JOURNAL OF LIGHTWAVE TECHNOLOGY. VOL. zyxwvutsrqp 8. NO. zyxwvutsrqponm 5. MAY 1990 zyxwvutsrqp 699 Chirping-Induced Waveform Distortion in 2.4-Gb /s Lightwave Transmission Systems JOHN c. CARTLEDGE, MEMBER, IEEE, AND GREGORY zyxwvu S. BURLEY, MEMBER, IEEE Abstract-The performance of high-speed lightwave transmission systems operating in the 1.55-pm-wavelength region with conventional single-mode optical fiber can be influenced by the dynamic wavelength chirping exhibited by directly modulated semiconductor lasers. A re- cent paper presented the first detailed analysis of the hit error ratio performance of lightwave systems influenced by chirping-induced wave- form distortion. This paper contributes additional numerical results which address the performance implications of the waveform distor- tion by considering the dependence of the receiver sensitivity on the decision time and decision threshold, the eye pattern, and the bit error ratio pattern. It is shown that the response of the linear receiver filter can influence the dependence of the receiver sensitivity on the decision time, and that there is a strong similarity between the eye pattern open- ing and the decision-time decision-threshold deformation of the hit er- ror ratio pattern. The effects of the laser bias current, gain compres- sion factor, and linewidth enhancement factor are also considered. I. INTRODUCTION IRECTLY modulated single-longitudinal mode D semiconductor lasers exhibit a dynamic shift of the peak emission wavelength due to carrier-induced changes in the refractive index of the active region. This linewidth broadening or chirping is characterized by a shift toward shorter wavelengths during the leading edge of a pulse (blue-shift) and a shift toward longer wavelengths during the trailing edge of a pulse (red-shift) [l]. The combina- tion of the laser chirping and group velocity dispersion of conventional single-mode optical fibers can cause a per- formance penalty for high-speed systems operating in the 1.55-pm-wavelength region, as the leading and trailing edges of a pulse spread away from the pulse center [2]- [7]. The degradation in the system performance depends upon the laser structure, modulation conditions, fiber length, and receiver filtering. Previously, we presented a method of evaluating the effect of laser chirping on the bit error ratio performance of lightwave systems and illustrated its usefulness by way of numerical examples [7]. The optical power and phase of the laser, in response to the injected current waveform, are determined numerically by solving rate equations Manuscript received July 26, 1989; revised October 17, 1989. This work was supported by the Natural Sciences and Engineering Research Council of Canada under Grant A4736 and by Bellcore. J. C. Cartledge is with Bellcore, Red Bank, NJ 07701, on leave from Queen’s University, Kingston, Ont., Canada. G. zyxwvutsrqponmlk S. Burley was with the Department of Electrical Engineering, Queen’s University, Kingston, Ont., Canada. He is now with the Siemens Com- munication Systems, Phoenix, AZ. IEEE Log Number 8933729. which describe the nonlinear laser dynamics. Solutions to the rate equations are obtained for all possible 2”’ reali- zations of an m-bit truncated data sequence, as a truncated pulse train method is used to evaluate the bit error ratio. For each solution, the intensity of the signal at the output of a fiber is obtained by convolution and modulus squared operations. The lightwave receiver is modeled by a pho- todetector, additive white Gaussian noise process, linear receiver filter, sampling device, and threshold compara- tor. For each realization of the truncated data sequence, the bit error ratio is evaluated assuming that the photo- current, obtained from the semiclassical model for the de- tection of weak optical signals, is Gaussian distributed. In this paper, we present the results of additional com- puter simulations, which were generated to assist the sys- tem design process by elucidating the performance impli- cations of chirping-induced waveform distortion. Of particular interest is the dependence of the system perfor- mance on the decision time (time at which the output of the receiver filter is sampled) and the decision threshold. This issue is addressed by considering a) the dependence of the receiver sensitivity on the decision time and deci- sion threshold, b) the eye pattern, and c) the bit error ratio pattern. 11. RESULTS AND DISCUSSION The parameters used to obtain the numerical results are given in [7]. The linewidth enhancement factor, gain compression factor, and laser bias current are important parameters which affect the laser chirping. The linewidth enhancement factor is a proportionality factor between changes in the optical gain and refractive index of the las- ing medium. The optical phase varies with the injection current because a change in the carrier density affects the refractive index and, hence, the optical path length. The gain compression factor models, phenomenologically, gain nonlinearities such as spectral hole-burning. The strength of the nonlinearity affects the damping of the re- laxation oscillations and the transient and adiabatic chirp [8]. Biasing the laser at or below the threshold current enhances the relaxation oscillations, thereby increasing the transient chirp. Unless stated otherwise, the linewidth enhancement factor (Y is 5 and the bias current is 38 mA ( 1.14 times the laser threshold current). A transmission bit rate of 2.4 Gb/s was chosen because of current ad- vanced product development and standardization activi- 0733-8724/90/0500-0699$01 .OO @ 1990 IEEE