Coherent light generators – Laser logic system
Patent
1988-08-10
1989-10-10
Scott, Jr., Leon
Coherent light generators
Laser logic system
372 96, H01S 330
Patent
active
048736902
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an optical switch. It finds particular application in optical logic and signal processing.
It is known to use semiconductor optical devices as switches in optical logic and signal processing. They are advantageous in that they can be designed to operate at low power levels, physically take up little space in a signal processing system, operate at wavelengths compatible with those common in optical communications, and potentially can be monolithically integrated with other optical components.
Either passive or amplifying devices can be used as switches. The inherent gain of an amplifying device, such as a laser, reduces the need for additional amplification in a system and therefore gives amplifying devices an advantage compared with passive devices. Further, amplifying devices can be designed to switch at power levels typically of the order of 10.sup.3 times lower than those required for passive devices, and are readily available.
A semiconductor laser commonly comprises a wafer grown from materials containing combinations of elements from the III and V groups of the Periodic Table. The layers of the wafer are selectively doped to provide a p-n junction, in the vicinity of which lies an active region. Photons can be generated in the active region by radioactive recombination of electron-hole (carrier) pairs under a driving current applied across the junction. By variation in the refractive index of the wafer materials and/or by control of the current distribution in the photodiode, the generated photons are guided to move in a waveguiding region along the photodiode. Feedback is provided to the waveguiding region for instance by reflective end facets of the laser (a Fabry-Perot laser) or by corrugations in an interface which lies near the active region (an example of a distributed feedback laser).
A factor in the choice of materials for optical devices is the fact that silica optical fibres, widely used in today's 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 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 optical switches with such favourable chracteristics 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.
If optical radiation is input to the active region of a semiconductor laser and a driving current applied, amplification of the radiation occurs even when the driving current is below the lasing threshold current necessary for lasing action to occur. The relationship between input and output radiation intensity is non-linear and can show bistability, the output intensity switching rapidly between two values as the input intensity reaches a relevant switching level. The non-linearity arises from changes in the refractive index of the material of the active region. The input radiation in undergoing amplification reduces the free carrier concentration and hence the gain. The refractive index varies with the gain according to the Kramers-Kronig relationship. In turn, the degree of amplification of the input radiation is dependent on a relationship between input wavelength and the refractive index of the active region material. Hence if the refractive index changes but the input wa
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Olsson et al., "Optoelectronic Logic Operations by Cleaved-Coupled Cavity Semiconductor Lasers", IEEE Journal of Quantim Electronics vol. QE-19, No. 11 Nov. 1983, pp. 1621-1625.
Sharfin et al., "Femtojoule Optical Switching in Nonlinear Semiconductor Laser Amplifiers"Applied Physics Letters, vol. 48, No. 5, 3 Feb. 1986.
British Telecommunications public limited company
Jr. Leon Scott
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