JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 34, NO. 13, JULY 1, 2016 3065 A Low-Loss Optical Switch Using Liquid Crystal Core Waveguide With Polymer Cladding Mukesh Sharma, Mangalpady R. Shenoy, Member, OSA, and Aloka Sinha Abstract—We demonstrate an electrically controlled optical switch based on nematic liquid crystal (LC) core waveguide, fab- ricated on an ITO-coated glass substrate, with upper and lower cladding layers of the negative photoresist AZ15nXT. The reori- entation of the LC molecules in the presence of an applied elec- tric field produces distinct guiding properties for the TE and TM polarizations of light. Considering full anisotropy of the LC, the thickness of the cladding layer is chosen suitably to minimize the propagation loss of guided modes. The fabricated device operates at a threshold voltage of 3.5 V pp , and behaves as an optical switch for TM polarization of light with an extinction ratio > 15 dB. We experimentally achieved a relatively low insertion loss ∼7.96 dB at the applied voltage of 10 V for a waveguide of length 10 mm. The proposed device has potential application as an electrically con- trolled optical switch or retarder, particularly for low frequency applications. Index Terms—Electrooptic devices, liquid crystals, liquid crystal devices, optical switch, polymers. I. INTRODUCTION P LANAR lightwave circuits (PLCs) and devices have po- tential use in integrated optics for light modulation, switch- ing, and multiplexing [1]–[3]. Among the available materials for fabrication of PLC devices, liquid crystals (LC) are attractive due to their inherent anisotropy and large electro-optic coeffi- cients [4]–[7]. In recent years, different LC based optical devices for waveguiding [8]–[13] and switching [14]–[22] applications have been demonstrated. In addition, LC core waveguides have also been proposed on ITO-coated glass [11]–[13], and silicon substrates with V-grooves [10], [14], and [17]. Low insertion loss and high extinction ratio along with low operating voltage are the requirements of an optical switching device. In order to overcome the overall losses of LC based waveguide devices, different kinds of device structures have been reported [10]– [22]. Wang et al. [12] demonstrated an electrically-tunable LC core channel waveguide for switching application; the LC core waveguide was encapsulated in semicircular grooves made on optical glass substrate with a propagation loss of 1.3 dB/cm. Cai et al. [20] reported an electrically-tunable LC core wave- guide attenuator based on hollow waveguides on silicon sub- strates with a high insertion loss of 47.26 dB. Donisi et al. [17] Manuscript received March 01, 2016; revised April 18, 2016; accepted April 20, 2016. Date of publication April 21, 2016; date of current version June 22, 2016. This work was supported by the Government of India, Ministry of Defence, Defence Research and Development Organization, New Delhi, for this project ERIP/ER/140047/M/01/1571. The authors are with the Department of Physics, Indian Institute of Tech- nology Delhi, New Delhi 110 016, India (e-mail: mukeshnice@gmail.com; mrshenoy@physics.iitd.ac.in; aloka@iitd.ac.in). 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/JLT.2016.2557962 reported an integrated optical switch based on LC on silicon substrate with an overall insertion loss of 14 dB. Recently, we proposed an electrically-controlled nematic LC core waveguide on an ITO-coated glass substrate that works as an optical switch and an optical attenuator for TM polarization of light with a low threshold voltage of 1V pp and an insertion loss of 17.93 dB [13]. In all the above reported structures, the LC core layer is in direct contact with the ITO layer, and therefore the evanescent field of the guided wave, which extends into the ITO-coated (conducting) glass substrates, results in absorption of the opti- cal field. This affects, and increases the overall insertion loss. To circumvent this problem, we propose the use of a polymer cladding layer between the LC core layer and the ITO-coated glass substrate. The thickness of the polymer cladding layer has to be chosen suitably since the evanescent optical field that extends from the core-cladding interface, has to be confined well-before it reaches the ITO-coated glass substrate [23]–[27]. As a result, the overall insertion loss of the LC core waveguide device is significantly reduced. In this paper, we present details of an electrically-controlled nematic LC core waveguide opti- cal switch, fabricated on a ITO-coated glass substrate with core layer of the LC 5CB, and upper and lower cladding layers of the negative photoresist AZ15nXT. In Section II, we discuss the design of the optical switch, including the effect of the thick- ness of the cladding layers on the guided-mode field profiles, and the fabrication process is described in Section III. Results and discussion are presented in Section IV. II. DEVICE DESIGN AND SIMULATION In our previous work [13], we used the nematic LC 5CB as the waveguide core material which was sandwiched between two ITO-coated glass substrates; the refractive indices of the glass cladding and the LC core were n glass =1.51, and n o =1.53, n e =1.71, respectively. In the reported LC core waveguide [13], light is guided both with and without the application of an exter- nal electric field, because n o and n e of the LC core material is more than the refractive index of the top and bottom glass plates. When the electric field is applied, the LC molecules re-orient in the direction of the field, and the two orthogonal polariza- tion components of the light (TE and TM) see two different refractive indices. The TM polarization would see the extraor- dinary refractive index (n e ), and the LC core waveguide would then support more number of modes, while the TE polarization would continue to see the ordinary refractive index (n o ), and therefore would support the same number of modes, as before the application of the electric field. The coupled power in the TM polarization is distributed among the various modes, and the output power drops significantly at the threshold voltage of 0733-8724 © 2016 IEEE. 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