Silica glass exhibits a permanent anisotropic response, referred to as polarization induced birefringence (PIB), when
exposed to short wavelength, polarized light. The magnitude of the PIB has been empirically correlated with the OH
content of the glass. Our recent studies pertaining to PIB have focused on careful characterization of PIB, with particular
emphasis on understanding all of the contributions to the measured birefringence signal and finally extracting only that
signal associated with birefringence arising from exposure to a polarized light beam. We will demonstrate that a critical
contributor to the total birefringence signal is birefringence that comes from exposure beam inhomogeneities. After
subtracting beam profile effects we are able to show that PIB is proportional to the OH content of the glass. Polarized
infrared (IR) measurements were performed on glasses that developed PIB as a consequence of exposure to polarized
157-nm light. These studies reveal that there is preferential bleaching of a specific hydroxyl (OH) species in the glass
with OH aligned parallel to the incident polarization undergoing more bleaching than those perpendicular. Further, we
observe a very strong correlation between the measured PIB of these samples and the anisotropic bleaching. From these
studies we propose a mechanism that can explain the role of hydroxyl in PIB.
Fused silica, when exposed to excimer laser light, exhibits permanent anisotropic birefringence and wavefront changes. These laser-induced changes depend on the silica composition and processing conditions. The optical anisotropy is most clearly observed in samples that are exposed with linear polarization. This polarization-induced effect has been known for several years, but has become much more important with the advent of immersion lithography and its associated very high numerical apertures. High numerical aperture optics require controlled polarization, notably linear polarization, in order to maintain phase contrast at the image. When birefringence and wavefront changes are induced by laser exposure, the image contrast at the wafer deteriorates. We interpret the changes in optical properties in terms of permanent anisotropic strain induced by laser damage, and the associated strain-induced optical effects. This is accomplished using the mathematics of tensors to account for anisotropic strain and optical anisotropy, and using finite element analysis to calculate the strain fields taking the sample and exposure geometries into account. We report the relations between underlying density and strain anisotropy changes and the induced birefringence and wavefront for a given experimental sample geometry. We also report some examples of the different degree of laser damage from silica with different compositions and processing conditions.
Silica glass exposed to pulsed UV excimer laser irradiation undergoes optical changes that can include either an optical path increase or a decrease. During a given exposure the sign of the induced optical path change can reverse as a function of pulse count. The reduced optical path and sign reversal are only observed in H2-containing glasses, and at high exposure fluence only optical path increase is observed. In past work we proposed an induced density change model invoking a dynamic equilibrium density to explain the high fluence experiments. Here we present a model that extends the density model to the low fluence regime by allowing the equilibrium density to be a function of the time-dependent break-up of the silica network during exposure. The network break-up is tracked by calculation of the induced SiH concentration in the glass. The agreement of optical path change obtained from experimental data with that deduced from the kinetic approach covers a wide range of exposure fluence and molecular hydrogen concentration. Using the model one can predict the change in optical path that arises from the excimer laser exposure.
Birefringence mapping of fused silica samples is used to measure density change in the material after exposure to excimer laser radiation. The proper techniques and methods that should be used to perform the exposure of the samples and the analysis of the birefringence results will be discussed. The quantitative analysis of birefringence measurements includes the correct subtraction of the initial birefringence of the sample and the comparison with a theoretical birefringence map calculated for a 1 ppm unconstrained density change under consideration of material and exposure parameters. Proper experimental conditions include the use of samples with low initial birefringence and a round circularly polarized laser beam with top-hat intensity profile.
We briefly review recent progress in the fabrication and characterization of air-core photonic band-gap fibers. These are silica fibers with an hexagonal array of air holes in the cladding, and a larger air hole creating the core. Improved structural uniformity transverse to the fiber axis and down the fiber axis has yielded fibers with better transmission characteristics. We have measured a minimum loss of 13 dB/km at 1500 nm for a 100 m length of our fiber. This is a marked improvement over previous loss measurements for air-core fibers of any kind. A comparison of observed spectra and calculated gap modes suggests that coupling between surface modes and core modes may be an important contributor to the remaining loss. We present a detailed analysis of the expected losses associated with mode crossings between the fundamental core mode and surface modes, showing that Lorentzian-shaped loss peaks are predicted.
As optical lithographers push to extend optical lithography technologies to create smaller features with higher NA, lower k1 values and shorter wavelengths, transmitted wavefront specifications for HPFSR fused silica blanks continue to tighten. HPFSR fused silica blanks are typically certified for acceptance using an interferometer operating at a wavelength of 632.8 nm. As the market demands increasingly tighter homogeneity specifications, it has become critical to understand the sources of variation in wavefront measurements. Corning has recently initiated a study to identify those sources of variation. One glass attribute being studied is the impact of residual stress on the wavefront. It is known that residual stresses can alter the refractive index of fused silica. To obtain the residual stress measurements, birefringence measurements were obtained at 632.8 nm for comparison to wavefront measurements at 632.8 nm. The relationship between residual birefringence and transmitted wavefront measurements, at 632.8 nm on Corning HPFSR fused silica blanks, is explored in this paper.
Laser irradiation of fused silica produces compaction. Irradiation at photolithographic wavelengths of 248nm or 193nm produces density increase of order parts per millon. While these changes are small, the accompanying increase in index of refraction may be large enough to degrade the performance of photolithographic exposure optics. This paper reports experimental of compaction by interferometry and theoretical analysis of the elastic response of the glass samples to extract the sample-independent unconstrained compaction. The power law behavior of compaction vs. exposure dose is presented, along with a Monte Carlo analysis of error bars on this curve.
The compaction (densification) of fused silica under low fluence (< 1 mJ/cm2/pulse), long term (hundreds of millions of pulses) 193 nm irradiation has been studied. With the use of a finite element analysis, the unconstrained densification, (delta) (rho) /(rho) , is extracted from the experimentally determined wavefront distortion. We find that the densification of silica in the low fluence exposure regime corresponds to what is predicted from the behavior described by: (delta) (rho) /(rho) equals 0.000117 (NI2)0.53 where N equals number of pulses, I equals intensity (mJ/cm2/pulse). Results are presented of high and low intensity studies that establish the utility of the above form as a predictive tool for densification in fused silica.
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