Optical Coherence Tomography: Technical Aspects - Biomedical Optical Imaging Technologies: Design and Applications - page 168

From this equation, represents the FWHM of the normalized spectral width of
the source, and then the corresponding free-space correlation length can be obtained
as follows:

0:44 2

2 2 ln 2 c

v D

2 ln 2

l c D

:

(5.23)

Since the coherence length gives the practical estimation of the width of the sig-
nal envelope, it is considered as the axial resolution in low-coherence interferometry.
Figure 5.2 b shows the signal at the photodetector for the case of high-coherence
source and low-coherence source. If the light source is highly coherent with
narrow linewidth (a laser with 0:01 nm, in which case l c 9 cm), then the
interference will occur for a wide range of relative path length delays of the
reference and sample arms. But, when the optical source is low coherent with broad
linewidth laser (a tungsten with 300 nm, in which case l c 3m), then the
interference occurs only when the path length in the reference and sample arms
matches within the coherence length of the source of the light source. Thus, in low-
coherence interferometry, the coherence length is the parameter that determines the
resolving ability of backscattering or backreflecting sites in the sample.

5.3 OCT Principle of Operation

As described above, the interferometry is the core of the OCT imaging method.
Different interferometric configurations can be used in OCT. One of the most
common interferometric schemes used in the development of the OCT system is
the Michelson interferometer, which can be implemented either with a free-space
optic or fiber-optic configuration. The fiber-optic implementations of the OCT are
seen as the most convenient for clinical applications because the fiber has the
potential to be rugged, compact, and integrated with a wide range of medical
instruments. However, there are also a few possible issues associated with the
utilization of fiber. Fibers show absorption, although this will be negligible for the
distances involved, but this limits the dispersion-free spectral width, resulting in
degradation of longitudinal resolution. Figure 5.3 shows the schematics of a typical
first-generation time-domain free-space OCT system based on heterodyne envelope
detection scheme.
The beam from the low-coherence optical source splits into two parts, a reference
and sample beam. The reference beam reflects from a reference mirror mounted
on the scanning reference optical delay and returns to the detector. The sample
beam reflects off from the different layers within the sample. And at the output
of the interferometer, both the reference and sample beams recombine. Due to the
broadband nature of the light source, the interference between the optical fields is
only observed when the reference and sample arm optical path lengths are matched
within the coherence length of the source. The light reflected from the sample and
the reference mirror are mixed at a photodetector, and the resulting current signal