Custom scalable fs OPA and ps narrowband spectral amplifier for high-resolution kHz-rate rovibrational fs/ps CARS

The advancement of kHz-rate hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) spectroscopy requires the efficient conversion of a broadband fs source to a narrowband ps probe for resolving key spectral features, multiple species, and/or nonequilibrium phenomena. In this work, a custom fs optical parametric amplifier (fs OPA) coupled with a diode-pumped Nd:YAG-based narrowband spectral amplifier (NSA) is developed to achieve a high ratio (<i>R</i>_{<i>p</i><i>s</i>-<i>f</i><i>s</i>}=0.47) of amplified narrowband ps probe pulse output to broadband fs source laser energy input. The fs OPA uses about 50 µJ at 800 nm to produce a white-light continuum (WLC), while about 850 µJ of 800 nm light is frequency-doubled to 400 nm to pump multiple stages of parametric amplification to generate a broadband idler output at 1064 nm. The 1064 nm beam is then spectrally narrowed and temporally stretched using an etalon, amplified in the NSA, and frequency-doubled to 532 nm. Detailed characterization of the WLC, spectral tunability, parametric amplification, and output beam quality is performed. The high-energy, narrowband output from the NSA is then used as a probe pulse for high-resolution <i>N</i>₂ rovibrational CARS thermometry in a <i>H</i>₂/<i>A</i><i>i</i><i>r</i> Hencken burner flame. This approach provides flexibility in the overall energy budget of fs laser sources by enabling efficient generation of ps probe pulses from a small, variable fraction of the fundamental laser source energy. This can enable highly sensitive single or multiple fs/ps CARS measurement strategies over a wide range of potential applications.