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Optics Letters

Soliton squeezing at the gigahertz rate in a Sagnac loop

C. X. Yu, H. A. Haus, Erich P. Ippen

Published May 15, 2001• 46 citations
Publisher

Abstract

We demonstrate what is to our knowledge the first all-fiber squeezing experiment. A balanced Sagnac loop is used, and a record 6.1+/-0.2dB of noise reduction below shot noise has been obtained without stabilization. A gigahertz Er-doped fiber laser and a high-power double-clad Er-Yb amplifier have been developed to suppress guided acoustic-wave Brillouin scattering and to make possible the all-fiber configuration.

DOI
10.1364/ol.26.000669

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We demonstrate what is to our knowledge the first all-fiber squeezing experiment. A balanced Sagnac loop is used, and a record 6.1+/-0.2dB of noise reduction below shot noise has been obtained without stabilization. A gigahertz Er-doped fiber laser and a high-power double-clad Er-Yb amplifier have been developed to suppress guided acoustic-wave Brillouin scattering and to make possible the all-fiber configuration.
Convenient methods of generating amplitude-squeezed (sub-Poissonian) light have recently been discovered, which rely on the use of soliton-like pulses in fibers. The idea of squeezing solitons in fibers is; what is new, however, is the use of femtosecond pulses in conjunction with asymmetric Sagnac loops. Since femtosecond solitons have a correspondingly high peak power, much shorter fiber lengths are required to obtain a given amount of squeezing, which directly translates into lowering of the guided acoustic-wave Brillouin scattering noise in the system. The use of an asymmetric Sagnac loop leads to a robust, practically very stable configuration for generating amplitude squeezing. In a related development, spectral filtering of femtosecond solitons emerging from a straight piece of fiber has also been shown to generate sub-Poissonian light.
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.
Direct photon-number squeezing is demonstrated, for the first time to our knowledge, in a nonlinear fiber-optic interferometer. Launching 126-fs solitons into a highly asymmetric Sagnac loop, the maximum photocurrent noise reduction was $3.9\ifmmode\pm\else\textpm\fi{}0.2\mathrm{dB}$ below shot noise, corresponding to $6.0\ifmmode\pm\else\textpm\fi{}0.9\mathrm{dB}$, when corrected for linear losses. The loop is a model system for squeezing generated by the interference of two pulses after nonlinear propagation through a fiber.