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What is frequency multiplying in lasers?


Frequency multiplying is a technique used to produce a wavelength that is one half (or one third or one quarter) of the fundamental wavelength of a laser. Using this technique the 1.06µm Nd:YAG beam can be produced 'frequency doubled' at 0.53µm, or with a wavelength of 0.266µm if 'frequency quadrupled', for example. The potential benefits of this are that the shorter wavelength will often be absorbed better by the material being processed and can also be focussed to a smaller spot size, thereby increasing available power density. These 'frequency multiplied' advantages however come at a cost of quite significantly lower average power output than at the fundamental frequency of the laser concerned, due to the inefficiency of the process. Commercial frequency doubling relies on non-linear crystals. As a laser beam passes through the crystal, non-linear interactions between the beam and the material (e.g. lithium borate with Nd:YAG), generate electromagnetic waves at twice the laser frequency. Generation of third and fourth harmonics using non-linear crystals is usually a multistep process. Generating a second harmonic, then mixing it with the fundamental wavelength in another non-linear crystal, so the frequencies combine, produces the third harmonic. To produce the fourth harmonic, the output of a second harmonic generator is passed through a second frequency doubler. Clearly generation of frequency tripled and quadrupled laser light is highly inefficient.

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