Biomedical Engineering Reference
In-Depth Information
Fig. 8.5.
Comparison of optical output spectra (
top
) with interference signals
(
lower left
) and envelopes (
lower right
) for a Kerr-lens modelocked titanium:sapphire
(Ti:Al
2
O
3
) laser vs. a superluminescent diode (SLD). The broad optical bandwidth
of the titanium:sapphire laser (260 nm) permits a free-space axial resolution of
1.5
µ
m. In comparison, the superluminescent diode with a 32 nm spectral bandwidth
permits an axial resolution of 11.5
µ
m. Figure reprinted with permission [16]
this purpose. In the mid 1990s, it was realized, and in recent years, demon-
strated that alternatives to this time-domain mode of scanning were tech-
niques that simultaneously collected light from all depths into the tissue as
the beam was positioned at each transverse position. The light sources for
these techniques are either a broadband source in which the collected light is
detected by a linear CCD array in a spectrometer (spectral-domain OCT, SD-
OCT) [39,40], or a narrow-band source which is rapidly swept over a range of
frequencies and the collected light is detected by a photodiode (swept-source
OCT, SS-OCT) [41, 42]. For both of these techniques, depth-dependent scat-
tering information is obtained by taking the inverse Fourier transform of this
acquired spectral data. Recently, these techniques have become preferred for
the improvement in acquisition rate and sensitivity that they provide over the
time-domain OCT systems.
Recent use of the SS-OCT has demonstrated real-time acquisition rates.
The primary advantages to using a swept-source is that all the optical power
is contained within a narrow wavelength range at any one point in time, and
