Abstract

Recently, bright sub-Poissonian pulses of light were obtained from an unbalanced nonlinear Sagnac interferometer.¹ The noise reduction was shown to be mainly due to the strong correlation between the pulse’s photon number and phase, that develops during propagation in the Kerr medium.² The success of these experiments critically depends on the use of ultrashort (femtosecond) soliton-like pulses in order to achieve substantial nonlinear phase shifts in short (few meters) lengths of fiber. For picosecond pulses, however, the required longer fiber lengths lead to accumulation of excess noise that is due to scattering on thermally-excited acoustic modes of glass fiber, also known as guided-acoustic-wave Brillouin scattering (GAWBS),³ which makes observation of quantum effects very difficult. Two schemes were developed in order to achieve broadband suppression of the GAWBS noise in squeezed-vacuum generation experiments that employed a balanced Sagnac interferometer: i) cooling the fiber to liquid-nitrogen temperatures⁴ and ii) generating two squeezed-vacuum pulses, separated by a short time delay, and detecting them with a relative phase shift of π by a two-pulse local oscillator.⁵ The second method, in order to achieve excess-noise cancellation, used time delays shorter than the inverse bandwidth of GAWBS, which required a GHz electro-optic modulator for fast phase switching.

© 1999 Optical Society of America