By Dr. Aileen Sansone and Dr. Chris Emslie
History will remember optical-fiber technology as one of the truly great inventions of the 20th century: it is the driver behind the telecommunications revolution and the very backbone of the Internet, telephony, and Cable TV. In fact, these mainstream deployments have been so influential that, by comparison, alternative applications have remained all but forgotten.
One specific increasingly prominent application for optical fibers is intrinsic fiber-sensing techniques in areas ranging from battlefields, through homeland security, to deep-sea surveying, and oil exploration.
Conduct a “vox pop” (“voice of the people”) anywhere in the developed world and people will associate fiber optics with telecommunications, medicine, and decorative lighting. Despite a history at least equal to that of fiber-optic telecommunications, fiber-optic sensing is unlikely to get so much as a mention, but perhaps things are about to change.
Advocates for fiber-optic sensing have much for which to thank the telecom “boom.” The boom created a technological comfort level that reduced the concerns of the conservative potential users of fiber sensors. The subsequent “bust” released a flood of specialists, components, and ancillary technologies that, allied with the changing demands of the military, is breathing new life into this previously esoteric area.
Intrinsic and extrinsic sensors
Fiber-optic sensors fall into two categories: “intrinsic” and “extrinsic.” In extrinsic sensors the fiber simply conducts light from a sensing head to a detector, thus enabling potentially delicate electronics to be “remoted” and, in some cases, providing a direct means to multiplex several sensors to one detector.
In extrinsic sensors, the interaction between light and the environment takes place outside the optical fiber. Intrinsic sensors rely on the interaction between the environment, the fiber, and the light itself to generate information about a specific measure.
Popular wisdom often states that optical fibers, unlike copper cables, are immune from environmental interference. In fact, the reverse is true - temperature, pressure, electromagnetism, rotation, vibration, stress, and strain all interact with an optical fiber to create subtle changes in its transmission characteristics. This property led many researchers to despair that the challenge was not so much to make a fiber-optic sensor, as to be able to determine exactly what it was sensing.
The key to the success of intrinsic sensors is a fiber’s fundamental ability to guide light around bends and over large distances. This ability enables systems designers to confine a long optical path within small physical volumes and magnify the effects of these subtle changes sufficiently to measure and quantify them.
The fiber-optic gyroscope, or FOG, was arguably the first application of an intrinsic fiber sensor. To date, the FOG certainly has enjoyed the greatest commercial success with deployments in civil and military aircraft, unmanned aerial vehicles, missile-guidance systems, helipad stabilization systems, and as a component in the navigation systems of some luxury automobiles.
Age-old phenomenon
In common with many other great inventions, the history of the FOG goes back a great deal further than the device itself. Physicist Francis Harress first observed the phenomenon by which a FOG operates in 1911. However, it was Georges Sagnac, who repeated Harress’s experiments in 1913, who lent the most universally known name to the phenomenon.
Further experimentation by Michelson and Gale in 1925, who used the Sagnac Effect to determine the absolute rotation rate of Earth, provided an early premonition of the significance of Harress’ discovery in the development of optical gyroscopes. While Michelson’s and Gale’s demonstration proved the concept of using light to measure rotation rate, their experimental setup was far from practical - it relied on a rectangular optical path 0.4 miles long and 0.2 miles wide. It was the advent of the laser in the early 1960s that opened the door to practical optical gyroscopes.
The first of this new generation of devices to make use of the Sagnac Effect was the ring laser gyro (RLG), which designers at Sperry-Rand Corp., first realized in 1963. The RLG comprises a triangular or rectangular optical circuit either machined from quartz, or fabricated from Zerodur (a glass-ceramic that has a zero coefficient of thermal expansion), with high-reflectivity mirrors strategically placed to guide and counterpropagate beams.
Ring laser gyros are rugged, precise, exceptionally reliable, and continue to be used extensively in aerospace and marine navigation. However, exotic materials and fine machining tolerances ensure that manufacture remains costly and, while RLGs may be sufficiently compact for most applications, the use of a free-space optical path and discrete mirrors places fundamental limits on the ultimate scope for miniaturization.
The development of low-loss optical fibers in the 1970s raised interest in substituting the RLG’s optical circuit with a coil of optical fiber. This concept would enable a new family of optical gyroscopes to compete with the precision of RLGs and offer ruggedness, small size, and low cost. It was the introduction of specialty fibers in the early 1980s that quickened the pace of development.
Optical fiber helps confine a long optical path within a small volume. This long optical path magnifies small optical effects, thereby increasing the sensitivity of the device. This advantage is clear when one considers that, had Michelson and Gale had access to optical fiber in 1925, the bulk of their experimental apparatus could have fit comfortably into a teacup - and not 200,000 square meters of open ground.
Not just a standard fiber
The sensing coils of typical FOGs vary from 20 to 60 millimeters in diameter and contain between 100 and 3,000 meters of fiber. Hydrophones tend to have much shorter coils, with a maximum of perhaps 100 meters, but diameters can be as small as 10 millimeters. If a standard telecommunications fiber were deployed in such coils two things would happen - light launched into one end of the coil would radiate before it reached the other, and the fiber would be liable to fracture due to the phenomenon of static fatigue.
Static fatigue is a phenomenon by which optical fibers can fracture spontaneously if subjected to bending stress or an invariant tensile load. The stress induces intrinsic and microscopic flaws located on the fiber surface to grow and ultimately break the fiber.
Fiber manufacturers typically help their products resist static fatigue either by increasing proof-test levels, or by reducing glass diameter. Increasing the refractive index of the core glass relative to that of the cladding may enhance the strength of guidance in an optical fiber; increasing the core index of a telecom-type fiber by only 0.4 percent would make its transmission characteristics suitable for FOG use and provide effectively lossless transmission in a coil 20 millimeters in diameter. An increase of 1.7 percent is required to enable similarly strong guidance in a 10-millimeter diameter hydrophone coil.
Proof testing involves straining the fiber to destruction to screen-out intrinsic flaws - the higher the proof-test level, the smaller the size of intrinsic flaw that will remain in the surviving fiber and the lower the probability that the fiber will fail under static fatigue.
The telecommunications industry standard for proof-test is 1 percent strain. Deploy such a fiber in a 20-millimeter-diameter FOG coil and it could fracture within months, in a 10-millimeter hydrophone coil, failure could be immediate. For this reason, specialty fibers are generally proof-tested to 2 or 3 percent strain. A more direct way to enhance lifetime is to limit bending stress levels by reducing the outside diameter of the fiber itself.
A typical FOG or hydrophone fiber has a diameter of 80 microns - a little more than two-thirds that of a standard telecommunications fiber of 125 microns. When bent, the induced stress within these fibers is around 40 percent lower than that of the larger fiber, slowing growth of intrinsic flaws and boosting lifetimes from mere months to 20 years or more.
The use of polarization-maintaining (PM) fibers in interferometric sensors greatly simplifies sensor design by avoiding the phenomenon of signal fade that is induced by polarization drift. Combining two light pulses, such as occurs in a FOG, makes the most of constructive interference when the pulses have the same state of polarization.
The most prolific design of PM fiber used in fiber sensors is the so-called bow-tie fiber, developed in 1982 at the University of Southampton, England. In this design a pair of bow-shaped stress- applying parts (or “SAPs”) are at either side of the core. These SAPs induce the birefringence that preserves the polarization state of light guided within the fiber.
It is fascinating to reflect that specialty fibers, developed more than 20 years ago to support sensor programs are now returning home and growing in sophistication and capabilities - after half a decade of captivity at the hands of the telecommunications industry.
The prime advantage of the fiber- optic gyroscope is its status as a true solid-state device that is exceptionally rugged, reliable, and maintenance-free. Designers demonstrated its ruggedness on both sides of the Atlantic more than a decade ago when FOGs were attached to rockets and cannon shells; not only did they survive acceleration to supersonic velocities within a few feet, but they also continued to function accurately throughout their ordeals.
When fiber cost-reduction efforts combine with advances in multifunctional optical-chip technology, and industry’s increasing familiarity with manufacturing with optical fibers, the advent of low-cost FOGs for smart munitions cannot be far away. This vision is in keeping with the defense strategies of the modern world - which aim to keep soldiers out of harm’s way.
Ear in the water
While the founding principles of the fiber gyro trace back to the early 20th century, the principles for underwater acoustic sensing go back more than 400 years. Interest in detecting sound-wave energy in the world’s oceans dates back at least to the 15th century, when Leonardo da Vinci wrote “If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you.” It was 1826, however, when Charles Sturm and Daniel Colladon made the first accurate measurement of the speed of sound in water. Sturm rang a submerged bell and Colladon used a stopwatch to note the length of time it took the sound to travel across Lake Geneva. Their measurement of 1,435 meters per second was only three meters per second off from the speed accepted today. This demonstrated that sound travels through water almost five times faster than through air.
Since that time, scientists have made considerable advances in underwater acoustics and in hardware to detect faint signals. The two primary types of underwater sound systems are direct listening and echo ranging.
Direct listening systems are passive and detect vessels and marine life passing near the underwater microphones called hydrophones. Echo ranging, or active-detection, uses hydrophones and signal-processing subsystems to measure sound that reflects from objects traveling through the water. Examples are fish finders, navigation systems, and geophysical exploration systems. Sonar, an acronym for sound navigation and ranging, was first used for active detection systems, but later applied to all types of underwater sound devices.
Typical sonar systems rely on piezo-ceramic hydrophones and DC-powered signal conditioning and data telemetry. Hydrophone arrays come in bottom-mounted grids, patterns on a submerged surface, or in long flexible streamers towed through the water.
Almost all these systems are expensive and delicate, particularly the towed arrays. Fiber-optic hydrophones, how-ever, have the potential to provide robust and relatively inexpensive sonar systems with thin fibers - coated to counter the effects of long periods in salt water - that accommodate tight bends.
Naval programs
Over the past several years the U.S. Navy has pursued several fiber-optic sensor and array development programs with the goal of enabling electrically passive underwater acoustic sensing. Fiber-optic-sensor multiplexing topologies have been developed for a number of applications, including towed arrays, hull arrays, and deployed bottom-mounted arrays.
These sensor multiplexing topologies all have the advantage of being electrically passive; the lasers and receivers that process the acoustic information from the sensors are located remotely from the sensor arrays and the sensors are interrogated via optical fiber links.
The sensors themselves are typically compliant mandrels that are wrapped with specialty optical fiber. The mandrels respond to the acoustic signals and introduce a phase shift in the light that is traveling through the fiber. These signals are then transmitted over optical fibers back to the signal-processing location.
While recent world events point out the desirability of intrusion detection for homeland defense applications, there are also a variety of applications for large arrays of multiplexed hydrophones in the commercial sector as well. Two of the largest applications of hydrophone arrays are geophysical exploration and oil-field monitoring.
Historically, the commercial sector uses arrays of ceramic hydrophones, yet as fiber-optic sensing technology matures it will become an attractive alternative to conventional ceramic hydrophones. The lines towed from ships connect to a large in-line array of hydrophones, and each ship can tow as many as 16 streamers simultaneously. Each streamer contains more than 1000 separate hydrophone channels.
In addition to the streamers of hydrophones, the geophysical exploration ship also tows an acoustic source. The source generates loud pulses of low frequency sound energy. These pulses penetrate the sea floor, reflecting off of various sub-bottom layers as well as the oil reservoir. The reflected pulses are detected by the towed streamers and processed to construct a picture of the strata below the sea floor in the survey area. This exploration and mapping technique allows oil companies to be very precise in the location of wells, minimizing unnecessary drilling.
Fiber-optic sensing has been an active area of research and development for nearly 30 years. After years spent starved for attention and investment, the fields of fiber sensors and fiber sensing are once more beginning to flourish. While these benefits lay largely forgotten during the boom years of telecom, defense at home and on the battlefield combined with the quest for new energy sources and a hard core of individuals with a true passion for the technology, fiber sensors will one day take their rightful place in public consciousness - alongside telephony, cable TV, medicine . . . and those funny little horsetail lamps.
Dr. Aileen Sansone is a senior fiber-optic systems engineer at Chesapeake Sciences Corp. in Stonington, Conn. She is working to develop fiber-optic hydrophone and array technologies to enable flexible and cost-effective sonar systems to meet a variety of Navy and commercial requirements. Dr. Chris Emslie is managing director of specialty-optical-fiber manufacturer Fibercore Ltd. in Southampton, England, and a 20-year veteran of specialty-fiber technology.
