Complex field mapping is a powerful tool to characterize the optical performance of astronomical instruments, and has become the standard for characterizing heterodyne array cameras. Recently, an adaptation of the heterodyne beam mapping technique was demonstrated on a single pixel of a direct detector instrument. We present a novel measurement apparatus and data acquisition techniques to efficiently reconstruct the complex field pattern of individual pixels across a direct detector focal plane array. These techniques are scalable to high pixel counts as the technology maturation and scientific requirements push to larger arrays. For this demonstration, we used an engineering model of the low-frequency band of the APEX microwave kinetic inductance detector camera with a center frequency of ν = 350 GHz. Amplitude and phase radiation patterns were measured from all 880 pixels of the test array in two orthogonal polarizations. We also discuss an updated postprocessing pipeline using the complex field data to characterize the optical performance of the array. Using the measured complex field pattern, we extract the co- and cross-polarization patterns and Gaussian beam parameters, and propagate the beam from the measurement plane to additional planes of interest across all pixels in the test array. Complex field measurements of direct detectors allow more precise characterization of beam parameters when compared to thermal measurements, particularly for individualized fitting in postprocessing not reliant on the accuracy of the probe system alignment. These techniques enable high-precision characterization of individualized beam parameters as well as the overall optical system to very large format arrays with modest computational processing power. These results demonstrate the diagnostic power of the presented measurement and analysis techniques.@en

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.@en

GUSTO will be a NASA balloon borne terahertz observatory to be launched from Antarctica in late 2021 for a flight duration of 100-170 days. It aims at reviewing the life cycle of interstellar medium of our galaxy by simultaneously mapping the three brightest interstellar cooling lines: [OI] at 4.7 THz, [CII] at 1.9 THz, and [NII] at 1.4 THz; along the 124 degrees of the galactic plane and through a part of the Large Magellanic Cloud. It will use three arrays of 4x2 mixers based on NbN hot electron bolometers (HEBs), which are currently the most sensitive mixers for high resolution spectroscopic astronomy at these frequencies. Here we report on the design of a novel 4.7 THz receiver for GUSTO. The receiver consists mainly of two subsystems: a 4×2 HEB quasi-optical mixer array and a 4.7 THz multi-beam LO. We describe the mixer array, which is designed as a compact monolithic unit. We show, for example, 10 potential HEB detectors with the state of the art sensitivity of 720 K measured at 2.5 THz. They have a small variation in sensitivity, being less than 3%, while also meet the LO uniformity requirements. For the multi-beam LO we demonstrate the combination of a phase grating and a single QCL at 4.7 THz, which generates 8 sub-LO beams, where the phase grating shows an efficiency of 75%. A preliminary concept for the integrated LO unit, including QCL, phase grating and beam matching optics is presented.@en

GUSTO will be a NASA balloon borne terahertz observatory to be launched from Antarctica in late 2021 for a flight duration of 100-170 days. It aims at reviewing the life cycle of interstellar medium of our galaxy by simultaneously mapping the three brightest interstellar cooling lines: [OI] at 4.7 THz, [CII] at 1.9 THz, and [NII] at 1.4 THz; along the 124 degrees of the galactic plane and through a part of the Large Magellanic Cloud. It will use three arrays of 4x2 mixers based on NbN hot electron bolometers (HEBs), which are currently the most sensitive mixers for high resolution spectroscopic astronomy at these frequencies. Here we report on the design of a novel 4.7 THz receiver for GUSTO. The receiver consists mainly of two subsystems: a 4×2 HEB quasi-optical mixer array and a 4.7 THz multi-beam LO. We describe the mixer array, which is designed as a compact monolithic unit. We show, for example, 10 potential HEB detectors with the state of the art sensitivity of 720 K measured at 2.5 THz. They have a small variation in sensitivity, being less than 3%, while also meet the LO uniformity requirements. For the multi-beam LO we demonstrate the combination of a phase grating and a single QCL at 4.7 THz, which generates 8 sub-LO beams, where the phase grating shows an efficiency of 75%. A preliminary concept for the integrated LO unit, including QCL, phase grating and beam matching optics is presented.@en

We compare the results of simulated and measured power efficiency and far-field beam pattern, for two reflective Fourier phase gratings, designed to generate 2 x 2 and 2 x 4 beams respectively from a single-beam, coherent source at 1.4 THz. The designed surface structures were manufactured on aluminum plates by a computer numerical control (CNC) micro-milling machine. Despite small differences between the designed and fabricated gratings, we measured power efficiencies of both gratings to be around 70%, which is in a good agreement with the simulated values. We also find a good agreement between the simulated and measured diffracted beam size and spatial distribution. We demonstrate the application of both gratings as multiple beam local oscillators to simultaneously pump (or operate) a 4-pixel array of superconducting heterodyne mixers.@en

This paper is a report on the demonstration and stabilization of a 2x4 NbN hot electron bolometer (HEB) array receiver for GUSTO based on a multiple beam local oscillator (LO). Through the combination of a FIR Gas laser and a Fourier phase grating we were able to generate a 8 beam pattern at 1.4 THz. For the array demonstration, it used a 2x2 HEB mixer array which was translated vertically to match the full 2x4 LO beam pattern. Here two lens with a 60 mm were applied to reduce the beam pattern divergence, allowing for a 12 mm spacing between the beams to be achieved at 130 mm from the grating. To stabilize the array receiver, we introduced a new stabilization method, requiring only a voice coil PID controlled loop between the beam originated from the gas laser and a given pumped pixel. The total power Allan time measurements indicate a five times improvement on a two-pixel system stability.@en

Modelling, manufacturing and characterization of two 4 and 8-pixel Fourier phase gratings operated at 1.4 THz are reported, mainly applicable as local oscillator multiplexers for heterodyne receivers. Comparing the measurements with full 3D simulations shows good agreements and provides good understanding. Power efficiency of around 70% is experimentally derived for both gratings. We demonstrate the application of both, as multiple beam local oscillators to simultaneously pump (or operate) an array of 4-pixel superconducting mixers.@en

Modelling, manufacturing and characterization of two 4 and 8-pixel Fourier phase gratings operated at 1.4 THz are reported, mainly applicable as local oscillator multiplexers for heterodyne receivers. Comparing the measurements with full 3D simulations shows good agreements and provides good understanding. Power efficiency of around 70% is experimentally derived for both gratings. We demonstrate the application of both, as multiple beam local oscillators to simultaneously pump (or operate) an array of 4-pixel superconducting mixers.@en

Modelling, manufacturing and characterization of two 4 and 8-pixel Fourier phase gratings operated at 1.4 THz are reported, mainly applicable as local oscillator multiplexers for heterodyne receivers. Comparing the measurements with full 3D simulations shows good agreements and provides good understanding. Power efficiency of around 70% is experimentally derived for both gratings. We demonstrate the application of both, as multiple beam local oscillators to simultaneously pump (or operate) an array of 4-pixel superconducting mixers.@en