A photonic method to estimate the angle-of-arrival (AOA) of microwave signals based on optical phase scanning is proposed and experimentally demonstrated. At remote antenna unit (RAU), one received high-frequency microwave signal is directly applied to a phase modulator (PM), and another received high-frequency microwave signal coupled with a low-frequency large-voltage sawtooth-wave signal is applied to another PM. The sawtooth-wave signal is used to scan the phase of the optical sideband from 0 to 2π. After transmission over a segment of fiber link, the AOA parameter can be estimated by processing the received low-frequency electrical signals at central office (CO). Such scheme can be applicable to either single-tone or wideband microwave signals. Experimental results show that the AOA measurement errors are less than 2.27° and 4.45° for a single-tone signal at 10 GHz and a 2-GHz broadband signal with the center frequency of 18 GHz, respectively.

A widely frequency-tunable parity-time (PT) symmetric optoelectronic oscillator (OEO) implemented based on a single polarization-dependent Sagnac loop is proposed and experimentally demonstrated. Two mutually coupled feedback loops having an identical geometry with balanced gain and loss are implemented to achieve PT-symmetry, which is essential for single-mode selection without using an ultra-narrow passband microwave filter. In the implementation, a Sagnac loop consisting of a polarization beam splitter (PBS) and two polarization controllers (PCs) is employed which functions equivalently as two mutually coupled feedback loops, with the gain, loss and coupling coefficient being independently controllable. Since only a single physical loop is used, the implementation is greatly simplified and the stability is highly improved. The operation of the proposed OEO is experimentally verified. Stable single-mode oscillation with a wide frequency tunable range from 2 to 12 GHz is achieved. The phase noise is -128 dBc/Hz at an offset frequency of 10 kHz.

A wavelength-tunable single-longitudinal-mode fiber ring laser based on parity-time (PT) symmetry for mode selection is proposed and experimentally demonstrated. The PT symmetry is implemented using a single physical fiber-optic loop, which functions equivalently to two mutually coupled optical loops with an identical geometry but balanced gain and loss. In the proposed system, a linearly polarized light wave is split into two, which are traveling in a Sagnac loop incorporating a polarization beam splitter (PBS) and two polarization controllers (PCs). By controlling the polarization states of the light waves injected into the PBS via tuning the PCs, the gain and loss of the two equivalent loops corresponding to the light waves traveling in the clockwise (CW) and counter-clockwise (CCW) directions are controlled. When the gain and loss are balanced and exceed the coupling coefficient, PT-symmetry breaking condition is met, and single-longitudinal-mode lasing is achieved. The proposed PT-symmetric fiber ring laser is experimentally demonstrated. Single-longitudinal-mode lasing with a tunable wavelength from 1530 to 1565 nm and a narrow linewidth as small as 390 Hz is achieved. The sidemode suppression ratio is 41.9 dB confirming the effectiveness of using PT symmetry for mode selection.

An approach to transmitting two independent microwave vector signals on a single optical carrier with one polarization state based on coherent detection and digital phase noise cancellation is proposed and experimentally demonstrated. At the transmitter, two independent microwave vector signals are modulated on an optical carrier via a dual-drive Mach-Zehnder modulator (DD-MZM). The modulated optical signals are transmitted over a single-mode fiber (SMF) and sent to a coherent receiver. At the receiver, the optical signals are detected where a local oscillator (LO) optical wave generated by a second free-running laser source is also applied. To recover the two microwave vector signals, a novel digital signal processing (DSP) algorithm is developed and applied to eliminate the joint phase noise terms from the transmitter and the LO laser sources as well as the unstable offset frequency between the two laser sources. An experiment is performed. The transmission of two independent 16 quadrature amplitude modulation (16-QAM) microwave vector signals at 4 GHz with a symbol rate of 1 GSymb/s over a 9-km SMF is demonstrated. The transmission performance in terms of error vector magnitudes (EVMs) and bit error rates (BERs) is also evaluated.

High-fidelity modeling is of great importance for digital twin (DT) technology. A Transformer-based modeling method for radio over fiber (RoF) link is developed and experimentally demonstrated, in which the Transformer is a kind of deep learning network. The modeling performance of the constructed model by using the proposed method is evaluated, which is better than the ones based on fully connected neural network (FCNN), convolution neural network (CNN), and generative adversarial network (GAN). In addition, the modeling consistency with different laser launch power, fiber length, carrier frequency, and signal-to-noise ratio (SNR) is also investigated. Experimental results show that the fitting waveforms predicted by the proposed method agree well with the actual waveforms at the receiver end. The normalized mean square errors of the fitting waveforms are lower than the acceptable bound of 0.02.