I think it is too short, try my suggestions, if you can get 10-20% efficiency, I think it might be the best case. PS, the diameter is too large, the intensity is around 20MW/cm^2, it is too low, try to focuse the fundamental to BBO by a lens.

What's the pulse duration? Generally, for a fs pulse, the SHG efficiency is low. I suggest you reduce the beam diameter and try again. According to my experience, compared with BBO, LBO maybe have a slightly higher conversion efficiency. If possible, you can try.

the pulse width is ~35fs

I think it is too short, try my suggestions, if you can get 10-20% efficiency, I think it might be the best case. PS, the diameter is too large, the intensity is around 20MW/cm^2, it is too low, try to focuse the fundamental to BBO by a lens.

thanks Li! I have used a lens with 150mm focal length and get the 400nm power of ~7 mW, i.e., an efficiency of ~1.3%. Not high but it is getting better. However, I have met another problem. I found that when using the BBO for a relative long time the BBO crystal seems to be invalid, that is, no 400nm pulse can be observed. If you wait for a long time, such as 24 hours, the crystal seems to work normal. Have you met this problem?

There are several things to look at.

1) Are you aware of the fact that the BBO crystal must be oriented in a particular way with respect to the input beam? (polarization, phase matching) If so, are you sure you have optimized these parameters in the best way possible?

2) A short pulse oscillator is indeed a tricky source for second harmonic generation. Do you care about the pulse duration? How short do you need the 400nm pulse? If it is not particularly important to have the shortest possible 400nm pulse, a thicker SHG crystal may give you better efficiency, with the tradeoff of slight loss of pulse and beam quality. In any case, though, the 150mm focal length may still be too long!

3) The issue with long-term operation does not occur with BBO, at least as far as I know (although similar things may happen with other nonlinear crystals). Are you sure that the laser is stable during long time? For instance, does it not lose modelocking?

It,s better to use thick BBO crystal to get a quite good efficiency. Use a short focal length lens  (~5 cm) to focus the input beam onto the BBO crystal. Slightly rotate the BBO crystal in such a way so that there occurs a proper phase matching. If you follow the process then you can get a quite high energy SHG pulses.  

If you are sure that from the following:-

• the phase maching conditions and the focused the laser in the crystal

•• the crystal sample quality is prepared for this second harmonics generation , may be for Q-switching laser operation.

,than still to looking for the best filtering between the out put two waves(the frequency doubter and the findemantal frequency).

If you get the efficiency   0.15 - 0.20 it wil be very satisfy.

good luck with the my best wishes. 

It is better to reduce the beam diameter.

I suggest, use a slightly larger BBO crystal and try to focus the in-coming beam at the central portion. Then adjust the angle with a micrometer; angle tune.

I am sure you will better results for the SHG.

By the way, the best BBO crystals are from Caseks, China. I have had a good experience.

best wishes,

Aslam 

As well as ,

 in order to get higher nonlinearity the BBO must be longer and the laser source need more intensive.

For the pulse width you are using the crystal thickness should be

Smaller than0.2 mm,  is too short in order to  get satisfy optical nonlinearity in side the crystal.

thank

In generally,The efficiency of the second harmonics generation SHG depends on  the Length of the crystal and on the input power of laser

but inversely dependence on the A diameter of the beam.

thank.

1) Is the 800nm beam focused? If not, focus it into the BBO. Test a few focal lenghts, there will be an optimal length.

2) Check the 800nm polarisation, or BBO crystal rotation - there is an optimum there.

3) How is the BBO cut? For 800nm/400nm SHG?

4) Make sure that however you split 800nm and 400nm after BBO, 400nm makes it through for your detection.

I am also using bbo crystal with 0.2 mm thickness and the efficiency is around 20 percent. I think there is something wrong with the quality of input beam such as the degree of collimation, polarization, divergence (M2 factor and so on, check everything.

or other possibility is that you are using the low quality BBO,

or the crystal is not cut at 29.2 degree.

Use a beam collimator and quite thick BBO (0.5 or 1mm) to get maximum SHG efficiency (~30-35 %).

I also have this problem. Have you solved it?

Dear Chao,

Deepak answered your question well. 10% efficiency is very small. By increasing the the thickness of BBO 1mm, a high efficient laser SHG can be achieved.

Please look at the reference given below.

http://www.photonics.es/images/publications/J09-087.pdf

Ashish

Hello Everyone,

I have a question about the damage threshold of LBO and BBO crystals using a femtosecond and high rep. rate lasers. More precisely I am using a HighQ-2 Ytterbium laser with 250 fs pulse duration and 63 MHz. After 2 weeks of use LBO (3.7 mm) and (2mm) I got the damage on the crystals. I collimated 1.5 W beam using a lens with focal length of 50 mm.

Thank you in advance