1032 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 8, APRIL 15, 2012 Theoretical and Experimental Results of High-Birefringent Fiber Loop Mirror With an Output Port Probe Ricardo Manuel Silva, Azam Layeghi, Mohammad Ismail Zibaii, Hamid Latifi, Jose Luis Santos, Member, OSA, and Orlando Frazão, Member, OSA (Invited Paper) Abstract—Theoretical and experimental results of three dif- ferent high-birefringent fiber loop mirrors with output ports are analyzed. For theoretical model, the Jones matrix analysis is used. The theoretical studies present similar results for all experimental configurations. The last configuration is tested as an interrogation system where the spectral response arises from the combination of the reference signal modulated by the sensor signal. The configu- ration is characterized in strain with the phase changes recovered from two quadrature phase signals, providing a sensitivity of 16 mrad/ with a resolution of 1.9 . Index Terms—High-birefringent fiber loop mirror (Hi-Bi FLM), interferometer, optical fiber sensor, strain. I. INTRODUCTION S INCE the end of the 1980s, the fiber loop mirror (FLM) has been shown to be an attractive device for optical fiber sensing [1]. The loop mirror is made up of a splice between the output ports of one directional optical coupler. In this case, the two waves travel with identical optical paths in opposite directions and a constructive interference is assured when the waves reenter the coupler. Afterward, all light is reflected back into the input port, while no light is transmitted to the output port. The reflectivity is limited by the losses of the splice, fiber, and coupler. When a section of highly birefringent (Hi-Bi) fiber is spliced inside the FLM, a path imbalance is introduced as Manuscript received May 30, 2011; revised August 31, 2011; accepted September 01, 2011. Date of publication September 22, 2011; date of current version March 02, 2012. R. M. Silva and O. Frazão are with INESC Porto—Instituto de Engenharia de Sistemas e Computadores do Porto, 4169-007 Porto, Portugal (e-mail: rm- silva@inescporto.pt; ofrazao@inescporto.pt). A. Layeghi was with Laser and Plasma Research Institute, Shahid Beheshti University, Tehran 1983963113, Iran. She is now with INESC Porto—Instituto de Engenharia de Sistemas e Computadores do Porto, 4169-007 Porto, Portugal (e-mail: a.layeghi@gmail.com). M. I. Zibaii is with Laser and Plasma Research Institute, Shahid Beheshti University, Tehran 1983963113, Iran, and also with INESC Porto—Instituto de Engenharia de Sistemas e Computadores do Porto, 4169-007 Porto, Portugal (e-mail: mizibaye@gmail.com). H. Latifi is with the Laser and Plasma Research Institute, Shahid Beheshti University, Tehran 1983963113, Iran (e-mail: latifi@cc.sbu.ac.ir). J. L. Santos is with the Department of Physics and Astronomy, Faculty of Sci- ences, University of Porto, 4169-007 Porto, Portugal (e-mail: josantos@fc.up. pt). Digital Object Identifier 10.1109/JLT.2011.2167499 a consequence of propagation of light along different polariza- tion eigenaxis, and thus, an interferometric channeled spectrum is observed. This configuration is an unbalanced Sagnac inter- ferometer. The fiber birefringence and the fiber length inside the loop determine the channel separation of the pattern fringe [2]. Over the last decade, several high-birefringent fiber loop mir- rors (Hi-Bi FLM) applications as sensors have been reported, through the inclusion of different types of Hi-Bi fibers [3]. Be- sides the gyroscope application [4], the Hi-Bi FLM has been used in temperature [2], [5] and strain [6] measurement, liquid level [7], displacement sensing [8], and refractometers [9], as well as a spectral filter for fiber Bragg grating demodulation [10]. Furthermore, the Hi-Bi FLM combined with other op- tical devices was also demonstrated for simultaneous measure- ment of strain and temperature [11]. Preliminary works using a Hi-Bi FLM with an output port probe were recently demon- strated [12]. These configurations presented similar sensitivi- ties to strain when compared with the conventional Hi-Bi FLM. One of the configurations was used as refractometer using the Fresnel reflection [12]. In this paper, the authors present a theoretical model of three different Hi-Bi FLMs with an output port configuration. The last experimental setup is studied as a strain sensor using a reference signal to reconstruct the phase signal of the sensor. II. THEORETICAL ANALYSIS Three configurations of Hi-Bi FLMs are schematically shown in Fig. 1. They consist of a loop of optical fiber formed between the output ports of two directional couplers along with an output port probe. The input light, constituted by one wave, travels from the optical source towards the first optical coupler (input port 1) and splits in two waves with half power, each traveling from output ports 3 and 4, following different optical paths. One arm of the FLM has a polarization controller (PC). The two waves couple into the second optical coupler for the output port 7 and reach the mirror. Afterward, they are reflected and arrive to the port 7, where the two waves are separated once more by the second optical coupler. Four waves are thus obtained at the output ports of the second coupler. Two waves are combined in the first coupler with opposite directions and the same optical path. Half of the light is then reflected back into the input port. This behavior is similar to the conventional Hi-Bi FLM [1]. The 0733-8724/$26.00 © 2011 IEEE