A photonic-assisted multiple microwave frequency measurement approach based on stimulated Brillouin scattering (SBS) and frequency-to-time mapping with high accuracy and a wide frequency measurement range is reported. A frequency sweeping signal is modulated on an optical carrier that is shifted by the Brillouin frequency shift as a frequency sweeping optical probe wave. The unknown microwave frequencies are modulated on the original optical carrier to pump a length of single-mode fiber. Thanks to the SBS effect, the Brillouin gain spectrum is detected by the frequency sweeping optical probe wave. After the optical signal from the probe branch is detected at a photodetector, the amplitude of the direct-current component will be much increased at the exact time when the SBS gain generated by the unknown signal overlaps with a specific optical wavelength of the frequency sweeping optical signal. Therefore, the unknown microwave frequencies are mapped to the time domain. More importantly, by introducing a two-step accuracy improvement, including the reference calibration and the curve smoothing, the measurement accuracy of the proposed approach is greatly improved compared with most of the available approaches. An experiment is performed. Microwave frequency measurement from 6 to 18 GHz is demonstrated with a measurement error of less than ±1 MHz.

A photonic approach for simultaneous wideband radio-frequency (RF) self-interference cancellation and frequency downconversion in an in-band full-duplex radio-over-fiber system is proposed based on a dual-polarization quadrature phase-shift keying (DP-QPSK) modulator. The upper dual-parallel Mach–Zehnder modulator (DP-MZM) in the DP-QPSK modulator is used to cancel the self-interference directly in the optical domain and generate two sidebands of the desired RF signal, whereas the lower DP-MZM generates two sidebands of the local oscillator. After detecting the combined optical signal at a photodetector, the desired RF signal is downconverted to an intermediate frequency (IF) signal with the self-interference cancelled. The proposed approach can overcome the reappearance of the interference signal and the power fading effect of the received signal caused by fiber dispersion. An experiment is performed. The cancellation depth for single frequency interference cancellation is around 58 dB, whereas that for the wideband interference cancellation is larger than 25 dB. The fading effect of the proposed IF transmission system is also evaluated compared with the traditional RF transmission system.

In this paper, all-optical short-time Fourier transform (STFT) based on stimulated Brillouin scattering (SBS) is proposed and further used for real-time time-frequency analysis of different radio frequency (RF) signals. In the proposed all-optical STFT scheme, SBS not only provides a band-pass filter for implementing the window function in conjunction with a periodic frequency-sweep optical signal but also obtains the frequency domain information in different time windows through the generated waveform via frequency-to-time mapping (FTTM). A periodic frequency-sweep optical signal is generated and then modulated at a Mach–Zehnder modulator by the electrical signal under test (SUT). During different sweep periods, the fixed Brillouin gain functions as a bandpass filter to select a specific range of the spectrum, which is equivalent to applying a sliding window function to the corresponding section of the temporal signal with the help of the frequency-sweep optical signal. At the same time, after the optical signal is selectively amplified by the SBS gain and converted back to the electrical domain, SBS also implements the real-time FTTM, which can be utilized to obtain the frequency domain information corresponding to different time windows through the generated waveforms via the FTTM. The frequency domain information corresponding to different time windows is formed and spliced to analyze the time-frequency relationship of the SUT in real-time. We demonstrate the concept experimentally. STFTs of a variety of RF signals are carried out in a 12-GHz bandwidth limited only by the equipment, and the dynamic frequency resolution is better than 60 MHz.

An on-chip two-step microwave frequency measurement method with high accuracy and an ultra-wide frequency measurement range is reported. A silicon photonic integrated micro-disk resonator (MDR) array is used to coarsely measure the signal frequency via an array of add-drop MDRs with smaller disk radii; then an MDR with a larger radius is used to finely measure the signal frequency, which is done by monitoring the optical powers of the optical signals from the through port and drop port of the MDRs. The proposed system features a very compact structure, ultra-wide frequency measurement range, and high frequency measurement accuracy, which is verified by a proof-of-concept experiment using two MDRs with radii of 6 and 10 μm. A frequency measurement of microwave signals from 1.6 to 40 GHz is implemented with a measurement error of less than 60 MHz. The stability of the system is also evaluated.

A novel photonic approach to generate high-speed and wideband frequency-hopping microwave signal via switching the bias point of a dual-drive Mach-Zehnder modulator (DD-MZM) is proposed and experimentally demonstrated. The core component of the system is a DD-MZM. By switching the bias point of the DD-MZM between the quadrature transmission point and the minimum transmission point, frequency hopping between two frequencies can be realized with an ultra-high frequency-hopping speed and a very wide bandwidth. The proposed approach has a compact structure and good tunability. A proof-of-concept experiment is performed to verify the principle of the proposed technique. Tunable frequency-hopping microwave signals between two frequencies of 4 and 8 GHz, 9 and 18 GHz, or 12 and 24 GHz are generated with a frequency-hopping speed up to 1 GHz.