Laser Beam Alignment Research Articles

Hardware accelerated optical alignment of lasers using beam-specific matched filters

Accurate automated alignment of laser beams in the National Ignition Facility (NIF) is essential for achieving extreme temperature and pressure required for inertial confinement fusion. The alignment achieved by the integrated control systems relies on algorithms processing video images to determine the position of the laser beam images in real time. Alignment images that exhibit wide variations in beam quality require a matched-filter algorithm for position detection. One challenge in designing a matched-filter-based algorithm is to construct a filter template that is resilient to variations in imaging conditions while guaranteeing accurate position determination. A second challenge is to process images for thousands of templates in under a second, as may be required in future high-energy laser systems. This paper describes the development of a new analytical template that captures key recurring features present in the beam image to accurately estimate the beam position under good image quality conditions. Depending on the features present in a particular beam, the analytical template allows us to create a highly tailored template containing only those selected features. The second objective is achieved by exploiting the parallelism inherent in the algorithm to accelerate processing using parallel hardware that provides significant performance improvement over conventional processors. In particular, a Xilinx Virtex II Pro field programmable gate array (FPGA) hardware implementation processing 32 templates provided a speed increase of about 253 times over an optimized software implementation running on a 2.2 GHz AMD Opteron core.

Line laser beam based laser-induced fluorescence detection system for microfluidic chip electrophoresis analysis

In this work, a new laser-induced fluorescence (LIF) detection system based on a line laser beam for microfluidic chip electrophoresis analysis was developed. This detection system had the advantages of simple optical structure, compactness, and ease in constructing. Highly sensitive detection was realized by detecting the fluorescence light emitted in the micro-channel through the vertical intersection between the line laser source and micro-channel. The filtered line source was established by a bevel laser beam and a micro-gap which could facilitate the alignment of line laser beam with the microfluidic channel. Both the theoretical analysis and experimental study demonstrated that the detection system with a 0.07 mm-width micro-gap had enough sensitivity and adequate separation efficiency. By this system, a detection limit (S/N > 12) of 1.284 × 10 −10 M fluorescein isothiocyanate was obtained, and the plate number could reach to 6100, which were comparable to those of optimized confocal or orthogonal LIF systems for microchip based capillary electrophoresis. The reproductibility of the detection system was evaluated by Sybr Green labeled DNA markers contained five fragments. Finally, the multi-PCR products including 165 bp, 266 bp, 378 bp and 881 bp fragments could be successfully achieved for baseline separation by this system within 4 min. The work undertaken can gear toward a semi-automated handheld system with substantial time and cost saving.

A Large-Area PVDF Pyroelectric Sensor for CO$_{2}$ Laser Beam Alignment

We present the design of a large-area (50 mm times 50 mm) polyvynilidene fluoride (PVDF) pyroelectric sensor array for industrial CO2 (lambda = 10.6 mum) laser beam positioning. The array dimensions were chosen to match the area typically monitored in the alignment procedure of external optics (beam steering moving arm system, for example) used to redirect the laser beam from the laser output window to a remote working station. The instrument is provided with a tilted, high reflection, ZnSe plate which partially transmits the laser beam onto the sensor array. From positioning simulations with a Gaussian laser intensity profile with a sigma = 3.2 mm standard deviation (equivalent spot size 3sigma cong 20 mm), the positional accuracy along the two orthogonal array dimensions was found to be better than 0.02 mm for an 8 times 8 array and one order of magnitude higher for a 16 times 16 array. The centroid position of a CO2 industrial laser beam was evaluated by integrating the pyroelectric current for a time comparable to the time duration (100-200 ms) of the laser pulses used in the alignment procedure.

Laser beam alignment by fast feedback control of both linear and angular drifts

A new way to enhance the directional stability of laser beams with alternating intensity by fast feedback control of both linear and angular drifts has been proposed for alignment of laser beams at higher accuracy. Both linear and angular drifts of laser beams, processed through light intensity modulation and primary alignment using single-mode optical fiber (SMOF), are separated using light path arrangement and detected using phase-lock technique, and are controlled using fast feedback control mechanisms according to their detected magnitudes, so that both linear and angular drifts are suppressed to enhance the directional stability of the emitting laser beams. Theoretical analyses and preliminary experimental results indicate that the approach proposed can be used to achieve an alignment accuracy of more than 10 −8 rad.

Active sub-Rayleigh alignment of parallel or antiparallel laser beams

We measure and stabilize the relative angle of parallel and antiparallel laser beams to 5 nrad/(square root of)Hz resolution by comparing the phases of radio frequency beat notes on a quadrant photodetector. The absolute accuracy is 5.1 and 2.1 microrad for antiparallel and parallel beams, respectively, which is more than 6 and 16 times below the Rayleigh criterion.

Orchestrating shots for the national ignition racility

The National Ignition Facility (NIF), currently under construction at the Lawrence Livermore National Laboratory, is a stadium-sized facility containing a 192-beam, 1.8 Megajoule, 500-Terawatt, ultra-violet laser system together with a 10-meter diameter target chamber with room for nearly 100 experimental diagnostics. When completed, NIF will be the world's largest and most energetic laser experimental system, providing an international center to study inertial confinement fusion and physics of matter at extreme densities and pressures. The NIF is operated by the Integrated Computer Control System (ICCS), which is a layered architecture of over 700 lower-level front-end processors attached to nearly 60,000 control points and coordinated by higher-level supervisory subsystems in the main control room. An Ada95 based shot automation framework has been developed and deployed during the past year to orchestrate and automate shots performed at the NIF using the ICCS. Ada95 provides language features that have enabled the construction of a complex, highly distributed control application. The Ada tasking model and protected types provide multi-threaded behaviors and synchronization mechanisms required by the ICCS. The Shot Automation framework is designed to automate 4-hour shot sequences including derivation of shot goals from an experiment definition, set up of the laser and diagnostics, automatic alignment of laser beams, and a countdown to charge and fire the lasers. A typical sequence consists of one or more preparatory verification shots leading to an amplified system shot that is followed by post-shot analysis and archiving. The framework provides for flexible model-based workflow execution with work divided into data driven packets called "macro steps". The Shot Director program is the top-level orchestrating component of the shot automation framework, which manages the state machine that defines the structure of the shot sequence. Collaboration Supervisors translate shot lifecycle state commands from the Shot Director into sequences of "macro steps" that are distributed to subsystem shot supervisors while maintaining the order of macro steps for each subsystem and supporting collaboration between different macro steps. Each macro step has phases for database-driven verification and a scripted execution. This provides a highly flexible framework for performing a variety of NIF shot types. Database tables define the order of work and dependencies (workflow) of macro steps to be performed for a shot. A graphical model editor facilitates the definition and viewing of an execution model. A Change Manager tool enables "de-participation" of individual devices, of entire laser segments (beams, quads, or bundles of beams) or of individual diagnostics. This software has been successfully deployed to the NIF facility and is currently being used to support NIF laser commissioning shots and the build-out of additional laser bundles. The software will also be used to automate future target and experimental shot campaigns.

Effects of laser beam alignment tolerance on lidar accuracy

One of the major lidar error sources not yet analyzed in the literature is the tolerance of the laser beam alignment with respect to the scanning mirror. In this paper, the problem of quantifying these errors is solved for rotating polygon mirror type lidar systems. An arbitrary deviation of the beam from its design direction–the vector of beam misalignment–can be described by two independent parameters. We choose these as horizontal and vertical components of the misalignment vector in the body frame. Either component affects both, horizontal and vertical lidar accuracy. Horizontal lidar errors appear as scan line distortions—along and across track shifts, rotations and scaling. It is shown that the horizontal component of misalignment results in a scan line first being shifted across the track and then rotated around the vertical at the new center of the scan line. Resulting vertical lidar error, being a linear function of the scan angle, is similar to that produced by a roll bias. The vertical component of the beam misalignment causes scan line scaling and an along track shift. The corresponding vertical error is quadratic with respect to the scan angle. The magnitude of these effects is significant even at tight alignment tolerances and cannot be realistically accounted for in the conventional calibration model, which includes only range, attitude and GPS biases. Therefore, in order to attain better accuracy, this model must be expanded to include the beam misalignment parameters as well. Addition of new parameters into the model raises a question of whether they can be reliably solved for. To give a positive answer to this question, a calibration method must utilize not only ground control information, which is typically very limited, but also the relative accuracy information from the overlapping flight lines.

Optimal position estimation for the automatic alignment of a high-energy laser

The alignment of high-energy laser beams for potential fusion experiments demands high precision and accuracy by the underlying positioning algorithms whether it be for actuator control or for monitoring the beam line for potential anomalies. The feasibility of employing on-line optimal position estimators in the form of model-based processors to achieve the desired results is examined. We discuss the modeling, the development, the implementation, and the processing of model-based processors applied to both simulated and actual beamline data.

Precise alignment of laser and electron beams in a periodic focusing system for multiple Compton backscattering

In the present paper, methods of precise alignment of laser and electron beams in a periodic focusing system for generating intense γ-rays by multiple Compton backscattering are discussed. In the system, a CO 2 laser beam and a 5.8 GeV electron beam are focused by a series of optical lenses and permanent quadrupole magnets, respectively. Since the beam sizes are as small as 20– 50 μm at 120 collision points, a precise alignment within about 3 μm is required. The electron beam can be aligned by the energy-scanned beam-based method, but this method is not appropriate for the laser beam since the wavelength is not changed easily. Instead, a new method of normal reflection using Michelson interferometers is considered. Diffraction analysis shows that the misalignment of lenses can be corrected by reducing the integrated intensity of the laser beam at the detector of the interferometer, which decreases monotonically with the reduction of alignment errors.

Laser–plasma interaction studies in the context of megajoule lasers for inertial fusion

Laser–plasma interaction (LPI) physics is one the major issues for the realization of inertial fusion. Parametric instabilities may be driven by the incident laser beams during their propagation in the underdense plasma surrounding the fusion capsule. These instabilities may result in various effects detrimental to a good energy transfer from the laser beams to the target: the backscattering of the incident beams, the generation of energetic electrons which might preheat the fusion fuel, and the spoiling of the laser beam alignment. The control of the linear growth of these instabilities, together with the understanding of their nonlinear saturation mechanisms are therefore of fundamental importance for laser fusion. During the past few years, a series of new concepts have emerged, deeply modifying our approach to LPI physics. In particular, LPI experiments are now carried out with laser beams which are optically smoothed by means of random phase plates. Such beams are characterized inside the plasma by randomly distributed intensity maxima. Filamentation instabilities may locally increase the laser intensity maxima and deplete the electron density, leading to an intricate coupling between various nonlinear processes. One of the most striking features of this intricate coupling is the resulting ability of the plasma to induce additional temporal and spatial incoherence to the laser beams during their propagation. The increased incoherence may in turn reduce the level of backscattering instabilities.

Calibration procedure and data processing for a TV Thomson scattering system

Calibration procedures for Thomson scattering systems based on television-like cameras, so-called TVTS systems, are described. The TVTS systems of the Rijnhuizen Tokamak Project (RTP), the TJ-II stellarator, and the Torus Experiment for Technology Oriented Research (TEXTOR) tokamak combine a 10–15 J ruby laser as a source with an intensified charge coupled device camera as a detector. A tungsten strip lamp in combination with an integrating sphere is used to calibrate all ∼105 pixels of the camera relatively to each other. Rayleigh scattering on hydrogen or nitrogen is used to perform an absolute calibration of the complete detection system. Great emphasis is placed on possible systematic errors on the determination of the electron temperature Te and density ne due to the calibration, such as tungsten lamp temperature, detoriation of the detection window, long term stability, laser beam alignment, and detector linearity. The long term stability of the system was tested by comparing different sets of calibration factors. Over a period of 1/2 yr the values of the calibration factors varied by less than 5%. Using the same Thomson scattering data but different sets of calibration factors the Te values varied even less than 1%. A two camera technique was used to search for possible unknown systematic errors in the determination of Te profiles. It appears that these systematic errors are about half of the observational error on Te. Density fluctuations can be determined with an accuracy equal to the statistical error of ∼3%, while the systematic error on ne appeared to be ∼10%, which can be corrected for using interferometer data. As a result, these TVTS systems can measure Te and ne profiles with ∼100 (RTP) or 120 (TEXTOR) spatial elements along the full plasma diameter with observational errors on Te of ∼6% in the range of 25 eV–6 keV, at ne=2×1019 m−3 and 10 J laser energy.

Development of a ground-based LIF instrument for measuring HOx radicals : Instrumentation and calibrations

An instrument for measuringtropospheric OH/HO2 radicals by laser-inducedfluorescence developed in our laboratory is presentedin detail. It is based on FAGE (fluorescence assay bygas expansion) technique and OH is both excited anddetected at 308 nm corresponding to its A-X(0,0) band.The alignment of the laser beam, the design of thesample gas inlet, and the devices for the fluorescencedetection are optimized so as to reduce the backgroundsignal while keeping the OH sensitivity as high aspossible. A thermalized position of the expanding gasbeam is probed in our system and we did not observe asevere decrease of the HOx sensitivities under humidconditions. An optical fiber is used for deliveringthe laser light to the fluorescence detection cellmounted outside at a high position. Thus the laserbeam alignment is by far simplified and is made highlyreproducible, once settled properly. For thecalibration, two methods are employed: a system withlaser absorption measurements of OH and a system ofsimultaneous photolysis of H2O and O2. Thecalibration factors are compared well within thecombined uncertainty. Using the latter system, theconversion efficiency of HO2 to OH by NO additionis measured to be around 90%. The detection limitsfor OH and HO2 (S/N = 2) are estimated to be3.3 × 106 and 3.6 × 106cm−3 at noon,respectively, with an integration time of 1 min. Theresults of test observations at our institute are alsopresented.

Pyroelectric PVDF sensor modeling of the temporal voltage response to arbitrarily modulated radiation

Our design of transducer arrays for custom pyroelectric sensors is mainly devoted to IR laser beam characterization and control. It benefits from some of the properties of PVDF film such as low cost, low weight, mechanical flexibility, chemical stability (inert), and compatibility of thick film interconnection technologies on metallized films. By using the temporal characteristics of the source intensity and starting from a standard equivalent one-dimensional model of a multilayer thick-film transducer in the frequency domain, we developed a computer model of the PVDF sensor that determines the temporal response to arbitrarily modulated radiation. The validation of the model accuracy has been carried out with a simulation procedure performed on a PVDF sensor designed for accurate beam alignment of low power laser beams. In this case, an iterative algorithm also was developed to estimate some thermal and physical properties of the front absorbing and the metallization layers that are generally barely known. We present a fitting procedure to determine these properties by using the temporal pyroelectric response to a square wave modulated laser diode that provides a reliable reference signal.

Computer integrated vision and neural network technologies for intelligent automatic laser butt joint welding

This paper discusses the integrated application of computer vision and neural technologies on laser welding to make welding processes more intelligent and automatic. Computer vision was employed to inspect a welding gap, to measure the welding gap size, and to trace the welding path for laser beam alignment. The NC code for the welding path was generated, and then the CNC workstation was located to ensure that the laser beam was properly aligned with the centre of the welding gap. The measured welding gap size and the material thickness were then processed in neural network models in order intelligently to predict welding parameters and the associated welding quality according to the knowledge base of the learnt data. This technology was developed to solve the difficulties of beam alignment and the nonlinearity of welding parameter determination caused by gap variation in butt joints. The applicability of intelligent and automatic laser welding for butt joints will be discussed.

New setup for end-fire coupling excitation of planar waveguides at low temperatures

This work describes a new experimental setup for end-fire coupling excitation of planar waveguides in the 300–10 K temperature range, which allows us to analyze their optical and spectroscopic properties through different techniques such as transmission, luminescence, or waveguide Raman spectroscopy. Two main novelties are relevant in this setup; (1) the use of long working distance microscope objectives, placed out of the cryostat, for both coupling and collecting guided light, and (2) the original set of mirrors, combined with different translation and rotation stages, which allows fine x,y,z,θ displacements of the beam focusing point preserving the laser beam alignment along the optical axis of the coupling objective. The setup has been tested in a series of Nd3+ doped ZBLA (ZrF4-BaF2-LaF3-AlF3) fluoride glass channel waveguides (from 2 to 40 μm width and 2 μm depth) obtained by ion-exchange and photolithography methods. The Nd3+ guided luminescence has been analyzed at low temperatures and compared to...

Rapid alignment of laser beams within optical systems

A method for rapid alignment of laser beams in optical systems is presented. It employs a Seward alignment module (SAM) to qualify both the spatial position and angular direction of the beam. Alignment procedures for turning mirrors and laser mounts are described. Application of the SAM on two complex laser-etching systems is presented.

Effects of processing on laser assisted thermoplastic tape consolidation

This investigation examines the relationships between processing conditions and the interlaminar strength of thermoplastic composite rings manufactured using laser-assisted on-line consolidation. The laser power and laser beam alignment were varied to determine an acceptable processing window. Relationships between interlaminar strength and these processing parameters were investigated using the Interlaminar-Shear-Device (ISD) experimental test method. Micrographs of the manufactured and failed specimens were taken to determine the quality of consolidation and the failure location, respectively. The void contents were reasonably high and failures generally occurred in the void regions resulting in lower interlaminar strengths compared with parts manufactured in an autoclave. In addition, the shear strength varied significantly for the different processing parameters used.

A compact Thomson scattering system

We have made and installed a multipulse Nd:YAG Thomson scattering system for measuring electron temperature and density profiles in the throat of the divertor of the Alcator C-Mod machine. The observing head is located in the vacuum vessel in a re-entrant chamber. It is optically fast and very compact. A system for providing feedback to maintain the laser beam alignment is a part of the observing head assembly. The head is designed to minimize eddy currents, and have a very rigid adjustable mounting to resist the forces generated by the eddy currents during plasma disruptions. A four wavelength polychromator using 25 element avalanche photodiode arrays for spatial resolution has been designed and built for this system. Two of these polychromators and a single observing head will provide 50 spatial resolution elements.

In Situ Laser Beam Alignment for Thomson Scattering System

An in situ laser beam alignment method was developed for a tokamak plasma Thomson scattering system. The laser beam position was swept rapidly while observing the scattered light signals from 28-channel polychromators, using which the tilt angle and position of the laser beam were optimized.

Inexpensive laser cooling and trapping experiment for undergraduate laboratories

We present detailed instructions for the construction and operation of an inexpensive apparatus for laser cooling and trapping of rubidium atoms. This apparatus allows one to use the light from low power diode lasers to produce a magneto-optical trap in a low pressure vapor cell. We present a design which has reduced the cost to less than $3000 and does not require any machining or glassblowing skills in the construction. It has the additional virtues that the alignment of the trapping laser beams is very easy, and the rubidium pressure is conveniently and rapidly controlled. These features make the trap simple and reliable to operate, and the trapped atoms can be easily seen and studied. With a few milliwatts of laser power we are able to trap 4×107 atoms for 3.5 s in this apparatus. A step-by-step procedure is given for construction of the cell, setup of the optical system, and operation of the trap. A list of parts with prices and vendors is given in the Appendix.