Next-Generation Fiber Optic Gyroscopes

The weight of fiber optic gyroscope (FOG)-based inertial navigation systems (INS) has now been reduced to less than 3 kg, and even below 2 kg, with diameters smaller than 200 mm. As the size of autonomous underwater vehicles (AUVs) decreases, FOGs are also becoming more compact. With AUVs diving deeper and executing more extensive data collection missions, FOGs will continue to play a crucial role in navigation systems that do not rely on Global Navigation Satellite Systems (GNSS), enabling fully autonomous GNSS-free navigation.

Figure 1 Typical Construction of Inertial Measurement Unit (IMU)

Since satellite navigation systems such as GPS are unavailable underwater, FOG sensors in inertial navigation systems (INS) enable dead reckoning by measuring changes in the AUV’s orientation. FOGs operate by detecting the phase shift between two counter-propagating light beams traveling through a closed optical loop. The beams are transmitted in opposite directions within a fiber optic coil. As the vehicle rotates, the counter-propagating beam experiences a slightly shorter path delay than the other due to the Sagnac effect. This phase shift difference is used to estimate the angular velocity. A complete inertial navigation system typically includes three FOGs, aligned perpendicularly to each other, and integrated with accelerometers to provide six-degree-of-freedom (6-DOF) motion sensing, capturing both rotational rates and linear accelerations.

The drive to minimize size, weight, power, and cost (SWaP-C) has spurred significant innovations in these systems.

Earlier this year, Exail introduced a new high-birefringence fiber for military-grade FOGs, offering the highest birefringence in the industry and the shortest beat length (1 mm at 633 nm). The latest inertial measurement units (IMUs), including their fiber optic coils, now feature diameters smaller than 30 mm. Enhanced fiber materials have extended the usable fiber length, maximizing accuracy while ensuring mechanical reliability. This fiber is available with a standard 80 µm cladding and an even smaller 60 µm cladding variant for compact FOG applications.

Figure 2: Phins 9 Compact from Exail weighs 1.2 kg and consumes less than 7 W of power

According to Maxime Le Roy, Underwater Navigation Systems Product Manager at Exail, the market is shifting towards smaller, fully capable AUVs for diverse operational scenarios. Exail recently delivered its latest model, the Phins 9 Compact, to Bedrock for deployment in a new modular AUV designed for rapid geophysical survey and environmental monitoring missions. This AUV is equipped with a multibeam echosounder, side-scan sonar, and magnetometer, rated for 300-meter depth, and offers a 12-hour endurance at 3 knots with all systems operational.

The Phins 9 Compact adds only 1.2 kg to the AUV’s payload and consumes less than 7 W of power. It features Doppler Velocity Log (DVL)-aided navigation, delivering a heading accuracy of 0.07° and pitch/roll accuracy of 0.01°. Le Roy stated, “Phins 9 Compact is an ideal solution for next-generation AUV manufacturers and e-ROV operators seeking to optimize power efficiency without compromising data processing capabilities.” Applications include survey-grade coastal and deep-sea seafloor mapping, inspection, maintenance, and defense operations.

Advanced Navigation’s Boreas A-Series DFOG

The latest addition to Advanced Navigation’s DFOG (Differential Fiber Optic Gyroscope) series is the Boreas A-Series, which includes the A90 and A70 models. Designed for subsea, maritime, land, and aerial applications in surveying, mapping, and navigation, these models are strategic-grade inertial measurement units (IMUs) featuring ultra-high-precision DFOG technology and high-performance closed-loop accelerometers.

All measurements are processed through a proprietary neural network algorithm that eliminates sensor errors and filters out disturbances. Advanced Navigation’s sensor fusion algorithm is more sophisticated than traditional Kalman filters, which primarily provide statistical estimations of unknown variables. This enhanced approach offers more precise error determination and correction, resulting in higher-performance navigation solutions.

The DFOG technology incorporates digital modulation techniques to measure dynamic errors within the fiber coil and compensate for them in real-time. The AI-driven learning capability accumulates sensor error data and its environmental conditions during and after measurement, enabling adaptive sensor error compensation tailored to primary operational conditions. This advancement allows for shorter fiber coils, significantly reducing SWaP, while maintaining the precision of longer coils.

Next-Generation Fiber Optic Coils

Xavier Orr, CEO and co-founder of Advanced Navigation, explained that the Boreas DFOG system supports fiber coil lengths ranging from 300 meters for compact systems to 5 kilometers for ultra-high-precision applications. The most widely used variant features a 1,000-meter fiber coil, precisely wound using a quadrupole winding method by specialized robotics at the company’s high-tech manufacturing facility in Sydney.

Figure 3: Advanced Navigation's Boreas A series is designed specifically for underwater, offshore, land, and aerial measurement, mapping, and navigation applications

Additionally, a custom photonic chip integrates sensitive optical components into a single chip, eliminating all fiber optic junctions, reducing SWaP, and enhancing reliability and performance. “At the core of Boreas DFOG technology is a customized optical or photonic chip that replaces five traditional discrete components. This innovation not only reduces SWaP but also enhances system durability against shock and vibration, increasing mean time before failure (MTBF) and overall reliability,” Orr explained.

The Future of FOG Technology

Orr emphasized that widespread GNSS jamming and spoofing threats have driven defense organizations to adopt advanced inertial navigation systems as an alternative to GNSS-dependent solutions. INS provides higher update rates than GNSS and remains operational in GNSS-denied environments. “Inertial navigation systems can output data at higher frequencies and provide heading, pitch, and roll measurements that GNSS alone cannot offer,” he stated.

Looking ahead, the next frontier for FOG technology will involve integrating all FOG components into a single photonic chip instead of assembling them on printed circuit boards. “This breakthrough will drastically reduce FOG systems’ SWaP-C, simplify manufacturing, and unlock commercial viability for broader market applications,” Orr concluded.