Biomedical Engineering Reference
In-Depth Information
and the optical output is unpolarized, which limits its application in OCT where
polarization-sensitive imaging is required.
5.4.5.3
Doped Fiber-Based Amplified Spontaneous Emission (ASE)
Sources
Doped fiber-based optical sources can provide a high-power and high-brightness
broadband superfluorescent optical spectrum that has many applications in many
areas including evaluation of fiber optical components by low-coherence interfer-
ometry, rotation sensing, spectroscopy, and medical imaging [
31
]. One attractive
way to generate broadband output is via the process of ASE in a rare-earth doped
fiber. Over the last few years, there has been very rapid progress in developing
high-efficiency output power from cladding-pumped fiber lasers and amplifiers.
Rare-earth doped fluoride, telluride, and silicon fibers realize broadband spectra
ranging from 1,300 to 1,600 nm. One drawback with doped fiber-based ASE is
the need for high-power single-mode pump for optimal coupling into the doped
single-mode core. Fiber-coupled single-mode semiconductor pump sources are
commercially available; however, they are typically very expensive.
5.4.5.4
Kerr-Lens Mode-Locked (KLM) Lasers
As described earlier, SLEDs are easy to use and generate smooth and stable spectra;
they can permit 20-7-m resolution imaging at a fixed wavelength and are attractive
because of their simplicity and low amplitude noise. However, achieving high-speed
imaging with a high signal-to-noise ratio requires more than the mW-level power
typically available from diode sources. Additionally, the limited optical bandwidth
of diodes precludes cellular-level-resolution imaging. With the development of
ultrafast laser, it is possible to achieve broadband spectrum. Recently, Kerr-lens
mode-locked (KLM) lasers are getting more attention in the field of OCT. The
titanium:sapphire laser is tunable from 0:7 mto1:1 m and can produce not only
broad spectral bandwidths for high-resolution imaging but also high output powers
for fast image acquisition [
32
,
33
]. The Ti
W
Al
2
O
3
laser is optimized for short coher-
ence length and was demonstrated to achieve sub-2-m resolution with a power
exceeding 100 mW. Although this source provided cellular-resolution imaging,
comparative studies have shown that the 800-nm center wavelength of Ti
W
Al
2
O
3
is not optimal for deeply penetrating biological tissue. An optimally penetrating
optical source will take advantage of the diminishing scattering cross section with
wavelength while avoiding the resonant molecular absorption of common tissue
constituents such as water and melanin. The chromium:forsterite laser has also been
used to generate high output powers and broad spectral bandwidths (5maxial
resolution) centered around 1,300-nm wavelength, for deeper imaging penetration
in highly scattering tissue [
34
]. Both of these laser sources, however, are large and
Search WWH ::
Custom Search