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We have demonstrated an efficient speckle reduction method for laser illumination using a micro-vibrated paper screen along the projection direction. Using this method, a micro-vibrated amplitude of 0.532 μm, or a 2π phase change is sufficient to de-correlate the generated speckle pattern for a static SHG green laser beam image. When the measured speckle contrast is lowered to about 5.0%, and comparable with an LED source, a speckle-free image can be achieved. This technique will be suitable for compact and portable laser illumination equipment, image display, signal sensing devices, and so forth. Wearable and near-to-eye laser display applications are also included.

In situ ablation of thin metal films on fused silica substrates by picosecond class lasers was investigated as a method of characterizing the beam at the sample plane. The technique involved plotting the areas enclosed by constant fluence contours identified in optical microscope images of the ablation sites versus the logs of the pulse energies. Inconel films on commercially available neutral density filters as well as magnetron sputtered gold films were used. It was also shown that this technique could be used to calibrate real-time beam profile diagnostics against the beam at the sample plane. The contours were shown to correspond to the boundary where part or all of the film was ablated.

The specific points raised here augment the full paper on this same subject by Measures [Appl. Opt. 13, 1121 (1974)]. In support of the feasibility of such a laser, an electrochemiluminescent system with ø(ecl) ~0.2 is cited from the literature (vs ø(ecl) ~1.4 x 10(-2)); and in relation to the feasibility calculations, the incompleteness of the Feldberg model is noted. Various other topics are also discussed.

In low-Earth orbit, the already existing population of small and medium debris (between 1 cm and several dozens of cm) is a concrete threat to operational satellites. A space-based laser space debris removal (SLDR) system that can remove hazardous debris around selected space assets appears to be a flexible and effective project. To achieve high-precision tracking and emitting, the optical system of the SLDR mission includes a target-detection telescope and emitting telescope, adopting a common light path structure. The optical design results, system performance, tolerance budget, and detailed stray light control design are presented in this paper. The large-aperture off-axis two-mirror beam-narrowing system characteristics are also discussed in terms of stray light control. This paper will present the lateral-displacement (LD) setting, two-stage fore baffle design, black baffle surface selection, and opening direction of the telescope door. The results showed that the stray light elimination reaches a 10 − 9 order, meeting design requirements.

The sidelobe phenomenon of acousto-optic tunable filter (AOTF)-based hyperspectral imaging is the result of the phase mismatching condition, which will reduce the quality of hyperspectral imaging data. This phenomenon can be observed in two specified scenes, namely, high brightness panchromatic target imaging and high brightness monochromatic target imaging. In this paper, the acousto-optic interaction in these two scenes is discussed using the phase matching condition. As a follow-up work of the previous related research, this paper studies laser interference in AOTF imaging, which is a typical example of the second case. A method to remove laser sidelobes in AOTF-based hyperspectral imaging is proposed, which combines a series of procedures including laser point extraction, reverse imaging before AOTF, ray tracing in AOTF, forward imaging process after AOTF, and laser elimination. Experimental results have verified the effectiveness of the proposed method.

This paper presents the results of application of the laser speckle correlation (LSC) technique for studying the combustion process of aluminum-based nanopowders. For assessing the results, a combined experimental scheme is proposed with simultaneous application of LSC analyses and another feasible method of nanopowder combustion study, i.e., laser monitoring. In this paper, we present the principle of using the LSC technique to characterize the surface changes of nanopowder during combustion. Calculating the correlation coefficient of image sequencing, it is possible to estimate the time parameters of combustion of aluminum nanopowder and an aluminum mixture with iron nanopowder. Comparing the results obtained with the LSC method and laser monitoring, we conclude that LSC is quite acceptable for investigating the combustion process of metal nanopowders. In contrast with laser monitoring, the LSC method allows us to determine the preheating period preceding the first combustion wave. In practice, the LSC method application for nanopowder combustion control is prospective because of the simple hardware implementation.

Taking the influence of alloying elements into consideration, a hardness prediction model based on a newly proposed parameter named actual carbon diffusion time (ACDT), which was calculated by combining temperature history with carbon diffusivity, is developed in this paper. The hardness distribution was directly derived from a 3D thermal model, which was constructed to simulate the laser surface hardening of AISI 1045, and from which the temperature history was also obtained. Using this 3D thermal model, the geometric characteristics after laser hardening (e.g., the width of the hardened layer, the depth of the phase change layer) were determined according to the martensitic transformation. The influences of the scanning speed and the laser power on the geometric characteristics were investigated, and the relations among the geometric characteristics after laser hardening were further determined. The laser hardening experiments with various parameters were conducted, and it was found that the hardness can be accurately predicted using the developed hardness prediction model (R-squared, 97.0%). The proposed parameter ACDT, as the core of the hardness prediction model in this paper, broadens the way for hardness prediction in laser hardening with the alloying effect involved.

In recent years, Raman spectroscopy techniques have been successfully applied to the area of deep-sea exploration. However, there are still some problems impeding the further application of Raman systems. For example, the large size of an underwater Raman system makes it difficult to deploy on the underwater vehicle. Meanwhile, the sensitivity is often a disadvantage, requiring improvement for detecting more trace components. To solve these problems, a new compact deep-sea in situ Raman spectroscopy system is presented in this paper. The whole system weighs 60 kg and is housed in an L800 mm×ϕ258 mm pressure vessel with an optical window on the front end cap. The main components include a 532 nm Nd:YAG laser, an optics module, a high-throughput spectrograph with 0∼4900 cm−1 spectral range and 8 cm−1 spectral resolution, a TEC-cooled 2000 pixel×256 pixel CCD detector, a PC104 embedded computer, and an electronics module. To evaluate the performance of the newly developed Raman system, systematic experiments have been carried out with solutions in laboratory, and the results have shown that the system limit of detection of SO42− is 0.4 mmol/L. The Raman system has been successfully deployed on a remote-operated vehicle on the Kexue research vessel in June 2015. The typical in situ detection results are presented in this paper, and it is shown that the Raman system is capable of detecting the Raman signal of SO42− and fluorescence of chlorophyll a (chl-a) and chromophoric dissolved organic matter (CDOM) in seawater. With 500 spectra accumulations and some data processing, the Raman signal of HCO3− is obtained. This is the first report of direct measurement of HCO3− by Raman system in in situ experiments. After further optimization, it is hoped to apply the Raman system in seafloor observation networks for long-time carbon cycling research.

In this paper, quantitative experiments were made to measure the concentration of key intermediate products (CO, N2O, and NO) and the gas temperature for combustion flow based on near-infrared and mid-infrared laser absorption spectroscopy. This paper used the developed diagnostic system to study two main ignition modes of a real 1-Newton thruster based on ammonium dinitramide (ADN): steady-state firing and pulse-mode firing over a feed pressure of 5–12 bar. The steady-state firing experiments distinguished the whole process into catalytic decomposition stage and combustion stage, experimentally demonstrating the combustion kinetics mechanism of an ADN monopropellant. Experiments for pulse-mode firing showed the measured multispecies concentration and temperature were consistent with pulse trains, verifying good performance for the thruster pulse-mode firing operation. The performance of the thruster was given based on the optical measurements, and characteristic velocity for the ADN-based thruster standard operation was higher than the corresponding 1-Newton hydrazine thruster.

The amplified spontaneous emission (ASE) effect is a major factor affecting large, high gain Yb:YAG thin disk lasers. In this paper, the analytical model is built to estimate the ASE photon flux density of thin disk crystals with different thickness anti-ASE caps. Theoretical analysis shows that it is unnecessary to use a critical thickness anti-ASE cap to suppress the ASE effect. At the same time, adjusting the coupler reflectivity of the resonator is an effective way to decrease the ASE effect and decrease the thickness of the anti-ASE cap to effectively suppress the ASE effect. This paper will better reveal how the anti-ASE cap suppresses the ASE effect.

In this erratum we clarify our previously published paper [Appl. Opt. 57, 4008 (2018)APOPAI0003-693510.1364/AO.57.004008], where we used a solar spectrum truncated to a maximum wavelength of 830 nm in the numerical modelling, but did not state this in the paper. Here, we present a graph of the numerically modelled absorption in the Nd:YAG rod as a function of the diffuse reflectivity of the chamber walls using the full solar spectrum, confirming that the theoretical maximum possible absorption we predict is in agreement with literature values.

This paper presents a modeling approach of a laser varifocal system for longitudinal Gaussian beam waist shifting. The developed approach is based on laser optics theory, which is considered to be classical optics generalized for coherent radiation. A basic optical system, consisting of two components, was developed using this theory. Components have fixed focal lengths and specific movements. An automated modeling algorithm was proposed, and an example was provided in text. Various systems shown in this paper form a Gaussian beam with required waist parameters and provide its longitudinal shifts, which exceed the length of the near-zone of a focused Gaussian beam. Such systems can be used in laser technologies, including micro- and nano-sized objects manipulation.

Phase shift between the injection current and amplitude modulation due to the characteristics of diode lasers is discussed in this paper. Phase shift has no apparent regularity, but it has an obvious effect on measurement results, especially for high-precision measurement. A new method is proposed to suppress the influence of this phase shift. Water vapor is chosen as the target gas for experiment in this paper. A new detection system with the new method applied is presented and shows much better performance than the traditional wavelength modulation spectroscopy detection system. Phase shift fluctuation between the injection current and amplitude modulation is suppressed from 0.72 deg to 0.07 deg; accuracy is improved from 0.88 ppm to 0.16 ppm.

This paper presents a proposal of an optically reconfigurable gate array using a colored configuration. The optically reconfigurable gate array consists of a very-large-scale integration (VLSI), a holographic memory, and four lasers with different wavelengths. The optically reconfigurable gate array VLSI includes a fine-grained programmable gate array as well as field programmable gate arrays. Four configuration contexts can be stored on the holographic memory and can be programmed onto the programmable gate array VLSI addressed by the four lasers. This paper presents the demonstration of the optically reconfigurable gate array using a colored configuration.

Speckle is the main obstacle for the use of laser light sources in projection technology. This paper focuses on speckle suppression by the reduction of temporal coherence which is provided by the broadband laser light. The investigation of the effect of laser spectrum width and multiple lasers on speckle contrast is discussed. A broader spectrum width of the laser light is attained by the use of multiple semiconductor laser diodes of the broad area type. Measurements of speckle contrast with and without angle diversity are performed for two and four laser diodes. The measurement of speckle contrast for a single laser diode is also presented for comparison. The experimental results show that multiple laser diodes provide lower speckle contrast as compared to a single laser diode. In addition, it is also shown in this paper that the wavelength distribution of independent laser diodes has an effect on speckle contrast. Two different types of blue laser diodes, Nichia NUB802T and Nichia NUB801E, which have slightly different central wavelengths, were used for the measurements. Four laser diodes with a combination of two types of laser diodes offer better speckle contrast reduction than four laser diodes of the same type due to an effective broader spectrum. Additional speckle contrast reduction is achieved through the angle diversity by using a dynamic deformable mirror.

This paper provides the results of work on a new method for metrological laser beam steering for the purpose of angular stabilization. Frequency stabilized metrological lasers are used for interference measurements of length and angle. Angular deviation of the beam may cause cosine errors when measuring displacements. This paper presents a method for metrological laser beam deflection with an air wedge of variable refractive index. The change in the index is achieved by changing the air pressure in the wedge volume. The implemented system produced a range of angular displacements of the 0.1 mrad (10−4 rad; 100 μrad) beam with a resolution of 10−7 rad.

This paper describes an alternative automated setup for phase-shifting and phase retrieval based on the wavelength tunability of a laser diode. For many industrial and scientific applications it is desirable that the laser gets stabilized. In this paper, on the contrary, the laser diode is turned on and off for a short period of time. The generated variations of voltage and temperature cause a modulation of its wavelength. This modulation is used for a simple demonstration of the phase-shifting phenomena and phase retrieval under the scheme of an unbalanced Twyman-Green interferometer and using the Carre algorithm. The on-off switching of the laser can be done manually, but for this proposal, it will be emulated by a pulse voltage signal from a data acquisition device. (C) 2015 Optical Society of America

Various existing target ranging techniques are limited in terms of the dynamic range of operation and measurement resolution. These limitations arise as a result of a particular measurement methodology, the finite processing capability of the hardware components deployed within the sensor module, and the medium through which the target is viewed. Generally, improving the sensor range adversely affects its resolution and vice versa. Often, a distance sensor is designed for an optimal range/resolution setting depending on its intended application. Optical triangulation is broadly classified as a spatial-signal-processing-based ranging technique and measures target distance from the location of the reflected spot on a position sensitive detector (PSD). In most triangulation sensors that use lasers as a light source, beam divergence-which severely affects sensor measurement range-is often ignored in calculations. In this paper, we first discuss in detail the limitations to ranging imposed by beam divergence, which, in effect, sets the sensor dynamic range. Next, we show how the resolution of laser-based triangulation sensors is limited by the interpixel pitch of a finite-sized PSD. In this paper, through the use of tunable focus lenses (TFLs), we propose a novel design of a triangulation-based optical rangefinder that improves both the sensor resolution and its dynamic range through adaptive electronic control of beam propagation parameters. We present the theory and operation of the proposed sensor and clearly demonstrate a range and resolution improvement with the use of TFLs. Experimental results in support of our claims are shown to be in strong agreement with theory.

The inter-satellite laser link is the backbone of the next inter-satellite information network, and ranging and communication are the main functions of the inter-satellite laser link. This study focuses on the inter-satellite laser ranging based on the pseudo-code correlation technology. In this paper, several typical laser-ranging methods have been compared and we determined that the laser pseudo-code ranging architecture is more suitable for the inter-satellite laser communication link. The pseudo-code ranging system is easy to combine with a digital communication system, and we used it to calculate integer ambiguity by modulating the time information. The main challenge of the ranging system is range precision, which is the main focus of this paper. First, the framework of the pseudo-code ranging system is introduced; the ranging architecture of dual one-way ranging is used to eliminate the clock error between the two transceivers, and then the uncertainty of the phase detector is analyzed. In the analysis, the carrier to noise ratio and the ranging code rate are constrained by the laser communication link margin and the electronic hardware limitation. Therefore, the relationship between the sampling depth and the phase detector uncertainty is verified. A series of optical fiber channel laser pseudo-code ranging experiments demonstrated the effects of sampling depth on the ranging precision. By adjusting the depth of storage, such as the depth of 1.6 Mb, we obtained a pseudo-range measurement precision of 0.33 mm (1σ), which is equivalent to 0.0001 times code subdivision of 10.23 Mcps pseudo-code. This paper has achieved high precision in a pseudo-range measurements, which is the foundation of the inter-satellite laser link.

After the demonstration of the occurrence of visual artifacts with an active imaging system in burst mode in a previous paper, the analysis of this phenomenon was realized. A visual artifact resulting from a remote zone in the scene can appear in the image of the real visualized zone when the duty cycle of laser pulses is close to 50%, as in the burst mode. Therefore, the elements of this remote zone will create confusion in the image, with erroneous estimated distances. These misinterpretations can be very embarrassing to those attempting to determine the distance of a target in the scene. From the modeling realized and validated in the previous paper, the behavior of the visual artifact was analyzed with two types of burst mode used in active imaging, the duration of the laser pulse being identical to the duration of the temporal aperture of the imager. In the first mode, the width of the visualized zone is set, depending on the distance. The second mode increases the width of the visualized zone so that the foreground of the zone is constantly visible. The results showed that the distance of the visual artifacts in variable mode increased much more quickly than the distance in fixed mode. In both modes, the most intense visual artifacts appear when the range of the visualized zone remains within the first kilometer. When this range is very short, the illuminance of the visual artifact in fixed mode is much more intense than the illuminance in variable mode. On the other hand, for long distances, the illuminance of the visual artifact in variable mode is greater than the illuminance in fixed mode, but decreases quickly beyond a certain distance, making it insignificant.

Nanofluids have been increasingly used in a wide range of thermal applications. Although these applications can benefit greatly from investigating the behavior of nanoparticles under different heating scenarios, there is a lack of experiments that can achieve this. To overcome this challenge, an optical “pump–probe”-type experiment is suggested in this paper. In experiments of this type, a set of “pumping” nanoparticles are specifically selected to absorb laser radiation. These particles represent a flexible tool for volumetric heating. A second set of “probing” nanoparticles can be tailored to scatter a separate optical probing signal. This work presents a selection procedure for nanoparticles of both types. The selection procedure is then demonstrated for a specific example where the pump and probe wavelengths are of 980 and 532 nm, respectively. Gold nanorods with diameters of 10 and a length of 58 nm are selected as the “most suitable” absorbing particles, while silver nanospheres with a diameter of 110 nm are selected as the “most suitable” scattering particles. These particles are synthesized and shown to experimentally match the desired optical properties. Overall, this paper proposes and demonstrates an approach by which it is possible to design and fabricate particles for a wide range of optical studies in semi-transparent nanofluids.

The range gated technique is a promising laser ranging method that is widely used in different fields such as surveillance, industry, and military. In a range gated system, a reflected laser pulse returned from the target scene contains key information for range reconstruction, which directly affects the system performance. Therefore, it is necessary to study the characteristics and effects of the target reflection factor. In this paper, theoretical and experimental analyses are performed to investigate the influence of target reflection on three-dimensional (3D) range gated reconstruction. Based on laser detection and ranging (LADAR) and bidirectional reflection distribution function (BRDF) theory, a 3D range gated reconstruction model is derived and the effect on range accuracy is analyzed from the perspectives of target surface reflectivity and angle of laser incidence. Our theoretical and experimental study shows that the range accuracy is proportional to the target surface reflectivity, but it decreases when the angle of incidence increases to adhere to the BRDF model. The presented findings establish a comprehensive understanding of target reflection in 3D range gated reconstruction, which is of interest to various applications such as target recognition and object modeling. This paper provides a reference for future improvement to perform accurate range compensation or correction.

A mid-infrared supercontinuum (SC) light source, which has important applications in many fields, has been extensively investigated in soft glass fibers. However, the poor instinct properties of soft glass fibers and the development of ultrashort pulse lasers left an opportunity for mid-infrared SC generation in silica fiber. Until now, silica fiber has been the commonly used medium for SC generation due to its outstanding properties. In this paper, mid-infrared SC generation in short silica photonic crystal fibers (PCFs) is investigated theoretically and systematically. In the case of a 1550�nm pump, the soliton self-frequency shift effect is utilized to extend the long wavelength edge of SC. Adopting a fiber that has a zero dispersion wavelength away from the pump pulse is a benefit for the suppression of blue spectral component and energy distribution in the long wavelength band. In the case of a 1950�nm pump, the generation of a red-shifted dispersive wave is an efficient way to extend the long wavelength edge of SC. Additionally, the coherence for femtosecond pulse pumping is discussed in this paper. Finally, the long wavelength edge of SC is beyond 3000�nm when a 1950�nm femtosecond pump pulse propagates in a PCF with negative dispersive slope around the pump pulse.

This paper presents a short pulse-width gain-switched Ho:YAG ceramic laser at 2089 nm resonantly pumped by a homemade Q-switched Tm:fiber laser at ∼1908 nm. We generated stable pulses of 44-103 ns duration and 0.13-2 kW peak power at 20 kHz of pulse repetition frequency when the incident pump pulse energy increased from ∼0.1 to 0.3 mJ. We also obtained an average output power of 1.76 W under an incident pump power of 6 W, corresponding to a slope efficiency of 41.8%. Finally, this paper discusses the prospects to further improve the results with even a shorter pulse-width and higher peak power.

Approaches that are, to our knowledge, novel, are proposed in this paper to improve the accuracy performance of self-mixing interferometry (SMI) for displacement measurement. First, the characteristics associated with signals observed in SMI systems are studied, based on which a new procedure is proposed for achieving accurate estimation of the laser phase. The studies also revealed the reasons for the inherent errors associated with the existing SMI-based techniques for displacement measurement. Then, this paper presents a new method for estimating the optical feedback level factor (denoted by C) in real time. Combining the new algorithms for estimating the laser phase and updating C value, the paper finally presents a novel technique for displacement measurement with improved accuracy performance in contrast to existing techniques. The proposed technique is verified by both simulation and experimental data.

This issue of Applied Optics features papers on the application of laser technology to chemical analysis. Many of the contributions, although not all, result from papers presented at the Fourth OSA Topical Meeting on Laser Applications to Chemical Analysis, which was held at Jackson Hole, Wyoming, March, 1994. This successful meeting, with nearly one hundred participants, continued the tradition of earlier LACA meetings to focus on the optical science of laser-based measurements of temperature and trace chemical assays in a wide variety of practical applications.

The tenth annual Symposium on Optical Materials for High Power Lasers (Boulder Damage Symposium) was held at the National Bureau of Standards in Boulder, Colorado, 12-14 September 1978. The symposium was held under the auspices of ASTM Committee F-1, Subcommittee on Laser Standards, with the joint sponsorship of NBS, the Defense Advanced Research Project Agency, the Department of Energy, and the Office of Naval Research. About 175 scientists attended, including representatives of the United Kingdom, France, Canada, Japan, West Germany, and the Soviet Union. The symposium was divided into sessions concerning the measurement of absorption characteristics, bulk material properties, mirrors and surfaces, thin film damage, coating materials and design, and breakdown phenomena. As in previous years, the emphasis of the papers presented was directed toward new frontiers and new developments. Particular emphasis was given to materials for use from 10.6 microm to the UV region. Highlights included surface characterization, thin film-substrate boundaries, and advances in fundamental laser-matter threshold interactions and mechanisms. The scaling of damage thresholds with pulse duration, focal area, and wavelength was also discussed. In commemoration of the tenth symposium in this series, a number of comprehensive review papers were presented to assess the state of the art in various facets of laser induced damage in optical materials. Alexander J. Glass of Lawrence Livermore Laboratory and Arthur H. Guenther of the Air Force Weapons Laboratory were co-chairpersons. The eleventh annual symposium is scheduled for 30-31 October 1979 at the National Bureau of Standards, Boulder, Colorado.

The Sixth ASTM-ONR-NBS Symposium on Laser Induced Damage in Optical Materials was held at the National Bureau of Standards in Boulder, Colorado on 22–23 May 1974. Over 150 attendees at the Symposium heard thirty-one papers on topics relating to laser induced damage in crystalline and nonlinear optical materials, at dielectric surfaces, and in thin film coatings as well as discussions of damage problems in the ir region due both to cw and pulsed irradiation. In addition, several reports on the theoretical analysis of laser–materials interaction relative to the damage progress were given, along with tabulations of fundamental materials properties of importance in evaluation of optical material response to high-power laser radiation. Attention was given to high-power laser system design considerations that relate to improved system performance and reliability when various damage mechanisms are operable in such systems. A workshop on the machining of optics was held, and nine papers on various facets of the topic were presented dealing with machining procedures, surface characterization of machined elements, coating of machined components, and the polishing and damage resistance of polished, coated, and bare metal reflectors.

During the 1973 Symposium on Damage in Laser Materials several specific topics received considerable attention. The papers presented to the approximately 135 specialists in attendance can logically be separated into categories on self-focusing damage, surface damage, coating damage, infrared component damage, and theory and fundamental properties. Highlighting the meeting were papers on small scale self-focusing, the importance of surface or near-surface absorbing impurities as a major extrinsic influence in reducing the intrinsic damage level of coated and uncoated surfaces, development of new instrumental methods for evaluation of material properties important in the damage process, initial attempts at cataloging these important properties, and, finally, several reports on the frequency and temporal aspects of laser-induced damage in a variety of materials, from the visible to the infrared, and from subnanosecond pulsed to continuous wave exposures.

A great variety of compounds occur in nature or have been synthesized in the laboratory that crystallize with the apatite structure. We have investigated a number of the apatites and found them to be excellent laser hosts for neodymium and holmium. The apatites described in this paper were grown using the Czochralski method, have low optical losses in the pump and emission spectral regions for neodymium and holmium, and the hosts have been developed to readily accept large concentrations of doping ions. This paper describes the crystal growth, physical properties, spectroscopy, and laser performance of this family of new laser materials.

In an accompanying paper [Appl. Opt. 40, 783-793 (2001)], we predict the existence of background signals from a frequency-doubled wavelength-modulated diode-laser system. We now demonstrate and characterize various nf harmonics of such background signals from a system producing light in the 422-nm region by use of a single-pass KNbO(3) crystal with respect to the modulation amplitude, the laser center frequency, and the crystal temperature. It is demonstrated that 2f detection is plagued by considerably larger amounts of background signal than is detection at other higher, even harmonics. This result implies that 4f or 6f detection is often to be preferred in comparison with 2f detection when frequency-doubled wavelength-modulation spectrometry (WMS) is to be used. This preference is illustrated by the detection of Ca in an acetylene-air flame. It is also shown that the background signals have a much stronger dependence on the modulation amplitude than do the analytical signals. This difference implies that the optimum detectability for frequency-doubled WMS is often reached for modulation amplitudes lower than those normally used. An analysis of the effect of a finite temperature stability of the doubling crystal on the drift of the background signals as well as on the detectability is included. The results verify the theoretical description given in our accompanying paper.

The application of excimer lasers in ablation, cleaning, and restoration for the recovery of paper and parchment manuscripts is a recently implemented technique. A report of the use of excimer lasers in a cleaning process by which mud was removed from Islamic papers and parchments is presented. It was found that, because of the close proximity of the binding energies of paper to paper and of paper to mud, it was difficult to maintain control of the ablation process. However, the substrate was not affected. Spectrocolorimetry was used as a technique to detect the effects of ablation on cleaned areas of the manuscripts in terms of change in color appearance and severity of aging postablation. The analysis was performed by comparison of treated and untreated areas. Mathematical modeling was developed to define a representative original color and a color-distribution parameter. Improvements in the measuring method were made to yield the required precision for evaluating differences in color produced by laser ablation and to follow the color evolution after ablation. Results show that the effects of restoration, aging, and the environmental conditions can be individually identified under certain conditions. The method has applications in other domains.

This paper demonstrates an all-fiber narrow linewidth amplifier with a seed based on narrow linewidth fiber Bragg gratings (FBGs). The fiber amplifier achieves a narrow bandwidth output of 823 W, with an opto-optic efficiency of 84.5%. The pair of FBGs in the seed configuration helps to assure a narrow linewidth of the laser as 0.08 nm. In the laser profile, we introduce a cladding stripper with a sectional structure, which realizes high pump power leakage with high efficiency. The paper also discusses the impact of seed linewidth and fiber length on the SBS threshold in a narrow bandwidth laser. Based on this analysis, we discovered ways to inhibit SBS onset and scale power output.

The ablation depths of a 200 nm thick nickel film by ultrafast lasers were simulated and improperly compared with the existing numerical results for a bulk nickel target in our paper [Appl. Opt.54, 1737 (2015)10.1364/AO.54.001737APOPAI1559-128X]. The ultrafast laser ablation of a 1 μm thick nickel film is, therefore, simulated here, which demonstrates that the conclusions in our published paper are not affected and that the adopted 1 μm film is sufficiently thick to be used for the simulation of the ablation behavior of a bulk nickel target subject to 1 ps laser irradiation.

The application of tunable diode laser absorption spectroscopy (TDLAS) to flames with non-homogeneous temperature and concentration fields is an area where only few studies exist. Experimental work explores the performance of tomographic reconstructions of concentration and temperature profiles from wavelength-modulated TDLAS measurements within the plume of an axisymmetric McKenna burner. Water vapor transitions at 1391.67 nm and 1442.67 nm are probed using calibration free wavelength modulation spectroscopy with second harmonic detection (WMS-2f). A single collimated laser beam is swept parallel to the burner surface, where scans yield pairs of line-of-sight (LOS) data at multiple radial locations. Radial profiles of absorption data are reconstructed using Tikhonov regularized Abel inversion, which suppresses the amplification of experimental noise that is typically observed for reconstructions with high spatial resolution. Based on spectral data, temperatures and concentrations are calculated point-by-point. Here, a least-squares approach addresses difficulties due to modulation depths that cannot be universally optimized due to a non-uniform domain. Experimental results show successful reconstructions of temperature and concentration profiles based on two-transition, non-optimally modulated WMS-2f and Tikhonov regularized Abel inversion, and thus validate the technique as a viable diagnostic tool for flame measurements., Comment: This paper was published in Applied Optics and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://dx.doi.org/10.1364/AO.53.008095. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law

Single-mode lasing for small size semiconductor laser is significantly important in the on-chip optical signal processing, data storage, and dense optical integrated systems. This paper presents new, to the best of our knowledge, single-mode quantum well microlasers by distributing periodical strain along the longitudinal laser cavity. The quantum transmission line modeling (Q-TLM) method is employed to establish the model for strained microlasers. The dynamic output of quantum well microlasers with longitudinal periodical strain (LPS) distribution is analyzed in the time and frequency domains, and it is found that the introduction of LPS significantly improves the single-mode output of quantum well microlasers by increasing the side mode suppression ratio (SMSR) from 8.44 to 28.29 dB. The study results confirm that well-controlled periodical strain along the longitudinal laser cavity provides an alternative routine for realizing single-mode lasing by strain engineering.

The realization of output of the controllable transverse mode in a laser resonator has always been the key problem in applications of lasers. At present, the theory of optical resonators for passive resonators is relatively mature, but the non-uniformity of gain media greatly affects the output of the laser transverse mode for astigmatic resonators in operation; especially for gas lasers, controllable high-order modes have not been studied. To realize the theory of an astigmatic passive resonator of a gas laser as a good approximation of an active resonator, this paper develops the theory of selecting the laser eigenmode through an astigmatic resonator, and verifies that the two-dimensional tilt of the cavity mirror can break the axial symmetry. Controllable output of the laser mode is realized in real time and conveniently for the first time, to the best of our knowledge. This scheme is not only common to all kinds of lasers, but also has important research significance for the high-order modes required for real-time and rapid regulation of gas lasers under operating conditions.

In this paper, we demonstrate a design for a laser picoprojector with a high rate of reduction in the speckle contrast value. The method utilizes a holographic diffuser, deformable mirror, and fast electrically tunable (FET) lens. Then we compare the reduction rate of the speckle contrast value for holographic diffusers with different divergence angles and an FET lens with different optical power ranges combined with a deformable mirror. The increase in reduction rate of speckle contrast value means that the minimum value of the speckle can be reached faster. There reduction rate increases from 4.76 to 5.74 for the 5 × 5 holographic diffuser, 3.45 to 5.66 for the 10 × 10 holographic diffuser, and 1.52 to 4.58 for the 30 × 30 holographic diffuser.

This paper presents the concept of constant-fluence amplification. It refers to the fact that laser fluence anywhere in the amplifier gain medium is identical to the design fluence E(design), which represents the maximum allowed fluence in consideration of the laser-induced damage threshold (LIDT). Based on this concept, we explore ideal amplifier model (IAM) and near-ideal amplifier chain (NIAC) and propose an optimal NIAC design method and analytically offer an indicator of the extraction efficiency. Finally, the paper discusses the influences of losses on the NIAC's extraction efficiency.

As a new type of carbon-based material, carbon nanofibers (CNFs) have attracted much attention due to their unique physical structure and optical properties. In this paper, we propose the application of CNFs as the saturable absorber (SA) and established a passively mode-locked thulium-doped fiber laser (TDFL) for verification. By mixing sodium carboxymethyl cellulose solution with CNFs, CNF SA was prepared, the nonlinearity of which was tested as follows: the modulation depth was ∼ 1.3 % , and the saturation intensity was 18 M W / c m 2 . By inserting the CNF SA into the TDFL ring cavity, mode-locked laser pulses of a central wavelength of 1954.47 nm and a 3 dB bandwidth of 5.93 nm were obtained. The spectral pulse width was 1.31 ps; the repetition frequency was 32.68 MHz; and the signal-to-noise ratio (SNR) was calculated to be ∼ 57 d B . To our knowledge, this is the first time that CNFs have been reported as SAs for mode-locked lasers in the 2 µm wavelength region. Our work provides a new reference for using carbon-based materials in the realization of ultrafast lasers, and the proposed CNFs are highly advantageous in the development of ultrahigh-speed optical modulators and next-generation high-performance nonlinear photonic devices.

In the paper, we prepare a lead zirconate titanate saturable absorber, which is used to demonstrate a passively Q -switched (PQS) Tm:YAP laser. In the PQS mode, an average output power of 0.81 W and pulse width of 1.69 µs at 175 kHz are obtained at 1991.9 nm with a pump power of 12.76 W, corresponding to an optical–optical conversion efficiency of 6.35%. In addition, the beam quality factors M 2 of the PQS Tm:YAP laser in the x and y directions are 1.17 and 1.15, respectively.

In this paper, the concentration effects of Au nanoparticles placed in dye-doped polymeric spherical microlasers are investigated. The microlasers (average diameter of ∼ 293 µ m ) are made with a mixture of UV curable polymer named Norland Blocking Adhesive (NBA) and Rhodamine 6 G. Four different ratios (between the Au nanoparticles and the NBA solutions) are investigated here, namely, 1000, 2000, 3000, and 4000 ratios. The Au nanoparticles (size of ∼ 5 n m ) are randomly scattered within the microlaser. The light is collected via a multimode optical fiber, ending to a spectrometer/CCD camera. It is found that the 3000 ratio case exhibited the lowest energy threshold value and the highest photonic emission slope. The 4000 ratio (lowest concentration) exhibited a behavior that was very similar to a microlasers with no Au particles. In terms of longevity of the laser modes, the 3000 ratio case exhibited the most stable emission, although the laser mode disappeared at an earlier time. The emission of the 2000 ratio case dropped drastically after a few seconds but increased after that before dropping again; in this case, the TE and TM laser modes were found to be in competition with each other due to the partial overlapping of the plasmonic emission with some of the resonant cavity modes and due to the thermal expansion effect. The quality factor was found to be of the same order of magnitude for all cases ( ∼ 10 4 ). Ultimately, this work allows us to select the optimum microlaser’s configuration in terms of Au nanoparticle concentration as well as laser mode emission and longevity for different mechanical and biomedical sensing applications.

This paper proposes a method of superimposing acceleration and optimizing the optical reconfiguration speed while requiring no increase of laser power. Using this technique, the optical reconfiguration speed is increased by superimposing multiple configuration contexts. Simultaneously, optimization of the number of configuration contexts and reconfiguration speed is possible. A full four-context optically reconfigurable gate array system consisting of an optically reconfigurable gate array VLSI, an easily rewritable liquid crystal holographic memory, and four vertical-cavity surface-emitting lasers was constructed to demonstrate this method. This paper clarifies the method's benefits using experimental results obtained from the demonstration system.

This paper demonstrates the application of a thermometry method in turbulent sooting non-premixed flames using filtered Rayleigh scattering (FRS). Fuel tailoring is used to establish a specific C 2 H 2 -based fuel mixture such that temperature can be determined accurately by a single FRS measurement over the entirety of mixture fraction space, or equivalently, for all relevant thermo-chemical states. Evaluation is performed in a hierarchy of flows to establish measurement precision and accuracy. Initial assessments in a series of heated fuel mixtures; non-sooting, near-adiabatic flat flames; and laminar non-premixed sooting flames show accuracy of the approach over a full range of expected temperatures and high single-shot measurement precision (e.g., 65 < S N R < 80 ) for temperatures between 1900 and 2200 K. Subsequently, 2D temperature measurements are evaluated in a turbulent non-premixed sooting flame. For the current fuel mixture, the local mixture-averaged Rayleigh scattering cross section is nearly constant over all composition space, and thus traditional laser Rayleigh scattering (LRS) is used as a measurement standard in the absence of soot. The results show excellent agreement between the proposed FRS thermometry approach and LRS in non-sooting regions. In the presence of soot, the proposed FRS-based approach shows no signs of interference. In addition, 2D temperature imaging shows high SNR (60–74) over all temperature conditions. Thus, we believe the proposed methodology successfully provides a high-resolution 2D temperature method under turbulent sooting flame conditions.

Optical diagnostics of gas-phase pressure are relatively unusual. In this work, we demonstrate a novel, rapid, and robust method to use laser-induced grating scattering (LIGS) to derive this property in real time. Previous pressure measurements with LIGS have employed a signal fitting method, but this is relatively time-consuming and requires specialist understanding. In this paper, we directly measure a decay lifetime from a LIGS signal and then employ a calibration surface constructed using a physics-based model to convert this value to pressure. This method was applied to an optically accessible single-cylinder internal combustion engine, yielding an accuracy of better than 10% at all tested conditions above atmospheric pressure. This new approach complements the existing strength of LIGS in precisely and accurately deriving temperature with a simple analysis method, by adding pressure information with a similarly simple method.

Lithium niobate-on-insulator (LNOI) has been emerging as a popular integration platform for optical communications and microwave photonics. An edge coupler with high coupling efficiency, wide bandwidth, high fabrication and misalignment tolerance, as well as a small footprint is essential to couple light in or out of the LNOI chip. Some edge couplers have been demonstrated to realize fiber-to-chip coupling in the last few years, but the coupling with distributed feedback (DFB) semiconductor laser is rarely studied. In this paper, we propose a multi-tip edge coupler with three tips to reduce the mode size mismatch between the LNOI waveguide and the DFB laser. The tilted sidewall, fabrication tolerance, misalignment tolerance, and facet reflection due to the effective index mismatch are discussed. It shows that the proposed multi-tip edge coupler can be practically used in the production of effective LNOI integrated chips.

In this paper, the results of experimental characterization of an optical system consisting of a chromatic confocal displacement sensor integrated with an optical laser head are presented. As a result of integration, the part of the optical path of the displacement sensor was combined with the optical path of the laser beam. Consequently, a working distance of 250 mm for the optical system was achieved. The main aim of the characterization was to determine the calibration curve, as well as the application parameters of the system. The methodology for spectral data processing with particular emphasis on the effectiveness of various extraction algorithms is presented. The Lorentzian fitting was considered as the optimal algorithm for the optical system. Consequently, a large measuring range of 10 mm was obtained with perfect linear tendency of the calibration curve. The optical system was characterized by high accuracy equal to ± 0.11 % of the measuring range, as well as 1 µm resolution. Moreover, the functionality of the system was verified on materials commonly used in laser processing and additive manufacturing. Finally, the system was validated through a comparative measurement with a laser profiler.