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Abstract
An Alexandrite laser on-peak pumped by a frequency doubled Raman Yb-fiber laser at 589 nm, which matches well with the absorption peak of Alexandrite crystal along b-axis, is demonstrated for the first time. With a pump power of 7.7 W, a maximum average output power of 2.51 W at 755 nm was achieved, with a beam quality factor better than 1.25. The slope efficiency was 41%, which is not the highest but moderate due to the experimentally confirmed high roundtrip loss of 2.3% resulting from the imperfect crystal coating quality and crystal quality. In addition, wide wavelength tuning from 727.2 nm to 787.3 nm was demonstrated by employing a 1 mm thick single plate birefringent filter (BRF). With the help of a 6 mm thick BRF, dual-wavelength operation was achieved at 755.2 nm and 764.2 nm, with a maximum average output power of 1.8 W. It is believed that much higher power Alexandrite laser with high beam quality and high efficiency could be expected considering the fact that 100-W level 589 nm laser is available now.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Alexandrite (chromium-doped chrysoberyl, Cr3+:BeAl2O4) lasers have attracted much attentions thanks to their special near-infrared emission wavelength (700∼858 nm) and very wide bandwidth [1,2], which enables the direct generation of visible ultrashort pulse [3,4], one-step ultraviolet laser generation [5,6], dual-wavelength lasing operation [7,8], applications in remote sensing [9,10] and cosmetology [11], and so on. It is well-known that Alexandrite is a broadly tunable vibronic solid-state laser material with a thermal conductivity of as high as 23 W/(mK) [2], which is almost twice of that of Nd:YAG and five times of that of the Cr-doped colquiriities (such as Cr:LiSAF, Cr:LiCAF, and Cr:LiSGaF). Alexandrite crystal exhibits strong birefringence, allowing direct linearly polarized laser output. Its stimulated emission cross section σ is just 0.7×10−20 cm2 [2], but strong pumping is allowed thanks to the high damage threshold of approximatively 270 J/cm2 [2]. As a result, a high enough gain could be guaranteed as usual. Besides, the upper state lifetime τ of Alexandrite is as long as 262 µs at room temperature [2], which is much longer than that of Ti:sapphire (3 µs) and thus beneficial for energy storage with lower brightness pumping sources. Importantly, its higher στ product suggests the potentiality of low laser threshold.
Alexandrite exhibits broad absorption from 350 nm to 690 nm with two peaks around 410 nm and 590 nm [12], and two narrow absorption peaks at 678.5 nm and 680.4 nm, which are the so-called R-line. Thus so far, some different pumping sources at different wavelengths have been adopted, mainly including flashlamps [1,2], Krypton ion lasers (647 nm) [13], dye lasers (615-680 nm) [14], blue laser diodes (LDs) (444 nm) [15], frequency doubled neodymium lasers at 532 nm [16], and red LDs (630-680 nm) [17–20]. Pumped by flashlamps, an average power of as high as 60 W was demonstrated but with a slope efficiency of just 1.5% [2]. S. T. Lai et. al. reported a slope efficiency up to 51% by employing a Krypton ion laser at 647.1 nm as pumping source with an average power of 1.9 W [13], whose output power, however, was just 0.6 W and limited by the pumping source itself. Further, a recorded slope efficiency of 64% was realized by using a dye laser at 640 nm as pumping source [14], which, however, is complex, unstable, and noxious. Recent years, with the development of high-power LD technology, LD pumped Alexandrite lasers have gained popularity, especially with blue LD and red LD. For instance, an average output power of 326 mW has been generated with a slope efficiency of 20%, pumped by a 3.5 W InGaN LD at 444 nm [15]. Yet, it is obvious that the quantum efficiency is low and the thermal effect is expected to stronger because of the shorter wavelength. By employing red LD at 639 nm, the highest average output power of 26 W has been reported, but not in TEM00 mode operation [17]. In addition, an output power of 2.6 W at 755 nm with a slope efficiency of 26% was achieved by using a 11 W 532 nm green laser [16]. An output power of 200 mW with a slope efficiency of 38% was obtained by a tapered diode at 680.5 nm [19]. Evidently, for all these reported cases, the pumping wavelengths are not matched with the maximum absorption peak of 590 nm in E//b polarization. Therefore, to the best of our knowledge, there is still no report of on-peak pumping at 590 nm for Alexandrite laser, because of the unavailability of 590 nm LD and direct high-power solid-state laser at 590 nm. While, it is fortunate that frequency doubled Raman Yb-fiber laser can provide high power 589 nm single frequency lasers [21,22]. Compared with red LD at 638 nm and laser source at 532 nm, the pumping laser source at 590 nm features high brightness, moderate quantum efficiency, and the highest absorption coefficient along the E//b polarization, which is about 2 times and 3 times higher than that for 638 nm and 532 nm. Additionally, it is foreseeable that on-peak laser pumping around 590 nm could be a better way for realizing kerr-lens mode locked Alexandrite laser, due to the diffraction limited beam quality and high absorption coefficient, which can allow the employment of shorter crystal length and relax the strict Kerr lens mode locking condition to some extent.
In this manuscript, an Alexandrite laser on-peak pumped by a frequency doubled Raman Yb-fiber laser at 589 nm is reported, for the first time to the best of our knowledge. The used frequency doubled Raman Yb-fiber laser at 589 nm can provide an average power of up to 7.7 W with diffraction limited beam quality. A maximum average power of 2.51 W at 755 nm with a slope efficiency of 41% was achieved. The beam quality factors M2 were measured to be 1.10 and 1.25 in the horizontal and vertical directions, respectively. Using a 1 mm thick birefringent filter (BRF), wavelength tuning from 727.2 nm to 787.3 nm was obtained. Stable dual-wavelength operation at 755.2 nm and 764.2 nm with a maximum average power of 1.8 W was achieved by employing a 6 mm thick BRF.
2. Experimental setup
Two kinds of cavity configurations were experimentally adopted to characterize the 589 nm laser pumped Alexandrite laser performance. The first one was a plane-plane linear cavity with a cavity length of 30 mm, which is schematically shown as the top inset of Fig. 1. The rear mirror (RM) was antireflection (AR) coated both for 589 nm and 638 nm and high-reflection (HR) coated for 700-800 nm band. The gain medium was a 0.2 at.% Cr-doped Alexandrite crystal with AR coating for 700-800 nm band, which was provided by Crystech Co. Ltd. and cut along c-axis. It was 3×3×10 mm3 in dimension. The crystal was wrapped with indium foil and mounted in a copper block maintained at 25 °C. A linearly polarized, frequency doubled Raman Yb-fiber laser with a maximum average power of 7.7 W was used as the pumping source (Precilasers, VFLH-589-10-cw), whose central wavelength was 588.4 nm, as shown in Fig. 1. The beam quality factor was better than 1.1 and the corresponding beam profile is shown as the bottom inset of Fig. 1. A half-wave plate (HWP) at 589 nm was used to adjust the pump polarization to be aligned with the b-axis of the Alexandrite crystal for maximum absorption. The pump beam was focused into the Alexandrite crystal with a radius of 125 µm and a Rayleigh range of about 83 mm in air by a lens with a focal length of 75 mm. About 100% of the incident pump power at 589 nm was absorbed. The middle inset of Fig. 1 shows the schematic diagram of the second employed cavity configuration, which was a folded cavity with longer cavity length to allow for the insertion of BRF. The full cavity length of the folded cavity was about 400 mm, where cavity length of L1 and L2 were 260 mm and 140 mm, respectively. M2 was a concave mirror with a radius of curvature of 500 mm and HR coating for 700-800 nm band. Three pieces of BRFs with thickness of 1 mm, 4 mm, and 6 mm were tested for wavelength tuning. Output couplers (OCs) with partial reflectivity of 95%, 97%, and 99% were tested during the laser experiments.
Fig. 1. Central wavelength of the frequency doubled Raman Yb-fiber laser. Inset (top): the schematic diagram of the experimental setup of a plane-plane linear cavity. Inset (middle): the schematic diagram of the experimental setup of a folded cavity. Inset (bottom): the typical beam profile of the pumping source at 589 nm.
3. Results
Figure 2(a) shows the Alexandrite laser performance obtained with the plane-plane linear cavity configuration. One can see that the output power almost increased linearly with the increase of 589 nm pump power. The slope efficiency was found to be 41%, 39%, and 34%, for OC with R=99%, R=97%, and R=95%, respectively. Although this trend was inconsistent with the typical four-level laser theory, similar effects were also reported in Alexandrite lasers pumped by red LDs around 638 nm [18,23] and tapered LDs at 678.5 nm and 680.5 nm [19], for which the pump excited state absorption (ESA) effects should be responsible [18,24]. This is due to the fact that the pump ESA effects reduce the pump quantum efficiency when a low reflectance OC is used. When the pump power was 7.7 W, a maximum output power of 2.51 W was achieved with the OC of R=99%, corresponding to an optical-to-optical conversion efficiency of 32.6% and a slope efficiency of 41%. In solid-state lasers, the quality of crystals and mirrors have significant contributions on the passive losses of a cavity, which further influences the slope efficiency. From the measured lasing threshold with different OC transmissions, one can get the round-trip losses of the cavity. In our case, the laser thresholds were found to be 3.3 W, 2.4 W, and 1.5 W for the OCs with reflectivity of 95%, 97%, and 99%, respectively. With the modified Findlay-Clay analysis used [19], the round-trip internal loss of the cavity was found to be 2.3% as shown in Fig. 2(b). The results yield a perfect straight line which is inconsistent with the theory that the pump ESA results in approximately quadratic increase in laser threshold with OC transmission [18]. It is likely due to the reason that there is some deviation between the actual and used value of OC transmission and fewer amounts of the OC transmission have been adopted. Such high loss can be greatly reduced by using an Alexandrite crystal with better coating and further improved crystal quality. The typical Alexandrite laser emission spectrum was recorded by an optical spectrum analyzer with a resolution of 0.5 nm, as shown in the right inset of Fig. 2(a). The spectrum was centered at 755.5 nm with a bandwidth of 7 nm in full width at half maximum (FWHM). The beam quality factor was measured to be 1.10 and 1.25 for the horizontal and vertical direction, as shown in the left inset of Fig. 2(a).
Fig. 2. (a) Output power versus 589 nm pump power with different OC transmissions for the Alexandrite laser with a plane-plane linear cavity. Inset (left): the measured M2 and the spatial beam profile at maximum output power with 589 nm pumping source. Inset (right): the recorded spectrum at maximum output power. (b) Measured lasing threshold with different OC transmissions. (c) Laser output power versus incident pump power for both cases with an OC of R=99%. Inset (left): the measured M2 values and the spatial beam profile at maximum output power with 638 nm pumping source.
To intuitively show the advantages of the high brightness on-peak 589 nm laser pumping source, another off-peak 638 nm LD was employed to compare the laser performance directly, based on the plane-plane linear cavity configuration. To do that, a fiber coupled 638 nm LD with a maximum power of 9 W (Huaguang Optoelectronics Co. Ltd.) was used. A polarizer was used to achieve a linear polarized output, whose polarization was aligned with the b-axis of the Alexandrite crystal using an HWP. The diameter of the fiber was 105 µm with a length of 0.5 m and its numerical aperture (NA) was 0.22. The beam quality factor was about 57. The available linearly polarized 638 nm pump power was up to 8 W owing to the coupled fiber with a short length. The pump beam from the fiber was focused into the Alexandrite crystal by a coupling system with a beam compression ratio of 2.4, resulting in a beam radius of about 125 µm which is the same with that used in the case of 589 nm on-peak pumping. The Rayleigh range was estimated to be around 1.3 mm in air. About 90% of the incident pump power at 638 nm was absorbed. The laser output power versus incident pump power with an OC of R=99% for both cases is shown in Fig. 2(c). The maximum output power, optical-to-optical conversion efficiency, and slope efficiency were 1.46 W, 18%, and 23% for 638 nm pumping. While in the case of 589 nm pumping, they were 2.51 W, 32.6%, and 41%, respectively. The beam quality factors at maximum output power of 1.46 W for 638 nm pumping were 1.59 and 1.92 in the horizontal and vertical directions, respectively. From all these results, one can see the clear difference for both cases, which could be partially attributed to the higher absorption efficiency for 589 nm laser. Importantly, the 83 mm Rayleigh range of the 589 nm laser was much longer than that of the 638 nm LD (∼1.3 mm), resulting in a better overlap between the pump and signal beams in the crystal which ensures a more efficient interaction and higher output efficiency. Therefore, the better beam quality of the 589 nm laser might be the main factor for achieving more excellent laser output characteristics.
Afterwards, cavity configuration shown in the middle inset of Fig. 1 was used for obtaining wavelength tuning operation. First, the output characteristics in the folded cavity without the birefringent element were explored. The measured laser output power versus the 589 nm pump power with an OC of R=99% is shown in Fig. 3(a). The threshold was about 1.2 W. The maximum output power was about 2.21 W when the pumping power at 589 nm was 7.7 W, corresponding to an optical-to-optical conversion efficiency of 28.7% and a slope efficiency of 28%. The typical spectrum with a resolution of 0.5 nm at the highest output power is shown as the black line in Fig. 3(b). The central laser wavelength was 757.2 nm with a bandwidth of 1.9 nm in FWHM, which is much narrower than that (7 nm) obtained in the case when a plane-plane linear cavity was used. This is because of a higher lasing threshold due to the longer cavity length in the folded cavity, which then restricts the spectrum of modes that eventually reach threshold.
Fig. 3. (a) Output power versus 589 nm pump power in a folded cavity. (b) The (dual) laser wavelength spectrum at the maximum pump power for the Alexandrite laser in a folded cavity without BRF, with a 4 mm thick BRF, and with a 6 mm thick BRF, respectively.
The dual-wavelength laser output was also investigated by using a 4 mm thick BRF and a 6 mm thick BRF, respectively. A series of laser spectrum was obtained by rotating the BRF. When a 4 mm thick BRF was introduced in the folded cavity, a dual-wavelength operation with a maximum output power of 1.7 W and a slope efficiency of 28% was achieved, as shown in Fig. 3(a). The typical dual-wavelength spectrum with the wavelengths at 749.2 nm and 762.9 nm is shown as the red line in Fig. 3(b). Its resolution is 0.1 nm. The spectral separation was about 13.7 nm. Dual-wavelength output characteristics of the Alexandrite laser by employing a 6 mm thick BRF were also obtained. A stable dual-wavelength operation at 755.2 nm and 764.2 nm with a spectral separation of 9 nm was generated, which is shown as the blue line in Fig. 3(b). The maximum output power was 1.8 W at the highest 589 nm pump power of 7.7 W, as shown in Fig. 3(a). The results in Fig. 3(b) also reveal that the spectral separation and the linewidth of the generated dual-wavelengths with the 4 mm BRF were wider and broader than that obtained in the case with the 6 mm BRF. It is consistent with the theory that the free spectral range of a BRF and the linewidth of the generated wavelength are inversely proportional to the thickness of the BRF [7].
The wavelength tunability was further investigated by introducing a 1 mm thick BRF, which was placed between the mirror M2 and OC and positioned at the Brewster angle to minimize the insertion losses. Figure 4 shows the results of laser output power versus laser wavelength. The laser wavelength was continuously tunable from 727.2 nm to 787.3 nm. A maximum output power of 1.42 W was reached at 755 nm. A wider tuning range might be obtained by optimizing the temperature of Alexandrite crystal and choosing a thinner BRF such as a 0.5 mm thick BRF which has a larger free spectral range.
Fig. 4. Wavelength tuning and the corresponding laser output power for the Alexandrite laser with a folded cavity using a 1 mm thick BRF.
4. Conclusion
In summary, an Alexandrite laser on-peak pumped by a frequency doubled Raman Yb-fiber laser at 589 nm was demonstrated, with a maximum output power of 2.51 W at 755 nm and a slope efficiency of 41%. The beam quality factor was better than 1.25. The wavelength tunability were verified with a 1 mm, 4 mm, and 6 mm thick BRF, respectively. With a 1 mm thick BRF, emission wavelength was tuned between 727.2 nm and 787.3 nm. In the case of a 4 mm thick BRF, dual-wavelength operation at 749.2 nm and 762.9 nm was achieved. A stable dual-wavelength operation at 755.2 nm and 764.2 nm was obtained by employing a 6 mm thick BRF. The maximum dual-wavelength output power was up to 1.8 W. The results obtained in this letter may provide a way for the development of higher power Alexandrite laser with high beam quality and high efficiency benefiting from the available 100-W level 589 nm laser, such as the 50 W 589 nm continuous-wave (CW) Raman fiber laser and the 208 W 589 nm all-solid-state pulsed laser [21,25]. The resulting heat loading at fracture for Alexandrite crystal is about 600 W/cm which is high enough to enable high power optical pumping, including CW and pulsed pumping without fracture [1,2].
Funding
Natural Science Foundation of Shandong Province (ZR2019MF039); founding for Qilu Young Scholars from Shandong University; founding for Distinguished Young Scholars from Shandong University; National Natural Science Foundation of China (11804192, 61475087); Ministry of Education of the People's Republic of China (6141A02022421, 6141A02022430); Fundamental Research Fund of Shandong University (2017JC023, 2018JCG02).
Disclosures
The authors declare no conflicts of interest.
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