2568 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 11, NOVEMBER 2004 Automatic Tunable Chromatic Dispersion Compensation at 40 Gb/s in ASK and DPSK, NRZ, and CSRZ 263-km Transmission Experiments D. Sandel, S. Bhandare, A. F. Abas, B. Milivojevic, R. Noé, Martin Guy, and Martin Lapointe Abstract—Chromatic dispersion (CD) in single-mode optical fiber distorts pulses and is a big obstacle for the installation of long-haul dynamically routed transmission systems, especially at 40 Gb/s. Here the automatic residual CD compensation of a 263-km fiber link in the range 300 to 700 ps/nm is demon- strated for nonreturn-to-zero amplitude-shift keying (NRZ-ASK), carrier-suppressed return-to-zero (CSRZ)-ASK, NRZ differential phase-shift keying (DPSK), and CSRZ-DPSK modulation formats at 40 Gb/s. A thermally tunable dispersion compensator minimizes residual CD, which is measured by a synchronous arrival time detection scheme. The measured transmission penalty with online CD compensation is between 1.2 dB for various link lengths and compensated CD values. Index Terms—Arrival time detection, chromatic dispersion (CD), optical fiber transmission, tunable dispersion compensator. I. INTRODUCTION T UNABLE chromatic dispersion (CD) compensation is needed in long-haul and dynamically routed transmission links, especially at 40 Gb/s. Various integrated optical disper- sion compensators [1]–[5] have been demonstrated but fiber Bragg gratings (FBGs) exhibit the largest dispersion and lowest insertion loss with an associated tunability. Recent advances in FBG technology now allow the realization of single and multichannel tunable devices [6]. Among many CD detection schemes, synchronous arrival time detection with a sensitivity limit of 100 attoseconds [7] is the most promising option because the scheme has an extremely low incremental cost, provides the sign of CD, responds in 1 ms and is usable for various modulation formats [8]. Prior to this, a similar method was reported by Takushima and Kikuchi [9], but arrival time detection was asynchronous. As a consequence, the required frequency deviation was larger, the measurement interval was longer, and the sign of dispersion remained ambiguous. The tolerance to residual CD with respect to in-line CD compensation ratio for various modulation formats including nonreturn-to-zero amplitude-shift keying (NRZ-ASK), carrier- suppressed return-to-zero (CSRZ)-ASK, NRZ differential phase-shift keying (DPSK), and CSRZ-DPSK was evaluated Manuscript received May 25, 2004; revised June 23, 2004. D. Sandel, S. Bhandare, A. F. Abas, B. Milivojevic, and R. Noé are with Department of Optical Communication and High Frequency Engineering, Uni- versity of Paderborn, 33098 Paderborn, Germany (e-mail: suhas@ont.upb.de; noe@upb.de). M. Guy and M. Lapointe are with TeraXion, Sainte-Foy, QC G1P 4N3, Canada (e-mail: mguy@teraxion.com). Digital Object Identifier 10.1109/LPT.2004.834889 Fig. 1. Chromatic dispersion compensation (CDC) setup for 40-Gb/s ASK and DPSK transmission experiments. numerically in [10] at 43 Gb/s. To our knowledge, we report for the first time an automatic CD compensation for all these modulation formats. We conduct 40-Gb/s transmission exper- iments, with a commercially available FBG-based thermally tunable dispersion compensator and synchronous arrival time detection. II. TRANSMISSION SETUP A small pump current modulation of a distributed-feedback (DFB) laser will not only modulate the optical power but also the optical frequency. At low frequencies, there can be a consid- erable phase lag between pump current and frequency modula- tion because the latter depends not only on the carrier density but likewise on the chip temperature which is modulated by the pump current with an intrinsic delay. In the presence of CD with a dispersion coefficient and a fiber length , a frequency ex- cursion causes an arrival time delay . It can be neglected in comparison with the eye closure as long as . In the following, this will be the case. It allows the small-signal modulation frequency to be chosen so high that it is outside the phase-locked loop bandwidth of the clock re- covery in the receiver. Fig. 1 shows the transmission setup. A DFB laser at 192.5 THz (1557.366 nm) is modulated with a 5-MHz sinusoidal source to provide 1.8% [root mean square (rms)] power modulation and 336-MHz (rms) frequency modulation. The 40-Gb/s pseudorandom binary sequence data obtained from a 16 : 1 1041-1135/04$20.00 © 2004 IEEE Authorized licensed use limited to: UNIVERSITATSBIBLIOTHEK PADERBORN. Downloaded on April 30,2010 at 12:33:05 UTC from IEEE Xplore. Restrictions apply.