In this paper, a packaged FBG based optical fiber sensor written by femtosecond laser pulses in highly birefringent micro-structured optical fiber (MS-FBG sensor) is presented and validated for simultaneous pressure and temperature monitoring. The MS-FBG sensor is capable of separating the temperature information from pressure information without the need for an additional transduction mechanism and this with a negligible pressure-temperature cross-sensitivity. However, in order to use the sensor for downhole applications, a ruggedized sensor housing is required that not only offers mechanical protection to the fiber, but also provides pressure transfer from the well fluid to the sensing element without inducing an additional pressure-temperature cross-sensitivity. In this article, the design of the sensor housing is reported as well as the lab-scale validation up to a temperature and pressure of 150 °C and 700 bar, respectively.
The applicability of fiber Bragg gratings written in highly birefringent Butterfly micro-structured optical fibers (MSFBG) for simultaneous High Pressure/High Temperature monitoring without a significant pressure/temperature cross sensitivity have recently been shown. This makes these MS-FBG sensors extremely interesting for downhole monitoring in the Oil and Gas industry. However, an important effect to be taken into account for these applications is the presence of hydrogen, as hydrogen is known to diffuse into the fiber structure and therefore might affect the wavelength responses of the sensor element. In this paper, the effect of hydrogen gas on the MS-FBG sensor readings by monitoring the wavelength changes of the MS-FBG sensor in a hydrogen rich environment have been investigated.
In this experiments, two MS-FBG sensors were placed in a hydrogen test chamber: one with its fiber end sealed for pressure sensing and the other with its fiber end kept open for referencing purposes. It could be demonstrated that both sensors show a similar wavelength shift after some time and that due to the hydrogen diffusion, the pressure in the airholes of the sealed MS-FBG sensor equalizes the hydrogen pressure in the chamber. Furthermore, it could be demonstrated that the refractive index seen by the waveguide of the fiber is also affected. Based on all these observations, the influence of the hydrogen on the temperature and pressure measurement performance of the MS-FBG sensor is estimated, and a mitigation scheme that partially compensates for this influence is discussed.
In this paper, we demonstrate that femtosecond laser pulse written fiber Bragg gratings (FBGs) fabricated in specialty highly birefringent micro-structured optical fiber (MSF) can be used for high pressure and high temperature monitoring in downhole applications. The design of the micro-structure allows encoding the pressure information into the spectral separation between the two Bragg peaks reflected by the obtained MS-FBG. We obtained a differential pressure sensitivity of 3.30 pm/bar over a pressure range from atmospheric up to 1400 bar and at temperatures between 40 °C and 290 °C. Owing to the negligible differential pressure-temperature cross-sensitivity of 6.06E-3 bar/°C, the proposed MSFBG sensor is an ideal candidate for pressure monitoring in the presence of high temperature transients.
The speed of light is an important physical parameter. Currently it is a common belief of the constance of the speed of light regardless of the relative velocity between the source and the observer. Because the speed of light is very fast, if the relative velocity is small compared with the speed of light, it is difficult to detect the effect of the relative velocity on the measurement of the speed of light. In this paper we present a method of comparing the speeds of starlight and the light emitting from a terrestrial source. We use a telescope to collect the light from the star having significant relative velocity with respect to the earth, e.g. Capella. Then we modulate the starlight and the light emitted from the local source into pulses i.e. these pulses leave the modulator simultaneously. After travelling 4.2 km, these pulses are detected by a receiver. If the starlight and the terrestrial light have the same speed, then these pulses must arrive at the receiver at the same time. Our results show that the arrival times of the pulses of starlight are different from that of the local light. For example, the Capella is leaving away from the earth. The Capella pulses arrive later than the local light pulses. It indicates that the speed of Capella starlight is slower than the common believed value, c. The presented method uses one clock and one stick, so the clock synchronization problem and any physical unit transformation can be avoided.