Phase-Sensitive Optical Coherence Microscopy (OCM) - Biomedical Optical Phase Microscopy and Nanoscopy - page 257

single-point imaging, similar to A-scan in ultrasound, line-scan, or full-field imaging. When
applying the techniques for full-field imaging, the FOV, which is governed by the
objective NA, plays a crucial role, as high lateral resolution means lower FOV, so part of
the full-field image is defocused. Various methods to extend the FOV in this case have
been proposed in the literature [4] .

13.3 Phase OCM

The interference spectral data that are collected by the OCM system contains both the
amplitude and the phase information of the light beam. When using Fourier transform on
the interference signal, a complex expression is resulted. OCM estimates the axial position
of the internal reflective structures in the sample by taking the magnitude of the Fourier
function in the space domain. Phase OCM goes one step further and uses the phase
information of the obtained Fourier transform as well. The phase information allows
measuring very small time-dependent variations in the OPD. This information is already
contained in the OCM image and does not require additional hardware setup.

A description will be given here for the SD phase-sensitive OCM technique that was proposed
by Izatt and Choma [1] and later enhanced [5 7] to allow increased OPD range measurements.

The intensity of the interference pattern on the spectrometer can be expressed as follows:

p

R R R S IðkÞ 5 SðkÞR R 1 SðkÞR S 1 2 SðkÞ cos ð 2 kΔd 1 θÞ (13.5)

where k is the wavenumber, S ( k ) is the spectral density of the light source, R R and R S are
the reflectivities from the reference surface and the sample surface, respectively, Δd is the

OPD between the sample and the reference signals, and θ is a constant phase shift.

The first two expressions in the equation are constant intensities for a given setup and sample,
so the intensity is modulated by the cosine expression as a function of the OPD. The
challenge is to determine the phase expression and extract the depth information from it.
The phase is defined as follows:

ϕðkÞ 5 2 kΔd 1 θ (13.6)

The system sensitivity in time depends on the stability of this phase expression. If the
system has a phase stability or phase jitter of Δϕ , the system SNR can be expressed as:

2

SNR 5 (13.7)

π

2

hΔϕ

2

i

To determine the OPD, the phase function needs to be unwrapped. Then, the slope of the
phase function is used and combined with the phase value to increase depth precision.