Optical Paramteric Oscillators (OPO) oscillate at a specific frequency by means of a non-linear interaction. A pump input laser with frequency \(\omega_p\) is converted through the non-linear interaction into a signal and idler frequencies, such that \(\omega_p = \omega_i + \omega_s\). Both frequencies can resonate in the cavity (doubly-resonant) or just one (singly-resonant).
The light generated by the OPO is “squeezed” - one quadrature is attenuated and the other amplified, which is useful for various scientific purposes.
By coupling two oscillators together, we create more modes. Unlike regular oscillators, where usually only one mode can oscillate due to mode competition, paramteric oscillators can (and must) oscillate in multiple modes. Active mode-locking can be used to create a source for broadband squeezed-light.
Demonstration of these effects using optical components is difficult and time consuming, we use radio components to simulate the same behavior in Radio Frequency domain (RF). RF components are easy and quick to set up, and can capture the same behavior.