Gyroscope Technology and Applications: A Review in the Industrial Perspective <s> Recent Advances in IFOGs and RMOGs <s> We report the demonstration of the first air-core photonic-bandgap fiber gyroscope. Because the optical mode in the sensing coil travels largely through air, which has much smaller Kerr, Faraday, and thermal constants than silica, far lower dependencies on power, magnetic field, and temperature fluctuations are predicted. With a 235-m fiber coil, we observe a minimum detectable rotation rate of ~2.7deg/h and a long-term stability of ~2deg/h, which are consistent with the Rayleigh backscattering coefficient of the fiber and comparable to that measured with a conventional fiber <s> BIB001 </s> Gyroscope Technology and Applications: A Review in the Industrial Perspective <s> Recent Advances in IFOGs and RMOGs <s> This article describes a novel design of the interferometric fiber-optic gyroscope (IFOG) program, that is, a structure of hybrid optical IFOG. It is characterized by using low-polarized light source and single-mode optical fiber coupler in optical head. This optical structure not only can reduce the effect of polarization crosstalk, but also simplifies the assembly process and improves production efficiency. In addition, through the application of all-digital closed-loop control and random modulation signal processing technology, the dead-zone problem in IFOG caused by electronic cross-coupling have been eliminated, and the SNR of gyro output is improved. A large number of experimental testing results have proved that the gyro's zero-bias instability achieves 0.01 ° / h, random walk coefficient reaches 0.001 ° / √h, and scale factor nonlinearity is less than 5ppm (room temperature). The gyro's performance can meet the requirements of navigation, guidance and control in various fields. <s> BIB002 </s> Gyroscope Technology and Applications: A Review in the Industrial Perspective <s> Recent Advances in IFOGs and RMOGs <s> Precision guidance in navigation systems requires highly accurate, compact, and low cost inertial measurement units (IMUs). The key active guided-wave component of the IMU is the phase modulator. In our approach, electro-optic polymers have been utilized in fabricating low loss phase modulators with low half-wave drive voltage using advanced hybrid waveguide fabrication processes and novel optical integration techniques. However, the interference between the primary wave and the backscatter waves generated by the phase modulator and/or the interference between the two counter-propagating backscatter waves at the detector of the IMU has been a major source of error in this approach. A novel technique was introduced in assessing the error caused by backscatter and an offset waveguide design was developed to suppress the interference of backscatter light. The novel design not only preserved the miniaturization, but also improved the insertion loss with the use of a shorter waveguide. The gyro level tests performed with the backscatter compensated modulators showed about 5 times improvement of the average bias uncertainty over gyros integrated with a standard symmetric phase modulator. <s> BIB003 </s> Gyroscope Technology and Applications: A Review in the Industrial Perspective <s> Recent Advances in IFOGs and RMOGs <s> Resonator micro optic gyro (RMOG) with a waveguide-type ring resonator is a promising candidate for applications requiring small, light and robust gyros. In optical passive ring resonator gyros, clockwise (CW) and counter clockwise (CCW) lightwaves are phase-modulated at different frequencies to reduce the backscattering induced noise. The effectiveness of this technique, however, is determined by the carrier suppression level. In this paper, the influence of the carrier suppression level on the gyro performance is examined experimentally for an RMOG system with a silica waveguide resonator. In our experiment, carrier suppression is applied onto both the CW and the CCW lightwaves at the same time to achieve higher total suppression. We show that carrier suppression as high as 100 dB can be achieved by optimizing the amplitude of the phase modulation. A bias stability of 0.46 /s in 50 seconds is demonstrated in an RMOG with a silica waveguide ring resonator having a ring length of 7.9 cm. This is the best result reported to date, to the best of our knowledge, for waveguide-type ring resonator gyros of this size. <s> BIB004 </s> Gyroscope Technology and Applications: A Review in the Industrial Perspective <s> Recent Advances in IFOGs and RMOGs <s> We demonstrate that by driving a fiber optic gyroscope (FOG) with a laser of relatively broad linewidth ( ~ 10 MHz), both the noise and the bias drift are reduced to very low levels (0.058°/√h and 1.1°/h, respectively), comparable to the performance of the same gyroscope conventionally driven with a broadband light source. When the laser linewidth is reduced to a low enough value ( ~ 2.2 kHz), the FOG exhibits a higher drift but an even lower noise, about 4 dB lower than with a broadband source, and only 3.5 dB above shot noise. The measured dependencies of the noise and drift on laser linewidth are in good quantitative agreement with the predictions of an advanced model of backscattering errors in a FOG interrogated with coherent light, which confirms that the noise and drift are predominantly limited by backscattering. The use of a laser comes with the additional benefit of a much greater wavelength stability compared to a broadband source, which is expected to translate directly into a much more stable scale factor than possible in conventional FOGs. Residual sources of drift and the prospects for reducing them in order to achieve inertial navigation performance are discussed. <s> BIB005 </s> Gyroscope Technology and Applications: A Review in the Industrial Perspective <s> Recent Advances in IFOGs and RMOGs <s> A low-noise low-delay digital signal processor is constructed on a single field-programmable gate array. An equivalent input noise as low as 3.752 nV/√Hz is demonstrated for the digital signal processor, which can detect an equivalent Sagnac effect of 0.003°/s in a resonant micro optic gyro (RMOG) with a 2.5-cm diameter ring resonator. With the processing time reduced from hundreds of seconds to 1.1 μs , this processor significantly increases the loop gain of the feedback loop and reduces the reciprocal noise in the RMOG. Owing to the fast speed of this processor, the lock-in frequency accuracy is reduced to 0.78 Hz (1σ), which is equivalent to a rotation rate of 0.004°/s. Relationship between this digitalized RMOG output signal and angular rate is obtained from ±0.25°/s to ±400°/s. The standard deviation of the residuals between RMOG output results and linear fit curve is 0.0236°/s. <s> BIB006 </s> Gyroscope Technology and Applications: A Review in the Industrial Perspective <s> Recent Advances in IFOGs and RMOGs <s> Resonator micro-optic gyro (RMOG) is a promising candidate for the next generation inertial rotation sensor based on the Sagnac effect. A current modulation technique used in an external cavity laser diode is proposed to construct the gyroscope system for the first time. The resonance curves before and after eliminating accompanying amplitude modulation are theoretically analyzed, calculated, and simulated; the demodulation curves with different modulation currents are formulated theoretically; and the optimum modulation current corresponding to the maximum sensitivity is obtained. The experiment results from the established RMOG experimental setup demonstrate that a bias stability of 2.7 deg/s (10 s integrated time) over 600 s, and dynamic range of ±500 deg/s are demonstrated in an RMOG with a silica optical waveguide ring resonator having a ring length of 12.8 cm. <s> BIB007 </s> Gyroscope Technology and Applications: A Review in the Industrial Perspective <s> Recent Advances in IFOGs and RMOGs <s> We demonstrate a novel dual-polarization interferometric fiber-optic gyroscope (IFOG), which needs only one coupler and no polarizer. Polarization nonreciprocity (PN) errors in common IFOGs will increase significantly if the polarizer is absent, or if only one coupler is used. In our setup, however, PN errors are effectively compensated by using two balanced polarizations. The 2 km coil, open-loop configuration obtains a bias instability of 0.02°/h in detecting the Earth's rotation rate. Its performance difference from the conventional two-coupler IFOG is only a stable bias, caused by coupler nonreciprocity. <s> BIB008
Different solutions have been proposed to improve Fiber Optic Gyroscopes performance in the last few years. In particular, a Resonant Fiber Optic Gyroscope (RFOG) uses dozens of meters of resonant fiber loops, thus reaching the same performance associated to kilometers of fiber coils into an I-FOG. The usage of Resonant Micro Optic Gyroscopes (RMOGs) allows obtaining good performance with smaller sizes, compared to FOGs. For all kinds of gyroscopes, bias drift is one of the most limiting factors. The first experimental configuration of an IFOG was proposed in  by Vali and Shorthill in 1976. Very important improvements have been done since the earliest solutions to date. Next, some of the recent advances in Fiber Optic Gyroscopes (FOGs) and Resonant Micro Optic Gyroscopes (RMOGs) will be illustrated. In 2006, the first air-core photonic-bandgap fiber gyroscope was reported BIB001 . Because the optical mode in the sensing coil largely travels through air (having low Kerr, Faraday and thermal constants than silica), a lower power and magnetic field dependences were obtained. With a 235 m fiber coil, a minimum detectable rotation rate of 2.7 • /h and a long-term stability of 2 • /h were achieved. In 2007, a new open-loop configuration of an IFOG was presented. A single mode telecommunication optical fiber and an EDFA pumped with DFB laser were used as sensing coil and broadband source, respectively  . The Sagnac phase shift was extracted by a phase tracking circuit with an RC band pass filter, an amplifier and a modulator chip. It measured a bias stability of 1.57 • /h and a sensitivity of 72 µV/( • /h). Moreover, a peak-to-peak noise of 8 • /h was derived. In 2009, a novel design of IFOG was presented by Yu et al. BIB002 . The proposed structure can reduce the effect of the polarization crosstalk and improves production efficiency. Through the application of all-digital closed-loop control and random modulation signal processing technology, the dead zone problem, typical of IFOG, caused by electronic cross-coupling, has been eliminated and the SNR of the gyro output has been improved. A zero-bias instability of 0.01 • /h has been achieved and the random walk coefficient has reached 0.001 • / √ h. In BIB003 , it is shown that electro-optic polymers have been used in fabricating low loss phase modulators with low half-wave drive voltage for an Inertial Measurement Unit based on an IFOG. A novel technique was introduced for assessing the error caused by backscatter and an offset waveguide design was developed to suppress the interference of backscattered light. An average ARW of about 0.006 • / √ h and a bias uncertainty of about 0.02 • /h were obtained. In 2010, Yahalom et al.  presented a new IFOG. The design was based on an innovative approach that enabled the production of a small and low-cost gyro with excellent noise and bandwidth characteristics. The goal was to develop an inexpensive sensor in less than 50 cm 3 . The gyro was configured as a "split gyro", where the light source, electronics and receiver are integrated in an external package and the sensor head was integrated in a robust and rigid package. The head sensor was 6.9 cm × 6.9 cm × 5 cm. They obtained a bias long-term stability of 0.2 • /h, and an ARW of 0.0022 • / √ h. In 2013, it was demonstrated that, by driving an IFOG with a laser of relatively broad linewidth (about 10 MHz), the noise would be reduced to 0.058 • / √ h, while the bias drift would be reduced to 1.1 • /h BIB005 . Researchers used a 150 m fiber coil, with a 3.5 cm coil diameter. Using a laser instead of a broadband light source, it offers increased scale factor arising from the frequency stability of the laser. However, driving a FOG with a laser leads to large noise and drift, because of the coherent backscattering, the Kerr effect and polarization non-reciprocity. It was demonstrated that, carefully selecting the laser linewidth and using a symmetric phase modulation scheme, one can reduce these sources of error to a very low level. In 2014, Wang et al. demonstrated a novel dual-polarization IFOG BIB008 , which only needs one coupler and no polarizer. With a 2 km coil and an open-loop configuration, a bias instability of 0.02 • /h and an ARW of 1.5 × 10 −3• / √ h were obtained. The usage of RFOGs (Resonant Fiber Optic Gyroscopes) instead of IFOGs, allows to reduce fiber length, thus leading to lower dimensions. In the era of miniaturization, the possibility of integrating optical waveguides leads to even smaller solutions. Thus, RMOG is a promising candidate for applications requiring small, light and robust gyros. In RMOGs, clockwise and counter-clockwise waves are phase-modulated at different frequencies to reduce backscattering induced noise. The effectiveness of this technique, however, is determined by the carrier suppression level. In the experiment reported in BIB004 , carrier suppression is applied onto both the CW and the CCW waves at the same time to achieve higher total suppression. Ma et al. obtained a bias stability of 0.46 • /s over 50 s with a silica waveguide ring resonator, having a ring length of 7.9 cm. In 2013, a 2.5 cm diameter ring was equipped with a low-noise, low-delay digital signal processor based on FPGA BIB006 , reaching a 0.67 • /s bias stability over 3600 s. The proposed solution can detect a minimum rotation rate of 0.25 • /s. In the same year, Lei et al. BIB007 proposed a current modulation technique in an external cavity laser diode to construct a gyroscope system for the first time. The experimental results from the established RMOG setup demonstrate a bias stability of 2.7 • /s (over 600 s) with a silica optical waveguide ring resonator having a ring length of 12.8 cm.