Quantum optical coherence tomography (Q-OCT) is an imaging technique that uses nonclassical (quantum) light sources to generate high-resolution images based on the Hong-Ou-Mandel effect (HOM). Q-OCT is similar to conventional OCT but uses a fourth-order interferometer that incorporates two photodetectors rather than a second-order interferometer with a single photodetector. The primary advantage of Q-OCT over OCT is insensitivity to even-order dispersion in multi-layered and scattering media. Several quantum sources of light have been developed so far. An example of such nonclassical sources is spontaneous parametric down-conversion that generates entangled photon pairs (twin-photon). The entangled photons are emitted in pairs and have stronger-than-classical temporal and spatial correlations. The entangled photons are anti-correlated in frequencies and directions. However, the nonclassical light sources are expensive and limited, several quantum-mimetic light sources are developed by classical light and nonlinear optics, which mimic dispersion cancellation and unique additional benefits.
Theory The principle of Q-OCT is fourth-order interferometry. The optical setup is based on a Hong ou Mandel (HOM) interferometer with a nonclassical light source. Twin photons travel into and recombined from reference and sample arm and the coincidence rate is measured with time delay.
The nonlinear crystal is pumped by a laser and generates photon pairs with anti-correlation in frequency. One photon travels through the sample and the other through a delay time before the interferometer. The photon-coincidence rate at the output ports of the beam splitter is measure as a function of length difference ( c τ q {\displaystyle c\tau _{q}} ) by a pair of single-photon-counting detectors and a coincidence counter. Due to the quantum destructive interference, both photons emerge from the same port when the optical path lengths are equal. The coincidence rate has a sharp dip when the optical path length difference is zero. Such dips are used to monitor the reflectance of the sample as a function of depth. The twin-photon source is characterized by the frequency-entangled state:
| ψ ⟩ = ∫ d Ω ζ ( Ω ) | ω 0 + Ω ⟩ 1 | ω 0 − Ω ⟩ 2 , {\displaystyle \left|\psi \right\rangle =\int \,d\Omega \zeta (\Omega )\left|\omega _{0}+\Omega \right\rangle _{1}\left|\omega _{0}-\Omega \right\rangle _{2},}
where Ω {\displaystyle \Omega } is the angular frequency deviation about the central angular frequency ω 0 {\displaystyle \omega _{0}} of the twin-photon wave packet, ζ ( Ω ) {\displaystyle \zeta (\Omega )} is the spectral probability amplitude. A reflecting sample is described by a transfer function:
H ( ω ) = ∫ 0 ∞ d z r ( z , ω ) e i 2 ϕ ( z , ω ) , {\displaystyle H(\omega )=\int \limits _{0}^{\infty }\,dzr(z,\omega )e^{i2\phi (z,\omega )},}
where H ( ω ) = r ( z , ω ) {\displaystyle H(\omega )=r(z,\omega )} is the complex reflection coefficient from depth z {\displaystyle z} , The coincidence rate C ( τ q ) {\displaystyle C(\tau _{q})} is then given by
C ( τ q ) ∝ Λ 0 − R e Λ ( 2 τ q ) , {\displaystyle C(\tau _{q})\propto \Lambda _{0}-Re{\Lambda (2\tau _{q})},}
where
Λ 0 = ∫ d Ω | H ( ω 0 + Ω ) | 2 S ( Ω ) {\displaystyle \Lambda _{0}=\int \,d\Omega |H(\omega _{0}+\Omega )|^{2}S(\Omega )} , and
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