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.

We report a high-accuracy multi-frequency identification scheme based on stimulated Brillouin scattering (SBS) and frequency-to-time mapping (FTTM). In our scheme, a dual-drive Mach-Zehnder modulator (DDMZM) is driven by an unknown microwave signal to realize single-sideband (SSB) modulation. The optical carrier and the -1st optical sideband are processed as pump light to generate SBS in a 1-km highly nonlinear fiber (HNLF). A linearly chirped microwave (LCM) signal is fed to a phase modulator (PM) to generate a phase-modulated linearly chirped optical signal, which acts as the probe light in the SBS process. Actually, the narrow bandwidth SBS spectra are converted to microwave photonic filters (MPFs) after photodetection. In this way, the unknown frequency information of the microwave signal is mapped into the time domain and measured by calculating the time interval of the output electrical pulses. In the proof-of-concept experiment, single and multi-frequency microwave signals within a measurement range from L-band to Ku-band are successfully implemented with measurement errors of less than ±6 MHz.

A photonic multi-format background-free binarily modulated microwave signal generator using a single dual-polarization dual-parallel Mach–Zehnder modulator is proposed and demonstrated. The proposed generator features good reconfiguration and switchability. By simply adjusting the direct current biases of the modulator, amplitude shift keying (ASK), phase shift keying (PSK) and frequency-shift keying (FSK) microwave signals can be generated. Benefiting from the complementary intensity modulation, there is no residual baseband modulated signal in the generated binarily modulated signals. The system does not involve any optical or electrical filters, which ensures good tunability of the generator. By changing the frequency of the driven microwave carrier and bit rate of the electrical coded signal, the frequency and rate of the generated multi-format signals can be adjusted accordingly. The multi-format switchable signal generator has great potential in multi-functional communication and radar systems.

A widely tunable narrow-linewidth laser consisting of a sampled-grating distributed Bragg reflector (SG-DBR) laser and an external silica-based multi-period-delayed feedback (MPDF) photonic circuit is fabricated and experimentally demonstrated. The measured results show that a 58-nm lasing wavelength tuning range together with a more than 40 dB side-mode suppression ratio can be achieved. Notably, among the wavelength tuning range, all linewidths remain below 1 kHz, which are narrowed by more than three orders of magnitude compared to the average intrinsic linewidth of 3 MHz for the original SG-DBR laser. Besides, since the proposed MPDF structure has an ultra-dense distribution of the multi-period feedback, the linewidth reduction capability can be maintained over the entire operating wavelength range does not require free spectral range (FSR)-alignment or any extra phase match. Such widely tunable narrow-linewidth laser might offer a compact, cost-effective and easily-implemented solution for next-generation applications including ultra-wideband dense wavelength division multiplexing (DWDM) systems and light detection and ranging (LiDAR) systems.

With the significant expansion of communication capacity in fiber-optic networks, the directly modulated laser serves as a viable candidate due to its low cost and small footprint. In recent years, distributed feedback (DFB) lasers with an integrated passive feedback waveguide were demonstrated to enhance bandwidth by introducing a photon–photon resonance or detuned-loading effect. However, the butt-joint regrowth process between the active layer and passive waveguide complicates the fabrication. Here we propose and demonstrate a DFB laser with an integrated active feedback waveguide, utilizing identical active layer technology to simplify the fabrication process, achieving a bandwidth of exceeding 50 GHz. Additionally, transmission experiments with 55 Gbps NRZ and 70 Gbps PAM4 signals are also demonstrated.

We propose and demonstrate a highly efficient photonic-assisted microwave signal harmonic frequency down-conversion using four-wave mixing (FWM) in a silicon waveguide with a reverse-biased P-i-N junction. In the proposed method, two Mach-Zehnder modulators (MZMs) are driven by a radio frequency (RF) signal and a low-frequency local oscillation (LO) signal, separately. Then, the modulated LO signal is injected into the silicon waveguide. Besides, the FWM phenomenon occurred in the integrated waveguide due to the doped regions of the junction can increase the efficiency of free carrier removal from the waveguide region. As the applied bias voltage on the silicon waveguide is increased, the generated harmonic sidebands are further enlarged. Correspondingly, the conversion efficiency is up to 12.8 dB with the bias voltage of 30 V. The silicon waveguide is fully packaged with a grating coupling fiber array and a wire-bonded printed circuit board (PCB) to make the module stable and compact. By simply adjusting the low-frequency electrical LO, the down converter based on FWM can realize accurate tunable and ultra-wide frequency operation. Due to the strengths of silicon integrated waveguide doped with reverse-biased P-i-N junction, the high-order LO modulated optical sidebands can be generated and used to down-conversion high-frequency RF microwave signal. This scheme can significantly reduce the high-frequency demand for LO source. In the experiment, the RF signals ranging from 8 GHz to 31 GHz are all down converted within 2 GHz by utilizing the 5.8 – 8.2 GHz LO signals, and signal-to-noise ratios (SNRs) larger than 50 dB are achieved. Moreover, the phase noise of the intermediate frequency (IF) signal is as low as -80.12 dBc/Hz at an offset frequency of 10 kHz and spurious free dynamic range (SFDR) is 105.5 dB · Hz ^2/3 with 3 ^rd -order FWM effect for the LO signal and 103.8 dB · Hz ^2/3 with 4 ^th -order FWM effect for the LO signal. This shows that the down conversion signal is reliable to further use in modern electrical systems such as microwave photonic radar.