Longitudinal Mode Competition in Chirped Grating Distributed Feedback Lasers.pdf

1402 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 35, NO. 10, OCTOBER 1999 Longitudinal Mode Competition in Chirped Grating Distributed Feedback Lasers Kim A. Winick, Senior Member, IEEE Abstract—Gain competition often inhibits the simultaneous las- ing of multiple longitudinal modes in homogeneously broadened laser systems. A stability analysis is developed to demonstrate that the two lowest order degenerate longitudinal modes in an index- coupled distributed feedback (DFB) laser will lase simultaneously when the index grating is chirped asymmetrically along the axis of the device. This chirped grating structure is shown to decrease gain competition by reducing the spatial overlap between the degenerate modes. Stable mode beating between the two lowest order lasing modes results, and this beating produces high-frequency output self-pulsations which can be used for millimeter-wave and soliton pulse train generation. An exact closed-form expression for the output intensity of an antisymmetrically chirped index-coupled DFB laser, as a function of the unsaturated gain, is also derived. The expression is valid for arbitrary levels of gain saturation. Index Terms— Chirped grating, distributed feedback (DFB) laser, mode competition, waveguide laser. I. INTRODUCTION I T IS WELL known that the two lowest order modes of a uniform index-coupled distributed feedback (DFB) laser are degenerate and symmetrically located about the grating stopband [1]. Experimental results indicate that single-mode operation of these devices is possible, though multimode behavior is often observed. Zhang et al. explained these observations by numerically solving the time-dependent DFB laser coupled-mode equations in the presence of envelope gain saturation and population-dependent refractive index vari- ations [2]. Using a similar approach, Liao and Winful [3] and Wake [4] recently predicted, theoretically, that an anti- symmetrically chirped single-section DFB laser can undergo sustained high-frequ