Single Photon Counting Devices
Single Photon Counting (SPC) and Time-of-Arrival (TOA) instruments are ideal for various near-infrared (NIR) applications, from single photon counting, to LADAR and LIDAR ranging, to time correlated single photon counting (TCSPC).
About Single Photon Counting
For the purposes of optical measurement and sensing, a single photon’s worth is the smallest amount of energy that can be extracted from the electromagnetic field, making a single photon the smallest amount of light that can be detected by absorption. Of course, even relatively strong optical signals can be reduced to a stream of “single†photons when viewed on a short time scale. Although the actual absorption of each individual photon is a random event that obeys Poisson’s distribution, a 1 W beam of 580 nm yellow light delivers one photon every 3.4×10-19 s on average. Of course, technologically speaking, no detector responds on such a short time scale, so the granular nature of strong optical signals manifests itself as shot noise on a continuous signal rather than discrete pulsing of the signal itself. Only in the limit of weak signals – 300 pW at 580 nm corresponds to an average of one photon every 1.14 ns – can one speak of a detector “counting†the arrival of individual photons. Extremely weak signals on this order are characteristic of laser radar returns from distant targets, the fluorescence of rarified species in spectroscopic experiments, and data in certain quantum information applications.
Most photon counting schemes for the visible (VIS, 400 – 720 nm), near infrared (NIR, 720 – 1800 nm), and short-wave infrared (SWIR, 1800 – 2500 nm) bands use the photon’s energy to liberate a mobile electron which is then accelerated in an electric field. The primary photoelectron eventually gains sufficient energy from the applied field to knock additional electrons free from a suitable medium, thereby creating a current pulse large enough to be detected by the circuit that monitors the detector.
