The response characteristics of direct current modulation on an injection-locked semiconductor laser are investigated. When a semiconductor laser is strongly injection locked in a stable locking state, the benefits of a significantly enhanced modulation bandwidth, a broadband noise reduction, and a large modulation dynamic range can be realized. A reduction on the frequency chirping can also be realized at a sufficiently large modulation index, though not at small modulation indices due to the effect of the laser noise. In a locking-unlocking bistable state, a large modulation current can unlock the laser. In a state near or beyond the Hopf bifurcation boundary, the dynamic instability of the laser can lead to high broadband noise and large frequency chirping. The effects of the distortion and the noise compression on the response of the current modulation with digital signals are investigated with eye patterns.
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