Tunable Wavelengthsby Four-Wave Mixing

Photonics specialists from Helsinki University of Technology and from Arctic Photonics of Jorvas, both in Finland, are using four-wave mixing (FWM) to produce tunable narrowband light in a microstructured optical fibre. The light has a pulse shorter than 1 ns, peak power of tens of watts, with signal wavelengths from blue to orange. It would be a good source for fluorescence spectroscopy.

To produce this narrowband light, photons at 532 and 1064 nm from an Nd:YAG laser pass through a lens into the small-core triangular lattice fibre, engineered to have its zero dispersion between the two pump wavelengths. The pump photons interact and produce a visible signal photon and a near-infrared idler photon, conserving both energy and momentum. An Ando Electric Co. Ltd. optical spectrum analyser measures the resulting wavelengths.

The researchers found that the signal and idler wavelengths are more widely spaced when the 532-nm pump light is polarized parallel to the fibre’s direction of maximum FWM output. The output waves have a shorter pulse than the pump light because of the third-order nature of their generation and because of group-velocity dispersion in the fibre.

(Applied Physics B – Lasers and Optics, June 2008, pp. 461-465)