A resonant fibre optical gyroscope, RFOG, comprising: an optical resonator coil comprising a first coil end and a second coil end; wherein the optical resonator coil comprises a hollow core optical fibre (26, 27, 28), wherein the hollow core optical fibre comprises a tubular outer jacket comprising an inner surface, a hollow core, and a cladding, wherein the cladding comprises a plurality of glass cladding capillaries, wherein the hollow core is bounded by inwardly facing portions of outer surfaces of the plurality of glass cladding capillaries, wherein the plurality of glass cladding capillaries are arranged in a ring around the hollow core, and wherein each cladding capillary is bonded to the inner surface of the tubular outer jacket; a resonance loop coupler optically coupled to the first coil end and the second coil end; a laser optically coupled to the resonance loop coupler and configured to generate a clockwise CW, optical signal configured to propagate around the optical resonator coil in a clockwise direction and a counterclockwise, CCW, optical signal configured to propagate around the optical resonator coil in a counterclockwise direction, wherein each of the CW optical signal and the CCW optical signal have a wavelength which the hollow core optical fibre is configured to guide by the antiresonant optical guidance effect; a CW detector optically coupled to the resonance loop coupler and configured to detect a portion of the CW optical signal emitted, through the resonance loop coupler, from the optical resonator coil; and a CCW detector optically coupled to the resonance loop coupler and configured to detect a portion of the CCW optical signal emitted, through the resonance loop coupler, from the optical resonator coil; wherein rotation of the optical resonator coil around winding axis of the optical resonator coil creates a difference between a CW resonance frequency of the CW optical signal and a CCW resonance frequency of the CCW optical signal, and wherein the difference between the CW resonance frequency and the CCW resonance frequency is proportional to a rate of angular rotation around the winding axis; wherein each of the CW optical signal and the CCW optical signal are linearly polarized in the hollow core optical fibre, and wherein the hollow core optical fibre has a holding parameter value of less than 1×10 -5 m -1, or less than 1×10 -6 m-1, or in a range of 1×10 -6 m -1 to 1×10 -5 m -1, or in a range of 1×10 -7 m -1 to 1×10 -6 m-1, or in a range of 1×10 -7 m -1 to 1×10 -5 m -1. an optical resonator coil comprising a first coil end and a second coil end; wherein the optical resonator coil comprises a hollow core optical fibre (26, 27, 28), wherein the hollow core optical fibre comprises a tubular outer jacket comprising an inner surface, a hollow core, and a cladding, wherein the cladding comprises a plurality of glass cladding capillaries, wherein the hollow core is bounded by inwardly facing portions of outer surfaces of the plurality of glass cladding capillaries, wherein the plurality of glass cladding capillaries are arranged in a ring around the hollow core, and wherein each cladding capillary is bonded to the inner surface of the tubular outer jacket; a resonance loop coupler optically coupled to the first coil end and the second coil end; a laser optically coupled to the resonance loop coupler and configured to generate a clockwise CW, optical signal configured to propagate around the optical resonator coil in a clockwise direction and a counterclockwise, CCW, optical signal configured to propagate around the optical resonator coil in a counterclockwise direction, wherein each of the CW optical signal and the CCW optical signal have a wavelength which the hollow core optical fibre is configured to guide by the antiresonant optical guidance effect; a CW detector optically coupled to the resonance loop coupler and configured to detect a portion of the CW optical signal emitted, through the resonance loop coupler, from the optical resonator coil; and a CCW detector optically coupled to the resonance loop coupler and configured to detect a portion of the CCW optical signal emitted, through the resonance loop coupler, from the optical resonator coil; wherein rotation of the optical resonator coil around winding axis of the optical resonator coil creates a difference between a CW resonance frequency of the CW optical signal and a CCW resonance frequency of the CCW optical signal, and wherein the difference between the CW resonance frequency and the CCW resonance frequency is proportional to a rate of angular rotation around the winding axis; wherein each of the CW optical signal and the CCW optical signal are linearly polarized in the hollow core optical fibre, and wherein the hollow core optical fibre has a holding parameter value of less than 1×10 -5 m -1, or less than 1×10 -6 m-1, or in a range of 1×10 -6 m -1 to 1×10 -5 m -1, or in a range of 1×10 -7 m -1 to 1×10 -6 m-1, or in a range of 1×10 -7 m -1 to 1×10 -5 m -1.