Dissipative Kerr solitons (DKSs) in microresonators have boosted the development of the chip-scale ultrastable microcomb sources, and thrived in both fundamental physics and a wide range of applications. Among various DKS states, single DKS, double DKS, and a perfect soliton crystal could be identified simply based on the optical spectrum. Especially, the double DKS state, due to its two-pulse-interference nature, has recently found its own application in microwave photonics, such as reconfigurable rf filters. However, the traditional method to generate double DKS usually yields stochastically relative positions of the DKS in the cavity, which limits the versatility of the application. Here a method to deterministically generate double DKSs with fixed relative positions assisted by the dual-pump driven scheme in a 97-GHz ${\mathrm{Si}}_3{\mathrm{N}}_4$ microresonator is demonstrated. Via the dual-pump scheme, not only has the single soliton been repeatably generated, double DKS with deterministic relative positions has also been realized through backward tuning. The effects of pump wavelength tuning and pump power on the relative positions are investigated. A direct bridge between the relative positions and the avoided mode crossing induced modulated cw background is established. This work not only provides insight to DKS dynamics in a dual-pump scheme, but also improves the versatility of double-DKS based applications in microwave photonics.

Dissipative Kerr soliton microcombs in microresonators have enabled fundamental advances in chip-scale precision metrology, communication, spectroscopy, and parallel signal processing. Here we demonstrate polarization-diverse soliton transitions and deterministic switching dynamics of a self-stabilized microcomb in a strongly-coupled dispersion-managed microresonator driven with a single pump laser. The switching dynamics are induced by the differential thermorefractivity between coupled transverse-magnetic and transverse-electric supermodes during the forward-backward pump detunings. The achieved large soliton existence range and deterministic transitions benefit from the switching dynamics, leading to the cross-polarized soliton microcomb formation when driven in the transverse-magnetic supermode of the single resonator. Secondly, we demonstrate two distinct polarization-diverse soliton formation routes – arising from chaotic or periodically-modulated waveforms via pump power selection. Thirdly, to observe the cross-polarized supermode transition dynamics, we develop a parametric temporal magnifier with picosecond resolution, MHz frame rate and sub-ns temporal windows. We construct picosecond temporal transition portraits in 100-ns recording length of the strongly-coupled solitons, mapping the transitions from multiple soliton molecular states to singlet solitons. This study underpins polarization-diverse soliton microcombs for chip-scale ultrashort pulse generation, supporting applications in frequency and precision metrology, communications, spectroscopy and information processing.

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.

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.

A scalable electrical method is proposed for measuring high-frequency responses of photodetectors (PDs) through harmonic up-conversion based on pilot-aided electro-optical stimulus. The proposed method provides wideband optical stimulus by the electro-optical harmonic up-conversion and subtracts the frequency response of the assisted electro-optical Mach-Zehnder modulator (MZM) based on the pilot insertion and extraction fixed at low frequency, which is free of any golden intensity modulator (IM) of known performance. Moreover, it achieves wideband and scalable high-frequency measurement of PDs with n -fold measuring frequency range, where the sweeping frequency up to f_s of the assisted MZM enables the measuring frequency range of n × f_s . In the experimental demonstration, a commercial PD is measured with an assisted MZM under a frequency-swept modulation up to 16.75 GHz and a fixed-low-frequency pilot signal at 80 MHz, and the frequency responses are evaluated up to 67 GHz. Moreover, the feasibility of the scalable scheme is proved by those cases of 1, 2, 3, and 4-fold measuring frequency range. The consistency between our method and the two-tone mixing method verifies that the scalable high-frequency measurement provides a simple but accurate method for the performance characterization of high-speed PDs.

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.