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

ABSTRACT \Ve present a new heterodyne technique for detecting weak signals beyond the standard shot-noise limit by employingamplitude-squeezed light as the local oscillator (LO) field. The amplitude-squeezed LO field at 428.8 nm is generated from ahighly 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 ofthe crystalto combine with the generated squeezed LO field. An enhancement of 0.7 dB (1.4 dB inferred) in signal-to-noise ratiobeyond the shot-noise limit is directly observed for this new heterodyne detection scheme. Practical applications of proposedtechnique in laser radar systems are discussed.Keywords: second harmonic generation, squeezed light 1. INTRODUCTION Second harmonic generation (SHG) is one of the nonlinear frequency conversion methods used as a means to extend coherentlight sources to shorter wavelengths. Having at the input of the nonlinear medium a fundamental field at a given frequency(no, one can obtain a field at doubled frequency 2wo with a quite high conversion efficiency, especially in case of pulse lasersbeing used as the fundamental field for pumping. The efficiency of the SHG conversion process is dependent on suchparameters as the input pump power, traveled distance inside the nonlinear medium, and the values of the medium's nonlinearcoefficients.i When one considers SHG from ultrashort pulses (such as pulse widths below half of a picosecond) in certainnonlinear media (such as KNbO3) processes like group velocity dispersion have to be taken into account.2One of the attractive features of SHG is the resulting suppression of the amplitude noise of the generated light. Amplitude-squeezed states of light have less quantum fluctuations in photon numbers of the light field than those of the coherent field(such as lasers).3 Unlike squeezed vacuum states of light, amplitude squeezed states of light can have intense optical power4,which allows direct replacement of coherent lasers in precision optical measurements where quantum shot-noise associatedwith the laser field needs to be suppressed.It has been observed that the degree of squeezing is proportional to the efficiency of SHG process,56 where the efficiency isdefined as the ratio of the generated second-harmonic light power to the input fundamental power. Therefore, configurationsof highly efficient SHG processes are very desirable for the generation of squeezed light. Using short pulse laser to pumpSHG process has a main advantage, which is the gain of its high peak power. Since the input power is one of the parametersthat the efficiency of SHG depends on, the high peak intensities that can be reached in short laser pulses contribute to theachievement of high conversion efficiencies in the nonlinear crystal. Recently, single-pass traveling-wave (TW) SHG hasbeen proposed and experimentally demonstrated in a bulk crystal7 and in quasi-phase-matched nonlinear waveguide8.Conversion efficiencies up to 60% were obtained in the experiments with nonlinear waveguides, and O.35-dB squeezing inthe generated second harmonic light was observed. We have shown that conversion efficiencies higher than 60% andamplitude squeezing of more than 1.0 dB could be obtained in a conceptually simple system of type I TW SHG in a bulkcrystal .