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Electronics Letters

All-optical high gain transistor action using second-order nonlinearities

Philip St.J. Russell

Published Jun 24, 1993• 52 citations
Publisher

Abstract

It is shown that the output stage of a phase-matched frequency doubler is highly sensitive to the phase and amplitude of a weak injected second harmonic signal. Such devices may be regarded as all-optical transistors with very high small signal gain, resulting in strong modulation of the pump light. Gains as high as 60 dB appear to be experimentally feasible in the recently reported highly efficient quasi-phasematched frequency doublers formed by periodic domain inversion in LiNbO3.

DOI
10.1049/el:19930820

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This thesis describes the implementation of the all-optical wavelength converters based on quasi-phase-matched (QPM) lithium niobate (LiNbO3) crystals and some related optical nonlinear materials for optical fiber communication systems and other applications. Such devices would be the enabling technology to realize high-capacity and transparent communication networks required in wavelength-division-multiplexing systems and highly efficient laser sources at desired wavelengths. The wavelength converters discussed in this thesis are based on the three-wave mixing processes attributed to the second-order nonlinear interactions (x2,). The main challenge with a LiNbO3 QPM wavelength converter is to implement a highly efficient device, which is capable of operating at a power of hundreds of mW. For this purpose, several wavelength conversion schemes have been demonstrated and theoretically investigated, including the fiber ring resonators and a novel cascaded χ(2).∖ The former based on the conventional cascaded x(2) contains erbium-doped fiber amplifiers and other passive components exclusively as pump sources (no additional pump source is required) with excellent characteristics such as high black-box efficiency. While the encouraging properties of the latter are those pump wavelengths can be set outside the communication window, and same efficiency can be achieved by employing two pump sources with lower output power as compared with the conventional cascaded one. By taking into account the higher-order dispersion effects, pulse propagation in QPM second harmonic generation waveguides is also analyzed. In addition, the photorefractive effect (PRE) in QPM waveguides is systematically studied by using a pump-probe method, since the LiNbO3 crystals are extremely vulnerable to high-power shorter-wavelength light illumination. Both temperature and wavelength dependent results are helpful in further understanding the physical insight of the PRE, and providing a guideline for practical application of the QPM LiNbO3 waveguides.

•Jul 1, 2006•0 citations
It is shown that the output stage of a phase-matched frequency doubler is highly sensitive to the phase and amplitude of a weak injected second harmonic signal. Such devices may be regarded as all-optical transistors with very high small signal gain, resulting in strong modulation of the pump light. Gains as high as 60 dB appear to be experimentally feasible in the recently reported highly efficient quasi-phasematched frequency doublers formed by periodic domain inversion in LiNbO3.
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.
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.
A device is proposed which is the optical analog of the transistor wherein a weak incoming optical signal directly controls a strong outgoing optical signal. The basic principle utilized in the proposed device is the strong dependence of second harmonic generation on the phase-matching condition. A weak incoming signal can perturb the indices of refraction in a birefringent material so that the second harmonic power derived from a pump beam is significantly affected. Large incremental power gain is shown to be possible. Tellurium is considered as an example.
This thesis describes the implementation of the all-optical wavelength converters based on quasi-phase-matched (QPM) lithium niobate (LiNbO3) crystals and some related optical nonlinear materials for optical fiber communication systems and other applications. Such devices would be the enabling technology to realize high-capacity and transparent communication networks required in wavelength-division-multiplexing systems and highly efficient laser sources at desired wavelengths. The wavelength converters discussed in this thesis are based on the three-wave mixing processes attributed to the second-order nonlinear interactions (x2,). The main challenge with a LiNbO3 QPM wavelength converter is to implement a highly efficient device, which is capable of operating at a power of hundreds of mW. For this purpose, several wavelength conversion schemes have been demonstrated and theoretically investigated, including the fiber ring resonators and a novel cascaded χ(2).∖ The former based on the conventional cascaded x(2) contains erbium-doped fiber amplifiers and other passive components exclusively as pump sources (no additional pump source is required) with excellent characteristics such as high black-box efficiency. While the encouraging properties of the latter are those pump wavelengths can be set outside the communication window, and same efficiency can be achieved by employing two pump sources with lower output power as compared with the conventional cascaded one. By taking into account the higher-order dispersion effects, pulse propagation in QPM second harmonic generation waveguides is also analyzed. In addition, the photorefractive effect (PRE) in QPM waveguides is systematically studied by using a pump-probe method, since the LiNbO3 crystals are extremely vulnerable to high-power shorter-wavelength light illumination. Both temperature and wavelength dependent results are helpful in further understanding the physical insight of the PRE, and providing a guideline for practical application of the QPM LiNbO3 waveguides.