In this paper we review the most important fabrication techniques for glass and plastic refractive microlenses and we quantitatively characterize in a systematic way the corresponding state-of-the-art microlenses which we obtained from selected research groups. For all our measurements we rely on three optical instruments: a non-contact optical profiler, a transmission Mach-Zehnder interferometer and a Twyman-Green interferometer. To conclude we survey and discuss the different fabrication techniques by comparing the geometrical and optical characteristics of the microlenses.
Ink-jet printing technologies are now being developed and used across a wide spectrum of optoelectronic and microelectronic manufacturing applications, because they provide opportunities both for significant cost reductions in existing components and for new component and device configurations. Examples of cost reductions in existing component configurations include printing of optical epoxies to fabricate precision microlens arrays for micro-mirror-based MEMS optical switches and fluxless printing of solder bumps for flip-chip BGA, μBGA and CSP (chin-scale package) manufacture. The most widely recognized, relatively new application of ink-jet printing has arisen in the manufacturing of PLED (Polymer-LED) displays. Potential uses of these technologies for creating new device and package configurations that provide both higher performance and lower cost include microlens printing directly onto VCSELs or the tips of optical fibers for increasing the efficiency of coupling and solder ball printing for making right-angle electrical connections, in order to enable further miniaturization of optoelectronic packages. Example ofnew device configurations which may be fabricated using these capabilities include chip-level optical switches, transceivers, transmitters and receivers.
A significant advance in technical capability has recently been achieved in the fabrication of refractive microlens arrays by microjet printing. This advance enables control of lens diameter and center-to-center distances to accuracies on the order of +/- 1 micrometers , and of focal length variations within an array to less than the +/- 1%. Such accuracies are especially important for microlens arrays used for MOEMs device interconnects to optical fibers because of the relatively long free space optical path lengths required for such applications. The new process also enables the printing of microlenses of a given diameter with aspect ratios and focal lengths varying over a wide range (e.g., f/1-f/5). The profile of plano-convex microlenses printed by this method have exhibited less than a quarter wavelength of deviation from spherical surface. The thermal durability of the optical epoxies used for microlens printing enables both cycling temperature up to 200 degree(s)C and continuous exposure to thousands of hours at 85 degree(s)C, without affecting microlens performance. The microjet printing of lensed fibers provides another solution for fiber beam shaping and collimation with low production cost.
Ink-jet printing technology is, in many ways, ideally suited for addressing a number of these MEMS device packaging challenges. The general advantages of this form of microdispensing derive from the incorporation of data-driven, non-contact processes which enable precise, picoliter-level volumes of material to be deposited with high accuracy and speed at target sites, even on non-planar surfaces. Being data-driven, microjet printing is a highly flexible and automated process which may readily be incorporated into manufacturing lines. It does not require application-specific tooling such as photomasks or screens, and, as an additive process with no chemical waste, it is environmentally friendly. In short, the advantages obtainable with incorporation of micro-jet printing technology in many fabrication applications range from increased process capability, integration and automation to reduced manufacturing costs.
Micro-optoelectronic mechanical systems (MOEMS) typically rely on free-space optical interconnects for fiber array in/out connections. The fiber output collimating and input focusing functions may be performed by using either individual gradient-index-of-refraction (GRIN) microlens rods or, more typically, arrays of microlenses formed on a glass substrate, to which the fibers are butte-coupled. We present methods for fabricating, with micron precision, various configurations of micro-optics for fiber collimation using low-cost, ink-jet printing technology. These configurations range from micro-deposition of droplets of optical epoxy into the tips of fibers, positioned in either individual collets or fiber ribbon connector ferrules, to the printing of arrays of collimating/focusing microlenses onto glass substrates. In the latter case the flexibility of the data-driven printing process enables unique capabilities, such as the variation of microlens geometries within an array, in order, for example, to compensate for the varying distances between the input fibers and the individual micro-mirrors within an array of a MOEMS device. The processes and optical modeling approaches used for fabricating such fiber collimation structures utilizing ink- jet printing technology will be discussed in detail, along with process control issues and optical performance data.
The microjet printing method is being used to fabricate microlenses for free-space optical interconnects in telecommunication devices. This low-cost, data-driven process, based on `drop-on-demand' inkjet technology, involves the dispensing and placing of precisely sized micro-droplets of 100%-solids, UV-curing optical epoxy pre- polymers at elevated temperatures onto optical substrates and components. The method offers significant cost advantages in the fabrication of components such as precision lens arrays and fiber ribbon collimators for use in certain configurations of DWDM multiplexing- demultiplexing devices and optical switches.
In this paper we present different configurations for a compact free-space optical interconnection (FSOI) module by combining two radial gradient refractive index lenses (GRIN) and/or two arrays of refractive microlenses. Based on our finding with ray-tracing and radiometric analysis we discuss how we have selected the proper optical system configurations and how we have chosen the different design parameters to optimally accommodate different types of opto- electronic emitters such as LEDs, micro-cavity LEDs and VCSELs. We hereby focused on maximizing optical coupling efficiencies and misalignment tolerances while minimizing inter-channel cross-talk. Furthermore we discuss the experimental optical characteristics of two such prototype modules that we completed together with the first experimental results of their use in parallel data communication demonstrator systems.
The purpose of this initial study was to begin development of a new, objective diagnostic instrument that will allow simultaneous quantitation of multiple proteases within a single periodontal pocket using a chemical fiber optic senor. This approach could potentially be adapted to use specific antibodies and chemiluminescence to detect and quantitate virtually any compound and compare concentrations of different compounds within the same periodontal pocket. The device could also be used to assay secretions in salivary ducts or from a variety of wounds. The applicability is, therefore, not solely limited to dentistry and the device would be important both for clinical diagnostics and as a research too.
The microjet printing method is being used to fabricate microlens arrays for use in massively parallel, VCSEL-based datacom switches and to deposit lenslets of various configurations onto the tips of single-mode telecom fibers. Applications in the latter case include collimation of the output beams for free space optical interconnection and increasing the fiber numerical aperture for collection of light from edge-emitting diode lasers. Additional applications of this technology include point of arrays of active sensor elements onto the tips of imaging fiber bundles and fabrication of microlenses with axial index of refraction gradients to reduce focal spot size, utilizing multiple print heads with differing fluids. This low-cost, data-driven process, based on 'drop-on-demand' inkjet technology, involves the dispensing the placing of precisely sized microdroplets of optical material onto optical substrates. The micro-optical elements are printed with 100 percent solid, UV-curing optical epoxies, utilizing printing devices that can dispense picoliter-volume droplets at temperatures up to 300 degrees C.
The microjet printing method of micro-optical element fabrication is being used to make arrays of high-performance hemi-elliptical and hemi-cylindrical microlenses for potential use in applications such as collimation of edge-emitting diode laser array beams. The printing method enables both the fabrication of very fast (e.g., f/0.75) microlenses and the potential for reducing costs and increasing flexibility in micro-optics manufacture. The process for fabricating anamorphic microlenses, including those of square or rectangular shape, involves the dispensing and placing of precisely sized microdroplets of optical material onto optical substrates, and then controlling their coalescence and solidification. By varying the number, diameter and spacing of adjacent microdroplets of optical materials deposited at elevated temperatures onto heated substrate, both the dimensional aspect ratios and the ratio of `fast'- to-`slow' focal lengths of a printed hemi-elliptical microlens may be varied over a very wide range. Arrays of hemi-elliptical and hemi-cylindrical microlenses on the order of 100 - 300 micrometers in width and 150 micrometers to 20 mm long, with focal length ratios (fast/slow) from 1 (circular) to 0 (cylindrical), have been printed. A model for predicting printed hemi-elliptical microlens focal lengths from printed lenslet geometry is illustrated, along with an interferometric method of detecting lenslet defects and aberrations.
Microjet printing methods are being utilized for data-drive fabrication of micro-optical elements such as refractive lenslet arrays, multimode waveguides and microlenses deposited onto the tips of optical fibers. Materials used for microjet printing of micro-optics to date have included optical adhesives and index-tuned thermoplastic formulations dispensed at temperatures up to 200 degree(s)C onto optical substrates and components. By varying such process parameters as numbers and locations of deposited microdroplets, print head temperature and orifice size, and target substrate temperature and surface wetability, arrays of spherical and cylindrical plano-convex microlenses have been fabricated with dimensions ranging from 80 micrometers to 1 mm to precision levels of just a few microns, along with multimode channel waveguides. Optical performance data such as lenslet f/#s and far-field diffraction patterns are presented, along with beam-steering agility data obtained with an optical telescope system assembled from microlens arrays printed by this process.