Biomedical Engineering Reference
In-Depth Information
a
b
25
200
20
High -OH
Multimode, step index
150
15
Low -OH
100
10
Multimode, graded index
50
5
0
0
200
600
1000
1400
1800
Singlemode, step index
Wavelength (nm)
Fig. 1.4
Fiber types (
a
) and the transmission properties of high-OH and low-OH fibers (
b
). The
attenuation properties of low-OH and high-OH fibers were reproduced from “The Book on the
Technologies of Polymicro” with permission from Polymicro Technologies
PCF transmits light in a different mechanism. The light is confined in the hollow
core by a photonic bandgap in the cladding which acts as a loss-free mirror. Because
the light is transmitted in the hollow air holes, this type of PCF has extremely low
dispersion and loss. It is suitable for short pulse and continuous wave laser delivery
and pulse shaping.
1.2.2.5
Polarization-Maintaining Fiber
Polarization-maintaining fiber (PMF) is a special type of fiber in which the
polarization of the light can be maintained throughout the light transmission in
the fiber. It is based on the birefringence of the optical fiber, in which the beam is
split into two orthogonal polarized beams. For the conventional fibers, because the
birefringence is randomized, the polarization of the output beam is also randomized.
In PMF, the birefringence is intentionally maintained by applying an external stress
to the fiber cross section to make it slightly asymmetric along the length and thus
prevent the cross coupling of the two orthogonal directions. There are a number
of designs including Panda, Bow-Tie, Elliptical Core, or Cladding (Nufern, East
Granby, CT, USA).
1.2.3
Spectrograph/Monochromator
Spectrometer refers to any spectroscopic instruments, whether it scans wave-
lengths individually or acquires the entire spectrum simultaneously or whether
it employs a prism, grating, or other mechanisms. It is used to measure the
light intensity dependence on wavelength. The commonly used configurations
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