A dual-cavity opto-mechanical microwave oscillator (OM-MO) for microwave photonic (MWP) generation with ultra-narrow linewidth based on radial (R) guided acoustic modes of a single-mode fiber (SMF) is proposed and investigated experimentally. The dual-cavity OM-MO consists of a 5 km SMF main ring, which provides forward stimulated Brillouin scattering (FSBS) gain, and a 300 m SMF subring that achieves single-frequency output of the R₀₇ guided acoustic mode based MWP (R₀₇-MWP) with Vernier effect. At 300 mW 980 nm pump threshold power, the 319.79 MHz R₀₇-MWP is generated by adjusting polarization controllers based on nonlinear polarization rotation effect, corresponding to the 7437th harmonic of the 43 kHz cavity round trip frequency. The 3 Hz ultra-narrow linewidth of R₀₇-MWP is achieved by decreasing the intrinsic linewidth of the passive ring resonator. The acoustic-mode and longitudinal-mode suppression ratios reach 22 and 36 dB, respectively. Within 20 min of the stability experiment, the power and frequency stability fluctuation of the R₀₇-MWP are ±1 dB and ±0.05 MHz, respectively. This ultra-narrow linewidth MWP generation technology has great potential in the communication field, especially in long-distance wireless communication transmission.

The interest in the field electron emission cathode nanomaterials is on the rise due to the wide applications, such as electron sources, miniature X-ray devices, display materials, etc. In particular, nanodiamond (ND) film is regarded as an ideal next-generation cathode emitter in the field emission devices, due to the low or negative electron affinity, small grain size, high mechanical hardness, low work function, and high reliability. Increasing efforts are conducted on the investigation of the emission structures, manufacturing cost, and field emission properties improvement of the ND films. This review aims to summarize the recent research, highlight the new findings, and provide a roadmap for future developments in the area of ND film electron field emitter. Specially, the optimizing methods of large-scale, high-quality, and cost-effective synthesis of ND films are discussed to achieve more stable surface structure and optimal physical properties. Additionally, the mainstream strategies applied to produce high field emission performance of ND films are analyzed in detail, including regulating the grain size/boundary, hybrid phase carbon content, and doping element/type of ND films; meanwhile, the problems existing in the related research and the outlook in this area are also discussed.

This article presents a flexibly tunable microwave photonic filter (MPF) with a dual ultra-narrow passband based on a dual-wavelength and narrow linewidth Brillouin laser. The dual passband of the filter not only exhibits ultra-high frequency selectivity but also allows for flexible and simultaneous tuning of the center frequency and interval of the passbands. In the proposed scheme, the core optical processor of the MPF consists of a dual-ring Brillouin laser resonator, which is composed of a 100-meter main fiber ring cascaded with a 10-meter secondary fiber ring. A dual-tone pump is generated through carrier-suppressed dual sideband modulation and injected into the dual-ring resonator to excite two Brillouin gains, which resonate separately in the dual-ring resonant cavity to generate a dual-wavelength single longitudinal mode Brillouin laser for processing optical RF signals. By selectively amplifying the modulated sideband signal through narrow linewidth Brillouin laser gain, the ultra-narrow dual passband are ultimately demodulated. The resulting dual filtering passband exhibits unparalleled sub-kHz bandwidth and flexible tuning capability. Experimental results demonstrate that the MPF proposed in this paper achieves an ultra-narrow dual passband of 83 Hz with a maximum side-mode suppression ratio of over 28 dB. Within the testing range of vector network analysis, the maximum tuning range of the center frequency is 20 GHz, and the passband interval can be freely tuned from 2-18 GHz. These results highlight the MPF’s ability to concurrently filter dual-channel signals with high precision and its significant potential application value in ultrafine microwave frequency spectrum resolution and processing fields, such as instantaneous frequency measurement and microwave photonic sensing.

We have demonstrated an erbium doped fiber amplifier (EDFA) gain equalizer for flattening the amplified spontaneous emission gain spectrum using a Sagnac loop incorporated with a single-section unpumped erbium-doped fiber. The gain spectra output from the transmission port and the reflection port of the Sagnac loop are studied in detail. The absorption effect of unpumped erbium-doped fiber and the resonance mode caused by the birefringence beat length in the loop are combined to make the gain spectrum be partially or completely flattened by adjusting the polarization controllers properly in the loop. On the one hand, the part of the spectrum is flatted within ±0.15 dB and ±0.145 dB in the wavelength range of 16 nm and 14 nm at the reflection port and transmission port respectively. On the other hand, the total spectrum is flatted within ±1.225 dB over a bandwidth of 36.5 nm in the whole C-band at the transmission port, while the two-peak with 29.6 nm interval occurred at the reflection port because of the 0.15-1.55 m birefringence beat length of single-mode erbium-doped fiber in the loop. The results are consistent with theoretical simulations. This flat technique has the potential to be used in communication technology, especially in wavelength division multiplexing to adjust multichannel gain simultaneously and multi-wavelength narrow-linewidth fiber lasers to achieve a wide range of stable light output.

What we believe to be a novel single longitudinal mode (SLM) triple-ring (T-R) self-seeded Brillouin fiber laser (BFL) featuring outstanding stability, an ultra-narrow linewidth, and a high optical signal-to-noise ratio (OSNR) is proposed and experimentally investigated in this study. The key innovation in this design is eliminating the need for an additional costly ultra-narrow linewidth pump source typically required in conventional BFL. Instead, the laser preferentially excites stimulated Brillouin scattering (SBS) in the fiber through the self-seeded cavity mode dominant in the cavity. This approach generates Brillouin stokes and leverages the Vernier effect of the T-R resonator structure to suppress multimode oscillations, ensuring the generation of a Brillouin laser with an ultra-narrow linewidth and enabling SLM operation. The experimental results show that with the output power of 980-nm LD fixed at 400 mw, the OSNR of the self-seeded BFL spectrum reaches 54 dB, and the side mode suppression ratio (SMSR) is 22 dB. The -3 dB linewidth of the self-seeded BFL can be measured by the heterodyne beat frequency method at 30 Hz, and the output power and wavelength fluctuations are lower than 1.318 dB and ±0.007 nm, respectively, during the sixty minutes observation period. Additionally, the wavelength of the self-seeded BFL can be flexibly tuned within the range of 1560 – 1575 nm. This innovative approach demonstrates significant theoretical and practical implications for the development of low-cost, high-performance BFL systems compared to traditional BFL methods.