Pulsed Laser Source Research Articles

Comparative Study of Achievable Quality Cutting Carbon Fibre Reinforced Thermoplastics Using Continuous Wave and Pulsed Laser Sources

Laser cutting of CFRP lightweight parts has the advantages of a contact-free, automatable and flexible processing for a prospective series production. For the development of strategies for laser cutting of carbon fibre reinforced plastics (CFRP), different scientific approaches exist to achieve a process with small heat affected zones (HAZ), and high cutting rates. Within this paper a cw laser, a nanosecond and a picosecond laser source emitting in the near infrared range have been used in combination with a scanning system to cut CFRP with a thermoplastic matrix. The influence of the scanning speed on the size of the HAZ and the corresponding tensile strength were investigated for each laser source. Furthermore, the authors compared the achievable HAZ and the effective cutting speeds of the different setups in order to evaluate the efficiency and quality of the chosen strategies. The results show that a nanosecond pulsed laser source with high average power is a good trade-off between attainable quality and cutting rate.

Design of a Simple Cryogenic System for Ultraviolet–Visible Absorption Spectroscopy with a Back-Reflectance Fiber Optic Probe

We report a convenient and inexpensive technique for the rapid acquisition of absorption spectra from small samples at cryogenic temperatures using a home built cryostat with novel collection optics. A cylindrical copper block was constructed with a coaxial bore to hold a 4.00 mm diameter electron paramagnetic resonance (EPR) tube and mounted on a copper feed in thermal contact with liquid nitrogen. A 6.35 mm diameter hole was bored into the side of the cylinder so a fiber optic cable bundle could be positioned orthogonally to the EPR tube. The light passing through the sample is reflected off of the opposing surfaces of the EPR tube and surrounding copper, back through the sample. The emergent light is then collected using the fiber optic bundle and analyzed using a dispersive spectrometer. Absorption spectra for KMnO4 were measured between 400 and 700 nm. Absorption intensity at 506, 525, 545, and 567 nm was found to be proportional to concentration, displaying Beer's law-like behavior. The EPR tube had an internal diameter of 3.2 mm; the double pass of the probe beam through the sample affords a central path length of about 6.4 mm. Comparing these measurements with those recorded on a conventional tabletop spectrometer using a cuvette with a 10.00 mm path length, we consistently found a ratio between intensities of 0.58 rather than the anticipated 0.64. These 6% smaller values we attribute to the curvature of the EPR tube and transmission/reflection losses. This system is particularly well-suited to studying the kinetics and dynamics of chemical reactions at cryogenic temperatures. The rapid response (100 ms) and multiplex advantage provided the opportunity of recording simultaneous time courses at several wavelengths following initiation of a chemical reaction with a pulsed laser source.

High throughput Time Resolved Fluorescence in a Microplate Reader

We describe a high-throughput time-resolved fluorescence (TRF) spectrometer, able to detect multiple fluorescence lifetimes across 384 wells with short (< 5 min.) read times using direct waveform recording. The instrument combines high-energy pulsed laser sources (5-10 kHz repetition rate, 1-3 ns pulse width) with a photomultiplier and high-speed digitizer (1 GHz, effectively 5 GHz with interleaving) to record a complete fluorescence decay waveform after each pulse. Single-well measurements of dyes with 200-fold signal averaging (0.1 s acq. time) yield lifetimes comparable in accuracy and precision to single photon counting (SPC.) Integrated software enables immediate analysis by fitting exponential decays or by calculating a model-independent truncated first moment. In a 384-well format changes in quencher concentration are readily seen, with the first moment calculation providing resolution comparable to exponential decay models. Further, we are able to resolve relative mole fractions in two-dye mixtures when pure samples are used as standards. In multiple-well experiments the variation in total measured fluorescence is comparable to steady-state instruments, while the precision in lifetime is better than 2%. These features will enable high-throughput TRF experiments to detect changes to structure and dynamics in solution, cells and reconstituted systems. Acknowledgements: Spectroscopy experiments were performed at the Biophysical Spectroscopy Center, University of Minnesota. Excellent computational resources were provided by the Minnesota Supercomputing Institute. This work was funded by NIH grants to DDT (R01 AR32961, P30 AR057220) and KJP (T32 AR007612.)

Laser Spot Welding of Copper-aluminum Joints Using a Pulsed Dual Wavelength Laser at 532 and 1064 nm

Abstract A modulated pulsed laser source emitting green and infrared laser light is used to join the dissimilar metals copper and aluminum. The resultant dynamic welding process is analyzed using the back reflected laser light and high speed video observations of the interaction zone. Different pulse shapes are applied to influence the melt pool dynamics and thereby the forming grain structure and intermetallic phases. The results of high-speed images and back-reflections prove that a modulation of the pulse shape is transferred to oscillations of the melt pool at the applied frequency. The outcome of the melt pool oscillation is shown by the metallurgically prepared cross-section, which indicates different solidification lines and grain shapes. An energy-dispersivex-ray analysis shows the mixture and the resultant distribution of the two metals, copper and aluminum, within the spot weld. It can be seen that the mixture is homogenized the observed melt pool oscillations.

Laser-Based Mass Spectrometric Determination of Aggregation Numbers for Petroleum- and Coal-Derived Asphaltenes

Petroleum- and coal-derived asphaltenes have been studied with three laser-based mass spectrometric techniques: laser desorption ionization–mass spectrometry (LDI–MS), in which a single laser desorbs and ionizes solid analytes; surface-assisted laser desorption ionization–mass spectrometry (SALDI–MS), in which a single laser desorbs and ionizes solid analytes from an activated surface; and laser desorption laser ionization mass spectrometry (L2MS), in which desorption and ionization are separated spatially and temporally with independent pulsed laser sources. We find that asphaltene nanoaggregates can be detected in LDI–MS and SALDI–MS under mild conditions of relatively low laser power, whereas L2MS avoids aggregation and fragmentation, detecting asphaltenes as monomeric molecules. A comparison of the L2MS and SALDI–MS results yields an estimate of the distribution of aggregation numbers (number of molecules comprising the nanoaggregate). The most probable aggregation number observed for nanoaggregates of petroleum asphaltenes is approximately 6–8 molecules, consistent with the Yen–Mullins model of asphaltenes and predictions from the island geometry for asphaltene molecules. Additionally, the nanoaggregates are found to be relatively monodisperse, because most aggregates observed have aggregation numbers within one molecule of the most probable. In contrast, the nanoaggregates of coal asphaltenes are found to be smaller and more polydisperse, with aggregation numbers ranging from 3 to 6 molecules. Under higher powers, SALDI–MS measurements show that nanoaggregates decompose to form small multimers and monomers, suggesting that the aggregates are bound noncovalently. Coal asphaltene nanoaggregates decompose at lower laser powers than petroleum asphaltene nanoaggregates, indicating that the coal asphaltene nanoaggregates are bound less strongly than petroleum asphaltene nanoaggregates.

A simple single-mode fiber loss measurement scheme in the C-band based on fiber loop-cavity ringdown spectroscopy

An extremely sensitive and simple fiber loop-cavity ringdown spectroscopy (FL-CRDS) setup has been designed based on a turn-key nanosecond pulse laser source operating at 1535nm. The system sensitivity is demonstrated to be approximately 0.01dB after extracting the characteristic macrobend loss curve of a standard single mode fiber (SMF-28). The experiment demonstrated that the oscillatory behavior in the rapid loss due to the increasing curvature could be seen for single turn bare fibers with radii of curvature from 10–20mm. Since the wavelength of laser lies in the range of the conventional window, known as C-band, the demonstrated scheme may suggest the use of these techniques in versatile applications where quick results are necessary without the system complexity.

Novel techniques in VUV high-resolution spectroscopy

Novel VUV sources and techniques for VUV spectroscopy are reviewed. Laser-based VUV sources have been developed via non-linear upconversion of laser pulses in the nanosecond (ns), the picosecond (ps), and femtosecond (fs) domain, and are applied in high-resolution gas phase spectroscopic studies. While the ns and ps pulsed laser sources, at Fourier-transform limited bandwidths, are used in wavelength scanning spectroscopy, the fs laser source is used in a two-pulse time delayed mode. In addition a Fourier-transform spectrometer for high resolution gas-phase spectroscopic studies in the VUV is described, exhibiting the multiplex advantage to measure many resonances simultaneously.

Prospects for storage and retrieval of a quantum-dot single photon in an ultracold87Rb ensemble

Epitaxially grown quantum dots (QDs) are promising sources of non-classical states of light such as single photons and entangled photons. However, in order for them to be used as a resource for long-distance quantum communication, distributed quantum computation, or linear optics quantum computing, these photons must be coupled efficiently to long-lived quantum memories as part of a quantum repeater network. Here, we theoretically examine the prospects for efficient storage and retrieval of a QD-generated single photon with a 1 ns lifetime in a multi-level atomic system. We calculate using an experimentally demonstrated optical depth of 150 that the storage (total) efficiency can exceed 46% (28%) in a dense, ultracold ensemble of $^{87}$Rb atoms. Furthermore, we find that the optimal control pulse required for storage and retrieval can be obtained using a diode laser and an electro-optic modulator rather than a mode-locked, pulsed laser source. Increasing the optical depth, for example by using Bose-condensed ensembles or an optical cavity, can increase the efficiencies to near unity. Aside from enabling a high-speed quantum network based on QDs, such an efficient optical interface between an atomic ensemble and a QD can also lead to entanglement between collective spin-wave excitations of atoms and the spin of an electron or hole confined in the QD.

Transcutaneous laser treatment of leg veins

Leg telangiectasias and reticular veins are a common complaint affecting more than 80% of the population to some extent. To date, the gold standard remains sclerotherapy for most patients. However, there may be some specific situations, where sclerotherapy is contraindicated such as needle phobia, allergy to certain sclerosing agents, and the presence of vessels smaller than the diameter of a 30-gauge needle (including telangiectatic matting). In these cases, transcutaneous laser therapy is a valuable alternative. Currently, different laser modalities have been proposed for the management of leg veins. The aim of this article is to present an overview of the basic principles of transcutaneous laser therapy of leg veins and to review the existing literature on this subject, including the most recent developments. The 532-nm potassium titanyl phosphate (KTP) laser, the 585-600-nm pulsed dye laser, the 755-nm alexandrite laser, various 800-983-nm diode lasers, and the 1,064-nm neodymium yttrium-aluminum-garnet (Nd:YAG) laser and various intense pulsed light sources have been investigated for this indication. The KTP and pulsed dye laser are an effective treatment option for small vessels (<1 mm). The side effect profile is usually favorable to that of longer wavelength modalities. For larger veins, the use of a longer wavelength is required. According to the scarce evidence available, the Nd:YAG laser produces better clinical results than the alexandrite and diode laser. Penetration depth is high, whereas absorption by melanin is low, making the Nd:YAG laser suitable for the treatment of larger and deeply located veins and for the treatment of patients with dark skin types. Clinical outcome of Nd:YAG laser therapy approximates that of sclerotherapy, although the latter is associated with less pain. New developments include (1) the use of a nonuniform pulse sequence or a dual-wavelength modality, inducing methemoglobin formation and enhancing the optical absorption properties of the target structure, (2) pulse stacking and multiple pass laser treatment, (3) combination of laser therapy with sclerotherapy or radiofrequency, and (4) indocyanin green enhanced laser therapy. Future studies will have to confirm the role of these developments in the treatment of leg veins. The literature still lacks double-blind controlled clinical trials comparing the different laser modalities with each other and with sclerotherapy. Such trials should be the focus of future research.

Imaging Microscopic Pigment Chemistry in Conjunctival Melanocytic Lesions Using Pump-Probe Laser Microscopy

To report the application of a novel imaging technique, pump-probe microscopy, to analyze patterns of pigment chemistry of conjunctival melanocytic lesion biopsies. Histopathologic specimens of eight previously excised conjunctival melanocytic lesions were analyzed with pump-probe microscopy. The technique uses a laser scanning microscope with a two-color pulsed laser source to distinguish hemoglobin, eumelanin, and pheomelanin pigment based on differences in transient excited state and ground state photodynamics. The pump-probe signatures of conjunctival melanins were compared with cutaneous melanins. The distributions of hemoglobin, eumelanin, and pheomelanin were analyzed, and pump-probe images were correlated with adjacent hematoxylin and eosin (H&E)-stained sections. The pump-probe signatures of conjunctival melanins are similar, but not identical to cutaneous melanins. In addition, there are qualitative and quantitative differences in the structure and pigment chemistry of conjunctival benign nevi, primary acquired melanosis of the conjunctiva (PAM), and conjunctival melanomas. The pump-probe images correlated well with histopathologic features observed in the adjacent H&E-stained sections, and provided a label-free means of discerning conjunctival anatomic features and pathologic benign or malignant tissue. Pump-probe laser microscopy shows promise as an adjuvant diagnostic tool in evaluation of ocular melanocytic lesions based on morphologic correlation with the histopathology results and pigment chemistry. This initial study suggests systematic differences in pigmentation patterns among conjunctival benign nevi, primary acquired melanosis, and melanomas. In addition, pump-probe microscopy has the potential for use as a noninvasive "in vivo" optical biopsy technique to aid clinical and surgical management of conjunctival melanocytic lesions.

Simulation based study of thermal effects on periodic nanostructures fabricated by laser interference

Laser interference based direct writing is a potential solution for wide range of fabrication of nanostructure. This paper deals with two dimensional heat conduction analysis and the simulation of the same on silicon and germanium surfaces. Simulation of direct writing of patterns with 193nm, 27nsec, single pulse laser source with power varied from 800mJ/cm2 to 1100mJ/cm2 are presented. The rise in the temperature on the surfaces as well as beneath the surface is analyzed on the basis of the results. We also report about the dependency of the thermal diffusion length on the homogeneity of the formed structures.

11 mJ all-fiber-based actively Q-switched fiber master oscillator power amplifier

We report a high energy all fiber format nanosecond pulsed laser source at ∼1064 nm in master oscillator power amplifier (MOPA) configuration. The seed source is an acousto-optic Q-switched fiber laser with a varied pulse duration and repetition rate. The output average power of the oscillator is ∼30 mW and two pre-amplifiers were developed to boost the average power to ∼3 W. Pulse energy of >11 mJ for ∼660 ns pulses at 3 kHz was achieved in the final power amplifier using a commercial 50/400 μm (core/cladding diameter) double cladding Yb-doped fiber (DCYF).

230 W average-power all-fiber-based actively Q-switched fiber master oscillator–power amplifier

We report a high-power all-fiber format pulsed laser source at ~1064 nm in a master oscillator-power amplifier configuration. The seed source is an acousto-optic Q-switched fiber laser with a varied pulse duration and repetition rate. The output power of the oscillator is ~1 W, and two double-cladding Yb-doped fiber amplifiers were used to boost the average power of the seed. >230 W average power was achieved for ~1.4 μs pulses at 100 kHz repetition rate. The optical-to-optical efficiency of the main amplifier is 72.81%.

Rapid high-resolution spin- and angle-resolved photoemission spectroscopy with pulsed laser source and time-of-flight spectrometer

A high-efficiency spin- and angle-resolved photoemission spectroscopy (spin-ARPES) spectrometer is coupled with a laboratory-based laser for rapid high-resolution measurements. The spectrometer combines time-of-flight (TOF) energy measurements with low-energy exchange scattering spin polarimetry for high detection efficiencies. Samples are irradiated with fourth harmonic photons generated from a cavity-dumped Ti:sapphire laser that provides high photon flux in a narrow bandwidth, with a pulse timing structure ideally matched to the needs of the TOF spectrometer. The overall efficiency of the combined system results in near-E(F) spin-resolved ARPES measurements with an unprecedented combination of energy resolution and acquisition speed. This allows high-resolution spin measurements with a large number of data points spanning multiple dimensions of interest (energy, momentum, photon polarization, etc.) and thus enables experiments not otherwise possible. The system is demonstrated with spin-resolved energy and momentum mapping of the L-gap Au(111) surface states, a prototypical Rashba system. The successful integration of the spectrometer with the pulsed laser system demonstrates its potential for simultaneous spin- and time-resolved ARPES with pump-probe based measurements.

A Finite-Difference Time-Domain Model for Quantum-Dot Lasers and Amplifiers in the Maxwell–Schrödinger Framework

We describe a finite-difference time-domain (FDTD) model of a long (edge-emitting) gain medium based on a quantum-dot (QD) in-a-well structure under the framework of the Maxwell-Schrodinger equations. The model includes the dynamic behavior of a QD gain medium including an excited state incorporated within carrier rate equations and considers the carrier density dependence of the refractive index. The model enables us also to calculate carrier diffusion effects, which, unlike in quantum well based structures, play an important role in QD devices, since carrier capture and escape processes modify the effective carrier diffusion length. We present results of basic static and dynamic lasers properties as well as of the interaction of a QD amplifier with short, 150 fs pulses. We identify four regimes of operation for the pulse-QD interaction, two of which are important: the linear-saturated regime and the Rabi-oscillation dominated regime. The latter leads to Rabi floppings with a period shorter than the pulse itself. The model can be easily employed for any complicated process such as four-wave mixing, saturable absorption, semiconductor pulse laser sources, etc.

Nanosecond and sub-nanosecond pulsed laser ablation of thin single and multi-layer packaging films

Translating single and multi-layer packaging films are exposed to 0.5–0.8ns laser pulses of wavelength 1064nm and 10–12.5ns laser pulses of wavelength 515nm. Ablation depths and threshold fluences are reported for single-layer polyethylene (PE), polypropylene (PP) and aluminium of thickness 20–50μm. Interaction and cut widths are reported for the same single-layer films and for four multi-layer films comprising aluminium-polypropylene and aluminium-paper. Ablation of the PE and PP films is only possible in the tested parameter range with 0.5ns, 1064nm pulses. Though a one order of magnitude reduction in the ablation threshold of aluminium is observed with 0.5–0.8ns, 1064nm pulses, the efficiency of material removal for fluences >8Jcm−2 is superior with 10–12.5ns, 515nm pulses. Multi-layer film response is found to be heavily dictated by the thickness of metallic layers. For multi-layer films with aluminium layers of thickness 7–9μm, adjacent layers are removed by inter-layer heat conduction from the aluminium layer, in some cases leading to very large cut widths. For multi-layer films with aluminium layers of thickness <0.1μm, direct ablation of all layers must take place for complete film penetration. The study provides quantitative results regarding process efficiency and quality for application of pulsed laser sources within the packaging industry.

Application of space–time duality to ultrahigh-speed optical signal processing

Manipulation and characterization of information using ultrafast optical signals is critical for numerous applications in telecommunications, biology, quantum information science, spectroscopy, and atomic and molecular physics. Femtosecond pulsed laser sources are available over a wide range of wavelengths and repetition rates, which enable the generation, transmission, and characterization of information at bandwidths beyond 1&#xA0;THz. In this article, we review the concept of space&#x2013;time duality as a system design tool for ultrafast optical processing and characterization. The combination of this design framework with recent advances in nonlinear optical devices enables the realization of highly complex signal processing systems that can generate, characterize, and manipulate arbitrary and non-repetitive optical waveforms at unprecedented processing speeds.

Nanosecond pulsed laser welding of high carbon steels

Abstract The present paper deals with the possibility to exploit low-cost, near infra-red, nanosecond pulsed laser sources in welding of high carbon content thin sheets. The exploitation of these very common sources allows to achieve sound weld beads with a good depth-to-width ratio and very small heat affected zones when the proper process parameters are involved. In particular the role of pulse frequency, pulse duration, peak power and welding speed on the characteristics of the weld beads is studied and the advantage of the application of short-pulse laser sources over traditional long-pulse or continuous wave one is assessed.

Energy relaxation of exciton-polariton condensates in quasi-one-dimensional microcavities

We present a time-resolved study of energy relaxation and trapping dynamics of polariton condensates in a semiconductor microcavity ridge. The combination of two non-resonant, pulsed laser sources in a GaAs ridge-shaped microcavity gives rise to profuse quantum phenomena where the repulsive potentials created by the lasers allow the modulation and control of the polariton flow. We analyze in detail the dependence of the dynamics on the power of both lasers and determine the optimum conditions for realizing an all-optical polariton condensate transistor switch. The experimental results are interpreted in the light of simulations based on a generalized Gross-Pitaevskii equation, including incoherent pumping, decay and energy relaxation within the condensate.

One- and two-dimensional photo-imprinted diffraction gratings for manipulating terahertz waves

Emerging technology based on artificial materials containing metallic structures has raised the prospect for unprecedented control of terahertz waves. The functionality of these devices is static by the very nature of their metallic composition, although some degree of tunability can be achieved by incorporating electrically biased semiconductors. Here, we demonstrate a photonic structure by projecting the optical image of a metal mask onto a thin GaAs substrate using a femtosecond pulsed laser source. We show that the resulting high-contrast pattern of photo-excited carriers can create diffractive elements operating in transmission, potentially providing a route to terahertz components with reconfigurable functionality.