Anyone know anything about frequency doubling lasers

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Re: Anyone know anything about frequency doubling lasers

Postby doug694 » Tue Dec 20, 2011 5:04 pm

Hi,

I have done quite a bit with frequency doubled lasers, and just recently managed to double to blue (a bit harder than green!).

If you are working with an 808nm pump diode there are roughly 2 ways you can use it to pump a cavity - which method you use will depend a bit on the diode.

If its a big fat bar diode in the 10s of watts upwards (!), you'd probably consider side-pumping a 'generic' Nd:YAG or Nd:YVO4 crystal (short cylinder or cuboid) with one or more of those. You would typically then have a bunch of options to play with on resonator mirrors and position of the KTP (or other nonlinear SHG crystal). This is more of a DIY situation where you can move things about. It is actually possible to put the SHG crystal inside or outside of the cavity, and to boost efficiency by either q-switching, or focusing the beam down inside the SHG. There are lots of ways to do it.

If it's a smaller diode - say 1->8 watts - you'd probably want to end-pump a crystal instead. This is far less flexible. It usually involves coated and often bonded crystals.

In both cases the HR normally flat. If the OC is a separate mirror, it is often curved because it helps with alignment and beam mode structure. But this isn't essential - flat/flat works if you are really patient and have a stable rail/platform and good mirror mounts to tweak.

In both scenarios, the pump beam generates a new laser beam in the pumped crystal, because a resonator exists in both cases. It's just that the resonator is less obvious in the end-pumped configuration, consisting of coatings on the end(s) of at least one crystal. But they are both lasers in the sense that the YAG/YVO4 is generating its own gain with its own mode structure.

The reason end-pumping usually involves bonded crystals with funny coatings, is simply that you're sending the pump beam into one of the crystal faces - so it needs to be anti-reflection (AR) @ the pump wavelength, but high-reflection (HR) at the wavelength(s) for which you are trying to achieve amplification. This is more complicated if you are frequency doubling, because you're dealing with 3 frequencies at a single face (pump, gain, doubled). The output face of the end-pumped crystal will tend to have the SHG crystal bonded onto it - simply because it can - removes alignment as a fabrication issue. The SHG crystal carries the 'output mirror' or OC coating, which is also multi-wavelength. It's a very small fairly complicated sandwich.

Generally, end-pumped crystals are fabricated to be exactly what they are going to be used for - it's not easy to adapt them or hack your own layout. (On this subject, I do know somebody who grew their own SHG crystal in a jam jar from phosphate/fertiliser and got green light out of it :-)

Side-pumping lets you use 'real' independent mirrors, so you can use crystals pulled from other lasers based on different designs, and still have some chance of getting a workign laser in the end.
doug694
 
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Re: Anyone know anything about frequency doubling lasers

Postby Jerry » Wed Dec 21, 2011 1:19 am

The lasers I have are 32 and 40 watt Coherent "FAP" fiber coupled. They have a 18 diode bar internally that focuses into the fibers. I have one with a bad fiber coupling and could be taken out to side pump with.
Jerry
 
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