Interest in the use of THz detectors outside the laboratory for space, atmospheric, and terrestrial applications has grown immensely in the past half-century. Of particular interest in recent years is the development of the quantum cascade laser (QCL) as a THz frequency source.
A number of approaches for tuning the emission from a THz QCL — which may use temperature, electrical (current/voltage), resonant cavity, or mechanical methods as a means of frequency/mode selection — are currently being investigated. All of these approaches have one or more negative aspects. These limitations adversely affect commercialization and use in a large variety of potential applications.
The innovation presented here is a THz QCL source (1 to 5 THz), based on a passive waveguide tuning mechanism, that can utilize the full bandwidth of a broadband THz QCL, and produce a significantly larger number of frequency channels.
The QCL is coupled to a grating router, which consists of an appropriately configured linear dielectric waveguide array capable of generating a high density of THz channels. The output of the grating router enters an on/off switching waveguide controller to enable selection of the desired single mode frequency. The outputs of the waveguide controller are combined into one signal using a multiplexer.
This technique offers many potential benefits, which include more THz frequency channels, reduced size, weight, complexity, and significant cost savings. An important NASA application is a multi-frequency local oscillator (LO) in a heterodyne detector that could significantly enhance the use of THz radiometry and spectroscopy for exploration and characterization of planetary, comet, and asteroid atmospheres and gaseous emissions, and for cosmic background and other astrophysics and cosmology science studies. The concept can be extended to function as a tunable heterodyne emission source for frequencies in the hundreds of GHz or less (<1 THz) by combining with a diode mixer either (a) two of the THz frequencies from a single tunable QCL, or (b) the output THz frequencies from two tunable QCLs.
In addition to high-resolution tuning, the innovation offers a number of advantages (simplicity, low cost, and environmental stability) over alternative QCL tuning approaches. As a result, the innovation potentially offers a pathway to a greatly expanded use of the THz QCL in commercial terrestrial and NASA Earth and space science applications such as sensing and spectroscopy.