Page 339 - Flexible Robotics in Medicine

330 Chapter 14

array of tools (e.g., a magnetic resonance imaging scanner). FBGs are also able to detect
changes in strain and temperature and can be sampled at rates of 20 kHz [13], making them
useful at real-time data collection and force sensing.

FBGs have been tested in a variety of medical applications. Li [13,14] demonstrated a
distributive tactile sensing mechanism by arranging FBGs in tubing instruments. Li
[15 20] wrapped a continuum robot with FBGs in a helical formation in order to provide
simultaneous torsion, force, and curvature measurements accurately.

14.2.2.1 Working principle

FBGs are created from laser exposure on the fiber’s core, which varies periodically,
primarily working like a wavelength-selective mirror (Fig. 14.1). When light is directed
onto the different regions of the fiber’s core, it is scattered, and the appropriate wavelengths
that satisfy the Bragg condition are added up constructively, creating a reflected central
wavelength λ Bragg termed the Bragg wavelength [16] and given by
λ Bragg 5 2nΛ;

where n is the refraction index, and Λ is the period of the index of refraction variation of
the FBG.

When the refractive index of the core or the grating periodicity is altered, the Bragg
wavelength is shifted as influenced by n and Λ.

Incident light
Intensity

Periodicity
Wavelength

Intensity Reflected light Fiber core

Fiber Bragg
grating
Wavelength
Intensity Reflected Strained fiber core
wavelength shift

Wavelength shift
Figure 14.1
Strain response from FBG.