Obtaining Single Shot High-Resolution Frequency Spectrum of an Ultra-Short Signal Using an Analog Optical Auto-Correlation Technique

It is very difficult to obtain a high-resolution real-time frequency spectrum from an ultra-short duration pulsed high frequency signals due to the lack of a high speed analog-to-digital converters (ADC). Even if such an ADC exist, there is a fundamental limit by the uncertainty principle for the ultra-short observation/detection/digitization time. This work demonstrated a Radio frequency (RF)-Photonic real time spectrum analyzing technique by sending a single 30ns short microwave signal pulse into a fiber optic recirculation loop circuit to perform analog auto-correlation to resolve a two-tone signal pulse separated by 25 MHz for a 50 GHz wideband spectrum. This optical correlation technique allow us to increase the observation/measurement time by keep the “live” signal in the loop system over long time and “stretching” time using the dispersion of the two RF sidebands for time-correlation. This technique provides a “snapshot” in the frequency domain of ultra-short or transient events without the need of a high-speed ADC, and is able to push the fundamental physical limit in spectrum analyzation of short duration signals.