Collimated Laser Beam Research Articles

Physics and Measurement of Aero-Optical Effects: Past and Present

The field of aero-optics is devoted to the study of the effects of turbulent flow fields on laser beams projected from airborne laser systems. This article reviews the early and present periods of research in aero-optics. Both periods generated impressive amounts of research activity; however, the types and amount of data differ greatly in accuracy, quality, and type owing to the development of new types of instrumentation available to collect and analyze the aberrated wave fronts of otherwise collimated laser beams projected through turbulent compressible flow fields of the type that form over beam directors. This review traces the activities and developments associated with both periods but particularly focuses on the development of modern high-bandwidth wave-front sensors used in the present research period. We describe how these modern wave-front data are collected and analyzed and the fluid mechanic information that can be gleaned from them; the use of these data in the fundamental study of turbulence is emphasized.

Scanning Laser Ophthalmoscopy

The aim of retinal imaging techniques is visualization of morphological changes at the cellular and tissue level. Various techniques are used for this purpose. The scanning laser ophthalmoscopy (SLO), a retinal optical imaging device based on standard scanning laser microscopy is an imaging technique that scans the fundus with a highly collimated narrow laser beam and measures the backscattered light intensity. Here, progress on developing SLO instruments and their applications in ophthalmology are reviewed.

Two-axis water-immersible microscanning mirror for scanning optics and acoustic microscopy

Fast multiaxis scanning is useful for not only optical but also acoustic microscopic imaging. Although they have been used for optical scanning, the application of (MEMS) scanning mirrors in acoustic microscopy is still very limited due to their small mirror plate size, and more importantly, inability to operate in liquids (as ultrasound coupling media). A microfabricated two-axis water-immersible scanning mirror for optical and acoustic microscopy is reported. It has an optical and acoustically reflective mirror plate (6 mm×4 mm) to provide numerical aperture for ultrasound beam steering. Electromagnetic and mechanical analysis and simulation were conducted to estimate the mechanical tilting angle and resonance frequency of both fast and slow axes, which matches well with the measurement results. The fast axis has a resonant frequency of 320 Hz in air and 220 Hz in water, which is more than 10 times higher than that of the slow axis (24 Hz in air and 14 Hz in water). Under a 100-mA driving current, the scanning angles of the fast axis reached ±9.5 deg in both air and water at the resonance frequency, respectively. The scanning angles of the slow axis reached ±15 deg in air and ±12.5 deg in water at resonant frequencies, respectively. Raster scanning of a collimated laser beam was achieved by driving both axes simultaneously close to their own resonance frequencies. The feasibility of using the two-axis water-immersible scanning mirror in scanning acoustic microscopy was also demonstrated.

Evaluating the influence of laser wavelength and detection stage geometry on optical detection efficiency in a single-particle mass spectrometer

Abstract. Single-particle mass spectrometry (SPMS) is a useful tool for the online study of aerosols with the ability to measure size-resolved chemical composition with a temporal resolution relevant to atmospheric processes. In SPMS, optical particle detection is used for the effective temporal alignment of an ablation laser pulse with the presence of a particle in the ion source, and it gives the option of aerodynamic sizing by measuring the offset of particle arrival times between two detection stages. The efficiency of the optical detection stage has a strong influence on the overall instrument performance. A custom detection laser system consisting of a high-powered fibre-coupled Nd:YAG solid-state laser with a collimated beam was implemented in the detection stage of a laser ablation aerosol particle time-of-flight (LAAP-TOF) single-particle mass spectrometer without major modifications to instrument geometry. The use of a collimated laser beam permitted the construction of a numerical model that predicts the effects of detection laser wavelength, output power, beam focussing characteristics, light collection angle, particle size, and refractive index on the effective detection radius (R) of the detection laser beam. We compare the model predictions with an ambient data set acquired during the Ice in Clouds Experiment – Dust (ICE-D) project. The new laser system resulted in an order-of-magnitude improvement in instrument sensitivity to spherical particles in the size range 500–800 nm compared to a focussed 405 nm laser diode system. The model demonstrates that the limit of detection in terms of particle size is determined by the scattering cross section (Csca) as predicted by Mie theory. In addition, if light is collected over a narrow collection angle, oscillations in the magnitude of Csca with respect to particle diameter result in a variation in R, resulting in large particle-size-dependent variation in detection efficiency across the particle transmission range. This detection bias is imposed on the aerodynamic size distributions measured by the instrument and accounts for some of the detection bias towards sea salt particles in the ambient data set.

Spatially-offset fluorescence spectroscopy (SOFS) using ring illumination and point collection for sub-surface measurements in layered tissue

To report development of a depth-sensitive fluorescence spectroscopy system based on the configuration of point collection fixed at the centre of an illumination ring on the surface of a target sample. The system makes use of an axicon for converting the collimated laser beam into ring shaped illumination and achieves the ability of subsurface interrogation by varying the radii of the illumination rings thereby introducing spatial offsets between collection and illumination. The system was validated on a non-biological phantom comprising a thin tissue paper overlaying a thick dye card and also on a biological tissue sample that was a chicken leg tissue consisting of a thin epithelial membrane on the top of a much thicker muscle tissue. It was found that while the fluorescence spectra corresponding to zero spatial offset are dominated by the fluorescence emission bands associated with the top layer of the layered samples, the fluorescence spectra measured with spatial offsets contain higher contribution of fluorescence signatures characteristics of the bottom layer as compared to the spectrum measured with the zero offset. The SOFS based system is able to interrogate subsurface depths beyond the reach of the conventional confocal fluorescence by simply varying the position of the axicon in the excitation arm. Due to the simplicity in its instrumentation, the approach has the potential to be developed into a portable system for in situ measurement in a tissue.

Two-step tomographic reconstructions of temperature and species concentration in a flame based on laser absorption measurements with a rotation platform

We present a system for accurate tomographic reconstruction of the combustion temperature and H2O vapor concentration of a flame based on laser absorption measurements, in combination with an innovative two-step algebraic reconstruction technique. A total of 11 collimated laser beams generated from outputs of fiber-coupled diode lasers formed a two-dimensional 5×6 orthogonal beam grids and measured at two H2O absorption transitions (7154.354/7154.353cm−1 and 7467.769cm−1). The measurement system was designed on a rotation platform to achieve a two-folder improvement in spatial resolution. Numerical simulation showed that the proposed two-step algebraic reconstruction technique for temperature and concentration, respectively, greatly improved the reconstruction accuracy of species concentration when compared with a traditional calculation. Experimental results demonstrated the good performances of the measurement system and the two-step reconstruction technique for applications such as flame monitoring and combustion diagnosis.

Laser-Stabilized Real-Time Plant Discrimination Sensor for Precision Agriculture

A novel proximal spectral-reflectance-based plant discrimination sensor for use in selective herbicide spraying systems is developed and its dynamic outdoor performance is experimentally assessed for two plants. For plant illumination, the sensor uses a new stabilized three-wavelength laser diode module that sequentially emits identically polarized laser light beams through a common aperture, along one optical path. Each laser beam enters a multi-spot beam generator, which produces 15 parallel, collimated laser beams spaced over a 230-mm span. The intensity of the reflected light from each beam is detected by a high-speed line scan image sensor. Plant discrimination is based on calculating two different normalised difference vegetation indices, and experimental results show that by improving the stability of the laser diodes, a plant discrimination rate greater than 90% can be achieved with a travelling speed of 7.5 km/h for canola and wild radish, which is a dominant weed in the canola crop field.

High-speed camera thermometry of laser droplet generation

This paper presents a high-speed thermal imaging method using a visible light camera, with application to the laser droplet generation process (LDG). In the experiment, a nickel wire-end was exposed to a collimated laser beam, and the subsequent process of wire melting, pendant droplet formation and its detachment were recorded by a high speed camera. Instantaneous temperature fields of the metal surface were calculated from the imaging data and were characterized by a very good spatial and temporal resolution (200×400 pixels at 13,837 frames per second). The droplet temperature could be accurately calculated between the melting point of nickel (1455°C) and approximately 1950°C, where image saturation started to occur. The remaining pendant droplet was shown to cool much more rapidly than the detached droplet, which is due to the heat conduction to the solid wire. Except for the time immediately after the droplet separation, the temperature distribution across the melt droplets was found to be quite uniform. Apart from the possibility of temperature field calculation, it was also demonstrated that the high-speed images of the LDG process can accurately capture contours and oscillation dynamics of melt droplets.

Instantaneous photoinduced patterning of an azopolymer colloidal nanosphere assembly

Colloidal azopolymer nanospheres assembled on a glass substrate were exposed to a single collimated laser beam. The combination of photo-fluidic elongation of the spherical colloids and light induced self-organization of the azopolymer film allows the quasi-instantaneous growth of a large amplitude surface relief grating. Pre-structuration of the sample with the nanosphere assembly supports faster creation of the spontaneous pattern. Confinement into the nanospheres provides exceptionally large modulation amplitude of the spontaneous relief. The method is amenable to any kind of photoactive azo-materials.

An optical lever by using a mode-locked laser for angle measurement

A new concept of optical lever for angle measurement having an extended angular measurement range with a mode-locked laser as the light source, which is significantly extended from the conventional photodiode (PD)-type optical levers with a single frequency laser, is proposed. In the proposed concept, a collimated laser beam of the mode-locked laser is made incident to a grating reflector to generate a group of first-order diffracted beams from the grating reflector. Differing from a conventional PD-type optical lever employing a laser beam with a single frequency as the light source, the angle measurement range can be significantly expanded for the sake of the group of widely-spread first-order diffracted beams. In addition, the proposed optical lever has a potential of assuring the traceability of angle measurement by linking it to the optical frequency comb based on the mode-locked laser, which is employed as the national standard of time and frequency. In this paper, as a first step of this research, a mode-locked femtosecond laser is employed as the light source of the proposed optical lever. To distinguish each of the first-order diffracted beams generated with the combination of the mode-locked femtosecond laser and the diffraction grating having a grating period of 1.67μm, a Fabry-Pérot etalon is employed in the setup of the optical lever to modulate the distance between two neighboring beams in the group of first-order diffracted beams. Experimental setups are developed, and basic experiments are carried out to verify the feasibility of the proposed optical lever.

Low-level birefringence measurement by cyclic-path polarization interferometer

A modified cyclic-path interferometer is employed for complete measurement of spatially varying birefringence. An expanded and collimated laser beam intercepted by a birefringent specimen is incident on a polarization-masked cube beam splitter, resulting in two mutually orthogonal polarization components propagating along clockwise and counterclockwise directions in the interferometer. These two wavefronts are made to interfere for four specific orientations of an analyzer. Suitable combinations of the interferograms result in determination of the direction of birefringence and its magnitude. Experimental results are presented.

Directional random laser source consisting of a HC-ARROW reservoir connected to channels for spectroscopic analysis in microfluidic devices.

Light sources are used in optofluidic devices for real-time system control and quantitative analysis of important process parameters. In this work, we present a random laser source using a hollow-core antiresonant reflecting optical waveguide (HC-ARROW) containing the gain media inside a reservoir to reduce dye bleaching, which is connected to microchannel waveguides to increase beam directionality. The device is pumped externally and emits a highly coherent and collimated laser beam.

Lensless in-line holographic microscope with Talbot grating illumination

We have developed a wide field-of-view lensless in-line holographic microscope (LIHM) capable of acquiring microscopic images with a compact design. In our imaging system, a Ronchi grating was illuminated by a collimated laser beam to generate a Talbot self-imaging grating illumination on the sample, and the in-line holograms were recorded by a CMOS imaging sensor behind the sample. An iterative reconstruction algorithm was then applied to reconstruct the sample image while eliminating the twin-image background that appears in traditional in-line holography. In the algorithm, the dark areas of the illumination grating were used as a known constraint to define the sample support that led to convergence of the iteration. The whole-sample image can be acquired by laterally shifting the grating. We demonstrated the performance of our iteration algorithm and imaging system by successfully acquiring images of polystyrene microspheres with 5 μm diameter and the wing of a green lacewing.

Optical particle detection in liquid suspensions with a hybrid integrated microsystem

In this work, we present an optical microsystem for particle flow detection, based on hybrid integration and silicon micromachining. Sensing principle is light obscuration. We demonstrate the feasibility of integrating commercial components such as light sources (850nm near infrared Vertical-Cavity-Surface-Emitting Lasers VCSELs) and optics (an array of microlenses) into a silicon fabricated structure to create an optoelectronic package of reduced dimensions (14mm×12mm) and low power consumption (≈7.2mW) that satisfies optical requirements and provides four collimated laser beams with reduced diameter spots (minimum radius of 124μm measured as 1/e2 standard). Besides this microoptical system, the detection system is completed with a double linear array imaging sensor of 256 pixels, designed and fabricated in 0.35μm CMOS technology, and a microfluidic platform. The designed set-up allows the simultaneous optical detection of singly aligned microbeads flowing in liquid suspensions through several parallel microfluidic channels, which share the pitch of the laser emitting sources, i.e. 250μm. While the throughput of the system is increased up to 5000 particles/s thanks to the possibility of performing parallel measurements, the double array of the imaging sensor improves the detection process discarding false detected events. Experimental results prove that the system is able to detect and distinguish microparticles of several sizes in a mixed solution, with detection limit set at 10μm diameter. In conclusion, the fabricated microsystem is presented as a miniaturized and robust alternative to current optical microflow cytometers based on the use of optical fibre, and the complete system, including fluidics and imaging sensor stages, represents a portable solution packaged in less than 5cm3. These features make it an interesting choice to consider in the field of point-of-care biosensing applications.

Monolithic fiber end cap collimator for high-power free-space fiber-fiber coupling.

In this paper, we present the design, construction, and testing of a monolithic fiber end cap collimator for high-power free-space fiber-fiber coupling applications. The collimator is based on a large-sized fiber end cap and a spherical lens design on the output facet. Values of the spot size and working distance are theoretically analyzed based on Gaussian approximation and ABCD transmission matrix. The free-space fiber-fiber coupling process is also analyzed for different lens curvature radii and coupling distances. In the experiment, a collimated laser beam is obtained with Rayleigh length of about 400mm. A high-power laser with 1.1kW output is tested on the end cap collimator, which only heats up by 7°C at the output facet without active cooling. Free-space fiber-fiber coupling between two 20/400μm fibers is achieved based on these collimators, with measured coupling loss lower than 0.3dB.

Generation of strong pulsed magnetic fields using a compact, short pulse generator

The generation of strong magnetic fields (∼50 T) using single- or multi-turn coils immersed in water was studied. A pulse generator with stored energy of ∼3.6 kJ, discharge current amplitude of ∼220 kA, and rise time of ∼1.5 μs was used in these experiments. Using the advantage of water that it has a large Verdet constant, the magnetic field was measured using the non-disturbing method of Faraday rotation of a polarized collimated laser beam. This approach does not require the use of magnetic probes, which are sensitive to electromagnetic noise and damaged in each shot. It also avoids the possible formation of plasma by either a flashover along the conductor or gas breakdown inside the coil caused by an induced electric field. In addition, it was shown that this approach can be used successfully to investigate the interesting phenomenon of magnetic field enhanced diffusion into a conductor.

A novel calibration method for non-orthogonal shaft laser theodolite measurement system.

Non-orthogonal shaft laser theodolite (N-theodolite) is a new kind of large-scale metrological instrument made up by two rotary tables and one collimated laser. There are three axes for an N-theodolite. According to naming conventions in traditional theodolite, rotary axes of two rotary tables are called as horizontal axis and vertical axis, respectively, and the collimated laser beam is named as sight axis. And the difference between N-theodolite and traditional theodolite is obvious, since the former one with no orthogonal and intersecting accuracy requirements. So the calibration method for traditional theodolite is no longer suitable for N-theodolite, while the calibration method applied currently is really complicated. Thus this paper introduces a novel calibration method for non-orthogonal shaft laser theodolite measurement system to simplify the procedure and to improve the calibration accuracy. A simple two-step process, calibration for intrinsic parameters and for extrinsic parameters, is proposed by the novel method. And experiments have shown its efficiency and accuracy.

Tuning of a high magnification compact parabolic telescope for centimeter-scale laser beams.

Off-axis parabolic telescopes, widely used in astronomy and laser optics, if perfectly tuned, are virtually free from aberrations along the parabola's axis direction, but their alignment is very critical. We present a detailed method to align a high magnification off-axis afocal parabolic telescope. The method is composed of two steps: an initial pre-alignment using autocollimators, followed by a fine tuning with a collimated laser beam. Due to the large telescope magnification, the outcoming beam cannot be measured without being refocused. The beam is therefore reflected on a flat mirror and sent back through the telescope. This double-pass configuration allows the measurement of the beam quality without the need for large additional optics. In the fine-tuning step, a numerical simulation is also used to identify the degrees of freedom to be adjusted. The experimental results presented are obtained with one of the mode-matching parabolic telescopes of the gravitational wave interferometric detector Advanced Virgo.

Inverse identification of radiative properties of a multi-component media in MPA modeling of radiative transfer

Inverse identification of the radiative properties of a two-component media based on multi-phase approach (MPA) formulation of the radiative transfer phenomenon is investigated. Packed bed of semitransparent spheres under a normal collimated laser beam is considered as the geometry of the mentioned problem. Discrete normal directional reflectance and transmittance of the packed bed is used as input data to the identification procedure. The sensitivity of these two parameters to radiative properties of the medium is studied first, followed by the inverse parameter estimation. The discrete ordinates method is used to solve the direct problem while interior trust region (ITRA) algorithm is utilized in the identification process. Results indicate the ability of the presented procedure to estimate the desired radiative properties of the medium within an acceptable error margin. Moreover, the effect of uncertainty of the normal directional reflectance and transmittance data on the accuracy of the estimated parameters is assessed.

Optical Pushing: A Tool for Parallelized Biomolecule Manipulation

The ability to measure and manipulate single molecules has greatly advanced the field of biophysics. Yet, the addition of more single-molecule tools that enable one to measure in a parallel fashion is important to diversify the questions that can be addressed. Here we present optical pushing (OP), a single-molecule technique that is used to exert forces on many individual biomolecules tethered to microspheres using a single collimated laser beam. Forces ranging from a few femtoNewtons to several picoNewtons can be applied with a submillisecond response time. To determine forces exerted on the tethered particles by the laser, we analyzed their measured Brownian motion using, to our knowledge, a newly derived analytical model and numerical simulations. In the model, Brownian rotation of the microspheres is taken into account, which proved to be a critical component to correctly determine the applied forces. We used our OP technique to map the energy landscape of the protein-induced looping dynamics of DNA. OP can be used to apply loading rates in the range of 10−4–106 pN/s to many molecules at the same time, which makes it a tool suitable for dynamic force spectroscopy.