The knowledge of wind fields for a global terrestrial coverage and accurate altitude sampling is one of the main keys for improvement of meteorological predictions and general understanding of atmosphere behaviour. The best way to recover this information is remote sensing from space using low Earth orbit satellites. The measurement principle is to analyse the Doppler shift of the flux emitted by the space instrument and backscattered by the atmosphere. One of the most promising principle for Doppler shift measurement is the direct detection which does not need local oscillators. what significantly simplifies the design of such a space-borne receiver. ESA-ESTEC initiated at early 95' a programme called "lncoherent Doppler Wind Lidar (IDWL) technologies" for the study and bread-boarding phase. MMS won this contract proposing an original concept based on the use of a Fizeau high resolution interferometer working in the UV band. coupled with an intensified CCD. This concept is patented by MMS, as well as the special CCD timing sequence that will be depicted below. The programme begun by a study of the space-borne instrument in order to identify main constraints and define the receiver as could be for a flight model. A detailed performance model was established and parametric analysis allowed to optimise the concept in order to reach required performances. This study phase finally provided the definition of a bread-board for expected performances demonstration. Moreover, the Laser Signal Simulator (LSS) which is used to simulate the Lidar echo in term of amplitude as well as frequency modulation was defined at this step. The performances of this test support equipment are of main importance for the validation of the demonstrator design and performances. The second part of the study aimed at defining the derailed design of the demonstrator and associated test support equipment as well as initiating preliminary validation experiments on most critical technologies, like Fizeau interferometer which needs particularly high thermal stability and spectral resolution. At the end of this design phase. the test bench equipment begun to be manufactured and equipment test results preliminary assessed the study phase results. After integration, the correct operation and control of the overall test bench were assessed and performance tests were undertaken . The final conclusion of this programme aimed at updating the performance simulation software in order to refine expected performances for the future flight instrument.
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.
First results and analysis of second harmonic generation with a Nd:YAG laser working in the long pulse, free running mode are presented in this paper. Second harmonic generation conversion efficiencies of up to 17.5% and pulse powers of 162W have been generated with a free running Nd:YAG laser and a KTP non-linear crystal. The conversion efficiency is limited by a saturation effect and optical damage occurring at ~50 times lower peak intensities than in the Q-switch mode. The saturation and damage mechanism involves creation of temporary 'color centers' induced by the second harmonic radiation and subsequent increased fundeamental wave absorption.
Industrial high precision micro-material processing with solid-state lasers needs the reliable and efficient generation of high brightness laser beams. The key problems for this goal are the control of the thermal effects (lensing) in the active material and the overlap efficiency between the resonator mode and the pumped active material volume. Modern solid state lasers with low thermal effects such as zig-zag slab lasers have non circular geometries difficult to adapt for high efficiency and brightness simultaneously. Resonators comprising a cylindrical telescope, resulting in an elliptical beam section in the active material of rectangular geometry but nevertheless a circularly symmetric output beam can increase the efficiency and beam quality and also compensate for eventual astigmatic effects of the active medium. These lasers yield therefore TEM00 output beams (pulsed, free running) with a beam quality of M<SUP>2</SUP><1.7, pulse powers up to several kW and intensities up to 500MW/cm<SUP>2</SUP> for a spot diameter of 10 to 15micrometers . Such lasers are ideally suited for industrial high precision cutting and drilling, but also for quasi-cw harmonic generation, where the beam quality influences directly the conversion efficiency via the limited angular phase matching acceptance angles. Laser cutting with fundamental mode Nd-YAG lasers at 1.06micrometers and at 532nm (SHG efficiencies up to 17%) yields a minimal kerf width down to 15 micrometers and heat affected zones of less than 2micrometers .
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.
Frequency doubled Nd:YAG lasers represent an attractive alternative to other laser tools for many material processing applications, but frequency doubling with pulsed Nd:YAG lasers has been performed until now only with pulses of tens of nanoseconds. In material processing with longer pulses (10-1000 microsecond(s) ), such as encountered in typical 1.06 micrometers industrial Nd:YAG applications, the laser-material interaction is different and, in particular, higher material ablation rates are performed. Furthermore, the green light material processing permits a better focusability and a higher absorption in most materials. However, frequency doubling with long pulse lasers is much more difficult and less efficient up to now. The main problems are the generation of a fundamental 1.064 micrometers beam of high quality necessary for the non-linear process, and the low damage threshold of the non linear materials in the long pulse regime. Therefore, a zigzag slab laser, which has a high beam quality and an inherently linear polarization of the beam, is an ideal candidate for non-linear processes. The optical damage threshold in the non-linear materials is the main limiting parameter. The 140 W instantaneous power obtained for a 200 microsecond(s) pulse duration in extra-cavity configuration allows us to finely process sheets up to 200 micrometers thick.
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.
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 mm<SUP>2</SUP>. 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 mm<SUP>2</SUP> 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 fast-steering two axis-tilt mirrors are key components in astronomy, laser communications, material processing applications, imaging systems, biomedical and ophthalmologic applications. The laser scanner presented in this paper can perform a variety of functions such as tracking, beam stabilization and alignment, pointing and scanning. The small overall volume of 30 X 40 X 50 mm<SUP>3</SUP> can lead to a very stable and compact system design. The fact that the steered mirror has a single point of rotation for the two tilt axes is a clear advantage for systems with scanning lenses designed for this purpose. The scanner is composed of one single mirror driven by two pairs of push-pull linear electromagnetic actuators. The suspension of the mirror is based on a cone-ball bearing with optimized friction and wear behavior. A Position Sensitive Detector integrated in the module is used for the closed loop feedback positioning. The mirror can be tilted by more than +/- 52 mrad (+/- 3 degree(s)) with an accuracy better than 50 (mu) rad. A differential resolution of the order of 5 (mu) rad and a settling time for maximum deflection of 9 ms is achieved. Due to the large active area of the mirror (30 mm X 40 mm), very small spot diameters can be reached (less than 1 micrometers ) using high quality laser beams. Therefore, the scanner can be used e.g. in high precision micro-material processing of semiconductor and sensor industry.
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.
High precision laser cutting permits a new approach for the conception and realization of complex miniaturized components and systems necessitating very high absolute dimensional and assembling precision. The laser cutting with a pulsed TEM00 Nd:YAG-slab laser results in mechanical tolerances and a reproducibility better than 3 micrometers over working areas of up to 30 X 30 mm<SUP>2</SUP>. This permits not only to realize mechanical 2D-parts with very high dimensional precision but also the introduction of features for precise assembling of complex structures with very close tolerances without the need of delicate assembling procedures and machines. We present, as an example, an electrically steerable miniature mirror mount. In a volume of 10 by 10 by 4 mm<SUP>3</SUP>, a mirror with a clear aperture of 4 mm, an angular excursion of +/- 2 mrad and an axial excursion of 15 micrometers can be realized. The mechanical resonance lies between 200 and 600 Hz, depending essentially on the mirror mass and the stiffness of the flexure elements. Such a device can be used e.g. in laser resonators as passive or actively controlled mirror, in resonant cavities for SHG and THG or as a small Fabry-Perot spectrum analyzer.
The application of YAG-SLAB lasers operating in fundamental and low order mode
to metal cutting has been investigated. The high beam quality and the relative
insensitivity of the beam shape to average power variations make slab lasers
strong candidates for high precision cutting applications. The advantage of very
fine focusability (focused beam diameters down to 30,um) and high intensity
(100 MW/cm2) for cutting applications are shown. Three limiting factors are
derived for high quality cutting : The threshold intensity for efficient laser
beam absorption, the depth of focus of the laser beam and the limited aspect
ratio (cutting thickness to kerf width ratio) achievable by material removal.
Experimental results of stainless steel cutting with fundamental and low order
mode beams are given for material thicknesses of 0.5 mm to 1.5 mm. The cuts show
kerf widths of 35im (aspect ratios of 21:1) and roughness values of Ra 1.6 pm
for 0.8 mm thick steel samples.