JPS61156894A - Distributed feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To enhance the reliability under operating conditions particularly of high temperature and high output with a large strength to reflection noise, by a method wherein the titled device has the reflectance of an end surface of higher reflectance made larger than that of a cleavage surface, and the reflectance of an end surface of lower reflectance made 10% or more. CONSTITUTION:A diffraction grating 2 is formed on an N-InP substrate 1, and an N-InGaAsP guide layer 3 of 1.3mum composition, a non-doped InGaAsP active layer 4 of 1.55mum composition, and a P-InP clad layer 5 are successively laminated thereon. This device is made hetero-structural by the process of burial growth, and one end surface is provided with an SiN film 7 and an Au high-reflection coating film 8. The reflectance at this time is 97%, and the reflectance in the emission end surface 6 is 31%; then, the characteristic of particularly high-temperature action largely improves by the difficulty of being affected by reflection noise.

Description

[Detailed description of the invention] The present invention relates to a distributed feedback semiconductor laser.

(Prior art and its problems) Distributed feedback semiconductor laser (DFB-LD) that stably oscillates in a single axis mode even during high-speed modulation at the Gb/s level
is being rapidly developed as a light source for long-distance, large-capacity optical fiber communications. Optimization of the end face structure is an important issue for DFB-LD, and DFB-L
It has a great influence on the characteristics of D. At a relatively early stage of development, emphasis was placed on how to suppress the 7-application Perot mode oscillation, which oscillates near the peak of the gain spectrum due to resonance in the reflectors on both end faces, and A prototype DFB-LD with a window structure and an AR coating was produced. Later, as development progressed, under appropriate manufacturing conditions such as preserving the diffraction grating deeply and performing crystal growth, for example, even if both end faces were formed by cleavage,
It has been found that the threshold value for the DFB mode is sufficiently smaller than the threshold value for the 7-app/Perot mode, and that the 7-app/Perot mode can be suppressed well. DFB-L with various end face structures that have been reported so far
A schematic diagram of the structure of D is shown in FIG. 2. FIG.
b) is a double-sided cleavage type that uses the cleavage surface 10 as it is, and (C) is an AR coating film 9 on one side. The other side is coated with high reflection coating films 7 and 8. Each structure has a light output level of 30 mW or more at 100°C.
It is becoming possible to operate at higher temperatures in cw.
In the figure, 1 is a substrate λ, a diffraction grating is a guide layer, 4 is an active layer, and 5 is a cladding layer.

Coupled with such improvement in initial characteristics, the reliability of the device, especially reliability under high output and high temperature operation, has become important. From this point of view, Figures 2 (a) and (c)
The DFB-LD having the structure shown in FIG. 1 can extract light output from the AR coating film 99A having a small reflectance, and since the back surface is a highly reflective end face, it is possible to effectively extract a large light output from the light output surface 6. It is effective because it can be done. However, in this structure, light from the outside is easily coupled to the internal electric field.

The drawback was that it was susceptible to reflected noise. When the relative noise intensity was actually evaluated by changing the amount of reflected light, it was found that there was a deterioration of about 20 dB compared to the one with a cleaved surface. Although the structure shown in FIG. 2(b) does not have this problem, it has limitations in terms of high temperature operation and high output operation.

(Object of the Invention) From the above-mentioned viewpoint, the object of the present invention is to provide a DF that has high resistance to reflection noise and high reliability especially under high temperature and high output operating conditions.
O (Structure of the Invention) The structure of the present invention consists of an active layer, a light guide layer whose energy gap is larger than that of the active layer, and a diffraction grating formed on one surface. In a distributed feedback semiconductor laser having a laminated structure between two cladding layers having at least (Function/principle of the invention) High reflectance on the back surface is required to achieve high temperature and high output operation. A coating is effective in which the threshold current is also reduced by more than 50% compared to, for example, a double-sided cleavage type. Therefore, since the operating current is also small, there is a greater degree of operational freedom in reliability tests, and reliability is improved. However, the second
If the reflectance of the light emitting end face 6 is 6-1 smaller as shown in Figure (a) (C1), it will become weak against reflection noise as described above. Therefore, if the reflectance of the light emitting end face 6 is increased, However, if the reflectance is 10% or more, for example, a normal isolator with an isolation rating of 20 to 3 QdB can be used without causing large noise to the reflected light. If the reflectance of the light emitting end face 6 is made too large, sufficient optical output cannot be obtained, and furthermore, the Fabry-Bello mode is likely to oscillate.

The inventor of this application conducted an experiment and found that the coupling coefficient was 50.
cPn or more, and if the relationship between the gain peak wavelength and the DFB oscillation wavelength is within an appropriate range, it is possible to operate at a sufficiently high output even with a reflectance as small as a cleavage surface, and the reed and Fabry-Perot modes can be suppressed. Understood O again AR
Threshold current is approximately 40% compared to cocoing.
For example, 5 mW or 10 mW at a temperature of 80°C or higher.
Reliability in the high temperature range has been significantly improved to 91%, which means that the operating current at the mW level is small.

(Example) A schematic structural diagram of a DFB-LD, which is an example of the present invention, is shown in FIG.
A diffraction grating 2 is formed on a P substrate l, and a 1.3μ
m composition fl-InGaAsP guide layer 3, 1.55
Non-doped InGaAsP active layer 4 with μm composition, p −
InP cladding layers 5 are sequentially laminated. The diffraction grating 2 has a period of 2400X and a depth of about 100OX, the guide layer 3 and the active layer 4 each have a thickness of 0.1 μm. In this case, the guide layer 3 has a thickness measured from the peaks of the diffraction grating 2. Some 0
Also, during crystal growth, the height of the diffraction grating 2 decreases mainly due to the influence of heat, but by lowering the growth temperature, the height of the diffraction grating 2 decreases by approximately 500
A double heterostructure DFB wafer with a depth of about After cutting into bars containing laser pellets, SiN film 7 and Au
A highly reflective coating film 8 was formed. The reflectance at this time is 97%. The light emitting end face 6 uses a cleavage surface as it is, and the reflectance at this emitting end face 6 is 31%. When we measured the characteristics of the DFB-LD obtained in this way, we found that it had an oscillation threshold current of 10 mA at room temperature (W), a differential quantum efficiency of 40%, a maximum output of 40 mW, and a maximum C'W operating temperature of 3130°C. Excellent characteristics were obtained o
Even if the diffraction grating 2 has a depth of about 500X, the coupling coefficient is 50
cl11 or more, and in this case, the gain peak wavelength is 100 to 2
In a region as short as 50X, the temperature ranges from around room temperature to the temperature at which oscillation is possible.

7 No appli-hero one mode oscillation was observed 0 However, as a result of the experiment, the diffraction grating was below 300X.

Alternatively, if the gain peak wavelength and the DFB oscillation wavelength are in a region other than the above, the Fabry-Perot mode may be observed. 7 apps/Perot mode can be suppressed.

The device manufactured in this way is capable of operating at a particularly high temperature compared to the conventional example), and in a severe reliability test of 80010 mW, an improvement of about five times was obtained compared to the best result of the conventional example. Of course, the resistance to reflection noise was also sufficiently good.

In the embodiment, the light emitting end face 6 is formed by cutting, but the invention is not limited to this, and the semiconductor material used may also be InGa. The semiconductor material is not limited to Sr/InP type, and other semiconductor materials such as GaAjAs/GaAs type may be used.

(Effects of the Invention) A feature of the present invention is that in the DFB-LD, the reflectance of one end face is increased and the reflectance of the light emitting end face is set to 10% or more. As a result, the DFB-L is less susceptible to reflection noise and has significantly improved high-temperature operating characteristics.
If the Dt obtained coupling coefficient and the difference between the gain peak and the DFB wavelength are within an appropriate range that can be sufficiently controlled by normal methods, the operating current in the high temperature region where the Fabry-Perot mode will not oscillate at all. It is possible to perform reliability tests at high temperatures, which was impossible with conventional methods, and it has a large degree of operational leeway even within the normal temperature range of about 50°C, compared to conventional methods. 〇 (C) shows a structural schematic diagram of a conventional DFB-LD, which has achieved reliability several times higher. In the figure: InP substrate, 2 is a diffraction grating, 3 is a guide layer, 4 is an active layer, 5 is an InP cladding layer, 6 is a light emitting end face, 7 is a SiN film, 8 is an Au coat film, 9 is a rubber layer The coating film, 10, has a cleavage surface, respectively.

i′ Sentaki Patent Attorney Susumu Uchihara ( Figure 1

Claims (1)

[Claims] The energy gap between the active layer and the active layer is larger than that of the active layer.
In a distributed feedback semiconductor laser having a laminated structure between two cladding layers, which has at least a light guide layer having a diffraction grating formed on one surface, the reflectances of a pair of opposing laser end faces are made to be different from each other. , the reflectance of the end face with higher reflectance is greater than the reflectance of the narrower face,
A distributed feedback semiconductor laser characterized in that the reflectance of the lower end facet is 10% or more.