Biophotonics_Concepts_to_Applications

an increase in imaging speed. There are two types of FD-OCT methods, as
described in Sects.10.1.2and10.1.3, respectively. Thefirst is calledspectral
domain OCT, which uses a spectrometer and a line-scanning CCD camera. The
other method is calledswept source OCT, which uses a frequency-swept light
source having a narrow bandwidth.

10.1.1 Time Domain OCT

An OCT system can be implemented either in free space with a dichroic beam-
splitter or with opticalfiber channels by using opticalfiber couplers. The basic
opticalfiber OCT system normally is based on low-coherence interferometry using
a Michelson interferometer setup as shown in Fig.10.3. In such setups, the use of
opticalfiber technology enables the OCT system to be compact and portable.
In the TD-OCT procedure shown in Fig.10.3,first light pulses from a broad-
band optical source are divided in half by a 3-dB opticalfiber coupler or splitter.
A key feature of a broadband light source is that its low coherence length allows a
well-defined imaging position in the tissue sample. In addition, it is necessary to use
single-modefibers (SMFs) in order to achieve a high image resolution. Thus the
two pulse halves from the optical coupler outputs are sent separately through SMFs
to the reference arm and to the sample arm. Returning reflections of the pulses from
the mirror at the end of the reference arm and from the sample are recombined by
means of the optical coupler and then are directed to the photodetector. Because
low-coherence light from a broadband source is used, constructive interference of
the light at the photodetector only occurs when the optical path lengths between the
two arms of the interferometer lie within the coherence length of the light source.

Optical splitter

Optical

source

Optical

detector

Movable

reference

mirror at zref

Tissue

sample

z 1 z 2 z 3

Scan data

from various

sample depths

Illumination pulse

Illumination

pulse

Reference

pulse

Reference

pulse

Demodulator

z 1 z

2

z 3

z 1 z 2 z 3

zre

f

Voltage

Outputs when zjmatches zref

Fig. 10.3 A typical time-domain (TD) OCT imaging setup

294 10 Optical Imaging Procedures