The PDM-UV photon counting detectors are all solid-state instruments that detect light from 300 nm to 1000 nm wavelength range. They have a peak photon detection efficiency of 43% at 430 nm and generate a TTL and a NIM output pulse per detected photon. Output pulses provide better than 100 ps (FWHM) timing resolution, better than 350 ps at FW1/10M and better than 600 ps at FW1/100M. Indeed, the PDM-UV Instrument Response Function (IRF) shows a very fast tail, as low as 75 ps of decay time at 670 nm, i.e. much faster than the typical tail that can be measured when using a standard Micro Photon Devices PDM detector.

The excellent photon detection efficiency in the UV region and the superior timing resolution are obtained by using a CMOS Single Photon Avalanche Diode (SPAD) having a specifically engineered thin junction. This SPAD is then monolithically connected, i.e. in the same silicon chip, to an integrated Active Quenching Circuit (iAQC), specifically designed and optimized for photon counting applications. The SPAD is thermoelectrically cooled, and its temperature is controlled to ensure stabilized performance despite ambient temperature changes. The module is also fully protected against light overload of short duration. The PDM-UV works in free running mode only.

The PDM-UV typical dead-time is 95 ns and therefore the maximum saturated count rate is about 10 Mc/s. With such dead-time and without applying any deadtime correction, the linearity error is below 5% if the detected count-rate is smaller than 500 kc/s; by applying well known dead-time correction formulas, and using the deadtime value indicated in the delivered test report, the linearity error has been experimentally verified to be below 5% for detection counting rates up to 5 Mc/s. The recommended maximum count-rate for the PDM-UV is therefore 3-4 Mc/s.

The PDM-UV series is available with two optical interfaces: a free space optical window (PDM-UV) and a version where a miniaturized optical focusing system has been integrated with the SPAD (PDM-UV-EC). The latter has been designed to greatly enhance module’s collection efficiency, has a nominal demagnification of 10× and focuses the collected light onto the 50 µm diameter SPAD. Light is collected at a collection plane placed in front of the module, nominally 13 mm away from the outermost glass surface. The equivalent collection area at the collection plane is not defined by design, but it depends on the incoming beam aperture and diameter; for nearly collimated beams, it is expected to be about at least 100 times the area of the 50 µm diameter detector, i.e., equivalent to having a 500 µm diameter sensor. Main figure of merit is the percentage of optical power that is collected and sent to the detector, i.e., the collection efficiency. For example, in case of an incoming beam with 20 degrees full angle and 400 µm diameter, the coupling efficiency would be around 50%. Every photon that will reach the detector will be detected with the SPAD PDE shown in the first page. Collection plane actual position may vary (+/- 3 mm) due to assembling tolerances and mainly with wavelength, so it should be found for each PDM-UV-EC, scanning along the Z axis and looking for the maximum signal out of the detector.