High refractive index contrast in fused silica waveguides by tightly focused,high-repetition rate femtosecond laser

A new domain of optical waveguide writing with record high refractive index contrast (0.022) is reported in fused silica by strong focusing of a 522 nm wavelength, 500 kHz repetition rate femtosecond laser with oil-immersion optics. The strongly confining waveguide supports a mode of only 7 μm mode field diameter at 1550 nm wavelength, opening the door for higher density integration in photonic circuits formed by femtosecond lasers. It is found that green and fundamental wavelengths have similar absorption in femtosecond laser waveguide writing in fused silica and that the advantage of the second harmonic is simply from an increased fluence through a smaller focal volume.     

Relationship between composition,density and refractive index for germania silica glasses

The density and the refractive index for various compositions of binary SiO₂GeO₂ glass prepared by a flame deposition technique, similar to that used to make low-loss optical waveguides, have been measured by standard Archimedes method and the Becke line method, respectively. The density as a function of composition is calculated considering the effective volume of the ions contained in the glass to be invariant. The refractive index as a function of composition is also calculated, based on the Lorenz-Lorenz equation, by computing the electronic polarizability of fused silica and germania. All calculated results are in good agreement with the observed data.     

Effect of hydroxyl impurity on temperature coefficient of refractive index of synthetic silica glasses

We have studied the temperature coefficient of the refractive index of synthetic silica glasses with various hydroxyl impurities. The refractive index was measured at 15 °C and 35 °C at 1.707–0.238 μm wavelengths. The temperature coefficient of a low-OH group (110 wt. ppm) containing glass increased from 8.0 ± 0.2 × 10^?6/°C (at 1.707 μm) to 14.0 ± 0.2 × 10^?6/°C (at 0.238 μm), although it increased respectively from 7.0 ± 0.2/°C to 12.0 ± 0.2 × 10^?6/°C for a high-OH group (1300 wt. ppm) containing glass. The three-term Sellmeier equation, having two terms with resonance photon energies in the vacuum ultraviolet region and one term in the infrared region, was used to analyze the wavelength dispersion of the refractive index. Increasing temperatures shifted the resonance energy in the second term by ?4.14 ± 0.4 × 10^?4 eV/°C for low-OH (110 wt. ppm) glass and ?2.64 ± 0.4 × 10^?4 eV/°C for high-OH (1300 wt. ppm) glass. The fundamental absorption edge in the vacuum ultraviolet region shifted by ?8.8 ± 0.7 × 10^?4 eV/°C for the low-OH glass and ?6.3 ± 0.7 × 10^?4 eV/°C for the high-OH glass in a region of 25–100 °C. Both high-OH glass shift rates were lower than low-OH glass shift rates. The lower temperature coefficient for the Si–OH-related band probably explains the smaller temperature coefficient for high-OH glass: the absorption band of Si–O–H structure is located at lower energy side close to the fundamental absorption band associated with the Si–O–Si structure.     

Use of EELS to study the absorption edge of fused silica

Electron energy loss spectroscopy (EELS) was used to study the absorption edge of various types of fused silica. The relationships between transmission (%T), fictive temperature (T_f), band gap energy (E₀), and slope of the Urbach tail (K) were investigated. The trends are in agreement with other published studies and provide initial validation of the new technique. The measured band gap energies, E₀, varied very little among the different silicas, and there was no obvious relationship between E₀ and either %T or T_f. The slopes of the Urbach tail, K, varied significantly and were related to both %T and structural disorder (influenced by both T_f and F concentration). The observed trends were an increase in transmission with an increase in K, and an increase in K with a decrease in structural disorder. The results of this work support the idea that structural disorder affects transmission near the absorption edge.     

Carbon coating of fused silica ampoules

A reliable method for depositing a layer of carbon on the inner walls of a fused silica ampoule is described and characterized. Carbon deposition rates were found to be 0.33 μm/h at mid-length and range from 0.26 to 0.55 μm/h at the ends of a 150 mm long ampoule. Deposition rate was found to vary along the length of the ampoule, but not along the radial perimeter.     

Densification modeling of fused silica under nanoindentation

Fused silica is the reference material used for estimating the area function of nanoindenter tips. Despite being a fundamental step in nanoindentation, little has been done to study its deformation. Under a complex state of stress during indentation, fused silica densifies pointing out that the hydrostatic stress contributes to its yielding. A linear Drucker–Prager model is successfully employed to describe fused silica deformation. Real tip geometry obtained from Atomic Force Microscopy (AFM) is utilized to numerically simulate the area calibration process. Our results indicate a significant discrepancy between the tip area input into our simulation and the one obtained by the calibration process. This implies that the estimated area is not an intensive property of the indenter tip but a convolution of the indenter geometry by the fused silica deformation characteristics and as such may produce erroneous values when used on other materials.     

Study on the gel casting of fused silica glass

Slip casting process is usually applied for the forming of fused silica products. Segregation always occurs and it will results in density deviation. By using gel casting process, green is fabricated by means of in situ polymerization with a three-dimensional network, holding the particles together and eliminating the tendency of migration. To prepare gel casting slurries, premix solutions were composed of acrylamide and N,N′-methylenebisacrylamide. Solid loading was kept 60% and the average particle size of silica powder 8–8.5 μm. Lactic acid was introduced as a dispersant to regulate the pH value 3–4. Mechanism of the dispersant was investigated by studying ζ-potential at different pH. Ammonium persulfate (NH₄)₂S₂O₄ was added as an initiator. Gelation took place with the help of initiator at 50–60 °C. Nanometer silica was introduced to boost sinterability so that the density and bending strength of fused silica ceramics have been increased to 2.1 g/cm³ and 40 Mpa, respectively.     

Hydrogen diffusion in fused silica at high temperatures

Hydrogen diffusion in a silica glass at high temperature (750–1500 °C) was investigated. The starting material was a silica glass that was melted in hydrogen atmosphere and contained a high concentration of hydroxyl and hydride. After heat treatment at various temperatures in air, the effective hydrogen diffusion coefficients were evaluated from the removal rates of hydroxyl and hydride. The obtained diffusion coefficients exhibited an unusual temperature dependence, with the lower diffusion coefficient being observed at higher temperature in the temperature range of 1000–1500 °C. This was attributed to the higher reactivity of hydrogen with the glass network at higher temperatures, effectively slowing down the hydrogen mobility. Below 1000 °C, the effective hydrogen diffusion coefficient exhibited a normal temperature dependence. This reversal of the temperature dependence of the effective hydrogen diffusion coefficient appears to be related to the reversal of the temperature dependence of the hydroxyl (or hydride) solubility in silica glass.     

Laser-induced defects in fused silica by UV laser irradiation

The structure of fused silica with irradiation of the third harmonic of the Nd:YAG laser was investigated by Fourier transform infrared, Raman and Photoluminescence spectroscopy. Some variation in the Si/O stoichiometry of silica in the ablated spot was induced. The primary defect species are oxygen deficient centers and oxygen interstitials. The frequency shift of the Si–O–Si vibration proves that the central force constant between oxygen and silicon atoms, and the band angle of Si–O–Si increases in the UV-laser ablation spot. Small silicon clusters within SiO_x appear to be a possible explanation for the 564 nm Fluorescence peak, and the 181 cm⁻¹ Raman peak.     

The thermodynamics of water and hydrogen solubility in fused silica

A statistical thermodynamic model of the solubility of water vapor in glass is developed as an extension of previous models of monatomic and diatomic gas solubility in glass. The model predicts the $\text{p}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ dependence of water vapor solubility in fused silica and indicates the binding energies of dissociated H and OH species to be about ?63 and ?99 kcal/mol, respectively. The OH binding energy is found to be slightly temperature dependent, namely E_OH(0) = ?81.3?0.0163T. The tendency of chemical solubility of H₂ to predominate over physical solubility above 500°C correlates with the model equations for H₂ solubility.     

Investigation of the devitrification and microwave penetrating properties of fused silica

Four kinds of fused silica (SiO₂) bulk materials were prepared by the hot-pressing method. Bulk SiO₂ made from sol-gel powders was optically transparent and exhibited the best microwave penetrating properties. Commercial SiO₂ obtained from melted quartz increased crystallization. The addition of polyvinyl alcohol (PVA) (aqueous solution) blocked the crystallization of commercial SiO₂ to a limited extent. In contrast, PVA powders accelerated the heterogeneous nucleation and growth of crystalline silica. In this case, the bulk material of SiO₂-PVA was visibly darkened and exhibited the worst microwave penetrating properties. X-ray diffraction patterns and electron probe X-ray microanalysis confirmed that crystallization was correlated with the optical transmission loss. Crystallized areas randomly dispersed in the continuous amorphous body of fused SiO₂, served as scattering centers which affected the optical transparency and also served as microwave absorbing or scattering centers which limited the microwave penetration. The connection between devitrification and microwave penetrating properties of fused SiO₂ could therefore be used as a reference to produce other microwave penetrating materials with better performance.     

On the dispersion of the refractive index in AIIBVI thin films

We propose a new calculation following traditional methods for deducing refractive index from the fringe pattern of the transmission spectrum of a thin transparent CdS, ZnS, and Zn_xCd_1−xS film surrounded by nonabsorbing media. The error in the refractive index measurements is 4–5%. We have found that polycrystalline CdS, ZnS, and Zn_xCd_1−xS evaporated films have refractive index different from those of single crystal material.     

Sub-surface mechanical damage distributions during grinding of fused silica

The distribution and characteristics of surface cracking (i.e., sub-surface damage or SSD) formed during standard grinding processes have been measured on fused silica glass using a surface taper polishing technique. The measured SSD depth distributions are described by a single exponential followed by an asymptotic cutoff in depth. The observed surface cracks are characterized as near-surface lateral and deeper trailing indent type fractures (i.e., chatter marks). The length of the trailing indent is strongly correlated with a given grinding process. It is shown that only a small fraction of the abrasive particles are being mechanically loaded and causing fracture, and most likely it is the larger particles in the abrasive particle size distribution that bear the higher loads. The SSD depth increased with load and with a small amount of larger contaminant particles. Using a simple brittle fracture model for grinding, the SSD depth distribution has been related to the SSD length distribution to gain insight into ‘effective’ size distribution of particles participating in the fracture. Both the average crack length and the surface roughness were found to scale linearly with the maximum SSD depth. These relationships can serve as useful rules-of-thumb for non-destructively estimating SSD depth and for identifying the process that caused the SSD.     

The fatigue of high-strength fused silica optical fibers in low humidity

The strength and dynamic fatigue of UV-acrylate coated silica optical fibers were measured as a function of relative humidity in the range ∼0.025-13% at 25 °C. The degradation kinetics of silica in low humidities was investigated and it was found that the reaction order was approximately first-order with respect to humidity. In our previous work, a second-order reaction was found in the humidity range 20-95% RH at 25 °C and the process for obtaining this reaction order was found to be independent of the mathematical form of the kinetic models used. The change in reaction order observed here verifies some earlier results based on the power law which implied a change in the reaction order from ∼2 (15-100% RH) to ∼1 at low humidity (<0.01%).     

Dynamic condensation of water at crack tips in fused silica glass

Water molecules play a fundamental role in the physics of slow crack propagation in glasses. It is commonly understood that, during stress-corrosion, water molecules that move in the crack cavity effectively reduce the bond strength at the strained crack tip and, thus, support crack propagation. Yet the details of the environmental condition at the crack tip in moist air are not well determined. In a previous work, we reported direct evidence of the presence of a 100 nm long liquid condensate at the crack tip in fused silica glass during very slow crack propagation (10^?9–10^?10 m/s). These observations are based on in situ AFM phase imaging techniques applied on DCDC glass specimens in controlled atmosphere. Here, we discuss the physical origin of the AFM phase contrast between the liquid condensate and the glass surface in relation to tip-sample adhesion induced by capillary bridges. We then report new experimental data on the water condensation length increase with relative humidity in the atmosphere. The measured condensation lengths were much larger than what predicted using the Kelvin equation and expected geometry of the crack tip.     

Compaction versus expansion behavior related to the OH-content of synthetic fused silica under prolonged UV-laser irradiation

The behavior of different types of synthetic fused silica under prolonged 193 nm excimer laser irradiation at very low energy densities was investigated. Irradiation experiments at energy densities below 50 μJ/cm² for tens of billions of laser pulses were performed. Effects of changes in the density in the irradiated region of the samples were measured at 633 nm, both with an interferometer as an optical path difference and as the distribution of stress induced birefringence. Samples of synthetic fused silica of type III and IIIa with different levels of hydroxyl-content and different levels of molecular hydrogen content were manufactured. Depending on the composition of the fused silica samples, compaction as well as rarefaction was observed. The OH-content in the sample is found to be the dominating factor in the expansion-versus-compaction behavior. A qualitative model is postulated in order to describe the dependence on the OH-content. The induced optical path difference as a function of energy density and number of laser pulses is described with a phenomenological model function.     

Absorption bands due to silver particles in the glasses containing halogen ions

The absorption bands due to silver particles formed in the glasses containing a small amount of halogen ions were measured and the peak positions were compared with those calculated by the Mie theory. The absorption peaks occurred at about 405–410 nm in the glasses without halogen ions. However, they shifted to far longer wavelengths in the halogen-containing glasses, occurring at about 470–510 nm in the glasses with Cl ions (0.26%) and at about 520–530 nm with Br ions (0.19%), respectively. The comparison of the experimental results with the calculated values on the peak position suggested that the absorption bands observed in the halogen-containing glasses heat treated for a prolonged time were due to nearly spherical silver particles formed in silver halide crystals.     

The dielectric properties of as-received and gamma irradiated fused silica

Audio frequency capacitance and conductance measurements have been carried out on 15 kinds of fused silica over the temperature range 5.5–380 K. The results are used to arrive at corresponding values for the complex dielectric constant. It is shown that the relative strengths of the three relaxations observed in as-received fused silica are characteristic of the type of fused silica. In addition, it is found that one of the relaxations can be destroyed by gamma rays and that the destruction is accompanied by the creation of a lower activation energy relaxation. A corresponding growth of optical absorption bands in the visible portion of the spectrum is observed. The gamma-ray sensitive relaxation is attributed to an aluminum-alkali center and the gamma-ray induced relaxation is attributed either to an aluminum-oxygen-hole center or to a “non-local” alkali ion. Finally, accurate values for the real part of the dielectric constant at various temperatures are presented.     

Rapid lifetime testing of fused silica for DUV laser applications

Lifetime testing of fused silica's absorption degradation upon 193 nm is shortened by enhancing the two-photon absorption (TPA) induced generation of E′ and NBOH defect centers per laser pulse. Increasing the irradiation fluence from typical marathon test values H < 1 mJ/cm² to H = 10 mJ/cm² gives a more efficient TPA process. In addition, the sample's temperature is lowered to ?180 °C during irradiation yielding an increased breaking efficiency of weak Si–O bonds per pulse. A small sample length of 10 mm combined with the laser induced deflection (LID) technique for direct absorption measurements prevents microchannel (MC) formation, a common break-down criterion in marathon tests.For a UV grade fused silica sample (type III) the end of absorption degradation is found after about 1.2 * 10⁷ laser pulses. Absorption measurements between 3 and 25 mJ/cm² before and after lifetime testing reveal that the laser induced absorption change decreases with decreasing fluence. The experimental results are in good agreement with a real marathon test at a fixed fluence.     

Effect of temperature on hardness and indentation cracking of fused silica

The fracture behavior induced by Vickers indentations in fused silica was investigated as a function of temperature. Indentations were performed from room temperature to 400 °C in air. The indentations and the crack pattern formed were analyzed by optical and scanning electron microscopy. The hardness at room temperature was 7.3 ± 0.3 GPa and decreased to 4.2 ± 0.1 GPa at 400 °C. Cone and radial cracks were observed at all temperatures. The radial crack length increased with temperature for a constant load. Lateral and median cracks were present under the indenter, and their expansion was constrained by cone cracks nucleated during the loading-unloading cycle. The threshold loads for cone and radial crack nucleation increased with temperature. The results are discussed in terms of the elastic modulus/hardness ratio variation with temperature.