An approach to generating microwave frequency combs (MFCs) with superior performance is proposed and experimentally demonstrated based on an optically injected semiconductor laser (OISL). The OISL operates at the period-one (P1) oscillation state under proper injection conditions. A sinusoidal voltage signal is used to modulate the P1 state for the initial MFC generation, and then two optoelectronic feedback loops are introduced to enhance the performance of the MFC: a short-delay feedback loop is firstly applied to improve comb contrast based on Fourier domain mode locking (FDML), and a long-delay feedback loop is added to reduce the comb linewidth based on the self-injection-locking technique. In the proof-of-concept experiment, a K-band MFC (18–26 GHz) with a line spacing of 8.45 MHz is obtained, where a comb linewidth of approximately 500 Hz and a comb contrast over 45 dB are simultaneously achieved. Additionally, each comb component exhibits superior performance in terms of phase noise, all below − 90 d B c / H z at 10 kHz offset, demonstrating an excellent coherence among these combs.

We propose an approach to generating nonlinear frequency-modulated (NLFM) microwave waveforms, which is based on controlled period-one (P1) dynamics of an optically injected semiconductor laser (OISL). When the optical injection is modulated, the OISL, which originally operates at a P1 oscillation state, acts as a microwave voltage-controlled oscillator (VCO). In the proposed system, the microwave frequency output depends closely on the optical injection strength controlled by the modulation voltage input, while the electrical modulation signal required to generate a desired NLFM microwave waveform can be calculated on the basis of the “voltage-to-frequency” transfer function of the established VCO system. Our simulations and experiments demonstrate that both single-chirp and dual-chirp NLFM microwave waveforms can be readily generated with a bandwidth up to 9 GHz. Considering peak-to-sidelobe ratio (PSLR) of the compressed pulses, the NLFM signals generated by the VCO exhibit a practical improvement of ∼13 dB when compared with LFM signals with the same bandwidth, and the tunability of the generated NLFM signals is also experimentally demonstrated.

With the exponential growth in demand for transmission based on optical communication technology, providing secure transmission is crucial for the orderly development of individuals and groups in contemporary society. In the context of free-space optical (FSO) communication, we propose and experimentally demonstrate a novel physical layer security transmission scheme based on phase encryption controlled by chaotic sequences. Utilizing the chaotic Chen's equation to generate pseudo-random sequences as keys, we control the modulation of PAM4 signals on phase modulator to achieve phase encryption and decryption of user data. We successfully achieved 10 Gbps NRZ signal transmission over a free-space distance of approximately 3 meters, with a tested bit error rate (BER) lower than the hard-decision forward error correction (HD-FEC) limit. Additionally, we analyzed the influences of various encryption depths, FSO conditions, and delay matching on the system's transmission performance. To the best of our knowledge, this is the first application of chaotic phase encryption in FSO communication scenarios, which will inspire future developments in secure optical communication.

A bandwidth-enhanced linear frequency-modulated (LFM) waveform generation scheme is proposed and demonstrated based on dynamic control of an optically injected semiconductor laser (OISL). The OISL operates at the period-one (P1) oscillation state under proper injection conditions. After photodetection, a tunable microwave signal is obtained with its frequency determined by the optical injection strength and the detuning frequency between the master and slave lasers. For a fixed detuning frequency, an LFM waveform can be generated by introducing an electrical control signal S(t) with a quasi-sawtooth profile to dynamically manipulate the injection strength of the OISL. Then, to overcome the bandwidth limitation by the achievable P1 frequency range under a given detuning frequency, both the injection strength and the detuning frequency are dynamically controlled to achieve a synthesized P1 frequency range, thus generating LFM waveforms with enhanced bandwidths. In our demonstration, LFM waveforms with a synthesized bandwidth of 8 GHz (12–20 GHz) and 24.8 GHz (12.6–37.4 GHz) are generated in the experiment and simulation, respectively.

A novel scheme for a high-performance microwave photonic filter (MPF) based on vector composite stimulated Brillouin scattering (SBS) is proposed and experimentally demonstrated. The proposed MPF achieves an ultra-narrow passband, free from the inherent bandwidth limitations of natural SBS, and exhibits a high out-of-band rejection. By utilizing composite SBS technology (two Brillouin loss spectra are superimposed upon a central Brillouin gain), the MPF's bandwidth is effectively narrowed. However, the superimposed losses lead to a decrease in SBS gain, resulting in reduced out-of-band rejection of the MPF. To achieve narrow bandwidth and high out-of-band rejection simultaneously, vector SBS technology is introduced. In vector SBS, suppression of unamplified spectral components and SBS gain compensation are implemented through polarization state and power adjustments of the pump wave. Benefiting from the suppression of unamplified spectral components, the proposed MPF demonstrates higher out-of-band rejection at the same SBS gain. Consequently, the MPF's response exhibits a remarkably steep slope due to the narrowed bandwidth and increased out-of-band rejection. In the proof-of-concept experiment, the 3-dB and 20-dB bandwidths of the MPF are narrowed to 5.5±0.1 MHz and 14.4±0.2 MHz, respectively, with a high out-of-band rejection of 55 dB. The slope of the MPF response reaches 22.24 dB/oct. Additionally, a tunable central frequency ranging from 11 GHz to 20 GHz is demonstrated. The effectiveness of the MPF is further validated by suppressing two strong interferences, placed 100 MHz away from the signal, by 56 dB while only a minimal 0.3 dB loss is observed in the signal power due to SBS gain.

We propose and demonstrate a high-precision phase noise analysis method based on a photonic-assisted microwave phase shifter (MPS) without nonlinear phase distortions (NPD). The proposed scheme utilizes a dual parallel Mach Zehnder modulator (DPMZM) and an optical bandpass filter (OBPF) to implement the photon-assisted microwave phase shifter. To avoid additional frequency noise induced by frequency fluctuations of the optical carrier, we employ a dispersion compensation photonic delay line as the delay line component. The MPS operates in the optical single sideband modulation (OSSB) mode with even-order sideband suppression, allowing continuous phase tuning over the entire $\mathbf { 360^{\circ }}$ range and eliminating NPD commonly encountered in traditional microwave photonic phase shifters. Experimental results demonstrate significant improvements in the phase and magnitude responses of the MPS without NPD compared to those with NPD. The proposed system exhibits long-term stability, with phase drift and amplitude drift of less than $\mathbf {5^{\circ }}$ and 1 dB, respectively, over continuous operation for more than 1500 seconds. We successfully measured microwave signals at the frequency of 8 GHz, 10 GHz, and 12 GHz with phase noise of -66.7, -65.5, and -62.1 dBc/Hz at 10 kHz offset frequency without resetting the proposed system. The method could be used to characterize electronic and photonic oscillators, which is highly reconfigurable and widely tunable.