Precise distance, time, and pulse width measurements are required for many areas of science and engineering. Time-of-flight methods are often used to measure remote distances. Optical signals are attractive because the small wavelength allows small transmit and receive telescopes. Short pulsed lasers are attractive sources that can provide high-distance measurement precision. Fast optical detectors are typically used at the receiver. Measurement precision is usually limited by the detectors’ temporal bandwidth. Picosecond and sub-picosecond pulses are difficult to measure and resolve. Optical autocorrelators have been used that employ nonlinear crystals. The optical autocorrelators require strong optical signals that can limit or preclude their use for remote, long-distance measurements.
An optical correlation receiver was developed that provides ultra-precise distance and/or time/pulse width measurements even for weak (single photons) and short (femtosecond) optical signals. The receiver enables precise distance/time or pulse width measurements using optical signals. The optical correlator is formed with a beamsplitter and two detectors, and a means of delaying one arm of the path between the beamsplitter and a detector. The optical correlator uses a low-noise-integrating detector that can resolve photon number (intensity).
A key part of the invention is in the processing of the electrical signals from the two detectors. The heart of the invention is to form the correlation product integral kernel by calculating the variance of the photon number (intensity) of the difference of the optical signals on the two detectors.
This work was done by Michael Krainak of Goddard Space Flight Center. GSC-17107-1