Single-mode quantum cascade lasers based on a folded Fabry-Perot cavity

We demonstrate single-mode quantum cascade lasers employing a folded Fabry-Perot cavity consisting of two straight sections connected by a semicircular section in a "hairpin" shape. These folded cavity lasers emitting at ∼4.5 μm are fabricated with identical processes as those for plain Fabry-Perot ridge lasers, and show a strong suppression of the comb of Fabry-Perot cavity modes, leading to tunable single-mode emission with up to 27 dB side mode suppression ratio and a single-mode operating current range of up to 60% above the threshold current when operated in pulsed mode; single-mode emission is achieved from 80 to ∼240 K.