The drawback of FOPO with CW pump is its relatively low pump power. The alternative scheme is to use optical pulse as the pump so as to get very high peak pump power. To build up the oscillation, the pump pulse train must be synchronized with intra-cavity pulse, as is called the regenerative mode locking [
15]. In this case, the repetition rate of the pulse should be the multiple of the longitudinal mode spacing of the cavity. Since the OPA gain provided by the HNL-DSF is instantaneous and there is no gain storage, every time when the intra-cavity pulse enters the HNL-DSF, it must propagate through the fiber simultaneously with a pump pulse so as to obtain the OPA gain [
16]. Due to the large peak pump power, the generated signal light can operate at wavelength far from the pump when the pump is located in the normal dispersion region [
17,
18]. Due to the large separation between the pump and the oscillating wavelength, walk off would occur as consequence of their difference in group velocities because of the dispersion of gain fiber. Hence the gain fiber is shorter than that of the CW pump case. Experimentally only 50-m HNL-DSF is enough for sufficient OPA gain. In the experimental setup, an intra-cavity tunable optical delay line is used to adjust the cavity length so as to synchronize with the pump pulse train [
18]. Different from the actively mode locked FOPO, intra-cavity optical filter is not necessary to select oscillating wavelength. Since it requires rigorous synchronization between the pump and the signal pulses, the cavity can select the oscillating wavelength adaptively with the effect of the intra-cavity dispersion. This characteristic can be utilized to realize wavelength tunability. Once the oscillation is built up, the rigorous synchronization between the pump and the signal pulse is realized. If a small detune is introduced intendedly, the cavity can adaptively select a different oscillating wavelength to insure the synchronization to be built up again based on the function of cavity dispersion. Either the cavity length or the repetition rate of the pump pulse can be adjusted to introduce detune and the synchronization can be built up again at new wavelength instantaneously. Figure 3 shows the tunability of FOPO simply by adjusting the cavity length. The generated signal can be tuned from 1413.5 to 1478.0 nm and the idler can be tuned from 1610.1 to 1695.8 nm. It can be observed that the generated wavelengths are in the nonconventional wavelength band.