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Japanese Journal of Applied Physics

Purification of Squeezed Vacuum Pulse Generated from a Sagnac Loop Fiber Using Linear Optics and Conditional Homodyne Detection

Kenichi Hirosawa, Yoshinori Momose, Hidetake Ushio, Yuji Fujiwara, Fumihiko Kannari

Published Mar 1, 2009• 3 citations
Publisher

Abstract

Purification of quadrature squeezed vacuum pulses at the wavelength of 1.5 µm generated from a Sagnac loop fiber interferometer is experimentally demonstrated using linear optics and conditional homodyne detection. The anti-squeezed quadrature variance degraded by guided acoustic wave Brillouin scattering in the fiber is significantly improved from 12.6 to 4 dB after the post selection, while the squeezed quadrature variance of -0.7 dB slightly decreases to -0.3 dB due to the linear optical loss caused by a beam splitter. This result agrees well with the theory.

DOI
10.1143/jjap.48.034001

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•Aug 1, 2009•0 citations
Purification of quadrature squeezed vacuum pulses at the wavelength of 1.5 µm generated from a Sagnac loop fiber interferometer is experimentally demonstrated using linear optics and conditional homodyne detection. The anti-squeezed quadrature variance degraded by guided acoustic wave Brillouin scattering in the fiber is significantly improved from 12.6 to 4 dB after the post selection, while the squeezed quadrature variance of -0.7 dB slightly decreases to -0.3 dB due to the linear optical loss caused by a beam splitter. This result agrees well with the theory.
We experimentally demonstrate generation of a squeezed vacuum at 800 nm with a Sagnac loop fiber interferometer. When negative dispersion is properly added to an input laser pulse to compensate for the fiber dispersion, the level of squeezing is improved. A squeezed vacuum of 0.45 dB is obtained at a dispersion of -0.0157 ps(2) for the 1.5 m-long fiber loop. Since the squeezed vacuum is degraded by guided acoustic-wave Brillouin scattering (GAWBS), the noise level of the squeezing is improved by -0.3 dB at a liquid nitrogen temperature. We also demonstrate generation of photon number squeezing at -1.3 dB.
Photon-number squeezing and quadrature squeezing were studied using ultrafast fibre nonlinear optics pumped by 1.5 µm femtosecond laser pulses. To obtain quadrature squeezing using an Erbium doped fibre amplifiers (EDFA) light source, we developed temporal homodyne technology to exclude the excess noise of an EDFA light source. We also performed a quadrature squeezing experiment using an optical parametric oscillator light source in stead of an EDFA light source exhibiting substantially high excess noise. Although the photon-number squeezing of -3dB was obtained from an asymmetric Sagnac fibre loop, the quadrature noise could not decrease to less than the Shot noise level without cooling the fibre, which is presumably due to GAWBS. To suppress guided acoustic wave Brillouin scattering, we refrigerated the fibre loop by liquid nitrogen, and squeezing at -0.7 dB was obtained by cooling the fiber at liquid nitrogen temperature.
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.