Time correlated single photon counting by time to digital conversion

A system for detection of chemical agents comprising:

• a laser producing laser pulses;
• a chemical sensing optical fiber configured to receive at least some portion of optical energy of the laser pulses;
• a laser event detector configured to receive some portion of the optical energy of the laser pulses and configured to produce a laser event detector electrical output;
• a delay generator configured to receive the laser event detector electrical output and configured to produce a delayed electrical output, wherein the delayed electrical output is based on a programmable delay;
• an event counter configured to receive the laser event detector electrical output and configured to produce an event output for each received laser event detector electrical output;
• a photon detector configured to receive Rayleigh backscattered light from the chemical sensing fiber and configured to produce a detected photon signal upon reception of a photon;
• a gate pulse generator configured to receive the delayed electrical output and configured to provide a gate pulse to the photon detector for turning on the photon detector for reception of photons during the duration of the gate pulse;
• a time to digital converter configured to produce a time interval measurement based upon elapsed time between the laser event detector electrical output and the detected photon signal;
• and
• a host processor configured to receive the event output and the time interval measurement and configured to produce a history of time interval measurements, whereby the history of time interval measurements correlates with the presence or absence of chemical agents at locations along the chemical sensing fiber.

A system for Raman detection from a target comprising:

• a laser producing laser pulses;
• a first optical apparatus configured to direct at least some portion of optical energy of the laser pulses towards the target;
• a laser event detector configured to receive some portion of optical energy of the laser pulses and configured to produce a laser event detector electrical output;
• a wavelength dispersion apparatus having an input and an output;
• a second optical apparatus configured to receive some portion of Raman scattered light from the target and configured to direct the Raman scattered light to the wavelength dispersion apparatus input;
• a delay generator configured to receive the laser event detector electrical output and configured to produce a delayed electrical output, wherein the delayed electrical output is based on a programmable delay;
• an event counter configured to receive the laser event detector electrical output and configured to produce an event output for each received laser event detector electrical output;
• a photon detector coupled to the output of the wavelength dispersion apparatus and configured to produce a detected photon signal upon reception of a photon;
• a gate pulse generator configured to receive the delayed electrical output and configured to provide a gate pulse to the photon detector for turning on the photon detector for reception of photons during the duration of the gate pulse;
• a time to digital converter configured to produce a time interval measurement based upon elapsed time between the laser event detector electrical output and the detected photon signal;
• and
• a host processor configured to receive the event output and the time interval measurement and configured to produce a history of time interval measurements, whereby the history of time interval measurements correlates with a transformation of a Raman spectra to a time domain representation of the Raman spectra.