1. The problem statement, all variables and given/known data Assume a Gaussian laser beam ( 1064 nm wavelength) having an FWHM-diameter(intensity) of 5 mm. By transmission through a thin nonlinear crystal, a new wave is generated that is proportional to the square of the incoming field (and therefore radiates at twice the frequency, or 532nm wavelength). This second harmonic (SH) beam co-propagates with the fundamental laser beam. What is the FWHM-diameter of the SH beam? 2. Relevant equations dFWHM = ω(z)√2ln2 3. The attempt at a solution λ =1064nm, dFWHM=5 mm##, ## λSH=532 nm . I can't solve this problem because I don't know how to relate this equation "dFWHM=ω(z)√2ln2= 5mm " with the FWHM-diameter of second harmonic (SH) beam?!