Coherent light generators – Particular component circuitry – Optical pumping
Patent
1988-07-06
1991-05-28
Arnold, Bruce Y.
Coherent light generators
Particular component circuitry
Optical pumping
372100, 372101, 372102, G02B 518, H01S 308
Patent
active
050188313
DESCRIPTION:
BRIEF SUMMARY
This invention relates to tunable optical resonant devices and in particular but not exclusively to long external cavity lasers.
In this specification, the term optical is intended to include those parts of the electromagnetic spectrum which is generally known as the visible region together with that part of the infra-red and ultra-violet regions at each end of the visible region.
For coherent optical communication systems it is desirable to have narrow line width laser sources while gas lasers are examples of such narrow line width laser sources, it is generally necessary to filter the output of semiconductor lasers which typically have broad linewidths of a few magahertz. Such filtration is wasteful and it is preferable to stimulate the laser with feedback of a particular wavelength in order to narrow the output linewidth. One of the ways of introducing selected feedback is by a distributed feedback laser, but at present the linewidths achievable are not satisfactory. Narrower linewidths of the order of 100 KHz have been achieved with long external cavity lasers where the light output from one facet of a semiconductor laser is collimated and reflected from a grating back through the collimating lens onto the laser facet. This feedback stimulation optimises further emmision of the same wavelength. An example of a tunable long external cavity laser is described in Electronics Letters 1983, 19, pp 110-112.
Typically the long external cavity laser consists of a semiconductor laser source, a collimating lens and a grating. Coarse tuning of the reflected wavelength is achieved by rotating the grating to alter the effective grating spacing and fine tuning is provided by adjustment of the path length between the laser and the grating. The path length may be altered by moving the grating closer to or further from the laser or by rotation of a tuning plate in the optical path. Rotation of such a tuning plate, which consists of a transparent plate of different optical density to air, causes the light to pass through at a greater or lesser angle and therefore to be in the plate for a different path length. An arrangement of this type is shown in FIG. 1.
The disadvantage of known long external cavity lasers is that there are three or four separate components each requiring alignment on the optical axis and a need to maintain that alignment during separate adjustments to the grating or grating and tuning plate. In the case of the three element system (no tuning plate) it is necessary to provide a mechanism capable of both rotating and translating the grating or in the case of the four element arrangement two rotating mechanisms are required.
One method of avoiding the need for a rotatable grating is described in a paper entitled "A proposal for wavelength-tuning and stabilization of GaAs lasers with a graded index fibre segment in a dispersive cavity"by J. C. Vanderleeden Opto-electronics, vol 6, no. 6, 1974 PP443-449. A quarter pitch graded-index (GRIN) lens has deposited on one end a grating perpendicular to its central optical axis. A laser diode is positioned in contact with the other end of the lens and laterally offset from the optical axis so that light from the laser diode impinges on the diffraction grating at an angle thereby providing the required selective feedback.
One disadvantage of this arrangement is that the laser diode must be offset a substantial distance from the optical axis of the lens. The acceptance solid angle of the lens decreases with increasing distance from the optical axis which for a lens with marginal f-number can increase the laser light losses, but also because a smaller area of the diffraction grating is then illuminated which adversely affects the resolution. Lens aberrations also increase as the offset increases.
The present invention is directed towards minimising the number of components that require alignment and to simplifying the tuning adjustment mechanisms in optical resonant cavity devices while permitting the optical source to be positioned at or close to the optical axis
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Cameron Keith H.
Wyatt Richard
Arnold Bruce Y.
British Telecommunications public limited company
Shafer R. D.
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