Fig. 1. Operational principle of χ(2) (top) and χ(3) (bottom) processes for undetected-photon pair generation. UP, undetected photons; DP, detected photons; DFG, difference frequency generation; PDC, parametric down-conversion; and FWM, four-wave mixing.

Fig. 2. (a) Schematic of tapered SCF as used for stimulated FWM. (b) Calculated GVD and FOD parameters for the SCF with a 915 nm core diameter designed for a 1550 nm pump source.

Fig. 3. (a) Experimental setup for amplitude imaging using undetected photons. For phase imaging, the PS and chopper are removed, and the detector is replaced by a camera. BS, beam splitter; PC, polarization controller; OB, object; OC, optical chopper; WDM, wavelength division multiplexer; TLF, tapered lens fiber; TF, tunable filter; and PS, phase shifter. (b) FWM spectra generated by the two SCFs during amplitude imaging. (c) Phase correlation measurement for the detected idlers, including a sinusoidal fit to the experimental data.

Fig. 4. (a) Pattern for amplitude imaging. (b) Relative amplitude image obtained with a scan step of 1 mm. The color scale shows amplitude levels.

Fig. 5. (a) Transparent object for phase imaging cut out from a pellicle film. (b) Relative phase image obtained with a scan step of 1 mm. The color scale shows phase shift levels.

Fig. 6. (a) Opaque object for amplitude imaging cut out from a thick aluminum foil. (b) Calculated GVD and FOD parameters for the untapered SCF with a 12 μm core diameter.