Method and apparatus for analyzing the spectrum of radio-frequency signals using unamplified fiber optic recirculation loops

An apparatus for generating a frequency spectrum of an RF signal comprising:

• a gate switch for generating an output signal pulse from a laser of wavelength lambda modulated by an input RF signal using a Mach Zehnder modulator;
• a fiber optical loop having a length of approximately 1 Km;
• for circulating a first pulse of the output signal pulse from the gate switch, for a predetermined number of cycles n where each cycle takes time t1;
• a low-loss high-speed fiber optical switch where the switch is approximately faster than a microsecond, coupled to the gate switch, in a first state for coupling the output signal pulse from the gate switch to a coupler said coupler having a coupling ratio of 1% to 99%, where the first percentage is 1% and the second percentage is 99%, the coupler tapping out a first percentage of the output signal pulse from the fiber optical loop as a replica pulse and coupling a second percentage of the output signal pulse into the first fiber optical loop, and a second state for coupling the fiber optical loop to the coupler;
• and
• a processor for correlating a predetermined amount of replicas of the pulse with each other to produce a set of data points comprising a plurality of multiplexed correlated pulses and transforming the data points into a channelized frequency spectrum of the input RF signal wherein further comprising:
• a control circuit configured to synchronize the gate switch and the low-loss high speed fiber optical switch.

A method for determining a channelized frequency spectrum from an RF signal comprising:

• conducting, in a first fiber optical loop, a first percentage of a pulse an RF modulated laser light generated by modulating a carrier laser with an RF input signal, for a predetermined number of cycles n, each cycle equal to a time period t1;
• conducting, in a second fiber optical loop, a second percentage the pulse, for a predetermined number of cycles k, each cycle equal to a time period t2, where t2*k=t1*n;
• switching state of a switch to a state configured to permit the second fiber optical loop after n*t1 cycles to couple the second percentage of the pulse into the first fiber optical loop to increase the intensity of the pulse in the first fiber optical loop;
• tapping out, after every cycle, a pulse replica of the pulse in the first fiber optical loop, forming a set of pulse replicas;
• generating a frequency spectrum from the set of pulse replicas by autocorrelating the pulse replicas to generate data points and applying a Fourier Transform to the autocorrelated data points.