Understanding How Femtosecond Laser Waveguide Fabrication in Glasses Works Page: 11 of 131
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1.1 Laser induced refractive index changes in glass
Touch something in the sun and it feels warm; leave something in the sun and its color
fades. These are basic observations of light affecting matter. Science works by making
such observations and then asking questions. Why does something in the sun feel warm?
Can the warmth of the sun be used for something?
In the early 1970s it was discovered that lasers could cause noticeable, permanent
refractive index changes in glass; or more simply put, the laser could make a spot [1.1].
Hill et al. used this "photoinduced" change for optical device fabrication. In 1978 his
group demonstrated that a blue laser could be employed to fabricate Bragg grating
reflectors inside glass fibers [1.2]. Further studies by Garside et al. [1.3-1.4], explained
that this refractive index change was induced by a nonlinear absorption process, in which
two blue photons are absorbed simultaneously. Unfortunately, the two photon process
was not practical for industrial scale device fabrication.
It was theorized that the linear absorption of a UV laser source could be used to alter the
glass more efficiently. The linear absorption of a UV laser is possible because a single
UV photon has the same energy as two blue photons. The experimental confirmation of
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Reichman, W J. Understanding How Femtosecond Laser Waveguide Fabrication in Glasses Works, thesis or dissertation, May 11, 2006; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc891320/m1/11/: accessed May 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.