Reducing the uncertainty in laser beam size measurement with a scanning edge method

Imaging system aberrations through optical windows with nonuniform laser heating

An optical window is a critical component of an imaging system. When operating in harsh environments with extreme heating, nonuniform temperature changes occur throughout the window and cause nonuniform refractive index changes and mechanical deformations due to thermal expansion, which can degrade the imaging system's performance. In this paper, we present results collected from an experimental setup developed to characterize these aberrations. This setup includes a C O 2 laser for sample heating, an infrared camera for measuring front and back surface temperatures, and a visible imaging system and a wavefront sensor for measuring degradations of a collimated beam from a point source transmitted through the heated window. Sapphire samples are laser heated with a Gaussian profile to temperatures in excess of 500 K with surface temperature gradients in excess of 15 K/mm. These measurements are compared with first principles models, which show quantitative agreement for window temperatures and qualitative agreement with the transmitted wavefront and imaged point source.

Sparse Lissajous scanning reflectance confocal microscope with an adjustable field of view and fast iterative Fourier filtering reconstruction

Medical imaging devices are becoming increasingly compact, necessitating optimization research into different methods of actuation. Actuation influences important parameters of the imaging device such as size, weight, frame rate, field of view (FOV), and image reconstruction for imaging devices point scanning techniques. Current literature around piezoelectric fiber cantilever actuators focuses on device optimization with a fixed FOV but neglects adjustability. In this paper, we introduce an adjustable FOV piezoelectric fiber cantilever microscope and provide a characterization and optimization procedure. To overcome calibration challenges, we utilize a position sensitive detector (PSD) and address trade-offs between FOV and sparsity with a novel inpainting technique. Our work demonstrates the potential for scanner operation when sparsity and distortion dominate the FOV, extending the usable FOV for this form of actuation and others that currently only operate under ideal imaging conditions.

Spatial zigzag evolution of cracks in moving sapphire initiated by bursts of picosecond laser pulses for ultrafast wafer dicing

Spatial zigzag evolution of cracks in moving sapphire wafer was observed after irradiation with sequences of picosecond laser pulses (bursts). The Gaussian beam was tightly focused inside the sapphire. The spatial position of laser initiated cracks moved in vertical and horizontal directions when a wafer was translated at a controllable speed perpendicular to the beam propagation direction. The cracking plane consisting of the periodically repeating inclined modifications and cracks was observed. The period of modifications and the inclination angle had a linear dependence on the wafer translation speed. The model of spatial zigzag crack evolution was created and the physical origin of modification growth at a measured speed of 1.3 ± 0.1 m s-1 is discussed. The zigzag cracking was applied for ultrafast stealth dicing and cleavage of the sapphire: dicing speed 300 mm s-1, wafer thickness 430 μm, laser power 5.5 W, repetition rate 100 kHz, sub-pulse duration 9 ps, the temporal distance between sub-pulses in burst 26.7 ns, and the number of sub-pulses 13.

A fiber-based beam profiler for high-power laser beams in confined spaces and ultra-high vacuum

Laser beam profilometry is an important scientific task with well-established solutions for beams propagating in air. It has, however, remained an open challenge to measure beam profiles of high-power lasers in ultra-high vacuum and in tightly confined spaces. Here we present a novel scheme that uses a single multi-mode fiber to scatter light and guide it to a detector. The method competes well with commercial systems in position resolution, can reach through apertures smaller than 500×500 µm² and is compatible with ultra-high vacuum conditions. The scheme is simple, compact, reliable and can withstand laser intensities beyond 2 MW/cm².

Advanced laser scanning for highly-efficient ablation and ultrafast surface structuring: experiment and model

Ultra-short laser pulses are frequently used for material removal (ablation) in science, technology and medicine. However, the laser energy is often used inefficiently, thus, leading to low ablation rates. For the efficient ablation of a rectangular shaped cavity, the numerous process parameters such as scanning speed, distance between scanned lines, and spot size on the sample, have to be optimized. Therefore, finding the optimal set of process parameters is always a time-demanding and challenging task. Clear theoretical understanding of the influence of the process parameters on the material removal rate can improve the efficiency of laser energy utilization and enhance the ablation rate. In this work, a new model of rectangular cavity ablation is introduced. The model takes into account the decrease in ablation threshold, as well as saturation of the ablation depth with increasing number of pulses per spot. Scanning electron microscopy and the stylus profilometry were employed to characterize the ablated depth and evaluate the material removal rate. The numerical modelling showed a good agreement with the experimental results. High speed mimicking of bio-inspired functional surfaces by laser irradiation has been demonstrated.

Experimental and theoretical investigations of nonlinear optical properties of 1,4-Diamino-9,10-Anthraquionone

Nonlinear optical properties of 1,4-Diamino-9,10-Anthraquinone dye in solution at different concentrations are investigated by utilizing single beam Z-scan technique using a low power continuous wave laser (λ=532 nm). The anthraquinone dye is found to exhibit self-defocusing and reverse saturable absorption behavior. Effect of concentration on nonlinear refractive index and nonlinear absorption coefficient are also studied. The nonlinear absorption coefficient (β) and nonlinear refractive index (n2) have been evaluated from the open aperture and closed aperture Z-scan data and are found to increase with increase in concentration. The order of magnitude obtained for nonlinear refractive index and nonlinear absorption coefficient are found to be 10(-6) esu and 10(-4) m/W, respectively. The optical limiting behavior and induced self-diffraction patterns are also observed. To have a theoretical insight of nonlinear optical properties of 1,4-Diamino-9,10-Anthraquinone, first hyperpolarizability (β) is also evaluated by using quantum chemical calculations employing DFT method using 6-311 G basis set. The results obtained confirm the nonlinear optical behavior of 1,4-Diamino-9,10-Anthraquinone dye.

Gaussian beam radius measurement with a knife-edge: a polynomial approximation to the inverse error function

A method for approximating the inverse error function involved in the determination of the radius of a Gaussian beam is proposed. It is based on a polynomial inversion that can be developed to any desired degree, according to an a priori defined error budget. Analytic expressions are obtained and used to determine the radius of a TEM(oo) He-Ne laser beam from intensity measurements experimentally obtained by using the knife edge method. The error and the interval of validity of the approximation are determined for polynomials of different degrees. The analysis of the theoretical and experimental errors is also presented.

Minimized spot of annular radially polarized focusing beam

We have experimentally demonstrated the measurement of a tighter focal spot generated by a radially polarized narrow-width annular beam with the double-knife-edge method. The reconstructed spot profiles indicate that sharper focus cannot be achieved by shrinking the annular aperture further. The smallest focal spot (0.0711λ(2)) is obtained in experiment with an annular factor of 0.91. An apodization function has been introduced with the consideration of the diffraction effect, which achieves good agreement with the experimental data. Our result shows that the diffraction effect should be considered with small topography structures of the incident beam.

In vivo evaluation of demyelination and remyelination in a nerve crush injury model

Nerves of the peripheral nervous system have, to some extent, the ability to regenerate after injury, particularly in instances of crush or contusion injuries. After a controlled crush injury of the rat sciatic nerve, demyelination and remyelination are followed with functional assessments and imaged both ex vivo and in vivo over the course of 4 weeks with video-rate coherent anti-Stokes Raman scattering (CARS) microscopy. A new procedure compatible with live animal imaging is developed for performing histomorphometry of myelinated axons. This allows quantification of demyelination proximal and remyelination distal to the crush site ex vivo and in vivo respectively.

Temporally focused femtosecond laser pulses for low numerical aperture micromachining through optically transparent materials

Temporal focusing of spatially chirped femtosecond laser pulses overcomes previous limitations for ablating high aspect ratio features with low numerical aperture (NA) beams. Simultaneous spatial and temporal focusing reduces nonlinear interactions, such as self-focusing, prior to the focal plane so that deep (approximately 1 mm) features with parallel sidewalls are ablated at high material removal rates (25 microm(3) per 80 microJ pulse) at 0.04-0.05 NA. This technique is applied to the fabrication of microfluidic devices by ablation through the back surface of thick (6 mm) fused silica substrates. It is also used to ablate bone under aqueous immersion to produce craniotomies.

Near-field Raman imaging using optically trapped dielectric microsphere

The stumbling block of employing Raman imaging in nanoscience and nanotechnology is the diffraction-limited spatial resolution. Several approaches have been employed to improve the spatial resolution, among which aperture and apertureless near-field Raman techniques are the most frequently used. In this letter, we report a new approach in doing near-field Raman imaging with spatial resolution of about 80 nm, by trapping and scanning a polystyrene microsphere over the sample surface in water. We have used this technique to resolve PMOS transistors with SiGe source drain stressors with poly-Si gates, as well as gold nanopatterns with excellent reproducibility.

Graphene thickness determination using reflection and contrast spectroscopy

We have clearly discriminated the single-, bilayer-, and multiple-layer graphene (<10 layers) on Si substrate with a 285 nm SiO2 capping layer by using contrast spectra, which were generated from the reflection light of a white light source. Calculations based on Fresnel's law are in excellent agreement with the experimental results (deviation 2%). The contrast image shows the reliability and efficiency of this new technique. The contrast spectrum is a fast, nondestructive, easy to be carried out, and unambiguous way to identify the numbers of layers of graphene sheet. We provide two easy-to-use methods to determine the number of graphene layers based on contrast spectra: a graphic method and an analytical method. We also show that the refractive index of graphene is different from that of graphite. The results are compared with those obtained using Raman spectroscopy.