Characterization of wide-field optics in the Terahertz regime imposes new and demanding requirements for testing systems. Basic optical parameters can be determined from scalar planar characterization, obtained using monochromatic or thermal sources located in the instrument focal plane. In contrast, important features, such as the spillover efficiency, wave front error, or aperture efficiency cannot be easily measured by such approaches. Moreover, when instruments have a curved focal plane, designed to match the hosting telescope, even basic parameters are difficult to extract from scalar planar measurements. In such cases, the use of phase and amplitude information is mandatory. From a complex planar measurement, the complete information of the optical system can be obtained, allowing the estimation of all relevant optical parameters. In this work, we present and demonstrate experimentally a technique to perform such measurement based on the use of continuous wave photonic terahertz sources. Here, we present our results at 350 GHz and 850 GHz, demonstrating the feasibility of performing measurements at different submillimeter frequencies using a single experimental setup. The proposed system was implemented to fully characterize a wide-field submillimeter camera based on kinetic inductance detectors designed to be deployed at the APEX Telescope in Chile.

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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.

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The development of astrophysics kilo-pixel imaging systems requires a dedicated cryogenics and optics setup to measure the performance of the detector arrays in terms of sensitivity, crosstalk, dynamic range, and spatial response. We have developed such testbed for the characterization of large format arrays of microwave kinetic inductance detectors (MKIDs), capable of measuring detector chips of 6 cm × 6 cm, operating in the 350 and 850 GHz band. The testbed is a wide field camera that produces an aberration free image of the chip outside of the cryostat. The cryostat is based upon a commercial pulse-tube cooled 3 K system with an He3-He3-He4 sorption cooler that reaches a base temperature below 250 mK. We will describe the thermomechanical solutions implemented in our system to minimize the thermal loading on the cold stage, needed to reach the low base temperature. We will also give the optical design including straylight control.

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With increasing array size, it is increasingly important to control stray radiation inside the detector chips themselves. We demonstrate this effect with focal plane arrays of absorber coupled Lumped Element microwave Kinetic Inductance Detectors (LEKIDs) and lens-antenna coupled distributed quarter wavelength Microwave Kinetic Inductance Detectors (MKIDs). In these arrays the response from a point source at the pixel position is at a similar level to the stray response integrated over the entire chip area. For the antenna coupled arrays, we show that this effect can be suppressed by incorporating an on-chip stray light absorber. A similar method should be possible with the LEKID array, especially when they are lens coupled.

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We present the design, fabrication, and full characterisation (sensitivity, beam pattern, and frequency response) of a background limited broadband antenna coupled kinetic inductance detector covering the frequency range from 1.4 to 2.8 THz. This device shows photon noise limited performance with a noise equivalent power of 2.5 × 10^-19W/Hz^1/2 at 1.55 THz and can be easily scaled to a kilo-pixel array. The measured optical efficiency, beam pattern, and antenna frequency response match very well the simulations.

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We present results from the first vector beam pattern measurement of microwave kinetic inductance detectors (MKIDs). Vector beam patterns require sampling of the E-field of the receiver in both amplitude and phase. MKIDs are inherently direct detectors and have no phase response to incoming radiation. We map the amplitude and phase patterns of the detector beam profile by adapting a two-source heterodyne technique. Our testing strategy recovers the phase information by creating a reference signal to trigger data acquisition. The reference is generated by mixing the slightly offset low-frequency signals from the output of the two synthesizers used to drive the submillimeter sources. The key requirement is that the time-series record always begins at the same set phase of the reference signal. As the source probe is scanned within the receiver beam, the wavefront propagation phase of the receiver changes and causes a phase offset between the detector output and reference signals. We demonstrated this technique on the central pixel of a test array operating at 350 GHz. This methodology will enable vector beam pattern measurements to be performed on direct detectors, which have distinct advantages reducing systematic sources of error, allowing beam propagation, and removing the far-field measurement requirement such that complicated optical systems can be measured at a point that is easily accessible, including the near field.@en

Microwave Kinetic Inductance Detectors (MKIDs) are becoming a very promising candidate for next generation imaging instruments for the far infrared. A MKID consists of a superconducting resonator coupled to a feed-line used for the readout. In the devices presented here radiation coupling is achieved by coupling the MKID directly to planar antenna. The antenna is placed in the focus of an elliptical lens to increase the filling factor and to match efficiently to fore-optics. In this paper we present the design and the optical performance of MKIDs optimized for operation at 350 GHz. We have measured a device consisting of 14 pixels, characterized the coupling efficiency, antenna-lens frequency response and beam pattern and compared these to theoretical simulations. The optical efficiency has been measured by means of a black body radiator mounted in an ADR cryostat, through the variation of the black body temperature a variable illumination of each pixel (from 0.1 fW to 2 pW) is achieved. The frequency response and beam pattern have been directly measured in a He3 cryostat directly via the cryostat window and without the use of intermediate optics.

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Microwave kinetic inductance detectors (MKIDs) promise high sensitivity, combined with device simplicity and intrinsic multiplexibility. We demonstrate in this paper the realization of an imaging system consisting of an array of 961 antenna-coupled MKIDs that is read out using a single pair of coax cables and an analogue-digital back-end with a 2 GHz readout bandwidth. We measure the optical NEP of the central pixels of the array and find NEP = 510 -19 w/Vhz. The electrical NEP is measured for all pixels, giving NEP = 3.3 ± 1.310 -19 W/VHz for 85% of the pixels. We further obtain an instantaneous dynamic range of a factor 1000 and a cosmic ray dead time of 0.5% in the lab, resulting in 25% dead time when operated in a spacecraft at L2.@en

We show the first experimental results which prove that superconducting NbTiN coplanar–waveguide resonators can achieve a loaded Q factor in excess of 800 in the 350 GHz band. These resonators can be used as narrow band pass filters for on-chip filter bank spectrometers for astronomy. Moreover, the low-loss coplanar waveguide technology provides an interesting alternative to microstrip lines or constructing large scale submillimeter wave electronics in general.@en