Wavelength Monitoring of Sensors: Two Approaches

Anne L. Fischer

Fiber sensors offer several significant advantages over conventional electrical sensors. They’re rugged passive components requiring no external power. They form an intrinsic part of the fiber optic cable that transmits the measurement signal, impervious to electromagnetic interference, so they can function in many hostile environments where conventional sensors would fail. And, because optical signals can travel through miles of fiber with minimal attenuation, the delicate monitoring equipment can be a great distance from the hostile environment of the sensor. Yet another advantage is their ability to be multiplexed, so that one instrument can monitor many sensors, driving down the cost of complex control systems.

Applications

Fiber sensors are finding their way into an ever-increasing number of applications as engineers and designers take advantage of their unique abilities. In petroleum engineering, for example, these devices are used for remote monitoring of the structure and function of an oil well to prevent hazards or malfunction. As the search for oil intensifies, wells are being drilled deeper, and monitoring equipment faces great demands in terms of pressure and temperature. Another novel use in the oil fields is in squeezing the last ounce of production from old wells by optimizing the amount of water or steam injection used to efficiently remove reserves.

Structural monitoring is another use. By measuring stress and strain in bridges, buildings and tunnels — where failure can mean disaster —these sensors provide crucial analysis. The devices are small enough to be embedded into the structure of buildings or vehicles, so they can passively monitor on a continuous basis, providing information that can be tracked over time to indicate important structural changes. These sensors may be the insurance underwriter’s delight, as they can provide early warning of weaknesses, so that minor repairs can be made before major disaster strikes.

The fiber Bragg grating is one of the most common fiber sensors. It is created by writing a periodic variation into the refractive index of an optical fiber, so that a lightwave in the fiber that is exactly twice as long as the refractive-index period will be reflected, and all other wavelengths will be transmitted. The operating principle of the fiber Bragg grating sensor is that a change in external conditions — temperature or mechanical force, for example — will expand or compress the fiber Bragg grating, altering the refractive-index period, and thereby changing the wavelength reflected.

There are two fundamental approaches to monitoring this change. In one technique, the fiber Bragg grating is illuminated with a broadband source, and the reflected wavelength is monitored with a spectrum analyzer. In the second technique, a narrowband tunable laser is swept across the appropriate spectral region, and a reflected signal is observed with a broadband detector only when the laser is precisely tuned to the sensor’s reflectivity. The following two articles take a closer look at these two contrasting techniques.