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Abstract
In this work, we present the visible laser performance of improved optical quality Czochralski-grown 4 at.% Pr3+-doped Sr0.7La0.3Mg0.3Al11.7O19 (Pr:ASL) single crystals in the deep red (726 nm), the red (645 nm) and the orange (620 nm) range using two different pumping sources. Using a high beam quality frequency doubled Ti:sapphire laser with 1 W output power as pump source, deep red laser emission was reached at a wavelength of 726 nm with 40 mW of output power and a laser threshold of 86 mW. The corresponding slope efficiency was 9%. At 645 nm in the red, up to 41 mW of laser output power were obtained with 15% slope efficiency. Moreover, orange laser emission at 620 nm was demonstrated with 5 mW output power and 4.4% slope efficiency. Using a 10 W multi-diode module as pumping source allowed to obtain the highest output power of a red and deep-red diode-pumped Pr:ASL laser to date. The respective output powers at 726 and 645 nm reached 206 mW and 90 mW.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Regarding the recent advances in the output power of InGaN based visible blue laser diodes, solid state lasers based on rare earth doped materials emitting in the visible range can be scaled to the multi-watt regime [1–3]. Among the trivalent lanthanides, the praseodymium III ion is interesting as a dopant for laser emission at several wavelengths in the deep red, red, orange and green (Fig. 1). However, in an unsuitable host material with a high crystal field strength [4], this ion can be subject to exited state absorption (ESA) from the 3Pj multiplet into the lowest energy levels of the 4f15d1 configuration. In addition, host materials with high phonon energies may trigger multiphonon relaxation between the upper laser level 3P0 and the next lower level 1D2 [4]. These two phenomena can decrease the efficiency of Pr3+-based lasers. For these reasons a careful selection of the host matrix is required to obtain efficient laser emission.
Fig. 1. Schematic representation of Pr3+ energy levels. The blue arrow illustrates potential excited state absorption from the upper laser level into the lowest levels of the 4f15d1 configuration . The green arrows illustrate multiphonon relaxation from the 3P0 level into the 1D2 level.
Fluoride materials, such as LiYF4 (YLF) [5] or LaF3 [6] were widely studied as laser hosts for Pr3+ ions for their low crystal field strength and phonon energy. However, a careful control of the growth atmosphere is a key point to obtain good optical quality fluorides crystals. Also, the segregation coefficient of praseodymium in the most commonly used fluoride hosts is low (around 0.2 for LiYF4 [7]), which leads to inhomogeneous distribution of the dopant ions in the grown crystal boules.
That is why, over the last few years, also oxide based hosts with suitably weak crystal field strengths [8], and low phonon energies have been investigated for praseodymium. Moreover, these usually exhibit better thermomechanical properties compared to fluorides. Indeed, the perovskite-type oxide YAlO3 (YAP) enabled laser emission at 622 nm, 662 nm and 747 nm with laser output power levels up to 300 mW in the deep red range at 747 nm using a frequency doubled optically pumped semiconductor laser (2ω-OPSL) as pumping source [9]. More than 1 W of output power was also obtained at 747 nm utilizing diode pumped of Pr:YAP crystals [10]. Another interesting oxide family for Pr3+-based lasers are the hexaaluminates such as SrAl12O19 (SRA) [11,12]. Unfortunately, SRA exhibits a non-congruent melting behavior which imposes difficulties on the crystal growth. A solution found by S. Sattayaporn et al. [13] was the use of a solid solution between SRA and the congruent hexaaluminate material LaMgAl11O19 (LMA). At suitable composition ratios, these still obtain a congruent melting behavior, but remain spectroscopic properties similar to those of SRA (Table 1). In fact, the Sr0.7La0.3Mg0.3Al11.7O19 (ASL) was found to be suitable for this purpose. Doped with Pr3+ ions, laser emission was obtained under 2ω-OPSL pumping with up to 300 mW of output power in the deep red range at 725 nm [14]. Red and deep red laser emissions based on diode-pumped Pr:ASL was also demonstrated but the performance was limited by the optical quality of the laser samples [15]. In the present work we investigate the laser emission of 4 at.% Pr3+-doped ASL crystals which is closed to reach an optimal optical quality using two different pumping sources: a high beam quality frequency doubled Ti:sapphire (2ω-Ti:Sa) laser and a 10 W multi-diode module, both emitting at 445 nm.
Table 1. Spectroscopic properties of several Pr3+ doped crystals
2. Crystal growth
Using the optimal composition, Sr0.7La0.3Mg0.3Al11.7O19, determined by S. Sattayaporn in her PhD work [17], the raw materials SrCO3, La2O3, Al2O3, Pr6O11 and MgO with 4N purity were mixed in the right proportions to obtain 4 at.% Pr3+ doped ASL. This doping level corresponds to an atomic concentration of 1.8 × 1020 ions/cm3. Even though this doping level is significantly above the limit of ∼1 × 1020 ions/cm3 found to be suitable for Pr3+ in YLF, in previous work no strong quenching of the upper level lifetime was found [14]. The powders were then pressed into pellets and annealed under air at 1550 °C for 24 h to form the single ASL phase. An iridium crucible (of 50 mm diameter and 50 mm height) was filled with the sintered pellets. The Pr:ASL crystal was grown using the Czochralski technique in an inductive furnace under argon atmosphere to prevent the oxidation of the iridium crucible. The growth was initiated using a Pr:ASL seed oriented along the crystallographic $\vec{c}$ axis and then pulled with a rate of 0.6 mm/h and with a rotation rate of 10 rpm.
At the end of the growth, a greenish transparent and crack-free crystal boule of 25 mm diameter and about 16 cm length was obtained (Fig. 2). Hexagonal facets along the growth axis are clearly visible. The crystal was then annealed at 1550 °C in air to remove stress from the crystal growth. No visible inclusions, bubbles nor cracks were observed in any part of the boule.
3. Optical quality inspections
ASL exhibits a tendency for cleaving perpendicular to its $\vec{c}$ axis as the SRA does [18]. Considering the hardness of this material of ∼9 on Mohs’ scale, we utilized cleaved samples for the inspection of the optical quality. First, the slices were observed under cross polarizers using a Keyence VHX 5000 microscope. A core effect with a diameter between 4.1 and 5.1 mm was observed in the center of the 25-mm-diameter crystal boule all along the growth axis (Fig. 3). This effect is due to the growth of facets (001) perpendicularly to the growth axis. However, the size of the core effect in this work is lower compared to the previous growth attempts (around 8 mm) [16]. The difference is mainly due to a decreased rotation speed from 20 rpm to 10 rpm, which resulted in a more convex shape of the liquid/solid interface during the crystal growth and decreased the size of the (001) facets.
Fig. 3. Cleaved sample from the top of the 4 at.% Pr3+ doped ASL crystal under crossed polarizers. The core effect can be seen in the center.
For further optical quality inspection we applied the shadowgraphy method. A white light source was collimated using a 150 mm lens after passing through a hole to limit the light expansion. The sample is placed inside the collimated light beam and the pattern of the transmitted beam is observed directly on a screen (Fig. 4).
This technique reveals defects which were not seen using the crossed polarizer microscopy. In fact, one can clearly see concentric growth striations and growth sectors perpendicular to the hexagonal facets in the shadowgraphy picture (Fig. 5(a)). Moreover, inhomogeneous areas are found on the outer side of the crystal slice, e.g., in the top left of Fig. 5(a). According to these findings, different laser samples of 4 mm, 5 mm, 6 mm, 7 mm and 9.5 mm length oriented along the c-axis of the crystal were fabricated from the homogeneous and defect-free areas of the Pr:ASL boule (Fig. 5(b)).
Fig. 5. Shadowgraphy picture of a Pr:ASL slice cut perpendicular to the $\vec{c}$ axis (a) and oriented uncoated laser samples (b). Darker spots and lines are surface defects due of the cleaved sample.
4. Laser operation of Pr(4 at.%):ASL
For the laser experiments, we used two different pump sources: a 1 W tunable 2ω-Ti:sapphire laser and a 10 W multi-diodes module from Shimadzu Corp., both emitting at 445 nm.
4.1 Frequency doubled Ti:sapphire laser pumping
A 2ω-Ti:sapphire laser tuned to the highest absorption peak of Pr:ASL at a wavelength of 445 nm was used as pump source for laser experiments with the uncoated Pr(4 at.%):ASL laser samples. At this wavelength, the available pump power amounted to about 1 W. We investigated the laser performances using several output coupling (OC) mirrors with 100 mm radius of curvature and transmissions T between 0.8 and 3.6% at the different lasing wavelengths. The total length of the cavity was 103 mm. The incident pump power at the entrance of the laser cavity was adjusted using a combination of a λ/2 waveplate and a polarizing beam splitter cube (PBS). The temperature of the Pr:ASL laser crystal placed in a copper heat sink was set to 10 °C during all experiments by means of a thermoelectric cooling system. The laser output power was recorded after filtering the residual pump beam. A sketch of the setup is shown in Fig. 6.
For the red emission at 645 nm (based on the 3P0 → 3F2 transition), the laser cavity utilized a plane pump mirror with an anti-reflection (AR) coating for the pump-range between 400 and 510 nm and a highly reflective (HR) coating for the range between 530 and 675 nm. To optimize the mode overlap, we tested different lenses with different focal lengths for an output coupler transmission of 3.6% at 645 nm using the 7 mm long Pr:ASL sample, which absorbed about 50% of the incident pump power. The best results were obtained using focusing lens of f = 100 mm (Fig. 7(a)). Thus, all the following laser experiments were performed using this focusing lens.
Fig. 7. Laser performances of 7 mm long Pr:ASL crystal at 645 nm under 2ω-Ti:Sa pumping at 445 nm using different focusing conditions for the pump beam (a) and different OC mirror transmissions (b).
For the transition at 645 nm we obtained the best results using 3.6% of output coupler transmission. In this case, 41 mW of laser output power were reached at 530 mW of absorbed pump power, corresponding to a slope efficiency η of 15% (Fig. 7(b)). Using T = 1.8% we observed a slightly lower slope efficiency of 14%, but due to the lower laser threshold the obtained output power amounted to 45 mW.
The slope efficiency is comparable to the one obtained by S. Sattayaporn et al. (13% slope efficiency) with a 6 mm long AR coated 4 at.% Pr:ASL for the same output coupling transmission, even though the maximum achievable slope given by the stokes efficiency is smaller for 445 nm excitation compared to the 486 nm pumping user in [14]. Moreover, the laser threshold of 300 mW observed in our work is lower to the 800 mW obtained in previous work [14], highlighting the better optical quality of the samples used in this work as can be seen in Fig. 8.
Fig. 8. Shadowgraphy picture of 6 mm long 4 at.% Pr3+ doped ASL laser samples used in previous work [14] (left) and of 7 mm long 4 at.% Pr:ASL sample used in this study (right).
Then we estimated the maximum internal losses inside the laser cavity from the slope efficiency as not enough data point are available for the Findlay-Clay method. The expression of the slope efficiency ηs given by the following Eq. (1):
with ηpump the pump efficiency, ηquantum = λexc/λlum the quantum defect, ηc = γOC/γ the output coupling efficiency with γOC = -ln(1-TOC) the losses from the output coupler mirror and 2γ = - ln(1-γi-TOC) and ηt the transfer efficiency [19]. Assuming a pump efficiency and a transfer efficiency to be ideal and no inversion dependent losses, the logarithmic internal losses can be linearized (considering low losses and small TOC):Finally, the internal losses Li can be estimated from γi using the Eq.3:
This estimation was calculated for our laser test as well as the others performed with 2ω-OPSL pumping of Pr:ASL [14] and Pr:SRA [20]. The comparison of internal losses is resumed in Table 2. In our case, the calculated internal losses of the laser cavity are 12% at 645 nm for frequency doubled Ti:Sa pumping of Pr:ASL crystal. These losses are comparable with the 11% obtained by S. Sattayaporn et al. with 2ω-OPSL pumping of Pr:ASL crystal [14]. However, as our Pr:ASL crystal is uncoated, some additional losses should take a part in the internal losses in our case. Thus, we can assume that the optical losses of our Pr:ASL crystal are lower than that of previous work [14], in accordance with shadowgraphy inspection (Fig. 8). Nevertheless, the calculated internal losses remain significant whereas it seems that the crystal quality of Pr:ASL reaches a kind of optimum so that only marginal gain could still be obtained. That is why the low losses of 1% obtained by D.T. Marzahl et al. with 2ω-OPSL pumped AR coated Pr:SRA [20] may point out the fact that some additional loss mechanisms not related to crystal optical quality, could appear in Pr:ASL.
Table 2. Internal losses of Pr3+ doped hexaaluminate crystals-based laser cavities pumped with high beam quality sources.
For the deep-red laser emission 3P0 → 3F4 at 726 nm, the pump mirror was AR coated in the range 440-550 nm and HR coated for 700-880 nm. Using an output coupler with T = 0.8% for 726 nm, up to 40 mW of laser output power were achieved with 9% slope efficiency and a laser threshold of 86 mW. Finally, orange laser emission at 620 nm was realized, obtaining 5 mW of output power corresponding to 4.4% slope efficiency at T = 0.9% using the same pump mirror than for the red laser tests. The laser results under 2ω-Ti:Sa pumping are summarized in Fig. 9.
Fig. 9. Laser performances of the 7 mm long Pr:ASL sample at different laser wavelengths under 2ω-Ti:Sa pumping at 445 nm using the respective best output coupler transmissions T.
4.2 Multi-diode module pumping
As the pump power of the 2ω-Ti:sapphire was limited to 1 W, we performed further laser experiments utilizing a 10 W multi-diodes module from Shimadzu Corp. in order to scale the laser output power of Pr:ASL. This module is composed of six individual laser diodes collimated next to each other inside the module (Fig. 10 inset) which emit at 445 nm (FWHM of 1.5 nm). The pump beam was focused in the laser gain medium using a 75 mm lens. A photography of the setup in operation is shown in the Fig. 10.
Fig. 10. Photography of the experimental setup for the laser tests of Pr:ASL with multi-diodes module pumping. The inset on the left shows the pump beam profile at the exit of the module.
For the 645 nm laser experiments, we used the same pump mirror as in the 2ω-Ti:sapphire pumped experiments but an OC mirror with 75 mm radius of curvature and T = 1% at 645 nm. The total cavity length was 80 mm in this case. First, we investigated the influence of the focusing conditions with the shortest crystal of 4 mm. We found a shorter focal length of f = 50 mm to be less suited than a f = 75 mm, as the laser threshold increased from 1 to 1.3 W and the slope efficiency decreased from 5.8 to 3.7%.
We also tested the influence of the crystal length. As seen in Fig. 11 the best results were obtained using the 4 mm long crystal. Up to 90 mW of laser output power were obtained with an absorbed pump power of 2.6 W. The corresponding slope efficiency was 5.8%. at a laser threshold of 1 W for this crystal. In the results presented in [15] under single-laser-diode pumping, the best slope efficiency reported is 8.5% with only 45 mW of output power at 645 nm but, some power rollover phenomenon was observed from 1 W of absorbed power, probably indicating thermal lensing effects that were not observed in our case up to 4 W absorbed power. This observation shows that the crystal quality of our samples seems suitable for power scaling of the laser emission of Pr:ASL. Recently, up to 19% slope efficiency were demonstrated with a Pr:SRA crystal pumped by a reshaped laser diode [21]. As Pr:SRA has similar spectroscopic properties than Pr:ASL, it indicates that further improvements of the Pr:ASL laser performances are feasible.
When increasing the length of samples from 4 to 9.5 mm under identical focusing conditions the laser threshold increased gradually from 1 W to 2.4 W for the longest sample of 9.5 mm. As Pr3+ lasers at 645 nm are four-level lasers with no reabsorption in unpumped areas, this is a clear indication for residual propagation losses at this laser wavelength in the Pr:ASL samples. Moreover, all thresholds are significantly higher than under 2ω-Ti:Sa pumping, where they amounted to ∼0.2-0.3 W, due to the lower beam quality of the multi-diode module (M2>20) as compared to the 2ω-Ti:Sa (M2∼1) which prevents from tight focusing.
The estimation of the cavity internal losses was also performed for the diode pumped Pr:ASL crystal of 4 mm length using the equations Eq. 2 and Eq. 3 and compared in Table 3 to the one that can be calculated for all the published laser results dealing with laser diode pumping of Pr3+ doped hexaaluminates. The internal losses estimated to 10% for our laser crystal are similar to the ones obtained with 2ω-Ti:Sa pumping (Table 2). They are comparable to those obtained with AR coated crystals by S. Zhou et al. with diode pumped Pr:ASL [15] pointing out the good performances obtained with uncoated crystal in this work. About Pr:SRA, a significant variability of estimated internal loss is observed between the two teams who worked on laser diode-pumped of AR coated Pr:SRA crystals. The internal losses estimated from the most recent works on Pr:SRA [21,22] gives results between 7 and 11% close to Pr:ASL, whereas the team who got execptional laser performances with Pr:SRA again get internal losses in the range 1-2% [11,12]. Deeper investigations are necessary to better size the laser potentialities of Pr3+ doped hexaaluminates and understand their eventual limitations.
Table 3. Internal losses of Pr3+ doped hexaaluminate crystals-based laser cavities pumped with blue laser diodes.
For the laser experiments at 726 nm, the pump mirror was changed to one with an AR coating for the 440-550 nm range and a HR coating for the 700-880 nm range. The OC mirror used had 75 mm radius of curvature, resulting in a cavity length of 80 mm, and T = 3%. For all crystal lengths, similar laser thresholds of about 1.45 W were found, pointing to significantly lower losses than for the 645 nm laser test case. The best laser performance was obtained using the 7 mm long Pr:ASL crystal with up to 206 mW of output power at 726 nm for 3.8 W of absorbed pump power, corresponding to a slope efficiency of 8.5% (Fig. 12). It should be noted that this sample also showed a reasonable performance at 645 nm, pointing to a high optical quality of this particular sample. This slope efficiency is similar to the one obtained by S. Zhou et al. using 3 at.% doped Pr:ASL with a slightly different composition (Sr0.5La0.5Mg0.5Al11.5O19) under single laser diode pumping [15]. However, enabled by the higher pump power available in this work, the deep-red laser output power obtained here is the highest laser output power reported for any Pr:ASL laser under diode pumping.
5. Summary
A 4 at.% Pr3+ doped ASL single crystal with improved optical quality was grown using the conventional Czochralski technique. Shadowgraphy observations of c-oriented slices of the crystal boule revealed growth defects such as core effect, striations and growth sectors. Thanks to an optimized rotation rate during the crystal growth, the core size was reduced compared to previous growth attempts of this material. The precise optical inspection enabled to identify homogeneous areas of the crystal to cut oriented samples for the laser tests.
Under 2ω-Ti:sapphire pumping at 445 nm, laser emission of Pr:ASL was obtained in the deep-red (726 nm), red (645 nm) and orange (620 nm). The highest slope efficiency of 15% was reached for the red emission using a 7 mm long Pr:ASL sample. At an absorbed pump power of 530 mW, the output power at 645 nm reached 41 mW. Orange emission of Pr:ASL with 5 mW of output power was also demonstrated for the first time using a 445 nm pumping source.
A laser power scaling of Pr:ASL was performed using a 10 W multi-diodes module at 445 nm as the pumping source. Lower slope efficiencies compared to 2ω-Ti:Sa pumping were obtained due to the lower pump beam quality. However, up to 206 mW of output power at 726 nm were demonstrated with the 7 mm long sample. To the best of our knowledge, this is the highest output power reached for a diode pumped Pr:ASL laser. Considering the good thermal conductivity of ASL of 7 Wm-1K-1 [13], all these results open a path for further power scaling of Pr:ASL-based red lasers using the newest >100 W-class blue diode modules.
The estimation of the internal losses of the laser cavity at 645 nm shows that the improvement of the Pr:ASL crystal quality allows to reach lower losses with AR coating as compared to previous Pr-doped ASL crystals. However, the estimated internal calculated losses are still higher than the lowest ones obtained with Pr:SRA pointing out that some additional mechanisms to investigate could appears in Pr:ASL.
Disclosures
The authors declare no conflict 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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