  The Wavelength-shifting Optical Module (WOM) is a novel optical sensor that
uses wavelength shifting and light guiding to substantially enhance the
photosensitive area of UV optical modules. It has been designed for the IceCube
Upgrade, a seven-string extension of the IceCube detector planned for the
2022/2023 South Pole deployment season. The WOM consists of a hollow quartz
cylinder coated in wavelength shifting paint which serves as detection area and
has two photomultipliers (PMTs) attached to the end faces. The light-collecting
tube increases the effective photocathode area of the PMTs without producing
additional dark current, making it suitable for low-signal, low-noise
applications. We report on the design and performance of the WOM with a focus
on the 12 modules in production for deployment in the IceCube Upgrade. While
the WOM will be deployed in IceCube, its design is applicable to any
large-volume particle detector based on the detection of Cherenkov light.
