Electrical mode-hopping noise in external-cavity semiconductor lasers and mode-hopping elimination by a nonoptical control loop

The longitudinal mode-hopping and the associated terminal electrical noise in AlGaAs buried heterostructure lasers in an external-cavity configuration are investigated. It is found that the electrical mode-hopping noise has a
1/f
-dependence when two external-cavity modes are involved in the mode-hopping. Furthermore, it is found that the optical feedback can suppress the electrical noise significantly below the noise level of the free-running laser. This corresponds to a reduction of the emission line width which is in agreement with the reduction of the electrical noise level. A novel, nonoptical, scheme for active elimination of mode-hopping attributable to thermal drift or mechanical disturbances in CW-operated external-cavity lasers is also proposed and demonstrated. Using this method, where the electrical noise is used as an error signal in a control loop, long-term single external-cavity mode operation and low electrical noise and optical intensity noise levels were maintained.