Single-mode quantum cascade lasers employing a candy-cane shaped monolithic coupled cavity

We demonstrate single-mode quantum cascade lasers emitting at ∼4.5 μm by employing a monolithic "candy-cane" shaped coupled-cavity consisting of a straight section connecting at one end to a spiral section. The fabrication process is identical to those for simple Fabry-Perot-type ridge lasers. Continuously tunable single-mode emission across ∼8 cm ^-1 with side mode suppression ratio up to ∼25 dB and a single-mode operating current range of more than 70% above the threshold current is achieved when the lasers are operated in pulsed-mode from 80 K to 155 K.