Abstract

We propose and demonstrate a travelling-wave nonlinear device for optical signal processing. It is based on the sequential use of a nonlinear phase shift (Kerr-like effect) and a phase sensitive parametric amplification (PSA). Even though it is under first step investigations, this arrangement has already featured interesting optical proprieties, among which: (1) amplification of a weak modulation encoded on a strong optical signal, better than the usual gain of the bare PSA. (2) Sign inversion of the weak modulation with absolute gain greater than unity. (3) Complete suppression of classical noise. For these demonstrations, the processed signal is generated by a pulsed frequency doubled Nd:YLF laser. A 0.2 % depth sine amplitude modulation at 18 MHz is imparted to the fundamental signal at 1054 nm. A periodically poled KTP (PP-KTP) crystal is used to achieve the nonlinear phase shift via cascaded second order processes, similar to an effective third order Kerr nonlinearity [1], When the PP-KTP is heated at 41°C, a nonlinear phase shift of π/6 is obtained for signal powers of about 30 mW. The modulated signal is then sent through a KTP based PSA pumped by the second harmonic (527 nm) of the laser. It is finally detected with an InGaAs photodiode. The AC photocurrent at 18 MHz is mixed down and sent to a boxcar for data acquisition. Figures 1(a) and (b) represent the measured gains of the amplitude modulation (squares) of the signal for a fixed nonlinear phase shift of π/6, as well as the gain of the bare PSA (triangles). Depending on the relative phase between the signal and the pump, and on the pump power, the modulation could be amplified (Fig.1.a) or inverted (Fig.1.b). In both cases the proposed configuration achieves better performance than the bare PSA. Moreover, It allows negative gains (sign inversion of the modulation) in a full parametric and travelling-wave interaction.

© 2000 IEEE