Abstract
We present an 8-beam local oscillator (LO) for the astronomically significant [OI] line at 4.7 THz. The beams are generated using a quantum cascade laser (QCL) in combination with a Fourier phase grating. The grating is fully characterized using a third order distributed feedback (DFB) QCL with a single mode emission at 4.7 THz as the input. The measured diffraction efficiency of 74.3% is in an excellent agreement with the calculated result of 75.4% using a 3D simulation. We show that the power distribution among the diffracted beams is uniform enough for pumping an array receiver. To validate the grating bandwidth, we apply a far-infrared (FIR) gas laser emission at 5.3 THz as the input and find a very similar performance in terms of efficiency, power distribution, and spatial configuration of the diffracted beams. Both results represent the highest operating frequencies of THz phase gratings reported in the literature. By injecting one of the eight diffracted 4.7 THz beams into a superconducting hot electron bolometer (HEB) mixer, we find that the coupled power, taking the optical loss into account, is in consistency with the QCL power value.
Highlights
Heterodyne spectroscopy provides valuable scientific information for modern astrophysics, in particular, the process of star formation and the life cycle of interstellar clouds in our own and nearby galaxies
We present an 8-beam local oscillator (LO) for the astronomically significant [OI] line at 4.7 THz
By injecting one of the eight diffracted 4.7 THz beams into a superconducting hot electron bolometer (HEB) mixer, we find that the coupled power, taking the optical loss into account, is in consistency with the quantum cascade laser (QCL) power value
Summary
Heterodyne spectroscopy provides valuable scientific information for modern astrophysics, in particular, the process of star formation and the life cycle of interstellar clouds in our own and nearby galaxies. One important observation is to map particular THz lines in star forming areas within our Milky Way at very high spectral resolution Such large-scale surveys with velocity-resolved imaging of key fine structure lines require multi-pixel heterodyne receivers [2] with the state-of-the-art mixer performance in order to increase the observation speed and the observing efficiency of the telescope. Since waveguide based multi-beam LOs have not been demonstrated at such a high frequency, the QCL [9] is the only practical solid-state source at 4.7 THz, which can provide multiple beams in combination with a phase grating. Given the power level and beam pattern of the current generation of THz QCLs suitable for use as an LO, such as third order DFB QCLs [10], they can typically only drive a single pixel superconducting hot electron bolometer (HEB) mixer. We demonstrate the grating bandwidth advantage using a FIR gas laser emission at 5.3 THz as the input
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