Non-collinear optical parametric amplification (non-collinear OPA, NOPA) has been used for more than a decade to generate ultra-broadband pulses in the visible frequency range (~500-750 nm). The basic idea behind the concept of NOPA is illustrated in Figure 1. Briefly, when the signal and pump are collinear it is only possible to amplify a relatively narrow range of frequencies at once. However, at certain non-collinear angle α between pump and signal beams, the phase matching curve becomes essentially flat in a broad frequency range over which simultaneous amplification of signal frequencies now is possible.[1-4]
Fig.1. Collinear (left panel) vs. non-collinear (right panel) optical parametric amplification.[3-5]
The extension of the OPA bandwidth in non-collinear geometry can be briefly explained by the fact that the bandwidth of the OPA process Δλ is inversely proportional to the so-called group-velocity mismatch (GVM) between the idler and signal pulses:
where νi (νs) is the signal (idler) group velocity, α (β) is the angle between the wave-vectors of the pump and signal (idler) beams. At certain values of angles α and β it is possible to match the projection of the group-velocity of faster-propagating idler onto the group-velocity vector of slower-propagating signal, thus greatly reducing the value of GVM. This extends the bandwidth of visible pulses generated in a BBO-OPA from ~300 cm-1 to ~5000 cm-1.[5]
Reference:
1. Wilhelm, T., Piel, J., and Riedle, E., Sub-20-fs pulses tunable across the visible from a blue-pumped single-pass noncollinear parametric converter. Optics Letters 22(19), 1494-1496 (1997)
2. Cerullo, G., Nisoli, M., Stagira, S., and De Silvestri, S., Sub-8-fs pulses from an ultrabroadband optical parametric amplifier in the visible. Optics Letters 23(16), 1283-1285 (1998)
3. Kobayashi, T. and Shirakawa, A., Tunable visible and near-infrared pulse generator in a 5 fs regime. Applied Physics B-Lasers and Optics 70, S239-S246 (2000)
4. Kobayashi, T. and Baltuska, A., Sub-5 fs pulse generation from a noncollinear optical parametric amplifier. Measurement Science & Technology 13(11), 1671-1682 (2002)
5. Kobayashi, T., Femtosecond noncollinear parametric amplification and carrier-envelope phase control, in Femtosecond Optical Frequency Comb: Principle, Operation and Applications, J. Ye and S.T. Cundiff, Editors. 2005, Springer. p. 133-175.