Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1990-11-15
1992-04-21
Eisenzopf, Reinhard J.
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359156, 359181, 359173, H04B 1000, H04J 100
Patent
active
051073587
DESCRIPTION:
BRIEF SUMMARY
The invention relates to methods and apparatus for transmitting information.
It is well known that conventional coherent optical detection systems only give optimum performance when the signal and local oscillator polarisations are identical. For practical systems this represents a problem because an optical signal propagating through a conventional single-mode fibre undergoes random polarisation changes and this causes the received polarisation state to be both indeterminate and time varying. To date this problem has been overcome using a variety of techniques including polarisation tracking receivers, polarisation diversity receivers, and polarisation scrambling.
It is a known property of many transmission media, including optical waveguides such as optical fibres, that although random changes in absolute polarisation may take place during transmission through the medium, these random variations are the same for each signal so that a pair of signals which are initially orthogonally polarised will remain orthogonal.
An article entitled "A polarization-insensitive coherent lightwave system using wide-deviation FSK and data-induced polarization switching" by L. J. Cimini et al, Electronic Letters Vol 23 December 1987 pp 1365-1366 discloses a polarisation-insensitive technique taking advantage of the stable orthogonal relationship of two polarisation states in which the frequency shift in an FSK signal induces polarisation switching by introducing a passive device with high birefringence in the path of the transmitted signal. Thus, as the signal switches from one frequency to another, its polarisation switches between mutually orthogonal polarisation states. However such a technique is useful only with FSK modulation.
In accordance with one aspect of the present invention, a method of transmitting information comprises launching a pair of orthogonal polarised optical signals having different frequencies, each of which is identically modulated with the information to be transmitted, into an optical waveguide which causes the signals to undergo random polarisation changes during propagation through the waveguide and detecting said signals after propagation through the waveguide by means of a coherent optical receiver.
Thus, at the receiver, irrespective of the polarisation of the local oscillator signal, two electrical interference signals will be generated after the optical mixing process with their amplitudes varying in antiphase only one of which can be zero at a given time.
In accordance with a second aspect of the present invention, an optical network comprises signal generating means for generating a pair of orthogonally polarised signals having different frequencies and being modulated in accordance with the information to be transmitted; and transmitting means for transmitting the modulated signals with the portions of each signal modulated with the same information being transmitted together.
The invention may be used with a wide variety of different types of signal but is particularly suitable for use with signals having frequencies within the optical band. In that case, the transmission medium will comprise an optical waveguide such as an optical fibre.
In one example, the signal generating means comprises carrier signal generating means for generating signals with two frequencies, combining means for combining the two signal with the polarisation of one signal orthogonal to the other, and modulating means for modulating the signals in accordance with the information to be transmitted.
The modulating means may be positioned downstream of the combining means although it would also be possible to provide modulating means for modulating each of the two signals separately, the signals subsequently being combined.
In a second example, the signal generating means comprises carrier signal generating means for generating two carrier signals with different frequencies, signal coupling means for coupling the two signals, the coupled signals being fed to the modulating means, and a birefringent device downstre
REFERENCES:
patent: 2707749 (1955-05-01), Mueller
patent: 3457414 (1969-07-01), Ragen et al. 3
patent: 5008958 (1991-04-01), Cimini, Jr. et al.
Electronics Letters, vol. 24, No. 6, 17 Mar. 1988 (Stevenage GB), L. J. Cimini et al., "Polarisation-Insensitive Coherent Lightwave System Using Wide-Deviation FSK and Data-Induced Polarisation Switching", pp. 358-360.
(a) Hodgkinson, "Receiver Analysis for Synchronous Coherent Optical Fibre Transmission Systems", J. Lightwave Tech., vol. LT-5, No. 4, Apr. 1987, pp. 573-586.
(b) Harmon et al., "Polarization Control in a Coherent Optical Fibre System Using a Dither Technique", SPIE, vol. 734, Fibre Optics '87 (1987), pp. 63-67.
(c) Hodgkinson et al., "Polarization-Insensitive Hetrodyne Detection Using Polarization Scrambling", Elec. Ltrs., vol. 23, No. 10, 7 May 1987, pp. 513-514.
(d) Kersey et al., "New Polarization-Insensitive Detection Technique for Coherent Optical Fibre Hetrodyne Communications", Elec. Ltrs., vol. 23, No. 18, 27 Aug. 1987, pp. 924-925.
(e) Glance, "Polarization Independent Coherent Optical Receiver", J. Lightwave Technology, vol. Lt-5, No. 2, Feb. 1987, pp. 274-276.
(f) Tzeng, "Polarization-Insensitive Coherent Receiver Using a Double Balanced Optical-Hybrid System", Elec. Ltrs., vol. 23, No. 22, 22 Oct. 1987, pp. 1195-1196.
(g) Kreit et al., "Polarization-Insensitive Optical Hetrodyne Receiver for Coherent FSK Communications", Elc. Ltrs., vol. 23, No. 4, 12 Feb. 1987, pp. 168-169.
(h) Cimini et al., "A Polarization-Insensitive Coherent Lightwave System Using Wide-Deviation FSK and Data-Induced Polarization Switching", AT&T, Bell Labs, PD18-1 through PD18-4, Elec. Ltrs, 1988.
Cook Andrew R.
Hodgkinson Terence G.
British Telecommunications public limited company
Eisenzopf Reinhard J.
Negash Kinfe-Michael
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