760 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 9, MAY 1, 2012 Transmission Over 608 km of Standard Single-Mode Fiber Using a 10.709-Gb/s Chirp Managed Laser and Electronic Dispersion Precompensation Abdullah S. Karar, Member, IEEE, John C. Cartledge, Fellow, IEEE , and Kim Roberts, Member, IEEE Abstract— An electronic dispersion precompensation scheme for a chirp-managed directly modulated laser is described and experimentally demonstrated for transmission at 10.709 Gb/s. A single look-up-table (LUT) for the drive current is designed to mitigate the effects of fiber dispersion and the intrinsic nonlinear modulation response of the laser. Experimental results show that an 11-bit LUT can compensate the dispersion of 608 km of a standard single-mode fiber with a required optical-signal-to-noise ratio of 14.5 dB at a bit-error ratio of 3.8 × 10 -3 . Index Terms— Chirp managed laser, digital signal processing, digital to analog converter, electronic dispersion compensation. I. I NTRODUCTION D IRECTLY modulated lasers (DMLs) offer a promising solution for cost-sensitive metro links due to their small footprint, low power dissipation and high output optical power. However, DMLs exhibit a high wavelength chirp, which limits the transmission distance to below 20 km at 10-Gb/s. In metro networks with transmission distances up to 600 km there is considerable interest in 10-Gb/s systems without bulky and expensive optical dispersion compensation modules (ODCM). The chirp managed laser (CML) provides a compact and convenient dispersion tolerant transmitter with a reach of 200 km. It utilizes a DML biased at about 5 times the threshold current and an optical spectrum reshaper (OSR) [1]. To extend the reach of CMLs beyond 200 km without ODCMs, disper- sion can be compensated electronically at the transmitter or receiver. Although electronic dispersion compensation (EDC) at the receiver can mitigate dispersion without pre-coding or prior knowledge of the fiber dispersion, it is limited to a reach of about 300 km due to the loss of phase information with direct detection [2]. To overcome this drawback, electronic dis- persion precompensation can be performed at the transmitter. For Mach–Zehnder modulators, the determination of the pre-compensating drive voltages is relatively straightforward Manuscript received January 18, 2012; revised February 4, 2012; accepted February 5, 2012. Date of publication February 13, 2012; date of current version April 11, 2012. A. S. Karar and J. C. Cartledge are with the Department of Electrical and Computer Engineering, Queen’s University, Kingston, ON K7L 3N6, Canada (e-mail: abdullah.karar@queensu.ca; john.cartledge@queensu.ca). K. Roberts is with Ciena Corporation, Nepean, ON K2H 8E9, Canada (email: kroberts@ciena.ca). 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.2012.2187781 since a simple model of the modulator is sufficiently accurate for the back-calculation. The two drive voltages for a modu- lator are used to generate the required amplitude and phase of the transmitted optical signal. Digital signal processing (DSP) combined with two 6-bit 22 GSa/s digital-to-analog converters (DACs) implemented within a single application specific integrated circuit (ASIC) enabled transmission of 10-Gb/s differential phase shift keying signals over 3840 km of standard single mode fiber (SMF) [3]. However, for a CML the description of the modulation dynamics is more complicated and the amplitude and phase of the transmitted optical signal cannot be generated inde- pendently by a single drive current. One possible solution is to perform digital pre-shaping through phase-shaped binary transmission. A 1-bit DSP driver with a uni-polar boosting function has been shown to extend the reach of a CML to 360 km [4]. An alternate solution involves shaping the drive current through a nonlinear processing unit which is optimized for a minimum bit error ratio (BER) at a specified transmission distance. This technique has been shown to extend the reach of a DML to 250 km (see [5] and references therein). In this letter, we apply this method to substantially extend the reach of a commercially available CML modulated at 10.709-Gb/s to 608 km using a 6-bit 21.418-GSa/s DAC. The presence of the OSR considerably improves the EDC algorithm as it de- couples the optical power and chirp at the output of the CML. This relaxes the optimization and enables longer reach. II. OFFLINE PROCESSING A schematic of the pre-compensating transmitter is shown in Fig. 1(a). The system uses the non-return-to-zero (NRZ) intensity modulation format. The input data bit stream is mapped to digital samples of the pre-compensating drive current, with 2 samples per bit, using a look-up table (LUT) as part of a DSP block. The LUT entries are pre-calculated using offline processing. To determine the entries of this LUT, the effect of the modulated power and chirp on pulse propagation and the nonlinear mapping between the input current and the output optical power of the DML must be considered. With only one control variable (i.e., the drive current), the near optimum power P (t ) at the output of the DML which results in a low BER at the receiver is first determined. Then the appropriate drive current I (t ) is found that generates that 1041–1135/$31.00 © 2012 IEEE