For the water vapour DIAL “WALES” the wavelength regions around 935 nm, 942 nm and 944 nm have been identified as the most suitable wavelength ranges. These wavelengths can be obtained using opticalparametric-oscillators (OPOs), stimulated Raman shifters and the Ti-Sapphire laser but none of these systems could deliver all the needed parameters like beam quality, efficiency, pulse length and energy yet. Also these systems are comparably big and heavy making them less suitable for a satellite based application.
A fourth possibility to achieve these wavelength ranges is to shift the quasi-3-level laser lines (938 nm and 946 nm) of the Nd:YAG laser by replacing aluminium and yttrium by other rare earth elements. Changes of the host lattice characteristics lead to a shift of the upper and lower laser levels.
These modified crystals are summarized under the name of "Mixed Garnet" crystals. Only the Mixed Garnet lasers can be pumped directly with diode laser and use a direct approach to generate the required laser pulses without frequency conversion. Therefore no additional non-linear crystals or special pump lasers are needed and a higher electric to optical efficiency is expected as well as single frequency operation using spectral tuning elements like etalons.
In a first phase such mixed garnet crystals had been grown and characterised. The outcome was the selection of the gadolinium-scandium garnet for the most suitable laser crystal. During a second phase the complete laser system with output energy about 18 mJ in single 20 ns pulses and up to 8 mJ in free running mode with a combined pulse width of 250 μs at 942 nm have been demonstrated.
The results of the first laser operation and the achieved performance parameter are reported.
KEYWORDS: Semiconductor lasers, High power lasers, Diodes, Astronomical imaging, Space operations, Nitrogen, Oxygen, Contamination, Single crystal X-ray diffraction, Reliability
Several ESA missions incorporate high power laser systems as core constituents of the payload. Instruments based on techniques such as Light Detection And Ranging (LIDAR) can offer extremely accurate measurements of great scientific and industrial interest such as atmosphere composition, wind speed, aerosol presence, topography and water depth, surface backscattering and many more. Other active optical techniques such as Laser Induced Breakdown Spectroscopy (LIBS) enable the remote testing of the composition of rocks. Rendezvous sensors can guide the approach and docking of spacecraft, or assist in the descent of a lander on a surface.
Scalar magnetometers measure the magnitude of the magnetic field, while vector magnetometers (mostly fluxgate magnetometers) produce three-component outputs proportional to the magnitude and the direction of the magnetic field. While scalar magnetometers have a high accuracy, vector magnetometers suffer from parameter drifts and need to be calibrated during flight. In some cases, full science return can only be achieved by a combination of vector and scalar magnetometers.
Laser based altimetry can be very beneficial for planetary exploration, especially in the absence of any significant atmosphere. This technique can provide accurate information on the surface profile (topographic mapping) in a fast and cost effective manner, allowing, within the characteristics of the spacecraft orbit, repeated global coverage of the planet surface. The key characteristics of planetary laser altimetry are therefore an adequate altitude resolution having a range appropriate for full coverage in a compact mission lifetime, an active measurement principle not requiring direct sun illumination, a relatively simple detection chain (as compared to radar based systems), a low resource budget (e.g. mass, power, envelope, data rate) and a relatively simple interface and integration with the spacecraft. A laser altimeter forms a key component for the ESA mission to Mercury, Bepi-Colombo, onboard the Mercury Planetary Orbiter, MPO. The European Space Agency (ESA) is promoting the study of a generic laser altimeter for planetary exploration. This definition study will use Mercury as a reference for the definition of the environmental and operational requirements.
The concept of a receiver technique for high spectral resolution lidars is presented. The optical amplifier placed in front of the detector acts like an active narrow-band interference filter. The wavelength is selected by the tuning element of the amplifier and, therefore, the application to a variety of lidars is possible. The application can be an incoherent Raman lidar as well as short wavelength Doppler lidars with direct detection. The advantages and disadvantages of optical amplifiers as preamplifiers are discussed.
KEYWORDS: Process control, Plasma etching, Control systems, Rule based systems, Data processing, Integrated circuits, Inspection, Etching, Metrology, Manufacturing
This paper discusses the development and implementation of a rule-based system which assists in providing automated process control for plasma etching. The heart of the system is to establish a correspondence between a particular data pattern -- sensor or data signals -- and one or more modes of failure, i.e., a data-driven monitoring approach. The objective of this rule based system, PLETCHSY, is to create a program combining statistical process control (SPC) and fault diagnosis to help control a manufacturing process which varies over time. This can be achieved by building a process control system (PCS) with the following characteristics. A facility to monitor the performance of the process by obtaining and analyzing the data relating to the appropriate process variables. Process sensor/status signals are input into an SPC module. If trends are present, the SPC module outputs the last seven control points, a pattern which is represented by either regression or scoring. The pattern is passed to the rule-based module. When the rule-based system recognizes a pattern, it starts the diagnostic process using the pattern. If the process is considered to be going out of control, advice is provided about actions which should be taken to bring the process back into control.
The implementation of statistical process control coupled with a failure mode evaluation analysis had failed to produce feature sizes consistently on the target of 5 micrometers . The process was in control but was not capable of delivering product consistently within specification. A Taguchi-type experiment was organized to maximize the process output. Several experiments were conducted varying just one factor at a time. This procedure identified (a) potential nonlinear response factors and (b) appropriate levels for the Taguchi experiment. Six important factors and one potential interaction (exposure and develop time) were identified. The L8 design of Taguchi's arrays was used and each experiment was repeated at both high and low humidities and different ambient temperatures. Significant factors and their appropriate levels were identified and the predicted response was 4.956 micrometers . The confirmatory runs averaged 5.005 microns.
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