Fig. 1. Experimental setup: scattered Stokes and anti-Stokes photons emerging from the port labelled “Out” are detected; FPC, fiber polarization controller; PBS, polarization beam splitter; G, gratting; QWP, quarter-wave plate; HWF, half-wave plate; “Signal-In” port is blocked during photon-counting measurement.

Fig. 2. Measured coincidence rates as a function of the number of scattered photons per pump pulse (labelled Single Counts/Pulse) in the anti-Stokes channel for (a) scattered photons co-polarized with the pump and (b) scattered photons cross-polarized with the pump. In both cases λ_p =1536nm and Ω/2π=1.25THz; the diamonds represent the total-coincidence counts produced by a single pump pulse, the triangles represent the accidental-coincidence counts produced by two adjacent pump pulses, and the line represents the calculated coincidence counts for two independent light sources. The insets show the number of scattered photons per pump pulse detected in the anti-Stokes channel as a function of the number of photons in the pump pulse (hollow circles). A second-order polynomial, N_a =s ₁ N_p +s ₂ $N_p^2$, is shown to fit the experimental data (dot-dashed line). The contributions of linear scattering, s ₁ N_p , (dashed line) and quadratic scattering, s ₂ $N_p^2$, (dotted line) are plotted separately as well. For the inset in (a): s ₁=0.00436 and s ₂=0.01046; for the inset in (b): s ₁=0.00381 and s ₂=0.00033.

Fig. 3. Same as in Fig.2, except λ_p =1525nm. For the inset in (a): s ₁=0.00688 and s ₂=4.38×10^-5; for the inset in (b): s ₁=0.005 and s ₂=0.

Fig. 4. Optical transmission spectrum of the double-grating filter. The pump at 1536 nm is rejected by more than 75 dB compared to the peak transmissions in the Stokes and anti-Stokes channels.

Fig. 5. Same as in Fig.2, except Ω/2π=0.5THz. For the inset in (a): s ₁=0.00317 and s ₂=0.0132; for the inset in (b): s ₁=0.00259 and s ₂=0.00025. In (a), taking into account the detection efficiency of 6% in the anti-Stokes channel, at a photon-pair production rate of 0.04 (0.067) per pulse the ratio between the total coincidence rate and the accidental coincidence rate is 13:1 (7.5:1).