I did not get full clarity in your question but any way broadening of temporal profile due to high intensity can happen while it passing through material due self phase modulation.  Group velocity Mismatch(GVM) comes at low intensity due to linear refractive index and at high intensity nonlinear refractive index will also included in the process.

in the generation of other frequency laser pulses form fundamental by nonlinear process, the generated pulse properties depends on the Fundamental Pulse Depletion, fundamental pulse shape.

may be after your reply only I can give better answer than this.

Dear A S V Rao, we have done SHG of high power ultrashort pulse using LBO and measured its FWHM  using Intensity autocorrelator. While working at different  output power of Green, we see variation in pulse width (FWHM increases at maximum output of Green). I believe for a fixed fundamental pulse width; SHG pulse width should remain constant. 

we will see the variation in the pulse width and spot size of the beam as function of incident intensity for Gaussian beam in the nonlinear process. Because it is intensity dependent process. At low intensity only peak intensity can produce SHG if it is above threshold intensity and when we are keep on increase the fundamental beam intensity then intensity towards edges of pulse crosses the threshold intensity as a result the range of pulse conversion form fundamental to SHG increases as a result we will see that some variation in the pulse width. same for spatial profile. If you want you can check for spot size variation also.

with reference to threshold intensity of SHG, SHG pulse width smaller than the fundamental pulse width  for low intensity andSHG pulse width larger than the fundamental pulse width for high intensity.

This is what my interpretation for your question.

Thanks A S V Rao.. Can you suggest some article/references discussing such phenomena.

SHG is a nonlinear process when I(omega 2) = k I(omega 1)xI(omega 1),

were omega 2 = 2 omega 1. The pump beam intensity is function of time, so when I(1) = 0.5xI(max), then I(2)= 0.5 x 0.5= 0.25

Since regularly we measure the pulse width to the 0.5 level, we will obtain the duration of SH pulse approx. twice shorter than the duration of pump beam.

Dear Samir kumar I do not have any paper for this phenomena. I gave answer with my knowledge what I got by reading books like laser spectroscopy books by ruller and demtoder and nonlinear optics books of R W boyd and sutherland and from my nonlinear experiments

FIrst, I must point out that some incorrect answers have been given here.

1) At least in typical nonlinear crystals (including BBO), there is no such thing as a threshold for SHG (there have been experiments where SHG from that detected SHG of sunlight! http://www.sciencedirect.com/science/article/pii/S0030401811007930)

2) In the simplest case of SHG (perfect phase matching, undepleted Gaussian-shaped pump), the SH pulse is expected to be shorter by a factor of sqrt(2), not a factor of 2

If the conversion efficiency is high, the most likely explanation is pump depletion. Depending on the level of pump depletion and neglecting any secondary effects, the duration of the second harmonic pulse may vary from smaller by a factor of square root of 2 (at very small pump depletion)  to exactly the duration of the pump pulse (100% pump depletion - not really achieved in reality). The takeaway here is, however, that the duration of the SH can and will vary depending on the pump energy/intensity, and there is not much that one can do about it.

If you really need to know the precise answer to your question, you would need to specify at least the fundamental pulse duration, crystal thickness, and conversion efficiency.

Thanks everyone.  

Dear Rimantas, fundamental pulse duration is 250 fs , Crystal length (LBO) is 4 mm and Conversion efficiency >45 %. As you said duration of SH can and will vary depending on the pump intensity. It would be very helpful if you suggest some article/references having discussion on  temporal pulse broadening. Thanks in advance.