Numerical optimization of integrating cavities for diffraction-limited millimeter-wave bolometer arrays

Abstract

Far-infrared to millimeter-wave bolometers designed to make astronomical observations are typically encased in integrating cavities at the termination of feedhorns or Winston cones. This photometer combination maximizes absorption of radiation, enables the absorber area to be minimized, and controls the directivity of absorption, thereby reducing susceptibility to stray light. In the next decade, arrays of hundreds of silicon nitride micromesh bolometers with planar architectures will be used in ground-based, suborbital, and orbital platforms for astronomy. The optimization of integrating cavity designs is required for achieving the highest possible sensitivity for these arrays. We report numerical simulations of the electromagnetic fields in integrating cavities with an infinite plane-parallel geometry formed by a solid reflecting backshort and the back surface of a feedhorn array block. Performance of this architecture for the bolometer array camera ͑Bolocam͒ for cosmology at a frequency of 214 GHz is investigated. We explore the sensitivity of absorption efficiency to absorber impedance and backshort location and the magnitude of leakage from cavities. The simulations are compared with experimental data from a room-temperature scale model and with the performance of Bolocam at a temperature of 300 mK. The main results of the simulations for Bolocam-type cavities are that ͑1͒ monochromatic absorptions as high as 95% are achievable with Ͻ1% cross talk between neighboring cavities, ͑2͒ the optimum absorber impedances are 400 ⍀͞sq, but with a broad maximum from ϳ150 to ϳ700 ⍀͞sq, and ͑3͒ maximum absorption is achieved with absorber diameters Ն1.5. Good general agreement between the simulations and the experiments was found.

Silicon nitride micromesh bolometer array for submillimeter astrophysics

“…The measured optical efficiency is lower than theoretically expected 18 and lower than the efficiency measured in similar millimeter-wave systems. 18,31 One possible explanation for the low efficiency is the high return loss by the feedhorns.…”

“…The waveguide terminates into a tuned ͞2 cavity, with the detector placed in the middle of the cavity to maximize absorption. The absorption of a bolometer in such a cavity fed with a circular waveguide is analyzed in Glenn et al 18 with numerical simulations, and we briefly summarize these results. Glenn et al found that the optical efficiency of the waveguide and cavity section is high ͑95%͒ assuming an absorber with a diameter of 2 and an electrical impedance of 400 ⍀͞square.…”

SHIFTS: simulator for the Herschel imaging Fourier transform spectrometer

“…and submillimetre emission from cold mirrors), various elements were not discussed [5,83,86,88,113,126]. The following chapter describes how the components described in this chapter were integrated into SHIFTS.…”

“…Each of the three interconnected efficiencies (aperture, feedhorn and main-beam) have mode and wavelength dependencies [5,66,83,85,113,114,[118][119][120][121][122]124,126].…”

Future Developments in Low Temperature Detectors for CMB and Submm Astronomy

“…The first move away from the single-feedhorn approach has been the use of monolithic detector arrays behind conical smooth-walled feedhorn arrays [63]. The feeds are typically /A to 2/A in size to maximize the use of focal plane area.…”