Gyroscope Technology and Applications: A Review in the Industrial Perspective <s> Recent Advances in RLGs <s> Soft magnet devices have a wide variety of applications in aerospace, military, and industrial sectors. Generally, devices used in industrial applications are required to operate satisfactorily under ambient temperatures of +10 degC to +50 degC under normal pressure, whereas those used in aerospace and military applications will face a wider range of temperatures from -60 degC to +80 degC and generally high pressure. In most of the soft magnetic materials, the use of a temperature compensating and stress-sensitive alloy will introduce changes in their normal performance. Therefore, an attempt is made to devise a method that ensures inherent temperature compensation and stress proof. Recent work in characterizing the performance of multielement high-entropy magnetic nanocomposite systems will be described. Specifically, we have developed soft magnetic alloys that exhibit tolerance of high stress and large temperatures changing above current operating ranges for four-mode ring laser gyro (RLG) devices <s> BIB001 </s> Gyroscope Technology and Applications: A Review in the Industrial Perspective <s> Recent Advances in RLGs <s> We report for the first time (to our knowledge) the experimental achievement of a single-frequency ring-laser gyroscope using a diode-pumped half-vertical-cavity semiconductor-emitting laser structure as a gain medium. Thanks to the control of mode competition by an active feedback loop, we observe a beat signal from recombined beams that has a frequency proportional to the rotation rate as predicted by the Sagnac effect. This promising result opens new perspectives for rotation sensing. <s> BIB002 </s> Gyroscope Technology and Applications: A Review in the Industrial Perspective <s> Recent Advances in RLGs <s> Nonlinear couplings induced by crystal diffusion and spatial inhomogeneities of the gain have been suppressed over a broad range of angular velocities in a solid-state ring laser gyro by vibrating the gain crystal at 168 kHz and 0.4 μm along the laser cavity axis. This device behaves in the same way as a typical helium-neon ring laser gyro, with a zone of frequency lock-in (or dead band) resulting from the backscattering of light on the cavity mirrors. Furthermore, it is shown that the level of angular random-walk noise in the presence of mechanical dithering depends only on the quality of the cavity mirrors, as is the case with typical helium-neon ring laser gyros. <s> BIB003 </s> Gyroscope Technology and Applications: A Review in the Industrial Perspective <s> Recent Advances in RLGs <s> Abstract To avoid the lock-in effect, a body dither is introduced for the ring laser gyro (RLG). A dither feedback signal is necessary to control the dither drive signal. In the current dither control method, a piezoelectric or electromagnetic transducer is often used to obtain the dither feedback. The read-out signal of the RLG contains all dither-related information; hence, dither feedback can be obtained from the read-out signal, and external feedback can be reduced. A novel dither controlling method without external feedback is introduced in the present paper. How to control the dither amplitude and track the resonant frequency of the dither motor is described in detail. A comparative experiment is conducted between the old dither control method and the new one. Results show that the output of RLG using the new method has the same accuracy as that using the old method, aside from having more compact circuit dimensions. <s> BIB004 </s> Gyroscope Technology and Applications: A Review in the Industrial Perspective <s> Recent Advances in RLGs <s> Large ring lasers have exceeded the performance of navigational gyroscopes by several orders of magnitude and have become useful tools for geodesy. In order to apply them to tests in fundamental physics, remaining systematic errors have to be significantly reduced. We derive a modified expression for the Sagnac frequency of a square ring laser gyro under Earth rotation. The modifications include corrections for dispersion (of both the gain medium and the mirrors), for the Goos-Hänchen effect in the mirrors, and for refractive index of the gas filling the cavity. The corrections were measured and calculated for the 16 m2 Grossring laser located at the Geodetic Observatory Wettzell. The optical frequency and the free spectral range of this laser were measured, allowing unique determination of the longitudinal mode number, and measurement of the dispersion. Ultimately we find that the absolute scale factor of the gyroscope can be estimated to an accuracy of approximately 1 part in 108. <s> BIB005
In the last ten years, important recent advances in RLGs have been achieved. To improve the performance of this kind of gyroscopes, different solutions have been proposed. In 2007, Cai et al. BIB001 developed soft magnetic alloys that exhibited high strength to external stresses and to large temperature changes, allowing a wide temperature operating range for a four-mode ring laser gyro. They obtained a 0.01 • /h zero excursion, with a 15-cm dimensioned differential laser gyro. In 2008, Mignot et al. BIB002 reported for the first time the experimental achievement of a single-frequency ring-laser gyroscope, using a diode-pumped half-vertical-cavity semiconductor-emitting laser structure as a gain medium. The experimental setup had an overall perimeter of 50 cm. They obtained a scale factor of 716 Hz/( • /s). In 2009, Schwartz et al. BIB003 could suppress nonlinear couplings induced by crystal diffusion and spatial inhomogeneities of the gain over a broad range of angular velocities in a solid-state ring laser gyro. The result was obtained by vibrating the gain crystal at 168 kHz and 0.4 µm along the cavity axis. The solid-state RLG used was made of a 22 cm long ring cavity, containing a 3 mm long diode-pumped Nd:YAG crystal. They showed that the level of angular random-walk noise in presence of mechanical dithering depends only on the quality of the cavity mirrors. The scale factor of the considered ring laser gyro was about 754 Hz/( • /s). Large ring lasers can exceed the performance of navigational gyroscopes by several orders of magnitude. In  , it is reported that an ultralarge ring He-Ne ring laser gyroscope (UG-2, 39.7 × 21 m 2 ) has been built under ground. Earth rotation is sufficient to unlock it, with a Sagnac frequency of 2.18 kHz. The residual Sagnac frequency error, caused by backscatter coupling, is measured as <2 parts in 10 8 . The best stability achieved for an averaging time of about 2000 s. The scale factor is 7.67 × 10 5 Hz/( • /s). Direct dither control without external feedback was used in BIB004 in 2012 to avoid the lock-in effect for a ring laser gyro. A new design, that makes the system more compact and inexpensive, was proposed. Experiments showed that the accuracy using this method (ARW = 6.31 × 10 −4• / √ h and bias instability = 3.86 × 10 −4• / √ h) is nearly the same as that using the prior method with PZT as dither feedback. While the most prevalent design of a RLG is the active gyroscope, in  a "passive" gyroscope is shown, in which the sensing cavity is tracked using external laser beams. This design is free from the deleterious lock-in effect observed in active systems and could be constructed using commercially available components. The core of the gyroscope was a squared free-space optical cavity of 75 cm side length. A sensitivity of about 5.7 × 10 −7 ( • /s)/ √ (Hz) above 500 mHz was achieved. Recently, a modified expression for the Sagnac frequency of a large square ring laser gyro undergoing Earth rotation has been derived in BIB005 . The modifications include corrections for dispersion of the gain medium and the mirrors, for the Goos-Hanchen effect in the mirrors and for refractive index of the gas filling the cavity. The corrections were measured and calculated for the 16 m 2 Grossring laser at the Geodetic Observatory Wettzell.