Recently there has been increasing interest in high quality ceramic laser gain materials, particularly for high-energy lasers, due to the successful application of high-volume advanced ceramics consolidation techniques to transparent oxide gain materials. In this paper, a brief comparison of manufacturing techniques is presented, including an overview of the co-precipitation process and the solid-state reaction process. Merits and risks of each will be presented from a processing viewpoint. Ceramic Nd:YAG in particular shows promise for high power laser design. The program reported here is also compiling a definitive database to compare ceramic and single crystal Nd:YAG materials. Uniform doping levels of up to 9 at% Nd³⁺ have been reported by Konoshima Chemical Co. in ceramic Nd:YAG, and studied by the US Army Research Laboratory and the US Air Force Research Laboratory. All ceramic Nd:YAG materials studied to date have exhibited similar, if not identical, spectroscopic parameters to those measured for single crystal samples. Thermal properties, laser damage thresholds and refractive indices for a range of temperatures and wavelengths are reported. Diode-pumped free running laser experiment results with highly concentrated (up to 8 at% Nd³⁺) ceramics and their comparison with our modeling results are presented. High pulse repetition frequency actively (AO) Q-switched laser experiments are in progress. While there are still challenges in the manufacturing of ceramic laser gain materials, and the benefits of the application of ceramic technology to laser material are yet to be fully realized, ceramic Nd:YAG shows promise and could provide new options to the laser design engineer.

Recent advances in the growth of rare earths doped into ceramic (poly-crystalline) materials have generated considerable interest for the next generation of tactical laser systems mainly because ceramics provide larger size, greater strength and lower cost factors in design than their single-crystalline counterparts. For many years, Nd:YAG has been the laser material choice for stability and high power Er has been an ion laser source of interest for defense due to its eye-safe emission at 1.5 μm and has applications in infrared counter-measures, illumination detection, remote sensing and communication technologies. A model Hamiltonian including atomic and crystal-field terms is diagonalized within the complete 4f¹¹ SLJMJ basis set which includes 364 states. Within the standard deviation obtained between 117 comparable calculated-to-observed Stark levels, one set of atomic and crystal-field parameters describes the splitting of the Nd³⁺ and Er³⁺ energy levels in either the ceramic or single-crystal host. We report a detailed crystal-field splitting analysis for a number of multiplet manifolds of Nd³⁺ and Er³⁺ in both the ceramic and single-crystal form of YAG (Y₃Al₅O₁₂). With few exceptions, analysis shows that the energy-level structure of Nd³⁺ and Er³⁺ is similar in the ceramic and single-crystal laser rods.

Adhesive-free bonded (AFB®) composite crystals have proven to be useful components in diode-pumped solid-state lasers (DPSSL). The combination of a lasing medium of higher index of refraction with laser-inactive cladding layers of lower index results in light- or wave-guided slab architectures. The cladding layers also serve to provide mechanical support, thermal uniformity and a heat sink during laser operation. Therefore, the optical and mechanical properties of these components are of interest for the design of DPSSL, especially at high laser fluencies and output power. We report on process parameters and material attributes that result in stress-free AFB® composites that are resistant to thermally induced failure. Formation of stress-free and durable bonds between two dissimilar materials requires heat-treatment of composites to a temperature high enough to ensure durable bonds and low enough to prevent forming of permanent chemical bonds. The onset temperature for forming permanent bonds at the interface sets the upper limit for heat treatment. This limiting temperature is dependent on the chemical composition, crystallographic orientation, and surface characteristics. We have determined the upper temperature limits for forming stress-free bonds between YAG and sapphire, YAG and GGG, YAG and spinel, spinel and sapphire, spinel and GGG, and sapphire and GGG composites. We also deduce the relative magnitude of thermal expansion coefficients amongst the respective single crystals as α_GGG > α_{sapp_c} > α_spinel > α_YAG > α_{sapp_a} from interferometric analysis.

In order to develop an efficient eye-safe laser, operating in the 1.53 μm region, we have written software that models the performance of a passively Q-switched Er:Yb:glass laser with a divalent cobalt Co²⁺:spinel saturable absorber. At present a 0D model, which uses a plane-wave approximation, has been completed. The model is based on a set coupled first order differential equations that describe the laser dynamics. These equations represent a two-level Yb³⁺ diode pump scheme, a five-level Er³⁺ gain medium, and a four-level Co²⁺ Q-switch. The model takes into account cooperative upconversion and excited state absorption (ESA) in both the gain and absorber media. Solutions to the rate equations and optimization examples are presented.

We present a novel high power all-fiber-based master oscillator power amplifier (MOPA) laser system operating in the C-band (1.5 mm) with pulses <5ns and a repetition rate range of 200 kHz. This system generates >4 Watts of average power and a maximum pulse energy of 20 mJ and peak power of 5 kW at 200 kHz using custom designed Er:Yb co-doped double-clad fibers. This system was also operated at reduced repetition rates of 6 kHz and pulse energy of 165 mJ was generated with a peak power of 28 kW. By shortening the seed pulses a peak power of up to 33.9 kW with a pulse energy of 73 mJ was achieved at 20 kHz. A beam quality of M²=1.2 was achieved, which makes this system very suitable for scanning lidar applications.

In this work, we demonstrate how a polarization switching technique can be used to create multiple fiber ends, and allow the radiation to pass twice through each amplifying section for more efficient energy extraction. The technique uses a polarizing beam splitter combined with polarization switching in each arm of the cavity to define a ring-like cavity with multiple gain sections that can be end pumped. Polarization-maintaining double-clad rare-earth-doped fiber with slightly multi-mode core was used as the gain sections. A laser system based on the in-cavity polarization switching design has been demonstrated with maximum measured 62% slope efficiency and close to 30W output. The relatively low output power is only limited by the available pump sources.

We demonstrated mutual injection locking and coherent beam combining of three individual Nd:YVO₄ laser modules. A beam splitter couples three lasers as well as combines their outputs. In the free running state, the divergence of combined beams is large. Under mutual injection locking, the divergence of the combined beams becomes substantially smaller than that in the free-running state and is as small as that of the individual laser beam. Mutual injection locking was also realized without active stabilization with large individual laser cavity length difference and low individual laser Q-factor.

Quantum cascade lasers based on planar quantum wells have emerged as a leading candidate for infrared laser sources. However, these lasers are ultimately limited by phonon emission, and exhibit useful optical gain only for the tranverse magnetic polarization. Quantum dot (QD) gain material to replace the planar gain regions is very attractive because the unipolar approach can then lead to both a phonon bottleneck, and surface emission. However, tunneling phenomenon is quite different for unipolar QD injection, and designs that follow the now standard approaches based on planar quantum wells are known to have unfavorable tunneling characteristics. In this paper we present a new device design based on QDs that can lead to important advantages for realizing high performance unipolar injection infrared lasers. The new quantum dot cascade laser design is based on controlling electron tunneling in the different quantum dimensional systems, from zero-dimensional to two-dimensional, to both block as well as enhance tunneling in a gain stage so as to obtain the population inversion necessary for infrared gain. This new device, the quantum dot cascade laser (QDCL), can operate with a phonon bottleneck, and therefore can exhibit improved high temperature performance in contrast to planar heterostructure unipolar devices. In addition, the zero-dimensional confinement can also provide transverse electric polarization in the radiation field, and therefore surface emission. Epitaxial growth experiments based on self-organized quantum dots to realize the new QDCL approach are presented and discussed.

We have demonstrated the feasibility of cooling high power solid-state lasers with diamond windows, whose thermal conductivity is about two orders of magnitude higher than sapphire's, the material conventionally used for this purpose. Since pumping and cooling were along the same axis, a Cartesian thermal gradient was achieved, while the zigzag scheme was used to minimize thermal lensing. An output power of 200Watt was achieved from a single Nd:YVO₄ slab in a zigzag configuration when pumped with 600Watt diodes at 808nm. The maximum output power previously reported in the literature with Nd:YVO₄ using conventional cooling schemes is only about 100W. A 2.3x4x24mm³ slab was pumped from its broad side (4x24 mm²) through a 0.3mm thick optical diamond window placed in close contact with the lasing crystal. The diamond window, held in a water-cooled copper housing acted as a heat conductor. The other broad side of the crystal was cooled directly by its water-cooled copper housing. The output of a two-head configuration was 295Watt. By using a RTP Q-switch, 124Watt average power was obtained at 15kHz with a pulse width of 17nsec, pumping at 650Watt. An additional larger head was developed to pump a Nd:YAG slab. The concept of the pumping and cooling is identical to the Nd:YVO₄ laser head. An output power of 1000Watt was achieved from a single Nd:YAG slab when pumped with 2500Watt diodes at 808nm. The slab dimensions are 3×12×90mm³.

The Air Force Institute of Technology's Center for Directed Energy has developed a software model, the High Energy Laser End-to-End Operational Simulation (HELEEOS), under the sponsorship of the High Energy Laser Joint Technology Office (JTO), to facilitate worldwide comparisons across a broad range of expected engagement scenarios of expected performance of a diverse range of weight-constrained high energy laser system types. HELEEOS has been designed to meet JTO's goals of supporting a broad range of analyses applicable to the operational requirements of all the military services, constraining weapon effectiveness through accurate engineering performance assessments allowing its use as an investment strategy tool, and the establishment of trust among military leaders. HELEEOS is anchored to respected wave optics codes and all significant degradation effects, including thermal blooming and optical turbulence, are represented in the model. The model features operationally oriented performance metrics, e.g. dwell time required to achieve a prescribed probability of kill and effective range. Key features of HELEEOS include estimation of the level of uncertainty in the calculated Pk and generation of interactive nomographs to allow the user to further explore a desired parameter space. Worldwide analyses are enabled at five wavelengths via recently available databases capturing climatological, seasonal, diurnal, and geographical spatial-temporal variability in atmospheric parameters including molecular and aerosol absorption and scattering profiles and optical turbulence strength. Examples are provided of the impact of uncertainty in weight-power relationships, coupled with operating condition variability, on results of performance comparisons between chemical and solid state lasers.

We report what is believed to be the first demonstration of direct resonant diode pumping of a 1.6-mm Er³⁺-doped bulk solid-state laser (DPSSL). The most of the results is obtained with pumping Er:YAG by the single mode diode laser packaged in fibered modules. The fibered modules, emitting at 1470 nm and 1530 nm wavelength with and without fiber grating (FBG) stabilization, have been used in pumping experiments. The very first results on high power DPSSL operation achieved with diode array pumping also will be presented. The highest absorbed photon conversion efficiency of 26% has been obtained for Er:YAG DPSSL using the 1470-nm single-mode module. Analysis of the DPSSL input-output characteristics suggests that the obtained slope efficiency can be increased at least up to 40% through the reduction of intracavity losses and pumping efficiency improvement. Diode pumped SSL (DPSSL) operates at a wavelength of 1617 nm and 1645 nm.

The measurement of positive small signal gain on the 1.315 micron spin orbit transition of atomic iodine following energy transfer from chemically generated NCl(a¹Δ) is reported. Previous instances of gain produced by energy transfer from NCl(a¹Δ) used DC discharges to generate F and Cl atoms; this report describes recent progress towards a true chemical laser device which uses a high temperature chemical combustor and a supersonic reactor to generate NCl(a¹Δ). These improvements represent a significant step towards the development and demonstration of a scalable All Gas-phase Iodine Laser (AGIL) device.

An airborne megawatt (MW) average power Free-Electron Laser (FEL) is now a possibility. In the process of shrinking the FEL parameters to fit on ship, a surprisingly lightweight and compact design has been achieved. There are multiple motivations for using a FEL for a high-power airborne system for Defense and Security: Diverse mission requirements can be met by a single system. The MW of light can be made available with any time structure for time periods from microseconds to hours, i.e. there is a nearly unlimited magazine. The wavelength of the light can be chosen to be from the far infrared (IR) to the near ultraviolet (UV) thereby best meeting mission requirements. The FEL light can be modulated for detecting the same pattern in the small fraction of light reflected from the target resulting in greatly enhanced targeting control. The entire MW class FEL including all of its subsystems can be carried by large commercial size airplanes or on an airship. Adequate electrical power can be generated on the plane or airship to run the FEL as long as the plane or airship has fuel to fly. The light from the FEL will work well with relay mirror systems. The required R&D to achieve the MW level is well understood. The coupling of the capabilities of an airborne FEL to diverse mission requirements provides unique opportunities.

The problems related to experimental measurements of LADAR calibration targets are often discussed in terms of expected signal returns and related statistics with insignificant regard to target dynamics. Laboratory measurements of specular and diffuse radiometric calibration targets have been undertaken in order to obtain well-controlled optical cross section data for model development and validations with a goal of 20% radiometric accuracy. Factors, such as speckle and laser noise, that have some bearing on this measurement have been recently quantified and indicate that calibration target dynamics may be an important variable in assigning overall radiometric accuracy.

One of the more exciting capabilities foreseen for future 3-D imaging laser radars is to see through vegetation and camouflage nettings. We have used ground based and airborne scanning laser radars to collect data of various types of terrain and vegetation. On some occasions reference targets were used to collect data on reflectivity and to evaluate penetration. The data contains reflectivity and range distributions and were collected at 1.5 and 1.06 μm wavelength with range accuracies in the 1-10 cm range. The seasonal variations for different types of vegetation have been studied. A preliminary evaluation of part of the data set was recently presented at another SPIE conference. Since then the data have been analyzed in more detail with emphasis on testing algorithms and future system performance by simulation of different sensors and scenarios. Evaluation methods will be discussed and some examples of data sets will be presented.

An airborne optical system requiring a large field-of-regard will often use a hemisphere or similarly-shaped "turret" to transmit or receive radiation. The aerodynamic flow, however, creates disturbances about the turret resulting in the formation of turbulent boundary and shear layers, and flow separation. The disturbed flow is characterized by optical phase distortions that vary rapidly in time and degrade system performance. With the advancement of Computational Fluid Dynamics (CFD) and computing power, the complex flow field around the turret can be accurately modeled, both spatially and temporally. The density field is extracted from the flow field solution and interpolated within the volume defining the transmitted or received beam. By applying the Gladstone-Dale constant to obtain index of refraction, and integrating along the desired line-of-sight, time-dependent OPD (optical path difference) maps are obtained. These are used as complex field phase modifiers in the wave optics system performance analysis. This methodology was applied to a hemisphere-cylinder turret protruding from a flat plate in Mach 0.3 flow. Time-accurate flow solutions capturing the flow separation and wake oscillations were obtained. Performance of both conformal and flat transmitting windows was assessed as a function of beam angle.

Schafer has demonstrated two different methods for actively cooling our Silicon Lightweight Mirror System (SLMS^TM) technology. Direct internal cooling was accomplished by flowing liquid nitrogen through the continuous open cell core of the SLMS^TM mirror. Indirect external cooling was accomplished by flowing liquid nitrogen through a CTE matched Cesic® square-tube manifold that was bonded to the back of the mirror in the center. Testing was done in the small 4- foot thermal/vacuum chamber located at the NASA/MSFC X-Ray Calibration Facility. Seven thermal diodes were located over the front side of the 5 inch diameter mirror and one was placed on the outlet side of the Cesic® manifold. Results indicate that the mirror reaches steady state at 82K in less than four minutes for both cooling methods. The maximum temperature difference of the eight diodes was less than 200 mK when the mirror was internally cooled and covered with MLI to insulate it from the large 300 K aluminum plate that was used to mount it.

We have demonstrated that direct excitation of 3rd Stokes Raman emission in crystal can produce short (few nanosecond) eye-safe pulses. Produced beam has very high quality and the pulse energy can be as high as tens of millijoules. For pulsed diode pumped solid state lasers the demonstrated repetition rate was 250 Hz but higher repetition rates are certainly achievable. It is important that tested schemes do not have strict requirements on laser pump parameters, namely beam divergence and frequency bandwidth. The obtained results are very relevant to the development of eye-safe lasers, such as the new generation of rangefinders, target designators, and laser tracking and pin-pointing devices, as well as remote 2D and 3D imaging systems.

In this paper the dependence of the crystal radius on the pulling rate (v), melt temperature (T₀) at the meniscus basis, pressure in the furnace (p) and die radius (r_0e) is found. This dependence is used to show in which kind the effect of the variation of one of these parameters can be compensated by varying an other one. Those values of v, T₀, p and r_0e are determined for which the crystal radius variations due to small uncontrollable variations of v, T₀, p around some average values cause minimal surface non-uniformity. Numerical results are given for a Nd:YVO₄ cylindrical bar grown in a furnace in which the vertical temperature gradient is k=33 K/mm.