ABSTRACT
Abstract: An optical fibre sensor for the simultaneous measurement of strain and temperature is presented. This is achieved using two sensing techniqucs within a singlc sensing element, namely a fibre Bragg grating located within a length of erbium-doped fibre, thereby achieving temperature and strain measurement at the same location. Grecn fluorescence, aris ing from the de-excitation of erbium ions, is used to provide the temperature through the t1uorcsccnce intensity ratio method of temperature sensing. This method has the advantage of a cross-sensitivity with strain that is essentially nil. Thus shifts in the Bragg grating output due to temperature changcs arc readily determined so that the applied strain can be calculated. The particular sensor arrangement dcscribed offers the addcd attraction of requiring only one excitation source to stimulate grecn fluorescencc and illuminate the grating. The sensor was characterised by placing it in a tube oven and attaching weights to the end orthe fibre with a pulley arrangement. Calibration of the sensor over various strain and temperature ranges resulted in temperatnre and strain accuracies better than I °C and around 15 ~lE, respectively,
1. Introduction
Strain measurement is important in many areas, such as in the monitoring of civil structures such as bridges, tunnels and dams. Improved safety and reliability through realtime monitoring will result in significant savings to the community. Optical fibre sensors are of much interest in thesc applications because of their small size and flexibility and their ability to be embedded into equipment and structures. Fibre Bragg gratings (FBG) arc very attractive as sensors since an applied strain is encoded as a wavelength shift [1]. Unfortunately Bragg grating strain sensors suffer an inherent cross sensitivity with temperature. Overcoming the cross-sensitivity in Bragg grating sensors continues to be a major interest of many groups [1-2,6], most of whom obtain both parameters simultaneously from two separate sensor elements having different temperature and strain responsivities. Often two essentially identical sensors are used in close proximity, with one
sensor having temperature-sensitivity only (e.g. two Bragg gratings, one not bonded to the structure [1 D. However, if the sensors are not exactly at the same location, measurements may be misleading, particularly if the temperaturc or strain varies significantly from place to place as often occurs. Ideally these sensors should be co-located, and various co-located strain/temperatun: sensors involving only gratings have been reported [I]. Thesc include a pair of Bragg gratings operating at ~50 and 1300 nm, a combination of a Bragg grating with a long period grating, and thc usc of both the first-and second-ordcr dim'action wavelengths of a single Bragg grating. Another approach is to combine a Bragg grating with another sensing technique: for example, by writing a grating into rare-earth-doped fibre. Recently such a sensor was reported which involved the combination of the fluorescence lifetimc of crbium ions and the Bragg wavelength, both of which are strain and temperature depcndent [2].
