Improved quantum-cascade lasers (QCLs) are being developed as electrically tunable sources of radiation in the far infrared spectral region, especially in the frequency range of 2 to 5 THz. (Heretofore, the wavelengths of QCLs have been adjusted by changing temperatures, but not by changing applied voltages or currents.) In comparison with gas lasers now used as far-infrared sources, these QCLs would have larger wavelength tuning ranges, would be less expensive, and would be an order of magnitude less massive and power-hungry. It is planned to use the improved QCLs initially as the active components of local oscillators in spaceborne heterodyne instruments for studying infrared spectral lines of molecules of scientific interest. On Earth, the QCLs could be used as far-infrared sources for medical glucose-monitoring and heart-monitoring instruments, chemical-analysis and spectral-imaging systems, and imaging instruments that exploit the ability of terahertz radiation to penetrate cloth and walls for detection of contra-band weapons.
The structures of QCLs and the processes used to fabricate them have much in common with those of multiple-quantum-well infrared photodetectors described in numerous previous NASA Tech Briefs articles. In one of four approaches being followed in the present development effort, the focus is upon designing and fabricating the structures to obtain heterogeneous cascades for different electric fields and different wavelengths in order to enable electrical tuning of laser emission wavelengths. Both the variation of the emission wavelength and the targeted range of the electric-field strength for each cascade would be kept small so the spectral gains of adjacent cascades at any given electric-field strength in the target range would overlap. This approach is expected to afford the desired variation of the gain maximum with electric-field strength, so that a change in applied bias voltage would result in a wavelength change.
In the second approach, layers of a QCL structure are to be graded to modify the shapes and depths of quantum wells, such that the electronic wave functions in the quantum wells and the transition energies between them would change with an intentional variation of the applied bias electrical field. A change of the bias applied to such a structure would result in a change in the energy and, hence, of the wavelength of the lasing transition.
In the third approach, the focus is on exploiting distributed-feedback and distributed-Bragg-reflector QCL architectures to achieve wavelength tuning through variation of applied electric current.
In the fourth approach, which is complementary to the other three, the focus is on increasing maximum operating temperatures and output power levels in continuous-wave operation. In a given QCL, this approach may involve one or more of the following changes: reducing the threshold current density, improving mounting and packaging to enhance removal of heat and reduce stresses, and optimizing designs of the active regions, injectors, and waveguide. Moreover, optimization of design for increasing maximum operating temperature and output power must include finding the most advantageous combination of the designs of the active regions and waveguide for optimal tuning performance.
This work was done by Sarath Gunapala, Alexander Soidel, and Kamjou Mansour of Caltech for NASA’s Jet Propulsion Laboratory.
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