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Nonlinear Optics: Materials, Fundamentals, and Applications. Technical Digest. Postconference Edition. TOPS Vol.46 (IEEE Cat. No.00CH37174)

Quantum fiber-optics: some recent experimental and theoretical developments

Prem Kumar

Published Nov 8, 2002• 0 citations
Publisher

Abstract

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.

DOI
10.1109/nlo.2000.883661

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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.
Summary form only given. Several schemes to generate squeezed light by use of the Kerr nonlinearity in optical fibers have been devised. The most recent experiments used asymmetric Sagnac loops to produce amplitude squeezing. In our experiment the Sagnac geometry is "unfolded" into a Mach-Zehnder, wherein the two arms of the interferometer correspond to the polarization modes of a single-mode PM fiber. This setup allows us to switch from an asymmetric to a symmetric interferometer, and gives us full control of the phase and amplitude of the two interfering pulses. With these degrees of freedom, we are able to maximize noise reduction and perform a full measurement of the noise properties of the soliton squeezed pulses.
We experimentally demonstrate a new scheme for generating amplitude squeezed solitons in an asymmetric fiber Sagnac loop. We measure by direct detection a 5.7±0.1 dB (73%) and, with corrections for linear losses 6.2±0.1 dB (76%), reduction in the photon number fluctuations below the shot noise level. The same scheme is also shown to generate a significant classical noise reduction and is limited by Raman effects in fiber. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
Summary form only given. We have shown for the first time that large photon-number squeezing of coherent pulses can be produced by a Sagnac loop for a significant range of input energies and that it is larger than predicted for directly spectrally filtered optical fiber solitons. Despite this, we pointed out the similarities between the two schemes. In essence, a Sagnac fiber loop is capable of producing optical pulses closer to true quantum solitons than their coherent counterparts from modelocked lasers.
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.
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.