School of Physics Lecture Demonstration Manual

Og-6 Voice Communication Using a Laser Beam (The Faraday Effect)

Published: Wednesday 15 March 2006 - Updated: Monday 28 March 2011


To demonstrate the transmission of sound via the modulation of a laser beam using the Faraday effect.



Click on pictures to enlarge.

Voice comm with Laser Diagram


The Faraday modulator cell A consists of a crystal surrounded by a coil. The crystal has a large Verdet constant of 0.05 minutes of arc per magnetic field cm. The polarised laser beam has its plane of polarisation rotated by an amount proportional to the magnetic field strength in the coil. The coil is modulated directly from the output of a radio receiver. The laser beam strikes a photocell B which has a polaroid filter which may be rotated to maximise the signal.

Safety notes

Electrical safety 

Laser safety 


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