Chirped QPM crystals can provided very large parametric gain bandwidths, more than sufficient for amplifying short femtosecond pulses.  We have developed a chirped OPA based on a linearly chirped crystal of MgO:APPLN, seeded by highly chirped, broadband (>100nm) femtosecond pulses derived from a low-power modelocked Er:fibre laser.

With the appropriate design of the MgO:APPLN crystal grating structure a parametric conversion bandwidth matching the spectral bandwidth of the seed pulses can be achieved, allowing the entire seed bandwidth to be parametrically amplified.

The figures below shows the schematic and actual layout of the OPA:

The results above compare the output from a CW-seeded and unchirped OPA with one using a chirped crystal and seeded by pre-chirped pulses from a fs oscillator. The FWHM bandwidth is ~100nm and the entire spectral coverage ranges from 1470 - 1640nm.

OPA was achieved by synchronising a chirped pulses from a femtosecond Er:fibre oscillator with  3.5 ns pulses from a Nd:YLF Q-switched laser.  The final output power was 2.5mW, corresponding to a maximum energy of 2.55µJ, a gain of 51dB, and an efficiency of 2.1%.  The output pulses had a FWHM bandwidth of 80nm and the seed pulses had a FWHM bandwidth of 30nm.

The synchronisation scheme is shown below:

In an extension of the work we have demonstrated a Q-switched OPO based on a chirped APPLN crystal pumped by a Q-switched Nd:YLF laser:

The OPO is a highly efficient monolithic, Q-switched, nanosecond optical parametric oscillator based on a magnesium-oxide-doped periodically poled lithium niobate crystal and containing multiple quasi-phasematched gratings. The crystal consisted of a single unchirped grating and five gratings containing progressively increasing amounts of longitudinal chirp (above). The monolithic design makes the device highly compact, stable, and robust, and it demonstrated a pump-to-signal conversion efficiency of around 50%:

The OPO generated 50 uJ pulses at 1.55 um with a spectral bandwidth of 20 nm. Sonogram traces are presented showing the effect of crystal chirp on the temporal and spectral performance.