JPH0417384A - Distributed feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To reduce a feedback light induction noise by providing a non- excitation region at a position coming in contact with an end face and utilizing a grating in the non-excitation region as a distributed reflector. CONSTITUTION:An InGaAsP guide layer 4, a p-type InP clad layer 5 and a p-type InGaAsP cap layer 6 having an n-type InP clad layer 2, an InGaAsP active layer 3 and a grating are in order formed on an n-type InP board 1, while an n-side electrode 7 is formed on the other side exposed surface of the board 1 and a p-side electrode 8 is formed on a cap layer 6 and a region 9, where the electrode 8 is not formed, is formed. Here, a current is not introduced into the active layer to make it a non-excitation region and inside this region, a grating on the top of a guide layer is made a distributed reflector. Further, a product KL of a coupling constant of the grating and the length of the non- excitation region is to be not less than 0.5.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention reduces return light induced noise, which is one of the major causes of deterioration of semiconductor laser characteristics, without coating the end face with a dielectric multilayer film. Distributed feedback semiconductor laser (hereinafter referred to as D
FB laser).

(Prior art) As an example of a conventional structure for reducing return light induced noise,
Figure 2 shows the structure of an InGaAsP/InP-based DFB laser.

In FIG. 2, 1 is an n-type 1nP substrate, 2 is an n-type 1nP cladding layer on the substrate 1, 3 is an active layer on the InP cladding layer 2, and 4 is an InG layer having a diffraction grating on the active layer 3.
aAsP-guide layer, 5 is p-type 1nP on guide layer 4
The cladding layer 6 is P-type 1nGaAs that covers the cladding layer 5.
P cap layer, 7 is an n-side electrode formed on the surface of the substrate 1, 8
10 is a p-side electrode formed on the cap layer 6, and 5i is formed on the end surface.
This is a dielectric multilayer film in which 02 and ALO3 are alternately layered and coated.

This laser operates by applying a positive voltage to the electrode 8 and a negative voltage to the electrode 7, and by passing a current through the active layer 3, electrons and holes are injected, and light is generated by their recombination. Since the wavelength selectivity of the light is further enhanced by the diffraction grating formed in the guide layer 4, the light oscillates in a single mode.

Moreover, the dielectric multilayer film 10 coated on the end face of this laser is
This is to change the reflectance of the end face, and by coating the end face with the dielectric multilayer film 10, it is possible to increase the reflectance of the end face and suppress reflected return light from entering the inside of the active layer. It has a structure.

However, when using a dielectric multilayer film as shown in Figure 2,
It is not easy to coat the end face while controlling the thickness, which also causes a decrease in yield.

Additionally, for dielectric multilayer coating, the laser must be placed face-up into the film deposition equipment, making it difficult to use this method for lasers on monolithic integrated circuits. It was difficult.

(Problems to be Solved by the Invention) The present invention has been made in view of the above-mentioned drawbacks, and an object of the present invention is to provide a DFB laser that can reduce return light induced noise with a high yield and a structure and process that are as simple as possible. It is in.

(Means for Solving the Problem) In the present invention, the end face of a DFB laser having a conventional structure is not coated with a dielectric multilayer film, and a region (hereinafter referred to as a non-excited region) in which no current is injected has a diffraction grating. ) to reduce the return light induced noise.

That is, the distributed feedback semiconductor laser of the present invention is a DFB laser having a structure in which an optical waveguide formed of an active layer and a guide layer has a diffraction grating. There is a non-excited region, and the product KL of the length of this region in the cavity direction and the coupling constant of the diffraction grating is set to be 0.5 or more.

However, the coupling constant refers to the ratio per unit length at which light traveling in one direction is reflected by the diffraction grating and converted into light traveling in the opposite direction, and is determined by the depth of the diffraction grating or the unevenness of the diffraction grating. It is determined by the ratio of the widths of

(Function) In the present invention, the active layer of a conventional DFB laser is
By separating the non-excited region into a region (where current is injected) and a non-excited region, and by providing the non-excited region in a position in contact with the end face, the dielectric multilayer film can be applied to the end face by utilizing the diffraction grating in the non-excited region as a distributed reflector. Even without coating, it is possible to suppress reflected return light from entering the excitation region of the active layer.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

Here, an InGaAsP active layer PDFB laser will be described as an example of the present invention, but the present invention is not limited to this example.

FIG. 1 is a cross-sectional view in the active layer direction of a laser according to an embodiment of the present invention, in which 1 is an n-type 1nP substrate, and 2 is an n-type 1nP cladding formed on one main surface of the substrate 1. 3 is an InGaAsP active layer formed on the cladding layer 2; 4 is a TnGaAs layer with a diffraction grating formed on the active layer 3;
P guide layer 5 is a p-type 1nP formed on the guide layer 4
A cladding layer 6 is p-type InGaAs that covers the cladding layer 5.
P cap layer, 7 is formed on the other exposed surface of substrate 1 ([1, 11 electrode, 8 is p formed on cap layer 6
(!], 111i pole. 9 is a region where the electrode 8 is not formed, where no current is injected into the active layer and becomes a non-excited region, so within this head, the diffraction grating on the upper part of the guide layer It acts as a distributed reflector and has the effect of reflecting returning light.

The process and basic operation of this example is similar to that of a conventional DFB.
It is similar to a laser, and emits light by applying a positive voltage to the electrode 8 and a negative voltage to the electrode 7, and passing a current through the active layer 3 (injecting electrons and holes).

In the present invention, a region 9 in which no electrode is formed is provided on one end surface side of the upper part of the cap layer 6. This is accomplished by changing the pattern of the mask.

Actually, the relative noise intensity of the laser when there is reflected return light when using the DFB laser of this example and when using a conventional DFB laser not coated with a dielectric multilayer film. When (RIN) was measured and compared, it was confirmed that the larger the product KL of the coupling constant of the diffraction grating and the length of the non-excited region becomes, the smaller RIN becomes, and the return light induced noise is reduced. If KL was 0.5 or more, RIN could be reduced by at least 5 dB/Hz or more compared to a DFB laser with a conventional structure.

However, the relative noise intensity is the ratio of the fluctuation of the optical output intensity to the average optical output intensity divided by the measurement frequency band.

Although the present invention has been described above using an example of an InGaAsP/InP DFB laser, the present invention is not limited to this example (the present invention is also effective when the laser is made of other materials such as GaAs).

The present invention is also effective in a so-called p-substrate type element in which the p-type and n-type conductivity types are reversed.

(Effects of the Invention) As explained above, the DFB laser of the present invention can be manufactured in the same manufacturing process as the conventional DFB laser, and without coating the end face with a dielectric multilayer film.
Return light induced noise can be reduced.

Furthermore, since it is possible to reduce return light induced noise due to reflected return light without having to cover the end face with a dielectric multilayer film, the manufacturing process can be shortened accordingly. It has the advantage that it can also be applied to lasers on monolithic integrated circuits, which has been difficult to do.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing the structure of an embodiment of the present invention, and FIG. 2 is a sectional view showing the structure of an example of a conventional DFB laser for suppressing return light induced noise. 1...N-type? nP substrate 2...n-type 1nP ground layer 3-1nGaAsP active layer 4-1nGaAsP guide layer 5...p-type 1nP cladding layer 6...p-type 1nGaAsP cap layer 7...n-side electrode 8... N-side electrode 9...Region 10 where no electrode is deposited...SiO□/Afz (h dielectric multilayer film. Fig. 1

Claims (1)

[Claims] 1. In a distributed feedback semiconductor laser having a structure in which an optical waveguide formed of an active layer and a guide layer has a diffraction grating, at a position in contact with one or both end faces of the optical waveguide, There is a region with a diffraction grating where no current is injected,
The length L of the region in the cavity direction and the coupling constant K of the diffraction grating
A distributed feedback semiconductor laser characterized in that a product KL of 0.5 or more is 0.5 or more. However, the coupling constant K represents the rate per unit length at which light traveling in one direction is reflected by the diffraction grating and converted into light traveling in the reflected direction.