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Nonlinear Frequency Conversion

Author: the photonics expert

Definition: the conversion of input light to light of other frequencies, using optical nonlinearities

More general term: nonlinear effects

More specific terms: frequency doubling, frequency tripling, frequency quadrupling, sum and difference frequency generation, optical rectification, supercontinuum generation, optical parametric oscillation and generation, stimulated Raman scattering, high harmonic generation

Category: article belongs to category nonlinear optics nonlinear optics

DOI: 10.61835/bbc   Cite the article: BibTex plain textHTML

Nonlinear frequency conversion means that an optical nonlinearity is utilized for converting part of the optical power of some input light to output light in a different wavelength region. This is often quasi-monochromatic light with a fixed wavelength or sometimes a tunable wavelength; in some cases, broadband light is generated. In almost all cases, the input light is provided in the form of a laser beam with a substantial optical intensity; only under such conditions, nonlinear frequency conversion processes can be highly efficient. The output is then also usually obtained in the form of a laser-like beam, i.e., with high spatial coherence.

Not all wavelength regions of interest are directly accessible with lasers. Therefore, it is common e.g. to generate visible or ultraviolet light by nonlinear conversion of near-infrared light from one or several lasers. Also, mid-infrared laser sources are often realized based on a near-infrared laser combined with some nonlinear frequency conversion apparatus.

Examples of nonlinear conversion processes are:

Many but not all of these processes can be efficient only with phase matching and with polarized light. Laser radiation is usually linearly polarized, but some devices (e.g. certain high-power fiber lasers and amplifiers) are not well suited for nonlinear frequency conversion because they do not emit with a stable linear polarization state, or because they have insufficient spatial or temporal coherence.

Efficient Conversion at High Optical Intensities

As nonlinear frequency conversion can be efficient only at sufficiently high optical intensities, the intensities often have to be increased with one or several of the following methods:

Applicable intensities are often limited by the damage threshold of the nonlinear materials. There are situations where that limitation prevents highly efficient frequency conversion. An example is frequency doubling of ultrashort pulses into the ultraviolet spectral region, where the large group velocity mismatch limits the interaction length while the damage threshold is relatively low.

Design Issues

The design of nonlinear frequency conversion devices can involve subtle issues. For devices based on parametric nonlinearities, there can be beam quality effects due to spatial walk-off, gain guiding, pump depletion and backconversion. Such effects can be investigated with numerical computer models, which can simulate the evolution of the spatial (and possibly temporal) profiles of the interacting beams. Particularly for the conversion of ultrashort pulses, there is a wide range of phenomena which should be properly understood in order to avoid problems. Numerical simulations may be essential for optimizing the performance.

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[1]G. D. Boyd and D. A. Kleinman, “Parametric interaction of focused Gaussian light beams”, J. Appl. Phys. 39 (8), 3597 (1968); https://doi.org/10.1063/1.1656831 (a seminal work with a comprehensive quantitative discussion)
[2]R. L. Sutherland, Handbook of Nonlinear Optics, 2nd edn., Marcel Dekker, New York (2003)
[3]A. V. Smith, SNLO software for simulating nonlinear frequency conversion in crystals, free download, http://www.as-photonics.com/snlo, from AS-Photonics
[4]A. V. Smith, Crystal nonlinear optics with SNLO examples, ISBN 978-0-692-40044-9, https://as-photonics.com/products/crystal-nlo-book/

(Suggest additional literature!)

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