Optical encoding and multiplexing of detector signals with dual threshold time-over-threshold

We have previously demonstrated a method of optical pulse processing for PET detectors in which position, timing, and energy data is encoded into a series of fast (~100 ps) optical pulses at 1550 nm. This optical encoding method can potentially support a very high multiplexing ratio, allowing the majority of the readout electronics to be concentrated into a single high speed digitizer channel that records and decodes the event data from every detector, making it suitable for high resolution PET systems with many detectors. In our previous prototype, single threshold time-over-threshold (ToT) was used to encode energy and timing as it was relatively simple to implement with few components, minimizing added timing jitter. However, single threshold ToT has major drawbacks that degraded both timing and energy resolution, resulting in poor performance overall. In this work we replace single threshold ToT with a dual threshold ToT method using a custom circuit with only a few high speed, low jitter components, greatly improving timing and energy performance. With dual threshold ToT we can fully encode and multiplex position, timing, and energy information from two detector channels on just a single optical fiber with excellent timing resolution (160 ps FWHM with 3×3×5 mm³ LSO and Hamamatsu SiPMs), suitable for time-of-flight.