A novel method of producing squeezed vacuum uses cross phase modulation between a linearly polarized pump signal and the orthogonal polarized vacuum. Here we report on such cross phase modulation using 1-nJ 150-fs pulses from a low noise stretched pulse laser. The nonlinear medium was a single mode fiber and the noise reduction was 3 dB.
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.
Squeezed vacuum generation is demonstrated using an ultrashort pulse fiber laser and a symmetric nonlinear polarization interferometer (NOPI). In this system, the squeezed vacuum can be generated only by passing the ultrashort pulse along the optical fiber. Noise reduction of -1.7 dB below shot noise level is observed. Considering the effect of quantum efficiency in this system, the corresponding magnitude of squeezing is estimated to be -2.7 dB. The system is constructed with all fiber devices and is compact, stable, and useful for practical applications.
Optical fibers offer considerable advantages over bulk nonlinear media for the generation of squeezed states. This thesis reports on experimental investigations of reducing quantum noise by means of squeezing in nonlinear fiber optic interferometers. Fibers have low insertion loss which allows for long interaction lengths. High field intensities are easily achieved in the small cores of single mode fibers. Additionally, the nonlinear process employed is self phase modulation or the Kerr effect, whose broad band nature requires no phase matching and can be exploited with ultra-short pulses of high peak intensity. All these advantageous features of fibers result in easily obtained large nonlinear phase shifts and subsequently large squeezing parameters. By the self phase modulation process a correlation is produced between the phase and amplitude fluctuations of the optical field. The attenuated or squeezed quadrature has a lower noise level than the initial level associated with the coherent state field before propagation. The resulting reduced quantum noise quadrature can be utilized to improve the sensitivity of a phase measuring instrument such as an interferometer.