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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Second Harmonic Generation in Spliced Poled Fibers

Wasyhun A. Gemechu, Umberto Minoni, D. Modotto, Alessandro Tonello, Vincent Couderc

Published Jun 21, 2021• 1 citation

Abstract

Optical poling is a very simple technique for inducing a quadratic nonlinearity in germanium-doped silica fibers [1] , [2] . By injecting into the fiber fundamental frequency (FF) pulses together with a low power second harmonic (SH) seed, a multi-photon quantum interference process naturally generates a quadratic nonlinearity whose period satisfies the quasi-phase matching (QPM) condition. The nonlinear grating writing process takes a few hours before showing a saturation of the SH conversion efficiency, which is always lower than the best values achievable by resorting to thermal poling [3] . Also, the maximum length of the poled region exhibits saturation, and this hampers the SH generation [4] .

DOI: 10.1109/cleo/europe-eqec52157.2021.9541899

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Optical poling is a very simple technique for inducing a quadratic nonlinearity in germanium-doped silica fibers [1] , [2] . By injecting into the fiber fundamental frequency (FF) pulses together with a low power second harmonic (SH) seed, a multi-photon quantum interference process naturally generates a quadratic nonlinearity whose period satisfies the quasi-phase matching (QPM) condition. The nonlinear grating writing process takes a few hours before showing a saturation of the SH conversion efficiency, which is always lower than the best values achievable by resorting to thermal poling [3] . Also, the maximum length of the poled region exhibits saturation, and this hampers the SH generation [4] .

The present calculation on squeezing capabilities of quadratic nonlinear media in which the phase matching condition is achieved artificially by a periodic poling of the nonlinear susceptibility shows that interesting performance can be obtained for highly integrable and nonlinear materials, using technologies already developed. The origin of squeezing in quasi-phase matched (QPM) media is the cascading of two second order nonlinearities, which at small second harmonic conversion rates has properties similar to a more familiar, purely third order nonlinear effect—Kerr effect.

We report on wavelength-shifting based on cascaded second-order nonlinearities1 in periodically-poled lithium niobate. Wavelength shifting from a signal s at λs=λp+Δ to a frequency-shifted signal f at λf=λp−Δ is obtained under the action of a pump p at λp: the effect is realized through the generation of the second harmonic (sh) of p and subsequent parametric amplification of s driven by sh. In the latter process f is also generated. Exploiting the advantages of quasi-phase-matching, efficient shifting is achieved at pump intensities about two orders of magnitude lower than in traditional second-order materials such as BBO.2

The effect of slow light on second-harmonic generation in a periodically poled ${\ensuremath{\chi}}^{(2)}$ nonlinear medium is investigated theoretically. A linear $\ensuremath{\pi}$ phase-shifted grating is used to slow the group velocity of the fundamental frequency, and the resulting field enhancement greatly increases the second-harmonic conversion efficiency. A second linear grating at the input end ensures that all output is in the forward direction. We show that almost 100% conversion efficiency can be achieved for continuous wave pumping at low intensities that generate negligible conversion in the absence of the slow-light grating.

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.