Second-harmonic generation with femtosecond pulses and its applications in sub-shot-noise detection

We present a new heterodyne technique for detecting weak signals beyond the standard shot-noise limit by employing amplitude-squeezed light as the local oscillator (LO) field. The amplitude-squeezed LO field at 428.8 nm is generated from a highly efficient (up to 60% conversion efficiency) single-pass second-harmonic generation in a nonlinear crystal (KNbO3) pumped by femtosecond (130 fs) pulses at 857.7 nm. The weak signal field is introduced from the entrance port of the crystal to combine with the generated squeezed LO field. An enhancement of 0.7 dB (1.4 dB inferred) in signal-to-noise ratio beyond the shot-noise limit is directly observed for this new heterodyne detection scheme. Practical applications of proposed technique in laser radar systems are discussed.