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I'm trying to understand how to combine a square array of laser diodes of the same wavelength and polarisation into a single beam.

Nichia laser array

It seems like there are only two options (beam splitter cubes and dichroic mirrors), neither of which make sense here.

Question

Is it possible to use a telescope arrangement? This should condense the square image to a very small size.

I'm a hobbyist looking to build a cheap CW laser to pump a Ti:Sapphire, and so ultimately all the energy will need to be directed to a point of < 1mm.

enter image description here

chrism2671
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  • Does this answer your question? Is it possible to construct a lens which focuses all the light rays from an extended object in one point? At least the reasoning is the same. – A. P. Mar 17 '21 at 15:39
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    Ok, to be fair, for pumping the Ti ions you don't need to combine the beams, but just focus them tightly. – A. P. Mar 17 '21 at 15:42
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    The question is why it should not be possible. Telescope: parallel beams in, parallel beams out. You should ask yourself, however, if this qualifies for what you call "combining". The beams will still be separated after the telescope. And most importantly, the individual lasers will not be coherent with respect to each other. Since the linewidths of laser diodes are wider, AFAIK, the effects of incoherence will be stronger than when you "combine" the beams of other types of lasers. – oliver Mar 17 '21 at 16:12
  • @A.P. I think it's possible to pump the TiSa with just a converging lens, I think my main concern was getting a usable beam out the other side, and I had figured a particularly tight beam with low divergence was required to engage Kerr Lens mode-locking. Looking at the Ti:Sa schematic, having widely separated beams, even if focused to a point, might be problematic. – chrism2671 Mar 17 '21 at 18:56
  • @oliver I think the lack of coherence is a non-issue for pumping; I believe the goal is just to maintain the Ti:Sa above a lasing threshold (although I have no experience to qualify this). – chrism2671 Mar 17 '21 at 18:56
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    @chrism2671 I guess the question to ask is if the size of the focused diode light is too large for you laser to operate in a single spatial mode. – A. P. Mar 17 '21 at 19:01
  • exactly, you would do better with a single diode. There are several papers on how TiSa's pumped by diodes. 515nm and 440nm diodes, should work, and you do not require more than 3W to have a working TiSa. Soft-aperture KLM will be a nightmare using this badly behaving pump diodes. You would be better using hard-aperture. If money is no issue, you can get a beam homogenizer which would basically combine the light. – José Andrade Mar 18 '21 at 19:51
  • another option is combination into a fibre...like they do with diode stacks for pump lasers. – José Andrade Mar 18 '21 at 19:52

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