Drew A. Copeland

Excimer kinetics and multiline model for the electron-beam pumped XeF(B–X) laser

A physical model of the time-, temperature- and wavelength-dependent behavior of an E-beam pumped XeF(B-X) laser is developed. Correlations with published laser- and fluorescence-efficiency data, laser spectra, gain, and absorption data are discussed.

Liquid metal heat sink for high-power laser diodes

We report on the development of a novel, ultra-low thermal resistance active heat sink (AHS) for thermal management of high-power laser diodes (HPLD) and other electronic and photonic components. AHS uses a liquid metal coolant flowing at high speed in a miniature closed and sealed loop. The liquid metal coolant receives waste heat from an HPLD at high flux and transfers it at much reduced flux to environment, primary coolant fluid, heat pipe, or structure. Liquid metal flow is maintained electromagnetically without any moving parts. Velocity of liquid metal flow can be controlled electronically, thus allowing for temperature control of HPLD wavelength. This feature also enables operation at a stable wavelength over a broad range of ambient conditions. Results from testing an HPLD cooled by AHS are presented.

Exact and approximate solutions of the utilization and yield equations for O2(1delta) generators

A mathematical model for the production of singlet delta oxygen from the reaction of a gas containing chlorine with the hydroperoxy ion in liquid basic hydrogen peroxide is reviewed. An exact solution for the Cl2 utilization, O2(1(Delta) ) yield, and efficiency of the generator is obtained in the well-stirred limit (WSL) for which the surface concentration of HO2- is constant. A universal set of performance curves is presented and the implications when assessing generator performance are discussed. When depletion of the surface concentration of HO2- is important, perturbation theory is used to obtain a solution for the generators utilization, yield, and efficiency which is a generalization of the corresponding WSL solution. A criterion for the validity of the perturbation solution is obtained and it is shown that the performance of a rotogenerator plateaus not too far above the value of disk rotation rate predicted by this criterion. Finally an integral method is used to obtain a simple, but approximate, solution of the utilization-yield equations which applies over a wide range of operating conditions.

Spectra of electron-beam pumped XeF lasers.

An analysis is presented of the lasing spectrum of the rovibronic XeF(B-X) transition as observed in an electron-beam pumped laser. The intricate spin-split rotational and vibrational structure in the 35.1- and 353-nm lasing emission is investigated, and insight is gained on the processes responsible for the vibrational, rotational, and spin inhomogeneities manifested in the spectra. Explanations of the data are developed in terms of near-resonant vibration-rotation energy transfer, rotational relaxation rate differences in the B and X states, rapid dissociation of the rotational resonances near the dissociation limit of the ground state, relatively slow collisional spin relaxation, and optical coupling of spin states. The implications for narrowband extraction in the 351- and 353-nm bands are noted. The rotational relaxation rates of the X and B states of XeF are estimated in the Appendix.

Simple model for optical extraction from a flowing oxygen-iodine medium using a Fabry-Perot resonator

A loaded-gain model of a CW flowing oxygen-iodine medium is described, and its principal assumptions and approximations are discussed. The model includes pumping of the upper laser level by O(2)(1 Delta), deactivation losses by water, and stimulated emission. The solution of the model in the absence of flux is obtained, from which a small-signal gain is determined. The model is solved for the output power from a Fabry-Perot resonator as a function of the medium and resonator parameters. It is shown that the maximum available power from the medium is determined by the O(2)(1 Delta) concentration at the nozzle exit plane.

Two-phase model of O2(1-delta) production with application to rotating disk generators

A model for the production of singlet delta oxygen, O 2 ( 1 (Delta) ), following the reaction of gaseous chlorine, Cl 2 , with liquid basic hydrogen peroxide, BHP, is described. The model includes diffusion of the Cl 2 gas into the liquid, diffusion of the hydroperoxy anions, HO - 2 , to the surface, reaction of the Cl 2 with the HO - 2 ions at a finite-rate, heterogeneous deactivation of the O 2 ( 1 (Delta) ) within the liquid, and homogeneous deactivation of the O 2 ( 1 (Delta) ) molecules in the gas. Transport equations are written for the chlorine, oxygen, and HO - 2 species concentrations in the liquid while ordinary rate equations are written for the chlorine and oxygen species in the gas. The appropriate initial and boundary conditions for these coupled, nonlinear equations are discussed. Several assumptions and approximations, justified because of the existence of several widely disparate temporal and spatial scales associated with the convection, diffusion, and reaction of Cl 2 with BHP, are discussed and applied to simplify these coupled equations.

Amplified spontaneous emission (ASE) models and approximations for thin-disk laser modeling

It is well–known that amplified spontaneous emission (ASE) can be a major source of upper laser level loss in high gain pulsed or steady–state solid state lasers. This paper briefly reviews the theory of ASE and, using a simple rate equation model of the upper laser level, a geometric, radiative transport equation to describe the ASE intensity, and the perturbation method of multiple time scales, demonstrates that the loss rate of the upper laser level due to ASE adiabatically follows the spontaneous emission source term. This result which includes gain saturation is applicable to both quasi–three level and four level lasers and rigorously justifies formally using the steady–state expression derived heuristically by Lowenthal and Eggleston1 to model ASE loss in pulsed laser media. Then, it is shown that the frequency integral occurring in the ASE loss term can be evaluated analytically for both a broad “flat–top” and a Lorentzian stimulated emission lineshape but must be evaluated numerically or using an approximation due to Tommasini and Balmer2 for a Gaussian stimulated emission lineshape. It is shown that at high gain loss due to ASE is mitigated by ASE line narrowing. For a thin disk laser an approximate expression for the rate of ASE loss (or ASE lifetime) can be obtained by evaluating the remaining volume integral using either the method of Speiser3 or of Vretenar et al4. A new approximate expression for the ASE loss rate is obtained which, unlike Speiser’s3 expression, accounts for ASE line narrowing and, unlike Vretenar et al’s4 expression, correctly scales with the cylindrical volume of the disk. Application to both 1D and 3D laser modeling is briefly discussed.

Pulsed amplifier model for optical extraction from a supersonic oxygen iodine medium

A time-dependent model of a single-pass amplifier which treats upper laser level pumping by O2(1 Delta), deactivation by water, and stimulated emission within the flowing oxygen-iodine medium is described. An efficient algorithm for solving the coupled, flowing medium, and optical extraction equations is presented. The model indicates that extraction efficiencies comparable to CW efficiencies can be obtained with pulse repetition rates comparable to the rate of flow through the cavity and pulse lengths of the order of the time it takes to extract the energy stored in O2(1 Delta).

Optical extraction model and optimal outcoupling for a CW quasi-three level thin disk laser

Using the well-known quasi-three level kinetics model of Beach1,2 and Bourdet3 an exact analytical solution of the coupled medium and geometric, plane-wave optical propagation equations for a longitudinally or face-pumped CW laser is obtained. Although the quasi-three level kinetics model ignores all medium losses, e.g. amplified spontaneous emission, upconversion, and excited state absorption, it is applicable to Yb:YAG devices. The optical extraction model, which accounts for both laser wave amplification and pump wave absorption saturation coupling, treats both one- and two-face pumping as well as single-, double-, and multiple-reflections of the pump wave between the faces of the disk. Analytical expressions for the laser output power, the absorbed pump power, the threshold pump power, as well as the pump absorption, optical-to-optical, optical extraction, and slope efficiencies are obtained. With a suitable modification of the pump absorption efficiency, multiple-pass pumping via pump beam reinjection as achieved with a parabolic reflector by Stewen et al4 can also be treated. Explicit equations for determining the spatial distributions of the pump and laser intensities along the optic axis of the resonator are presented. Finally, explicit transcendental equations to determine the resonator outcoupling fraction which maximizes either the optical-to-optical or the optical extraction efficiency as a function of mirror loss, gain per pass, and pump power are derived. As an example the theory is applied to the Yb:YAG gain medium.

Gain tailoring model and improved optical extraction in CW edge-pumped disk amplifiers

The effect of gain tailoring upon the optical extraction and OPD in a CW edge-pumped disk amplifier is examined using a two-dimensional model of diode pumping coupled with a two-dimensional, geometric model of optical extraction by a Gaussian profile beam from a Yb:YAG medium1. The gain medium is described by the well-known quasi-three level model of Beach2,3. Gain tailoring is accomplished by focusing the diode pump beam using cylindrical lenses. The diode pump beam, optical extraction, and gain medium models are described after which the pump absorption efficiency, energy deposition uniformity, output energy, and maximum peak-to-valley (PV) OPD are examined as a function of the pump lens focal length and output aperture radius as well as amplifier input seed energy, number of roundtrip amplifier passes, and diode pump power. It is shown that using pump beam focusing to tailor the gain radially deposits more energy in the central region of the disk and thus results in improved optical extraction because a Gaussian input optical beam preferentially accesses the central region of the disk. With gain tailoring one can achieve the same amplifier output energies as without gain tailoring but using less pump power and/or amplifier seed energy, resulting in reduced disk heating and diode-pump waste heat. Although the maximum PV OPD is larger, the central region of the thermally-induced OPD remains relatively uniform, allowing one to increase the output energy with only modest increases in the effective OPD.