An optical interrogation system for a sensing fiber, the system comprising: interferometric apparatus to probe a single multimode core of the sensing fiber over a range of predetermined wavelengths of light, the single multimode core having at least three light propagating modes, the interferometric apparatus comprising at least three interferometers including at least three reference branches and at least three measurement branches, the at least three measurement branches having different optical delays and comprising an array of associated single-core-single-mode detecting fibers, each single-core-single-mode detecting fiber to couple light in a light propagating mode of the at least three light propagating modes, the interferometric apparatus further comprising acquisition circuitry to detect measurement interferometric data associated with reflected light coupled from the at least three light propagating modes of the single multimode core back into the array of single-core-single-mode fibers for each predetermined wavelength in the range; and data processing circuitry to: process the measurement interferometric data to determine modal cross coupling coefficients associated with the at least three light propagating modes of the single multimode core; determine a cross-sectional index perturbation of the single multimode core from the modal cross coupling coefficients; and determine strain, bend, and twist parameters of the sensing fiber from the determined cross-sectional index perturbation as compared with a baseline cross-sectional index perturbation. interferometric apparatus to probe a single multimode core of the sensing fiber over a range of predetermined wavelengths of light, the single multimode core having at least three light propagating modes, the interferometric apparatus comprising at least three interferometers including at least three reference branches and at least three measurement branches, the at least three measurement branches having different optical delays and comprising an array of associated single-core-single-mode detecting fibers, each single-core-single-mode detecting fiber to couple light in a light propagating mode of the at least three light propagating modes, the interferometric apparatus further comprising acquisition circuitry to detect measurement interferometric data associated with reflected light coupled from the at least three light propagating modes of the single multimode core back into the array of single-core-single-mode fibers for each predetermined wavelength in the range; and data processing circuitry to: process the measurement interferometric data to determine modal cross coupling coefficients associated with the at least three light propagating modes of the single multimode core; determine a cross-sectional index perturbation of the single multimode core from the modal cross coupling coefficients; and determine strain, bend, and twist parameters of the sensing fiber from the determined cross-sectional index perturbation as compared with a baseline cross-sectional index perturbation. process the measurement interferometric data to determine modal cross coupling coefficients associated with the at least three light propagating modes of the single multimode core; determine a cross-sectional index perturbation of the single multimode core from the modal cross coupling coefficients; and determine strain, bend, and twist parameters of the sensing fiber from the determined cross-sectional index perturbation as compared with a baseline cross-sectional index perturbation.