OPTOELECTRONICS LETTERS Vol.10 No.6, 1 November 2014 A high performance all-optical set-reset flip-flop based on SOA-MZI ∗ Jahanshir Sohrabtash, Abbas Zarifkar ∗ , and Mohammad Hossein Sheikhi Department of Communications and Electronics, School of Electrical and Computer Engineering, Shiraz University, Shiraz 7134851154, Iran (Received 1 September 2014) ©Tianjin University of Technology and Springer-Verlag Berlin Heidelberg 2014 A set-reset all-optical flip-flop (SR-AOFF) based on semiconductor optical amplifier Mach-Zehnder interferometer (SOA-MZI) is proposed. Simulation results show that low switching energy in the femto joule range, the transition time of less than 20 ps, high stability and high extinction ratio (ER) of 30 dB can be achieved, while AOFF output is power insensitive approximately. Document code: A Article ID: 1673-1905(2014)06-0430-4 DOI 10.1007/s11801-014-4156-x ∗ E-mail: zarifkar@shirazu.ac.ir An all-optical flip-flop (AOFF) is a key building block in the next generation photonic transmission and com- putation systems. AOFF also has the potential to be used as a fundamental module for more advanced func- tionalities, like optical random access memory (RAM) cells [1,2] , clock generation [3] and optical packet switch- ing [4,5] . Since AOFFs are sequential circuits, their out- puts depend on not only the information at the inputs but also the information in the previous state. The out- put of AOFF must have two states at least. These states can be distinguished by different output power in con- ventional reset-set flip-flop or different output wave- lengths in optical memories [6] . Several researches have been carried out to implement AOFF [7-10] . The ring laser can be extended to other AOFF schemes, where the coupled nonlinear optical elements are semiconductor optical amplifier (SOA) Mach-Zehnder interferometers (MZIs) [11] . Other AOFF implementations incorporate the adjustment of the lasing threshold in a distributed feedback (DFB) laser [12] or the polarization bistability in a cylindrical-shaped single-mode vertical-cavity surface emitting laser (VCSEL) [13] . Among the interferometric structures, SOA-MZI which uses cross-phase modula- tion (XPM) effect is the most promising candidate due to its attractive features of low energy requirement, simplicity, stability and compactness [14] . Clavero et al [15] presented a set-reset AOFF (SR-AOFF) based on SOA- MZI and passive feedback loop, which has an extinction ratio (ER) more than 20 dB and transition time less than 100 ps, while the required energy for set and reset pulses is less than 2 pJ. The basic structure of this con- figuration with some changes was used to make a T- flip-flop [16] and a D-flip-flop [17] . The frequency domain transfer function of this structure was analyzed in Ref.[18]. It is very power sensitive, so the precise power adjustments for set and reset pulses are necessary by proper operation. In this paper, an SR-AOFF based on SOA-MZI is re- ported with operation bit rates at 10 Gbit/s and 40 Gbit/s, rejected forbidden state (set=1, reset=1), transition time less than 20 ps, ER about 30 dB and very low power sensitivity. Also the output of AOFF is stable against the changes of the SOAs’ parameters. The schematic diagram of the proposed architecture for SR-AOFF is shown in Fig.1. Fig.1 Schematic diagram of the architecture for SR- AOFF A continues wave (CW) optical signal with wave- length of λ cw is launched into the input port of SOA- MZI via input coupler. The set signal at wavelength of λ set is launched into the lower arm of the MZI. Because of the existing phase shift of π between the interferome- ter branches, in absence of the set signal, the destructive interference takes place, so no CW signal is obtained at the interferometer output. When a set pulse is injected into the lower arm of the MZI, it enters SOA-2, and