Optical interposers are promising as a robust, reliable, and scalable technology for high-density coupling between the dissimilar platforms of optical fiber and silicon photonics (SiP) chips. To extend this concept, femtosecond laser micro-structuring was harnessed to develop a multi-level, mirror-waveguide optical circuit platform in fused silica glass. The flexible laser writing facilitated compact, low-profile vertical interconnection between multi-core fibers and SiP circuits, exploiting total internal reflection mirrors and vertical grating couplers. Various design strategies of laying out 3D waveguide fanouts, multi-core fiber sockets, and turn-mirrors were explored in 40 channel systems. The flexible interposer technology is scalable to higher channel counts, while maintaining a small footprint, thus offering a broad solution to challenges in areas of optical interconnects and photonic packaging.
Femtosecond laser light has been shaped with a spatial light modulator (SLM) to generate optically thin, aberration-free sheets of uniform intensity for inscription of fiber Bragg gratings (FBGs) inside the core of SMF-28 telecommunication fiber. The combination of flexible beam shaping and sheet-by-sheet writing offers facile means in controlling the coupling to cladding or radiation modes while facilitating spectral tuning flexibility that is not available with interference-based techniques. Spectral responses of uniform first order FBGs fabricated with single-pulse exposures are presented.
Femtosecond laser welding was extended to optical silica fiber (SMF-28) by focusing through fused silica substrates and ferrules to form all-glass weld seams. Laser radiation was focused into the fiber cladding to create a welding zone, which drove molten glass to fill as much as a 3 𝜇𝑚 gap around a contact line to form a crack free pseudo-continuous welding seams along the contact line. The strong weld seams up to 30 𝜇𝑚 wide were generated in fiber-to-plate and fiber-to-ferrule geometries without inducing photochemical or thermal degradation of a fiber Bragg grating (FBG) positioned only 62.5 𝜇m from the weld zone. Welding was optimized by real-time monitoring of the FBG thermo-optical shift during laser scanning. Four-point bending tests confirmed a high mechanical strength while thermal annealing showed stable mechanical and FBG responses up to 1000 ˚C. Femtosecond laser writing and welding thus demonstrated a flexible means for photonics fabrication and packaging of FBGs, enabling reliable, high frequency vibration sensing suited for high temperature and strain environments.
The demand for greater link capacity in datacenters has pushed silicon photonic (SiP) optical interconnects from linear edge coupler arrays to two-dimensional grids of vertical grating couplers. To meet the challenge for low-profile, fiber-to-chip coupling, 3D optical waveguide circuits were integrated with total internal reflection (TIR) mirrors, enabling efficient horizontal fiber coupling and vertical SiP coupling through a fused silica interposer. TIR air disks of 30 µm diameter and ~3 µm thickness were fabricated up to 320 µm circuit depth by femtosecond laser irradiation followed by chemical etching (FLICE). The micro-mirror offered 0.54 to 1.2 dB reflection loss for waveguide-to-waveguide coupling within the interposer, as measured across the 1460 to 1625 nm telecom bands. The efficient TIR mirror lays the groundwork for flexible design of 3D photonic interposers to meet high-density interconnection requirements of SiP circuits to multicore fiber arrays for the telecom industry.
Beam shaping of femtosecond lasers was applied in the Kerr-lensing and aberration regime to enable high-aspect-ratio filament tracks to form uniformly through the silica cladding and core waveguide of single-mode fiber (SMF28/450). One- and two-dimensional filament arrays were embedded along the waveguide to form weak to strong photonic stopbands. The filament shape enhanced transverse light scattering into narrow azimuthal radiation zones. Tailoring of chirp and 2D patterns further facilitated high-resolution (~350 pm) spectral focusing onto a CCD camera, defining a compact “Spectrometer-in-fibre” over the visible spectrum. At higher exposure, the filaments opened into narrow nano-channels (200-400 nm diameter) presenting a novel Bragg grating for refractive index sensing of the ambient environment. This lab-in-fiber technology presents a robust, flexible, and ubiquitous communication platform for nano-scale sensing across expansive networks or into tightly confined, sinuous spaces.
Optically clear and elastic conductors are critical for the next generation of fully imperceptible stretchable electronics that are not only optically transparent, but also invisible under typical lighting conditions and reading distances. Such conductors have a central role in a wide range of emerging applications such as wearable computing, soft bioelectronics, and biologically-inspired robotics. Here, we introduce a materials architecture and laser-based microfabrication technique to produce electrically conductive circuitry (sheet resistance = 2.95 Ω/sq; conductivity = 5.65×105 S/m) that are soft, elastic (strain limit > 100%), and optically transparent. The circuitry is composed of a grid-like array of visually imperceptible liquid metal (LM) lines on a clear elastomer. The laser fabrication approach allows for fully imperceptible electronics that have not only high optical transmittance (>85% at 550 nm) but also are invisible under typical lighting conditions and reading distances. This unique combination of properties is enabled by using a direct laser writing technique that results in LM grid patterns with a line width and pitch as small as 4.5 μm and 100 μm, respectively – yielding grid-like wiring on a transparent polydimethylsiloxane (PDMS) elastomer substrate that has adequate conductivity for digital functionality but is also well below the threshold for visual perception. The fabricated LM wiring can be readily interfaced with conventional circuit components (e.g., lead wiring, or LED chips) to enable optically clear digital electronics. The electrical, mechanical, electromechanical, and optomechanical characterization of fabricated LM circuits shows that the high conductivity and transparency are preserved at tensile strains of ~100%.
Kerr-lens self-channelling of femtosecond laser light offers a novel high-aspect geometry for laser processing inside transparent materials. In glass materials, the laser filaments enable white-light continuum generation, scribing, nanochannel formation, and refractive index modification. In the present work, refractive index matching oils were applied around optical fiber to eliminate astigmatic aberration and thereby form highly symmetric and uniform filaments selectively in the cladding or core waveguide of standard single-mode optical fibre (SMF-28). Under tight focusing, long filaments exceeding 20 um length were formed with single pulses to sub-micron diameter. Arrays of 0.5 um spaced filaments are verified by formation of strong fiber Bragg gratings (FBGs). Flexible positioning of the filament arrays within the fiber core offers wide scope for coupling to cladding and radiation modes and creating new types of in-fibre optical devices.
Spatial Light Modulators (SLM) are emerging as a power tool for laser beam shaping whereby digitally addressed phase shifts can impose computer-generated hologram patterns on incoming laser light. SLM provide several additional advantages with ultrashort-pulsed lasers in controlling the shape of both surface and internal interactions with materials. Inside transparent materials, nonlinear optical effects can confine strong absorption only to the focal volume, extend dissipation over long filament tracks, or reach below diffraction-limited spot sizes. Hence, SLM beam shaping has been widely adopted for laser material processing applications that include parallel structuring, filamentation, fiber Bragg grating formation and optical aberration correction.
This paper reports on a range of SLM applications we have studied in femtosecond processing of transparent glasses and thin films. Laser phase-fronts were tailored by the SLM to compensate for spherical surface aberration, and to further address the nonlinear interactions that interplay between Kerr-lens self-focusing and plasma defocusing effects over shallow and deep focusing inside the glass. Limits of strong and weak focusing were examined around the respective formation of low-loss optical waveguides and long uniform filament tracks. Further, we have employed the SLM for beam patterning inside thin film, exploring the limits of phase noise, resolution and fringe contrast during interferometric intra-film structuring.
Femtosecond laser pulses of 200 fs pulse duration and 515 nm wavelength were shaped by a phase-only LCOS-SLM (Hamamatsu X10468-04). By imposing radial phase profiles, axicon, grating and beam splitting gratings, volume shape control of filament diameter, length, and uniformity as well as simultaneous formation of multiple filaments has been demonstrated. Similarly, competing effects of spherical surface aberration, self-focusing, and plasma de-focusing were studied and delineated to enable formation of low-loss optical waveguides over shallow and deep focusing conditions.
Lastly, SLM beam shaping has been successfully extended to interferometric processing inside thin transparent film, enabling the arbitrary formation of uniform or non-uniform, symmetric or asymmetric patterns of flexible shape on nano-scale dimensions without phase-noise degradation by the SLM patterning. We present quantized structuring of thin films by a single laser pulse, demonstrating λ/2nfilm layer ejection control, blister formation, nano-cavities, and film colouring. Closed intra-film nanochannels with high aspect ratio (20:1) have been formed inside 3.5 um thick silica, opening new prospects for sub-cellular studies and lab-in-film concepts that integrate on CMOS silicon technologies.
F2-laser writing of silica (SiO2) optical waveguides has been successfully demonstrated on the surface of silicone [(SiO(CH3)2)n] rubber by the photochemical modification of silicone into silica. The 2-mm-thick silicone rubber was exposed to the 157-nm F2-laser beam through a thin (~0.2 mm) air layer. A proximity Cr-on-CaF2 photomask with 8- to 16-micron-wide slits controlled the exposure size to define the width of the silica waveguide. Optimum laser conditions to generate crack-free waveguides with good transparency were found by varying the laser fluence, pulse repetition rate and total exposure. The optimized waveguides were found to guide both red (~635-nm) and infrared (~1550-nm) wavelengths with light end-fired from standard single-mode fiber.
Beam shaping at the output of optical fibers is required in a variety of applications including optical sensors, telecommunication devices and medical applications. We present a laser micro-machining technique for the fabrication of micro-lenses directly upon the end face of silica fibers using a F2-laser processing station. Ablation is performed in a mask projection arrangement with 25x demagnification. A mask with an occluded circular beam shape is imaged perpendicular to the fiber axis. The fiber is rotated axially while the laser cuts through the fiber, yielding a spherically shaped tip with radius defined by the mask dimensions. Strong 157 nm absorption by the silica glass facilitates precise structuring without micro-crack formation. The quality of the fiber-lenses is characterized by AFM, SEM and by analysing the beam profile at the fiber output.
Lasers microprocessing is attractive for the custom fabrication of novel lab-on-a-chip designs. However, processing of glass biochips is challenging for most lasers because of the weak light interactions inherent in such transparent substrates. The F2-laser generates a high 7.9-eV photon energy that drives strong absorption in glasses, while the short 157-nm wavelength offers high-resolution patterning on the 100-nm scale. With these benefits, F2-laser ablation is well suited to the fabrication of high aspect ratio microfluidic channels and other biochip functions. F2-laser radiation also produces a strong photosensitivity response in fused silica and other glasses that enable the fabrication of buried optical waveguides, Bragg grating filters and other refractive index structures inside the glass. In this paper, we combine laser micromachining and refractive index profiling to enable single-step integration of photonic functions with microfluidic functions on a single chip. Optical waveguides were written to intercept microfluidic channels for optical sensing of cells and other bio-materials. An integrated biophotonic sensor is demonstrated for polystyrene spheres. The sensor is optically characterized for insertion loss, propagation loss, and particle sensitivity. The demonstration and analysis of this simple device offers insight into the capabilities and potential applications for laser fabricated glass lab-on-a-chip devices. Moreover, the groundwork is laid for rapid laser prototyping of custom-designed microfluidic biochips interlaced with integrated-optical circuits to define a new generation of highly functional bio-sensor and lab-on-a-chip devices.
The 157nm F2-laser drives strong and precisely controllable interactions with fused silica, the most widely used material for bulk optics, optical fibers, and planar optical circuits. Precise excisions of 10 to 40 nm depth are available that meet the requirements for generating efficient visible and ultraviolet diffractive optical elements (DOE). F2-laser radiation was applied in combination with beam homogenization optics and high-precision computer controlled motion stages to shape 16-level DOE devices on bulk glasses and optical fiber facets. A 128×128 pixel DOE was fabricated and characterized. Each level had distinguishable spacing of ~140 nm and surface roughness of ~38 nm. The far-field pattern when illuminated with a HeNe laser agreed well with the simulation results by an Iterative Fourier Transform Algorithm (ITFA). Improvements to increase the 1st order diffraction efficiency of 22% are offered.
F2-laser ablation at 157 nm was used for generating sub-micron surface relief structures on fused silica to define binary diffractive phase elements (DPE). A pattern array of 128 x 128 pixels was excised using the F2 laser in combination with a high resolution processing system comprising of CaF2 beam-homogenization optics and a high-resolution Schwarzschild reflective objective. A square projection mask provided precise excisions in less than 10 x 10 μm2 spots, having sub-μm depths that were controlled by the laser fluence and the number of laser pulses to provide for the required phase delay between ablated and non-ablated pixels. Thus a diffractive phase element (DPE) optimized for first order in the UV spectral range was made. A four-level DPE design computed by the Iterative Fourier Transform Algorithm (IFTA) will be described for generating an arbitrary irradiation pattern without the point symmetry of a two level design.
Chrome-on-quartz optics serve multi-functions including high-resolution masks for UV lithography. At the next node down, F2-laser lithography demands alternate high-transparency substrates such as wide-bandgap CaF2. We present here a direct-write method of F2-laser ablation for selective removal of 110-nm thick chrome films on CaF2 and SiO2 (fused silica) substrates. Laser-processing parameters are presented for micropatterning of the chrome film with minimal damage to the underlying substrates. Damage thresholds, ablations rates, incubation processes, and surface morphology of the chrome and CaF2 are described together with methods that reduce ablation debris and collateral damage. Laser-patterned masks are tested in a 157-nm optical projection system at 30-mJ/cm2 fluence for sub-micron laser structuring of glass and other materials.
Submicron surface-relief gratings were fabricated on fused silica by laser ablation with nanosecond (ns) pulses from a high-resolution F2-laser processing station. The grating relief was generated by imaging a transmission amplitude grating with a Schwarzschild objective of 25x demagnification. The chrome-coated CaF2 mask had been structured by laser ablation at 193 nm to form a line and space pattern of 20-μm period. The F2-laser generated gratings on fused silica were characterized by SEM, AFM and diffraction of a HeNe laser beam, yielding a grating period of 830 nm and a corrugation depth of 250 nm. Surface-relief gratings on optical materials are required for various applications such as grating demultiplexers for telecommunication components, light couplers for planar optical waveguides, Bragg reflectors, or alignment grooves for liquid crystals. Laser ablation is a rapid and flexible method to generate custom grating designs on a variety of materials.
Microfluidic devices, or "lab-on-a-chip" systems for single cell analysis represent a new field of micro-total analysis systems (μTAS) that could not only perform a task quicker, and more accurately than conventional methods, but could also incorporate additional tools to the study of biological variability in a population by allowing researchers to directly examine the contents of a multitude of single cells from the population under study. Realizing such a device, presents several engineering challenges to the fields of micro-machining, micromanipulation and analytical bio-chemistry. The device needs to be able to accurately and efficiently select, manipulate and analyze volumes represented by a single cell without diluting the contents. For this purpose, optical tweezers and scissors were implemented to select single cells on a microchip, bring the cell to a desired location, and lyse the cell using the optical scissors. Channels were engineered in the device using a molecular fluorine (F2) laser. Each channel’s cross-section is approximately the size of an individual cell (10μm wide and deep). This paper, describes the manipulation of cells on a microchip using optical tweezers and the injection of the cellular contents by optical scissors from a single cell into 10μm channels.
The energetic 7.9-eV photons of the F2 laser directly access bandgap states in germanosilicate glasses to provide a strong and direct channel for inducing refractive index changes in optical fibers and planar waveguides. In this paper, we review our F2-laser photosensitivity studies with an aim to assess prospects for shaping useful photonics structures directly inside the germanosilicate waveguides. We describe strong photosensitivity responses in standard telecommunication fibers and planar optical waveguides without the need for hydrogen loading, and compare with responses provided by traditional ultraviolet lasers. Because of the strong 157-nm absorption in the germanium-doped guiding layers, large non-uniform changes to refractive index are noted that offer opportunities for trimming phase errors and correcting waveguide birefringence in planar optical circuits. With hydrogen soaking, modest 157-nm pre-irradiation was found to 'lock-in' a permanent photosensitivity enhancement in the germanosilicate guiding core, permitting the formation of strong (40-dB) and stable fiber Bragg gratings with 248-nm KrF laser light. The 157-nm 'lock-in' mechanism is associated with Si-OH and Ge-OH defect formation and permanently enhances the ultraviolet photosensitivity response by several orders of magnitude above that for an untreated fiber without the aging related disadvantages of conventional hydrogen soaking. The unique opportunities for F2-laser photosensitivity applications in shaping and trimming photonic components will be outlined in this presentation.
The F2-laser Nano fabrication Facility at the University of Toronto delivers high-fluence 157-nm radiation at high resolution to micro fabricate high-finesse silica-based optical components. The 7.9-eV photons drive strong material interactions near the band-edge states of fused silica and related glasses that help avoid microcrack formation, a common limitation of longer wavelength laser. The strong interactions provide for small and smooth excisions, offering depth control on a scale of tens of nanometers. A 157-nm beam homogenization system and a 25x Schwarzschild lens provided a uniform on-target fluence of 9 J/cm2 in a 0.25 mm by 0.25 mm field. Larger work are was enabled by synchronously driving the projection mask and target motion stages. The 0.4 NA lens supported the formation of high- aspect channel walls and surface-relief features as small as approximately 500 nm. Both mask projection and direct writing technique were employed. The novel aspects of the optical beam delivery system are presented together with results on fabricating micro-channels, cutting optical fiber, fabricating surface relief grating and cylindrical lens. The results demonstrate broad application directions for fabricating telecommunication devices, general optical and photonic components, and biological devices.
Buried optical waveguides have been fabricated directly in pure bulk fused silica with a novel high-resolution 157-nm optical processing system. Refractive index changes of > 10-4 were induced within the small focal volume of large numerical-aperture optics, removing the need for ultrafast laser interactions. Single-mode guiding was inferred from Gaussian-like near-field and far-field intensity distributions of 635-nm guided light. The results demonstrate a useful extension of writing buried 3D refractive index structures inside glasses with nanosecond duration UV lasers.
Ongoing efforts to engineer a system capable of selecting and labeling single cells using optical micromanipulation tools and performing electrophoretic separation on the contents of a single cell using the 'lab-on-a-chip' format are presented. At the heart of this design, are channels with 10micrometers diameter cross-sections, etched using a molecular fluorine laser. Individual cells are moved on the microchip using optical tweezers. These single cells are brought into contact with a liposome containing fluorescent tags. The liposome and cell are fused using optical scissors; resulting in a cell with labeled components. This cell is lysed using the optical scissors, and high voltage is applied to separate the contents. This design will allow us to directly look at protein and mRNA expression from a single cell without amplifying the contents of interest, as well as to obtain the population averages and their variations from the analysis of a sufficient number of individual cells.
Persistent spectral hole burning (PSHB) was performed for several porphyrin molecules imbedded in polymer hosts and rare-earth ion Sm2+ doped in BaFCl0.5Br0.5 matrix. Power saturation and broadening effects for hole depth and width were observed. The dynamical process of hole formation was measured and explanation by population rate equations was given. Quantum efficiency of photochemical hole burning was investigated. For BaFCl0.5Br0.5:Sm2+ the capacity of data storage was estimated according to the result of multiple hole burning.
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