Optical contact and van der Waals interactions: the role of the surface topography in determining the bonding strength of thick glass plates

Oblique incidence of ultrafast laser for glass butt welding

Lap welding restricts the connection state of glasses by vertically incident ultrafast laser at the interface to be welded, which cannot meet the increasingly flexible welding needs. In this Letter, glass butt welding is achieved by oblique incidence of an ultrafast laser, expanding the applicability of ultrafast laser glass welding. Furthermore, the propagation path of the laser beam after oblique incidence into a glass is solved based on geometric optics, the dynamic development of the molten pool in a glass is observed through a high-speed camera, and the mechanism of glass butt welding is elucidated. Finally, the influence of laser pulse energy, glass tilt angle, and defocus amount on welding strength is investigated, achieving a maximum shear strength of 11.5 MPa.

A construction method of the quasi-monolithic compact interferometer based on UV-adhesive bonding

Quasi-monolithic interferometers play a crucial role in high-precision measurement experiments, including gravitational wave detection, inertial sensing, vibrometry, and seismology. Achieving high stability and accuracy in such interferometers requires a method for bonding the optical components to a baseplate. While optical contact bonding and silicate bonding are common methods, UV adhesives offer advantages such as controlled curing and low geometrical requirements for optical components and baseplates. This paper presents a detailed construction method for a quasi-monolithic compact interferometer based on UV-adhesive bonding. We built two types of interferometers using this method: a 100 × 100 × 20 mm3 Mach-Zender homodyne interferometer with unequal arm lengths of about 100 mm for laser frequency noise monitoring and a heterodyne interferometer as a displacement sensing head, sizing 20 × 30 × 20 mm3. Our Mach-Zender interferometer achieved a phase noise level of 2μradHz at 1 Hz and an equivalent laser frequency noise monitoring sensitivity of about 1kHz/Hz at 1 Hz. The compact heterodyne interferometer sensing head showed a sensitivity level of 1pm/Hz in translation and 0.2nrad/Hz in two tilts above 0.4 Hz. Our tests demonstrate that quasi-monolithic compact interferometers based on UV-adhesive bonding can achieve high sensitivity levels at the pico-meter and nano-radian scales.

Design and expected performance of a variable-radii sinusoidal spiral x-ray spectrometer for the National Ignition Facility

A novel high-resolution x-ray spectrometer for point-like emission sources has been developed using a crystal shape having both a variable major and a variable minor radius of curvature. This variable-radii sinusoidal spiral spectrometer (VR-Spiral) allows three common spectrometer design goals to be achieved simultaneously: 1. reduction of aberrations and improved spectral (energy) resolution, 2. reduction of source size broadening, and 3. use of large crystals to improve total throughput. The VR-Spiral concept and its application to practical spectrometer design are described in detail. This concept is then used to design a spectrometer for an extreme extended x-ray absorption fine structure experiment at the National Ignition Facility looking at the Pb L₃ absorption edge at 13.0352 keV. The expected performance of this VR-Spiral spectrometer, both in terms of energy resolution and spatial resolution, is evaluated through the use of a newly developed raytracing tool, xicsrt. Finally, the expected performance of the VR-Spiral concept is compared to that of spectrometers based on conventional toroidal and variable-radii toroidal crystal geometries showing a greatly improved energy resolution.

3D Manufacturing of Glass Microstructures Using Femtosecond Laser

The rapid expansion of femtosecond (fs) laser technology brought previously unavailable capabilities to laser material processing. One of the areas which benefited the most due to these advances was the 3D processing of transparent dielectrics, namely glasses and crystals. This review is dedicated to overviewing the significant advances in the field. First, the underlying physical mechanism of material interaction with ultrashort pulses is discussed, highlighting how it can be exploited for volumetric, high-precision 3D processing. Next, three distinct transparent material modification types are introduced, fundamental differences between them are explained, possible applications are highlighted. It is shown that, due to the flexibility of fs pulse fabrication, an array of structures can be produced, starting with nanophotonic elements like integrated waveguides and photonic crystals, ending with a cm-scale microfluidic system with micro-precision integrated elements. Possible limitations to each processing regime as well as how these could be overcome are discussed. Further directions for the field development are highlighted, taking into account how it could synergize with other fs-laser-based manufacturing techniques.

Quantification of Carbonic Contamination of Fused Silica Surfaces at Different Stages of Classical Optics Manufacturing

The chemical composition of ground and polished fused silica glass surfaces plays a decisive role in different applications of optics. In particular, a high level of carbon impurities is often undesirable for further processing and especially for gluing or cementing where adhesion failure may be attributed to carbonic surface-adherent contaminants. In this study, the surface carbon content at different stages of classical optics manufacturing was thus investigated. Two different standard processes—grinding and lapping with two final polishing processes using both polyurethane and pitch pads—were considered. After each process step, the chemical composition and roughness of the surface were analysed using X-ray photoelectron spectroscopy and atomic force microscopy. An obvious correlation between surface roughness and effective surface area, respectively, and the proportion of carbon contamination was observed. The lowest carbon contamination was found in case of lapped and pitch polished surfaces.

Influence of Numerical Aperture on Molten Area Formation in Fusion Micro-Welding of Glass by Picosecond Pulsed Laser

Focusing condition such as numerical aperture (N.A.) has a great influence on the creation of molten area and the stable welding process in fusion micro-welding of glass. In this study, a picosecond pulsed laser of 1064 nm in wavelength and 12.5 ps in pulse duration was tightly focused inside a borosilicate glass using objective lenses of numerical apertures 0.45, 0.65, and 0.85 with spherical aberration correction. Influence of numerical aperture on molten area formation was experimentally investigated through analysis of focusing situation in glass, and movement of absorption point, and then molten area characteristics were discussed. It is concluded that N.A. of 0.65 with superior focusing characteristics can form a large and continuous molten area without cracks, which enables achievement of stable joining of glass material by picosecond pulsed laser.

Determination of the refractive index and thickness of a hydroxide-catalysis bond between fused silica from reflectivity measurements

Hydroxide-catalysis bonding is a high precision jointing technique producing strong, transparent and thin bonds, the use of which in the delicate fused silica mirror suspensions of aLIGO have been instrumental in the first detections of gravitational radiation. More sensitive future gravitational wave detectors will require more accurate (ideally in situ) measurements of properties such as bond thickness. Here a non-destructive technique is presented in which the thickness and refractive index of a bond are determined from measurements of optical reflectivity. The reflectivity of a bond made between two fused silica discs using sodium silicate solution is less than 1⋅10^-3 after 3 months. The thickness decreases to a constant value of around 140 nm at its minimum and the refractive index increases from 1.36 to 1.45. This proves that as well as determination of bond thickness in situ this bonding technique is highly interesting for optical applications.

Optical characterization of gaps in directly bonded Si compound optics using infrared spectroscopy

Silicon direct bonding offers flexibility in the design and development of Si optics by allowing manufacturers to combine subcomponents with a potentially lossless and mechanically stable interface. The bonding process presents challenges in meeting the requirements for optical performance because air gaps at the Si interface cause large Fresnel reflections. Even small (35 nm) gaps reduce transmission through a direct bonded Si compound optic by 4% at λ=1.25  μm at normal incidence. We describe a bond inspection method that makes use of precision slit spectroscopy to detect and measure gaps as small as 14 nm. Our method compares low-finesse Fabry-Perot models to high-precision measurements of transmission as a function of wavelength. We demonstrate the validity of the approach by measuring bond gaps of known depths produced by microlithography.

Avoiding the requirement for pre-existing optical contact during picosecond laser glass-to-glass welding

Previous reports of ultrafast laser welding of glass-to-glass have indicated that a pre-existing optical contact (or very close to) between the parts to be joined is essential. In this paper, the capability of picosecond laser welding to bridge micron-scale gaps is investigated, and successful welding, without cracking, of two glasses with a pre-existing gap of 3 µm is demonstrated. It is shown that the maximum gap that can be welded is not significantly affected by welding speeds, but is strongly dependent on the laser power and focal position relative to the interface between the materials. Five distinct types of material modification were observed over a range of different powers and surface separations, and a mechanism is proposed to explain the observations.

Gap bridging in joining of glass using ultra short laser pulses

Glass welding by ultra-short laser pulses provides hermetic welding seams with high mechanical stability. The required distance between the samples must be extremely small (<100nm), otherwise cracks will form inside the seam reducing its stability. However, to achieve such small gaps the roughness of the samples must be small enough necessitating additional polishing. Additionally, Van-der-Waals forces grow substantial at such distances thereby effectively preventing sample movement and an easy and precise sample alignment. Here we present a method utilizing ultra-short laser pulses which exploits a volume expansion of irradiated glass enabling the joining of glass plates across gaps of up to 1µm.

High speed, high strength microwelding of Si/glass using ps-laser pulses

A novel microwelding procedure to join Si-to-glass using ps-laser pulses with high repetition rates is presented. The procedure provides weld joint with mechanical strength as high as 85 MPa and 45 MPa in sample pairs of Si/aluminosilicate (Si/SW-Y) and Si/borosilicate (Si/Borofloat 33), respectively, which are higher than anodic bonding, at high spatial resolution (< 20 µm) and very high throughput without pre- and post-heating. Laser-matter interaction analysis indicates that excellent weld joint of Si/glass is obtained by avoiding violent evaporation of Si substrate using ps-laser pulses. Laser welded Si/glass samples can be singulated along the weld lines by standard blade dicer without defects, demonstrating welding by ps-laser pulses is applicable to wafer-level packaging.

Monitoring the thinning dynamics of soap films by phase shift interferometry. The case of perfluoropolyether surfactants

An interferometric method to monitor the thinning process of vertical soap films from a water solution of surfactant materials is reported. Raw data maps of optical path difference introduced by the film are obtained by conventional phase shift interferometry. Off-line re-processing of such raw data taking into account the layered structure of soap films leads to an accurate measurement of the geometrical thickness. As an example of data acquisition and processing, the measuring chain is demonstrated on perfluoropolyether surfactants; the section profile of vertical films is monitored from drawing to black film state, and quantitative data on the dynamics of the thinning process are presented. The interferometric method proves effective to the task, and lends itself to further investigate the physical properties of soap films.

Modified Roberts-Langenbeck test for measuring thickness and refractive index variation of silicon wafers

We describe a method to simultaneously measure thickness variation and refractive index homogeneity of 300 mm diameter silicon wafers using a wavelength-shifting Fizeau interferometer operating at 1550 nm. Only three measurements are required, corresponding to three different cavity configurations. A customized phase shifting algorithm is used to suppress several high order harmonics and minimize intensity sampling errors. The new method was tested with both silicon and fused silica wafers and measurement results proved to be highly repeatable. The reliability of the method was further verified by comparing the measured thickness variation of a 150 mm diameter wafer to a measurement of the wafer flatness after bonding the wafer to an optical flat.

Ultrashort pulsed fiber laser welding and sealing of transparent materials

In this paper, methods of welding and sealing optically transparent materials using an ultrashort pulsed (USP) fiber laser are demonstrated which overcome the limit of small area welding of optical materials. First, the interaction of USP fiber laser radiation inside glass was studied and single line welding results with different laser parameters were investigated. Then multiline scanning was used to obtain successful area bonding. Finally, complete four-edge sealing of fused silica substrates with a USP laser was demonstrated and the hermetic seal was confirmed by water immersion test. This laser microwelding technique can be extended to various applications in the semiconductor industry and precision optic manufacturing.

Characteristics of laser absorption and welding in FOTURAN glass by ultrashort laser pulses

The nonlinear absorptivity of FOTURAN glass to ultrashort laser pulses is evaluated by experimental measurement and thermal conduction model at different parameters including energy and repetition rate of the laser pulse, translation speed and thermal properties of the sample. The mechanical strength of an embedded laser-melted sample and an overlapped weld sample is determined by a three-point-bending test and a shear test, respectively. The results are related to the average absorbed laser power Wab. We found the mechanical strength of an overlapped weld joint to be as high as that of the base material for low Wab, if the sample pair is pre-bonded to provide optical contact.

Sample preparation method for glass welding by ultrashort laser pulses yields higher seam strength

Glass welding by ultrashort laser pulses allows joining without the need of an absorber or a preheating and postheating process. However, cracks generated during the welding process substantially impair the joining strength of the welding seams. In this paper a sample preparation method is described that prevents the formation of cracks. The measured joining strength of samples prepared by this method is substantially higher than previously reported values.

Calcium-assisted glass-to-glass bonding for fabrication of glass microfluidic devices

Glass is a desired material for many microfluidics applications. It is chemically resistant and has desirable characteristics for capillary electrophoresis. The process to make a glass chip, however, is lengthy and inconvenient, with the most difficult step often being the bonding of two planar glass substrates. Here we describe a new glass bonding technique, which requires only washing of the glass surfaces with a calcium solution and 1-2 h of bonding at 115 degrees C. We found calcium uniquely allows for this simple and efficient low-temperature bonding to occur, and none of the other cations we tried (e.g., Na (+), Mg (2+), Mn (3+)) resulted in satisfactory bonding. We determined this bond is able to withstand high applied field strengths of at least up to 4 kV x cm (-1). When intense pressure was applied to a fluid inlet, a circular portion of the coverslip beneath the well exploded outward but very little of the glass-glass interface debonded. In combination with the directed hydrofluoric acid etching of a glass substrate using a poly(dimethylsiloxane) (PDMS) etch guide, we were able to make glass chips with better than 90% yield within 6 h. This technique is compatible with inexpensive unpolished glass and is limited in resolution by the PDMS etch guide used and the intrinsic properties of isotropic etching.

Optical bonding using silica nanoparticle sol-gel chemistry

A simple method is described to bond optical components using silica nanoparticle sol-gel chemistry. The silica nanoparticles polymerize into highly branched networks that link the surfaces together. The nanoparticle mediated bonding has several advantages to currently used optical joining technologies. The bonding is a room-temperature process and does not require any clean room facilities. The bonded interface has a high mechanical strength and low scattering. The bonding is resistant to organic solvents on silylation with hydrophobic surface groups. This method achieves 100% successful bonding rates between soda-lime glass slides. The bond-setting time can be tailored to allow time for precision optical alignment.

Rapid bonding of Pyrex glass microchips

A newly developed vacuum hot press system has been specially designed for the thermal bonding of glass substrates in the fabrication process of Pyrex glass microchemical chips. This system includes a vacuum chamber equipped with a high-pressure piston cylinder and carbon plate heaters. A temperature of up to 900 degrees C and a force of as much as 9800 N could be applied to the substrates in a vacuum atmosphere. The Pyrex substrates bonded with this system under different temperatures, pressures, and heating times were evaluated by tensile strength tests, by measurements of thickness, and by observations of the cross-sectional shapes of the microchannels. The optimal bonding conditions of the Pyrex glass substrates were 570 degrees C for 10 min under 4.7 N/mm(2) of applied pressure. Whereas more than 16 h is required for thermal bonding with a conventional furnace, the new system could complete the whole bonding processes within just 79 min, including heating and cooling periods. Such improvements should considerably enhance the production rate of Pyrex glass microchemical chips. Whereas flat and dust-free surfaces are required for conventional thermal bonding, especially without long and repeated heating periods, our hot press system could press a fine dust into glass substrates so that even the areas around the dust were bonded. Using this capability, we were able to successfully integrate Pt/Ti thin film electrodes into a Pyrex glass microchip.

Polarization-insensitive high-dispersion total internal reflection diffraction gratings

We report the first experimental realization of total internal reflection (TIR) diffraction gratings. Performance of less than 0.7-dB insertion loss (IL) for both TE and TM polarizations and 0.5-dB polarization-dependent loss (PDL) are predicted over a 50-nm spectral bandwidth with simultaneous fabrication tolerances on the depth and the duty cycle of binary gratings of +/-5% and +/-14%, respectively. Nineteen gratings were fabricated that met these specifications, yielding IL and PDL values less than 0.6 and 0.2 dB, respectively, across the entire 50-nm bandwidth. Measurements made under the Littrow configuration resulted in high efficiency and low PDL across a 100-nm bandwidth, with up to 100% diffraction efficiency within the experimental measurement error.