How to find the FWHM-diameter of the SH beam?

[telekom90]

Homework Statement

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?

Homework Equations

d_FWHM = ω(z)√2ln2

The Attempt at a Solution

λ =1064nm, d_FWHM=5 mm$,$ λ_SH=532 nm .
I can't solve this problem because I don't know how to relate this equation "d_FWHM=ω(z)√2ln2= 5mm " with the FWHM-diameter of second harmonic (SH) beam?!

 

[blue_leaf77]

The key word is

that is proportional to the square of the incoming field

.