684 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 48, NO. 3, JUNE 1999 Prototype Fiber Optic Liquid Crystalline Sensor for Pressure Monitoring Tomasz R. Woli´ nski, Member, IEEE, Andrzej W. Doma´ nski, Witold Konopka, and Wojtek J. Bock, Senior Member, IEEE Abstract—This paper presents results of initial tests performed on a new prototype fiber optic liquid crystalline sensor for hydrostatic pressure monitoring which employs pressure-induced deformations in twisted nematic liquid crystal cells. The proto- type pressure sensor is based on polarization effects occurring in the reflective configuration of the liquid crystal cells under the Mauguin limit. Results indicate that the prototype sensor offers high response to pressure with reduced temperature sensitivity and, depending on the replaceable liquid crystalline sensing element, can be adjusted for monitoring of low hydrostatic pressures up to 4 MPa. Index Terms— Fiber optic sensors, liquid crystals, pressure measurement. I. INTRODUCTION T HE use of liquid crystal (LC) materials in applications to fiber optic sensing of hydrostatic pressure in both lower (of the order 1 MPa) and higher (up to 100 MPa) pressure regions was recently reported [1]–[5]. Strong rotatory power of some LC’s has become a basis of a new method of pressure sensing in the lower pressure region [4], [5] and resides in detecting changes in helicoidal pitch (characterizing the chiral nematics structure) under pressure. This paper introduces a new prototype fiber optic liquid crystalline sensor for hydrostatic pressure monitoring. The sensor employs pressure-induced deformations in twisted ne- matic liquid crystal cells and is based on polarization effects occurring in the reflective configuration. This configuration is of special importance from the prospective applications point of view, especially in pipelines, where both input and output fibers should be situated at the same side of the sensing head. II. TWISTED NEMATIC CELLS FOR PRESSURE MONITORING Optical properties of chiral nematic liquid crystals are very specific and are determined by helicoidal pitch ( ), the arrangement of its axis, and the polarization of the incident light. Strong rotatory power occurring in chiral nematic liquid crystals can be exploited in a new method of hydrostatic pressure sensing. The effect manifests itself in rotation of linear polarization of light coming through a liquid crystalline Manuscript received June 5, 1996; revised February 22, 1999. This work was supported in part by the Warsaw University of Technology, University of Qu´ ebec at Hull, and by the Polish Committee for Scientific Research (KBN). T. R. Woli´ nski, A. W. Doma´ nski, and W. Konopka are with the Institute of Physics, Warsaw University of Technology, 00-662 Warszawa, Poland. W. J. Bock is with the Optoelectronics Laboratory, University of Qu´ ebec at Hull, Hull, P.Q., J8X 3X7 Canada. Publisher Item Identifier S 0018-9456(99)04997-9. film (Mauguin limit) and resides in detecting changes in heli- coidal pitch of a chiral nematics under the pressure-induced deformation. Sensitivity of such a pressure sensor depends on , on birefringence , and on elasticity of the LC layer. Liquid crystalline films with high values of are appropriate materials for pressure sensing since they possess strong rotatory power proportional to that can be easily modulated by deformation of film thickness. These materials are commonly used in liquid crystalline displays such as twisted nematic (TN) cells or super-twisted nematic (STN) cells and are thermally stable in a wide region of temperature. In the TN cell, chiral nematic LC is introduced between two substrates treated to impose parallel molecular alignment but twisted with the angle 90 . A monochromatic wave propagating through a TN (STN) cell along its helicoidal axis can be described as a super- position of two normal modes. In a rotational coordinate system (one of the axes is parallel to the helicoidal axis) the normal modes are in general elliptically polarized, and the axes of their polarization ellipses rotate at the same rate as the light propagates through the twisted LC medium. However, if the total birefringence of the LC medium is much larger than initial twist (rotatory power) induced by the helicoidal structure, the light remains approximately linearly polarized for all (along the helix) and the plane of polarization follows the twist of the LC directors. Since , this leads to the Mauguin limit—originally discussed by Mauguin in 1911 [7] and also known as adiabatic approximation: (1) This condition can be easily understood from the following: linear birefringence of the medium must be much greater than circular birefringence induced by twist, i.e., chiral nematic rotatory power equal to . Since and , so (2) For the TN cell director rotates of an angle between two limiting glass walls separated by the distance , hence and the condition (3) is evident (3) In practical situations, the Mauguin limit can be approxi- mated by a condition or (4) 0018–9456/99$10.00  1999 IEEE