Frequency spectrum detection system

1. A frequency spectrum detection system, comprising: a frequency-scan light source, a phase modulator, an optical filter, an optical fiber, a photodetector, a power divider, an electric amplifier, a combiner, an electric filter, and an oscilloscope, wherein,

a ring-shaped optoelectronic oscillator resonant cavity is defined by the frequency-scan light source, the phase modulator, the optical filter, the optical fiber, the photodetector, and the electric amplifier together, the optoelectronic oscillator resonant cavity being configured to generate a frequency-scan signal with adjustable bandwidth and adjustable center frequency when a Fourier domain mode-locking condition is satisfied;

the combiner is configured to receive a signal to be measured, and to combine the signal to be measured with a frequency-scan signal generated by self-excited oscillation in the optoelectronic oscillator resonant cavity to form a combined electrical signal, the combined electrical signal being input into an electrical signal input terminal of the phase modulator;

the phase modulator is configured to modulate the combined electrical signal, which is input through the electrical signal input terminal, onto a frequency-scan optical signal emitted from the frequency-scan light source, and is configured to output a double-sideband phase-modulated optical signal;

the optical filter is configured to selectively attenuate or amplify one sideband of double sidebands of the double-sideband phase-modulated optical signal;

the photodetector is configured to detect a signal filtered by the optical filter;

the photodetector, the power divider, the electric amplifier, the combiner, and the phase modulator are connected through cables; and

the power divider, the electric filter, and the oscilloscope are connected through cables.

2. The frequency spectrum detection system according to claim 1 , wherein the frequency-scan light source is a current-driven frequency-scan semiconductor laser or a single-sideband-modulation-based frequency-scan light source, and the frequency-scan light source has an emission wavelength which is changed periodically.

3. The frequency spectrum detection system according to claim 1 , wherein the optical filter is a notch optical filter, or an optical filter based on a stimulated Brillouin scattering effect gain spectrum.

4. The frequency spectrum detection system according to claim 1 , wherein,

the frequency-scan light source, the phase modulator, the optical filter, and the photodetector together form a microwave photonics filter, and

the microwave photonics filter has a frequency-scan period that matches a time delay due to a single pass of signals in a loop of the optoelectronic oscillator resonant cavity, which satisfies the following Fourier domain mode-locking condition:

nT=T r ;

wherein, n is a positive integer, T is a period of change of the microwave photonics and T r is the time delay due to a single pass of the signals in the loop of the optoelectronic oscillator resonant cavity.

5. The frequency spectrum detection system according to claim 1 , wherein the frequency spectrum detection system is configured to implement frequency spectrum detection in different frequency measurement ranges by changing a magnitude relationship between a frequency of the frequency-scan signal, a frequency of the signal to be measured and a center frequency of the electric filter.

6. The frequency spectrum detection system according to claim 1 , wherein the frequency spectrum detection system further comprises a polarization controller for controlling a polarization state of optical signals.

7. The frequency spectrum detection system according to claim 1 , wherein the frequency spectrum detection system further comprises an optical amplifier or a second electric amplifier for amplifying signals.

8. The frequency spectrum detection system according to claim 1 , wherein the loop of the optoelectronic oscillator resonant cavity comprises a single loop or multiple loops.