Thomas Sidler

Design, simulations and experimental investigations of a compact single mirror tip/ tilt laser scanner

In this paper, we present the design aspects of a compact tip/tilt, fast steering laser scanner. The scanner is composed of one single mirror with a large active area, driven by linear electromagnetic actuators. The suspension of the mirror is based on a cone-ball bearing. Designing such a scanner requires a multi-disciplinary approach of the optics, mechanics, electromagnetic and control theories. Experimental results are presented as well to demonstrate the performance and effectiveness of the scanner developed. These results also serve to verify the analytical and simulated results.

Monolithic shape memory alloy microgripper for 3D assembly of tissue engineering scaffolds

This paper describes a microgripper used for the micro-assembly of an artificial scaffold for tissue engineering. The porous sponge-like scaffold is a three dimensional construct built by tiny unit parts of biodegradable polymer. This application requires the assembly of several parts by applying a suitable level of force. In this framework, a monolithic shape memory alloy (SMA) microgripper was developed. It consists of two small fingers for grasping, an active part that changes its shape when heated and a parallel elastic structure used as a bias spring. The main aspect of the design is that all these elements are included within a single piece of material, but have different mechanical properties and serve as different functions. Using a new technology of Shape Memory Alloy laser annealing developed at EPFL, a local shape memory effect is introduced on the active part while leaving the remaining areas in a state where no shape memory effect occurs, i.e., in a cold-worked state. The parallel elastic structure is used to provide a pullback force on cooling as well as to guide the finger movement. An electrical path is integrated to heat the active part and drive the gripper by Joule effect. This paper focuses on the principle of the micro-gripper, its design, calculations and describes the fabrication process. Some first experimental results are also presented.

Robotic micro-assembly of scaffold/cell constructs with a shape memory alloy gripper

Describes an integrated approach to design and fabricate scaffold/cell constructs for tissue engineering. With this approach it becomes possible to produce scaffolds with controlled distribution of living cells and growth factors, a critical condition for successful grafting. Our idea consists of building a scaffold/cell construct by robotic micro-assembly of microscopic polymer building blocks. The paper introduces the rationale and concept of this interdisciplinary project and presents some realized steps. A 3D contact FEM simulation has been carried out to study the forces involved on the scaffold elements and micro-gripper during assembly. An error analysis has been performed to evaluate the accumulated error when building a scaffold/cell construct. A dedicated monolithic shape memory alloy micro-gripper has been realized and tested, which is able to handle parts in the range of 50-100 microns.

Study of the beam path distortion profiles generated by a two-axis tilt single-mirror laser scanner

Beam distortion profiles are studied for scanning devices that have a single mirror with two rotational degrees of freedom (DOF), also named tip/tilt scanners. The case of a fast steering scanner used for high power material processing applications is studied. The scanner has a bandwidth of 700 Hz, a range of motion of 652 mrad (63 deg), and a resolution ,5 mrad. The main dominant parameters that affect the dis- tortion profile are identified. Furthermore, a vector analysis is derived to represent these distortions, and equations of the correction factors used to compensate for the systematic errors are proposed. A final accuracy better than 0.05% is obtained when these compensation factors are taken into account. The derivation proposed here can be extended to any scanning device with a single mirror with two-rotation DOF.

High-power long-pulse second harmonic generation and optical damage with free-running Nd:YAG laser

Frequency doubling with a free-running long-pulse Nd:YAG laser and LBO or KTP nonlinear crystals yields conversion efficiency of up to 17.5% and 162 W peak power in the second harmonic. This efficiency is obtained for a TEM/sub 00/ beam with rectangular temporal pulse shape of 50 to 400 /spl mu/s. To our knowledge, this is the highest second-harmonic generation (SHG) efficiency reported for the long-pulse free-running configuration. The efficiency is limited by optical damage with much lower threshold than in the Q-switch domain. The damage is preceded by a saturation effect of the SHG efficiency. Both wavelengths (fundamental and second-harmonic) are necessary for the creation of the catastrophic damage. We present first evidence for a mechanism that involves creation of transient absorption centers by the second-harmonic radiation due to multiphoton absorption. Absorption of the fundamental wave at these centers leads to local heating and ultimately catastrophic damage.

Flexible automated assembly of micro-optical elements (optical SMD)

An automated assembly technique for small optical components has been developed. It concerns components such as, e.g., laser diodes and LEDs, fibers, lenses beamsplitters, polarizers, mirrors, crystals, prisms, diffractive elements or photodiodes. It is based on the flexible 2-dimensional arrangement of a universal tripod holder (10 by 10 by 4 mm) on a planar mounting plate. Its particular mechanical structure allows to align the optical elements on-line and to attach them to the mounting plate in a one step procedure. The different elements are aligned with an accuracy of plus or minus 1 micrometer and attached one after the other. Very good position stability (plus or minus 0.7 micrometer, plus or minus 0.2 mrad) during the attachment procedure has been achieved by laser point welding. They are optically interconnected by free-space propagation of a light beam with diameter of up to 8 millimeters. Mass production has been shown with a collimator as test vehicle. The collimator is composed of two elements (laser diode and collimating lens) and is mounted entirely automatically by two co-working robots. Easy prototyping has been shown with the realization of the optical position sensing system featuring a high precision linear magnetic bearing. Flexibility, simple handling, high packaging density and low cost make this new assembly technique satiable to both mass production and prototyping of small opto electronical devices.

Local annealing of shape memory alloys using laser scanning and computer vision

A complete set-up for local annealing of Shape Memory Alloys (SMA) is proposed. Such alloys, when plastically deformed at a given low temperature, have the ability to recover a previously memorized shape simply by heating up to a higher temperature. They find more and more applications in the fields of robotics and micro engineering. There is a tremendous advantage in using local annealing because this process can produce monolithic parts, which have different mechanical behavior at different location of the same body. Using this approach, it is possible to integrate all the functionality of a device within one piece of material. The set-up is based on a 2W-laser diode emitting at 805nm and a scanner head. The laser beam is coupled into an optical fiber of 60(mu) in diameter. The fiber output is focused on the SMA work-piece using a relay lens system with a 1:1 magnification, resulting in a spot diameter of 60(mu) . An imaging system is used to control the position of the laser spot on the sample. In order to displace the spot on the surface a tip/tilt laser scanner is used. The scanner is positioned in a pre-objective configuration and allows a scan field size of more than 10 x 10 mm2. A graphical user interface of the scan field allows the user to quickly set up marks and alter their placement and power density. This is achieved by computer controlling X and Y positions of the scanner as well as the laser diode power. A SMA micro-gripper with a surface area less than 1 mm2 and an opening of the jaws of 200(mu) has been realized using this set-up. It is electrically actuated and a controlled force of 16mN can be applied to hold and release small objects such as graded index micro-lenses at a cycle time of typically 1s.

Compact multisensor laser scanning head for processing and monitoring microspot welding

In order to improve the reliability of micro-spot welding of metal parts in production such as e.g. in electron guns for TV picture tubes, real-time information about the evolution of the welding process should be available to allow on-line modification of the laser parameters. Such information can be derived from a set of sensors that are mounted on a laser-scanning head. Different sensors are used to monitor the optical fiber output power to determine the evolution of temperature during the spot welding process, to measure plasma emission and back-reflected laser light. A vision channel and a CCD camera are used to control the position of the laser spot on the parts to be processed. The compact scanning head is composed of a tip/tilt laser scanner, a collimating lens and a focusing lens. The scanner is fast steering, with a bandwidth of 700Hz, and can tilt by +/- 3.5 degree(s) with a repeatability better than 50(mu) rad. The settling time for maximum deflection is less that 10ms. The scanning lens is a newly developed focusing lens designed to replace commercial cumbersome scanning lenses such as F-(theta) lenses, which have large volume, weight and price. This lens is based on the well-known Cooke triplet design and guarantees a constant shape of the spot all over the scan surface and is specially well suited for high power beam delivery. The scan field achieved by the system is limited to 25mm x 25mm. The laser used for this application is a pulsed Nd:YAG laser delivered by an optical fiber to the optical head. However, the system can be adapted to different types of lasers. Laser micro-spot welding on copper substrate has been performed in the frame of the Brite-Euram project MAIL. Smaller tolerances (a factor of 2 less) on the spot diameters were obtained in the case of a sensor controlled operation compared to the case where sensor control is not used.

Automated surface mounting of miniaturized optical elements

An automated surface mounting assembly procedure has been developed for mounting miniaturized optical elements. It is based on a low cost universal holding device, free space optical interconnection, and on-line adjustment of the elements. It is characterized by submicron mounting precision and excellent mechanical properties. Flexibility in the layout, simple handling, high packaging density render this technique suitable to low cost prototyping and mass production of optoelectronic devices. The possibility of mass production will allow a new class of optical systems to be conceived for the customer market.

Reliable laser micro-spot welding of copper

Nd:YAG laser welding of high reflectivity metals is difficult because of the highly non-linear light-material interaction yielding a narrow process window and poor reliability. However, achieving high reliability is mandatory for applying this technique in industrial production lines. The welding control can be improved by real-time monitoring of the process evolution with sensors. Such sensor signals are particularly useful for weld classification and for laser power control in off-line or in closed-loop feedback configurations. The latter possibility is difficult to implement in pulsed lasers and requires a careful sensor choice. Here, we report on laser lap micro-spot welding of thin copper sheets using a pulsed Nd:YAG laser. The welding was performed under atmospheric conditions on pure, 50 μm thick, slightly oxidized copper sheets with pulse durations and energies of less than 8 ms and 8 J, respectively. The process was experimentally analyzed by detecting normal laser reflection, heat emission, and instantaneous laser power with high time resolution. The meaningful signal parameters have then been selected for a closed loop feedback control. The variance of top and bottom weld spot diameters could be reduced by more than a factor of 8 in the case of closed loop control.