Autonomous vehicles require gyroscopes for navigation and stabilization of the platform in demanding battlefield, aerospace and undersea environments. Autonomous vehicles place tough demands on gyroscope technology, including extreme ruggedness, small-size, light-weight, low-cost, and low power operation. Also, slew-rates for gyros in autonomous vehicle applications can be very high and exceed the capabilities of many gyro systems.
Optical gyroscopes such as fiber optic gyroscopes (FOGs) and ring laser gyroscopes (RLGs) are typically used when global positioning systems (GPS) or micro-electromechanical systems (MEMS)-based gyroscopes are insufficiently precise or when GPS signal access is denied or may be jammed. The duration of time required for operation in a GPS-denied environment, combined with the motions executed by the platform and the azimuth accuracy requirement over the mission duration, drive the requirements on the gyroscope drift rate. A larger drift rate leads to faster degradation of angle errors.
For autonomous navigating vehicles operating in GPS-denied environments, such as underwater or in urban settings with the possibility of jamming, the requirements on drift-rate can be significantly better than 1 degree per hour. This requirement, in conjunction with the environmental ruggedness requirements, practically eliminates the possibility of using anything but RLGs or FOGs.
The closest alternative technology with high performance on the order of 1 degree/hour or better is the RLG, and these are widely deployed in many aerospace and precision munition applications. However, RLGs suffer from reliability concerns due to their hermetically-sealed gas lasers, high drive voltages and larger size, weight and power. In addition, the need for mechanical “dithering” of RLGs makes them ill-suited for many applications, especially covert or undersea. Micro-Electromechanical Systems (MEMS)-based gyroscopes are another alternative typically used for lower-precision applications with drift rates on the order of 10 degrees/hour or higher, but they have significant limitations for precise, rapid, and rugged applications.
It should be noted that available open-loop FOG technology is not considered an option for most navigation applications, due to performance limited typically to the order of 1-10 degrees/hr. This may be suitable for platform stabilization, but is not typically suitable for precision navigation applications in GPS-denied environments. Open-loop FOGs are gradually being replaced by MEMs solutions which can achieve similar levels of accuracy at greatly reduced cost, size, weight and power.
Since FOGs have no moving parts, “instant-on” capability with no warm-up required, and their sensitivity can be increased by lengthening the fiber sensing spool, they are nearly ideal for many applications, including commercial and military aircraft, guided munitions, terrestrial vehicles, submarine environments, industrial and military robotics and unmanned vehicles, and real-time stabilization. For high-precision applications in high-dynamic environments with severe thermal excursions, shock and vibration, FOGs are a highly-desirable navigation solution, but high-precision FOGs have heretofore been too expensive for most volume applications. The recent focus of technology development has been on bringing the cost of high-precision FOGs down to the point where they are a viable alternative to RLG technology.
FOGs (and other optical gyros such as RLGs) exploit the Sagnac effect which is the difference in time-of-flight for counter-propagating light in a circular optical path caused by rotational acceleration. Light propagating clockwise through a fiber loop will have the same optical path length as light that propagates counter-clockwise if the loop is at rest. However, if the loop is rotated while the light is propagating through the fiber, the beginning and ending points of the loop are shifted during the optical propagation, with the result that one beam will have farther to travel than the other, depending on which direction the loop is rotated. The path difference can be measured as a phase difference by interfering the beams from the two directions. Thus a fiber loop can be used as a rotation sensor.
In order to increase the precision of a FOG loop, the optical fiber is wound many times to form a coil. In extremely high accuracy applications, several kilometers of fiber may be used in each coil. For each axis, a FOG uses a transmitter to generate a beam of light into a fiber. In high-performance systems, a closed feedback loop is formed in which the light is split into two serrodyne-modulated beams using a lithium-niobate phase modulator. These modulated beams are fed into opposite ends of the PM fiber coil loop and counter-propagate. The returning light passes through the phase modulators again and is then recombined and detected. The phase modulators serve to move the interfered signal away from noise at DC, enhancing the sensitivity of the FOG. By detecting the interference fringes, the FOG measures rotation in that axis, and by combining all three axes, complete information on attitude in space is obtained. FOGs typically use a low-coherence optical source so that noise induced from multiply-reflected light as well as Rayleigh backscatter and non-linear effects are reduced.
The use of a lithium-niobate FOG modulator employing the annealed-proton-exchange process creates an inherently strong polarizer which is ideal for interferometric sensing applications. The lithium niobate device also has extremely high bandwidth (~1 GHz), which permits a “closed-loop” electronics implementation which maintains a fixed phase offset on a single interference fringe that is synchronized with the light transit time through the fiber coil. This form of closed loop operation greatly improves the precision of the gyro by linearizing its operation and eliminating many of the effects of deficiencies in the optical circuit.
Previously, the relatively large size and high cost of the optical components required to realize a closed-loop FOG (transceiver source and detector elements, lithium-niobate optical phase modulators, and polarization-maintaining (PM) fiber) has limited their use to mostly the high-performance applications in low volumes, such as spacecraft and aircraft navigation and attitude control.
21st-Century Compact FOG Design
A modern FOG combines the transmitter and receiver functions into one integrated transceiver package for each axis as shown in Figure 2 (a complete FOG system usually includes three sensors aligned with pitch, roll and yaw). Similarly, the splitter and phase modulator pairs are combined into one modulator package for each axis. Current development efforts are focused on reducing the size and cost of this assembly, while maximizing sensitivity and immunity to shock, vibration, and temperature extremes.
In order to develop such a compact transceiver without sacrificing performance, a new lens mounting technology was developed that allows sub-micron lens positioning in the context of an optical package. Emcore manufactures its FOG transceiver using similar technology to that employed in the manufacture of telecommunications transceiver products used for large-volume CATV, digital and fiber-to-home applications. Leveraging the massive prior investment in telecom packaging technology is key to keeping the cost of the FOG components as low as possible, and enabling a new generation of affordable FOGs.
The recent availability of low-cost PM fiber, high-power super-luminescent diodes (SLDs) at telecom wavelengths, low-cost and low-drift lithium-niobate modulators, and rugged hybrid photonic packaging techniques have ushered in a new generation of FOG components that break the long-standing price, size, and performance barriers for FOGs, andtherefore will cause a proliferation of FOG applications.
For example, PM fiber prices have dropped in the past few years from more than $5/meter to under $1/ meter, and single-axis FOG transceivers are now available with dimensions of 0.6×0.5×0.3inches(15×13×8mm) enabled by new hybrid integrated optical manufacturing and high-power super-luminescent diode (SLD) technologies. The cost and size benefits of these integration breakthroughs are enabling smaller and lower-cost FOGs that represent a viable alternative to RLGs for “smarter” flight of airplanes and missiles, as well as improvements in land-based navigation systems and autonomous vehicles.
An example of such a highly-integrated FOG unit is shown in Figure 3. this fully-integrated unit is 3" in diameter, 0.5" in height, operates from a single 5V supply, and includes all the optical components and DSP-based electronics, and fiber coil in a rugged housing. The unit has drift rate performance of less than 1 deg/hour and is capable of rotation rates up to 1 KHz. This level of integration and performance is achieved through the use of an InP semiconductor optical transceiver, a lithium-niobate optical modulator, and closed-loop DSP-based electronics. The design is easily scalable to longer fiber lengths for higher-accuracy applications. Fiber lengths from 200m to 1.2 Km have been demonstrated utilizing exactly the same optical components and DSP-electronics, with only simple changes to the firmware of the DSP required to accommodate different fiber lengths.
Performance Data for Integrated, Low-Cost FOGs
For a number of applications, such as line-of-sight stabilization for optical systems and systems that find heading precisely through detection of the Earth’s rotation, attitude noise can be the dominant concern. Emcore’s FOG transceiver, utilizing patented technology, provides high optical efficiency (typically four times greater than traditional designs). This higher optical power results in lower noise at the FOG’s optical detector and thus the gyro output.
The Allan variance plot shown in Figure 4 shows the performance of an Emcore EMP-1 FOG (200 m coil length) and an EMP-1.2k FOG (1.2 Km coil length). The slope of an Allen Variance plot is indicative of the angle random walk (ARW) noise and the area where the plot flattens is indicative of the drift stability.
Autonomous vehicles require gyroscopes that are high-performance, light-weight, low-power, rugged and low cost. The recent development of FOG components that leverage the tremendous prior investment in telecom network component technology has brought recent dramatic reductions in the cost of high-performance FOGs that are well-suited to the challenges of autonomous vehicles and weapons systems. This new generation of gyros is already finding increasing acceptance in a wide range of applications, and these FOGs are expected to enable new, higher levels of performance in autonomous vehicle and weapons-system applications.
This article was written by Ronald T. Logan Jr. and K.K. Wong, Emcore Corporation, Alhambra, CA. For more information, Click Here