Fig. 1. Nonlinear phasematching diagram for the process 2ω_p→ω_s+ω_i, calculated from the measured dispersion curve of a certain PCF for input powers P_p =14 W (blue curve); P_p =140 W (red curve); P_p =1400 W (green curve).

Fig. 2. Calculated frequency offset and bandwidth for pair photons generated by MI (pump offset λ _pump-λ ₀=1.3 nm) and FWM (λ _pump-λ ₀=-11.7 nm) for a given PCF at P_p =10 W. The Raman gain shape is also shown (from [11]). Each gain curve is normalized individually.

Fig. 3. Electron microscope image of the PCF. Λ=2.97 µm, d/Λ=0.39, λ₀=1065 nm

Fig. 4. Output spectrum of the PCF when pumped with low-power Q-switched laser pulses at 1047 nm. The OPO wavelengths at 834/1404 nm are clearly visible.

Fig. 5. Optical layout. Laser, 1047nm Nd:YLF laser, 250mW CW; WP, halfwave plate; PBS, polarizing beamsplitter cube; P1, P2, SF11 dispersing prisms; O1, ×20 microscope objectives; PCF, 1.5 m, 3 m or 6 m of modified dispersion PCF; M1, protected silver mirror (R>95%); M2, near IR dielectric mirror (R>98%); F1, 850 nm interference filter, bandwidth 70 nm, T=75%; F2, long wave pass filter, cut-on wavelength 1220 nm; O2, ×10 microscope objectives; SMF, fibre patchcords (SMF28); D Si, Silicon single photon detector; D Ge, cooled Germanium single photon detector.

Fig. 6. Time interval histogram showing the coincident photon detection peak

Fig. 7. Germanium detector count rate as a function of wavelength. The low wavelength cut-off is from the 1200 nm pump blocking filter and the long wavelength cut-off is due partly to dropping Raman signal but also due to falling detector efficiency. The red curve shows a finely sampled experiment around the pair photon peak at 1392 nm.