Telecommunications – Receiver or analog modulated signal frequency converter – Signal selection based on frequency
Patent
1988-02-19
1989-09-19
Griffin, Robert L.
Telecommunications
Receiver or analog modulated signal frequency converter
Signal selection based on frequency
455610, 455612, H04B 900
Patent
active
048688967
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention is directed to an optical superheterodyne receiver which is particularly adapted for the reception of phase shift modulated light.
DESCRIPTION OF THE PRIOR ART
Optical superheterodyne receivers are known from the Journal of Lightwave Technology, Vol. LT-3, No. 5, October, 1985, pages 1110 through 1122 and No. 6, December, 1985, pages 1238 through 1247 and from Electronics Letters, Sept. 12, 1985, Vol. 21, No. 19, pages 867 and 868. Upon utilization of a potentially modified "Costas loop", an optical superheterodyne receiver that is actually composed of two receiver branches is constructed in the second publication. The reception signal and local oscillator signal are mixed in a traditional way in the first receiver branch; in the second receiver branch, the local oscillator signal experiences a 90.degree. phase rotation before the mixing. In this case, optical directional couplers are required that are referred to as optical hybrids; a directional coupler at whose two outputs the superimposed lightwaves of the signal and of the local laser appear is referred to as a 90.degree. hybrid. One of the output signals of the 90.degree. hybrid thereby contains a component having an additional 90.degree. phase rotation. According to the first publication, the influence of the laser intensity noise can be eliminated by using a balance receiver. A phase-locked publication; however, a 90.degree. hybrid is likewise utilized, its two output signals being separately demodulated after the conversion into photo currents and being subsequently summed. Differential phase shift keying (DPSK) is thereby used.
AEU, Vol. 37 (1983), No. 5/6, pages 203 through 206 discloses the realization of 90.degree. and 180.degree. hybrid branching for optical signals having wavelengths of about 10 .mu.m. The known 90.degree. and 180.degree. hybrids are thereby realized with discrete optical elements, so that an extremely involved structure derives overall. A proposal for a 180.degree. hybrid in integrated optics is also known from the publication cited above, the light path therein being construsted on a lithium niobate substrate.
Presentation TUB5 of D W. Stowe at the Optical Fiber Conference, New Orleans 1983, discloses that an optical 90.degree. hybrid may be realized coupling by four 2.times.2 couplers; the demands made of the monitor of the optical path length between the couplers, however, are extremely high.
90.degree. hybrids composed of a 3.times.3 fiber coupler having connected photodiodes are known from the work of R. G. Priest, IEEE Journal of Quantum Electronics, Vol. QE-18 (1982), pages 1601 through 1603, of K. P. Koo, A. B. Tveten and A. Dandridge in Appl. Phys. Lett. 41 (7), Oct. 1, 1982, pages 616-618 and of A. Dandridge et al in Intermat. Conference on Optical Fiber Sensors, London 1983, pages 48-52 relating to sensor technology and, in particular, to fiber interferometers. A symmetrical 3.times.3 coupler having a respective 120.degree. phase shift between the outputs is used for the realization of the 90.degree. hybrid The theoretical derivation of the principle by R. G. Priest is based on an ideal, loss-free coupler.
SUMMARY OF THE INVENTION
The object of the present invention is comprised in developing an optical receiver of the species initially cited that, given little outlay, is exployable in optical communications transmission with light waveguides and which eliminates the need for an intermediate frequency amplification.
This object is inventively achieved by an optical superheterodyne receiver of the species initially recited which utilizes a triple optical coupler. The outputs of the triple optical coupler are manipulated to provide a data signal and control signal. By virtue of such manipulation, the control signal is devoid of a D.C. component. The control signal may thus be used to more accurately control the local laser.
By contrast to the known 90.degree. hybrids of symmetrical 3.times.3 fiber couplers, the inventiion also advantageously enables the employment
REFERENCES:
patent: 4718121 (1988-01-01), Epworth
patent: 4732447 (1988-03-01), Wright et al.
Smith; "Techniques for Multigigabit Coherent Optical Transmission"; Journal of Lightwave Technology; vol. LT-5; No. 10; Oct. 1987; pp. 1466-1478.
Abbas et al.; "A Dual-Detector Optical Heterodyne Receiver for Local Oscillator Noise Suppression"; Journal of Lightwave Technology; vol. LT-3; No. 5; Oct. 1985; pp. 1110-1122.
Kazovsky; "Balanced Phase-Locked Loops for Optical Homodyne Receivers: Performance Analysis; Design Considerations; and Laser Linewidth Requirements"; Journal of Lightwave Technology; vol. LT-4; No. 2; Feb. 1986; pp. 182-195.
Dandridge et al.; "Fiber Optic Interferometric Sensor Development at NRL"; Naval Research Laboratory; Washington, D.C.; International Conference on Optical Fibre Sensors; Apr. 26-28, 1983; London, IEEE; pp. 48-52.
Davis et al.; "Coherent Optical Receiver for 680 Mbit/s Using Phase Diversity"; Electronic Letters; Jan. 1986; vol. 22; No. 1; pp. 9-11.
Hodgkinson et al.; "Demodulation of Optical DPSK Using In-Phase and Quadrature Detection"; Electronic Letters; Sep. 1985; vol. 21; No. 19; pp. 867-868.
Leeb, "Realization of 90.degree.-and 180.degree. Hybrids for Optical Frequencies", 37 AEV 203 (1983).
Priest, "Analysis of Fiber Interferometer Utilizing 3.times.3 Fiber Coupler", IEEE Journal of Quantum Electronics, vol. QE-18, No. 10 (10/82).
Stowe, "Passive Quadrature Demodulation for Interferometric Sensors", Optical Fibre Conference, New Orleans (1983).
Griffin Robert L.
Siemens Aktiengesellschaft
Telesz Jr. Andrew J.
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