# Ultrafast Mid-IR Laser Pulses Generation via Chirp Manipulated Optical Parametric Amplification
## 4 Experiments on Broadband Pumped DC-OPA
### BBP-OPA in NIR Region  
The experimental setup of NIR BBP-OPA is depicted in Figure 10. A 800-nm Ti:sapphire femtosecond laser system (Legend Elite Duo - COHERENT:inc, US) with a pulse duration of 35 fs and energy of 300 \(\upmu\)J is employed. The NBP is focused by a f = 1.3 m convex lens and sent into an array of 4 fused silica (FS) plates for spectral broadening based on self-phase modulation effect in the plates. The position and angle of incidence of the FS plates are carefully adjusted to the realized maximum spectral bandwidth for the pump pulse. The 210-\(\upmu\)J BBP is obtained with a FWHM bandwidth of 73 nm, which is 2.7 times broader compared to the NBP (27 nm). The seed pulse is generated from white light continuum (WLC) in a 3-mm sapphire crystal. The BBP and WLC are synchronized and combined in a piece of 2.4-mm BBO crystal cut at \(\theta\) = 20\({}^{\circ}\). The phase-matching condition is optimized for a signal centered at 1400 nm, as shown in Figure 4. A very small noncollinear angle between the pump and seed beams is employed for the convenience of separating the output pulses. Figure 9: **(a)** Tunable idler spectra centered at 1730 nm (blue), 1790 nm (red), 1840 nm (yellow), 1920 nm (purple), 1990 nm (green), and 2050 nm (cyan). **(b)** Conversion efficiencies and TL durations of the tunable idler pulses. The BBP is characterized with a second harmonic generation frequency-resolved optical gating (SHG-FROG), and the results are shown in Figure 11. The FWHM bandwidth of the pump spans 770-850 nm range, which is broad enough to support high-efficiency phase-matching for signal components from 1170 nm to 1660 nm according to Figure 4. The measured pump pulse duration is 87 fs, with a near-quadratic spectral phase (green dashed line in Figure 11b) corresponding to a GDD of 450 fs\({}^{2}\). The seed GDD is around 240 fs\({}^{2}\) resulted from the transmissive optical components on the path, setting the pump-seed chirp ratio \(A=C_{p}/C_{s}\) at 1.8 and a broadband signal is therefore expected. The amplified signal spectrum is plotted as the red line in Figure 12a, covering the 1000-2000 nm range with a FWHM bandwidth over 400 nm. The signal spectrum from NBP-OPA system is measured and shown as the blue line in Figure 12a, and the achieved FWHM bandwidth is only 141 nm. The difference between the spectra can be well explained by the analysis based on Figure 4. According to the chirp-compensation theory, more signal wavelength components can be phase matched with more pump wavelengths. Since the BBP has a 2.7 times broader bandwidth than the NBP, it is reasonable to realize a signal bandwidth from the BBP system that is \(400/141\approx 2.8\) times broader than the signal from the NBP system. As mentioned above, high wavelength tunability is also expected from the BBP-OPA, therefore it is further investigated and plotted in Figure 12b. The signal wavelength is tuned by slightly rotating the BBO crystal and changing the pump-seed delay, which is equivalent to shifting the blue line in Figure 1 so that the high efficiency phase-matching wavelength is adjusted. The signal spectra in Figure 12b reveal high tunability throughout the non-degenerate NIR region from 1100 nm to 1500 nm, meanwhile the broad bandwidth is preserved. Figure 11: Spectral-temporal properties of BBP pulse. (**a**) Intensity (blue line) and temporal phase (green dashed line) of the pulse. (**b**) Retrieved (purple line) and measure (red dotted line) spectrum and spectral phase (green dashed line). Figure 10: Schematic design of broad band pumped-OPA (BBP-OPA) in the NIR region.