Simple interferometric method for preventing mode hopping in tunable external-cavity semiconductor lasers

A problem in frequency tuning of narrow-linewidth external-cavity lasers is mode hopping between neighboring external-cavity modes. Mode hopping occurs in both the grating-tuned external-cavity laser1,2 and the monolithic integrated tunable distributed Bragg reflector (DBR) laser.3 We demonstrate an interferometric method for mode-hopping detection and an automatic control circuit that maintains single-mode operation while the frequency of a grating external-cavity laser is tuned. The same method can be used to control a monolithic integrated external-cavity DBR laser.