Advanced Spectroscopy Technique for Biomedicine - Biomedical Optical Imaging Technologies: Design and Applications - page 8

because of its capability to maintain the single-mode quality of the transmitted laser
light.

1.2.2.2 Multimode

Contrary to single-mode fiber, multimode fiber refers to the fibers that allow
multimodes of light to be transmitted. The core diameter of multimode optical fiber
is usually larger than about 10 m and varies from 50 to 2;000 m for most off-
the-shelf products. Multimode fiber is the most commonly used fiber in biomedical
spectroscopy. There are two types of multimode fibers: step-index multimode fiber
and graded-index multimode fiber. The ways of light propagations in single-mode
and multimode fibers are shown in Fig. 1.4 a.

1.2.2.3 Low-OH/High-OH Fibers

For optical fibers, certain hydroxyl (-OH) absorption bands may limit the per-
formance. The optical attenuation properties of low-OH and high-OH fibers are
quite different, as shown in Fig. 1.4 b. Materials with high OH (600-800 ppm) offer
excellent transmission in the UV wavelength range and can transmit light from 190
to 1,064 nm. It is suitable for broadband and laser delivery in the UV range. Low-OH
(<1 ppm) optical fiber, with a minor absorption peak around 1,385 nm, is suitable
for longer visible and near-IR wavelength range, usually from 500 to 2,300 nm.
Now, low-loss broadband optical fiber is also available for the wavelength range
from 275 to 2,100 nm. These are step-index multimode fibers. The core sizes are
between 50 and 1;000 m.

1.2.2.4 Photonic Crystal Fiber

Photonic crystal fibers (PCFs) are a special kind of optical fibers [ 15 ], which
contains an array of air-filled holes running along their optical axis. Comparing
to conventional optical fibers, PCFs have far more complicated index profile due
to the index contrast between air holes and the core materials and the capability to
vary the size and position of the air holes. PCFs can be single mode, polarization
maintaining, highly nonlinear, highly dispersive, and highly birefringent.
Based on the operating principles and geometry, PCFs can be divided into
two distinct types: solid-core PCFs and hollow-core PCFs. The optical properties
of these two types of PCFs are quite different. Solid-core PCF guides light by
total internal reflection at the boundary of the high-index core and the low-index
cladding, similar to the conventional optical fibers. The refractive index of the
cladding is altered by the arrangement of the position, distance, and shape of the
air holes. This type of PCF is highly nonlinear and dispersive and can be used for
supercontinuum generation [ 18 ] and single-mode laser light delivery. Hollow-core