A magnetic field sensor comprises a magnetically responsive light propagating component configured to cause a polarization of light propagating inside the component to be rotated in response to an applied magnetic field, wherein the magnetically responsive light propagating component is formed of a bulk material doped with a dopant, the dopant including at least gadolinium, the dopant concentration being at a sufficiently low concentration such that the dopant is uniformly dispersed in the bulk material to provide a high Verdet constant. The magnetic field sensor also comprises a detector, and a polarization-maintaining light input device to couple the light into the magnetically responsive light propagating component. The detector is configured to measure a property of light output from the magnetically responsive light propagating component to determine a change in polarization of the light, the change caused by the presence of a magnetic field.