Spectral bandwidth enhancement and pulse compression in a nanosecond monolithic optical parametric oscillator using chirped quasi-phasematching

Broadband operation is not commonly associated with nanosecond optical parametric oscillators (OPOs), and for most applications, eg. spectroscopy, narrowband operation is the desired outcome. Exceptions are found in applications involving free-space propagation, eg. active ranging, three-dimensional laser terrain-mapping and burst-illumination imaging, which require pulses with energies sufficient to facilitate detection after propagation over distances of hundreds of metres to kilometres, and spectral bandwidths wide enough to minimise speckle effects. Broadband operation has previously been applied in femtosecond OPOs using chirped QPM (Arbore et al., 1997) to achieve low threshold and widely tunable operation (Beddard et al., 2000) but, to our knowledge, the application of chirped QPM to nanosecond OPOs has not been widely explored. Free-space imaging can be implemented with few-muJ pulses (Busck and Heiselberg, 2004), and here we describe the performance of a Q-switched OPO designed to produce a broadband 1.55mum output using chirped QPM gratings.