Abstract
The linewidth enhancement factor α of a semiconductor laser has been widely studied in recent years. In optical communications, the stable signal transmission is attributed to the lower α. In tradition, α is considered as a constant value. Thus, we propose to use the external control to modulate α under the influences of optical feedback with different feedback strengths, external cavity lengths, and feedback phases. By optically injecting the laser, the Hopf bifurcation curve corresponding to the upper frequency boundary of the stable locking region can be obtained. In the experiment, a pellicle beamsplitter mounted on a PZT stage placed on a linear translation stage is used as the reflector, where the external cavity length can be adjusted continuously from the long cavity regime to the short cavity regime with phase accuracy. The boundary between the long cavity regime to the short cavity regime is determined by the relaxation oscillation frequency. α is found to be strongly affected by the feedback strength both in the long and short cavity regimes. Moreover, while α is insensitive to the feedback phase in the long cavity regime, it can be tuned continuously when varying the phase in the short cavity regime. With a moderate feedback strength, the α is found to increase as the feedback strength increases. Moreover, while the α is insensitive to the feedback phase in the long cavity regime, it can be tuned continuously in the short cavity regime when varying the phase. A normalized variation range of 21.59% is obtained experimentally at an external cavity length of 1.5 cm, which can be further enhanced with a shorter cavity. With an external cavity length of 0.5 cm, a normalized variation range of 63.41% is predicted with the simulation. Under such condition, α as low as 2.25 is found which is notably lower than its free-running value.