Five Minute Fiber Expert- The Fiber Optic Gyroscope (FOG)

Five Minute Fiber Expert 4 - The Fiber Optic Gyroscope (FOG)

Last time you learned about our BIG Product, Bow-Tie PM Fiber – this month, it’s the turn of the BIG Application. So let’s answer five simple questions:

1. What is a gyroscope?

2. What is a fiber optic gyroscope?

3. What makes a fiber optic gyroscope better?

4. Why does a FOG need Bow-Tie PM fiber?

5. What are the key PM fiber design elements?

Put simply, a gyroscope is a device that measures rotation. So, by using a gyroscope, you can find out where something is pointing (e.g. an aircraft) or how level it is (e.g. 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. Aircraft, 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’s 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 it.

A FOG achieves the same result, but using polarized light and something called the Sagnac Effect. The Sagnac Effect states that if light travels simultaneously in both directions around an enclosed optical system 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 analyze 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 military 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 spinning-mass gyro can take up to 30 seconds to spin-up and stabilize. 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. Satellite manufacturers often ask for long lengths of continuous fiber, sometimes up to 5,200 m lengths of HB1500G variants – this is because the fiber is used in the very accurate FOGs used in satellites launched by the European Space Agency (ESA) that make sure that the solar cells are always pointing at the Sun. And if those solar cells stop pointing at the Sun, the satellite dies – so I guess we play a pretty important part in ensuring the success of ESA missions!

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 5-Minute Expert #3), high numerical apertures to ensure that the fiber continues to guide strongly in these small coils (see 5-Minute Expert #1) and reduced fiber diameter, to improve lifetime and save space (see 5-Minute Expert #1). All of which go a long way to explaining our current development direction of ultra low-profile, ultra-high birefringence Bow Tie PM Fiber.

Fibercore’s product range can be found at http://www.fibercore.com/

Five Minute Expert: ‘Bow Tie’ Polarization Maintaining Fiber

‘Bow Tie’ Polarization Maintaining Fiber

Well, here it is, ‘The Big One’. So what makes it different from standard, telecommunications fibers, and how does it work?

As you can see, it got its name because it was invented in 1982, so long ago that Fabricators wore bow-ties – because an ordinary tie could too easily have become wrapped around the lathe ...…OK, so I made this bit up, but at least you were listening!

The distinctive cross-section of a bow-tie fiber is created by the two segments of boron-doped glass that flank the core (you may hear these segments being called SAPs or stress-applying parts). As the fiber cools during the drawing process, the boron makes the S

APs contract more than the rest of the fiber, placing the core in tension. This tension stretches the glass structure along the axis running parallel to the stress and compresses it along the axis running perpendicular to it and, in doing so, changes its optical properties. Light moves less easily through the compressed, densified structure, causing it to travel more slowly – and conversely, more easily and more quickly through the stretched structure. In this way, the fast and slow axes are created – the fast running perpendicular and the slow, parallel to the SAPs. This is called birefringence.

A lot of people (including some of our customers) think that the elliptical shape of the core is caused by stress – it isn’t. The core shape is formed in the preform, during the collapse phase of the fabrication process, when the glass is molten and cannot support stress – the birefringence can only start to happen after the glass has solidified.

Birefringence can be a useful thing to have in a fiber because it causes light waves to travel at different speeds, depending on their orientation relative to the SAPs – so if your optical source is polarized, i.e. if all of the light waves it generates have the same orientation, and you line them up to the fast or to the slow axis, then the polarization state of the source will be maintained – and you have a polarization maintaining or ‘PM’ fiber.

Fibercore’s product range can be found at http://www.fibercore.com/

Five Minute Fiber Expert – Hydrophones

Previously, you heard that Fibercore SM fibers are typically, skinnier, stronger and more strongly guiding than the fibers used in conventional telecommunications – but why do they need to be? Good question!

The most widespread and for Fibercore, ultimately the most important use for these fibers is acoustic sensors –sensors that detect sound waves. And whilst similar technology can be used to good effect on the land, most of the applications that we encounter involve the detection of sound waves in the water – hydrophones. Hydrophones do a wide variety of jobs. They can protect sensitive shoreline installations like oil-refineries or docks, by identifying the acoustic signatures of swimmers or speedboats. Mounted on the flanks of nuclear submarines, they can be used in conjunction with sonar to help assess depth, position and detect the presence of other vessels. They can assist the management of subsea oil reservoirs, by detecting the characteristic vibrations caused by oil flows – and arrays of hydrophones (or ‘streamers’) can even be towed behind survey ships in their quest to identify new oil reserves through the detailed seismic analysis of geological structures on or beneath the ocean floor.

Lots Of Different Applications – One Fundamental Design (courtesy of our own Dr John Wooler,who worked on hydrophones at Qinetiq before joining Fibercore).

Basically, a hydrophone sensor comprises a rod (or mandrel) encapsulated in a special, dense foam and wound with perhaps 100 m or so of singlemode optical fiber (typically one of our SM1500(X.X/80) series). When the sound waves hit the sensor, they cause the foam to expand and contract and, in doing so, the fiber is made to stretch and then relax – this modulation of the fiber changes the characteristics of the light guided within it and these changes can be analysed to determine critical characteristics of the acoustic wave.

So …

Why does our fiber need to be 80 μm (or even 50 μm) rather than 125 μm?

Because skinny fibers stretch more easily, making the sensor more sensitive and skinny fibers are under less stress when coiled and so they last longer – hydrophones in nuclear subs need to last at least 30 years.

Why does our fiber need to be much stronger than telecoms fiber – proof tested to 3% and not 1%?

Not only do these fibers have to last for 30 years or more when coiled, they are also tensioned onto the mandrel (ie pre-loaded) to ensure that they follow the compliant layer exactly as it expands and contracts.

Why does our fiber need to guide so much more strongly than a telecoms fiber?

Because, in common with many fiber sensors, designers need to get as much fiber as possible into the smallest possible volume, as a longer optical path length improves the sensitivity. Also for towed-arrays, or ‘streamers’, the individual hydrophones need to be as small as possible (typically around 10 mm diameter) in order to minimize both the turbulence they cause in the water (which could muffle-out the acoustic waves they are trying to detect) and the amount of energy it takes to drag these streamers through the water. You may have noticed that the survey ship in the picture is unusually short and wide at the stern (a bit like an iron) – that’s because it’s the kilometer or so of hydrophone streamers behind the ship that effectively complete the more usual, streamlined hull shape.

Fibercore’s product range can be found at http://www.fibercore.com/

Five Minute Fiber Expert: Fibercore SM Fibers

At trade shows, people from the Telecoms Industry often ask us what makes a ‘Specialty Fiber’ special? This question has given us the idea of featuring one range of Fibercore products each month in a short article designed to provide the answers – starting with SM.

Q. What makes Fibercore SM fibers special?

A. Three things – operating wavelength, resistance to bend-induced attenuation and strength

Operating Wavelength:

Standard telecommunication fibers are typically designed to operate in the second and third telecommunications windows between 1310 nm and 1550 nm. Fibercore SM-series fibers are designed for various wavelength ranges from 476 – 532 nm (SM450) to 1550 nm + (SM1500) to enable different visible and near infra-red optical sources (lasers, LEDs, SLDs etc.) to be used. These different operating wavelength ranges are achieved by changing the diameter of the core – smaller diameters optimize transmission for shorter wavelengths.

Resistance to Bend-Induced Attenuation:

Standard telecommunications fibers are typically used in a straight line – with bend diameters less than about 30 cm rarely encountered. Fibercore SM fibers, on the other hand, are often used in very small diameter coils. For example the biggest use of SM is fiber hydrophones, which can be as small as 10 mm in diameter – they need to be to reduce hydrodynamic drag when they are pulled through the water and to pack the longest optical path length into the smallest possible volume. Light just cannot travel through a telecoms fiber bent to 10 mm – but some of our SM fibers (mainly the SM1500 series) have very high numerical apertures (NAs) to provide exceptionally strong guidance and virtually lossless transmission under these conditions. Typical telecoms fibers have NAs of around 0.13 – compared with 0.3 for SM1500(4.2/80).

Strength:

The small bend-diameters that Fibercore SM-series can experience puts them under enormous stress which, over time, can cause them to break – a phenomenon called static fatigue. We prevent this in two ways – by reducing the fiber diameter and by increasing the proof-test level, relative to telecoms fibers. Standard Corning SMF28 telecoms fiber, with an outside diameter of 125 μm and proofed to 1% strain (around 100 kpsi stress) could fail immediately if coiled to 10 mm. By comparison, SM1500(4.2/80) at 80 μm diameter and proofed to 3% strain (300 kpsi stress) could last 25 years and SM1500(4.2/50) even longer.

Fibercore’s SM product range can be found at http://www.fibercore.com/