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Tough fiber sensor survives extremes

01 Mar 2004

A fiber pressure sensor boasts a measurement limit that cripples most optical sensors.

A new type of optical pressure sensor that is able to operate in extreme conditions has been developed by scientists in the US. The team from Mississippi State University says its optical fiber sensor can measure pressures of up to 18 million Pa (2620 psi).

What’s more, plunging the sensor head into liquid nitrogen at -196°C or heating it up to 538°C has no discernable effect on its performance. The hope is that with further refinement the sensor will suit jet propulsion tests where conditions can reach extremes of 15,000 psi and -254°C.

The sensor is the brainchild of Chuji Wang and Susan Scherrer from the University’s Diagnostic Instrumentation and Analysis Laboratory (DIAL). They decided to take the principles behind a well known laboratory technique called cavity ring-down spectroscopy (CRDS) and apply it to optical fiber sensing.

The result is a device called a “fiber ringdown sensor” which works by measuring the decay time of a laser pulse travelling around a long loop of optical fiber. “So far, all ring-down techniques have been limited to spectral measurements of trace gases and liquids,” say Wang and Scherrer in their paper (Optics Letters 29 352). “To our knowledge no one has reported an attempt to introduce the ring-down concept for fiber pressure sensor development.”

Wang says that the new sensor design offers many advantages over designs based on fiber Bragg gratings (FBGs) and fiber Fabry-Perot interference (FPPI). “I confidently predict that this new idea will bring a new generation of physical sensors that can measure pressure, temperature and strain for a variety of applications,” Wang told Optics.org. “Our type of pressure sensor will potentially have an extremely high upper measuring limit and the harshest temperature tolerance with which FPPI- and FBG-based sensors cannot compete.”

The sensor consists of two identical 2x1 fiber couplers (99:1 splitting ratio) and some standard singlemode optical fiber (Corning SMF-28) that are connected together into a 65 m long loop.

A 1650 nm light pulse from a temperature-controlled diode laser is injected into the loop via one of the couplers and circulates around the loop, becoming slightly weaker after each round trip. The strength of the signal and its decay time is monitored continuously at the free port of the other coupler.

The key to the sensor’s operation is that the decay time of the light pulse depends very strongly on the attenuation of the optical fiber loop. As a result any phenomenon which changes the transmission of the fiber, such as an external force (pressure), can be measured by monitoring the decay time.

Experiments applying weights to an 8 mm long section of the fibre indicate that the sensor is able to measure a pressure of up to 1.8x107Pa. Stripping the fiber’s protective jacket reduces its measuring range but increases its sensitivity.

As for his future plans Wang says that the design has already been patented and he is exploring the possibilities for commercialising the technology and making sensor arrays. More details of the work are described in a paper submitted to Applied Optics.

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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