# Why are optical parametric oscillator so big?

Optical parametric oscillators have an optical resonant cavity. Why is this cavity so big when the wavelength of light is so small? What will happen if optical cavity length is shortened?

• I'm not an expert so I don't post an answer, but I think the reason comes from the fact that modulation of optical media is usually extremely weak, so we need a large path length to get an appreciable modulation of the light. In other words, for a given applied voltage, the change of index of refraction of interesting optical media is very tiny. Feb 26 '16 at 0:05

In that case, the cavity length needs to be cf/N, with c the speed of light, f the pulse repetition rate, and N a counting number. $N=1$, and therefore the longest cavity length, is the most straightforward configuration, because the higher the value of N, and the shorter the cavity, the more difficult it is to align the cavity length get two pulses to overlap.