  Coherence-domain imaging systems can be operated in a single-photon counting
mode, offering low detector noise; this in turn leads to increased sensitivity
for weak light sources and weakly reflecting samples. We have demonstrated that
excellent axial resolution can be obtained in a photon-counting coherence
domain imaging (CDI) system that uses light generated via spontaneous
parametric down-conversion (SPDC) in a chirped periodically poled
stoichiometric lithium tantalate (chirped-PPSLT) structure, in conjunction with
a niobium nitride superconducting single-photon detector (SSPD). The bandwidth
of the light generated via SPDC, as well as the bandwidth over which the SSPD
is sensitive, can extend over a wavelength region that stretches from 700 to
1500 nm. This ultra-broad wavelength band offers a near-ideal combination of
deep penetration and ultra-high axial resolution for the imaging of biological
tissue. The generation of SPDC light of adjustable bandwidth in the vicinity of
1064 nm, via the use of chirped-PPSLT structures, had not been previously
achieved. To demonstrate the usefulness of this technique, we have constructed
images for a hierarchy of samples of increasing complexity: a mirror, a
nitrocellulose membrane, and a biological sample comprising onion-skin cells.
