GaAs nanowires with a 100% wurtzite structure are synthesized by the vapor-liquid-solid method in a molecular beam epitaxy system, using gold as a catalyst. We use resonant Raman spectroscopy and photoluminescence to determine the position of the crystal-field split-off band of hexagonal wurtzite GaAs. The temperature dependence of this transition enables us to extract the value at 0 K, which is 1.982 eV. Our photoluminescence excitation spectroscopy measurements are consistent with a band gap of GaAs wurtzite below 1.523 eV.
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.
Abstract. In the field of atmospheric research, lidar is a powerful technology that can measure gas or aerosol concentrations, wind speed, or temperature profiles remotely. To conduct such measurements globally, spaceborne systems are advantageous. Pulse energies in the 100-mJ range are required to achieve highly accurate, longitudinal resolved measurements. Measuring concentrations of specific gases, such as CH 4 or CO 2 , requires output wavelengths in the IR-B, which can be addressed by optical-parametric frequency conversion. An OPO/ OPA frequency conversion setup was designed and built as a demonstration module to address the 1.6-μm range. The pump laser is an Nd:YAG-MOPA system, consisting of a stable oscillator and two subsequent Innoslab-based amplifier stages that deliver up to 500 mJ of output pulse energy at 100 Hz repetition frequency. The OPO is inherited from the OPO design for the CH 4 lidar instrument on the French-German climate satellite methane remote-sensing lidar mission (MERLIN). To address the 100-mJ regime, the OPO output beam is amplified in a subsequent multistage OPA. With potassium titanyl phosphate as nonlinear medium, the OPO/OPA delivered more than 100 mJ of output energy at 1645 nm from 450 mJ of the pump energy and a pump pulse duration of 30 ns. This corresponds to a quantum conversion efficiency of about 25%. In addition to demonstrating optical performance for future lidar systems, this laser will be part of a laser-induced damage thresholds test facility, which will be used to qualify optical components especially for the MERLIN. © The Authors.Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
We present a theoretical and experimental analysis of a pulsed 1645 nm optical parametric oscillator (OPO) to prove the feasibility of such a device for a spaceborne laser transmitter in an integrated path differential absorption (IPDA) lidar system. The investigation is part of the French-German satellite mission MERLIN (Methane Remote Sensing Lidar Mission). As an effective greenhouse gas, methane plays an important role for the global climate. The architecture of the OPO is based on a conceptual design developed by DLR, consisting of two KTA crystals in a four-mirror-cavity. Using numerical simulations, we studied the performance of such a setup with KTP and investigated means to optimize the optical design by increasing the efficiency of the OPO and decreasing the fluence on the optical components. For the experimental testing of the OPO, we used the INNOSlab-based ESA pre-development model ATLAS as pump laser at 1064 nm. The OPO obtained 9.2 mJ pulse energy at 1645 nm from 31.5 mJ of the pump and a pump pulse duration of 42 ns. This corresponds to an optical/optical efficiency of 29%. After the pump pulse was reduced to 24 ns, a similar OPO performance could be obtained by adapting the pump beam radius. In recent experiments with optimized optical design the OPO obtained 12.5 mJ pulse energy at 1645 nm from 32.0 mJ of the pump, corresponding to an optical/optical efficiency of 39%. Two different methods were applied to study the laser damage thresholds of the optical elements used
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