1988-12-14
1991-04-09
Miller, Stanley D.
350332, 350347E, 350352, 350403, G02F 113
Patent
active
050059521
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to polarisation controllers.
2. Description of Related Art
Polarisation controllers are used in optical communication systems utilising monomode optical fibres. In such systems, in order to enhance the receiver sensitivity compared to the direct detection methods used at present in optical fibre systems, it is desirable to use coherent detection techniques. Such techniques however require the matching of the state of polarisation of the optical signal at the output of the transmission fibre to that of the local oscillator beam at the receiver. The problem arises for long lengths of optical fibres, i.e. kilometres, that the state of polarisation of the light emerging from the fibre will vary with time due to the inherent birefringence of the fibre, defects and strains either inherent or introduced by bending etc. of the fibre, temperature and pressure changes along the length of the fibre etc.
Thus, for example, a linear polarised input beam will emerge from the fibre with elliptical polarisation.
Polarisation controllers must therefore be used in such systems in order to match the state of polarisation of the output and local oscillator beams. A number of polarisation controllers have been proposed over the years, a review of such controllers being given in Journal of Lightwave Technology, volume LT-3, No. 6 published in December 1985. These include electromagnetic fibre squeezers, rotatable fibre coils, Faraday rotators, electro-optic crystals, phase plates and rotatable fibre cranks. All these proposed polarisation controllers suffer from disadvantages, however, the fibre squeezers, 1 rotatable fibre coils, Faraday rotators and electro-optic crystals not being capable of coping with endless, unbounded variations in the state of polarisation, whilst the phase plates and rotatable fibre cranks suffer from a slow temporal response. Whilst electro-optic crystals, for example pairs of lithium niobate crystals, have been used in practical systems, these suffer from the additional disadvantage of high operating voltages, typically 160 to 260 volts, and the long optical path length through the crystals leading to high insertion losses of typically 3 to 6 dB.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a polarisation controller suitable for use in an optical communication system wherein some of the disadvantages inherent in previously proposed polarisation controllers are at least alleviated.
According to a first aspect of the present invention a polarisation controller comprises at least one stack of nematic liquid crystal cells arranged such that radiation incident on the stack will pass through each cell in the stack in sequence; and means for applying synchronised fields across the cells so as to change the phase retardation of the radiation transmitted through each cell by a chosen amount, the optical axes of the cells being oriented with respect to each other such that the polarisation of radiation incident on the controller is caused by the controller to change from a first state to a second predetermined state.
According to a second aspect of the present invention a polarisation controller for converting the polarisation state of incident radiation of a first arbitrary polarisation state to a second arbitrary polarisation state comprises a stack of three nematic liquid crystal cells arranged such that the incident radiation will pass through the cells in sequence, the slow axes of the first and last cells within the stack being either parallel or perpendicular to each other, the slow axis of the intermediate cell being inclined at 45.degree. to the slow axis of the first cell; and means for applying synchronised fields across said cells so as to produce the required polarisation conversion.
According to a third aspect of the present invention a polarisation controller for converting the polarisation state of incident radiation the polarisation of which is continuously varying to a predetermine
REFERENCES:
patent: 3558214 (1971-01-01), de Lang et al.
patent: 3558215 (1971-01-01), de Lang et al.
patent: 3753608 (1973-08-01), Bernal
patent: 3890628 (1975-06-01), Gurtler
patent: 3956626 (1976-05-01), Ross
patent: 4394069 (1983-07-01), Kaye
patent: 4443065 (1984-04-01), Funada et al.
patent: 4466702 (1984-08-01), Wiener-Avnear et al.
Bruno Rossi, Optics, Addison-Wesley, 1957 pp. 269-291.
"Polarization-State Control Schemes for Heterodyne or Homodyne Optical Fiber Communications", by Takanori Okoshi, in Journal of Lightwave Technology, vol. LT-3, No. 6, Dec. 1985, pp. 1232-1236.
Clark Michael G.
Samuel Ifor D. W.
Gross Anita Pellman
Miller Stanley D.
The General Electric Company p.l.c.
LandOfFree
Polarization controller does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Polarization controller, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polarization controller will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2030359