Biomedical Engineering Reference
In-Depth Information
oscillator into a few centimeters of PCF produces more than one octave broad
supercontinuum. The fact that only an oscillator with a few nJ of energy is required
to generate supercontinuum makes the PCF-based sources reliable, affordable, and
relatively simple to operate. These sources make use of optical nonlinear effects
in specially designed PCF to create light with a wide spectrum that can span the
visible and near infrared, typically from around 400 to 2,000 nm. Due to their
spectral properties, they are suitable for OCT imaging with submicrometer axial
resolution [
39
]. SC light sources generating two separate spectral bands were
also introduced into OCT, allowing separate scanning at 800 nm and 1,300 nm
with axial resolutions of
3 and
5m, respectively [
40
,
41
]. Commercially, SC
sources are available from Fianium and Koheras. Koheras A/S (NKT Photonics,
Birkerod, Denmark) offers the SuperK Extreme, which it says is the brightest visible
supercontinuum source available. The source, with the highest power the company
offers, has a broad spectrum, from 460 to 2,400 nm, and provides a spectral power
density of more than 3 mW nm
1
in the visible wavelength range. Combining the
source with a tunable filter allows a user to tune up to eight simultaneous laser
lines independently across the visible spectrum. The SC500-FC supercontinuum
source from Fianium (Southampton, UK) is a low-cost, mode-locked fiber laser
generating supercontinuum radiation in the spectral band from 500 to 1,700 nm,
with an average power in excess of 250 mW. The feasibilities of the Fianium and
Koheras SCs were demonstrated for both time-domain and frequency-domain OCT
configurations [
42
-
44
].
5.4.5.6
Swept-Source Lasers
Swept-source lasers are a tunable light source emitting one wavelength at a
time, rapidly swept over a broad spectral range. The promising capabilities of
frequency-swept light sources for OCT imaging have gained intense interest in
their development and enabled a quantum leap in the speed and sensitivity of
OCT technology development. There are three major concepts to achieve high-
speed tuning depending on the method used for wavelength selection inside the
laser cavity of the wavelength-swept source: one based on a fast rotating polygonal
mirror [
45
,
46
], the second based on a diffraction grating on a mechanically resonant
galvo-scanner [
47
], and the third using a fiber Fabry-Perot tunable filter (FFP-TF)
[
48
,
49
]. For very high tuning speeds and to overcome limitations given by the
buildup time of lasing in the cavity, the technique of Fourier domain mode locking
(FDML) has been introduced. High repetition rate up to 370 kHz has been already
demonstrated for FDML swept sources [
50
-
54
]. Recent advancements in the field
of ultrahigh-speed swept-source optical source enables Fourier domain detection
approach using frequency-swept lasers and provides a powerful, complementary
approach for spectral/Fourier domain detection.
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