Design and Performance of 30/40 GHz Diplexed Focal Plane for BICEP Array

We demonstrate a wide-band diplexed focal plane suitable for observing low-frequency foregrounds that are important for cosmic microwave background polarimetry.

The antenna elements are composed of slotted bowtie antennas with 60% bandwidth that can be partitioned into two bands.

Each pixel is composed of two interleaved 12$\times$12 pairs of linearly polarized antenna elements forming a phased array, designed to synthesize a symmetric beam with no need for focusing optics.

The signal from each antenna element is captured in-phase and uniformly weighted by a microstrip summing tree.

The antenna signal is diplexed into two bands through the use of two complementary, six-pole Butterworth filters.

This filter architecture ensures a contiguous impedance match at all frequencies, and thereby achieves minimal reflection loss between both bands.

Subsequently, out-of-band rejection is increased with a bandpass filter and the signal is then deposited on a transition-edge sensor bolometer island.

We demonstrate the performance of this focal plane with two distinct bands, 30 and 40 GHz, each with a bandwidth of $\sim$20 and 15 GHz, respectively.

The unequal bandwidths between the two bands are caused by an unintentional shift in diplexer frequency from its design values.

The end-to-end optical efficiency of these detectors are relatively modest, at 20-30%, with an efficiency loss due to an unknown impedance mismatch in the summing tree.

Far-field beam maps show good optical characteristics with edge pixels having no more than $\sim$ 5% ellipticity and $\sim$10-15% peak-to-peak differences for A-B polarization pairs.

;Comment: 17 pages, 13 figures.

Accepted for publication in ApJS