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Optics Letters

Squeezing in fibers with optical pulses

K. Bergman, H. A. Haus

Published May 1, 1991• 196 citations
Publisher

Abstract

A novel method of squeezing with optical pulses in a fiber ring reflector is demonstrated experimentally. Squeezing of greater than 5 +/- 0.3 dB has been observed. The pump is separated from the squeezed radiation with a fiber ring reflector and can be reused, in principle fully, as the local oscillator. The detection is at low frequencies (35-85 kHz) and is unaffected by guided-acoustic-wave Brillouin scattering.

DOI
10.1364/ol.16.000663

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Experiments in our laboratory have arrived at squeezing in a pulse-excited fiber ring. The pulse excitation suppresses stimulated Brillouin scattering, and at the low frequencies used for the measurement, which are made possible by the pump separation, guided acoustic-wave Brillouin scattering is ineffective. It is likely that large squeezing can be realized by using solitons in the same fiber-ring configuration, because solitons assure better squeezing phase profiles. These findings encouraged us to "design" a fiber gyro that performs below the standard quantum limit. It consists of two fiber-ring reflectors; the first acts as a squeezer, and the second acts as a Sagnac interferometer. We use the theory of quantization of solitons.1 The pulse injected into the first fiber ring is a sech adjusted so as to travel as a pair of countertraveling solitons in the squeezing ring.2 The ring acts like a Mach-Zehnder interferometer folded back on itself. The pump is reflected from the pump port, and the vacuum port has squeezed vacuum emerging from it. The pump is recycled in the second Sagnac ring to act as the signal for the rotation sensing. The squeezed vacuum from the first ring is injected into the vacuum port of the Sagnac interferometer. Because practical design considerations dictate fiber lengths on the order of 100 m for both rings, the nonlinearity in the Sagnac ring cannot be neglected. It produces its own squeezing but in the wrong phase for detector noise reduction. However, by proper adjustment of the squeezing in both interferometers, it is possible to counteract the harmful squeezing effect in the Sagnac interferometer and still arrive at a net reduction of the detector noise.

Annual Meeting Optical Society of America•Jan 1, 1990•0 citations
A novel method of squeezing with optical pulses in a fiber ring reflector is demonstrated experimentally. Squeezing of greater than 5 +/- 0.3 dB has been observed. The pump is separated from the squeezed radiation with a fiber ring reflector and can be reused, in principle fully, as the local oscillator. The detection is at low frequencies (35-85 kHz) and is unaffected by guided-acoustic-wave Brillouin scattering.
Squeezing of 100 ps pulses from a Nd:YAG laser operating at 1.3 μm in a fiber ring has led to a reduction of noise 5 dB below the shot noise level. This reduction occurs in a spectral window between 40 and 90 kHz, a frequency range particularly convenient for instrumentation. Guided acoustic wave Brillouin scattering (GAWBS) was found to be the process limiting our squeezing to 5 dB. The G AWBS measurements will be presented. In the meantime promising ways of reducing the GA WBS have been found and will be described. Fiber ring squeezing permits the reuse of the pump as local oscillator (in principle in full if Faraday rotators are used). A simple system of a fiber ring squeezer with an interferometer in cascade is being constructed to perform a phase measurement below the standard quantum limit. Experimental results will be presented.
Squeezing with degenerate parametric amplification sacrifices the power needed for the pump.1 Squeezing by using the third- order nonlinearity does not necessarily sacrifice the pump power, because it remains at the same frequency as the squeezed radiation. Because the third-order nonlinearity is weak, long interaction lengths must be used, i.e., optical fibers. Past experiments with quasi excitation in optical fibers were plagued by stimulated Brillouin scattering (SBS) and guided acoustic wave acoustic Brillouin scattering (GAWBS).2 The use of pulses raises the peak power and the SBS threshold, allowing use of shorter fibers. A Mach-Zehnder interferometer configuration separates the pump from the squeezed vacuum.3 This has two advantages: the pump power can be reused as the local oscillator power, and the squeezed radiation can be measured closer to zero frequency (30 kHz) because the pump fluctuations are absent.
Observation of squeezed noise, 5 +/- 0.3 dB below the shot noise level, generated with pulses in a fiber ring interferometer is reported. The interferometric geometry is used to separate the pump pulse from the squeezed vacuum radiation. A portion of the pump is reused as the local oscillator in a homodyne detection. The pump fluctuations are successfully subtracted and shot noise limited performance is achieved at low frequencies (35-85 KHz). A possible utilization of the generated squeezed vacuum in improving a fiber gyro's signal to noise ratio is discussed.
Hie generation and detection of quadrature squeezed states of light with a fiber ring reflector is reviewed. A 0.85µm polarization maintaining (PM) fiber is used as the nonlinear medium, and it is pumped by a 1.064µm mode-locked Nd:YAG laser. By the optical Kerr effect in the PM fiber, the input 1.064 mm light is squeezed. The combination of the smaller core diameter and the shorter source wavelength is effective in obtaining a higher peak power density, and consequently, a larger squeezing parameter at a low pump power. The squeezed vacuum is generated by combining the 50m long PM fiber ring with a 1:1 PM fiber coupler, and is detected by a balanced homodyne detector. The squeezed light noise is measured at 200kHz, and a squeezing parameter of 3.7dB is obtained at the input peak power of 5W. A simple calculation model based on the wave optics is used to estimate the squeezing parameter as a function of the input peak power. The calculated results agree well with the experimental ones. The squeezing in PM fibers is limited by the polarized guided acoustic wave Brillouin scattering (GAWBS), which is the phase noise originated from thermally induced fluctuations in the refractive index of the fiber. To avoid the noise due to GAWBS, a shorter PM fiber is preferable.
Experiments in our laboratory have arrived at squeezing in a pulse-excited fiber ring. The pulse excitation suppresses stimulated Brillouin scattering, and at the low frequencies used for the measurement, which are made possible by the pump separation, guided acoustic-wave Brillouin scattering is ineffective. It is likely that large squeezing can be realized by using solitons in the same fiber-ring configuration, because solitons assure better squeezing phase profiles. These findings encouraged us to "design" a fiber gyro that performs below the standard quantum limit. It consists of two fiber-ring reflectors; the first acts as a squeezer, and the second acts as a Sagnac interferometer. We use the theory of quantization of solitons.1 The pulse injected into the first fiber ring is a sech adjusted so as to travel as a pair of countertraveling solitons in the squeezing ring.2 The ring acts like a Mach-Zehnder interferometer folded back on itself. The pump is reflected from the pump port, and the vacuum port has squeezed vacuum emerging from it. The pump is recycled in the second Sagnac ring to act as the signal for the rotation sensing. The squeezed vacuum from the first ring is injected into the vacuum port of the Sagnac interferometer. Because practical design considerations dictate fiber lengths on the order of 100 m for both rings, the nonlinearity in the Sagnac ring cannot be neglected. It produces its own squeezing but in the wrong phase for detector noise reduction. However, by proper adjustment of the squeezing in both interferometers, it is possible to counteract the harmful squeezing effect in the Sagnac interferometer and still arrive at a net reduction of the detector noise.