What are Fiber Optic Gyroscopes?
Fiber Optic Gyroscope (FOG) systems use the Sagnac effect to measure the angular velocity or rotation rate on an object (e.g. aircraft).
Two counter-propagating beams traveling through a FOG coil undergo a differential phase shift when subjected to a rotation in the plane of the coil. The magnitude of this phase shift is proportional to the rotation rate and hence it can be used to measure the angular velocity.
The FOG offers significant advantages over other IMU technology:
- Size
- Accuracy
- Low maintenance
- Long lifetime
A FOG is not sensitive to acceleration, vibration, or dithering, which means they are capable of very high signal to noise ratios (SNR), as required for the highest performance gyroscopes. They have no moving parts, which reduces the probability of mechanical failure. The technology is also scalable and can be designed into very compact volumes.
Figure 1. Design schematic of a PM FOG system.
Fibercore has developed a range of optical fibers specifically for use in FOG systems. These include an extensive range of Bow-Tie design, highly birefringent gyroscope (HB-G) fibers for use as the FOG sensor coil fiber, as well as a new range of short beat length (HBG-SB) fibers optimized for the next generation of FOG coil designs. Fibercore also offers a number of supporting products, for use in other components of the FOG system, such as ZingTM polarizing fibers (HB-Z), standard PM fiber (HB), telecoms PM fiber (HB-T) and IsoGainTM erbium doped fiber for broadband ASE source generation.
HiBi Fiber for FOG Coils
Polarization Maintaining (PM) fibers for FOG systems are generally of a reduced cladding diameter design (typically 80μm) in order to: maximize fiber length in a restricted volume and improve mechanical reliability in tight coils. In order to minimize macro and micro-bend losses within compact coils, Fibercore’s range of HiBi fibers is also of a high NA design.
The HB-G range is the current standard for PM FOG fibers. Growing demand for high accuracy gyroscopes and reduced coil volumes can also be met with Fibercore’s new range of ultra-high birefringent, short beat length range of HBG-SB fiber. Both designs utilize Fibercore’s Bow-Tie structure to create the internal birefringence and resulting PM axes of the fiber.
Fibercore also offers a range of reduced coating diameter products, to further maximize the possible fiber length within restricted coil dimensions. For example, both the HB1500G-HI and HB1500G-SB/010 products encapsulate the 80μm cladding glass within a coating package of just 155μm outer diameter. The HB1500G-SB/013 product reduces the coating package further still, with an outer coating diameter of 135μm.
Radiation Tolerant FOG Fibers
Fibercore also offers FOG fiber products that are resistant to radiation damage. Products such as HB1500G-RT and HB1500G-RT-SB are designed with a radiation-hardened core and cladding composition in order to minimize Radiation-Induced Attenuation (RIA). This makes them a great choice for the high radiation environments that are often found with space applications, such as satellite attitude control, solar panel pointing on Mars rovers, or even applications in the nuclear sector.
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ZingTM Polarizing Fiber
Typically, a single polarization state is desired within a FOG system in order to produce a stable interference pattern of the Sagnac phase shift. Fibercore has developed a range of all-fiber ZingTM Polarizing Fibers, such as HB1550Z(11/80), which are capable of stripping energy from the fast axis, leaving only light in the slow axis. By having an all-fiber device, low insertion loss, small form factor, and high-reliability polarizers can be achieved. This is particularly important for FOG systems where low sensitivity to mechanical vibration is preferable.
Depolarizer
FOG systems also commonly use depolarizers as well as polarizers. If a light source has an uncontrolled polarization state, for example, a superluminescent diode coupled into non-PM SM fiber, then it is necessary to depolarize the light before polarizing. Otherwise, the polarization state in the SM fiber can rotate, causing power fluctuations on the output of the polarizer. By splicing two PM fibers together with the stress axis offset by 45°, a Lyot depolarizer can be created. To maximize the performance of a Lyot depolarizer, it is important to choose the correct lengths of the two pieces of PM fiber. This is non-arbitrary and depends on many factors including the birefringence of the fiber, wavelength, bandwidth, and spectral shape of the light source. Fibercore’s HB series, HB-G, and HB-T series of PM fibers are ideal for use as Lyot depolarizers.
Erbium Doped Fiber
Erbium Doped Fiber (EDF) can be used to make Amplified Spontaneous Emission (ASE) light sources capable of emitting broadband incoherent light. Using a low coherence broadband light source can considerably improve the signal to noise (SNR) ratio on a FOG system, as it acts to reduce the sensitivity to backscattered light, which would otherwise have the potential to impede the Sagnac phase shift measurement. Fibercore’s IsoGainTM range of erbium doped fibers offers a wide range of absorption rates, cladding diameters, and pump wavelengths. For high conversion efficiencies IsoGainTM fibers, such as I-4(980/125) offer the best option. For short fiber lengths and smaller packing sizes, Fibercore’s high absorption EDF range, such as I-15(980/125)HC is ideal. Fibercore also offers the I-25H(1480/125) product, a reduced 80μm cladding EDF with high NA, suitable for even the smallest high-reliability coils.