Biophotonics_Concepts_to_Applications

1500 nm. As shown in Fig.3.6, these attenuation mechanisms produce a low-loss
region in silicafibers in the spectral range of 700–1600 nm, which matches the
low-absorption biophotonics window illustrated in Fig.1.6. The attenuation spike
around 1400 nm is due to absorption by residual water ions in the silica material.
Greatly reducing these ions during thefiber manufacturing process results in alow-
water-contentfiberorlow-water-peakfiberin which the attenuation spike has been
greatly reduced.
Standard commercially available multimodefibers have core diameters of 50,
62.5, 100, 200μm, or larger. These multimodefibers are used in applications such
as light delivery to specific tissue areas, photobiomodulation, opticalfiber probes,
and photodynamic therapy (PDT). Single-modefibers have core diameters around
10 μm, the exact value depending on the wavelength of interest. Applications of
single-mode opticalfibers include uses in clinical analytical instruments, in endo-
scopes or catheters, in various imaging systems, and in healthcare sensors.

3.5 Specialty Solid-Core Fibers

Specialty solid-corefibers are custom-designed for functions such as the following
[ 5 ]:

• Manipulating lightwave signals to achieve some type of optical
  signal-processing function


• Extending the spectral operating range of thefiber


• Sensing variations in a physical parameter such temperature or pressure


• Analyzing biomedicalfluids

Attenuation (dB/km)

10

100

0.01

0.1

1

Wavelength (μm)

0.5 0.6 0.8 1 1.2 1.5 2 5

Scattering

loss

Ultraviolet

absorption loss

Infrared

absorption loss

Measured

fiber loss

Fig. 3.6 Typical attenuation
curve of a silicafiber as a
function of wavelength
(J. Biomed. Opt. 19(8),
080902 (Aug 28,
2014). doi:10.1117/1.JBO.19.
8.080902)

3.4 Conventional Solid-Core Fibers 69