Demonstration of Dispersion-Canceled Quantum-Optical Coherence Tomography

We present an experimental demonstration of quantum-optical coherence tomography. The technique makes use of an entangled twin-photon light source to carry out axial optical sectioning. It is compared to conventional optical coherence tomography. The immunity of the quantum version to dispersion, as well as a factor of 2 enhancement in resolution, is experimentally demonstrated.

Figure 1

Schematic of QOCT. A monochromatic laser of angular frequency ${\omega }_p$ pumps a nonlinear crystal (NLC), generating pairs of entangled photons. BS stands for beam splitter and ${\tau }_q$ is an adjustable temporal delay. ${\mathrm{D}}_1$ and ${\mathrm{D}}_2$ are single-photon-counting detectors that feed a coincidence circuit indicated by the symbol $\otimes$. The outcome of an experiment is the coincidence rate $C({\tau }_q)$.

Figure 2

Experimental arrangement for quantum/classical optical coherence tomography QOCT/OCT. A monochromatic ${\mathrm{K}\mathrm{r}}^{+}$-ion laser operated at ${\lambda }_p=406\mathrm{n}\mathrm{m}$ pumps an 8-mm-thick type-I ${\mathrm{L}\mathrm{i}\mathrm{I}\mathrm{O}}_3$ nonlinear crystal (NLC) after passage through a prism, P, and an aperture (not shown), which remove the spontaneous glow of the laser tube. BD stands for beam dump (to block the pump), BS for beam splitter, M for mirror, A for 2.2-mm aperture, F for long-pass filter with cutoff at 725 nm, and D for single-photon-counting detector (EG&G, SPCM-AQR-15). The quantities ${\tau }_q$ and ${\tau }_c$ represent temporal delays. For QOCT scans, the dotted components ${\mathrm{M}}_1$, ${\mathrm{M}}_2$, and ${\mathrm{D}}_3$ are removed, ${\tau }_q$ is swept, and the coincidence rate $C({\tau }_q)$ is measured within a 3.5-nsec time window. For OCT scans, beam 1 is discarded (beam 2 serves as the short-coherence-time light source), ${\tau }_c$ is swept, and the singles rate $I({\tau }_c)$ is recorded.

Figure 3

QOCT and OCT normalized interferograms for a $90\mathrm{\text{−}}\mu \mathrm{m}$ fused-silica window in air (as shown at the top of the figure). The abscissa is the scaled temporal delay $c\tau /2$, which represents displacement of the delay line ($\tau$ therefore represents both ${\tau }_q$ and ${\tau }_c$). (a) Coincidence rate $C({\tau }_q)$ normalized to ${\Lambda }_0$ (the QOCT normalized interferogram). (b) Singles rate $I({\tau }_c)$ normalized to constant background (the normalized OCT interferogram).

Figure 4

QOCT and OCT normalized interferograms for a $90\mathrm{\text{−}}\mu \mathrm{m}$ fused-silica window buried beneath two cascaded 5-mm-thick windows of highly dispersive ZnSe. As shown at the top of the figure, the ZnSe is slightly canted with respect to the incident beam (arrow) to divert back-reflections. The abscissa is the scaled temporal delay $c\tau /2$, which represents displacement of the delay line ($\tau$ therefore represents both ${\tau }_q$ and ${\tau }_c$). (a) Coincidence rate $C({\tau }_q)$ normalized to ${\Lambda }_0$ (the QOCT normalized interferogram). (b) Singles rate $I({\tau }_c)$ normalized to constant background (the normalized OCT interferogram).