Optical Path Length Changes Research Articles

Photo-induced optical changes in amorphous AsS films

Photo-induced optical changes were investigated in thin film structures of As x S 100− x with x between 10 and 43. For As 40S 60 the magnitude of the change in optical path length at 6328 Å for the reversible photodarkening process is about one-third of that for the irreversible one. The relaxation time of the former is approximately twice that of the latter. In specimens with excess chalcogen, the influence of annealing becomes weaker with increase in sulphur content. The change in optical path length for the virgin films has a minimum at a composition of about As 25S 75. These characteristics suggest that sulphur plays an active role in the photo-induced transformation.

Laser Doppler interferometry for measuring small absorption coefficients

A laser Doppler interferometric technique for measuring small absorption coefficients of infrared-transmitting materials is described. The interferometer is calibrated as a remote temperature sensor by monitoring optical path length changes of the sample as a function of temperature. The sample is then irradiated by a CO2 laser and the change in optical path due to absorbed laser energy is observed interferometrically as a function of time. From this measurement and the calibration data, a temperature-vs-time curve is generated from which the absorption coefficient of the sample at 10.6 μ may be calculated.

Experimental Determination of the Effect of Temperature on Refractive Index and Optical Path Length of Glass

With the Fizeau interferometer method, the change in optical path length with temperature is measured at six wavelengths for twenty new Corning optical glasses. The sample, a polished block with a few seconds of wedge, is held in a double-walled chamber with temperature control provided over the range -20 degrees C to +80 degrees C by a circulating water-glycol mixture. A Gaertner interferometer viewing apparatus is modified by addition of a monitor head that permits simultaneous photoelectric fringe detecting and viewing. Measurement of thermal expansion with the Plummer-Hagy high precision dilatometer permits the extraction of the temperature coefficient of absolute refractive index from the optical path measurement. The coefficient of refractive index relative to standard air at the temperature of the glass is found by further data reduction using Edlén's equations. The experimental values of the index change as a function of temperature are curve-fitted by a FORTRAN program, and the slopes are computed to give instantaneous values of the coefficients. The average residual for the fit is about 1.0 x 10(-6). The precision of the absolute coefficient values is estimated to be +/-0.05 x 10(-6). Representative data for one glass are presented to illustrate the results available from this work.

Inclusions in Laser Materials.

One of the severe problems encountered in high-power-solid-state laser systems is the thermal damage to laser rods and optical elements. One such type of damage is thought to arise from metallic or dielectric inclusions; i.e., impurities with physical and optical properties which differ substantially from those of the host material. Such inclusions may absorb an appreciable amount of the incident radiation and thereby may undergo thermal expansion. This produces major stresses within the host material. Estimating such thermal properties requires the consideration of solutions to the heat diffusion equation and to the thermal stress equations with appropriate boundary conditions. The optical path length change for a probing light ray passing near the inclusion, the radial and tangential stress components, and the changes of the refractive index for radially polarized and tangentially polarized light due to the thermal stress field are computed. The dependence of the maximum value of the tensile stress upon the size of the inclusion and upon the physical properties of the host is examined. The feasibility of using optical techniques to detect metallic and dielectric inclusions in laser materials before they cause damage also is studied. The computations suggest that the use of laser pulse widths of the order of microseconds or longer may be more promising for the detection of small incipient absorbing centers than the use of nanosecond pulse widths.

A Holographic Interferometer for Measuring Radiation Energy Deposition Profiles in Transparent Liquids

An apparatus has been designed for real-time and double-exposure holographic interferometry to determine radiation absorbed dose distributions in transparent liquids. The change in refractive index of the liquid due to a temperature rise after irradiation is measured interferometrically. In a cylindrically symmetrical radiation field, the dose distribution can be computed from data supplied by the reconstruction of the holographic interferogram taken as side-view profile of the change in optical pathlength. Relatively inexpensive components such as a low-powered He-Ne laser together with a conventional photographic shutter and low-cost mirrors and lenses were used. The mathematical procedure for unfolding the three-dimensional dose distribtion is described, and an example is given for use with a high-intensity, pulsed, 2-MV electron source.

Lloyd Mirror Laser Interferometer for Diffusion Layer Studies

A Lloyd mirror laser interferometer is described which determines the refractive index gradient in a diffusion layer. The interference fringe movement is detected by an array of n-p-n planar silicon photodevices the output of which is recorded by oscillographs. Various modes of measurement are discussed which are suitable for transparent or opaque electrodes. The presently used measuring mode is based on the determination of the reflected ray's optical path length change at various angles of incidence. The refractive index gradient is obtained by an iterative solution of the derived equation for the optical path length change on a digital computer. The time dependence of the fringe movement during electrodeposition from CuSO4 solutions is discussed in addition to the effect of incidence angle and cathode displacement. The use of this interferometer is not limited to electrochemical processes and it can be used for the study of refractive index gradients in heat and other mass transfer boundary layers at solids.

End-Region Analysis for Stress and Optical Pathlength Change in Composite Laser Rods

A general solution is presented for the end-region stress in a composite cylindrical rod where differential thermal expansion or contraction occurs between the core and cladding. Results describe local conditions set up in the ends of clad laser rods during manufacture and permit an analytical assessment of the optical path distortion which can arise in this geometry from residual strain. The formulations are obtained by superposition of the infinite rod solution and a solution to the boundary value problem posed by an axial force distribution, of equal but opposite magnitude to the infinite rod stress, applied to the surface of a stress-free cylinder. Expressing the applied tractions as a Fourier-Dini expansion at the axial boundary and the radial stress as a Fourier expansion at the radial boundary leads to a double infinite series form for the principal stresses. Implementation of Fejer's theorem was helpful in smoothing the numerical results; however, summing as many as 100 series terms with the computer was necessary to gain satisfactory convergence. Principal stress-strain distributions are presented for a typical glass rod with a core-cladding diameter ratio of 0.7 to 1 and contraction ratio below the transformation temperature of 0.95 to 1. Results show the end-region domian extends less than 1 rod diameter. Optical pathlength variation across the core aperture for radial and tangential beam polarizations was computed to be 0.58 and 0.51 μ, respectively.

Pulsed holographic interferometry vs Schlieren photography.

A hologram, sometimes called a holographic interferogram, provides a fringe pattern revealing changes in optical path length that occur between exposures of a scene. The method is thus obviously applicable to gas flow analysis. Appearing to offer approximately the same sensitivity as schlieren techniques, it requires no tedious alignment. To afford a direct comparison of results obtainable with the two methods, the experiments described herein were conducted. Holographic as well as schlieren techniques were employed in measuring the density distribution of different twodimensional air flow patterns: one from a supersonic jet impinging on an inclined flat plate; the other from an ejector. Figure 1 diagrams the experimental setup used for making the interferograms. A ruby laser operating in a single axial and transverse mode served as the light source. Beamsplitter BS split the laser beam into a reference beam and an illumination beam that provided scene backlighting through diffusing glass D. Path length matching was not required since the laser operated in a single axial mode and had a coherence length greater than a meter. The test scene occupied an approximate 4X 4-in. area, whereas hologram plate dimensions were 4 X 5 in. The holographic interferogram of the supersonic jet (see photo, Fig. 2a) was made by first firing the laser under conditions of no air flow, then opening the flow control valve for the second laser shot. The holographic interferogram of the gas ejector, photographed in Fig. 3a, was obtained by taking the first laser shot with flow and the second with no flow. Because the fringe patterns of the interferogram are produced by air density changes that vary the optical path length in the test section between exposures, the effects of dirt, optically nonuniform windows, and other stationary distortions of optical path length tend to cancel out. Figure 3 shows the effect of dirt on test chamber windows. The dirt did not noticeably affect the hologram (Fig. 3a) but caused air flow boundaries in the schlieren photograph (Fig. 3b) to be obscured. Since the double-exposure technique is sensitive only to optical path length changes, there is no necessity

The use of atmospheric dispersion in optical distance measurement

The development of lasers, new electro-optic light modulation methods, and improved electronic techniques have made possible significant improvements in the range and accuracy of optical distance measurements, thus providing not only improved geodetic tools but also useful techniques for the study of other geophysical, meteorological, and astronomical problems. One of the main limitations, at present, to the accuracy of geodetic measurements is the uncertainty in the average propagation velocity of the radiation due to inhomogeneity of the atmosphere. Accuracies of a few parts in ten million or even better now appear feasible, however, through the use of the dispersion method, in which simultaneous measurements of optical path length at two widely separated wavelengths are used to determine the average refractive index over the path and hence the true geodetic distance. The design of a new instrument based on this method, which utilizes wavelengths of6328 A and3681 A and3 GHz polarization modulation of the light, is summarized. Preliminary measurements over a5.3 km path with this instrument have demonstrated a sensitivity of3×10 −9 in detecting changes in optical path length for either wavelength using1-second averaging, and a standard deviation of3×10 −7 in corrected length. The principal remaining sources of error are summarized, as is progress in other laboratories using the dispersion method or other approaches to the problem of refractivity correction.

HOLOGRAPHIC INTERFEROMETRY WITH ULTRAVIOLET LIGHT

Simultaneous red and ultraviolet holographic interferograms were recorded on a single Eastman 649F plate by radiation existing after the passage of a Q-switched ruby laser beam through a KDP crystal. The ultraviolet record had twice the sensitivity to optical path length changes as the red record.

Erratum: Temperature-Induced Changes in Optical Path Length for a Nd-Doped Glass Rod during Pumping

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Temperature-Induced Changes in Optical Path Length for a Nd-Doped Glass Rod during Pumping

Frame-camera interferogram data for a neodymium-doped glass rod taken during the laser pumping process have been analyzed in terms of changes in optical path length due to: (1) explicit temperature-induced changes in length and index of refraction, and (2) temperature-induced stress birefringence. Criteria for compatibility of the interferogram data, the formalism, and independently determined values for the phenomenological constants involved are set up and met, after which the temperature over the rod cross section obtains. The minimum in the temperature distribution found accounts for the observed split fringes, and our qualified estimate of the temperature rise at the center of the rod is ∼16°C. Estimates of the changes in optical path length due to the various effects are made, and values for the temperature coefficients of expansion and refractive index of 0.97×10−5/°C±6% and −7.2×10−7/°C±60%, respectively, are submitted.

Change of optical path length in laser rods within the pumping period

Change of optical path length in laser rods within the pumping period