Biomedical Engineering Reference
In-Depth Information
13.2 OCM Principles
A typical OCM system records the interference pattern between light that has interacted
with the sample and a reference wave, where the light source used is a low-coherence
source. By analyzing the interference pattern, a reflectivity profile along the axial
dimension can be constructed, similar to A-scan in ultrasound.
The performance of an OCM system is mainly determined by its longitudinal (axial) and
transverse resolutions, data acquisition specifications, including digitization resolution and
speed
[3]
.
OCM achieves high axial image resolution independent of focusing conditions because the
axial and transverse resolutions are determined independently. The transverse resolution as
well as the axial field of view (FOV) is governed by the focal spot size as in conventional
microscopy. However, the axial resolution is determined by the coherence length of the
light source.
The transverse resolution,
Δx
,
y
, is determined by the focused beam spot size which,
according to the common Rayleigh criterion, is given by:
0
:
61
λ
NA
Δx
;
y
5
(13.1)
where
is the central wavelength of the source and NA is the numerical aperture of the
objective lens.
λ
The axial FOV, assuming a detector with infinitely small pixel size, is given by:
πλ
2NA
2
FOV
5
(13.2)
From
Eqs. (13.1) and (13.2)
, it can be deduced that increasing the NA of the objective will
result in higher transverse resolution (
Δx
,
y
are smaller) but with the tradeoff of decreased
axial FOV.
The axial resolution is inversely proportional to the bandwidth of the light source (assuming
Gaussian-shaped spectrum profile)
1
:
2
2
2ln
ð
2
Þλ
0
:
44
λ
Δz
5
(13.3)
πΔλ
Δλ
where
Δλ
is the full-width-half-maximum (FWHM) spectral bandwidth of the light source.
1
In FD-OCM with tunable source, the axial resolution is determined by the scanning spectral range, and the
bandwidth determines the working range.
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