Optical Frequency Multipliers
There are various kinds of optical devices which can (partially) convert light to other light where the optical frequency is an integer multiple of the original optical frequency. Such optical frequency multipliers (or optical harmonic generators) are based on one or several optical nonlinearities.
The term optical frequency multipliers is less common than the more specific terms frequency doublers, triplers etc.
Common Frequency Multipliers
The following types of frequency multipliers are common:
- Of the simplest kind are frequency doublers, based on second-harmonic generation in transparent nonlinear crystal materials with a χ(2) nonlinearity. It is particularly common to generate green light by frequency doubling of infrared light in the 1-μm spectral region, where many lasers operate. However, frequency doublers can also be realized in other spectral regions.
- By combining a frequency doubler with another device for sum frequency generation of the harmonic wave with some of the original pump light, one achieves frequency tripling. Generally, different nonlinear crystals are used for doubling and some frequency generation. The most common application is to produce ultraviolet light, e.g. by frequency tripling from 1064 nm wavelength to 355 nm.
- Frequency quadrupling can be achieved by combining two frequency doublers. The most common application is to generate UV light at 266 nm from 1064-nm light.
- The fifth harmonic of a laser beam can be generated by frequency quadrupling with subsequent sum frequency generation.
Generally, efficient optical frequency multipliers can be operated only with laser light having a sufficiently high optical power and beam quality, because the required nonlinear processes can be reasonably efficient only for sufficiently intense and coherent light. While frequency doubling alone can often be done with a high conversion efficiency – sometimes above 80% –, the conversion efficiency for higher harmonics is usually substantially lower.
Considering the particle nature of light, frequency multiplication processes imply that lower-energy input photons are converted into a correspondingly smaller number of higher-energy photons.
In most cases, optical frequency multiplication is performed with quasi-monochromatic light. This is not only because laser sources happen to be quasi-monochromatic in many cases, but also because phase matching in the used nonlinear crystals can usually be achieved only in a quite limited optical frequency interval.
When applying optical frequency multiplication to monochromatic light, one obtains light with precisely integer multiples of the original optical frequency. The involved processes are fully coherent, i.e., for every optical cycle of the input light one obtains two optical cycles, for example, in frequency doubling. This effect can be exploited in optical frequency metrology, e.g. for optical frequency chains.
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