Coherent light generators – Particular resonant cavity – Mirror support or alignment structure
Patent
1989-10-23
1991-10-15
Sikes, William L.
Coherent light generators
Particular resonant cavity
Mirror support or alignment structure
372 20, 372 33, 372 34, 372 99, 372101, 372102, 357 74, 359224, H01S 3101
Patent
active
050581244
DESCRIPTION:
BRIEF SUMMARY
This invention relates to semiconductor device assemblies, and in particular to external cavity semiconductor laser packages and laser amplifier packages suitable for incorporation into an optical communications system, and to apparatus for electrically controlling movable optical processing elements within a device package.
Coherent optical communications systems potentially offer greater sensitivity, but for detection in such systems it is necessary to have a continuously tunable, narrow line width, single mode source. Such a source is an external cavity laser in which one semiconductor laser facet is anti-reflection coated to reduce internal feedback and a selected wavelength is reflected back into the laser in order to stimulate dominance of that wavelength in the laser output. The reflection is usually from an externally located diffraction grating forming the external cavity. In order to tune to different wavelengths the grating can be rotated to present a different period or the grating can be moved longitudinally to alter the cavity length. When the grating is rotated the passband of reflected wavelengths changes and thus if rotated sufficiently the passband may become centred on a different mode or become centralised between two modes. Changing the cavity length alters the mode positions, and so a mode centred on the grating passband will move over the passband with variation in longitudinal grating position. Thus it is possible by simultaneous rotation and longitudinal movement to shift the wavelength of the grating passband (by rotation) but remain centred on the same mode (by changing the cavity length) and avoid mode hopping. It is necessary to avoid mode changes during detection because the control circuits are tuned to fine adjustment and cannot cope with the comparatively massive changes that occur with mode change.
In laboratory test equipment, adjustments to the grating position have been made manually by micrometer screw adjustment and the components are all movable and realignable. While such an arrangement is satisfactory for investigative purposes, for implementation in actual communications systems it is necessary to have automatic control, a durable package that is permanently aligned and a hermetically sealed environment for the laser. Similar problems arise in packaging receiver elements such as tunable laser amplifiers.
The present invention is directed towards providing practical packaging including elements requiring hermetic sealing, for systems use.
Accordingly a first aspect of the invention provides a device assembly comprising a light emitting element and an optical processing element, the light emitting element being disposed within a hermetically sealed package having a transparent portion for the passage of light to said optical processing element disposed outside the hermetic package, wherein said optical processing element is mounted on apparatus comprising a plurality of piezoelectric stacks arranged such that selective elongation and/or contraction of the piezoelectric stacks provides rotational and translational movement of the optical processing element.
Another aspect of the invention provides apparatus for moving an optical processing element within a device assembly, the apparatus comprising a plurality of piezoelectric stacks arranged such that selective elongation and/or contraction of the piezoelectric stacks provides rotational and translational movement of the optical processing element.
In a preferred embodiment the light emitting element is a semiconductor laser and the optical processing element is a grating defining an external cavity. Another embodiment comprises a laser amplifier with a tunable filter: the filter may be of reflection or transmission type.
The invention is now described by way of example with reference to the accompanying drawings in which:
FIG. 1 is a plan view of a first embodiment of the invention;
FIG. 2 is an end view of a grating alignment mount of the embodiment of FIG. 1;
FIG. 3 is a side view of the embodiment of FIG. 1.
FIG.
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Al-Chalabi Salah A.
Brain Michael C.
Cameron Keith H.
Mellis John
Wyatt Richard
British Telecommunications public limited company
Hansen Galen J.
Sikes William L.
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