide distributed feedback of light in the active region where current is injected, whereas DBR lasers are different in that distributed reflectors are formed on both sides (or one side) of the active region remote from the active region. Basically, they are common in utilizing the distributed feedback of light by the diffraction grating. Therefore, hereinafter, DF
The B laser is used as a general term for a diffraction grating type semiconductor laser including a DBR laser. The DFB laser is an edge-emitting type that emits output light in a direction parallel to the substrate surface, like many other semiconductor lasers. In this edge emission type, it is difficult to make the vertical divergence angle and the horizontal divergence angle of the output light the same, and thus the coupling efficiency of the output light to a fiber or the like is not good. Also, the assembly is difficult, and it is particularly unsuitable for constructing a high-capacity fiber communication or other high-performance optical integrated circuit system for performing three-dimensional optical coupling.

On the other hand, in recent years, various surface emitting semiconductor lasers for extracting laser output light in a direction perpendicular to the substrate surface have been proposed. Surface emitting semiconductor lasers are broadly classified into two types, a vertical type and a horizontal type. The vertical type constitutes a Fabry-Perot resonator in a direction perpendicular to the substrate surface, and continuous oscillation at room temperature has also been reported. In the horizontal type, a resonator is formed horizontally on the substrate surface in the same way as a normal laser, and light is extracted in the vertical direction using a diffraction grating, etc.
Typically, there is a DFB laser using a secondary diffraction grating.

[0004]

The vertical type surface emitting semiconductor laser has a problem that the manufacturing process is complicated, and unlike the horizontal type, the reflecting mirror is not a cleavage plane, so that it is difficult to obtain a mirror having a high reflectivity. There is. On the other hand, a diffraction grating type surface emitting semiconductor laser is advantageous in that a conventional DFB laser manufacturing technique can be used almost as it is.
However, in the structure of a normal DFB laser, the light emitting surface is several tens.
There is a problem that the length is as small as 0 μm, so that it is difficult to efficiently couple the fiber with the fiber. In a normal DFB laser, for example, one of the two cleavage planes is used as a 100% reflection mirror, and the other is used as an emission end coated with an antireflection film.
There is also a problem that the quality cannot be determined unless the cleavage is performed finally.

The present invention has been made in view of the above circumstances, and makes it possible to extract a circular focused light beam and efficiently couple it to a fiber, and to judge pass / fail without performing cleavage. It is an object of the present invention to provide a surface-emitting type semiconductor laser using such a diffraction grating.

[0006]

According to the present invention, a lower cladding layer, a waveguide layer including an active layer, and an upper cladding layer are sequentially formed on a semiconductor substrate, and a distributed feedback of light is performed along the waveguide layer. A semiconductor laser on which a diffraction grating is formed, wherein the diffraction grating is a secondary diffraction grating that draws a concentric pattern, and has a laser resonator in a direction parallel to the substrate surface in a direction parallel to the substrate surface in a direction parallel to the substrate surface. The obtained laser light is taken out in the direction perpendicular to the substrate surface by the diffraction effect, and the circular output light beam taken out on the substrate surface in the region where the diffraction grating is formed is a focused light beam. For forming a grating lens having a concentric pattern.

According to the present invention, a laser resonator is formed in a 360 ° direction using a secondary diffraction grating that draws a concentric pattern. Focusing on an arbitrary cross section, it is similar to the conventionally proposed surface emitting DFB laser, in which the light excited by the active layer is distributed and fed back by the diffraction grating and oscillates, and at the same time, partly diffracts. And taken out in the direction perpendicular to the substrate surface. Further, a part of the light diffracted in the vertical direction is further Fresnel-reflected on the substrate surface, and is coupled to the waveguide layer again by the diffraction grating. The above operation is performed in all directions of 360 ° centering on the center of the diffraction grating, and a circular output light beam having a light intensity peak at the center of the diffraction grating pattern without using the reflection by the cleavage plane. This is a circular focused light beam that is focused at the lens focal position by the grating lens formed on the substrate surface. The output is in single longitudinal mode by the DFB.

Therefore, according to the present invention, a circular output light beam is obtained, which is converged by being condensed by a grating lens, so that a high coupling efficiency is obtained when coupling to a fiber or the like. Further, since the reflection of the cleavage plane is not used for laser resonance, the quality of the chip can be determined regardless of the cleavage. Further, it is easy to manufacture a planar laser array, which facilitates construction of a three-dimensional optical integrated circuit.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. 1A and 1B are a sectional view and a plan view of a surface emitting DFB laser according to an embodiment of the present invention. On an n-type semiconductor substrate 1 (preferably on which a buffer layer is formed), an n-type lower cladding layer 2, i
A type active layer 3 and a p-type upper cladding layer 4 are sequentially formed. In the case of this embodiment, the active layer 3 has an MQW structure made of a material having a higher refractive index than the lower clad layer 2 and the upper clad layer 4, and this becomes the waveguide layer almost as it is.

In this embodiment, the active layer 3 is formed over the entire area of the substrate, and a second-order diffraction grating 5 is formed near the interface with the upper cladding layer 4 in a concentric pattern. That is, the active layer 3 is located below the entire diffraction grating 5 that draws a concentric pattern. The concentric diffraction grating 5 is produced, for example, as follows. A thin waveguide layer 8 is formed on the wafer on which the active layer 3 is formed, a resist is applied on the waveguide layer 8, and electron beam exposure is performed. In electron beam exposure, an electron beam is scanned while adjusting the exposure amount distribution. Then, the resist is developed to form a resist pattern having a repetitive triangular waveform in a certain cross section, and the waveguide layer 8 is etched using the resist pattern.

The upper clad layer 4 is laminated on the diffraction grating 5 thus formed, and the ohmic contact layer 6 is further formed thereon. A grating lens having a concentric pattern is formed thereon so as to overlap the region of the diffraction grating 5. 10 are formed. The grating lens 10 is formed of, for example, a glass layer in a manner basically similar to that of the diffraction grating 5. At this time, by controlling the exposure scanning time of the electron beam, a so-called chirped grating in which the pitch becomes smaller toward the outside is obtained. Then, an anode electrode 7 is formed outside the grating lens 10, and a cathode electrode 9 is formed on the entire back surface of the substrate 1.

Specifically, in the case of an InP-based DFB laser, InP is used for the substrate 1, the lower cladding layer 2 and the upper cladding layer 4, and the active layer 3 has a slightly different composition ratio x, y. making MQW structure _{in x Ga 1-x As y} P 1-y layer which has a plurality layers alternately stacked. The waveguide layer 8 is made of InG
aAsP layer.

In the DFB laser according to this embodiment, any cross section passing through the center of the diffraction grating 5 has a cross-sectional structure shown in FIG. 1A, and a DFB resonator is formed in a 360 ° direction centered on the center of the diffraction grating 5. It was done. Then, a part of the light obtained by the laser resonance is extracted by the secondary diffraction grating 5 in a direction perpendicular to the substrate surface. Part of the light diffracted upward by the diffraction grating 5 causes Fresnel reflection on the substrate surface and is returned to the active layer 3, whereby a standing wave at which the excitation light has a peak at the center of the concentric pattern diffraction grating 5. Stands. And grating lens 1 with concentric pattern
With 0, a circular focused light beam 11 is obtained as shown in FIG.

According to this embodiment, the laser oscillation of a single longitudinal mode can be obtained by the DFB resonator, and the convergent pattern diffraction grating 5 and the grating lens 10 form a circular convergent light beam 11, which is applied to the fiber. It becomes possible to combine efficiently. Also, since the reflection of the cleavage plane is not used for laser resonance, without cleavage,
The quality of the chip can be determined at the wafer stage. Further, it is easy to manufacture a planar laser array, and a three-dimensional high-performance optical integrated circuit can be constructed.

FIG. 2 shows an embodiment in which the embodiment of FIG. 1 is slightly modified. In the embodiment shown in FIG. 1, the diffraction grating 5 is formed on almost the entire surface of the substrate so as to avoid the region immediately below the anode electrode 7 where the current is concentrated. On the other hand, FIG. The diffraction grating 5 having a size reaching the end face of the substrate is formed in a wrapped manner. According to this embodiment, the same effect as that of the previous embodiment can be obtained.

FIGS. 3 (a) and 3 (b) show still another embodiment, and are sectional views of a so-called distributed reflector structure (DBR structure) in which an active region into which current is injected and a region of the diffraction grating 5 are separated. And a plan view. In the case of this embodiment, a portion of the upper cladding layer 4 of the wafer formed up to the upper cladding layer 4 is selectively etched in the same manner as in the previous embodiment to form the waveguide layer 8.
Is exposed, and a diffraction grating 5 having a concentric pattern is formed here as in the previous embodiment. Thereafter, a p-type upper cladding layer 4 is further formed, and a grating lens 10 having a concentric pattern is formed at a position overlapping the region of the diffraction grating 5 of the upper cladding layer 4. An anode electrode 7 is formed in a region adjacent to the grating lens 10 via an ohmic electrode 6.

Also in this embodiment, the light excited in the active region immediately below the anode electrode 7 and guided to the region of the diffraction grating 5 is distributed and fed back by the diffraction grating 5 in the 360 ° direction, and the output by laser oscillation is obtained. Light is extracted above the diffraction grating 5. A part of the light amplified in the region of the diffraction grating 5 is distributed and reflected similarly to a normal DBR laser, and is returned to the active region immediately below the anode electrode 7. This allows
A circular focused light beam 11 similar to that of the previous embodiment can be obtained.

[0018]

As described above, according to the present invention, a circular output light beam is taken out in the direction perpendicular to the substrate surface by using a secondary diffraction grating having a concentric pattern, and this light is focused by a grating lens. As a beam, a surface-emitting type semiconductor laser that can be easily coupled to a fiber can be obtained.

[Brief description of the drawings]

FIG. 1 shows a configuration of a DFB laser according to one embodiment of the present invention.

FIG. 2 shows a configuration of a DFB laser according to another embodiment.

FIG. 3 shows a configuration of a DFB laser according to another embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Semiconductor substrate, 2 ... Lower cladding layer, 3 ... Active layer, 4
... upper clad layer, 5 ... diffraction grating, 6 ... ohmic contact layer, 7 ... anode electrode, 8 ... waveguide layer, 9 ... cathode electrode, 10 ... grating lens, 11 ... circular focused light beam.

Claims (1)

[Claims] A semiconductor laser in which a lower cladding layer, a waveguide layer including an active layer and an upper cladding layer are sequentially formed on a semiconductor substrate, and a diffraction grating for distributed feedback of light is formed along the waveguide layer. Wherein the diffraction grating is a secondary diffraction grating that draws a concentric pattern, and forms a laser resonator in a 360 ° direction in a plane parallel to the substrate surface with the center of the concentric pattern as a center.
The obtained laser light is extracted in a direction perpendicular to the substrate surface by the diffraction effect, and a concentric pattern for forming a circular output light beam extracted from the substrate surface in a region where the diffraction grating is formed as a focused light beam. A surface-emitting type semiconductor laser characterized by forming a grating lens as described above.