Understanding How Femtosecond Laser Waveguide Fabrication in Glasses Works Page: 12 of 131
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this theory by Meltz et. al. in 1989 [1.5] opened the door to using photoinduced refractive
index changes for practical device fabrication. However, strong laser absorption was
required for the process to work, limiting its initial applicability to certain doped glasses.
As a result research efforts evolved to develop new UV lasers that could affect existing
glasses; and to develop new photosensitive, doped glasses that would absorb existing
1.2 Femtosecond laser induced refractive index changes
The development of femtosecond lasers in the 1990s provided a method to induce
refractive index changes in most glasses, including undoped glasses. A femtosecond (fs)
pulsed laser has pulses that are orders of magnitude shorter than conventional lasers, only
lasting 10s or 100s of femtoseconds (10-15 s). These pulses have very high peak
intensities, with relatively low energies. This combination allows for efficient highly
nonlinear processing, in which several near-infrared photons are absorbed simultaneously
to modify a glass. Nonlinear, femtosecond laser processing has three important
advantages compared with linear, UV laser processing: It creates less collateral damage.
It can be used to fabricate devices inside bulk materials. And as mentioned earlier, it
works in most glasses.
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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/m1/12/: accessed May 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.