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Abstract
We present design and performance data of two diode-pumped Alexandrite lasers developed explicitly as laser emitters in mobile potassium resonance lidar systems. The lasers yield an output power of up to 1.75 mJ at a repetition rate of 500 Hz with a beam quality of $M{{^2}} \lt {1.1}$ in both spatial directions. Reliable single longitudinal mode operation with a unrivaled narrow linewidth of 3.3 MHz at a potassium resonance line at 769.898 nm is achieved. The wavelength can be switched from pulse to pulse in a range of several gigahertz so the potassium line can be scanned. The lasers are finally integrated in highly efficient lidar systems with a power consumption of 500 W for the whole lidar system. The extremely high spectral requirements are investigated and the performance for different working points regarding repetition rates and pump durations is investigated. Several weeks of remotely controlled operation of the prototype in a field campaign were conducted without changes of the output parameters. Approximately 1000 h of reliable single longitudinal mode operation was achieved during the campaign and measurements of Doppler–Mie wind observations in the stratosphere and of the potassium layer in the mesopause were conducted simultaneously even at daytime.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. INTRODUCTION
The importance of understanding and monitoring atmospheric physics has increased dramatically in recent years due to the impact of the anthropogenic climate change. One crucial research field is the understanding of wind fields and temperature distributions in the atmosphere [1]. The mesosphere and lower thermosphere (MLT) at altitudes between 80 and 110 km are of special interest because temperature data in this region is scarce and modeling of temperature developments show discrepancies to the available data [1,2]. Additionally, breaking and therefore energy distribution of atmospheric gravity waves occurs at this altitude [3,4], being the driving effect of short-term changes in temperature with feedback on the lower atmosphere on global scale.
One approach to provide temperature data is to measure the Doppler-broadened and -shifted resonance lines of metal atoms present at this altitude, e.g., potassium (770 nm or 766 nm) [5–10], iron (386 nm or 372 nm) [11–14], and sodium (589 nm) [15–20], by means of a Doppler resonance lidar which requires a narrow bandwidth of the laser emission of the order of several megahertz (MHz) to resolve the shape of the resonance line [21,22]. A different approach to derive temperature data in the mesosphere is a Boltzmann lidar [23,24], which obtains the temperature by measuring the temperature-dependent population density of specific electronic states in the potassium atom. An advantage of this technique is that a linewidth of the laser emitter of several hundred MHz is sufficient. Therefore, in contrast to Doppler resonance lidar, no single longitudinal mode (SLM) operation of the laser beam source is necessary, which simplifies the laser technology. Nevertheless, both lasers have to be spectrally stabilized and aligned to the same field of view. However, two different laser beam sources, telescopes and lidar systems, are required to address the different electronic transitions which makes the instrument twice as complex and error-prone as well as enlarges the space and energy consumption, which is obstructive when operating the lidar system at remote locations. Additionally, the information on the wind field is not available with this technology.
As the effects of gravity waves are especially distinctive at polar regions, lidar systems have to be operated at remote locations like Arctic or Antarctica. Therefore, efficient and maintenance-free lidar systems that can be remotely controlled are favorable for long-term field campaigns. At present, only the IAP has operated a mobile Doppler resonance lidar providing Doppler measurements in the MLT for two decades at various places around the world as a Doppler potassium and later on as a Doppler iron lidar [5–8,25–27]. The laser emitter in the lidar is a flashlamp-pumped Alexandrite ring laser with a very narrow bandwidth of 30 MHz but low electro-optical efficiency and therefore high cooling effort.
Diode-pumped, $Q$-switched Alexandrite ring lasers in SLM operation are an ideal fit as a laser emitter used in resonance lidar systems since they enable comparable high electro-optical efficiency and excellent beam quality. The all-solid-state laser system in combination with a long lifetime of the pump diodes allows for maintenance-free operation. Because of the low emission cross section, the generated $Q$-switch pulses exhibit pulse durations of several hundred nanoseconds, allowing for a narrow linewidth in the regime of several MHz, which is crucial for analyzing resonance lines. The wavelength spectrum of Alexandrite between 700 and 800 nm [or 350 and 400 nm with second harmonic generation (SHG)] covers several atomic lines interesting for lidar applications. The relevant wavelengths for a ground-based resonance lidar for the MLT are the potassium resonance line at 770 nm and the iron resonance lines at 372 and 386 nm, with each atomic line being located within a Fraunhofer line, so that the solar background is significantly reduced if narrowband spectral filtering is applied.
The wavelength range can also be reached with other laser concepts, though each of them has several disadvantages in comparison to diode-pumped Alexandrite lasers.
Dye lasers were already deployed as a beam source in a potassium lidar [28], but because of the short pulses generated with dye lasers a narrow linewidth in the range of ${\sim}{{10}}\;{\rm{MHz}}$ is not achievable and the high maintenance effort when operating dye lasers does not allow for maintenance-free operation.
Ti:sapphire lasers, as all-solid-state lasers, can easily reach 770 nm [29] but are not yet used as emitters in resonance lidars with the requirement of very narrow bandwidth. Additionally, because of the short lifetime of the upper laser level diode-pumping of Ti:sapphire lasers is not feasible. Therefore, dye lasers and Ti:sapphire lasers are pumped with frequency-doubled Nd:YAG lasers, limiting the efficiency and raising the complexity.
Another approach to reach the envisioned resonance line is to use Nd:YAG lasers and different nonlinear processes, which is already pursued to address the resonance line of sodium at 589 nm for resonance lidars [19,20]. To address the resonance wavelength for potassium, a frequency-doubled Nd:YAG could be used to pump an optical parametric oscillator (OPO) emitting at 770 nm, which also has not yet been realized for a resonance lidar. This concept also suffers from low efficiency and high complexity.
Recently conducted investigations in the field of diode-pumped Alexandrite lasers for lidar applications show promising results. Diode-pumped Alexandrite lasers with focus on resonance lidar [30–32] and altimetry lidar for vegetation monitoring (red edge) [33,34] were developed, although the requirements and hence the resonator concepts and laser properties for these applications differ strongly. Power scaling of the beam source can be conducted by usage of a pump diode module with a larger pump energy [35] or by deploying one or several amplifier stages. A first attempt for an amplifier based on diode-pumped Alexandrite is reported in [36], showing the possibility for an energy-scaling, though still suffering from significant loss of beam quality. Yet another attempt is the usage of an amplifier based on LED-pumping with a Ce:YAG slab as wavelength converter and beam guidance [37].
A first demonstration of the feasibility of a diode-pumped Alexandrite laser that fulfils all spectral requirements for a beam source in a potassium Doppler resonance lidar system was presented in [30], though the laser beam source and the lidar technology were still preliminary systems. The lidar instrument was mobile but still large enough to ensure the integration of the laser and lidar setup that was built for laboratory use. But mainly as a proof of principle it did not allow for automated measurements and the low average power of the beam source required a long integration time for atmospheric measurements. Further development to a more stable and balanced design was carried out and led to a more sophisticated diode-pumped Alexandrite laser [31,32] that was operated in a laboratory environment. Now, the focus of the laser development presented in this publication is the transition of the laboratory systems presented in [31,32] to a mobile and yet robust breadboard system while maintaining all the laser parameters relevant for Doppler resonance measurements.
This transition allows for usage of the laser beam source in an innovative mobile resonance lidar system with dimensions of about ${1}\;{\rm{m}^3}$ [38] that can be operated remotely and maintenance-free in continuous long-term measurement campaigns. High stability against misalignment is a key requirement for a laser beam source that has to be operated under rough environmental conditions. Optimal stability can be achieved, e.g., by implementing soldered mirror mounts and optical components. However, that kind of soldering technology is very elaborate, complex, and therefore expensive and was initially developed for spaceborne lasers [39]. Thus, in our approach for a robust yet cost-efficient laser beam source, the resonator was designed and experimentally validated for high misalignment stability [32]. Therefore, in combination with a careful selection of materials and basic measures for the mechanical setup, commercially available mounts, that have been qualified in [32], are sufficient.
The enhanced efficiency of the diode-pumped Alexandrite laser compared to the flashlamp-pumped lasers, formerly used in the existing mobile potassium and iron resonance lidar systems [6,12], allows for the development of maintenance-free, compact, and therefore easier to transport lidar systems. A first design proposition that was given in [40] is realized, with the laser described in this publication being integrated as the laser emitter of the lidar system. Many components of the lidar system are 3D-printed to lower costs and speed up building time. Thus, first field campaigns should not only prove the robustness of the laser beam source itself but also of the lidar technology required for unattended Doppler measurements.
In this publication we present the design of the lidar system as a whole and focus on the optical and mechanical design of the pump and resonator unit of the Alexandrite laser within this compact space. Subsequently, the experimental results of the pump and resonator unit in the laboratory as well as inside the lidar system are presented. A focus is set on the spectral properties of the laser most relevant for the usage as an emitter in Doppler resonance lidar and their measurement and optimization by means of a new cavity control technology. Furthermore, the ideal working point regarding repetition rates and pump durations and the losses of the resonator components are investigated.
Fig. 1. CAD model of the mobile lidar system. All electronics necessary for controlling the laser and the signal analysis as well as the cooling interface are included.
Finally, we give insight into the performance of the lidar system during several field campaigns conducted at the IAP in Kühlungsborn (54°07′N, 11°46′O) between November 2019 and April 2020 and show exemplary results of atmospheric measurements such as Doppler–Mie wind observations in the stratosphere and simultaneously the potassium layer in the mesopause even at daytime.
2. OPTICAL DESIGN
The main aim of the laser development is a beam source for a compact (dimensions ${\sim}{{1}}\;{\rm{m}} \times {{1}}\;{\rm{m}} \times {{1}}\;{\rm{m}}$), mobile, and autonomous lidar system that can be operated remotely and without maintenance. Several such lidar systems can be operated as a Doppler lidar network which would enable overlapping measurement ranges and therefore the ability to monitor dynamic processes such as the propagation of gravity waves through several atmospheric layers on a large scale [38]. The lidar system is well insulated and shock absorbing to provide a vibration-free environment with high temperature stability. The lidar system is equipped with an automated roof hatch and can be controlled remotely via the Internet while attached to a standard 230 V wall-plug. Automatic operation is possible if laser safety is ensured by means of a radar or other devices. A weather station monitoring the environment and measuring cloud heights and an additional camera allow automatic operation. Starting from stand-by-mode, measurements are obtained within 1 min. Shutting down the pulsed laser due to external reasons such as laser safety takes only from one pulse to the next without negative influence on the laser.
The available space around the telescope tube containing the 50 cm in diameter main mirror is partitioned into three vertical sections. The lower section contains the pump diode driver, seeder driver, and controller and is equipped with an uninterruptible power supply, which is important for safe laser operation and shutdown. Additionally, the electronic of the integrated cooling and the PC for instrumentation control and signal analysis are located in the lower section. The middle section contains the pump unit of the laser emitter and the optics and electronics necessary for analyzing the backscattered signal. The upper section contains the resonator unit of the laser emitter as well as the beam path for beam shaping and monitoring of the output beam. The middle and upper sections are temperature-controlled by a constant airflow with high temperature stability so that no misalignment occurs when the cube is operated at varying ambient temperatures. Additionally, the air in the upper section is filtered to reduce containment of the optics by dust particles.
Seeing within the telescope has a large impact on the signal strength at daytime due to heating by the Sun. To compensate for that, the integrated telescope is actively cooled to ambient temperature. The lowest section of the lidar system is also temperature stabilized but because of the heat produced by the PC and other equipment the temperature stability in this section is lower compared to the middle and especially the upper section. At polar regions temperature stabilization of the lidar is achieved by an integrated heat-exchange system whereas operation at middle latitudes or tropical regions requires an external chiller supporting the temperature stabilization to approximately 22°C within the system.
A large fraction of the structural parts of the lidar system is 3D-printed with a carefully chosen combination of standard, engineering, and high-performance thermoplastics to lower the costs and weight of the lidar system. Details on the design and manufacturing of the lidar system as well as on the development of a Doppler lidar network consisting of several lidar systems as presented here can be found in [38]. A CAD model of the lidar system is shown in Fig. 1.
The laser emitters consist of a fiber-coupled pump unit and a resonator unit. The units are built on two separate customized honeycomb breadboards. The fiber-coupling of the pump light allows for a spatial decoupling of both the pump and resonator unit, so they can be independently placed within the limited space of the lidar system (see Fig. 1) and exchanged in case of failure of one of the modules. Additionally, the homogenization of the pump profile yields higher optical–optical energy efficiency of the resonator that compensates the losses of the fiber-coupling. A detailed analysis of the benefits of fiber-coupling for the Alexandrite laser is shown in [41].
A. Pump Unit
The pump unit contains two commercially available water-cooled laser diode modules emitting at 636 nm and tailored beam shaping optics for fiber-coupling. Because of the low beam quality of the diode modules in slow-axis (for a detailed analysis of the diode modules see [30]) beam shaping and symmetrization of the beam quality is necessary for a sufficient fiber-coupling efficiency. The optical design of the pump unit is similar to the one presented by our group in [41].
The pump light of each module is linearly polarized, and the polarizations are oriented perpendicular to each other by means of half-wave plates. The beam of each module is magnified by a factor of 3 in slow-axis with a cylindrical telescope and polarization-coupled with a polarizing beam combiner cube. Afterwards, the combined beams are symmetrized in their beam qualities in both directions by means of a step mirror that is customized for rearranging the beam distribution. The resulting beam is magnified by a factor of 3 in fast-axis before being focused into an optical fiber with a fiber diameter of 400 µm, a numerical aperture of 0.22, and uncoated facets. A schematic drawing of the pump unit is shown in Fig. 2.
To increase the robustness against misalignment and reduce contamination by dust particles, the beam line is enclosed, and the optics are placed in specially designed, integrated mechanical mounts. The receptable for the high-power sub-miniature-A (SMA) connector is chosen to guarantee a high reproducibility for fiber connections without loss of coupling efficiency. Also, the water and power connectors ensure a fast and easy replacement of the pump unit within the lidar cube. A photo of the pump unit with the labeled subunits (outer cover removed) is shown in Fig. 3.
B. Resonator Unit
The optical design of the ring resonator is similar to the design presented in [31,32,41], although the folding of the resonator was altered so the footprint matches the space requirements inside the lidar system.
The resonator has a total length of approximately 2000 mm and comprises two plane dichroic pumping mirrors with high transmission for the pump wavelength, several plane folding mirrors, two concave mirrors (with a radius of curvature of ${-}{{900}}\;{\rm{mm}}$) with high reflectivity for the laser wavelength, and a plane output coupler with 3% transmission for the laser wavelength. Active-low $Q$-switching is done by a setup of a Brewster-angled Pockels cell (${\rm KD}^*{\rm P}$), two thin-film polarizers (TFPs), and two half-wave plates for polarization adjustment. A Faraday rotator in combination with the TFPs ensures unidirectional operation of the ring resonator.
The Alexandrite crystal is cut in c-direction and has a rectangular shape with an aperture of ${2}^*{2}$ ${\rm{mm}}^2$ and length of 7 mm (dopant concentration 0.2 at. %). The facets of the Alexandrite crystal are coated with an anti-reflection coating for 638 nm and for 760–780 nm. The crystal is mounted in a copper heat sink that can be temperature controlled by a Peltier element that is mounted underneath the heat sink. Indium foil is placed between the heat sink and the crystal for an optimized thermal contact.
Fig. 4. Schematic drawing of the resonator design. The optical setup is similar to the one presented in [31,32,41], although the folding of the resonator was changed.
Behind the incoming fiber from the pump unit, the pump light is collimated and refocused into the laser crystal by two achromatic lenses with a focal length of $f = {{75}}\;{\rm{mm}}$. To absorb all of the unpolarized pump light inside the Alexandrite crystal, the part of the pump light that is transmitted through the crystal is back-folded and absorbed with the second pass through the crystal with transmission losses of about 5%.
Taking into account the thermal lens induced by the pump light, the resonator is designed for a laser beam radius of approximately 230 µm in the crystal which results in a good overlap with the pump beam (see Section 3.A). The beam radius on the critical optical components such as the Pockels cell or the Faraday rotator is approximately 700 µm, preventing laser-induced damage. The intracavity components were specially selected for low losses considering the low emission cross section of Alexandrite. A schematic drawing of the resonator is shown in Fig. 4.
The resonator is seeded with a commercial fiber-coupled external-cavity diode laser with a linewidth of approximately 100 kHz that operates at the desired wavelength in cw operation. The diode laser light is collimated with a fiber-collimator and part of it is used for a reference port for further lidar subsystems such as Doppler-free spectroscopy. The other part of the beam is shaped by a spherical telescope to match the resonator mode in its size and divergence and coupled into the resonator through the output coupler. The seed laser power at the output mirror is approximately 1 mW, which is sufficient for reliable SLM operation. One of the plane folding mirrors is mounted on a linear piezo actor so the resonator length can be adjusted to be resonant for the seed wavelength.
The utilized novel cavity control technology to stabilize the cavity with a single laser pulse and with the additional capability to tune the laser from pulse to pulse is an advancement of the ramp-delay-fire technology presented in [42]. Frequency jitter and shift are caused by variations of the resonator length at the time of the pulse emission. By applying a nonlinear driving voltage to the piezo actor, the resonator length is also tuned nonlinear, and the exact length is known at any time. This is even of greater importance for a change of the wavelength with every pulse as necessary in a scanning lidar [12]. Also, the pulse-buildup time after the trigger of the $Q$-switch has to be taken into account. Therefore, after each pulse the optical feedback of the laser pulse is analyzed and the piezo voltage curve for the next pulse is altered accordingly by the cavity control electronic. This way, the linewidth and the frequency jitter of the laser pulse are minimized, as also described in [38]. The mean shift between the pulsed laser and seeder laser is also controlled by this technology, allowing zero frequency shift between the pulsed laser and seeder on average, as shown in Section 4. The frequency stability of the pulsed laser from pulse to pulse (jitter) at the target frequency is finally given by the stability of the seeder laser, which is of the order of a few hundred kilohertz, depending on external disturbances of the seeder laser under field conditions.
By triggering the Pockels cell accordingly, SLM operation at the given wavelength is guaranteed for every single laser pulse. The piezo actor module is chosen for its high linearity, minimal vibrations, and its tilting during the travel that is small compared to the laser tolerances which were measured previously with a comparable resonator (see, for example, [32]).
A photo of the resonator unit without its cover is shown in Fig. 5. Since the laser has to be operated maintenance-free for several thousand hours, the resonator unit is completely enclosed and the air inside the resonator housing is constantly exchanged and filtered. One of the plane resonator folding mirrors is mounted in a piezo-driven mirror mount, so readjusting the resonator remotely is possible while the resonator is integrated inside the lidar system. All opto-mechanics, electronics, and cables that are installed in the resonator unit are free from outgassing. To prevent disturbances during fast switching, the Pockels cell driver is mounted inside the resonator unit close to the Pockels cell. The driver is custom-designed and permits very fast (${\sim}{{10}}\;{\rm{ns}}$) active-low switching of the Pockels cell with minimal power consumption.
Fig. 5. Photograph of the resonator unit in the same orientation as the schematic drawing in Fig. 4 with the sketched beam path.
Fig. 6. Measured peak power of the pump module for different diode currents (left) and caustic after the optical fiber (right). The calculated beam quality is $M{{^2}} = {{160}}$ in both directions.
As indicated in Fig. 5, the part of the seed laser light inside the resonator that is reflected at the Brewster-cut Pockels cell is measured by a fast photodiode for controlling the cavity length to be resonant for the seed wavelength. An additional photodiode monitors the pulse width, pulse buildup time, and the pulse energy of the laser pulse during $Q$-switched operation. The output laser pulse is shaped by a spherical telescope with one of its lenses mounted on a piezo-driven linear stage so that the divergence of the output beam and therefore the focus of the laser inside the field of view in the atmosphere can be controlled remotely. Additional space behind the pump beam back-folding unit is reserved for further options, e.g., for SHG and further beam diagnosis. All the connectors necessary for controlling and powering the Pockels cell driver, the photodiodes, the piezo mirror mount, and the piezo linear stage as well as the seed laser fiber-collimator are implemented in an interface plate that allows for fast connecting of the resonator unit to the lidar system electronics. Plug-in connectors for the crystal oven and the piezo actor for cavity control are placed on the right side of the resonator unit so that the cable length can be kept short for these connections.
3. LASER PERFORMANCE IN THE LABORATORY
Two pump units and two resonator units are built which are each interchangeable. We will first show the results for the fiber-coupled pump units in Section 3.A and afterwards the results of the resonator units in Section 3.B.
A. Performance of the Fiber-Coupled Diode Pump Unit
The pump units show almost identical energetic and spatial parameters. The peak power of the pump beam behind the fiber at maximum diode current is 148 and 153 W for the two pump modules with a beam quality of ${M^2} = {{160}}$ in both directions (within the measurement error of beam diameters of high multimode beams) for both modules, which is slightly improved in comparison to ${M^2} = {{200}}$ shown in [41] although using the same fiber. A measurement of the peak power and beam caustic of the first diode module is shown in Fig. 6. The waist of the pump beam at the crystal’s position is 210 µm (radius), resulting in an effective pump beam waist of 247 µm inside the chosen Alexandrite crystal.
The first pump unit was already operated for over 1000 h and was transported several times for the field campaign without change of the output parameters behind the fiber.
B. Performance of the Alexandrite Laser
The laser resonator is stable for a wide range of thermal lensing without replacement of any resonator components or resonator mirrors and therefore can be operated with pump durations between 90 and 200 µs and repetition rates between 150 and 625 Hz. Care has to be taken that the duty cycle of the diode modules does not exceed 10% to prevent a decreased lifetime of the modules. Higher repetition rates around 625 Hz lead to higher average powers with lower pulse energies. A pulse energy of 1.6 mJ is measured at a repetition rate of 625 Hz and a pump duration of 160 µs, which equals an average power of 1 W. Larger pulse energies can be generated for lower repetition rates with longer pump durations, up to a maximum of 1.91 mJ at a pump duration of 160 µs and a repetition rate of 150 Hz, though the average power decreases thereby to 0.29 W.
Fig. 7. Measured pulse energy of the Alexandrite laser for different pump energies (left) and at maximum pump energy in a range of 200 min (right).
Fig. 8. Measured caustic of the Alexandrite laser in $Q$-switched, SLM operation (left). The measured beam quality is $M{\rm{^2}} \lt{1.1}$ for both directions. Internal losses of the resonator of 1.60% are measured by applying a Findlay–Clay analysis with five different output coupling mirrors (right).
For the envisaged lidar application, with measurements at altitudes of up to 120 km, an operating point with a repetition rate of 500 Hz is chosen for both prototypes to avoid an overlap of the traveling laser pulses. The two lasers are operated with different pump pulse durations of 100 and 130 µs to investigate long-term degeneration effects related to longer pump durations and therefore higher thermal stress of the pump diodes and inside the Alexandrite crystal.
The laser beam sources yield a pulse energy of 1.75 mJ (${{t}_p} = {{130}}\;{\unicode{x00B5}}{ \rm{s}}$, ${{T}_{\rm{crystal}}} = {{90}}^\circ {\rm{C}}$) and 1.4 mJ (${{t}_p} = {{100}}\;{\unicode{x00B5}}{\rm{s}}$, ${{\rm{T}}_{\rm{crystal}}} = {{125}}^\circ {\rm{C}}$) in $Q$-switched SLM operation at a repetition rate of 500 Hz with laser pulse durations of 780 ns and 850 ns, respectively. The difference in pulse duration is a consequence of the slightly different laser gain for the varying pump pulse durations. Taking into account the different pump pulse energies for the two pump durations, the optical–optical efficiency is 8% for ${{t}_p} = {{130}}\;{\unicode{x00B5}}{\rm{s}}$ and 8.6% for ${{t}_p} = {{100}}\;{\unicode{x00B5}}{\rm{s}}$. Exemplarily, a measurement of the pulse energy of the first resonator for different pump energies is shown on the left side of Fig. 7. An also exemplary measurement of the pulse energy at its maximum over time is shown on the right side of Fig. 7. The pulse-to-pulse energy deviation (RMS) is typically 0.2% although in the plot a slight rise of the output energy over 3 h is measured that is likely due to a slow thermal effect changing the adjustment of the resonator during the specific measurement.
For both resonator units, the beam quality is ${M^2} \lt{1.1}$ for both spatial directions which is essential for a small field of view (in our case the field of view is 33 µrad) and a low solar background when measuring the potassium resonance line in the MLT during daylight. A caustic of the laser output beam without further, especially cylindrical, beam shaping is shown in the left side of Fig. 8 and shows no astigmatism due the homogeneous pump beam profile and the careful resonator design. A Findlay–Clay measurement [43] is carried out, with five different output couplers that have reflectivities of ${{R}_{\rm{out}}} = {0.99}$, 0.98, 0.97, 0.96, and 0.95, respectively, and results in a value for the internal losses of $\delta = {1.60}\%$. The analysis was carried out only for the first laser. For both lasers, an output coupler degree of 3% yields the highest pulse energy with a resonator-internal fluence ${\lt}\!{{100}}\;{\rm{J/c}}{{\rm{m}}^2}$ at the optical components and is chosen for the final setup. This fluence is reasonably low for the given pulse length and ensures a long lifetime.
Table 1. Required and Measured Parameters of the Alexandrite Laser
By tuning the wavelength of the seed laser, the shape of the potassium resonance peak at 769.898 nm can be scanned in a range of 2 GHz by fast switching of the output wavelength. For a measurement of the temperature in the mesosphere with a sufficient accuracy, a laser linewidth of ${\lt}{{30}}\;{\rm{MHz}}$ is required, since a temperature change of 1 K equals a change of the linewidth of the potassium resonance line of approximately 0.86 MHz/K [6]. Therefore, the temperature accuracy of the lidar system depends on the linewidth and line shape of the Alexandrite laser and measurements of the temperature with sufficient accuracy are only possible when each pulse is in ${{\rm{TEM}}_{00}}$ and SLM operation, and the line shape and linewidth are known precisely. During observations these parameters are measured, as shown in Section 4, within the lidar for each single atmospheric measurement and considered during analysis.
Both Alexandrite lasers show comparable energetic, spatial, spectral, and temporal parameters which fulfill all the requirements for a laser emitter in a potassium resonance lidar (see Table 1). These results show the reproducibility of the laser parameters, if the same quality of critical optical components is given, which is essential as several lidar systems will be deployed.
4. PERFORMANCE OF THE LASER AS LIDAR EMITTER
The requirements of a laser operated as a lidar emitter, especially in a field campaign, go beyond the ones of a laboratory setup. The performance must be guaranteed over several hundreds or thousands of measurement hours without the possibility to have physical access for realignment. For the field campaign a transportation with, despite all damping efforts, possible shocks and vibrations is necessary, which should have no effect on the laser. The same applies to the operation under moderately changing environmental conditions, mainly temperature, during the 24/7 measurements.
When operating at maximum laser output power, the lidar system has an overall power consumption of 500 W, half of which is used for the housekeeping, PC, and cooling in the cube. This result represents a huge improvement compared to the state-of-the-art mobile Alexandrite resonance lidar presented in [12], which had a power consumption of 30 kW (with cooling). Further reduction of the power consumption in the future can be achieved by switching to less energy consumptive components (e.g., drivers for the piezo-driven mirror mount) and exchanging the PC for a customized electronic.
A. Operation within the Lidar Instrument
There are no changes in any beam parameter after the implementation of the laser beam source into the lidar system. Also repeated disconnecting and reconnecting of the optical pump fiber shows no effect on the laser output.
After several transportations of the lidar system between the ILT in Aachen and the IAP in Kühlungsborn, each time the laser parameters are measured again and show no change in reference to the parameters measured prior. Additionally, the alignment of the beam path in the signal analysis module of the lidar system was unchanged by the transportation, which shows the stability of the 3D-printed mounts used here.
A photograph of the lidar system outside the IAP is shown on the left side of Fig. 9. On the right side of Fig. 9 a photograph of the laser inside the lidar system is shown. The cover plate of the resonator unit was replaced by a transparent cover for the photograph so that the interior of the resonator unit can be seen.
Fig. 9. Photo of the lidar system measuring during the field campaigns in Kühlungsborn (left). The right side shows the resonator unit of the Alexandrite laser in operation. The cover plate of the resonator was replaced by a transparent cover for the photograph.
To measure the laser linewidth, the transmission through a confocal etalon with a linewidth of 9.5 MHz, also included in the lidar system, is measured for each laser pulse and frequency. Since the spectral shift of the etalon in reference to the potassium resonance line has to be known for Doppler measurements, the etalon is stabilized on one of the crossover lines of the potassium resonance peak by means of Doppler-free spectroscopy, as shown in Fig. 10 (left). The frequency jitter is reduced to a few hundred kilohertz by the cavity control technique described above and with more details in [38]. The laser linewidth is then mainly defined by the spectral chirp of the laser pulse which is a result of the change of the resonator length during the 700 ns long laser pulse. This chirp can be compensated by a suitable fast compensation technology within the next iteration of the cavity control technique.
Fig. 10. Confocal etalon with known linewidth, which is stabilized on a potassium crossover line (left), is scanned by varying the wavelength of the Alexandrite laser over 50 MHz in 1 MHz steps. The measured linewidth of the spectrum behind the filter is 12.5 MHz (right). Taking into account the Lorentzian shape and the spectral width of the confocal etalon of 9.5 MHz, the resulting linewidth of the Alexandrite laser is 3.3 MHz.
The measured spectrum of the laser emission, shown on the right side of Fig. 10, corresponds to the convolution of the spectral shape of the confocal etalon and of the Alexandrite laser. In this case from a test operation still indoors, the measured linewidth of the spectrum behind the filter is 12.5 MHz with the spectral width of the confocal etalon being 9.5 MHz (the spectral width of the confocal etalon is determined by scanning the etalon with the seed laser with its linewidth of about 100 kHz alone simultaneously). Therefore, taking into account the Lorentzian shape, the linewidth of the Alexandrite laser is approximately 3.3 MHz. This is the narrowest linewidth of an Alexandrite laser emitter that is reported up to now and most important for a lidar that is based on a narrow bandwidth laser and filter for the splitting of Mie and Rayleigh signals, as described in [38]. We will show the novel capabilities that derive from that technology in the laboratory and with measurements under field conditions.
The same confocal etalon is used as the spectral filter during the atmospheric measurements to measure both the backscattered spectra from the atmosphere and directly from the pulsed laser and seeder laser. The beams are measured along the same optical path as far as possible but with different spatial beam parameters (e.g., diameter and divergence) as for the atmospheric signal a large 50 cm telescope mirror illuminates the interferometers within the system. For any interferometer, the obtained interference pattern depends not only on the interferometer but also on the beam properties, which are never completely identical for two light sources. Consequently, slightly different interference patterns are obtained, also depending on the alignment of all optical components within the system. A major advantage of a confocal etalon is its insensitivity against minor adjustment changes. Laboratory tests show that larger adjustments change the shape of the filter curve and spectral broadening occurs. The change in the shape is a sensitive measure of the alignment and shows up in the data analysis if occurring.
For more precise measurements of the pulsed laser linewidth and line shape, an interferometer with higher Finesse is preferable, but the main goal of the etalon is atmospheric measurements via the large telescope as we will show in the section of the field campaign. For high sensitivity for atmospheric measurements, the spectral width of the etalon should be at least 2–3 times broader than the linewidth of the laser. As a consequence, the derived spectral width and shape is only an upper limit of the properties of the laser but still known with very high accuracy for each pulse of the laser compared to the required knowledge for data analysis.
The observed shift between the pulsed laser and seeder laser is to a large degree given by the actual shift of the pulsed laser against the seeder laser and not strongly influenced by misalignment of the etalon. The measurement shown in Fig. 10 is used to minimize the frequency shift via a novel cavity control system and is also available during data analysis to compensate for the remaining small bias of the order of 100 kHz in this example. A frequency measurement for each single laser pulse is not needed any more but is additionally available from the cavity control system (not shown) as monitor signal. For comparison, a line-of-sight Doppler shift of 1 m/s corresponds at this wavelength for Rayleigh and Mie scattering to a shift of 2.6 MHz. Even without considering the measured frequency shift, wind measurements are only biased of the order of 4 cm/s compared to typical atmospheric winds of several m/s or even more than 100 m/s occurring at mesospheric heights. The novel cavity control system allows frequency tuning from pulse to pulse with such high accuracy that no improvement is achieved any more by a direct frequency measurement of the pulsed laser since the remaining average bias is known and is already considered in the analysis. Moreover, this ensures a high spectral purity, which can be extracted from a measurement of the energy in the wings, as shown on the right side of Fig. 10, and is better than 99.9999%.
Fig. 11. Exemplary atmospheric measurement on 19 January 2020 with the lidar system during the field campaign, showing the height- and time-resolved line of sight Doppler wind measurement (left) and the calculated spectral width of the backscattered light (right) in the lower stratosphere over about 8 h.
For Doppler temperature measurements in the mesosphere the linewidth and also the line shape is required, and the spectral purity alone does not specify the spectral requirements. For existing lidar systems, both are typically not measured but assumed during analysis. The technology described here allows simultaneous measurements of all required properties with very high accuracy during each single atmospheric measurement and allows stabilization of the narrowband filters to high precision during atmospheric measurements.
B. First Field Campaign with the Lidar System
During the field campaigns from November 2019 to April 2020 the lidar cube was placed several times at the lidar and radar facility at the IAP in Kühlungsborn (54°07′N, 11°46′O) for overall 2 months. During the standard operation a readjustment of the laser was not necessary since no change of the output parameters was observed. The temperature inside the lidar system was usually kept at around 22°C, although internal temperatures between 17°C and 30°C were tested without change or degrading of the laser parameters. Even though for these extended temperature ranges the laser had to be readjusted remotely by means of the piezo-mounted mirror within the resonator. These tests are crucial for long-term operations at harsh environments even though later, the standard operation will only be performed within stable inside temperature close to the set point. Overall, the laser was operated for around 1000 h without degradation of output power or change of the linewidth or beam quality. The output parameters of the pump unit were measured as well after the field campaign with no degeneration of the output parameters compared to the status before the integration in the lidar system.
As an example of atmospheric measurements performed during the field campaign, we show in Fig. 11 on the left side the Doppler wind in the lower stratosphere, obtained from Mie scattering as described in detail for this example by [38]. By tuning the pulsed laser from pulse to pulse over a frequency range of ${\rm{\pm 50}}\;{\rm{MHz}}$, we obtain a discrete spectrum of 50 frequencies from the atmosphere. Finally, we calculate the wind as described in [38] by comparing the atmospheric spectrum to the spectrum of the pulsed laser or seeder laser every 200 m utilizing a moving average with an integration time of 20 min and 10 min time steps. The wind is derived by fitting the observed spectrum with a Gaussian function, considering additional possible spectral broadening effects due to turbulence and wind gradients, compared to the laboratory measurements shown before. Because of such additional contributions, we expect a spectral broadening depending on altitude and time. During this particular case, a strong horizontal wind is observed at approximately 10 km altitude and confirmed by European Centre for Medium-Range Weather Forecasts (ECMWF) as shown by [38] for a single profile. Since the instrument was only tilted by approximately 2°, the observed line-of-sight wind is small compared to future measurements with additional telescopes in different directions. The right side of Fig. 11 shows the calculated spectral width of the backscattered light.
At altitudes above 12 km, relatively stable conditions exist, and the observed spectral width is on average approximately 14.5 MHz and therefore close to the laboratory measurements even though the large telescope illuminates the interferometer and additional spectral broadening by turbulent motion within the atmosphere is expected. At altitudes of strong wind shears, the spectrum is at least a sum of different wind speeds and therefore Doppler-shifted spectra within the altitude range of 200 m. This can also occur in time and is clearly visible at 23:00 LT at approximately 7 km altitude. In such cases, the spectrum broadens and values up to 18 MHz occur. Since the obtained atmospheric spectra depend additionally on the aerosol motions within the observed volume, a precise analysis of the spectral width and shape may allow either measurement of turbulence at stratospheric heights and/or particle size at mesospheric heights. A much more detailed description of the influence of turbulences and wind shears on the spectral broadening is given in [38].
Additionally, as a demonstration of the potential of the system, measurements up to the higher mesosphere were conducted [38], as shown in Fig. 12. Even at daytime, the potassium layer between 80 and 110 km can be detected, although only interference filters with 0.3 nm bandwidth, but no special daylight filter with bandwidths in the pm-range (${\sim}{{1}}\;{\rm{GHz}}$), were included. As a consequence, the lidar has no benefit from the Fraunhofer line for potassium that is usually used [7]. Additionally, such a narrow bandwidth filter can also be used simultaneously for Doppler Rayleigh measurements and was tested previously but requires stable temperatures within the system during operation. Since the goal of the test campaign was to test the whole optical, mechanical, and electronic system under various extreme conditions, this additional interferometer, acting as a daylight filter with a FWHM of ${\sim}{{1}}\;{\rm{GHz}}$, was not included. This additional narrow spectral filter would reduce the solar background by 2 orders of magnitude. Even though no such narrow bandwidth filter was applied, it was possible to observe the potassium layer with longer integration and a solar background below ${{1}}{{{0}}^4}$ photons/s (blue channel in Fig. 12) with a field of view of 33 µrad. The signal behind the confocal etalon (red channel in Fig. 12) from the measurements in Fig. 10 (9.5 MHz bandwidth) has only ${\sim}{{1}}{{{0}}^2}$ photons/s as background and would be totally solar-blind with the narrow bandwidth (${\sim}{{1}}\;{\rm{GHz}}$ bandwidth) filter mentioned above. The dark count of the avalanche photodiodes (APDs) used for both channels is of the order of 20–50 counts/s. More technical details and the principle optical lidar setup can be found in [38].
Fig. 12. Daytime measurement with confocal etalon (9.5 MHz) but without the narrow bandwidth filter (1 GHz) used for Doppler Rayleigh measurements and solar background rejection and 200 m binning. Only two channels/detectors are required from the troposphere to the thermosphere for Doppler Rayleigh/Mie/resonance observations. The linear background of both channels has been subtracted. For APD2 the remaining noise is only noticeable as few single photons above 40 km as this channel is almost solar-blind.
To our knowledge, this was never achieved for potassium before and is a consequence of a better solar background reduction compared to former potassium lidar systems. Theoretical calculations show that the solar background can be lower compared to the former potassium lidar [7] by a factor of 4.3 due to the replacement of the Faraday filter with a solid-state etalon of finesse 80. Even though no solid etalon was used during this campaign, for the previously mentioned reasons, the measurements shown here indicate that such an improvement with a solid-state etalon is feasible, as the signal to background ratio is in agreement with the theoretical calculations.
5. SUMMARY AND OUTLOOK
In this work we present a rugged diode-pumped Alexandrite laser in single longitudinal mode operation that was used as a beam source in a mobile potassium resonance lidar including an extended field campaign. The laser consists of a fiber-coupled pump unit and a resonator unit which are both based on an optical design that was already qualified in the laboratory. The mechanical setup was altered to increase robustness and ensure a dust-free beam path.
Two systems with identical designs were built and yielded a laser pulse energy of 1.75 mJ and 1.4 mJ, respectively, with beam qualities of $M{\rm{^2}} \lt{1.1}$ at a repetition rate of 500 Hz. With a narrow bandwidth confocal etalon, a linewidth of 3.3 MHz is measured, and high frequency stability is proven. This is to our knowledge the narrowest bandwidth of an Alexandrite laser published up to now and was enabled by a novel cavity control technology.
The laser systems were integrated into mobile potassium resonance lidars that can be operated remotely with a power consumption of only 500 W for the entire lidar system. A first field campaign from November of 2019 to April of 2020 was successfully conducted with at least 1000 h of operation without degradation of the laser parameters. As an example, we have shown Doppler Mie wind measurements at tropospheric and stratospheric heights. The spectral width of such an atmospheric measurement is compared to laboratory results showing an expected broadening of a few MHz due to either turbulence or wind gradient in altitude or time. Additionally, the potassium layer in the mesosphere could be measured during the field campaign even at daylight, although no special daylight filters were integrated in the lidar system.
The experimental results show the high technical readiness level of the diode-pumped Alexandrite laser as a laser emitter in a Doppler resonance lidar. In the future, further power scaling can be achieved by increasing the available pump power. Additionally, frequency-doubling of the Alexandrite laser allows for addressing the iron resonance line at 386 nm, where the favorable combination of a low backscatter coefficient, a high density of the Fe atoms, a narrow resonance line, and a strong Fraunhofer line allows nearly solar background-free observations during daylight [13]. Also, with a frequency-doubling the resonance line of ${\rm{N}}_2^ +$ (391 nm) is addressable to extend the measurement altitude even further (up to 300 km) [44].
In parallel, several lidar systems will be set up to work as a Doppler lidar network to allow for overlapping measurement of large-scale effects. Therefore, monitoring dynamic processes such as the propagation of atmospheric gravity waves through several atmospheric layers will be possible.
The results shown in this publication show the potential for remote and even spaceborne lidar systems based on the diode-pumped Alexandrite laser technology.
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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