The unique demands of space-based lasers for atmospheric remote sensing require the development of high efficiency, narrow line width lasers throughout the .280 pm to 10.0 μm region. The combination of laser performance requirements, large number of candidate laser materials, and complex mathematical models necessitates the development of an integrated, computerized software system consisting of a database, laser models, and a user-friendly software interface. Such a three part software system for laser design is under development. The three parts include: a database of laser, optical, and nonlinear materials; laser component, amplifier, resonator, and oscillator models; and a menu-driven interface.

Solid-state laser models require data for laser, optical, and nonlinear materials. Depending on the particular concern being addressed by a specific model, a broad range of parameters, including, but not limited to, crystalline, optical, thermal, and mechanical properties may be required. Many materials have been investigated as potential laser. or nonlinear materials: new materials continue to be developed. Topical concerns, such as atom-to-atom energy transfer and up-conversion, force laser researchers to reassess old data in a new light. The breadth of the data requirements, the large number of materials, the necessity for future expansion, and the changing analytical requirements necessitate a versatile and flexible approach to data storage and retrieval. A computerized, interactive data base which retrieves material data in a way that is useful to the laser designer solves the problem. Such a data base, one part of a larger software package for designing new lasers and assessing the performance of existing lasers, is being developed in the Flight Electronics Division at the NASA Langley Research Center.

The coherent states of the electromagnetic field modify the fundamental properties of Bloch electrons such as effective mass, band gap, charge, and coupling coefficient to phonons. The effects of this renormalization depend on the phases as well as on the intensities of coherent modes. Among the renormalization effects are shifts of spectral edges for absorption and emission to higher frequencies, decrease in radiative interband transitions, and increase in free carrier absorptions and emissions. The coherent field renormalization shows that the electron-hole system is thermodynamically unstable with respect to the growth of coherent modes when its population is inverted.

Laser-design and manufacturing considerations encountered in a micro-lithography application for a solid-state laser system are considered. A solid-state laser utilized in the application incorporates a zig-zag slab laser amplifier in a master oscillator/power amplifier staging configuration that can meet specifications for reliability and performance required for the semiconductor manufacturing industry. The laser staging makes extensive use of anamorphic beam-shaping and image-relaying to extract energy from the slab amplifier efficiently. The mechanical design of the laser structure employs multiple angularly multiplexed beams through a single-slab amplifier in order to minimize the total system volume. Relay spatial filters and maintenance issues are reviewed.

The paper describes performance of a novel Nd:YAG laser marker equipped with a liquid crystal device(LCD) mask instead of the conventional metal stencil. The laser marker is characterized by its capability of changing mask patterns, such as alphanumeric characters, freely using a microprocessor terminal within a short period.

We report a blue emission upconversion solid-state laser based on Tm³⁺:YLF. Under double resonance excitation at 780.8 nm (near-ir) and 648.8 nm (red), the Tm³⁺ ion is sequentially excited from the ³H₆ ground state to the ¹D₂ excited state through the ³H₄ intermediate level. The laser output at 450 and 453 nm corresponds to the ¹D₂ -> ³F₄ transitions of Tm³⁺ ions in YLF.

The frequency doubling of laser radiation at 1064 nm is studied in order to characterize efficient harmonic materials capable of delivering second-harmonic average power at the multiwatt level. Three nonlinear materials are considered: Mg:LiNbO3, potassium titanyl phosphate (KTP), and lithium triborate (LBO). No photoreactive damage is observed in Mg:LiNbO3; however, it exhibits broadening of the temperature tuning curves and distortion of the harmonic beam. An average output power in excess of three watts is extracted from KTP, but the material shows optically induced nonuniformities in the n(z) refractive index. LBO as a harmonic converter achieves 2.2 W at 532 nm, though the fundamental beam has to be tightly focused in the crystal.

The development and operation of a kilowatt scale KD(asterisk)P second harmonic generator is described. The design utilizes a simple and compact two-plate alternating-Z configuration with a transverse thermal gradient. The calculated single pulse conversion efficiency is between 50 and 75 percent for beams whose divergence is up to 15 times the diffraction limit. Offline subaperture tests demonstrated that the effect of induced thermal gradients on conversion efficiency was less than 5 percent for optical loads expected for kilowatt operation. Full aperture tests with a 300 watt average power Nd:glass zigzag slab laser gave conversions of 35 percent.

The laser properties of the LiCaAlF₆:Cr³⁺ (Cr:LiCAF) material are reviewed. The impact of the absorption and emission spectra on the laser performance is discussed. Laser-pumping and flashlamp-pumping experiments have shown that Cr:LiCAF has potential for high efficiency, although the presence of scattering losses remains a significant problem. The "gradient freeze" growth technique has been found to generate lower-loss material compared to Czochralski growth. The thermal lensing of Cr:LiCAF has been measured to be small and is in rough agreement with the magnitude expected on the basis of the intrinsic thermo-optical properties.

Development of an automated steady-state process for Czochralski growth of flat interface Nd:YAG laser crystals is described. Steady-state is achieved through addition to the melt to maintain constant melt depth and composition. Interface flattening is accomplished through a controlled spin-up of crystal rotation during which characteristic changes in melt flow and crystal weight are observed. Combined steady-state/flat-interface growth has been demonstrated for sections up to 30 mm diameter x 110 mm long. In batch mode, without melt addition, flat interface YAG and Nd:YAG up to 52 mm x 220 mm, and undoped YAG up to 85 mm x 75 mm have been grown. In batchmode, gradual reduction of rotation rate is required to maintain interface contour and cross-sectional shape of the crystal as the melt height decreases.

The properties of lasers based on the double molybdates and tungstates of alkaline rare-earth metals activated by trivalent neodymium ions are analyzed for free lasing and Q-switching modes. The physical, optical, and spectral-luminescence properties of crystals are covered. The energy and spectral characteristics of these lasers, and the dependence of their variations on the concentration of the activator are investigated. It is observed that Nd-doped potassium-gadolinium tungstate and potassium-yttrium tungstate lasers are more efficient under pulsed low-frequency pumping than Nd-doped yttrium-aluminum garnet lasers. It is concluded that such a phenomenon is caused by higher values of the laser transition cross sections and by higher optical homogeneity.

Optical, spectral, and generation characteristics as well as the degradation threshold experienced under laser radiation are investigated for alexandrite crystals grown by horizontally directed crystallization (HDC) in molybdenum containers. Various properties and qualities of HDC alexandrite crystals are compared with those of crystals obtained by the Czochralski technique. It is demonstrated that the HDC alexandrite crystals have a lower uncontrolled iron impurity and weaker UV-absorption than the alexandrites grown by the Czochralski method, which allows the pumping lamp irradiation to be used more effectively. A lower concentration of opaque inclusions and a higher degradation treshold under laser radiation are noted, as well as the dependency of energy characteristics of HDC alexandrite lasers on periodical growth-structure orientation in relation to the cavity axis.

A 6.3 mm diameter by 15 mm long a-axis Nd:YLiF4 (YLF) laser rod was side-pumped by a 475 W GaAlAs diode array. A 42.9 percent optical slope efficiency was achieved while pumping in a region of low absorption in the 2H9/2, 4F5/2 band at 806 nm. Findlay-clay loss analysis and pump absorption profiles are discussed.

The construction and operation of a diode-pumped Nd:YLF monolithic mini-laser operating at 1.053 μm is described along with its application of injection seeding a CW flashlamp pumped Nd:YLF Q-switched oscillator to produce single frequency pulses. Alignment of the mini-laser axis to the "c" axis of the Nd:YLF crystal allows lasing at 1.053 μm in a monolithic configuration. The Q-switched oscillator when seeded produces single frequency pulses 90% of the time.

A billion-shot flashlamp developed under a NASA contract for spaceborne laser missions is presented. Lifetime-limiting mechanisms are identified and addressed. Two energy loadings of 15 and 44 Joules were selected for the initial accelerated life testing. A fluorescence-efficiency test station was used for measuring the useful-light output degradation of the lamps. The design characteristics meeting NASA specifications are outlined. Attention is focused on the physical properties of tungsten-matrix cathodes, the chemistry of dispenser cathodes, and anode degradation. It is reported that out of the total 83 lamps tested in the program, 4 lamps reached a billion shots and one lamp is beyond 1.7 billion shots, while at 44 Joules, 4 lamps went beyond 100 million shots and one lamp reached 500 million shots.

The high-resolution absolute-intensity spectrum of a Vortek-arc lamp is obtained in order to evaluate the lamp as a candidate for a pump source for crystal-laser amplifiers. Spectra are also obtained at lower resolution for Ar-Kr and Ar-Xe mixtures. These spectra are convolved with the Nd:YAG absorption cross section assuming YAG crystal thickness of 0.5 cm and doping of 2 x 10 to the 20th/cu cm. Emphasis is placed on wavelength and intensity calibrations, assembling the spectral regions, and low-resolution spectra. A spectral overlap with Nd:YAG bands is discussed, as well as pulsed operation, lamp-tube filtering, and electrical efficiency. It is noted that pulsing of the lamp should be possible and may provide further improvements in laser operation.

A ground state depleted (GSD)^1,2 laser has been demonstrated in the form of a Q-switched oscillator operating at 912 nm. Using Nd³⁺ as the active ion and Y2SiO5 as the host material, the laser transition is from the lowest lying stark level of the Nd³⁺⁴F_3/2 level to a stark level 355 cm^-1 above the lowest lying one in the ⁴I_9/2 manifold. The necessity of depleting the ground ⁴I_9/2 manifold is evident for this level scheme as transparency requires a 10% inversion. To achieve the high excitation levels required for the efficient operation of this laser, bleach wave pumping using an alexandrite laser at 745 nm has been employed. The existence of a large absorption feature at 810 nm also allows for the possibility of A1GaAs laser diode pumping. Using KNbO₃, noncritical phase matching is possible at 140°C using d₃₂ and has been demonstrated. The results of Q-switched laser performance and harmonic generation in KNbO₃ will be presented. Orthosilicate is a monoclinic biaxial crystal. An oriented spectroscopic evaluation consisting of a Judd-Ofelt analysis of oriented absorption spectra and the measurements of oriented emission spectra has been completed and will be presented. Results of modeling using these spectroscopically determined parameters will be compared with the actual laser performance. The performance of this laser at 911 nm which allows accessing Cs atomic resonance filters through harmonic doubling will also be discussed. Orthosilicate can be grown in large boules of excellent optical quality using a Czochralski technique. Because of the relatively small 912 nm emission cross section of 2-3 x 10^-20cm² (orientation dependent) fluences of 10-20 J/cm² must be circulated in the laser cavity for the efficient extraction of stored energy. This necessitates very aggressive laser damage thresholds. Results from the Reptile laser damage facility at Lawrence Livermore National Laboratory (LLNL) will be presented showing Y₂SiO₅ bulk and AR sol-gel coated surface damage thresholds of greater than 40 J/cm² for 10 nsec, 10 Hz, 1.06 μ pulses.

The design and operation of a laser diode-pumped Nd:YAG ring laser are described. This laser is shown to be capable of efficient generation of both the fundamental and second harmonic frequencies, and can operate either CW or Q-switched. The single frequency power obtained was 542 mW at the fundamental and 19 mW at 532 nm. Q-switched operation produced 22.4 micro-J per pulse at 5 kHz in the fundamental and 7.3 micro-J per pulse at 532 nm.

The design and operation of a laser diode-pumped Nd:YAG ring laser are described. This laser is shown to be capable of efficient generation of both the fundamental and second harmonic frequencies, and can operate either CW or Q-switched. The single frequency power obtained was 542 mW at the fundamental and 19 mW at 532 nm. Q-switched operation produced 22.4 micro-J per pulse at 5 kHz in the fundamental and 7.3 micro-J per pulse at 532 nm.

The first operation of a direct diode-pumped tunable chromium-doped solid state laser is reported. A small alexandrite crystal (Cr:BeAl2O4) was placed in a hemispherical cavity and longitudinally pumped by two 680 nm laser diodes. The threshold pump power was found to be 12 mW using the R1 line at 680.4 nm for the pump transition. A tunable dye laser was employed to probe the dependence of the threshold power on wavelength over the broad absorption band to the blue of the R lines, and pump threshold powers ranging from 15 mW at 630 nm to 55 mW at 670 nm were recorded. With no dispersive element in the cavity the output was broadband and its spectral width depended on pump power. At threshold, the measured bandwidth was 2.1 nm and increased to 3.1 nm at a pump level approximately 10 times threshold. The slope efficiency for diode pumping was measured to be 25 percent.

The design elements of a diode-pumped Nd:YAG planar-ring oscillator (PRO) are presented, along with the theory describing the operation of PROs. Design considerations based on the theoretical findings are assessed, and it is pointed out the phase shift of the effective waveplate can be tuned to account for any practical deviations of the phase shift in the output-mirror coating from the desired phase shift. PROs based on these design considerations are reviewed. Ring crystals of two different dimensions are fabricated, and results for one of each size are compared. Focus is placed on the relaxation oscillation frequency and beat-note stability. Single-frequency output of greater than 200 mW from a monolithic diode-pumped PRO is reported.

Single-longitudinal-mode (SLM), pulsed operation is demonstrated on a tunable Ti:Al2O3 oscillator that utilizes a glancing-incidence cavity configuration. The oscillator is tunable over 720-915 nm, and the output has a bandwidth that is near transform-limited at equal to or less than 500 MHz. Beam walk-off and diffraction effects define cavity configurations for which SLM operation is possible. Stable SLM operation, without any active stabilization, is achieved with a 6.5-cm long cavity in which the separation between the tuning mirror and the grating is kept small (1.5 cm). The oscillator is actively stabilized by a feedback mechanism that monitors small changes in the direction of the output beam that result from small wavelength deviations. Single-longitudinal-mode operation over several hours has been demonstrated. The temporal jitter of the oscillator is reduced to + or - 1.2 ns while maintaining SLM operation.

Laser performance results and Cr(3+)-Tm(3+) transfer efficiency studies are used to demonstrate that YAG is the preferred host for the Cr:Tm:Ho laser, and that highest efficiencies are obtained at relatively low Cr(3+) concentrations. A slope efficiency of 5.1 percent was achieved from a flashlamp-pumped room-temperature 2-micron Cr:Tm:Ho:YAG laser.

The 2-micron spectral region containing several strong molecular absorption bands such as carbon dioxide and water is reviewed. Tunable CW-laser action from resonantly pumped trivalent thulium is considered. A thulium-laser operating scheme is presented, and attention is given to the laser transition and laser cavity. The tuning range of a CW Tm:YAG laser at room temperature is analyzed, along with its output spectrum. Tunable laser performance in YAG codoped with trivalent thulium and trivalent holmium is investigated, and it is concluded that the output efficiency is better for the codoped laser than for the singly doped Tm(3+) laser in the range of 2.09 - 2.12 micron, but the overall tuning range and efficiency is better for the singly doped laser

The laser performance of Nd:Cr:GSGG and Nd:YAG was investigated and compared for laser efficiency, thermal focusing, and depolarization effects. Laser efficiency was studied for Nd:Cr:GSGG and Nd:YAG under similar conditions. Laser efficiency was measured as a function of electrical energy and output mirror reflectivity. Maximum laser efficiency was calculated by determining the losses in the laser cavity. Thermal focusing and birefringence loss of Nd:Cr:GSGG and Nd:YAG have been examined by varying the average pump power. The average pump power changed by adjusting both the energy per pulse and the pulse-repetition frequency. Substantial thermal focusing differences for Nd:Cr:GSGG are explained.

The design and performance characteristics of a TEM(00) mode Nd:YLF laser oscillator which can produce over 35 watts CW at 1053 nm are described. Performance data for Q-switched operation and for mode-locked operation are also presented. The performance of the laser was analyzed using a plane-wave extraction model, and the results are presented.

Realization of practical multi-kilowatt Nd:garnet lasers will require the scale-up of crystal dimensions as well as more powerful pumping sources. A high average power zigzag slab crystal amplifier testing facility has been established at LLNL which employs two 100 kW_e vortex stabilized arc lamps, cooled reflectors and a cooled, spectrally filtered, crystal slab mounting fixture. The operational characteristics of the first crystal laser to be tested in this setup, a Nd:GGG zigzag oscillator, are presented. A Nd:GGG crystal of dimensions 18x7x0.5 cm³, doped at 2x10²⁰ cm^-3 Nd³⁺ atomic density, was pumped by up to 40 kW of filtered argon line emission. A small-signal single pass gain (losses excluded) of 1.09 was measured with a probe laser when the DC input to the lamps was 43 kW_e. Our power supply was then modified to operate in a pulsed mode and provided one to three milliseconds pulses at 120 Hz. An average optical output power of 490 watts was obtained at a lamp input power of 93 kW_e in an unoptimized resonator. The laser output power declined after a few tens of seconds since the slab tips were not properly cooled. A birdhouse specular lamp reflector and a contoured diffuse reflector were tested; in both cases the pump illuminated crystal surface was smaller than the total crystal face area. Fluorescence imaging of the zigzag amplifier's output aperture registered a smoother, more uniform pumping profile when the diffuse reflector was used. Uniformity of pumping results in decreased resonator loss and yields higher laser output power. Thermo-optic distortions observed in these preliminary tests are analyzed with the aid of computer simulations of the thermal fields, stresses, and surface displacements of our crystal slab.

The passive mode-locking of solid-state lasers in the stable regime of a stationary single pulse is investigated from the point of view of designing reliable sources of picosecond, subpicosecond, and femtosecond light pulses. The direction of the transient evolution and steady regime of laser generation is determined from the Liapunov functional, and the self-phase modulation of ultrashort pulses and the shortest pulse duration are discussed. The output power, spectrum, and transient evolution of the spectral shape of mode-locked pulses of a ruby laser and Nd:glass laser are analyzed, and it is shown that pulse widths are in good agreement with the theoretical predictions.

Technological advances in the field of compact visible laser sources are reviewed. Red laser diodes are considered first, and it is pointed out that they are visible to the human eye, but with a reduced brightness; these devices are expected to fall in price and play a major role in higher-density optical data-storage systems as well as bar-code scanners and laser printers. Diode-pumped solid-state lasers are then outlined, and direct diode doubling is discussed. Two major advances in this area, the availability of nonlinear materials and the availability of single longitudinal mode laser diodes with powers of 100 mW, are discussed. In the area of blue laser diodes, emphasis is placed on the material of choice ZnSe.

Frequency doubling of the 1047-nm Lithium Neodymium Tetraphosphate (LNP) laser via intracavity second harmonic generation has been demonstrated. Using MgO:LiNbO3 as the nonlinear crystal, 1.2 mW of 523-nm output was observed at an input power of 190 mW.

Cooperative multi-ion energy transfer and two-step excitation processes have been used to achieve population inversion and laser action in erbium-doped YLiF₄ and BaY₂F₈. Pulsed and cw experiments have been carried out to provide details of the pumping pathways and to identify systems having particularly favorable values for the upconversion transfer coefficient. Examples of laser emission at four wavelengths shorter than the cw pump will be described for YLiF4. Pulsed IR laser output has been demonstrated in BaY₂F₈:Er, where the time dependence confirms efficient cooperative energy transfer associated with the ⁴I_13/2 level. Spectroscopic and kinetic data are reported.