Biophotonics_Concepts_to_Applications

10.1.2 Spectral Domain OCT

Whereas time domain OCT uses a low-coherence interferometer with a scanning
reference delay arm to detect echo delay signals from tissue layers in the time
domain,spectral domain OCT(SD-OCT) performs detection in the Fourier domain
by measuring the interferometer spectrum [ 12 , 13 ]. The advantage of the SD-OCT
method is the speed of obtaining an image is increased greatly, because data from
all reflection points along a given axial measurement are obtained simultaneously.
The basic setup for SD-OCT is shown in Fig.10.7. In this procedure the optical
source is similar to that used in TD-OCT, but the mirror in the reference arm
remains stationary. In SD-OCT the spectral components of the recombined beams
for a specific sample illumination point are dispersed onto an optical detector array
by a spectrometer. This process forms a spectral interferogram (interference pat-
tern), which simultaneously yields reflectance information from all backscattering
points as a function of depth along a given axial pass through the sample. The
reflection signals from two or more interfaces may be collected simultaneously
because these signals result from different spectral oscillation components. Similar
to TD-OCT, transverse scanning across the sample then produces 2D or 3D images.

10.1.3 Swept Source OCT

Swept source OCT(SS-OCT) uses a narrowband light source that can be tuned
rapidly to measure spectral oscillations at evenly spaced wavenumbers [ 14 , 15 ].
This method sometimes is calledoptical frequency domain reflectometry,wave-
length tuning interferometry,oroptical frequency domain imaging. An example
setup for SS-OCT is shown in Fig.10.8.
Here the light source is a tunable laser that can be swept at a nominally 20 kHz
sweep rate across a spectral bandwidth of 120–140 nm. Suppose the SS-OCT
system acquires signals from M evenly spaced wavenumbers with a separation
δk=Δk/M, whereΔk is the total optical bandwidth over which the source is swept.
Typical values of M range from 100 to 1000. Then the SS-OCT system will have a
scan depth range(an A-line range) zdepthgiven by

Tissue

sample

Fixed mirror

Broadband Reference arm

optical

source

Optical splitter

SMF

SMF

Grating

CCD

Signal

output

to PC

Spectrometer

Fig. 10.7 Basic setup for a spectral domain OCT system

10.1 Optical Coherence Tomography 301