Biomedical Engineering Reference
In-Depth Information
Fig. 8.1.
Schematic illustrating the concept of low coherence interferometry. Using a
short coherence length light source and a Michelson-type interferometer, interference
fringes are observed only when the path lengths of the two interferometer arms are
matched to within the coherence length of the light source in a time-domain OCT
system. The full-width half-maximum of the envelope of the autocorrelation function
is equal to the coherence length (∆
l
c
) and axial resolution in OCT. This envelope
also represents the axial point-spread-function of an OCT system
light reflected from a reference path of known path length, which spatially
determines the location of the reflection in depth. Interference of the light
between the two arms of the interferometer can occur only when the optical
path lengths of the two arms match within the coherence length (axial resolu-
tion) of the optical source. If the reference arm optical path length is scanned,
as in a time-domain OCT system, different delays of backscattered light from
within the sample are measured, and a single column of depth-dependent data
is collected. The interference signal is detected at the output port of the in-
terferometer, digitized, and stored on a computer. Following a depth (
z
)scan,
the incident beam is scanned in the transverse direction (
x
) and multiple ax-
ial measurements are performed. A two-dimensional data array is generated,
which represents the optical backscattering through a cross-sectional plane in
the specimen (Fig. 8.2). Similarly, the OCT beam can be translated in the
third (
y
) dimension, and a series of two-dimensional cross-sectional images
can be collected to form a three-dimensional volume. The logarithm of the
backscatter intensity is then mapped to a false-color or gray-scale and dis-
played as an OCT image. Typically, the interferometer in an OCT instrument
can be implemented using fiber optic couplers, and beam-scanning can be
