1950 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 13, JULY 1, 2008 On-Chip NRZ-to-PRZ Format Conversion Using Narrow-Band Silicon Microring Resonator-Based Notch Filters Linjie Zhou, Hui Chen, Student Member, IEEE, and Andrew W. Poon, Member, IEEE Abstract—We report experimental demonstration and modeling of all-optical on-chip non-return-to-zero (NRZ) to pseudo-re- turn-to-zero (PRZ) format conversion using narrow-band silicon microring resonator-based notch filters. Our proof-of-prin- ciple experiment using a carrier-injection-based tunable silicon microring resonator demonstrates NRZ-to-PRZ conversion at 3.6 Gbit/s. Our Fourier-transform-based modeling reveals in detail the format conversion dependence on the microring resonance Q factor, extinction ratio, phase response, and NRZ signal transition times, assuming signal format conversion of 40 Gbit/s. Index Terms—All-optical clock recovery, integrated optics, notch filters, optical resonators, optical signal processing, silicon photonics, waveguides. I. INTRODUCTION S ILICON microresonators with their key merits of compat- ibility with complementary metal–oxide–semiconductor (CMOS) microelectronics fabrication processes, narrow-band wavelength agility, micrometer-scale device footprint, and accessibility with integrated wire waveguides offer one of the building blocks for large-scale-integrated photonic circuits on a silicon chip. Recently, various optical signal-processing devices and circuits based on silicon microresonators have been proposed and demonstrated including multistage high-order microring-resonator add–drop filters [1], ultracompact optical buffers [2], carrier-dispersion-based modulators [3]–[6], and wavelength converters [7]. Previously, we proposed an all-optical on-chip non-return-to- zero (NRZ) to pseudo-return-to-zero (PRZ) format conversion using silicon second-order coupled-microring resonator-based notch filters [8]. Our approach potentially constitutes a highly integrated linear optics method to high-data-rate NRZ clock re- covery on a silicon chip. It is well known that NRZ-to-PRZ format conversion is essential for clock recovery from NRZ data format, which in Manuscript received September 14, 2007; revised November 2, 2007. Pub- lished August 29, 2008 (projected). This work was supported by the Research Grants Council of The Hong Kong Special Administrative Region, China, under Projects HKUST6112/03E and 618505. L. Zhou was with the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Hong Kong SAR, China. He is now with the Department of Electrical and Computer Engineering, University of California at Davis, Davis, CA 95616 USA (e-mail: ljzhou@ucdavis.edu). H. Chen and A. W. Poon are with the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Hong Kong SAR, China (e-mail: chenh@ust.hk; eeawpoon@ust.hk). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2007.913742 principle carries no clock component. Conventional approaches to converting NRZ data format to PRZ data format (which carries a clock component) include using i) nonlinear semicon- ductor optics upon self-phase modulation in a semiconductor optical amplifier (SOA) [9]–[11] and ii) linear optics such as fiber Bragg grating (FBG)-based notch filters for suppressing the optical NRZ carrier frequency component [12], Fabry–Perot optical bandpass or comb filters for transmitting the weak clock components [13]–[16], and polarization-maintaining fibers [17]–[19] and integrated asymmetric Mach–Zehnder interfer- ometer (AMZI) for generating relative delays [20]–[22]. However, nonlinear semiconductor optics methods impose high input signal power and slow operation speed limited by SOA carrier dynamics. Also an additional bandpass filter is needed in order to filter the frequency-chirped SOA signals to form the PRZ pulses, suggesting complicated designs and processes for on-chip integration. In contrast, linear optics approaches feature low complexity, and no active components are involved. Nonetheless, most conventional linear optics ap- proaches are either fiber-based or based on bulk optical filters, and thus they are not readily applicable to on-chip optical signal processing. Although AMZI converters have been integrated on a silicon chip [22], the long AMZI arms needed to generate 10’s ps relative delays between them still impose a relatively large device footprint. In this paper, we detail our systematic experimental study and modeling of the proposed all-optical NRZ-to-PRZ format con- version using narrow-band silicon microring resonator-based notch filters. The rest of the paper is organized as follows. Section II briefly reviews the principle of using linear filtering for format conversion, Section III details our proof-of-principle experiment, Section IV presents the modeling and analysis, and Section V concludes this paper. II. PRINCIPLE Fig. 1 illustrates the principle of all-optical NRZ-to-PRZ format conversion using a microring resonator-based notch filter. The concept is common to other linear optic filtering ap- proaches to signal processing. Like the conventional FBG-based notch filter approach [12], the microring notch filter can sup- press the optical NRZ carrier frequency component at the microring resonance and thereby effectively enhance the RF clock signal in the sidebands. Fig. 1(a) illustrates the format conversion in time domain. An optical NRZ waveform is launched into the bus waveguide with the carrier wavelength at the microring notch filter center 0733-8724/$25.00 © 2008 IEEE