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The results obtained within the framework of the present project can be tentatively grouped as follows. The first part (Sections 2, 3) of the present project is devoted to a detailed description of the schemes for subpicosecond USP amplification in a chain of large-aperture KrF amplifiers with electron-beam pumping, as well as lasing dynamics of combined (amplitude-modulated) high-energy 100-ns long UV laser pulses in the regime of regenerative amplification of a USP train. Experimental and theoretical studies of filamentation of the USP UV and IR laser beams in atmospheric air, of the processes of air ionization, electron loss and revival in the laser-produced air plasma are described in the second part (Sections 4-6). The third part (Sections 7, 8) demonstrates the advantages of amplitude-modulated pulses in the production of extended plasma channels in the atmosphere and laser guiding of high-voltage electric discharges., The original document contains color images.

In the first year we focused on Multiphoton processes and their amplification in a nanocomposite medium to achieve upconversion lasing. This work was accomplished by collaboration with Professor Anderson Gomes s group at Recife, Brazil. A significant research was accomplished and the resulting work was submitted to Optics Express, a high impact journal, for publication. We have received the reviewers s comment. The two reviews who were asked to evaluate, have both given a positive review and asked for some revision. This work reports the operation and characterization of an upconversion random laser emitting at 560nm, when directly pumped by three photon excitation at the near IR wavelength of 1350nm in a colloidal dye solution in the weakly scattering regime. Using a special dye with a high three-photon cross-section and TiO2 nanoparticles (250 nm diameter), optimized upconverted emission was obtained for particle densities of 2 x 109/cm3. A strong dependence on the nanoparticle concentration and the pumping area was verified. The presence of spikes with linewidths 0.4 nm in the emitted spectrum is the signature of coherent emission from this three-photon pumped random laser., The original document contains color images.

Laser cooling of atoms and molecules is an enabling technology for a number of applications, including atomic clocks, navigational sensors and quantum information processing. So far applications of this powerful technique have been limited to a handful of species as the conventional laser cooling requires closed (cycling) transitions. Here we will theoretically investigate the possibility of extending laser cooling techniques to a much wider class of multilevel atoms and molecules. The basic idea is to employ controllable coherent trains of laser pulses (frequency combs). Recently the power and spectral coverage of frequency combs have grown considerably with projected average powers above 10 kW. We will take advantage of this emerging technology. In the first project, we will exploit spectral selectivity of the combs and develop a time sequence protocol to move population across multiple levels with the goal of optimizing Doppler cooling for multi-level systems. In the second project, we will investigate stimulated optical force to slow heteronuclear molecules using ro-vibrational transitions inside the ground electronic state. Further, we propose to apply concepts from quantum-control and learning algorithms for finding optimal pulse sequences for cooling molecules even when the knowledge about their internal structures is imprecise.

We carried out both fundamental and developmental research on a novel DBR-free membrane reflector based VCSELs (MR-VCSELs), with the demonstration of 1550 nm lasing operation at room temperature. The laser consists of an InGaAsP QW based heterostructure, sandwiched in between two single-layer photonic crystal Fano resonance silicon nanomembrane reflectors. Detailed theoretical and experimental work has been carried out towards high performance single layer broadband membrane reflector demonstrations, MR-VCSEL cavity design, and optically pumped laser demonstration at room temperature. The thermally engineering membrane reflectors have also been investigated for high performance lasers.

We performed experiments to test the hypothesis that partial counter-doping of sulfur-hyperdoped silicon would create a partially filled intermediate band, which could be used for sub-bandgap photodetection. We fabricated counter-doped Si:S:B of well-controlled crystal quality and dopant concentration-depth profile, and we made three independent measurements of photoconductivity: one with contacts, and two without contacts. These measurements were per-formed in collaboration with the Persans group at RPI, the Buonassisi group at MIT, and the Warrender group at Ben t Labs. In all cases, the sub-bandgap photoresponse is negligibly small. We conclude from these results that the lifetime of photo-excited carriers is very small-- less than about 10 ns. Sub-bandgap photodetection using counter-doped silicon does not appear to be promising. The optoelectronic response appears to be less desirable than those of certain deep-level transition metals in Si, which appear much more promising for future investigation.

We have created a new class of lasers known as Hollow-core Optical Fiber Gas LASer (HOFGLAS) combining the advantages of fiber lasers with those of gas lasers. Pulsed HOFGLAS has been studied with C2H2 and HCN gas inside the hollow core of a kagome structured photonic crystal fiber, and near- ideal efficiencies were realized. The gases are optically pumped near 1.5 microns using 1 ns-pulses and yield laser emissions near 3 microns. Furthermore, we have improved fiber loss near 3 microns to below the dB/m-levels. In addition, we have demonstrated a CO2 waveguide (silver coated hollow capillary) laser lasing at 4.3 microns when pumped at 2.0 microns with 5-nanosecond pulses, and demonstrated a slope efficiency in terms of absorbed power of 22%. We have explored the feasibility of cw lasing from a hollow fiber filled with molecular iodine (I2) when pumped at 532 nm, and have spectrally resolved the intensity dependence of fluorescence from a 10-cm photonic crystal fiber of 80-microns diameter filled with I2. In an effort to achieve near-IR sources for testing fiber transmission, we have demonstrated a novel thulium/holmium fiber laser near 2 microns., The original document contains color images.

For the first time, fiber Raman lasers produced near-infrared wavelengths from directly diode-pumped by high-power multimode diode sources and on a fiber OPO at red wavelengths. The fiber Raman laser reached 20 W of output power at 1019 nm, pulsed operation at 835 nm, and M square = 2 at 1019 nm from a double-clad fiber Raman laser. These three results are all world records or world firsts. It was also found that the fiber OPO suffers from below-par gain at the anti-stoles wavelength of 718 nm. An OTDR is constructed to see how the 718 nm power evolves in the gain fiber. This is likely the first time this has been done. We find that the power grows exponentially for the first part of the fiber. After that, the growth is much slower. Even in the high-growth part, the gain is only 10% of what theory predicts. The reason for this is unclear, but we suspect it is caused by fiber longitudinal fiber variations, which modify the dispersion and therefore disrupt the phase-matching. Attempts to control the dispersion by heating a section of the fiber showed that the conversion did change, but for the worse rather than for the better in these preliminary attempts., The original document contains color images.

We demonstrate longwave infrared (LWIR) generation with an optical parametric oscillator (OPO) based on quasi-phasematched orientation-patterned gallium arsenide (OPGaAs). The OPGaAs OPO was directly pumped with a Q-switched lambda=2.054 micron Tm,Ho:YLF laser. OPGaAs samples representing three different grating periods were used to explore the LWIR OPO performance yielding outputs ranging from lambda=2.5-2.7 micron (signal) and lambda=8.8-11.5 micron (idler). Slope efficiencies for the combined signal and idler outputs reach as high as 26% while slope efficiencies for only the idler reached 8%. Spectral measurements of OPO output confirm good agreement with theoretical calculations., Prepared in collaboration with the University of Dayton, OH.

Tomographic particle image velocimetry (tomographic-PIV) is a recently developed measurement technique used to acquire volumetric velocity field data in liquid and gaseous flows. The technique relies on line-of-sight reconstruction of the rays between a 3D particle distribution and a multi-camera imaging system. In a turbulent flame, however, index-of-refraction changes resulting from local heat-release may inhibit reconstruction and thereby render the technique infeasible. The objective of this study was to test the efficacy of tomographic-PIV in a turbulent flame. An additional goal was to determine the feasibility of acquiring usable tomographic-PIV measurements in a turbulent flame at multi-kHz acquisition rates with current generation laser and camera technology. To this end, a setup consisting of four CMOS cameras and a dual-cavity Nd:YAG laser was implemented to test the technique in a lifted turbulent jet flame. While the cameras were capable of kHz-rate image acquisition, the laser operated at a pulse repetition rate of only 10 Hz. However, use of this laser allowed exploration of the required pulse energy and thus power for a kHz-rate system. The imaged region was 29 28 2.7mm in size. The tomographic reconstruction of the 3D particle distributions was accomplished using the multiplicative algebraic reconstruction technique. The results indicate that high quality tomographic-PIV measurements in a turbulent flame are possible with laser pulse energies of 25mJ, which is well within the capability of current-generation kHz-rate diode-pumped solid state lasers., The original document contains color images.

A field deployable Tunable Diode Laser Atmospheric Spectroscopy (TDLAS) device has been used to investigate the atmospheric absorption of Diode Pumped Alkali Laser (DPAL) wavelengths at a distance of 2,000 meters in a maritime environment. The spectral regions surrounding the cesium (Cs) and rubidium (Rb) DPAL emission lines were examined in order to determine the effects of absorption by the (000)---(003) and (000)---(102) water vapor vibrational bands over a distance of 2 km. This data was compared with data previously collected at shorter path lengths and the scaling of absorbance with various factors was analyzed. Spectral data for the Cs spectral region were analyzed to determine temperature and concentration estimates with statistical errors of between 0.1 and 1.14 deg. C and 11 and 1x10(to the 14th power). Temperature and pressure estimates were within 6% and 25%, respectively, of those reported by the weather station at the test site. The rotational dependence of broadening rates was investigated. By analyzing the widths of two widely spaced rotational lines, it was found that the observed linewidths were approximately 25% higher than those reported in existing spectral databases with a scaling ratio of the observed rates approximately 5% below the ratio of the reported values., The original document contains color images.

The Baranger model is used to compute collisional broadening and shift of the D1 and D2 spectral lines of M + Ng, where M = K, Rb, Cs and Ng = He, Ne, Ar, using scattering phase shift differences which are calculated from scattering matrix elements. Scattering matrix elements are calculated using the Channel Packet Method where the collisions are treated non-adiabatically and include spin-orbit and Coriolis couplings. Non-adiabatic wavepacket dynamics are determined using the split-operator method together with a unitary transformation between adiabatic and diabatic representations. Scattering phase shift differences are thermally weighted and integrated over energies ranging from E = 0 Hartree up to E = 0.0075 Hartree and averaged over values of total angular momentum that range from J = 0.5 up to J = 400.5. Phase shifts are extrapolated linearly to provide an approximate extension of the energy regime up to E = 0.012 Hartree. Broadening and shift coefficients are obtained for temperatures ranging from T = 100 K up to T = 800 K and compared with experiment. Predictions from this research find application in laser physics and specifically with improvement of total power output of Optically Pumped Alkali Laser systems., The original document contains color images.

This technical note explores the design of the monolithic neodymium (Nd): yttrium aluminum garnet (YAG) laser used in the diode-pumped laser ignition system (DPLIS). Emphasis is placed on the divergence of the output beam and the dependence of the output power on misalignment of the mirrors polished on the two ends of the rod. A tradeoff between divergence and misalignment tolerance is identified and quantified. With input from the manufacturer on fabrication cost, the optimum design can be determined., The original document contains color images.

Numerical simulations are used to solve the coupled partial differential equations describing stimulated Brillouin scattering (SBS) built up from random thermal phonons as a function of time and the longitudinal spatial coordinate in an optical fiber. In the case of a passive fiber, a laser beam is incident with constant power, but its frequency is linearly ramped at 1.55 microns at a rate of up to 10(to the 16th power) Hz/s. High chirp rates lead to an increased Brillouin spectral bandwidth and decreased gain. The resulting SBS suppression is well described by an adiabatic model and agrees with experimental results. For an 18-m active fiber pumped at 1.06 microns and chirped at up to 2 x 10(to the 16th power) Hz/s, the suppression enables output laser powers in the kilowatt range while maintaining a narrow instantaneous linewidth., Published in the IEEE Journal of Quantum Electronics, v48 n12 p1542-1546, December 2012. Prepared in collaboration with U.S. Naval Academy, Annapolis, MD, and California Institute of Technology, Pasadena, CA.

Laser weapons may find broad military applications against many types of targets from tactical to strategic missions on the ground, sea, air, and in space. The highest priority and nearest term applications are the defense against ballistic missiles, cruise missiles, rockets, artillery, and mortars. Recent progress in developing diode-pumped solid-state lasers has been dramatic and enables electrically driven systems. However, thermal management and associated beam quality issues continue to stress the design and may limit employability and effectiveness., Published in SPIE Newsroom, 2012.

The MIT Single-Cycle Pulse and Isolated Attosecond Pulse Generation Program enabled to demonstrate, for the first time, a novel pulse synthesis technology based on ultrabroadband multi-color OPCPAs and explore the generation and control of isolated attosecond XUV pulses using the synthesized pulses. We developed several relevant ultrafast laser technologies and attosecond measurement systems, such as picosecond cryo-cooled Yb:YAG pump laser amplifiers, 2D and 4DSI pulse characterization techniques, optical balanced cross-correlator, CEP-jitter measurement apparatus, adiabatic DFG technique for high-energy broadband seeding, and a FROG-CRAB setup for attosecond XUV pulse characterization. Experimental demonstration of HHG with the 2.1 m source shows cutoff extension in Ar to 160 eV and promises the feasibility of coherent control of the HHG process with the synthesized sub-cycle electric-field transients after a further upgrade of the 800-nm OPCPA to the mJ level, as supported by our theoretical study. Since the scaling of energy and pulse duration is clearly possible by using higher-energy OPCPA pump lasers and/or by coherently adding different-color OPCPA systems, we believe that the high-energy pulse synthesis technology demonstrated under this grant has opened up a new high-energy ultrafast laser technology that is especially suited to attosecond and strong-field science., The original document contains color images.

We have demonstrated of a new class of optically pumped gas lasers inside a hollow-core photonic crystal fibers. Here, a molecular gas is confined to a hollow-core photonic crystal fiber whose transmission spans several octaves to reach the mid-infrared spectral region. The gas is pumped in the near-infrared (1.5 mum) and the laser produces mid-infrared light (3 mum) offering a potentially robust, efficient, and compact means of producing step-tunable eye-safe mid-infrared radiation well suited to a multitude of applications. During the course of the grant we have demonstrated for the first time mid-infrared lasing in both an acetylene and hydrogen cyanide filled kagome structured hollow-core photonic crystal fiber. These lasers are the first in a new class of infrared lasers based on the combination of hollow-fiber and optically pumped-gas technologies., The original document contains color images. Supported in part by ARO under contract W911NF-08-1-0332 and Joint Technology Office under contract W911NF-05-1-0507.

Interest in photonic crystal nanocavities is fueled by advances in device performance, particularly in the development of low-threshold laser sources. Effective electrical control of highperformance photonic crystal lasers has thus far remained elusive due to the complexities associated with current injection into cavities. A fabrication procedure for electrically pumping photonic crystal membrane devices using a lateral p-i-n junction has been developed and is described in this study. We have demonstrated electrically pumped lasing in our junctions with a threshold of 181 nA at 50 K. The lowest threshold ever demonstrated in an electrically pumped laser. At room temperature, we find that our devices behave as single-mode light-emitting diodes (LEDs), which when directly modulated, have an ultrafast electrical response up to 10 GHz corresponding to less than 1 fJ/bit energy operation. The lowest for any optical transmitter. In addition, we have demonstrated electrical pumping of photonic crystal nanobeam LEDs and have built fiber taper coupled electro-optic modulators. Fibercoupled photodetectors based on two-photon absorption are also demonstrated as well as multiply integrated components that can be independently electrically controlled. The presented electrical injection platform is a major step forward in providing practical low power and integrable devices for on-chip photonics., Prepared in IEEE Journal of Selected Topics in Quantum Electronics, v18 n6 p1700-1710, November/December 2012.

The output of high power fiber amplifiers is typically limited by stimulated Brillouin scattering (SBS). An analysis of SBS with a chirped pump laser indicates that a chirp of 2.5 x 10 to the 15th power Hz/s could raise, by an order of magnitude, the SBS threshold of a 20-m fiber. A diode laser with a constant output power and a linear chirp of 5 x 10 to the 15th power Hz/s has been previously demonstrated. In a low-power proof-of-concept experiment, the threshold for SBS in a 6-km fiber is increased by a factor of 100 with a chirp of 5 x 10 to the 14th power Hz/s. A linear chirp will enable straightforward coherent combination of multiple fiber amplifiers, with electronic compensation of path length differences on the order of 0.2 m., Published in Optics Express, v20 n14 p15872-15881, 2 July 2012.

Report developed under topic #A10-076, contract W911NF-11-C-0044. The purpose of the work is to make a proof-of-concept demonstration on THz generation by using a dual-frequency laser in a nonlinear crystal. We have achieved all the objectives set for Phase I. In particular, we designed CW and Q-switched Yb:YAG lasers, purchased all the components necessary, assembled them into laser systems, optimized the setups, and tested the performances of the CW and Q-switched laser systems. Such laser systems were pumped by diode lasers at 940 nm. We achieved the broadband emission from the CW laser system. By placing an etalon in the laser cavities, we generated a pair of the output wavelengths around 1 micron from the CW and Q-switched laser systems. As a result, we successfully demonstrated the dual-frequency operation of the CW and Q-switched laser systems. Using the Q-switched dual-frequency laser, we generated THz pulses in a GaP crystal based on difference-frequency generation. We also confirmed the polarizations of the THz output pulses. Based on our calculations, the THz output powers can reach 8.5 mW through optimizations. We developed a concrete plan for identification and detection of biological agents, which is regarded as the major application of the THz spectrometer., The original document contains color images.

The laser characteristics of waveguide sample manufactured by World Laboratory Co., Ltd. were evaluated. Maximum output of 120 W was obtained from approximately 264 W of absorbed power. Greater than 50% slope efficiency was achieved--the target of in this effort--without cracking the sample. However, in the region with 264 W or more absorbed power, saturation of the output power occurred. The saturation of this output is thought to be caused by a thermal cooling factor. To obtain higher output power, introducing a high-power sample cooling system is indispensable. Oscillation experiments of still-higher output are planned for a follow-on series of experiments.

A complimentary experimental/numerical investigation on the effect of counter-propagating pulsed lasers on molecular nitrogen was conducted. The experiment verified published theoretical predictions of the effect of laser intensity and gas pressure on the magnitude of induced density perturbations in the gas using a coherent Rayleigh-Brillouin scattering technique. The investigation further verified the use of a modified version of the SMILE DSMC code for more robust prediction of the effect of a non-resonant pulsed optical lattice on a neutral gas. The ambient pressure of molecular nitrogen was varied from 100 torr to 760 torr, and the pump laser energy was varied from 2 mJ to 25 mJ per pulse. The resulting scattered signal from the experiment was measured and compared with numerical predictions. Assuming that the signal of the experiment is proportional to the probe intensity and the square of the density perturbations induced by the pump lasers, the results of the experiment qualitatively support both theoretical predictions and numerical simulations., To be presented at the AIAA Thermophysics Conference (42nd) held in Honolulu, HI on 27-30 June 2011.

The central feature of the project was the development of a high-energy (30 J) high-power (petawatt) laser amplifier, which is seeded by an existing 100-terawatt, 30-fs laser system. This 2-pass, 10-cm aperture, Ti:Sapphire amplifier pumped by four high-energy (total of 100-J energy), high-repetition-rate (0.1 Hz), frequency-doubled Nd-Glass pump lasers, and a new, larger grating-based optical compressor, were installed in the Extreme Light Laboratory and successfully tested. The amplifier gain was measured, and shown to match the predictions of calculations based on an analytical model, which predict 1-PW peak power at maximum pumping. Transverse lasing was measured, and shown to be eliminated by the use of index-matching fluid surrounding the Ti:Sapphire crystal. The bandwidth of the amplifer was measured, and shown to be capable of supporting pulse compression to 25-fs duration. An optical protection system for the laser was also designed, installed, and successfully integrated?to prevent catastrophic damage to the amplifier by accidental bandwidth collapse.

Efficient, low-threshold and compact semiconductor laser sources are under investigation for many applications in high-speed communications, information processing and optical interconnects. The best edge-emitting and vertical-cavity surface-emitting lasers have thresholds on the order of 100 microA (refs 1,2), but dissipate too much power to be practical for many applications, particularly optical interconnects. Optically pumped photonic-crystal nanocavity lasers represent the state of the art in low-threshold lasers; however, to be practical, techniques to electrically pump these structures must be developed. Here, we demonstrate a quantum-dot photonic-crystal nanocavity laser in gallium arsenide pumped by a lateral p-i-n junction formed by ion implantation. Continuous-wave lasing is observed at temperatures up to 150 K. Thresholds of only 181 nA at 50 K and 287 nA at 150 K are observed-the lowest thresholds ever observed in any type of electrically pumped laser., Published in Nature Photonics, v5 p297-300, May 2011. Prepared in cooperation with Lawrence Berkeley National Laboratory, and with University of California, Berkeley.

This research seeks to gain greater insight on the mechanics of The Diode Pumped Alkali Laser through analytic modeling techniques. This work is an extension to a previous model developed by Dr. Gordon Hager, and focuses on the addition of pump-beam bandwidth. Specifically, it seeks to determine the effect that broadband pumping has on laser performance. The model incorporates all the fundamental parameters within the laser system, including alkali concentrations, collision partner concentrations, pump bandwidth, length and temperature of gain medium, transmission, and reflectivity. Baseline operating conditions set Rubidium (Rb) concentrations ranging from 1012 - 1014 atoms/cm3, corresponding to operating temperatures ranging from 50 -150 C. Ethane or Methane concentrations are varied corresponding to partial pressures from 100 - 500 Torr. The system is evaluated for incident beam intensity ranging from 0 - 30000 W/cm2, for both lasing and non-lasing system analysis. Output laser beam intensities scale well with input beam intensity and the model predicts optical to optical efficiencies of over 70%., The original document contains color images.

Diode Pumped Alkali Lasers (DPAL) have been scaled to greater than 100 W and exhibit slope efficiencies exceeding 80%, offering application for tactical laser weapons. The hybrid DPAL system combines efficient diode pumping with the good beam quality and thermal characteristics of gas lasers. Thermal effects on alkali concentration have been observed to degrade performance, while low speed flowing systems are in development. However, spatial gradients in temperature and concentrations have not previously been observed. In the present work, a 0.8 W/cm2 pump laser at the D1 frequency heats the medium in a T=50-100 deg C cesium heat pipe with 5 Torr nitrogen used for quenching. A 31 muW/cm2 diode laser probes the spectral absorbance of the cesium cell on the D2 transition with radial spatial resolution. The 300 kHz linewidth probe laser is scanned 20 GHz across the optically thick hyperfine structure, revealing absorbances of 1-5. The absorbance outside of the pumped volume is modulated by up to a factor of 2 when the pump beam is blocked, suggesting significant temperature gradients. The radial temperature profile is observed across the 1.5 cm pipe with resolution of 2 mm. The variation of pump power, nitrogen pressure, and heat pipe temperature has been provided showing distinct trends. Cesium D2 lineshapes have been obtained for several heat pipe spatial locations with the pump laser actively heating the gaseous medium., The original document contains color images.

This report summarizes the entire research period. In this program, we performed experiments using a femtosecond mid-infrared pump-probe system implemented for QCL samples operating at 4.6 and 5.3 m. We employed femtosecond time-resolved pump-probe measurements to probe the nature of the transport through the laser structure via the dynamics of the gain. The gain recovery was determined by the time-dependent transport of electrons through the cascade heterostructure; as the laser approaches and exceeds threshold, the gain recovery shows a dramatic reduction due to the onset of quantum stimulated emission. Since the electron transport through each state in the cascade is determined by the state lifetime, the transport in a cascade laser is driven by the photon density in the cavity. The gain recovery is qualitatively different from that in conventional atomic, molecular and interband semiconductor lasers due to the superlattice transport in the cascade. We also studied the effects of pulse propagation in the laser, including group velocity dispersion and coherent pulse reshaping due to ultrafast Rabi flopping of the gain medium., The original document contains color images.

The small signal modulation of a vertical external cavity surface emitting laser (VECSEL) is examined. The modulation transfer function (MTF) of the cavity is measured for multiple photon lifetimes operating between Class A and Class B regimes, where the photon and carrier lifetimes are of the same order. Three coupled ordinary differential equations with similarities to an electrically-injected quantum-well laser with a separate confinement heterostructure are used to mathematically describe the time-dependant VECSEL response. We present a series of measurements that provide important laser parameters such as internal device losses and differential gain. The VECSEL operating in this regime is an overdamped oscillator and has free-running characteristics that are not unlike quantum-dot and quantum-cascade lasers., Published Proceedings of the Physics and Simulation of Optoelectronic Devices XVIII, San Francisco, CA on 25 January 2010.

Several parameters are proposed for describing the statistical thermodynamic component of the exchange of photons between a pump and a laser beam. They are based on the occupation probability of absorbing and emitting, pump and laser levels, and are complementary to the optical cross sections. The "occupation factor," f0, is appropriate for describing an optical amplifier in the small signal regime. f1 is appropriate for describing an amplifier in the large signal regime, e.g., a laser. They serve to facilitate a quantitative comparison of laser gain media, operating temperatures, and choice of pump and laser wavelengths. After a simple scaling, both occupation factors have a numerical value that coincides well, in most cases, with conventional usage of the terms two-, three-, and four-level laser. They can thus serve as an unambiguous, quantitative alternative to the quasi-two-, quasi-three-, and quasi-four-level terminology. The proposed definitions are general enough to apply to many types of gain media, but are particularly useful for comparing systems with discrete levels, pumped with a narrowband source, in near-resonance with the laser wavelength. Several low-quantum-defect combinations of pump and laser wavelengths are analyzed for Er3+, Nd3+, Yb3+, and Ho3+ in YAG, as a function of temperature., The original document contains color images. Published in IEEE Journal of Quantum Electronics, v45 n10 p1213-1220, October 2009.

Over the third year of the grant, our research program in active plasmonics has concentrated on developing ultracompact all-optical and electro-optic modulators based on metal-insulator-metal Fabry-Perot resonators. Highlights over the past year include: *Realization of tunable color filter pixels based on plasmonic metal-insulator-metal waveguides and *Demonstration of a plasMOStor, a Si CMOS plasmonic electro-optic modulator. Another significant development was the collaborative work (with Yariv and Scherer groups at Caltech) of an electrically pumped hybrid evanescent Si/InGaAsP laser. Diest and Atwater developed the InP/Si bonding technology needed to realize active electrically pumped InP-based lasers on Si., The original document contains color images.

Diode Pumped Alkali Lasers (DPALs) offer a promising approach for high power lasers in military applications that won't suffer from the long logistical trails of chemical lasers or the thermal management issues of diode pumped solid state lasers. This research focuses on characterizing a DPAL type system to gain a better understanding of using this type of laser as a directed energy weapon. A rubidium laser operating at 795nm is optically pumped by a pulsed titanium sapphire laser to investigate the dynamics of DPALs at pump intensities between 1.3 and 45 kW/cm2 . Linear scaling as high as 32 times threshold is observed, with no evidence of second order kinetics. Comparison of laser characteristics with a quasi-two level analytic model suggests performance near the ideal steady-state limit, with the exception of mode matching. Additionally, the peak power has scaled linearly as high as 1 kW, suggesting aperature scaling to a few cm2 is sufficient to achieve tactical level laser powers. The temporal dynamics of the 100ns pump and rubidium laser pulses are presented, and the continually evolving laser efficiency provides insight into the bottlenecking of the rubidium atoms in the 2P3/2 state. Lastly, multiple excited states of rubidium and cesium were accessed through two photon absorption in the red, and yielded a blue and an IR photon through amplified stimulated emission. Threshold is modest at 0.3mJ/pulse, and slope efficiencies increase dramatically with alkali concentrations and peak at 0.4%, with considerable opportunity for improvement. This versatile system might find applications for IR countermeasures or underwater communications., The original document contains color images.

Laser induced fluorescence was used to study how pump pulse duration and alkali recycle time effects maximum power output in a Diode Pumped Alkali Laser (DPAL) system. A high intensity short pulsed pump source was used to excited rubidium atoms inside a DPAL-type laser. The maximum output power of the laser showed a strong dependence upon the temporal width of the pump pulse in addition to the input pump intensity. A linear relationship was observed between the maximum output power and the pulse width due to the effective lifetime of the excited state, defined as the time it takes for the alkali to be excited to the 2P3/2, relax down to the 2P1/2 state, and finally lase. This effective lifetime, calculated to be 0.139 ns, allowed for a calculation of the number of times each alkali atom in the pump volume could be used for lasing during a pulse. The number of recycles ranged from approximately 15 during the shorter 2 ns pulses up to 50+ times during the 7-8 ns pulses. The maximum output of the system scaled linearly with the number of cycles available., The original document contains color images.

Optical amplifiers have been a topic of research over several decades. Throughout the early nineties research was focused on erbium doped fiber amplifiers, a technology which reached commercialization in the mid nineties and enabled the revolution within optical communication leading to long haul all optical communication links. Indirectly the erbium doped fiber amplifiers also enabled the progress that we witnessed within Raman amplifiers in the late nineties due to the development of suitable high power pump lasers enabled by rare earth doped fibers. Lately, research within optical amplifiers has been directed toward parametric optical amplifiers encouraged by a potential large gain bandwidth and low noise properties of these amplifiers. Currently erbium doped fiber amplifiers is the most mature optical amplifier technology and the safe choice, offering high gain and good noise properties from signal wavelengths from 1530 nm to 1610 nm. Raman amplifiers have proven superior noise properties when used as distributed amplifiers, compared to erbium doped fiber amplifiers. In addition Raman amplifiers, discrete as well as distributed offer wide bandwidth of operation since multiple pump lasers at different wavelengths may be combined to broaden the gain bandwidth. Finally, and maybe most important, the operation wavelength is solely determined by the wavelength of the pump laser, and Raman amplifiers from 1300 nm to 1610 nm have been demonstrated. Recent research has suggested parametric amplifiers as a promising alternative to erbium doped fiber amplifiers and Raman amplifiers offering low noise properties, wide bandwidth of operation and in addition wavelength conversion. Raman assisted parametric amplifiers have also been suggested., The original document contains color images.

This paper develops a three level model for a continuous wave diode pumped alkali laser by creating rate equations based on a three level system. Differential equations for intra-gain pump attenuation and intra-gain laser growth are developed in the fashion done by Rigrod. Using Mathematica 7.0, these di erential equations are solved numerically and a diode pumped alkali laser system is simulated.The results of the simulation are compared to previous experimental results and to previous computational results for similar systems. The absorption pro le for the three level numerical model is shown to have excellent agreement with previous absorption models. The lineshapes of the three level numerical model are found to be nearly identical to previous developments excepting those models assumptions. The three level numerical model provides results closer to experimental results than previous systems and provides results which observe effects not previously modeled, such as the effects of lasing on pump attenuation., The original document contains color images.

The remote sensing of gases in complex mixtures at atmospheric pressure is a challenging problem and much attention has been paid to it. The most fundamental difference between this application and highly successful astrophysical and upper atmospheric remote sensing is the line width associated with atmospheric pressure broadening, 5 GHz in all spectral regions. In this paper, we discuss quantitatively a new approach that would use a short pulse infrared laser to modulate the submillimeter/terahertz (SMM/THz) spectral absorptions on the time scale of atmospheric relaxation. We show that such a scheme has three important attributes. 1) The time resolved pump makes it possible and efficient to separate signal from atmospheric and system clutter, thereby gaining as much as a factor of 10 6 in sensitivity. 2) The 3-D information matrix (infrared pump laser frequency, SMM/THz probe frequency, and time resolved SMM/THz relaxation) can provide orders of magnitude greater specificity than a sensor that uses only one of these three dimensions. 3) The congested and relatively weak spectra associated with large molecules can actually be an asset because the usually deleterious effect of their overlapping spectra can be used to increase signal strength., Published in the IEEE Journal of Quantum Electronics, v45 n2 p163-170, February 2009. Prepared in cooperation with Wright State University, Dayton, OH; and the Army Aviation and Missile Research, Development, and Engineering Center, Redstone Arsenal, AL. Federal Purpose Rights. The original document contains color images.

The development of a reliable and effective laser source for pumping mesospheric sodium to generate an artificial guidestar has been well documented. From the early achievements with 589nm high-power dye lasers at the Keck and Lick observatories to the ground-breaking 50W CW FASOR (Frequency Addition Source of Optical Radiation) Guidestar at the Air Force?s Starfire Optical Range (SOR), there has been intense interest in this technology from both the academic and military communities. Beginning in the fall of 2008, the Air Force Research Laboratory?s Advanced Electric Lasers Branch began a project to build, test, verify and deliver an upgraded version of the SOR FASOR for use at the AF Maui Optical Station (AMOS) in the summer of 2010. This device, called ?FASOR-X? will be similar in design to the existing SOR device and produce 50W of diffraction limited, linearly polarized narrow linewidth 589nm light by combining the output of two injection-locked Nd:YAG ring lasers (operating at 1064nm and 1319nm) using resonant sum-frequency generation in a lithium triborate crystal (LBO). This upgraded system will include features such as modularized sub-components, embedded control electronics, and a simplified cooling system. The first portion of this upgrade project is to test and build the two injection laser cavities while validating the improved control mechanisms FASOR-X will employ. In parallel with this effort, the technical plans for the modularization and re-packaging of the FASOR will be finalized and coordinated with the staff at Maui. This paper summarizes the results of these efforts to date and provides updates on the AMOS FASOR-X status. Additionally AFRL plans for next-generation guidestar pump sources will be discussed., Presented at the 2009 Advanced Maui Optical and Space Surveillance Technologies Conference, 1-4 Sep, Maui, HI.. The original document contains color images.

An optically pumped semiconductor laser resonant periodic gain structure, grown lattice-mismatched on an AlAs/GaAs Bragg mirror, exhibits a peak pulsed power of 70 W when pumped with a pulsed 1064 nm neodymium doped yttrium aluminum garnet laser., Published in Applied Physics Letters, v95 article 081112, 2009.

Since the year 2000, Selex Galileo activity on Space Frequency Standards has been pursuing the development of a Passive Hydrogen Maser (PHM), presently the primary clock of the Galileo Navigation Constellation. A consortium has been established in 2007 with one of the best scientific laboratories in the European Time-Frequency community: the Istituto Nazionale di Ricerca Metrologica - INRiM (I) in order to demonstrate that the POP technique is suitable for space applications. The name of the project, Maser POP, comes from a very promising technique studied and refined by INRiM in order to achieve performance close to that of a passive hydrogen maser, but with dimensions and power consumption closer to a rubidium clock. Under an Italian Space Agency ASI contract, a feasibility study has been completed in 2008 concerning the manufacture of a clock breadboard and the preliminary design of the three units composing the clock: Space Physics Unit, Optical Unit, and Electronic Unit. This paper summarizes the outcome of this study in terms of Breadboard (BB) results (where a frequency stability on the order of 1.2x10(-12)tau(-0.5) has been measured) and the critical areas that shall be correctly addressed for the project objective's full achievement. The development plan of the Maser POP is presented together with an overview of expected characteristics, in terms of mass and power consumption, realistically optimized taking into account the needs of the space market. A preliminary schedule is also presented concerning future activities aiming at the development of an Engineering Model (EM). This plan includes the qualification of the clock against the Galileo environmental requirements. Also, these further activities will be supported by the Italian Space Agency (ASI) in the frame of the Space Atomic Clock Development program., Presented at the Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting (41st), Santa Ana Pueblo, NM, 16-19 Nov 2009 and published in proceedings of the same p519-530. Prepared in cooperation with the Istituto Nazionale di Ricera Metrologica (INRiM), Italy and Agenzia Spaziale Italiana (ASI), Italy. The original document contains color images.

A simple analog locking circuit was shown to stabilize the beat signal between a 2.408 THz quantum cascade laser and a CH2DOH THz CO2 optically pumped molecular laser to 3-4 kHz (FWHM). This is approximately a tenth of the observed long-term (t ~ sec) linewidth of the optically pumped laser showing that the feedback loop corrects for much of the mechanical and acoustic-induced frequency jitter of the gas laser. The achieved stability should be sufficient to enable the use of THz quantum cascade lasers as transmitters in short-range coherent transceivers., Published in Optics Express, v17 n9, 27 April 2009.

The technique of Picosecond Time-Resolved Laser-Induced Fluorescence (PITL1F) was expanded to enable two-point hydroxyl (OH) series concentration time series measurements of turbulent jet flames. Measurements were obtained with two-point separation along the radial direction of the jet (delta r), i.e., the propagation direction of the horizontal laser beam, as well as axial two-point separations, i.e. along the axis of the jet, i.e., the z-direction. As the jet configuration is statistically axisymmetric, so that statistical analyses along the azimuthal direction can be considered insignificant, this allows complete coverage of the jet flame. Investigation of axial two-point statistics requires a different laser configuration than that of the previous radial two-point system: either a single vertical laser beam or two horizontal beams with adjustable vertical separation. The combustor used in this study has a closed base which does not permit vertical alignment of a laser; hence, the two-beam configuration was adopted. With the installation of our new diode-pumped laser system, which generates nearly ten times more UV power than before, the laser can now be split into two beams while still providing a sufficient signal level at each probe volume. This paper describes experimental details for this new approach and its demonstration within a well-studied turbulent hydrogen-nitrogen nonpremixed flame, the H3 flame (H2/N2 = 50:50, Re = 10,000)., The original document contains color images. All DTIC reproductions will be in black and white. Prepared in cooperation with Connecticut University, Storrs, CT.

A laser-diode bar incorporated into an external cavity with a volume Bragg mirror produced 30 W of cw output power within a 20 pm (10 GHz) spectral linewidth (FWHM) centered at 780 nm. The device output power exceeded 90% of that for the free-running laser-diode bar. The emission wavelength was tuned over a 400 pm range without broadening laser spectrum width. Absorption of 90% of the laser radiation by a 25 mm vapor cell containing Rb that has been pressure broadened with 300 torr of ethane was demonstrated., Published in Optics Letters, v33 n7 p702-704, 1 April 2008.

This paper models the absorption phenomena of light in a three level diode pumped alkali laser system. Specifically this model calculates for a user defined set of system parameters the attenuation of the input pump beam and characteristics of the bleached wave. Using Wolfram's Mathematical 6.0 software all necessary physics for an accurate description of absorption was modeled from first principles: energy levels, cross sections, spin-orbit kinetic processes, saturation frequencies, pump attenuation, and differential transmittance, which is a representation of the bleached wave. A specific DPAL scenario was simulated, 455K system temperature, alkali concentration of 6.1 - 10(exp 13), and a system pressure of 200 torr of He and 600 torr of Ethane. For a range of initial input intensities the linear approximation to the beam attenuation predicted consistently a differential transmittance value of 70%. It was concluded that the linear approximation is a good indicator of the distance a bleached wave penetrates an absorbing cell. This model was also benched marked against the quasi two level model. In the limit of high system pressure the simulated model converged to the same population inversion as that of the quasi two level regime. Finally, within the quasi two level regime a closed analytic equation was developed to describe under what conditions the system would lase., The original document contains color images.

We report laser-induced cooling with thulium-doped BaY2F8 single crystals grown using the Czochralski technique. The spectroscopic characterization of the crystals has been used to evaluate the laser cooling performance of the samples. Cooling by 3 degrees below ambient temperature is obtained in a single-pass geometry with 4.4 Watts of pump laser power at lambda= 1855 nm., Published in Optics Express, v16 n3 p1704-1710, 4 Feb 2008. Additional AFOSR grant F49620-02-1-0057. Prepared in collaboration with Los Alamos National Laboratory, Los Alamos, NM, and INFN, Pisa, Italy.

The recent merge of precision optical frequency metrology and ultrafast laser technology has made a profound impact to a number of scientific disciplines, including fundamental physical tests, precision spectroscopy in chemistry, biology, and material science, metrological support in space science, and coherent quantum control. For the area of frequency metrology on which this proposal is focused, we expect a significant increase in the stability and accuracy of frequency standards is needed and is ready to be developed, especially in light with the identification of optically based transitions with extremely high quality factors. Phase coherent connection between the optical and microwave spectral regions has now been established with direct optical frequency synthesis, with a qualitatively new regime being developed owing to the introduction of precise femtosecond laser based optical frequency comb technology. At the present time, optical fiber communication lines (OFCL) are widely used. One way for increasing the OFCL transmission capacity is the spectral separation of channels. For this, a stable chain of working optical frequencies is necessary. It is convenient to use the radiation from a stabilized femtosecond laser as such a frequency chain. The prospects for using such femtosecond synthesizers for OFCL are confirmed by the fact that there are many publications on this theme. .In first stage we propose to develop an ideal alternative system based on femtosecond diode-pumped ytterbium-laser. The use of diode pumping for crystalline lasers makes it possible to construct compact, highly efficient, as well as economical laser systems. The relatively low thermal load and compactness of the laser system help to create stable optical resonators and to obtain highly reliable laser operating parameters. A femtosecond comb system based on a Yb-laser will be much more compact and economical than its Ti:S counterpart., The original document contains color images.

We report on the development of a high-power tunable yellow-orange laser. It is based on intracavity frequency doubling of a widely tunable, highly strained InGaAs-GaAs vertical-external-cavity surface-emitting laser operating near 1175 nm. Over 5 W of continuous-wave output power is achieved and is tunable over a 15-nm band centered at 587 nm. This compact low-cost high-power yellow-orange laser provides an innovative alternative for sodium., Published in IEEE Photonics Technology Letters, v20 n20 p1700-1702, 15 Oct 2008. Prepared in collaboration with the Arizona Center for Mathematical Sciences, University of Arizona, Tucson, AZ; Department of Physics and Material Sciences Center, Philipps Universitaet Marburg, Marburg, Germany and Arete Associates, Longmont, CO. The original document contains color images.

Many of the difficulties of creating compact, high power laser systems can be overcome if the heat dissipating properties of chemical lasers can be combined with the efficiency of diode lasers. Recently, the novel idea of using solid state diode lasers to pump gaseous gain media, such as is done in diode pumped alkali lasers (DPALs), has been proposed and early experiments have shown promising results. However, a number of technical issues need to be overcome to realize high output power from these lasers. In order to achieve higher power, the efficiency of coupling between pump laser energy and the chemical cell must be increased, and eventually multiple high power diode pumps must be combined and synchronized so that their energy can pump the chemical cell. Additionally, an inter-cavity adaptive optics system may be a requirement to be able to propagate these lasers with high efficiency. DPAL systems are complex and require a significant amount of data fusion and active feedback to control and optimize their performance. There are a wide range of components including pump lasers, gain cells and monitoring points needed to study and refine the overall laser system. In support of this dynamic development environment, we have developed a hardware framework using commercial off the shelf (COTS) components which supports the rapid assembly of functional system blocks into a cohesive integrated system. Critical to this system are a simple communication protocol, industry standard communication pipes (USB, Bluetooth, etc), and flexible high level scripting. Simplifying the integration process has the benefit of allowing flexible "on the fly" modifications to adapt the system as needed and enhance available functionality. The modular nature of the architecture allows scalability and adaptability as more pieces are added to the system., Advanced Maui Optical and Space Surveillance Technologies Conference held 16-19 September 2008 in Maui, HI. The original document contains color images.

We demonstrate the ability to generate ultra-high-frequency sequences of broadly wavelength-tunable, high-intensity laser pulses using a custom-built optical parametric oscillator pumped by the third harmonic output of a "burst-mode" Nd:YAG laser. Burst sequences consisting of 6-10 pulses separated in time by 6-10 microsecond are obtained, with average total conversion efficiency from the 355 nm pump to the near-IR signal and idler wavelengths of approx. 33%. Typical individual pulse output energy for the signal and idler beams is in the range of 4-6 mJ, limited by the available pump energy. Line narrowing is demonstrated by means of injection seeding the idler wave using a low-power external-cavity diode laser at 827 nm. It is shown that seeding reduces the time-averaged linewidth of both the signal and idler outputs to approx 300 MHz, which is near the 220 MHz Fourier transform limit. Line narrowing is achieved without recourse to active cavity stabilization., Sponsored in part by the National Science Foundation, Division of Chemical and Transport Systems. Prepared in collaboration with Innovative Scientific Solutions, Inc., Dayton, OH; the Department of Mechanical Engineering, Iowa State University, Ames, IA; and the Propulsion Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH. Published in Applied Optics, v47 n1 p64-71, 1 Jan 2008.

We present the results of the experimental study and comparison of Yb-free, Er-doped, all-fiber, alignment free, single frequency (SF) fiber amplifiers operating under 980-, 1470- and 1530-nm pumping for the core- and clad-pumping architectures. In the single-mode core-pumped configuration Er-doped fiber amplifiers demonstrated 52% and 60% pump to output efficiencies for 980 and 1480 nm pump wavelength, respectively, producing over 140 mW of SF output power at seed wavelength approximately 1560 nm and over 180 mW at seed wavelength 1605 nm for 300 mW of pump power. At the same time, when clad pumped, Er-doped 20/125 DC LMA gain fiber demonstrates laser efficiencies of 22.4% pumped at 980 nm - up to 20 W of fiber-coupled diode laser pumping. The same LMA fiber demonstrates 33% optical-to-optical efficiency (46% slope efficiency versus absorbed power) when cladding-pumped with 1520-1530-nm fiber-coupled laser diode modules. Detailed analysis of these experiments is presented., Sponsored in part by High Energy Laser Joint Technology Office. Prepared in collaboration with NovaWave Technologies, Inc., Redwood City, CA and Princeton Lightwave Inc., Cranbury, NJ. Presented at the SPIE Defense and Security Symposia in Orlando, FL on 16-20 Mar 2008. Published in Laser Source Technology for Defense and Security IV, SPIE Proceedings, v6952 article 695205, Apr 2008. The original document contains color images. All DTIC reproductions will be in black and white.

We detect trace residues of RDX and TNT by surface photofragmentation - fragment detection in real time at ambient conditions. An infrared or visible laser generates signature, NO molecular fragments, and a second, low energy, ultraviolet laser facilitates their detection by resonance enhanced multiphoton ionization. We present the effects of pump laser wavelength, laser energy, delay timing between the pump and probe beams, and analyze concentration on signal intensity. Sensitivity analyses yield RDX and TNT detection limits in the low nanogram/(square centimeter). The detection of energetic materials is a serious concern in the war on terror, either on the war front or in our homeland. With the widespread use of improvised explosive devices and the increased potential for homeland attack, a sensor that detects energetic materials that is robust, accurate, and easy to deploy would undoubtedly reduce the risk to warfighters and civilians. As a result, there has been much interest in developing such a sensor., See also ADM002187. Presented at the Army Science Conference (26th) held in Orlando, Florida on 1-4 December 2008. Published in the Proceedings of the Army Science Conference (26th), 2008. The original document contains color images.

High power laser diodes have important military applications, particularly for material processing, and for pumping solid state lasers. The latter in turn can be used for neutralizing explosives, intercepting rockets, missiles, mortar shells, etc. The Joint High Power Solid State Laser program makes exclusive use of laser diode pumping. Science Research Laboratories, Inc. (SRL) has developed electronic circuitry to extend the lifetime and/or output power of high power laser diodes. Their results have reported enhancements ranging from a factor of three for continuous wave (cw) operation to ten for pulsed operation. Our primary objective was to independently verify their results. Our testing shows that the prototype device works, and that the claims are valid., The original document contains color images.

In a laser-pumped x-ray free electron laser (FEL) an intense laser field replaces the magnetic wiggler field of a conventional FEL. Depending on the intensity and quality of both the electron beam and pump laser, the Thomson backscattered radiation can be coherently amplified. in a conventional FEL, the generation of x-rays requires electron beam energies in the multi-GeV range. In a laser-pumped x-ray FEL, electron beam energies in the multi-MeV range would be sufficient. To generate coherent x-rays with this mechanism, a number of physics and technology issues must be addressed. Foremost among these are the stringent requirements placed on the electron beam quality and brightness as well as the pump laser intensity and pulse energy. The seed radiation for the laser-pumped FEL is the laser-induced spontaneous radiation. The evolution of incoherent radiation into coherent radiation as well as the power gain lengths associated with the coherent x-rays as a function of electron beam energy spread are analyzed and discussed. There is excellent agreement between our analytical results and GENESIS simulations for the radiated power, gain length, conversion efficiency, line-width and saturation length., The original document contains color images. All DTIC reproductions will be in black and white. Prepared in collaboration with Icarus Research, Inc., Bethesda, MD.