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Electronics Letters

Phase-insensitive, single wavelength, all-opticaltransistor based on second order nonlinearities

Z. Wang, David J. Hagan, Eric W. Van Stryland, Gaetano Assanto

Published Jun 6, 1996• 12 citations
Publisher

Abstract

The authors describe how the phase-sensitivity problem of optical transistors based on second order nonlinearities may be avoided using two type II SHG crystals. The first crystal couples the mutually incoherent signal and pump, and the second crystal amplifies this signal. All inputs and outputs may be at the same wavelength.
DOI
10.1049/el:19960725

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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.
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.
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.
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.
An optical transistor capable of simultaneous amplification and switching is theoretically proposed via cascaded second-order nonlinear interactions in a resonant structure. Two distinct operational schemes are analyzed. A single-frequency scheme employs cascaded second harmonic generation and inverse second harmonic generation (SHG/iSHG) using two Type-I SHG interactions, whereas a dual-frequency scheme employs cascaded SHG and optical parametric amplification (SHG/OPA). Exact theoretical solutions and numerical calculations show cascadable amplification and digital on/off switching. A new, to our knowledge, optical phenomenon of nonlinear transparency is predicted by the theoretical solutions and confirmed by the numerical solutions in each scheme of the cascaded SHG/iSHG and SHG/OPA. The single- and dual-frequency configurations satisfy the cascadability and fan-out criteria with power transfer ratios of α TR =4.838 and 52.26 and power amplification factors of β AF =48.38 and 522.6, respectively. These results indicate transistor-like performance at input powers in the milliwatt range, which are readily supplied by laser diodes. The proposed structure provides a proof-of-principle theoretical route to optical transistors, which may serve as a basis for future all-optical communication and computing.