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Sensing & Measurement

Patents in fiber-optic sensing

The durability and sensitivity of fiber-optic sensors have made them a logical choice for extreme environments.
17 February 2009, SPIE Newsroom. DOI: 10.1117/2.2200902.0001

Fiber optics, although used for years in the telecommunications industry, have now become an exciting new sensor choice for harsh environments. Since optical sensors are essentially a passive mechanical component, they are insensitive to all forms of electrical interference, making them ideal for sensing in noisy, electrical environments. Glass and plastic fibers also do not spark, so these types of sensors can be used in mining, pharmaceutical, and chemical operations as well as in grain bins and oil refineries. If fibers break, there is no risk of personnel being shocked during repair.

Because fiber optics are made from glass, they are by nature extremely resilient and thus perform reliably in high temperature and corrosive environments. Most glass fibers can withstand operating temperatures as high as 450 degrees Fahrenheit, and with special coatings can withstand up to 1200 degrees. For less extreme temperatures, plastic fiber optics are a great option, as they allow a bending radius as small as 1 mm, mimicking the flexibility of an electric wire. When exposed to harsh chemicals or solvents, glass fibers are preferable, but plastic fibers can be coated with Teflon, nylon, or polypropylene for enhanced performance. Other inherent advantages of fiber optic sensors are that they are lightweight and very small, require little power, have high sensitivity and bandwidth, and are environmentally rugged.

Fiber optic sensors are now used for measurement of rotation, acceleration, electric fields, temperature, pressure, acoustics, vibration, position, strain, humidity, viscosity, and chemicals. A review of the patents reveals many exciting new applications for fiber optic sensors, a few of which will be discussed in greater detail below.

US 7,277,605 Silicon fiber optic sensors
This patent describes a fiber optic sensor microfabricated using silicon wet etching, giving it very small size and high sensitivity. By microfabricating on a wafer, the optical fibers are exactly positioned relative to an optical cavity, and a predetermined attachment point sets the gauge length. Since the gauge length and optical cavity size are precisely known, noise is reduced, and thus the sensor is both accurate and highly sensitive at low signals.

US 6,072,922 Cryogenic fiber optic temperature sensor
This patent provides a means of coating an optical fiber so that strain can be sensed even at cryogenic temperatures when material movement is extremely minimized. The coating acts as an amplifier and thus the thermal strain is increased enough to physically measure. The coating could also be optimized for less extreme temperatures as well, allowing for measurement of even minute strains.

US 7,336,862 Fiber optic sensor for detecting multiple parameters in a harsh environment
The glass used in fiber optic sensors can be doped with impurities to provide bands that can be thermally excited under operational conditions. '862 describes a fiber optic core with periodic or quasiperiodic microcrystalline grating structure, which allows one fiber optic sensor to measure a plurality of parameters. Basically, the single sensor becomes a distributed sensing system, with all signals integrated back at a signal processor.

US 7,397,976 Fiber optic sensor and sensing system for hydrocarbon flow
In pipelines and wells, it is important to measure the hydrostatic pressure, but most methods interrupt the natural flow. In this patent, long fibers are axially held within a pipeline. If there is any buildup or erosion of the pipe walls, it can be detected by the change in strain in the fiber. The fiber optic sensors are coated to magnify any change in pressure, and they also respond to changes in temperature as a change in strain.

US 2005023434 (A1) Electro-optic sensor
The semiconductor industry needs to fabricate silicon wafers at high speed with uniformity and reliability. In this patent application, the inventor discloses a fiber optic sensor system that first generates a modulation signal, then mixes in an acoustic signal using a pulsed laser, which is then down-converted to lower signals that are picked up by a photodetector. This allows for nondestructive, very rapid measurement of small features on the silicon wafers, and the sensor lends itself to a completely automated detection and measurement method.

Analyst Deborah Schenberger, Ph.D., brings 16 years of industry and two years of academic experience and insight to medical device companies, particularly those developing implants, to help them to achieve product approval through the FDA, ISO, and CE registration processes. She also analyzes mechanical devices and machines, using her expertise to assess mechanisms for novelty by working across industries to find other applications that may help provide design solutions. She is well versed in patents and intellectual property, bioinstrumentation, MEMS, biomedical sensors, and nanotechnology. Dr. Schenberger is section chair for the American Society of Mechanical Engineers, and a member of the American Society of Biological and Agricultural Engineers (ASABE), the Biomedical Engineering Society (BMES), and the Institute of Biological Engineering (IBE).

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