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We report a compact, 130-W single-stage master oscillator power amplifier with a high peak power of 51.3 kW and a narrow spectral linewidth of 0.1 nm. The seed source is a single-mode, passively mode-locked solid-state laser at 1064 nm with an average power of 2 W. At a repetition rate of 73.5 MHz, the pulse duration is 30 ps. After amplification, it stretches to 34.5 ps. The experiment enables the optical-to-optical conversion efficiency to reach 75%. To the best of our knowledge, this is the first report of such a high-power, narrow spectral linewidth, high peak power picosecond-pulse fiber amplifier based on a continuous-wave, mode-locked solid- state seeding laser. No amplified spontaneous emission and stimulated Raman scattering were observed when the pump was increased. [ABSTRACT FROM AUTHOR]

Uniform near-infrared (NIR) and short-wave infrared (SWIR) illuminators are desired in low ambient light detection, recognition, and identification of military applications. Factors that contribute to laser illumination image degradation are high frequency, coherent laser speckle and low frequency nonuniformities created by the laser or external laser cavity optics. Laser speckle analysis and beam uniformity improvements have been independently studied by numerous authors, but analysis to separate these two effects from a single measurement technique has not been published. In this study, profiles of compact, diode laser NIR and SWIR illuminators were measured and evaluated. Digital 12-bit images were recorded with a flat-field calibrated InGaAs camera with measurements at F/1.4 and F/16. Separating beam uniformity components from laser speckle was approximated by filtering the original image. The goal of this paper is to identify and quantify the beam quality variation of illumination prototypes, draw awareness to its impact on range performance modeling, and develop measurement techniques and methodologies for military, industry, and vendors of active sources. [ABSTRACT FROM AUTHOR]

This paper suggests an optical printed circuit board (OPCB) having new optical coupling structures, including a laser-drilled and under-filled structure (LD-UFS) and a vertical waveguide structure (VWS). The suggested OPCB has the features of high-speed data transmission as well as highly efficient optical coupling because it was fabricated with low-dielectric and transparent electrical PCB materials through a PCB compatible process. To evaluate and compare the optical and electrical performances of the suggested OPCB with those of other OPCBs, the various types of OPCBs were fabricated and measured. The optical coupling losses of the LD-UFS and the VWS were measured with excellent results of 9.8 and 7.8 dB, respectively, which are lower than that of the basic structure. The electrical 3-dB bandwidth of the OPCB was also evaluated up to more than 40 GHz. [ABSTRACT FROM AUTHOR]

A compact two-dimensional laser scanner based on piezoelectric actuators is presented. The scanner consists of two single-axis laser scanners placed perpendicular to each other, which exhibit the advantages of small size, large angle, high scanning speed, and high linearity. The mechanical structure and principle of the scanner are introduced and the performance of the scanner is experimentally investigated. The result shows that the maximum angle of the scanner is approximately 9.315 deg with a main resonant frequency of 1242 Hz. An open-loop controller based on a hysteresis compensation algorithm and analog notch filter is proposed. Its non-linearity is reduced to ± 0.5% after compensation. High frequency scanning and the step response of the scanner are also studied to demonstrate the performance and effectiveness of the scanner. [ABSTRACT FROM AUTHOR]

In the early 1990s, Church and Takacs pointed out that the specification of surface figure and finish of x-ray mirrors must be based on their performance in the beamline optical system.We demonstrate the limitations of specification, characterization, and performance evaluation based on conventional statistical approaches, including root-mean-square roughness and residual slope variation, evaluated over spatial frequency band-widths that are system specific, and a more refined description of the surface morphology based on the power spectral density distribution. We show that these limitations are fatal, especially in the case of highly collimated coherent x-ray beams, like beams from x-ray free electron lasers (XFELs). The limitations arise due to the deterministic character of the surface profile data for a definite mirror, while the specific correlation properties of the surface are essential for the performance of the entire x-ray optical system. As a possible way to overcome the problem, we treat a method, suggested by Yashchuk and Yashchuk in 2012, based on an autoregressive moving average modeling of the slope measurements with a limited number of parameters. The effectiveness of the approach is demonstrated with an example specific to the x-ray optical systems under design at the European XFEL. [ABSTRACT FROM AUTHOR]

We demonstrate a channel-reuse bidirectional 10-Gb/s/λ long-reach DWDM-PON and an optical beat noise-based automatic wavelength control method for a tunable laser used in a colorless optical network unit. 100-GHz channel spacing 55- and 100-km full-duplex bidirectional 320 Gb/s (32 x 10 Gb/s) capacity transmissions are achieved without and with optical amplification. Transmission performance is also measured with different optical signal to Rayleigh backscattering noise ratios and different central wavelength shifts between upstream and downstream in the channel-reuse system. [ABSTRACT FROM AUTHOR]

Laser radar (LADAR) is a three-dimensional (3-D) imaging system which provides very high-resolution 3-D images and is used for automatic recognition of 3-D targets. In order to achieve a high recognition ability for this system, a new LADAR image descriptor is proposed. This descriptor extracts features from the LADAR images by using special chromatic processors called invariant half-height overlapping triangular processors. The descriptor developed has a high discrimination capability and is robust to the effects that disturb LADAR images. The performance of the proposed LADAR descriptor is evaluated using simulated and real LADAR images. The results show the robustness of the proposed descriptor in the presence of noise, low resolution, view change, rotation, translation, and scaling effects. The results also show the high discrimination capability for the new descriptor over the traditional techniques such as invariant moments descriptor, which is used to benchmark the simulation and the experimental results. [ABSTRACT FROM AUTHOR]

We experimentally study two Ti:sapphire optical frequency comb femtosecond regimes, respectively, with a linear and a nonlinear dependence of the carrier-envelope offset frequency (fCEO) on pump intensity. For both regimes, we study the effect of single- and multimode pump lasers on the fCEO phase noise. We demonstrate that the femtosecond regime is playing a more important role on the fCEO phase noise and stability than the pump laser type. [ABSTRACT FROM AUTHOR]

We report on the continuation of a comparative study of different fused silica materials for ArF laser applications. After selecting potentially suited fused silica materials from their laser-induced absorption and compaction obtained by a short-time testing procedure, accelerated lifetime tests have been undertaken by sample irradiating at liquid nitrogen temperature and subsequent direct absorption measurements were made using the laser-induced deflection technique. The obtained degradation acceleration strongly differs between fused silica materials, showing high and low oxygen hole (OH) contents, respectively. As a result, a difference in the absorption degradation mechanism between high and low OH-containing fused silica is proposed. Consequently, two different scenarios for an acceleration of the absorption degradation are derived. [ABSTRACT FROM AUTHOR]

The modeling of laser interaction with metals for various applications requires a knowledge of absorption coefficients for real, commercially available materials with engineering grade (unpolished, oxidized) surfaces. However, most currently available absorptivity data pertain to pure metals with polished surfaces or vacuum-deposited thin films in controlled atmospheres. A simple laboratory setup is developed for the direct calorimetric absorptivity measurements using a diode-array laser emitting at 780 nm. A scheme eliminating the effect of convective and radiative losses is implemented. The obtained absorptivity results differ considerably from existing data for polished pure metals and are essential for the development of predictive laser-material interaction models. [ABSTRACT FROM AUTHOR]

The cracks and scratches inevitably generated by previous grinding and polishing significantly lower the ability of laser resistance of optical substrates. In this study, the artificial indentations, scratches, and structural defects imbedded with metal nanoparticles are fabricated. The laser-induced damage characteristics of such defects in different types and sizes are investigated qualitatively and quantitatively under 1064-nm laser irradiation. Moreover, the etching effect on improving the laser-induced damage threshold (LIDT) of artificial defects under different etching conditions is analyzed. LIDT is then evaluated according to the etching depth and the morphologies of artificial defects. [ABSTRACT FROM AUTHOR]

Laser calorimetry is based on the measurement and evaluation of the temperature increase caused by absorption in the sample exposed to laser radiation. A temperature distribution develops in the irradiated sample as a result of dependence on the thermal diffusivity of the sample. Therefore, finding a correlation between the temperature increase and absorption becomes a complex task. This challenge was met by keeping the sample geometry at a standard size and simulating the thermal distribution for a number of optical materials. Using this method, Laser Zentrum Hannover e.V. (LZH) developed a calorimetric test setup that offers fully calibrated absorptance values for sample diameters of 25 mm (or 1 in.) with a total error of below 13% and a relative measurement error of below 5%. However, this technique is limited to the above-mentioned sample geometry. This work presents an approach to adjust the measurement configuration to numerous sample sizes for standard circular laser components. Finite element analysis and experimental verification are presented for exemplary values of the samples' diameters. Based on the different sample mount concept, this procedure allows utilizing flexibility in test wavelength and angle of incidence, combined with the sensitivity level sufficient for current optical materials. [ABSTRACT FROM AUTHOR]

In order to obtain accurate position of the inner key components in the experimental advanced super-conducting tokamak (EAST), a combined optical measurement method which is comprised of a laser tracker (LT) and articulated coordinate measuring machine (CMM) has been brought forward. LT, which is an optical measurement instrument and has a large measurement range and high accuracy, is employed for establishing the precision measurement network of EAST, and the articulated CMM is also employed for measuring the inner key components of EAST. The measurement uncertainty analyzed by the Unified Spatial Metrology Network (USMN) is 0.20 mm at a confidence probability of 95.44%. The proposed technology is appropriate for the inspection of the reconstruction of the EAST. [ABSTRACT FROM AUTHOR]

A reflective beam shaper is designed for the purpose of compensation of the low-order aberrations in high-power slab lasers. A proportional-integral-derivative (PID) control algorithm is used to control the optical parameters of a beam shaper for active compensation of low-order aberrations. Simulations of the PID algorithm show that different combinations of defocus, 0-deg astigmatism, and 45-deg astigmatism, which are the main contributors of beam aberrations in slab lasers, can be well compensated by variation of distance and rotation angle of mirrors. For a beam with large wave aberrations [peak-to-valley (PtV) = 87.7λ, root-mean-square (RMS) = 19λ], the adjustment of distance between mirrors is less than 100 mm, and the rotation angle about the z-axis is <3.2 deg, and the wavefront distortion is reduced to a level (PtV = 0.50λ, RMS = 0.09λ) that can be further corrected with one deformable mirror. The effectiveness and performance of low-order aberration compensation with the reflective beam shaper are also verified by experiments. [ABSTRACT FROM AUTHOR]

Modulation bandwidth and frequency chirping of the optical injection-locked (OIL) microring laser (MRL) in the cascaded configuration are investigated. The unidirectional operation of the MRL under strong injection allows simple and cost-saving monolithic integration of the OIL system on one chip as it does not need the use of isolators between the master and slave lasers. Two cascading schemes are discussed in detail by focusing on the tailorable modulation response. The chip-to-power ratio of the cascaded optical injection-locked configuration has decreased by up to two orders of magnitude, compared with the single optical injection-locked configuration. [ABSTRACT FROM AUTHOR]

Ultrafast Bessel beams are ideal sources for high aspect ratio submicron structuring applications because of their nondiffracting nature and higher stability in nonlinear propagation. We report here on the interaction of ultrafast Bessel beams at various laser energies and pulse durations with transparent materials (fused silica) and define their impact on photoinscription regimes, i.e., formation of positive and negative refractive index structures. The laser pulse duration was observed to be key in deciding the type of the structures via excitation efficiency. To understand the relevant mechanisms for forming these different structures, the free carrier behavior as a function of laser pulse duration and energy was studied by capturing instantaneous excitation profiles using time-resolved microscopy. Time-resolved imaging and simulation studies reveal that low carrier densities are generated for ultrashort pulses, leading to soft positive index alterations via presumably nonthermally induced structural transitions involving defects. On the other hand, the high free carrier density generation in the case of longer pulse durations leads to hydrodynamic expansion, resulting in high aspect ratio submicron-size wide voids. Delayed ionization, carrier defocusing, and lower nonlinear effects are responsible for the confinement of energy, resulting in efficient energy deposition on-axis. [ABSTRACT FROM AUTHOR]

An automated data acquisition and processing system is established to measure the force applied by an optical trap to an object of unknown composition in real time. Optical traps have been in use for the past 40 years to manipulate microscopic particles, but the magnitude of applied force is often unknown and requires extensive instrument characterization. Measuring or calculating the force applied by an optical trap to nonspherical particles presents additional difficulties which are also overcome with our system. Extensive experiments and measurements using well-characterized objects were performed to verify the system performance. [ABSTRACT FROM AUTHOR]

We proposed and experimentally demonstrated a delay-match sampling method to measure and compensate the laser phase error in optical frequency-domain reflectometry system. By using the error signal extracted from a simple auxiliary Mach-Zehnder interferometer with only a 10-ns delay, the laser phase error is effectively compensated. Considerable improvement is achieved in spatial resolution from 200 m to 7 cm at a measurement distance over 10 times the round-trip laser coherence length. [ABSTRACT FROM AUTHOR]

A backlight unit is constructed by laying out a plastic optical fiber (POF) in a curved trench fabricated in a light-guide plate. First, the light leaks out of the POF at curved sections and enters the plate. Next, the light is extracted from the plate by some microstructures fabricated on the surfaces of the plate. Coupled to a laser diode, its optical power can be efficiently and uniformly delivered over a large area via the POF. In this experiment, we fabricated a 10 cm x 10 cm x 3 mm prototype unit with off-the-shelf components. It becomes see-through when the space around the POF is filled with index-matching oil. One can build an arbitrary-shaped planar light source by tiling multiple cells and connecting them by a POF. The light inside the POF is depleted as it propagates downstream. This can be compensated by decreasing the radii of curvature. Microstructures on the light-guide plate can distort the passage of ambient light. For a see-through unit, we can distribute them sparsely and/or use absorbers. A see-through backlight unit is a relatively unexplored device, and itmight pave the way for new applications. [ABSTRACT FROM AUTHOR]

One-millimeter-thick Al 1050 sheets were cut using a 2-kWfiber laser operating in continuous-wave (CW) mode. An experimental approach that consisted of fitting the regression models by means of response surface methodology was adopted. The effects of cutting speed, assist gas pressure, and focal position on roughness arithmetic mean value were investigated. The desirability function was applied for the simultaneous optimization of cut quality and operating costs. The full potential of the CW mode high processing speeds and of the better absorptivity of 1-µm laser radiation for highly reflective materials are employed at the same time. Cutting aluminum with fiber laser increases the cutting speed and gives a cut quality comparable with results obtained with CO2 and Nd:YAG lasers that represent the most established laser sources for this application. [ABSTRACT FROM AUTHOR]

A diode-side-pumped actively Q-switched first-Stokes eye-safe laser with a Nd:YAP/YVO4 crystal is reported. Under the pump power of 250 W, a stable Raman laser at 1525 nm with an average output power of 4.5Wand pulse width of 60 ns at the pulse repetition frequency of 4.5 kHz is achieved. Another laser crystal Nd: YAG is used to carry out the contrast experiment. The output performances of these two lasers are measured and compared. The result reveals that the first-stokes Raman laser generated by the Nd:YAP/YVO4 crystal has a lower power threshold and higher Raman conversion efficiency because of the large value of the stimulated emission cross section and the fluorescence lifetime at 1342 nm of the Nd: YAP crystal. [ABSTRACT FROM AUTHOR]

Laser range-gated imaging systems can obtain images of targets hidden around the corner, with an intermediary reflective surface with certain specular reflection characteristics. This imaging mode is called non-line-of-sight imaging. This paper describes a simulation of the target signal illuminance and disturbance radiation on the photosensitive surface of a non-line-of-sight imaging system based on modeling of an intermediary reflective surface. Meanwhile, an image contrast model of a non-line-of-sight imaging system is constructed. Simulation of the image contrast for a laser range-gated imaging system as a non-line-of-sight imaging equipment was carried out by analyzing the effects of varying the target signal illuminance and intermediary reflective surface reflection. Our simulation results show that the reflection characteristics of the intermediary reflective surface have a significant effect on the non-line-of-sight imaging. Although ordinary active laser imaging can realize non-line-of-sight imaging for an intermediary reflective surface with significant specular reflection characteristics, a laser range-gated imaging system is indispensable for non-line-of-sight imaging with commonly used intermediary reflective surfaces without significant specular reflection characteristics. The image contrast model of non-line-of-sight imaging constructed in this paper provides insights into the theoretical analysis and system design, as well as practical application of non-line-of-sight imaging. [ABSTRACT FROM AUTHOR]

A laser steering systembased on the Ti-diffusion lithiumniobate (LiNbO3) waveguides is presented. A phase shifter based on the LiNbO3 waveguide is designed. This waveguide can provide a continuous phase shift for laser-phased-array (LPA) by changing the voltage loaded on it. The theory of irregular LPA based on the Ti-diffusion LiNbO3 waveguide phase shifter is studied numerically and experimentally. Beam steering with an angle of 1.37 deg is gained by a 1 x 3 array setup that agrees well with the theory. [ABSTRACT FROM AUTHOR]

A resonantly pumped monolithic Ho:YAG nonplanar ring oscillator (NPRO) with single-frequency and dual-wavelength laser outputs is demonstrated. The Ho:YAG NPRO is resonantly pumped by a 1907-nm Tm:YLF laser. In single-frequency operation, the output power is 3.09 W, with a slope efficiency of 61%and an optical efficiency of 48%with respect to the pump power. Up to 10 W dual-wavelength laser output at 2091 and 2097 nm is also obtained from the Ho:YAG NPRO with the maximum pump power, with a slope efficiency of 61% with respect to the pump power. [ABSTRACT FROM AUTHOR]

Laser imaging through a turbid medium is complicated by scattering. Backscattered photons reduce image contrast as weak target returns compete against a large background of backscattered light. Forward scattering broadens the interrogating laser beam, thereby reducing the spatial resolution of the target. Prior research has shown that intensity modulation (<100 MHz) can be used to "wash-out" the backscatter, resulting in better discrimination of the target and higher contrast. We show that the higher modulation frequencies (>100 MHz) can be also used to suppress forward scattered light, thereby increasing spatial resolution. [ABSTRACT FROM AUTHOR]

It has been demonstrated experimentally that pulsed pumping can significantly improve thermal management, thus upgrade the output power of an optically pumped semiconductor disk laser (SDL). The transient heat conduction equation is solved by the use of the finite element method, and the maximum temperature rise of the active region in an InGaAs quantum well SDL under pulsed pumping is focused. Based on the numerical results, the influences of width, repetition rate, and shape of pump pulses on the maximum temperature rise are discussed, the optimized design of width, repetition rate, and shape of pump pulses are concluded, and the theoretical results are in good agreement with the reported experiments. [ABSTRACT FROM AUTHOR]

Linewidth optimization of a fiber grating Fabry-Perot (FGFP) laser is performed numerically. In addition to the external optical feedback (OFB), the effect of temperature, injection current, cavity volume, gain compression factor, and external cavity parameters [i.e., coupling coefficient (Co) and external cavity length (Lext)] on linewidth characteristics are investigated. The effects of external OFB and temperature on linewidth characteristics are calculated according to their effect on threshold carrier density (Nth). The temperature dependence (TD) of linewidth characteristics is calculated according to the TD of laser parameters instead of the well-known Pankove relationship. Results show that the optimum external cavity length (Lext) is 3.1 cm and the optimum range of operating temperature is within ±2°C from the fiber Bragg grating (FBG) reference temperature (To). In addition, the antireflection (AR) coating reflectivity value of 1 x 10-2 is sufficient for the laser to operate at narrow linewidth and low fabrication complexity. The linewidth can be reduced either by increasing the laser injection current or the strength of external OFB level. [ABSTRACT FROM AUTHOR]

A location technique based on power spectrum analysis for the phase-sensitive optical time-domain reflectometry is proposed. The frequency characteristics of the backscattered signal at a time interval over the sensing fiber are provided to discriminate the disturbance region from other regions. Compared with conventional location techniques, the proposed method significantly enhances the signal-to-noise ratio (SNR). Therefore, it can provide a high-performance and cost-effective solution avoiding the use of the laser with super low-frequency drift. Although the frequency drift of the laser utilized in the experiment is 230 MHz/min, the average SNR is improved to be 20.3 dB and the maximum location error is 100 m over the monitored length of 9 km during 20 experiments. [ABSTRACT FROM AUTHOR]

Laser gated viewing is a prominent sensing technology for optical imaging in harsh environments and can be applied for vision through fog, smoke, and other degraded environmental conditions as well as for the vision through sea water in submarine operation. A direct imaging of nonscattered photons (or ballistic photons) is limited in range and performance by the free optical path length, i.e., the length in which a photon can propagate without interaction with scattering particles or object surfaces. The imaging and analysis of scattered photons can overcome these classical limitations and it is possible to realize a nonline-of-sight imaging. The spatial and temporal distributions of scattered photons can be analyzed by means of computational optics and their information of the scenario can be restored. In particular, the information outside the line of sight or outside the visibility range is of high interest. We demonstrate nonline-of-sight imaging with a laser gated viewing system and different illumination concepts (point and surface scattering sources). [ABSTRACT FROM AUTHOR]

Lincoln Laboratory of Massachusetts Institute of Technology has developed a technique known as dynamic photoacoustic spectroscopy (DPAS) that could enable remote detection of trace gases via a field-portable laser-based system. A fielded DPAS system has the potential to enable rapid, early warning of airborne chemical threats. DPAS is a new form of photoacoustic spectroscopy that relies on a laser beam swept at the speed of sound to amplify an otherwise weak photoacoustic signal. We experimentally determine the sensitivity of this technique using trace quantities of SF6 gas. A clutter-limited sensitivity of ~100 ppt is estimated for an integration path of 0.43 m. Additionally, detection at ranges over 5 m using two different detection modalities is demonstrated: a parabolic microphone and a laser vibrometer. Its utility in detecting ammonia emanating from solid samples in an ambient environment is also demonstrated. [ABSTRACT FROM AUTHOR]

Laser-induced thermal damages of a silicon wafer surface subjected to continuous near-infrared laser irradiation were investigated. Silicon wafer specimens were illuminated by a continuous-wave fiber laser beam (1070-nm wavelength) with irradiances from 93 to 186 W/cm², and the surface morphology of each specimen was analyzed using optical microscopy. With increasing irradiance, straight cracks in the <110> direction appeared first, and partial melting and complete melting were subsequently observed. The mechanism of these laser-induced thermal damages in the silicon wafer surface was discussed with numerical analysis based on the heat transfer and thermoelasticity model. The irradiances initiating the cracking and melting were predicted by determining the irradiances in which the calculated thermal stress and temperature exceeded the corresponding limits of the fracture strength and melting point, respectively. These predictions agreed well with the experimental findings. Laser-induced thermal damages of the silicon wafer surface subjected to a con- tinuous near-infrared laser irradiation were identified based on these investigations. [ABSTRACT FROM AUTHOR]

We characterized the dynamic response of a Bragg grating-based fiber laser sensing system. The sensing system comprises of a narrow line width fiber laser based on p-phase-shifted fiber Bragg grating formed in an active fiber, an unbalanced fiber Michelson interferometer (FMI), which performs wavelength-to-phase mapping, and a phase detection algorithm, which acquires the phase change from the interferometric output signal. The novel phase detection algorithm is developed based on the combination of the two traditional phase generated carrier algorithms: differential-cross-multiplying and arctangent algorithms, and possesses the advantages of the two algorithms. The modulation depth fluctuation of the carrier does not affect the performance of the sensing system. A relatively high side mode suppression ratio of above 50 dB has been achieved within a wide range of carrier amplitude from 1.6 to 5.0 V which correspond to the modulation depth from 1.314 to 4.106 rad. The linearity is 99.082% for the relationship between the power spectral density (dBm/Hz) of the detected signal and the amplitude (mv) of the test signal. The unbalanced FMI is used to eliminate the polarization effect. [ABSTRACT FROM AUTHOR]

The detection and classification of small surface targets at long ranges is a growing need for naval security. Simulations of a laser radar at 1.5 µm aimed for search, detect, and recognition of small maritime targets will be discussed. The data for the laser radar system will be based on present and realistic future technology. The simulated data generate signal waveforms for every pixel in the sensor field-of-view. From these we can also generate two-dimensional (2-D) and three-dimensional (3-D) range and intensity images. The simulations will incorporate typical target movements at different sea states, vessel courses, effects of the atmospheric turbulence and also include different beam jitter. The laser pulse energy, repetition rate as well as the receiver and detector parameters have been the same during the simulations. We have also used a high resolution (sub centimeter) laser radar based on time correlated single photon counting to acquire examples of range profiles from different small model ships. The collected waveforms are compared with simulated wave forms based on 3-D models of the ships. A discussion of the classification potential based on information in 1-D, 2-D, and 3-D data separately and in combination is made versus different environmental conditions and system parameters. [ABSTRACT FROM AUTHOR]

Laser-induced breakdown spectroscopy (LIBS) has been used to study the surface hardness of special aluminum alloys containing zeolite. The aluminum alloy has acquired pronounced changes in its metallurgical properties due to the zeolite inclusion. The surface hardness of the samples under investigation is determined by measuring the spectral intensity ratios of the ionic to atomic spectral lines in the LIBS spectra of samples having different surface hardness values that have been conventionally measured before for comparison. The presence of aluminum silicate mineral in the studied alloys enabled material volume to expand under compression. This feature gave new results in the measurement of hardness via LIBS. It has been proven that the trend of the alloy density change complies with the increase of ionic to atomic spectral line intensity ratio. [ABSTRACT FROM AUTHOR]

Theoretical models of laser-induced damage mechanisms in optical materials are reviewed: inclusion-initiated thermal explosion (extrinsic mechanism) and impact ionization (II) and photoionization (intrinsic mechanisms). Different approaches to II theory based on quantum kinetic equation, Boltzman equations, and rate equations are briefly described. A relative contribution of II and photoionization predicted by these models at different laser pulse durations, including femtosecond-range, are discussed and compared with available experimental data. Basing on an analysis of published theoretical and experimental results, a today's state of understanding fundamental laser damage mechanisms is concluded. [ABSTRACT FROM AUTHOR]

We demonstrate a saturable absorber (SA) based on cladding-filled graphene in a specially designed and manufactured photonic crystal fiber (PCF) for the first time. The saturation absorption property is achieved through the evanescent coupling between the guided light and the cladding-filled graphene layers. To boost the mutual interaction, the PCF is designed to contain five large air holes in the cladding and small-core region. Employing this graphene-PCF SA device, we construct an erbium-doped all-fiber laser oscillator and achieve mode-locked operation. This device can pave the way for high power and all-fiber applications of photonics with graphene with some unique advantages, such as single-mode operation, nonlinearity enhancement, high-power tolerance, environmental robustness, all-fiber configuration, and easy fabrication. [ABSTRACT FROM AUTHOR]

Based on the elastic-plastic deformation theory, status between abrasives and workpiece in magnetorheological finishing (MRF) process and the feasibility of elastic polishing are analyzed. The relationship among material removal mechanism and particle force, removal efficiency, and surface topography are revealed through a set of experiments. The chemical dominant elastic super-smooth polishing can be fulfilled by changing the components of magnetorheological (MR) fluid and optimizing polishing parameters. The MR elastic super-smooth finishing technology can be applied in polishing high-power laser--irradiated components with high efficiency, high accuracy, low damage, and high laser-induced damage threshold (LIDT). A 430 x 430 x 10 mm fused silica (FS) optic window is polished and surface error is improved from 538.241 nm [peak to valley (PV)], 96.376 nm (rms) to 76.372 nm (PV), 8.295 nm (rms) after 51.6 h rough polishing, 42.6 h fine polishing, and 54.6 h super-smooth polishing. A 50 x 50 x 10 mm sample is polished with exactly the same parameters. The roughness is improved from 1.793 nm [roughness average (Ra)] to 0.167 nm (Ra) and LIDT is improved from 9.77 to 19.2 J/cm² after MRF elastic polishing. [ABSTRACT FROM AUTHOR]

We describe a method based on multichannel time-delay estimation with linear fitting correction for laser time-of-flight (TOF) measurement. The laser TOF measurement system is constructed with a laser source, a stop receiver channel, a reference receiver multichannel, an analog to digital converter (ADC) sampling unit, and a digital signal processing unit. Limited by the sampling rate, the precision of laser TOF measurement is restricted no more than the ADC sampling period in conventional methods. As this problem is considered, multichannel correlation time-delay estimation with linear fitting correction is devised. It is shown that the measuring precision is better than 2 ns with multichannel time-delay estimation and not influenced by signal-to-noise ratio. The experimental results demonstrate that the proposed method is effective and stable. [ABSTRACT FROM AUTHOR]

A prototype of a 160 GHz millimeter-wave (mm-wave) generator is proposed and analyzed. In the scheme, two lasers with 100 GHz frequency interval serve as sources. Then, a frequency 16-tupling feed-forward modulation technique is employed to generate two-phase correlated sidebands with a 160 GHz interval. The desired sidebands can be selected by using optical interleavers. A 160 GHz mm-wave signal free of phase noise can be achieved. [ABSTRACT FROM AUTHOR]

We use a chaotic laser, instead of thermal light, as the light source in temporal ghost imaging. This laser is generated by employing an external optical feedback. The imaging magnification is varied by adjusting the group-delay dispersion parameters of the fibers. The temporal ghost imaging result is the convolution between the transmission function of the object and the temporal correlation functions of the chaotic laser. The simulation experiment, which uses a controllable time switch as the object, shows the effectiveness of our scheme. This scheme could find applications in the time-domain tomography of pulses. [ABSTRACT FROM AUTHOR]

The utilization of adaptive equalization in the design of atmospheric laser link transceiver architectures that can be used for television and broadcast signal interconnect between the external place of event and the master control room is suggested. At the transmitter side of the proposed transceiver; an array of atmospheric laser sources, digital signal processing, and optical radiators are used to send light waves in free space. At the receiver side, an adaptive finite impulse response least mean square (LMS) equalizer with activity detection guidance (ADG) and tap decoupling (TD) is used to mitigate the effect of channel impairments. The performance of the suggested adaptive equalizer is compared with that of the conventional adaptive equalizer based only on the standard LMS algorithm. The simulation results revealed that the adaptive LMS equalizer with ADG and TD is a promising solution for the inter-symbol interference problem in optical wireless communication systems. [ABSTRACT FROM AUTHOR]

The effects of cutting speed and assist gas pressure on laser cutting of 1-mm thick Al 1050, AZ31, and Ti6Al4V lightweight alloys are experimentally investigated. Fiber laser cutting of these materials is not broadly investigated and the acquisition of a new level of knowledge is of fundamental importance for applications like sheet metal trimming in automotive industry. The main process outputs are in depth compared with results reported in literature and obtained by cutting with CO2 and Nd:YAG lasers. The good cut quality, the high productivity, and the easy delivery of the beam obtained at the same time, corroborate the advantage of using fiber lasers for thin sheets lightweight alloys cutting. [ABSTRACT FROM AUTHOR]

In some computer vision applications, it is necessary to calibrate the geometry relationships of nonoverlapping cameras. However, due to lacking a common field of view, the calibration of this camera topology is quite difficult. A calibration method for nonoverlapping cameras is proposed and investigated. The proposed method utilizes several light planes, which can be generated by a line laser projector or a rotary laser level, as the calibration objects. The fact that local light planes available in different cameras are identical in global coordinates is used to recover the geometries. Results on both synthetic and real data show the validity and performance of the proposed method. The given method is simple and flexible, which can be used to calibrate geometry relationships of cameras located in large-scale space without expensive equipment such as theodolites and laser trackers. [ABSTRACT FROM AUTHOR]

A new and simple method for measuring the refractive index of liquid substances is presented. In this method, a laser beam impinges transversely on a glass tube (cylindrical cell) filled with the liquid to be measured. The laser beam incident on the cylindrical cell is deviated when it propagates through the wall of the cell and the liquid contained in it. By measuring the deviation of the principal ray of the laser beam when it emerges from the cylindrical cell, we can determine the refractive index of the liquid. To show the feasibility of the method, we measured the refractive index of pure water with a He-Ne laser. [ABSTRACT FROM AUTHOR]

This paper's text is taken from a monograph written by the author in 1988. Some of the material contained here has been referenced in the open literature attributed to a private communication. This reprinting of the content of the monograph is and should be viewed as a historic paper documenting the author's thoughts and knowledge as of 1988 and does not reflect developments since that time. [ABSTRACT FROM AUTHOR]

Detailed modeling results of 100 J class laser systems with respect to the output energy, beam propagation, nonlinear phase accumulation, wavefront aberrations, and adaptive optics performance obtained in MIRÓ and MATLAB codes are presented here. The laser system is based on a cryogenically cooled Yb3+:YAG multislab amplifier with two identical amplifier heads and operates at 10 Hz repetition rate with an average power above 1 kW. [ABSTRACT FROM AUTHOR]

We propose and investigate a refractive index (RI) sensor consisting of a combination of single-multi-single mode fiber and a micro-slot structure in the multimode fiber fabricated by femtosecond laser as the sensing head. The beam propagation method is utilized to simulate the microslot structure. Simulation results show the size of the microslot significantly influences the RI sensitivity. At an ablation depth of 170 µm and a width of 30 µm, the refractometer shows a high sensitivity of 1327.27 nm per RI unit (RIU) and a better stability in the RI range of 1.33 to 1.35. Moreover, higher sensitivity in a narrow RI range can be achieved by optimizing the microslot structure. Experimental investigations demonstrated a sensitivity of 912.31 nm/RIU for an RI range of 1.33 to 1.35 for a refractometer with a microslot depth of 170 µm and width of 24 µm, which is essentially in agreement with the simulated results. [ABSTRACT FROM AUTHOR]

We applied a Monte Carlo simulation for studies of single-shot laser-induced breakdown in pure transparent dielectrics for the case when the photon energy is close to or exceeds mean electron energy. Avalanche ionization rate dependence on intensity was obtained by solving both the kinetic Fokker--Planck type equation and the quantum kinetic equation. Threshold scaling with pulse width was obtained in the range from 3 ps to 30 ns for several values of laser frequency and initial lattice temperature. Lattice heating influence (during the laser pulse action) on avalanche ionization rate was studied. [ABSTRACT FROM AUTHOR]

We present a direct comparison of two techniques for the determination of laser damage threshold. The two techniques are compared for their accuracy and repeatability and analyzed to understand the intrinsic biases of each test. We show that the damage frequency method underestimates the true value and has poor repeatability and the binary search method over-estimates the threshold, but is the more repeatable method. [ABSTRACT FROM AUTHOR]

The laser-induced deflection (LID) technique, a photo-thermal deflection setup with transversal pump-probe-beam arrangement, is applied for sensitive and absolute absorption measurements of optical materials and coatings. Different LID concepts for bulk and transparent coating absorption measurements, respectively, are explained, focusing on providing accurate absorption data with only one measurement and one sample. Furthermore, a new sandwich concept is introduced that allows transferring the LID technique to very small sample geometries and to significantly increase the sensitivity for materials with weak photo-thermal responses. For each of the different concepts, a representative application example is given. Particular emphasis is placed on the importance of the calibration procedure for providing absolute absorption data. The validity of an electrical calibration procedure for the LID setup is proven using specially engineered surface absorbing samples. The electrical calibration procedure is then applied to evaluate two other approaches that use either doped samples or highly absorptive reference samples. [ABSTRACT FROM AUTHOR]