Ring laser |
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Two independent single mode lasers are arranged so that laser light passes around a ring of mirrors (three or four) in opposite directions. The lasers must operate independently without coupling. Rotating the ring about a perpendicular axis through its center Doppler shifts the laser light in opposite directions. The laser light in the ring beats, with a frequency (f1-f2) that varies upwards from a few Hertz depending on the speed of rotation. Small three-mirror ring lasers are used as guidance systems in long range passenger aircraft. A one meter square ring laser was set up by the Physics Department at the University of Canterbury NZ, in a 1940's (Second World War) bunker 60 meters below ground in the Cashmere Hills. The ring laser was mounted on a 10 cm thick granite table with four short (10 cm) 3 cm diameter steel 'legs' embedded into the solid rock. The ring laser had a beat frequency of ~60 Hz due to the rotation of the Earth. Pulling gently on a rubber-band attached to the corner of the table caused a measurable change in beat frequency! |