# Sum harmonic and sum frequency generation

Can two collinear beams of two different wavelengths generate the sum frequency or they need to pass each other at a certain angle. For a monochromatic light how does the sum harmonic gets generated while for two different beams a certain angle is necessary?

for any frequency generation process, we need two conditions: frequency match: $$\omega_1+\omega_2=\omega_3$$
phase matching: $$k_1+k_2=k_3$$