Unmanned Systems Technology 036

same as an electronic integrated circuit except that the former is designed to provide for optical functions, including the emission of light needed for interferometry. Given the size, weight and cost disadvantages of FOGs compared with high-end MEMS gyros, some developers have sought to migrate the core principles and componentry of conventional FOGs onto PICs. That process requires comprehensive r&d to overcome key operational barriers, such as how to use shorter wavelengths to hit critical performance and price targets. Also, making the technology commercially viable has involved further r&d into how best to design the systems on chips for high performance and manufacturability, and how to arrange the entirely new kind of production line necessary for this kind of FOG. The result is a FOG that maintains or improves on the performance and accuracy of conventional products, while having a much-simplified manufacturing process that can be automated and accelerated for bulk production – to a degree that is competitive with the volume and price of high-end MEMS IMUs. Much of these improvements come from determining how to integrate the many passive functional FOG components – an estimated 80% of the waveguiding optical components – monolithically into the chips, as part of the wafer substrate from which they are manufactured. For example, the lengths of the fibre optic couplers in a conventional FOG are proportional to the wavelength and the desired split ratio, and their sensitivity to heat and shock means they need their own housings. These factors make them among the largest components in a FOG, so integrating the couplers monolithically considerably reduces the volume burden. The size and weight footprints of many more components – polarisers, light sources and detection circuitry, for example – are similarly consolidated and reduced in a PIC. Furthermore, compared with the many bonds and splices needed for the 39 IMUs, gyros and accelerometers | Focus propagates), and the smallest possible bend radius (meaning the highest possible flexibility). The last of these is the biggest factor in the size of the overall IMU, as optical fibre can only bend so far before suffering strain, damage and losses. Improvements in fibre bend radius thus stand to be major game-changers as and when they become available. In addition to the coils of fibre, a light source is needed. Careful selection of diode is key, as too short a wavelength (such as those from laser diodes) can induce additional interferences in the coil that would conflict with the FOG’s ability to accurately calculate angular rate. Laser diodes however can be (and are) used when coupled with an erbium- doped fibre. The erbium will increase the wavelength of the emitted light from 980 to 1550 nm, the latter being consistent with high-performance FOGs. This approach is more complex than using three super-luminescent diodes, for example, but their higher output enables a single laser diode to be split across three coils, ensuring more consistent performance throughout the FOG IMU as well as potentially reducing weight. Naturally these components must also be selected to optimise for weight, size and power consumption, but when packaged together they typically result in a system that is too large, heavy and too powerful to justify their use on small platforms. That might change in the future, with FOGs in the unmanned space shrinking to 40-100 mm at their greatest dimension. Improvements in costs and lead times will also be needed however, to reach high- volume professional markets. For larger platforms though, such as the fast-growing markets of long- endurance UUVs, heavy-lifting UAVs and urban air taxis, FOGs are increasingly being used to enable the necessary accuracy for safety-critical navigation and extremely precise pointing of sensors and gimbals. Integrated FOG systems A new generation of FOG technology also stands to offer considerable improvements over standard optical gyroscope configurations. The core technology is the photonic integrated circuit (PIC), which is much the Unmanned Systems Technology | February/March 2021 Both MEMS and FOG IMUs have been subject to rigorous engineering over the past few years to improve their manufacturability and consistency between batches (Courtesy of EMCORE)