How to avoid deterioration of a BBO (beta barium borate) crystal within minutes or hours?

We are using a BBO crystal to do SFG with 800 nm and 400 nm. After adjusting we start with 20 mW output power for the UV light (266 nm). After 1-2 hours the power goes down to 8-12 mW.

The major problem is, that at the same time our time zero shifts by 20-40 fs. If we now move the crystal a tiny bit, the power comes back to 20 mW and time zero also shifts back to its original value.

Does anyone have an idea of where this comes from and how to avoid it?

Apr 12, 2013 · Flag

Topics

• Optical Devices

• Nonlinear Optics

• Laser Spectroscopy

• Femtosecond lasers

• Laser

• Peter Diehr · 3.52 · University of Michigan

You may be putting too much power through the crystal; contact the provider for the specifications for your crystal.

I've used a pair of BBO crystals for this same task - 800 to 400, then 266. Conversion efficiency is increased by higher intensities, so I used a long-focal length lens (2 m), with the pair of crystals at about the 1 m mark, and the "near focal" point for the UV near the 2 meter mark. I used this same setup for several years with 150 fs commercial laser. I used the UV to drive an ultrafast photo-electron gun; I used the intensity of the electron beam to monitor the UV stability ... I never had any problems such as you report.

3 days ago

• Bruce Marsh · 27.47 · 114.15 · European Organization for Nuclear Research

I dont know what you mean by the time zero shift but the UV induced BBO performance degradation is a well known effect. Since it occurs at a light intensity that is well below the damage threshold of BBO, it is probably attributed to a cloudy coating that appears on the crystal surfaces due to photo-deposition of some impurities (from the air) on the crystal surface. The coating then results in absorption of the fundamental or UV beam, and therefore actual material damage can occur. There have been some studies to investigate the actual composition of the milky coating (e.g http://www.opticsinfobase.org/oe/fulltext.cfm?uri=oe-21-6-7285&id=250905). At the ISOLDE resonance ionization laser ion source at CERN, we often generate ns pulses of UV light down to 215 nm with an average power of up to 100mW, the BBO crystals and the lenses downstream quickly develop this coating. Since we use dye lasers with ethanol solvents, we suspect that the coating may contain decomposed hydrocarbon residue due to the greater than normal quantity of ethanol vapour in our lab. The lenses are easily cleaned with acetone and the crystals can be translated, as you have already been doing. We are planning to implement measures to limit the speed of this UV induced effect, such as surrounding the crystal with clean nitrogen or argon. You may be able to do something similar.

2 / 0 · 3 days ago

• A. Wilson · 8.12 · 36.28 · National Institute of Standards and Technology

We observe a similar effect generating 20 mW average power at 235 nm (tripling 705 nm). In our case, the power drop recovers overnight. This isn't a problem for our research, so we haven't tried to fix it. However, I do have a couple of suggestions. We also generate cw 313 nm light (up to a 1W) using cavity-enhanced SHG, and the BBO is mounted in a way that the crystal is "bathed" in a gentle flow of oxygen. This seems to eliminate (or at least drastically reduce) the need to move the crstal periodically. Aside from keeping the crystal clean, perhaps the oxygen plays a role in reducing damage of the surface - we're not sure. Another suggestion is that you place a chacoal filter (and fan?) inside the SFG box. The idea here is that it could be UV-activated chemistry on the BBO surfaces. We have seen UV-activated films (i.e. flims just where the beam is) forming on optics when the mounts were not thoroughly cleaned to remove machine oil. Before installing the filter, I suggest you clean everything using UHV vacuum style methods, and make sure the system is housed in a fairly well sealed enclosure. I hope these ideas help.

2 / 0 · 3 days ago

• Aaron Rury · 11.24 · 13.76 · California Institute of Technology

HI Marco,

in using nonlinear optical parametric amplifiers in grad school, I found that we needed to heat our BBO crystals to maintain nonlinear conversion performance. Heating the crystals stopped deposition of water on their surfaces and degradation of the surface environment.

I can't remember which product we used for this application, but your crystal provider may be able to help track down a product to help you.

1 / 0 · 3 days ago

• 

Peter Diehr · 3.52 · University of Michigan

Our lab was very dry ... 3 days ago

• A. Wilson · 8.12 · 36.28 · National Institute of Standards and Technology

We avoid the familiar humidity-related fogging problems by living in Boulder and using an oxygen purge. I doubt this is your problem, since moving the crystal won't help with this - the fogging is usually uniform. Have you looked at the crystal under a microscope? Can you see "burn" patches? If there are burn patches that can be removed with solvent, you have a contamination problem.

1 / 0 · 3 days ago

• Paulo Lopes · University of Aveiro

If you restore the previous power by moving the crystal... maybe it's not properly fixed!
There's a chance that power recovery is due to air replacement when you open the crystal case... are you using oxygen flow around BBO ?
Paulo,
Portugal. 1 / 0 · 3 days ago

• Jing-Yuan Zhang · 25.56 · 60.12 · Georgia Southern University

I have been using BBO for more than 20 years. Two suggestions for you: 1. heat up the BBO to 60-80 degree C using a temperature controlled oven; or 2. purged the BBO surface with pure nitrogen continuously when it is in use or put BBO in an air-sealed cell.. 1 / 0 · 3 days ago

• Marco Peccianti · 45.15 · 347.18 · National Research Council

Hello Marco

I'm just an user but I use BBOs quite regularly fairly closet to their damage limits.
Could you provide specifications about the laser source (energy per pulse, rep rate) and the spot size on the BBO?

Is the BBO a type I for 800nm? Modified 3 days ago by the author ,3 days ago

• Norman Barnes · 23.69 · 43.87 · NASA

This is speculation, however a similar effect was noted with LiNbO3. Your sum frequency is close enough to the absorption band that it may excite some electrons to the conduction band. If so, these electrons could establish an electric field. Because BBO is electro optic, as is LiNbO3, the electric field could affect phase matching in a deleterious way. If the production of conduction band electrons is slow enough and the decay of conduction band electrons is fast enough, this effect would not be noted immediately. Heating the BBO crystal may hasten the decay of conduction band electrons and diminish the effect. Of course, the phase matching would have to be adjusted to compensate for the higher temperature. 1 day ago

• Marko Wietstruk · 17.51 · 81.78 · Freie Universität Berlin

Thank you for all these fast and profound answers. Up to now we have no box around our crystal and therefor no nitrogen flowing around it. We have just a flow box for our whole laser table. We will first try to build a box to fill it with Nitrogen. Thanks a lot.

21 hours ago

• Jeremias Seppä · 9.89 · 8.64 · Centre for metrology and accreditation MIKES

We use heating, probably with a TEC unit, to keep water out, but in an Ti:S laser we use some granules that pick up/catalyze (organic?) particles from inside the Ti:S laser box, they shift color between yellow and purple when they age, and they seem to help quite much keeping the Ti:S crystal clean. I can't remember what they are called.