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UV Nd:YAG Lasers at 355-nm Wavelength
Frequency tripling or third harmonic generation (THG) is a technique used to produce a wavelength
that is one-third (1/3) of the fundamental wavelength of a laser. For the 1064-nm fundamental of
Nd:YAG, the third harmonic wavelength is 355 nm. 355 nm lies in the ultraviolet (UV) region of
the electro-magnetic spectrum, and it offers many advantages over 1064 nm and 532 nm for laser
processing, among which are:
- The shorter wavelength couples (or is absorbed) more efficiently with many target
materials than does the 1064-nm fundamental wavelength, and for some materials
even more efficiently than the second harmonic 532-nm wavelength.
- The shorter wavelength has the potential to produce a beam focus spot which is 67 %
smaller than that produced by the fundamental wavelength, and 33 % smaller than
That produced by the second-harmonic wavelength.
- Short wavelength lasers are valuable tools for scientific study.
- Material removal is an ablation process rather than a thermal process.
To produce the 355-nm UV wavelength, the fundamental 1064-nm wavelength must be mixed with the
second-harmonic 532-nm wavelength, in the proper proportions and phase relationships, inside the
THG crystal. Wavelength conversion efficiency is proportional to the level of the peak power
density of the input beams, so a means must be found to concentrate the 1064- and 532-nm beams
into the THG crystal to produce an acceptable level of UV output.
There are two ways to do this:
1. Focus coaxial 1064-nm and 532-nm beams into a THG crystal that is installed outside of the
laser optical resonator. This is called extra-cavity third harmonic generation.
2. Install the SHG/THG generator inside the laser optical resonator. This is called intra-cavity
third harmonic generation.
The fundamental design problem with extracavity THG results in the requirement for periodic replacement
of the THG crystal. It becomes, in effect, a consumable part.
Lee Laser avoids this problem by installing the THG crystal inside the laser optical resonator. The
high level of peak power density that is needed to achieve good wavelength conversion is provided by
the high circulating power of the 1064- and 532-nm beams inside the laser optical resonator. There
is no need to focus the beams into the THG crystal, so the high peak-power-density condition on the
surface of the THG crystal never exists. Lifetime of the THG crystal is greatly extended, and there
is no requirement for re-collimating optics.
Lee Laser’s greatly simplified THG generator eliminates the costly design features that are required
for extracavity THG. All of this is explained in more detail in Lee Laser’s technical bulletin,
UV Nd:YAG Laser at 355-nm Wavelength.
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