1077-260X (c) 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSTQE.2019.2948051, IEEE Journal of Selected Topics in Quantum Electronics > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract—Ultrafast all-optical memory at 250 Gb/s using Mach-Zehnder interferometers with symmetrical quantum-dot semiconductor optical amplifiers, acting as all-optical AND gate and regenerator in a loop configuration, is theoretically demonstrated for 500 loop circulations using a return-to-zero modulated format. The memory operation is examined and evaluated by the quality factor (QF), which is a more sensitive method of measuring memory quality. The obtained numerical results show that the proposed memory can be operated up to 250 Gb/s for 500 circulations with high QF. Index Terms—Logic devices, Optical memories, Quantum dots, Semiconductor optical amplifiers I. INTRODUCTION Optical packet switching (OPS) is a widespread technology in telecommunication systems because it processes the signal in the all-optical domain in respect of its characteristics of higher operation rate, power efficiency, and transparency [1]. However, a high-speed memory is necessary for developing OPS networks in order to avoid packet collisions during packet routing. Moreover, memory is used in applications of long-term data storage/buffer [1]. The types of memories are divided into two main categories, i.e., pulse preserving and pulse regeneration. The pulse preserving is the simplest method to store optical data by sending the signal down additional lengths of optical fiber. However, the storage period is limited by the signal degrading effects such as dispersion and attenuation. The alternative is the pulse regeneration where the effects of long fiber lengths are reduced to a potentially negligible level. The optical data can be stored for a long-term using a loop structure where the optical signal is injected and kept recirculating for several circulations [2]. Typically, the operation of all-optical memory is based on the logic operations, which are the main core unit for the signal processing systems. The operation Manuscript received Sep. 03, 2019. A. Kotb is with the Guo China-US Photonics Laboratory, Changchun Institute of Optics, Fine Mechanics, and Physics, Chinese Academy of Sciences, Changchun 130033, China and with the Department of Physics, Faculty of Science, University of Fayoum, Fayoum 63514, Egypt. (e-mail: amer@ciomp.ac.cn). C. Guo is with the Guo China-US Photonics Laboratory, Changchun Institute of Optics, Fine Mechanics, and Physics, Chinese Academy of Sciences, Changchun 130033, China and with the Institute of Optics, University of Rochester, Rochester, NY 14627, USA. (e-mail: guo@optics.rochester.edu). principle of these logic gates depends on the optical nonlinearity characteristics of the optical devices such as a nonlinear loop mirror, nonlinear optical fiber, photonic crystal, and semiconductor optical amplifier (SOA) [3]-[7]. The SOA, on the other hand, is a very competitive device due to its strong nonlinearity, low-power consumption, compact size, wide bandwidth, and potential integration with other optoelectronic devices [3]-[5]. These features have made SOAs popular candidates as nonlinear elements in investigating the performance of Boolean functions [4]-[23]. Despite these attractive features, the SOA operating speed is hardly exceeded ~ 100 Gb/s due to the slow SOA phase and gain dynamics and hence cannot be upgraded as data rates get higher [24]. On the other hand, placing quantum-dots (QDs) in the SOA active region in order to reduce the carrier relaxation time (i.e. 300 fs–10 ps) has been followed so far to overcome the SOA limitation problem. Moreover, compared to bulk SOAs, QDSOAs exhibit higher saturation power [25], [26], wider gain bandwidth [27], [28], lower noise figure [25], [29], lower temperature dependence [30], [31], lower amplified spontaneous emission (ASE) noise [32], and above all, faster gain recovery time [33], [34]. These unique properties exhibited by QDs made the QDSOAs effectively suitable for investigating ultrafast all-optical Boolean functions with better performance at higher data rates up to 2 Tb/s [4], [35]-[48]. Therefore, a lot of efforts have been devoted to realizing all-optical long-term memory using (QD)SOAs [49]-[54]. Best to our knowledge, these great efforts are not convincing in terms of the stability requirement of long-term memory [49]-[52] and only 42 and 20 loop circulations were demonstrated in Refs. [51] and [52], respectively. Furthermore, the operation speed of the memory is limited by the slow recovery time of SOA when using two Mach-Zehnder interferometers (MZIs) with four SOAs operated at 80 Gb/s in a loop configuration used for on-off-keying modulation format as in [53], or when using a single SOA in parallel with one MZI with two QDSOAs operated at 100 Gb/s used for pulse amplitude format as in [54], the errors generated by the finite SOAs recovery time will become apparent after a few circulations [53], [54]. Thus, here, the performance of ultrafast all-optical memory using MZIs with symmetrical QDSOAs, acting as all-optical AND gate and regenerator in a loop configuration, used to store 250 Gb/s for 500 circulations is theoretically demonstrated. In this study, the memory loop includes two parallel MZIs with four symmetrical QDSOAs in order to increase the number of memory circulations up to 500 at 250 Gb/s with better performance. The MZI effectively combines many attractive characteristics, such as the compact size, high-temperature stability, possibility to control the phase in each arm, low Theoretical demonstration of 250 Gb/s ultrafast all-optical memory using Mach-Zehnder interferometers with quantum-dot semiconductor optical amplifiers Amer Kotb and Chunlei Guo