Ultra-fast transitions buried within long recovery times are difficult to measure and study directly. As an example, consider the spectroscopic measurement of a non-radiative upper level transition of a laser material possessing a long fluorescence time. The slow time constant of the overall relaxation process requires a pump with a low repetition rate. The speed of the transition in question requires an ultra-fast probe. It is difficult to obtain synchronous multi-wavelength pulses that satisfy both femtosecond duration and kilohertz repetition rates.
We circumvent these issues by cavity dumping a femtosecond optical parametric oscillator. The signal is accessed at the appropriate rate to act as the pump, and the use of quasi-phase-matching provides synchronous sub-100 fs probe pulses at various wavelengths. We study the transient behavior of the singly-resonant optical parametric oscillator and develop a model to predict recovery profiles after loss events. Finally, we explore the feasibility of applying the new technique to Cr4+:YAG crystals to query the never before measured time constant of the 3E to 3B2 transition.