Coherent light generators – Particular active media – Semiconductor
Patent
1988-10-04
1990-10-16
Davie, James W.
Coherent light generators
Particular active media
Semiconductor
372 96, H01S 319
Patent
active
049641344
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor laser structures and finds application in optical communications.
2. Description of Related Art
The radiation used in optical communications is not necessarily strictly in the visible region. If silica optical fibres are used as the transmission medium, infra-red radiation is of special usefulness since the loss minima occur in such fibres at 1.3 .mu.m and 1.55 .mu.m approximately. (The above wavelengths are in vacuo wavelengths as are all wavelengths herein except where otherwise specifically stated).
Semiconductor lasers are known which will emit optical radiation at wavelengths including the above. In general a semiconductor laser structure will include a p-n junction across which a driving current can be applied (the conventional current from p to n), an "active region" in which electrons and holes combine, producing photons by stimulated emission, and a feedback structure to cause oscillation of radiation in the active region. The active region has to relate suitably in band gap and refractive index to the other semiconductor regions of the structure in order to achieve a suitable degree of "confinement" of these processes to the active region. Lasers operating to produce optical radiation in the wavelength range from 1.1 to 1.65 .mu.m, especially from 1.3 to 1.6 .mu.m, usually comprise regions of indium phosphide and of the quaternary material (In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y). By suitable choices of x and y it is possible to lattice-match the various regions while varying the band gaps of the materials. (Band gaps can be determined experimentally by for instance photoluminescence). Additionally, both indium phosphide and the quaternary materials can be doped to p- or n- type as desired.
Semiconductor lasers comprising regions of gallium arsenide and gallium aluminium arsenide are also used for communications purposes. They operate near to 0.9 .mu.m.
The feedback structure might comprise for instance reflective end facets of the laser (Fabry-Perot or FP lasers). Alternatively it might comprise a grating of corrugations which lie near the active region (distributed feedback or DFB lasers). The corrugations may be shifted to a position external to the laser structure, adjacent the emission axis (a distributed Bragg reflector or DBR laser), or a reflective interface may be used in a cavity external to the laser.
Semiconductor lasers may vary otherwise in structure. For instance a double heterostructure (DH) laser may comprise an (initially undoped) active region of a first material, lying between a lower n-doped layer and an upper p-doped layer each of a different material. In a multiple quantum well (QW) laser, the active region comprises one or more very thin layers of active material, and where a plurality of such layers are provided, they are separated by barrier layers. The thin active layers are each so thin that that dimension is comparable to the De Broglie wavelength for electrons, and the discreteness of the density distribution of energy states in the active material becomes apparent. Quantum well layers have thicknesses (known as well width) typically of the order of 10 nm and the barrier layers may be still thinner.
Semiconductor laser structures are used in devices other than optical sources. For instance, if the driving current applied is less than a lasing threshold driving current, a laser structure may still be useful as an optical amplifier, amplifying an optical radiation input signal. Further, a laser structure may be used as an absorption modulator, a device coupled to a laser output port which can be switched between optically opaque and transparent conditions with respect to the laser output and so modulate it.
Laser structures vary considerably from each other in a number of operating characteristics. A known characteristic, which affects the modulation performance of directly modulated lasers, as well as laser emission linewidth, is the linewidth enhancement factor ".alp
REFERENCES:
patent: 4257011 (1981-03-01), Nakamara et al.
Electronics Letters, vol. 21, No. 13, 20 Jun. 1985, (Stevenage, Hertz, GB), N. K. Dutta et al.: "Fabrication and Performance Characteristics of InGaAsP Ridge-Guide Distributed-Feedback Multi-Quantum-Well Lasers", pp. 571-573.
IEEE Journal of Quantum Electronics, vol. QE-21, No. 10, Oct. 1985, (IEEE, New York, US), Y. Arakawa et al.: "Theory of Gain, Modulation Reseponse, and Spectral Linewidth in AlGaAs Quantum Well Lasers", pp. 1666-1674.
Electronics Letters, vol. 20, No. 1, 5 Jan. 1984, (London, GB) M. G. Burt: "Linewidth Enhancement Factor for Quantum-Well Lasers", pp. 27-29.
Electronics Letters, vol. 19, No. 6, 17 Mar. 1983, (Hitchin, Herts, GB), M. G. Burt: "Gain Spectra of Quantum-Well Lasers", pp. 210-211.
Electronics Letters, vol. 19, No. 22, 27 Oct. 1983, (Hitchin, Herts, GB), I. D. Henning et al.: "Measurements of the Semiconductor Laser Linewidth Broadening Factor", pp. 927-929.
Japanese Journal of Applied Physics, vol. 24, No. 7, part 2, Jul. 1985, (Tokyo, JP), N. Ogasawara et al.: "Linewidth Enhancement Factor in GaAS/AlGaAs Multi-Quantum-Well Lasers", pp, L519-L521.
Applied Physics Letters, vol. 42, No. 8, 15 Apr. 1983, American Institute of Physics, (New York US), K. Vahala et al.: "On the Linewidth Enhancement Factor .alpha. in Semiconductor Injection Lasers", pp. 631-633.
Green, C. A., Dutta, N. K.; Watson. W., Applied Physics Letters, vol. 50, No. 20, pp. 1409-1410, 1987.
Uomi, K.; Ohtoshi, T.; and Chinone N., Japan Journal Applied Physics, 24 L539, 1985.
Arakawa, Y,; Vahala, K.; Yariv, A., Applied Physics Letter vol. 45, p. 950, 1984.
Arakawa, Y.; Yariv, A., IEEE J. Quantum Electronics, QE-22, p. 1887, 1986.
Henry, C. H.; IEEE J. Quantum Electronics, QE-18(2), Feb. 1982, pp. 259-264.
Asada, M.; Kaneyama, A.; Suematsu, Y;-IEEE J. Quantum Electronics, QE-20, Jul. 1984.
Uomi, K.; Ohtoshi, T.; and Chinone, N.; "Ultra High Relaxation Oscillation Frequency (-50 GHz) in Modulation Doped Multiquantum Well (MD-MQW) Lasers: Theoretical Analysis", Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo, 185, pp. 184-185.
Adams Michael J.
Westbrook Leslie D.
British Telecommunications public limited company
Davie James W.
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