Duanduan Wu

Bessel-vortex beams, which combine non-diffracting propagation and orbital angular momentum (OAM), are promising for infrared photonics. However, existing generation methods suffer from system complexity and poor mid infrared compatibility. Here, we propose a monolithic curved fork grating (CFG) design fabricated via femtosecond laser direct writing in chalcogenide glass. Leveraging the high refractive index of the glass (As₂₇S₆₃), which provides a distinct etch depth benefit, we demonstrate, for the first time, direct writing of a device generating 1550 nm Bessel vortex beams on a single substrate. Through parametric optimization and employing processing strategies such as line array filling and double layer writing with two pass scanning, binary phase gratings were successfully fabricated. The devices generate Bessel vortex beams with topological charges (TCs) ranging from 1 to 5, achieving diffraction efficiencies of approximately 30% for both ±1st orders and a non diffracting distance of ~700μm. Performance surpasses that of fused silica counterparts, highlighting the material advantages enabled by the high refractive index. This work presents a viable route toward integrated, high performance structured light generation in the infrared, with inherent potential for extension into the mid infrared regime.

We demonstrate a 1.6 µm single-longitudinal-mode (SLM) fiber laser achieving an ultra-narrow linewidth of 185 Hz, enabled by a hybrid approach combining a saturable absorber filter (SAF) and distributed Rayleigh feedback (DRF). An optimized length of 2.1-m unpumped erbium-doped fiber acts as an SAF to enforce SLM operation, while a 17.5-m ultra-high numerical aperture (NA = 0.41) fiber provides effective DRF for linewidth compression, replacing tens of kilometers of standard single-mode fiber. Under a maximum pump power of 775 mW, the laser operates at 1608.75 nm, achieving an output power of 6.9 mW with a corresponding slope efficiency of 2.3% and an optical signal-to-noise ratio greater than 83 dB. The relative intensity noise of the laser is below −143 dB/Hz above 10 MHz. The peak wavelength and output power stability are also monitored over two and a half hours. To the best of our knowledge, this is the first implementation of linewidth compression utilizing the novel hybrid integration of SAF and DRF based on ultra-high NA fibers in the 1.6 µm band. This study offers a compact and efficient solution for high-coherence laser sources in L-band applications such as coherent communications, precision sensing, and next-generation LiDAR.

This work establishes a comprehensive framework for nonlinear suppression in gain-switched Raman fiber lasers (RFLs) through dispersion-engineered fibers and spectral control strategies. Combining numerical modeling with experimental validation, we decode the triadic interplay among second-order dispersion (15 ps²/km≤β₂ ≤ 30 ps²/km), nonlinear coefficients (4 W^-1km^-1≤γ≤6 W^-1km^-1), and fiber length (L = 30 m) in governing nonlinear phase accumulation. Key breakthroughs include: 1) identification of the β₂-γ equilibrium window that simultaneously suppresses modulation instability (MI), self-phase modulation (SPM) and four wave mixing (FWM) and enhances second-order Stokes conversion efficiency. 2) Experimental realization using UHNA7 fiber (γ=5.27 W^-1km^-1@1660 nm, β₂ = 30 ps²/km, L = 30 m) achieving 60 dB radio frequency (RF) signal-to-noise ratio (SNR), 4.6 ns pulse duration, and 99.2% spectral purity. 3) FBG-based spectral purification that compresses 3 dB bandwidth from 13.0 nm to 1.0 nm. The proposed methodology bridges fundamental nonlinear optics with practical laser engineering, offering: 1) a universal design metric for Raman gain optimization, 2) experimentally validated guidelines for MI/SPM/FWM suppression, and 3) spectral-temporal control architecture extendable to multistage RFL systems. This paradigm shift enables high performance gain switched RFLs, paving the way for precision nonlinear photonic systems.

We demonstrate a low-threshold, broadband-tunable cascaded Raman fiber pulsed laser utilizing highly GeO₂-doped silica and As₂S₃ fibers as Raman gain media. As a result of the high Raman spectral purity of the first-order Raman laser and the high Fresnel reflection coefficient of the As₂S₃ fiber resonant cavity, a second-order nanosecond pulsed Raman laser with a center wavelength of 2335 nm is achieved, with a peak threshold power of 95 W with a 1970nm nanosecond pump source and corresponding to 8.4 W with a 2150 nm first-order Raman laser as the pump source. The maximum single-pulse energy and spectral purity of the second-order Raman laser are measured at 5.1 μJ and 47.2%, respectively. Furthermore, through precise tuning of the central wavelength in the gain-modulated pulsed source, the second-order Raman laser achieved a wavelength tuning range spanning 74 nm, from 2303 nm to 2377 nm. To the best of our knowledge, this achievement establishes the broadest wavelength tuning range reported to date for cascaded Raman fiber lasers operating in the 2.3 μm spectral region.