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Setup of the external cavity terahertz QCL. A small gold mirror mounted on a vacuum feedthrough is placed in front of the coated laser facet at a distance of few tens of microns. Light is collected from the other facet with a picarin lens.
characteristics of a long laser in the external cavity setup, as measured from the uncoated facet at two different heat-sink temperatures. In the inset we report the laser multimode emission spectrum collected at under a drive current of .
Laser emission spectra of the long laser as recorded at for different mirror positions showing continuous tuning of the lasing mode. The drive current was fixed at about and the spectra are shown on the same vertical scale. The emission peak widths are limited by the FTIR resolution of . Inset: laser emission frequency as a function of mirror position for this laser (squares) and for a second one long (circles). Solid lines are linear fits with the slope fixed at the values deduced from the theoretical expression in Eq. (3), using in each case and of the lowest frequency data point; note, however, that the slope is largely independent of , which is about two orders of magnitude smaller than .
(Color online) Broadband tuning of the terahertz QCL by mode hopping over a range of . All spectra were recorded under the same drive current and temperature conditions ( and ), and are shown on the same vertical scale. The different Fabry-Perot modes were obtained by acting on the manipulator to slightly tilt the mirror alignment and changing the cavity length in a range of about .
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