Fibre assisted single photon spectrograph

An experimental detection scheme is demonstrated for directly measuring the spectrum of a parametric down-conversion (PDC) source by the use of a highly dispersive fibre. A temporally narrow pulse envelope is spectrally stretched in time by chromatic group velocity dispersion. In order to maximise this effect the high group velocity dispersion of a commercially available dispersion compensating fibre is employed. If the pulse contains only a single photon on average, detection with a conventional APD yields precise information about the arrival time of the photon at the end of the fiber. When carefully calibrated one can accurately reconstruct the original spectral distribution from the measured temporal distribution. Harnessing a single mode fiber for this type of spectrometer brings the benefits of low loss due to efficient guidance, simplicity of alignment and the necessity of only a single detector to scan the spectrum. A fibre assisted single photon spectrograph is employed to measure the correlation spectrum of a bi-photonic state. State generation is achieved by a type-II parametric down conversion process at 1550 nm in a KTP waveguide structure.