The European Space Agency is developing its first spaceborne LIDAR for global monitoring of wind velocities. ALADIN, to be launched on board ADMAeolus in 2008, is a pulsed Nd:YAG laser with about 120 mJ of pulse energy at 355 nm and a repetition rate of 100 Hz during bursts. Within the projected mission duration of three years, this gives a lifetime requirement of close to 5 billion pulses.
While laser-induced damage thresholds of optics in vacuum (possibly contaminated by small amounts of organic compounds) can differ from atmospheric conditions, their damage behaviour is generally poorly understood. The European Space Agency has therefore established a test campaign to measure the power handling of all the instrument optics with several European laboratories participating.
In the Optics and Opto-Electronics laboratory at ESTEC, a laser-induced damage threshold (LIDT) test facility has been set up with a 50 Hz Nd:YAG test laser. The pulse energy is 700 mJ at 1064 nm. This allows us to recreate the laser pulse conditions to which the ALADIN optics will be exposed. The flattop beam profile of the test laser irradiates the optics with uniform fluences and relatively large spots (up to 1mm across) at damaging intensities.
Damage tests are performed with up to 1 million pulses per test spot according to the S-on-1 test ISO-11254 standard, requiring typically 10 days to test one sample. With such extended tests, we can predict the laser-induced damage threshold over the ALADIN lifetime with improved accuracy.
The idea of deploying a lidar system on an Earthorbiting satellite stems from the need for continuously providing profiles of our atmospheric structure with high accuracy and resolution and global coverage. Interest in this information for climatology, meteorology and the atmospheric sciences in general is huge. Areas of application range from the determination of global warming and greenhouse effects, to monitoring the transport and accumulation of pollutants in the different atmospheric regions (such as the recent fires in Southeast Asia), to the assessment of the largely unknown microphysical properties and the structural dynamics of the atmosphere itself.
Spaceborne lidar systems have been the subject of extensive investigations by the European Space Agency since mid 1970’s, resulting in mission and instrument concepts, such as ATLID, the cloud backscatter lidar payload of the EarthCARE mission, ALADIN, the Doppler wind lidar of the Atmospheric Dynamics Mission (ADM) and more recently a water vapour Differential Absorption Lidar considered for the WALES mission. These studies have shown the basic scientific and technical feasibility of spaceborne lidars, but they have also demonstrated their complexity from the instrument viewpoint. As a result, the Agency undertook technology development in order to strengthen the instrument maturity. This is the case for ATLID, which benefited from a decade of technology development and supporting studies and is now studied in the frame of the EarthCARE mission. ALADIN, a Direct Detection Doppler Wind Lidar operating in the Ultra -Violet, will be the 1st European lidar to fly in 2007 as payload of the Earth Explorer Core Mission ADM. WALES currently studied at the level of a phase A, is based upon a lidar operating at 4 wavelengths in near infrared and aims to profile the water vapour in the lower part of the atmosphere with high accuracy and low bias. Lastly, the European Space Agency is extending the lidar instrument field for Earth Observation by initiating feasibility studies of a spaceborne concept to monitor atmospheric CO2 and other greenhouse gases.
The purpose of this paper is to present the instruments concept and related technology/instrument developments that are currently running at the European Space Agency. The paper will also outline the development planning proposed for future lidar systems.
Recent results obtained on integrated optical gyroscopes are presented in this paper. The sensors configuration, based on high-Q resonators both in silica-onsilicon and InP technologies, is discussed and the estimation of the resonators performance through an accurate optical characterization is reported. On the basis of the numerical and experimental achievements, we demonstrate that resonant micro optical gyros can potentially exhibit resolution values in the range 1-10 °/h, which is demanded by several emerging space applications.
Spaceborne lidars carry much promise for Earth observation and interplanetary missions to measure atmospheric parameters (wind velocity, optical extinction or species concentrations) and planet topologies. As the first European lidar mission, the European Space Agency is developing a Doppler wind lidar, ALADIN, to be launched on board ADM-Aeolus in 2008. ALADIN is a pulsed laser, emitting about 120 mJ of pulse energy in the UV. The mission duration is envisaged to be three years, which corresponds to several billion emitted pulses, thus imposing very stringent criteria on the longevity of the system. Laser-induced damage is one of the most significant issues here, in particular since laser-induced damage in space vacuum is still poorly understood. The European Space Agency has therefore established a test campaign to measure the power handling of all the instrument optics with laboratories in Germany, Italy, the Netherlands, the United Kingdom and France participating. Measurements are conducted at three wavelengths (1064nm, 532nm and 355nm) and with the introduction of several contaminants. The presentation covers laser-induced damage risk mitigation, the ESA test campaign and some test results.
AEROSPATIALE, prime contractor, presents the main results related to the activities performed in order to demonstrate the feasibility of a coherent 2 micrometers lidar instrument capable of measuring water vapor and wind velocity in the planetary boundary layer, and to determine the main subsystem critical items: selected instrument configuration and associated performances, 2 micrometers laser configuration with phase conjugation, coherent receiver chain architecture, and frequency locking and offsetting architecture. The second phase of this study will be dedicated to breadboard the most critical elements of the instrument at 2 micrometers in order to technologically consolidate the feasibility of such an instrument.
AEROSPATIALE, prime contractor, presents the main results related to the activities performed in order to demonstrate the feasibility of a coherent 2 micrometer lidar instrument capable of measuring water vapor and wind velocity in the planetary boundary layer, and to determine the main subsystem critical items: (1) selected instrument configuration and associated performances, (2) 2 micrometer laser configuration with phase conjugation, (3) coherent receiver chain architecture, (4) frequency locking and offsetting architecture. The second phase of this study will be dedicated to breadboard the most critical elements of the instrument at 2 micrometers in order to technologically consolidate the feasibility of such an instrument.
The transmitter laser is recognised to be one of the most critical technologies for space-based Doppler windlidar .
We present initial evaluation of the performance of an e-beam sustained device in the 1OJ, 10 Hz class. Lifetime issues
are addressed in a subsidiary paper. We describe the design of the device and the results of a number of characterisation studies:
1) General nonoptical tests of gas circulation and heat exchanger efficiency. 2) Performance optimisation to maximise multimode efficiency as a function of energy loading, main discharge
E/N and gas composition, all tests allowed for optimisation of cavity extraction. 3) Characterisation of the novel plasma anode electron gun with respect to beam uniformity, secondary electron
concentration, and current constancy. 4) Optical characterisation to examine operating wavelength, pulse shape, beam profile in the near and far-field,
output energy and electrical to optical conversion efficiency, and frequency behaviour during the pulse.
The European Space Agency program for Development of a CO2 Laser for Spaceborne Doppler Wind Lidar Applications addresses both performance and lifetime aspects. Lifetime issues are of particular importance due to the 109 pulse life requirement for a spaceborne laser operating continuously at 10 Hz for a period of three years. Particularly critical lifetime issues for an e-beam sustained laser have been identified as the electron transmitting metal foil separating the electron gun and the laser, and the gas life. Four areas of study have been undertaken to address the foil and gas lifetime issues: Parametric Study of Gaseous Catalysis to determine the range of operating conditions under which oxidation of CO by high energy electrons can be expected to offset dissociation of CO2, thus eliminating the need for solid catalyst. Extended Sealed Runs to demonstrate long life in a representative laser system of the actual size required. Several runs of 107 pulses, and one run of 6.5 X 107 pulses, have been performed. The Foil Thermal Profile has been monitored using a pyroelectric vidicon camera to determine the maximum temperature reached by different candidate foil materials under representative conditions. High Temperature Foil Fatigue tests of 109 pulses have been carried out to simulate the effect of the laser pressure pulse, by performing fatigue tests on foil materials at high temperature.
The European Space Agency is supporting the development of key technologies for future spaceborne Doppler wind lidar instruments. The focus for this work is the ALADIN (Atmospheric Laser Doppler Instrument) program which is directed towards the establishment of a coherent CO2-laser based lidar system to improve operational meteorology and climate science. Technology support for this program has to date centered on the development of a 10 J class electron-beam sustained CO2 laser. The stringent alignment tolerances necessary for coherent spaceborne lidar systems have been addressed by the manufacture and test of a single-axis lag-angle and image-motion compensator. Receiver related work has to date concentrated on advanced signal processing algorithms. An alternative approach to coherent DWL (Doppler Wind Lidar) that shows promise for the longer term is a 2 micrometers lidar based upon InGaAs detection and the all-solid-state Tm:Ho:Host laser. These technologies, which may also find application within a spaceborne water-vapor DIAL (Differential Absorption Lidar), have been the subject of experimental activities in relation to spectroscopy, cw lasing and heterodyne efficiency. Also under development is incoherent DWL operating in the ultra-violet which, whilst not capable of the ultimate sensitivity of coherent lidar, is attractive for its reduced alignment tolerances and can be considered for climate and atmospheric research.
In meteorological and climatological fields, the scientific community will increasingly need global measurements of key atmospheric parameters with high spatial resolution (horizontal as well as vertical): the spaceborne lidars are the most suitable instruments for those missions. While backscatter lidar (ATLID, currently studied as ESA) is presently first candidate for space deployment, the next generation of lidars will be DIAL and Doppler wind lidars, presenting a higher level of complexity, mainly due to the large power and complex signal processing required. The present considered wind lidars are based on CO2 lasers, whose space compliance still needs confirmation, while alexandrite lasers are considered for water vapor and temperature measurements, but they need flashlamp pumping which poses a lot of several thermal constraints and lifetime problems: on the other side, the recent developments achieved in solid-state technology allow to envisage diode pumping as most promising possibility for both previous applications.
Active laser remote sensing from space is considered an important step forward in the understanding of the processes which regulate weather and climate changes. The planned launching into polar orbit in the late 1990s of a series of dedicated Earth observation satellites offer new possibilities for flying lidar in space. Among the various lidar candidates, ESA has recognized in the backscattering lidar and Doppler wind lidar the instruments which can most contribute to the Earth observation program. To meet the schedule of the on-coming flight opportunities, ESA has been engaged over the past years in a preparatory program aimed to define the instruments and ensure timely availability of the critical components. This paper reviews the status of the ongoing developments and highlights the critical issues addressed.