Method and apparatus for measurements dependent on the faraday effect using polarized counter-propagating light

A method of measurement comprising

• passing polarized light beams in counter propagating directions through optical guide means which exhibit Faraday rotation of polarized light in the presence of a magnetic field,
• applying a component of a magnetic field longitudinally of the directions of light propagating in the guide means, the guide means having high circular birefringence compared with the Faraday rotation of the polarization of the light beams due to said component,
• deriving first light-dependent signals from light received after passing through the guide means in one propagation direction only and second light-dependent signals derived from light received after passing through the guide means in the other propagation direction only, the first and second signals being dependent on the polarization states of light received and allowing an output signal to be derived which is dependent on Faraday rotation but not on reciprocal polarization effects, and
• deriving the output signal from the first and second light-dependent signals as representative of a quantity to be measured.