Incoming Laser Beam Research Articles

Criteria for selection of point-ahead angle compensation strategy in intersatellite laser ranging interferometry of TianQin based on research of tilt-to-length coupling noise

TianQin is expected to deploy three spacecraft in Earth orbit at the altitude of about 1 × 108 m to form an equilateral triangular constellation with arm lengths of about 1.7 × 108 m. It is aimed at detecting gravitational waves in the frequency range from 0.1 mHz to 1 Hz by means of high sensitive laser ranging interferometry with pathlength measurement noise below 1 pm/Hz1/2. The long-range propagation of beam in the intersatellite laser ranging interferometry between the three spacecraft results in a non-negligible time delay of 0.58 s. Therefore, there is an angular difference between the optimal outgoing and incoming laser beam of laser ranging interferometry on each spacecraft, which is denoted as the point-ahead angle. Due to the relative motion between the three spacecraft, the point-ahead angle of TianQin constellation is approximately 22.96 μrad with a dynamic variation range of ±24.71 nrad. Point-ahead-angle mechanism is a fine steering mirror used for point-ahead angle compensation to diminish the beam pointing error, thus to maintain the intersatellite laser link as well as to diminish the tilt-to-length coupling noise, which are vital to the gravitational wave detection. Both the static and dynamic point-ahead angle compensation strategies can be used in TianQin and the selection of the two is based on minimizing the total point-ahead-angle-related tilt-to-length coupling noise. In this paper, a criteria for selection of the two strategies are proposed by establishing a model of point-ahead-angle-related tilt-to-length coupling noise as well as a corresponding evaluation function for calculation and comparison. The results indicate that the dynamic compensation strategy is the optimal choice only when the first-order tilt-to-length coupling coefficient of point-ahead-angle mechanism is less than 6.4 × 10−9 m/rad, otherwise the static compensation strategy should be used. The proposed criteria, as well as the model and the requirement for point-ahead-angle mechanism, can serve as a common theoretical basis in laser ranging interferometry applications.

Design and Implementation of an Energy Selector for Laser-Accelerated Protons

Highly intense bunches of protons and ions with energies of several MeV/u can be generated with ultra-short laser pulses focused on solid targets. In the most common interaction regime, target normal sheath acceleration, the spectra of these particles are spread over a wide range following a Maxwellian distribution. We report on the design and testing of a magnetic chicane for the selection of protons within a limited energy window. This consisted of two successive, anti-parallel dipole fields generated by cost-effective permanent C-magnets with customized configuration and longitudinal positions. The chicane was implemented into the target vessel of a petawatt laser facility with constraints on the direction of the incoming laser beam and guidance of the outgoing particles through a vacuum port. The separation of protons and carbon ions within distinct energy intervals was demonstrated and compared to a ray tracing code. Measurements with radiochromic film stacks indicated the selection of protons within [2.4, 6.9] MeV, [5.0, 8.4] MeV, or ≥6.9 MeV depending on the lateral dispersion. A narrow peak at 4.8 MeV was observed with a time-of-flight detector.

Creating an Array of Parallel Vortical Optical Needles

We propose a method for creating parallel Bessel-like vortical optical needles with an arbitrary axial intensity distribution via the superposition of different cone-angle Bessel vortices. We analyzed the interplay between the separation of individual optical vortical needles and their respective lengths and introduce a super-Gaussian function as their axial profile. We also analyzed the physical limitations to observe well-separated optical needles, as they are influenced by the mutual interference of the individual beams. To verify our theoretical and numerical results, we generated controllable spatial arrays of individual Bessel beams with various numbers and spatial separations by altering the spectrum of the incoming laser beam via the spatial light modulator. We demonstrate experimentally how to implement such beams using a diffractive mask. The presented method facilitates the creation of diverse spatial intensity distributions in three dimensions, potentially finding applications in specific microfabrication tasks or other contexts. These beams may have benefits in laser material processing applications such as nanochannel machining, glass via production, modification of glass refractive indices, and glass dicing.

Design and Development of a Laser Warning Sensor Prototype for Airborne Application


 
 
 Due to recent developments in high-energy laser systems, the laser is becoming one of the most potential choices in battlefield applications. Laser of a laser range finder used to find target distance may be of nanosecond pulse width and a single pulse may be sufficient to gather the instantaneous range information. A laser target designator is a similar laser with higher energy and with programmable pulse repetition frequencies5,7. Detection of such a specific battlefield laser radiation along with recognizing friend or foe is required for countermeasures. Designing a laser detection system that is capable of detecting such low-power level laser pulses of nanosecond pulse width at a long distance is a critical design and a challenging task. Again detecting a wide wavelength band that can start from 500 nm to around 1700 nm range using a single detector or device is also a challenging task. In this work, a sensor system is being designed and a prototype is developed to cover such a long band detection using a single detector for high-energy lasers. Also, in addition to detecting hostile code, the direction of an incoming laser beam is tried to incorporate into this sensor. The sensor can be utilized to detect unknown or non-friendly laser illumination from within a specific angular cone and distance.
 
 

3D tracking velocimetry of L-PBF spatter particles using a single high-speed plenoptic camera

Particle spatter is an unavoidable by-product of the Laser-Powder Bed Fusion (L-PBF) process, as the high intensity of the incoming laser beam generates high vapor fluxes on the meltpool, allowing metal particles to be ejected into the process environment. This is detrimental to the final manufactured part, as it risks the incorporation of defects. It is therefore important to study this spattering behavior and to apply the knowledge gained to further improve the L-PBF process. This work introduces the use of a high-speed plenoptic camera to acquire 3D spatter particle trajectories generated via L-PBF line tracks. Spatter particles are tracked in the volume above the laser-metal interaction zone at 1000 fps and their velocity calculated. It is found that the calculated speeds of the spatter particles are within the expected range for this process, and that the behavior is a complex 3-dimensional process.

Integrated optical system design for large-field-of-view and broad-spectrum laser warning.

We introduce an integrated large-field-of-view optical system, including a diffraction grating and a multiband narrowband filter, for laser detection. Simulation of the system in the ZEMAX optical design software package indicated that it was capable of detecting laser radiation in the visible light to near-infrared spectral range (500-1700 nm) from azimuth angles up to 120°, and pitch angles up to 96°. In practical experiments conducted at typical laser wavelengths (532 nm, 1064 nm, 1550 nm), the angle error in the full field of view was better than 0.5°, while the laser center wavelength detection accuracy was better than 7 nm, confirming the system's ability to identify the angle of incidence and wavelength of an incoming laser beam.

Measuring refractive index gradient of sugar solution

To measure refractive index at a particular altitude in a solution with vertical refractive index gradient, a transparent wedge-shaped container was constructed altogether with the development of mathematical formula derived from the Snell’s law. The refractive index of the solution can be calculated by measuring the angles of incoming and outgoing laser beams relative to respective normal line. By varying height of the laser beam, the refractive index as a function of height of a sugar solution was obtained. This technique is applied to investigate Fata Morgana which is a kind of superior mirage resulting from bending of light in a medium with density gradient.

Analysis of GRACE Follow-On Laser Ranging Interferometer Derived Inter-Satellite Pointing Angles

Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) was launched on May 22, 2018. It carries the Laser Ranging Interferometer (LRI) as a technology demonstrator that measures the inter-satellite range with nanometer precision using a laser-link between satellites. To maintain the laser-link between satellites, the LRI uses the beam steering method: a Fast Steering Mirror (FSM) is actuated to correct for misalignment between the incoming and outgoing laser beams. From the FSM commands, we can compute the inter-satellite pitch and yaw angles. These angles provide information about the spacecraft's relative orientation with respect to line-of-sight (LOS). We analyze LRI derived inter-satellite pointing angles for 2019 and 2020. Further, we present its comparison with the pointing angles derived from GRACE-FO SCA1B data, which represents the spacecraft attitude computed from star cameras and Inertial Measurement Unit (IMU) data using a Kalman filter. We discuss the correlations seen between the laser based attitude data and the spacecraft temperature variations. This analysis serves as the basis to explore the potential of this new attitude product obtained from the Differential Wavefront Sensing (DWS) control of a FSM.

Characterization of angularly resolved EUV emission from 2-µm-wavelength laser-driven Sn plasmas using preformed liquid disk targets

The emission properties of tin plasmas, produced by the irradiation of preformed liquid tin targets by several-ns-long 2 µm-wavelength laser pulses, are studied in the extreme ultraviolet (EUV) regime. In a two-pulse scheme, a pre-pulse laser is first used to deform tin microdroplets into thin, extended disks before the main (2 µm) pulse creates the EUV-emitting plasma. Irradiating 30- to 300 µm-diameter targets with 2 µm laser pulses, we find that the efficiency in creating EUV light around 13.5 nm follows the fraction of laser light that overlaps with the target. Next, the effects of a change in 2 µm drive laser intensity (0.6–1.8 × 1011 W cm−2) and pulse duration (3.7–7.4 ns) are studied. It is found that the angular dependence of the emission of light within a 2% bandwidth around 13.5 nm and within the backward 2π hemisphere around the incoming laser beam is almost independent of intensity and duration of the 2 µm drive laser. With increasing target diameter, the emission in this 2% bandwidth becomes increasingly anisotropic, with a greater fraction of light being emitted into the hemisphere of the incoming laser beam. For direct comparison, a similar set of experiments is performed with a 1 µm-wavelength drive laser. Emission spectra, recorded in a 5.5–25.5 nm wavelength range, show significant self-absorption of light around 13.5 nm in the 1 µm case, while in the 2 µm case only an opacity-related broadening of the spectral feature at 13.5 nm is observed. This work demonstrates the enhanced capabilities and performance of 2 µm-driven plasmas produced from disk targets when compared to 1 µm-driven plasmas, providing strong motivation for the use of 2 µm lasers as drive lasers in future high-power sources of EUV light.

Low-cost laser detection system with a 360-deg horizontal field of view

A low-cost laser detection system based on coherence detection has been developed and is able to detect weak, continuous laser sources even against bright background light. The system is composed of a Mach–Zehnder interferometer with one arm modified with a piezo-mounted mirror to modulate the path length. We introduce methods to determine the laser wavelength and to extend the horizontal field of view of the detector. To widen the field of view, a cone mirror is added to the system while the additional use of a camera allows the direction of the incoming laser beam to be studied. The wavelength from three different lasers is estimated with the use of the modulation amplitude of the piezo mirror. The preliminary results demonstrate that a 360-deg horizontal field of view can be achieved and that the direction of the laser beam can be determined with an estimated angular precision of ±5 deg. Moreover, the wavelength can be determined with a precision of ±10 nm. The system trades sensitivity for a larger field of view with the resultant detection sensitivity equal to 70 nW (or 1 μW · cm − 2) at 635 nm.

Morphological characteristics of nanoholes induced by single-shot femtosecond laser ablation of borates and aluminate silicates

Single-shot femtosecond laser ablation experiments with linearly and circularly polarized light were conducted in order to investigate the morphological characteristics of surface nanostructures in lithium borate crystals and glasses, a strontium borate crystal, lanthanide containing borate crystals, and aluminate silicate crystals: Li2B4O7 (LTB) and LiB3O5 (LBO) crystals and Li2O⋅2B2O3 (LTB) and Li2O⋅3B2O3 (LBO) glasses; SrB4O7 (SBO) crystal; Li6Gd(BO3)3 (LGB) and LaSc3(BO3)4 (LSB) crystals; and Ca2Al2SiO7 (CAS) and CaSrAl2SiO7 (CSAS) crystals. In the present study, the material and laser polarization dependance of the morphology of nanoholes was examined in these crystals and glasses. A single nanohole or two holes (a primary hole and a secondary hole) were observed in the borate and aluminate silicate crystals. The size of the nanohole is not restricted by the diffraction limit but instead is dependent on the laser fluence and the materials. It is suggested that the formation of these secondary nanoholes in the studied crystals is attributed to a spontaneous reshaping of the incoming Gaussian pulse into a Gaussian–Bessel pulse. In the LTB and LBO crystals, nanoholes (both primary and secondary holes) with subwavelength sides exhibit a quadrilateral (approximately square or rectangular) morphology, regardless of linear or circular polarization. The sides of the quadrilateral nanoholes lie approximately in the {h h 0} planes on the LTB crystal and in the ({h 0 0} and {0 0 l}) planes on the LBO crystal. We found that the nanohole morphology did not reflect the spatial distribution of the laser intensity. These phenomena were the first observations on the anisotropic morphology of nanoholes. These morphologies do not correspond to the circular symmetric pattern of the Gaussian intensity distribution of the incoming laser beam. This is contrary to the expectations based on the generally accepted laser ablation mechanism. The quadrilateral nanoholes could be an inherent morphology in the LTB and LBO crystals. The morphology of the quadrilateral holes in the LTB and LBO crystals is considered to reflect the continuous BO33− and/or BO45− network structure in their respective tetragonal or orthorhombic unit cells, in which self-tapped excitons are formed in an initial process under multiphoton excitation. In contrast, the SBO, LGB, LSB, CAS, and CSAS crystals and the LTB and LBO glasses exhibit circular nanoholes with subwavelength diameters independent of the laser polarization, the structure, or the composition. The isotropic morphology of nanoholes in these samples reflects the circular pattern of the Gaussian intensity profile of the focused laser beam.

Intracavity optical trapping of microscopic particles in a ring-cavity fiber laser

Standard optical tweezers rely on optical forces arising when a focused laser beam interacts with a microscopic particle: scattering forces, pushing the particle along the beam direction, and gradient forces, attracting it towards the high-intensity focal spot. Importantly, the incoming laser beam is not affected by the particle position because the particle is outside the laser cavity. Here, we demonstrate that intracavity nonlinear feedback forces emerge when the particle is placed inside the optical cavity, resulting in orders-of-magnitude higher confinement along the three axes per unit laser intensity on the sample. This scheme allows trapping at very low numerical apertures and reduces the laser intensity to which the particle is exposed by two orders of magnitude compared to a standard 3D optical tweezers. These results are highly relevant for many applications requiring manipulation of samples that are subject to photodamage, such as in biophysics and nanosciences.

CALIBRATING PHOTOGRAMMETRIC AIRBORNE CAMERA SYSTEMS WITH DIFFRACTIVE OPTICAL ELEMENTS

Abstract. This paper presents a laboratory approach for geometric calibration of airborne camera systems. The setup uses an incoming laser beam, which is split by Diffractive Optical Elements (DOE) into a number of beams with precisely-known propagation directions. Each point of the diffraction pattern represents a point at infinity and is invariant against translation. A single image is sufficient to allow a complete camera calibration in accordance with classical camera calibration methods using the pinhole camera model and a distortion model. The presented method is time saving, since complex bundle adjustment procedures with several images are not necessary. It is well suited for the use with frame camera systems, but it works in principle also for pushbroom scanners. In order to prove the reliability, a conventional test field calibration is compared against the presented approach, showing that all estimated camera parameters are just insignificantly different. Furthermore a test flight over the Zeche Zollern reference target has been conducted. The aerotriangulation results shows that calibrating an airborne camera system with DOE is a feasible solution.

Accounting for laser beam characteristics in the design of freeform optics for laser material processing

Abstract Recently, freeform optics have been introduced for application adapted beam shaping in laser heat treatment. There, intensity distributions are generated that induce previously defined temporal and spatial temperature profiles. To this end, a two-step simulation strategy is necessary, where in the first step the intensity distribution must be derived for which in the second step the freeform optics is calculated. To provide a design that can successfully be integrated in an experimental setup, the incoming laser beam’s characteristics must be accounted for in the derivation of the adapted intensity distribution as well as in the freeform optics design. Here, the two most relevant quantities are the beam’s maximum output power as well as the divergence angle. In this work, strategies are presented that account for the beam’s maximum output power in the derivation of the adapted intensity distribution. Furthermore, stabilizing methods are introduced to enhance the performance of a previously introduced freeform optics design algorithm that takes into account the laser beam’s finite divergence angle but suffers from numerical noise and oscillation problems. A simulation example that uses both techniques is given for (nano)ceramic thin-film laser processing.

Brilliance improvement of laser-produced extreme ultraviolet and soft x-ray plasmas based on pulsed gas jets

Two methods improving the brilliance of laser-induced plasmas emitting in the extreme UV (EUV) and soft x-ray (SXR) regions were investigated, using three different gases (nitrogen, krypton, and xenon) from a pulsed gas jet. Utilizing a newly designed piezoelectric valve, up to almost ten times higher gas pressures were applied, resulting in increased target densities and thus, higher conversion efficiencies of laser energy into EUV and SXR radiation. Secondly, geometrically reducing the angle between the incoming laser beam and the observed plasma emission minimizes reabsorption of the emitted short wavelength radiation. Combining both methods, the source brilliance is increased by a factor of 5 for nitrogen. Furthermore, a compact EUV focusing system for metrological applications is presented utilizing the optimized plasma source. An energy density of 1 mJ/cm2 at wavelength λ = 13.5 nm in the focal spot of an ellipsoidal mirror is achieved with xenon as the target gas being sufficient for material removal of PMMA samples with an ablation rate of 0.05 nm/pulse.

Alternative Design of Dammann Grating for Beam Splitting With Adjustable Zero-Order Light Intensity

Conventional Dammann grating is a binary-phase (0, π) transmission grating capable of splitting an incoming laser beam into multiple outgoing beams with equal intensity. In this paper, we present an alternative design for Dammann-type beamsplitter with a non π-phase shift value. In the design, the simulated annealing algorithm is taken to optimize the grating structure and phase difference simultaneously. We find that this non 0-π phase grating exhibits interesting properties. When the etching depth is slightly lower than the theoretical depth, the zero-order light intensity is enhanced compared to other diffraction orders. Conversely, when the etching depth is slightly greater than the theoretical depth, the zero-order light intensity is attenuated. This allows us to flexibly adjust the brightness of the zero-order diffractive light. In contrast, the deviation of the etching depth will always result in a brighter zero-order light for the conventional Dammann grating. Simulations and experiments are performed to verify the feasibility of the proposed grating.

Inline laser power measurement by photon momentum.

We present a novel measurement scheme and instrumentation for quantifying laser power by means of photon momentum. The optical design is optimized such that the incoming laser beam is minimally perturbed and is available for other purposes along the incoming beam axis. Additionally, the geometry of the instrument gives access to the small but measurable transmittance between two passive mirrors that face the force sensor. The force sensor is based on a commercially available weighing instrument ("scale") that has a temporal response of approximately 5 s and a readability of approximately 1 μg (∼2 W). Our measurement results demonstrate beam profile and power for 500 W, but the mirror and mass (or force) calibration are suitable for very high power up to 50 kW and beyond. The optics are based on commercially available, off-the-shelf mirrors optimized for the angle of incidence and maximum reflectance at the wavelength of 1070 nm. The size of the complete instrument has an input aperture of Ø75 mm, but this constraint is only a matter of optimizing the beam path and box geometry.

The unexpected role of evolving longitudinal electric fields in generating energetic electrons in relativistically transparent plasmas

Superponderomotive-energy electrons are observed experimentally from the interaction of an intense laser pulse with a relativistically transparent target. For a relativistically transparent target, kinetic modeling shows that the generation of energetic electrons is dominated by energy transfer within the main, classically overdense, plasma volume. The laser pulse produces a narrowing, funnel-like channel inside the plasma volume that generates a field structure responsible for the electron heating. The field structure combines a slowly evolving azimuthal magnetic field, generated by a strong laser-driven longitudinal electron current, and, unexpectedly, a strong propagating longitudinal electric field, generated by reflections off the walls of the funnel-like channel. The magnetic field assists electron heating by the transverse electric field of the laser pulse through deflections, whereas the longitudinal electric field directly accelerates the electrons in the forward direction. The longitudinal electric field produced by reflections is 30 times stronger than that in the incoming laser beam and the resulting direct laser acceleration contributes roughly one third of the energy transferred by the transverse electric field of the laser pulse to electrons of the super-ponderomotive tail.

Diffractive optics for axial intensity shaping of Bessel beams

Bessel beams (BBs) appear to be immune to diffraction over finite propagation distances due to the conical nature of light propagation along the optical axis. This offers promising advantages in laser fabrication. However, BBs exhibit a significant intensity variation along the direction of propagation. We present a simple technique to engineer the axial intensity of the BBs over centimeter-long propagation distances without expansion of the incoming laser beam. This method uses two diffractive optical elements (DOEs), one converts the input Gaussian intensity profile to an intermediate intensity distribution, which illuminates the second DOE, a binary axicon. BBs of a desired axial intensity distribution over a few centimeters length can be generated.

Nonlinear optical tweezers for longitudinal control of dielectric particles

A model of the optical tweezers using thin layer of organic dye as the addition nonlinear lens in configuration is proposed. The expressions of the focal length of nonlinear lens, intensity distribution of reshaped Gaussian beam, and total longitudinal optical force acting on dielectric particles are theoretically derived using paraxial approximation. The influence of the average power of incoming Gaussian laser beam, thickness of thin layer, and nonlinear coefficient of refractive index on properties of nonlinear optical tweezers are numerically observed. The results also are discussed to find out the conditions that the dielectric particle could be stable trapped and finely controlled by tuning of average power of incoming laser beam.