Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2002-02-12
2004-08-31
Wong, Don (Department: 2828)
Coherent light generators
Particular resonant cavity
Specified cavity component
C257S021000
Reexamination Certificate
active
06785320
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the control of the polarisation of light emitted by Vertical Cavity Surface Emitting Lasers or VCSELs.
2. Discussion of Prior Art
Recently there has been increased interest in VCSELs because they have several potential advantages over conventional edge emitting semiconductor lasers, such as planar construction, the emission of light perpendicular to the surface of the semiconductor substrate and the possibility of fabrication in an array. Edge emitting lasers have the advantage of emitting polarised light, however they emit an elliptical beam of polarised light which requires the use of a lens to couple the elliptical beam to a circular optical fibre. By comparison VCSELs which in their simplest form have a semiconductor structure which is axially symmetric about the vertical axis of the VCSEL will emit a circular beam of light which is unpolarised. A circular beam can be directly coupled into a circular optical fibre without the use of a lens, or at least using a simplified lens structure.
For many proposed applications for VCSELs, such as sources for spatial light modulators, it is desirable, and in some cases necessary to have single mode operation of VCSELs with a well defined polarisation of light, that is, the direction of the electric field vector of the light emitted by the VCSEL has to be well defined and must not change with current or temperature. To achieve this a differential gain has to be introduced for two orthogonal polarisations of light generated in the gain region of the VCSEL.
This problem has been tackled by altering the semiconductor structure of the VCSEL from a simple axially symmetrical structure to a more complex structure in which the VCSEL comprises a waveguide which preferentially supports one orthogonal polarisation. This approach is used in JP10223973 and EP772269 and results in a more complicated semiconductor structure which can reduce the circular symmetry of the beam of light generated by the VCSEL. EP772269 also discloses the use of a non-symmetrical emission port on a VCSEL to promote one orthogonal polarisation, but again this will reduce the circular symmetry of the beam of light generated by the VCSEL. U.S. Pat. No. 5,727,014 also discloses the use of a non-symmetrical emission port on a VCSEL, which port is surrounded by an electrode of the VCSEL.
In U.S. Pat. No. 5,412,680 the active layer of the VCSEL comprises at least one strained semiconductor layer having a preferential direction of electrical conductivity along a direction parallel to the mirrors of the VCSEL so that the VCSEL emits light having a polarisation substantially parallel to this preferential direction. However, this straining of the semiconductor material adds complexity to the structure of the VCSEL. Furthermore, the strained layer must be relatively thin and such thin layers are difficult to reproduce accurately in bulk manufacture, resulting in VCSELs whose properties are not very reproducible. Alternatively, the active layer of the VCSEL can be elongated so that the polarisation of the radiation emitted by the VCSEL is parallel to the longitudinal axis of the active layer. In GB 2,311,166 a multi-layer polymeric Bragg reflector is stretched to orient polymer molecules to define a direction of polarisation.
In JP09181391 the VCSELs are grown with their axes of symmetry inclined to the vertical in order to promote the generation of one orthogonal polarisation. However, the inclined structure of the VCSELs complicates the fabrication process used to form the individual VCSELs because undercutting will be required.
In JP09283859 and JP09283860 ring electrodes on one end surface of a VCSEL are used to switch between two orthogonal polarisations.
SUMMARY OF THE INVENTION
The present invention aims to provide a VCSEL which overcomes at least some of the problems discussed above. In particular the present invention aims to provide a VCSEL which emits a circular beam of polarised light and yet which maintains a simple structure to ease fabrication.
According to a first aspect of the present invention there is provided a vertical cavity surface emitting laser (VCSEL) comprising a one dimensional grating structure located at an end of the VCSEL for selectively promoting the gain of a first polarisation of light within the VCSEL as compared to the gain of a second orthogonal polarisation of light within the VCSEL. The VCSEL will therefore tend to lase at the first polarisation.
Thus, a polarisation controlled VCSEL is provided using a standard VCSEL structure with only one additional structure added to one of its ends. Thus, fabrication of a VCSEL according to the present invention can be simplified relative to the polarisation controlled VCSELs already known in the prior art. Furthermore, the arrangement according to the present invention will not reduce the symmetry of the circular beam emitted by the VCSEL. The present invention also enables arrays of polarisation controlled VCSELs of the same polarisation to be fabricated by fabricating the same one dimensional grating structure over the entire array of VCSELs in a single processing step.
Preferably, the one dimensional grating structure is located at an end of the VCSEL as this generates a structure that is simple and relatively easy to fabricate.
In a preferred embodiment the one dimensional grating structure is located at the top end of the VCSEL. This is preferred particularly if the grating structure is made of metal because it is presently not possible to grow the layers of semiconductor material that make up a VCSEL over a layer of metal.
The polarisation controlled VCSELs according to present invention can be arranged to emit light from their top end surface or from their bottom end surface as required.
In one embodiment the one dimensional grating structure can reflect both the first and second orthogonal polarisations of light back into the cavity of the VCSEL. The gain of the first polarisation can then be selectively promoted by arranging light of the first polarisation reflected by the grating structure to interfere constructively with other light of the first polarisation reflected back into the VCSEL cavity (eg. due to the arrangement of the layers of a Bragg mirror, and/or by arranging for light of the second orthogonal polarisation reflected by the grating structure to interfere destructively with other light of the second polarisation reflected back into the VCSEL cavity.
In an alternative embodiment the one dimensional grating structure can be arranged to preferentially reflect the first polarisation of light back into the cavity of the VCSEL. The VCSEL then will tend to lase at the first polarisation which is preferentially reflected back into the cavity of the VCSEL, provided it is reflected back in such a way that it is in phase with the light of the first polarisation which is also reflected back into the laser cavity, for example by the layers of a Bragg stack mirror. This is because there will be a higher electric field intensity within the cavity, ie. a higher gain, at this first polarisation.
The one dimensional grating structure can be arranged to preferentially absorb the second orthogonal polarisation of light, so that the VCSEL lases with the first preferentially reflected polarisation of light.
The one dimensional grating structure can be arranged to preferentially transmit the first orthogonal polarisation of light with the advantage that if the VCSEL is arranged to lase with this first polarisation, light can be coupled out of the VCSEL via the one dimensional grating structure.
In embodiments of the present invention in which the VCSEL also includes a Bragg stack adjacent to the one dimensional grating structure for reflecting light back into the cavity of the VCSEL, it is preferred that the structure of the VCSEL is arranged such that the grating structure and the Bragg stack reflect the first polarisation of light back into the VCSEL cavity substantially in phase to promote constructive interferenc
Amos Richard M
Lewis Meirion F
Wilson Rebecca A
Nguyen Phillip
Nixon & Vanderhye P.C.
QinetiQ Limited
Wong Don
LandOfFree
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