Magnetic field measuring device

A magnetic field measuring device comprising:

• a laser light source for delivering laser light having x and y axes components having a first frequency and a second frequency, respectively;
• a beam splitter for dividing said laser light into first and second portions;
• a first optical analyzer provided to receive the first portion delivered from the beam splitter for delivering a component polarized at an angle of 45.degree. with respect to the x and y axes, and the optical intensity of which being modulated at a frequency corresponding to the difference of said first and second frequencies, and having reference phase;
• a quarter wave plate provided to receive the second portion delivered from the beam splitter for delivering an output, the principal axis direction of which being set at an angle of 45.degree. with respect to x and y axes;
• a Faraday rotator for receiving the output of said quarter wave plate for rotating the same by an angle corresponding to a magnetic field applied thereto;
• a second optical analyzer for extracting from the output of the Faraday rotator a predetermined polarization component, and the optical intensity of which being modulated at a frequency equal to the difference between the first and second frequencies, and phase modulated by twice the rotated angle produced by said Faraday rotator;
• first and second photoelectric converters for converting outputs of the first and second optical analyzers into first and second electric signals, respectively;
• and
• means for comparing the phase difference between said first and second electric signals thereby to produce an output directly proportional to the magnetic field applied to the Faraday rotator.