About the Research Group
The goal of this research group is the development of world-leading active ring laser gyroscopes for measuring subtle variations in the rotation rate of the earth. This is important for research in geophysics, geodesy, general relativity and other areas of fundamental physics.
The ring laser research group is a collaboration between the University of Canterbury Physics and Astronomy Department who operate the Cashmere Cavern Laboratory and the Technical University of Munich and the Forschungsgruppe Satelliengeodäsie (German Federal Institute for Cartography and Geodesy) who operate the Gross Ring Facility.
About Ring Lasers
A ring laser gyroscope is a special kind of laser for measuring rotation.
In the 1960s lasers were described at scientific conferences as "a solution in search of a problem". Nowadays lasers are a multibillion dollar business. Every supermarket checkout, every CD player and many builders' toolkits hold one.
A typical laser works by maintaining a light beam bouncing back and forward between two mirrors. What we see as a laser beam is the small fraction of light that leaks out at each reflection. A ring laser has the mirrors arranged in a triangle or square forming a closed path or 'ring'. This allows two essentially independent laser beams to travel around the ring, one clockwise, the other counterclockwise.
Suppose the whole apparatus is rotated clockwise. Imagine a beam of light beginning at one mirror. Since light travels at the same speed in both directions it will take longer for the clockwise traveling beam to reach its starting position compared to the counterclockwise beam since the starting position has moved since the beam departed. This is illustrated in the left most animation below. This effect alone causes a phase shift between the two beams and was discovered by french physicist Georges Sagnac who first demonstrated the principle in 1913 . It is this phase shift that is measured in a passive Sagnac interferometer such as our own fibre-optic gyro.
Because a laser is a resonant cavity there must always be a whole number of wavelengths of light around the closed loop. As a result the wavelengths must stretch or shrink in response to rotation. This causes a subtle change in the colour or each beam depending on the rotation speed. By mixing these two beams together we can make a very accurate measurement of the Sagnac frequency. The accuracy is such that over one second we can measure a change in angle roughly equivalent to the angle seen between the two sides of a human hair viewed from a distance of 500 km. Compared to the best fibre gyros this is not especially remarkable, though what sets us apart is that this accuracy can be maintained for many hours.
The principle of wavelength change due to rotation is illustrated in the animation shown below to the right. Here, the laser is depicted rotating back and forward. The dots represent packets of light; if you follow one with your eye you can see it change from red to blue depending on the rotation of the laser. Notice that at the instant when the laser has zero rotational speed both beams have the same colour (black). This illustrates a condition known as lock-in. When this condition occurs the device becomes useless as a rotation sensor.
About the Research
So a ring laser can measure the absolute rotation of whatever environment it is placed in. Small ring lasers are used for navigation in aircraft, submarines and spacecraft. If you have flown in a modern airliner, you have most probably been guided by a small laser gyroscope in addition to the global positioning system. In navigation ring lasers must be used in conjunction with 'dead reckoning', the technique of determining your position from knowledge of your starting position and all the accelerations you have been subject to since beginning. Since small errors add up over time the development of more accurate ring lasers means more accurate navigation information.
In our case, it is the much smaller daily rotation of the earth about it's axis that is our main interest. It turns out that the larger the ring laser and the better quality the mirrors, the more sensitive the device becomes as a rotation sensor - but it also becomes much harder to construct and operate. In recent years we have produced larger and larger devices, our UG-2 laser is the largest in the world.
The rotation rate of the Earth makes it a very good clock, but not perfectly so. The rotation axis twists and wobbles since the Earth is not a perfect sphere; atmospheric motion reacts back on the Earth. (Try keeping your body absolutely still as you wave your arms). As a result, one day is a few thousandths of a second longer or shorter than the next.
Improving our knowledge of how the Earth twists and turns in space at the level of parts per billion remains an important scientific goal. This knowledge is important for precise navigation, since it maps the terrestrial reference frame (where you want your position accurately) into the celestial reference frame (where the navigation satellites belong).
Another method of determining extremely precise information on Earth motion comes from the field of intercontinental radio astronomy and satellite laser ranging. After decades of steady development it is now possible to use these techniques to see local motions near earthquake faults and even continental drift after only a few months of observations. It helps to combine results from different technologies. As a sensitive, stand-alone and absolute rotation sensor, a ring laser can give complementary information to astronomical measurements and also short-term information not available using these methods. Short term information is particularly interesting to those who study earthquakes.
One recent and interesting result from the ring laser group is our recent landmark direct measurement of polar wobble , an effect due to the moons gravity that causes the earth rotation axis to be perturbed daily by 60 cm at the poles. Another interesting measurement is that of earth tides , an effect of the same origin as the regular ocean tides we are all familiar with however on solid earth rather than water. Another interesting measurement just on the horizon is the measurement of the subtle periodic movement of mountain ranges caused by wind pushing on them.