Mode interference and UV-induced mode conversion in few-mode fibers

Resonant and non-resonant effects in few-mode fibers have found use in versatile devices. From sensing perspective, extensive research has been done on fringe sensitivity of nonresonant two-mode interference. Resonant mode converter gratings are promising candidates for the fields of mode division multiplexing and active devices based on few-mode fibers. This thesis explores multi-parameter sensing using two effects of two-mode interference (TMI), namely fringe shift and shift of group-velocity equalization (GVE) wavelength. The following sensing parameters were achieved: a) TMI fringe shift can be used for measuring the effect of changes in temperature and strain on intermodal dispersion. For exposure of the fiber with a scanning laser spot of constant velocity, the change in intermodal dispersion due to such particular exposure can also be measured using TMI fringes. b) GVE wavelength shift can be used for straightforward sensing of temperature and strain, without suffering from any dependency on the fiber length like TMI fringes. Using the shift of GVE wavelength during exposure of the full fiber with a scanning laser spot, the change in core index can be estimated due to a simple model. This allows measurement of small index changes due to irradiation, which is not possible with FBGs due to imperceptible strength of FBGs for small index changes and lower sensitivity. Combining the GVE and FBG resonance wavelengths provides a method for differentiating strain and temperature. The GVE wavelength also has an approximately three times higher temperature sensitivity compared to FBG resonance at similar wavelengths. The strain sensitivity of GVE and FBG resonance wavelengths are of similar magnitude but of opposite sign. TMI fringe shift can also be used to measure the extra phase added between two modes by each mark of laser irradiation. Two methods have been developed for completely characterizing the intermodal dispersion, which were verified experimentally for the LP01-LP02 dispersion. Intermodal and intramodal reflection peaks of two excited modes from an FBG written in the few-mode fiber can be used to approximately estimate the offset of the intermodal dispersion of the pristine fiber. Combining this with TMI phase unwrapping provides estimate of complete intermodal dispersion including offset. Measuring the resonance wavelength for a mode converter written using marks already characterized using TMI provides a precise estimate of the offset of the intermodal dispersion of the pristine fiber. From the estimate of extra phase added by each mark and intermodal dispersion of the pristine fiber, the intermodal dispersion experienced by resonantly interacting modes in an MC can be predicted. Fabrication of broadband mode converter gratings at any desired wavelength is an ongoing field of research. Different strategies were evaluated for making broadband MCs: a) Partial core irradiation with a tightly focussed laser spot was simulated with simple approximate models. In was concluded that the losses might be too high due to poor mode overlaps. b) It was determined using simulations that phase-shifted mode converter gratings can provide broader than 35 nm bandwidth at 20 dB conversion strength using reasonable irradiation parameters. The method was verified by achieving 41 nm bandwidth at 16 dB conversion strength.