KEYWORDS: Polarization, Laser processing, Spatial light modulators, Laser systems engineering, Digital Light Processing, Internet of things, Holography, Phase shift keying, Optical signal processing, Manufacturing
The arbitrary dynamic control of both amplitude and polarization distributions is attracting strong interest in laser processing field to manage the quality and to collect valuable polarization characteristics of processing materials in smart manufacturing. We present a holographic method to generate arbitrary polarization state of multiple beams by synchronizing two phase-only liquid crystal spatial light modulators (SLMs) with imaging feedback system for hologram designing of each polarization state. This research work will help to accelerate the use of liquid crystal SLMs for high-throughput and optimized additive manufacturing.
We developed an ultra-precise retardation-measurement system based on optical-heterodyne interferometry with a 3σ repeatability of λ/360, 000 for zero retardation, where the frequency shift for the optical-heterodyne interferometry was generated by a rotating half-waveplate, and both polarizations for the retardation measurement were always exactly on a common path. Using this system, the direction of the c-axis of a sapphire window was determined by analyzing the incidence-angle dependence of the retardation. The possible resolution of the c-axis direction of the sapphire window was estimated to be 0.9 arcmin from the retardation-measurement repeatability. This c-axis determination method will be applicable to, for example, high-precision sapphire-mirror production/evaluation for gravitational-wave detection.
We developed a liquid-crystal spatial light modulator having a 30 mm active area and a multilayered dielectric mirror for industrial infrared lasers to establish an innovative manufacturing and fabrication technique in the smart-manufacturing post-pandemic era. The reconstruction of computer-generated holograms was achieved to demonstrate the concept of this device in the IR region. The incident phase performance characteristics of this device under high-power laser irradiation were obtained using a 1030 nm ultrashort pulse laser. The work presented here will accelerate the use of liquid-crystal SLMs in high-precision laser processing of the process-resistant materials and high-throughput processing for additive manufacturing.
In this study, we developed a liquid-crystal spatial light modulator with high laser power capacity for industrial ultrafast pulse lasers to demonstrate innovative manufacturing and fabrication techniques using a cyber-physical system. The incident phase performance characteristic of this device was obtained with a 60 W, 1035 nm ultrafast laser. This research work will help to accelerate the use of liquid crystal spatial light modulators for high-precision laser processing of resistant materials and high-throughput for additive manufacturing.
Sophisticated control of beam patterns is attractive for applications including LiDAR, surveying, and 3D measurements. Light sources with beam pattern control on chips would enable simplicity and portability to systems, and this technology would prove useful in many fields. Therefore, we propose integrable spatial-phase-modulating surface-emitting lasers (iPMSELs) in which static arbitrary two-dimensional beam patterns are emitted from needle-tip sized sources. We present a demonstration of various static two-dimensional beam patterns including characters, multi-spots, lines, and even gray-scale pictures.
The basic structure of iPMSELs is similar to that of ordinary laser diodes. A novel phase modulating layer is introduced near the active layer. The holes in the phase modulating layer are systematically arranged in positions slightly shifted from the lattice point of square-lattice photonic-crystal. The layer contributes to two important operating mechanisms, “in-plane resonance” due to zero-group velocity at the photonic-band edge and “spatial-phase modulation” of output beam patterns due to the positional shift of holes designed using computer generated holograms. However, the prototype device shows not only target beam patterns but also subsidiary beam patterns including a strong central spot beam (zero order beam) attributable to vertical diffraction.
To address the issue, we improved the design and successfully removed the beam, demonstrating periodic beam patterns useful for 3D measurements. We also present a demonstration of electrical switching of beam patterns using arrayed iPMSELs where eight devices are integrated onto a TO-8 base. This enables applications including beam scanning or indications.
Beam shaping techniques with diffractive optical elements have garnered considerable attention for laser material processing and microscopy because of their high efficiency of light utilization. Particularly, the design of top-hat beams with several shapes including circular and rectangular is required to facilitate a high-throughput system for line scanning and surface peeling applications. In this study, we propose a diffractive beam shaping method for the generation of a tophat beam with arbitrary shapes under tight focusing conditions. We implemented the iterative Fourier-transform algorithm (IFTA) with an error function in the form of a Gaussian distribution in the input laser beam to calculate an optimized phase distribution for generating a top-hat beam with arbitrary shape. This phase distribution was generated with a phase-only spatial light modulator and relayed with an optical system to the pupil of an objective lens with a numerical aperture of 0.75. The point spread function under the focal spot was observed with a microscopic imaging system placed opposite to the beam focusing optics. We experimentally demonstrate that the size of the focused top-hat beam is twice the size of the airy disk under tight focusing conditions. Further, we measure the profile of the generated beams. The proposed method with a spatial light modulator offers an adaptive control on the uniformity of the generated distribution, which fluctuates according to the effect of slightly different laser conditions on the diameter and profile of the input beam.
We demonstrated direct surface-emitting of Laguerre–Gaussian beams with wavefront modulated lasers. This integrable phase-modulating surface-emitting lasers has potential to emit arbitrarily configured beam patterns without requiring any optical elements or scanning devices. The fabricated devices are on-chip-sized, making them suitable for integration. We introduce a phase-modulating resonator in a semiconductor laser, which analogically behaves as phaseonly holograms, kinoform, to allow the concurrent realization of lasing and phase modulation. Particularly, this is promising in the use for free-space optical communications due to the fact that coaxial propagation of orbital angular momentum (OAM) properties with different OAM mode states are mutually orthogonal.
Laser dicing with tightly focused nanosecond pulsed laser light inside a semiconductor wafer is a dry, debris-free dicing method achieved by the generation of thermal microcracks. This method has two practical issues: a dicing speed that is limited by the repetition rate of the pulsed laser and potential damage to integrated circuits on the wafer from excessive laser intensity due to insufficient beam divergence. By correcting aberrations and generating multiple beams via wavefront modulation, multiple focused beams inside the wafer will become sufficiently divergent to avoid undesirable potential laser damage. We confirmed these improvements by dicing sapphire wafers with a pulsed laser and a high-numerical-aperture objective lens.
We report here a holographic high speed accessing microscope of sensory-driven synaptic activity across all inputs to single living neurons in the context of the intact cerebral cortex. This system is based on holographic multiple beam generation with spatial light modulator, we have demonstrated performance of the holographic excitation efficiency in several in vitro prototype system. 3D weighted iterative Fourier Transform method using the Ewald sphere in consideration of calculation speed has been adopted; multiple locations can be patterned in 3D with single hologram. Standard deviation of intensities of spots are still large due to the aberration of the system and/or hologram calculation, we successfully excited multiple locations of neurons in living mouse brain to monitor the calcium signals.
Conventional methods of compensating for self-distortion in liquid-crystal-on-silicon spatial light modulators (LCOS-SLM) are based on aberration correction, where the wavefront of the incident beam is modulated to compensate for aberrations caused by the imperfect optical flatness of the LCOS-SLM surface. Previously, we proposed an effective method to compensate for the distortion by displaying a compensation phase pattern obtained from interferometry However, the phase distribution of an LCOS-SLM varies with changes in ambient temperature and requires additional correction. The ambient temperature of LCOS-SLMs can vary under certain circumstances, i.e. equipped inside systems for field use or long-term operations. In this presentation, we discussed a novel phase compensation method under temperature-varying conditions based on an orthonormal Legendre series expansion of the phase distribution from viewpoint of multiple beam holographic generation. We found several Legendre coefficients that follow quadratic functions of ambient temperature. This prompted us to propose an algorithm for correcting the temperature dependency by displaying a phase pattern using two simple steps: an initializing step and a temperature correction step. We investigated the temperature dependency by controlling the ambient temperature with an incubator and successfully corrected for self-distortion in a temperature range of approximately 68°F to 122°F, giving an optical flatness of <λ /10. Our approach has the potential to be adopted in tight-focusing applications which require wavefront modulation with very high accuracy. Additionally, the concept of this method is extensible to the thermal behavior of other optical devices, such as lenses and mirrors, which have the possibility of causing unexpected aberrations.
Dynamics of micrometer-sized dielectric objects can be controlled by optical tweezers with scanning light, however, the trapped objects fail to track the scan when drag exceeds the trapping by too quick movement. On the other hand, optical vortices (OVs), which have a property of carrying angular momenta, can directly control torque on objects rather than their position. Laguerre-Gaussian (LG) beams are the most familiar examples of OV and have been studied extensively so far. Revolution of the objects trapped by the LG beams provides typical models of nonequilibrium statistical system, but stable mid-water trapping by the LG beams becomes essential to evaluate physical properties of the system without extrinsic hydrodynamic effects,. Nevertheless, off-axis revolutions of small objects trapped in mid-water by the LG beams have not yet been established with secure evidences. Here we report stable off-axis trapping of dielectric spheres in mid-water using high-quality LG beams generated by a holographic complex-amplitude modulation method. Direct evidence of the three-dimensional off-axis LG trapping was established via estimating the trapping position by measuring the change of revolution radii upon pressing the spheres onto glass walls. Resultantly, the axial trapping position was determined as about half the wavelength behind the beam waist position. This result provides a direct scientific evidence for possibility of off-axis three-dimensional trapping with a single LG beam, moreover, suggests applications as powerful tools for studying energy-conversion mechanisms and nonequilibrium nature in biological molecules under torque.
We have developed a confocal fluorescence laser scanning microscopy (CFLSM) incorporating a liquid crystal on silicon spatial light modulator (LCOS-SLM). To achieve high-resolution and high-contrast imaging for deeper part of the tissue with CFLSM, high numerical aperture objective lenses are required to tightly focus excitation light to meet Rayleigh limit(criterion) for the specimens. However, mismatch of refractive index at the boundary of interfacing materials, such as atmosphere, glass cover, and biological tissues, causes spherical aberration. Recently, we proposed a numerical method for correcting spherical aberration. In this method a pre-distorted wavefront pattern for aberration correction is calculated by ray tracing from a hypothetical focal point inside a specimen to the pupil plane. The resulting microscope can correct such spherical aberration. We observed 6.0μm fluorescent micro-beads dispersed three-dimensionally in agarose gel to confirm effectiveness of aberration correction. We reconstructed a three-dimensional image by taking 20 images by changing the depth with 1 μm interval and stacking them. It was apparent that the longitudinal/depth resolution was improved and that the intensity of fluorescence image was increased with aberration correction. While this method is applicable to other laser scanning microscopes, it has potential to enhance the signals for various super-resolution microscopic techniques, such as stimulated- emission-depletion (STED) fluorescence microscopy.
In the laser wafer dicing technique of stealth dicing (SD), a laser beam that is tightly focused inside a silicon wafer is scanned multiple times at different depths. The focused beam creates multilayered cracks that allow dry, debris-free dicing. To reduce the dicing time, it is desirable to produce longer cracks with each scan. However, when the laser beam is focused in a deep region of the wafer, the beam is blurred, and its power density decreases owing to spherical aberration caused by a refractive index mismatch between air and the wafer. Consequently, the generated cracks become shorter. We present an approach to making longer cracks deep within the wafer by correcting the spherical aberration. This correction is made using an SD machine incorporating a phase-only spatial light modulator to apply aberration correction patterns, which are calculated by a method based on inverse ray tracing. Experimental results using 300-µm wafers show that, when the aberration was corrected, the cracks formed during multidepth scans became longer even deep within the wafer and that the dicing speed with correction is more than twice that without correction. This is because each scan produced longer cracks, so fewer scans were necessary. We also demonstrated that the quality of dicing was improved.
“Stealth Dicing” laser processing is a dry and debris-free semiconductor wafer dicing method achieved by generating
thermal micro-cracks inside a wafer with a tightly focused laser beam. This method has two practical issues: (1) the
dicing speed is limited by the repetition rate of the pulsed laser, and (2) integrated circuits on the opposite side of the
wafer from the laser light are potentially damaged by excessive laser intensity required to compensate for insufficient
beam convergence. The insufficient beam convergence is a result of spherical aberration due to a refractive index
mismatch between air and the wafer. These problems can be resolved by incorporating a phase-only spatial light
modulator (SLM) into the laser dicing system. The SLM produces two types of wavefront configurations simultaneously
for two different functions. One is for multi-beam generation with a phase grating pattern. This improves the dicing
speed by a factor equal to the number of diffracted beams. The other is for aberration correction of the multiple beams
using a pre-distorted wavefront pattern. By correcting aberrations, the focused multiple beams inside the wafer will
become sufficiently convergent to avoid undesirable laser damage. We demonstrated these improvements by dicing
sapphire wafers with a pulsed laser and a high-numerical-aperture objective lens.
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