Advice for the repair of a lab-style Melles Griot laser

[aimpertro]

Hey guys,

I have recently acquired a defective Melles Griot laser of the model 58-GHS-305-048. It is built to have an output power of 2.5W at 532nm. The gain crystal was cracked, while the pump diode (probably a 20W version) operates perfectly fine, and the doubling crystal as well as the output coupler are in good shape. I have bought a 3x3x4mm replacement Nd:YVO4 crystal (0.5% doping concentration), which has the HR@532+1064nm mirror deposited on its rear face. Because the original cavity was set up at an angle (refer to sam's laserfaq: http://www.repairfaq.org/sam/mghpgdl2.jpg), I have removed the SHG crystal and output coupler as well.

Now, I need to place the Nd:YVO4 crystal in the cavity and realign the other parts. Does anyone have advice on how to do it? My first thought was to shine a 532nm laser pointer through the front side and align all components one after the other so that the reflected beams all overlap. I didn't want to fire the pump diode up with the crystals in place, I am a little bit afraid of destroying the crystal with a wrong positioning..

Thanks in advance,

Alex :thanks:

 

[diachi]

You need to modify the crystal assembly (or fabricate an entirely new one) in order to mount your vanadate crystal. The original vanadate in these is much smaller than what you have and is optically contacted/optically bonded so you can't just stick another vanadate crystal where the old one was - that is not something you can do at home with any degree of success. It needs good thermal contact to prevent thermal stress on the crystal which would result in cracks or bad output stability. You will need to fabricate a new mounting solution for your vanadate at the least.

Alignment will be a PITA without the correct tools, there is no way to adjust these on the vertical axis, only on the horizontal axis. You must also use an alignment jig to adjust the OC mirror, then glue it in place once it is aligned. Same goes for the KTP, the mount for that has no way to adjust and is glued in place, although there is some space inside the mount that would allow movement of the KTP.

You didn't mention what coating the other face of your vanadate has? The original crystal is HT@808nm/HR@1064nm on the rear (input) face closest to the diode and is HT@1064nm/HR@532nm on the output side. Is yours the same?

I have two of these, both with de-laminated/cracked gain crystals, one completely separated from the rear surface, the other cracked beyond repair (Did manage to get 1W of green for a short while after an alignment).

Do you have safety glasses for 532nm/808nm/1064nm? Pump diodes in these are ~20W or so, there is a lot of invisible laser radiation spilling out when they are powered up without the lid on. Very dangerous. Any experience with building/aligning/repairing DPSS lasers like this before? Experience with high power lasers well into Class IV?

You need to keep everything clean too, DO NOT touch or get dirt on the fiber output, when you power it up the end will be permanently damaged if it is not perfectly clean. Same goes for the focusing lens after the fiber output.

 

[aimpertro]

Thanks for your fast reply!

First of all, I have safety glasses with an appropriate OD for all three wavelengths as well as experience in working with high-power lasers (university physics laser lab). The reason I am approaching this project is to also be familiar with the fabrication of solid-state lasers at the lowest level. I have previously built a smaller DPSS laser where the cavity consisted of optically contacted gain and harmonic crystals with the mirrors deposited on the faces, therefore the crystal assembly had to be aligned only in respect to the pump light to achieve a good transversal mode structure. This is the first time where all cavity components have to be adjusted individually.

I have already feared that I require a new mounting solution for the vanadate. I suppose a milled copper mount together with some indium foil, enclosing all four crystal sides should be appropriate?

The vanadate coating is the same and should work with the underlying configuration, and also the pump power is within the allowed range.

You mention an alignment jig for the output coupler. Is such a tool commercially available or can be improvised? Moreover, I have found no helpful resources on alignment procedures for these kind of lasers, not even on sam's laser faq.