Squeezed vacuum up to -1.7 dB is generated with chi(3)nonlinearity caused by cascaded chi(2)nonlinearity in a quasi-phase-matched (QPM) PPLN crystal placed in a Sagnac interferometer with 1550 nm femtosecond laser pulses.
Squeezed vacuum pulses up to $\ensuremath{-}1.7\text{ }\text{dB}$ at telecom-band $(\ensuremath{\sim}1550\text{ }\text{nm})$ is generated with femtosecond laser pulses using cascaded ${\ensuremath{\chi}}^{(2)}$ optical nonlinearity in a periodically poled lithium niobate crystal placed in a Sagnac interferometer. In spite of group velocity mismatch at short laser pulse pumping, sufficient cascaded nonlinear wave mixing is obtainable at the wavelength shifted from the phase matching wavelength for second harmonic generation. The theoretical model prediction agrees well with the experimental results.
Squeezed vacuum (SV) generation through third-order optical nonlinearity chi(3)is useful for generating a quantum entanglement pair based on continuous variable. However, third-order nonlinearity is usually small. Therefore, the typical interaction length should be large and ultrashort laser pulses should be used. Then, it becomes important to avoid excess phase noise, especially guided-acoustic-wave Brillouin scattering (GAWBS) in optical fibres. In fact, so far, GAWBS has been preventing from generating SV pulses at sufficiently high squeezing levels. In this paper, we examine insusceptible chi(3)squeezing to GAWBS.
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.