Fig. 1. Experimental setup for the fiber-feedback OPO at mid-infrared. An Yb-doped fiber laser (YDFL) at 1032 nm is used to pump the periodically poled lithium niobate (PPLN) crystal for generating the signal and idler lights. The near-infrared signal beam is coupled into a section of polarization-maintaining (PM) fiber. After passing an Er-doped gain fiber, the signal is coupled back into the nonlinear crystal to form a ring cavity. Alternatively, the crystal can be replaced by a chirped-poling lithium niobate (CPLN) to access a broadband spectrum for the parametric gain. In this case, the gain fiber should be removed to fully exploit the wide tuning range. DM, dichroic mirror; Col., collimator; WDM, wavelength division multiplexer; LD, laser diode.

Fig. 2. (a) MIR power dependence on the LD and pump powers. (b) MIR power as a function of the LD power for the gain fiber in the presence of various pump powers for the nonlinear conversion. (c) MIR power versus the pump power for various LD powers. The behavior for single-pass configuration is given for comparison as denoted by OPG.

Fig. 3. Pulse instability at the FOPO and OPG configurations. Recorded pulse traces for the (a) OPG and (b) FOPO at pump powers of 0.8 and 0.5 W, respectively. (c) Measured pulse-to-pulse fluctuation for the near-infrared (NIR) signal and mid-infrared (MIR) idler pulses.

Fig. 4. Passive synchronization of the pump, signal, and idler pulses. (a) Measured repetition rates for the pump and signal pulses during a period of 1 h. The repetition rate is offset by 20.113 MHz for clarity. (b) Relative repetition rate for the synchronized pulse trains, indicating a standard deviation of 98 mHz. (c) Evolution of repetition rates for the pump and idler pulses, and (d) for the corresponding differential values. (e), (f) Tolerance range for obtaining synchronized pulses versus the pump power (e) and LD power (f).

Fig. 5. Generated spectra and output power as a function of the cavity-length variation. (a), (b) Spectral evolution for the signal (a) and idler (b) beams as tuning the delay line in the OPO cavity. (c), (d) Central wavelength and output power as a function of the delay for the signal (c) and idler (d) beams. The solid lines are calculated from the theoretical model.

Fig. 6. Recorded optical spectra for the (a) signal and (b) idler light as tuning the delay line in the case of a CPLN nonlinear crystal used in the OPO cavity. (c), (d) Central wavelength and output power as a function of the delay for the signal (c) and idler (d) beams. The solid lines are calculated from the theoretical model.