Optical EngineeringInstability in self-pulsation in laser diodes and its effect on all-optical synchronization
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The effect of short- and long-term frequency instability in self- pulsation on all-optical synchronization using a twin-section laser diode is experimentally investigated. Short-term frequency instability broadens the unlocked full width at half maximum (FWHM) of the fundamental of the ii spectrum of the self-pulsating laser diode. We show experimentally that the value of the unlocked FWHM, and thus the level of short-term instability, has a direct effect on the optical power required to maintain synchronization. A novel means of reducing the FWHM is presented, based on a reflective transmission line stub connected to the absorber of the twin-section self-pulsating laser diode in use. A reduction of up to 5 dB in the average optical power required for effective synchronization is observed. Long-term frequency instability can prevent synchronization from taking place because of the limited lock-in frequency range of a self-pulsating laser diode. It is shown that for the devices used here, the dominant cause of long-term instability is temperature. A new method of sensing the temperature in a twin-section laser, called absorber current temperature sensing, which reduces the measured unsycnchronized frequency drift by more than 5:1, is demonstrated. The results have important implications for the design of new all-optical synchronization subsystems, based on self-pulsating laser diodes.