JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 15, AUGUST 1, 2008 2641 Microwave-Domain Analog Predistortion Based on Chirped Delay Lines for Dispersion Compensation of 10-Gb/s Optical Communication Signals Leonardo Ranzani, Pierpaolo Boffi, Member, IEEE, Rocco Siano, Member, IEEE, Sébastien Rondineau, Member, IEEE, Zoya Popovic, Fellow, IEEE, and Mario Martinelli, Member, IEEE Abstract—Optical chromatic dispersion compensation is achieved by analog predistortion of the signal in the microwave domain exploiting a chirped delay line at the transmitter. We designed a microwave device that guarantees high compactness of the solution and increased performance in terms of uncompen- sated propagation reach. Error-free, 10-Gb/s transmission over 225 km of uncompensated fiber is experimentally demonstrated without the use of coherent detection, digital equalizers, or optical dispersion compensators. Index Terms—Compensation, microwave circuits, modulation, optical fiber communication. I. INTRODUCTION C HROMATIC dispersion compensation in optical fiber communication systems is still an open issue. In par- ticular, in the case of dispersion-uncompensated metropolitan and regional networks, transmission over standard single mode fiber (SMF) at 10 Gb/s is strongly limited to about 80–100 km for conventional nonreturn-to-zero (NRZ) intensity-modulated, direct-detected (IM-DD) signals. Dispersion compensation can be generally achieved by either optical or electrical techniques. Optical techniques, such as the use of dispersion-compensating fibers (DCF) or chirped fiber Bragg gratings, are generally expensive and not easily reconfigurable for varying dispersion conditions. Dispersion-tolerant modulation formats can be used, such as the so-called phase-shaped binary transmission (PSBT) duobinary format, but they allow the achievement of up to about 200-km SMF propagation without compensation [1]. As an alternative, electrical dispersion compensation (EDC) techniques have been developed [2]–[5]. EDC makes use of digital equalizers, such as transversal filters [2], digital feed- back equalizers [3], or maximum likelihood sequence detectors [4], [5] placed at the optical receiver to compensate for the total accumulated optical dispersion. Using EDC, dispersion compensation for NRZ IM-DD propagation over hundreds of Manuscript received January 30, 2008; revised April 19, 2008. Current ver- sion published October 10, 2008. L. Ranzani, P. Boffi, and M. Martinelli are with Dipartimento di Elettronica e Informazione, Politecnico di Milano, Milan, Italy (e-mail: leonardo.ran- zani@polimi.it; boffi@elet.polimi.it; martinelli@elet.polimi.it). R. Siano is with Corecom, Milan, Italy (e-mail: siano@corecom.it). S. Rondineau and Z. Popovic are with the Electrical Engineering De- partment, University of Colorado, Boulder, CO 80309 USA (e-mail: se- bastien.rondineau@gmail.com; zoya@colorado.edu). 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.2008.925638 kilometers of SMF has been experimentally demonstrated at 10 Gb/s. Electrical techniques are typically cheaper than optical ones. Therefore, they are promising for low-cost metropolitan area networks applications. However, electrical equalization is usually characterized by less efficient performance, owing to the channel nonlinearity introduced by square-law photodiodes. Because of this detection nonlinearity, optical phase informa- tion is lost after detection, making dispersion compensation difficult to achieve. EDC operating in the microwave regime using mi- crostriplines to achieve analog equalization has also been proven [6]. However, this solution requires the use of coherent detection of the optical signal in order to save amplitude and phase information after square-law detection. An alternative technique to compensate for optical chromatic dispersion is based on electronic digital predistortion at the trans- mitter [7]. This technique shows better performance than elec- tronic equalization, but it requires the use of high-speed digital electronics in the transmitter. Moreover, the length of the predis- torted bit sequence increases quadratically with the bit rate, thus increasing the memory requirements at high bit rates. In [8], analog predistortion at the transmitter has been proposed in order to solve the aforementioned problems. The needed predistortion is achieved by using a microstripline and then transferring to the optical carrier by using an I/Q electrical demodulator followed by a dual-parallel Mach-Zehnder mod- ulator (DPMZM). The DPMZM is made up of a combination of three nested electro-optic modulators, which allows mod- ulation of, in a proportional way, the in-phase and quadrature components of an optical signal. Unfortunately, the solution based on simple microstriplines is intrinsically bulky and lossy, and therefore, more compact microwave components should be used in order to increase system performance and achieve better integration. In this paper, analog predistortion by using original chirped delay lines (CDLs) is described. A CDL can be built by peri- odically modulating the line width and, therefore, the charac- teristic impedance of a microstripline. The operation of these devices is similar to the Bragg gratings behavior in the optical regime. By chirping the impedance modulation period along the line, similarly to what is done to the refractive index in optical Bragg gratings, signal dispersion is obtained [9]. System prop- agation is experimentally evaluated with the proposed predis- tortion compensating about 225 km for a standard NRZ IM-DD signal at 10 Gb/s. 0733-8724/$25.00 © 2008 IEEE Authorized licensed use limited to: UNIVERSITY OF COLORADO. Downloaded on November 13, 2008 at 15:06 from IEEE Xplore. Restrictions apply.