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Optics Letters

Sub-shot-noise measurement with fiber-squeezed optical pulses

K. Bergman, Christopher Richard Doerr, H. A. Haus, Masataka Shirasaki

Published Apr 15, 1993• 57 citations
Publisher

Abstract

A novel scheme employing two pulses separated by a short time delay is used to cancel the phase noise from guided-acoustic-wave Brillouin scattering in a fiber ring interferometer. The dual-pulse-excited fiber ring is used to generate squeezed vacuum that, when injected into a measuring Mach–Zehnder interferometer, improves its sensitivity by 3 dB beyond the shot-noise limit.

DOI
10.1364/ol.18.000643

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Reduction of guided-acoustic-wave Brillouin scattering noise in a squeezer

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A new scheme for interferometric measurements that uses squeezed light produced in an optical fiber ring is proposed. Guided-acoustic-wave Brillouin scattering noise is suppressed by cancellation of the phase fluctuations caused by this scattering in two consecutive pulses that are pi phase shifted with respect to each other.

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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.

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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.

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Optics Express•Jan 10, 2011•3 citations
A novel scheme employing two pulses separated by a short time delay is used to cancel the phase noise from guided-acoustic-wave Brillouin scattering in a fiber ring interferometer. The dual-pulse-excited fiber ring is used to generate squeezed vacuum that, when injected into a measuring Mach–Zehnder interferometer, improves its sensitivity by 3 dB beyond the shot-noise limit.
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.
We propose and experimentally demonstrate a quantum-enhanced fiber Mach-Zehnder interferometer for low-frequency phase measurement beyond the shot-noise limit using a high-frequency squeezing technique. The local oscillator field is amplitude-modulated in the MHz range and is then demodulated to avoid the technical noise that occurs at low frequencies. After measurement of the phase noise at a frequency of tens of kHz, an improvement of ∼2 dB relative to the shot-noise level is achieved. Additionally, the amplitude modulation depth has no significant effect on the phase noise improvement of the interferometer when deployed in our experimental configuration. The current scheme introduces a quantum technique into the fiber-based measurements, particularly for the low frequency range, and this scheme has potential applications in the high-precision fiber sensing of temperature, strain, and various other parameters.
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.
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.
We experimentally demonstrate the generation of a squeezed vacuum pulse at 810 nm with a fiber polarization interferometer. During femtosecond laser pulse propagation through an optical fiber in the normal dispersion regime, only self-phase modulation within a short length contributes to pulse squeezing since the laser pulse is immediately broadened. Guided acoustic-wave Brillouin scattering (GAWBS) noise that increases in proportional to the fiber length is also lower with shorter fibers. Consequently, a maximum noise reduction of 2.1 dB (4.8 dB when corrected for losses) is obtained using a 40-cm-long single mode optical fiber.