JP2687526B2 - Distributed feedback semiconductor laser and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention relates to a single-axis mode oscillation distributed feedback semiconductor laser used for optical fiber communication and the like.

[Conventional technology]

Distributed feedback semiconductor lasers that have a diffraction grating in the active waveguide that contributes to laser oscillation are capable of single-axis mode oscillation even during modulation, and are being actively researched and developed as key devices for ultrafast optical fiber communications. There is. Using a distributed feedback semiconductor laser as a light source, transmission experiments of several Gbit / s and several tens of km level have been conducted, and its usefulness has been verified. Especially near the center of the active waveguide, the phase of the diffraction grating is λ / 4.
(Λ is the wavelength in the medium of the laser oscillation light) The λ / 4 phase-shifted semiconductor laser, which is deviated, oscillates at the black wavelength, and the gain difference between the main mode and the sub mode can be made large, which has attracted attention.

[Problems to be solved by the invention]

In manufacturing a distributed feedback semiconductor laser, conventionally, a coupling coefficient K between the laser oscillation light and the diffraction grating is axially (direction in which light propagates, that is, a direction perpendicular to the end face of the semiconductor laser).
Produced an almost uniform diffraction grating. However,
In a λ / 4 phase-shift distributed feedback semiconductor laser, the electric field strength is concentrated near the phase-shift region, so hole burning occurs at the time of optical output, mode instability is caused, and the spectral line width of laser oscillation light increases. Or, in some cases, two-mode oscillation may occur. Such hole burning is described in Electronics Letters, Vol. 22, No. 20, page 1046. Currently, semiconductor lasers are often used with an optical output of about 10 mW, which is relatively small, and there is no effect of hole burning, but in the future, as optical communication becomes longer, high output of 20 mW to 50 mW will be produced. As required
It is essential to develop a high-performance distributed feedback semiconductor laser that stably oscillates in a single-axis mode even at high output. Therefore, suppressing the hole burning is extremely important for improving the performance of the distributed feedback semiconductor laser.

Up to now, as a means for suppressing the hole burning, for example, as reported by Usami et al. In C-155, Volume C-1, Volume C-1 of the 63rd National Congress of the Institute of Electronics, Information and Communication Engineers of Japan. The method of dividing the electrodes in the resonator axial direction and changing the injection current of each electrode to suppress the hole burning has been tried, but it is complicated and lacks practicality.

An object of the present invention is to provide a λ / 4 phase shift distributed feedback type semiconductor laser capable of suppressing hole burning by a relatively simple method and stably oscillating in a single axis mode even at high output.

[Means for solving the problem]

In a phase shift distributed feedback semiconductor laser in which a diffraction grating is formed along the active waveguide and the phase of the diffraction grating is shifted near the center of the active waveguide, the depth of the groove of the diffraction grating is Of the phase-shift distributed feedback semiconductor laser, which is shallow near the center of the active waveguide and gradually deepens as it approaches both end faces of the active waveguide,
The above problems can be solved.

[Action]

The gist of the present invention is the cavity axis direction (direction along the active waveguide, that is, direction perpendicular to the emitting end face of the semiconductor laser).
The hole burning is suppressed by unevenly distributing the coupling coefficient K. FIG. 5 shows the relationship between the electric field distribution in the cavity axis direction of the conventional distributed feedback λ / 4 phase shift semiconductor laser and the diffraction grating shape (groove depth). As shown in FIG. 5, a distributed feedback type λ / 4 having a uniform coupling coefficient K (about 50 cm ⁻¹ ) in the conventional resonator axial direction.
In the phase shift semiconductor laser, the electric field strength becomes strong at the central position shift portion, and hole burning easily occurs at high output.

The electric field distribution of the semiconductor laser of the present invention that the coefficient K in the vicinity of the resonator center gradually increased to reduce by about 70cm ^-1 towards the end face direction of about 30 cm ^-1 is shown in Figure 4. It can be seen that the electric field distribution becomes flat as compared with FIG. 5 because the coupling coefficient K is made nonuniform along the resonator axis. This has a great effect on suppressing hole burning, and it can be seen that the present invention in which the coupling coefficient K is changed along the resonator axis is effective for suppressing hole burning.

〔Example〕

The present invention will be described in detail below with reference to the drawings illustrating embodiments.

FIG. 1 shows a first embodiment of the present invention and is a cross-sectional structural view of a λ / 4 phase shift distributed feedback semiconductor laser. First n
-On the InP substrate 6, the diffraction grating 5 with the groove depth varied along the cavity axis direction is formed. Such a diffraction grating 5
FIG. 2 shows a method of forming the.

By coating a photoresist on the n-InP substrate 6 and exposing it to electron beam (EB), a stripe pattern is formed in which the width is narrower from the center of the substrate to the end as shown in FIG. The photoresist 9 is formed. The substrate 6 is etched by using this photoresist as a mask (FIG. 2 (b)). After that, by removing the photoresist, it is possible to form a diffraction grating 5 having a shallow groove in the central portion of the resonator and a deep groove in the vicinity of the end face (FIG. 2 (c)).
The same method as the conventional method was used for forming the λ / 4 phase shift region.

Thus, the InGaAsP having a composition of 1.3 μm is formed on the substrate 6 on which the diffraction grating 5 having the λ / 4 phase shift region is formed near the center.
Light guide layer 4, InP buffer layer 3, 1.55 μm composition InG
An aAsP active layer 1, an InGaAsP meltback prevention (AMB) layer 8 having a composition of 1.3 μm, and an InP clad layer 2 are sequentially grown to form a laminated structure having an active optical waveguide including an active layer, a buffer layer and an optical guide layer. After that, this laminated structure was etched to form a strap, and high resistance layers were formed on both sides of this to form a buried structure. Finally, to improve heat dissipation,
Resonator length 1000 μm for narrowing line width with long resonator
Then, a silicon nitride film 7 was formed as an antireflection film on the end face thereof to obtain a semiconductor laser (FIG. 1). When this semiconductor laser was evaluated, the maximum value was obtained at room temperature CW.
A high output of 180mW, which was unprecedented, was obtained. A distributed feedback semiconductor laser with an unprecedented high performance was obtained, with an oscillation spectrum linewidth of 500 KHz and SMSR of 40 dB or more. Normal resonator length
In the semiconductor laser of the present invention cut out at about 300 μm,
An oscillation threshold current of 10 to 20 mA and a differential quantum efficiency of about 0.25 W / A were obtained with good reproducibility.

The diffraction grating 5 of such a λ / 4 phase shift distributed feedback semiconductor laser can also be manufactured by the method shown in FIG. That is, after forming a normal λ / 4 phase shift diffraction grating 5 with a uniform groove depth on the n-InP substrate 6, a photoresist for protecting the diffraction grating 5 from etching on both ends of the resonator. 9 is formed (FIG. 3 (a)). After that, etching is performed so that the depth of the diffraction grating 5 is shallow only in the central portion (FIG. 3 (b)), and the photoresist 9 is removed to form a diffraction grating (FIG. 3 (c)).

In the embodiment, the material system can be realized with GaAs, and the active region can be formed with a quantum well structure.

Although the diffraction grating is formed in the light guide layer in the embodiment, it may be formed in another layer, for example, the active layer. The diffraction grating should just be in the area | region where light propagates.

〔The invention's effect〕

A feature of the present invention is that in the λ / 4 phase shift distributed feedback semiconductor laser, hole burning is suppressed by reducing the coupling efficiency K of the diffraction grating near the center of the resonator. As a result, an excellent distributed feedback semiconductor laser having an optical output and a narrow oscillation spectrum line width can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic view of a λ / 4 phase shift distributed feedback semiconductor laser which is an embodiment of the present invention, FIG. 2 and FIG. 3 are examples of a method for producing a diffraction grating of the semiconductor laser according to the present invention. 4A and 4B show the electric field distribution in the axial direction when the depth of the diffraction grating is made non-uniform, and FIG. 5 shows the electric field distribution in the axial direction of a normal diffraction grating when the depth of the groove is uniform. It is a figure. 1 ... Active layer, 2 ... Clad layer, 3 ... Buffer layer,
4 ... Optical guide layer, 5 ... λ / 4 phase shift diffraction grating, 6
... n-InP substrate, 7 ... silicon nitride film, 8 ... meltback prevention layer, 9 ... photoresist.

Claims (2)

(57) [Claims] 1. A phase-shift distributed feedback semiconductor laser in which a diffraction grating is formed in an active waveguide that participates in laser oscillation, and the phase of the diffraction grating is shifted near the center of the active waveguide. The depth of the groove is shallow near the center of the active waveguide and becomes gradually deeper as it approaches both end faces of the active waveguide. 2. A step of applying a photoresist to the surface of a semiconductor substrate, and then patterning it in stripes with a width gradually narrowing from the center of the resonator to the end thereof,
The semiconductor substrate is etched using the patterned photoresist as a mask, and the photoresist is removed after the etching is completed. A method of manufacturing a distributed feedback semiconductor laser, comprising at least a step of forming a semiconductor substrate and a step of forming a semiconductor multilayer laminated structure including an active layer on the semiconductor substrate having the diffraction grating formed thereon.