Tikalon Blog by Dev Gualtieri

Fiber-Optic Sensors

September 2, 2019

Soldiers in antiquity likely signaled to each other using sunlight reflected from their metal shields, and soldiers closer to our time used signaling mirrors called heliographs for the same purpose. Optical communications technology became considerably advanced in 1880 when telephone inventor, Alexander Graham Bell (1847-1922), and his associate, Charles Sumner Tainter (1854-1940), demonstrated their photophone by transmitting speech on a light beam a distance of about 700 feet (213 meters) between building in Washington, DC. Bell considered this invention, U.S. Patent No. 235,199,[1] to be more important than his invention of the telephone. Four of Bell's 18 patents were for his photophone.

Alexander Graham Bell in a Moffett Studio portrait circa 1914-1919.

Bell is best known as the inventor of the telephone, although others claimed the same invention.

Since Bell and other inventors of his time did not have electronic amplification devices even as primitive as the vacuum tube, they needed to be inventive indeed.

Bell's first idea, before vibrating diaphragms, was to use an array of vibrating metal reeds like the construction of an harmonica. The reeds would be excited at their specific resonant frequencies to switch currents that would excite identical reeds on the other end of a circuit.

(Wikimedia Commons image, via the Library and Archives Canada (image C-01733), modified for artistic effect)

The photophone's technology is now called free-space optical communication, which is the optical equivalent of shouting across an open field. You might have an example of this in your own home in the form or a Television remote control (some remotes, as the one for the Amazon Fire TV Stick, use low power Bluetooth, but they may also have an infrared option for television control). Improvements in laser, optical detector technology, and digital error-correction, make free-space an effective means to bridge small distances in data networks. Applications for this include building-to-building hops on college and business campuses, and in cities, as well as bridging roads and waterways. As you might imagine, fog is a problem for free-space, so links are limited to still respectable distances of less than a kilometer.

Light will also travel in a transparent medium. In the early 19th century, physicists, Jean-Daniel Colladon (1802-1893) and Jacques Babinet (1794-1872), created fountain displays in which light was directed through jets of water. Bundles of glass fibers or plastic fibers were used for transmission of images in the 1950s, and in 1961, Elias Snitzer of American Optical theorized that a fiber with an extremely small diameter, a single mode optical fiber, would transmit just a single waveguide mode, thereby eliminating phase interference from other modes along long distances.[2]

At that point, the principal problem was optical loss in long fibers. As you can imagine, the world would appear quite dark when viewed through a living room window a mile thick. The essential problem was iron impurity in the glass, and this problem was solved by Donald Keck and Peter Schultz of Corning Glass who used chemical vapor deposition to create preforms from which optical fibers were drawn to make fiber-optic cable.[3] Such low-loss cable, along with advanced laser sources and detectors, has enabled the worldwide Internet that we enjoy today.

Figures from US Patent No. 3,711,262, "Method of producing optical waveguide fibers," by Donald Keck and Peter Schultz, January 16, 1973. (Via Google Patents).[3]

Optical fibers are useful not just for communications, but for sensing as well. If you pull on an optical fiber, the path length of the light is changed, so a strain sensor is easily made. If the fiber is wrapped around a material with high thermal expansion, a temperature change will stretch the fiber and produce a thermometer. Likewise, an optical fiber will detect pressure when wrapped around a pressure vessel that elastically deforms with a pressure change. Fiber-optic hydrophones can be built on these same principles.

One important application is the measurement of extremely high voltage. Since the glass fibers are intrinsically insulating, there's a safe isolation from the voltage source, and the voltage is sensed by the Kerr effect, the change in the refractive index in response to an applied electric field, in the fiber material. While the prior examples are sensors that utilize an intrinsic property of the optical fiber, fibers can be combined with other elements to form sensors. Many years ago I worked on the combination of magneto-optical garnet and optical fibers to create a variety of magnetic sensors for aerospace applications. These included simple switches and wheel speed sensors.

Fiber-optics has several advantages when applied to aerospace systems, including weight reduction and robustness against electromagnetic interference and lightning strikes. Temperature-resistant fibers made from sapphire can extract light from the hottest parts of a gas turbine engine to allow a determination of interior engine temperature. Birefringent materials allow extrinsic sensing with optical fibers of many variables that facilitate engine control.[4] Birefringence can be used for measurement of temperature, pressure, strain, force, torsion, and vibration.

The most interesting fiber-optic sensor is the fiber-optic gyroscope, the fiber version of a ring laser gyroscope. In compliance with the Sagnac effect, named after French physicist, Georges Sagnac (1869-1928), who designed this device in 1913 as a means of testing the theory of special relativity, counter-propagating light beams in a closed path will have a phase shift as a consequence of angular velocity. An interferometer measures the shift to indicate the angular velocity.

Schematic diagram of a Sagnac interferometer, the principle of the ring laser gyroscope.

Schematic diagram of a Sagnac interferometer, the principle behind the ring laser gyroscope (RLG) and the fiber-optic gyroscope. A ring laser gyroscope has an internal (typically helium-neon) laser source. A coil of multiple turns of optical fiber amplifies the effect in a smaller volume in the fiber-optic gyroscope.

There's a lot of materials science involved in the construction of a ring laser gyroscope, principally in the containment structure. The principal material for an RLG body is Zerodur®, a lithium-aluminosilicate glass-ceramic developed by Schott AG. Zerodur contains crystallites in a glass matrix, and this gives it a frosty appearance. It has an extremely low thermal expansion coefficient (less than 0.10 x 10⁻⁶/kelvin over a wide temperature range), so it's ideal for an RLG, where small changes in path length would produce errors.

(Modified Wikimedia Commons image by Krishnavedala.)

Recently, a team of electrical engineers from the Hong Kong Polytechnic University (Hong Kong, China) has developed fiber-optic sensors to continually monitor accelerations and vibrations on trains to detect vehicle or track problems. These sensors utilize an optical fiber structured to produce birefringence in a Sagnac interferometer.[5-6] Their research is published in an open access article in Optics Express.[5]

Says Zhengyong Liu, the lead author of the paper describing this work, “An all-optical sensing network has many advantages as it is immune to electromagnetic interference, has long transmission distance and the sensors don't require electricity... However, there is a need for fiber-optic sensors that are optimized to measure different parameters in railway systems."[6]

The Hong Kong fiber-optic accelerometer is based on a polarization-maintaining photonic crystal fiber that's formed into a disk shaped coil just 15 millimeters in diameter. This coiled fiber is glued onto a stainless steel substrate, and a cylindrical mass is affixed above it. Vibrations cause the mass to press the fiber, and this causes a phase shift in its transmitted light that's read by an interferometer (see figure).[5-6]

The Hong Kong Polytechnic University fiber-optic vibration sensor. The polarization-maintaining photonic crystal fiber (PM-PCF) is driven by a single-mode fiber (SMF). A cross-section of the photonic crystal structure is shown on the right. (Reformatted open access schematic diagram of the fiber-optic vibration sensor from The Hong Kong Polytechnic University.[5] Click for larger image.)

The Hong Kong team designed and fabricated the sensing fiber to have air holes to provide the photonic crystal structure, and also to have a small diameter, low bending loss, and high birefringence.[6] The resulting accelerometer sensor has double the response time of conventional fiber sensors that incorporate a Bragg grating as the sensing structure.[6] The fiber-optic sensor has no moving parts, and it is immune to the electrical noise that exists in the high-voltage train environment.[6]

Field tests were done on an in-service train in comparison with a Bragg grating fiber accelerometer and a piezoelectric accelerometer.[6] The accelerometer sensor based on the polarization-maintaining photonic crystal fiber had a sensitivity of ~8 pm/g, and a resonant frequency exceeding 2.5 kHz.[5] The high frequency response allows detection of the impulse associated with a defect or crack present on the railway track.[5]

The Hong Kong researchers believe that their sensor could be combined with an artificial intelligence system to prevent catastrophic train derailments and other accidents.[6] Hwa-yaw Tam, the research team leader remarks that "Each year, train accidents lead to severe injuries and even deaths... Our fiber accelerometers could be used for real-time monitoring of defects in the railway track or the train to pinpoint problems before an accident occurs."[6] Zhengyong Liu adds that “In addition to railway monitoring, these new accelerometers can be utilized in other vibration monitoring applications, for example, structural health monitoring for buildings and bridges and vibration measurements of aircraft wings."[6]

Fiber-Optic Sensors

References: