Coherent light generators – Laser logic system
Patent
1988-08-11
1990-10-09
Scott, Jr., Leon
Coherent light generators
Laser logic system
372 38, 372 31, 372 96, H01S 330
Patent
active
049625020
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an optical signal processing device. It finds particular application in optical logic and communications systems, for instance as an optical signal limiter, or as a noise filter.
It is sometimes useful in optical logic or communications systems that the intensity of an optical signal is limited to a maximum value which is at least substantially constant. Such a limited signal could for instance be used as a bias signal to ensure reproducable peak laser power in pulsed laser systems, to control pulse shapes, as a digital input signal which avoids saturating a photodetector, or to equalise signal amplitudes in a multiplex system. Further, a device which limits optical signal intensities can operate as a noise filter since modulation superimposed on an optical signal can be cut out.
It is known to use semiconductor optical devices in optical logic and signal processing. They are advantageous in that they can be designed to operate at the order of low power levels, for instance at about 10 mW which might be available in optical logic and signal processing. They physically take up little space, operate at wavelengths compatible with those common in optical logic and communications, and potentially can be monolithically integrated with other optical components.
A factor in the choice of materials for optical devices is the fact that silica optical fibers, which are the basis of present optical communications systems, have loss minima at 0.9 .mu.m, 1.3 .mu.m and 1.55 .mu.m approximately. Accordingly there is an especial need for devices which show favourable characteristics when operated using optical radiation in the wavelength range from 0.8 to 1.65 .mu.m, and especially in the ranges from 0.8 to 1.0 .mu.m and from 1.3 .mu.m to 1.65 .mu.m. (These wavelengths, like all the wavelengths herein except where the context indicates otherwise, are in vacuo wavelengths). Materials which have been found suitable for the manufacture of devices which show such favourable characteristics comprise the III-V semiconductor materials, including gallium arsenide, indium gallium arsenide, gallium alluminium arsenide. indium phosphide and the quaternary materials, indium gallium arsenide phosphides (In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y). With regard to the quaternary materials, by suitable choices of x and y it is possible to lattice-match regions of different ones of these materials to neighbouring III-V materials in a device while being able to select the associated band gap equivalent wavelength.
A known device for use in conjunction with a laser to shape pulses is a non-linear Fabry-Perot (FP) etalon. A FP etalon is a simple optical cavity which can be fabricated from semiconductor materials, having planar reflecting endfaces of about 30.DELTA. reflectivity, and (optionally) waveguiding properties. Limiting action is achieved by tuning the peak output of the etalon at low signal input to the source laser frequency as the input intensity rises, the etalon detunes, keeping the transmitted power approximately constant.
Alternatively, passing a signal through a non-linear medium can be used to limit the signal. It is known to use a device wherein the far field of the output signal spreads at increasing input powers. By using an aperture stop, the increasing input powers can be at least approximately balanced out.
Another device suggested but not demonstrated for use as an optical signal limiter is a multiple quantum well guided wave structure, also fabricated from semiconductor materials. Such a device is discussed in "Nonlinear Guided Wave Applications", Optical Engineering 24 (4) 1985, by C. T. Seaton, Xu Mai, G. I. Stegeman and H. G. Winful. Optical limiting action is obtained because either one or both of the bounding media are characterized by a self-defocusing nonlinearity.
However, known devices tend to suffer from one or more disadvantages. For instance, they might only operate at an input signal intensity higher than those suitable for optical logic or communications systems. Another dis
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British Telecommunications public limited company
Jr. Leon Scott
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