Biophotonics_Concepts_to_Applications

As is described below, this means that the two returning pulse envelopes must
overlap at the photodetector in order to have a maximum received optical signal. By
scanning across the face of the reference arm mirror, a single 2D-scan from a slice
of reflection data at a specific axial depth is acquired from the sample. This is
illustrated in Fig.10.4. In time domain OCT the path length of the reference arm is
varied longitudinally in time. Thus once a particular two-dimensional scan has been
obtained, then by slightly moving the position of the reference mirror another
2D-scan slice through the tissue sample is obtained at a different axial depth.

Example 10.2Consider a setup for a TD-OCT A-line scan of a human eye.

What are the characteristics of an A-line scan through the center of the eye?

Solution: The A-line scan will show large reflectance peaks at interfaces

between materials having different refractive indices. Example boundaries are

at the air-cornea, cornea-lens, lens-vitreous humor, and vitreous humor-retina

interfaces, as is indicated in Fig.10.5.

The principles of OCT operation can be understood by examining the interac-
tions of the electromagnetic waves returning from the sample and the reference
arms. For simplicity, first the approximation can be made that the source is
monochromatic and emits plane waves. Thus the incident wave Esourcefrom the
source propagating in the z direction is written as

Esource¼E 0 exp½iðkzxtފ ð 10 : 1 Þ

where k = 2π/λwithλbeing the wavelength of the source,ωis the angular fre-
quency of the electricfield, and E 0 is the amplitude of the wave with the subscript 0
indicating that a monochromatic source is used.

2D-scan done over

this optical slice

2D-scan at a

specific depth

Illumination to

slice depth

Reflectance from

slice depth

Tissue sample

Optical slice depth determined by

the reference mirror position

Fig. 10.4 Various positions of the OCT reference mirror yield 2D scans at different depths

10.1 Optical Coherence Tomography 295