Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
2002-10-15
2004-10-05
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S477000
Reexamination Certificate
active
06801320
ABSTRACT:
BACKGROUND OF THE INVENTION
Typically, optical polarization state measurement methods are based on measurements of the individual Stokes vector components, i.e., measurements of the optical power transmitted through 0° linear, 45° linear and circular polarizers. The analogous technique has been proposed in the heterodyne architecture by I. Roudas et al. in “Coherent heterodyne frequency-selective polarimeter for error signal generation in higher-order PMD compensators,” OFC 2002, pp. 299-301, where the polarization state of the local oscillator is sequentially switched between 0° linear, 45° linear and circular polarization states to provide heterodyne measurements of the signal amplitude in the selectable polarization states. The polarization switching that is required slows down the polarization measurement process.
Another heterodyne technique determines the polarization state by determining the amplitude and relative phase of the two detected heterodyne signals. This technique has been used by K. Oka et al., “Evaluation of phase fluctuations of orthogonal optical eigen modes guided in an axially vibrating birefringent single-mode fiber”, Journal of Lightwave Technology, Vol. 8, No. 10, 1482-1486, 1990 to determine fiber birefringence and by C. Chou et al., “Amplitude sensitive optical heterodyne and phase lock-in technique on small optical rotation angle detection of chiral liquid”, Japanese Journal of Applied Physics, Part 1, Vol. 36, No. 1A, 356-359, 1997 to measure optical activity in chiral liquids at fixed optical frequencies. However, this heterodyne technique has not been used with swept local oscillator sources.
SUMMARY OF THE INVENTION
An optical heterodyne system is inherently sensitive to the polarization of the heterodyned signals. In accordance with the invention, a polarization state is measured by using a polarization diversity receiver employing a polarization beam splitter to output two heterodyne signals. The amplitude and relative phase of the two detected heterodyne signals uniquely determine the polarization state. However, a problem arises when the local oscillator (LO) is swept over a frequency range and not kept at a fixed frequency. The polarization state is no longer uniquely determined but jumps between the hemispheres of the Poincare sphere creating a polarization state ambiguity.
Modification of the detection method and apparatus in accordance with the invention eliminates the polarization state ambiguity arising from the two images that result from the mixing process thus allowing unambiguous determination of the polarization state.
REFERENCES:
patent: 6671056 (2003-12-01), Szafraniec
Heismann, Fred et al., “Automatic Compensation of First-Order Polarization Mode Dispension in a 10Gb/s Transmission System”, ECEC 98, Sep. 20-24, 1998, Madrid, Spain, pp. 529-530.
Sunnerud, Henrik et al., “A Comparison Between Different PMD Compensation Techniques”, Journal of Lightwave Technology, vol. 20, No. 3, Mar. 2002, pp. 368-378.
Poti, Luca et al., “Experimental Demonstration of a PMD Compensator with a Step Control Algorithm”, IEEE Photonics Technology Letters, vol. 13, No. 12, Dec. 2001, pp. 1367-1369.
Song, Shuxian et al., “A Poincare Sphere Method for Measuring Polarization-Mode Dispersion Using Four-Wave Mixing (FWM) in Single-Model Optical Fiber”, pp. 79-82.
Calvani, Riccardo et al., “Real-Time Heterodyne Fiber Polarimetry with Narrow- and Broad-Band Sources”, Journal of Lightwave Technology, vol. LT-$, No. 7, Jul. 1986, pp. 877-883.
Roudas, I. et al., “Coherent Heterodyne Frequency-Selective Polarimeter for Error Signal Generation in Higher-Order PMD Compensation”, Wednesday Afternoon, OFC 2002, pp. 299-301.
Baney, Douglas M. et al., “Coherent Optical Spectrum Analyzer”, IEEE Photonics Technology Letters, vol. 14, No. 3, Mar. 2002, pp. 355-357.
Abbas, Gregory L., et al., “A Dual-Detector Optical Heterodyne Receiver for Local Oscillator Noise Suppression”, Journal of Lightwave Technology, vol. LT-#, No. 5, Oct. 1985, pp. 1110-1122.
Agilent Technologie,s Inc.
Connolly Patrick
Krause-Polstorff Juergen
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