1. Introduction

The potential of Alexandrite as active laser medium for spaceborne earth observation missions based on laser instruments has been actively investigated in recent years [1–3]. The ability to tune the laser wavelength due to the broad emission band covering wavelengths between ∼700-858 nm [4,5] or generate (ultra-)short pulses (down to 70 fs [6]) are benefits of Alexandrite for many remote sensing applications. The Alexandrite emission range further allows to reach the UV spectral region by a single SHG frequency conversion process. Moreover, the absorption characteristics offer the opportunity for direct pumping by laser diodes, e.g. in the red [7,8], green [9,10] and blue [11] spectral region, which potentially enables a compact and simple laser system.

Alexandrite lasers with their wide emission band offer the capability to be tuned to match the red-edge of chlorophyll, a spectral region, where the reflectance of vegetation shows a sharp increase towards longer wavelengths [12]. The reflectivity properties in this spectral region are known to react very sensitively on the health status of the vegetation. Laser systems based on Alexandrite are therefore interesting for applications like altimetry or vegetation monitoring. Satellite-based laser instruments for vegetation monitoring allow for a reduced pulse energy compared to atmospheric sensing applications due to their better signal return (few hundred µJ vs. several mJ output energy). Studies for atmospheric sensing have been dealing with the development of single frequency operating Alexandrite lasers for Potassium LIDARs [1,2] or frequency doubled Alexandrite-based Iron LIDARs to measure wind and temperature in the Mesosphere [13]. Here, pulse durations up to 100 ns are sufficiently short [14]. For vegetation monitoring, pulse durations of a few nanoseconds and a high pulse repetition frequency (PRF) of multi-kHz are desirable for a good distance resolution and high sampling rates [14]. Q-switching systems do not deliver these short pulses. Here, the shortest pulse duration reached was 40 ns with a flashlamp-pumped Q-switched Alexandrite laser system despite optimization of the crystal temperature and output coupling reflectivity [15]. A first approach to generate few nanosecond short pulses with large peak power at high repetition rates was reported by Thomas et al. in 2016 [14] using cavity-dumped Q-switched operation with continuous-wave (CW) and pulsed (QCW) red diode-pumping. The optimum output parameters of 510 µJ pulse energy with 2.9 ns pulse duration at a PRF of 1-3 kHz were achieved with a QCW dual-end-pumped diode laser. The system was limited to a PRF of 4 kHz. Reaching higher PRF (≥10 kHz), which are required e.g. for spaceborne laser altimeter instruments [16], QCW laser diodes are no longer suitable as pump source due to their limited duty cycle. To retain the advantage of direct pumping with laser diodes, CW pumping is a desirable alternative. However, only a limited laser output energy of 170 µJ at a PRF of 4-5 kHz and 130 µJ at 10 kHz with CW end-pumping has been demonstrated so far [14].

In this work, we address the development of a cavity-dumped Q-switched Alexandrite laser with CW double-pass diode pumping to achieve high-performance, highly efficient and short-pulse laser operation. For this purpose, first investigations were carried out with an L-shaped CW Alexandrite laser resonator pumped in a single- and double-pass configuration to provide details regarding the intra-cavity losses as well as the dependence of the laser performance on the crystal temperature (Section 2). Based on these results, an enhanced cavity-dumped Q-switched laser was implemented, and the results are presented in Section 3.

2. High power diode-pumped CW Alexandrite lasers

In this section, we investigate the CW operation of an Alexandrite laser with an L-shaped cavity design. This configuration is the first step towards the further development of a cavity-dumped Q-switched system. At first, the Alexandrite laser – schematically shown in Fig. 1(a) – is characterized regarding output power, spectrum and beam quality. The intra-cavity losses are determined using Findlay-Clay analysis.

 

Fig. 1. (a) Sketch of the double-pass pumped Alexandrite laser in CW regime: M: mirror, QWP: quarter wave plate, PO: pump optics, BM: bending mirror, IC: input coupler, OC: output coupler; (b) Current-power characteristic of the diode laser. Insets: Spectrum at a set current of 12 A and super-Gaussian beam profile at the focus position.

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Another important aspect, especially if using Alexandrite as active laser medium, constitutes the dependence of the laser performance on the crystal temperature. As reported in literature [11,17,18], the crystal temperature plays a crucial role in optimizing the laser output power due to temperature-dependent effects of the intrinsic Alexandrite crystal properties, e.g. absorption and emission cross sections or the fluorescence lifetime. Investigations have shown that the influence of the crystal temperature on the laser output performance also depends on the overall laser system. This includes the used crystal holder and the resulting thermal contact, the chosen pump module and its output characteristics as well as the mirror reflectivity [19,20]. Therefore, we characterized our system regarding the crystal temperature influence on the laser output performance for different outcoupling reflectivities.

2.1 Results of the CW Alexandrite laser characterization

The experiments were performed with two similar Alexandrite laser crystals, only differing in their Cr³⁺ doping concentration. The first crystal (named ‘crystal 1’ hereafter), which was used for the measurements described in the following two sections, had a doping concentration of 0.20 at%, whereas the doping concentration of the second crystal (‘crystal 2’) was slightly higher with 0.22 at% chromium ions. Crystal 2 was used for almost all measurements presented in Section 3. Both laser crystals were 10 mm long and had an aperture of 4 × 4 mm². The end faces were plane-parallel and coated with an anti-reflection coating for the lasing (760 ± 20 nm) and the pump wavelength range (638 ± 20 nm). The c-cut crystals were embedded with their b-axis orientated horizontally in a thermoelectric cooler (TEC) controlled copper mount, which was used to tune and control the crystal set temperature between 25 °C and 120 °C. Furthermore, the crystal holder could be tilted and rotated by 2° for further optimization.

A sketch of the CW Alexandrite laser configuration is shown in Fig. 1(a). Pumping was achieved using a high-power fiber-coupled laser diode with a fiber core diameter of 400 µm and NA = 0.22, delivering a maximum output power of 43 W at a wavelength of around 638 nm (compare Fig. 1(b)). The unpolarized pump laser output was focused into the crystal by two aspherical lenses with a resulting beam focus diameter of 320 µm and an M² of ∼200. The resulting super-Gaussian beam profile (shown in the inset of Fig. 1(b)) has shown in literature the potential for highly efficient laser operation [21]. The complete pump optic, including the fiber output coupling, was mounted on a linear stage, which enabled a precise adjustment of the focus position inside the crystal along the optical axis without changing the focus size or caustic. The confocal parameter of the pump beam (∼2.15 mm) matched well with the absorption length (∼2 mm) of the used Alexandrite crystal. A measured amount of 56% of the pump power was absorbed in a single pass through the crystal. A retro-reflection of the transmitted pump beam, which was mainly polarized orthogonal to the b-axis, was used to maximize the absorption in the crystal. For this, the transmitted pump beam was back reflected by a highly reflecting mirror and refocused by double-passing a pair of aspherical lenses as well as a quarter wave plate (QWP) to rotate the polarization by 90°. Thereby, the refocused light mainly consisted of a polarization parallel to the b-axis orientation again, which increased the total amount of pump light absorption to ∼90%.

The L-shaped cavity was formed by three plane mirrors. The dichroic input-coupling mirror (IC) was highly transmitting (R < 0.5%) for the pump and highly reflecting (R > 99.95%) for the laser wavelength. The output coupler (OC) had a reflectivity of 98.8% for the laser wavelength. A bending mirror (BM), which was also highly transmitting for the pump, highly reflecting for the laser wavelength and adjusted to 45° towards the optical axis, was used to realize the double-pass pump configuration. Moreover, an intra-cavity plano-convex lens with a focal length of 125 mm was used to ensure a good overlap of the pump volume and the laser mode in the crystal. The laser beam diameter inside the crystal was simulated to be 290 µm almost independently of the pump power. Moreover, the lens generated a nearly collimated beam towards the OC, which is desirable for the later integration of a Pockels cell. In this cavity design, the beam size in the BBO crystal was simulated to be minimum 480 µm and increased up to 1000 µm for increasing pump power due to an increasing dioptric power of the thermal lens. The increasing beam size counteracts a damage of the BBO. All cavity simulations were done assuming an M² of 1. For worse beam quality, the beam diameters will be correspondingly larger. The resulting cavity length was around 340 mm.

A comparative study of the performance of the single- and double-pass Alexandrite laser was undertaken to evaluate the enhancement via the double-pass pump configuration. The crystal temperature was set to 25 °C during these measurements. A highest laser output power of 7.8 W was achieved in single-pass at an absorbed pump power of 26.3 W. The resulting slope efficiency was 34%. By retro-reflection of the pump beam, the amount of absorbed pump power could be increased to 36 W and the corresponding laser output power to 11.6 W. Thereby, the slope efficiency was slightly improved to 37% (see Fig. 2(a)). The results show that the double-pass pump configuration does not significantly increase the slope efficiency of the laser with respect to the absorbed pump power, but improves the total amount of absorbed pump power by around 30%, as expected. Thereby, the laser output power could also be increased by about 30%. Hence, the subsequent results presented in the following sections were all acquired with the double-pass pump configuration due to its superior performance.

 

Fig. 2. Output power of the CW Alexandrite laser: (a) Laser power for single- (black) and double-pass (red) diode pumping with R = 98.8% output coupling reflectivity. (b) Laser output power as a function of the absorbed pump power for different output coupling reflectivities.

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Considerably higher slope efficiencies can be found in literature (up to 54.4%) for an Alexandrite laser in CW regime [20]. The main reason for that is the short cavity configuration used therein and thus the smaller number of optical components, which benefits from very low round-trip losses of around 0.2%. Since the extended, L-shaped cavity presented within this paper was chosen to further enhance the setup towards cavity-dumped Q-switching, higher losses were expected due to the additional components like the BM or the intra-cavity lens, which were unavoidable in our case. To determine the round-trip losses occurring in the L-shaped cavity, a Findlay-Clay analysis was performed by measuring the lasing threshold for five different output couplers (see Fig. 3(a)). The corresponding output power curves are depicted in Fig. 2(b). According to this analysis, the roundtrip loss amounts to L = 0.8%, which leads to a cavity loss factor T/(T + L) of 60% with transmission of T = 1.2%. Considering a photon quantum efficiency of 84%, a beam overlap efficiency at the pump focus position of around 90% (for small pump powers) and a pump quantum yield of 100% [4], a maximum slope efficiency of ∼45% can be expected [22]. The calculated value shows, that a higher slope efficiency is potentially possible in the presented laser configuration. However, it has to be emphasized that the assumed beam overlap efficiency was only considered at the focal point of the pump beam and its effective value over the crystal length is smaller.

 

Fig. 3. (a) Findlay-Clay analysis for the double-pass pump configuration. (b) Beam quality parameter M² measured for six different absorbed pump powers. The insets show two typical beam profiles (nearly perfect TEM₀₀ beam at low pump power and distorted profile containing also higher order modes at high absorbed pump power).

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The laser beam quality was measured at six different pump power levels with the results shown in Fig. 3(b). The laser operated in TEM₀₀ mode with an M² of ∼1.1 in the vertical and horizontal direction up to an absorbed pump power of ∼8 W and a laser power of ∼1.3 W. For higher pump power, the laser beam quality decreased and the M²-value increased almost linearly up to M_x² = 4.2 and M_y² = 4.4 at an absorbed pump power of ∼33 W. The degradation of the beam quality can be explained by increasing pump-induced lensing and refractive index variation effects (e.g. thermal and population lensing), which caused a decrease of the fundamental mode size in the laser crystal (determined via simulations) and a degradation of the beam overlap inside the Alexandrite laser crystal for higher absorbed pump powers [23]. This leads to the onset of higher order modes, which can also be seen in the distorted beam profile shown in the inset of Fig. 3(b). Please note, that in contrast to this, extending the setup to a cavity-dumped Q-switched laser system led to a TEM₀₀ mode operation over the whole tested pump power range. These results will be presented below in Section 3.1.

2.2 Dependence of the CW laser performance on the crystal temperature

The Alexandrite laser performance in CW-regime was also investigated regarding the impact of the crystal set temperature. The laser output power and the peak laser wavelength were recorded for crystal set temperatures ranging from 30 °C to 100 °C at a fixed absorbed pump power of 16.5 W. Additionally, the output coupling reflectivity was varied, as shown in Fig. 4(a) and (b).

 

Fig. 4. Experimental results of the crystal temperature variation in the L-shaped cavity design: (a) Laser output power and (b) peak wavelength as a function of the crystal set temperature, both measured for four different output coupling reflectivities.

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As can be seen in Fig. 4(a), a decrease of the laser output power was detected in all cases when the crystal set temperature increased. Furthermore, a high reflectivity of the output coupler caused a steeper decrease of the output power curve. The peak laser wavelength as a function of the crystal set temperature is depicted in Fig. 4(b). The peak wavelength shifted to longer wavelengths for higher set temperatures for all output couplers. Thereby, it could also be observed that the wavelength range of the laser emission generated with high output reflectivities was on the one hand located in a longer wavelength area (>765 nm for R = 99.5%). On the other hand, the tuning range due to crystal temperature changes was wider, e.g. for R = 99.5% the wavelength was tunable by 21 nm and for R = 96.4% only by 12 nm. In total, the wavelength could be tuned by about 33 nm just by varying the crystal set temperature and the output coupler reflectivity. This wavelength shift was expected due to the change of the peak net emission cross section with temperature and was also observed and investigated in literature [18,19].

3. Cavity-dumped Q-switched Alexandrite laser in the L-shaped cavity design

After characterizing the laser performance in CW operation, the system was extended to the final cavity-dumped Q-switched Alexandrite laser, for which the first approach from literature [14] was adopted and enhanced to achieve a larger PRF range up to 20 kHz and to optimize the laser output energy towards a highly efficient operation.

The final system shown in Fig. 5(a) was implemented by expanding the setup of the CW Alexandrite laser described in Section 2.1. The diode pump source, pump optics including the double-pass configuration as well as the crystal holder, which were explained in detail in the previous section, were kept for this laser configuration. Alexandrite crystal 2 with the same dimensions but with a slightly higher doping concentration of 0.22 at% compared to crystal 1 was used. The OC was replaced by a mirror with high reflectivity (HR) for the laser wavelength. Furthermore, a BBO Pockels cell in combination with a QWP was integrated into the system to realize Q-switching by operating the Pockels cell at its quarter-wave voltage. During the switching-on time of the Pockels cell, the laser pulse inside the cavity built up only experiencing internal cavity-losses, which were estimated to be ∼2.1% per round trip. Cavity-dumping of the system was achieved by integrating a thin film polarizer (TFP) in front of the Pockels cell and rapidly switching off the high voltage at the maximum of the resonator-internal pulse build-up (see Fig. 5(b)). The duration of the resulting laser output pulse at the TFP arises out of a combination of the Pockels cell switching-off time and the round-trip time and is independent of the gain or repetition rate. In this case, the Pockels cell had a switching off time of 3 ns (90/10) and the cavity round-trip time was ∼2.1 ns.

 

Fig. 5. (a) Schematic drawing of the cavity-dumped Q-switched Alexandrite laser with double-pass diode pumping: HR: highly reflecting mirror, TFP: thin film polarizer; (b) Illustration of the pulse build-up in the cavity as well as the cavity-dumped pulse with respect to the Pockels cell switching behavior.

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During the measurements, the delay between switching-on the high voltage for the Pockels cell and switching it off was optimized to reach the maximum output laser energy extracted via the TFP. This procedure was carried out for every set pump power since the delay corresponds to the build-up time of the intra-cavity flux to reach its maximum, which decreases for increasing pump power.

3.1 Characterization of the cavity-dumped Q-switched Alexandrite laser

Initial characterization was carried out at a crystal set temperature of 25 °C and a PRF of 5  kHz. The resulting laser output energy is plotted in Fig. 6(a) as the black curve. It increased linearly up to 355 µJ at an absorbed pump power of 22 W and slightly fluctuated afterwards. The highest output energy of 443 µJ was achieved at an absorbed pump power of 25 W. The laser pulse duration was measured to be 2.8 ns in all cases and a typical temporal profile is shown in Fig. 6(b). This results in a corresponding maximum peak power of ∼158 kW. For higher pump power, the laser cavity turned into an unstable operation. This behavior is probably caused by thermal effects inside the laser crystal, e.g. due to an increasing dioptric power of the thermal lens.

 

Fig. 6. Characterization of the cavity-dumped Q-switched laser performance; (a) Laser output energy for a fixed pump focus position (black curve) and an adjusted pump focus position (red curve); (b) Temporal pulse shape with 2.8 ns duration (FWHM).

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For further improvement of the laser output energy, the pump focus position in the direction of the optical axis of the laser crystal was additionally adjusted for each pump power to obtain a better mode overlap and adapt to the small changes in the absorption length at each measurement point due to the temperature-dependent absorption cross sections. As it can be seen in the resulting laser output energy in Fig. 6(a) (red curve), this procedure could enhance the energy characteristic to a linear course over the whole pumping range. The maximum laser output energy was increased to more than 500 µJ with the same pulse duration, which results in a record peak power of 180 kW reached within a cavity-dumped Q-switched Alexandrite laser with CW diode pumping (more than twice of the value found in literature for a comparable system [14]). Moreover, the optical-to-optical efficiency could be improved from 5% [14] to 8.2%. The laser wavelength was 755 nm and only a small shift of ∼2 nm could be observed by increasing the pump power. This behavior differs from the measurements in CW regime and was caused by the optical coating of the TFP, which has a narrow-band reflectivity minimum for p-polarized light at around 755 nm retaining the resulting laser wavelength within this spectral region.

Furthermore, the beam profile was characterized at an absorbed pump power of 25 W, and the M²-measurement is illustrated in Fig. 7. The spatial profile, shown in the inset of Fig. 7 for the focus position, revealed a TEM₀₀ mode beam with the caustic measurement giving M²_x = 1.33 and M²_y = 1.42. The beam quality remained almost the same over the whole pump power range and differed from the beam quality decrease observed in the previously presented CW regime.

 

Fig. 7. Caustic measurement of the resulting laser beam at 25 W absorbed pump power.

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Satellite-based lasers for spaceborne earth observation missions like vegetation monitoring are continuously operated over a long period of time and thus require a stable laser performance over several years. To get a first impression of the long-term performance of the presented system and to identify a potential degradation of the laser crystal and its coating due to the generally high fluence inside the cavity, a stability measurement was performed over 500 minutes (150 Mega-shots) at a laser output energy of 350 µJ and a PRF of 5 kHz. A longer timeframe was not chosen to avoid an uninterrupted laser operation over several working days due to laboratory safety reasons. The spectra and the size of the cavity-dumped beam at an arbitrary position were recorded in addition to the output laser energy to better assign possible changes of the laser performance during the measurement. The crystal temperature was kept at 30 °C. The peak wavelength was 755.3 nm with a bandwidth of 1.9 nm. No changes were observed in the spectral characteristics over the entire measurement. The results of the laser pulse energy and beam diameter measurement are presented in Fig. 8. The laser yielded a pulse energy of 350  µJ with a standard deviation of 1% over the 150 Mega-shots. Small drifts were existent in the first 45 minutes as well as in the last 40 minutes, which can be attributed to changes in the room temperature as well as to a specific warm-up time of the laser system to reach a stable condition, and depend on e.g. the thermal control and the laboratory conditions. This could also be observed in the beam size.

 

Fig. 8. Measurement of the laser performance over 500 minutes (150 Mega-shots) at 5 kHz.

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The measurements have demonstrated that no degradation of the laser output occurred during the considered time. Since no damage could be observed either on the Alexandrite crystal nor the optical components, a longer operation time should also be possible, which is auspicious for the potential use in altimetry LIDAR applications.

3.2 Dependence of the laser performance on PRF and crystal set temperature

Keeping the same optimization procedure (adjustment of pump focus position and delay time), the cavity-dumped laser output was characterized as a function of the repetition rate, which was varied between 1 kHz and 20 kHz. The absorbed pump power was set to 27 W. The pulse duration was still 2.8 ns for all repetition rates. As it can be seen in Fig. 9, the pulse energy was above 475  µJ up to a PRF of 5 kHz and decreased nearly linearly to 150 µJ at 20 kHz. The pulse energy evolution for higher PRF can be explained by the upper state lifetime of Alexandrite, which is around 260 µs at 25 °C. This corresponds to the inverse of the PRF at around 3 kHz, which leads to an expected maximum in this frequency range and a nearly linear decrease afterwards, since the population in the upper state cannot fully recover to reach the maximum inversion between each pulse. The small pulse energy decrease observed for PRF below 3 kHz can be explained by excess heat generation inside the laser crystal as the pumping time becomes significantly longer than the fluorescence lifetime. This can further lead to an increase of pump excited state absorption, which represents an additional loss mechanism, especially in this PRF region. Nevertheless, the double-pass pump configuration is favorable for a reduction of the pump excited state absorption, as it has been shown in [24].

 

Fig. 9. Variation of the cavity-dumped Q-switched pulse energy with pulse repetition rate, measured for a fixed absorbed pump power of 27 W.

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This is the first time that a cavity-dumped Q-switched Alexandrite laser was demonstrated for PRF over 10 kHz.

Finally, the temperature-dependent laser performance was investigated for five different PRF (1 kHz, 5 kHz, 10 kHz, 15 kHz and 20 kHz) with crystal set temperatures in the range of 25 °C to 120 °C, but here with crystal 1.

Figure 10(a) depicts the variation of the laser output energy with crystal set temperature for a constant, rather low absorbed pump power of 12.3 W. Similar to the CW Alexandrite laser, the laser output power always decreased for PRF of 1 kHz and 5 kHz, if the temperature is increased, due to the decreasing fluorescence lifetime and the related loss of population inversion. The slope of the energy curves decreased towards higher PRF, and for PRF greater than 5 kHz, a maximum appeared at higher temperatures (e.g. for 10 kHz at ∼50 °C). The laser output energy at higher PRF increased initially for elevated temperatures due to the increase of the emission cross section and the simultaneous consistent amount of population inversion. This behavior changed for higher pump powers, as it can exemplarily be seen in Fig. 10(b) for a PRF of 10 kHz and a pump power of 18.4 W. No maximum at higher temperatures existed anymore, and the shape of the curve became more similar to the curves for PRF ≤ 5 kHz. Due to the increasing role of excited state absorption and ground state absorption in the lasing wavelength region of around 755 nm for elevated temperatures, additional losses occur for higher pump powers because of a larger heat load into the crystal. The counteracting effect of the increasing emission cross section is thus too small to have a greater impact than the increasing losses. Since the used temperature controller was not able to decrease the crystal temperature below 25 °C, no measurements could be performed in this temperature range. Nevertheless, the results suggest that a further increase of the output energy could be possible, if the temperature could be decreased. This aspect can be further investigated in future.

 

Fig. 10. Investigation of the influence of the crystal set temperature on the cavity-dumped Q-switched laser performance. (a) Laser pulse energy as a function of the crystal set temperature at different pulse repetition rates and a fixed absorbed pump power of 12.3 W; (b) Laser pulse energy as a function of the crystal set temperature at 10 kHz repetition rate for two different absorbed pump power; (c) Example of the output spectrum at two different crystal set temperatures.

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Furthermore, all five energy curves showed a flattening above a crystal set temperature of 90  °C, and the output energy stayed nearly constant for higher PRF. This behavior can be explained by having a look on the spectral changes within the measurement. Two examples of measured spectra at different temperatures are shown in Fig. 10(c). On the one hand, the main laser wavelength shifted about 2 nm towards longer wavelengths up to 90 °C, and on the other hand, a second wavelength peak appeared at around 773 nm for higher temperatures, i.e. the laser operated on two wavelengths. The amount of energy contained in this 773 nm peak is represented as the colored area in Fig. 10(b) and grew together with a simultaneous decrease of the other wavelength component. The explanation is given again by considering the coating characteristics of the TFP, which has a second reflectivity minimum at higher wavelengths for p-polarized light, in combination with the temperature-related effects in Alexandrite. In general, a shift towards longer emission wavelengths is observed for elevated temperatures due to the spectral change of the maximum net emission cross section, as it was also discovered for the CW Alexandrite laser (see Fig. 4(b)). In this case, the laser experiences too much losses for wavelengths higher than 755 nm due to the TFP, and no shift of the output spectrum occurs at first. If the crystal set temperature reaches the value at which the wavelength of the maximum net emission cross section matches the location of the second reflectivity minimum of the TFP, enough gain is built up to additionally generate laser emission within this wavelength range.

The observed dual-wavelength operation has a frequency offset of 8.5 THz, which can possibly be used in fields such as spectroscopy, imaging, communications and quantum information, and has been generated so far using a birefringent filter [25] or using crystal birefringence control [26]. The combination of a TFP with two reflectivity minima at the desired wavelengths and the natural temperature dependence of the generated Alexandrite laser wavelength (compare Section 2.2) turns out to be another possibility to achieve dual-wavelength operation. Since this behavior could also be observed with this cavity arrangement in CW regime, future investigations in this direction could push forward the application of generated THz waves from Alexandrite lasers.

4. Conclusion

In this work, we presented a highly efficient CW double-pass diode-pumped cavity-dumped Q-switched Alexandrite laser with ns pulse duration and PRF in the range of 1-20 kHz.

Initial measurements were performed with a CW Alexandrite laser to optimize the cavity design. The highest slope efficiency was 36% using an output reflectivity of 98.8% and coincided well with the theoretical limit. Higher values could only be reached by decreasing the determined roundtrip loss of 1.3%, which is challenging due to the required coating characteristics of the used optical components. The arrangement has thereby shown good prerequisites for further modification towards cavity-dumped Q-switching.

A CW diode-pumped cavity-dumped Q-switched Alexandrite laser was demonstrated with record output energies of more than 500 µJ with a pulse duration of 2.8 ns at a PRF of 5 kHz. The corresponding peak power of 180 kW represents, to the best of our knowledge, a record value reached within such an Alexandrite-based laser configuration. The spatial beam profile was TEM₀₀ mode with M²_x = 1.33 and M²_y = 1.42. A stability measurement at an output energy of around 350 µJ with 5 kHz PRF showed no degradation over 150 Mega-shots, which is auspicious for the potential use in altimetry LIDAR applications. Furthermore, laser operation at PRF up to 20 kHz has been demonstrated for the first time. Variation of the crystal temperature has shown that low crystal set temperatures (25 °C) led to the maximum output energies for high pump powers over the whole PRF range, which is advantageous for space applications with limited electrical power for heating. Dual-wavelength operation could be observed for elevated temperatures due to the TFP coating characteristics and opens up a new possibility to generate dual-wavelength output with THz frequency offset (8.5 THz), which can be used to generate THz radiation.

Our investigations provide an overview of important parameters to improve and optimize the laser performance of cavity-dumped Q-switched Alexandrite lasers and therefore enable a further step towards space- or airborne applications such as altimetry or vegetation monitoring. From the temperature dependencies measured in the previous section, a further improvement of the system output laser performance is expected to be achieved operating at even lower crystal temperatures. However, this upgrade will also require encapsulating and purging the overall system to avoid condensations, which can be realized, e.g. via a hermetically sealed housing as typically used for space laser instruments. For the intended LIDAR application, additional elements for laser wavelength control will have to be introduced, e.g. a birefringent filter. This will enable tuning of the laser wavelength precisely to the one needed for the application. Furthermore, it will ensure a narrowing of the spectrum, which will be required for e.g. efficient phase-matched second harmonic generation. These aspects could be part of future developments and improvements of the system.