JPS63185A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To inhibit field intensity difference in a resonator and index difference even under the state in which oscillation is grown, to ensure mode selectivity and to obtain an optical output having a stable single wavelength by forming a plurality of and an odd number of phase shifts corresponding to phase difference of 1/2pi to light waves having a Bragg wavelength to the corrugations. CONSTITUTION:With corrugations 4, photo-resists such as a positive resist and a negative resist are applied alternately on the upper surface of an N-type InP clad layer 3 in a striped manner prior to the epitaxial growth of an N-type InGaAsP guide layer 5, and exposed through a two-beam interference method by helium-cadmium (He-Cd) laser beams and a mask is shaped, and phase shifts of 1/2LAMBDA are each formed at the center (1/2L) and positions (S, L-S) at a distance S from respective end surface at a period LAMBDA 200nm corresponding to the order m=1. Where cavity length L=300nm, S=100nm and a coupling coefficient kappa=60cm<-1> are determined, and kappaL=1.8 is decided. Accordingly, even when kappaL of a distributed feedback type laser is large, the mode selectivity of the laser is also ensured after oscillation growth, thus stably acquiring single wavelength beams having a large output.

Description

[Detailed Description of the Invention] [Summary] The present invention provides, in a distributed feedback semiconductor laser, a plurality of odd number phase shifts corresponding to a phase difference of 2π for a black wavelength light wave in its corrugations. Even in a state where oscillation has grown, field strength differences and refractive index differences within the resonator are suppressed to ensure mode selectivity and to obtain stable optical output with a single wavelength.

[Industrial application field]

The present invention relates to a semiconductor laser, and more particularly to the structure of a distributed feedback semiconductor laser that obtains a single wavelength optical output.

° In order to promote the sophistication and diversification of optical communication systems that use light as an information medium, expectations are placed on distributed feedback (DFB) lasers that are suitable for single-wavelength oscillation, but conventional If high output is sought in a DFB laser, the stability of its oscillation mode tends to deteriorate, and improvements are desired.

[Conventional technology]

As a semiconductor laser suitable for controlling the oscillation wavelength, that is, the longitudinal mode, a DFB laser that selectively performs feedback using a diffraction grating provided in an optical waveguide region is the most promising.

This diffraction grating has a maximum reflectance of the light wave at the black wavelength λ, but upon reflection by the diffraction grating, the phase of the light wave at the black wavelength λ changes by Aπ with one reflection.
During one round trip, the phase change is π, that is, reversed, so oscillation does not occur at this black wavelength, and during propagation from one end face of the resonator to the other end face, a phase of approximately Positive feedback occurs at the wavelength that causes the shift, and laser oscillation occurs. Among these oscillation conditions, the dark value current of the two wavelengths n = 0 is the smallest, and the longitudinal mode of a normal DFB laser is in these two wavelengths.
It becomes an unstable mode that includes a wavelength λ2 or a wavelength λ2, or that tends to fluctuate from one to the other.

In order to unify the longitudinal mode of a DFB laser having the above-mentioned properties, a structure as shown in FIG. 3 is known. In FIG. By forming a first-order diffraction grating 24 at the interface between the guide layer 25 and the cladding layer 23, and shifting the phase of the corrugation run of the diffraction grating 24 by two to its period on the left and right sides as seen from the center, the required amount of energy can be obtained. gives a phase difference,
It is growing.

To actually manufacture this structure, for example, a positive resist is applied to one side and a negative resist is applied to the other side, using the position where the phase is shifted by %Δ as an interface, and this is exposed using the two-beam interference method to form a resist mask. , or a phase mask method is applied.

[Problem that the invention seeks to solve]

As mentioned above, single wavelength oscillation is achieved by providing a phase shift corresponding to a phase difference of 1/2π with respect to the black wavelength light wave in the corrugation, for example, in the case of a first-order diffraction grating, each phase shift to that period is provided. However, even in a DPB laser with this structure, the coupling coefficient of the diffraction grating is relatively large (the product of the resonator length (light waveguide region length) L is 1.5).
In low threshold lasers of about 260 MHz or more, multimodes may develop as oscillation grows, and countermeasures are required.

This phenomenon occurs as the oscillation grows and the optical output increases, as shown in Figure 4 (al) shows the field intensity (light intensity) distribution, and Figure 4 (b) shows the refractive index distribution, both in the cavity length direction. The field strength becomes particularly strong near the corrugation phase shift position in the optical waveguide region, and stimulated emission increases at this position, reducing the carrier density and causing a slight difference in the refractive index distribution within the resonator, resulting in mode selectivity. That is, the difference in minimum threshold values between modes is reduced.

[Means for solving problems]

The above problem is solved by the semiconductor laser according to the present invention, in which a corrugation that selectively returns light waves is provided with a plurality of odd number phase shifts corresponding to a phase difference of 2π with respect to light waves of black wavelength.

[For production]

According to the present invention, by arranging an odd number of the above-mentioned 2π phase shifts, the total value Δψ of the phase adjustment amount seen from the light wave is set as Δψ=±−±nπ (n is an integer), and a single longitudinal mode of the black wavelength λB is set. As shown in Figure 2 (al, (bl), the field strength difference and refractive index difference in the resonator length direction are shown by solid lines. As shown in the example, mode selectivity is ensured by being significantly suppressed compared to the conventional example shown by the broken line.

Note that if these multiple Aπ phase shifts are arranged symmetrically with respect to the center of the optical waveguide direction, the field strength and refractive index are averaged,
It is highly effective in suppressing fluctuations.

〔Example〕

The present invention will be specifically explained below using examples.

FIG. 1 is a schematic side sectional view of an embodiment of the present invention, where 1 is n
A skewer-shaped InP substrate, 3 is an n-type TnP cladding layer, 4 is a corrugation, and 5 is, for example, a luminescence peak wavelength λg=
1.0-1 n-type 1nGaAsP guide layer with a thickness of about 0.2-1, 6 is, for example, the luminescence peak wavelength λg = 1.3
-1 Non-doped InGaAsP with a thickness of about 0.15-
Active layer 7 is p-type! nP cladding layer, 8 is p+ type InG
In the aAsP contact layer, 9 is an n-side electrode, and 10 is a p-side electrode.

The corrugation 4 of this embodiment is formed by forming an n-type [n-type 1nP prior to the epitaxial growth of the nGaAsP guide layer 5].
For example, a positive resist and a negative resist are applied alternately in stripes on the upper surface of the cladding N3, and helium-cadmium (H
e-Cd) A mask is formed by exposure using a two-beam interferometry method using a laser beam, and a period A=20 corresponding to order m-1 is formed.
At 0 nm, the position of distance S from the center (%L) and each end face (
SSL-S) is provided with a phase shift to 〃, respectively. However, for example, the resonator length L −3O0 nm, S5-1
O0n, coupling coefficient = 60 cm-', and L = 1.8
It becomes.

In this example, the field distribution and refractive index distribution after the oscillation grows are improved as shown in FIG. 2, and the single longitudinal mode of black wavelength λ8 is kept stable even at high output.

Note that the corrugation 4 in the above embodiment has a period A# of 200 nm corresponding to the order m=1, and is provided with a phase shift of %A, but when using a corrugation with a period Δ"1100n corresponding to the order m=2, , χΔ” phase shift is provided. Corrugations with such a phase shift can be formed by drawing a pattern on a resist using, for example, an electron beam, an ion beam, or the like.

〔Effect of the invention〕

As explained above, according to the present invention, the L
Even when the oscillation is large, the mode selectivity is maintained even after oscillation growth, making it possible to stably obtain high-output single-wavelength light. , will greatly contribute to advances in capacity expansion, etc.

[Brief explanation of drawings]

Fig. 1 is a diagram showing an embodiment of the present invention, Fig. 2 is a diagram showing an example of the distribution of field strength and refractive index in the cavity length direction according to the present invention, and Fig. 3 is a schematic diagram of a conventional example of corrugation phase shift. FIG. 4 is a diagram showing an example of the conventional distribution of field strength and refractive index in the cavity length direction. In the figure, 1 is a male-type 1nP substrate, 3 is an n-type InP cladding layer, 4 is a corrugation, 5 is an n-type InGaAsP guide layer, 6 is an InGaAsP active layer, 7 is a p-type 1nP cladding layer, 8 is a p+ type InGaAsP contact 9 is an n-side electrode, and 10 is a p-side electrode. Grass 2 Turn leather 2 Figure (b) $4@

Claims (1)

[Claims] 1) A corrugation that selectively returns light waves is provided with a plurality of odd number phase shifts corresponding to a phase difference of 1/2π with respect to light waves of black wavelength. semiconductor laser. 2) Claim 1, wherein the phase shift is arranged symmetrically with respect to the center of the optical waveguide direction.
Semiconductor laser described in section.