Measurement of the static and temporal variation of Earth’s gravity field yields important information on water storage, seasonal and sub-seasonal water cycles, their impact on water levels and delivers key data to Earth’s climate models. The satellite mission GOCE (ESA) and GRACE (US-GER) resulted in in a significant improvement on our understanding of the system Earth. On GRACE and GRACE Follow-On two satellites are following each other on the same orbit with approx. 200 km distance to each other. A microwave inter-satellite ranging system measures the variation of the intersatellite distance from which the gravity field is derived. In addition, on GRACE Follow-On, which has been launched May 22<sup>nd</sup> 2018, a laser interferometer is added as an experiment to demonstrate the capability of this system to improve the ranging accuracy by at least one order of magnitude. To significantly improve the gravity field measurement accuracy, ESA is investigating the concept of a ‘Next generation gravity mission’ (NGGM), consisting of two pairs of satellites and a laser interferometer as the sole inter-satellite ranging system. Based on the heritage of the development of the laser ranging interferometer for GRACE Follow-On and the former and ongoing studies for NGGM, several concepts for the laser metrology instrument (LMI) for NGGM, namely the on- and off-axis variants of the transponder and the retroreflector concept have been investigated in detail with respect to their application for an inter-satellite distance of approx. 100 km. This paper presents the results of the detailed tradeoff between different concepts, including laser link acquisition, ranging noise contributors, instrument performance analyses, technology readiness levels of the individual instrument units and an instrument reliability assessment.
The European Data Relay System (EDRS) is operational, optically transferring data from currently four LEO Earth observation satellites to the geo-stationary EDRS-A spacecraft at 1.8 Gbps. The demand has increased to extend these point-to-point optical links towards a full optical network in space and enable high data rate links between space assets and between ground and space. This article presents the ESA developments towards high data rate optical free space feeder links. The performance of an optical link from a ground station to a geostationary relay spacecraft experiences major limitations by atmospheric turbulence. To overcome this limitation, a free-space optical link experiment over 13 km is being set up. It shall assess the gain in irradiance and corresponding reduction of the scintillation index by pre-distortion of the optical “uplink” beam based on the measured wave-front disturbances of the “downlink” beam using an adaptive optics system. A second experiment will answer the question if the isoplanatic angle covers the point ahead angle in a ground to GEO link. This was/will be done by correlation measurements on double stars separated between 3.6 and 4.1 arcsec in varying elevation angles and atmospheric turbulence conditions. A third experiment shall address the potential gain and limitations of the implementation of Wavelength Division Multiplexing (WDM) into optical inter-satellite links. WDM being a standard technique to increase the data handling capacity of fibre networks by injecting multiple data streams into one single fibre using only one set of transmit and receive optics.
For a spaceborne lidar a highly reliable, long living and efficient laser source is absolutely essential. Within the frame of the development of a laser source for the backscatter lidar ATLID, which will be flown on EarthCare mission, we setup and tested a predevelopment model of an injection-seeded, diode pumped, frequency tripled, pulsed high power Nd:YAG MOPA laser operating nominally at 100 Hz pulse repetition frequency. We also tested the burst operation mode. The excellent measured performance parameter will be introduced. <p> </p>The oscillator rod is longitudinally pumped from both sides. The oscillator has been operated with three cavity control methods: "Cavity Dither", "Pound-Drever-Hall" and "Adaptive Ramp & Fire". Especially the latter method is very suitable to operate the laser in harsh vibrating environment such in airplanes. <p> </p>The amplifier bases on the InnoSlab design concept. The constant keeping of a moderate fluence in the InnoSlab crystal permits excellent possibilities to scale the pulse energy to several 100 mJ. An innovative pump unit and optics makes the laser performance insensitive to inhomogeneous diode degradation and allows switching of additional redundant diodes. <p> </p>Further key features have been implemented in a FM design concept. The operational lifetime is extended by the implementation of internal redundancies for the most critical parts. The reliability is increased due to the higher margin onto the laser induced damage threshold by a pressurized housing. Additionally air-to-vacuum effects becomes obsolete. A high efficient heat removal concept has been implemented.
The Gravity Recovery and Climate Experiment (GRACE) has produced a wealth of data on Earth gravity, hydrology, glaciology and climate research. To continue that data after the imminent end of the GRACE mission, a follow-on mission is planned to be launched in 2017, as a joint USGerman project with a smaller Australian contribution. The satellites will be essentially rebuilt as they were for GRACE using microwave ranging as the primary instrument for measuring changes of the intersatellite distance. In addition and in contrast to the original GRACE mission, a Laser Ranging Interferometer (LRI, previously also called ‘Laser Ranging Instrument’) will be included as a technology demonstrator, which will operate together with the microwave ranging and supply a complimentary set of ranging data with lower noise, and new data on the relative alignment between the spacecraft. The LRI aims for a noise level of 80 nm/√Hz over a distance of up to 270km and will be the first intersatellite laser ranging interferometer. It shares many technologies with LISA-like gravitational wave observatories. This paper describes the optical architecture including the mechanisms to handle pointing jitter, the main noise sources and their mitigation, and initial laboratory breadboard experiments at AEI Hannover.
With GRACE (launched 2002) and GOCE (launched 2009) two very successful missions to measure earth’s gravity field have been in orbit, both leading to a large number of publications. For a potential Next Generation Gravity Mission (NGGM) from ESA a satellite-to-satellite tracking (SST) scheme, similar to GRACE is under discussion, with a laser ranging interferometer instead of a Ka-Band link to enable much lower measurement noise. Of key importance for such a laser interferometer is a single frequency laser source with a linewidth <10 kHz and extremely low frequency noise down to 40 Hz / √Hz in the measurement frequency band of 0.1 mHz to 1 Hz, which is about one order of magnitude more demanding than LISA. On GRACE FO a laser ranging interferometer (LRI) will fly as a demonstrator. The LRI is a joint development between USA (JPL,NASA) and Germany(GFZ,DLR). In this collaboration the JPL contributions are the instrument electronics, the reference cavity and the single frequency laser, while STI as the German industry prime is responsible for the optical bench and the retroreflector. In preparation of NGGM an all European instrument development is the goal.
ESA’s Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission and the American-German Gravity Recovery and Climate Experiment (GRACE) mission map the Earth’s gravity field and deliver valuable data for climate research.
The French-German Methane Remote Sensing LIDAR Mission (MERLIN) planned for launch in 2020 aims to provide a global methane concentration map. The instrument is a differential absorption LIDAR (DIAL) system measuring the column-weighted dry-air mixing ratios of methane with a horizontal resolution of 50 km employing an absorption line at 1645 nm .
The Gravity Recovery and Climate Experiment Follow-On (GRACE FO) is a space borne mission to map variations in the earth’s gravity field with an even greater accuracy than the first GRACE mission. GRACE FO is a collaborative project of NASA (USA) and GFZ (Germany) scheduled for launch in 2017. On GRACE the gravity field is reconstructed from a measurement of the distance variation between two satellites following each other in 200 km distance by use of a microwave ranging instrument. On GRACE FO a laser ranging interferometer (LRI) is added as a demonstrator in addition to the microwave. Moving from microwave range to optical wavelengths provides an improvement in distance measurement noise from some μm/√Hz to 80 nm/√Hz down to 0.01 Hz frequency. The criteria on the beam delivery system are demanding, in particular with respect to laser beam quality, wave front deviation and pointing as well as thermal and mechanical stability. Conventionally such a system can be manufactured with at least two special mounted lenses or an aspheric lens aligned with respect to the fiber end. However, the alignment of this optical system must be maintained throughout the mission, including the critical launch phase and a wide temperature range in orbit, leading to high alignment effort and athermal design requirements. The monolithic fiber-collimator presented here provides excellent optical and thermal and mechanical performance. It is a part of the LRI and located on the Optical Bench Assembly (OBA) which has already been described in [1, 3].
The Gravity Recovery and Climate Experiment (GRACE) is a successful Earth observation mission launched in 2002 consisting of two identical satellites in a polar low-Earth orbit . The distance variations between these two satellites are measured with a Micro Wave Instrument (MWI) located in the central axis. In data postprocessing the spatial and temporal variations of the Earth’s gravitational field are recovered, which are among other things introduced by changing groundwater levels or ice-masses [2, 3, 4, 5]. The Laser Ranging Interferometer (LRI) on-board the GRACE Follow-On (GFO) mission, which will be launched in 2017 by the joint collaboration between USA (NASA) and Germany (GFZ), is a technology demonstrator to provide about two orders of magnitude higher measurement accuracy than the initial GRACE MWI, about 80 nm/√Hz in the measurement band between 2 mHz and 0.1 Hz. The integration of the LRI units on both GFO S/C has been finished in summer 2016. The design as well as the functional, performance, and thermal-vacuum tests results of the German LRI flight units will be presented.
In scope of the ESA funded “High stability Laser” activity, a single-mode and single-frequency fiber power amplifier with 500 mW output power at 1064 nm wavelength has been developed. It is part of an elegant breadboard (EBB) which consists additionally of an ultra-stable Fabry-Perot reference for frequency stabilization. The monolithic fiber amplifier is seeded by a non-planar ring oscillator (NPRO) with a linewidth below 10 kHz. The amplifier is stabilized in power via pump diode modulation and achieves a RIN performance of < 0.01/sqrt(Hz) in the range from 10<sup>-3</sup> Hz to 10 Hz and a polarization extinction ratio of >30 dB.
The basic principles of faraday isolation and thermal effects in the rod geometry under high power operation have been
reviewed. The temperature dependency of the verdet-constant and the magnetization of NdFeB have been identified as
the limiting factor of isolation ratio for an unchilled high power isolator device. The focal shift due to the isolator with
rod-geometry is independent from beam diameter and strongly depending on beam quality. A highly compact isolator for
unpolarized radiation up to 400 W fundamental mode for industrial application surrounding with minimal thermal
lensing has been developed. At 400 W and 10-40 °C a transmission of 95-97 %, an isolation larger 20 dB, a thermal
focus shift of less than 2 rayleigh ranges and a M<sup>2</sup> smaller than 1.2 have been achieved.
For spaceborne lidar like the atmospheric backscatter lidar (e.g. ATLID on the ESA EarthCARE mission) highly reliable and efficient laser sources are needed. As pre-development model we realized a Nd:YAG MOPA diode pumped at 100 Hz. With more than 21 % optical-optical efficiency the amplifier based on the InnoSlab design raises the 8 mJ pulse energy from the single frequency rod oscillator to more than 70 mJ. Frequency-tripling leads to more than 25 mJ at 355 nm and a beam quality of M<sup>2</sup> < 1.7. The total optical-optical efficiency of more than 7.5 % exceeds the efficiency of comparable current lidar transmitter systems at least by a factor of 2. The laser is designed to cope with diode degradation or failure. Moderate pulse intensities in the InnoSlab amplifier offer excellent possibilities to scale the pulse energy to several 100 mJ in a most reliable and efficient way.
Lidar Systems for the measurement of three-dimensional wind or cloud and aerosol formations in the earth atmosphere
require highly stable pulsed single frequency laser systems with a narrow line width. The lasers for ESAs ADM-Aeolus
and EarthCARE missions require frequency stabilities of 4 and 10 MHz rms at a wavelength of 355 nm and a line width
below 50 MHz at 30 ns pulse duration. Transferred to the fundamental wavelength of the laser systems the stability
requirement is 1.3 and 3.3 MHz, respectively. In comparison to ground based lidar systems the vibrational load on the
laser system is much higher in airborne and spaceborne systems, especially at high frequencies of some hundred Hertz or
even some kHz. Suitable frequency stabilisation methods have therefore to be able to suppress these vibrations
sufficiently. The often used Pulse-Build-up method is not suitable, due to its very limited capability to suppress vibration
frequencies of the order of the pulse repetition frequency.
In this study the performance of three frequency stabilisation methods in principle capable to meet the requirements, the
cavity dither method, the modified Pound-Drever-Hall method and a modified Ramp-Fire method - named Ramp-Delay-
Fire - is theoretically and experimentally investigated and compared.
The investigation is performed on highly efficient, passively cooled, diode end-pumped q-switched Nd:YAG oscillators,
which are breadboard versions of the A2D (ADM-Aeolus) and possible ATLAS (EarthCARE) oscillators. They deliver
diffraction limited output pulses with up to 12 mJ pulse energy at a pulse duration of 30 ns and 100 Hz pulse repetition
A resonator setup applying a double-sided diode end-pumped configuration and an electro-optical Q-switch for efficient
generation of 4 mJ pulses (< 60 ns fwhm) at 935 nm from Nd:YGG is presented, to our knowledge for the first time. The
optical-optical efficiency is 9 % (absorbed pump light to laser out). High quality crystals have been investigated,
showing high damage threshold, high efficiency and good optical properties permitting Q-switched mode of operation.
Experimental small signal gain data coincide with spectroscopic measurements. For vapour detection frequency stable
single mode operation is required. Injection seeding with a single frequency cw-signal has been successfully achieved.
Frequency control mechanisms are currently under investigation. The direct generation of 935 nm radiation simplifies
future LIDAR systems significantly compared to current approaches based on OPO, Raman or Ti:Sapphire technology.
A Master-Oscillator-Power-Amplifier (MOPA) design combining rod and slab laser technology for high pulse energy, high average power and near diffraction limited beam quality for industrial use has been developed. To achieve the good beam quality at high average and high pulse power, an advanced birefringence compensation scheme, which ensures a high mode overlap while simultaneously minimizing the power densities on optical surfaces, has been developed and applied. The prototypes deliver an average power of up to 860 W with M<sup>2</sup> < 2 or 1.3 kW with M<sup>2</sup> < 12 at 10 kHz repetition rate and 5-16 ns pulse duration. At 1 kHz up to 420 mJ pulse energy can be achieved. The prototypes are fully computer controlled and can be operated from 0 to 100 % output power and from single shot to 10 kHz. They are currently operated for plasma generation in a laboratory surrounding and have run for more than one thousand hours without failure up to now. An analytical solution of the thermally induced refractive index profile in dependency of a radially symmetric pump light distribution including the effect of thermally induced birefringence, temperature dependency of the thermal conductivity and the second derivative of the refractive index with the temperature (d<sup>2</sup>n/dT<sup>2</sup>) has been derived. This allows a fast calculation of thermally induced aberrations without the use of FEA. Experimental results are compared to predictions from analytical and FEA modelling. Based on experimental and theoretical results, scaling limits of rod based MOPAs are predicted.