Abstract: A system for fibre-optic Sagnac interferometry, the system comprising: an optical source; an optical splitter configured to split light from the optical source into a first optical beam and a second optical beam; an optical circuit comprising a first modulation unit, a second modulation unit, and an optical fibre operatively coupled between the first and second modulation units, wherein the optical circuit is operatively coupled to the optical splitter such that the first and second optical beams traverse the optical circuit in opposite directions, the first optical beam being modulated by the first modulation unit before being modulated by the second modulation unit, and the second optical beam being modulated by the second modulation unit before being modulated by the first modulation unit, wherein the first modulation unit is configured to modulate light passing through it with a first modulation code, and the second modulation unit is configured to modulate light passing through it with a second modulatio