Hydrogel-based fiber-optic sensor detects water ingress in cables
Dave Wilson
By combining three technologies--optical time-domain reflectometry, chemically sensitive water swellable polymer and fiber-optic cable--opto-electronics division researchers at the University of Strathclyde in Glasgow, Scotland, have designed a sensor that makes distributed measurements of various chemicals, including water.
According to Craig Michie, university senior research fellow, many applications could benefit from such distributed measurements. For example, early detection and location of water ingress could prevent premature damage to fiber and copper cables in communication and power distribution ducts.
Optical fiber sensors that modify a backscatter signal in the presence of a target under measurement prove to be convenient devices for performing distributed measurements. In this environment, OTDRs are able to resolve changes in backscatter signals of 0.01 decibel with a spatial resolution of less than one meter.
With such accurate measurement techniques available, university researchers decided to investigate the development of mechanically perturbing, or bending, fiber to modify its backscatter signal in the presence of water to develop an accurate sensor.
Microbend transducers have been available for years. But the university`s sensor approach involves the achievement of mechanical bending by employing a range of polymer materials, or hydrogels, as the active detection medium.
Hydrogels are materials that swell in water without dissolution. The selected hydrogel for the sensor design is a poly (ethylene oxide)-co-poly (propylene oxide) block co-polymer polyurethaneurea (PUU) gel that undergoes a volumetric expansion of 5% to 250% when wet. The present formulation swells volumetrically by 40% when wet.
The idea for the sensor emerged from joint discussions among Michie, Brian Culshaw and two other researchers--Neil Graham and Chris Moran--in the university`s chemistry department. "When they explained what the hydrogel did, it seemed an obvious thing to do," says Michie. The successful sensor design has since been patented by this research team.
Sensor structure
To fabricate the sensor, the hydrogel and a graded-index optical fiber are arranged in a geometrical configuration that allows for the swelling of the gel. Acting through a microbend transformer, the configuration influences the loss of light within the fiber. The induced losses can be readily detected, located and measured using an OTDR instrument.
For its composition, the hydrogel material is dissolved in an alcohol solvent and deposited onto a central supporting former. A coated rod is then held in contact with the optical fiber by a helically wound thread with a 2-millimeter winding pitch. In the presence of water, the hydrogel swells and exerts a microbending force onto the fiber through a Kevlar wrap.
The microbending effect causes a loss of power, thus enabling the detection of water. A coating layer of 40-micron-thick hydrogel produces a signal loss of approximately 110 decibels per kilometer when the gel is in a swollen condition.
The sensor responds rapidly when initially wetted, producing a loss of 60 dB/km in less than 30 seconds. The equilibrium swelling of the gel, where the fiber reaches its maximum loss condition, is attained after approximately 40 seconds. The swelling is reversible when the water is removed. Depending on the surrounding conditions, the sensor can take as long as 10 minutes to dry and recover its original state.
The device`s construction enables the adaptation of the fiber-optic sensor to detect chemicals without modifying the basic design. The active component in the sensor is the hydrogel. Gels have been shown to be sensitive to various parameters, including pH, amino compounds, ionic strength, photo-irradiation and temperature. In fact, the university researchers are presently using a sensitive gel to fabricate a pH detector for medical applications.
Gels that swell in water only in the presence of a particular ion, as well as gels that swell in the presence of a specific value of pH, have been constructed at the university. These gels are compatible with the processes used to construct the sensor cable.
According to Michie, the university is negotiating a licensing agreement with Ericsson to commercialize the sensor. In addition to the successful trials of the sensor in civil engineering applications, Michie states the primary research goal is to develop the product for use in the communications industry.
In such applications, the sensor would be used to detect water ingress into communication cables. "Obviously, water will corrode either electrical conductors or fiber-optic cables, says Michie. "If you detect what`s happening, you can repair it. If you don`t, then the first thing you know about it will be when the entire communications system goes down."
Locating water ingress in fiber-optic cables has proven challenging. Presently, the sensor`s signal strength works for just a few kilometers of cable. This limitation rules out usage in transoceanic cables at this time. However, says Michie, "It`s likely the sensor will find use in critical areas where cables run under rivers or where it`s important to monitor an important cable section."
Grouting application
The sensor has been applied successfully in experimental trials aimed at assessing its suitability as a distributed water monitor. This sensor, as part of a monitor, can determine the extent of grouting fill of reinforced tendon ducts in civil engineering structures.
In these structures, steel tendons located in the ducts are used to apply a reinforcing compressive loading force to sections of a structure. The ducts are then filled with a cement-like grout to provide a protective seal from environmental influences. A major structural problem, until this sensor approach, has been the lack of a technique to effectively assess how the grout fills in along the duct length. Without adequate grout, the steel tendons could be left exposed to water ingress.
Through the use of fiber-optic sensors, the position of grout along a tendon duct can be accurately determined. When buried in grout, the sensor`s hydrogel becomes exposed to any water ingress and locally deforms the fiber. This approach provides a means of assessing the quality of the grouting process during construction. q
Dave Wilson writes from London.