An optical-fiber measurement system includes an optical transceiver comprising an optical transmitter and an optical receiver. A multi-core optical fiber has a proximal end with a first optical core coupled to the transceiver and a second optical core coupled to the transceiver, and a distal end with the first optical core coupled to a sample path that is configured to convey light collected from a sample positioned external to the multi-core optical fiber and the second optical core coupled to a reference path such that the sample path and the reference path experience mostly a same disturbance along the multi-core optical fiber. The optical receiver is configured to interferometrically detect light from the sample path and light from the reference path.