Development of crossover-free fiber optic gyroscope sensor coils

Draper Laboratory's experience with Interferometer Fiber Optic Gyro (IFOG) technology started via an Internal Research and Development effort in 1978. This work led to developing fiber optic gyros in which significant advances in IFOG component technologies, assembly, integration, and test were achieved. In excess of 30 patents have been issued to Draper as a result of this pioneering effort. More recently, Draper collaborated with JPL to transition their fiber optic breadboard gyro to a space qualifiable instrument for interplanetary long duration missions. During the initial phase, brassboard gyros were designed and fabricated that demonstrated performance that bettered NASA's CRAF-Cassini spacecraft objectives. In the second phase, the Engineering Model gyro was developed to meet mission qualification tests. Concurrent with this later phase, analysis, tests and qualification activities were performed to validate that gyro components would realize the required 16 year life. Two major inventions (patent pending) were conceived; one provides for continuous adaptive scale factor stability and the other integrates the optical source and photodetector into a single component [Source Integrated Detector (SID)], thereby eliminating a coupler and several splices thus reducing loss by 6 db. Using the above technologies and the implementation of common mode rejection of laser noise, Draper has defined a low noise high performance gyro in which rate power spectral density (PSD) is projected to be < 7 X 10-7 (deg/hr)2/Hz. In those applications, where the PSD < 4 X 10-5 (deg/hr)2/Hz is sought, a small size three component (SID, integrated optics circuit and fiber coil) gyro is easily implemented.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The use of an optical fiber gyroscope in automotive navigation is reported. The performance requirements for a navigational gyroscope are discussed in terms of the input range, resolution, and bias and scale errors. The OFG-3, an automotive optical fiber gyroscope prototype, is introduced, and its error sources are related to the gyroscope designs. The gyroscope is made of polarization maintaining fibers and incorporates optical phase modulation and synchronous demodulation techniques. The gyroscope electronics restrict the resolution and the bias error, while the optics dominate the scale error. The microcomputer in the electronics compensate for the scale error, and the OFG-3 meets navigational requirements. Automotive navigation has been demonstrated using the gyroscope in combination with a map-matching algorithm.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The degradation of optical symmetry caused by the thermal strain of the fiber coil results in the thermal bias drift of fiber optic gyro (FOG). This paper presents a method for quantitative analysis of the optical symmetry of the fiber coil and accurate prediction of thermal bias drift in FOG. Firstly, a thermal strain model is established to illustrate the strain characteristics of the layer structure in the fiber coil under temperature fields, emphasizing the importance of thermal strain on optical symmetry. Then, an equivalent asymmetric index (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">α_Tk</i>) is introduced to quantitatively evaluate the optical symmetry of the fiber coil. By deriving equations for optical symmetry and thermal bias drift caused by thermal strain, a novel method is proposed to predict and optimize the thermal bias drift. This method utilizes high-performance OFDR to measure temperature and strain in a 3 km fiber coil to predict thermal bias drift. It also discusses the relationship between the length of the tail fiber and optical symmetry, predicting the precise length required to optimize thermal bias drift. Subsequently, the same fiber coil is placed in the FOG to verify the accuracy of the thermal bias drift prediction. Adjusting the length of the tail fiber reduces the thermal bias drift from 0.33 °/h to 0.13 °/h.

The fiber optic gyroscope (FOG)based on Sagnac effect has became to one of the most important sensors in developing due to light in quality, high accuracy, compact in dimension and long life and has played a very important role in both military and civil use. It is the most difficult problem that the FOG has an obvious bias drift caused by temperature change and temperature grade, so its application is limited to a great extent. Fiber coil is one of the most critical components in FOG. Here, the characteristic of temperature error of the fiber optical coil was analyzed. At first, by studying the glass transition of coating adhesive in the fiber coil, the element model of the fiber coil with the glass transition of coating adhesive in FOG was built. Then the discrete mathematics model of SHUPE error with the glass transition of coating adhesive in FOG was built. Finally, based on the temperature models mentioned above, the effects caused by the glass transition of coating adhesive on temperature performance of fiber optic gyroscope were analyzed. Theoretical analysis and experimental results show that effect caused by the glass transition of coating adhesive had seriously affected the temperature performance of FOG. By optimizing the glass transition temperature of coating adhesive, the SHUPE error of fiber coils can be reduced. At the same time, the amplitude uniformity of the SHUPE error can be improved greatly to reduce the difficulty in temperature compensation.

Shupe error, a kind of bias error over temperature-varying environments, still exists even with strict quadrupolar winding pattern, and it affects the environment adaptability of fiber optical gyroscope (FOG) and limits its application. A novel configuration to realize optical compensation for Shupe error in FOG is promoted, employing the opposite polarity of Shupe error in two FOGs working with polarization-maintained (PM) fiber’s fast and slow axis, respectively, in a single PM fiber coil. Analysis is also made for polarization non-reciprocity (PNR) in both FOGs, based on which the optimum coupling angles between integrated optic chip and PM fiber coil are obtained to guarantee that Shupe error arising from both FOGs can be counteracted to zero, and PNR error is suppressed to be minimum at the same time.

The Sagnac phase shift between two counterpropagating beams is the basis for all optical gyroscopes [11, although it is detected in a variety of ways. In the ring laser gyro and various "closed loop" optical gyroscopes, the scale factor (e.g. counts per unit rotation rate) is fixed by the area of the optical medium whereas the phase reading fiber optic gyroscope (FOG) has a scale factor which increases with increasing length of optical fiber. Thus development of low-loss fibers holds the possibility of extremely sensitive gyroscope operation, and has made the FOG a competitive optical gyroscope. Although most of the first decade of FOG development has concentrated on improving sensitivity, attention is now turning to other aspects of performance.

In this work, the fiber optic gyroscope (FOG) with an elliptic fiber coil (EFC) is applied to the inertial measurement of petroleum well trajectory for the special condition of the borehole diameter limitation in the inclinometer while drilling (IWD). The three-dimensional (3D) Shupe error mathematical model and finite element model of the EFC are established by the quadrupole symmetrical technique, which is based on the Shupe effect of fiber coil. Then, the simulation experiments and the temperature experiments of the EFC are carried out to verify the correctness of the mathematical model. Under this prerequisite, the influence of the structure parameters of the EFC on the Shupe error is analyzed. In order to compare the rotation rate error between the EFC and the circular fiber coil (CFC), the temperature experiments of the EFC and the CFC are carried out with the same temperature excitation when the fiber length, the layers of fiber coil, and the radius of fiber coil of the EFC are the same as those of the CFC. The results show that the bias stability of the EFC is 35% to 39% smaller than that of the CFC before the temperature compensation, and after the temperature compensation, the bias stability of the EFC is 49% to 70% smaller than that of the CFC.

Reduced phase error of a fiber optic gyroscope using a polarization maintaining photonic crystal fiber

Gyroscopes and accelerometers are unique devices, the critical input sensors for inertial navigation and surveying systems. Among the many types of gyroscopes only a few have proved adequate for precise inertial applications. These are the ring laser gyroscope (RLG), electrostatic suspended gyroscopes (ESG), air-bearing floated-gimbal spinning mass gyroscopes, and dynamically tuned rotor gyroscopes (DTG). All have been developed to a high art among precision instruments, with supremely low noise and bias errors. Extensive acceptance of inertial navigation systems in commercial flight service, particularly using RLGs, has also required and stimulated development of practical and competitve reliability, cost, and lifetime.A few currently emerging gyroscope technologies might play a role in future inertial systems, but it is not yet clear whether they can advance to be competitive in performance and cost with current gyroscopes. Most notable is the fiber-optic gyroscope (FOG), another form of optical gyroscope, which in the last several years has advanced to a state suitable for engineering prototypes in aircraft attitude heading and reference systems. Another is the hemispherical resonator gyroscope (HRG), which uses the precession of vibrations on the rim of a “wine glass” shaped quartz resonator.

In modern aerospace technology, strapdown inertial navigation systems (SINS) are widely used, using fiber-optic (FOG) or ring laser (CLG) gyroscopes. During the operation of such systems, the sensitivity axes are rotated relative to the basic coordinate system. The resulting angles between the axes of the base coordinate system and the axes of sensitivity of the navigation system (non-orthogonality) are one of the factors leading to an increase in the measurement errors of the device, which affects the measurement accuracy. During operation, the system is affected by vibrations of various nature, the impact of which can contribute to the appearance of non-orthogonality. The purpose of this work is to determine the maximum permissible vibration amplitudes affecting the SINS body according to the permissible values ​​of the deviation of the FOG sensitivity axes for two variants of the SINS layout. An approach to determining the permissible amplitudes of an external harmonic impact on the unit of a strapdown inertial navigation system based on fiber-optic or ring laser gyroscopes is considered. A design scheme, mathematical and finite element models for calculating natural frequencies and forced oscillations of a strapdown inertial navigation system unit have been developed. In various frequency ranges, numerical calculations have determined the boundary values ​​of the amplitudes of the external harmonic impact on the base of specific configurations of the SINS assembly. It has been established that dangerous states take place in the region of the 1st natural frequency of the system, as well as near higher frequencies. Comparison of the results for design options 1 and 2 allows us to conclude that in order to weaken the effect of vibrations on the accuracy of the SINS unit, it is advisable that the lowest natural vibration frequencies for the SINS assembly be as high as possible (more than 1000 Hz).&#x0D; Key words: vibration; fiber optic gyroscope; strapdown inertial navigation system; finite element method; natural frequencies and modes of vibration.

Firstly, this paper determines that the main cause of the Shupe error is the temperature distribution gradient in the fiber coil that changes with time and space by studying the Shupe error mathematical model of fiber optic gyroscope (FOG). To solve this problem, according to the structural form and composition of the fiber coil, the equivalent physical property parameters of the fiber coil are analyzed and obtained, and the equivalent model of the fiber coil is established. Then, a variety of structural forms with different protective layers are designed for the optical fiber coil under the condition that space allows, such as thermal conduction layer and thermal insulation layer, and the finite element transient temperature field simulation analysis is performed for each structural form. The simulation results show that the optimal protective layer structure is that the inner layer is thermal conduction layer, and the outer layer is thermal insulation layer. Compared with the structure without protective layer outside the fiber coil, the temperature difference within the fiber coil is reduced from 9.32℃ to 0.5℃, and the average temperature rise of the fiber coil is reduced from33.34℃ to 4.26℃. Finally, a local heating experiment is performed on the FOG with such protective layer structure, and the results show that the zero bias stability of the FOG is improved from the original 0.0118°/h to 0.0055°/h, which is of great significance for improving the temperature performance of FOG.

Fiber optic gyroscope (FOG) is a new type of angular velocity sensor in the field of inertia technology. Its non-moving parts and all-solid-state characteristics leads to many advantages compared with traditional electromechanical gyro and ring laser gyro. Scale factor is a key parameter of FOG, which affects the angle measurement accuracy. Closed-loop feedback phase modulation is an important method to enhance the scale factor stability in FOG. The multifunctional integrated optical circuit (MIOC) is the actuator of closed-loop feedback, and the mean wavelength stability of transmission spectrum at full temperature affects the scale factor stability of FOG. With the improvement of FOG's accuracy, the effect of the mean wavelength shift on FOG's performance are becoming more and more important, which has been an important factor limiting the scale factor stability enhancement. Based on the optical waveguide theory, we found out alignment deviation (axial gap, transverse dislocation and axial angle deviation) is the main factor of the mean wavelength stability, and verified by coupling experiments. Moreover, the mean wavelength is the most sensitive to the transverse dislocation. Take z=10 μm for example, the mean wavelength increases by 134 ppm relative to the wavelength of 1.31 μm when x increased from 0 to 2 μm. Secondly the axial gap, the mean wavelength increase by 19 ppm when z increased from 0 to 20 μm. Thirdly the angle deviation plays a minor role, and the mean wavelength decreases by 5 ppm when θ increased from 0° to 2°. This study is of great significance to optimize MIOC, and to provide device guarantee for the enhancement of scaling factor stability of FOG.

With the development of laser and optical fiber, the north finding system (NFS) with fiber optic gyroscopes (FOGs) has become an active trend in inertial technology research. This paper demonstrates the error analysis of dynamic north-finding with a fiber optic gyroscope and a new way for the FOGs noise measurement in the dynamic implementation. Moreover, in order to suppress the north-finding error caused by FOG bias drift, especially for the small azimuth angle, we propose a modified dynamic north-finding scheme by taking advantage of the dynamic method. The experiments verify the validity of the proposed error analysis for the dynamic implementation. Experimental results show that for a FOG with bias instability of 0.005°/hr, the average (Avg) north-finding bias error is less than 0.03° (0.5 mrad) around 2° azimuth angle by the proposed modified scheme. And compared with the conventional dynamic north-finding, this scheme reduces the estimated error (1σ) of azimuth angle and the bias error (Avg) by 22% and 23.8%, respectively, at the small azimuth angle. For a poorer-performance FOG with 0.0262°/hr bias instability, the proposed method is more effective and reduces 32.4% estimated error (1σ) of azimuth angle.

Fiber optic gyroscopes (FOGs) are vital components for manned, unmanned, and autonomous platforms, as well as applications requiring extremely accurate data for stabilization, pointing, orientation of payloads, and navigation and control. FOG-based inertial systems also provide highly accurate data for all types of navigation, positioning, and georeferencing systems. Substantial improvements in the performance of FOGs and FOG-based inertial systems at KVH are directly related to advancements in the design and manufacture of optical fiber, as well as in manufacturing operations and signal processing. Open-loop FOGs, such as those developed and manufactured by KVH Industries, offer tactical-grade performance in a robust, small package. The success of KVH FOGs and FOG-based inertial systems is due to a series of key innovations including the development of proprietary D-shaped fiber with an elliptical core, as well as KVH's ThinFiber. KVH continually improves its FOG manufacturing processes and its signal processing, with the goal of improving accuracies across its entire FOG product line. KVH acquired its FOG capabilities, including its patented E·Core® fiber, when the company purchased the assets of Andrew Corporation's Sensor Products Group in 1997. E·Core fiber is unique in that the light-guiding fiber core — critical to the FOG's performance — is elliptically-shaped. The elliptical core results in an optical path with low loss and high polarization-maintaining ability. In 2011, KVH developed its ThinFiber, a 170-micron diameter dual acrylate-coated fiber that retains the full performance characteristics of E·Core fiber. ThinFiber has enabled the development of compact, high-performance open-loop FOGs, which are also used in a line of FOG-based inertial measurement units (IMUs) and inertial navigation systems (INS).

The advantages and the characteristics of the fiber optic gyroscopes (FOGs) for the space application, compared with the traditional mechanical gyroscope and the laser gyroscope, have been mentioned. There are differences of the FOG designing between application in space and other conditions especially the radiation-resistant, low power cost and less weight of the FOG. The key techniques of the fiber optic gyros technology have been proposed and some test results have been discussed.