Far-field Laser Research Articles

Laser ultrasonic frequency-domain imaging and phase weighted optimization based on full matrix capture

Far-field laser technology has greatly promoted the progress of nondestructive ultrasonic imaging of bulk structures. However, under thermoelastic excitation, the body waves exhibit a relatively low signal-to-noise ratio, resulting in images with low resolution and contrast. Based on the motivation, this paper developed a frequency-domain phase weighted imaging method to improve the quality of laser ultrasonic defect imaging. Firstly, laser ultrasonic scanning was performed on the sample with artificial transverse hole defects. The cylindrical lens focused line source was used to improve the intensity of the body wave signals, and ensure that there was no damage on the material surface under high laser energies. Then, the frequency-domain phase shift migration (PSM) algorithm was used to perform multimode imaging of defects, achieving frequency-domain synthetic aperture focusing technique (F-SAFT) and total focused method (F-TFM) imaging based on full matrix capture. Furthermore, the phase circular statistical vector (PCSV) was proposed for weighted optimization, which improved the image quality, suppressed the background noise and multimode artifacts. Finally, the imaging quality of several algorithms were discussed. The results indicate that frequency-domain images were superior to time-domain results. After phase weighting, the imaging quality can be further improved, and the detection blind zone was significantly reduced. This work will contribute to the rapid and high-quality defect imaging of laser ultrasonic.

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.

Line-shaped laser lithography for efficient fabrication of large-area subwavelength nanogratings

Subwavelength nanogratings for optical modulation have attracted significant attention for efficient large-area fabrication. We developed a far-field laser nanofabrication method, called line-shaped laser lithography, to ablate thin films with separated line-shaped ultrafast laser pulses on substrates into multiple nanogratings. The millimeter pulse length and high-speed scanning for a single-pulse ablation ensure exceptionally high large-area fabrication efficiency. Moreover, nanogratings with a sub-100 nm linewidth (e.g., a 57 nm linewidth ( ∼ λ / 9 ) nanograting on 10 nm Au film) were generated based on the Marangoni effect. An ultra-uniform large-area ( 6 × 15 m m 2 ) nanograting with a width of 132 nm and a length-to-width ratio of ∼ 45 , 500 was achieved in only 1.5 s. In particular, diversified nanogratings can be obtained by designing scanning strategies, including moiré, blaze-like, Dammann, and 2D nanogratings, revealing light modulations in diffraction and beam splitting.

High precision center measurement of far-field laser based onimproved Zernike moments

High precision center measurement of far-field laser based onimproved Zernike moments

Laser ultrasonic imaging of submillimeter defect in a thick waveguide using entropy-polarized bilateral filtering and minimum variance beamforming

Recent quantum leap in far-field laser techniques has advanced noncontact implementation of nondestructive ultrasonic imaging, in pursuit of enhanced accessibility, detectability and practicability. Nevertheless, when laser-generated thermoelastic waves are extended to thick waveguides, they manifest fairly low signal-to-noise ratios (SNRs), along with severe wave diffusion, consequently lowering image resolution and contrast. With these motivations, a laser-ultrasonics imaging approach is developed, in conjunction with i) entropy-polarized bilateral filtering (Entropy-P-BF) for signal denoising, and ii) minimum variance (MV) beamforming for defect imaging, targeting at precise characterization of a submillimeter defect (with its characteristic dimension being smaller than the wave diffraction limit) in a thick waveguide. The entropy-polarized bilateral filtering denoises laser-induced ultrasonic wave signals via a two-dimensional convolution, the weight matrices of which are continuously updated according to local noise and uncertainty. With an elevated SNR, MV beamforming subsequently conducts an apodized beamforming to image the defect. Experimental validation is conducted by imaging a void-type defect, 0.7 mm only in its diameter, in a jet aero-engine turbine disk. Results prove that the developed approach is capable of characterizing a submillimeter defect accurately in a thick waveguide with thickness ∼25 times the wavelength of laser-induced shear wave, regardless of a fairly low SNR (<1dB).

Fast fabrication and judgement of tip-enhanced Raman spectroscopy-active tips

The quality of the scanning tip is crucial for tip-enhanced Raman spectroscopy (TERS) experiments towards large signal enhancement and high spatial resolution. In this work, we report a controllable fabrication method to prepare TERS-active tips by modifying the tip apex at the atomic scale, and propose two important criteria to in-situ judge the tip's TERS activity for tip-enhanced Raman measurements. One criterion is based on the downshift of the first image potential state to monitor the coupling between the far-field incident laser and near-field plasmon; the other is based on the appearance of the low-wavenumber Raman peaks associated with an atomistic protrusion at the tip apex to judge the coupling efficiency of emissions from the near field to the far field. This work provides an effective method to quickly fabricate and judge TERS-active tips before real TERS experiments on target molecules and other materials, which is believed to be instrumental for the development of TERS and other tip-enhanced spectroscopic techniques.

Year-long atmospheric characterization effort for laser weapon testing on the Potomac River Test Range at the Naval Surface Warfare Center Dahlgren Division

Characterizing far-field high energy laser (HEL) propagation during laser weapon system (LWS) test events in an open-air environment is challenging due to many variables that affect laser transmission. These influencing variables can be divided into two different categories, the first of which is the test architecture and LWS configuration. Missions to be accomplished by the system under test (SUT) have varying degrees of difficulty, which include fast-moving dynamic targets. The SUT may also have various operating modes, wavelengths, and shifting internal optics. The second category is the underlying physics occurring from inside the laser cavity and along the optical path to the target. Phenomena including—but not limited to—cavity stability, optical physics, optical turbulence, atmospheric absorption and scattering, and thermal blooming will affect laser propagation and the characterization methodology for an LWS. It is important to consider all these factors as weapon systems are tested and scored to requirements and performance specifications for assessing lethality and effectiveness. Therefore, a year-long measurement study was conducted on the Potomac River Test Range (PRTR) to assess the optical turbulence conditions of the laser range in support of LWS test and evaluation (T&E) events. Experimental data is analyzed for averaged optical turbulence seasonal trends on a complex nonuniform range. The results are used to coordinate optimal test event timing, predict a laser’s far-field properties at the laser termination site, and generate atmospheric forecasts in conjunction with HEL and atmospheric modeling software. The results will contribute to improving future HEL testing methodology and examining accuracy of models producing optical turbulence calculations.

Atom recoil in collectively interacting dipoles using quantized vibrational states

The recoil of atoms in dense ensembles during light matter interactions is studied using quantized vibrational states for the atomic motion. The recoil resulting from the forces due to the near-field collective dipole interactions and far-field laser and decay interactions are explored. The contributions to the recoil and the dependence on the trap frequency of the different terms of the Hamiltonian and Lindbladian are studied. These calculations are compared with previous results using the impulse model in the slow oscillation approximation. Calculations in highly subradiant systems show enhanced recoil indicating that recoil effects cannot be ignored in such cases.

Fuzzy fraction-order stochastic parallel gradient descent approach for efficient fiber coupling

In a high-speed free-space optical communication system, a far-field space laser is usually coupled with a single-mode fiber via the optical antenna and then transmitted to a detector by an optical fiber. However, atmospheric turbulence will affect the coupling effect of the optical fiber. A fuzzy fraction-order stochastic parallel gradient descent (SPGD) is proposed based on the traditional SPGD algorithm to achieve efficient fiber coupling. In this algorithm, the fraction-order differential is integrated into the SPGD for the purpose of improving the speed of the fiber coupling. Furthermore, the fuzzy control method is combined with the fraction-order SPGD approach, and fuzzy rules are designed for the gradient. The fractional order and learning rate are adjusted adaptively in each iteration. In the simulation, the fuzzy fraction-order SPGD can converge to the extreme point quickly and has excellent robustness.

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.

Far-field laser spot image detection for use under atmospheric turbulence

In wireless laser communication, the effects of atmospheric turbulence cause laser spots to appear as irregular speckles in the far-field plane, which may make automatic acquisition, pointing, and tracking systems unable to locate a spot position. We propose an innovative and precise anti-interference, spot-detection algorithm for far-field irregular speckles based on the intensity characteristics of the far-field spots. Before obtaining the spot center coordinates through two-dimensional vector space circle fitting, the proposed algorithm employs computer graphics combined with mathematical morphology to fill the connected domains and extracts light-spot edges using the Canny operator, thus resulting in an accurate detection of far-field spots. The algorithm validation, using an automated experimental platform (atmospheric turbulent refractive-index structure parameter C n 2 is 10 − 15 m − 2/3), yields a detection threshold accuracy of 30 to 32 pixels (each pixel represents 3 μrad) for 10.2 km far-field spot images. The corresponding deviation satisfies the two-dimensional aiming platform for a minimum step distance of 17.45 μrad, verifying the feasibility and effectiveness of the algorithm.

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.

Evaluating the Potential of Laser Beam Quality Improvement by Adaptive Optics System

AO (adaptive optics) systems have been extensively used to improve the beam quality in high-energy lasers; however, few studies have focused on how much the beam quality could be improved. A novel evaluation criteria and factor is presented in this paper. The factor, defined as power in ring (PIR), is expressed by distinguishing the low- and high-spatial frequency components in the far-field laser intensity distribution. Beams with different PIR values are generated in our model, and then they are compensated by AO systems. Calculation result shows that the PIR factor could evaluate a laser beam’s improvement potential by an AO system quantitatively.

Novel Ionic-Liquid-Type Lubricant for Heat-Assisted Magnetic Recording System

In this paper, a new thermally stable ionic-liquid (IL)-type lubricant has been developed. Since heat is applied intensively on the surface of magnetic disks in heat-assisted magnetic recording (HAMR) systems, a thermally stable lubricant is expected to prolong the life of HAMR drives. To verify this concept experimentally, the depletion of ultrathin film lubricants was evaluated for IL- and perfluoropolyether (PFPE)-type lubricants by far-field laser irradiation. For PFPE, lubricant depletion decreased with increasing bond ratio, whereas lubricant depletion for IL did not depend on its bond ratio. The lubricant depletion quantities of ILs were small and comparable to that of PFPEs with the highest bond ratio.

Active Far-Field Control of the Thermal Near-Field via Plasmon Hybridization.

The ability to control and manipulate temperature at nanoscale dimensions has the potential to impact applications including heat-assisted magnetic recording, photothermal therapies, and temperature-driven reactivity. One challenge with controlling temperature at nanometer dimensions is the need to mitigate heat diffusion, such that the temperature only changes in well-defined nanoscopic regions of the sample. Here we demonstrate the ability to use far-field laser excitation to actively shape the thermal near-field in individual gold nanorod heterodimers by resonantly pumping either the in-phase or out-of-phase hybridized dipole plasmon modes. Using single-particle photothermal heterodyne imaging, we demonstrate localization bias in the photothermal intensity due to preferential heating of one of the nanorods within the pair. Theoretical modeling and numerical simulation make explicit how the resulting photothermal images encode wavelength-dependent temperature biases between each nanorod within a heterodimer, demonstrating the ability to actively manage the thermal near-field by simply tuning the color of incident light.

Photonic Crystal Diode Laser Array Integrated With a Phase Shifter for Narrow Beam Divergence

A narrow far-field edge-emitting photonic crystal diode laser array integrated with a phase shifter emitting in the 980-nm range has been designed and fabricated. A phase shifter structure is integrated on the chip to convert the out-of-phase mode to the in-phase mode. The real effective-refractive-index difference produced by the phase shifter is measured according to the comparison of the experiment and the simulation results. The effects of phase shifters with different lengths on the near and far fields are investigated. With the phase and the amplitude modulation of the phase shifter, the out-of-phase mode is successfully converted to the in-phase mode. Thus, a single-lobe far-field pattern is achieved in the lateral direction. The far-field divergence angles of the lateral and vertical directions are 1.5° and 12°, respectively. Such a device is promising for narrow far-field emission with a high output power.

Far-field constant-gradient laser accelerator of electrons in an ion channel

We predict that electrons in an ion channel can gain ultra-relativistic energies by simultaneously interacting with a laser pulse and, counter-intuitively, with a decelerating electric field. The crucial role of the decelerating field is to maintain high-amplitude betatron oscillations, thereby enabling constant rate energy flow to the electrons via the inverse ion channel laser mechanism. Multiple harmonics of the betatron motion can be employed. Injecting electrons into a decelerating phase of a laser wakefield accelerator is one practical implementation of the scheme.

Analytical prediction of laser mediated polymer melt and damage width

Far-field (remote) laser net-shape scanning has revolutionary potential across numerous applications which involve localized heating of materials. It offers a very high degree of manufacturing flexibility in concert with process repeatability, traceability and low cycle energy usage when compared to traditional tooling-based solutions if the material response can be accurately predicted.The functional mechanism of such processes is localized heating; in this work, an analytical model of the line width of phase change occurring between a 3mm thick virgin polypropylene, PP, sheet and a visually transparent 25µm thick PP film is presented.Validation of the model is provided empirically by the scanned application of a CO2 laser exhibiting a Gaussian beam profile onto reference materials at varying incident spot diameters, powers and traverse velocities.This work is of value for process parameter prediction, as this analytically based method is computationally light, enabling its real-time implementation in manufacturing environments.

线阵光纤激光相干合成角度扫描控制方法研究

Laser beam combining technique allows increasing the power of laser radiance far beyond what is obtained from a single conventional laser. Moreover, this technique is also able to control the far-field laser combining intensity distribution. Based on the theory, the principle of coherent combining of lasers from multi-core linear fiber array was studied. By using the coherent synthesis model, the coherent combining effect of multi-core linear fiber array was numerically simulated by MATLAB when wavefront of the lasers with and without random phase noise respectively, and the feasibility of angle scanning using actively phase-controlling was analyzed. The coherent combining angle scanning control system was built based on MOPA laser linear array fibers. The experiment shows that active phase-controlling of the laser linear array is able to realize the angle scanning of the main maximal stripe, and the scanning range is between two secondary large stripes. The experimental results are consistent with the theoretical calculation, and that has a guiding significance to the application of coherent combining of linear fiber array lasers.