Optics & Infrared Sensing
External Cavity Tunable Quantum Cascade Laser (QCL)
The broadly tunable external cavity quantum cascade laser (ECQCL) developed at Pacific Northwest National Laboratory (PNNL) brings proven tunable diode laser sensing techniques into the highly desirable mid-infrared spectral region. The utility of mid-infrared spectroscopy for trace gas sensing is well established, as are the advantages of laser-based sensing over incoherent techniques such as FTIR. With development of the broadly tunable QC laser, the same sensor platform can now detect a wide range of gases with the high sensitivity of laser-based techniques. Furthermore, broadly tunable QC lasers enable laser-based sensing of large complex molecules with broad absorption features, as well as smaller molecules with atmospherically broadened absorption features.
The tunable ECQCL uses a diffraction grating to select and tune the laser output wavelength. We have designed, constructed, and tested several ECQCLs at PNNL under funding from the NNSA Office of Nonproliferation Research and Development. These lasers operate in the 8-10 µm wavelength range with tuning ranges up to 100 cm-1. The ECQCL requires QC devices with minimal processing (FP devices), so high performance devices and new wavelengths are considerable easier to obtain than distributed feedback (DFB) lasers.
The ECQCL system is completely cryogen-free, requiring only a thermoelectric cooler for temperature stabilization (as with all tunable diode lasers). The ECQCL developed at PNNL currently fits on a 12 inch square breadboard, as shown in the photograph to the right. Engineering efforts are underway to reduce this size to less than 24 cubic inches, with further size reductions planned for future versions.
The ECQCLs constructed at PNNL have demonstrated average output powers of up to 10 mW, which is adequate for many different laser-based sensing techniques. Work is currently underway to demonstrate a tunable laser with over 50 mW of output power, which will be useful for photoacoustic spectroscopy or transmission measurements over long distances.
The ECQCL can be operated in either pulsed mode or continuous wave mode. We have used pulsed mode operation to demonstrate photoacoustic spectroscopy with quartz tuning fork transducers as an extremely sensitive and compact sensing platform [insert link to paper]. The solid lines in the figure to the left show absorption spectra of two different Freons (Freon 125 and Freon 134a) with 10 ppm concentration measured using the external cavity laser and quartz tuning fork transducers. The dashed lines are reference spectra from the NWIR spectral library. The high accuracy of this measurement clearly demonstrates the abilities of the external cavity laser for sensing of large and complex molecules.
In continuous wave operation, we have demonstrated both direct transmission and wavelength modulation spectroscopy of N2O. Part (a) in the figure to the right shows the measured absorption spectrum of a N2O line resolved with 82 MHz resolution. Part (b) shows the 2-f signal using wavelength modulation techniques. These techniques have been previously demonstrated with narrowly tunable QC lasers for high sensitivity detection of small gas molecules. With the broadly tunable laser we can perform the same sensing techniques, but with detection of any line within the 50 cm-1 tuning range, enabling detection of many different gas species with the same laser.