  Magnetic fields play an important role in plasma dynamics, yet it is a
quantity difficult to measure accurately with physical probes, whose presence
disturbs the very field they measure. The Faraday rotation of a polarized beam
of light provides a mechanism to measure the magnetic field without disturbing
the dynamics, and has been used with great success in astrophysics and high
energy density plasma science, where physical probes cannot be used. However,
the rotation is typically small, which degrades the accuracy of the
measurement. Paradoxically, the main source of error is the probe beam itself.
Since polarization cannot be measured directly, detectors rely on a polarizer
to measure a small change in beam intensity instead. In this work, we show how
suppress the beam intensity that is not part of the Faraday rotation signal by
taking forming an optical derivative. Since the rotation measurement is now
strictly proportional to the beam intensity, the system allows to amplify the
rotation measurement simply by increasing the laser power.
