Biophotonics_Concepts_to_Applications

As described in Chap. 7 , an important biophotonics application of a FBG is to
sense a variation in a physical parameter such as temperature or pressure [ 5 ]. For
example, an external factor such as strain will slightly stretch thefiber. This
stretching will lengthen the periodΛof the FBG and thus will change the value of
the specificreflected wavelength. Similarly, rises or drops in temperature will
lengthen or shorten the value ofΛ, respectively, thereby changing the value ofλB,
which then can be related to the change in temperature.

Example 3.9Consider a FBG that was made to reflect a wavelength

λB= 1544.2 nm at 20 °C. Suppose that the Bragg wavelength changes linearly

with temperature with an incremental wavelength changeΔλB= (0.02 nm/°C)ΔT

per incremental temperature changeΔT. (a) Find the Bragg wavelength when

T = 35 °C. (b) What is the Bragg wavelength when T = 10 °C?

Solution: The linear dependence ofλBon temperature can be expressed as

kBðÞ¼T 1544 :2nmþð 0 :02 nm=CÞðT 20 CÞ

(a) At T = 35 °C the Bragg wavelength becomes

kBð 35 CÞ¼ 1544 :2nmþð 0 :02 nm=CÞð 35  20 CÞ¼ 1544 :5nm

(b) At T = 10 °C the Bragg wavelength becomes

kBð 10 CÞ¼ 1544 :2nmþð 0 :02 nm=CÞð 10  20 CÞ¼ 1544 :0nm

3.5.2 Fibers Resistant to UV-Induced Darkening

Conventional solid-core germanium-doped silicafibers are highly sensitive to
ultraviolet (UV) light [ 5 ]. The intrinsic attenuation is higher in the UV region
compared to the visible and near-infrared spectra, and there are additional losses
due to UV-absorbing material defects, which are created by high-energy UV
photons. These additional UV-induced darkening losses are known assolarization
and occur strongly at wavelengths less than about 260 nm. Thus although newly
manufactured conventional silicafibers offer low attenuation in the 214–254 nm
range, upon exposure to an unfiltered deuterium lamp the transmission of these
fibers drops to about 50 % of the original value within a few hours of continuous
UV irradiation. Consequently, conventional silica opticalfibers can be used only for
applications above approximately 300 nm.

3.5 Specialty Solid-Core Fibers 71