title: Selective excitation of higher-order modes in etched gelatin-coated few-mode fiber and demonstration of high relative humidity measurement.
abstract: We report here our experimental studies on efficient excitation of higher-order modes in a few-mode fiber realized through selective chemical etching of single-mode fiber, depositing a dynamic refractive index profile coating (gelatin) and designing a well-performing high relative humidity (RH) (73%-99% RH) sensor with sensitivity as high as ∼1.2  dBm/% RH and fast response (∼125  ms). The design relies on fixing quasi-adiabatic transition of the single-mode-few-mode-single-mode configuration to launch light at angles corresponding to desired higher-order modes, pushing more power into the coating. Our systematic study using the devised hybrid theoretical ray- and wave-optic model uniquely interprets the variety of experimental results.
authors.forename: Saba N
authors.initials: SN
authors.lastname: Khan
authors.forename: Partha Roy
authors.initials: PR
authors.lastname: Chaudhuri