BOTDA Brillouin optical time domain analysis DAS History of engineering & technology Mach-Zehnder interferometer Raman spectroscopy SiO2-Au-TiO2 heterostructure Technology: general issues anti-resonant effect anti-resonant hollow-core fibers capillary-based hollow core fibers chalcogenide glasses (ChGs) critical wavelength crosstalk detection distributed acoustic sensing distributed fiber sensor doped fiber amplifiers dual-core photonic crystal fiber extinction ratio few-mode fiber fiber Bragg grating fiber end cap fiber gas lasers fiber grating sensors fiber lasers fiber mode crosstalk fiber optics fiber sensor fiber-optic sensors