Doppler lidars with daylight capability are challenging systems because of the small field of view (FOV), spectral filtering, and other additional subsystems required compared to observations at night. A universal Doppler Lidar platform with all the required technologies for automatic operation is assembled. By combining a novel narrow-bandwidth pulsed laser (FWHM ~3 MHz) and a matched interferometer (FWHM ~7.5 MHz) the backscattered signals from molecules and aerosols are separated into two channels within the receiver. By tuning the frequency of the pulsed laser from pulse to pulse with sub-MHz accuracy relative to the interferometer, Doppler-aerosol measurements with largely reduced Rayleigh-signal allow aerosol measurements from ground to ~25 km altitude, including Doppler-wind measurements from the Doppler-shift. The compact lidar (1m^3) is built for automatic 24/7 operation and comprises a novel diode-pumped alexandrite ring-laser, a 50cm-telescope and a receiver for solar background free aerosol measurements. The combination of cost-efficient design and fast assembling of such a system allows the construction of a Doppler Lidar network with identical units. Such a network can address a wide range of horizontal and vertical scales.
We present the mission proposal for a spaceborne multi-frequency lidar that is able to perform simultaneous Doppler Rayleigh, Mie and resonance measurements to gain temperature and wind data from ground to thermosphere. The suitability of the technology is demonstrated with a novel ground-based lidar with a diode-pumped alexandrite laser that acts as a unit for a lidar network able to cover the atmosphere up to 100 km over a large area at day- and nighttime in polar regions. The performance of a spaceborne lidar at the iron resonance line at 386 nm is derived from simulations with parameters similar to Aeolus. The multi-frequency lidar achieves the same resolution and uncertainties in wind measurements as Aeolus but additionally provides the wind and temperature up to 120 km. The reduction of the solar background by three to five orders of magnitude by means of the usage of narrow bandwidth filters and laser allows for significantly lower laser pulse energies (~ 10 mJ) and higher repetition rates (~500 Hz). Thereby the laser as a key-component and with the highest risk of failure is reduced in complexity, as no amplification stages are necessary to yield the high pulse energy. The roadmap to space is discussed in light of the heritage from Aeolus and with the diode-pumped alexandrite laser that is in line with the FULAS (Future Laser System) platform that is also the basis for the emitter of Merlin. The necessary development steps are identified and the current activities to address them are described.
First atmospheric measurements with a diode pumped alexandrite ring laser in a ground-based general-purpose Doppler lidar demonstrates high-resolution spectral measurements with minimum hardware requirements. Two decades of Doppler measurements by flash lamped pumped alexandrite ring laser within the worldwide only existing mobile Doppler resonance lidar have shown the capability of such scanning lidar to measure any spectrum from the troposphere to the thermosphere without a frequency reference at the wavelength of desire. An improved second-generation system with a size of 1 m3, including a 50 cm telescope, is currently under development for ground-based automatic observations at harsh environments, such as Antarctica. Minimizing the size, complexity, maintenance time and improving the optical setup pave the way of this technology for future space missions. As an example for a possible future space mission, we discuss a mesospheric potassium resonance Doppler lidar by extrapolating the signal from ground-based measurements.
In this work the first diode-pumped Alexandrite ring laser in Q-switched single-longitudinal mode (SLM) operation is presented. A carefully designed and complex ring resonator with several functional components is combined with an innovative pumping scheme with high-power red laser diodes.
The spectral and energetic suitability of a first laser was demonstrated in hundreds of operating hours and, with a novel mobile lidar system, by the first measurements in the atmosphere by means of a diode-pumped Alexandrite laser, yielding data from the stratosphere to the mesosphere.
An improved laser yields a pulse energy of 1.7 mJ at a repetition rate of 500 Hz with an excellent beam quality of M2 < 1.1. By seeding the resonator with a narrow-band diode laser, SLM operation with a linewidth below 4 MHz is achieved. The electro-optical efficiency of 2 % is the highest archived for all Alexandrite lasers in SLM operation and reasonable for space-operation.
The performance analysis as well as benchmarking with the space-qualified mounting technology point out the TRL and the remaining effort of development of the technology.
In this work a diode-pumped Q-switched Alexandrite laser operating in single longitudinal mode (SLM) at the potassium resonance line is presented. The self-developed laser diode pump device is fiber-coupled ( = 400μm, NA=0.22) and delivers a pump energy of 18 mJ at 636 nm with a pulse duration of 120 μs and a repetition rate of 500 Hz. Pump light not absorbed in single-pass through the 7 mm long crystal is recollimated, polarization adjusted and refocused into the crystal. The Alexandrite laser yields a pulse energy of 1.7 mJ at a repetition rate of 500 Hz with a high pulse-to-pulse stability of 0.2 % (rms) and a beam quality of M2 < 1.1 in both spatial directions. The output beam is round and stigmatic without further beam shaping. The electro-optical efficiency of the laser system is 2 % which is approximately two magnitudes higher than of comparable flashlamp-pumped Alexandrite laser systems. By seeding the resonator with a SLM diode laser and actively stabilizing the cavity length, SLM-operation at the resonance line of potassium at 769.898 nm with a linewidth of approximately 10 MHz is achieved. Thereby the laser fulfills all the requirements for a resonance-lidar system. The investigations presented in this publication show the feasibility for pumping a complex ring resonator with a fibercoupled pump module in the red spectral region. This presents an important step to compact lidar systems for autonomous measurements under rough environmental conditions.
We present design and performance data of a diode-pumped Q-switched Alexandrite ring laser in the millijoule regime, which is longitudinally pumped by laser diode bar modules in the red spectral range. As a first step, a linear resonator was designed and characterized in qcw operation as well as in Q-switched operation. Based on these investigations, two separate linear cavities were set up, each with one Alexandrite crystal longitudinally pumped by one diode module. The two cavities are fused together and form a ring cavity which yields up to 6 mJ pulse burst energy in the qcw regime at 770 nm.
We present design and first performance data of a broadly tunable Alexandrite laser longitudinally pumped by a newly developed high brightness single emitter diode laser module with output in the red spectral range. Replacing the flashlamps, which are usually used for pumping Alexandrite, will increase the efficiency and maintenance interval of the laser. The pump module is designed as an optical stack of seven single-emitter laser diodes. We selected an optomechanical concept for the tight overlay of the radiation using a minimal number of optical components for collimation, e.g. a FAC and a SAC lens, and focusing. The module provides optical output power of more than 14 W (peak pulse output in the focus) with a beam quality of M2 = 41 in the fast axis and M2 = 39 in the slow axis. The Alexandrite crystal is pumped from one end at a repetition rate of 35 Hz and 200μs long pump pulses. The temperature of the laser crystal can be tuned to between 30 °C and 190 °C using a thermostat. The diode-pumped Alexandrite laser reaches a maximum optical-optical efficiency of 20 % and a slope efficiency of more than 30 % in fundamental-mode operation (M2 < 1.10). When a Findlay-Clay analysis with four different output couplers is conducted, the round-trip loss of the cavity is determined to be around 1 %. The wavelength is tunable to between 755 and 788 nm via crystal temperature or between 745 and 805 nm via an additional Brewster prism.
In this work, a detailed analysis and redesign of a tunable UV laser is presented. The laser is part of measurement system of “IEK 8, Forschungszentrum Jülich” for airborne LIF analysis of the OH-radical concentration. The design concept of the laser comprises a frequency doubled Nd:YAG laser as pump source, a dye as active medium to emit light at 616 nm, and a NLO crystal as intracavity frequency doubler. The output wavelength is tunable by a combination of dispersion prisms and an etalon. During measurement campaigns, the laser is mounted on top of Zeppelin NT and therefore is exposed to temperatures ranging from 10 to 40 °C and ambient pressures from 800 to 1000 hPa. In former flights the output power of an existing laser decreased rapidly and the wavelength was unstable during the flights and therefore hinders continuous measurements. The analysis of the existing laser combines a theoretical study of tolerance requirements with experimental testing of opto-mechanical components and of the entire laser system in a climatic test chamber. The performance of the laser is measured over the expected temperature range. It is shown that changing the baseplate temperature by a few Kelvin stops laser emission completely. The optical mounts that are used in the laser and worthwhile alternatives were tested separately in the climatic chamber. The stability of the best mounts exceeds those currently used by a factor of 50. A new laser has been built based on the results of the analysis and further experiments for an optical redesign. This laser was on a field campaign for several weeks and worked reliably.
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