Spectral domain low-coherence interferometry is a powerful way to measure the magnitude and echo time delay of backscattered light with very high sensitivity.
resolution for the light’s echo time delays. In low-coherence
interferometry, axial resolution is given by the width of the
field autocorrelation function, which is inversely proportional
to the bandwidth of the light source.
Transverse resolution in OCT is the same as in optical
microscopy, and it is determined by the diffraction-limited
spot size of the focused optical beam. The diffraction-limited
minimum spot size is inversely proportional to the numerical aperture or the focusing angle of the beam. In OCT, the
coherence-gated information about the elementary volume of
the scatterers within the obscuring scattering specimen can be
obtained from either the time-domain measurement (
TD-OCT) or the frequency domain measurement.
In the TD-OCT system, the reference mirror is scanned
to match the optical path from reflections within the sample.
First-generation commercial OCT instruments have been
developed for ophthalmology based on the TD-OCT configuration. However, the main drawback is that the depth
scanning is realized by scanning the reference arm axially.
Moreover, this mechanical scanning has limited repeatability
and gives rise to motion artifacts due to mechanical jitter.
These effects deteriorate imaging quality, especially at high
speeds. FD-OCT is a variant interferometric imaging modality;
it has widely attracted interest in the field of biomedical imaging
because of its higher sensitivity and imaging speed compared to
its time domain counterpart. The principle of FD-OCT relies
on the transformation of the OCT time-varying signal along the
optical axis—termed the A-scan—into the frequency domain.
FD-OCT has the advantage of not requiring any moving
parts in order to obtain an axial scan. In FD-OCT, a spectral
fringe pattern is recorded either by a spectrometer with a line-scan camera (spectral domain OCT, SD-OCT) or by a single
detector with a rapidly tunable laser as a light source (swept
source OCT, SS-OCT). The figure on the facing page shows
a simple free-space implementation of the SD-OCT system
based on a Michelson interferometer; it illustrates how the
system reconstructs the depth-resolved information from the
anterior chamber of an eye.
In SD-OCT, researchers acquire a broadband interference
signal with spectrally separated detectors with a dispersive
optical element such as a grating and a linear detector array.
To suppress autocorrelation, self-cross correlation and camera
noise artifacts, investigators ensemble-average all the spectral interferograms in each slice along the x-direction at each
wavelength to obtain a reference spectrum. This background
spectrum is then subtracted from each A-scan.
Researchers then remap the subtracted spectral interferograms from -space to k-space by using the spline interpolation method. Due to the Fourier relation (Wiener-Khintchine
theorem between the auto correlation and the spectral power
density), the depth-resolved information can be immediately
reconstructed without moving the reference arm, by using a
Fourier-transformation from the remapped spectra.
OMAG is a functional extension of FD-OCT. The key difference between OMAG and FD-OCT is that, in OMAG, the
spatial interferogram in the lateral direction (B-scan) is modulated with a Doppler frequency that can separate the moving
and static scattering components within the sample. This
Doppler modulation frequency can be introduced by mounting the reference mirror in the reference arm onto a linear
piezo-translation stage, which moves the mirror at a constant
velocity across the B-scan (i.e., x direction scan), or simply via
inherent scattering fluid flow within the sample.
We achieved this by mounting the reference mirror in the
reference arm onto a linear piezo-translation stage that moved
the mirror at a constant velocity across the B-scan (i.e., the x
direction scan). However, the latest version of OMAG utilizes
the spatial modulation frequency provided by the inherent
blood flow rather than reference arm modulation.
The figure below shows the OMAG setup. The light source
was a broadband, superluminescent diode with a full-width-
Pilot laser for beam guiding
50: 50 fiber