Coherent light generators – Particular beam control device – Modulation
Patent
1989-10-16
1991-04-30
Gonzalez, Frank
Coherent light generators
Particular beam control device
Modulation
372 8, 372 46, 372 50, H01S 3103
Patent
active
050124749
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the controlled switching of the polarisation states in the optical output of a semiconductor optical device.
The radiation used in optical communications is not necessarily in the visible part of the electromagnetic spectrum and the words "optical" and "light" when used in this specification are not to be interpreted as implying any such limitation. For example, longer wavelengths are preferred for transmission through silica optical fibres because the loss minima occur in such fibres in the infra-red region, at 1.3.mu.m and 1.55.mu.m approximately.
Electronically controlled polarisation modulators or switches are important components for high-speed optical communications and data transmission systems. Currently bulk and waveguide polarisation switches based on the use of the electro-optic effect are the most commonly used passive devices.
Polarisation bistability in semiconductor lasers shows promise for use in all-optical signal processing systems, in routing systems and in optical computing and has aroused much interest. Polarisation bistability arises from the ability or certain types of diode lasers to emit light with either its transverse electric field polarised parallel to the junction plane (TE) or with its transverse magnetic field polarised parallel to the junction plane (TM), depending on relative mode losses determined by the device structure. In most semiconductor diode lasers, the TE mode sees a higher reflectivity at the facets than the TM mode, resulting in predominantly TE polarised light output. However, polarisation switching has been achieved by varying the relative TE-TM mode cavity losses, for example by generating stress inside the active region through the application of external pressure. Y C Chen and J M Liu have reported (Applied Physics Letters, Volume 45, Number 6, 15 Sept. 1984, Pages 604 to 606) polarisation switching in InGaAsP/InP buried heterostructure lasers operating near their polarisation transition temperature of -68.degree. C. Chen an Liu report that polarisation switching of the laser output is achieved by a relative change in the net gain of the TM and TE modes through a small perturbation (of the order of 1.degree. C.) of the junction temperature induced by the injection current. Although Chen and Liu suggest that with sufficient internal strain in the device's active layer the polarisation transition occurs at room temperature, and indicate that for InGaAsP/InP lasers the strain can be created easily by introducing a small amount (of the order of 10.sup.31 4) of the lattice mismatch between the InGaAsP active layer and the InP cladding layers and InP substrate, neither they nor anyone else appear to have been able to achieve such current driven polarisation switching at room temperature.
Polarisation switching has also been induced by the injection of TM polarised light from a master laser into a TE-emitting slave laser causing the latter to switch to a TM-emitting state, but switch-back and locking of the TE mode have not been observed (A Sapia, P Spano. and B Daino, Applied Physics Letters, Vol 50, No 2, 12 Jan. 1987, pages 57-59).
Mori, Shibtat and Kajiwara of Matsushita have reported (extended abstracts of 18th, 1986, International conference on solid state devices and materials, Tokyo, pp 723-724) what appears to be polarisation bistability controlled by applied current in an InGaAsP buried heterostructure laser simultaneously injected with a 0.7 mW TM wave from a similar laser.
To date, despite their tremendous commercial importance neither optically triggered bistable switching nor room temperature direct polarisation switching have been reported.
According to a first aspect, the present invention provides a bistable polarisation source comprising a semiconductor injection laser capable of providing optical gain in two distinguishable waveguiding paths, the waveguding paths extending substantially parallel to one another at a small enough separation to enable the fields of optical radiation in the two paths to overlap, the
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Linton Richard S.
White Ian H.
British Telecommunications public limited company
Gonzalez Frank
Hansen Galen J.
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