Precise Monitoring of the Earth Rotation by Ring Lasers

A suitable laser gyroscope for geodetic purposes consists of a square shaped cavity, which contains a helium-neon gas mixture and a mirror at each corner. The size of the area spanned ought to be as big as possible. In such a ring resonator there are two counter-rotating beams. When either beam goes from one mirror round the interferometer and back to that mirror, rotation will lengthen or shorten the time of passage. However in a laser the whole path must contain a whole number of wavelengths. Therefore the two counter-rotating beams spontaneously split in frequency to ensure this. The difference in frequency is called Sagnac-frequency and amounts to

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where A is the encircled oriented surface of the ring laser, P the perimeter, f_24_2.gif (85 Byte) the laser wavelength and f_24_3.gif (127 Byte) the imposed rotation. If such an instrument is located on a rigid foundation on the surface of the earth, it allows the precise measurement of the instantaneous Earth Rotation f_24_4.gif (1077 Byte) Earth.

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Principle of Ring Laser

Four mirrors form a closed light path in the shape of a square. A small part of this path houses the excited gain medium, which turns this resonator into a Helium/Neon-Laser for two counter-rotating beams. When this assembly is rotated as indicated, the cavity has different effective lenghts for the two directions of propagation. This results in a splitting of the cavity frequencies, which is proportional to the speed of rotation. The difference frequency can be obtained by interfering the beams with each other.

The ultimately desired resolution of a few parts per billion of the value of the Earth Rotation places enormous technical demands on the gyroscope. As the perimeter of the ring laser and the encircled surface area must be held constant at a high degree of accuracy (because they appear in the Sagnac equation above), the resonator is formed from ZERODUR. This material almost does not change with variations in the ambient temperature (relative change in dimensions:>3*10-9 per degree Celsius). Moreover, the instrument is placed in an underground cave with very stable temperature conditions. Numerous other technical demands have to be satisfied as well.

Canterbury Ring Laser CI

The "Canterbury Ring" (CI) demonstrated the feasibility of big laser gyroscopes. Spanning a surface area of roughly 0.75 m2 this He-Ne-laser obtained a beat frequency of nearly 71 Hz with a resolution of 54 µHz over 14 hours. The mechanical design of this instrument does not allow routine measurements over long periods of time.

11_2.jpg (13770 Byte) The "Canterbury Ring" during operation. The red glow comes from the plasma excitation of the Helium/Neon gas mix-ture. The beat frequency between the two optical beams is approximately 70.7 Hz. The uncertainty of the maximum is as small as 54 micro Hertz for a measure-ment of 14 hours duration. 11_3.gif (11374 Byte)

Second Generation Ring Laser CII

The new instrument CII is a prototype of a ring laser which is suitable for geodetic measurements and is to be running continuously for a year or more. All the known design problems were avoided in this approach. It is made from a monolithic block of ZERODUR, forming an ultra-high vacuum quality cavity, supporting a mechanically rigid design and a long-lasting gas mixture. This instrument has been made by the Company C. ZEISS in Oberkochen (Germany).

11_4.jpg (12310 Byte) The new instrument CII was designed to allow for long-term stable measurements. This photograph shows the ring laser after completion at C.Zeiss (Oberkochen, Germany). Like the first instrument, CII is using high frequency to excite the plasma. This photograph shows a close-up of the RF section. Laser gain is generated by the glowing plasma. 11_5.jpg (13973 Byte)

Banks Peninsula and Cavern

Both instruments, CI and CII are now operated side by side in an old underground war bunker at Banks Peninsula, close to Christchurch in New Zealand. The stable ambient temperature and the solid volcanic rock foundation offer ideal conditions for ring laser operations. This project is the result of the collaboration between the "Forschungsgruppe Satellitengeodäsie (Germany)", the "University of Canterbury (New Zealand)" and the "Oklahoma State University (USA)".

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The location of operation of both instruments is the "Cashmere Cavern", an old war bunker on Banks Peninsula, near Christchurch in New Zealand. The stable temperature conditions and the solid rock formations give an ideal environment for such ring lasers.

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Letzte Änderung: 25.07.2002