Frequency-correlation requirements on the biphoton wave function in an induced-coherence experiment between separate sources

Abstract

There is renewed interest in using the coherence between beams generated in separate down- converter sources for new applications in imaging, spectroscopy, microscopy and optical coherence tomography (OCT). These schemes make use of continuous wave (CW) pumping in the low para- metric gain regime, which produces frequency correlations, and frequency entanglement, between signal-idler pairs generated in each single source. But can induced coherence still be observed if there is no frequency correlation, so the biphoton wavefunction is factorable? We will show that this is the case, and this might be an advantage for OCT applications. High axial resolution requires a large bandwidth. For CW pumping this requires the use of short nonlinear crystals. This is detrimental since short crystals generate small photon uxes. We show that the use of ultrashort pump pulses allows improving axial resolution even for long crystal that produce higher photon uxes.