A new generation of quasi-optical (QO) systems with multiple refractive and reflective components are required for several (sub)-millimeter applications such as the next generation of communication systems, sensing and security imagers, and instrumentation for far-infrared astronomy. Currently, there are no numerically efficient and accurate modeling tools available to design such systems tailored to their moderate sizes, in terms of wavelength, and wide operational bandwidths. In this article, a numerically efficient and accurate technique is proposed for designing QO systems containing a cascade of homogeneous dielectric lens components. The methodology is based on sequential geometrical optics (GO) combined with an analysis in reception mode. Thanks to the computational efficiency of the proposed technique, we were able to embed it as the kernel for multiobjective optimizers to design complex multilens QO systems. Here, two indicative examples of such geometries targeting specific requirements and scenarios are showcased. The overall optimization technique is validated against in-house and commercial physical optics (PO) codes, as well as full-wave simulations where possible, with excellent agreement in both efficiency terms and far-field beam patterns. Moreover, the proposed technique achieves orders of magnitude faster execution times with respect to the currently available commercial tools.

Studying the polarization and spectral distortion of the cosmic microwave background (CMB) in tandem with intensity fluctuations of the cosmic infrared background allows us to verify our assumptions on cosmic inflation and investigate the dynamics and evolution of galaxy clusters in the past 10 billion years. Because of its broadband emission and being an all-sky extended source, observing the entire CMB in detail is a very time-consuming and expensive exercise. Fortunately, in the past few years, the on-chip superconducting spectrometer technology has moved out of the lab and into the telescope. With its compact size and background-limited sensitivity, this family of instruments is particularly well-suited for fast and large area observations in a relatively unexplored range of the electromagnetic spectrum. However, recent examples of this technology do not yet reach the requirements needed for large spectroscopic and polarimetric surveys of the CMB. We formulate several of these requirements and introduce novel on-chip components and fabrication techniques. We introduce a crossover to enable distinguishing signal polarization, minimize signal loss by locally optimized lithography of a coplanar waveguide, lower the spectral resolution of microstrip filters by deposition of a dielectric layer, and increase the yield of the spectrometer array by removing individual line shorts. These together have culminated in the successful fabrication of a 14-spaxel integral field unit.

We developed, characterized, and verified an alignment procedure for the DESHIMA 2.0 instrument, an ultra-wide-band spectrometer operating between 200 and 400 GHz, at the ASTE telescope. To this end, we mounted the warm optics, consisting of a modified Dragonian dual reflector system, on a motor-controlled hexapod. Crucial in the alignment procedure is our sky chopper, which allows fast beam switching. It has a small entrance and exit aperture coupling to the (cold) sky, which creates a measurable signal with respect to the warm cabin environment. By scanning the instrument beam across the entrance aperture of the sky chopper using the hexapod, we found the hexapod configuration that produced the lowest signal on our detectors, implying that the beam is coupled fully to the cold sky and not the warm cabin. We first characterized the alignment procedure in the laboratory, where we used a vat containing liquid nitrogen as the cold source behind the sky chopper. Then, we applied the alignment procedure to DESHIMA 2.0 at ASTE. We found that the alignment procedure significantly improved the aperture efficiency compared with previously reported values of the aperture efficiency of DESHIMA at ASTE, which indicates the veracity of the alignment procedure.

The PRobe far-Infrared Mission for Astrophysics (PRIMA) is an astrophysics mission concept currently under study. The instrument comprises four focal plane arrays (FPAs), each with over 1000 pixels, consisting of lens-coupled kinetic inductance detectors designed for ultra-high sensitivity spectroscopy and imaging. PRIMA covers the far-infrared band from 24 to 250 μm. We present the fundamental properties of lens-coupled absorbers, including aperture efficiency, throughput, and beam patterns. Compared with bare absorbers, lens-coupled absorbers exhibit reduced sensitivity to noise from both the instrument enclosure and the sky background. We analyze FPA sampling strategies in terms of relative observing speed, considering both detector noise–limited and background noise–limited scenarios. In the background noise–limited case, near-maximum-gain sampling is optimal only when the point spread function (PSF) in the reflector focal plane is aligned with the lens. For arbitrary PSF incidence, a near-Nyquist sampling configuration provides the best average performance. These calculations are based on a computationally efficient quasi-analytical technique and assume a hex-packed array of circular lenses, recently developed for PRIMA.

The PRobe far-Infrared Mission for Astrophysics (PRIMA) is under study as a potential far-IR space mission, featuring actively cooled optics, and both imaging and spectroscopic instrumentation. To fully take advantage of the low background afforded by a cold telescope, spectroscopy with PRIMA requires detectors with a noise equivalent power (NEP) better than 1 × 10 ^-19 W Hz ^-12. To meet this goal, we are developing large format arrays of kinetic inductance detectors (KIDs) to work across the 25–250 μm range. Here, we present the design and characterization of a single pixel prototype detector optimized for 210 μm. The KID consists of a lens-coupled aluminum inductor-absorber connected to a niobium interdigitated capacitor to form a 2 GHz resonator. We have fabricated a small array with 28 KIDs, and we measure the performance of one of these detectors with an optical loading in the 0.01–300 aW range. At low loading, the detector achieves an NEP of 9 × 10 ^-20 W Hz ^-1/2 at a 10 Hz readout frequency. An extrapolation of these measurements suggests this detector may remain photon noise limited at up to 20 fW of loading, offering a high dynamic range for PRIMA observations of bright astronomical sources.

Context. Integrated superconducting spectrometers (ISSs) for wide-band submillimeter (submm) astronomy use quasi-optical systems for coupling radiation from the telescope to the instrument. Misalignment in these systems is detrimental to the system performance. The common method of using an optical laser to align the quasi-optical components requires an accurate alignment of the laser to the submm beam from the instrument, which is not always guaranteed to a sufficient accuracy. Aims. We develop an alignment strategy for wide-band ISSs that directly uses the submm beam of the wide-band ISS. The strategy should be applicable in both telescope and laboratory environments. Moreover, the strategy should deliver similar quality of the alignment across the spectral range of the wide-band ISS. Methods. We measured the misalignment in a quasi-optical system operating at submm wavelengths using a novel phase and amplitude measurement scheme that is capable of simultaneously measuring the complex beam patterns of a direct-detecting ISS across a harmonic range of frequencies. The direct detection nature of the microwave kinetic inductance detectors in our device-under-test, DESHIMA 2.0, necessitates the use of this measurement scheme. Using geometrical optics, the measured misalignment, a mechanical hexapod, and an optimisation algorithm, we followed a numerical approach to optimise the positioning of corrective optics with respect to a given cost function. Laboratory measurements of the complex beam patterns were taken across a harmonic range between 205 and 391 GHz and were simulated through a model of the ASTE telescope in order to assess the performance of the optimisation at the ASTE telescope. Results. Laboratory measurements show that the optimised optical setup corrects for tilts and offsets of the submm beam. Moreover, we find that the simulated telescope aperture efficiency is increased across the frequency range of the ISS after the optimisation.

We report measurements characterizing the performance of a kinetic inductance detector array designed for a wavelength of 25 microns and very low optical background level suitable for applications such as a far-infrared instrument on a cryogenically cooled space telescope. In a pulse-counting mode of operation at low optical flux, the detectors can resolve individual 25-micron photons. In an integrating mode, the detectors remain photon noise limited over more than 6 orders of magnitude in absorbed power from 70 zW to 200 fW, with a limiting noise equivalent power of 4.6×10-20 W Hz-1 at 1 Hz. In addition, the detectors are highly stable with flat power spectra under optical load down to 1 mHz. Operational parameters of the detector are determined including the efficiency of conversion of the incident optical power into quasiparticles in the aluminum absorbing element and the quasiparticle self-recombination constant.

Large format focal plane arrays (FPAs) of dielectric lenses are promising candidates for wide field-of-view submillimeter imagers. In this work, we optimize the scanning gain of such imagers via shaping lens surfaces. We develop an optimization procedure using a field correlation technique between the fields generated by a reflector on the top of the lenses and those generated by the lens feeds. Based on this procedure, an FPA of quartz lens antennas combined with leaky-wave feeds is designed to efficiently illuminate the reflector, achieving a directivity of 50.5 dBi up to scanning 20.3°. The obtained scanning gain loss of 2.6 dB is much lower than that associated with the direct fields coming from the reflector (about 6 dB). The proposed FPA is validated by full-wave simulations with excellent agreement. We have fabricated and measured an example shaped quartz lens optimized for the scanning angle of 20.3° at 180 GHz. The comparison between the simulations and the measurements also shows excellent agreement.

This article presents a feasibility study of single feed per beam quasi-optical (QO) antennas for enabling incoherent multiple-input multiple-output (MIMO) array front-end architectures at 270 GHz. The objective is to reach ultrafast and radiated energy efficient point-to-point (PtP) wireless links by exploiting the multimode capacity of radiative (Fresnel region) near-field links. In this article, we present a feasibility study of the number of independent links achievable with QO MIMO incoherent arrays. For this purpose, we present theoretical curves of the level of EM co-coupling and interference between the multiple modes versus the link distance. The study focuses at the 252-325 GHz spectral bandwidth defined by the new IEEE 802.15.3d standard. A specific and new MIMO array architecture operating at 270 GHz based on a 2 × 2 array of parabolic reflectors is proposed for a link distance of 100 m. The proposed PtP MIMO system is capable of generating 16 dual-polarized modes in a 70 GHz bandwidth with signal-to-interference ratio >17 dB and a power co-coupling coefficient of -3 dB without the need for interference cancelation techniques. Combining this architecture with wideband front ends could potentially lead to an aggregated data rate in the order of terabit per second in a PtP wireless line-of-sight link, not previously achieved experimentally to the best of authors' knowledge.

A system concept for online alignment verification of millimeter-wave, corneal reflectometry is presented. The system utilizes beam scanning to generate magnitude-only reflectivity maps of the cornea at 650 GHz and compares these images to a precomputed/measured template map to confirm/reject sufficient alignment. A system utilizing five off-axis parabolic mirrors, a thin film beam splitter, and two-axis galvanometric mirror was designed, simulated, and evaluated with geometric and physical optics. Simulation results informed the construction of a demonstrator system which was tested with a reference reflector. Similarity metrics computed with the aligned template and 26 misaligned positions, distributed on a 0.5 mm x 0.5 mm x 0.5 mm mesh, demonstrated sufficient misalignment detection sensitivity in 23 out of 26 positions. The results show that positional accuracy on the order of 0.5 mm is possible using 0.462 mm wavelength radiation due to the perturbation of coupling efficiency via beam distortion and beam walk-off.

We present a phase and amplitude beam pattern measurement technique using harmonic mixers. This allows a simultaneous multi-frequency phase sensitive characterization of a low resolution and wideband (220-420GHz) on-chip spectrometer using microwave kinetic inductance detectors. We investigate the beam quality, in particular the beam pointing and inferred telescope coupling and hence aperture efficiency. The measurements match the goal requirements for the DESHIMA-2 instrument for the ASTE telescope. The technique would be of interest for any (direct detector) spectrometer with a wide instantaneous bandwidth, particularly ones with dispersive components.