An optical sensor comprising: at least one optical coupler ; an optical waveguide comprising a plurality of loops, the optical waveguide in optical communication with the at least one optical coupler and mechanically decoupled from the at least one optical coupler such that the at least one optical coupler and the optical waveguide can undergo rotation relative to one another about an axis of symmetry of the plurality of loops, the optical waveguide configured to receive a first optical signal from the at least one optical coupler during said rotation, wherein the first optical signal has a group velocity and a phase velocity while propagating through at least a portion of the optical waveguide, the group velocity being less than the phase velocity, wherein an interference between the first optical signal and a second optical signal is affected by the rotation about the axis of symmetry, wherein the at least one optical coupler comprises a first port and a second port , wherein the optical sensor further comprises an optical detector (D) configured to detect interference between the first optical signal and the second optical signal, the optical sensor being characterised in that the optical waveguide comprises a first end and a second end , the first port optically coupled to the first end during said rotation and the second port optically coupled to the second end during said rotation; wherein the optical sensor further comprises either (i) a first gap between the first port and the first end such that optical signals traveling between the first port and the first end propagate in free space, wherein a pathlength of the first gap changes during said rotation, or (ii) a first lengthwise stretchable optical waveguide between the first port and the first end, wherein a length of the first lengthwise stretchable optical waveguide changes during said rotation. at least one optical coupler ; an optical waveguide comprising a plurality of loops, the optical waveguide in optical communication with the at least one optical coupler and mechanically decoupled from the at least one optical coupler such that the at least one optical coupler and the optical waveguide can undergo rotation relative to one another about an axis of symmetry of the plurality of loops, the optical waveguide configured to receive a first optical signal from the at least one optical coupler during said rotation, wherein the first optical signal has a group velocity and a phase velocity while propagating through at least a portion of the optical waveguide, the group velocity being less than the phase velocity, wherein an interference between the first optical signal and a second optical signal is affected by the rotation about the axis of symmetry, wherein the at least one optical coupler comprises a first port and a second port , wherein the optical sensor further comprises an optical detector (D) configured to detect interference between the first optical signal and the second optical signal, the optical sensor being characterised in that the optical waveguide comprises a first end and a second end , the first port optically coupled to the first end during said rotation and the second port optically coupled to the second end during said rotation; wherein the optical sensor further comprises either (i) a first gap between the first port and the first end such that optical signals traveling between the first port and the first end propagate in free space, wherein a pathlength of the first gap changes during said rotation, or (ii) a first lengthwise stretchable optical waveguide between the first port and the first end, wherein a length of the first lengthwise stretchable optical waveguide changes during said rotation. <EOS>