Ring laser gyroscope

A laser gyro or ring laser is a device that uses a laser whose resonator forms a closed loop ( ring resonator ). Laser gyro measure highly sensitive and wear-free rotational motion in a plane. The laser gyro has in common with a gyroscope gyroscope or the property that it can display rotation rate in the room, so stirred his name. However, it has no moving parts.

Construction

Determining the rotation in space to the three spatial axes requires three ring laser, which are arranged in mutually perpendicular planes.

A laser gyro or ring laser consists of two light rays that pass through together in the opposite direction a closed path. It used as a light source, a laser is the only way a monochromatic and stable frequency radiation can be generated. For the light path several mirrors are used. Come take a long mirror, wound glass fiber as a waveguide for use, it is called a fiber-optic gyroscope. Shares of the counterpropagating beams are brought to the overlay. Loop, the light at rest, the two signals are identical, on the other hand causes a rotation of the loop, the plane normal to a phase shift which is detected in the interference pattern.

Principle

You build a laser resonator, which is not usually stretched as usual, but with the help of several mirror or a coiled fiber is annular. This gives two fundamentally independent of each other light waves with opposite direction of rotation. By the Sagnac effect, the optical lengths of the annular periphery of the two shafts vary slightly during the rotation of the entire structure. Thus, the frequencies at the same longitudinal laser mode differ minimally.

If you couple shares both beams and letting it interfere, one can observe the difference frequency as the beat: depending on the rotational direction of the interference fringes move to the left or right.

This effect is much easier to measure than the small phase shift in the Sagnac interferometer and thus can be easily evaluated by measurement - measuring the amplitude modulation of the interfering beams with a photo diode, and thus counts the difference frequency of the beams.

Theory

For the difference between the frequency where:

So

With area A, angular velocity, the circumferential and wavelength l. For the calculation of the phase shift see Sagnac interferometer.

Lock-in effect

At low rates of rotation occurs a fundamental problem, the lock-in effect occur not only reflection and transmission, but also scattering at each mirror. A small part of the scattered light coupled into the opposite direction of rotation.

Thus both laser oscillations influence. This means that at rotation rates below the lock-in threshold, both laser oscillations have exactly the same frequency. At higher rotation rates, the frequency difference is still less than calculated according to the above theory.

In principle, this nonlinearity would be no problem as long as you want only measure rotation rates above the lock-in threshold - you could count out the effect. However, the lock-in threshold is not constant, it depends on the amount of scattering, and the dispersion is affected, inter alia, on the number of particles of dust on the mirrors. In addition, the back-scattered at the four mirrors waves interfere with each other. This leads, depending on the phase position to gain or attenuation.

Characterized the back-scatter, and thus the lock-in effect is very strongly dependent on the distance of the mirrors. You must continue to be built and very stable on a base plate with an extremely small thermal expansion. In addition, the temperature must be very well kept constant.

To bypass the lock-in effect ( dithering) is added, for example in aerospace technology, the entire laser gyros either with constant angular velocity rotation ( rate bias technique) or in a dither vibration. This one also measures at rest support system rotation and is always far above the lock-in threshold.

Applications

Laser gyroscopes are used in the aerospace industry, but also in military land vehicles and naval vessels on as a navigation aid and are generally part of an inertial navigation system (INS ). In civilian commercial aircraft, they are increasingly supplemented by GPS receiver, as by means of GPS certain positions are short term long term more accurately and precisely by means of INS certain positions. The systems thus complement each other.

In military aircraft and civilian commercial aircraft, they still have a meaning as an additional security if the GPS system fail ( Global Positioning System ) system or should be disturbed. In the military sector is also of advantage that the laser gyro requires no run- time as a mechanical gyroscope ( gyro, gyroscope). However, gyroscopes or gyros are often still redundant, since these are mechanical and therefore retain their directional information in case of power failure.

In the private aviation is often satisfied for reasons of cost with mechanical rate gyros.

Ring lasers are also used in geodesy to measure the rotational component of earthquakes and the continuous measurement of the Earth's rotation. Here one uses a stationary ring laser and endeavors by expensive air conditioning to keep the lock-in threshold constant.

In commercial applications, the laser gyro finds its use in the measurement of pipelines, the stabilization of laser scanners or cameras on aircraft for airborne measurement of objects such as buildings or landscape topologies (LIDAR or ortho - photogrammetry) or in the precise navigation of drilling in the area of Horizontal drilling under the ground (so-called no- dig pipeline ).

Even underwater robot to navigate with laser gyros, when it comes to extremely high accuracy for autonomous missions over many hours or days.