A 4 x 128 SPAD array with a 78-ps 512-channel TDC for time-gated pulsed Raman spectroscopy

A time-gated 4 x 128 single photon avalanche diode (SPAD) line array with a 512-channel time-to-digital converter (TDC) has been designed and fabricated in a 0.35 mu m high voltage CMOS technology for pulsed Raman spectroscopy. In Raman spectroscopy the SPAD array can be designed as a line array with a high fill factor because the electronics can be placed on either side of it. The resolution of the designed TDC is 80 ps in the first four bins, to achieve an accurate time position of the Raman photons, while the last four bins are increased in width to achieve a larger dynamic range. The measured standard deviation of the variation in the width of the first four bins of the 512-channel TDC was less than 10 ps. In addition, cross-talk measurements were carried out to investigate the probability of cross-talk between adjacent SPADs in an array of high density. These showed a cross-talk probability of 0.12 % with a pitch of 32 mu m between adjacent SPADs. The highest probability of cross-talk was found to occur with a short delay.