MERLIN (Methane remote sensing LIDAR mission) is a joint DLR/CNES mission, which will measure column densities of methane in the Earth atmosphere. The heart of the instrument is the laser transmitter subsystem, developed and built by Airbus (Ottobrunn, Germany) in cooperation with the Fraunhofer Institute for Laser Technology (Aachen, Germany), being in charge of the laser’s optomechanical assembly. The project is currently in Phase D with an expected instrument delivery date to the satellite prime in 2026 and launch in 2028. The laser system features key technologies, as already demonstrated successfully in the frame of the FULAS project, to enable reliable long term and high-stability laser performance operation under space conditions. The technologies are optimized with respect to thermal and mechanical stability and developed with special attention on LIC (laser induced contamination) issues by aiming for a fully inorganic design, avoiding any critical organic and outgassing materials. This publication provides an insight into the system design. Furthermore first results from the ongoing qualification model assembly and integration activities are presented, including evidence of the technology maturity for space, based on subassembly and component level qualifications as well as representative bread board activities. Additional presentation content can be accessed on the supplemental content page.
The Franco-German »MERLIN Project« was initiated in 2010. The small satellite MERLIN (Methane Remote Sensing LiDAR Mission) will map the methane in the earth’s atmosphere. Fraunhofer ILT has developed the LiDAR laser source and is currently integrating the Engineering Qualification Model. The laser consists of a laser oscillator pumped by fiber coupled diode laser modules, an INNOSLAB amplifier and KTP-based frequency converter. The amplifier pump is based on qcw stacks which are homogenized in the slow axis direction and focused in the fast axis direction. We will present the design of the pump optics and results of reliability tests. In addition, we will give an outlook on the development of a laser source for a future wind LiDAR mission.
For the satellite-based methane lidar instrument MERLIN a reliable laser source is needed that emits laser pulses at two wavelengths of around 1645 nm to measure the methane concentration of earth’s atmosphere with an Integrated Path Differential Absorption LIDAR (IPDA). To generate those pulses, the laser (LASO) consists of a seeded, actively qswitched, diode pumped Nd:YAG master oscillator power amplifier (MOPA) and a subsequent seeded and frequencycontrolled optical parametric oscillator (OPO). Due to the passive thermal control of the instrument the laser has to withstand a large non-operational and operational temperature range and also high mechanical loads while at the same time a compact envelope is required. Together with the demanding requirements on the laser performance a very robust optical design is needed. To meet those requirements, Fraunhofer Institute for Laser Technology (ILT) uses optomechanical mounts that were developed in a previous project and show very low tilting over a large operational temperature range, even after nonoperational temperature cycling and applying mechanical loads. The mounts are soldered and free of organic substances as the LASO is enclosed in a pressurized housing (LASH). Any outgassing could lead to a decay or damage of the optics and thus a failure of the laser. During the development of the optomechanical mounts many tests were performed to quantify the statistical behavior under mechanical and thermal loads. Based on those results and additional mechanical simulations, Monte-Carlo-Analyses have been performed to analyze the performance of the laser and to verify the fulfilment of the requirements.
A test campaign for assessing the radiation hardness of different Erbium-doped garnet crystals including Er:YAG and a
compositionally tuned Er:YAG/Er:LuAG mixed garnet is reported. Tests with proton and gamma radiation have been
performed with parameters mimicking a 3-year low-earth-orbit satellite mission like MERLIN or ADM-Aeolus. For each
test sample broadband transmission spectra in the wavelength range of 500 nm – 1700 nm and characteristic laser curves
from a test laser oscillator have been measured. Radiation-induced losses have been calculated from the obtained data.
The results indicate that gamma radiation is the dominant loss source with about 0.5 %/cm radiation-induced losses for
the nominal dose of the chosen mission scenario.
We present the world’s first laser at 515 nm with sub-picosecond pulses and an average power of 445 W. To realize this
beam source we utilize an Yb:YAG-based infrared laser consisting of a fiber MOPA system as a seed source, a rod-type
pre-amplifier and two Innoslab power amplifier stages. The infrared system delivers up to 930 W of average power at
repetition rates between 10 and 50 MHz and with pulse durations around 800 fs. The beam quality in the infrared is
M2 = 1.1 and 1.5 in fast and slow axis. As a frequency doubler we chose a Type-I critically phase-matched Lithium
Triborate (LBO) crystal in a single-pass configuration. To preserve the infrared beam quality and pulse duration, the
conversion was carefully modeled using numerical calculations. These take dispersion-related and thermal effects into
account, thus enabling us to provide precise predictions of the properties of the frequency-doubled beam.
To be able to model the influence of thermal dephasing correctly and to choose appropriate crystals accordingly, we
performed extensive absorption measurements of all crystals used for conversion experiments. These measurements
provide the input data for the thermal FEM analysis and calculation. We used a Photothermal Commonpath
Interferometer (PCI) to obtain space-resolved absorption data in the bulk and at the surfaces of the LBO crystals. The
absorption was measured at 1030 nm as well as at 515 nm in order to take into account the different absorption behavior
at both occurring wavelengths.
We present the world’s first laser at 515 nm with sub-picosecond pulses and an average power of 445 W. To realize this beam source we utilize an Yb:YAG-based infrared laser consisting of a fiber MOPA system as a seed source, a rod-type pre-amplifier and two Innoslab power amplifier stages. The infrared system delivers up to 930 W of average power at repetition rates between 10 and 50 MHz and with pulse durations around 800 fs. The beam quality in the infrared is M² = 1.1 and 1.5 in fast and slow axis. As a frequency doubler we chose a Type-I critically phase-matched Lithium Triborate (LBO) crystal in a single-pass configuration. To preserve the infrared beam quality and pulse duration, the conversion was carefully modeled using numerical calculations. These take dispersion-related and thermal effects into account, thus enabling us to provide precise predictions of the properties of the frequency-doubled beam. To be able to model the influence of thermal dephasing correctly and to choose appropriate crystals accordingly, we performed extensive absorption measurements of all crystals used for conversion experiments. These measurements provide the input data for the thermal FEM analysis and calculation. We used a Photothermal Commonpath Interferometer (PCI) to obtain space-resolved absorption data in the bulk and at the surfaces of the LBO crystals. The absorption was measured at 1030 nm as well as at 515 nm in order to take into account the different absorption behavior at both occurring wavelengths.
The passive-alignment-packaging technique presented in this work provides a method for mounting tolerance-insensitive
optical components e.g. non-linear crystals by means of mechanical stops. The requested tolerances for the angle
deviation are ±100 μrad and for the position tolerance ±100 μm. Only the angle tolerances were investigated, because
they are more critical. The measurements were carried out with an autocollimator. Fused silica components were used
for test series. A solder investigation was carried out. Different types of solder were tested. Due to good solderability on
air and low induced stress in optical components, Sn based solders were indicated as the most suitable solders. In
addition several concepts of reflow soldering configuration were realized. In the first iteration a system with only the
alignment of the yaw angle was implemented. The deviation for all materials after the thermal and mechanical cycling
was within the tolerances. The solderability of BBO and LBO crystals was investigated and concepts for mounting were
developed.
More than 370 W average power at 515 nm were achieved by frequency doubling of sub-picosecond pulses of an
Innoslab based Yb:YAG MOPA system. At 50 MHz repetition rate this corresponds to a pulse energy of 7 μJ in the
green. The pulse duration of 680 fs and the transformation limit of the infrared source could be maintained, thus the laser
provides more than 10 MW pulse power. The beam quality at 280 W was M2 < 1.4, at more than 370 W it was measured
as M2 < 1.7. Extensive numerical modeling was applied to design the conversion stage.
An Innoslab based Nd:YV04 MOPA system with pulse energy of 7.25 mJ at 40 kHz repetition rate and pulse
duration of 11.4 ns has been used for third harmonics generation in Lithium Triborate (LBO) crystals. We report
UV pulses of 8.9 ns duration at pulse energy of 1.65 mJ, which means an average power of 66 W. We have been
able to show UV beam qualities (M2) of 1.7/2.4 (stable/instable direction with 90/10 knife edge method), while
IR beam quality is 1.8/5.2. A sinc2-shape transversal distribution of beam intensity has been used in instable
direction of the Innoslab MOPA system for conversion. Due to high average power and short pulse length at
355 nm the laser meets the demands for high-throughput micro material processing as stereolithography or edge
isolation of solar cells. The thermal dependence of the conversion efficiency (due to heating power of the beam)
has been investigated theoretically, using a time resolved numerical simulation model for the nonlinear process in
both LBO crystals. Scaling effects of the absorption coefficients of LBO and the pulse power on the conversion
efficiency are presented in this article.
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