The Vibrofiber sensor is a Fabry-Perot cavity formed between two broad band fiber gratings creating interference
fringes. It was introduced three years ago to monitor the vibration and temperature rise of the stator end winding in a
power generator.(1) This paper will discuss the use of Vibrofiber to monitor the deflection of the bridge under adverse
conditions: wide temperature swings, excess load, strong winds, earth quake, etc. The fringes in these cavity sensors
have features like peaks and valleys which are sensitive to temperature and strain. When the bridge becomes overloaded,
we are interested in knowing the extent of the deflection. In addition, we might want to locate the cause of the overload.
A simple Sagnac FBG interferometer has been invented to provide such diagnostics. A pair of long fibers with such
cavity sensors can be installed on the underside of the target bridge segment between two supporting columns. The
objective is to monitor the deflection together with any distortion of the bridge deck. Each of the 2 long fiber segments
has a pair of cavity sensors, one measures the deflection as a result of the excess strain, and the other measures
temperature and provides compensation for the deflection data. An array of cavity sensors with different center
wavelengths will be used to support the typical multi-segment bridge structure. The interrogation unit is based on a
tunable laser that can hop to different ITU grids. A separate DFB laser will run a grating based Sagnac interferometer,
measuring weight in motion, identifying the speed and the make of vehicle in traffic and provide deflection diagnostics.
Overloaded trucks and speeding vehicles can be captured and tagged for corrective actions. The interrogation unit is
equipped with wireless Ethernet communication enabling the monitoring of many bridges from a central location and
similarly warning can be initiated to alert the central traffic control ahead of any problems.