How to derive the Manley-Rowe relation for the process of the second harmonics generation

Derive the Manley-Rowe relation for the process of the second harmonics generation.

Manley-rowe relation ~ The Manley-Rowe relations are mathematical expressions developed originally for electrical engineers to predict the amount of energy in a wave that has multiple frequencies. - http://en.wikipedia.org/wiki/Manley%E2%80%93Rowe_relations

Second-harmonic generation ~ Second harmonic generation( SHG); also called frequency doubling) is a nonlinear optical process, in which photons interacting with a nonlinear material are effectively “combined” to form new photons with twice the energy, and therefore twice the frequency and half the wavelength of the initial photons.-http://en.wikipedia.org/wiki/Second-harmonic_generation
It is example of nonlinear phenomena(I, 3). In the SHG process, the intense wave at the frequency w propagates in the medium with second-order nonlinearity( VI, 1).