Abstract
Nonlinear conversion of unpolarized beams to lower frequencies is generally inefficient in c(2) materials, as it is challenging to achieve phase-matching for input ordinary and extraordinary beams simultaneously in the normal dispersion regime. Here, we show that cubic Raman crystals having doubly and triply degenerate (E and F type) modes provide a method for efficient nonlinear frequency downconversion of an unpolarized beam and yield a linearly polarized output state. Using Mueller calculus, optimal crystal directions for such polarization conversion are determined. Using diamond, an example of an F-class Raman crystal, we have verified that such conversion is possible with near quantum-defect-limited slope efficiency and a linear polarization contrast of more than 23.9 dB.
Highlights
IntroductionNonlinear optical frequency conversion by harmonic generation (e.g., second harmonic, sum frequency and difference frequency generation), optical parametric generation and stimulated scattering are fundamental tools for diversifying the emission spectrum of lasers and their applications[1,2,3]
Nonlinear optical frequency conversion by harmonic generation, optical parametric generation and stimulated scattering are fundamental tools for diversifying the emission spectrum of lasers and their applications[1,2,3]
There is no equivalent phase-matching for optical parametric generation of longer wavelengths from an unpolarized input as there is an inherent conflict in satisfying Type I (e.g., o → e + e) and Type II/III (e → o + o) phase matching conditions simultaneously in the normal dispersion regime
Summary
Nonlinear optical frequency conversion by harmonic generation (e.g., second harmonic, sum frequency and difference frequency generation), optical parametric generation and stimulated scattering are fundamental tools for diversifying the emission spectrum of lasers and their applications[1,2,3]. If the orthogonal polarization components of the unpolarized input act in a concerted fashion to generate linearly polarized output, conversion efficiencies may be twice that otherwise obtained. We show that stimulated Raman scattering (SRS) in certain cubic crystals simultaneously relaxes the dispersion condition and provides suitable off-axis tensor components for transferring power from orthogonal input polarizations onto a single linear output polarization mode. This allows the optical-to-optical conversion efficiency for unpolarized inputs to be as high as that typically obtained when using linear polarization and potentially as high as the Stokes quantum efficiency. We calculate crystal propagation directions that enable polarization conversion and demonstrate the concept using the example of a Raman laser in diamond
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