This is a reminiscence of where things stood in the laser damage problem at the time when the laser reached its 10th birthday. All of the different types of lasers that we know today had been demonstrated and those that could reach high power were shown to destroy the optics used in the resonators or to manipulate the high power beams. If high power lasers were to be useful the problem of laser damage had to be solved but first it had to be understood.
A high-power green picosecond laser based on the ‘Z-slab’ Nd:YAG amplifier is presented. The edge-pumped zigzag amplifier was designed to achieve high energy scaling with good beam quality. In a master oscillator power amplifier system 120 W average power was produced at 1064 nm, which was frequency doubled to 84 W at 532 nm. The maximum pulse energy was 400 μJ at 1064 nm and 280 μJ at 532 nm. The repetition rate was variable from 250 to 1000 kHz with M2 < 1.4. In burst mode with 1-10 pulses, over one millijoule total burst energy was demonstrated at 1064 nm.
We demonstrate power scaling of an Nd:YAG picosecond master oscillator power amplifier system to over 200 W. The ‘z-slab’ amplifier design is a power scalable, edge-pumped zigzag slab amplifier architecture, and it is demonstrated here in two alternative multi-stage implementations at 1064 nm using a picosecond seed laser. In a simple design, an average power of 225 W was generated with up to 450 μJ pulse energy at 11 ps pulse duration. In a compact multi-pass design, 150 W was generated with M2 < 1.75.
Microwave assisted chemical vapour deposited bulk diamond products have been used in a range of high power laser systems, due to low absorption across a range of wavelengths and exceptional thermal properties. However the application of polycrystalline products has frequently been limited to applications at longer wavelengths or thermal uses outside of the optical path due to the birefringence and scatter that are intrinsic properties of the polycrystalline materials. However, there are some solid state structures, including thin disc gain modules and amplifiers, that will gain significantly in terms of potential output powers if diamond could be used as a heat spreader in the optical path as well as a heat spreader on the rear surface of the disk. Therefore single crystal grades of diamond have been developed that overcome the limitations of the polycrystalline material, with low absorption, low scatter and low birefringence grades for demanding optical applications. We will present new data, characterising the performance of these materials across infra-red and visible wavelengths with absorption coefficient measured by laser calorimetry at a range of wavelengths from 1064 nm to 452 nm.
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.
Epitaxial single-crystal chemical-vapor-deposited diamond with (100) crystal orientation is obtained from Element Six (Ascot, United Kingdom) and Apollo Diamond (Boston, Massachusetts). Both companies supply 5×5-mm squares with thicknesses of 0.35 to 1.74 mm. Element Six also provides disks with a state of the art diameter of 10 to 11 mm and a thickness of 1.0 mm. The absorption coefficient measured by laser calorimetry at 1.064 µm is 0.003 cm−1 for squares from Element Six and 0.07 cm−1 for squares from Apollo. One Apollo specimen has an absorption coefficient near those of the Element Six material. Absorption coefficients of Element Six disks are 0.008 to 0.03 cm−1. Each square specimen can be rotated between orientations that produce minimum or maximum loss of polarization of a 1.064-µm laser beam transmitted through the diamond. Minimum loss is in the range 0 to 11% (mean=5%) and maximum loss is 8 to 27% (mean=17%). Element Six disks produce a loss of polarization in the range 0 to 4%, depending on the angle of rotation of the disk. Part of the 0.04 to 0.6% total integrated optical scatter in the forward hemisphere at 1.064 µm can be attributed to surface roughness.
The material in which a volume Bragg grating is made will always have some absorption at the grating's design wavelength. Thus, when exposed to a high power laser beam the grating will absorb some power, be heated such that a temperature gradient is formed and, consequently, become distorted. We developed an accurate model to calculate the reflection of a high power laser beam by a volume Bragg grating that experiences such distortion. We used the beam propagation method (BPM) to calculate the laser beam propagation in the grating numerically, and the BPM calculations are iterated to account for the counter propagation of the laser beam in the volume Bragg grating. We devised a new method to assure convergence in the iteration of the BPM calculations when the grating diffraction strength is very large. We also established a new formulation of the wave equation to include the grating period distortion in the BPM formulation. The surface distortion and temperature induced background index change are also included in the model. This model has been validated to be correct and very accurate. We applied it to calculate the reflection of a high power laser beam by a distorted volume Bragg grating which has large diffraction strength. Our calculation shows that a small amount of grating structure distortion could introduce significant changes of both the phase and intensity patterns of the reflected laser beam. Understanding such changes is critical to the application of volume Bragg grating to high power laser systems.
Epitaxial single-crystal chemical-vapor-deposited diamond was obtained from Element Six Ltd. (Ascot, UK) and from
Apollo Diamond (Boston, MA). Both companies provided 5 x 5 mm squares with thicknesses ranging from 0.5 to 1.5
mm. In addition, Element Six provided 10-mm-diameter disks with a thickness of 1.0 mm. The absorptance of all
specimens at 1064 nm was measured by laser calorimetry, with good agreement between independent measurements at
the University of Central Florida and at QinetiQ (Malvern, UK). Depolarization at 1064 nm and ultraviolet absorption
properties are also reported.
Dramatic spectral narrowing of normally broad band lasers, Ti:Sapphire,Cr:LiSAF, and alexandrite was achieved by simply replacing the output mirror with a reflective, volumetric Bragg grating recorded in photo thermal refractive (PTR) glass. The output power of each laser was changed very slightly from that obtained using dielectric coated output mirrors with the same output coupling as the Bragg grating while spectral brightness increased by about three orders of magnitude.
We evaluate the performance potential of a diode pumped Nd: YAG rod laser by finding the absorbed pump distribution using ASAP, pump induced thermal lensing, gain medium surface distortion and stresses using FEMLAB and depolarization losses using MATLAB. Beam propagation in the optically distorted Nd:YAG rod and the free space part of the cavity, and the output laser beam were determined with a computational scheme we developed which employs the beam propagation method combined with sparse matrix technology. We propose a special cavity design that can select the spatial eigen mode shape of the laser and simultaneously compensate for pump induced thermal lensing, gain medium surface distortion and birefringence. The converged solutions calculated this special cavity design give both high extraction efficiency and good output beam quality. Sensitivity of the output beam to mirror tilt, thermal induced mirror distortion, and errors in the cavity length or the optical distortions in the rod were also calculated.
Usually when optically pumped, dye-doped cholesteric liquid crystals (CLC) generate circularly polarized laser light from both directions of the lasing cell along the cholesteric helical axis. In reality, only the laser light from one direction can be utilized. In this paper, we demonstrate a simple method for doubling the laser output of a dye-doped CLC laser. The extracted laser output is nearly doubled. In experiment, we use a 6-ns, frequency-doubled Nd:YAG laser to pump the CLC lasing sample at ~20 degree incident angle. A reflector: a metal mirror or a cholesteric liquid crystal reflector is placed on the backside of the CLC sample. The reflector is in proximity contact with the CLC sample and the laser action occurs only in one direction. For the metal mirror reflector, the two orthogonal circularly polarized beams are mixed by incoherent superposition. While for the cholesteric liquid crystal reflector (same handedness as the lasing cell and highly reflective of the laser light), the enhanced laser output could also be achieved due to further stimulated amplification but the output is dominated by a single polarization state. For both cases the laser output is associated with a loss of coherence. Hence a nearly unpolarized CLC laser or a partially coherent CLC laser with nearly doubled output intensity is obtained.
We developed a computer model for simulating real solid state laser systems, by solving the paraxial wave Eq.s in the multi-physics modeling software FEMLAB. The reflection of the laser on the curved cavity mirror is calculated by an analytical method. This model was verified to be able to give very accurate results, by applying it to empty stable and unstable resonators, and a face pumped Yb:YAG disk stable resonator laser.
In this paper we present calculations of the effects on thermal lensing of bonding undoped host crystal to the ends and edges of edge-pumped slab lasers. Using ray tracing and finite element analysis we simulate the distribution of absorbed pump power, 3D temperature, stress, and surface displacements. We numerically calculate the induced lensing due to thermal and stress gradients, and deformation induced end effects. In slabs with undoped material bonded to the edges through which pump light enters, the induced lensing has an "m"-shaped profile while undoped Brewster ends reduce deformation induced lensing.
We present practical modeling for edged-pumped high power solid-state slab lasers. Both analytic and ray tracing methods were used to analyze the distributions of absorbed pump power using real broad spectrum diode laser pump sources. The resulting thermal and stress distributions were calculated with finite element analysis. Our analyses include the temperature dependence of the gain medium’s thermal conductivity and expansion coefficient. Nd:YAG and Yb:YAG were then compared for use as gain media for high power solid state slab laser.
Analysis of the process of up-conversion suggest scaling rules for up-conversion based optically written displays. These rules have been demonstrated experimentally and give us confidence in our designs of such displays.
Laser action of Nd3+ doped GdLiF4 (GLF) has been demonstrated for the first time for both pulsed and CW laser pumped operation. A slope efficiency of 60% was obtained in either manner of operation. Detailed comparison studies show that the spectroscopic properties and laser performance of Nd:GLF are very similar to those of Nd:YLF. GLF, on the other hand, can be doped with much higher Nd3+ concentration.
The Czochralski crystal growth of pure Cr, Er and Ho doped as well as Cr,Er and Cr,Ho codoped yttrium silicate Y2SiO5 (YSO) is reported. The growth conditions for producing the inclusion free single crystals of high optical quality are discussed. Spectroscopic properties of the singly doped and codoped material at room temperature are compared. Energy transfer processes from Cr4+ ions to Ho3+ and Er3+ ions in YSO host have been demonstrated for the first time.
Absorption and emission spectra and the fluorescence decay of Nd3+ doped Ca5(PO4)3F (FAP) are reported. A slope efficiency of 67% was obtained in pulse operation pumped by a long pulse Cr:LiSAF laser. Diode-laser-pumped lasing of Nd:FAP has been demonstrated for the first time. A slope efficiency of 25% was obtained without any optimization.
Detailed room temperature absorption and emission spectra and the dynamics of the upconversion energy transfer were investigated in Yb, Ho codoped KYF4. It was found, in the concentration range studied, that the energy transfer rate from Yb3+ to Ho3+ ions is linearly dependent on Ho3+ concentration. The back transfer from Ho3+ to Yb3+ ions was demonstrated and the transfer rate was found to be superlinearly dependent on Yb3+ concentration. Discussions are presented on the optimization of Yb3+ and Ho3+ concentrations for the potential use of Yb, Ho codoped KYF4 as an upconversion green laser.
The spectroscopic properties of Er3+ in the fluoride hosts LiYF4 and KYF4 or YLF and KYF respectively, are presented as they relate to cascade lasing of the 4I11/2 to 4I13/2 and 4I13/2 to 4I15/2 transitions. In this manner simultaneous lasing at near 2800 and 1600 nm is obtained.
Polarized spectroscopic studies are reported of Nd3+ doped KLiYF5 and Nd3+ doped KLiGdF5--a new anisotropic mixed fluoride. Emission in the 4F3/2 yields 4I11/2 channel is strongly polarized. Explanations of the spectroscopic behavior are presented in terms of the local rare earth environment. Lifetime measurements of the 4F3/2 state of Nd3+ ions in these monoclinic hosts and the dependence of fluorescence lifetime on dopant ion concentration are reported. Low threshold pulsed laser action was realized in both hosts with high efficiency, pumping with a long pulse Cr:LiSAF laser tuned to 797 nm.
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