JPS63186A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To ensure mode selectivity, and to obtain an optical output having a stable single wavelength by disposing a plurality of phase adjustment means for light waves in an optical guide region and specifying the total value of the quantity of phase adjustment by the phase adjustment means. CONSTITUTION:Two widened sections in length (l) and width W2 are each arranged at positions at a distance S from respective end surface regarding the center in the direction of the waveguide of an optical guide region in the optical guide region in overall length L and width W consisting of a guide layer 5 and an active layer 6, L=400mum, l=30mum, W=1.8mum, W2=2.5mum, S=0.3L, kappa=60cm<-1>, and kappaL=2.4 are determined regarding these each size and a coupling coefficient kappa, and the total value DELTApsi of the quantity of phase adjustment is represented by DELTApsi=2lDELTAbeta=pi/2+ or -npi(n represents an integer) by the difference DELTAbeta=beta2-beta1 of the propagation constant beta2 of the widened sections and the propagation constant beta1 of other sections. Accordingly, even when the kappaL of a distributed feedback type laser is large, the mode selectivity of the laser is ensured even after oscillation growth, thus stably acquiring single wavelength beams having a large output.

Description

[Detailed Description of the Invention] [Summary] The present invention provides a distributed feedback semiconductor laser in which a plurality of light wave phase adjustment means are disposed in the optical waveguide region thereof, and the total value of the phase adjustment amount by the phase adjustment means is adjusted. By setting Δψ to Δ, ψ = ±−±nπ (n is an integer), even when oscillation grows, field strength differences and refractive index differences within the resonator are suppressed to ensure mode selectivity and stability. This provides a single wavelength optical output.

[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 a medium for information signals, expectations are placed on distributed feedback (DFB) lasers that are suitable for single-wavelength oscillation, but conventional DFB lasers If high output is sought, the stability of the 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 Bragg wavelength λ, but upon reflection by the diffraction grating, the phase of the light wave at the Bragg wavelength λ changes by 2π with one reflection.
In one round trip, the phase change is π, that is, reversed, so oscillation does not occur at this Bragg wavelength, and during propagation from one end face of the resonator to the other end face, a phase change of approximately ± (2 ± n) π is generated with respect to the diffraction grating. Positive feedback relay laser oscillation is performed at the wavelength that causes the shift. 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 the wavelengths λ9 and λ2 or easily fluctuates from one to the other.

A structure as shown in FIG. 3 is known for making the longitudinal mode of a DFB laser having the above-mentioned properties single. In the figure, 26 is an active layer, 25 is a guide layer, 23 and 27 are cladding layers, and a first-order diffraction grating 24 is formed at the interface between the guide layer 25 and the cladding layer 23. , the required phase difference is provided by shifting the phase of the corrugations of the diffraction grating 24 to that period.

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 jumps by 28 as an interface, and this is exposed using a two-beam interference method to form a resist mask. Accurately realizing this structure is not easy.

In order to easily realize single wavelength oscillation, the applicant of this patent previously provided the following DFB laser in Japanese Patent Application No. 59-210588. That is, according to the invention, portions having different propagation constants are selectively formed in the optical waveguide region having corrugations, and the difference in propagation constant between the portions having different propagation constants and other portions is Δβ, and By setting the length of the portion where the propagation constant is different as L2 and Δβl, z=±−±nπ (n is an integer), single-longitudinal mode oscillation at the Bragg wavelength can be obtained.

FIG. 4 is a schematic perspective view of one embodiment, in which an n-type)nGaAsP guide layer and an InGaAsP guide layer in the semiconductor substrate 31 are shown.
Of the optical waveguide region 36 which is made of an AsP active layer and has a width W, a portion 36A having a length L2 is set as a width W2 to provide a propagation constant difference Δβ.

[Problem that the invention seeks to solve]

Although single wavelength oscillation can be easily realized by the above invention, even in a DFB laser with this structure, the coupling coefficient of the diffraction grating is relatively large, and the product of L and the length of the resonator (optical waveguide region) is 1.
In low-threshold lasers of about 5 to 2.0 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 the field intensity (light intensity) distribution in Fig. 5(a) and the refractive index distribution in Fig. The field strength becomes particularly strong at the phase adjustment part such as the widening of the optical waveguide region, and stimulated emission increases at this position, which lowers the carrier density. This causes a slight difference in the refractive index distribution within the resonator, which improves mode selectivity. That is, the difference in minimum threshold values between modes is reduced.

[Means for solving problems]

The problem is that a plurality of light wave phase adjustment means are disposed in an optical waveguide region having corrugations, and the total value Δψ of the amount of phase adjustment by the phase adjustment means is as follows: Δψ=±π/2±nπ ( n is an integer).

[For production]

According to the present invention, for example, by arranging a plurality of phase adjusting means as described above in a distributed manner, the different intensity distributions and refractive index distributions in the resonator length direction shown in FIGS. As illustrated by the solid line, the difference in intensity and the difference in refractive index are significantly suppressed compared to the conventional example shown by the broken line, thereby ensuring mode selectivity.

Note that if the plurality of phase adjustment means are arranged symmetrically with respect to the center of the optical waveguide direction, the effect of averaging the field strength and refractive index and suppressing fluctuations is great.

In addition, the total value Δψ of the amount of phase adjustment by multiple phase adjustment means
It is naturally necessary to set Δψ=±π/2±nπ (n is an integer) in order to obtain a single longitudinal mode. [Example] When forming a plurality of parts with different propagation constants, the difference in propagation constant from other parts and the length of the parts with different propagation constants are respectively Δβ and Li, and the total value of the phase adjustment amount Δψ is calculated. The present invention will be specifically explained below using examples.

FIG. 1 shows an embodiment of the present invention, FIG. 1(a) is a schematic side sectional view thereof, FIG. 1(b) is a schematic perspective view of the optical waveguide region,
Figure (C) is a diagram showing the field distribution.

In the same figure (al, 1 is an A-type 1nP substrate, 3 is an n-type I
nP cladding layer, 4 is corrugation, and 5 is, for example, the luminescence peak wavelength λg = 1.2. Irrn, an n-type 1nGaAsP guide layer with a thickness of about 0.2-1, 6 is, for example, a luminescence peak wavelength λg=1.3-1 and a thickness of 0.1
A non-doped InGaAsP active layer with a thickness of approximately 5 μm, 7 a p-type 1nP cladding layer, 8 a p+ pear-shaped 1nGaAsP contact layer, 9 an n-side electrode, and 10 a p-side electrode.

In this embodiment, the total length consists of a guide layer 5 and an active layer 6,
In an optical waveguide region having a width W, two widened portions each having a length l and a width W2 are arranged at a distance S from each end face symmetrical with respect to the center of the waveguide direction, and each of these dimensions and coupling is For example, L = 400tm, l
=30um.

W-1,8-1W,=2.5Ilrn.

S=0.3L.

= 60 cm-', K L = 2.4,
Based on the difference Δβ=β2−β between the propagation constant β2 of the widened portion and the propagation constant β of the other portions, the total value Δψ of the phase adjustment amount is set to π Δψ=21Δβ=−±nπ.

In this embodiment, the field distribution after the oscillation has grown is improved as shown in FIG. 1 (C1), and the single longitudinal mode with the Bragg wavelength λ8 is kept stable even at high output.

In this embodiment, the difference in propagation constant Δβ is given by the difference in the width of the optical waveguide region, but as provided by the prior invention, for example, the difference in the thickness of the guide layer or the active layer, the selection of the semiconductor material, etc. It is also possible to give a difference in the propagation constant.

〔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. 4 is a schematic diagram of a conventional example in which a difference in propagation constant is provided, and FIG. 5 is a diagram showing an example of the conventional distribution of field strength and refractive index in the resonator length direction. In the figure, 1 is a Y-type 1nP substrate, 3 is an n-type InP cladding layer, 4 is a corrugation, 5 is an n-type 1nGaAsP guide layer, 6 is an InGaAsP active layer, 7 is a p-type 1nP cladding layer, 8 is a p-type InGaAsP ': 1 contact layer, 9 is n
10 indicates a p-side electrode. (ρ) (b) Single 2 Z Leather 3 figures Single 4 factors (b) Hot Tochopu hji's Xuhuan's Gu'' 1 guard 1 figure $5 figure

Claims (1)

[Claims] 1) A plurality of light wave phase adjustment means are disposed in an optical waveguide region having corrugations, and the total value Δψ of the amount of phase adjustment by the phase adjustment means is Δψ=±π/2±nπ (n is an integer) A semiconductor laser characterized by: 2) The semiconductor laser according to claim 1, wherein the phase adjustment means is arranged symmetrically with respect to the center of the optical waveguide direction. 3) The phase adjustment means selectively forms portions with different propagation constants in the optical waveguide region, and sets the propagation constant difference between the portions with different propagation constants and other portions to Δβ_i, and the propagation constant to 2. The semiconductor laser according to claim 1, wherein the total value Δψ of the phase adjustment amount is Δψ=Σ_iΔβ_iL_i, where L_i is the length of the different portion.