The National Ignition Facility (NIF) is intentionally operated with the final fused silica glass (SiO2) optics exposed to fluences and intensities with the potential to induce damage that will grow with additional laser exposure. Therefore, the NIF operates a recycle loop to refurbish final optics by mitigating any initiated surface damage to arrest its growth. However, the morphology of filamentary damage, caused by local self-focusing in the bulk of a silica optic, adds additional complexity to optics mitigation and provides a limitation to optic reusability. This study evaluates techniques for mitigating isolated and clustered filamentary damage. Optical microscopy before and after NIF installation was used to determine the efficacy of filamentary mitigation after a series of high fluence and intensity laser exposures. The challenges and success rate of the methods are compared for various filamentary damage mitigations strategies.
Exit surface damage on high value fused silica final optics on the NIF is sometimes too large to be mitigated with the currently used technique of removing the damage by CO2 laser machining a cone into the surface. To extend the service life of the optic, a 2 cm diameter shadow is created at the damage using a programmable spatial light modulator at the front end of the laser system. The use of this shadow technique is limited by the obscuration due to the large size of the shadow. An alternative approach is to create the shadow by machining a cone on the input surface opposite the damage. This reduces the shadow a rea, and thus the obscuration by several orders of magnitude. Additional benefits in service life of optics would be realized if the shadow cone size could be increased from current 600 m diameter. There are fabrication challenges encountered when the cone size is increased. To overcome this problem, the shadow performance of a hexagonal array of four 600 m diameter cones has been tested. We report on shadow leakage, bulk damage, and exit surface intensification issues presented by this array and techniques to address those issues.
In this work, an update will be provided on the deployment of shadow cone blockers on NIF grating debris shields. These cones have been demonstrated to reduce the fluence in excess of 11 J/cm2 of an incident 351 nm nanosecond-scale pulse to below the growth threshold of most damage sites. Furthermore, shadow cones will be characterized in terms of their transmission reduction and intensification at downstream surfaces, giving insight into the technique’s broader potential applications and limitations.
An effective damage mitigation strategy is necessary to operate laser systems at energy densities above the damage growth threshold of their optical components. On the National Ignition Facility, growth of laser-induced damage has conventionally been arrested in situ by employing spatially registered cm-scale “spot blockers” in the laser beam to shadow mm-scale damage sites. Spot blockers come at a cost, however, as they obscure a portion of the laser light delivered to the target and thus require an increase in beam energy to compensate for this loss. This increase adds incremental stress to all optics in the beamline. Most spot blockers assigned to an optic are eliminated as part of the repair process when the optic is removed from NIF. However, defects too wide or deep to repair travel with the optic, along with the need for the blocker, throughout its life. Due to obscuration budgetary constraints, these permanent blockers reduce the optic’s usable lifetime. In this work, we propose an alternative method for mitigating a growing damage site by placing a scattering structure of comparable size to the site upstream to shadow the site. This solution obscures much less of the laser light and increases the lifetime of the optic compared to current mitigation strategies.
The final optics in the National Ignition Facility (NIF) are protected from target debris by sacrificial (disposable) debris shields (DDS) comprised of 3-mm thick Borofloat. While relatively inexpensive, Borofloat has been found to have bulk inclusions which, under UV illumination, damage, grow, and occasional erupt though the surface of the DDS. We have shown previously that debris generated from Input Surface Bulk Eruptions (ISBE) are a significant source of damage on NIF. Inclusion-free fused silica debris shield (FSDS) have been installed in between the DDS and the final optics on some NIF beam lines to test their efficacy in mitigating damage initiation. We will show results of the damage performance of the FSDS and its role in protecting the final optics. These results will help in our economic analysis of the potential benefits of using FSDS to protect NIF final optics.
Particles generated from laser-induced damage during operation of fusion class lasers can be a source of damage precursors for neighboring optics. Such particles have been identified on the National Ignition Facility as ejecta from 1) laser damage on absorbing glass protecting the installation hardware from known stray focusing reflections in the optical path and 2) bulk damage in the borosilicate target debris shield that grows and erupts on the input surface. The dependence of the particle generation and damage initiation rate of this newly recognized damage source on laser shot parameters is not yet known, making it difficult to predict how this source would affect facility optic lifetime for projected laser operation. In this work, we measure the 351-nm fluence-dependence on the size distribution of glass particles generated by ejection from absorbing and borosilicate shield glasses onto a neighboring fused silica window exit surface. In addition, we track the fates of these ejecta and measure their probabilities of removal, damage initiation, and damage growth upon subsequent laser exposure. Thousands of particles can be ejected and deposited onto the exit surface of the fused silica window following a single pulse. Damage initiation following exposure of large borosilicate particles was observed above a fluence of 6 J/cm2. A laser-driven strategy to remove particles before a high fluence pulse is explored.
National Ignition Facility (NIF) is the world’s most energetic laser system, capable of delivering over 1.8 MJ of energy at UV. After operating for over 10 years and working continuously to improve the damage resistance of the optics, there seems to be a disconnect between the offline measured damage performance of optics and the actual lifetime of an installed optic. Recently, we have discovered a source of laser-induced optic damage that originated from particles ejected from damage of a neighboring disposable optic. We were able to replicate this contamination-driven laser-induced damage experimentally in an offline facility and have developed a phenomenological model based on the results which includes particle generation, particle cleaning, and particle damage as a function of damage size as well as laser fluence. The model was able to accurately predict the multi-shot process of the offline experiment. Since then, we have used this model to predict online damage performance on NIF on a series of very high energy shots to test the validity of model with surprising results that shows the success of the model along with new features that will need to be address.
This work was performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. (LLNL-ABS-745711)
The National Ignition Facility (NIF) uses an in-situ system called the Final Optics Damage Inspection (FODI) system to monitor the extent of damage on installed optical components. Among this system's uses is to alert operators when damage sites on a Grating Debris Shield (GDS) require repair (≈300 microns) and triggers the removal of the damaged optic. FODI, which can reliably detect damage sites larger than 50 microns, records the size and location of observed sub-critical damage observed on the optic, so each of these sites can be repaired before the optic is next installed. However, by only identifying, and hence repairing sites larger than ≈50 microns, optics are left with numerous smaller sites, some fraction of which resume growing when the host optic is reinstalled. This work presents a method of identifying and repairing damage sites below the FODI detection limit that have a significant probability of growth. High resolution images are collected of all likely damage candidates on each optic, and a machine learning based automated classification algorithm is used to determine if each candidate is a damage site or something benign (particle, previously repaired site, etc.). Any damage site greater than 20 microns is flagged for subsequent repair. By repairing these smaller sites, recycled optics had a 40% increased lifetime on the NIF.
Operating the National Ignition Facility (NIF) near its power and energy performance limits has revealed a new damage initiation mechanism in the final UV optics. The typical damage event involves the last three optics in the NIF beamline: the final focusing lens, the grating debris shield, and the target debris shield. It occurs on high power shots from intensifications from small phase defects (pits) on the exit surface of the focusing lens that travel through the grating debris shield before reflecting off the AR-coated target debris shield about 75 cm downstream, then propagate back upstream and damage the input surface of the grating debris shield optic which is 15 cm downstream of the focusing lens. Ray tracing has firmly established the direct relationship between the phase defects on the final focusing lens and the damage on grating debris via the reflection from the target debris shield. In some cases, bulk filamentary damage is also observed in the 1-cm thick fused silica grating debris shield. It is not fully understood at this point how there can be enough energy from the reflected beam to cause damage where the forward-going beam did not. It does not appear that interaction between the forward-going beam and the backward-going reflected beam is necessary for damage to occur. It does appear necessary that the target debris shield be previously exposed to laser shots and/or target debris. Furthermore, there is no evidence of damage imparted to the target debris shield or the final focusing lens. We will describe all the conditions under which we have (and have not) observed these relatively rare events, and the steps we have taken to mitigate their occurrence, including identification and elimination of the source phase defects.
KEYWORDS: Particles, Silica, National Ignition Facility, Pulsed laser operation, High power lasers, Laser systems engineering, Laser induced damage, Scanning electron microscopy, Laser damage threshold, Contamination
Current fused silica surface processing, aimed at reducing known absorbing precursor concentration, has brought laboratory-tested ultraviolet laser-induced damage rates to nearly nil at fluences up to 10 J/cm2 . Yet this damage rate reduction has not been fully realized in large facility operation. A recently discovered source of damage in the facility is from particles ejected from damage of a neighboring optic under laser exposure and deposited onto the substrate surface. This state was observed to provide a means to couple energy from a subsequent laser pulse into the contaminated substrate and cause damage characterized by fracture. In this work, we explore the rate at which particles are removed from the surface and the rate at which particles lead to damage as a function of laser fluence and particle characteristics. This analysis allows for a derivation of an optimal pre-exposure fluence of a contaminated optic which maximizes particle removal probability while minimizing surface damage probability. For fluences up to 9.5 J/cm2 (351 nm, 5 ns square pulse), both particle removal and damage probabilities generally increased with particle size and laser fluence, with damage threshold around 6.5 J/cm2 . Two possible mechanisms that facilitate particle-induced damage on the substrate surface from laser-generated and deposited ejecta will be discussed, namely i) enhanced thermal contact from molten or partially molten ejecta and ii) fracture generated upon impact of solid ejecta with high kinetic energy.
A major source of kidneys for transplant comes from deceased donors whose tissues have suffered an unknown amount of warm ischemia prior to retrieval, with no quantitative means to assess function before transplant. Toward addressing this need, non-contact monitoring of optical signatures in rat kidneys was performed in vivo during ischemia and reperfusion. Kidney autofluorescence images were captured under ultraviolet illumination (355 nm, 325 nm, and 266 nm) in order to provide information on related metabolic and non-metabolic response. In addition, light scattering images under 355 nm, 325 nm, and 266 nm, 500 nm illumination were monitored to report on changes in kidney optical properties giving rise to the observed autofluorescence signals during these processes. During reperfusion, various signal ratios were generated from the recorded signals and then parametrized. Time-dependent parameters derived from the ratio of autofluorescence under 355 nm excitation to that under 266 nm excitation, as well as from 500 nm scattered signal, were found capable of discriminating dysfunctional kidneys from those that were functional (p < 0.01) within hours of reperfusion. Kidney dysfunction was confirmed by subsequent survival study and histology following autopsy up to a week later. Physiologic changes potentially giving rise to the observed signals, including those in cellular metabolism, vascular response, tissue microstructure, and microenvironment chemistry, are discussed.
The primary sources of damage on the National Ignition Facility (NIF) Grating Debris Shield (GDS) are attributed to
two independent types of laser-induced particulates. The first comes from the eruptions of bulk damage in a
disposable debris shield downstream of the GDS. The second particle source comes from stray light focusing on
absorbing glass armor at higher than expected fluences. We show that the composition of the particles is
secondary to the energetics of their delivery, such that particles from either source are essentially benign if they
arrive at the GDS with low temperatures and velocities.
Functional changes in rat kidneys during the induced ischemic injury and recovery phases were explored using multimodal autofluorescence and light scattering imaging. The aim is to evaluate the use of noncontact optical signatures for rapid assessment of tissue function and viability. Specifically, autofluorescence images were acquired in vivo under 355, 325, and 266 nm illumination while light scattering images were collected at the excitation wavelengths as well as using relatively narrowband light centered at 500 nm. The images were simultaneously recorded using a multimodal optical imaging system. The signals were analyzed to obtain time constants, which were correlated to kidney dysfunction as determined by a subsequent survival study and histopathological analysis. Analysis of both the light scattering and autofluorescence images suggests that changes in tissue microstructure, fluorophore emission, and blood absorption spectral characteristics, coupled with vascular response, contribute to the behavior of the observed signal, which may be used to obtain tissue functional information and offer the ability to predict posttransplant kidney function.
A comprehensive study of laser-induced damage associated with particulate damage on optical surfaces is presented. Contaminant-driven damage on silica windows and multilayer dielectrics is observed to range from shallow pitting to more classical fracture-type damage, depending on particle-substrate material combination, as well as laser pulse characteristics. Ejection dynamics is studied in terms of plasma emission spectroscopy and pump-probe shadowgraphy. Our data is used to assess the momentum coupling between incident energy and the ejected plasma, which dominates the laser-particle-substrate interaction. Beam propagation analysis is also presented to characterize the impact of contaminant-driven surface pitting on optical performance.
We study the formation of laser-induced Hertzian fractures on silica output surfaces at high incident influences initiated by surface bound metal particles. Hertzian fracture initiation probability as a function of incidence influence is obtained for two particle materials. The resulting modified damage density curve shows prototypical features determined by the surface-bound particles population. The data is further used to calculate the coupling coefficient between incident energy and the ejected plasma momentum.
We study the formation of laser-induced shallow pits (LSPs) on silica output surfaces and relate these features to optical performance as a function of incident laser fluence. Typical characteristics of the LSPs morphology are presented. Closed-form expressions for the scattered power and far-field angular distribution are derived and validated using numerical calculations of both Fourier optics and FDTD solutions to Maxwell’s equations. The model predictions agree well with the measurements for precise profile micro-machined shallow pits on glass, and for pitting caused by laser cleaning of bound metal micro-particles at different fluences.
Laser induced damage (breakdown) initiated on the exit surface of transparent dielectric materials using nanosecond pulses creates a volume of superheated material reaching localized temperatures on the order of 1 eV and pressures on the order of 10 GPa or larger. This leads to material ejection and the formation of a crater. The volume of this superheated material depends largely on the laser parameters such as fluence and pulse duration. To elucidate the material behaviors involved, we examined the morphologies of the ejected superheated material particles and found distinctive morphologies. We hypothesize that these morphologies arise from the difference in the structure and physical properties (such as the dynamic viscosity and presence of instabilities) of the superheated material at the time of ejection of each individual particle. Some of the ejected particles are on the order of 1 µm in diameter and appear as “droplets”. Another subgroup appears to have stretched, foam-like structure that can be described as material globules interconnected via smaller in diameter columns. Such particles often contain nanometer size fibers attached on their surface. In other cases, only the globules have been preserved suggesting that they may be associated with a collapsed foam structure under the dynamic pressure as it traverses in air. These distinct features originate in the structure of the superheated material during volume boiling just prior to the ejection of the particles.
Multilayer interference optical mirror coatings are traditionally fluence-limited by nodular inclusions. Planarization of these defects modifies the geometrically and interference-induced light intensification to increase the laser resistance of mirror coatings. Previous studies using engineered defects on the substrate or buried in the middle of the coating stack have focused only on understanding the improvement in laser resistance. However, real coating defects are distributed throughout the coating. To better understand differences between the critical fluence-limiting defects of both planarized and non-planarized mirror coatings, laser damage pit depths were determined as a function of laser fluence.
Crater formation that accompanies laser-induced damage is the result of material ejection following the rapid, localized
heating to temperatures on the order of 1 eV. The objective of this work is to compare the material ejection behavior in
fused silica and KDP crystals as captured using time-resolved shadowgraphy. These two materials are of fundamental
importance in ICF class laser systems but they also represent materials with significantly different physical properties.
We hypothesize that these different properties can affect the material ejection process.
The stimulated Raman scattering gain coefficient in KDP/DKDP crystals for any orientation with respect to the crystal
axis, propagation and polarization of the pump beam can be estimated using a) the spontaneous Raman scattering cross
section of the material, b) the spectral profile of the Raman line and, c) the Raman scattering tensor. Of particular
interest in ICF class laser systems are the parasitic Transverse Stimulated Raman Scattering effects. In this work we
provide experimental results that help advance our ability to estimate these effects at operational conditions for second,
third and fourth harmonic generation.
The dynamics of material response following initial localized energy deposition by the laser pulse on the material's surface is still largely unknown. We describe a time-resolved microscope system that enables the study of the sequence of events and the individual processes involved during the entire timeline from the initial energy deposition to the final state of the material, typically associated with the formation of a crater on the surface. To best capture individual aspects of the damage timeline, this system can be configured to multiple imaging arrangements, such as multiview image acquisition at a single time point, multi-image acquisition at different time points of the same event, and tailored sensitivity to various aspects of the process. As a case example, we present results obtained with this system during laser-induced damage on the exit surface of fused silica.
The processes involved at the onset of damage initiation on the surface of fused silica have been a topic of extensive
discussion and thought for more than four decades. Limited experimental results have helped develop models covering
specific aspects of the process. In this work we present the results of an experimental study aiming at imaging the
material response from the onset of the observation of material modification during exposure to the laser pulse through
the time point at which material ejection begins. The system involves damage initiation using a 355 nm pulse, 7.8 ns
FWHM in duration and imaging of the affected material volume with spatial resolution on the order of 1 μm using as
strobe light a 150 ps laser pulse that is appropriately timed with respect to the pump pulse. The observations reveal that
the onset of material modification is associated with regions of increased absorption, i.e., formation of an electronic
excitation, leading to a reduction in the probe transmission to only a few percent within a time interval of about 1 ns.
This area is subsequently rapidly expanding with a speed of about 1.2 μm/ns and is accompanied by the formation and
propagation of radial cracks. These cracks appear to initiate about 2 ns after the start of the expansion of the modified
region. The damage sites continue to grow for about 25 ns but the mechanism of expansion after the termination of the
laser pulse is via formation and propagation of lateral cracks. During this time, the affected area of the surface appears to
expand forming a bulge of about 40 μm in height. The first clear observation of material cluster ejection is noted at about
50 ns delay.
Tissue that has undergone significant yet unknown amount of ischemic injury is frequently encountered in organ
transplantation and trauma clinics. With no reliable real-time method of assessing the degree of injury incurred in tissue,
surgeons generally rely on visual observation which is subjective. In this work, we investigate the use of optical
spectroscopy methods as a potentially more reliable approach. Previous work by various groups was strongly suggestive
that tissue autofluorescence from NADH obtained under UV excitation is sensitive to metabolic response changes. To
test and expand upon this concept, we monitored autofluorescence and light scattering intensities of injured vs. uninjured
rat kidneys via multimodal imaging under 355 nm, 325 nm, and 266 nm excitation as well as scattering under 500 nm
illumination. 355 nm excitation was used to probe mainly NADH, a metabolite, while 266 nm excitation was used to
probe mainly tryptophan to correct for non-metabolic signal artifacts. The ratio of autofluorescence intensities derived
under these two excitation wavelengths was calculated and its temporal profile was fit to a relaxation model. Time
constants were extracted, and longer time constants were associated with kidney dysfunction. Analysis of both the
autofluorescence and light scattering images suggests that changes in microstructure tissue morphology, blood
absorption spectral characteristics, and pH contribute to the behavior of the observed signal which may be used to obtain
tissue functional information and offer predictive capability.
Optical components within high energy laser systems are susceptible to laser-induced material modification when the
breakdown threshold is exceeded or damage is initiated by pre-existing impurities or defects. These modifications are the
result of exposure to extreme conditions involving the generation of high temperatures and pressures and occur on a
volumetric scale of the order of a few cubic microns. The response of the material following localized energy deposition,
including the timeline of events and the individual processes involved during this timeline, is still largely unknown.
In this work, we investigate the events taking place during the entire timeline in both bulk and surface damage in
fused silica using a set of time-resolved microscopy systems. These microscope systems offer up to 1 micron spatial
resolution when imaging static or dynamic effects, allowing for imaging of the entire process with adequate temporal
and spatial resolution. These systems incorporate various pump-probe geometries designed to optimize the sensitivity for
detecting individual aspects of the process such as the propagation of shock waves, near-surface material motion, the
speed of ejecta, and material transformations. The experimental results indicate that the material response can be
separated into distinct phases, some terminating within a few tens of nanoseconds but some extending up to about 100
microseconds. Overall the results demonstrate that the final characteristics of the modified region depend on the material
response to the energy deposition and not on the laser parameters.
Laser-induced damage on the surface of optical components typically is manifested by the formation of microscopic
craters that can ultimately degrade the optics performance characteristics. It is believed that the damage process is the
result of the material exposure to high temperatures and pressures within a volume on the order of several cubic microns
located just below the surface. The response of the material following initial localized energy deposition by the laser
pulse, including the timeline of events and the individual processes involved during this timeline, is still largely
unknown. In this work we introduce a time-resolved microscope system designed to enable a detailed investigation of
the sequence of dynamic events involved during surface damage. To best capture individual aspects of the damage
timeline, this system is employed in multiple imaging configurations (such as multi-view image acquisition at a single
time point and multi-image acquisition at different time points of the same event) and offers sensitivity to phenomena at
very early delay times. The capabilities of this system are demonstrated with preliminary results from the study of exitsurface
damage in fused silica. The time-resolved images provide information on the material response immediately
following laser energy deposition, the processes later involved during crater formation or growth, the material ejecta
kinetics, and overall material motion and transformation. Such results offer insight into the mechanisms governing
damage initiation and growth in the optical components of ICF class laser systems.
The use of reduced nicotinamide adenine dinucleotide (NADH) fluorescence to gain metabolic information on kidneys in response to an alteration in oxygen availability has previously been experimentally demonstrated, but signal quantification has not, to date, been addressed. In this work the relative contribution to rat kidney autofluorescence of the capsule versus cortex under ultraviolet excitation is determined from experimental results obtained using autofluorescence microscopy and a suitable mathematical model. The results allow for a quantitative assessment of the relative contribution of the signal originating in the metabolically active cortex as a function of capsule thickness for different wavelengths.
Optical properties of near-surface kidney tissue were monitored in order to assess response during reperfusion to long
(20 minutes) versus prolonged (150 minutes) ischemia in an in vivo rat model. Specifically, autofluorescence images of
the exposed surfaces of both the normal and the ischemic kidneys were acquired during both injury and reperfusion
alternately under 355 nm and 266 nm excitations. The temporal profile of the emission of the injured kidney during the
reperfusion phase under 355 nm excitation was normalized to that under 266 nm as a means to account for changes in
tissue optical properties independent of ischemia as well as changes in the illumination/collection geometrical
parameters in future clinical implementation of this technique using a hand-held probe. The scattered excitation light
signal was also evaluated as a reference signal and found to be inadequate. Characteristic time constants were extracted
using a fit to a relaxation model and found to have larger mean values following 150 minutes of injury. The mean values
were then compared with the outcome of a chronic survival study where the control kidney had been removed. Rat
kidneys exhibiting longer time constants were much more likely to fail. This may lead to a method to assess kidney
viability and predict its ability to recover in the initial period following transplantation or resuscitation.
We explore an optical spectroscopy approach to monitor the progression of ischemia and reperfusion in situ using a rat
model. The system utilizes the sensitivity of NADH emission to changes in cell metabolism during ischemia and
reperfusion. In addition, the emission from tryptophan is employed as a normalization against changes in other optical
properties of the tissue. Ischemia was induced in one kidney followed by at least 60 minutes of reperfusion. During
both phases, autofluorescence images of the exposed surfaces of both the ischemic kidney and the normal (control)
kidney were acquired and the respective average emission intensities were determined. Preliminary results indicate that
the kinetics of the ratio of the emissions under these two excitations is related to the injury time.
Currently no clinical tool exists that measures the degree of ischemic injury incurred in tissue and assesses tissue function following transplantation. In response to this clinical problem, we explore optical spectroscopy to quantitatively assess ischemic injury. In our method we monitor the autofluorescence intensities under excitation suitable to excite specific tissue fluorophores. Specifically, a first excitation probes NADH, a biomolecule known to
change its emission properties depending on the tissue's metabolic state. A second excitation is used to mainly probe tryptophan, a biomolecule expected to be minimally affected by metabolism. We postulate that the ratio of the two autofluorescence signals can be used to monitor tissue behavior during ischemia and reperfusion. To evaluate this approach, we acquire autofluorescence images of the injured and contralateral control kidney in vivo in a rat model under excitation at both wavelengths during injury and reperfusion. Our results indicate that this approach has the potential to provide real-time monitoring of organ function during transplantation.
We explore imaging of tissue microstructures using autofluorescence and light scattering methods implemented through a hyperspectral microscope design. This system utilizes long working distance objectives that enable off-axis illumination of tissue thereby allowing for excitation at any optical wavelength without requiring change of optical elements within the microscope. Spectral and polarization elements are easily and rapidly incorporated to take
advantage of spectral variations of spectroscopic optical signatures for enhanced contrast. The collection efficiency has been maximized such that image acquisition may be acquired within very short exposure times, a key feature for transferring this technology to a clinical setting. Preliminary studies using human and animal tissues demonstrate the feasibility of this approach for real-time imaging of intact tissues as they would appear in the operating room.
Potentially transplantable kidneys experience warm ischemia, and this injury is difficult to quantify. We investigate optical spectroscopic methods for evaluating, in real time, warm ischemic kidney injury and reperfusion. Vascular pedicles of rat kidneys are clamped unilaterally for 18 or 85 min, followed by 18 or 35 min of reperfusion, respectively. Contralateral, uninjured kidneys serve as controls. Autofluorescence and cross-polarized light scattering images are acquired every 15 s using 335-nm laser excitation (autofluorescence) and 650±20-nm linearly polarized illumination (light scattering). We analyze changes of injured-to-normal kidney autofluorescence intensity ratios during ischemia and reperfusion phases. The effect of excitation with 260 nm is also explored. Average injured-to-normal intensity ratios under 335-nm excitation decrease from 1.0 to 0.78 at 18 min of ischemia, with a return to baseline during 18 min of reperfusion. However, during 85 min of warm ischemia, average intensity ratios level off at 0.65 after 50 min, with no significant change during 35 min of reperfusion. 260-nm excitation results in no autofluorescence changes with ischemia. Cross-polarized light scattering images at 650 nm suggest that changes in hemoglobin absorption are not related to observed temporal behavior of the autofluorescence signal. Real-time detection of kidney tissue changes associated with warm ischemia and reperfusion using laser spectroscopy is feasible. Normalizing autofluorescence changes under 335 nm using the autofluorescence measured under 260-nm excitation may eliminate the need for a control kidney.
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