Longitudinal mode competition in chirped grating distributed feedback lasers

Gain competition often inhibits the simultaneous lasing of multiple longitu dinal modes in homogeneously broadened laser systems. A stability analysis is developed to demonstrate that the two lowest order degenerate longitudin al modes in an index-coupled distributed feedback (DFB) laser will lase sim ultaneously when the index grating is chirped asymmetrically along the axis of the device. This chirped grating structure is shown to decrease gain co mpetition by reducing the spatial overlap between the degenerate modes. Sta ble mode beating between the two lowest order lasing modes results, and thi s beating produces high-frequency output self-pulsations which can be used for millimeter-wave and soliton pulse train generation. An exact closed-for m 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.