# Fast Ferroelectric Liquid Crystal Based Optical Switch: Simulation and Experiments
## 1 Introduction  
With the rapid pace in technological advances and the widespread deployment of optical networks over the past few years, fiber-optic communications have a significant role due to their higher bandwidth and greater reliability than copper cables [1; 2]. The demand for unlimited band width encourages the generation of novel technologies to make networks much faster, more reliable, and more scalable [3; 4]. In transparent optical networks, bit-error-rate (BER) largely depends on intensity of cross-talk. To achieve low BER (\(10^{-9}\)), required cross-talk is \(<\)-30 dB [5]. For a high bit rate transmission, optical switches are essential and this transmission must be matched with the switching capacity. All-optic switch fabrics allow switching directly in to the optical domain to avoid electronic bottlenecks. The high cross-talk and slow response time are, therefore, serious issues to be overcome in the fast growing communication networks. Development of power-efficient, low cross-talk, and much faster optical switches is crucial in tackling this problem. The optical switch performance can be assessed by measurement of the transparency to bit rate and data format, scalability, insertion loss, and cross-talk. In photonic switches, to control the properties of optical signals, it is most effective to use materials such as LiNbO\({}_{3}\)[6], semiconductor optical amplifiers (SOA) [7; 8; 9], liquid crystal (LC) [10; 11], etc., which can change their refractive index upon photo irradiation. Among the materials, LCs exhibit large optical anisotropy, and a large change in refractive index by changing their alignment, which is obviously advantageous for photonic applications [12; 13]. Due to the permanent electrical dipole moment, molecules can take up a certain mean relative orientation in the LC phase. It is possible via the application of suitable electric field across a LC cell to act on the orientation of molecules, and thereby alter the optical properties of the material [14; 15; 16; 17; 18; 19; 20]. LC optical switches are based on the change of polarization state of incident light by LC as a result of an external electric field. The change of polarization with polarization-selective beam splitters offers optical space switching.  
LC based optical switches exhibits attractive characteristics, such as low power consumption, no moving parts, low voltage driving, free space switching, fast response time, and cost efficiency [21; 22; 23; 24; 25]. The operating range of conventional nematic LC switches is in the order of 1-100 ms, which limits network speed and shows slightly excessive cross-talk [26; 27]. Unpowered nematic switches mayquickly become disordered and fail to direct signals according to the most recently selected path, which means they do not have a memory and numerous stability states. Ferroelectric liquid crystals (FLCs) discovered by [PERSON] in 1975 [28] have become a subject for intensive research and have observed increasing high technology applications for displays and data processing. In 1998, [PERSON] et al. reported a low cross-talk highly scalable \(2\times 2\) ferroelectric liquid crystal (FLC) switch for all-optical fiber networks [29]. The evolution of FLC switches in fiber networks deliver much faster response times (\(\uptau\)) and have a memory capability such that optical signals will be directed along a pre-selected path until the switch position is actively changed.  
The purpose of present work is the simulation and experimental investigation of the most important performance parameters of FLC modulator. The molecular tilt angle (\(\theta\)) of a KCFLC 10R sample shows 22.5\({}^{\circ}\). The cell thickness was optimized to get a better contrast ratio as cross-talk = 10 log (1/K), where K is the contrast ratio. For optimal FLC parameters, the best achieved cross-talk obtained at room temperature was \(\sim\)19 dB. The response time as a function of voltage showed very good results (\(<\)10 \(\upmu\)s).