This PDF file contains the front matter associated with SPIE Proceedings Volume 11042 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

There have been concerted efforts to develop high-energy diode-pumped alkali vapor lasers (DPAL). These hybrid gas phase / solid-state laser systems offer possibilities for constructing high-powered lasers that have high beam quality. Considerable progress has been made, but there are technical challenges associated with the reactivity of the metal atoms. Rare gas atoms (Rg) excited to the np⁵ (n+1)s ³P₂ configuration are metastable and have spectral properties that are closely similar to those of the alkali metals. Optically pumped lasers have been constructed using excitation of the np⁵ (n+1)p ← np⁵ (n+1)s transitions. Pulsed lasing has been observed for Ne*, Ar*, Kr* and Xe*. Helium was used as the collisional energy transfer agent that established population inversions. These systems have the advantage using inert reagents that are gases at room temperature, with excellent potential for closed-cycle, multi-wavelength operation. The primary technical difficulty for the rare gas laser is the discharge production of sufficient Rg* metastables in the presence of >200 Torr of He. We have developed a high frequency pulsed discharge that yields >10¹³ cm^-3 Ar* in the presence of He at pressures up to 730 Torr. Using this discharge, a diode pumped Ar* laser providing 4.1 W of continuous wave output has been demonstrated, with an optical conversion efficiency of 31%. Development of the pulsed discharge system and CW lasing demonstrations with Xe* are reported.

Activities on high power laser at Kawasaki Heavy Industries, Ltd. (KHI) are reviewed. We started R and D of high power laser technologies from the development of Chemical Oxygen-Iodine Laser (COIL) for industrial application in 1886. With advent of the 21st century, we shifted the application of our COIL technologies, and successfully received the contract of major high power laser R and D program from the Japan Ministry of Defense (J-MoD) in 2010. The COIL has many attractive features such as high power, good beam quality, etc., but the system is a bit complicated due to the chemical pumping. Currently, we are developing single-mode fiber laser and beam combing technologies for the downsizing of military high power laser.

Super-broadband mid-IR laser systems are under development in the Gas Lasers Lab of the Lebedev Physical Institute. These systems consist of different sorts of molecular gas lasers and various nonlinear crystals converting multi-line emission of those lasers into broadband radiation on sum and difference frequencies. As frequency converters, both main mid-IR nonlinear crystals such as ZnGeP₂, GaSe, AgGaSe₂ and new ones such as BaGa₂GeSe₆, PbIn₆Te₁₀ are applied. As radiation sources, repetitively-pulsed Q-switched lasers pumped by both longitudinal DC discharge and transverse slab RF discharge emitting microsecond pulses, and also actively mode-locked e-beam sustained discharge lasers emitting nanosecond pulses are applied. It was experimentally demonstrated for the developed laser systems to emit broadband radiation on hundreds spectral lines covering mid-IR range from ~1.7 to ~17 micron due to sum and difference frequency generation including third harmonic generation.

An opportunity of developing a laser with optical pumping from metastable ArI levels produced by a relativistic electron beam in high-pressure He-Ar gas mixtures was studied. It was shown that high population and long lifetime of the 1s5 level of Ar atom and broad width (~ 10 nm) of the absorption spectral line made it possible to obtain the most optimal lasing conditions in the afterglow of electron beam pulse within the classical three-level pumping scheme. Rate constants for collisional quenching of low-lying 4s-levels of excited argon atom in He-Ar mixtures in the pressure range from 4.0 to 0.75 atm were measured. An increase of photoluminescence on the 2p₁₀-1s₅ transition (912.3 nm) upon application of transverse optical pumping with a laser diode array (811.5 nm) was observed. An application of a semi-transparent cavity resulted in a significant increase of this photoluminescence signal in the afterglow indicating amplified spontaneous emission.

In this communication, we present the results of experiments with optically pumped rare gas laser (OPRGL) with a dielectric barrier discharge (DBD) as the source of metastable Ar atoms and a pulsed OPO system as the optical pump. In a longitudinal pumping scheme, the threshold input intensity to achieve lasing was 3.9 kW cm^-2. Lasing was observed in the pressure range from 100 to 750 Torr. However, lasing was possible only during the limited time of the DBD applied voltage cycle.

The optical and temporal characteristics of a cw hydrogen fluoride chemical laser with spectral lines is investigated. The distributions of populations on the energy levels of transition line are altered by the cascade effect. The output power of the target line is improved due to this process. Also the laser operational parameters is optimized, in order to demonstrate single line oscillation and laser output enhancement.

The kinetic behaviors of 6p[1/2]₀, 6p[3/2]₂ ,and 6p[5/2]₂ were examined under the ultrahigh pumped power. These processes were detected by the way of time-resolved fluorescence and ASE spectra. A theory of energy-pooling is presented under the focused condition. There are three types of energy-pooling processes. The first type is energy-pooling ionization. The obvious ionization can be observed whenever the laser prepared state is the 6p[1/2]₀, 6p[3/2]₂, or 6p[5/2]₂ state. The second type is energy-pooling with big energy difference. The energy-pooling collision between the two 6p[1/2]₀ atoms can produce one 5d[3/2]₁ atom and one 6s’[1/2]₀ atom when the prepared state is 6p[1/2]₀. The third type is energy-pooling with small energy difference. The way of generation of five secondary 6p states is energy-pooling instead of collision relaxation.

A large sample has been employed to obtain total reaction cross sections and product energies (translational, vibrational and rotational) state distributions for the title reaction, using Quasi-Classical Trajectory calculations and the Gaussian Binning methodology on Wu-Schatz-Lendvay-Fang-Harding and Neural Networks potential energy surfaces. It has been observed that, depending on a first selection, Vibrational-Gaussian Binning, a second selection, Rotational-Gaussian Binning, induces a preference for odd or even values in the Total Angular Momentum of de HOD product molecule. It is interesting to remember that a similar phenomenon is observed in the CO₂ triatomic molecule, utilized in the correspondent gas laser. The dynamics and kinetics of polyatomic chemical reactions (e.g., OH + D₂ → HOD* + D and its isotopic variants), of relevance in the excitation/emission processes of current and future chemical lasers, could be studied theoretically from the results obtained in this article.

The two-level generation model taking into account kinetic and optical losses has been applied to analyze and predict output performances of a continuous-wave oxygen-iodine laser (OIL) with chemical, electric discharge, and optical pumping. A comparative analysis of the calculation results and the experiments for cw OILs with different pumping types has shown the applicability of the model in practically important cases when low losses were ensured. The medium extraction efficiency degree can reach 90% of the potentially attainable for lasers pumped by all mentioned sources. The possibilities of increasing the cw OILs power efficiency have been considered on the basis of operating modes mapping.

The authors have developed and executed the computer simulation code to analyze the energy distribution of gas mixture for CO laser by using Macintosh computer The details, such as dependences of energy distributions on electric field and gas mixture ratio are described in this paper below.

Experiments operating a Cs D₂ line (852.1 nm: 6²P_3/2→6²S_1/2) laser, pumped by blue wing absorption of different thermal Cs-rare gas ground state pairs, were analyzed. Using a 10 cm sealed gas cell, the D₂ line laser performance is highest for Cs-Ar at 513K, and for Cs-Kr and Cs-Xe at 473 K. Overall, the highest optical-to-optical efficiency measured was 1.1% with a linear slope efficiency of 1.5% for the Cs-Ar collision pair at 513 K. All three Cs-rare gas mixtures show a D₂ line laser performance increase with temperature towards a peak efficiency, followed by a decrease as temperature is increased beyond the peak performance point. At lower temperatures ≤453 K the efficiency was highest with Cs-Xe, at 473 K highest with Cs-Kr, whereas at higher temperatures ≥ 493 K the efficiency was highest with Cs-Ar. Measurement of the reduced absorption coefficient for the peak of the blue satellite of the different collision pairs resulted in values of 1.2∙10^-36 cm⁵ for Cs-Ar at 836.7 nm, 2.0∙10^-36 cm⁵ for Cs-Kr at 841.1 nm, and 3.0∙10^-36 cm⁵ for Cs-Xe at 842.7 nm. Simulations of the data using the detailed BLAZE Multiphysics software suite were also performed, which aid in the understanding of the basic physics behind the XPAL system. The simulations indicate that energy pooling significantly inhibits laser performance at higher temperatures above 500 K.

We report on the results of ongoing work in our lab on flowing-gas Cs DPAL. Some of the results have been already reported [Yacoby et al, Opt. Express 26, 17814 (2018)] but we briefly describe them here again to give a full picture of the work. Experimental and theoretical study of continuous wave Cs DPAL with gas circulation (He and CH₄), flow velocities of 1-4.5 m/s and pump powers of 30-65 W, is reported. For the theoretical part of the study we used a 3D computational fluid dynamics model, solving the gas dynamics and kinetics equations relevant to flowing-gas laser operation. Maximum CW output power of 24 W and slope efficiency of 48% were obtained. The experimental and theoretical dependence of the lasing power on the flow velocity are in good agreement. The gas temperature rise in the laser cell was measured. The lasing power was not affected by the flow velocity at this range of pump powers and flow velocities due to the fact that the gas temperature rise was only several degrees. It was estimated – using a “fitting” method – that the quenching cross-section of the excited levels of Cs to the ground state is ~ 0.05 Ų .

Unstable optical resonators employing graded reflectivity mirrors (GRMs) for diode-pumped alkali laser (DPAL) are theoretically studied. In this study, a wave optics resonator model was coupled with a simplified gas flow model. The platform modeled was a 100-W class Cs DPAL proposed by us. As the pump beams are obliquely incident to the active medium, a wide gain region appears compared with that of the axially pumping scheme. Although an unstable resonator was expected to be suitable to provide a high-quality output beam, it was found to be inapplicable for our relatively small-scale DPAL apparatus. Instead, we demonstrated that the application of a GRM unstable resonator provide a nearly diffraction-limited beam (M² < 2) with minimal penalty for the output power. The effect of the gas flow on the beam quality is also discussed.

The beam quality and efficiency of diode-pumped alkali lasers are strongly dependent on parameters such as the pump beam geometry, the resonator configuration, the gain length and other features. In the present work we studied experimentally and theoretically the dependence of some of these parameters on the laser performance. An optical model of multi-transverse mode operation of alkali vapor lasers [Auslender et al, Opt. Express 25, 19767 (2017)], modified for the current experiments, is applied to the experimental results. The values of the laser power and M² predicted by the model are in good agreement with the experimental results for different shapes and powers of the pump beam.

After the multiphoton ionization of sodium-argon mixture, time-resolved atomic emission spectrum is used to experimentally study the unusual phenomenon of the obviously different broadening between Na D₁ and D₂ lines spectra. The primary reason for the unusual broadening of Na D₂ line is that the spectral line of Ar I 588.9 nm overlays with Na D₂ line (589.0 nm) after ionization, and the serious self-absorption on Na D₂ line is the secondary reason. Although there is difference of population between 3²P_3/2 and 3²P_1/2 states, the experiment result demonstrates that the difference between Na D lines in radiation channel will not affect the broadening of spectral profile.

We report on the generation of picosecond pulses at 515 nm and deep ultraviolet pulses at 257 nm and 206 nm. They are generated as second, fourth and fifth harmonic frequencies of the high power diode-pumped Yb:YAG thin-disk laser at the fundamental wavelength of 1030 nm. The laser at the fundamental wavelength is based on a chirped-pulse amplification of oscillator pulses in two-stage regenerative amplifier based on thin-disks as active media. The diode pumping at the zero phonon line is used. The pulses are produced at ~100 kHz repetition frequency, 2-10 ps pulse duration and ≤2 mJ pulse energy. The fundamental beam is doubled in an LBO crystal at an efficiency of 42%. Subsequently the fourth harmonic frequency (257 nm) is produced by frequency doubling of the second harmonic frequency in a CLBO crystal. The unconverted part of the fundamental beam after SHG is used with the fourth harmonic beam as 1ω+4ω frequency sum in a further CLBO crystal for the fifth harmonic frequency (206 nm) generation. Two issues are important in the efficient optical conversion: nonlinear absorption given mainly by two-photon absorption (TPA) and proper timing of interacting pulses. In our simulations we take into account TPA and study the consequences of different timing of 1ω- and 4ω-pulses on the CLBO when generating the 5th harmonic. It was found that 1.4 ps delay of the 1ω-pulse after the 4ω-pulse is necessary to get the highest 5ω-output. Also the 5ω-pulse duration is affected by the timing.

We have demonstrated a high peak power electro-optically cavity-dumped (CD) laser and a high energy picosecond regenerative amplifier (RGA) at 1342 nm. Their output power, pulse energy, pulse widths, and peak power were reported, respectively. For CD operation, the maximum pulse energy was 0.308 mJ at 1 kHz with the pulse width of 23.3 ns. The corresponding peak power was 13.2 kW. The maximum pulse energy of 0.243 mJ was extracted from RGA at 1 kHz under the seeded pulse energy of 250 pJ, resulting in its total optical gain of 9.7×10⁵ . Its pulse width was estimated to be 43.8 ps, corresponding to a peak power of 5.55 MW.

A blue random laser based on solid waveguide gain films with silver nanoparticles (NPs) and SiO₂ NPs is demonstrated. Located surface plasmon resonance (LSPR) property and multiple scattering are proven playing an important role simultaneously.

Amplification coefficient gratings, written in a gain medium with laser beams produce the equivalent effect on spectral properties of radiation to static Bragg gratings, so relations of angular and spectral selectivity of volume static gratings can be used for dynamic gratings. This allows control influence on spectral properties.

Stimulated Raman Scattering (SRS) is important method of laser frequency conversion. Optical frequency-comb is a special kind of SRS which output multiple Stokes beam simultaneously, and lasers with mutiple wavelength have broad applications. In this paper, the optical frequency-comb generated by SRS of CO₂ is presented and the spectral range covers from 0.4 μm to 1.5 μm. Research also indicates that the characteristics of optical frequency-comb depends on the wavelength of pumping laser. For instance, the SRS photon conversion efficiency pumped by 1064 nm laser is high at 1248nm, 1510nm but that pumped by 532 nm laser is high at 574nm, 624nm 683nm. The different features are compared and analyzed by the use of the mechanism of four-wave mixing and the change of SRS gain coefficient with Stokes wavelength.

The emission of commercial solid-state lasers is shifted by Raman lasers to selected wavelengths suitable for trace gas detection. High power Raman lasers based on Ba(NO₃)₂, diamond and silicon detect CO₂, O₃ and H₂O. Raman lasers base on the physical effect of stimulated Raman scattering. Two new Raman crystals (Spodumene and LuAlO₃) are investigated and their SRS-spectra is shown.

A high-pressure CO₂ laser with large discharge x-section of 22×22 mm and gain length of 800 mm has been developed for use as regenerative or multi-pass amplifier in high energy ps laser systems. A test system with reduced electrode length was constructed to establish initial design parameters for the full-size system. The design of the final amplifier is described, and measured performance characteristics are presented. The system can be operated at gas pressures of up to 12 bar and provides stable discharges with a small signal gain of >3 %/cm. Further scaling of the laser to x-sections of up to 50×100 mm will be discussed and preliminary results of a discharge test system for development of novel discharge excitation techniques will be presented.

We study guided self-imaging schemes useful to obtain Passive Laser Resonators Coupling (PLRC) for Coherent Beam Combining (CBC). Self-imaging schemes are well established methods for PLRC, but they are usually considered limited by large values of edge diffractive losses. In facts the Talbot effect (exact diffractive self-imaging) is obtained with infinite arrays only. This problem can be opposed by using a guided self-imaging that extends the finite array by means of a kaleidoscopic effect. This can be done in 2D with square or hexagonal symmetry or generating an infinite 1D array by winding up a linear array on a circle. In this case cyclic boundary conditions exactly produce an infinite array in the azimuthal direction. We report on numerical simulations performed to assess the coupling efficiency with particular attention to the alignment tolerance. Calculations are carried out with reference to a system of several fiber lasers. Our results indicate this as a viable solution for fiber lasers CBC.

A latest results obtained in THL-100 hybrid femtosecond laser system operating in the visible spectral range are presented and the ways of the peak power increase of the laser beam are discussed.

In self-amplified spontaneous emission free electron laser devices, the combined effect of slippage and energy spread impose significant limitations on the power performance. We provide a scaling formula capable of parameterizing the effect in terms of the electron beam parameters and discuss mechanisms leading to the output power optimization.

A feasibility of in situ quantitative multielemental analysis during parts production by additive manufacturing technology (coaxial laser cladding) has been demonstrating for the first time using laser induced breakdown spectroscopy (LIBS). Compact LIBS probe was developed to equip the laser cladding head installed at industrial robot for analysis of key components (carbon and tungsten) during the synthesis of high wear resistant coatings of nickel alloy reinforced with tungsten carbide particles. It was demonstrated that the only acceptable choice for LIBS sampling was the melt pool due to non-uniform distribution of tungsten carbide grains in the upper surface layer. We have not observed any impact of melt pool ablation by LIBS probe on cladding process and clad properties according to clad cross-sections study by optical and scanning electron microscopies. The feasibility of in-situ LIBS quantitative elemental analysis of key components (carbon and tungsten) has been demonstrated during the cladding process. LIBS technique was demonstrated to be a good tool for real-time detection of cladding process failures. For example, failures with powder flows during cladding process resulted in undesirable variation of components concentrations and such problems were identified in real time by LIBS measurements.

We present a high energy UV, 308 nm Xenon-Chloride excimer laser system providing a precisely controlled ultra-fast (nanosecond scale) thermal processing of semiconductor devices. The main elements of the system are described and key performance indicators are presented.

Thin organic samples and silicon nitride membrane were measured by means of a single-shot near edge x-ray absorption fine structure (NEXAFS) spectroscopy, using a laboratory laser produced plasma soft x-ray (SXR) source. High power nanosecond laser pulse from an Nd:YAG laser is interacting with a double stream gas puff target, forming krypton/helium plasma. Efficient emission in the "water window" spectral region and specially designed grazing incidence SXR spectrometer allow one to obtain simultaneously emission spectrum of the source and transmission spectrum of the investigated sample. Calculated absorption spectrum is then independent of source energy fluctuations and mechanical instabilities. Fine structures near the carbon K-α edge of polyethylene terephthalate (PET) film and ascorbic acid sample, as well as features near nitrogen K-α edge of silicon nitride membrane are revealed. Features near carbon absorption edges are compared with a multishot NEXAFS experiment and numerical simulations. Spectral resolution is comparable with early synchrotron measurements.

Photoionization and ablation of an ensemble of solid metal dust particles under the influence of high-power infrared laser q-switched pulses are under experimental investigation.

Laser-induced periodic surface structures (LIPSS) formation was observed inside a microhole produced in an optical fiber, when the femtosecond laser pulse number and the laser energy were optimized to be 150 and 30μJ, respectively. We have also found that LIPSS were formed only at the bottom of the microhole, which may be responsible for the strong evanescent field produced by the femtosecond laser propagating inside the microhole.

Optically pumped CO₂ lasers can operate with high efficiency, high repetition rate and large bandwidths, suitable for producing ultra-short pulses at terawatts to petawatts, in contrast to conventional discharge-pumped CO₂ lasers, which are restricted by the requirements of discharge dynamics in high-pressure gas. We show how an optically pumped CO₂ laser can be realised and we consider its application in laser-driven acceleration. There is potential to replace conventional transversely excited atmospheric CO₂ lasers with diode-pumped solid-state lasers as a pump laser for a high-pressure CO₂ gain medium, making it suitable for amplifying ultra-short pulses. We show that by driving a laser plasma wakefield accelerator with an ultra-short pulse CO₂ laser, a very high charge, high average current, high energy accelerator can be constructed. This could have a major impact on the application of these novel accelerators and radiation sources based on them.

A resistive target probes were employed to obtain unique characteristics of the dynamic of the laser-matter interaction through the observation of a return current neutralizing the positive target charge caused by electrons definitely leaving the plasma. Experimental observations show that three phases of the laser-produced plasma are well recognized, i.e. the plasma ignition phase, active plasma phase, and afterglow phase already for the lowest laser intensity at the focal point of 10⁸ Wcm^-2 . Then, at higher intensities the first two phases strongly dominate and the target current reaches a value of 10 kA. A throughput of plasma barrier for the fastest electrons leaving the plasma is defined, and its value is experimentally estimated. We also present key experiments proving that the duration of the electrical charge on the target is much longer than the duration of the laser-plasma interaction for the entire intensity range.

Here we explore ways of transforming laser radiation into incoherent and coherent electromagnetic radiation using laserdriven plasma waves. We present several examples based on the laser wakefield accelerator (LWFA) and show that the electron beam and radiation from the LWFA has several unique characteristics compared with conventional devices. We show that the energy spread can be much smaller than 1% at 130-150 MeV. This makes LWFAs useful tools for scientists undertaking time resolved probing of matter subject to stimuli. They also make excellent imaging tools. We present experimental evidence that ultra-short XUV pulses, as short as 30 fs, are produced directly from an undulator driven by a LWFA, due to the electron bunches having a duration of a few femtoseconds. By extending the electron energy to 1 GeV, and for 1-2 fs duration pulses of 2 nm radiation peak powers of several MW per pC can be produced. The increased charge at higher electron energies will increase the peak power to GW levels, making the LWFA driven synchrotron an extremely useful source with a spectral range extending into the water window. With the reduction in size afforded by using LWFA driven radiation sources, and with the predicted advances in laser stability and repletion rate, ultra-short pulse radiation sources should become more affordable and widely used, which could change the way science is done.

The main characteristics of the ENEA Discharge Produced Plasma (DPP) Extreme Ultraviolet (EUV) source are presented together with results of irradiations of various materials. The DPP EUV source, based on a Xe–plasma heated up to a temperature of 30 ÷ 40 eV, emits more than 30 mJ/sr/shot at 10 Hz rep. rate in the 10 ÷ 18 nm wavelength spectral range. The DPP is equipped with a debris mitigation system to protect particularly delicate components needed for patterning applications. The ENEA source has been successfully utilized for sub–micrometer pattern generation on photonic materials and on specifically designed chemically amplified resists. Details down to 100 nm have been replicated on such photoresists by our laboratory–scale apparatus for contact EUV lithography. Preliminary EUV irradiations of graphene films aimed at modifying its properties have been also performed.

Radiation hardness assurance at component or subsystem level is one of the major concerns in all phases of a spacebased project. The radiation effects and methodology used for laboratory testing are discussed.

Mid-term opportunities to launch a high-power spaceborne laser are highly restricted due to technical, economic and political issues of it`s design, satellite deployment and system performance. The key technologies, providing the midterm development of a preproduction prototype of a spaceborne laser for small scale debris removal from the low-Earth orbits, are analyzed. A novel approach of nuclear power supplying of the spaceborne laser is presented. It is claimed that nuclear power supplying and electric jet engine technologies implemented in a spacecraft with an onboard laser would provide not only all the energetic requirements, but also orbital maneuvering strong capabilities. The latter would ensure more consistent follow-on rendezvous conditions with space objects, and hence, more efficient low orbital debris removal scenarios.

Lidar fluorosensors, i.e. laser radars based on laser induced fluorescence (LIF), have been extensively operated by the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) to real-time monitoring of water bodies. LIF spectra contain unique signatures of phytoplankton pigments, chromophoric dissolved organic matter and dispersed impurities (crude oils). In this paper, we present technological innovations and oceanographic campaigns carried out by the Diagnostics and Metrology Laboratory (FSN-TECFIS-DIM) of ENEA. In particular, the lidar fluorosensors developed in 2017 within the RITMARE and RIMA projects will be described, and the preliminary results of the High North 18 and Flex 2018 campaigns will be reported.

Lighting is no longer just turning on a lamp. Solid State Lighting allows reaching high energy savings and visual perception, but the path to success was not easy: in the early years when LED were available on the market, several concerns slowed down their widespread implementation. The large spread of Lighting Systems based on LED arrived only when products quality increased thanks also to the development of new metrology paradigms able to certify LED performances peculiarities. Currently LED promises to widespread in every lighting application increasing not only energy savings, but night-time safety, health and wellbeing thanks to lighting systems based on connectivity and smart controls, but new design paradigms and metric headed to new objectives are necessary, therefore there will be no need to measure only energetic and light performances. We are moving toward an approach more sensitive to Human Needs: energy savings and light efficiency are of no more the only interest to focus on, we need to measure performance of a lighting system in terms of not only SI quantities. We need now a consolidated metrology and design approach to Smart System for complexity and suitable to compare lighting quality in term of human perception and wellbeing.

The archaeological cloth known as the Shroud of Turin is a controversial object for both scientific and religious reasons. After a brief introduction on the scientific data about the age of the cloth and the microscopic complexity of the images embedded on it, we discuss the unique reddish color of blood stains on the Shroud, which caught the attention of several scholars in the last decades. Various hypotheses were proposed to explain the blood stains reddish color, and the experimental tests produced uncertain results because data were not sufficient or were obtained in vitro. We have tested the strength of two hypotheses, namely, the long term influence of ultraviolet (UV) light on high-bilirubin blood and the presence of carboxyhemoglobin, respectively by RGB color analyses of high-bilirubin blood after irradiation by ns excimer laser pulses and UV lamp and by the study of the spectral reflectance of the blood stains on the Shroud.