I. Introduction
Optical parametric oscillators (OPOs) have been regarded as a practical technique to generate coherent radiation at nonconventional wavelength regions [1]. Conventionally, the nonlinear effect of crystals is utilized for parametric oscillation. The performances of CW and pulsed OPOs can be significantly improved by using doubly resonant configuration [2], [3]. Recently, fiber optical parametric oscillators (FOPOs), which utilize the nonlinear effect in optical fibers, eliminate the precise alignment required in crystal OPO and allow further integration with other fiber components, are particularly attractive [4]. In the communication band, CW and pulsed FOPOs have been reported by using photonic crystal fibers (PCFs) [5], highly nonlinear fibers (HNLFs) [6], or dispersion-shifted fibers (DSFs) [7]–[10] as the gain medium. At visible and 1 wavelength regions, CW and pulsed FOPOs based on PCFs have also been widely investigated [11]–[17]. High conversion efficiency FOPOs have been reported [18], however, the high conversion efficiency is usually achieved in one gain band which is selected to synchronize with the pump [19]. The conversion efficiency to the signal and idler gain bands is expected to be enhanced simultaneously by a doubly resonant configuration. A CW single-longitudinal-mode (SLM) FOPO operated at communication band with doubly resonant configuration has been reported, and the threshold pump power can be reduced significantly [20]. However, to the best of our knowledge, a pulsed FOPO with doubly resonant configuration has not been demonstrated.