Transversely loaded Bragg grating pressure transducer with mechanically enhancing the sensitivity

Abstract

LntroductionFiber optic sensors and systems in the oil and gas industry have potentially significant advantages compared to conventional sensor systems, particularly for downhole permanent monitoring applications. The typical environmental requirements in well bore applications are 20kpsi and 175 degree C and measurements in these hostile environments need much higher pressure and temperature capability [ 11. Fiber optics sensors are electrically passive and do not require downhole electronics, resulting in potential operations at high temperatures with high reliability [2].Fiber Bragg gratings (FBGs) can operate at high temperatures, and are candidates for multifunctional and multi-point sensors for well and reservoir monitoring. Fiber optic Bragg grating sensors have been used to measure longitudinal and transverse strain [3-51, as well as longitudinal strain and temperature [6-81. Schlumberger demonstrated a Bragg grating pressure sensor with a single mode side-hole fiber [9] and superior p ressure response with low temperature sensitivity up to 300 degree C compared to the Bragg grating written onto conventional single mode optical fiber [lo]. The transverse load is applied to the core by the air holes in the fiber cladding under high pressure, resulting in stress induced birefringence and a dual peak spectral output. The peak to peak separation is sensitive to pressure, and veryinsensitive to temperature.In this paper, we describe a transversely loaded Bragg grating pressure transducer with mechanically enhanced the sensitivityutilizing fiber Bragg gratings written onto a single mode fiber [l I], and demonstrate pressure sensitivity up to 5k psi with low temperature sensitivity.Transducer structure and experimental results Figure 1 shows a schematic view of the transducer structure. The transducer and an optical fiber are set in an atmospheric chamber, and transverse stress is applied to the optical fiber by a piston. The force applied to the fiber is proportional to the area of the piston; therefore, the transverse load can be adjusted by changing the area. Two optical fibers without coating material are set between two quartz plates, one w i t h a FBG, and a dummy fiber, added to keep the symmetry and alignment of the plates. The balance of the transverse load between the FBG and the dummy fiber depends on the position of the fibers; therefore, the pressure sensitivity of the transducer changes slightly depending on assembly conditions. The surfaces of 0-7803-7289-1/02/$17.0002002 IEEE 535

A pressure sensor based upon the transverse loading of a sub-section of an optical fibre Bragg grating

“…A number of mechanical enhancement schemes have been proposed, including a triangular cantilever [6], a bubble housing [7] and a piston [8]. The piston-based systems rely upon the applied pressure inducing birefringence in the optical fibre, causing a splitting of the Bragg reflection peak.…”

Sensitivity Enhancement of Long-gage FBG Sensors for Macro-strain Measurements

“…For different type of fiber optic sensors, the design concepts and manufacture methods for sensitivity enhancement are usually quite different. Past efforts have reported many innovative techniques for developing fiber grating pressure sensor of high sensitivity, including the configuration using a grating housed in a glass bubble [5] or partly shielding a fiber with a metal tube [6], the transversely loaded pressure transducer with mechanically enhanced sensitivity [7], and those utilizing polymeric materials to recoat fiber gratings [8,9]. It is worth noting that polymer packaging techniques have been regarded as a particularly efficient manner to improve the sensitivity of fiber grating sensors for various measurements [10,11].…”

Analysis of a demodulation system for Fiber Bragg Grating sensors using two fixed filters

“…The operating principle of these sensors is based on the Bragg wavelength shift that occurs when the grating is strained or subjected to changes in temperature. FBG sensors have also been incorporated in various transducers where an indirect evaluation of the quantity of interest is based on these two physical parameters [4][5][6]. In order to read the wavelength-encoded information, several techniques, some of them already commercially available [2], have been proposed and demonstrated [1].…”