Biomedical Engineering Reference
In-Depth Information
Fig. 8.3.
Schematic representation of an OCT system implemented using fiber op-
tics. The Michelson interferometer is implemented using a fiber-optic coupler. Light
from the low-coherence source is split and sent to a sample arm with a beam deliv-
ery instrument and a reference arm with an optical path-length scanner. Reflections
from the arms are combined and the output of the interferometer is detected with
either a photodiode or a linear CCD array in a spectrometer. Components for both
a time-domain and a spectral-domain OCT system are shown
the source. The dependence of the axial resolution on the bandwidth of the
optical source is plotted in Fig. 8.4. To achieve high axial resolution (approach-
ing 1
m), therefore, requires extremely broad bandwidth optical sources. The
curves plotted in Fig. 8.4 are for three commonly used wavelengths, 800, 1,300,
and 1,500 nm. From (8.1), higher resolutions can be achieved with shorter
wavelengths. However, shorter wavelengths are more highly scattered in bio-
logical tissue, and frequently result in less imaging penetration.
Evident from this discussion, the axial and transverse resolutions in OCT
are not directly related, as in conventional microscopy. However, the transverse
resolution in an OCT imaging system is determined by the focused spot size,
according to the principles of Gaussian optics, and is given by
µ
∆
x
=
4
λ
π
f
d
,
(8.2)
where
d
is the beam diameter on the objective lens and
f
is the focal length
of the objective lens. Large numerical aperture optics can be used to fo-
cus the beam to a small spot size and provide high transverse resolution.
