Generation, optimization, and application of ultrashort femtosecond pulse in mode-locked fiber lasers

We propose a broad band and high absorption adjustable ideal absorber. The whole structure is Cr-Ge-Si 3 N 4 -Ti from bottom to top. The simulation results show that three resonant absorption modes are generated at 9.15 μm, 10.68 μm and 15.17 μm, with absorption rates of 92.1%, 97.3% and 95.2%, respectively. The bandwidth of the absorber is broadened by adding a lossy dielectric layer. Then, the control variables of the structural geometry parameters of the absorber were studied respectively to obtain the best optimization parameters. Since then, comparisons of different underlying microstructures have confirmed the uniqueness of our structure. Finally, we simulate the absorption of the absorber in different polarization modes with varying incident angles. In addition, the performance of previous work and our absorber are compared. These studies show that our absorber has active tunability, polarization and incident angle sensitivity, and high absorption bandwidth. • The results of FDTD theory show that the absorber based on MDM structure has high absorption bandwidth and three resonant absorption peaks in the mid-infrared band, which indicates that the absorber has wavelength selectivity. • By tuning the geometric parameters of the absorber structure, the active tuning of the absorber in a wide range is realized. • The absorption of the absorber under the change of incident angle and polarization mode is simulated, and it is determined that the absorber is insensitive to polarization and incident angle. • By adding a lossy dielectric layer, the bandwidth of the absorber can be broaded. Long wavelength infrared (LWIR) wideband absorption is an important target in the research of middle infrared band. In this work, we study the absorption characteristics of Cr-Ge-Si 3 N 4 -Ti metamaterial absorber based on nanowire cross surrounding. The absorber has polarization independence when the light wave is incident vertically. Due to the coupling of surface plasmon resonance and the intrinsic absorption of lossy material, the numerical simulation calculation shows that the bandwidth with absorptivity greater than 90 % reaches 7.23 μm, and the average absorptivity in the LWIR band (8.98–16.21 μm) is 94.1 %. The absorption spectrum can be adjusted independently by changing the geometric parameters of the structure. Moreover, the absorber has excellent incident angle insensitivity in TE and TM modes. The designed absorber is simple in structure, easy to configure, and suitable for infrared imaging, thermoelectronic devices and thermal detection applications. 

Ultra long infrared metamaterial absorber with high absorption and broad band based on nano cross surrounding

As a two-dimensional planar material with low depth profile, a metasurface can generate non-classical phase distributions for the transmitted and reflected electromagnetic waves at its interface. Thus, it offers more flexibility to control the wave front. A traditional metasurface design process mainly adopts the forward prediction algorithm, such as Finite Difference Time Domain, combined with manual parameter optimization. However, such methods are time-consuming, and it is difficult to keep the practical meta-atom spectrum being consistent with the ideal one. In addition, since the periodic boundary condition is used in the meta-atom design process, while the aperiodic condition is used in the array simulation, the coupling between neighboring meta-atoms leads to inevitable inaccuracy. In this review, representative intelligent methods for metasurface design are introduced and discussed, including machine learning, physics-information neural network, and topology optimization method. We elaborate on the principle of each approach, analyze their advantages and limitations, and discuss their potential applications. We also summarize recent advances in enabled metasurfaces for quantum optics applications. In short, this paper highlights a promising direction for intelligent metasurface designs and applications for future quantum optics research and serves as an up-to-date reference for researchers in the metasurface and metamaterial fields. 

pdf/recent-advances-in-metasurface-design-and-quantum-optics-37vvgte4.pdf

Recent advances in metasurface design and quantum optics applications with machine learning, physics-informed neural networks, and topology optimization methods

Mode-locked fibre lasers are important sources of ultrashort pulses, where stable pulse generation is achieved through a balance of periodic amplitude and phase evolutions. A range of distinct cavity pulse dynamics have been revealed, arising from the interplay between dispersion and nonlinearity in addition to dissipative processes such as filtering. This has led to the discovery of numerous novel operating regimes, offering significantly improved laser performance. In this Topical Review, we summarise the main steady-state pulse dynamics reported to date through cavity dispersion engineering, including average solitons, dispersion-managed solitons, dissipative solitons, giant-chirped pulses and similaritons. Characteristic features and the stabilisation mechanism of each regime are described, supported by numerical modelling, in addition to the typical performance and limitations. Opportunities for further pulse energy scaling are discussed, in addition to considering other recent advances including automated self-tuning cavities and fluoride-fibre-based mid-infrared mode-locked lasers.

Dispersion engineering of mode-locked fibre lasers

A single-longitudinal-mode triple-ring Brillouin fiber laser (TR-BFL) with a saturable absorber ring (SAR) resonator is proposed and investigated experimentally. Triple ring is composed of a 1-km-long single-mode fiber (SMF) ring, a 100-m-long SMF ring, and an SAR with 8-m-long unpumped Erbium-doped fiber, respectively. Compared with the existing techniques, a better performance of TR-BFL can be obtained because of the 1-km-long cavity length and the triple-ring configuration with an SAR, which suppresses the weaker unwanted mode and does not require power control accurately and import additional loss. At the same time, a frequency-locking system based on Pound–Drever–Hall technology is used to reduce the external perturbations and the instability caused by 1-km-long cavity and triple-ring configuration, respectively. In the experiment, 185 linewidth-reduction ratio is obtained while still maintaining 72-dB optical signal-to-noise ratio. Meanwhile, approximately 65-Hz linewidth is measured with a delayed self-heterodyne method using a 100-km delay fiber, and 5% power fluctuation at 1-mW laser output power is observed every 1 min in 50 min.

Single-Longitudinal-Mode Triple-Ring Brillouin Fiber Laser With a Saturable Absorber Ring Resonator

We show numerically and experimentally the transition between two distinct mode-locking regimes via spectral filtering of frequency-chirped pulses in passively mode-locked net-normal dispersion fiber lasers (NDFLs), resulting in the realization of similariton-dissipative soliton fiber lasers. Two operating modes, amplifier similaritons (ASs) and dissipative solitons (DSs), can switch to each other by solely altering the filter bandwidth in the laser oscillator. The laser favors AS operation for small filter bandwidth and it jumps to DS operation by increasing the filter bandwidth. A narrow filter narrows pulses both in the time domain and in the frequency domain due to the large chirp, and when the pulse re-enters in the gain fiber, it can evolve into an attractive similariton in a definite gain fiber. For a broadband filter, the weak filtering effect indirectly enhanced the self-phase modulation in the laser oscillator, so as to force the laser working in the regime of DSs. Contrary to the previous reports on pulse shaping in NDFLs based on the shape of spectral filters or filter-induced dispersion change, to the best of our knowledge, this is the first time that the proposition that the adjustable-bandwidth filter can be implemented as the key element for realizing switchable AS-DS fiber lasers has been put forward.

Spectral filtering effect on mode-locking regimes transition: similariton-dissipative soliton fiber laser

We present an L-band dissipative soliton (DS) fiber laser, which can deliver 87.5 fs pulses at 1.6 μm band. Numerical simulations are used to confirm the DS generation, and prove the pivotal component of the invisible filter with proper bandwidth in the formation of DS pulses. Such a robust, compact ultrafast laser source with higher pulse energy is hence an excellent seed source for L-band amplifiers. The mechanism revealed in the simulations is helpful to develop a unified theory for understanding various mode-locking regimes in normal dispersion lasers.

Sub-90 fs dissipative-soliton Erbium-doped fiber lasers operating at 1.6 μm band

We have experimentally demonstrated superluminal propagation through a 500 m optical fiber using stimulated Brillouin scattering (SBS), which is the longest superluminal propagating distance reported so far, to the best of our knowledge. Brillouin-induced loss resonance for fast light generation of pump signals is achieved in a single frequency Brillouin lasing oscillator with a highly nonlinear fiber as the SBS medium. The single frequency operation is realized by embedding the saturable absorber fiber loop with an unpumped erbium-doped fiber. Consequently, signals with a pulse width of 710 ns experience a maximum advancement of 1330 ns. Furthermore, the effects of pulse width and the duty cycle of pump signals are also investigated.

Superluminal propagation through 500 m optical fiber via stimulated Brillouin scattering.

Anomalous generalized Brewster effect and ultra-broadband reflectionless Brewster metasurfaces

An all-fiber 1.3-μm femtosecond pulse source has been demonstrated via Cherenkov radiation in highly nonlinear fiber seeded by an ultrafast Er-doped fiber laser. Through the dispersion management in total process, the generated pulses can be as short as 29 fs. This pulse source has a 15% conversion efficiency accounting for the input power and the single pulse energy reaches 76 pJ. Furthermore, a simulation has been carried out to illustrate the pulse evolution on the spectral and temporal progress in fibers. To the best of our knowledge, this is the first report of all-fiber 1.3 μm ultrashort pulse source with the duration below 30 fs.

All-Fiber Generation of Sub-30 fs Pulses at 1.3-μm via Cherenkov Radiation With Entire Dispersion Management

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