JPH038386A - Coupling distributed-feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a coupling distributed-feedback type semiconductor laser which does not produce any made instability frequently by providing a phase shift section at three locations and deciding the length of each shift section for a specified valve where the amount of shift is lambda/4 and four-divided, stating with the ends. CONSTITUTION:An activated layer 2 is installed between two clad layers 1. The amount of shift in a phase shift section at three locations is lambda/4 in all cases. The length of the four divided sections are specified as L(1+d)/4, L(1+d)/4, L(1-d)/4, L(1-d)/4. The term lambda stands for laser wavelength while L stands for a total length of semiconductor laser. 0.25<d in particular is selected for the value in case of -1<d<1. This construction makes it possible to prevent the scattering of characteristics whenever the laser is changed since oscillation occurs when the difference between an oscillation frequency and Bragg frequency is delta=0. Furthermore, the deviation alpha of a threshold gain alphawith the next mode is increased, thereby providing a coupling distributed- feedback type laser having a narrow spectrum and equalizing the amount of three phase shift in all cases. It is, therefore, possible to facilitate the manufacturing process.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a coupled distributed feedback semiconductor laser, and particularly to a coupled distributed feedback semiconductor laser.
This invention relates to a semiconductor laser with a stable oscillation frequency and a narrow spectrum width.

[Prior art]

Distributed feedback semiconductor laser (D
In FB lasers, when the resonator length is lengthened to narrow the spectral width, spatial hole burning occurs and the mode becomes unstable. In order to improve this, the fourth
The combined phase shift DFB structure shown in the figure is effective.

In addition, as an improved version of Fig. 4, as shown in Fig. 5, 3
A structure has been proposed in which the phase shift amount φ at a location is shifted from λ/4.

Furthermore, as an improved version of Fig. 5, as shown in Fig. 6, 3
The phase shift of the point is λ(1+q)/4, λ/4, λ
(1-(1)/4, where -1< q < 1, and the length of each portion divided into four by this phase shift is L(1+d
)/4. L(1-d)/4. L(1-d)/4゜L(1
+d)/4 has been proposed.

In addition, in FIG. 4, FIG. 5, and FIG. 6, 1 is a cladding layer and 2 is an active layer.

[Problem to be solved by the invention]

However, in the structure shown in Fig. 4, the mode in which the difference δ between the first oscillation frequency and the Bragg frequency is O is not the lowest order mode, but the two modes with δ≠0 are the lowest order, so this lowest order mode is suppressed. There is a problem that the laser must be operated in a certain manner, and in the structure shown in FIG. 5, since δ≠O, the oscillation frequency depends on the DFB feedback strength. This determines the groove depth of the grating, which is difficult to control precisely.

This structure has a problem in that the oscillation frequency varies from sample to sample.

Further, the structure shown in FIG. 6 has a problem in that the amount of phase shift of each part must be individually and precisely controlled.

In addition, in FIGS. 4 to 6, λ is the wavelength of the laser beam, L is the total length of the semiconductor laser, and -1×d<1,
Including d=o.

The present invention has been made to solve the above problems.

The purpose of the present invention is that the difference between the oscillation frequency and the Bragg frequency δ=
The object of the present invention is to provide a coupled distributed feedback semiconductor laser in which the zero mode exists at the lowest order and mode instability due to spatial hole burning is less likely to occur.

Another object of the present invention is to provide a structure that allows easy manufacture of a coupled distributed feedback semiconductor laser.

The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[Means to solve the problem]

In order to achieve the above object, the present invention provides a coupled distributed feedback semiconductor laser having three phase shift sites, the phase shift amounts of which are all equal to λ/4, and the phase shift portions of the three locations are equal. The length of each portion divided into four by the shift is L(1+d)/4. L(1+d)/
4. The most important feature is that L(1-d)/4°L(1-d)/4, where d is a number between -1 and 1 excluding -1, l, and O.

Further, the above-mentioned d is set to 0.25 or more.

[Effect]

According to the above-mentioned means, there are three phase shift parts,
The amount of phase shift is equal to λ/4, and the length of each portion divided into four by the phase shift at these three locations is L(1+d)/4. L(1+d)/4.
L(1-d)/4. With the structure of L(1-d)/4, the laser oscillates in the mode of δ=0, so it is possible to eliminate variations in characteristics caused by changes in the coupled distributed feedback laser.

Furthermore, since Δα becomes large, a coupled distributed feedback laser with a narrow spectrum width can be obtained.

Further, since the structure has the same three phase shift amounts, it is possible to easily manufacture a coupled distributed feedback laser.

Further, when the coupled distributed feedback laser is used in an optical transmission system, an optical sensor system, etc., variations in oscillation characteristics are reduced, and the stability of the entire system can be improved.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be specifically described using the drawings.

In addition, in all the times for explaining the example.

Components having the same function are given the same reference numerals, and repeated explanations thereof will be omitted.

FIG. 1 is a cross-sectional view of the main parts for explaining the schematic configuration of a coupled phase-shifted DFB laser according to an embodiment of the present invention.

In the coupled phase-shifted DFB laser of this embodiment, as shown in FIG. 1, an active layer 2 is provided between two cladding layers 1. The phase shift portions at three locations all have the same phase shift amount of λ/4, and the lengths of the four divided portions are L(1+d)/4. L(1+
d)/4. L(1-d)/4. It is L(1-d)/4.

However, λ is the wavelength of the laser beam, L is the total length of the semiconductor laser,
d is a number between -1 and 1 excluding -1, 1 and O.

Generally, the mode instability due to the spatial hole burning effect of a DFB laser becomes smaller as the mode pattern becomes flatter and as the difference in threshold gain α from the next mode becomes larger.

The conventional structure shown in FIGS. 5 and 6 attempts to eliminate mode instability by flattening the mode distribution, but the structure of this embodiment has a threshold gain α of the next mode. The mode instability is removed by increasing the difference between the two.

The modes of a coupled phase-shifted DFB structure with a phase-shifted portion are determined by solving the following coupled mode equation.

Here, R and S are wave functions of light traveling to the right and left, and are parameters representing the strength of optical feedback, and are quantities proportional to the depth of the groove of the coupled distributed feedback structure.

The relationship between α and δ of each mode is determined by solving the above equations (1) and (2) under boundary conditions.

FIG. 2 shows the calculation result of the relationship between α and δ (α and δ are normalized by L in the figure) when L=2.0, with d as a parameter.

According to FIG. 2, in this structure, a mode always exists at δ=0.

Generally, the inventor found that in the structure of this embodiment shown in FIG. 1, a mode always exists at δ=O regardless of the value of d.

From the relationship between α and δ as shown in FIG. 2, the difference in α between the lowest mode and the next mode, that is, ΔαL (an amount normalized by Δα) can be determined. Further, the flatness of each mode can be defined by the following function F.

F=f (Work(z)−■.)2dZ・・・・・・・・・(
3) However, 1 intensity I (Z)=IRI"+ISl", and I.

is the average value of intensity I(Z).

In the structure of this embodiment shown in FIG. 1, FIG. 3 shows the dependence of ΔαL and F on the parameter d when ΔαL and F are optimized. d=1 corresponds to a conventional single phase-shifted DFB laser with one phase shift portion, and d=O corresponds to a conventional coupled distributed feedback laser having the structure shown in FIG.

According to FIG. 3, the flatness F of the mode pattern is a monotonous function with respect to d, but ΔαL is maximum when d is around 0°75. The value of ΔαL at this time is 1.35, which is about 1.9 compared to the conventional type with d=1.
It's double.

Moreover, ΔαL is positive in the range of d from 0.25 to 1.

With the structure of the present invention that can increase ΔαL, the influence of spatial hole burning can be reduced, and even a Nagai shaker can be stably operated.

Due to the Nagai shaker effect, the spectral width when operating a laser with this structure is 1/1 compared to the case of a single phase shift.
It becomes 3.

The present invention has been specifically explained above based on examples, but
It goes without saying that the present invention is not limited to the embodiments described above, and can be modified in various ways without departing from the spirit thereof.

〔Effect of the invention〕

As described above, according to the present invention, since the laser oscillates in the δ=0 mode, it is possible to eliminate variations in characteristics caused by changes in the coupled distributed feedback laser.

Furthermore, since Δα becomes large, a coupled distributed feedback laser with a narrow spectrum width can be obtained.

Further, since the structure has the same three phase shifts, it is possible to easily manufacture a coupled distributed feedback laser.

Further, when the coupled distributed feedback laser is used in an optical transmission system, an optical sensor system, etc., variations in oscillation characteristics are reduced, and the stability of the entire system can be improved.

[Brief explanation of drawings]

FIG. 1 is a sectional view of the main parts for explaining the schematic configuration of a coupled phase-shifted DFB laser according to an embodiment of the present invention, and FIG. 2 is a case in which L=2.0 with the structure shown in FIG. 1. Figure 3 shows the relationship between αL and δL when the parameter d is changed. Figure 3 shows ΔαL when L is optimized for the structure in Figure 1
A diagram showing the dependence of F and F on the structural parameter d. Figure 4. Figure 5. FIG. 6 is a cross-sectional view for explaining the problems of conventional coupled distributed feedback semiconductor lasers. In the figure, 1... cladding layer, 2... active layer.

Claims (2)

[Claims] (1) A coupled distributed feedback semiconductor laser having three phase shift portions with a phase shift amount of λ/4, wherein the length of each portion divided into four by the phase shift at the three portions is from the end. In order: L(1+d)/4, L(1+d)/4,
L(1-d)/4, L(1-d)/4, where λ
is the wavelength of the laser beam, L is the total length of the semiconductor laser, and d is -1
A coupled distributed feedback semiconductor laser characterized in that the number is between -1 and 1 excluding 1 and 0. (2) In the statement of claim (1), the d is 0.
．． A coupled distributed feedback semiconductor laser characterized in that the value is set to 25 or more.