The objective of this research is to study both theoretically and experimentally the mechanisms and limitations of rapid electrical control of the emission frequency in single-mode quantum cascade lasers using bias control of the modal effective refractive index. Rapidly tunable single-mode quantum cascade lasers with all-electrical control of the emission frequency will be used to demonstrate a prototype of a free-space optical frequency-modulation link. Intellectual Merits The proposed research is aimed at further exploration of the unique physics and design flexibility of quantum cascade lasers. We will study a delicate interplay between the effectiveness of voltage control of the devices frequency and their performance characteristics. This work will create knowledge needed of the realization of mid-infrared semiconductor lasers that can be frequency modulated at speeds well above 1 GHz. Broader Impacts The proposed research contains elements of many disciplines, including quantum mechanics, optics, high-frequency electronics, and semiconductor microfabrication and provides an excellent educational environment for graduate and undergraduate students. Knowledge developed during the research will be incorporated into graduate-level courses taught by the principle investigators, and disseminated to the research community through publications, technology transfer, and the research group websites. The devices developed over the course of this project are expected to enable the creation of high-fidelity optical communication links based on frequency-modulation in the 8-12 microns atmospheric transparency window.
Rapidly-tunable room-temperature quantum cascade lasers (QCLs) operating in the 8-to-12 microns atmospheric transparency window are expected to enable the creation of long-distance all-weather free-space optical communication links based on frequency-modulation (FM). Such links can have several orders of magnitude higher signal-to-noise ratio compared to amplitude-modulation links for the same power of the laser source. Additionally, rapidly-tunable mid-infrared QCLs are also expected to find applications in systems for ultra-fast chemical sensing and reaction control. In this project, we have investigated different approaches to enable rapid FM in mid-infrared QCLs. Three approaches to achieve desired FM performance were investigated with only one approach producing satisfactory results. The successful approach is based on coupling a mid-infrared QCL with an off-the-shelf telecommunication diode laser. The light output from a diode laser is coupled into a QCL to produce changes in electron-hole concentration in the QCL active region and waveguide layers. The electron-hole concentration change leads to the effective refractive index change for a QCL laser mode, which in turn leads to change in the QCL emission frequency. We have experimentally demonstrated this approach can produce FM modulation at frequencies up to 300 MHz in 8.5 microns wavelength single-mode distributed feedback QCLs operating in pulsed mode at room temperature. Our QCLs can operate at relatively low threshold current densities of below 2.5 kA/cm2, which indicate that they may also be operated continuous wave at room temperature, once proper thermal packaging is implemented. The development of optically-modulated continuous-wave FM QCLs are expected to lead to further research towards development of mid-infrared optical FM data links, based on heterodyne detection principle. Eventually, such systems may lead to the development of a network of free-space all-weather long-distance optical communication with very high data bandwidth. The results of this research have been disseminated to scientific and technology community through 3 peer-reviewed journal publications, 2 conference proceedings papers, and 2 conference presentations. In Oct. 2013, we were granted a US patent on the technology to achieve rapid frequency tuning of QCLs. We have also been invited to write a review paper on our method for the special semiconductor lasers issue of in the IEEE Journal of Selected Topics in Quantum Electronics, which is expected to be published in November/December 2015.
