A Simple Interferometric Method for Preventing Mode-Hopping in Tunable External-Cavity Semiconductor Lasers.
11 July 2007
A problem in frequency tuning of external-cavity lasers is mode-hopping between neighboring external cavity modes. An interferometric method is proposed for monitoring mode-hopping in external-cavity semiconductor lasers. The mode-hopping events in a 1.3micron grating-tuned external-cavity laser were analyzed using this method. The average frequency of mode- hopping for a 7.5-cm external-cavity length was estimated and was expressed as f sub c = 2.7 x 10 sup 6 x exp[-1.7/(I/I sub th -1)] (Hz) for 0.07 <= (I/I sub th -1) <= 0.8 (I = injection current and I sub th = threshold current). The mode-hopping monitoring signal was negatively fed back to the laser using an automatic control circuit which maintained single-mode operation while the wavelength of the grating external-cavity laser was tuned.