Unidirectional Laser Research Articles

Unidirectional lasing in up–down and left–right directions based on unidirectional bound states in the continuum

Unidirectional lasing in up–down and left–right directions based on unidirectional bound states in the continuum

Numerical investigation of laser powder bed fusion of glass

Additive manufacturing of glass using laser powder bed fusion has been recently developed, demonstrating its potential to be applied in small scale applications such as flow reactors for the chemical engineering and pharmaceutical manufacturing industries. While previous research demonstrated that complex 3-dimensional shapes can be manufactured, built parts are often brittle, exhibit high porosity and lack transparency. This study employs a transient, heat transfer finite element analysis to shed light on the thermal response of laser—glass powder bed interaction and the impact of processing parameters. Through this understanding, the research seeks to identify practical strategies that can be employed to improve the quality and properties of the built parts. Bulk solid and powder soda lime silica glass properties are used as input in the model, while the laser heat flux and scan strategy, conversion of powder feedstock to bulk solid glass and heat losses from convection and radiation effects are introduced in the model through Fortran coding. The study showed that effective powder consolidation, resulting in well-defined geometrical features, is achieved for temperatures near the glass melting point. Additionally, uniform consolidation depths and widths can be achieved by increasing laser power, elevating substrate temperature and reducing scan speed within certain limits, whilst ensuring hatch spacing is below the corresponding single scan track width for unidirectional adjacent laser trajectories.

Formation condition of lacy ferrite during solidification and subsequent transformation in austenitic stainless steels solidified with primary ferrite

The formation conditions of lacy ferrite in austenitic stainless steels solidified with primary ferrite was clarified through crystallography in order to increase the amount of lacy ferrite which contributed to the improvement of properties. Lacy ferrite was formed when the misorientation between ferrite and austenite from the Kurdjumov–Sachs (K-S) relationship was ≤ 7° on the close-packed plane and ≤ 9° in the close-packed direction during solidification. In contrast, vermicular ferrite was formed if the misorientation exceeded these values. The temperature gradient during solidification was controlled unidirectionally by two-pass laser welding. Following the performance of two-pass welding with a highly unidirectional laser, it was confirmed that the formation ratio of lacy ferrite to total ferrite significantly increased in the second-pass weld metal by remelting the first-pass weld metal, in which the heat flow direction and preferential growth direction of ferrite and austenite were controlled to be almost parallel. The growth ratio while maintaining the K-S relationship established between ferrite and austenite in the first-pass weld metal, and/or the ferrite formation ratio that satisfies the K-S relationship, increased in the second-pass weld metal because the growth of both the primary ferrite and secondary austenite was crystallographically independent of each other during solidification.

Unidirectional Lasing from Mirror-Coupled Dielectric Lattices.

This paper reports how a hybrid system composed of transparent dielectric lattices over a metal mirror can produce high-quality lattice resonances for unidirectional lasing. The enhanced electromagnetic fields are concentrated in the cladding of the periodic dielectric structures and away from the metal. Based on a mirror-image model, we reveal that such high-quality lattice resonances are governed by bound states in the continuum resulting from destructive interference. Using hexagonal arrays of titanium dioxide nanoparticles on a silica-coated silver mirror, we observed lattice resonances with quality factors of up to 2750 in the visible regime. With the lattice resonances as optical feedback and dye solution as the gain medium, we demonstrated unidirectional lasing under optical pumping, where the array size was down to 100 μm × 100 μm. Our scheme can be extended to other spectral regimes to simultaneously achieve strongly enhanced surface fields and high quality factors.

Spatio-spectral 4D coherent ranging using a flutter-wavelength-swept laser

Coherent light detection and ranging (LiDAR), particularly the frequency-modulated continuous-wave LiDAR, is a robust optical imaging technology for measuring long-range distance and velocity in three dimensions (3D). We propose a spatio-spectral coherent LiDAR based on a unique wavelength-swept laser to enable both axial coherent ranging and lateral spatio-spectral beam scanning simultaneously. Instead of the conventional unidirectional wavelength-swept laser, a flutter-wavelength-swept laser (FWSL) successfully decoupled bidirectional wavelength modulation and continuous wavelength sweep, which overcame the measurable distance limited by the sampling process. The decoupled operation in FWSL enabled sequential sampling of flutter-wavelength modulation across its wide spectral bandwidth of 160 nm and, thus, allowed simultaneous distance and velocity measurement over an extended measurable distance. Herein, complete four-dimensional (4D) imaging, combining real-time 3D distance and velocity measurements, was implemented by solid-state beam scanning. An acousto-optic scanner was synchronized to facilitate the other lateral beam scanning, resulting in an optimized solid-state coherent LiDAR system. The proposed spatio-spectral coherent LiDAR system achieved high-resolution coherent ranging over long distances and real-time 4D imaging with a frame rate of 10 Hz, even in challenging environments.

Novel Bidirectional Output Ytterbium-Doped High Power Fiber Lasers: From Continuous to Quasi-Continuous.

Traditional ytterbium-doped high-power fiber lasers generally use a unidirectional output structure. To reduce the cost and improve the efficiency of the fiber laser, we propose a bidirectional output fiber laser (BOFL). The BOFL has many advantages over that of the traditional unidirectional output fiber laser (UOFL) and has a wide application in the industrial field. In theory, the model of the BOFL is established, and a comparison of the nonlinear effect in the traditional UOFL and the BOFL is studied. Experimentally, high-power continuous wave (CW) and quasi-continuous wave (QCW) BOFLs are demonstrated. In the continuous laser, we first combine the BOFL with the oscillating amplifying integrated structure, and a near-single-mode bidirectional 2 × 4 kW output with a total power of above 8 kW is demonstrated. Then, with the simple BOFL, a CW bidirectional 2 × 5 kW output with a total power of above 10 kW is demonstrated. By means of pump source modulation, a QCW BOFL is developed, and the output of a near-single mode QCW laser with a peak output of 2 × 4.5 kW with a total peak power of more than 9 kW is realized. Both CW and QCW output BOFL are the highest powers reported at present.

Rate equation analysis for deterministic and unidirectional lasing in ring resonators with an S-shaped coupler

Ring resonators are traditionally popular optical devices that apply to various components in photonic ICs. They also play an important role in the on-chip generation of many novel optical states in topological systems and non-Hermitian systems. Unidirectional lasing of ring resonators is used in many such systems to create exotic states of light including optical vortexes and optical skyrmions, but the unidirectional behavior has not been fully understood. Previous research has constructed a simplified model to explain the steady state behaviors of unidirectional ring resonators, but the carrier dynamics and spontaneous emission were omitted. In this work, we give a numerical analysis of unidirectional ring resonators with an S-shaped coupler. We identified the importance of the gain saturation to robustness against backscattering and high unidirectionality by comparing to the model without saturation. We also discuss the effect of asymmetrical coupling on the deterministic realization of unidirectionality.

Determination of Enantiomeric Excess by Optofluidic Microlaser near Exceptional Point.

Enantiomeric excess (ee) is an essential indicator of chiral drug purification in the pharmaceutical industry. However, to date the ee determination of unknown concentration enantiomers generally involves two separate techniques for chirality and concentration measurement. Here, a whispering-gallery mode (WGM) based optofluidic microlaser near exceptional point to achieve the ee determination under unknown concentration with a single technique is proposed. Exceptional point induces the unidirectional WGM lasing, providing the optofluidic microlaser with the novel capability to measure chirality by polarization, in addition to wavelength-based concentration detection. The dual-parameters detection of optofluidic microlaser empowers it to achieve ee determination of various unknown enantiomers without additional concentration measurements, a feat that is challenging to accomplish with other methods. Featuring the sensitivity enhancement and miniature structure of the WGM sensors, the obtained chiroptical response of the present approach is ≈30-fold higher than that of the conventional optical rotation-based polarimeter, and the reagent consumption is reduced by three orders of magnitude.

2 × 4.5 kW bidirectional output near-single-mode quasi-continuous wave monolithic fiber laser

Quasi-continuous wave (QCW) laser has a very broad application in the industrial field, especially in additive manufacturing, surface treatment, laser cutting, laser cleaning, and laser drilling. Compared with the unidirectional fiber laser, the bidirectional output can be achieved two ports high power output with only one resonator, which can greatly reduce the industrial cost. However, there are few researches on QCW fiber lasers with bidirectional output. Here, we optimized and demonstrated a bidirectional output QCW laser with output power of 2 × 4.5 kW based on a double-clad ytterbium-doped fiber with a core/cladding diameter of 25/400 μm. The peak power at both ends reached 4515 W and 4694 W, respectively. The Raman suppression ratio at both ends of A and B is about 12 dB, and the beam quality factor M2 is about 1.37 and 1.42, respectively. The corresponding optical-to-optical efficiency is 79%. To the best of our knowledge, this is the highest peak power of QCW laser with near-single-mode beam quality in a bidirectional structure laser.

Research of Transverse Mode Instability in High-Power Bidirectional Output Yb-Doped Fiber Laser Oscillators

Bidirectional output fiber laser oscillators can realize two high-power laser outputs employing only a single-laser resonant cavity and hold the advantages of being low cost and of compact size. However, like other fiber lasers, their power improvement is limited by transverse mode instability (TMI). To achieve higher power output, in this paper, the characteristics and corresponding suppression method of the TMI in bidirectional output fiber laser oscillators were investigated for the first time. Firstly, the TMI threshold was obtained when the fiber laser oscillator was pumped by 976 nm LDs and 981 nm LDs, separately, and the difference between the two pumping conditions was researched in detail. After that, a comparison study between the bidirectional and unidirectional output fiber laser oscillators pumped by 981 nm LDs was carried out. In the experiment, the effect of pump distribution on the TMI threshold was also considered. The results show that the TMI threshold of the bidirectional-output laser pumped by 981 nm LDs is much higher than that pump by 976 nm LDs, which means that the effective TMI suppression methods in the unidirectional output laser are also applicable in the bidirectional output laser. In addition, it is found that the TMI threshold of a bidirectional output fiber laser is much lower than that of a unidirectional output fiber laser.

Tunable Narrowband Carbon Quantum Dots Laser Based on Self‐Assembled Microstructure

AbstractPhotonic integrated circuits (PICs) are coupled with the laser to create active devices for optical communication, optical interconnection, and optical computing. Consequently, the light source remains a focal point of extensive research and development. Currently, the utilization of carbon quantum dots as a gain medium in photonic cavity lasers is accomplished and suggested as a viable approach for achieving active PICs. Nevertheless, challenges persist in achieving narrowband lasing beam output in one direction that is real‐time tunable while minimizing energy losses. These issues represent an active area of ongoing research. This article presents a self‐assembled microstructure and proposes a method for obtaining tunable, narrowband, unidirectional lasing beams. The objective is to stimulate further development in integration of PICs with lasers, extending beyond these applications, to realize substantial performance gains, eco‐friendly solutions, and efficient mass production.

Unidirectional operation criterion in monolithic nonplanar ring oscillators.

Monolithic nonplanar ring oscillators (NPROs) under an applied magnetic field can operate unidirectional single-frequency lasing due to the loss differences among its four eigenpolarizations, where the minimum was empirically estimated to be 0.01%. However, this value has never been verified because the applied magnetic field is not uniformly distributed, making it hard to resolve both theoretically and experimentally. Here, we propose a method to resolve the applied magnetic field through an NPRO by combining finite-element analysis and experimental verification. By introducing the non-uniform magnetic field information to the eigenpolarization theory, the loss differences can be calculated by path integration along the optical path in the NPRO. The critical point, where the bidirectional lasing is emerging, is identified by the relative amplitude noise (RAN) of the laser and by the beating signal between the clockwise (CW) and counterclockwise (CCW) lasing. With this method, we determine that unidirectional operation is possible with loss differences as low as 0.0001% and 0.0003%, corresponding to two different NPRO designs with out-of-plane angles of 90° and 45°, respectively, which increases the precision of the loss differences for unidirectional single-frequency lasing by more than one order of magnitude. Our findings will greatly facilitate NPRO laser design with lowered magnetic field intensity requirements.

Dual Regime Mode-Locked and Q-Switched Erbium-Doped Fiber Laser by Employing Graphene Filament-Chitin Film-Based Passive Saturable Absorber.

We investigate the dynamics of high energy dual regime unidirectional Erbium-doped fiber laser in ring cavity, which is passively Q-switched and mode-locked through the use of an environmentally friendly graphene filament-chitin film-based saturable absorber. The graphene-chitin passive saturable absorber allows the option for different operating regimes of the laser by simple adjustment of the input pump power, yielding, simultaneously, highly stable and high energy Q-switched pulses at 82.08 nJ and 1.08 ps mode-locked pulses. The finding can have applications in a multitude of fields due to its versatility and the regime of operation that is on demand.

Theinfinity-loop microresonator: A new integrated photonic structure working on an exceptional surface

Exceptional points, where eigenvalues and eigenvectors coalesce, impact the behavior of different photonics components that show, e.g., enhanced sensing, coherent perfect absorption, unidirectional lasing, and chirality. However, only a few passive geometries have been developed that work on these points. Here, we introduce a novel non-Hermitian structure based on a microresonator shaped as the infinity symbol twice coupled to a bus waveguide: the infinity-loop microresonator. Unlike other structures working on an exceptional surface, the infinity-loop microresonator can achieve either high- or low-contrast unidirectional reflection with a negligible or identical reflection for counterpropagating light. It allows an easy walking through the Riemann sheet by simply controlling the phase of the light propagating in the bus waveguide, which makes it a tunable component to build more complex topological structures. Furthermore, the infinity-loop microresonator allows sensors that show the features of both an exceptional point device and a diabolic point device simultaneously.

Unidirectional hybrid diode laser through the integration of a hook-shaped traveling-wave semiconductor optical amplifier and Taiji ring resonator.

We demonstrate a unidirectional ring diode laser based on hybrid integration of a hook-shaped traveling-wave semiconductor optical amplifier (SOA) and a Taiji ring resonator. The additional crossover bending waveguide inside the silicon nitride Taiji ring introduces a non-reciprocal loss in the laser cavity while the gain is provided by a multiple depth etched hook-shaped SOA. We present the detailed design flow for both active and passive components of the unidirectional hybrid diode laser. This work paves the way to use a hook-shaped SOA-based hybrid platform for various applications including optical sensing, all-optical switching, photonic memories, and topological optics.

Development of a 671 nm injection-locked CW Ti:sapphire laser

We developed an injection-locked Ti:sapphire laser at a wavelength of 671 nm where the fluorescence gain of the Ti:sapphire crystal is quite low. We obtained an output of more than 500 mW at a pump power of 10 W. The injection-locked lasing operates at a single-frequency and unidirectionally lasing with no intracavity optical components. The spectral property and the stability condition for the unidirectional lasing of the injection-locked Ti:sapphire laser are reported. The developed laser will produce the powerful light source required for the laser cooling of lithium atoms.

Extremely Efficient DFB Lasers with Flat-Top Intra-Cavity Power Distribution in Highly Erbium-Doped Fibers.

High-performance erbium-doped DFB fiber lasers are presently required for several sensing applications, whilst the current efficiency record is only a few percent. Additionally, a flat-top intra-cavity power distribution that is not provided in traditional DFB lasers is preferred. Moreover, cavity lengths of <20 cm are attractive for fabrication and packaging. These goals can be achieved using highly erbium-doped fiber (i.e., 110 dB/m absorption at 1530 nm), providing high gain with proper engineering of coupling coefficients. In this paper, for a given background fiber loss, first the optimum intra-cavity signal powers for various pump powers are numerically calculated. Then, for a fully unidirectional laser, optimum coupling profiles are determined. Design diagrams, including contour maps for optimum cavity lengths, maximum output powers, maximum intra-cavity signal powers, and quality factors considering various pump powers and background fiber losses, are presented. The laser pump and intra-cavity signal distribution are also calculated for a realistic, feasible modified coupling profile considering a strong unidirectionality. The DFB laser is finally simulated using generalized coupled-mode equations for such modified profiles. The efficiency of more than 22% can be realized, which is the highest reported for DFB lasers based only on erbium-doped fiber.

Unidirectional emission of GaN-on-Si microring laser and its on-chip integration

Abstract GaN-based microring lasers grown on Si are promising candidates for compact and efficient light sources in Si-based optoelectronic integration and optical interconnect due to their small footprints, low mode volume, low power consumption, and high modulation rate. However, the high symmetry of circular microcavity leads to isotropic emission, which not only reduces the light collection efficiency, but also affects other adjacent devices during data transmission. In this study, the unidirectional lasing emission of room-temperature current-injected GaN-based microring laser was realized by coating metal Ag on the microring sidewall and integrating a direct coupled waveguide. The light was efficaciously confined in the cavity and only emitted from the waveguide, which avoided optical signal crosstalk with other adjacent devices. Furthermore, we integrated a microdisk at the other end of the waveguide as a photodetector, which could effectively detect the output power of the microring laser from the direct coupled waveguide. Therefore, a preliminary on-chip integration of GaN-based microring laser, waveguide and photodetector on Si substrate was successfully demonstrated for the first time, opening up a new way for on-chip integration and optical interconnect on a GaN-on-Si platform.

Dominated mode switching and nanoparticle detection at exceptional points

We present a theoretical investigation of dominated mode switching and nanoparticle detection at a wavelength of 2 µm that uses coupled whispering gallery mode (WGM) resonators operating at exceptional points (EPs). The simulations show that with assisted gain and loss the system can be steered to operate at EPs by adjusting the nanoparticle-introduced backscattering. The unbalanced contributions of the clockwise and counterclockwise modes can be observed when the system approaches the EPs, and the dominated mode is switchable by transiting the system from one EP to another. Owing to the square root topology of EPs, the enhanced frequency splitting can be obtained when the system is subjected to a perturbation, which results in a more than five times sensitivity enhancement compared to a traditional sensor. Furthermore, a simulation also reveals that an increased gain–loss contrast in the resonators contributes to an enhanced frequency splitting, and thus a higher sensitivity enhancement factor. Our investigations validate the feasibility of a WGM resonator operating at EPs for unidirectional laser emitting and nanoparticle sensing.

Molecular dynamics perspective of the effects of laser thermal configurations on the dislocation and mechanical characteristics of FeNiCrCoCu HEA through powder bed fusion process

The concept of alloying multiple principal elements at high concentrations has led to the development of High Entropy Alloys (HEAs) with exceptional mechanical properties, making them the focus of major recent scientific endeavors. Geometrically complex HEAs with tailored microstructural characteristics can be produced using additive manufacturing technologies such as powder bed fusion (PBF). However, an in-depth study on the effect of process thermal conditions during PBF is required to effectively fabricate HEAs with desirable mechanical characteristics. Thus, in our present molecular dynamic (MD) study we have explored the implication of PBF process thermal conditions on the mechanical characteristics of FeNiCrCoCu HEA by systematically varying laser scan speed from 0.4 {\AA}/ps to 0.1 {\AA}/ps, unidirectional and reversing laser passes from 1 to 4, and laser power from 100 microwatts to 220 microwatts. Our investigation suggests that reducing the laser scanning speed up to a critical velocity of 0.2 {\AA}/ps considerably improves mechanical strengths, with further reduction creating severe surface defects. Decreased ultimate tensile strength (UTS) is associated with the annihilation of the bulk sessile dislocations during tensile straining marking an early yield failure. Alternately, the material's strength could be improved by annealing with several unidirectional laser passes over the same target region, resulting in enhanced UTS due to subtler yield points. Increasing laser power aids in ameliorating material density ultimately leading to higher UTS even in non-dislocation-free structures. These findings will assist researchers to understand the underlying effects and optimize process thermal parameters to fabricate superior HEAs utilizing additive manufacturing.