Nonlinear Distortion Analysis of Directly Modulated ... - 연세대학교

III. Nonlinear Distortion Suppression of InjectionLocked DFB LasersThe direct laser modulation with RF-range frequency has been attracted inanalog fiber-optic systems, especially SCM systems because of its low costand simplicity. However, as shown in section II, when lasers are modulatedwith RF-range electrical signals, the laser diode (LD) nonlinearities become akey issue in the system performance because it can impose signal distortions.These distortions cause inter-channel interference, which limits the number ofchannels as well as transmission distance. To cope with this distortion problem,several methods are introduced such as feedforward compensation [7],predistortion [14, 15], and electro-optical feedback methods [6]. In 1997, it hasbeen theoretically shown that the intrinsic dynamic distortion can besubstantially suppressed by using optical injection locking to increase therelaxation oscillation frequency [16]. In 1999, X. J. Meng et. al. has shown theexperimental result of distortion suppression by optical injection locking [5].The injection locking method is that when an external laser source (MasterLaser: ML) light is injected into an operating laser (Slave Laser: SL), the SL islocked to the ML, and the operation of the SL follows the ML. When lockingoccurs, relaxation oscillation frequency of the SL increases. In RF-range (>1GHz) laser modulation, the distortion products are greatly influenced by laserrelaxation oscillation. Therefore, the relaxation oscillation frequency increasecan reduce the laser nonlinear distortion products. Furthermore, the injectionlocking technique has another merits such as laser narrow linewidth, chirpreduction, and mode partition noise reduction. Therefore, by using injection21