Fig. 1. Schematic illustration of the proposed OC-FOPA scheme.

Fig. 2. Simulated dependence of conversion efficiency and spectrum bandwidth of the mid-IR idler pulses on the crystal length, for (a), (c) the conventional fs-OPA and (b), (d) the proposed OC-FOPA schemes. It should be noted that for all intensity–length pairs of each OPA scheme, constant conversion efficiency and a similar idler spectrum [(c), (d)] can be achieved in the absence of temporal pulse slipping. The initial signal intensity was fixed at 1% of the pump intensity.

Fig. 3. (a), (b) Small-signal idler spectrum bandwidth versus various grating chirp rates and (c), (d) spectrum details near the maximum value with a fixed spatial chirp coefficient (1+u)1/2 of 5, for different pulse duration cases. In the calculations, pump and signal have equivalent elliptical beam profiles, and the major and minor axes are 5 and 1 mm, respectively. The PPLN crystal length is fixed at 5 mm. Insets: The (1+u)1/2-dependent optimum chirp rates. τpump and τsignal represent the pulse duration of the pump and signal pulses, respectively. (a), (c) τpump=35  fs and τsignal=100  fs; (b), (d) τpump=100  fs and τsignal=100  fs.

Fig. 4. (a) Dependence of the photon conversion efficiency on the spatial chirp coefficient (1+u)1/2, (b) the amplified idler spectra and the corresponding dispersion-compensated pulse envelopes, (c)–(f) the individual extracted spatial–spectral and spatial–temporal profiles. In the calculations, pump and signal have equivalent elliptical beam profiles, and the minor axes are fixed at 1 mm for various spatial chirp coefficients. The idler spectrum is obtained by linear superposition of each individual spectrum component across the spatial chirp axis x.

Fig. 5. (a) Amplified idler spectra and (b) dispersion-compensated pulse envelopes at different seeding intensities. To minimize the GVM effects, the spatial chirp coefficient (1+u)1/2 was fixed at 20. The same simulation parameters were employed as those given in Fig. 4. The seeding intensity was set to 0.1%, 1%, 5%, and 10% of the pump intensity, corresponding to an overall efficiency of 9%, 50%, 59%, and 54%, respectively.