Terahertz Quantum Cascade Laser Local Oscillator

NASA has a need for airborne or space-based observatories and remote sensors in order to penetrate the opaque atmosphere between 1 and 10 THz. For observations >2 THz, technologically mature microwave sources typically have microwatt power levels which are insufficient to act as LOs for a heterodyne receiver. LongWave Photonics is proposing to develop a compact, frequency agile, frequency locked, single mode quantum cascade laser (QCL) system. The distributed feedback grating (DFB) QCL arrays pack multiple devices on a single semiconductor die with individual devices lasing at different frequencies. The source will be frequency agile over 150 GHz with center frequencies ranging from 2 to 5 THz range. The DFB QCL array will be packaged in a high-reliability Stirling cycle cooler. The source will be frequency locked to a gas reference cell which has multiple absorption lines. The lines are much more closely spaced than the IF bandwidth of the detector, allowing continuous frequency coverage over the tunable range. Phase I LO power is expected to be > 1 mW with > 10 mW in Phase II. Methods for amplitude stabilization will be investigated.

NASA applications include the use of the QCL as an LO for >2 THz receivers on the Stratospheric Observatory for Infrared Astronomy (SOFIA) as well as for future remote sensing missions. Here the narrow line width (<100 kHz) of the QCLs can be used to resolve Doppler-limited low pressure gasses (~MHz linewidth). The DFB QCL array LO would be a frequency agile, compact replacement for any gas-laser LO. The utility of this work is enhanced by the tunable range, which is expected to be ~150 GHz, allowing for a wide range of gasses to be detected with a single instrument.

Applications for this technology include non-NASA-funded remote sensing instruments. Applications include gas absorption spectroscopy in fundamental research. In this application, there are currently no available frequency agile QCL sources above 2 THz with sufficient resolution to measure Doppler-limited linewidths. Long-term applications include industrial uses for trace gas detection. For industrial applications, the use of high-reliability, compact Stirling cycle coolers would greatly increase the usability of these QCL devices, which have traditionally required liquid nitrogen cooling or larger cryocooling systems.

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