# Design of XOR Photonic Gate Using Highly Nonlinear Fiber
## 1 Introduction  
The evolution of all-optical signal processing is increasing in demand towards the implantation of ultra-high-speed photonic networks, such as optical packet switching. The infrastructure and physical layer of such networks must be enhanced by a design that can terminate the bottleneck introduced by electronic devices and the imperfection of its functionalities. One of the components required for this realization is an all-optical logic gate. These gates can further provide advanced optical signal processing functionalities such as packet switching, optical signal regeneration, header addressing, encryption and decryption, data encoding, header recognition, parity checking, label swapping, wavelength conversion and header extraction. In general, conventional designs for photonic logic gates are based upon nonlinear effects in optical fiber such as four wave mixing, Self-Phase Modulation (SPM), Cross-Phase Modulation (XPM), and cross gain modulation in semiconductor optical amplifiers (SOA) and highly nonlinear fiber (HNLF). These nonlinear effects exist mostly in SOA and HNLF, and can be used in many applications. Namely, the Kerr nonlinear effect is defined as the third order nonlinear effect, which occurs when the refractive index changes the optical power, hence results in other secondary nonlinear phenomena such as four wave mixing, cross gain modulation, SPM and XPM. These effects can be used to implement many applications such as wavelength conversion, packet header optical processing, optical solition propagation, and optical phase conjugation as studied in [1; 2]. Furthermore, the aforementioned applications constitute a key feature for next generation transmission networks. Hence it can be based on fiber mediums that have highly efficient nonlinear effects. Also, reduced attenuation levels are mandatory to increase effective nonlinear interactions. Now conventional solutions, in particular, SOA designs for all-optical logic gates design [3; 4; 5; 6] generally result in poor optical signals due to the pattern effect. This distortion is mainly caused by the intrinsic slow gain recovery time of the SOA which results in limited values of data rates. In contrast, HNLF presents a promising alternative for SOA attributable to its highly nonlinear coefficient, tuneable zero dispersion at C and L bands, lower attenuation, low zero dispersion slope, and low splice loss to conventional single mode fibers [7; 8]. Various studies have looked at the design of photonic logic gates based on HNLF. Further, work in [9] presents an XOR gate design using polarization rotation effects. Also, work in [10] proposes a logic gate based on the Kerr effect as well in an effort to overcome SOA-based gates. However, these schemes are limited to 10 Gbps and suffer from a high HNLF length of 2 km with low nonlinear coefficients. Moreover, work in [11] leverages photonic crystals to present logic gates using a two-staged structure, i.e., a structure comprised of waveguides and ring resonators. However, the use of ring resonators imposes high control levels of the signal power because high light intensity is required for changing the refractive index of nonlinear materials here. In light of the above, the work here aims to utilize HNLF for all-optical XOR photonic logic gates to overcome the deficiencies associated with SOA, i.e., enhanced power efficiency, and ring resonators. The simplified gate structure here (based on HNLF component only) achieves high data rates with a relatively high nonlinear coefficient, as compared to existing methods in [9; 11]. Namely, a simplified XOR gate design is proposed based on a single stage HNLF. Also, the proposed design achieves high data rates (20 Gbps) with high extinction ratios. The HNLF performs the same logical operations as SOA by acting as a passive device, and hence utilizing low optical power levels. Furthermore, the proposed XOR configuration provides more flexibility to the optical transmission networks.