JPS60160680A - Surface light-emission type dfb laser - Google Patents

Abstract

PURPOSE:To stabilize an oscillation wavelength, and to reduce threshold current density by adopting DFB structure for a surface light-emission type laser. CONSTITUTION:When two substances having na, nb refractive indices are inserted alternately between two substances having n1, n2 refractive indices and the thickness da, db of the na, nb layers is represented severally by da=lambda/4na and db=lambda/4nb, reflectivity to vertical incident beams is represented by R1b= [{n1.(nb/na)<m>-n2}/{n1.(nb/ na)<m>+n2}]<2> (where m is the number of na and nb layers and lambda is a wavelength in a vacuum). Consequently, reflectivity R1b can be set properly by fitly selecting na, nb and m in the structure. That is, an oscillation wavelength is determined stably by the length of one period of a section having the periodic structure of a refractive index in a surface light-emission DFB laser having this structure as a reflection section, and the reflectivity of a resonator having an effect on threshold current density is improved largely, thus enabling oscillation at a low threshold.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to the improvement of a so-called surface-emitting type laser in which output light is emitted perpendicularly to the surface of a substrate, and in particular, by incorporating a DFB structure, its characteristics, The main objective is to improve the threshold current value density and the stability of one oscillation wavelength.

[Background of the invention]

A surface-emitting laser is a laser that emits light perpendicular to the substrate surface, and is expected to have the following applications1. .

■ Single-mode short-cavity laser ■ Large radiation area, Sayama beam angle laser ■ Two-dimensional laser array ■ Monolithic optical integrated circuit Conventional surface-emitting laser fabric bellows optical resonator The reflective surface of ゛ has an epitaxial crystal surface and a substrate surface. Alternatively, both epitaxial crystal surfaces were used. With laser,
The magnitude of the reflectance on both sides of the resonator of the optical intensifier is directly related to the magnitude of the threshold current value density. In edge-emitting lasers, reflectance is increased by using crystal wall surfaces as reflective surfaces.

As described above, in a surface emitting laser, the interface between the epitaxial layer and the air, the interface between the substrate and the epitaxial layer, and the interface between the substrate and the air are used as reflective surfaces, and a sufficient reflectance cannot be obtained compared to an edge emitting laser. As a result, oscillation at room temperature is particularly
, the threshold current value density becomes a large value.

Further, the oscillation wavelength is determined by the bandgap of the material forming the active layer, and the oscillation wavelength becomes unstable at high modulation.

[Purpose of the invention]

An object of the present invention is to stabilize the oscillation wavelength and further reduce the threshold current density by incorporating a DFB structure into a surface emitting laser.

[Summary of the invention]

In edge-emitting lasers, a DFB structure is employed as one means of stabilizing the oscillation wavelength. Specifically, wave-shaped unevenness is periodically formed using materials having different refractive indexes along the optical waveguide in or near the active layer to reflect light.

The present invention aims to solve the above-mentioned drawbacks of the surface emitting laser regarding the oscillation wavelength and the reflectance in the resonator by incorporating a DFB structure.

The specific structure is significantly different from the above-mentioned edge-emitting WDFB laser, in which a rectangular conductor thin film with a changed refractive index is periodically formed in the reflective section perpendicular to the direction of the leading wave. The length of one period is λ/ 2 n, where λ is the wavelength in vacuum and n is the effective refractive index of the semiconductor fli film. The method of changing the refractive index is preferably sinusoidal, but the method described below may also be used.

A set of semiconductor thin films having different refractive indexes (refractive index na Hnb ) each having a thickness of λ/4n1λ/4nh is formed periodically. 1st
Figure a shows two substances with refractive index nl and nl, and figure 1b shows
It is a schematic diagram when two substances with refractive indexes na and Qh are alternately inserted between them. The thicknesses d, dh of the natnb layer are d,=λ/4n, respectively.

d4=λ/4n1. The reflectance for vertically incident light is FL+-=((nt nt)/(nt+nt))”R in both figures, respectively.
1b=((n I 11 (nb/n-) '-n*)
/ (nt @(n-/n-)"10 nt))"": number of na layers and nb layers In the structure shown in Figure 1b, reflectance Rib can be set appropriately by appropriately selecting Hllh and m. In other words, in a surface-emitting DFB laser having this structure in the reflection part, the oscillation wavelength is 1 of the part with the period 14tt structure of the refractive index.
The reflectance of the resonator, which is stably determined by the period length and also affects the threshold current density, is
In FIG. 1a, this is greatly improved compared to the case where na is air), making it possible to oscillate at a low threshold.

[Embodiments of the invention]

Hereinafter, one embodiment of the F1 invention will be described with reference to FIG. Second
The figure is a cross-sectional view of the surface emitting laser of the present invention. 11 is n
-GaAs substrate (n = lXl0"cm-" film thickness a
= 2oo μm), 12.13 is a light reflecting layer, each made of GaAs (n = lXl0"cm-" d = 5
80 people), Gao, + λ/4-11As (n=lX1
0"', d = 660 people) were formed in three layers each.

14.16 is Gao・4A41. a As cladding layer (
Layer 14 has n = IXl 0"cm-", d = 1
μrn, layer 16 p = I x 1018 cm-”, d
= iμm). 15 is Gao, oA4. +Al! Active layer (n = 5X10"cm-', d = 0.1
tt m), and 17 is a Gao-tA4sAs antireflection film (p=IXIO”crn-3, d=660 people)
It is. 18 is GaAs (n= 5X10”crrr
", d = 1.5 μm) light absorption layer. 19° and 20 are n and I) electrodes, and 21 is 5iO for current confinement.
z It is an oxide film.

When a forward bias is applied between the electrodes 19 and 20, recombination light emission occurs in the active layer 15 sandwiched between the depleted layers 14 and 16. Oscillation in the horizontal direction of the active layer 15 does not occur due to gain loss due to the light absorption layer 18, but oscillation occurs in the vertical direction between the light reflection layer 13 and the antireflection film 17.

The manufacturing process of this device is as follows.

■ Crystal growth on GaAs substrate by MOCvD method,
Using trimethylgallium, trimethylaluminum, arsine, dimethylzinc, and hydrogen selenide as raw materials, growth temperature 6
I did it at 50c.

■ Mesa etch (Fig. 2 18), ■ GaAs buried growth by liquid phase epitaxy, growth temperature SOOC, ■ 5iOz oxide film formation, electrode formation. The oscillation wavelength was 660 nm.

〔Effect of the invention〕

According to the present invention, the oscillation wavelength of a surface emitting laser can be stabilized and the threshold current density can be reduced.

[Brief explanation of drawings]

Figure 1A is a schematic diagram showing the reflection characteristics of two substances with different refractive indexes G, Figure 1B is a diagram showing the reflectance characteristics when a multilayer film is further introduced between Figure 1A, and Figure 2 is a diagram showing the reflectance characteristics of the present invention. 1 is a cross-sectional view showing the structure of a GaAlks surface-emitting DFB laser according to an embodiment of the present invention. 11-n-GaAs substrate, 12-n-GaAa reflective film, 13-n-Gao-+Ato, oA8 reflective film, 14-n-Gao-aA4. aAs cladding layer, 15
-n-. Gao-os A41. oaAs active layer, 16-p
GaO, 4A4.6AS cladding layer, 17...pG
a6. tA4+, o As antireflection film, 18...G
a As light absorption layer, 19...n-electrode, 20...
p-electrode, 21...8i0. Oxide film.

Claims (1)

[Claims] 1. In a surface emitting semiconductor laser, the film thickness λ/2n (λ: wavelength in vacuum, n: refractive index of the film) with respect to the refractive index.
1. A surface-emitting DFB laser characterized by having a structure in which a semiconductor thin film having a period of 2 is formed on a surface opposite to a laser light extraction surface to reflect light. 2. As the periodic structure of the surface-emitting DFB laser according to claim 1, one semiconductor O# film having different refractive index is used.
A surface-emitting DFB laser characterized by having a structure in which a pair of pairs are stacked on top of each other.