960 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 13, JULY 1, 2010 Simultaneous Implementation of Photonic Logic OR and AND Gates for CSRZ-OOK Signals Songnian Fu, Wen De Zhong, Senior Member, IEEE, P. Shum, Senior Member, IEEE, and Chinlon Lin, Fellow, IEEE Abstract—A simultaneous implementation of photonic OR and AND logic gates for 10-Gb/s carrier-suppressed return-to-zero on–off-keying (CSRZ-OOK) signals is demonstrated using de- generate and nondegenerate four-wave mixing in a 60-m highly nonlinear photonic crystal fiber (HNL-PCF). Two different wave- length combinations of three-input lights, two input CSRZ-OOK data together with a CSRZ-OOK clock, are proposed to flexibly select the output wavelength of two logic AND gates and one logic OR gate. Experimental demonstrations at 10-Gb/s CSRZ-OOK format with clear waveforms, correct spectra, and bit-error rate verify the logic integrity of the scheme. Although three input wavelengths with a fixed spacing must be reserved in advance, the proposed scheme is still promising for future ultra-high-speed optical signal processing applications, due to the femtosecond response of the HNL-PCF nonlinearity. Index Terms—All-optical signal processing, four-wave mixing (FWM), modulation format, photonic logic gate. I. INTRODUCTION P HOTONIC logic gates are crucial elements in future optical networks to implement various optical signal processing functions such as data encryption, header recogni- tion, parity checking, and label swapping. Boolean logic OR and AND gates are essential logic gates and are the basis for forming more complex all-optical functional blocks, modules, or subsystems. In the past few years, many approaches using highly nonlinear fibers (HNLFs), semiconductor optical am- plifiers (SOAs), and waveguides have been proposed to realize a single logic function [1]–[3] or implement reconfigurable logic functions by varying the input power, the polarization, or the operation wavelength [4]–[6]. The task becomes more complex when two or more logic gates need to be realized simultaneously. All-optical multiple logic gates with XOR, NOR, OR, and NAND functions have been simultaneously realized for nonreturn-to-zero on–off-keying (NRZ-OOK) signals using a two-parallel SOA-Mach–Zehnder interferometer (SOA-MZI) structure [7]. Recently, a scheme of simultaneously realizing all-optical logic AND and NOR gates for NRZ differential phase-shift keying (NRZ-DPSK) has been proposed and demonstrated based on a delayed interferometer (DI) and Manuscript received November 29, 2009; revised March 31, 2010; accepted April 09, 2010. Date of publication April 19, 2010; date of current version June 09, 2010. This work was supported by Project 052 101 0107 of the Agency for Science, Technology and Research (A*STAR), Singapore and Open Fund of Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), Ministry of Education, China. The authors are with the Network Technology Research Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, Sin- gapore 637553, Singapore. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2010.2048424 Fig. 1. Experimental setup of photonic logic unit. LD: laser diode. two SOAs [8]. However, the output formats of the signals are changed from DPSK to OOK. Meanwhile, most existing schemes of photonic logic gates are restricted to conventional NRZ and RZ formats. The carrier-suppressed return-to-zero (CSRZ) OOK format is another widely used modulation format, as the CSRZ-OOK is a good candidate for the long-haul trans- mission due to its resilience to the fiber nonlinearity and group velocity dispersion [9]. However, because of the phase alterna- tion between 0 and at each bit transition, it is challenging to realize photonic logic gates for the CSRZ-OOK format. Except for the AND logic gate using nonlinear effects of periodically poled lithium niobate (PPLN) [10], no other photonic logic for CSRZ-OOK has ever been reported, not even simultaneous implementation of multiple photonic logic gates. In this letter, based on both degenerate four-wave mixing (D-FWM) and nondegenerate FWM (ND-FWM) arising in highly nonlinear photonic crystal fiber (HNL-PCF), we propose and demon- strate an all-optical scheme to simultaneously achieve logic OR and AND functions for CSRZ-OOK signals. Compared with the SOA-based approaches, our scheme can be potentially operated at terabits per second thanks to the almost immediate response of fiber nonlinearity. Moreover, two input wavelength combinations are proposed in order to flexibly select the output wavelength of two logic AND gates and one logic OR gate. Such a simultaneous multifunction logic unit is relatively compact and promising for future optical signal processing applications. II. OPERATIONAL PRINCIPLE AND EXPERIMENTAL SETUP Fig. 1 shows the experimental setup of the proposed pho- tonic logic unit. Continuous-wave (CW) lights from two lasers (LD 1 and LD 3) at the wavelength of and are com- bined by a coupler, and then externally modulated by two cascaded Mach–Zehnder modulators (MZMs) to generate two CSRZ-OOK signals. MZM 1 is used to generate NRZ signals driven by a pseudorandom binary sequence (PRBS) electrical NRZ signal provided by a pulse pattern generator (PPG). MZM 2 serves as a pulse carver to transform the NRZ 1041-1135/$26.00 © 2010 IEEE