Abstract
We present the time-resolved comparison of pulsed 2nd order ring cavity surface emitting (RCSE) quantum cascade lasers (QCLs) and pulsed 1st order ridge-type distributed feedback (DFB) QCLs using a step-scan Fourier transform infrared (FT-IR) spectrometer. Laser devices were part of QCL arrays and fabricated from the same laser material. Required grating periods were adjusted to account for the grating order. The step-scan technique provided a spectral resolution of 0.1 cm(-1) and a time resolution of 2 ns. As a result, it was possible to gain information about the tuning behavior and potential mode-hops of the investigated lasers. Different cavity-lengths were compared, including 0.9 mm and 3.2 mm long ridge-type and 0.97 mm (circumference) ring-type cavities. RCSE QCLs were found to have improved emission properties in terms of line-stability, tuning rate and maximum emission time compared to ridge-type lasers.
Highlights
Over the last two decades Quantum Cascade Lasers (QCLs) developed from novel and high sophisticated lasers [1] to important light sources for use in mid-infrared (IR) spectroscopy
We present the time-resolved comparison of pulsed 2nd order ring cavity surface emitting (RCSE) quantum cascade lasers (QCLs) and pulsed 1st order ridge-type distributed feedback (DFB) QCLs using a stepscan Fourier transform infrared (FT-IR) spectrometer
We have presented the step scan technique as a powerful tool for the characterization of midIR DFB QCLs
Summary
Over the last two decades Quantum Cascade Lasers (QCLs) developed from novel and high sophisticated lasers [1] to important light sources for use in mid-infrared (IR) spectroscopy. While in the beginning of their development only multi-mode emitting QCLs with a FabryPérot (FP) cavity were available, technical improvements such as the external cavity (EC) [2] or the integration of a Bragg grating (distributed feedback - DFB) [3] enabled single mode emission This makes them well suited for gas phase spectroscopy of small molecules as the observed ro-vibrational absorption bands are typically sharp and highly resolved [4,5]. Frequency upconversion requires non-linear optical crystals and a pump source to shift the mid-infrared QC laser radiation up to near-infrared where it can subsequently be analyzed by a conventional optical spectrum analyzer [12] This method is not applicable to laser devices with broader spectral tuning ranges, as the bandwidth of the non-linear crystal can be limiting. Both were already successfully applied for the characterization of external cavity QCLs [13,14]
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