Abstract

There is a critical need for high-power mid-infrared diode lasers to be used in such military and commercial applications as IR countermeasures, IR illumination, and long-range chemical sensing. To date, the highest reported cw output power from a semiconductor diode emitting in the 3-5 μm spectral region has been 215 mW/facet. This was obtained from a 250-μm stripe at 80 K,¹ and cw operation has never been observed in a III-V diode laser above 175 K.² Although output powers exceeding 1 W are readily attainable from near-IR (λ ≈ 1 μm) lasers operating at or near ambient temperature, mid-IR emitters are inherently at a disadvantage due to the inverse scaling of the differential slope efficiency (dP/dI) with wavelength. That is, while the same current is required to inject one electron-hole pair as in a near-IR diode laser, the energy of the photon that results is 3-5 times smaller. A recent breakthrough has been the demonstration that this fundamental limitation may be circumvented by employing a cascade geometry. The unipolar quantum cascade laser (QCL) of Faist et al.,³ which achieves lasing due to optical intersubband transitions, can in principle emit as many photons for each injected electron as there are periods in the structure. However, high cw operating temperatures and large cw output powers have not yet been reported, in part because the threshold current density is inevitably rather large owing to a rapid nonradiative phonon relaxation of the population inversion.

© 1997 Optical Society of America