# Ultrafast Mid-IR Laser Pulses Generation via Chirp Manipulated Optical Parametric Amplification
## 4 Experiments on Broadband Pumped DC-OPA
### Preliminary Results on Mid-IR BBP-OPA  
Despite the ultrabroad bandwidth and excellent tunability of BBP-OPA in the NIR region, the signal wavelength is inherited from the white light continuum. Since the WLC can only be extended to \(\sim\)2 \(\upmu\)m [50], it is technically difficult to directly amplify the WLC and obtain broadband pulses centered at \(>\)1.6 \(\upmu\)m. In this case, the idler pulse with longer wavelength and passive CEP stability can be utilized for further investigation. The schematic design of the BBP-OPA system for generating Mid-IR pulses is presented in Figure 14. The main difference between this system and  
Figure 12: (**a**) Signal spectra obtained from NBP-OPA (blue) and BBP-OPA (red). (**b**) Signal spectra centered at 1200 nm (blue), 1300 nm (red), and 1400 nm (yellow). Inset: beam profile of the output signal.  
Figure 13: Spectral-temporal properties of uncompressed signal pulse. (**a**) Intensity (blue line), phase (green dashed line) of the pulse, and TL pulse profile (red line). (**b**) Retrieved (purple line) and measure (red dotted line) spectrum and spectral phase (green dashed line).  
the previous one is that instead of using a positively chirped BBP pulse, a pair of chirped mirrors is employed to introduce negative GDD to the BBP, therefore a broadband Mid-IR idler can be expected according to the theoretical analysis. Furthermore, the recollimation of BBP is accomplished by a silver-coated concave mirror instead of a convex lens (L4 in Figure 10) in order to avoid undesired temporal and spatial distortions.  
According to the theory, unlike the signal pulse, the idler pulse in chirp-manipulated OPA is more sensitive to the initial pump parameters. Taking that into account, we will study the idler property with various pump chirps in this section, which is realized by inserting a bulk FS material on the pump path for adjustable positive GDDs. In this way, the GDD of both NBP and BBP can be precisely manipulated by using a bulk FS of different thickness or rotating the material. In this proof-of-principle experiment, the preliminary results are obtained using NBP and BBP pulses chirped with a similar GDD around \(-400\) fs\({}^{2}\).  
The pump energy of 100 uJ is used in both cases. Owing to the optimization of the pump quality, the conversion efficiencies in the NBP and BBP systems are both enhanced compared to the previous experiment. Output signal energies of 6.0 uJ and 5.6 uJ and idler energies of 3.8 uJ and 3.4 uJ are obtained in NBP and BBP systems, respectively. The higher efficiency is achieved in NBP-OPA due to the higher peak intensity of the pump pulse.  
Even though the efficiency in BBP-OPA is slightly lower, the achieved bandwidth can easily compensate the energy loss. As plotted in Figure 15, the signal spectra from two systems are similar, whereas the idler spectra show an observable difference. The idler bandwidth from NBP-OPA is only 175 nm, and the idler bandwidth from BBP-OPA reaches 434 nm, which is 2.5 times larger than NBP-OPA. The spectrum is measured up to 2100 nm due to the limitation of available spectrometer (Ocean Optics NIRQuest512-2.2) in our lab. The ultrabroad bandwidth of the idler supports a sub-2-cycle TL pulse duration of 10.6 fs, which makes it a much more useful tool than the idler from NBP-OPA in potential applications, despite its slightly lower pulse energy.  
Figure 14: Schematic design of BBP-OPA in the Mid-IR region.  
Figure 15: (**a**) Signal spectra obtained from NBP-OPA (red line) and BBP-OPA (blue line). (**b**) Idler spectra obtained from NBP-OPA (red line) and BBP-OPA (blue line).