Design and characterization of W-band and D-band calibration sources for the AliCPT-1 experiment

TL;DR

This work presents the design and laboratory validation of two rotating polarized calibration sources for AliCPT-1, covering W-band ($75-110$ GHz) and D-band ($120-170$ GHz) to enable accurate far-field beam mapping and sidelobe characterization. The sources employ dual channels (amplified thermal noise and VCO) on a fast, adaptive, GPS-synced sweep with precise polarization control, mounted on a mast above the telescope to satisfy far-field and sidelobe mapping requirements. Key results show output powers up to +14 dBm (W-band) and +11 dBm (D-band), dynamic ranges >60 dB, long-term stability at the sub-percent level, analyzable spectra with a rapid MP-FTS (2.7 GHz resolution), and robust polarization performance with high isolation (W-band ~$-30$ dB; D-band ~ $-18$ dB). Field validation in May 2025 confirms the D-band source performance aligns with lab measurements, with no scan-synchronous artifacts, establishing a reliable calibration foundation for AliCPT-1’s quest to measure primordial B-modes.

Abstract

Ali Cosmic Microwave Background Polarization Telescope (AliCPT-1) is the first Chinese cosmic microwave background experiment aiming to make sensitive polarization maps of the potential B-mode signal from inflationary gravitational waves. The telescope was deployed on the Tibet Ali site at 5250 m above sea level in early 2025. Before and after each observation season, the instrument performance must be carefully calibrated, including the far field beam performance, far sidelobe, spectral response, polarization angle, and cross-polar beam response. To characterize these optical performances, several calibrators have been developed. We developed a W-band source and a D-band source for the AliCPT-1 telescope's beam characterizations. We present the design and performance of the two calibration sources.

Figures (10)

• Figure 1: The source would be mounted on a 28 m tall mast at Ali astronomical observatory C1 point, which is 1400 m far away from B1 point where AliCPT laboratory is.
• Figure 2: Schematic of the W-band source for AliCPT-1 FFBM/FSLM calibration. The transmitter, which is enclosed in the inner box, contains the source components, power supply and control system. It is the core functional unit of the instrument that emits 75-110 GHz linear polarization radiation. The transmitter is mounted to a rotation stage so the angle of the polarization can be rotated in a range of $0 \sim 360\degree$. The rotation stage with the transmitter is bolted to an L-shaped standoff, which is rigidly bolted to an aluminum base plate. A tiltmeter is also bolted to the same base plate, so that the orientation of the polarization radiation can be monitored. The base plate will be mounted to the platform atop the mast.
• Figure 3: The W-band and D-band transmitters photoed before the inner box was covered.
• Figure 4: Electrical Diagram of the W-band (75-110GHz) and D-band (120-170GHz) transmitters. The microwave components of D-band transmitter are shown in gray and those of W-band transmitter are shown in blue gray.
• Figure 5: The S21 data for the two-stage WTAs (W-band: top; D-band: bottom) are normalized to their mechanical zero (micrometers fully retracted at WTA1-500, WTA2-500) to establish a consistent 0 dB reference, accurately showing the relative attenuation. WTA1 (input side, connected to frequency multiplier or Port 1 of VNA ) and WTA2 (output side, connected to antenna or Port 2 of VNA) are labeled in the legend with suffixes indicating their micrometer readings.