Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1994-08-22
1996-07-09
Turner, Samuel A.
Optics: measuring and testing
By particle light scattering
With photocell detection
356350, G01B 902
Patent
active
055350003
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to fiber optics. More particularly, the invention relates to fiber optic interferometric detectors which employ light squeezing.
BACKGROUND OF THE INVENTION
Light of any nature, including continuous wave and pulsed light (hereinafter collectively referred to as light beams), comprises shot noise. Shot noise is quantum noise. Shot noise exists because the rate of photons in a light beam is not uniform but is random to a certain extent. The smallest increment with which the phase or amplitude of a light beam can be determined, i.e., the accuracy of phase or amplitude determination, is limited by shot noise. For example, in a fiber optic interferometer in which two light pulses are compared to each other by a balanced detector after they counter propagate through an optical fiber loop, the accuracy of the balanced detector in detecting a phase difference between the two pulses is limited by the shot noise of the system.
Optical squeezing is a method for reducing the effect of shot noise. When light is introduced into an optical fiber at very high intensity, i.e., on the order of 1 kilowatt or greater, the index of refraction of the fiber varies slightly for different intensities. This difference in index of refraction causes the speed at which photons of different intensities travel through the fiber to be different, resulting in relative phase shifts for light pulses of different intensity.
In an optical fiber in which squeezing is not occurring, the probable phase and amplitude of an intense light beam follows a generally gaussian distribution. The amplitude distribution of the vacuum state would be generally circular, as shown by circle 11a in the phasor diagram of FIG. 1A, and the field of the light could be anywhere within the circle. If the light had an amplitude of X and a phase of .THETA. then the phasor diagram would be as shown in FIG. 1B and the field of the light may be anywhere within circle 11b. However, in an optical fiber experiencing squeezing, the probable phase and amplitude of light is altered due to nonlinear light effects in the fiber.
The phasor diagram of a squeezed vacuum is elliptical, as shown at 13a in FIG. 2A. FIG. 2B illustrates the situation for squeezed light of amplitude X and phase .THETA. at 13b. The orientation of the major axis of the ellipse is a function of the phase shift. A light beam which has a phase .THETA. such that it is oriented generally parallel to the minor axis of the ellipse, such as vector 25 in FIG. 2B, has less quantum noise in the photon number than unsqueezed light, i.e., sub-shot noise. If, on the other hand, the phase of the light was oriented generally parallel to the major axis of the ellipse, as illustrated by vector 27 in FIG. 2C, that light would have more quantum noise in the photon number than unsqueezed light.
Reference can be made to Bergman, K. and Haus, H. A., Squeezing in Fibers With Optical Pulses, Optics Letters, vol. 15, No. 9, May 1, 1991, as well as Shirasaki, M. and Haus, H. A., Squeezing of Pulses in a Non-Linear Interferometer, J. Opt. Soc. Am., vol. 7, No. 1, January 1990 for more thorough discussions of squeezing in optical fibers.
Further, a type of interferometric detection scheme using squeezed light is suggested in Shirasaki, M. and Haus, H. A., Non-Linear Guided-Wave Phenomena: Physics and Applications, vol. 2, OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1989), p. 232, and Shirasaki, M. and Haus H. A., Journal of the Optical Society of America, B7, 30 (1990).
Guided-acoustic-wave Briliouin scattering (GAWBS) noise is a noise imparted to a light beam by thermal vibration in an optical fiber. GAWBS generally occurs at very high frequencies, on the order of 20 MHz-1 GHz. The thermal vibration of GAWBS noise alters the index of refraction of the fiber. Light pulses in different parts of the fiber will be subject to different GAWBS noise and, therefore, different indices of refraction, thus introducing a phase shift between different pulses. In appli
REFERENCES:
Reduction of Guided-Acoustic-Wave Brillouin Scattering Noise in A Squeezer, Shirasaki et al., Optics Letters, Sep. 1, 1992, vol. 17, No. 17, pp. 1225-1227.
Squeezing of Pulses in a Nonlinear Interferometer, Shirasaki and haus, J. Opt. Soc. of Am. B., vol. 7, No. 1, Jan. 1990 pp. 30-34.
Squeezing in Fibers With Optical Pulses, Bergman and Haus, Optics Letters, May 1, 1991, pp. 663-665.
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