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Much can and has been said about the relationship between the police and Black Americans. This topic has been thrust into mainstream conversation in recent years, with copious efforts aimed at fixing the problem. However, little seems to have changed. In this paper, I will utilize Calvin Warren’s Black Nihilism as a guiding lens in my evaluation of Washington state-level policy. I argue by examining Washington’s policies and crime data that Warren’s primary thesis—that the politics of hope is an unending, unsuccessful endeavor—is true not just theoretically, but in practice as well. Specifically, Washington’s recent milquetoast attempts at police reform highlight a fundamental misunderstanding of the structural problems inherent in the relationship between the police state and the Black community, resulting in never-ending policy failure.

Pulsed lasers with ultrashort pulse durations have become ubiquitous in a variety of applications, including laser eye surgery. Therefore, the role of nonlinear optical effects, such as supercontinuum generation, needs to be considered when evaluating their potential hazard. We used a NIR femtosecond laser to generate a supercontinuum within an artificial eye. We recorded the visible spectra of the supercontinuum generated and calculated the energy contained within the visible band. Our results indicate that for certain exposure conditions, the supercontinuum’s energy within the visible range surpasses the maximum permissible energy allowed for visible wavelengths by the laser safety standards.

Lasers with ultrashort pulse durations have become ubiquitous in various applications, including ocular surgery. Therefore, we need to consider the role of nonlinear optical effects, such as supercontinuum generation during propagation within the ocular media, when evaluating their potential hazard. We used a NIR femtosecond laser to generate a supercontinuum within an artificial eye. We recorded the visible spectra of the supercontinuum generated and calculated the energy contained within the visible band. Our results indicate that for wavelengths between 1350 nm and 1450 nm the energy contained within the visible band of the generated white light supercontinuum may surpass current safety exposure limits, and pose a risk of injury to the retina.

Advances in fiber laser technology have resulted in the increased use and availability of several high-power laser systems operating in the mid-infrared band with short switching times and high modulation rates. The current American National Standard for the Safe Use of Lasers (ANSI Z136.1-2014) defines the calculation of the maximum permissible exposure (MPE) on the skin in terms of the exposure duration a single pulse or the total exposure time limit, and is based on continuous-wave laser skin exposure minimum visible lesion (MVL) data. This study determined the MVL data thresholds in Yucatan miniature pig skin for multiple-pulse 1940-nm laser exposures with pulse repetition frequencies (PRFs) of 100, 200, and 1000 Hz, and trains of 300, 1000, or 3000 pulses. The individual pulse duration in each exposure train was 500 µs. We report the MVL thresholds as the median effective dose (ED50) based on varying individual pulse energy. The results highlight the effect of PRF on the thresholds for multiple-pulse cases. Comparison with the existing ANSI Z136.1 MPE limits provides a calculation of the safety margin for each parameter case.

Neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers are among the most commonly used lasers with a wide variety of applications from biomedicine to manufacturing. The ubiquity of these lasers increases the likelihood of accidental ocular injury resulting in permanent visual impairment. We performed dosimetry studies to determine retinal damage thresholds and hemorrhagic lesions in the porcine eye with Qswitched Nd:YAG lasers. The Yucatan miniature pig model exhibited similarities in ocular anatomy to human eyes. The Nd:YAG laser, tuned to 1064 nm with a pulse width of seven nanoseconds, delivered laser energy to the retina. Retinal imaging modalities including fundus photography, real-time video, confocal scanning laser ophthalmoscopy (cSLO), and spectral domain optical coherence tomography (SD-OCT) provided visualization of retinal morphology at multiple time points. Retinal damage thresholds were grouped into three categories: minimum visible lesion (MVL), contained hemorrhagic lesion (CHL), and vitreal hemorrhagic lesion (VHL). Probit analysis determined the effective dose for 50% probability of damage (ED50) for each lesion category. The threshold to produce a MVL was 0.193 mJ based on 24-hour assessments of the retina. The one-hour hemorrhagic lesion thresholds were 0.408 mJ and 1.52 mJ for CHL and VHL, respectively

Computational models predicting cell damage responses to transient temperature rises generated by exposure to lasers have implemented the damage integral (Ω), which time integrates the chemical reaction rate constant described by Arrhenius. However, few published reports of empirical temperature histories (thermal profiles) correlated with damage outcomes at the cellular level are available to validate the breadth of applicability of the damage integral. In our study, an analysis of photothermal damage rate processes in cultured retinal pigment epithelium cells indicated good agreement between temperature rise, exposure duration (τ), and threshold cellular damage. Full-frame thermograms recorded at high magnification during laser exposures were overlaid with fluorescence damage images taken 1 h postexposure. From the image overlays, pixels of the thermogram correlated with the boundary of cell death were used to extract threshold thermal profiles. Assessing photothermal responses at these boundaries standardized all data points, irrespective of laser irradiance, damage size, or optical and thermal properties of the cells. These results support the hypothesis that data from boundaries of cell death were equivalent to a minimum visible lesion, where the damage integral approached unity (Ω=1) at the end of the exposure duration. Empirically resolved Arrhenius coefficients for use in the damage integral determined from exposures at wavelengths of 2 μm and 532 nm and durations of 0.05–20 s were consistent with literature values. Varying ambient temperature (Tamb) between 20°C and 40°C during laser exposure did not change the τ-dependent threshold peak temperature (Tp). We also show that, although threshold laser irradiance varied due to pigmentation differences, threshold temperatures were irradiance independent.

Recent developments in high-energy regenerative amplifiers and broadly tunable optical parametric amplifiers (OPA) opened new spectral windows to study the impact of ultrashort laser pulses on biological tissues. These sources can generate extraordinarily high peak power capable of causing laser-induced breakdown. However, current laser safety standards (ANSI Z136.1-2014) do not provide guidance on maximum permissible exposure (MPE) values for the skin with pulse durations less than one nanosecond. This study measured damage thresholds in excised porcine skin in the mid-infrared (MIR) region of the electromagnetic spectrum. The laser system, comprised of a high-energy regenerative amplifier and OPA, was tuned to wavelengths between 4000-6000 nm to coincide with heightened absorption for both water and collagen. The laser operated at a fundamental repetition rate of 1 kHz and a nominal pulse width of 150 fs. The beam was focused at the sample surface with a 36X aluminum reflective objective and scanned over a 4 mm2 area for each exposure condition. Spectral domain optical coherence tomography (SD-OCT) imaging of the tissue provided a volumetric assessment of tissue morphology and identified changes in the backscattering profile within the laser-exposed regions. The determination of laser damage thresholds in the MIR for ultrafast lasers will guide safety standards and establish the appropriate MPE levels for exposure to sensitive biological tissue. These data will help guide the safe use of ultrafast MIR lasers in emerging applications across a multitude of industries and operational environments.

Computational models are useful tools for simulating the thermal response of biological tissue to laser exposure. These models typically include a solution to a bio-heat equation and require a set of physical properties for each distinct tissue as inputs. In applications such as cauterization and surgery, the irradiated tissues may undergo severe heating, resulting in substantial denaturation and a probable change in one or more of their associated physical properties. While the wellestablished temperature-dependent behavior of water can approximate the changes in tissue thermal properties during heating, there is minimal available research on the dynamics of optical properties of tissue. This study characterized optical absorption and scattering of porcine skin tissues that had undergone temperature increases up to 90°C. We excised thin samples of porcine dermis and subcutaneous fat, placed them in a custom sealed tissue mount with a built-in temperature sensor, and raised them to various temperature intervals through submergence in a controlled hot water bath. Following heating, a series of goniometric spectrophotometry measurements of transmittance and reflectance, focusing on the near-infrared band, allowed for calculation of optical absorption and scattering coefficients as a function of tissue temperature and rate process model damage. The results allow for more accurate representation of tissue characteristics in computational models.

Understanding the optical properties of water is critical to both laser-tissue interactions as well as setting ocular laser safety standards. The nonlinear properties of water are responsible for supercontinuum generation; however, these effects are poorly understood for wavelengths longer than 1064 nm. A previous study suggested that the supercontinuum generation may convert retinal-safe femtosecond near-infrared pulses with wavelengths longer than 1064 nm into visible wavelength pulses that are above the maximum permissible exposure limit as defined by ANSI Z136.1-2014. To address this knowledge gap, we extend the Z-scan technique in distilled water to wavelengths between 1150 nm to 1400 nm, where linear absorption is strong. Utilizing wavelength tunable, nominally 100 fs laser pulses, we observe wavelength dependence of the nonlinear optical properties of water. The nonlinear refractive index at 1150 nm was consistent with measurements taken at 532 nm in previous studies, and was observed to increase at longer wavelengths. The nonlinear absorption was positive for wavelengths between 1150 nm and 1350 nm and reversed to saturable absorption at 1400 nm. Saturable absorption poses a previously unanticipated eye safety risk as current ocular laser safety standards assume strong absorption at 1400 nm. These results expand our current understanding of the nonlinear optical properties of water to wavelengths in the 1150 nm to 1400 nm region, and inform efforts to revise national and international exposure limits to account for retinal hazards due to nonlinear effects.

Computational models are capable of simulating the expected thermal response of biological tissue to laser irradiation. A typical laser tissue model accounts for optical energy deposition, heat transfer, and damage assessment, with the latter often represented by calculation of the Arrhenius integral. Previous studies have successfully employed these methods to predict skin damage thresholds at laser wavelengths with high absorption in water, and usually for single continuous-wave exposures. However, there remains a need for a robust and accurate predictive model in low-absorption, high-scattering cases, such as for exposures in the near-infrared region near 1000 nm. This study presents a framework for modeling laser irradiation of skin tissue at 1070 nm for both continuous-wave and pulsed exposures with durations ranging from 10−2 to 101 s. The authors derive an optical absorption coefficient for the epidermis that agrees with expected chromophore distribution and report the modeled skin thermal responses alongside surface thermography data from in vivo porcine exposures as validation of simulation accuracy. Comparisons of modeled damage thresholds calculated by the Arrhenius integral with documented experimentally determined minimum visible lesion ED50 data exhibit a high degree of agreement. The authors also provide new Arrhenius rate process coefficients of A = 2.74 × 1094 s−1 and Ea = 5.90 × 105 J/mol, determined from experimental thermal profiles with a unique method, that demonstrate more accurate threshold predictions than those used in previous modeling studies. The techniques outlined by this study provide a useful tool in assessing potentially hazardous near-infrared laser exposure scenarios.

In cases where a laser source produces a pattern that is asymmetric on the retina, a modeling-based approach can be used to calculate a retinal thermal response to predict damage and make meaningful comparisons to exposure limit trends. The SESE (Scalable Effects Simulation Environment) model is a full 3D thermal finite-volume model that can simulate multiple independently controlled laser sources with unique wavelength, spatial profile, pulse duration, and power. The model is well suited to evaluate asymmetric sources since there is not an assumed axial symmetry and the spatial profile of the laser can be adjusted as a function of time. Several examples of asymmetric sources include multi-fiber bundle exposures, scanning beams, and scintillating sources. We use the SESE model to predict trends in retinal injury threshold for these cases and to help inform the laser safety community by providing estimates for safe and unsafe levels of exposure. We also introduce factors that inform hazard levels based on relative exposure conditions.In cases where a laser source produces a pattern that is asymmetric on the retina, a modeling-based approach can be used to calculate a retinal thermal response to predict damage and make meaningful comparisons to exposure limit trends. The SESE (Scalable Effects Simulation Environment) model is a full 3D thermal finite-volume model that can simulate multiple independently controlled laser sources with unique wavelength, spatial profile, pulse duration, and power. The model is well suited to evaluate asymmetric sources since there is not an assumed axial symmetry and the spatial profile of the laser can be adjusted as a function of time. Several examples of asymmetric sources include multi-fiber bundle exposures, scanning beams, and scintillating sources. We use the SESE model to predict trends in retinal injury threshold for these cases and to help inform the laser safety community by providing estimates for safe and unsafe levels of exposure. We also introduce factors that inform hazard levels based on ...

Computational models are capable of quantifying the expected thermal response of biological tissue to laser irradiation. A typical laser-tissue model accounts for optical energy deposition, heat transfer, and damage assessment, with the later often represented by calculation of the Arrhenius integral. Previous studies have successfully employed these methods to predict skin damage thresholds at laser wavelengths with high absorption in water, and usually for single continuous-wave exposures. However, there remains a need for a robust and accurate predictive model in low-absorption, high-scattering cases, such as for lasers in the near-infrared (NIR) region near 1 µm, where a large volume of tissue is heated simultaneously. This study presents a framework for modeling laser irradiation of skin tissue at 1070-nm for both continuous-wave and pulsed exposures with durations ranging from 10−2 to 101 seconds. We report the modeled skin thermal responses alongside thermal camera recordings of in-vivo porcine exposures as validation of simulation integrity. Comparisons of modeled damage thresholds calculated by the Arrhenius integral with past experimentally-determined minimum visible lesion ED50 data demonstrate a high degree of accuracy. The techniques outlined by this study provide a useful tool in assessing potentially hazardous near-infrared laser exposure scenarios while informing future investigations into modeling skin laser exposure at these wavelength regions.Computational models are capable of quantifying the expected thermal response of biological tissue to laser irradiation. A typical laser-tissue model accounts for optical energy deposition, heat transfer, and damage assessment, with the later often represented by calculation of the Arrhenius integral. Previous studies have successfully employed these methods to predict skin damage thresholds at laser wavelengths with high absorption in water, and usually for single continuous-wave exposures. However, there remains a need for a robust and accurate predictive model in low-absorption, high-scattering cases, such as for lasers in the near-infrared (NIR) region near 1 µm, where a large volume of tissue is heated simultaneously. This study presents a framework for modeling laser irradiation of skin tissue at 1070-nm for both continuous-wave and pulsed exposures with durations ranging from 10−2 to 101 seconds. We report the modeled skin thermal responses alongside thermal camera recordings of in-vivo porcine exp...

Ultrafast lasers are capable of generating extremely high peak powers and inducing non-linear optical phenomena that may be hazardous to sensitive ocular tissue. The generation of broadband supercontinuum light from ultrafast near infrared (NIR) lasers is of particular interest because of the potential damaging effects on both the cornea and retina. Freshly excised porcine eyes and an artificial model eye proved to be suitable systems to investigate non-linear optical phenomena due to the similarity in size and optical properties to the human eye. A tunable optical parametric amplifier (OPA) with a nominal pulse with of 100 femtoseconds generated all laser exposures in the 1200-1500 nm range. A fiber-based spectrometer introduced into the back of excised porcine eyes through a small incision in the sclera and placed at the retinal plane measured spectral broadening of the NIR femtosecond laser pulses. Changes in the spectral profile of the supercontinuum light were observed at multiple pulse energies and monitored dynamically as the position of the beam focus was adjusted with respect to the corneal surface. Further consideration of non-linear optical hazards may be required to ensure the safe use of ultrafast lasers as they become more prevalent in life sciences, telecommunications, industrial processing, and biomedical applications.Ultrafast lasers are capable of generating extremely high peak powers and inducing non-linear optical phenomena that may be hazardous to sensitive ocular tissue. The generation of broadband supercontinuum light from ultrafast near infrared (NIR) lasers is of particular interest because of the potential damaging effects on both the cornea and retina. Freshly excised porcine eyes and an artificial model eye proved to be suitable systems to investigate non-linear optical phenomena due to the similarity in size and optical properties to the human eye. A tunable optical parametric amplifier (OPA) with a nominal pulse with of 100 femtoseconds generated all laser exposures in the 1200-1500 nm range. A fiber-based spectrometer introduced into the back of excised porcine eyes through a small incision in the sclera and placed at the retinal plane measured spectral broadening of the NIR femtosecond laser pulses. Changes in the spectral profile of the supercontinuum light were observed at multiple pulse energies and ...

Understanding the nonlinear properties of water is essential for laser surgery applications, as well as understanding supercontinuum generation in water. Unfortunately, the nonlinear properties of water for wavelengths longer than 1064 nm are poorly understood. We extend the application of the Z-scan technique in water to determine its nonlinear refractive index (n2) and nonlinear absorption (β) for wavelengths in the 1150–1400 nm range, where linear absorption is also significant. We observe the wavelength-dependent variation of the nonlinear properties of water around the water absorption band.

Ultrafast lasers have become a fixture in many biomedical, industrial, telecommunications, and defense applications in recent years. These sources are capable of generating extremely high peak power that can cause laser-induced tissue breakdown through the formation of a plasma upon exposure. Despite the increasing prevalence of such lasers, current safety standards (ANSI Z136.1-2014) do not include maximum permissible exposure (MPE) values for the cornea with pulse durations less than one nanosecond. This study was designed to measure damage thresholds in corneal tissue phantoms in the near-infrared and mid-infrared to identify the wavelength dependence of laser damage thresholds from 1200-2500 nm. A high-energy regenerative amplifier and optical parametric amplifier outputting ~100 femtosecond pulses with pulse energies up to 2 mJ were used to perform exposures and determine damage thresholds in transparent collagen gel tissue phantoms. Three-dimensional imaging, primarily optical coherence tomography, was used to evaluate tissue phantoms following exposure to determine ablation characteristics at the surface and within the bulk material. The determination of laser damage thresholds in the near-IR and mid-IR for ultrafast lasers will help to guide safety standards and establish the appropriate MPE levels for exposure sensitive ocular tissue such as the cornea. These data will help promote the safe use of ultrafast lasers for a wide range of applications.

Thermal damage rate processes in biological tissues are usually characterized by a kinetics approach. This stems from experimental data that show how the transformation of a specified biological property of cells or biomolecule (plating efficiency for viability, change in birefringence, tensile strength, etc.) is dependent upon both time and temperature. Here, two disparate approaches were used to study thermal damage rate processes in cultured retinal pigment epithelial cells. Laser exposure (photothermal) parameters included 2-μm laser exposure of non-pigmented cells and 532-nm exposures of cells possessing a variety of melanosome particle densities. Photothermal experiments used a mid-IR camera to record temperature histories with spatial resolution of about 8 μm, while fluorescence microscopy of the cell monolayers identified threshold damage at the boundary between live and dead cells. Photothermal exposure durations ranged from 0.05-20 s, and the effects of varying ambient temperature were investigated. Temperature during heat transfer using a water-jacketed cuvette was recorded with a fast microthermister, while damage and viability of the suspended cells were determined as percentages. Exposure durations for the heat transfer experiments ranged from 50- 60 s. Empirically-determined kinetic parameters for the two heating methods were compared with each other, and with values found in the literature.

Purpose. To investigate fundamental mechanisms of regimes of laser induced damage to the retina and the morphological changes associated with the damage response.Methods. Varying grades of photothermal, photochemical, and photomechanical retinal laser damage were produced in eyes of eight cynomolgus monkeys. An adaptive optics confocal scanning laser ophthalmoscope and spectral domain optical coherence tomographer were combined to simultaneously collect complementaryin vivoimages of retinal laser damage during and following exposure. Baseline color fundus photography was performed to complement high-resolution imaging. Monkeys were perfused with 10% buffered formalin and eyes were enucleated for histological analysis.Results. Laser energies for visible retinal damage in this study were consistent with previously reported damage thresholds. Lesions were identified in OCT images that were not visible in direct ophthalmoscopic examination or fundus photos. Unique diagnostic characteristics, specific to each damage regime, were identified and associated with shape and localization of lesions to specific retinal layers. Previously undocumented retinal healing response to blue continuous wave laser exposure was recorded through a novel experimental methodology.Conclusion. This study revealed increased sensitivity of lesion detection and improved specificity to the laser of origin utilizing high-resolution imaging when compared to traditional ophthalmic imaging techniques in the retina.

Remote chemically specific detection of trace impurities in the atmosphere from distances on the order of kilometers is an important problem from both an environmental and a national defense viewpoint. A new scheme is discussed consisting of the remote generation of a backward propagating stimulated Raman pulse. This pulse is then used to drive a coherent anti-Stokes Raman scattering scheme, resulting in a strong chemically specific signal propagating back to the detector.

The market for ultrafast lasers (picosecond and femtosecond laser pulses) is rapidly expanding and recent forecasts estimate a total market of over $1.4 billion USD by 2019. Ultrafast lasers have many applications in biomedicine, telecommunications, defense, materials processing, and general research. Despite the growing adoption of ultrafast laser technology, current safety standards (ANSI Z136.1-2014) do not include maximum permissible exposure (MPE) values for skin using laser applications with pulse durations less than one nanosecond. Moreover, the wavelength dependence of ultrafast laser exposure in the mid-infrared region of the spectrum has not been explored. Increasing prevalence of ultrafast lasers will likely lead to an increase in the number of adverse events from accidental exposures.Our objective was to use a tunable femtosecond regenerative amplifier and an optical parametric amplifier (Spectra-Physics, Irvine, CA) to investigate laser damage characteristics and exposure thresholds for tissue phantoms mimicking the cornea and skin. The laser system covers a range of wavelengths from 1200-2900 nm with pulse energies from tens of microjoules to three millijoules. Multiple parameters including wavelength, spot size, and pulse repetition were evaluated.Additional data observing ultrafast laser exposure to ocular tissue and skin are still needed to establish MPE values for the safety standards. This initial study will help guide future experiments, standards, and promote the safe use of ultrafast laser technology across a wide range of applications and disciplines.The market for ultrafast lasers (picosecond and femtosecond laser pulses) is rapidly expanding and recent forecasts estimate a total market of over $1.4 billion USD by 2019. Ultrafast lasers have many applications in biomedicine, telecommunications, defense, materials processing, and general research. Despite the growing adoption of ultrafast laser technology, current safety standards (ANSI Z136.1-2014) do not include maximum permissible exposure (MPE) values for skin using laser applications with pulse durations less than one nanosecond. Moreover, the wavelength dependence of ultrafast laser exposure in the mid-infrared region of the spectrum has not been explored. Increasing prevalence of ultrafast lasers will likely lead to an increase in the number of adverse events from accidental exposures.Our objective was to use a tunable femtosecond regenerative amplifier and an optical parametric amplifier (Spectra-Physics, Irvine, CA) to investigate laser damage characteristics and exposure thresholds for tissu...

Laser safety control measures are often based upon the laser hazard class for the system. The Class 3R accessible emission limit is commonly used as a threshold hazard class for the assignment of laser eye protection, medical surveillance, and other administrative and engineering control measures. In this paper, we review the ANSI Z136.1-2014 exposure limits and class 3R accessible emission limits, and compare historical bioeffects data in order to examine risks for exposure at the 3R level. Focus areas of most concern are the consideration of exposure to extended-source and multi-pulse exposure limits. This information is of use to laser safety programs when making decisions regarding a specific laser system’s control measures assignment and is assembled to inform future revisions to national and international safety standards.Laser safety control measures are often based upon the laser hazard class for the system. The Class 3R accessible emission limit is commonly used as a threshold hazard class for the assignment of laser eye protection, medical surveillance, and other administrative and engineering control measures. In this paper, we review the ANSI Z136.1-2014 exposure limits and class 3R accessible emission limits, and compare historical bioeffects data in order to examine risks for exposure at the 3R level. Focus areas of most concern are the consideration of exposure to extended-source and multi-pulse exposure limits. This information is of use to laser safety programs when making decisions regarding a specific laser system’s control measures assignment and is assembled to inform future revisions to national and international safety standards.

Optical imaging of fast events and processes is essential for understanding dynamics of complex systems. A bright flash of illuminating light is required to acquire sufficient number of photons for superior image quality. Laser pulses can provide extreme brightness and are typically employed to achieve high temporal resolution; however, the high degree of coherence associated with the lasing process degrades the image quality with speckle formation. Random lasers are low-coherence sources of stimulated emission and do not suffer from speckle, but are rather broadband and have a relatively low output power limiting the scope of their potential applications. In this report, we demonstrate the use of random Raman lasing as a novel imaging light source with unprecedented brightness for a speckle-free and narrowband light source. We showcase the advantages of a random Raman laser to image the nanosecond scale dynamics of cavitation formation in water and quantitatively compare these images to those taken with incoherent fluorescent emission and coherent laser light as illumination source.

Wide-field microscopy, where full images are obtained simultaneously, is limited by the power available from speckle-free light sources. Currently, the vast majority of wide-field microscopes use either mercury arc lamps, or LEDs as the illumination source. The power available from these sources limits wide-field fluorescent microscopy to tens of microseconds temporal resolution. Lasers, while capable of producing high power and short pulses, have high spatial coherence. This leads to the formation of laser speckle that makes such sources unsuitable for wide-field imaging applications. Random Raman lasers offer the best of both worlds by producing laser-like intensities, short, nanosecond-scale, pulses, and low spatial coherence, speckle-free, output. These qualities combine to make random Raman lasers 4 orders of magnitude brighter than traditional wide-field microscopy light sources. Furthermore, the unique properties of random Raman lasers make possible the entirely new possibilities of wide-field fluorescence lifetime imaging or wide-field Raman microscopy. We will introduce the relevant physics that give rise to the unique properties of random Raman lasing, and demonstrate early proof of principle results demonstrating random Raman lasing emission being used as an imaging light source. Finally, we will discuss future directions and elucidate the benefits of using random Raman lasers as a wide-field microscopy light source.

Photothermal damage rate processes in biological tissues are usually characterized by a kinetics approach. This stems from experimental data that show how the transformation of a specified biological property of cells or biomolecule (plating efficiency for viability, change in birefringence, tensile strength, etc.) is dependent upon both time and temperature. However, kinetic methods require determination of kinetic rate constants and knowledge of substrate or product concentrations during the reaction. To better understand photothermal damage processes we have identified temperature histories of cultured retinal cells receiving minimum lethal thermal doses for a variety of laser and culture parameters. These “threshold” temperature histories are of interest because they inherently contain information regarding the fundamental thermal dose requirements for damage in individual cells. We introduce the notion of time-integrated temperature (Tint) as an accumulated thermal dose (ATD) with units of °C s. Damaging photothermal exposure raises the rate of ATD accumulation from that of the ambient (e.g. 37 °C) to one that correlates with cell death (e.g. 52 °C). The degree of rapid increase in ATD (ΔATD) during photothermal exposure depends strongly on the laser exposure duration and the ambient temperature.

Anderson localization, also known as strong localization, is the absence of diffusion in turbid media resulting from wave interference. The effect was originally predicted for electron motion, and is widely known to exist in systems of less than 3 dimensions. However, Anderson localization of optical photons in 3 dimensional systems remains an elusive and controversial topic. Random Raman lasing offers the unique combination of large gain and virtually zero absorption. The lack of absorption makes long path length, localized modes preferred. The presence of gain offsets what little absorption is present, and preferentially amplifies localized modes due to their large Q factors compared with typical low Q modes present in complex media. Random Raman lasers exhibit several experimentally measured properties that diverge from classical, particle-like, diffusion. First, the temporal width of the emission being 1 to a few nanoseconds in duration when it is pumped with a 50 ps laser is a full order of magnitude longer than is predicted by Monte Carlo simulations. Second, the random Raman laser emission is highly multi-mode, consisting of hundreds of simultaneous lasing modes. This is in contrast to early theoretical results and back of the envelope arguments that both suggest that only a few modes should be present. We will present the evidence that suggests a divergence from classical diffusion theory. One likely explanation, that is consistent with all of these anomalies, is the presence of high-Q localized modes consistent with Anderson localization.

Osteoporosis is a bone disease characterized by reduced mineral content with resulting changes in bone architecture, which in turn increases the risk of bone fracture. Raman spectroscopy has an intrinsic sensitivity to the chemical content of the bone, but its application to study bones in vivo is limited due to strong optical scattering in tissue. It has been proposed that Raman excitation with photoacoustic detection can successfully address the problem of chemically specific imaging in deep tissue. In this report, the principal possibility of photoacoustic imaging for detecting mineral content is evaluated.

A custom confocal imaging system was built and used to record a probe beam's spatiotemporal response to a thermal lens induced by a near-IR laser radiation source in a water-filled artificial eye. The IR laser radiation input power levels were varied between 150 and 890 mW at wavelengths of 1110, 1130, 1150 and 1318 nm in order to determine the strength of the resulting thermal lens as a function of time, input power, and wavelength. A high-frame-rate camera captured the probe beam's logarithmic excitation and exponential decay caused by the thermal lens (supplemental video data are provided). Data showed that for equivalent input powers and beam geometries, thermal lensing was strongest for the 1150-nm laser radiation wavelength followed by 1130, 1318 and 1110 nm.

Background and Objectives Retinal damage thresholds from 100-millisecond exposures to laser radiation for wavelengths between 1,100 and 1,350 nm have never previously been established. We sought to determine the retinal damage threshold for 100-millisecond exposures of near-infrared (NIR) laser radiation wavelengths at 1,110, 1,130, 1,150, and 1,319 nm. These data were then used to create trends for retinal damage thresholds over the 1,100–1,350 nm NIR region based upon linear absorption of laser radiation in ocular media and chromatic dispersion of the eye. Materials and Methods The paramacula and macula areas of the retina in Macaca mulatta (rhesus) subjects were exposed for 100 milliseconds to NIR laser radiation wavelengths using a Coherent OPO laser for 1,110, 1,130, and 1,150 nm and a Lee laser for 1,319 nm. Probit analysis was used to establish the estimated damage threshold in the retina for 50% of exposures (ED50). Using trends of transmitted energy to the retina, refractive error of the eye and linear absorption of the retina, a scaling factor (SF) method was created to fit the experimental data, predicting retinal damage thresholds over the 1,100–1,350 nm region. Results The experimental retinal damage threshold, ED50, for 100-millisecond exposures for laser radiation wavelengths at 1,110, 1,130, and 1,319 nm were determined to be 193, 270, and 13,713 mW of power delivered to the cornea, respectively. The retinal damage threshold for the 1,150 nm wavelength was statistically undetermined due to laser-power limitations, but was achieved in one out of three subjects tested. Conclusion The SF predicts the experimental 100- millisecond NIR ED50 value for wavelengths between 1,100 and 1,350 nm. Lasers Surg. Med. 41:382–390, 2009. © 2009 Wiley-Liss, Inc.

Although laser-induced breakdown has the lowest energy threshold in the femtosecond domain, and appears to be responsible for production of threshold retinal lesions by ultrashort pulse lasers, previous findings suggested a role for multiphoton absorption in retinal damage. In this study, we investigated if near infrared ultrashort laser pulses produced DNA damage by exciting short wavelength-absorbing chromophores via multiphoton absorption. The experiments were carried out in nonpigmented and artificially pigmented hTERT-RPE cells, a line of human-derived retinal pigment epithelial (RPE) cells. Cells were exposed to 0.25 s pulses of CW and mode-locked 810 nm laser emissions, at irradiances equivalent to 0.5 and 1.0 times the ED50 for threshold damage in the primate eye, and DNA strand breaks were measured using the comet assay (single cell gel electrophoresis). At 0.5 X ED50, increased DNA strand breakage occurred after the ultrashort (mode-locked) exposures, but not the CW exposures, compared to contro...

The current national consensus standard for laser safety in the United States is the American National Standard for Safe Use of Lasers (ANSI Z136.1). The most recent standard, Z136.1-2000, incorporates a wealth of recent bioeffects data and established a number of new maximum permissible exposure (MPE) limits for laser safety. The standard also includes recent procedures for the computation of MPE values from large or extended diffusely scattering sources, which must be understood by health physicists, laser safety officers, and others in the field of occupational safety. Here we present the fourth in a series of tutorial articles, written to clarify laser safety analysis procedures under this standard, with an emphasis on the MPE computation methods related to extended sources, and the determination of nominal hazard zones.

Melanosome microcavitation is the threshold-level retinal pigment epithelium (RPE) damage mechanism for nanosecond (ns) pulse exposures in the visible and near-infrared (NIR). Thresholds for microcavitation of isolated bovine RPE melanosomes were determined as a function of temperature (20 to 85°C) using single ns laser pulses at 532 and 1064 nm. Melanosomes were irradiated using a 1064-nm Q-switched Nd:YAG (doubled for 532-nm irradiation). For comparison to melanosome data, a similar temperature (20 to 65°C) dependence study was also performed for 532 nm, ns pulse exposures of black polystyrene microbeads. Results indicated a decrease in the microcavitation average radiant exposure threshold with increasing sample temperature for both 532- and 1064-nm single pulse exposures of melanosomes and microbeads. Threshold data and extrapolated nucleation temperatures were used to estimate melanosome absorption coefficients in the visible and NIR, and microbead absorption coefficients in the visible, indicating that melanin is a better absorber of visible light than black polystyrene. The NIR melanosome absorption coefficients ranged from 3713 cm−1 at 800 nm to 222 cm−1 at 1319 nm. These data represent the first temperature-dependent melanosome microcavitation study in the NIR and provide additional information for understanding melanosome microcavitation threshold dependence on wavelength and ambient temperature.

Currently, no light source exists which is both narrow-band and speckle-free with sufficient brightness for full-field imaging applications. Light emitting diodes (LEDs) are excellent spatially incoherent sources, but are tens of nanometers broad. Lasers on the other hand can produce very narrow-band light, but suffer from high spatial coherence which leads to speckle patterns which distort the image. Here we propose the use of random Raman laser emission as a new kind of light source capable of providing short-pulsed narrow-band speckle-free illumination for imaging applications.

Thresholds for microcavitation of isolated bovine retinal melanosomes were determined as a function of temperature using single nanosecond laser pulses at 532 nm and 1064 nm. Melanosomes were irradiated using a 1064-nm Qswitched Nd:YAG (doubled for 532-nm irradiation). Time-resolved microscopy was accomplished by varying the delay between the irradiation beam and an illumination beam allowing stroboscopic imaging of microcavitation events. Results indicated a decrease in microcavitation fluence threshold with increasing sample temperature for both 532-nm and 1064-nm single pulse exposures. The nucleation temperature at both wavelengths was extrapolated through the linear relationship between the temperature increases and the decrease in fluence threshold. In addition, absorption coefficients of melanosomes for visible and near-infrared wavelengths were estimated using the calculated nucleation temperatures.

Laser damage thresholds were determined for expos ure to 2.5-ms 532-nm pulses in an established in vitro retinal model. Single and multiple pulses (10, 100, 1000) were delivered to the cultured cells at three different pulse repetition frequency (PRF) values, and overt damage (membrane breach) was scored 1 hr post laser exposure. Trends in the damage data within and across the PRF range identified significant thermal additivity as PRF was increased, as evidenced by drastically reduced threshold values (< 40% of single-pulse value). Microthermography data that were collected in real time during each exposu re also provided evidence of thermal additivity between successive laser pulses. Using thermal profiles simulated at high temporal resolution, damage threshold values were predicted by an in-house computational model. Our simulated ED 50 value for a single 2.5-ms pulse was in very good agreement with experimental results, but ED 50 predictions for multiple-pulse trains will require more refinement. Keywords: laser-induced damage, retinal pigment epithelial cells, damage threshold, microthermography, computational model

Summary Femtosecond mode-locked lasers are now being used routinely in multiphoton fluorescence and autofluorescence spectroscopy, are just beginning to be used in refractive surgery, and may be used in the future diagnosis of skin cancer. Pulses from these lasers induce non-linear effects in resultant tissue interactions. Using a modified confocal microscope with dispersion compensation and accurate measurements of beam diameter, a very low threshold was measured for photochemical oxidation in cultured cells. The measured threshold showed non-linear photo-oxidation at a peak irradiance and photon-flux density of 8.4 × 108 W cm−2 and 3.4 × 1027 photons cm−2 s−1, respectively (90-fs pulse). The impact of these findings is significant to those using ultrashort lasers because they provide a tangible reference point (microscope-independent) for the generation of photo-oxidative stress in laser-exposed tissues, and because they highlight the importance of dispersion compensation in minimizing collateral tissue damage.

Background and Objectives To gain refinement in safe-exposure limits, indicated by the maximum permissible exposure (MPE) limits, the minimum visible lesion thresholds for three spot sizes (5–15 mm) and four exposure durations (0.25–2.5 seconds) were determined for the skin at 2,000 nm continuous wave laser irradiation. Study Design/Materials and Methods A series of experiments were conducted in vivo on female Yucatan mini-pigs to determine the ED50 damage thresholds for 2,000 nm continuous wave laser irradiation. The study employed Gaussian laser beam exposures with spot diameters (1/e2) of 4.83, 9.65, and 14.65 mm and exposure durations of 0.25, 0.5, 1.0, and 2.5 seconds as a function of laser power. The effect of each irradiation was evaluated within 1 minute after irradiation and the final determination was made at 48 hours post-exposure. Probit analysis was conducted to estimate the dose for 50% probability of laser-induced damage (ED50), defined as persistent redness at the site of irradiation for the mini-pig skin after 48 hours. Results The MPE spot size and exposure duration trends for 2,000 nm laser exposure is consistent for exposure diameters less than 3.5 mm. However, for larger exposure diameters of 4.83, 9.65, and 14.65 mm and exposure duration longer than 0.25 second, the current MPEs are bigger than one tenth of our damage thresholds. For Gaussian laser profile, which is common for many laser output irradiance distributions, lower energy is required to generate a lesion on skin for smaller spot sizes and shorter exposure duration. On the other hand, for spot sizes greater than 4.83 mm and exposure duration over 0.25 second, the average radiant exposure at threshold is inversely proportional to spot size. The irradiance-time and temperature-time power law at the threshold were investigated as well and showed that the irradiance-time power law was a close approximation to estimate laser irradiance at ED50 damage threshold. Conclusions The thresholds study shows that consideration for lowering the MPE standards should be explored as the laser beam diameter becomes larger than 3.5 mm. Based on the limited experimental data, the duration and size dependences of the ED50 damage thresholds could be described by an empirical equation: Irradiance at the threshold = (5.669−1.81×spot diameter)×exposure duration−0.794. Lasers Surg. Med. 37:373–381, 2005. © 2005 Wiley-Liss, Inc.

BACKGROUND AND OBJECTIVES Terawatt (TW) lasers have become commonplace since the development of the chirped-pulse amplification method using Ti:sapphire and Nd:glass laser rods. We have measured the minimum visible lesion (MVL) thresholds for porcine1The animals involved in this study were procured, maintained, and used in accordance with the Federal Animal Welfare Act and the "Guide for the Care and Use of Laboratory Animals" prepared by the Institute of Laboratory Animal Resources-National Research Council. Brooks City-Base, TX has been fully accredited by the Association for Assessment and Accreditation of Laboratory Animal Care, International (AAALAC) since 1967. (Yucatan mini-pig) skin using TW laser pulses. STUDY DESIGN/MATERIALS AND METHODS Our system produced laser pulses at 810 nm and sub-50 femtoseconds. These 1-2 TW laser pulses created multiple self-focusing (SF) filaments during propagation and were directed on the flanks of mini-pigs under anesthesia. We measured the pulse energies necessary to determine the ED(50) skin damage thresholds. RESULTS The MVL ED(50) threshold at 1 hour was 8 mJ and increased to 21 mJ after 24 hours. Histological sections were obtained after 1-hour and 24-hour readings. CONCLUSIONS The damage patterns on the skin indicated the number of filaments in the laser pulse. Many of the pulses produced only superficial damage that disappeared in 24 hours and that nearly three times the pulse energy was required to cause subdural or cellular damage. With further research, non-thermal tissue ablation using TW laser pulses could provide a viable alternative to current techniques of laser use in dermatology.

Over the past few years, a comprehensive rewrite of the American National Standard for Safe Use of Lasers (ANSI Z136.1) has been conducted [American National Standards Institute, Z136.1-2000 American National Standard for Safe Use of Lasers (American National Standards Institute, New York, 2000)]. Important parts of many laser safety evaluations are the determination of the maximum distance at which the laser presents a hazard and the computation of optical density requirements for eye protection. In the ANSI Z136.1-2000 Standard, two approaches for the computation of hazard distance are presented. We specifically address the derivation of these two methodologies including the effects of atmospheric attenuation and optically aided viewing. In addition, these methodologies may be applied to the computation of visual interference distance as described in the ANSI Z136.6-2000 Standard [American National Standards Institute, Z136.6-2000 American National Standard for Use of Lasers Outdoors (American National Standards Institute, New York, 2000)]. This publication represents the third in a series of tutorial articles designed to clarify laser hazard analysis procedures under these ANSI Standards [R. J. Thomas, B. A. Rockwell, W. J. Marshall, R. C. Aldrich, S. A. Zimmerman, and R. J. Rockwell, Jr., J. Laser Appl. 13, 134–140 (2001); 14, 57–66 (2002)].

Random Raman laser emission is demonstrated to be an excellent source of narrow-band, high intensity, short duration light for speckle-free imaging. Spatial coherence measurement and strobe photography of cavitation bubbles are presented.

Advanced imaging techniques and proteomic technology continue to push the boundaries for diagnosing and understanding retinal laser lesion exposures. We conducted retinal imaging in the rhesus macaque to compare the appearance of suprathreshold laser lesions in the macula from both photothermal (532 nm, 100 ms) and photomechanical (532 nm, 9 ns) insult using five different imaging systems: three clinically approved systems; Heidelberg Spectralis SD-OCT-SLO, Heidelberg HRT3 cSLO, and Topcon Fundus camera, and two experimental systems; multispectral using a fundus camera and hyperspectral detection using a PSI Inc. LSLO. In addition to imaging, blood plasma samples were acquired before and after (6 and 24 hrs) laser exposure to search for biomarkers occurring from two different laser damage mechanisms. Imaging results should help identify the best potential imaging systems for capturing retinal laser lesions from photothermal or photomechanical injury. The proteomics results will assist in understanding the elicited molecular pathways involved in damage response to the two types of retinal laser insult, and may be used to hallmark approaches for potential treatment options.Advanced imaging techniques and proteomic technology continue to push the boundaries for diagnosing and understanding retinal laser lesion exposures. We conducted retinal imaging in the rhesus macaque to compare the appearance of suprathreshold laser lesions in the macula from both photothermal (532 nm, 100 ms) and photomechanical (532 nm, 9 ns) insult using five different imaging systems: three clinically approved systems; Heidelberg Spectralis SD-OCT-SLO, Heidelberg HRT3 cSLO, and Topcon Fundus camera, and two experimental systems; multispectral using a fundus camera and hyperspectral detection using a PSI Inc. LSLO. In addition to imaging, blood plasma samples were acquired before and after (6 and 24 hrs) laser exposure to search for biomarkers occurring from two different laser damage mechanisms. Imaging results should help identify the best potential imaging systems for capturing retinal laser lesions from photothermal or photomechanical injury. The proteomics results will assist in understanding the...

Background and Objectives In order to provide a direct comparison of the effects of mode-locked systems to those with continuous-wave (CW) or nonpulsed output, we have performed an experiment with lasers possessing otherwise identical output characteristics. This in vivo minimum visible lesion study compares retinal effects of mode-locked and CW lasers complete with histopathology of the treated areas. Study Design/Materials and Methods Titanium:Sapphire lasers produced 800-nm output for either mode-locked (76 MHz repetition rate, 120 femtoseconds) or CW exposures. Alternating CW and mode-locked laser exposures were delivered to the paramacular retinal region of rhesus subjects. Laser exposure duration was set to one-quarter second for both types of exposures. Through ophthalmoscopic examination of the fundus, a minimal visible lesion (MVL) threshold for damage was established. Results Approximately 75 test sites for each type of exposure were examined. The laser dosage thresholds and 95% confidence intervals for minimal visible damage at 24 hours postexposure were found to be 5.9 mJ (5.23–6.6 mJ) and 5.84 mJ (5.23–6.58 mJ) for mode-locked and CW exposures, respectively. Conclusions Results are compared with published studies conducted at similar exposures. These nearly identical damage thresholds indicate a primarily thermal tissue damage mechanism. Comparative histopathology of acute and chronic lesions of both exposure types is also presented. Lasers Surg. Med. 31:9–17, 2002. © 2002 Wiley-Liss, Inc.

Multiple-pulse lasers are routinely used in the laboratory for research, manufacturing, medical procedures, and in military applications. In order to provide a safe work environment for personnel using these lasers, safety standards have been established and have been in use for many years. These safety standards have addressed laser pulses of nanosecond duration and longer. Recently, safety standards have been updated to address laser pulses as short as 100 femtoseconds in duration. In order to tie these "ultrashort" laser pulses to hazard trends in currently established standards for multiple-pulse exposures with repetition rates less than several kilohertz, this experiment was conducted. Reported herein are minimum visible lesion thresholds in the paramacula of the primate retina using an 800-nm wavelength laser with 1,000 pulses per second, at 130 femtoseconds (fs) pulse duration. The minimum visible lesion (MVL) thresholds were determined at 1 h and 24 h post exposure for 1, 10, 100, 1,000, and 10,000 pulses and are compared with thresholds reported by other researchers. These new data are evaluated relative to the current safety standards for retinal exposure limits as a function of the number of pulses for femtosecond-pulse duration. Data from this study show that the retinal ED50 thresholds/pulse in the paramacula decrease by almost a factor of four as the number of pulses goes from one to ten and then decrease very little for an increase of three decades more in the number of pulses. The MVL-ED50 at the threshold decreased from 0.55 microJ for a single pulse to 0.15 microJ/pulse for 10 pulses and then only to 0.11 microJ/pulse for 10,000 pulses.

Over the past few years, a comprehensive rewrite of the American National Standard for Safe Use of Lasers (ANSI Z136.1) has been conducted. The ANSI Z136.1 is a user standard, as opposed to a manufacturer standard like the Federal Laser Product Performance Standard. As such, differing philosophies for hazard classification and application of safety control measures apply, based upon the user’s perspective. Here we present the second in a series of tutorial articles designed to clarify laser hazard analysis procedures under this new ANSI Standard. The procedure presented allows the reader to effectively determine the appropriate hazard classification for a small-source laser by using known laser output parameters in a step-by-step analysis.

The task of identifying explosives, hazardous chemicals, and biological materials from a safe distance is the subject we consider. Much of the prior work on stand-off spectroscopy using light has been devoted to generating a backward-propagating beam of light that can be used drive further spectroscopic processes. The discovery of random lasing and, more recently, random Raman lasing provide a mechanism for remotely generating copious amounts of chemically specific Raman scattered light. The bright nature of random Raman lasing renders directionality unnecessary, allowing for the detection and identification of chemicals from large distances in real time. In this article, the single-shot remote identification of chemicals at kilometer-scale distances is experimentally demonstrated using random Raman lasing.

Random lasers are a developing class of light sources that utilize a highly disordered gain medium as opposed to a conventional optical cavity. Although traditional random lasers often have a relatively broad emission spectrum, a random laser that utilizes vibration transitions via Raman scattering allows for an extremely narrow bandwidth, on the order of 10 cm−1. Here we demonstrate the first experimental evidence of lasing via a Raman interaction in a bulk three-dimensional random medium, with conversion efficiencies on the order of a few percent. Furthermore, Monte Carlo simulations are used to study the complex spatial and temporal dynamics of nonlinear processes in turbid media. In addition to providing a large signal, characteristic of the Raman medium, the random Raman laser offers us an entirely new tool for studying the dynamics of gain in a turbid medium., Unlike conventional lasers that require a uniform resonant cavity to operate, random lasers use a highly disordered gain medium in which scattering is dominant. Hokr et al. report Raman lasing from a bulk three-dimensional disordered medium whose intensity exceeds that of other random lasers by many orders of magnitude.

The time-temperature effects of laser radiation exposure are investigated as a function of wavelength. Here, we report the thermal response of bulk tissue as a function of wavelength from 700 to 1064 nm. Additionally, Monte Carlo simulations were used to verify the thermal response measured and predict damage thresholds based on the response.