Introduction

If an optical gain medium is available, one can in principle convert it into an oscillator by providing optical feedback to form an optical resonator. If the gain medium is a fiber OPA, the resulting device is a fiber optical parametric oscillator (fiber OPO). There are many possibilities for designing the feedback system, and hence a variety of possible fiber OPOs. In the simplest versions the pump is supplied externally and is non-resonant, i.e. it passes through the OPO in one direction, interacting with the signal and idler within the nonlinear medium.

Let us assume that the optical cavity is resonant at the signal wavelength but not at the idler wavelength; we then have a singly resonant oscillator (SRO). If the cavity has a high quality factor Q, little or no signal escapes from it, and so the useful output of the SRO may have to be the idler. If the idler is not available, however, the output is a fraction of the signal, exiting through a partially transparent mirror or via a fiber coupler; in such a case the coupling introduces a signal loss, and so Q may not be very high. Most fiber OPOs that have been implemented to date are of the SRO type.