Efficient four wave mixing from a picosecond fibre laser in photonic crystal fibre

In recent years nonlinear frequency conversion of light in Photonic Crystal Fibre (PCFs) has attracted widespread interest. In particular, efficient conversion of power from available pulsed pump sources (emitting at e.g 1064 nm) to shorter wavelengths is required for many applications including imaging and spectroscopy. These applications have been addressed by supercontinuum generation, with the required wavelengths being sliced from the broad spectrum generated. Here we focus on efficient generation of discrete wavelengths around 700 nm by four-wave mixing (FWM) offering high power in a narrow band. Fibre dispersion plays a key role in phase matching conditions for nonlinear processes such as FWM, and PCFs offer unique ability to tailor the dispersion properties for a particular application through proper modification of the pitch (Λ) and the relative airhole diameter (d/Λ). Various fibres have already been designed to provide a range of short signal wavelength between 686 nm and 975 nm and good conversion efficiency were demonstrated when fibres are pumped with a 1064 nm Q-switched microchip laser emitting 0.6 ns pulses [1]. Parametric oscillators using FWM in PCF have been developed [2] with fs or ps output pulses. Single pass conversion of high average power has recently been demonstrated, but required a seed to achieve good efficiency [3]. Here we use the simplest system of single-pass generation in short fibres, with a non-transform limited pump laser and without a seed. We demonstrate 29% internal conversion efficiency for frequency conversion from a picosecond fibre laser at 1064 nm to signal wavelengths in the near infrared using FWM and leading to few hundred milliwatts average power at 742 nm and 686 nm without any signal broadening.