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Nonlinear Guided Waves and Their Applications

Experimental processing of light pulses with quadratic nonlinearity in type II phase-matched conditions: switching, phase modulation, phase conjugation, transistor operation

L. Lefort, Alain Barthélémy

Published Jan 1, 1998• 0 citations
Publisher

Abstract

The scope of this paper is to revisit the use of quadratic crystals cut for second harmonic generation (SHG) with type II phase-matching in view of all-optical processing of light signal. The investigations on this topic mainly focused in the past years on the possibilities opened by cascading of quadratic nonlinearities in phase-mismatched conditions [1]. The goal of this paper is to remind the reader that various types of optical processing are achievable with sum and difference frequency generation in phase-matched position without the requirement for cascaded nonlinearities. In a first part we will report few examples of the use of difference frequency generation (or down conversion) for signal processing. In a second part we will show that type II SHG with unbalanced fundamental inputs may lead to coherent amplification or transistor like operation.

DOI
10.1364/nlgw.1998.nfa.1

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The scope of this paper is to revisit the use of quadratic crystals cut for second harmonic generation (SHG) with type II phase-matching in view of all-optical processing of light signal. The investigations on this topic mainly focused in the past years on the possibilities opened by cascading of quadratic nonlinearities in phase-mismatched conditions [1]. The goal of this paper is to remind the reader that various types of optical processing are achievable with sum and difference frequency generation in phase-matched position without the requirement for cascaded nonlinearities. In a first part we will report few examples of the use of difference frequency generation (or down conversion) for signal processing. In a second part we will show that type II SHG with unbalanced fundamental inputs may lead to coherent amplification or transistor like operation.
The resurgence of interest in cascaded second order nonlinearities[1,2] has resulted in several schemes for applying cascading in second-harmonic generating (SHG) crystals to optical transistor-like devices [3-7], Such devices allow a weak signal beam to impose a large amplitude modulation on a strong “pump” beam, through mixing in a χ(2) crystal. However, all of the devices reported to date require the pump and signal beams to be temporally coherent, as the process is strongly dependent on the phase between these two beams. There have been some suggestions[6,7] that type II phasematched SHG may remove phase sensitivity, but the devices proposed still require that the signal must be in phase with approximately half of the pump intensity. In this letter, we describe how one may properly exploit type II phase matching to produce a truly phase-insensitive all-optical transistor (AOT) based on second order nonlinearities.
In a search for all-optical signal processing elements operating at a picosecond time scale nonlinear phenomena in quadratic media, extensively studied since many years in a different context were revisited lately. However, all devices proposed and implemented to date (see [1,2] and Refs, therein) have certain drawbacks complicating their use as all-optical amplifying/switching devices, viz., low amplification because of a strong control signal in intensity-controlled devices, smooth switching characteristics of Mach-Zehnder interferometers, and a requirement of fast phase control in phase-sensitive devices [3]. A key requirement of all devices proposed until now is the coherence of pump and signal beams because the χ(2)-process strongly depends on the relative phase. Here we demonstrate that this phase-insensitive amplification/switching can be achieved by a three wave interaction even with reduced pump power requirements. In what follows we focus on the so-called vectorial or type II SHG where two orthogonally polarized fundamental frequency (FF) components A1 and A2 interact with the only second harmonic (SH) wave B polarized parallel to one of the fundamentals [2].
Cascaded second-order nonlinearities (CSON) and parametric downconversion (PD) are suggested as basis for phase insensitive all-optical transistor (AOT) action using type-II second harmonic generating (SHG) crystals. Both devices are tested using, in the CSON-based AOT, two type II KTP crystals in tandem and, in the PD-based, a SHG crystal and a type II KTP crystal. Both configurations were independent of the phase of the signal, however, the output of the first was coherent, while that of the second was incoherent. Therefore, the PD device output is a coherently amplified version of the input, while in the cascaded device the signal imposes a modulation of the power supply. Experimental fluence gains agree well with numerical modelling.
We demonstrate and compare two phase-insensitive all-optical transistors (AOT's) based on frequency-degenerate quadratic three-wave interactions. In particular, we demonstrate gain using KTP in a type II geometry. Both AOT's exploit the phase insensitivity inherent to three-wave parametric processes when only two fields are input, providing amplification of a small signal at the operating frequency via the interaction with a second-harmonic wave. The first scheme is based on successive up- and down-conversion (i.e., cascading) while the second relies on parametric down-conversion. We obtain gains of 5 and 160 in the two configurations, respectively, with a significant background and output coherent to the pump in the first case, no background and coherence between output and signal in the second.
Both static and dynamical analyses of quasi-phase-matched second-harmonic generation by backward propagating interaction, where the second-harmonic wave is generated in reflection, are presented. It is shown that bistability appears in the generated second-harmonic power if the amount of phase mismatch is suitably chosen. The bistability also appears in the output fundamental wave, since the cascaded second-order nonlinear process gives rise to the effective self-modulation of the fundamental wave, which can be applied to all-optical signal processing. The output second-harmonic and fundamental powers can be switched between two bistable states by controlling the intensity or phase of the input fundamental wave.>