Biomedical Engineering Reference
In-Depth Information
light sources has been a topic of great interest. So far, several approaches have
been proposed to achieve high axial resolution, such as superluminescent diode,
multiplexed superluminescent diode, femtosecond laser, continuum generation
from photonic crystal fiber, amplified spontaneous emission (ASE), and xenon
illuminator. In general, these solutions either have high cost or exhibit bumpy
spectrum, or both. The following section briefly reviews various state-of-the-art
broad-bandwidth light sources as potential light sources for the applications in OCT.
5.4.5.1
Superluminescent Diodes (SLEDs)
In principle, operations of SLEDs are very similar to the laser diode (LD), with the
lasing action based on amplified simulated emission. However, in SLEDs, lasing
action cannot be built up due to insufficient feedback for oscillations. The unique
property of SLEDs is the combination of laser diode-like output power and bright-
ness with broad LED-like optical spectrum. Such combination is allowed by high
optical gain and wide gain spectrum in semiconductor laser materials. SLEDs are
very compact, relatively cheap, and very friendly to use. Current SLEDs operate at
various wavelengths, such as 0.8, 1, 1.3, and 1:55 m, and all of these wavelengths
have been used for OCT in different applications. At 830nm range, the SLEDs
are based on AlGaAs emitters, and single SLEDs with up to 70-nm bandwidths
and 3
15-mW fiber pigtailed output power (corresponding to around 3:3-m
axial resolution in the tissue) are commercially available (Superlum Ltd, Russia).
For ranges above 1m, the SLED emitters are based on InGaAs, and the typical
bandwidths are around 50
60 nm, which limits the axial resolution to >10m
in the tissue. However, using multiplexed SLED technology (synthesized optical
source), SLEDs with a bandwidth more than 130 nm and optical power up to 15 mW
are commercially available (Denselight Semiconductors, Singapore). While in some
specific areas, like ultrahigh-resolution OCT, there are strong competitors to SLDs,
namely, femtosecond lasers/supercontinuum sources, for most of practical applica-
tions, SLDs are now considered as the most attractive emitters due to their small
size, easy use, and much cost-effectiveness compared to alternative light sources.
5.4.5.2
Multiple Quantum Well Semiconductor Optical Amplifiers
Multiple quantum well (MQW) optical amplifiers are another class of promising
semiconductor sources widely used in OCT because of their inherently large
saturation power and bandwidth. MQWs are generally considered for the use in
analogue and digital communication links and are commercially available with
wavelength ranging from 1,300 to 1,600 nm. Extremely broadband MQWs based on
InGaAsP/InP ridge bent-waveguide semiconductor optical amplifiers with several
nonidentical multiple quantum wells can be designed and fabricated on InP substrate
[
28
-
30
]. However, there are few potential disadvantages for these sources, most of
these QWM sources operate at the far-infrared region of the optical spectrum,
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