Electrical locking bandwidth of a diffraction grating external cavity mode-locked semiconductor laser

Mode-locking in a semiconductor laser is a highly resonant quantum process which takes place when the large modulation signal applied to the laser is equal to the frequency separation between the external cavity modes. This cavity is formed by placing an external diffraction grating at some distance from the laser diode antireflection coated facet. Short optical pulses, rich in harmonic content, are obtained at the output. If the drive signal is detuned from its optimum value the optical modes will no longer be locked. In this case reduced harmonic power with a significant noise floor and a broad optical spectrum is obtained. Mode-locking works efficiently around a certain locking bandwidth which has been related to the pull-in time (T_pi). In the paper the authors show how this locking bandwidth can be measured by an electrical spectrum analyser and related to the optical system bandwidth and the pull-in time