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Abstract
We present design and performance data of an energy-scaled diode-pumped Alexandrite laser in single longitudinal mode operation developed as a beam source in a mobile general-purpose Doppler lidar. A maximum pulse energy in Q-switched operation of 4.6 mJ and a maximum average power of 2.7 W were achieved for a repetition rate range from 500 to 750 Hz with excellent beam quality of ${{\rm M}^2} = {1.1}$. Two rugged and compact demonstrator lasers were built and integrated into mobile lidar systems, where a bandwidth of approximately 3 MHz is measured. Measurements of atmospheric winds and temperatures were conducted during several field campaigns from summer 2022 to spring 2023.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. INTRODUCTION
In light of the growing impact of the anthropogenic climate change, gaining data of wind velocities and temperature distribution in the atmosphere in altitudes above 5 km has seen increasing importance for detailed models of atmospheric physics [1,2]. Lidar systems able to monitor wind velocities and temperatures at these altitudes are receiving increased attention, especially after the huge success of the AEOLUS mission profiling Earth’s wind from space [3]. AEOLUS yields data from altitudes up to 30 km that lead to much higher accuracy in midterm weather forecasts [4,5].
The success of the AEOLUS mission especially intensified the interest in mobile lidar systems to monitor temperature and wind fields in regions formerly unattainable by stationary lidar systems. Several mobile lidar systems for monitoring wind and/or temperature data were developed in recent decades using Mie scattering [3,6,7], Rayleigh scattering [8–10], or resonance scattering [11,12], while some of those also use a combination of several scattering mechanisms (e.g., [3,6,11,12]). In general, Mie scattering with aerosols can be used for altitudes with sufficiently high aerosol density, mostly up to 25 km, with the advantage of high resolution. Rayleigh scattering by air molecules can be used for altitudes without aerosols, e.g., 25–80 km. Finally, resonance scattering by layers with metal atoms in the mesosphere and lower thermosphere (MLT) enables measuring temperature and wind in higher altitudes of 80–120 km, though for a measurement with high accuracy a bandwidth of the laser emitter of below 30 MHz is required [13] and the absolute frequency must be maintained with high accuracy [14]. Monitoring inelastic scattering in the form of Raman lidars is another approach to monitor wind fields (e.g., [15]), though due to the lower cross section for inelastic scattering on molecules, large pulse energies have to be deployed, reducing the suitability of this technique for mobile lidar systems.
The usage of a combination of several scattering mechanisms in a single lidar instrument can be achieved depending on the laser emitter. However, narrowband spectral filtering well below the full-width-half-maximum (FWHM) of Rayleigh scattering is required to separate the Mie and Rayleigh signals [3,16–18], since a mixture of both will lead to higher uncertainties [8].
For the construction of a compact and mobile lidar system with high accuracy, it is highly favorable to use a beam source with a high electro-optical efficiency to reduce energy consumption. A narrow bandwidth ${\lt}{10}\;{\rm MHz}$ is essential to separate the Mie and Rayleigh signals with a high separation ratio, and tunability of the laser wavelength is required to address specific atomic lines [6,19]. In recent years the Leibniz Institute of Atmospheric Physics (IAP) and the Fraunhofer Institute for Laser Technology (ILT) have developed a technology for a compact multi-frequency Doppler lidar instrument within a volume of ${\sim}{1}\;{{\rm m}^3}$ [16,19]. The key component of the system is a diode-pumped Alexandrite laser emitting at the potassium resonance line at 770 nm with a bandwidth of ${\lt}{5}\;{\rm MHz}$ and a pulse energy of up to 1.7 mJ [20]. In the same publication, alternative technologies for achieving these key parameters are discussed.
The combination of narrow laser and filter bandwidth with spectral measurements at many frequencies allows for separating the Mie and Rayleigh signals from the lower atmosphere [17,19]. The Alexandrite laser presented in [20] already achieved all parameters necessary to measure temperature and wind in the mesopause by measuring the Doppler broadening and shift of the potassium resonance in the MLT as well as temperature and wind velocities in the lower atmosphere by measuring Mie and Rayleigh scattering [17,19]. Further scaling of the pulse energy while maintaining the spectral, temporal, and spatial characteristics allows for a shorter integration during the atmospheric measurements and therefore higher temporal or altitude resolution of the lidar system. Additionally, higher pulse energies and therefore higher average power of the laser emitter are necessary for measurements in higher altitudes up to the thermosphere [16,19].
In this publication we present a power-scaled laser beam source that fulfills all the required parameters for a general-purpose lidar shown in [16]. Since the power-scaled laser beam source has to be integrated into the present lidar system with the volume of ${\sim}{1}\;{{\rm m}^3}$, a mobile and rugged prototype is developed with an adapted optical design while the footprint of the laser beam source has to remain unchanged compared to the beam source presented in [20]. Two such prototypes are built and characterized. Special emphasis is set on the spectral properties of the prototypes that are closely monitored during several field campaigns from summer 2022 to spring 2023.
2. OPTICAL DESIGN
Scaling of the output energy can be achieved by either using a higher pump power and therefore adapting the oscillator accordingly or keeping the oscillator unchanged and applying a master oscillator power amplifier (MOPA) scheme. In the latter case special attention is required to keep the beam quality of the output beam close to ${{\rm M}^2} = {1}$, which is paramount for the lidar applications envisioned and challenging using standard side-pumped configurations [21]. A possible solution is the well-known and proven Innoslab concept developed at ILT [22] that keeps the spatial properties of the amplified beam unchanged even for higher pump powers. However, the low optical gain of Alexandrite makes the use of Alexandrite amplifiers challenging [23,24].
As the pump energy of the pump diodes presented in our previous publication [20] is rather moderate, the approach of higher pump energy used for pumping a modified oscillator is only worth investigation for energy-scaling and is shown in this publication. In the case of diode-pumped Alexandrite lasers in Q-switched operation, simply raising the pump energy or pump power to achieve higher pulse energies resulted in a drop of the beam quality in several cases [23,25,26]. For this reason, a careful redesign of the resonator has to be conducted, since achieving excellent beam quality of ${{\rm M}^2} \lt {1.5}$ is crucial for two main aspects of the designated lidar system, namely reaching a narrow bandwidth and using a small field of view (FOV) in the atmosphere.
In this publication, a new fiber-coupled pump unit with up to 375 W peak power, which is 2.5 times higher than the former pump unit, is used. Since the additional pump energy must be deployed and extracted without changing the spatial, spectral, and temporal properties of the Alexandrite laser, the pump energy density inside the crystal is kept approximately unchanged in comparison to the value in [20]. The radius of the pump radiation inside the Alexandrite crystal is 400 µm with a measured beam quality value of ${{\rm M}^2} = {430}$. Taking into account the resulting effective pump radius (integrated mean of the pump beam radius weighed with the absorption in the crystal) of 516 µm, the pump energy density inside the crystal remains almost unchanged in comparison to the value in [20]. For a detailed description of the concept of the effective pump radius, see, e.g., [27].
Because of the different pump waist radius, a new resonator design must be developed to adapt the resonator mode size inside the crystal to the new pump waist size while still maintaining the same footprint of the complete laser system. As stated in our previous publications [20,28–30], because of the birefringence of the Alexandrite crystal, a resonator in ring geometry must be designed to achieve single longitudinal mode (SLM) operation without spatial hole burning. Special emphasis must be put on the beam size of the laser mode inside the laser medium as well as on the optical elements. While inside the crystal a good compromise between optimal overlap of pump and laser radiation for TEM00 operation as well as high intensity for increased gain has to be found [27], the beam width on the sensitive optical elements, e.g., Q-switch and Faraday rotator, should be as large as possible to avoid optical damage. At the current operation point no optical damage was observed, though the exact values for the laser induced damage threshold are yet unknown.
The newly designed resonator, shown in Fig. 1, contains one Alexandrite crystal that is end-pumped by the new fiber-coupled pump light. As the absorption of the pump light is strongly polarization-dependent and the pump light from the fiber is unpolarized, a pump light back-folding unit similar to the one presented in [20,28] is used. A symmetric ring resonator with a total length of 2 m is designed with two pairs of curved mirrors with radii of curvature of 1500 and 1200 mm each. A schematic drawing of the resonator is shown in Fig. 1.
Fig. 1. Schematic setup of the ring cavity with numbered cavity elements: optical fiber guiding the pump light (1), pump light collimation and focusing lenses (2), pump light back-folding unit (3), Alexandrite crystal (4), flat, dichroitic pumping mirrors (5), flat folding mirrors (6), curved mirrors ${\rm ROC} = {1500}\;{\rm mm}$ (7), curved mirrors ${\rm ROC} = {1200}\;{\rm mm}$ (8), flat folding mirror on piezo actor for stabilization of the cavity length (9), flat output coupler (10), Faraday rotator (11), half-waveplates (12), Q-switch (13), and thin-film polarizer (14).
Fig. 2. (a) Pulse energy versus pump energy with included beam profile at maximum pulse energy. (b) Energy stability at maximum pump energy in Q-switched operation at 500 Hz.
The radius of the laser mode is approximately 320 µm inside the Alexandrite crystal and 600 µm on the internal optical components such as the Faraday rotator or the Q-switch. The resonator stability range allows for a change of the repetition rate up to 750 Hz with slightly shorter pump durations without changing the optical design.
By means of a Faraday rotator in combination with waveplates and polarizers, unidirectional laser emission is guaranteed. By seeding the resonator with a narrow bandwidth (∼several 100 kHz) cw diode laser and actively controlling the cavity length for each laser pulse, SLM operation is achieved [31]. Therefore, one of the resonator mirrors is mounted on a piezo module.
3. RESULTS OF LABORATORY EXPERIMENTS
The pump light has a wavelength of 636 nm and is delivered by a fiber with 800 µm core diameter and a numerical aperture of 0.22. The pump light is collimated with a lens with a focal length of 75 mm and refocused into the crystal with a similar lens. A maximum peak power of 375 W is measured for a pump current of 23 A and a repetition rate of 500 Hz that can be enhanced to 750 Hz without changes of the pump energy or spatial parameters. A detailed description of the optical properties of the pump source is shown in Section 5 in [19].
A. Energy-Scaled Oscillator
The first experimental setup of the ring resonator in the laboratory yields an output pulse energy of up to 4.6 mJ at the potassium resonance line at 770 nm with a stigmatic Gaussian beam profile, as already shown in Section 5 in [19]. The laser pulse energy versus pump pulse energy with implemented beam profile at maximum pump energy is shown in Fig. 2(a), with the temperature of the Alexandrite crystal stabilized at 50°C. Because of the susceptibility of Alexandrite to different parasitic effects [24,32], parameters such as the crystal temperature, or the alignment of the resonator, must be optimized for each point of operation of the Alexandrite laser. Since such an optimization was only conducted for the highest pump energy, areas without laser operation are observable in the curve shown in Fig. 2(a).
Figure 2(b) shows the pulse-to-pulse energy stability of the output beam at maximum pump energy with an unenclosed laser yielding 4.6 mJ with a standard deviation of 0.6% at a repetition rate of 500 Hz resulting in an average power of 2.3 W. This pulse energy is, to the best of our knowledge, the highest pulse energy achieved with a diode-pumped Alexandrite laser. The oscillator is operated in Q-switched operation with a pump energy of 40.8 mJ and a pump duration of 107 µs resulting in an optical–optical efficiency of 11.3%.
A caustic of the output beam is shown in Fig. 3(a) and yields a beam quality of ${{\rm M}^2} = {1.1}$ in both spatial directions, proving that the efforts to sustain transversal ground mode were successful. The pulse duration is approximately 980 ns. The temporal shape is shown in Fig. 3(b). All values were simultaneously measured at the maximum pulse energy of 4.6 mJ.
Fig. 3. (a) Caustic and (b) temporal shape of the Alexandrite laser in Q-switched operation at a repetition rate of 500 Hz and a pulse energy of 4.6 mJ. (c) Findlay Clay analysis of the resonator for four different output coupler reflectivities of ${\rm R} = {0.992}$, ${\rm R} = {0.97}$, ${\rm R} = {0.963}$, and ${\rm R} = {0.95}$ (adjusted for real reflectivities according to the measurement curve given by the manufacturer). The calculated optical losses for the resonator are $1.1\% \pm 0.4\%$.
Special emphasis was given to high reflectivities and transmittance of the customized optical components used in the resonator. Intracavity optical losses of ${1.1}\% \pm {0.4}\%$ are measured by conducting a Findlay–Clay analysis [Fig. 3(c)]. The comparison to the losses of 1.6% in our previous publication [20] shows the progress of the quality of the optical components (five transmissive and 12 reflective in the resonator) used.
Applying an advanced ramp-and-fire (ARF) method to control the cavity length of the resonator while seeding with a narrowband seed laser allows for achieving SLM operation. ARF allows fast and precise frequency tuning from pulse to pulse about a range of several gigahertz as required for a multi-frequency lidar and is a further development of the ramp-and-fire method presented in [31]. Since the same cavity control technique is used for the laboratory laser as in the prototype lasers (see Section 4.B), it can be concluded that the spectral properties are similar, though no measurement of the actual bandwidth is conducted for the laboratory laser because the spectral filters necessary to measure such narrow bandwidth are only available in the lidar system (see Section 4.B).
B. Raising the Repetition Rate of the Oscillator
The backscattered signal strength is directly proportional to the average power of the lidar emitter. The required pulse energy depends on the background from the sky, which can be lowered by the FOV and narrow bandwidth spectral filters in the receiver that are matched to the bandwidth of the laser and suppress solar background. In our case, the combination of a very small FOV (22 µrad) and narrow filtering (${\sim}{\rm MHz}$) allows for measurements without solar background. Therefore, raising the average power by raising the repetition rate is a further way for power scaling that additionally does not raise the fluence on the critical optical components. The maximal repetition rate is limited by the designated altitude of the atmospheric measurement since the subsequent laser pulse would interfere with the backscattered signal. For the envisaged maximal altitude of 150 km, the repetition rate must not exceed 1 kHz.
For the repetition rate scaling experiments, no changes of the resonator design are made, although the pump duration was slightly lowered for higher repetition rates down to a pump duration of 79 µs at 750 Hz while optimizing the crystal temperature for each working point. The resulting average power for repetition rates from 500 to 750 Hz is shown in Fig. 4. As can be seen in the figure, higher repetition rates lead to higher average power up to a maximum of 2.7 W, which is the highest average power achieved with a diode-pumped Alexandrite laser in Q-switched operation. For all repetition rates the measured beam quality is ${{\rm M}^2} \le {1.1}$. Due to the lower pump duration, the pulse energies are slightly lower for higher repetition rates, although the optical–optical efficiency could be maintained and even risen in comparison to the original working point at 500 Hz (11.3% at 500 Hz compared to 12.0% at 750 Hz).
Fig. 4. Pulse energy and average output power of the Alexandrite laser for different repetition rates from 500 to 750 Hz in Q-switched operation.
4. PROTOTYPE DEVELOPMENT AND INTEGRATION IN A MOBILE LIDAR SYSTEM
Over the last four years, we built four laser prototypes that were integrated into lidar systems, the first two of which were already presented in [20]. Based on the optical design of the power-scaled Alexandrite laser in the laboratory, two further rugged prototypes for remote long-term operation with a general-purpose compact Doppler lidar within a volume of ${\sim}{1}\;{{\rm m}^3}$ [16] are developed. The beam parameters of these prototypes (prototypes 3 and 4) are measured after integration of the prototypes into the lidar system with special emphasis on the spectral properties of the beam sources. Additionally, the prototypes are used to further develop the lidar system and to conduct several atmospheric measurements during several field campaigns starting in summer 2022 including more than 1000 h of laser operation.
Fig. 5. Laser prototype without cover. The beam line is visualized with the solid red line, while the seed radiation is marked with dashed lines and the pump beam with large arrows.
Fig. 6. (a) Long time measurement of the output energy for prototype 4 at 750 Hz. The measurement is taken with closed cover, which explains the lower pulse-to-pulse deviation compared to the measurement shown in Fig. 2(b). (b) Caustic of prototype 4 at maximum pump energy and a repetition rate of 750 Hz. A beam quality of ${{\rm M}^2} = {1.1/1.1}$ is derived from the caustic. The embedded beam profiles in the focus and far field are not scaled equally.
A. Optical and Mechanical Design
The footprint of the laser baseplate is identical to the one presented in [20]. The laser beam source with a closed but not air-tight housing has dimensions of ${375}\;{\rm mm} \times {675}\;{\rm mm} \times {175}\;{\rm mm}$. Since the laser must be operated remotely and in harsh environmental conditions that can lead to misalignment of the resonator due to vibrations or shock, one of the curved mirrors is mounted in a piezo mirror mount that allows for remote alignment.
All the optical mounts used in the resonator are commercial off-the-shelf components to reduce the complexity and costs of the prototypes. The mechanical design is identical for the two prototypes built. A photograph of one of the laser prototypes without cover is depicted in Fig. 5.
B. Performance of the Rugged Prototype inside the Lidar System
To lower the fluences on the resonator optics and therefore ensure long time operation, both prototypes are operated with reduced pump parameters leading to reduced pulse energies in comparison to the 4.6 mJ measured with the laboratory system. All presented output parameters are measured in Q-switched SLM operation after the integration in the lidar systems.
Prototype 3 is operated with a pulse duration of 87 µs and a pump energy of 34.6 mJ. A pulse energy of 3 mJ at a repetition rate of 500 Hz with a beam quality of ${\rm M}_{x/y}^2 = {1.1/1.0}$ and an optical-to-optical efficiency of 8.7% is measured. Due to the enclosing of the resonator a pulse-to-pulse stability of the pulse energy of 0.37% is achieved. To make up for the reduced pulse energy, prototype 4 is operated at a repetition rate of 750 Hz with a pump pulse duration of 85 µs and a pump energy of 32.85 mJ. A pulse energy of 3.2 mJ is measured with a pulse-to-pulse stability of the pulse energy of 0.2% [see Fig. 6(a)]; therefore an increase in average power of more than 50% is achieved compared to prototype 3. Considering the pump energy this leads to an optical-to-optical efficiency of 9.75%. A measured caustic at maximum pulse energy is shown in Fig. 6(b) and yields a beam quality of ${\rm M}_{x/y}^2 = {1.1/1.1}$.
A narrow bandwidth and the knowledge of the exact spectral shape are essential for measuring the potassium resonance line and therefore the temperature and wind in the MLT with high accuracy. Changes of the spectral shape from pulse to pulse can induce a systematic error in the temperature and wind measurement. Additionally, the spectral shape and bandwidth are crucial for separating the Mie and Rayleigh signals and therefore for the wind measurement in the lower atmosphere. Therefore, the spectral properties of each laser pulse are directly measured by spectral filtering of the outgoing laser pulses with the spectral filters integrated in the lidar system for analyzing the backscattered signals. Here, the combination of a broadband solid etalon with an FWHM close to the Doppler width of the atmospheric molecular line (${\sim} {1}\;{\rm GHz}$) and a narrowband confocal etalon (FWHM 7.8 MHz) used in the lidar system [16,17] are used to determine the bandwidth of the laser pulse of 3 MHz. Only if the spectral shape of each outgoing laser pulse is known are the atmospheric measurements shown in Fig. 7 possible. A detailed description of the measurement technique used to determine the bandwidth for the outgoing laser pulses is given in [20].
Fig. 7. (a) Daytime measurement of aerosol scattering. The wind velocity can be calculated by taking into account the spectral shift of the backscattered signal (see, e.g., [16] or [20]). (b) Separation of Mie and Rayleigh scattering by selecting frequencies close to the Mie peak [yellow shade in (c)] and wings [green shade in (c)]. (c) Example of the spectral width measured by scanning a confocal etalon with known spectral properties. The measured bandwidth is 3 MHz.
The laser parameters were verified during several field campaigns in Kühlungsborn, Germany, from summer 2022 to spring 2023. Detailed information about the results of the atmospheric measurements during the field campaign can be found in [33].
Figure 7 shows an exemplary Doppler Mie measurement in the vertical direction on a clear day (12:00 to 15:00 UT) without any clouds. For all data obtained during our campaigns Doppler Mie can be separated from Rayleigh scattering due to the unique combination of the narrow bandwidth of the laser and the narrowband filter. Even though this measurement has been taken under a selected condition with exceptional clear sky conditions, Mie scattering is clearly visible at all altitudes with a signal strength well above the Rayleigh background.
Figure 7(a) shows the measured spectrum of the backscattered light at each altitude. The altitude resolution is 50 m, and 200 frequencies with a binning of 500 kHz were used to determine the spectrum. The remaining Rayleigh scattering is visible in blue colors at the wings. Figure 7(c) shows the integrated signal from 19 to 21 km altitude as an example for a stratospheric Doppler Mie measurement. As a demonstration we show in Fig. 7(b) various signals by integrating frequencies around the Mie peak and the wings with the same number of frequency channels and therefore identical signal strength. With this straightforward approach it is obvious that at all altitudes below about 20 km the signal from Mie scattering is 2 times larger compared to the Rayleigh signal and can be observed up to an altitude of ${\sim}{27}\;{\rm km}$, even though not visible in the spectrum in Fig. 7(a) if the signal is divided into 200 frequency channels. The observed line shape is close to a Voigt profile, which will be discussed in more detail in a future publication. The slight shift with the altitude in Fig. 7(a) correspond to a change in the vertical wind from the troposphere to the stratosphere.
Fig. 8. 48-h measurement of (a) frequency of the backscattered Mie-signal. The white lines represent shutdowns of the lidar systems because of air traffic. (b) Frequency shift of the mean frequency during the measurement.
Fig. 9. Measurement of the temporal variation of the potassium layer during nighttime for a 7-h timeframe. The signal strength has a logarithmic scale showing the large difference in Rayleigh signal to resonance signal strength.
The measured spectrum is a convolution of the laser line shape, the confocal etalon, and spectral broadening occurring during scattering (for ${\rm Mie} \ll {1}\;{\rm MHz}$), with a FWHM of 11.47 MHz (Voigt Fit). Considering the bandwidth of the etalon of 7.8 MHz and assuming a Lorentzian line shape for the laser, the resulting bandwidth of the laser pulse is approx. ${\sim}{3}\;{\rm MHz}$. The bandwidth is limited by the spectral chirp of the laser pulse induced by the change of the resonator length during the 1000 ns long laser pulse. First laboratory experiments to reduce the chirp and therefore further lower the bandwidth show very promising results that will be published in a different publication. The bandwidth during many months during our field campaigns remained unchanged.
We note that the time dependent frequency shift between pulsed laser and seeder laser is measured during atmospheric measurements and considered in the data analysis. The seeder laser is frequency stabilized by Doppler-free saturation spectroscopy in a potassium gas cell, and therefore the frequency shift between pulsed laser and seeder laser can be directly measured. Figure 8 shows a 48-hour measurement in the vertical direction at a fixed altitude of 7.5 km with the observed Mie spectrum at each time step and the measured frequency shift below ${\sim}{3}\;{\rm MHz}$, proving the high frequency stability of the laser even during long time operation even under challenging environmental conditions.
Figures 7 and 8 show the Mie channel of the lidar instrument. In Fig. 9 an example of the Rayleigh channel that also includes the resonance scattering and thereby showing the temporal variation of the potassium layer during nighttime is shown. Rayleigh scattering is measured at altitudes below 65 km, and resonance scattering on the atoms of the potassium layer occurs between 85 and 100 km. The atmospheric measurements conducted with the lidar system during the winter 2022 campaign are presented in much more detail in [33].
The measured output parameters for the prototypes and the laboratory setups presented in this publication as well as in our previous publications [20,29] are summarized in Table 1.
Table 1. Performance of Different Alexandrite Lasers
5. SUMMARY AND OUTLOOK
In this work we present an energy-scaled diode-pumped Alexandrite laser in Q-switched SLM operation at 770 nm designed as an emitter in a general-purpose lidar for measuring wind velocities and temperature distributions from ground to 100 km altitude. Energy-scaling is achieved by using a new pump beam source with a peak power of 375 W, therefore scaling the peak pump power by a factor of 2.5 in comparison to our previous setup. A maximum pulse energy of 4.6 mJ with unchanged excellent beam quality of ${{\rm M}^2} = {1.1}$ is achieved for a repetition rate of 500 Hz. The repetition rate is scaled up to 750 Hz without changing the resonator, although the pump duration has to be lowered resulting in a slightly lower pulse energy of 3.6 mJ at 750 Hz.
These first results of energy-scaling of the oscillator and the availability of a potentially suitable amplifier technology show that a diode-pumped Alexandrite laser might fulfill the energetic specifications while conserving the temporal, spatial, and spectral properties for further lidar applications with higher measurement altitudes or a future spaceborne mission as a successor of AEOLUS.
After the successful experiments in the laboratory, two rugged mobile prototypes were constructed based on the optical design of the energy-scaled oscillator and implemented in two mobile lidar systems with a volume of ${\sim}{1}\;{{\rm m}^3}$. To enhance the lifetime of the laser beam sources, the pump parameters are lowered resulting in a pulse energy of 3 mJ at 500 Hz for the first prototype and 3.2 mJ at a repetition rate of 750 Hz for the second prototype. Several successful measurement campaigns from summer 2022 to spring 2023 were conducted. To enhance the capabilities for monitoring dynamical phenomena in the atmosphere, the lidar can be reconfigured with up to five telescopes to get a lidar with several fields of view. First experiments with such a lidar system were successful, and the results are shown in [33].
Since addressing the iron resonance line at 386 nm benefits from a deeper Fraunhofer line with less solar background, higher backscattering signal due to the strong wavelength dependence of the Rayleigh scattering signal strength, and a symmetric atomic line shape [19], a frequency conversion of the laser light into the UV spectrum is the next step in the laser development.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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