Far-field Laser Beam Research Articles

Twisted Silica Few-Mode Hollow GeO2-Doped Ring-Core Microstructured Optical Fiber

This work presents the first instance of a silica few-mode microstructured optical fiber (MOF) being successfully fabricated with a hollow GeO2-doped ring core and by strongly inducing twisting up to 790 revolutions per meter. Some technological issues that occurred during the manufacturing of the GeO2-doped supporting elements for the large hollow cores are also described, which complicated the spinning of the MOFs discussed above. We also provide the results of the tests performed for the pilot samples—designed and manufactured using the untwisted and twisted MOFs described above—which were characterized by an outer diameter of 65 µm, a hollow ring core with an inner diameter of 30.5 µm, under a wall thickness of 1.7 µm, and a refractive index difference of Δn = 0.030. Moreover, their geometrical parameters, basic transmission characteristics, and the measurements of the far-field laser beam profile patterns are also provided.

Six-Core GeO2-Doped Silica Microstructured Optical Fiber with Induced Chirality

This work presents a fabricated silica few-mode microstructured optical fiber (MOF) with a special six GeO2-doped core geometry, an outer diameter of 125 µm (that corresponds to conventional commercially available telecommunication optical fibers), and improved induced twisting up to 500 revolutions per 1 m (under a rotation speed of 1000 revolutions per meter with a drawing speed of ~2 m per minute). The article discusses some technological aspects and issues of manufacturing the above-described twisted MOFs with complicated structures and geometry as GeO2-doped silica supporting elements for them. We present results of some measurements performed for fabricated samples of chiral silica six-GeO2-doped-core few-mode MOFs with various orders of twisting and both step and graded refractive indexes of “cores”. These tests contain research on MOF geometrical parameters, attenuation, and measurements of the far-field laser beam profile.

Calculation of spot entroid based on physical informed neural networks

To determine the centroid of far-field laser beam spot with high precision and accuracy under intense noise contamination, a positioning algorithm named centroid-PINN is proposed, which is based on physical information neural network. A U-Net neural network is utilized to optimize the centroid estimation error. In order to demonstrate this new method, Gaussian spots polluted by two kinds of noises, i.e. ramp noise and white noise, are generated by simulation to train the neural network. The neural network is tested by two kinds of spots, i.e. Gaussian spot and Sinc-like spot. Both are predicted with high accuracy. Compared with traditional centroid method, the centroid-PINN needs no parameter tuning, especially can cope with ramp noise interference with high accuracy. This work will be conducive to developing the far-field laser beam spot measurement device, and can also serve as a reference for developing the Shack-Hartmann wavefront sensor.

Trapping metallic particles using focused Bloch surface waves.

Metallic particles are promising for applications in various areas, including optical sensing, imaging and electric field enhancement-induced optical and thermal effects. The ability to trap or transport these particles stably will be important in these applications. However, while traditional optical tweezers can trap metallic Rayleigh particles easily, it is difficult to trap metallic mesoscopic/Mie particles because of the strong scattering forces that come from the far-field trapping laser beam. Here we demonstrate that metallic particles can be trapped stably using focused Bloch surface waves that propagate in the near-field region of a dielectric multilayer structure with a photonic band gap. Focused Bloch surface waves can be excited efficiently using an annular beam with azimuthal polarization and a high-numerical-aperture objective. Numerical simulations were performed to calculate the optical forces loaded on a gold particle by focused Bloch surface waves and the results were consistent with those of the experimental observations.

A novel radial beam smoothing scheme based on optical Kerr effect

Laser-beam illumination uniformity is a key issue in inertial confinement fusion facilities. In order to fulfill the requirement of improving illumination uniformity, a radial smoothing (RS) scheme is proposed. For smoothing the focal-spot pattern on a short time scale compared with the hydrodynamic response time of the target, the optical Kerr effect with extremely response time is taken into consideration. The basic principle of RS based on optical Kerr effect is that by using the interaction between optical Kerr medium and periodic Gaussian pulses to modulate a periodic spherical phase, to modulate periodic sphericel phase added at the wavefront of laser transmission wave, change the focal-spot size of the laser beam in far field, and further induce the fast radial redistribution of the speckles inside the focal spot in far field, and further induce the fast radial redistribution of the speckles inside the focal spot in far field. This fast radial redistribution of the speckles smoothes the intensity modulation of the focal spot on the target and eventually achieves the beam smoothing in the radial direction. The application of RS in the beamline is detailed. The optical Kerr medium is inserted in the front-end of the bemline, before the laser beam is injected into the main amplifier. The periodic Gaussian pulse for pumping the optical Kerr medium is obtained by the pulse stacking system based on fibers. The pulse width of stacked Gaussian pulse and the time delay between Gaussian pulses are set to be on a picosecond time scale or subpicosecond time scale. The induced refractive index of the optical Kerr medium by the pump laser fits spherical distribution with periodic variation, and results in the radial distribution of the speckles in focal plane. By establishing the theoretical model of the radial beam smoothing scheme implemented with continuous phase plate (CPP), the focusing characteristics of laser beam with RS and CPP are discussed in detail. The influences of the selection of optical Kerr medium and the characteristics of the radial redistribution on the radial smoothing effect are simulated and analyzed. Results indicate that the RS based on optical Kerr effect could efficiently achieve the periodic radial redistribution of the speckles on focal plane, and therefore improves the illumination uniformity in the radial direction while eliminating the stripe pattern presented in far field by one-dimensional smoothing spectral dispersion (SSD). The smoothing performance of RS is different from that of the conventional SSD due to its radial smoothing direction. Moreover, the combined application of RS with continuous phase plate could achieve a better smoothing level with a shorter time. The utilization of radial smoothing scheme in high power laser system may significantly improve the laser-beam irradiation with little influence on the performance of the beamline.

Laser ablation of silicon using a Bessel-like beam generated by a subwavelength annular aperture structure

Using a femtosecond laser incident to an oxide-metal-oxide film engraved with a subwavelength annular aperture (SAA) structure, we generated a Bessel-like beam to ablate silicon. Experimental results show that the silicon can be ablated with a 0.05 J/cm(2) input ablation threshold at 120 fs pulse duration. We obtained a surface hole possessing a diameter less than 1 μm. Optical performance, including depth-of-focus and focal spot of the SAA structure, were simulated using finite-different time-domain calculations. We found that a far-field laser beam propagating through a SAA structure possesses a submicrometer focal spot and high focus intensity. Our method can be easily adopted for surface machining in microfabrication applications.

Improved beam characteristics of solid-target soft x-ray laser amplifiers by injection seeding with high harmonic pulses

Injection seeding of solid-target soft x-ray laser amplifiers with high harmonic pulses is shown to dramatically improve the far-field laser beam profile and reduce the beam divergence. Measurements and two-dimensional simulations for a 13.9 nm nickel-like Ag amplifier show that the amplified beam divergence depends strongly on the seed and can therefore be controlled by selecting the divergence of the seed. The near-field beam size of both the seeded and unseeded lasers is shown to be determined by the size of the gain region and the divergence of the amplified beams.

Strong Extinction of a Far-Field Laser Beam by a Single Quantum Dot

Through the utilization of index-matched GaAs immersion lens techniques, we demonstrate a record extinction (12%) of a far-field focused laser beam by a single InAs/GaAs quantum dot. This contrast level enables us to report for the first time resonant laser transmission spectroscopy on a single InAs/GaAs quantum dot without the need for phase-sensitive lock-in detection.

レーザビーム伝送制御システムの開発

A laser beam delivery control system has been designed to align a laser beam axis and monitor its beam properties such as beam position, beam diameter, power, peak power and 3-D profile. A laser machining system for long term operation will make this system necessary because those beam qualities will be changed. The developed system is composed of two beam sensors, two angle control bending mirrors with actuators, and a host controller. A rotating wire method was applied for this sensor. In this system, the accuracy of the beam diameter was ±0.5 mm compared with the measurement from a burn pattern of acrylic resin. A far-field laser beam position of 17 m away from an oscillator could have been measured and controlled automatically within an accuracy of ±1 mm under on-line conditions. Thus this developed system is very promising for evaluating beam qualities.