Beam displacement and distortion effects in narrowband optical thin-film filters.

A three-dimensional model for beam propagation through optical interference filters is presented. The model predicts a wavelength-dependent lateral beam displacement of tens or hundreds of micrometers in narrowband filters at an angle of incidence of only 3 degrees to 5 degrees . The effects of filter bandwidth, wavelength offset, angle of incidence, and beam size are investigated. The effect is experimentally confirmed for a 100 GHz filter at a 3.5 degrees angle of incidence.

Path of a beam of light through an optical coating.

A simple model for the path of a light beam through an optical coating is the path of a ray predicted by Snell's law. By determining the exit point of a beam for various types of coating, one finds that the simple model is a good approximation in the case of antireflection coatings, but not for coatings of other designs. An approximate method for determining the correct path of a beam through the coating is derived and the path is illustrated using a Gaussian incident beam and tracing the position of the peak field of the beam as it traverses the coating.

Single-layer model for surface roughness.

Random roughness of an optical surface reduces its specular reflectance and transmittance by the scattering of light. The reduction in reflectance can be modeled by a homogeneous layer on the surface if the refractive index of the layer is intermediate to the indices of the media on either side of the surface. Such a layer predicts an increase in the transmittance of the surface and therefore does not provide a valid model for the effects of scatter on the transmittance. Adding a small amount of absorption to the layer provides a model that predicts a reduction in both reflectance and transmittance. The absorbing layer model agrees with the predictions of a scalar scattering theory for a layer with a thickness that is twice the rms roughness of the surface. The extinction coefficient k for the layer is proportional to the thickness of the layer.

Correlation between substrate preparation technique and scatter observed from optical coatings.

We present experimental evidence of the dependence of coating scatter on a substrate preparation technique for fused silica substrates. Samples included conventionally polished, superpolished, andfloat-polished substrates. We used scatterometry and total internal reflection microscopy to investigate the effects of substrate preparation on the performance of zirconium oxide thin films. Results indicate that scatter from coatings dominates the scatter signature of the coated optic. They also demonstrate that substrate preparation can affect the level of scatter produced in optical coatings. In addition it is observed that the substrates with the lowest scatter do not necessarily result in the coatings with the lowest scatter.

Feature issue on optical interference coatings.

The feature issue on Optical Interference Coatings, stimulated by the June 1992 Topical Meeting, covers the wider field of optical surface treatments after polishing. It is the latest in a series that has been running every four years since 1976.

Direct observation of waveguided scattered light in multilayer dielectric thin films.

When a focused laser beam falls on a multilayer thin-film coating, light is scattered from the volume over which the beam intersects the coating. Some of the light may be scattered into directions that correspond to guided modes of the thin-film structure. We report the observation of scattered light at locations removed from the region of incidence of the light on the coating, resulting from the secondary scatter of the guided scattered light. The shape of the pattern observed by secondary scatter clearly resembles the pattern expected for Rayleigh scatter from a point source. This observation has important implications for the design of scatter-measuring instruments as well as for the theoretical treatment of scatter from multilayer coatings.

Stress reduction in ion beam sputtered mixed oxide films.

Thin films deposited by ion beam sputtering typically have a high compressive stress. This paper demonstrates that this stress can be reduced by cosputtering two materials. Thin film mixtures of zirconia (ZrO(2)) and silica (SiO(2)) were prepared with a range of compositions using ion beam sputtering. The refractive index was found to vary almost linearly with composition. The large stress observed in zirconia films was found to be reduced significantly by the addition of silica.

Comparison of several shortwave pass filter designs.

The simplest shortwave pass filter design consists of repetitions of the basic period of the form (0.5L H 0.5L), where L and H refer to quarterwaves of low and high index materials at a particular wavelength. Such a filter has a passband limited by the first- and third-order rejection bands. The width of the passband can be extended to shorter wavelengths by further subdividing the basic period using materials of intermediate indices. Alternate shortwave pass filter designs can be obtained by approximating a continuous index profile in which the natural log of the refractive index follows a sinusoidal profile. Films with such a continuous index profile are commonly called rugate filters. Four discrete index approximations to the rugate filter are compared. They are found to differ with respect to the bandwidth of the shortwave passband.

Refractive index determination using an orthogonalized dispersion equation.

An orthogonal form of a general three-term dispersion equation is presented which is useful for determination of the refractive index of transparent optical materials from measured spectral data. The orthogonal basis functions are dependent on the spectral range of the data and whether the spacing of the data is uniform with respect to wavelength or wavenumber. The application of the technique to the reduction of ellipsometric measurements of a single layer thin film of zirconia on a fused silica substrate is presented. The measured data were fit to a Conrady dispersion equation and to the corresponding orthogonal basis set. The convergence to the solution was an order of magnitude faster for the orthogonal form of the dispersion equation.

Comparison of the properties of titanium dioxide films prepared by various techniques.

Fourteen university, government, and industrial laboratories prepared a total of twenty pairs of single-layer titanium dioxide films. Several laboratories analyzed the coatings to determine their optical properties, thickness, surface roughness, absorption, wetting contact angle, and crystalline structure. Wide variations were found in the optical and physical properties of the films, even among films produced by nominally the same deposition techniques.

Scandium oxide coatings for high-power UV laser applications.

Scandium oxide has proved to be a damage-resistant high-index material in laser coatings for use at 248 nm. The results of damage threshold measurements on laser reflectors and antireflection coatings of various designs, material combinations, and deposition temperatures are presented. The most significant effects are observed for overcoat layers on high reflectors and undercoat layers on antireflection coatings. Thresholds >6 J/cm(2) for 20 nsec pulses were observed in both cases.

Influence of deposition parameters on laser-damage threshold of silica-tantala AR coatings.

A series of 4-layer silica-tantala antireflection coatings was deposited under 18 different combinations of substrate temperature (175, 250, and 325 degrees C), oxygen pressure (0.5,1.0, and 2.0 x 10(-4) Torr), and rate of deposition (1.5 and 5 A/sec). Measurements of laser-damage threshold for 1064-nm, 1-nsec pulses, average absorption, net stress, and reflectivity were then made on these coatings. Coatings deposited at the lowest temperature had the highest damage thresholds. Damage thresholds were found not to be directly related to average absorption or net stress. Coatings deposited on fused silica substrates which had been polished by a bowl-feed process had generally higher damage thresholds than coatings deposited on conventionally polished fused silica or on BK-7 glass polished by either conventional or bowl-feed processes. Baking coatings in air for 4 h at 400 degrees C generally reduced average absorption and net stress, changed the net stress from compression to tension and, in some cases, increased the damage threshold.