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Applied Physics Letters

Fast Fourier transform spectrometer readout for large arrays of microwave kinetic inductance detectors

S. J. C. Yates, Andrey M. Baryshev, J. J. A. Baselmans, Bernd Klein, R. Güsten

Published Jul 27, 2009• 46 citations
Publisher

Abstract

Microwave kinetic inductance detectors have great potential for large, very sensitive detector arrays for use in, for example, submillimeter imaging. Being intrinsically readout in the frequency domain, they are particularly suited for frequency domain multiplexing allowing ∼1000 s of devices to be readout with one pair of coaxial cables. However, this moves the complexity of the detector from the cryogenics to the warm electronics. We present here the concept and experimental demonstration of the use of fast Fourier transform spectrometer readout, showing no deterioration of the noise performance compared to the low noise analog mixing while allowing high multiplexing ratios.

DOI
10.1063/1.3159818

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Microwave kinetic inductance detectors have great potential for large, very sensitive detector arrays for use in, for example, submillimeter imaging. Being intrinsically readout in the frequency domain, they are particularly suited for frequency domain multiplexing allowing ∼1000 s of devices to be readout with one pair of coaxial cables. However, this moves the complexity of the detector from the cryogenics to the warm electronics. We present here the concept and experimental demonstration of the use of fast Fourier transform spectrometer readout, showing no deterioration of the noise performance compared to the low noise analog mixing while allowing high multiplexing ratios.
Microwave Kinetic Inductance Detectors (MKIDs) have great potential for large very sensitive detector arrays for use in, for example, sub-mm imaging. Being intrinsically readout in the frequency domain, they are particularly suited for frequency domain multiplexing allowing $\sim$1000s of devices to be readout with one pair of coaxial cables. However, this moves the complexity of the detector from the cryogenics to the warm electronics. We present here the concept and experimental demonstration of the use of Fast Fourier Transform Spectrometer (FFTS) readout, showing no deterioration of the noise performance compared to low noise analog mixing while allowing high multiplexing ratios.
The frequency comb generator is an essential component in the readout of superconducting detectors. This component is responsible for generating the resonant frequencies necessary to detect photon interaction with the detectors. This component however has not received the much needed attention in terms of readout systems optimization. Conventional implementations rely on either inverse fast Fourier transform (IFFT) engines, which scale poorly in power with increasing frequency resolution requirement but allow flexible tone updates, or lookup tables (LUTs), which demand large memory for high frequency resolution. Microwave Kinetic Inductance Detectors (MKIDs) are superconducting detectors which have gained significant interest for cryogenic astrophysical instrumentation due to their high sensitivity, intrinsic multiplexing capability, and scalable fabrication. However, their simplified detector architecture shifts complexity to the readout electronics, making the development of compact, low-power frequency comb generators especially valuable. The design of the compact frequency comb generator targets applications involving MKIDs which typically detect photons in the submilliter bands. In this thesis, I present the design and hardware realization of a compact frequency comb generator. The design has been verified on an Field Programmable Gate Array (FPGA) using the Xilinx's RFSoC ZCU111 evaluation board. The architecture consumes less than 5% of FPGA resources and less than 5 Watts of power with the capability of handling multiple independent channels of the compact frequency comb generator on the evaluation board. These results demonstrate the potential of this approach to enable scalable, energy-efficient MKID readout systems for large-format detector arrays in next-generation cryogenic experiments.
A microwave kinetic inductance detector (MKID) is a superconducting pair-breaking detector used for astronomical observations in the sub-millimeter through visible range. Radiation is detected as a phase and amplitude signal proportional to the frequency shift of the device. In the case of arrays of MKIDs, the read-out is performed using the frequency division multiplexing technique. However, there is currently no commercial system or low-cost standard electronics available for this task. In this work, we take the first steps towards developing a low-cost readout system for MKIDs based on the Red Pitaya board. We have developed a firmware for the efficient generation of probe tones, and show figures of merit characterizing the performance of the readout in a hardware prototype. The design is flexible, letting us to adapt it to new frequencies and amplitudes requirements of the MKID arrays.
Superconducting microwave resonators have the potential to revolutionize submillimeter and far-infrared astronomy, and with it our understanding of the universe. The field of low-temperature detector technology has reached a point where extremely sensitive devices like transition-edge sensors are now capable of detecting radiation limited by the background noise of the universe. However, the size of these detector arrays are limited to only a few thousand pixels. This is because of the cost and complexity of fabricating large-scale arrays of these detectors that can reach up to 10 lithographic levels on chip, and the complicated SQUID-based multiplexing circuitry and wiring for readout of each detector. In order to make substantial progress, next- generation ground-based telescopes such as CCAT or future space telescopes require focal planes with large-scale detector arrays of 104–106 pixels. Arrays using microwave kinetic inductance detectors (MKID) are a potential solution. These arrays can be easily made with a single layer of superconducting metal film deposited on a silicon substrate and pattered using conventional optical lithography. Furthermore, MKIDs are inherently multiplexable in the frequency domain, allowing ∼ 103 detectors to be read out using a single coaxial transmission line and cryogenic amplifier, drastically reducing cost and complexity. An MKID uses the change in the microwave surface impedance of a superconducting thin-film microresonator to detect photons. Absorption of photons in the superconductor breaks Cooper pairs into quasiparticles, changing the complex surface impedance, which results in a perturbation of resonator frequency and quality factor. For excitation and readout, the resonator is weakly coupled to a transmission line. The complex amplitude of a microwave probe signal tuned on-resonance and transmitted on the feedline past the resonator is perturbed as photons are absorbed in the superconductor. The perturbation can be detected using a cryogenic amplifier and subsequent homodyne mixing at room temperature. In an array of MKIDs, all the resonators are coupled to a shared feedline and are tuned to slightly different frequencies. They can be read out simultaneously using a comb of frequencies generated and measured using digital techniques. This thesis documents an effort to demonstrate the basic operation of ∼ 256 pixel arrays of lumped-element MKIDs made from superconducting TiNx on silicon. The resonators are designed and simulated for optimum operation. Various properties of the resonators and arrays are measured and compared to theoretical expectations. A particularly exciting observation is the extremely high quality factors (∼ 3 × 107) of our TiNx resonators which is essential for ultra-high sensitivity. The arrays are tightly packed both in space and in frequency which is desirable for larger full-size arrays. However, this can cause a serious problem in terms of microwave crosstalk between neighboring pixels. We show that by properly designing the resonator geometry, crosstalk can be eliminated; this is supported by our measurement results. We also tackle the problem of excess frequency noise in MKIDs. Intrinsic noise in the form of an excess resonance frequency jitter exists in planar superconducting resonators that are made on dielectric substrates. We conclusively show that this noise is due to fluctuations of the resonator capacitance. In turn, the capacitance fluctuations are thought to be driven by two-level system (TLS) fluctuators in a thin layer on the surface of the device. With a modified resonator design we demonstrate with measurements that this noise can be substantially reduced. An optimized version of this resonator was designed for the multiwavelength submillimeter kinetic inductance camera (MUSIC) instrument for the Caltech Submillimeter Observatory.
Microwave kinetic inductance detectors (MKIDs) represent a class of low-temperature superconducting detectors employed for observing astronomical phenomena spanning the submillimeter through visible bands. Renowned for their intrinsic frequency-division multiplexing capability at microwave frequencies, MKIDs offer the advantage of eliminating the need for complex cryogenic multiplexing electronics. However, the overall readout architecture remains complex, necessitating sophisticated room-temperature electronics. Optimizing overall performance poses a significant challenge. This article introduces a novel approach to behavioral modeling and simulation for the MKID readout system utilizing Simulink and the RF Blockset library. The models incorporate major nonidealities of electronic components to investigate their impacts on system performance. In addition, the model of the MKID array is included based on measured transmission data, ensuring high simulation accuracy. The simulation tool facilitates the study and high-level design of the MKID readout system before delving into hardware implementation.
In the next decades millimeter and sub-mm astronomy requires large format imaging arrays and broad-band spectrometers to complement the high spatial and spectral resolution of the Atacama Large Millimeter/submillimeter Array. The desired sensors for these instruments should have a background limited sensitivity and a high optical efficiency and enable arrays thousands of pixels in size. Hybrid microwave kinetic inductance detectors consisting of NbTiN and Al have shown to satisfy these requirements. We present the second generation hybrid NbTiN-Al MKIDs, which are photon noise limited in both phase and amplitude readout for loading levels P850GHz < 10 fW. Thanks to the increased responsivity, the photon noise level achieved in phase allows us to simultaneously read out approximately 8000 pixels using state-of-the-art electronics. In addition, the choice of superconducting materials and the use of a Si lens in combination with a planar antenna gives these resonators the flexibility to operate within the frequency range 0:09 < v < 1:1 THz. Given these specifications, hybrid NbTiN-Al MKIDs will enable astronomically usable kilopixel arrays for sub-mm imaging and moderate resolution spectroscopy.