Hi, 

In order to get the highest efficiency you must maintain  phase matching between the fundamental and second harmonic for as long as possible in the non linear crystal. 

The crystal should be on the order of 10mm long to achieve the highest conversion with ns pulses. If the crystal is to thin, too much fundamental emerges unconverted on the back.

You may also focuse the fundamental  softly to get higher photon flux and peak power. The limit is the damage threshold for BBO which you should not get to close to. 

Ahnother issue is the polarisation of your beam. In order to be efficient the polarisation of your beam should be precisely along the crystal direction of highest second order susceptibility. You may put a half waveplate before the SHG stage and rotate it for maximum SHG output.

Try iterate optimizing the position of the crystal along the beam propagation direction and the crystal angle.   

Dear Mr. Hodak,

Thank you for your quick response.

I have optimized the angle and the position.

Do you know any papers that reported the experimental data and the theory calculated data?

My experimental data is about half lower than the calculated data.

What Jose said is right, also make sure you are using the right crystal with the right cut for your wavelength

Another important thing, does your calculations assume transform limited pulses? How sure you are about how dispersed your pulses are? SHG is a nonlinear process, so it is highly dependant on the phase of your pulses, so any dispersion would give less SHG

I am sorry I do not know of any paper where they give practical aspects of SHG. This book may be somehow useful: Fundamentals of Nonlinear Optics, Peter E. Powers. You may want to look at these papers also: Measurements of Second Harmonic Generation of the Ruby Laser Line in Piezoelectric Crystals

Albert Savage and Robert C. Miller 

85% efficiency for cw frequency doubling from 1.08 to 0.54 μm

Z. Y. Ou, S. F. Pereira, E. S. Polzik, and H. J. Kimble

Optics Letters Vol. 17, Issue 9, pp. 640-642 (1992)

Phase matching limitations of high efficiency second harmonic generation

IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-20, NO. 10, OCTOBER 1984

How accurate is optical second-harmonic generation?

R. Wynands, O. Coste, C. Rembe, and D. Meschede

Optics Letters Vol. 20, Issue 10, pp. 1095-1097 (1995)

Hi, do you exploit intra- or extra-cavity SHG, pls give these details.

extra cavity single pass SHG. 

Thanks: then I would check if the matching optics really match your input beam profile, divergence, and the BBO, and then to estimate the contribution of the BBO third-order nonlinearity into the synchronism fulfillment. Power's book is pretty good one, and that contribution you may also get from an original paper of A Rubenchik published while ago in JOSA B 

In my experiemnt, the beam diameter (FW@1/e2) is aboult 2.3 mm and slightly convergence. Now the maximum efficiency is 55%

你做的腔外倍频吗？理论上很多东西是估算的，有些药考虑M2因子的。你有没把M2因子加进去。

我做的是腔外倍频。我不知道怎么加M2因子啊。

Dear Mr. Jose Hodak

Is there  back conversion in SHG process?

参考英文版的固体激光工程，第10章，10.44 这个公式之前的几张内容

J. Hodak answer is very good. About papers, try "FM sideband injection locking of diode lasers", "Spectral, phase noise and phase modulation characteristics of AM sideband injection-locked semiconductor lasers" and "Amplitude-modulation sideband injection locking characteristics of semiconductor lasers and their application".