STUDY OF LINEAR AND NON-LINEAR TRANSMISSION OF AN OPTICAL FIBER SAGNAC INTERFEROMETER AS A BIDIRECTIONAL DEVICE
J. Ramos-Beltrán1, G. Beltrán-Pérez2, J. Castillo-Mixcóatl2, S. Muñoz-Aguirre2, P. Zaca-Morán3 y C. Felipe4 Benemérita Universidad Autónoma de Puebla, (México) 1 Facultad de Ciencias de la Electrónica 2 Facultad de Ciencias Fisco-Matemáticas 3 Fisicoquímica de Materiales, ICUAP 4Departamento de biociencias e ingeniería, CIIEMAD, Instituto Politécnico Nacional, (México) email@example.com
Abstract: The optical fiber Sagnac interferometer is a versatile system that has been investigated for a variety of applications such as optical switchers, filters, demultiplexers and passive modelocked laser. In many cases, this arrangement is designed using a symmetrical coupler with twoof their ports connected making a loop and generally the analysis have been focused in the transmission of the signal propagated in only one direction. Therefore in the present work a complementary study of the system considering the analysis for the lineal and non-lineal transmission as a bidirectional device has been performed. The experimental setup consists of different optical fiber lengthsinside the cavity loop (between 100 and 500 m) with highly twisted singlemode fiber, a quarter wave retarder placed asymmetrically in one arm and a 50/50 coupler. The results have shown that for low optical powers, it is possible to adjust the system transmission in both propagation directions with the rotation of the retarder wave. On the other hand, in high optical power levels, this arrangementshowed that the transmission increases slowly for the case when both the input and the output beams have the same polarization. This behavior can be used for pedestal suppression in a light pulse. Furthermore, for the case when the output signal polarization is orthogonal respect to the input one, the transmission changes quite fast. This effect can be used for applications such as the passivemode-locked. Keywords: Sagnac Interferometer, non-lineal transmission, bidirectional device
1. INTRODUCTION: In the last decade, the technology based on fiber optics has been quickly advancing due to its many applications such as, fiber optics lasers in continuous and pulsed emission, optical communications where the fiber is used as information transmission medium. They are also used for thedevelopment of fiber optic sensors to detect many physical parameters such as, temperature, pressure, strain, etc. The interest is due to the fact that using fiber optics, higher transmission rates can be achieved. Moreover, they can be used in places where it is difficult to access and they are immune to electromagnetic interferences. Another attractive device is the fiber optics Sagnacinterferometer (FOSI), which due to its versatility it has many important applications. There are many works that
have been reported about the FOSI where different applications have been showed, such as: optical switching [1-2], demultiplexing  and passive mode-locking, where it has been used as saturable absorber, that means, at low power levels the transmission is quite small and almost all theenergy is reflected by the device, whereas for high power levels it can be transparent. However up to now there is a lack of work about the FOSI as a bidirectional device and its possible applications such as feedback medium in a Fabry-Perot laser.. In previous work  there are reported results of the analysis of the Sagnac interferometer transmission when it was used as a nonlinear optical loopmirror (NOLM). Using a symmetric coupler, highly twisted and a quarter wave retarder plate (QWRP) in the loop, in this work they showed that twisting the QWRP, the transmission variation is from 2% to 82%. However results about the reflectance and bidirectionality of FOSI are not presented. The FOSI is formed by a bidirectional for four-port coupler (P1, P2, P3 and P4), where the P1 and P2 ports are...
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