1796 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 57, NO. 8, AUGUST 2008 Photonic Liquid Crystal Fibers for Sensing Applications Tomasz R. Woli´ nski, Member, IEEE, Aleksandra Czapla, Slawomir Ertman, Marzena Tefelska, Andrzej W. Doma´ nski, Jan Wójcik, Edward Nowinowski-Kruszelnicki, and Roman D ˛ abrowski Abstract—The paper presents our latest experimental results on the influence of temperature, an external electric field, and hydrostatic pressure on propagation properties of the photonic crystal fibers infiltrated with liquid crystals of low and medium material anisotropies. Measurand-induced shifts of the photonic bandgap wavelengths give information about the value of temper- ature, voltage, and pressure. Moreover, temperature-dependent positions of the photonic bandgap wavelengths in the transmission spectrum can serve to determine the thermal characteristics of the liquid crystal ordinary refractive index. Index Terms—Liquid crystals (LCs), optical fiber devices, optical fiber measurement applications, optical fibers, sensors. I. I NTRODUCTION M ICROSTRUCTURED optical fibers (MOFs) are a new class of optical fibers with a regular structure of micro- holes running along the axial direction, and they have been the subject of intense research over the past decade. MOFs with the periodic microstructure are called photonic crystal fibers (PCFs). There are two different types of PCFs: 1) with a solid core and 2) with a hollow core. In solid-core PCFs, the refractive index of the silica core is higher than the effective refractive index of the cladding, so the total internal refrac- tion (TIR) phenomenon is the mechanism responsible for the guiding of light. [Fig. 1(a)]. In hollow-core PCFs, the refractive index of the air core is always lower than the effective refractive index of the cladding, so light guiding by the TIR mechanism is not possible. In these fibers, only selected wavelengths can be guided by the photonic bandgap (PBG) effect [Fig. 1(b)]. PCFs have been intensively explored to achieve high bire- fringence (HB). Birefringence in PCFs usually results from accidental asymmetries in the cladding-hole lattice or from the intentional manipulation of the core and/or cladding structure [1]–[3]. Manuscript received July 9, 2007; revised March 4, 2008. T. R. Woli´ nski, S. Ertman, M. Tefelska, and A. W. Doma´ nski are with the Faculty of Physics, Warsaw University of Technology, 00-662 Warsaw, Poland. A. Czapla is with the Faculty of Physics, Warsaw University of Technology, 00-662 Warsaw, Poland, and also with the Université du Québec en Outaouais, Gatineau, QC J8X 3X7, Canada (e-mail: czapla@if.pw.edu.pl). J. Wójcik is with the Maria Curie Sklodowska University, 20-031 Lublin, Poland, and also with the Military University of Technology, 00-908 Warsaw, Poland. E. Nowinowski-Kruszelnicki and R. D ˛ abrowski are with the Military Uni- versity of Technology, 00-908 Warsaw, Poland. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIM.2008.922077 Fig. 1. Mechanism of light propagation in PCF. (a) TIR. (b) PBG. In specific applications, it is important to consider sensitivity of propagation and polarization properties to environmental perturbations [4], [5]. An interesting case is to infiltrate PCFs with liquid crystals (LCs) that induce significant changes to the guiding properties. In empty solid-core PCFs with air holes, the refractive index of the core is higher than the refractive index of the cladding, and the whole spectrum of the input light is guided by the modified total internal reflection (mTIR) mechanism, which is well known and is similar to the waveguiding effect within a conventional fiber. An LC introduced into a PCF significantly changes its guid- ing properties; the effective refractive index of cladding in- creases and, in most cases, is higher than the refractive index of the core. Hence, the light can be guided due to the PBG effect. This guiding mechanism relies on coherent backscattering of the light into the core, whereas only selected wavelengths of input light can propagate within the fiber. LCs seem to be particularly interesting substances to infil- trate PCFs, since their optical properties strongly depend on thermal, electric, magnetic, and optic fields. The LC-infiltrated PCF structures, which we call photonic LC fibers (PLCFs), open up a wide range of new possibilities for tuning light prop- agation properties due to the fact that LCs are materials with a strongly adjustable anisotropy [6]–[9]. Moreover, temperature switching between two propagation mechanisms (mTIR and PBG) in a single PLCF has been demonstrated [10]. The paper reports the latest experimental results on tem- perature, hydrostatic pressure, and electrically induced tuning of the PLCF propagation and polarization properties, in view of potential applications in all-optical fiber optic sensors, of temperature, an electric field (E-field), or hydrostatic pressure. 0018-9456/$25.00 © 2008 IEEE