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Lasers
Published in Robert G. Hunsperger, Photonic Devices and Systems, 2017
Another very useful type of resonator is the ring resonator. The simplest ring resonator uses three mirrors, as shown in Fig. 17a. Two traveling waves propagating in opposite directions can exist in this cavity, providing a pair of output beams. This configuration is used in the ring laser gyroscopes used for navigation. If a ring resonator laser whose perimeter p encloses an area A rotates at an angular frequency Ωrot in the plane of the ring, the movement of the mirrors produces opposite Doppler shifts in the two traveling waves. As a result, there will be a frequency difference Δv = AΩrot/pλ between the two output beams. The sensitivity of such a device is very high. For example, a ring laser with 10-cm spacing between the mirrors operating at λ = 633 nm will exhibit an easily measured 5-Hz frequency difference (measured at the North Pole) due simply to the earth’s rotation.
Inertial navigation systems
Published in Mike Tooley, David Wyatt, Aircraft Communications and Navigation Systems, 2017
The original inertial navigation systems used electromechanical gyros; these were subsequently replaced by a more reliable and accurate technology: the ring laser gyro (RLG). Ring laser gyros use interference of a laser beam within an optic path, or ring, to detect rotational displacement. An IRU contains three such devices (see Figure 17.8) for measuring changes in pitch, roll and azimuth. (Note that laser gyros are not actually gyroscopes in the strict sense of the word—they are in fact sensors of angular rate of rotation about an axis.) Two laser beams are transmitted in opposite directions (contrarotating) around a cavity within a triangular block of cervit glass; mirrors are located in two of the corners. The cervit glass (ceramic) material is very hard and has an ultra-low thermal expansion coefficient. The two laser beams travel the same distance, but in opposite directions; with a stationary RLG, they arrive at the detector at the same time.
All About Wave Equations
Published in Bahman Zohuri, Patrick J. McDaniel, Electrical Brain Stimulation for the Treatment of Neurological Disorders, 2019
Bahman Zohuri, Patrick J. McDaniel
Interferometers became popular toward the end of the 19th century and there are several different kinds, each based roughly on the principle we’ve outlined above and named for the scientist who perfected it. Six common types are the Michelson, Fabry-Perot, Fizeau, Mach-Zehnder, Sagnac, and Twyman-Green interferometers and they are all described as follows: The Michelson interferometer (named for Albert Michelson, 1853–1931) is probably best known for the part it played in the famous Michelson-Morley experiment in 1881. That was when Michelson and his colleague Edward Morley (1838–1923) disproved the existence of a mysterious invisible fluid called “the ether” that physicists had believed filled empty space. The Michelson-Morley experiment was an important stepping-stone toward Albert Einstein’s theory of relativity.The Fabry-Perot interferometer (invented in 1897 by Charles Fabry, 1867–1945, and Alfred Perot, 1863–1925), also known as an etalon, evolved from the Michelson interferometer. It makes clearer and sharper fringes that are easier to see and measure.The Fizeau interferometer (named for French physicist Hippolyte Fizeau, 1819–1896) is another variation and is generally easier to use than a Fabry-Perot. It’s widely used for making optical and engineering measurements.The Mach-Zehnder interferometer (invented by German Ludwig Mach and Swiss man Ludwig Zehnder) uses two beam splitters instead of one and produces two output beams, which can be analyzed separately. It’s widely used in fluid dynamics and aerodynamics—the fields for which it was originally developed.The Sagnac interferometer (named for Georges Sagnac, a French physicist) splits light into two beams that travel in opposite directions around a closed loop or ring (hence its alternative name, the ring interferometer). It’s widely used in navigational equipment, such as ring-laser gyroscopes (optical versions of gyroscopes that use laser beams instead of spinning wheels).The Twyman-Green interferometer (developed by Frank Twyman and Arthur Green in 1916) is a modified Michelson mainly used for testing optical devices.
Autonomous underwater vehicles - challenging developments and technological maturity towards strategic swarm robotics systems
Published in Marine Georesources & Geotechnology, 2019
N. Vedachalam, R. Ramesh, V. Bala Naga Jyothi, V. Doss Prakash, G. A. Ramadass
Precision gyroscopes for measuring the vehicle attitude changes in the angular DOF include the ring laser gyroscopes (RLG) and the interferometeric fibre optic gyroscope (I-FOG). Their technologies are in a highly advanced stage with extremely low bias stability, low angular random walk and capable of determining the true north using the true north seeking algorithms (Lefevre 2014a; Zhang and Liu 2017).These gyroscopes are immune to earth’s magnetic field and hence capable of providing effective measurements in the Polar Regions where magnetic field lines are near-vertical impairing use of traditional magnetic compasses (Lefevre 2014b). The RLG has attained full technological maturity with the ultimate challenge of “lock-in effect” experienced at very low rotation rates being overcome by the mechanical dithering technique. But RLG needs a larger volume to increase the length of the optical cavity, which makes it expensive and bulky (Lefevre 2013). The I-FOG characterized by their smaller footprint, light weight, wide dynamic range and faster response have attracted significant interest. The residual limit of the I-FOG bias stability is due to the ambient temperature time-transient, which when controlled could result in a long term bias stability of about 10−5°/h and capable of meeting the strategic grade requirements for underwater navigation (Ramadass et al. 2017). The advancements in the navigation grade accelerometer technologies and vehicle velocity measurements based on Doppler velocity has given a significant confidence to realize precise Doppler velocity-aided NS (Ellingsen 2008).