Laser Calorimetry Research Articles

Photothermal measurement of bulk and surface absorption of transparent infrared optical elements

Photothermal probe beam deflection spectroscopy is used to separately measure bulk and surface absorption of transparent IR optical elements. Different approaches of this technique are considered. One approach consists in variation of the laser beam heat modulation frequency in the condition that the heating beam component is transverse to the path of the probe beam. Another approach consists in varying the heat and probe cross section measuring point position within the tested optical sample. It is theoretically and experimentally shown that in the second case, the spatial resolution and measurement locality are determined by the size of laser beam spots and the thermal diffusion length in a sample. The relative contribution of surface and bulk absorption to summary absorption can be derived from the ratio of the probe beam deflection signal, measured both near the surface and within the sample volume far from the surface, to thermal wavelength. Absolute calibration of absorption measurements can be carried out using laser calorimetry. A comparison between deflection and calorimetry data on volume absorption in a number of ZnSe laser optical elements is made and shows a good correlation.

Surface and bulk absorption characteristics of chemically vapor-deposited zinc selenide in the infrared

<p>Chemically vapor-deposited zinc selenide exhibits outstanding properties in the infrared and has been established as a prime material for transmissive optics applications. Here we present and discuss data relating to the surface and the bulk absorption forward-looking infrared- (FLIR-) grade chemically vapor-deposited ZnSe, at wavelengths (2-20 µm) and temperatures (100-500 K) of current interest.</p><p>This investigation is based on both spectral emittance measurements and infrared transmission spectroscopy performed in the context of a systems development program. Surface effects can be detected at wavelengths of up to 14 µm and usually predominate at wavelengths of less than 8 µm. Fractional surface absorptions are temperature independent from approximately 200 to 400 K and can be fitted to a Fourier series, at wavelengths ranging from 3.5 to 13.5 µm. The bulk absorption coefficient (βv) is strongly dependent on temperature as well as wavelength, but it can be approximated by a bivariate polynomial expressin that yields recommended values. At wavelengths λ ≲ 10 µm, βv decreases with increasing temperature; it is shown that a wavelength-independent Debye-Waller factor provides a correct description of the temperature dependence, thus pointing to infrared-active localized modes. At wavelengths λ ≳ 14 µm, βv increases with temperature and exhibits temperature dependencies (T(1.7), T(2.6)) that reflect three- and four-phonon summation processes. Finally, an analysis of the temperature dependence of βv at 10.6 µm demonstrates that the intrinsic lattice dynamical contribution to bulk absorption at this wavelength should be close to 4 × 10(-4) cm(-1), in accord with the results of earlier laser calorimetry tests performed on exceptionally pure laser-grade chemically vapor-deposited ZnSe.</p>

Potentiality of the photothermal surface-displacement technique for precisely performed absorption measurement of optical coatings

The influence of sample properties on the measured signal has to be investigated, so that measurements of the optical absorption in thin-film coatings by means of the photothermal surface-displacement technique may be precisely performed. Carrying out simultaneously absorption measurements on metal coatings at λ=10.6 μm by using laser calorimetry as well as the surface-displacement technique, the aforementioned influence and the validity of assumptions of the theoretical model, respectively, can be proven. It can be demonstrated that, in contrast to the model, thermophysical coating properties contribute to the probed surface displacement.

Absorption losses in MgO-doped and undoped potassium niobate

Measurements of absorption losses in KNbO3 and MgO-doped KNbO3 were made in the blue and near-infrared wavelength regions using laser calorimetry. In the doped samples, the absorption loss for blue light was found to be 40% lower than that of the undoped samples. An enhancement in the infrared loss, caused by the presence of blue light, was observed in doped and undoped samples, but found to be much greater in the MgO-doped crystals. The blue-enhanced infrared absorption was characterized using two-wavelength laser calorimetry. Temporal variations in the second-harmonic power, caused by nonuniform heating of the active volume by the absorbed infrared and second-harmonic power, are described.

Absorption measurements of mixed silver halide crystals and fibers by laser calorimetry

The absorption coefficients of mixed silver halide crystals and fibers were measured by different laser calorimetric methods: adiabatic for crystals and isothermal for fibers. The composition dependence of the bulk absorption coefficients β(b) of crystals and polycrystalline fibers at 10.6-µm (CO(2) radiation) was investigated. It was found that the crystal surface absorption β(s) is dependent on the surface polishing quality. All experimental results of β(b) are smaller than those reported in the literature.

Potentiality of the photothermal surface displacement technique for precisely performed absorption measurement of optical coatings

The influence of sample properties on the measured signal has to be investigated, so that measurements of the optical absorption in thin-film coatings by means of the photothermal surface-displacement technique may be precisely performed. Carrying out simultaneously absorption measurements on metal coatings at λ=10.6 μm by using laser calorimetry as well as the surface-displacement technique, the aforementioned influence and the validity of assumptions of the theoretical model, respectively, can be proven. It can be demonstrated that, in contrast to the model, thermophysical coating properties contribute to the probed surface displacement.

Study of the optical properties of diamond-like coatings prepared by Laser-Arc

The Laser-Arc as a controlled pulsed arc plasma source was used to prepare diamond-like coatings (DLC) for optical applications. DLC films with a thickness of up to 500 nm were deposited on different substrate materials (ZnSe, NaCl, KCl, Ge) using special preparation techniques for enhancing the substrate-film adhesion. Coating was performed at room temperature in a vacuum of 10 −4 Pa. A special moving substrate holder was used to deposit an all-round protective DLC that covered the complete surface of the salt substrates including the rims. The optical properties of the coating were determined by spectrophotometry and laser calorimetry. On the basis of the photometric transmission measurements, the refractive indices were calculated. The optical absorption at 10.6 μm ranges from 200 to 900 cm −1 and depends on the film thickness and on the substrate material. The results demonstrate that the controlled arc evaporation by Laser-Arc is suitable for the preparation of DLC with optical quality.

Measurement of bulk absorption coefficients of chalcogenide and chalcohalide glasses at 10.6 μm using CO2 laser calorimetry

Abstract The bulk absorption coefficients of chalcogenide and chalcohalide glasses at 10.6 μm have been measured using CO2 laser calorimetry. The results indicate that processing of as-received chemicals is necessary to lower the impurity oxide content of the glasses and consequently the absorption coefficient at 10.6 μm.

Dynamics of the 5 eV optical absorption in SiO2 glass

The optical absorption at 5 eV in SiO2 glass was observed, using laser calorimetry, to change reversibly depending on the intensity of UV light. The generation and bleaching of an absorption band at 5 eV by two- and one-photon absorption processes, respectively, can explain these reversible changes. This observation supports the structural model of unrelaxed oxygen deficiency center for the 5 eV absorption band in silica.

A Study on the Relationship between Laser Beam Polarization and Absorptivity. 1st Report. Effect of Surface Roughness.

The relationship between the incident angle of a laser beam with linear polarization and absorptivity on S40C steel at a wavelength of 10.6μm was investigated by both laser calorimetry and theoretical calculation based on the Fresnel equations. A simplified triangulate surface model and a highly polished specimen were used to determine the effect of surface roughness on absorptivity. A comparison was also made with a normally machined specimen to confirm the effect. With regard to the absorption characteristics, it was found that the calculated and the measured absorptivities for an ideally flat or highly polished surface were nearly the same over the entire range of incident angles examined. Absorptivity was strongly dependent on incident angle. A pronounced absorptivity peak was observed near the Brewster's angle which was approximately 86° from the normal. When the surface was asymmetrically triangulated, the angle at which peak absorptivity occurred was either larger or smaller depending on the incident direction of the laser beam. The peak absorptivity value on triangulated specimen was lower than that on the polished specimen. These tendencies agree closely with the predictions made by theoretical calculation and with the results for the normally machined specimen.

Infrared optical characteristics of type 2A diamonds

The absorption coefficient of two samples of type 2A diamond was measured by laser calorimetry at 10.6 microm and found to be 0.033 and 0.0415 cm(-1). Transmission properties from 2.5 to 20 microm are also presented.

Determination of intrinsic mid-infrared absorption of a fluoride glass

The intrinsic mid-infrared optical absorption of a ZrF(4)-BaF(2)-LaF(3)-AlF(3)-NaF (ZBLAN) glass has been determined by combining laser calorimetry and spark-source mass spectrographic analysis. The total optical absorption of four samples of varying purity was measured at 3.9 microm with laser calorimetry. The extrinsic contribution to the total absorption for each glass was calculated from the measured concentration of rare-earth impurity ions and known extinction coefficients. The intrinsic absorption of this glass at 3.9 microm determined from the difference between the total measured absorption and the calculated extrinsic absorption is 6.11 (+/-0.61) x 10(-4) cm(-1).

Separation of intrinsic and extrinsic optical absorption in a fluoride glass

The contribution of impurity ions to the total optical absorption of a heavy metal fluoride glass has been determined at 532 and 1064 nm. Four ZrF4-BaF2-LaF3-AlF3-NaF glasses were prepared from various purity raw materials. The absorption coefficients of these glasses range from 0.92 to 45.4×10−4 cm−1 at 1064 nm and from 7.43 to 11.1×10−4 cm−1 at 532 nm as determined by laser calorimetry. The concentrations of Fe, Ni, Cu, and Co ions in each glass were determined by graphite furnace atomic absorption spectroscopy. These two measurements enable the absorption, due to transition metal ions to be differentiated from the intrinsic absorption of the glass. At 1064 nm, the absorption coefficient of these glasses is controlled entirely by the transition metal ion content. However, at 532 nm, the absorption by the transition metal ions accounts for 4–42% of the total absorption depending on impurity concentration. The intrinsic absorption of this fluoride glass calculated from these data at 532 nm is (7.69±0.99)×10−4 cm−1.

Laser-induced damage threshold and absorption measurements in rare-gas-halide excimer laser components

We have developed a new system to measure the damage threshold with high reproducibility. We controlled the total energy on the sample by an optical system, without changing a spot-size on the sample surface. We made high-reflection coatings with and without over-coat, and anti-reflection coatings with and without under-coat for 248 nm KrF excimer laser light. In high-reflection coating, non-quarter-wavelength design was also tested. Damage threshold of coatings for 193 nm ArF excimer laser were also tested. We measured the absorption of coatings by the interferometric laser calorimetry. Revealing a cause of damage, we made a qualitative and quantitative analysis, so that the main cause of the damageof the optical coatings for 248 nm light was the residual absorption of the coatings.

Synthesis and Characterization of Diamond-Like Carbon Films using Ion Beam Technique

ABSTRACTDiamond-like carbon (DLC) coatings have been deposited on several infrared transmitting substrates utilizing the ion beam deposition technique. Optimum deposition parameters have been established as a function of source gas composition, source pressure, ion-impact energy, substrate material and cleaning substrate procedures. Extensive characterization of the DLC films was also performed. Rutherford backscattering and proton recoil techniques were used to analyze carbon and hydrogen content and impurities. These films contain 70% atomic carbon and 30% atomic hydrogen. Transmission electron microscopy was used to analyze the crystallinity, void structure and surface microstructure, which were found to be amorphous and dense. Optical properties, such as refractive index and extinction coefficient, were measured using transmission/reflection spectroscopy, ellipsometry and laser calorimetry. Environmental testing was performed using various acids and solvents. The thermal stability and moisture penetration on these DLC films were extensively investigated. The effect of high ion energy radiation on DLC films was studied. Details of the preparation method and characterization of DLC films are presented.

Fluorescence quantum efficiency and optical heating efficiency in laser crystals and glasses by laser calorimetry

A photocaloric technique is described for determining the fluorescence quantum efficiencies and optical heating efficiencies of optically active ions in laser materials. Optical absorption within the sample results in a temperature increase until the heat produced by the absorbed power is balanced by heat leakage to the surroundings. The fluorescence quantum efficiency and optical heating efficiency are determined from a measure of the absorbed power, the steady-state temperature, and the time constant associated with sample cooling following laser excitation. An alternative analysis utilizing only the absorbed power and the steady-state temperature as a function of excitation frequency is also shown to yield quantum efficiencies consistent with the first method. Theory and experiment are demonstrated by measuring the fluorescence quantum efficiency and optical heating efficiency for trivalent chromium in gadolinium scandium gallium garnet. Measurements are also reported for several neodymium-doped phosphate laser glasses.

Pulsed laser calorimetry

The method of adiabatic pulsed laser calorimetry is considered. It is shown that this method of determination of the volume absorption coefficient of optical materials heated by laser pulses of different duration makes it possible to find the dimensions of submicron defects and to analyze the nature of nonlinear absorption without damage to the optical material.

Absorption-influenced Laser Damage Resistance of Ta 2 O 5 Coatings

Results from experiments on laser-induced damage and absorption of evaporated Ta2O5 quarter-wave single-layer films are presented at u = 530 nm and 1060 nm, respectively. The absorption measurements were carried out by laser calorimetry. Emphasis is layed on the lack of correlation between absorption and laser damage in the 10m3 range of absorption itself and the statistical-like behaviour of the damage threshold.

IR transparency of the glass of ZnCl 2-KBr-PbBr 2 system

IR transparency and some properties of halide glass of composition 48ZnCl 2-48KBr-4PbBr 2 (mol%) have been investigated as the basic study on the development of IR glass fiber for CO 2 gas laser. The glass transition temperature Tg, crystallization temperature T c, softning temperature T s, and linear thermal expansion coefficient α of the glass were found to be 45–46°C, 100°C, 54°C, and 570×10 −7 / °C, respectively. The refractive index of the glass for He-Ne laser emission (632.8 nm) was about 1.63. The amount of impurity which decreases the transmissibility for CO 2 laser beam due to the absorption in the wavelength region 10–11 μm could be reduced by preparing the glass from a batch containing NH 4 Cl under reactive atmosphere of CCl 4 or CBr 4 in a glove box filled with He gas of dew point −62∼ −45°C. The minimum value of absorption loss of the glass for CO 2 gas laser measured by laser calorimetry was about 20 dB/m.

Measurement of the thickness dependence of absorption in HfO2 and ZnS single-layer films

Abstract A method for measuring the bulk absorption in optical single-layer films using both laser calorimetry and photoacoustic techniques is presented in this paper. The absorption and damage thresholds for evaporated dielectric single-layer films such as HfO 2 and ZnS were measured at wavelengths λ of 515 nm (530 nm) and 1060 nm respectively. When the laser power at the interfaces of the single-layer films vanishes, they do not contribute to the absorption. Therefore the absorption due to the bulk alone can be measured together with the thickness dependence of the related absorption data. A correlation between the absorption and damage thresholds was also found.