Broadly-tunable external cavity quantum-cascade lasers

Abstract

External cavity quantum-cascade lasers (QCLs) emitting in the long-wave infrared (LWIR) around 10 µm and in the mid-wave infrared (MWIR) around 5.2 µm have been realized. The coupling facet of the QC chips was anti-reflection coated and the optical feedback was provided by a diffraction grating arranged in a Littrow configuration. The active regions of the gain elements were based on bound-to-continuum designs having broad gain curves ( ∼ = 300 cm −1 full width at half maximum). The LWIR laser was operated in pulsed mode on a thermoelectric (TE) cooler. It could be tuned over a frequency range of 150 cm −1 from 9.1 to 10.55 µm with a peak power ≥ 30 mW. The MWIR laser was operated in continuous-wave (CW) mode on a TE cooler. It could be tuned over more than 170 cm −1 from 4.95 to 5.4 µm and was single-mode over more than 140 cm −1 . Its output power was in excess of 5 mW over 130 cm −1 . This broad tunability (10 times more than that of distributed-feedback QCLs) enables new applications of QCLs in high-resolution infrared spectroscopy.

Improved two-mode dynamic model for external-cavity quantum cascade lasers under strong optical feedback

“…The device we select to model is an EC-QCL system with a bound-to-continuum active region design where operates at a center wavelength of 9.75 μm under the pulse mode at room-temperature [16,21]. Using a 1.5 mm long and 32 μm wide laser in the EC setup, the resulting tuning range of the device was from 9.11 μm (1097.7 cm −1 ) to 10.56 μm (947 cm −1 ) with a tuning of over 150 cm −1 around a free running wavelength of 9.75 μm (1025.64 cm −1 ) [21]. The key optical features of EC-QCL system with the model parameters are schematically illustrated in figure 1.…”

“…2 denotes the photon density for the certain EC optical mode, and we call ( ) l f t, ext the complex feedback coefficient. In the following, the total losses for the FP and EC optical modes can be determined as [21,28]:…”

“…Since the multiple reflections does not include, the model is valid in the limitation of weak feedback level [23]. These problems (the single mode operation and weak feedback limitation) are partially resolved in the second basic model [21,22].…”

“…Although the effects of multiple feedback terms has been modeled in [21,22] using the procedure of Hui and Tao [29] whereby the laser-cavity mirror loss ( ) ( )…”

“…Although, the impact of optical feedback on the characteristics of interband lasers has been extensively investigated in the past decades [18][19][20], but in case of the EC-QCLs there are still many aspects that should be examined and analysed. To date, the effect of an external cavity subject to weak optical feedback has been deeply studied in the EC-QCLs, both without considering the phase-amplitude coupling [21,22], and also by adopting phase-amplitude coupling via delayed feedback terms [23][24][25]. But, the influence of multiple reflections on the device characteristics, which emerges in the presence of strong feedback regimes, has not investigated so far using the rigorous model.…”