Biomedical Engineering Reference
In-Depth Information
retina
ND
Movable stage
CL2
Imaging plane
L1
L2
SLD
CL1
galvano scanner
L3
Grating
L5
PC
L4
slit
2D-Detector array
Fig. 5.14
Optical scheme of line-field spectral-domain optical coherence tomography.
CL
cylin-
drical lens,
ND
neutral density filter,
SLD
superluminescent diode, and
PC
personal computer
with a sweeping frequency of a few hundreds of kilohertz has been achieved, which
drastically improves the imaging speed of SS-OCT to compete with SD-OCT and
TD-OCT in terms of imaging speed [
83
].
Figure
5.15
shows the typical implementation of SS-OCT systems, which employ
a broadband, rapid frequency-swept laser source and InGaAs photodetectors to
perform Fourier domain OCT imaging without the use of a spectrometer [
45
,
50
-
54
,
77
]. With such a wavelength-swept source, interference signals at individual
wavelengths can be measured sequentially with high spectral resolution. This
spectrally resolved data acquisition is central to frequency-domain ranging. This
method offers significantly higher sensitivity than the time-domain ranging method
used in conventional OCT. Furthermore, frequency-domain ranging does not require
reference delay scanning and can therefore be applied to increase imaging speed.
Since SD-OCT systems typically employ silicon-based CCD detectors, they can-
not operate at wavelengths longer than
1,000 nm, since the detector efficiency
degrades rapidly beyond this point. Moreover, the unavailability of relatively cheap
spectrometer at 1,300 nm, which is very essential for OCT imaging of epithelial
tissues and many nonbiological samples to achieve an image penetration depth of
1-3 mm. With a pressing need for high imaging speed at 1,000-1,300 nm in various
applications, wavelength-swept sources, particularly rapidly tuned lasers, have
developed substantially in recent years and have emerged as important and practical
light sources for OCT. One particular advantage of SS-OCT over SD-OCT is worth
mentioning, that is, that a balanced detection scheme can be used in SS-OCT (see
Fig.
5.15
) to cancel out the strong background light in the interferometer, therefore
enhancing the dynamic range of the system.
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