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We report our progress in fabrication ceramic Praseodymium doped Calcium Lanthanum Sulfide for potential mid-IR laser gain material. Fabrication and spectroscopy of this new gain material will be reported.

Various rare earth doped single crystal YAG and sesquioxide fibers have been drawn using a state-of-the-art Laser Heated Pedestal Growth system. All crystalline core/clad fibers where thermal and optical properties are superior over glass based fibers have been successfully fabricated using various crystal growth and deposition methods. We report on the various fabrication methods, optical characterization of these clad fibers.

Single crystal fibers are currently being developed for high power single frequency lasers in 1-2 µm region. Crystal fibers offer several advantages over traditional glass fibers such as silica fiber due to their higher thermal conductivity and higher stimulated Brillouin scattering (SBS) thresholds, along with excellent environmental stability and higher doping concentrations. Yb3+ and Ho3+ doped single crystal YAG fibers with diameters down to 17 µm and lengths >1m long have been grown using our state-of-the-art Laser Heated Pedestal Growth system. Single and double cladded rare earth doped crystal fibers have been fabricated using glasses where optical and physical properties were precisely matched to the core single crystal fiber. We also show successful fabrication of all crystalline core/clad fibers where thermal and optical properties are superior over glass based fibers. Various fabrication methods, optical characterization and gain measurements on these clad fibers will be reported.

We report on fabrication and spectroscopy of ceramic Praseodymium doped Calcium Lanthanum Sulfide for potential mid-IR laser gain material.

A novel method combining elemental sulfur and selenium was developed, yielding crystalline sulfur–selenium compounds. The compounds were melted, and an organic comonomer added. Once the organic comonomer was consumed, the viscous compound was vitrified and allowed to cool yielding organic–inorganic hybrid polymers that are termed Organically Modified Chalcogenide (ORMOCHALC) polymers.

Optical absorption spectra, emission spectra, and fluorescence lifetimes were measured for a number of visible and infrared transitions in Ho 3+ doped KPb 2 Cl 5 . Judd–Ofelt parameters were obtained by using the conventional approach, as well as by a modified approach that minimizes the fractional difference, rather than the absolute difference, between calculated and measured oscillator strengths. Both procedures gave good overall agreement between measured and calculated lifetimes, with an average difference of ≈15%. This confirms the expectation from the multiphonon energy gap law that most observed transitions in Ho 3+ :KPb 2 Cl 5 are highly radiative. Emission cross section spectra for the transitions 5 F 3 → 5 I 8 , 5 G 5 → 5 I 7 , 5 F 3 → 5 I 7 , 5 F 4 / 5 S 2 → 5 I 7 , and 5 I 7 → 5 I 8 were obtained from measured emission spectra, with the peak cross section values scaled by the Judd–Ofelt calculated oscillator strengths.

Novel high-power ytterbium YAG lasers are described. These lasers incorporate the principle of anti-Stokes fluorescence cooling to reduce or eliminate detrimental heating. Lasers with net heating and net cooling are demonstrated. By balancing the spontaneous and stimulated emission, we have reduced the net thermal loading to below 0.01% of the laser's average output power. Design, testing, and analysis are reported for lasers up to 500 W average power and pulsed operation up to 30 s. Issues and limitations of this approach are discussed.

Polarized excited-state absorption spectra for the 4 I 13/2 → 4 I 9/2 transition in Er:KPb 2 Cl 5 were determined from the measured 4 I 9/2 → 4 I 13/2 emission spectra using the reciprocity relation of [D.E. McCumber, Phys. Rev. 136 (1964) A954]. From these spectra, the microparameters for energy transfer were found to be C DA = 2.6 × 10 −42 cm 6 /s for ( 4 I 13/2 , 4 I 13/2 ) → ( 4 I 15/2 , 4 I 9/2 ) and C DD = 6.9 × 10 −39 cm 6 /s for ( 4 I 13/2 , 4 I 15/2 ) → ( 4 I 15/2 , 4 I 13/2 ). The macroscopic upconversion coefficient was measured by comparing fluorescence intensities from the 4 I 13/2 and 4 I 9/2 levels, and found to be C up = (2.6 ± 0.4) × 10 −20 cm 3 /s, in good agreement with the value calculated from the microparameters C DA and C DD . Energy transfer upconversion between two ions in the 4 I 13/2 level is orders of magnitude weaker than in other typical hosts, and this is predicted to lead to dramatically reduced heat loading in a resonantly pumped 1.5 μm Er laser.

Waste heat generation is a generic problem in high-power solid-state laser systems. One way to reduce heat loading while improving efficiency is to reduce the laser's quantum defect. This paper presents a simple analysis of low quantum defect laser materials. In these laser materials, the effects of fluorescent cooling and weak loss processes should not be ignored. Simple expressions are developed for efficiency and heating in a steady-state purely radiative material. These expressions are then extended to include weak losses and fluorescence reabsorption. Evaluation of these relations using ytterbium-doped YAG is used to illustrate several optimization schemes and the impact of realistic losses.

Laser-grade single crystals of Er3+-doped potassium lead chloride (KPb2Cl5) have been grown by a novel modification of the Bridgman method. This new method allows for consistent seeding of a single-crystal growth from polycrystalline material while avoiding any problems with supercooling. An extended in situ annealing step allows strain to relax out of the crystal, reducing cracking problems and improving the quality of the resultant material. These crystals showed significant quantities of depolarized scattering in the visible, concentrated in stacks of parallel planes, but very little scattering in the mid-infrared. Two solid–solid phase transitions were found in this material, and passage through the higher-temperature transition was linked to the formation of the depolarization planes. Measurements were also made of the indices of refraction of all three crystal axes in the near-infrared and mid-infrared, and the orientation dependence of the 4.5 μm fluorescence of Er3+ in this material was examined.

We report a novel design for a high-power ytterbium disk laser. This laser utilizes radial diode pumping of a back surface cooled active-mirror geometry. Wing absorption of the pump light at 0.99 /spl mu/m allows efficient laser operation at 1.05 /spl mu/m with a low quantum defect. Laser performance and thermal loading were characterized for a wide range of conditions. Optimized operation of the laser yielded 490 W in a quasi-continuous-wave mode. Electrical efficiency of the laser was found to be 9.4%, while heating of the laser disk was only 3.2% of the absorbed optical power. Fluorescence re-absorption is identified as the principal heat generation mechanism in this laser. A simplified extension to the conventional rate model is proposed for lasing in radiation-trapped systems. This model allows power flow calculations in a radiation-trapped laser system using a single parameter determined from fluorescence decay waveforms. The revised model agrees with heat load measurements.

A discussion of the spectroscopic properties of the low-phonon energy laser host material potassium lead chloride, KPb 2 Cl 5 , doped with trivalent erbium is presented. In this paper we present room temperature spectroscopic measurements and subsequent analysis based on the Judd–Ofelt model. Additionally, Stark level energies of the Er 3+ ions in the crystal were determined from spectroscopic measurements performed at cryogenic temperatures.

We report on fabrication and optical properties of double clad single crystal Yb:YAG core fiber. For the first time, net gain is demonstrated in a cladding pumped Yb:YAG single crystal fiber structure.

It has been previously shown that it is theoretically possible to build a solid-state laser that generates no internal heat. This is accomplished through a detailed balance of the stimulated and spontaneous emission. Such a device is called a radiation-balanced laser. Here, we analyze laser operation when conditions deviate from perfect balance. Sensitivity and stability analyses are presented for perturbations in the field parameters and temperature. We show that the radiation balance is a stable equilibrium under both transient and steady-state perturbations. Limits are derived on the magnitude of allowable spatial perturbations, and techniques for spatial mode matching are discussed. Numerical simulations of 1-/spl mu/m laser operation of Yb-doped potassium gadolinium tungstate are provided as an example.

We present a discussion of the low-phonon energy host material potassium lead chloride, KPb 2 Cl 5 , doped with neodymium. Crystal growth, spectroscopic measurements, and an analysis of the magnetic and electric dipole transitions based on the Judd–Ofelt model are presented. Concentration quenching of the fluorescence is also examined. Based on the spectroscopic data and the known resonances in energy spacing of the first three excited states in neodymium, we suggest the possibility of a 3-for-1 cross-relaxation process. This process may efficiently populate the first excited state, 4 I 11/2 , which we show to be metastable with lifetime on the order of a few milliseconds. Population inversion of this level should be possible enabling a laser based on the 5.5-μm, 4 I 11/2 to 4 I 9/2 transition.

Short pulse laser-induced fluorescence experiments on the lowest-energy electronic states (υ

We have developed a simulation of a multiple quantum well optical correlator to investigate the effects of saturation on the correlation signal. We present the results of the modeling effort here.

In this paper, we present our recent progress in the development of rare-earth (Yb3+ or Ho3+) doped Lu2O3 and Y2O3 sesquioxides for high power solid state lasers. We have fabricated high quality transparent ceramics using nano-powders synthesized by a co-precipitation method. This was accomplished by developments in high purity powder synthesis and low temperature scalable sintering technology developed at NRL. The optical, spectral and morphological properties as well as the lasing performance from our highly transparent ceramics are presented. In the second part of the paper, we discuss our recent research effort in developing cladded-single crystal fibers for high power single frequency fiber lasers has the potential to significantly exceed the capabilities of existing silica fiber based lasers. Single crystal fiber cores with diameters as small as 35μm have been drawn using high purity rare earth doped ceramic or single crystal feed rods by the Laser Heated Pedestal Growth (LHPG) process. Our recent results on the development of suitable claddings on the crystal fiber core are discussed.

In this paper, we present our recent results in the development of Ho 3+ doped sesquioxides for eye-safe solid state lasers. We have synthesized optical quality Lu 2 O 3 nanopowders doped with concentrations of 0.1, 1.0, 2.0, and 5% Ho 3+ . The powders were synthesized by a co-precipitation method beginning with nitrates of holmium and lutetium. The nanopowders were hot pressed into optical quality ceramic discs. The optical transmission of the ceramic discs is excellent, nearly approaching the theoretical limit. The optical, spectral and morphological properties as well as the lasing performance from highly transparent ceramics are presented.

We report the first demonstration of laser emission from the /sup 3/F/sub 3/ to /sup 3/F/sub 2/ transition in the rare earth praseodymium. This new 7-/spl mu/m solid-state laser operates with an optical efficiency of 2.3% at 293 K and 10% at 150 K using an upconversion pumping mechanism. A discussion of the dynamics in this new laser material is included.

Electric and magnetic dipole transition probabilities, fluorescence branching ratios, integrated emission coefficients, fluorescence efficiencies, and fluorescence lifetimes are determined for the lower lying states of Pr:LaCl/sub 3/ using the Judd-Ofelt theory and calculated multiphonon emission probabilities. The results are used to characterize demonstrated mid-IR laser transitions and predict new potential mid-IR laser transitions in Pr:LaCl/sub 3/.

In this paper, we present our recent progress in developing single crystal fibers for high power single frequency fiber lasers. The optical, spectral and morphological properties as well as the loss and gain measured from these crystal fibers drawn by Laser Heated Pedestal Growth (LHPG) system are also discussed. Results on application of various cladding materials on the crystal core and the methods of fiber end-face polishing are also presented.

Using achromatic gratings techniques we have written dynamic holograms on an optically addressed spatial light modulator with a temporally and spatially incoherent white light source. The resolution and depth-of-field of these holograms are theoretically derived and experimentally verified. We demonstrate writing speeds over an order of magnitude faster than video framing rates.

In this paper, we present our recent results in developing cladded-single crystal fibers for high power single frequency fiber lasers significantly exceeding the capabilities of existing silica fiber based lasers. This fiber laser would not only exploit the advantages of crystals, namely their high temperature stability, high thermal conductivity, superior environmental ruggedness, high propensity for rare earth ion doping and low nonlinearity, but will also provide the benefits from an optical fiber geometry to enable better thermal management thereby enabling the potential for high laser power output in short lengths. Single crystal fiber cores with diameters as small as 35m have been drawn using high purity rare earth doped ceramic or single crystal feed rods by Laser Heated Pedestal Growth (LHPG) process. The mechanical, optical and morphological properties of these fibers have been characterized. The fibers are very flexible and show good overall uniformity. We also measured the optical loss as well as the non-radiative loss of the doped crystal fibers and the results show that the fibers have excellent optical and morphological quality. The gain coefficient of the crystal fiber matches the low quantum defect laser model and it is a good indication of the high quality of the fibers.

First operation of the 4F₉/₂ → 6H₁₃/₂ laser transition in dysprosium doped yttrium aluminum garnet is reported. Efficient room temperature operation at 583 nm was obtained using 447 nm GaN diode lasers pumps. Gaussian single-mode operation was demonstrated with a non-optimized slope efficiency of 12%. Millisecond pulsed operation generated 150 mW with power limited by the pump diodes' brightness.

First operation of the 4F 9 / 2 → 6H 13 / 2 laser transition in dysprosium doped yttrium aluminum garnet is reported. Room temperature operation at 582.7nm was obtained using 447nm GaN diode lasers pumps. Gaussian single-mode operation was demonstrated with a non-optimized slope efficiency of 12%. Millisecond pulsed operation generated 150mW with power limited by the pump diodes brightness. An emission cross section of 4.1E-21cm2 at 582.7nm was determined by laser threshold analysis.

We report new laser channels for the rare earth praseodymium. Laser emissions at 1.6 and 5.2 /spl mu/m were studied using a 2.0 /spl mu/m Tm:YAG laser to pump the /sup 3/F/sub 2/ level in Pr/sup 3+/:LaCl/sub 3/. Efficient 5 /spl mu/m operation was demonstrated at 130 K. Temperature dependence of the 5 /spl mu/m laser threshold was determined in the range from 130 to 250 K. The complex dynamics and potential utility of these new rare earth lasers are discussed. >

Excitation of the 3 F 2 level in 5% praseodymium-doped lanthanum trichloride (Pr 3+ :LaCl 3 with a diode-pumped Tm:YAG laser at 2.02 μm results in broad fluorescence centered at 5 μm that has a room temperature lifetime of 8 ms. There is also an up-conversion process that results in population of the 3 F 3 state, producing 1.6 and 2.4 μm fluorescence.

Detrimental heating in a high power Yb:YAG laser was eliminated by incorporating anti-Stokes optical cooling.

The first observations, to the authors' knowledge, of transient thermal lensing in a ZnGeP(2) crystal achieved with 2.09-µm laser excitation of 800-µs, 70-mJ pulses at 30 Hz and by burst-mode Q-switched pumping are presented. The laser power transmitted through an aperture was approximated by an adiabatic model with currently accepted values for the thermal properties of ZnGeP(2) and corresponded to focal-length changes from infinity to 10 cm during each 800-µs pulse. Similar results were seen when the crystal was operated as an optical parametric oscillator. This transient thermal lens severely limits ZnGeP(2) as a material for use in optical parametric oscillators for these modulated, high-power operating conditions.

We have pumped Nd(3+):YAlO(3) lasers with a 40-stripe, 500-mW, 807-nm diode laser. Maximum output powers at 1.34 microm of 26 mW multimode and 20 mW in a single transverse mode were achieved. The several longitudinal modes had a total spectral width of ~0.4 nm.

We are investigating materials for direct blue solid-state lasers assuming UV excitation with GaN based laser diodes. Room temperature spectroscopy is reported relevant to a proposed quasi-three level laser from the 4F 9/2 level in trivalent dysprosium. Modeling based on these measurements suggests this is a promising new laser transition. Keywords: Rare earth, spectroscopy, laser materials, diode pumping 1. INTRODUCTION Oceanic transmission presents an optimal band in the blue-green portion of the visible. Naval operations desire compact and practical laser sources in the band from 450 to 500nm. Of particular interest are solid-state sources capable of moderate average powers as well as high peak powers. The conventional approach to this type of source has been frequency conversion of a near-IR laser. While much progress has been made using this approach, the resulting laser systems lack the desired ruggedness and efficiency. Toward that goal, we are exploring direct diode-pumped laser materials in the blue-green. These laser systems are no w possible with the advent of blue-UV GaN laser diodes. Optical pumping with GaN laser diodes is similar in most respects to near-IR pumping with conventional GaAs laser diodes; however there are some important differences. The shorter wavelengths lead to higher intrinsic brightness from the GaN devices. The larger band gaps result in higher operating voltages (5-6V) as well as weaker temperature tuning rates (50-60pm/degC). Impressive wavelength agility (320-500nm) has already been demonstrated. Average power is currently limited to less than 1W from a single emitter with efficiencies around 20%. Multiple emitters can be combined to increase power and fiber coupling is possible. Significant improvements in performance and cost are expected as this technology matures.[1,2] Rare earth transitions in the UV-blue range have been studied principally as phosphors or scintillators. Laser based studies have largely focused on IR pumped upconversion excitation. This study attempts to fill in the information gaps required for blue laser modeling and design. This paper pres ents our experimentation and analysis of dysprosium doped YAG and YLF.

The authors study the loss process in a uniformly pumped amplifier experiment. The impact of the loss mechanism on laser performance is discussed and its dependence on excited state density is considered. An extensive set of measurements was performed on flashlamp pumped YAG crystals doped with chromium, thulium, and holmium. Near-room-temperature laser operation on the 2.1 mu m holmium transitions was characterized using a frequency selective resonator. Small signal gain and stored energy lifetimes were analyzed to determine the strength and dependence of loss mechanisms. The analysis of the thermal lensing experiments demonstrated a high degree of excess heat generation in this material. >

A study of the 3- mu m laser transitions in Ho:YAlO/sub 3/ and Nd,Ho:YAlO/sub 3/ using both flashlamp and laser pumping is discussed. Fluorescence measurements for the Nd,Ho:YAlO/sub 3/ rod indicate that the neodymium is effective in quenching the lower holmium laser level and in sensitizing the upper level. Intracavity laser pumping of Ho:YAlO/sub 3/ produced laser lines at 2.92 and 2.85 mu m and demonstrated high efficiency. >

A novel diode-pumped disk laser yielded 490 W in a quasi-cw mode. Electrical efficiency of the laser was found to be 9.4%. Heating of the laser disk was only 2.7% of the absorbed optical power.

Thulium is investigated as a sensitizing ion for the mid-infrared transitions of praseodymium and neodymium doped into low-phonon energy crystals. Pumping thulium with 800 nm laser diodes enhances emission of mid-infrared fluorescence between 4000 and 5500 nm in potassium lead chloride co-doped with Tm3+ - Pr3+ and Tm3+ - Nd3+.

Summary form only given. We have investigated high peak and average power operation of 2 /spl mu/m lasers as a pump source for parametric frequency conversion. Our goal is to produce a compact, reliable and tunable source of mid-IR radiation. A Tm,Ho:YAG Q-switched laser resonator system was developed to provide this pump source. >

The goal of our research program is to extend the useful range of rare earth solid state lasers into the mid-infrared range; 3

Potassium lead chloride is found to exhibit superior spectroscopic and mechanical properties as a low phonon host for trivalent erbium.

The temperature dependence between 300 and 500 K is reported for Tm3+ fluorescence in the low-phonon host YCl3. An increase in 1200 nm fluorescence as the temperature rises is evidence of an endothermic cross-relaxation process.

Concentration-dependent cross relaxation is reported for thulium ions in the low-phonon energy host YCl3. Absorption, emission, and fluorescence lifetimes are reported.

An examination is conducted of Q-switching in flashlamp-pumped Cr,Tm,Ho:YAG 2.1 mu m lasers. Experiments show that short-pulse operation in this material can exhibit comparable efficiencies to the long-pulse operation mode. Losses in overall efficiency are shown to be largely due to mode restriction in the resonator. A three-pulse extraction format is shown to improve slope efficiency by more than 200% compared to single Q-switched pulse extraction. The optimal interpulse interval was determined, revealing the time scale of the Tm sensitization process. Spectral shifts in the short-pulse operation mode were observed. Simultaneous dual-wavelength emission was found to be the most common mode of operation for the Q-switched laser. >

The tuning characteristics of the /sup 5/I/sub 6/-/sup 5/I/sub 7/ transition in a Ho:YAlO/sub 3 /laser, intracavity pumped by a 1.079 mu m Nd:YalO/sub 3/ laser, were studied. Operation on seven distinct lines between 2.844 and 3.017 mu m was found, and the threshold and relative slope efficiency of each line was measured. Several of these lines were previously unreported. >

We present a novel technique for recording and processing the speckle from a displaced object. We use an optically addressed, multiple-quantum-well spatial light modulator (MQW-OASLM). This device offers high resolution, speed, and sensitivity. It allows us to measure dynamic speckle patterns at rates up to tens of kHz.

Analysis of excitation decay in the holmium 2 μm system has led to the development of a new macroscopic dual population model for the migration limited upconversion process.

Optical correlators have, for over three decades, been considered as attractive alternatives to digital image processing for a variety of pattern recognition tasks1. Until recently however, there has been a lack of appropriate materials with which to implement these devices. A practical correlator requires an efficient, compact source of coherent light, a fast “optical memory” for displaying templates and, for joint-transform correlators, a sensitive, high resolution optically addressed spatial light modulator (OASLM). In this paper we discuss an optical correlator that uses a laser diode as the light source along with a GaAs/AlGaAs multiple quantum well optically addressed spatial light modulator (MQW-OASLM)2. The template images are supplied either by fixed images on film or by a ferroelectric liquid crystal spatial light modulator (FLC-SLM).

We report, for the first time, the laser oscillation from 2% Ho3+:Lu2O3 hot pressed ceramic. We have synthesized optical quality Lu2O3 nano-powders doped with concentrations as high as 5% Ho3+. The powders were synthesized by a co-precipitation method beginning with nitrates of holmium and lutetium. The nano-powders were hot pressed into optical quality ceramic discs. The optical transmission of the ceramic discs is excellent, nearly approaching the theoretical limit. The optical, spectral and morphological properties as well as the preliminary lasing performance from highly transparent ceramics are presented.

Spatial light modulators have been demonstrated by a number of groups using GaAs/AlGaAs quantum well materials. These optical modulators utilize the enhanced electro-absorption and electro-dispersion associated with quantum confined excitons. Many of these devices use pixelated electrodes to apply a spatially varying electric field across the quantum well layer. We have investigated non-pixelated devices in which the applied field is varied through spatially modulated illumination. These optically addressed spatial light modulators (OASLM) are simple to fabricate and have demonstrated higher spatial resolution than pixelated devices. [1-4] Here we present recent experiments designed to determine the lifetime of the transient space charge distributions generated in GaAs/AlGaAs OASLM's.

Recent demonstrations of perpendicular geometry GaAs/AlGaAs multiple quantum well (MQW) structures as optically addressed spatial light modulators (OASLM) have shown that these devices can offer high speed [1] and high resolution [2].