Multimode Laser Research Articles

Parallel random LiDAR with spatial multiplexing of a many-mode laser.

We propose and experimentally demonstrate parallel light detection and ranging (LiDAR) using random intensity fluctuations from a highly multimode laser. We optimize a degenerate cavity to have many spatial modes lasing simultaneously with different frequencies. Their spatio-temporal beating creates ultrafast random intensity fluctuations, which are spatially demultiplexed to generate hundreds of uncorrelated time traces for parallel ranging. The bandwidth of each channel exceeds 10 GHz, leading to a ranging resolution better than 1 cm. Our parallel random LiDAR is robust to cross-channel interference, and will facilitate high-speed 3D sensing and imaging.

Spontaneous mode locking of a multimode semiconductor laser under continuous wave operation

Self-starting mode-locking is observed in a laser based on a compact III-V diode-pumped quantum-well surface-emitting semiconductor laser technology with a saturable-absorber-free but dispersive cavity. Continuous wave generation of picosecond pulses at a rate of 100 GHz is demonstrated by recording microwave intensity noises, beat frequency, time-resolved optical spectra, and intensity autocorrelation. Coherence of the pulse train is obtained through the frequency noise measurement of the demodulated beat note, demonstrating a timing jitter as low as 110 fs, near the quantum limit. Using a theoretical model based on a generalized Haus master equation, we demonstrate the existence of this mode locked state without the need for saturable absorption. The fundamental physical mechanism is the interplay between self-phase modulation and anomalous dispersion like in cavity soliton together with light–matter interaction-induced time symmetry breaking.

Chaotic mode-competition dynamics in a multimode semiconductor laser with optical feedback and injection.

Photonic computing has attracted increasing interest for the acceleration of information processing in machine learning applications. The mode-competition dynamics of multimode semiconductor lasers are useful for solving the multi-armed bandit problem in reinforcement learning for computing applications. In this study, we numerically evaluate the chaotic mode-competition dynamics in a multimode semiconductor laser with optical feedback and injection. We observe the chaotic mode-competition dynamics among the longitudinal modes and control them by injecting an external optical signal into one of the longitudinal modes. We define the dominant mode as the mode with the maximum intensity; the dominant mode ratio for the injected mode increases as the optical injection strength increases. We deduce that the characteristics of the dominant mode ratio in terms of the optical injection strength are different among the modes owing to the different optical feedback phases. We propose a control technique for the characteristics of the dominant mode ratio by precisely tuning the initial optical frequency detuning between the optical injection signal and injected mode. We also evaluate the relationship between the region of the large dominant mode ratios and the injection locking range. The region with the large dominant mode ratios does not correspond to the injection-locking range. The control technique of chaotic mode-competition dynamics in multimode lasers is promising for applications in reinforcement learning and reservoir computing in photonic artificial intelligence.

Enhanced nonlinear optomechanics in a coupled-mode photonic crystal device

The nonlinear component of the optomechanical interaction between light and mechanical vibration promises many exciting classical and quantum mechanical applications, but is generally weak. Here we demonstrate enhancement of nonlinear optomechanical measurement of mechanical motion by using pairs of coupled optical and mechanical modes in a photonic crystal device. In the same device we show linear optomechanical measurement with a strongly reduced input power and reveal how both enhancements are related. Our design exploits anisotropic mechanical elasticity to create strong coupling between mechanical modes while not changing optical properties. Additional thermo-optic tuning of the optical modes is performed with an auxiliary laser and a thermally-optimised device design. We envision broad use of this enhancement scheme in multimode phonon lasing, two-phonon heralding and eventually nonlinear quantum optomechanics.

Fabry–Perot Oscillation and Resonance Energy Transfer: Mechanism for Ultralow-Threshold Optically and Electrically Driven Random Laser in Quasi-2D Ruddlesden–Popper Perovskites

The recently emerged metal-halide hybrid perovskite (MHP) possesses superb optoelectronic features, which have obtained great attention in solid-state lighting, photodetection, and photovoltaic applications. Because of its excellent external quantum efficiency, MHP has promising potential for the manifestation of ultralow threshold optically pumped laser. However, the demonstration of an electrically driven laser remains a challenge because of the vulnerable degradation of perovskite, limited exciton binding energy (Eb), intensity quenching, and efficiency drop by nonradiative recombinations. In this work, based on the paradigm of integration of Fabry-Perot (F-P) oscillation and resonance energy transfer, we observed an ultralow-threshold (∼250 μWcm-2) optically pumped random laser from moisture-insensitive mixed dimensional quasi-2D Ruddlesden-Popper phase perovskite microplates. Particularly, we demonstrated an electrically driven multimode laser with a threshold of ∼60 mAcm-2 from quasi-2D RPP by judicious combination of a perovskite/hole transport layer (HTL) and an electron transport layer (ETL) having suitable band alignment and thickness. Additionally, we showed the tunability of lasing modes and color by driving an external electric potential. Performing finite difference time domain (FDTD) simulations, we confirmed the presence of F-P feedback resonance, the light trapping effect at perovskite/ETL, and resonance energy transfer contributing to laser action. Our discovery of an electrically driven laser from MHP opens a useful avenue for developing future optoelectronics.

Non-linear dynamics of multimode semiconductor lasers: dispersion-based phase instability and route to single-frequency operation.

Emission dynamics of a multimode broadband interband semiconductor laser have been investigated experimentally and theoretically. Non-linear dynamics of a III-V semiconductor quantum well surface-emitting laser reveal the existence of a modulational instability, observed in the anomalous dispersion regime. An additional unstable region arises in the normal dispersion regime, owing to carrier dynamics, and has no analogy in systems with fast gain recovery. The interplay between cavity dispersion and phase sensitive non-linearities is shown to affect the character of laser emission with phase turbulence, leading to regular self-excited oscillations of mode intensity, self-mode locking, and single-frequency emission stabilized by spectral symmetry breaking. Such physical behavior is a general phenomenon for any laser with a slow gain medium relative to the round trip time, in the absence of spatial inhomogeneities.

Cladding pumped bismuth-doped fiber amplifiers operating in O-, E-, and S-telecom bands.

Bismuth-doped fibers (BDFs) are considered nowadays as an essential part of the development of novel optical amplifiers, which can provide a significant upgrade to existing fiber optic telecommunication systems, securing multiband data transmission. In this paper, a series of BDF amplifiers (BDFAs) for O-, E-, and S-telecom bands based on a cladding pumping scheme using low-cost multimode semiconductor laser diodes at a wavelength of 0.7-0.8 µm were demonstrated for, it is understood, the first time. The developed BDFAs are characterized by a high peak gain of >25-30 dB in the corresponding telecom bands and a relatively low noise figure of 5-6 dB. Comparative analysis shows that most of the parameters of cladding pumped BDFAs are close to those of the best core pumped ones. This research opens up new opportunities for utilizing Bi-doped fibers as a key element of cost-effective and ready-to-work BDFAs for various practical applications.

Polarization-maintaining large-mode-area solid-core anti-resonant fiber for high-power fiber lasers

A polarization-maintaining large-mode-area solid-core anti-resonant fiber is investigated in this work with double-layer anti-resonant elements for improved capability on fundamental mode guidance and higher order mode suppression, and structural and material asymmetry on inner-layer cladding tubes for high birefringence. Mode coupling between fundamental core mode in one polarization direction and mode on two high-index inner cladding tubes is used to enhance the fiber birefringence under large core diameter. Eventually, an optimized design of polarization-maintaining large-mode-area solid-core anti-resonant fiber with the currently largest mode field area of 3026 μm2 and birefringence of 1.20 × 10−5 can be anticipated with the confinement losses for all the higher order modes exceeding 10 dB/m. Moreover, confinement loss below 0.1 dB/m can still be obtained for the fundamental core mode in both polarization direction even under the mode-coupling effect. Active polarization-maintaining anti-resonant fiber is analyzed and the refractive index of the central rare-earth-doped region is optimized for the first time for both well guided signal and pump laser. Large pump absorption coefficient can thus be anticipated from the multimode laser core-pumped active large-mode-area fiber, which facilitates short active fiber length needed for sufficient laser gain. The fiber proposed here offers great potential for the development of high-performance fiber laser sources including high-power single-frequency fiber amplifiers and high-peak-power ultrashort pulsed fiber amplifiers.

Ambient Hydrocarbon Detection with an Ultra-Low-Loss Cavity Raman Analyzer.

The detection of ambient outdoor trace hydrocarbons was investigated with a multipass Raman analyzer. It relies on a multimode blue laser diode receiving optical feedback from a retroreflecting multipass optical cavity, effectively creating an external cavity diode laser within which spontaneous Raman scattering enhancement occurs. When implemented with ultra-low-loss mirrors, a more than 20-fold increase in signal-to-background ratio was obtained, enabling proximity detection of trace motor vehicle exhaust gases such as H2, CO, NO, CH4, C2H2, C2H4, and C2H6. In a 10-min-long measurement at double atmospheric pressure, the limits of detection obtained were near or below 100 ppb for most analytes.

Directional amplified spontaneous emissions from Ag nanohole array with high diffraction orders.

Surface plasmon excitations in metallic hole arrays have been extensively studied in the context of light-matter interaction, since the generated Bloch surface plasmon polariton (Bloch-SPP) exhibits unique properties of nanoscale light confinement, near-field enhancements, and long-range metal surface propagation. In this work, we experimentally demonstrate a plasmonic device that exhibits highly directional emission in visible light; using Ag film with a thickness of 100 nm deposited on a subwavelength porous alumina array as a plasmonic cavity, four-level rhodamine 6G (R6G) is selected as the gain material. It is suggested that a Bloch-SPP with high diffraction orders on a Ag nanohole array can generate a strong local electric field and a high Purcell factor at a nanohole edge. Moreover, directional five-fold enhanced amplified spontaneous emission (ASE) with polarization dependence is observed under a low threshold of 199.9 W/cm2 in the visible light region, which comes from the optical feedback provided by the 2D periodic nanohole array. This work opens up a wide range of applications for real-time tunable wavelength, controlled multimode laser, fluorescence detection, etc.

Multimode Vortex Lasing from Dye–TiO2 Lattices via Bound States in the Continuum

Spatially extended bound states in the continuum (BICs) can provide high-quality optical feedback for vortex lasing. This work reports that three out of the four supported BIC modes in 2D square lattices of dye-covered TiO2 nanoparticles can lase simultaneously under pulsed pumping, while the remaining one can be tuned into the gain bandwidth by adjusting the nanoparticle size. All the BIC modes in the in-plane symmetric lattices exhibited doughnut-shaped lasing patterns, regardless of the pump polarization, but with a polarization-dependent threshold. Multiple vortex beams can be generated by illuminating with multiple pump spots. When changing the nanoparticle shape from square to trapezoidal, the symmetry breaking resulted in predominantly single-lobed lasing at symmetry-breaking-induced band edges and dramatically suppressed two-lobe lasing at BIC modes. Such on-chip multimode lasing with different polarization patterns thus enables multiplexing in not only wavelength, but also polarization.

Sub-100 fs Kerr-lens mode-locked femtosecond Yb:CaYAlO<sub>4</sub> laser with GHz repetition rate

Femtosecond lasers with GHz repetition rate play an important role in scientific and industrial applications such as spectroscopy, optical frequency combs and GHz-Burst pulse trains for micro-machining in the ablation-cooled regime. Kerr-lens mode-locked (KLM) technique and passively mode-locking based on semiconductor saturable absorber mirror (SESAM) are the primary methods to generate GHz femtosecond all-solid-state lasers (ASSLs). Kerr-lens mode-locked Ti:Sapphire lasers have made significant progress benefited from the high-power green pump lasers, and repetition rate up to 10 GHz has been obtained with the average power of 1.2 W. In the early 21st century, ytterbium ion (Yb<sup>3+</sup>) doped laser crystals and ceramics with emission wavelengths near 1 μm received attention due to their high conversion efficiency and broad gain-bandwidth. Combining the customized SESAM and high-power multimode fiber-coupled laser diodes (LDs), GHz Yb-doped ASSLs with watt-level average power may be easily attained and have made rapid progress. However, GHz KLM lasers have strict requirements for the cavity design and pump sources. For satisfying mode matching and enhancing the soft aperture effect within the gain medium, a high-brightness pump source with excellent beam quality (<i>M</i><sup> 2</sup> ～ 1) is desired, such as the single-mode fiber coupled LD, however, the maximum pump power of which is only ～1 W. As a result, the average power of GHz KLM femtosecond laser is typically limited to few tens of milliwatts, which limits the further applications. In this work, we report the first GHz high-power KLM Yb:CaYAlO<sub>4</sub> laser by using a high-power single-mode fiber laser instead of the low-power single-mode fiber coupled LDs as the pump source. On the basis of <i>ABCD</i> matrix, a simple four-mirror bow-tie ring cavity is built so that the laser mode can match well with the focused pump spot in the crystal. At the pump power of 8 W, stable unidirectional KLM is achieved, the laser has the average power of 2.1 W with a pulse duration of 88 fs and a repetition rate of 1.8 GHz, corresponding to the peak power of 11.57 kW. The high peak power and extremely short pulse duration are crucial for coherent octave-spanning supercontinuum generation. The powerful GHz KLM laser with sub-100 fs pulse duration provides an attractive source for realizing the optical frequency combs and micro-machining applications.

A scalable and fully tuneable VCSEL-based neural network

We experimentally demonstrate an autonomous, fully tuneable and scalable neural network of 350+ parallel nodes based on a large area, multimode semiconductor laser. We implement online learning strategies based on reinforcement learning. Our system achieves high performance and a high classification bandwidth of 15KHz for the MNIST dataset. Our approach is highly scalable both in terms of classification bandwidth and neural network size.

Clinical evaluation and assessment of laser irradiation on the stability of orthodontic implant - A prospective experimental study

ABSTRACT Background: Currently, orthodontic implants have reached a peak where they are considered a dependable modality to provide temporary supplemental anchoring in orthodontic therapy. When absolute anchoring is a necessity or in cases of minimally cooperative patients, these devices can help manage skeletal anchorage. However, its failure is a serious multi-factorial issue that happens during orthodontic treatment. The stability of the mini-implant is crucial to the outcome of orthodontic intervention. Approaches to increasing the stability of the mini-implant were researched. Hence, this study was carried out to compare and contrast and clinically assess the integrity of orthodontic implants over time. Subjects and Methods: Split mouth technique of treatment was carried out on 16 patients, i.e., one side of the mandible was considered as the experimental group (implant site irradiated with laser after placement), and the other was considered as the control side (implant site not irradiated with laser). Titanium mini-implants of the dimensions 1.5 mm diameter and 6 mm length were employed in the present study. They were positioned in the inter radicular space between the first molar and second premolar in the mandibular posterior region, 7 mm apical to the alveolar crest. During the whole process, the laser utilized was a multimode GaAs diode laser with a wavelength of 980 nm. It had 0.5–10 W output power which was adjustable with the frequency of 1–20 kHz and its main body input voltage was DC12 to further analyze the stability of the implant which in turn would aid in success assessment, the resonance frequency concept was utilized. The readings were recorded (T0) after insertion, (T1) 24 h after insertion, (T2) 2 weeks after insertion, (T3) 4 weeks after insertion, (T4) 6 weeks after insertion, and (T5) 8 weeks after insertion. The higher the implant stability quotient values the greater the stability and hence the optimal loading time. Results: The test employed for statistical analysis was Mann–Whitney U, Kruskal Wallis, and analysis of variance test. After analysis of all the readings, it was found that low-level laser therapy has a significant role in the stability of orthodontic mini-implant. Conclusion: The findings from this study suggest that low-level laser irradiation at the time of implant placement controls the inflammatory reaction around the implant and improves its stability.

Controlling chaotic itinerancy in laser dynamics for reinforcement learning.

Photonic artificial intelligence has attracted considerable interest in accelerating machine learning; however, the unique optical properties have not been fully used for achieving higher-order functionalities. Chaotic itinerancy, with its spontaneous transient dynamics among multiple quasi-attractors, can be used to realize brain-like functionalities. In this study, we numerically and experimentally investigate a method for controlling the chaotic itinerancy in a multimode semiconductor laser to solve a machine learning task, namely, the multiarmed bandit problem, which is fundamental to reinforcement learning. The proposed method uses chaotic itinerant motion in mode competition dynamics controlled via optical injection. We found that the exploration mechanism is completely different from a conventional searching algorithm and is highly scalable, outperforming the conventional approaches for large-scale bandit problems. This study paves the way to use chaotic itinerancy for effectively solving complex machine learning tasks as photonic hardware accelerators.

Seven-Rod Pumping Concept for Highly Stable Solar Laser Emission

A seven-rod solar laser head was conceptualized and numerically studied to improve the tracking error compensation capacity and power stability in end-side-pumping schemes. It was composed of a first-stage heliostat–parabolic mirror system, a second-stage fused silica aspheric lens and a third-stage conical pumping cavity, within which seven Nd:YAG rods were mounted. Highly stable solar laser emission, with a power loss inferior to 5% for tracking errors up to ±0.4°, could potentially be enabled with seven 4 mm diameter, 13 mm length rods. The tracking error width at 10% laser power loss was about 1.0°, which is 1.65 times higher than the experimental record, attained by a dual-rod side-pumping prototype. Furthermore, a total multimode laser power of about 41.2 W could also be achieved, corresponding to 23.3 W/m2 collection and 2.5% solar-to-laser power conversion efficiencies, which are 1.65 and 1.36 times higher than those obtained with the dual-rod side-pumping prototype. They are also 1.27 and 1.12 times higher than the multirod experimental records in multimode regime for the same rod material.

Оптическая генерация тока и магнитного поля в динамических электротехнических комплексах

Alternative power supply of dynamic electrical complexes (DEC) with optoelectronics is a modern direction in electrical engineering and automation. Optoelectronic means are able to simultaneously deliver electricity and information to the consumer, which helps control the elements of automated and robotic systems. Most powerful sources of intense optical radiation are lasers and solar radiation concentrators. Intracavity control of the radiation of multimode laser sources by using Q-switches can initiate the intelligent operational mode for the DEC control system. When radiation intensity reaches 107 W/cm2, melting and evaporation of the solid target surface begin. When radiation intensity exceeds 1010 W/cm2, ionization occurs and the substance turns into the near-surface plasma. The effect of plasma formation during the interaction of intense optical radiation with the material of a solid body inside a capacitor closed to an inductor makes it possible to generate a current and a high-intensity magnetic field using the “capacitor-coil” method (CC-method). Higher radiation intensity causes shock excitation of semiconductor atoms, breaking the valence bonds in them, creating the electron-hole pairs, reversible avalanche breakdown and turning plasma into a solid. Using the generation of current and magnetic field by the CC method in the digital control system of DEC is of practical interest. There are shown the results of theoretical and experimental studies of spatial-energy characteristics of a laser plasma excited inside a capacitor closed to an inductor during the current and a magnetic field generation depending on the laser radiation divergence. The possibility of using a CC-converter of optical radiation into the current of the armature winding of a brushless electric motor is considered.

Off-Angle Amplified Spontaneous Emission of Upconversion Nanoparticles by Propagating Lattice Plasmons.

Lanthanide-doped upconversion nanoparticles (UCNPs) are appealing for light emitting applications because their high internal conversion efficiency facilitates the amplified spontaneous emission (ASE) under low pumping. In addition, the integration of photonic crystals and microcavities with optical quantum emitters provides a unique opportunity to manipulate their light emissions and generate coherent light sources for quantum photonics. Here, this work describes a two-dimensional (2D) plasmonic lattice of Al nanocone array (Al NCA), which can confine the light at the tip. Light confinement by the enhancement effect supports narrow linewidth resonances as optical feedback for the ASE of UCNPs doped with sensitizer Yb3+ ions/emitter Ho3+ ions/relaxator Ce3+ ions. An off-angle ASE with an enhancement of 19-fold from UCNPs is achieved by propagating lattice plasmons from the Al NCA. Moreover, this upconverting ASE can be switched on or off by adjusting the polarization state of the incident pump light, and photonic band engineering can be used to manipulate it intentionally. This composite plasmonic system opens prospective applications for the ASE as directional emission, real-time tunable wavelengths, controlled multimode lasing, and optical switches.

Photonic Crystal Surface Emitting Diode Lasers with λ near 2 µm

Epitaxially regrown electrically pumped photonic crystal surface emitting lasers (PCSELs) operating near 2 µm were designed and fabricated within a III-V-Sb material system. A high-index-contrast photonic crystal layer was incorporated into the laser heterostructures by air-pocket-retaining epitaxial regrowth. Transmission electron microscopy studies confirmed uniform and continuous AlGaAsSb initial growth over the nano-patterned GaSb surface, followed by the development of the air-pockets. The PCSEL threshold current density had a minimal value of ~170 A/cm2 in the 160–180 K temperature range when the QW gain spectrum aligned with the Γ2 band edge of the photonic crystal. The devices operated in a continuous wave regime at 160 K. The divergence and polarization of the multimode laser beam emitted from the 200 µm × 200 µm PCSEL aperture were controlled by filamentation.

PT symmetric single-mode line-defect photonic crystal lasers with asymmetric loss design.

The exploration of parity-time (PT) symmetry in micro-/nano-cavity lasers has recently gained immense research interest. The PT symmetric phase transition to single-mode lasing has been achieved by arranging the spatial distribution of optical gain and loss in single or coupled cavity systems. In terms of photonic crystal (PhC) lasers, a non-uniform pumping scheme is usually employed to enter the PT symmetry-breaking phase in a longitudinal PT symmetric system. Instead, we use a uniform pumping scheme to enable the PT symmetric transition to the desired single lasing mode in line-defect PhC cavities based on a simple design with asymmetric optical loss. The flexible control of gain-loss contrast is realized by removing a few rows of air holes in PhCs. We obtain single-mode lasing with a side mode suppression ratio (SMSR) of around 30 dB without affecting the threshold pump power and linewidth. The output power of the desired mode is six times higher than that in multimode lasing. This simple approach enables single-mode PhC lasers without sacrificing the output power, threshold pump power, and linewidth of a multimode cavity design.