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SD-OCT Base Unit Chapter 3: Description
Rev C, June 25, 2021 Page 17
Chapter 3 Description
Tutorial
Frequency Domain Optical Coherence Tomography (FD-OCT) is based on low-coherence interferometry,
which utilizes the coherent properties of a light source to measure optical path length delays in a sample.
To obtain cross-sectional images with micron-level resolution using OCT, an interferometer is set up to measure
optical path length differences between light reflected from the sample and reference arms.
There are two types of FD-OCT systems, each characterized by its light source and detection schemes;

Time-encoded Frequency Domain OCT (teFD-OCT), also named Swept Source OCT (SS-OCT).

Spatially-encoded Frequency Domain OCT (seFD-OCT),
also named Fourier Transform Domain OCT (FD-OCT) and Spectral Domain OCT (SD-OCT).
Thorlabs uses the abbreviations SD-OCT for the spatially encoded camera-based OCT systems and SS-OCT
for the time-encoded systems.
In both types of systems, a fiber coupler splits light into the sample and reference arms of an interferometer
setup.
Back reflected light, attributed to variations in the index of refraction within a sample, recouples into the sample
arm fiber and then combines with the light that has traveled a fixed optical path length along the reference arm.
The resulting interferogram is either measured by a spectrometer (SD-OCT) or by balanced photodetectors
(SS-OCT).
The frequency of the interferogram measured by the sensor is related to the depth location of the reflector in
the sample. As a result, a depth reflectivity profile (A-scan) is produced by taking a Fourier transform of the
detected interferogram. 2D cross-sectional images (B-scans) are produced by scanning the OCT sample beam
across the sample; by doing so, a series of A-scans are collected to create the 2D image. Similarly, when the
OCT beam is being scanned in a second direction, a series of 2D images is collected to produce a 3D volume
dataset.
Figure 15 FD-OCT Signal Processing