Friday, 2 September 2011

The Fiber Optic Gyroscope

Fibercore Ltd will be presenting at the forthcoming 'Inertial Sensors & Systems - Symposium Gyro Technology' on the 20th September 2011 to discuss the use of reduced clad fibers in Fiber Optic Guroscopes. But what is a Gyroscope and why is a FOG different?



Put simply, a gyroscope is a device that measures rotation. So, by using a gyroscope, you can find out where something is pointing (eg an aircraft) or how level it is (eg a hovering helicopter, a high-speed train carriage, or even a pair of binoculars). This is why gyros are used to help navigate ships, aircraft and even some land vehicles and in systems used for automatically stabilizing things.



To be effective you need one gyro for each degree of freedom (or 'axis') in which the 'platform' can move. Aeroplanes, for example, can move in three dimensions - so need three gyros to cover roll, pitch and yaw.



Traditional gyroscopes have been around for more than 100 years and work on the 'spinning mass' principle. Those of you who had a gyro top as a child, or have played with one of those wrist-strengthening balls, will recall that when you spin the rotor, the gyro gets a mind of its own and wants to remain upright. Well it is the force that the gyro exerts trying to right itself (aka 'gyro' torque') that can be used to determine how far you have tried to rotate.



A FOG achieves the same result, but using polarized light and something called the Sagnac Effect. The Sagnac Effect states that if light travels in both directions around an enclosed optical system, simultaneously, and the optical system experiences a rotation, the light will undergo a Doppler-Shift (remember how police sirens change pitch as the car speeds past you? ... same thing) with the result that the two beams will recombine out-of-phase, creating interference. If you analyse this interference, you can find out the degree and the rate of rotation.



In essence, a simple FOG looks a bit like this:

The true benefit of FOG over a traditional, spinning-mass gyro is that it has no moving parts - no moving parts means nothing to wear out and nothing to service. As a result, FOGs are tougher, more reliable and demand far less maintenance. In fact one Fibercore customer found that the US Army could use one of their systems for an average 30 times longer before repair, simply by switching from conventional gyros to FOG! Another advantage of FOG is that it is ready to work immediately, whereas a spining-mass gyro can take up to 30 seconds to spin-up and stabilise. 30 seconds might not seem that much - but those of you old enough to remember the Cold War will also remember that we would only have had three minutes to wait from detection to destruction. Right up until the early 1980s - 30 seconds was a long time!


The optical fiber in a FOG enables a very long optical path length to be confined into a small volume and so magnify the small phase-shift caused by the Sagnac Effect - and the longer the path length, the more accurate the FOG.



The fiber used in FOGs needs to preserve polarization because, in order to generate stable interference, two light waves have to have the same orientation - if they crossed at right angles, they wouldn't even know the other one was there! Typically people want to get the longest optical path length into the smallest possible space and this leads to small diameter coils with multiple layers - demanding lots of birefringence (short beat-lengths) to make sure that the micro bending created within these layers does not counteract the stress inside the fiber (see our blogg on 'Bow-Tie Polarization Maintaining Fiber'), high numerical apertures to ensure that the fiber continues to guide strongly in these small coils and reduced coil diameter, to improve lifetime and save space. All of which go a long way to explaining our current development direction of ultra-low profile, ultra-high birefringence Bow-Tie PM Fiber.



If you would like to know more, look out for Dr Andy Gillooly at the Symposium or contact Fibercore Ltd at sales@fibercore.com.


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