Research On High-sensitivity Photonic Crystal Fiber Pressure Sensor

Optical fiber sensors have been usedfor the measurement of a great number of parameters including temperature, strain, vibration and acoustic waves. Compared with their electronic counterparts, they have presented advantages of high sensitivity and resolution, compactness, immunity to electromagnetic interference, and especially the ability to work under environments with high temperature, flammable, high pressure and/or strong electromagnetic interference.Pressure measurement is a fundamental technique in industrial and down-hole applications. However, fiber optic pressure sensors have presented relatively low sensitivities, due to the high Young’s modulus of silica glass. In the recent years, the emerging photonic crystal fibers(PCF) have received great interests because they have flexible transverse structures and tailorable optical properties, based on which fiber optic sensors with high performances have been implemented.In this thesis, we have proposed two approaches based on photonic crystal fibers to fabricate highly sensitivity fiber optic pressure sensors. First, a pressure sensor is fabricated by selectively filling a hole of the PCF with a high-index liquid. The high pressure sensitivity is achieved taking advantage of the difference in elastic properties between the silica glass and the liquid. Second, a Fabry-Perot interferometric pressure sensor is fabricated by splicing a simplified hollow-core microstructured optical fiber with conventional singlemode fibers. The pressure sensitivity is enhanced by HF-etching the outer cladding to obtain a higher cavity elongation with applied pressure.The contents of this thesis are as follows:(1)Illustration of the working principle of the optical fiber sensor and the transmission properties of photonic crystal fiber, as well as photonic crystal fiber sensors.(2)We present the fabrication of a highly sensitive pressure sensor by selectively filling a high-index liquid into a single hole of an index-guided PCF. The core mode can be coupled to the liquid rod when the phase-matching is satisfied, which results in a dip in the transmission spectrum. Due to the difference in elastic properties between the silica glass and the liquid, the effective index of the core mode increases with the applied pressure while the liquid mode hardly changes. As a result, a pressure sensitivity of-0.621 nm/MPa has been achieved.(3)A Fabry-Perot pressure sensor is implemented by splicing a simplified microstructured optical fiber with conventional singlemode fibers. Pressure measurement can be performed by monitoring the spectral change of the interferometric fringes. Theoretical analysis suggests that the pressure sensitivity is determined by the elongation induced by the applied pressure and can be enhanced by reducing the thickness of the outer cladding of the fiber. In the experiment, the sensitivity has been increased from-17.5 to-32 pm/MPa by reducing the thickness of the outer supporting cladding via HF etching.(4)In mainly to the thesis summarizes the experimental subject, summarizes the research work of the innovation points, and for the future research are prospected.