Nils C. Fernelius

Extension of the Rosencwaig‐Gersho photoacoustic spectroscopy theory to include effects of a sample coating

The Rosencwaig‐Gersho theory for the photoacoustic signal from a homogeneous solid has been extended to include effects of a coating layer with different thermal and optical properties than the substrate. The cases treated include one with an optically transparent coating and one with appreciable optical absorption.

Helmholtz resonance effect in photoacoustic cells

Some types of photoacoustic spectroscopy sample cells exhibit acoustic resonances which are not explained by organ pipe or Bessel mode analysis. It is demonstrated here that one type of acoustic resonance encountered is explained by a Helmholtz resonator analysis. Its properties are studied as a function of the dimensions of the channel connecting the sample chamber and the microphone housing chamber.

Photoacoustic signal variations with chopping frequency for ZnSe laser windows

Experimental photoacoustic (PA) signal amplitude and phase angle data were obtained as a function of chopping frequency on a 10.6‐μm antireflective coated ZnSe window using a CO2 laser. The PA signal amplitude had an f−n frequency dependence with n=1.04. The PA phase angle varied by 20° between 1000 and 50 Hz; 7° between 1000 and 100 Hz. Theoretical calculations were made using the Bennett‐Forman, Rosencwaig‐Gersho, and two‐layer Rosencwaig‐Gersho theories. No theory yielded a 20° phase change between 1000 and 50 Hz. The homogeneous sample Rosencwaig‐Gersho theory did not fit either amplitude or phase angle data. Assuming a 1‐μm‐thick ThF4 layer on top of the ZnSe, the two‐layer Rosencwaig‐Gersho theory yielded a coating optical absorption coeffficient, βc =2.7 cm−1 to give a 7° phase change. This then gave an amplitude dependence of n?1.03. The Bennett‐Forman theory agreed with PA amplitude results for r=βsurface /βbulk ≳0.05 cm. A 7° phase change was obtained for r=0.03 cm. The best fit of the Bennett‐F...

A Photoacoustic Spectroscopy Study of Some Thallium Iodide Powders

Photoacoustic spectroscopy has been used to study TlI powders used in thin film coating applications. The spectra obtained show differences between what was regarded as good coating material from that which was unacceptable. Good material displayed a sharp absorption edge at 440 nm and no tailing off at the transparent side of the edge. The ratio of signals on the opaque side of the edge divided by that on the transparent side was largest for good material. Much of the poorer material contained absorbed I2 within the material. It was found that material exposed to the atmosphere would evolve I2 gas. Differenence spectra between freshly mounted TlI and evolved materials closely resemble absorption spectra of I2 gas.

Photoacoustic chopping frequency studies on uncoated zinc selenide.

Experimental photoacoustic (PA) signal amplitude and phase data were obtained as a function of chopping frequency on chemical vapor deposition (CVD) grown zinc selenide. The light source was a CO(2) laser operating at 10.6 microm with about a cw 24-W power level. The PA amplitude had an f(-n) dependence with n = 1.04. The PA phase angle plot yielded a phase difference between 1000 and 500 Hz and 50 Hz of 13 degrees ; between 1000 and 500 Hz and 100 Hz, of 6 degrees . Laser rate calorimetry on these samples gave an average beta(eff) = beta(B) + 2beta(s)/l = 4.32 x 10(-3) cm(-1), where l is the sample thickness, beta(B) is the bulk optical absorption coefficient in cm(-1), and beta(s), is the dimensionless surface absorption. The average value of l was 0.79 cm. Theoretical calculations were made using several PA theories and parameters relevant to CVD ZnSe. A modified version of the Rosencwaig-Gersho (RG) theory was derived to explicitly include surface absorption. The RG theory calculations yielded results that differed greatly from experiment. Calculations using the McDonald-Wetsel composite-piston model, an extension of the RG theory, yielded results closer to experiment. A comparison of various calculations using the Bennett-Forman theory indicated that the best consensus value for the surface-to-bulk optical absorption ratio, r identical with beta(S)/beta(B), was r = 0.025 cm. Using this value yields beta(B) = 4.06 x l0(-3) cm(-1) and beta(s) = 0.000102. Thus for these samples 94% of the measured beta(eff) is due to beta(B). This value for r is about an order of magnitude smaller than that obtained from a study on similar samples using the multithickness technique. Calculations using the modified RG theory were almost identical to the results of the Bennett-Forman theory for the same value of r.

Photoacoustic chopping frequency studies on ZnSe laser windows using a CO 2 laser

Experimental photoacoustic spectroscopy (PAS) amplitude and phase results on an AR-coated ZnSe laser window were obtained at 10.6 μm as a function of chopping frequency. Theoretical calculations related to this situation were performed using the Bennett-Forman, Rosencwaig-Gersho, and two layer Rosencwaig-Gersho theories.

Some Optical Properties Of Materials Measured At 1.3 õm

This work surveys the properties of materials at 1.3 um. The interest in this wavelength has been generated by the development of second generation optical fibers and by the development of the iodine (I*) laser, which operates at 1.315 um. Most of the data reported here were taken by Nd:YAG lasers modified to operate at 1.319pm. Spectral scans of some less common materials are presented. The effective optical absorption coefficient Beff measured by laser rate calorimetry is given for all materials. The thermo-optic coefficient an/aT at 1.3 um is given for a number of materials. If 1.3 um results are lacking, values at 1.15 um are given. Among the lower absorbing materials are KCI, LiF, NaCI, CaF2, SiO2, YLF (LiYF4), sapphire (Al203), MgF2, BaF2, and Q-98 phosphate glass. Results on a number of Schott infrared and laser glasses are given, including LG-670, LG-660, LG-750, LG-810, LG-760, LG-680, IRG-7, IRG-9, IRG-3, and IRG-N6. Results are given also on ZnS (normal and water clear), YAG (Y3A1501 2), ZrO2, SrF2, ZnSe, MgO, and CdTe. Some other glasses studied were hafnium fluoride glass, As2S3, CORTRAN 9753 and 9754, and Barr & Stroud calcium aluminate glasses BS 39B and BS 37A. The ten lowest absorbing materials had effective optical absorption coefficients Leff between 0.13 and 0.5 X10-3 cm-1. The lowest a n/aT value measured was 0.06X10-5/°C on Schott LG-660 alkali-Zn-silicate glass.

Characterization Of Candidate Laser Window Materials

The paper presents the results of a characterization study of selected materials for possible use as laser windows. The preliminary materials selection was based on known or calculated optical properties at the wavelengths of interest and the availability of the materials in sufficient quantity and size. The characterization includes optical, mechanical, and thermal property determinations necessary for the systems designer to make proper selection of a window material based on system requirements. The materials presently under consideration include A1203 (sapphire), Si02 (fused silica), Y3A15012 (YAG), MgO, ZnS, ZnSe, CaF2, BaF2, SrF2, LiYF4 (YLF) and various fluoride glass compositions. The optical properties for the wavelengths of interest will be presented and the problems encountered when designing with brittle materials will be discussed.

Photoacoustic and laser rate calorimetry studies of the bulk and surface optical absorption coefficients of laser window materials

Abstract The surface-to-bulk optical absorption ratio, r, was obtained on a group of similar CaF2 samples at 1.3 μm wavelength using three different experimental techniques. Laser rate calorimetry results on a series of samples with varying thickness gave r = β s β B = 0.136 cm . The two slope rate calorimetry technique was applied to a long bar and no second slope was seen. This implies that r is less than 0.3 cm. Analysis of photoacoustic chopping frequency variation studies using the Bennett-Forman theory give r = 0.07 cm about half the multithickness result. Thus all three techniques yielded consistent results.

Multiwavelength Laser Rate Calorimetry On Various Infrared Window Materials

The effective optical absorption coefficient obtained by the slope method of laser calorimetry is presented for a number of materials at a variety of laser wavelengths. The materials studied include MgF2, NaC1, KC1, LiYF4, CaF2, LiF, YAG, A1203 (sapphire), Si02, BaF2, CORTRAN 9753 and 9754 glass, ZBT glass, GGG, Mg0•A1203 (spinel), ZnSe, MgO, Si, As2S3, ZnS, and calcium aluminate glasses. The laser wavelengths studied were 1.319, 2.7, 3.8, 5.3, 9.27 and 10.6 μm.