Photonic Crystals (PhCs) is a category of periodic optical nanostructures having different dielectric constants. As it is inevitable in the next generation of signal processing and communication networks to use optical waves and photons as a data carrier instead of electrons, new designs should be created with unique specifications. PhCs are one of the structures that have these superb abilities to control and guide the optical waves inside the structure. PhCs provide opportunities to overcome the drawbacks of micro electronic technology, such as the slow signal processing, the large size and the huge power consumption of the integrated circuits. Thus, PhCs are great candidates for optical networks, optical communications and applications of optical signal processing (Joannopoulos et al.
2011). Many PhCs based optical components have been reported such as: multiplexers (Rao et al.
2021; Fasihi and Bashiri
2020; Zhao et al.
2019), de-multiplexers (Mohammadi et al.
2019,
2021a; Masilamani and Punniakodi
2020; Radhouene et al.
2020; Mohammadi and Seifouri
2019; Kannaiyan et al.
2017), full adders (Mohammadi et al.
2021b; Naghizade and Saghaei
2021a; Maleki et al.
2021a,
b; Goswami et al.
2021; Vali-Nasab et al.
2019), analog-to-digital converters (Naghizade and Saghaei
2021b; Shamsi and Moradi
2020; Geng and Zhao
2020), optical filters (Foroughifar et al.
2021), flip-flops (Zamanian-Dehkordi et al.
2018; Abbasi et al.
2012), comparators(Seraj et al.
2020; Feng et al.
2021; Cheng and Dadras Jeddi Pishkhani
2021; Zhu et al.
2019; Jile
2020), optical logic gates (Kumar and Sen
2020; Kotb and Guo
2020; Saranya and Anbazhagan
2020; Caballero et al.
2020; He et al.
2020; Mostafa et al.
2019a; Hussein et al.
2018a,
b; Olyaee et al.
2018; Fu et al.
2013; Caballero and Neto
2021) and all-optical encoders (Makvandi et al.
2021; Rajasekar et al.
2020; Mehdizadeh et al.
2017; Chhipa et al.
2022; Haddadan and Soroosh
2019; Haddadan et al.
2020; Saranya and Shankar
2021; Latha et al.
2022a,
b; Arunkumar et al.
2022; Fallahi et al.
2021; Khatib and Shahi
2020; Saranya and Rajesh
2020; Mostafa et al.
2019b). The encoder is a device with 2
N input and N output ports. Encoders are generally used in communication systems like automatic health monitoring systems, war field, medical industry and electronics industry. PhCs are classified according to the dimensions of periodicity as one dimensional (1D) PhC, two dimensional (2D) PhC or three dimensional (3D) PhC. The reason behind preferring 2D PhC for designing a photonic device instead of 1D PhC is because of having a complete photonic band gap unlike the 1D PhC. Also, the 2D PhC simulation requires less memory and less computational resources compared with 3D PhC (Kannaiyan et al.
2017). To fabricate a 2D PhC structure, microfabrication techniques are utilized like electron-beam lithography and focused ion-beam etching methods (Olyaee et al.
2018). The 2D PhC structure can be a periodic array of air pores in a dielectric background, or a periodic array of dielectric rods in an air background. The arrangement can be in many forms, such as a hexagonal (triangular) lattice or a square lattice. A square lattice of dielectric rods is preferred over a triangular lattice of air pores because of the former’s simple nanostructure, accurate band gap calculations and weak propagation loss, and also because the light can be confined easily during propagation inside the structure (Kannaiyan et al.
2017). So, 2D PhC square lattices of dielectric rods are used in our proposed designs. According to the unique property of the photonic band gap (PBG), we can confine and guide the flow of light through the structure by introducing defects (Joannopoulos et al.
2011). There are many types of defects that can guide the propagation of light inside the structure. Point defects are introduced by removing one rod or by changing its radius, shape or dielectric constant in order to localize the light. Line defects are another type of defects which are introduced by removing a line or multiple lines of rods or by changing the dielectric constant or the radius of the line rods to form a waveguide through which the light is directed to the desired destination (Kannaiyan et al.
2017). The line defects take several structures to serve a designed function. For example, the line defects can take the form of Ring Resonators in which part of the signal propagates in clockwise (CW) direction and the other part propagates in counterclockwise (CCW) direction. According to the phase difference between the CW and CCW signals, the interference could be either constructive or destructive. Researchers proposed different designs for 4 × 2 PhC Encoders, using linear and non-linear materials, and using point defects, line defects, and/or ring resonators (Makvandi et al.
2021; Rajasekar et al.
2020; Mehdizadeh et al.
2017; Chhipa et al.
2022; Saranya and Shankar
2021; Latha et al.
2022a,
b; Arunkumar et al.
2022; Fallahi et al.
2021; Khatib and Shahi
2020; Saranya and Rajesh
2020; Mostafa et al.
2019b), beside using cross-connected waveguides (Haddadan and Soroosh
2019), graphene stack in the center of a photonic crystal for approaching the encoding operation (Haddadan et al.
2020).
In this work, we propose two new designs for 4 × 2 PhC encoders. Our proposed designs have simple structures that can be fabricated easily. They do not contain any non-linear material. Also, the proposed structures are designed using only silicon, which is the most commonly used material in optical integrated circuits. Our designs have small footprints, reasonable contrast ratios and good response times.
This paper is organized as follow: Sect.
2 discusses the general design steps for PhCs devices. Section
3 describes the design of the first proposed encoder (ENC_1), its working principles and its simulation results. Section
4 describes the design of the second proposed encoder (ENC_2), its working principles and its simulation results. Finally, Sect.
5 presents the conclusion including a comparison between the proposed designs and recent previously published ones.