MANUSCRIPT NOT RECEIVED REPETITIVELY PULSED BEAM DIAGNOSTICS D. B. Nichols, D.J. Morris, and W. B. Shepherd Seattle, WA 98124 T.M. Donovan, J.L. Stanford, and C.D. Marrs China Lake, CA 93555 and C. F. Zahnow Maxwell Laboratories Inc. ABSTRACT Increasingly detailed information about the progression of damage to optical surfaces during repetitively-pulsed irradiation has accentuated the need for corresponding detailed information about the incident beam profiles. The invariance of spatial profile from pulse to pulse typical of small lasers cannot be invoked in many larger lasers. The product of amplitude resolution times spatial resolution times pulse rate produces high data rate requirements during repetitively-pulsed testing. This paper describes the utilization of one pulse-laser beam profiling system1 in optics tests at four laboratories, to illustrate single-pulsed DF, repped DF, single-pulsed XeF, and repped XeF, laser beam diagnostics. This work was supported by SPAWARS SYSCOM PMW-145, through the Naval Weapons Center, China Lake, CA. 1 D.B. Nichols, P.D. Texeira, and T.M. Donovan, "High-Energy Pulsed Laser Beam Profiling System," Eighteenth Annual Symposium on Optical Materials for High Power Lasers, Boulder, Colorado, 3-5 November, 1986. MANUSCRIPT NOT RECEIVED EMERGENCE OF "CONSENSUS STANDARDS" IN LASER DAMAGE ACCEPTANCE S.C. Seitel and E.A. Teppo J.W. Arenberg Santa Monica, California 90403 ABSTRACT Many government-sponsored Nd:YAG laser system programs now require comprehensive laserdamage testing for optics vendor qualification and parts acceptance. The most visible artifacts of these test programs are the test specifications. Initially idiosyncratic and often incomplete, recent examples of production test specifications show remarkable similarity of nomenclature, test parameter set, exposure procedure, calibration verification, "damage" definition and pass/fail criteria. For one segment of the industry, at least, reasonable order appears to be emerging from chaos. We review the emergence and refinement of these apparent "consensus standards". Specific examples of incomplete and ambiguous early specifications are given with discussion of how the loopholes allow bad parts to reach hardware. We show that careful consideration of key issues helps make the specification (and test program) consistent, complete, and system-relevant; and we present a "generic" specification which results from this process. Not by coincidence, our format embodies the important features of many recently issued Nd:YAG production test specifications. Finally, we ask whether the success of the "white-label" Nd:YAG specification might serve as a model for other segments of the industry, and offer it as our contribution toward one original and often-restated goal of this Symposium: standardization of test methods for better interchange of information between laboratories. Recently there has been a growing interest in the use of particle suspensions as nonlinear media for use in optical limiters for sensor protection. Using a tight focusing geometry we have observed the onset of limiting at input powers of as low as = 20 W using nanosecond 0.53 μm input pulses. We present evidence that is consistent with scattering from rapidly expanding microplasmas initiated by thermionic emission from the carbon particles that are heated by linear absorption of the input light. A simple calculation of the temperature of the 50 nm diameter carbon particles gives sufficient to produce significant thermionic emission to initiate avalanche ionization monitored transmission, absorption (using photoacoustic detection) and scattering (looking at side scattered light), we find that nonlinear scattering is the dominant mechanism limiting the transmission. Manuscript Received 1. Colloidal Materials as Optical Filtering Media K. J. Kogler, R. G. Pastor, R. L. Burton, R. A. Spence, and N. P. Murarka IIT Research Institute Particles 1 μm or less in diameter are said to be colloidal sized particles. When the colloids are metals dispersed in a dielectric matrix, the transmittance of the system may possess an absorption band. The shape, depth, and resonant wavelength of the stop band are determined by the size of the metal particles, the volume fraction of the metal particles in the dielectric, and the complex index of refraction of both the metal colloids and the dielectric matrix. The purpose of our experimental work was to produce colloids of vanadium dioxide embedded in a dielectric matrix and to study its optical properties. During this investigation, we also produced colloidal size particles of silver and gold and studied their optical properties. Key words: metal colloids; optical filters; plasmon resonances; sphere masks; Introduction The motivation for the optical materials development discussed herein is the demonstration of a switchable blocking filter based on the plasmon resonant absorption phenomenon exhibited by small metallic particles (on the order of hundreds of nanometers). The concept pursued is the substitution of VO2 particles of similar dimensions for the metal with expectation of achieving an absorption résonance that would be switched by the phase transition of VO2 to a metal-like state when heated. This approach improves upon a continuous thin film of VO by providing rejection of a narrow band of wavelengths only and offers potential for improving dynamic range (ratio of switching threshold to damage threshold). Furthermore, plasmon resonance of metallic particles occurs at visible wavelengths suggesting the applicability of VO2 to that part of the spectrum. 2. Theory and Approach The observation of the resonant absorption phenomenon in noble metal island films is well documented in the literature [1,2]. Several theories have been developed to explain the absorption phenomenon with none providing good quantitative agreement. Many of them stem from the basic theory of Garnett [3] which at least qualitatively predicts the presence or absence of absorption maxima and their shift with wavelength in terms of the observed structure of the films and the bulk optical properties of the metal. The simplicity of the theory and its utility in predicting absorption maxima and their spectral location made this an attractive tool for validating the VO2 colloidal concept. Relationships derived by Garnett to determine an effective index of a composite consisting of dispersed metal particles of bulk index and extinction coefficients n and k are given by eq (1) and (2). *Numbers in brackets indicate the literature references at the end of this paper. where Q is the volume of metal per unit volume of medium and necessarily 0 < Q < 1, and (2) (3) (4) These equations were applied to qualitatively predict resonance absorption in VO, particles by collecting optical constant dispersion data from spectral measurements on VO, thin films above and below the phase transition temperature. The values of transmittance and reflectance for light passed in both directions through the film were used to determine the reflectance of the films on an infinitely thick substrate (with no back surface reflectance). From these data and dispersion data for the substrate, it was possible to obtain a good fit for a multiparameter model. For room-temperature measurements (VO,-transmitting state) good fits were obtained by modeling the dispersion with five Lorentzian oscillators. The high-temperature data (VO,-blocking state) was well fitted with a model that incorporated one free carrier oscillator and four Lorentzian oscillators. The best fit parameters were then used to generate the dispersion curves for the index and extinction coefficient. The fitting procedure described above was repeated for three VO2 films. The data generated for all three films were quite similar. The data for the three were averaged to improve accuracy. 3. Results Figures 1 and 2 show some typical results of the fitting procedure, in the case of the 137 nm thick film. Figure 1 shows the reflectance data and the best fit results at room temperature. Figure 2 is a similar plot for high temperature. At room temperature the film is transmitting over most of the data range. Therefore, the reflectance shown in figure 1 is due to the combined effects of dispersion from multiple oscillators and interference phenomena. By comparison, the reflectance in figure 2 is very close to the bulk reflectance of VO2 at 90°C since the transmittance over most of the data range is less than one percent. The results of the analysis of the room-temperature data are presented in figures 3 and 4. Figure 3 shows the refractive index as a function of wavelength for the average of three films. The three low-temperature data sets do not match exactly but are all within ±0.04 of their average value. The extinction coefficient results are shown in figure 4. The lowtemperature data, in this case, were all very close together and demonstrate that absorption decreases with increasing wavelength for VO2 at low temperature. This is the opposite of the high-temperature case. For the high-temperature data, both n and k exhibit a large spread between the three samples. This may be due to the dominant effect of the free carrier oscillator on the reflectance, since the free carrier optical properties are highly dependent on electrical conductivity, which may vary from film to film due to differences in film quality. The extinction coefficient in the high-temperature regime is found to increase with increasing wavelength, as expected. |