JPS6370590A - Distributed feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To form gain distribution in the axial direction in conformity with the shape of the field distribution of a dominant mode, and to improve the yield of a single axial mode operating element by changing the composition of an active layer, a diffraction grating of which is shaped in a laser resonator, in a stepped manner or continuously in the direction of the resonator. CONSTITUTION:A diffraction grating 10 having approximately 1000Angstrom depth and a period of 2000Angstrom is formed onto an n-type InP substrate 1 (SN-doped and carrier concentration of 1X10<18>cm<-3>) having a (100) face azimuth. An n-type InGaAsP guide layer 2 (approximately 0.15mum film thickness, Sn-doped and carrier concentration of 7X10<17>cm<-3>) is grown on the diffraction grating and an active layer 12 having a band- gap equivalent wavelength lambdag=1.4mum extending over 100mum of the central section of a DFB laser having 300mum cavity length, and active layers 11 and 13 having lambdag=1.33mum (non-doped and approximately 0.1mum film thickness) are grown extending over 100mum sections of both sides of the active layer 12 and a p-type InP clad layer 4 (Zn-doped, carrier concentration of 1X10<18>cm<-3> and 0.7mum film thickness) is grown. Two parallel grooves in 3mum depth and approximately 8mum width are shaped, holding a mesa stripe in approximately 1.5mum width in the (110) direction, thus forming double channel planner buried type structure.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a distributed half-type semiconductor laser used as a light source for an optical fiber communication system.

[Conventional technology]

Distributed Feed Back semiconductor laser with a diffraction grating inside the laser cavity
La5erDiode (hereinafter referred to as DFB laser) is
Because it is a light source capable of single-axis mode oscillation, it has been rapidly developed in recent years and is being put into practical use as a light source for long-distance, large-capacity optical communications and for optical measuring instruments.

However, when a DFB laser chip of this DFB laser is cut out from a wafer, stable single-axis mode operation is rare, and most of the elements cause multi-axis mode oscillation or mode skipping.

For example, in an FB laser as shown in FIGS. 2(a) and 2(b), a diffraction grating 10 is provided on an n -1nP substrate 1, an active layer is formed on an n -1nGaAsP guide As, and a p -1nP layer 4 and p-1nGaAsP
A layer 5 is provided, and electrodes 6.7 are provided at both ends of the element. This DFB laser has 2,000 to 2,500 very short period intracavity diffraction gratings 10 as shown in Fig. 2 (b).
), it is impossible to specify at which position in one period on the end face the diffraction grating 10 of the end face is cut.
This is because the oscillation threshold gain difference (ΔαthL) between the main mode and the submode differs depending on the position. Further, the electric field distribution shapes of the main mode and the sub-mode also greatly depend on where the diffraction grating is cut at the end face.

An example of the results of calculating the electric field distribution shapes of the main mode and the submode is shown in the electric field intensity distribution diagrams of FIGS. 3 and 4.

These diagrams show the case where the diffraction grating of an element with a 10% AR code on the front side (left side) is cut at the front side (Figure 3) and when the front side is an end face (Figure 4), and the solid line is the electric field of the main mode. The intensity distribution, the broken line indicates the electric field intensity distribution of the secondary mode. As shown in these figures, sub-mode oscillation may occur.

[Problem that the invention seeks to solve]

As shown in these figures, in conventional DFB lasers, the gain distribution in the axial direction is uniformly given within the active layer, so depending on the end facet phase within the resonator, the electric field distribution of not only the main mode but also the sub-mode may be adjusted. There is a problem in that the gain distribution is easily given, which may encourage sub-mode oscillation, and many elements perform multi-axis mode oscillation, resulting in poor yield.

The purpose of the present invention is to solve these problems, create active layers with different compositions (band gaps) in the direction of the cavity axis, and create a gain distribution commensurate with the electric field distribution of the main mode, thereby achieving single-axis mode oscillation. DF that increases the yield of devices that
The objective is to provide a B laser.

[Means for solving problems]

The configuration of the DFB laser of the present invention is characterized in that the composition of the active layer in which a diffraction grating is provided in the laser cavity changes stepwise or continuously in the direction of the cavity. .

[Effect]

As shown in FIGS. 3 and 4, the electric field distribution of the main-secondary modes largely depends on the phase of the end face of the diffraction grating 10. When the electric field distributions in the resonator for the main and secondary modes are similar, when the injected carriers contribute to the formation of the electric field distribution in the main mode, it becomes difficult to form the electric field distribution necessary for secondary mode oscillation, and the secondary mode oscillation is suppressed. On the other hand, when the electric field distribution in the resonator between the main mode and the sub-mode differs greatly, carriers that do not contribute to the electric field distribution in the main mode provide the gain necessary for forming the electric field distribution in the sub-mode. Therefore, by providing a gain distribution commensurate with the main mode electric field distribution, stable single mode oscillation can be obtained regardless of how the end face phase of the diffraction grating is cut.

Conventionally, as a method of controlling the gain distribution within the resonator of a DFB laser, a method of changing the current distribution by dividing electrodes or the like has been considered. However, when this current density changes, not only does the gain change, but also the refractive index changes due to the plasma effect of the injected carriers, making the propagation constant of light waves in the axial direction non-uniform. As a result, the single-axis mode selectivity deteriorated, and the effect of gain distribution control was not necessarily expected.

On the other hand, according to the configuration of the present invention, since the injection current density is constant in the axial direction, there is no change in the propagation constant due to the plasma effect of carriers.

Furthermore, as shown in Figure 5, there is almost no difference in refractive index near the bandgap phase wavelength due to slight differences in composition.
The refractive index can be considered to be constant in the axial direction. It is also the refractive index at the corresponding wavelength. For light with a wavelength shorter than the bandgap equivalent wavelength, it can be considered that the refractive index is approximately equal to the refractive index at the bandgap equivalent wavelength.

Furthermore, an example of a change in the gain constant due to a change in composition is shown in the sixth section.
As shown in the figure. By changing the composition in this way, the gain constant curve changes significantly, so it can be seen that only the gain distribution can be controlled by controlling the composition.

〔Example〕

The present invention will be explained in detail below with reference to the drawings.

FIG. 1 is a cross-sectional view of one embodiment of the present invention, in which the Ijμm band DF
This figure shows a B laser provided with active layers having different compositions. In this example, first, (1, O.

O) n-type 1nP substrate 1 with plane orientation (Sn-doped, Kya!J
He-
It is formed by an interference exposure method using a Cd laser. On such a substrate 1, an n-type 1nGaAsP guide layer 2 (film thickness 0.15 μm, So doped, carrier concentration 7×10
"cm-3)" is grown by a liquid phase growth method or the like.

Next, 1 in the center of the DFB laser with a cavity length of 300 μm.
Over a length of 00 μm, the bandgap equivalent wavelength λ,
= 1.4 μm active layer 112 is selectively grown by a liquid phase growth method or the like. After that, 100 on both sides of this active 112
In the μm part, the bandgap equivalent wavelength λ□=1j3μ
m active layers 11 and 13 are selectively grown (all of these active layers are non-doped and have a film thickness of 0.1 μm).

Furthermore, a p-type 1nP cladding layer 4 (Zn-doped, carrier concentration I x 1018.-3, film thickness 0.7 μm) is grown in the 0th order, and two parallel layers with a depth of 3 μm and a width of about 8 μm are grown in the (110) direction. Grooves are formed across mesa stripes approximately 1.5 μm wide to form a double channel brainer buried structure to complete the laser fabrication.

(See Proceedings of the 1985 National Conference of the Institute of Electronics and Communication Engineers by Mito et al. 857).

When a current is injected into the laser thus fabricated, the gain distribution can be greatly controlled in the axial direction without causing almost any change in the refractive index in the axial direction.

Figure 6 shows the wavelength dependence characteristics of the gain constant of a quaternary mixed crystal with bandgap equivalent wavelengths λ, = ]j3 μm and λ, = 1.4 μm. In this case, the injected carrier concentration is 2 × 10
At a Bragg wavelength of 1 j μm, which corresponds to the period of the 0 diffraction grating, which is 18 crs-', the active layer 12 (^, = 1.4 μrn) in the center of FIG. 1 has a gain constant of about 75 cm-'. In addition, the active layers on both sides 11.13 (λ
, =lj3μm) has a gain constant of about 30cta-', and it can be seen that a large gain distribution is provided in the axial direction.

On the other hand, regarding the change in refractive index with respect to wavelength of Ga1nAsP, which is lattice-matched to InP, the refractive index on the shorter wavelength side than the bandgap equivalent wavelength shown in Figure 5 is considered to be given by the circled point in the figure. According to this, the refractive index of the active layer 12 (λ, = 1.4.czm) in the center in FIG.
The refractive index n is 3.54 for light having a wavelength of λ=1.3 μm, and the fluctuation in the refractive index in the axial direction can be almost ignored.

〔Effect of the invention〕

DFB lasers have main -
The shape of the electric field distribution in the secondary mode changes greatly, but even if the end face phase of the diffraction grating is cut off at an arbitrary point (randomly), the main mode will be unimodal in about 80% of cases, with a peak at the center. Calculations have shown that the shape has a peak of . Therefore, with the configuration of the present invention, if active layers with different compositions are created in the axial direction and the axial gain distribution can be formed to match the electric field distribution shape of the main mode, the yield of devices operating in a single axial mode can be greatly improved. can do.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view showing the structure of an embodiment of the present invention, and Figs. 2 (a) and (b) are cross-sectional views showing the structure of a conventional DFB laser and the phase state of the diffraction grating at the end face thereof. Enlarged views, Figures 3 and 4 have 10% AR on the front (left side)
An electric field strength distribution diagram inside the resonator when the coated rear surface is cut at α·-1π, FIG. 5 is a diagram of refractive index change characteristics with respect to wavelength of Ga1nAsP that is lattice-matched to InP,
FIG. 6 is a wavelength dependence characteristic diagram of the gain constant in a quaternary mixed crystal having a composition of bandgap equivalent wavelengths λ, -1j3 μm and λ, = 1.4 μm. 1----n-1nP substrate, 2-n-1nGaAs
P guide layer, 11...Band gap equivalent wavelength λ, =
Active layer having a composition having a wavelength of 1.33 μm, 12... Active layer having a composition having a band gap equivalent wavelength λ, = 1.4 μm, 13... Band gap equivalent wavelength λg = 1j
Active layer of composition having 3 noz m, 4...p -1n
P cladding layer, 5...p-1nGaAsP cap layer, 6,7... electrode. After the first year of the year, the age of 6 years and 7 years of age is blue.

Claims (1)

[Claims] A distributed feedback semiconductor laser characterized in that the composition of an active layer in which a diffraction grating is provided in a laser cavity changes stepwise or continuously in the direction of the cavity.