Optical Coherence Tomography: Technical Aspects - Biomedical Optical Imaging Technologies: Design and Applications - page 182

5.5.1 Time-Domain OCT Systems

The time-domain OCT operates as described in Sect 5.3 ; the wavenumber-
dependent photodetector current I D in Eq. 5.10 is captured using a single or multiple
detectors where the reference mirror is scanned to match the optical path from
reflections within the sample. Depending on the scanning modality and the signal
detection scheme employed, the time-domain OCT can be further classified into
three: (1) point-scan (flying-spot OCT), (2) line-field (linear OCT), and (3) full-
field (wide-field) OCT.

5.5.1.1 Time-Domain Point-Scan OCT (Flying-Spot OCT)

The first generation of commercial OCT instruments has been developed for
ophthalmology based on the TD-OCT configuration. However, the main drawback
of TD-OCT is that the depth scanning is realized by scanning the reference arm
axially. Moreover, the mechanical scanner pair for lateral scanning gives rises to
motion artifacts due to mechanical jitter and limited repeatability. These effects
adversely deteriorate imaging quality, especially at high speed and at high resolution
for subcellular-level imaging. A single scan of the reference mirror thus provides
a one-dimensional reflectivity profile of the sample. Similarly, two- or three-
dimensional images are produced by scanning the beam across the sample and
recording the optical backscattering vs. depth at different transverse positions. This
scheme is also called point-scan OCT or flying-spot OCT.
Figure 5.11 a shows the typical schematics of a time-domain flying-spot (point-
scan) OCT system. As described in Sect. 5.3 , in its most common version, it
comprises a broadband light source emitting light of high spatial and low temporal
coherence, which is coupled into a 2 2 fiber optics-based Michelson interferometer.
The Michelson interferometer divides the light beam and directs it into two
arms of the interferometer. The sample arm beam directs into the object to be
analyzed. It usually contains collimating optics enabling formation of a narrow
beam, which penetrates the object. In order to reconstruct two-dimensional cross-
sectional images of the object, the beam is galvanometrically scanned across the
sample surface. Light backscattered or reflected from the various structures returns
to the interferometer and is brought to interference with light reflected back from
the reference arm mirror (RM), which scanned back and forth through the required
depth of imaging. The standard fiber-optic Michelson interferometer is not the
most efficient design for practical implementation because a major disadvantage
of this configuration is that the DC signal and intensity noise generated by the light
from the reference arm add to the interference signal. In order to eliminate this
problem, an alternative configuration called balanced heterodyne configurations,
as shown in Fig. 5.11 a, has been commonly used, in which these background
noise components are canceled by subtracting the photocurrents generated by two
photodetectors. Then the resulting interfering light is detected by a photodiode (PD).