Biophotonics_Concepts_to_Applications

Axial resolution

Δz=lc

2

=2ln2

π

λ 02

Δλ

Objective lens:

focal length f, diameter D,

numerical aperture NA

Δx=^4 λ^0

π

f

D

=^2 λ^0

π

1

NA

Depth of field

To the scanning system with

a maximum scan angle θmax

Lateral resolution

Lateral field of view: FOVlat= 2 f θmax

Δzf=^2

π

λ 0

NA^2

Fig. 10.6 Definitions of
various resolutions for an
OCT observation (Modified
with permission from Izatt
and Choma [ 6 ])

Dz¼

lc

2

¼

2ln2

p

k^20

Dk

ð 10 : 11 Þ

whereΔλis the FWHM spectral width of the probing source. Thus, the axial
resolution is directly proportional to the square of the source central wavelengthλ 0
and inversely proportional to the source spectral bandwidthΔλ. Note thatthe axial
resolution in air is equal to half of the coherence length of the light sourcebecause
of the round-trip propagation of the reference and sample beams. From Eq. (10.11)
it can be seen that optical sources with broad spectral bandwidths are desired in
order to achieve a high axial resolution. Higher resolutions also could be achieved
by decreasing the center wavelength from the standard 1300-nm value. A drawback
to this idea is that shorter wavelengths are more highly scattered in biological tissue,
which results in less penetration of imaging light into the tissue.

Example 10.4Consider the two super-luminescent diodes described in

Example 10.3. What is the axial resolution for each of these sources?

Solution: Using Eq. (10.11) the axial resolution for the 850-nm source is

Dz¼

2ln2

p

ð 850 Þ^2

50

¼ 6 : 38 lm

For the 1310-nm source, the axial resolution is

Dz¼

2ln2

p

ð 1310 Þ^2

85

¼ 8 : 91 lm

The lateral point-spread function at the full-width half-maximum power in the
focal plane of an OCT system defines the standardconfocal lateral resolutionor

10.1 Optical Coherence Tomography 299