Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
1997-12-18
2001-02-13
Davie, James W. (Department: 2881)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S036000, C372S046012
Reexamination Certificate
active
06188711
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to lasers, and more particularly, to an improved method for controlling the polarization of surface emitting laser diodes in an array of such diodes.
BACKGROUND OF THE INVENTION
Vertical Cavity Surface-Emitting Lasers (VCSELs) are currently being considered for use in CD-ROM drives, DVD heads, and laser printers. For all of these applications, the optics which deliver the laser power to the disc or drum have a certain degree of polarization sensitivity, such that if the polarization of the light source drifts, the drift will translate into detrimental output power variations. In addition, the optics can be complex enough in these systems that knowledge of the polarization direction is necessary to optimize the optics design. For these reasons, the light sources for such applications must have a stable fixed polarization, and the polarization direction must be the same for every laser fabricated.
VCSELs often have a fixed polarization while operating in the fundamental mode. However, the output light is, in general, a mixture of polarization states with ratio of the light in the fixed polarization state to the other polarization state typically of the order of 10:1. In addition, the polarization state is not always stable. For example, the polarization direction can often flip by 90° as current is increased beyond threshold and as the laser switches between modes.
In addition to these instabilities, the polarization tends to be randomly oriented from device to device. This is due to the circular symmetry of typical VCSEL designs. Various methods have been applied to lock the polarization direction by adding some asymmetry to the cavity such as elliptical current confinement, or elliptical optical confinement, or both. However, these solutions have met with limited success in that the percentage of finished devices that remain locked in the desired polarization direction is significantly less than 100%. In addition, the beam shape is altered by some of the methods from the desirable circular output beam into a less desirable elliptical shape.
Broadly, it is the object of the present invention to provide an improved VCSEL.
It is a further object of the present invention to provide a VCSEL in which the output light is locked into one polarization state.
It is a still further object of the present invention to provide a VCSEL in which the polarization state is the same for all devices.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
SUMMARY OF THE INVENTION
The present invention is a Vertical Cavity Surface-Emitting Laser (VCSEL) assembly in which the polarization is locked to a specified direction that is the same for all VCSELs. A VCSEL according to the present invention includes a VCSEL having a top mirror region, a bottom mirror region, a light generation region between the top and bottom mirror regions, a conducting substrate and a bottom electrode. The bottom mirror region is sandwiched between the conducting substrate and the light generation region, and the conducting substrate is sandwiched between the bottom electrode and the bottom mirror region. The assembly also includes a mounting substrate having top and bottom surfaces, the VCSEL being mechanically coupled to the mounting substrate. The mounting substrate includes a means for defining a first axis. The assembly includes a means for causing the mounting substrate to flex about the first axis thereby inducing a strain in the light generation region which locks the polarization into a mode determined by the first axis. In the preferred embodiment of the present invention, the first axis is defined by a channel in the mounting substrate. The mounting substrate is caused to flex by the application of an adhesive layer applied between the mounting substrate and a mounting surface. The adhesive layer has a thermal coefficient of expansion different from the mounting substrate. In one embodiment, the adhesive layer includes a solder layer that fills the channel. Upon cooling, the solder layer contracts causing the mounting substrate to flex. In another embodiment, trenches whose direction defines the first axis are located in the top mirror region of the VCSEL.
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Babic Dubravko I.
Corzine Scott W.
Tan Michael R. T.
Yuen Albert T.
Agilent Technologie,s Inc.
Davie James W.
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