Semiconductor laser device

Details Semiconductor laser device Semiconductor laser device

2002-03-06

2003-11-25

Davie, James (Department: 2828)

Coherent light generators

Particular beam control device

Optical output stabilization

C372S036000, C372S031000

Reexamination Certificate

active

06654393

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application Nos. 2001-62175, filed on Mar. 6, 2001 and 2001-62176, filed on Mar. 6, 2001; the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser used for a light source of optical communication, optical transmission technology and optical information recording.
2. Description of the Related Art
Recently semiconductor lasers are widely put to practical use as a light source in the field of optical communication, optical transmission and optical information recording, etc., because of coherency of emitted light, of competence for high speed operation or of very small size. The semiconductor laser is mounted on a metallic member such as a lead frame or a metal block so that it can emit a stimulated light thanks to a current flowing from outside thereof and can ensure a passage for heat-radiation because the light intensity varies sensitively in proportion to heat change. To mitigate the difference of thermal expansion coefficient between the metal and the semiconductor material constituting the semiconductor laser, the semiconductor laser is mounted on the metallic member after it is mounted on a substrate material called ‘sub-mount’ of such as silicon (Si) or aluminum nitride (AlN). Besides, the semiconductor laser is realized by a medium with an amplification rate larger than 1 intervening in a resonator comprised of a plurality of reflection mirrors. An edge emission type semiconductor laser, which utilizes cleaved surfaces of a crystal as the reflection mirrors for the resonator and can take a long distance in the amplifying media through which the light passes, has been mainly developed. Although a surface emission type laser, which emits a radiation in the direction normal to the substrate of high reflection mirror made of such as a semiconductor or a multi-layered dielectric material, is partially put to practical use, there are many problems that the technology is not yet sufficient but mostly still at development stage with regard to a certain material, in other words, to a certain emitted radiation wavelength. Therefore, almost all the lasers utilized in various application products are edge emission type.
However, following problems arise when the edge emission type semiconductor laser is mounted on the sub-mount. A light beam emitted out of the edge diffracts largely and diffuses, because active region of the semiconductor laser is formed as a waveguide structure whose cross section is very small so as to improve the efficiency of amplification, preventing the light from escaping out of the amplification region and therefore causing a loss. In general, because a thin region with a thickness of the degrees of the wavelength can be formed in the direction perpendicular to the component substrate by means of, for example, crystal growth technology, confinement of the light to the region of the degrees of the wavelength is carried out. On the other hand, a confining region is formed by a flat structure in relation to the parallel direction thereto, so that it is difficult to confine the light within the degrees of the wavelength. In addition to the above reason, the light is confined in a region wider than the wavelength in order to prevent the component resistance from increasing. Therefore, diffraction angle expands largely in the perpendicular direction as compared with the parallel direction. For example, the expanding angle of the light beam, i.e. the direction in which the light intensity becomes 1/e
2
of the light intensity on the optical axis, is approximately 10 degrees to the optical axis in the parallel direction, whereas the angle is approximately 30 degrees in the perpendicular direction. When the component is mounted on the flat sub-mount, the light beam reaches the mounted surface in the vicinity of the component (for example, at 200 micrometers for the light emitting portion positioned at 100 micrometers above the mounted surface), resulting in reflection, scattering, absorption, etc., which causes deformation of the light beam due to occurrence of so-called ‘an eclipse’, in a part of the light beam. The phenomenon has a bad influence on connecting it to an optical pickup or an optical fiber utilizing a light beam. Consequently, when the component is mounted, it is necessary to adopt a means such as a structure where the laser component is mounted in the vicinity of the edge of the sub-mount so as to prevent the light from being eclipsed. Therefore, the relation of the position of the semiconductor laser to the position of the mounted surface should be limited, so that flexibility of the mount gets worse.
One method to solve the problem mentioned above is proposed as follows (e.g. Japanese unexamined patent disclosure No. Hei05-315700): The structure of the proposal is that the semiconductor laser component is mounted on a silicon substrate as the sub-mount, and the light beam outputs upward above the substrate by reflecting the beam on a oblique wall surface standing on the substrate. The output light according to this structure is reflected upward in the vicinity of the semiconductor component before it diffuses in a large scale, so that the output light can be taken out with the beam form being practically kept because the eclipse at the mounted surface is small without taking particularly the relation of the position to the mounted surface in consideration.
Meanwhile, the semiconductor laser varies sensitively the light output thereof in accordance with change of the circumambient temperature. Consequently, it is desirable that both the semiconductor laser and the mount substrate should be mounted together on a component being able to control the temperature, for example Peltier device. However because the mount substrate and the sub-mount even have some heat capacitance, though it is small, a method by which feedback control for the driving current circuit of the laser component is carried out by monitoring the actual output light is adopted, if accurate light output control is required. This is called ‘automatic power control (APC)’.
Because the edge emission type semiconductor laser has both end surfaces formed by e.g. cleaved surfaces working as the resonator mirrors, the output lights are symmetrically emitted in both front and rear directions as far as reflectance of the edges is specially not controlled. Although the aforementioned APC can be constructed by monitoring the rear-side-emitted light with a photo detector, utilization efficiency of the light decreases because the monitored light does not contribute to the signal source. Therefore, a method to improve the utilization efficiency of the light as much as possible by means of raising the reflectance of the rear-end surface with a multi-layered dielectric film, etc., is carried out for a system where raising an output or an efficiency thereof is required. In such a case, the emitted light out of the rear side, which can be used for the monitor, decreases, and consequently the SN ratio becomes too low to carry out the accurate APC. For reasons of the above, it is necessary to monitor a part of the front-side output light (signal light). Such control system is called ‘front APC (hereinafter denoted by FAPC)’.
FIG. 7
shows an example of the structure where the semiconductor laser is used for the FAPC system. The aforementioned Japanese unexamined patent disclosure No. Hei05-315700 and another Japanese unexamined patent disclosure No.2001-15849 propose a following structure. Namely, as shown in
FIG. 7
, the structure comprises a semitransparent film formed on a sub-mount
100
and a photo detector for a front monitor, which is formed by p-n junction by means of diffusion process, located behind the film. Mark
101
denotes a semiconductor laser, and a means, such as a half mirror
103
, etc. for splitting the output light is positioned

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