Rayleigh length extension in long-distance free-space optical communications based on lens group optimization.

In the field of high-speed data transmission, wireless optical communications provide a paradigm shift from the conventional tethered connections, offering promising bandwidth and minimal latency. The cornerstone of such systems lies in their ability to precisely control the propagation of Gaussian beams, which are favored due to their inherent properties of minimal divergence and high spatial coherence over long distances. Efficient transmission hinges on the proper manipulation of these beams' spatial characteristics, particularly the waist radius and the associated Rayleigh length, which together delineate the beam's diffraction and spread. This manuscript methodically explores the theoretical and practical aspects of Gaussian beam focusing through lens systems, aiming to elucidate the pivotal relationship between the optimally adjusted focal parameters and the resultant augmentation of the Rayleigh length. Through rigorous diffraction integral simulations and a keen analysis of constraints posed by finite apertures, the study articulates strategies to considerably enhance the Gaussian beam's propagation characteristics, thereby bolstering the reliability and efficacy of wireless optical communication systems.

Microbial etch: A novel construction method of functionalized biochar for enhanced uranium extraction in radioactive wastewater.

Nuclear energy is playing an increasingly important role on the earth, but the nuclear plants leaves a legacy of radioactive waste pollution, especially uranium-containing pollution. Straw biochar with wide sources, large output, low cost, and easy availability, has emerged as a promising material for uranium extraction from radioactive wastewater, but the natural biomass with suboptimal structure and low content of functional groups limits the efficiency. In this work, microbial etch was first came up to regulate the biochar's structure and function. The surface of the biochar becomes rougher and more microporous, and the mineral contents (Ca, P) indirectly increased by microbial etch. The biochar was modified by calcium phosphate and exhibited a remarkable uranium extraction capacity of 590.8 mg g-1 (fitted value). This work provides a cost-effective and sustainable method for preparing functionalized biochar via microbial etch, which has potential for application to uranium extraction from radioactive wastewater.

Femtosecond Laser-Fabricated Photonic Chips for Optical Communications: A Review.

Integrated optics, having the unique properties of small size, low loss, high integration, and high scalability, is attracting considerable attention and has found many applications in optical communications, fulfilling the requirements for the ever-growing information rate and complexity in modern optical communication systems. Femtosecond laser fabrication is an acknowledged technique for producing integrated photonic devices with unique features, such as three-dimensional fabrication geometry, rapid prototyping, and single-step fabrication. Thus, plenty of femtosecond laser-fabricated on-chip devices have been manufactured to realize various optical communication functions, such as laser generation, laser amplification, laser modulation, frequency conversion, multi-dimensional multiplexing, and photonic wire bonding. In this paper, we review some of the most relevant research progress in femtosecond laser-fabricated photonic chips for optical communications, which may break new ground in this area. First, the basic principle of femtosecond laser fabrication and different types of laser-inscribed waveguides are briefly introduced. The devices are organized into two categories: active devices and passive devices. In the former category, waveguide lasers, amplifiers, electric-optic modulators, and frequency converters are reviewed, while in the latter, polarization multiplexers, mode multiplexers, and fan-in/fan-out devices are discussed. Later, photonic wire bonding is also introduced. Finally, conclusions and prospects in this field are also discussed.

Generation of Bessel beams via femtosecond direct laser writing 3D phase plates.

Featuring diffracting-free propagation and self-reconstruc-tion, Bessel beams (BBs) have sparked great interest in the scientific community. These properties give the potential for application in optical communications, laser machining, and optical tweezers. However, generating such beams with high quality is still challenging. Here, by using the femtosecond direct laser writing (DLW) based on two-photon polymerization (TPP) technique, we convert phase distributions of ideal BBs with different topological charges into polymer phase plates. The experimentally generated zeroth- and higher-order BBs are propagation-invariant up to 800 mm. Our work may facilitate the applications of non-diffracting beams in integrated optics.

Adaptive turbulence compensation and fast auto-alignment link for free-space optical communications.

Free-space point-to-point optical communication often suffers from atmospheric turbulence and device vibration where the environment is harsh. In this paper, by introducing an adaptive system composed of turbulence compensation and fast auto-alignment installation, we propose and experimentally demonstrate an optical communication system that is effective against turbulence and vibration. Turbulence compensation can increase the coupling efficiency by at least 3dB, while fast auto-alignment can reduce the spatial range of beam vibration caused by device vibration by 72.22%. Since the photodiode detector (PD) is sensitive to optical power, reducing the loss of the link improves the communication quality of the system. Bit-error rate (BER) of 10-Gbaud 16-ary quadrature amplitude modulation (16-QAM) signal transmission in the link is also measured under different transmitted power, having ∼8dB power penalty improvement with the adaptive system. In addition, turbulence compensation for higher-order modes such as optical vortex (OV) beams is also implemented, showing a promising prospect in space-division multiplexing (SDM) applications.

Strong Second- and Third-Harmonic Generation in 1D Chiral Hybrid Bismuth Halides.

Breaking the symmetry of a crystal structure can enable even-order nonlinear activities, including second-harmonic generation (SHG). The emerging chiral hybrid organic-inorganic metal halides feature unique optical and electronic properties and flexible crystal structures, making them a class of promising nonlinear optical materials. However, their nonlinear response performances are currently inferior to traditional nonlinear crystals, because of the lack of research on resonant enhancement and third-harmonic generation (THG). Herein, we designed chiral hybrid bismuth halides with naturally nonsymmetrical structure to enable SHG. Simultaneously, these chiral compounds preserve 1D crystal structures to create strong free exciton, broad self-trapped exciton (STE), and discrete band energy levels, which facilitate the resonant enhancement of SHG and THG susceptibilities. These new chiral films showcase superior effective SHG susceptibility (χ(2) ∼ 130.5 pm V-1 at an interesting wavelength of 1550 nm), exceeding that of the reference, a commercial LiNbO3 (χ(2) ∼ 83.4 pm V-1) single-crystal film. Furthermore, their THG intensities are even higher than their SHG intensities, with effective THG susceptibility (χ(3)) being ∼9.0 × 106 pm2 V-2 at 1550 nm (37 times that of the reference monolayer WS2). Their high SHG and THG performances indicate the promising future of these 1D chiral hybrid bismuth halides toward nonlinear optical applications.

Experimental demonstration of a few-mode Raman amplifier with a flat gain covering 1530-1605 nm.

We design and experimentally demonstrate a few-mode Raman amplifier over the C+L band with a flat on-off gain of ∼4 dB, for the first time, to the best of our knowledge. A state-of-the-art gain bandwidth of 75 nm (1530-1605 nm) is achieved. The wavelength-dependent gain for both LP01 and LP11 modes is about 0.6 dB, and the maximum mode-dependent gain is less than 0.3 dB. Mode-division multiplexed optical transmissions are performed over 75 km few-mode fiber assisted by the proposed Raman amplifier. Due to the low noise Raman amplification, more than 3.3 dB optical signal-to-noise ratio enhancement has been obtained with a significantly improved bit-error rate of about two orders.

Feedback-enabled adaptive underwater twisted light transmission link utilizing the reflection at the air-water interface.

Line-of-sight link is widely used in common free-space optical (FSO) laser communications between two fixed locations. While in practical underwater wireless optical communications (UWOC), the environment is relatively complicated. In some scenarios there exist irremovable obstacles, which block the line-of-sight optical link. Fortunately, the air-water interface can function as a natural mirror to enable non-line-of-sight optical link using the total internal reflection. Very recently, twisted light beams carrying orbital angular momentum (OAM) have attracted researchers' great attention to improve the transmission capacity in UWOC. Here, we propose and experimentally demonstrate a non-line-of-sight underwater twisted light transmission link utilizing the total internal reflection at the air-water interface. To overcome the beam fluctuation and drift caused by the change of interface states, we develop a proof-of-concept adaptive feedback system to provide a stable output. Moreover, we study the degrading effects of the slight wind effect, the salinity (turbidity) effect, and the vertical thermal gradient-induced turbulence effect. The results show that the water wave caused by the slight wind causes the most beam drift, the thermal gradient causes the most distortions, and the salinity causes the most power loss.