NRL is developing new materials that transmit across wide wavelength ranges and will present recent results. MILTRAN is a new optical ceramic that transmits visible through LWIR and is well suited as an internal lens element. NRL-series moldable glasses transmit SWIR through LWIR and may be bonded to each other in an adhesive-free thermal process. NRL-200-series glasses transmit visible through MWIR and expand the glass map for multispectral lens designs. These new materials enable greater flexibility for designers of lenses for advanced defense applications and potentially reduce the size, weight and cost of next-generation optics.
NRL is developing new materials that transmit across wide wavelength ranges and will present recent results. MILTRAN is a new rugged optical ceramic that transmits visible through LWIR and is 3.5 times harder than ZnS and is well suited as an internal lens element. NRL-series moldable glasses transmit SWIR through LWIR and may be bonded to each other in an adhesive-free thermal process. NRL-200-series glasses transmit visible through MWIR and expand the glass map for multispectral lens designs. These new materials enable greater flexibility for designers of lenses for advanced defense applications and potentially reduce the size, weight and cost of next-generation optics.
At the Naval Research Laboratory, we have developed several rugged oxide materials for optics applications. These materials include cubic sesquioxides (Y2O3 and Lu2O3) and spinel ceramics which transmit in the UV, visible, SWIR and MWIR wavelength region. These materials have superior mechanical and optical properties compared to the currently used materials on various Navy / DoD systems. The transparent and rugged ceramics are fabricated by the Hot press/HIP method using ultra-pure powder synthesized in-house and the resulting ceramics typically have excellent optical quality with no birefringence. In this paper, we present the optical properties of these rugged transparent ceramics and lens designs using these materials will be also discussed.
High power laser systems require the use of optical isolators to prevent coupling of reflected light into the pump laser. Terbium Gallium Garnet (TGG) and Potassium Terbium Fluoride (KTF) are materials used as optical isolators and while they have been grown for many years, advances in crystal growth and processing make a new set of measurements of the Verdet coefficients of these materials desirable. We present new measurements of the Verdet coefficients of TGG and KTF from 0.405 μ to 1.55 μ and derive expressions for the spectral behavior of the Verdet coefficients.
High power laser systems operating at mid IR wavelengths are required for medical applications, environmental monitoring, and military applications. All of these systems require optical isolators to avoid feedback into the pump laser cavity. We present measurements of the Verdet coefficient of germanate glass with Dy concentrations varying from 20-50% at wavelengths between .4 and 1.5 microns. The results indicate a linear increase of the Verdet coefficient with impurity concentration and a Sellmeier like dependence on wavelength.
We study theoretically and numerically high density of states for hyperbolic bilayered metamaterials (HMM). It reveals that density response of HMMis reminiscent of Fermi electronic band structure of metal or semiconductors. By the method of Green function a van Hove type singularity is found in photonic density spectra of HMM with saddle
point localization on photonic Fermi surface (FS) of metamaterial Similar to the electronic systems, the photonic FS
close to Van Hove singularity experiences instabilities induced by the changes in volume fractions of its constituents
that leads to the Lifshitz type zero-temperature phase transition between FS of types I and II hyperbolic states at the
protected by topology critical point.
High power laser systems require optical isolators to avoid feedback into the pump laser cavity. To date, most of these devices have been based on the inverse Faraday effect in which the plane of polarization of the pump beam is rotated nonreciprocally in response to a magnetic field to prevent reflected light coupling back into the pump laser. Recently, new materials have been developed which have large Verdet coefficients and are able to withstand high optical power. We report measurements of the Verdet coefficient of potassium terbium fluoride and propose a design for an isolator based on this material.
The refractive index of fully dense, infrared-transparent polycrystalline alumina (PCA) with a mean grain size of ∼0.6 μm is reported for the wavelength range 0.85 to 5.0 μm over the temperature range T=296 to 498 K. The temperature-dependent Sellmeier equation is n2−1=(A+B[T2−To2])λ2/[λ2−(λ1+C[T2−To2])2]+Dλ2/(λ2−λ22), where λ is expressed in μm, To=295.15 K, A=2.07156, B=6.273×10−8, λ1=0.091293, C=−1.9516×10−8, D=5.62675, λ2=18.5533, and the root-mean square deviation from measurements is 0.0002. This paper describes how to predict the refractive index of fully dense isotropic PCA with randomly oriented grains using the ordinary and extraordinary refractive indices (no and ne) of sapphire spatially averaged over the surface of a hemisphere. The refractive index of alumina at 296 and 470 K agrees within ±0.0002 with the predicted values. Similarly, the ordinary and extraordinary optical constants ko and ke are used to predict the absorption coefficient of alumina. The refractive indices no and ne of sapphire grown at Rubicon Technologies by the Kyropoulos method were measured at 295 K and agree with published Sellmeier equations for sapphire grown by other methods within ±0.0002.
The refractive index of polycrystalline α-alumina prisms with an average grain size of 0.6 μm is reported for the wavelength range 0.9 to 5.0 and the temperature range 293 to 498K. Results agree within 0.0002 with the refractive index predicted for randomly oriented grains of single-crystal aluminum oxide. This paper provides tutorial background on the behavior of birefringent materials and explains how the refractive index of polycrystalline alumina can be predicted from the ordinary and extraordinary refractive indices of sapphire. The refractive index of polycrystalline alumina is described by
where wavelength λ is expressed in μm, To = 295.15 K, A = 2.07156, B = 6.273× 10-8, λ1 = 0.091293, C = –1.9516 × 10-8, D = 5.62675, and λ2 = 18.5533. The slope dn/dT varies with λ and T, but has the approximate value 1.4 × 10-5 K-1 in the range 296–498 K.
Potassium terbium fluoride is a recently developed magneto-optic material which has been proposed for use as an optical isolator. We have performed measurements of the refractive index, thermo-optic coefficient, and stress-optic coefficient of this material. We present a temperature dependent Sellmeier equation along with calculations of temperature and refractive index profiles at various pump power levels in a diode pumped laser. The data are critical to the design of laser systems in which optical isolators are employed.
A novel plasmonic modulator-switch for the long-wave infrared (LWIR) region is presented. The device consists of a thin metal film, an underlying photoconductive substrate, input and output reflection gratings located on top of the film on opposite ends of the device, and a limited aperture detector located over the out-couple grating. LWIR incident at a given angle is in-coupled, generating surface plasmons (SPs). Since the underlying metal film is thinner than the SP penetration depth, the SPs are couple on both the top and the bottom of the thin film and propagate on both sides of the metal film toward the out-coupling grating. When free carriers in the photoconductive substrate are excited by laser illumination, the electrical properties of the substrate are changed. This change in substrate electrical properties is sensed by the propagating SPs and thus changes the wavevector of the SPs. The SP wavevector change will cause the out-coupled radiation magnitude and angle to change. Thus, the radiation incident on the detector is modulated implementing a plasmonic modulator-switch.
Full-wave electromagnetic simulations were performed on the device. The reflected power at various angles was calculated for a fixed incident angle at λ = 10 μm using various geometries and substrate materials. The substrate materials modelled include III-V compound semiconductors and Si. The dielectric functions for these materials were computed as functions their free carrier concentration to simulate excited and unexcited states. This paper reports on how device performance was affected by variation of these geometric and material parameters.
Orientation patterned gallium phosphide (OP-GaP) is a new nonlinear optical (NLO) crystal which exhibits the highest nonlinear coefficient (d14=70.6 pm/V) and the longest infrared cut-off (12.5 μm) of any quasi-phase-matched (QPM) material that can be pumped at 1-μm without significant two-photon absorption. Here we report the first 1064nm-pumped OPO based on bulk OP-GaP. Multi-grating OP-GaP QPM structures were grown by producing an inverted GaP layer by polar-on-nonpolar molecular beam epitaxy (MBE), lithographically patterning, reactive ion etching, and regrowing by MBE to yield templates for subsequent bulk growth by low-pressure hydride vapor phase epitaxy (LP-HVPE). The pump source was a diode-end-pumped Nd:YVO4 monoblock laser with an RTP high-voltage Q-switch (1064 nm, 1W, 10kHz, 3.3 ns) which was linearly polarized along the <100> orientation of the AR-coated 16.5 x 6.3 x 1.1 mm3 OP-GaP crystal (800-μm thick HVPE layer, 20.8 μm grating period only 150 μm thick) mounted on a copper blocked maintained at 20°C by a thermo-electric cooler. The OPO cavity was a linear resonator with 10-cm ROC mirrors coated for DRO operation (85%R at signal, 55%R at idler). The pump 4σ-diameter at the crystal face was 175 μm. The observed OPO signal (idler) threshold was 533 mW (508 mW) with a slope efficiency of 4% (1%) and maximum output power 15 mW (4 mW). The signal (1342 nm) and idler (4624 nm) output wavelengths agreed well with sellemier predictions. Orange parasitic output at 601.7nm corresponded to 9th order QPM sum frequency mixing of the 1064-nm pump and the 1385-nm signal.
Infrared absorption of high-quality, commercial, polycrystalline MgAl 2 O 4 spinel is ∼40% greater in the range of 3.8 to 5.0 μm than the value predicted by the computer code OPTIMATR®, which has been used for window and dome design for more than 20 years. As a result, spinel and a -plane sapphire windows designed to support the same external pressure with the same probability of survival have approximately the same infrared absorptance in the range 3.8 to 5.0 μm. c -Plane sapphire has greater absorptance than spinel in the range 3.8 to 5.0 μm. Spinel has two weak absorption bands near 1.8 and 3.0 μm. At 1.064 μm, the laser calorimetric absorption coefficient of spinel is 10 to 50 times greater than that of sapphire. New measurements of specific heat capacity, thermal expansion, thermal conductivity, elastic constants, and refractive index (including dn/dT ) of spinel are reported.
The refractive indices of Yb:YAG crystals for 0, 1, 3, 5, 7, and 10 at. % Yb were measured from 0.4-5.2 microns. The coefficients for the Sellmeier fit are reported. The absorption at room temperature was measured from .185-3.3 microns. Using the index and spectroscopic data, the oscillator strengths for the samples were calculated, and their dependence on dopant concentration examined.
KEYWORDS: Refractive index, Thermal optics, Temperature metrology, Data modeling, Oscillators, Thermal effects, Ceramics, High power lasers, Prisms, Absorption
The refractive indices and thermo-optic coefficients for varying concentrations of Er3+ doped polycrystalline yttria
were measured at a variety of wavelengths and temperatures. A Lorenz oscillator model was employed to model the
room temperature indices and thermo-optic coefficients were calculated based on temperature dependent index
measurements from .45 to 1.064 microns. Some consequences relating to thermal lensing are discussed.
We have investigated room temperature fluorescence in the 500-900nm spectral region from high optical quality, polished and uncoated KTP crystals from three different commercial vendors. The crystals were all cut into 5mm x 5mm x 5mm cubes with their dielectric axes along the cube edges. The pump source was a tripled Nd:YAG laser operating at 355nm and 7mJ energy having 3ns pulse width and 100Hz pulse repetition rate. Samples from two vendors showed low fluorescence of similar magnitude while samples from the third vendor showed nearly two orders of magnitude higher value in the peak fluorescence near 800nm. In addition, all samples showed a weaker secondary fluorescence band peaking near 600nm. A low fluorescence sample from one of the vendors also showed typical "gray tracking" at these pump radiation conditions. We have also measured lifetimes of 2.9±0.7 µs and 4.9±0.1 µs for the centers responsible for fluorescence near 845nm and 595nm respectively in the KTP sample showing highest fluorescence and "gray tracking" in this group of samples. The manufacturing processes used to produce high optical quality and low fluorescence KTP materials are proprietary to the commercial vendors and were not provided. Possible origin and sources of fluorescence in these materials are discussed consistent with those published in the literature.
The building blocks available to the laser source designer for various applications such as LIDAR are continuing to progress, especially with regard to the availability of significantly improved nonlinear optical (NLO) crystals and their coatings. These crystals enhance the performance of laser systems, providing wavelength shifting and tunability with ever increasing output power across the spectral range extending from ultraviolet through the terahertz region. Progress in the development of NLO crystal growth and processing techniques at the Air Force Research Laboratory, comprising both in-house and contractual components, will be presented. The optical characteristics of many of the 'workhorse' crystals continue to improve. For example, zinc germanium phosphide (ZGP), which became the material of choice for 2-micron pumped wavelength-tunable laser sources with average powers greater than 1 watt, continues to improve in terms of transparency and laser damage threshold. In addition, the search continues for NLO crystals that may be pumped by Nd:YAG lasers and generate longer-wavelength laser output through 5 microns. Several compounds are presently being investigated including AgGaGeS4 and AgGaGe5Se12. Related to these bulk-crystal developments, surface preparation techniques and motheye surface structures continue to be developed. Since future laser trends are toward all-fiber systems, a new effort began earlier this year on nonlinear optical fiber for super continuum generation. Finally, NLO crystals are being studied for the generation and detection of terahertz radiation. In the presentation, recent advances in the materials development will be reviewed, and the direction of future efforts in this area will be forecast.
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.
The ceramic form of yttrium aluminum garnet (YAG) was studied to determine its suitability for high power lasers. The high Nd3+ doping, the large material size, and the variable doping level with position in the sample, all achievable in ceramics as opposed to single crystal, may lead to higher power solid state lasers than those currently available. We have compared the optical properties of ceramic YAG doped with 0-9 at% Nd3+ to single-crystal, 1 at% Nd:YAG material. Measurements included scattering, thermo-optic behavior, absorption, fluorescence, and laser damage.
Measurements of absorption and emission features showed a small but approximately linear increase in line width with increasing Nd3+ concentration. Nd3+ fluorescence lifetime was rapidly concentration quenched with the 240-μs lifetime for the 1 at% material decreasing to 30 μs for the 9 at% ceramic material. Bulk and surface laser damage thresholds were measured for undoped and 1% Nd-doped ceramic YAG samples using ns-duration laser pulses at 1.064 μm. Both bulk and surface damage threshold values were found to be at least as high as that of single crystals. Measurements of the refractive index and thermo-optic coefficients showed no difference between the single crystal and ceramic materials at 1% Nd3+ doping levels. The scattering in the ceramic material was less than half that of the single crystal.
These results suggest that for most optical characteristics, the ceramic material is equal to and in some cases superior to the single crystal material.
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