This PDF file contains the front matter associated with SPIE Proceedings Volume 9729 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Multiple variants of the Diode Pumped Alkali Laser (DPAL) have recently been demonstrated at the Air Force Research Laboratory (AFRL). Highlights of this ongoing research effort include: a) a 571W rubidium (Rb) based Master Oscillator Power Amplifier (MOPA) with a gain (2α) of 0.48 cm^-1, b) a rubidium-cesium (Cs) Multi-Alkali Multi-Line (MAML) laser that simultaneously lases at both 795 nm and 895 nm, and c) a 1.5 kW resonantly pumped potassium (K) DPAL with a slope efficiency of 50%. The common factor among these experiments is the use of a flowing alkali test bed.

Cs DPAL operation using Ethane, Methane and mixtures of these hydrocarbons with noble gases He and Ar as a buffer gases for spin-orbit relaxation was studied in this work. The best Cs DPAL performance in continuous wave operation with flowing gain medium was achieved using pure Methane, pure Ethane or a mixture of Ethane (minimum of 200 Torr) and He with a total buffer gas pressure of 300 torr.

Modeling of static and flowing-gas subsonic, transonic and supersonic Cs and K Ti:Sapphire and diode pumped alkali lasers (DPALs) is reported. A simple optical model applied to the static K and Cs lasers shows good agreement between the calculated and measured dependence of the laser power on the incident pump power. The model reproduces the observed threshold pump power in K DPAL which is much higher than that predicted by standard models of the DPAL. Scaling up flowing-gas DPALs to megawatt class power is studied using accurate three-dimensional computational fluid dynamics model, taking into account the effects of temperature rise and losses of alkali atoms due to ionization. Both the maximum achievable power and laser beam quality are estimated for Cs and K lasers. The performance of subsonic and, in particular, supersonic DPALs is compared with that of transonic, where supersonic nozzle and diffuser are spared and high power mechanical pump (needed for recovery of the gas total pressure which strongly drops in the diffuser), is not required for continuous closed cycle operation. For pumping by beams of the same rectangular cross section, comparison between end-pumping and transverse-pumping shows that the output power is not affected by the pump geometry, however, the intensity of the output laser beam in the case of transverse-pumped DPALs is strongly non-uniform in the laser beam cross section resulting in higher brightness and better beam quality in the far field for the end-pumping geometry where the intensity of the output beam is uniform.

We report a method for locking the output wavelength and reducing the spectral linewidth of diode lasers by feeding back light to the emitters from a wavelength selective external optical cavity. Ten years ago our team developed a stepped-mirror that allowed a single external cavity to lock the wavelength of a stack of diode array bars by equalizing path lengths between each emitter and the grating. Here we report combining one such step-mirror external cavity with an array of power dividers, each sending a portion of this feedback power to a separate diode array bar stack.

A two color pumping scheme has been demonstrated for the Rb D₂ line laser (780 nm). Photoexcitation of ground state Rb-rare gas collision pairs at 760 nm (peak of Rb D₂ blue satellite in Xe) and ~794.5 nm simultaneously improves the slope efficiency of single color pumping by a factor of 1.8. Furthermore, each D₂ laser photon removes 102 cm^-1 of thermal energy from the laser gain medium.

A numerical simulation code for a diode pumped alkali laser (DPAL) was developed. The code employs the Fresnel- Kirchhoff diffraction integral for both laser mode and pump light propagations. A three-dimensional rate equation set was developed to determine the local gain. The spectral divergence of the pump beam was represented by a series of monochromatic beams with different wavelengths. The calculated results showed an excellent agreements with relevant experimental results. It was found that the main channel of the pump power drain is the spontaneous emission from the upper level of the lasing transition.

Alkali vapor lasers commonly use methane (CH₄) or ethane (C₂H₆) to induce energy transfer between the optically pumped level (n²P_3/2) and the upper laser level (n²P_1/2). A complication is that the alkali metal eventually reacts with the hydrocarbons. The reaction becomes exothermic for alkali atoms that have been excited by energy pooling processes such as Rb(5p)+Rb(5p)→Rb(*)+Rb(5s), where * indicates 6s, 6p or 4d. We have used laser pump-probe methods to examine the reactions of Rb(n²P) with CH₄, and C₂H₆ for states with n=6 and 7. Pump-probe measurements indicated a loss of Rb due to a reaction. Surprisingly, the RbH product was not detected with CH₄, and C₂H₆. High-level ab initio calculations have been used to study the reactive interactions between Rb, methane and ethane.

The optically pumped rare-gas metastable laser is a chemically inert analogue to diode-pumped alkali (DPAL) and alkaliexciplex (XPAL) laser systems. Scaling of these devices requires efficient generation of electronically excited metastable atoms at number densities in excess of 10¹² cm^-3 in a continuous-wave electric discharge in flowing gas mixtures with helium diluent. This paper describes continuing investigations of the use of linear microwave micro-discharge arrays to generate metastable argon atoms, Ar (4s, 1s₅) (Paschen notation), in flowing mixtures of Ar and He at atmospheric and reduced pressures, in optical pump-and-probe experiments for laser development. We describe initial experimental investigations of several key aspects of concepts for scaling to higher output powers. This includes initial data on the dependence of argon metastable production and optically pumped gain on micro-discharge gap size, pressure, and discharge power. We have observed clearly measureable gain at pressures down to 85 Torr. We have also developed an overlapping dual-array micro-discharge-flow configuration, to conduct detailed measurements of Ar(1s₅) production and loss. Spatially resolved measurements of Ar(1s₅) distributions in discharge-flow provide preliminary indications of 20-50 μs collisional lifetimes of argon metastable atoms after they exit the active microplasma. This information is relevant to modeling the recycling of Ar(1s₅) in the optically pumped laser, and to the scaling architecture of the optically pumped system. The dual-discharge investigations demonstrate the potential for volume scaling of the active gain medium in a simple multi-discharge, flow-through configuration.

A metastable argon laser operating at 912 nm has been demonstrated by optically pumping with a pulsed titanium sapphire laser to investigate the temporal dynamics of an Advanced Noble Gas Laser (ANGL). Metastable argon concentrations on the order of 10¹¹ cm^-3 were maintained with the use of a radio frequency (RF) capacitively coupled discharge. The end-pumped laser produced output powers under 2 mW of average power with pulse lengths on the order of 100 ns. A comparison between empirical results and a four level laser model using longitudinally average pump and inter-cavity intensities is made. An alternative, highly-efficient method of argon metastable production for ANGL was explored using carbon nanotube (CNT) fibers.

The production of relatively high densities of Ar* metastables (>10¹² cm^-3) in Ar/He mixtures, at total pressures close to 1 atm, is essential for the efficient operation of an optically pumped Ar* laser. We have used emission spectroscopy and diode laser absorption spectroscopy measurements to observe the production and decay of Ar* in a parallel plate pulsed discharge. With discharge pulses of 1 μs duration we find that metastable production is dominated by processes occurring within the first 100 ns of the gas break-down. Application of multiple, closely spaced discharge pulses yields insights concerning conditions that favor metastable production. This information has been combined with time-resolved measurements of voltage and current. The experimental results and preliminary modeling of the discharge kinetics are presented.

Optically pumped all-rare-gas laser (OPRGL) with unique properties was recently proposed. To study this promising laser system it is necessary to have reliable diagnostics for the active medium. A set of pressure broadening coefficients, for self- and foreign- gas collision partners, is needed for measurements of the number density of metastable atoms and temperature in a rare gas discharge plasma by means of spectroscopy. However, literature analysis had shown that pressure broadening coefficients for rare gas lines in mixtures that are of interest for OPRGL’s are surprisingly hard to find, or were not yet measured. Diode laser absorption spectroscopy was employed for measurements of pressure broadening coefficients for the Krypton 811.3 nm line in an RF discharge. A multi-quantum well diode laser (L808P030, Thorlabs) with an original short external cavity was used as a source of probe radiation. The natural isotopic distribution of Kr was taken into account, and an appropriate fit function was constructed. This permitted the determination of pressure broadening coefficients using the natural mixture of isotopes. The coefficients for the Kr 811.3 nm line at 300 K, measured for the first time, were ξKr-Ne = (1.50 ± 0.05) ×10^-10 s^-1cm³ for broadening by Neon, and ξKr-Ar = (3.5 ± 0.3) ×10^-10 s^-1cm³ for broadening by Argon.

Diode laser pump source with narrow emitting spectrum for optically pumped metastable rare gas laser (OPRGL) of argon was achieved by employing a complex external cavity coupled with volume Bragg grating (VBG). A commercially available c-mount laser diode with rated power of 6 W was used and studied in both the free running mode and VBG external cavity. The maximum output power of 3.9 W with FWHM less than 25 pm and peak wavelength locked around 811.53 nm was obtained from the VBG external cavity laser diode. Precise control of VBG temperature enabled fine tuning of the emission wavelength over a range of 450 pm. Future researches on OPRGL of argon will benefit from it.

The kinetics of the processes in an O₂/I₂/Ar/H₂O gas flow that is irradiated simultaneously by light at wavelengths near 500 nm and 1315 nm, is considered. Radiation at 500 nm is used to photodissociate about 1% of the iodine molecules. The radiation at 1315 nm excites atomic iodine to the ²P_1/2 state. Singlet oxygen molecules are produced via the energy exchange process I(²P_1/2)+O₂(X³Σ) → I(²P_3/2) + O₂(a¹Δ), while I(²P_1/2)+O₂(a¹Δ) energy pooling produces b¹Σ oxygen. I(²P_1/2) and O₂(b¹Σ) then accelerate the dissociation of I₂. This active medium may reach ~40 W/cm^–2 at an optical efficiency of 50%.

Experiments and modeling of CH₃I dissociation in the plasma generated by a 40 MHz RF discharge were performed. A discharge chamber of an original design, consisting of quartz tubes between two planar electrodes, permitted the production of iodine atoms with number densities up to 2×10¹⁶ cm^-3. In this discharge chamber, contamination of the walls of the tubes did not hinder discharge stability, providing a good iodine production rate. Addition of oxygen into Ar:CH₃I mixture resulted in a substantial increase in iodine extraction efficiency. When the discharge power reached 200 W, complete CH₃I dissociation in a Ar:CH₃I:O₂ mixture was observed. The fraction of discharge power spent on iodine atom production at a 0.17 mmol/s CH₃I flow rate was 16%.

We report on an investigation of novel 2 μm thulium (Tm)-based laser accelerator driver (LAD) offering efficient generation of high-energy pulses with high-peak power at high pulse repetition rate (PRF), high efficiency, and with near-diffraction-limited beam quality (BQ). Laser acceleration of electrons by ultrashortpulse laser-generated plasmas offers accelerators of much reduced size and cost compared to conventional accelerators of the same energy, thus replacing the traditional mammoth-size and costly accelerator research facilities with room-size systems¹. A LAD operating at 2 μm wavelength offers ponderomotive forces four times that of 1 μm wavelength and six times that of a traditional 0.8 μm wavelength LAD. In addition, the Tm bandwidth of nearly 400 nm offers > 15% tunability and generation of ultrashort pulses down to <30 fs. The “2-for- 1” pump quantum efficiency of the Tm ion enables > 20% wall-plug efficiency. This work presents a preliminary analysis of Tm-based LAD configurations.

The excellent biocompatibility property of Grade 5 titanium alloy has made its desirability largely increasing in the field of biomedical. The titanium alloy (Ti6Al4V) was modified with the addition of 3 weight percent (wt %) copper via a laser deposition process using the Ytterbium fiber laser with a wavelength of 1.047 μm. Therefore, this paper presents the effect of laser power on the microstructural behaviour and strength of the modified Ti6Al4V+Cu alloy. The laser powers were varied between 600 W and 1600 W respectively while all other parameters such as the scanning speed, powder flow rates and gas flow rates were kept constant. The melt pool and width of the deposited alloy increases as the laser power was increased. The α-lamella was observed to be finer at low laser power, and towards the fusion zone, Widmanstettan structures were fused and become smaller; and showing an evidence of α-martensite phases. The strength of the modified alloy was derived from the hardness values. The strength was observed to increase initially to a point as the laser power increases and afterwards decreased as the laser power was further increased. The improved Ti6Al4V+Cu alloy can be anticipated for biomedical application.

Diode-pumped CW and passively Q-switched lasers of Nd:GdLuAG mixed garnet at 1123 nm were demonstrated. The maximum average output power of CW operation was 4.13 W. For Q-switched operation, the average output power was 800 mW, the corresponding single pulse energy was 133.8 μJ. The Nd:GdLuAG laser emitting at 1123 nm was obtained for the first time to the best of our knowledge, which proves that the Nd:GdLuAG mixed garnet has a better ability of energy storage than Nd:YAG in 1123 nm oscillation.

Identifying methane anomalies responsible for the temperature increase, by hiking trails in the Arctic requires great human labor .According to the tentative forecast by the year 2100 Arctic permafrost will greatly deteriorate, which will have numerous consequences. Indeed, release of less than 0.1% of the organic carbon stored in the upper 100-meter permafrost level (approximately 10000 ppm of carbon in the СН4 form) can double concentration of atmospheric methane, which is roughly 20 times more potent greenhouse gas than the CO2. Necessary to create a Raman lidar for monitoring of emissions of methane hydrate from the permafrost.

We suggest using modulated self-injected radiation to achieve lasing regimes similar to Q-switching in a CO₂ laser without loss of average output power. The feasibility of this solution is verified both theoretically and experimentally. Our theoretical model of CO₂ laser is based on a standard six-temperature model supplemented with terms accounting for self-injection of laser output radiation. We show that temporal modulation of self-injected radiation with power several percent that of the laser output power in external optical systems achieves pulsed-periodical generation regime. The model is experimentally verified using a commercial CO₂ laser. With modulation less than 5% of the output power pulsed-periodical lasing was realized. Pulse duration obtained was 300 ns with repetition rate of 20 kHz and power maximum-to-average rate around 20.