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Abstract
We report laser operation of Pr:YAlO3 pumped by a frequency-doubled optically pumped semiconductor laser. Continuous-wave laser oscillations at around 622 nm, 662 nm, and 747 nm were demonstrated in plano-concave or/and plano-plano cavities. The maximum slope efficiencies were found to be 37%, 35%, and 59%, respectively, which are record-high values for Pr:YAlO3 lasers. Furthermore, lasing at 622 nm was demonstrated at room temperature for the first time to the best of our knowledge.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Trivalent praseodymium (Pr3+) is the most developed lanthanide active ion for producing visible lasers without frequency conversions to date [1]. This is mainly due to the fact that Pr3+ provides abundant emission lines ranging from cyan to deep-red spectral regions with large peak emission cross-sections, which are typically in the order of 10−19 cm2. Most of the Pr3+-activated visible lasers are based on fluoride materials, such as LiYF4 [2], KY3F10 [3], BaY2F8 [4], Y0.5Gd0.5F3 [5], LaF3 [6], CaF2 [7], and SrF2 [8], whereas oxide materials, which are much less difficult to be fabricated, are more favorable for real applications and industrialization [9]. The perovskite YAlO3 (YAP) is not only one of the few oxide host materials (together with hexa-aluminates [10] and LuAlO3 [11]) but also a promising matrix for producing Pr3+-activated visible lasers. YAP features a small maximum phonon energy (∼585 cm−1) [12] which suppresses multiphonon relaxation from the upper laser level of 3P0, a relatively large thermal conductivity (∼11 W·m−1·K−1) [13], and weak crystal-field strength, which prohibits detrimental excited-state absorption from the upper laser level to the 5d states in visible laser operations [14,15]. The principal pump and emission transitions of Pr3+ in the YAP host are schematically shown in Fig. 1. Pr:YAP can be pumped by typical laser diodes (LDs) emitting at around 445 nm or frequency-doubled optically pumped semiconductor lasers (2ω-OPSLs) emitting at around 490 nm. Strong emission lines are located at around 620 nm, 660 nm, and 750 nm, which are good supplements to the laser wavelengths of around 605 nm, 640 nm, and 720 nm in Pr:LiYF4, the most promising fluoride crystal for Pr3+-lasers by far [1].
Fig. 1. Energy diagram of Pr3+ in YAP host showing pump and laser transitions in the visible spectral region.
The continuous-wave (cw) laser properties of Pr:YAP were first systematically studied by G. Huber’s team [16,17]. Pumped by an argon-ion laser emitting at 476.5 nm (3H4→3P1+1I6), several visible laser emissions could be generated. The best result was obtained with the 3P0→3F4 transition at 747 nm, yielding a slope efficiency of 25%. With the development of blue-emitting LDs, LD-pumped Pr:YAP lasers have been studied extensively in the past decade, including cw operations from the green to deep-red spectral ranges [18–20], second harmonic generation [21], Q-switching [22], and mode-locking [23]. More recently, efficient Pr3+-lasers have been demonstrated by 2ω-OPSL pumping in both fluoride and oxide gain media [2,10,24,25]. Benefiting from the excellent beam quality of the 2ω-OPSL, these lasers in general exhibit outstanding performances. In addition, due to its longer wavelength, the 2ω-OPSL pump scheme results in an elevated Stoke efficiency and less thermal effects compared to LD pumping. Thus, highly efficient Pr:YAP lasers can be expected via 2ω-OPSL pumping.
In this paper, we present the visible laser properties of a b-cut Pr:YAP using a 2ω-OPSL pump with maximum output power of 3 W at 488.0 nm. Cw operations have been demonstrated with the 3P0→3H6, 3P0→3F2, and 3P0→3F4 transitions using plano-plano or/and plano-concave cavities at ambient temperature. The 622-nm laser was studied for the first time at room temperature and re-absorption losses were observed at the lasing wavelength. Record-high slope efficiencies of 37%, 35%, and 59% were obtained for laser operations at 622 nm, 662 nm, and 747 nm. The experimental setups and characteristics of these lasers will be discussed in detail.
2. Experimental: results and discussion
The commercially fabricated b-cut Pr:YAP crystal used for the following experiments has a nominal Pr3+ doping ratio of 0.5 at%, diameter of 5 mm, length of 6 mm and no anti-reflection coating was applied on the crystal surfaces. The 2ω-OPSL pump source (Coherent Genesis CX-STM) provides a near diffraction-limited beam (${M^\textrm{2}} < 1.1$), nearly linear polarization (polarization ratio > 100:1), and maximum optical output of 3 W. The beam radius (1/e2) of the pump was measured to be 1.1 mm by a beam profiling camera (The Imaging Source DMK 41BF02). The center wavelength of 2ω-OPSL does not vary with temperature or output power.
To analyze the spectral overlap between the pump source and the gain medium, spectroscopic measurements were performed. Figure 2 illustrates the polarized ground-state absorption spectra (Shimadzu UV-3600 Plus UV-VIS-NIR Spectrophotometer) of the b-cut Pr:YAP crystal and the emission spectrum (ADVANTEST Q9381A optical spectrum analyzer) of the 2ω-OPSL pump. Although the emission wavelength of the 2ω-OPSL pump does not match well with the two main absorption peaks around this wavelength, which are 482.2 nm (3H4→3P1, E∥c) and 491.4 nm (3H4→3P0, E∥c), the single-pass absorption efficiency in E∥c polarization at the pump wavelength was calculated to be 22%, which is consistent with the measured value of 22–24%.
Fig. 2. Polarized absorption spectra of the Pr:YAP crystal and the emission spectrum of the 2ω-OPSL.
The pump system and resonator setup for the 2ω-OPSL pumped Pr:YAP lasers are schematically shown in Fig. 3. In all of the laser experiments, the pump beam was focused by a plano-convex lens ($f = 250\textrm{ }\textrm{mm}$) into the gain medium. This provides a suitable Rayleigh length around 8.7 mm, which is comparable to the length of the active medium (6 mm). The pump beam radius at the focusing point was calculated to be ca. 40 µm. The Pr:YAP crystal was placed as close as possible to the input coupler and water-cooled in a copper crystal holder. The temperature of the cooling water was ca. 16°C. To obtain the largest absorption efficiency, a half waveplate was utilized to match the pump polarization to the crystallographic c-axis. Via suitable combinations of the input and output couplers, laser oscillations at a single wavelength could be realized. All the input mirrors were AR coated (<0.5%) at the pump wavelength and HR coated (>99.8%) at the lasing wavelength. Bandpass filters centered at 620 nm, 660 nm, and 750 nm (Thorlabs FB series) with narrow FWHM of 10 nm enabled accurate measurements of the laser output power.
2.1 Laser operation at 622 nm
The 3P0→3H6 transition offers a relatively large peak cross-section of ca. 1.3×10−19 cm2 at 621.5 nm (see also the emission spectra in Fig. 7) whereas it suffers re-absorption by the gain medium due to the energy overlap with the ground-state absorption transition of 3H4→1D2. In fact, lasing of this transition was only observed in cryogenic condition in the case of LD pumping [26]. The ground-state absorption coefficient at the lasing wavelength of our Pr:YAP crystal was measured to be around 0.03 cm−1. Thus, an additional round-trip loss of ca. 4% could be expected.
Benefitting from the good beam quality of the 2ω-OPSL pump that can effectively reduce the laser threshold power, we succeeded in laser operation at 622 nm in Pr:YAP at room temperature. The cw laser operations were carried out with two planar output couplers which have high transmittance at the other potential lasing wavelengths. The cavity lengths were kept constantly at 9 mm, as well as for the other plane-parallel resonators in this study. The laser characteristics are shown in Fig. 4. Due to the detrimental re-absorption at the lasing wavelength, the absorbed threshold pump powers were found to be around 370 mW and 420 mW by using output mirrors with ${T_{\textrm{OC}}}$ of 1.2% and 7.0%, respectively. The threshold power of the former is higher than that in the 662-nm laser operation employing the same ${T_{\textrm{OC}}}$ value of 1.2% (see also Fig. 5), although the ${\sigma _{\textrm{em}}}$ at 622 nm is larger (1.3×10−19 cm2 vs. 0.8×10−19 cm2). Since the ground-state absorption losses become less dominating at high ${T_{\textrm{OC}}}$, the slope efficiency in terms of absorbed power increased drastically from 1% to 37% by increasing the ${T_{\textrm{OC}}}$ from 1.2% to 7.0% and a maximum output power of 74 mW was achieved.
Fig. 4. Output characteristics of the Pr:YAP lasers emitting at 621.5 nm. The inset shows the typical laser beam profile at maximum output power (Planar, ${T_{\textrm{OC}}} = 7.0\%$).
Fig. 5. Output characteristics of the Pr:YAP lasers emitting at 662.2 nm. The inset shows the typical laser beam profile at maximum output power (Planar, ${T_{\textrm{OC}}} = 1.2\%$).
2.2 Laser operation at 662 nm
Lasing at 662 nm in plano-concave cavities were demonstrated by two output couplers having 1.2% and 2.3% transmittance from 400 to 660 nm, respectively. It is worth pointing out that the 622-nm emission line also falls in this spectral range as well as the HR range of the input mirror, while laser oscillation was only observed at 662 nm. This suggests that the actual optical gain of the latter is higher. Due to the high reflectivity of the output couplers at the pump wavelength and the uncoated surfaces of the YAP crystal, second-pass absorption from the output mirrors as well as from the rear surface of the crystal were taken into consideration. By assuming that lasing only took place in the overlap area between the initial pump beam and the reflection beam, the total absorption efficiency was estimated to be 28%.
Figure 5 presents the output power at 662 nm versus the absorbed pump power for these Pr:YAP lasers and a typical beam profile image. The elliptical laser beam shape is due to the anisotropy of the biaxial crystal. The optimized cavity lengths were found to be slightly larger than the radius of curvature (${r_{\textrm{OC}}}$) in the case of hemi-spherical resonators. The slope efficiencies were measured to be 20% and 24% for ${T_{\textrm{OC}}}$ of 1.2% and 2.3%, respectively. A maximum output power of 142 mW could be extracted from the ${T_{\textrm{OC}}} = 2.3\%$ resonator. Lasing with a planar output mirror with ${T_{\textrm{OC}}}$ of 1.2% as well yielded an elevated slope efficiency of 35%, which is the highest value for our laser demonstrations at this wavelength, whereas the threshold pump power doubled. These 2ω-OPSL-pumped Pr:YAP lasers at 662 nm exhibit better performances than that pumped by a LD, which gave a slope efficiency around 10% and threshold power around 550 mW with regard to absorbed power (see also Table 1) [27]. This is because the 2ω-OPSL provided a near Gaussian beam which enabled better mode matching with the oscillating laser beam.
Table 1. Comparison between the laser properties of the 2ω-OPSL-pumped and LD-pumped Pr:YAP lasers.
2.3 Laser operation at 747 nm
The 3P0→3F4 transition of Pr3+ provides a unique lasing wavelength of 747 nm in the YAP matrix while it peaks at around 720 nm in the other host materials. The laser operations at 747 nm were carried out with both plano-concave and plano-plano cavities as well. With regard to the former, as is shown in Fig. 6(a), a low laser threshold power of 23 mW can be obtained with ${T_{\textrm{OC}}}$ of 1.1% while the threshold power was around 600 mW in the case of LD pumping (${T_{\textrm{OC}}} = 2\%$) [18]. By increasing the ${T_{\textrm{OC}}}$ to 2.8%, although the threshold increased as well, we achieved a maximum output power of 315 mW without any thermal rollover and a slope efficiency of 55%, which is superior to the highest value of 45% achieved by LD pumping (see Table 1) [18]. The low threshold power and high slope efficiency indicate negligible intracavity losses. The good laser performances also benefit from the effective mode matching between the pump and the laser beam. The beam radius of the 2ω-OPSL pump was calculated to be varying from 40 µm to 42 µm inside the crystal, which is comparable to the calculated lasing mode radius of ca. 50 µm (cavity length of 76.5 mm).
Fig. 6. Output characteristics of the Pr:YAP lasers emitting at 747.1 nm in (a) plano-concave cavities and (b) plano-plano cavities. The inset shows the typical laser beam profile at maximum output power (${T_{\textrm{OC}}} = 2.8\%$ in (a) and ${T_{\textrm{OC}}} = 5.1\%$ in (b)).
Similar to the 662-nm laser operation, larger slope efficiency values could be achieved in plane-parallel resonators while the laser threshold power increased. The maximum slope efficiencies obtained by employing ${T_{\textrm{OC}}}$ of 0.9%, 1.8%, and 5.1% were found to be 59%, 59%, and 54%, respectively [Fig. 6(b)]. The highest value of 59% is close to the quantum defect limit efficiency of 65%. It is also higher or comparable to those of the other 2ω-OPSL-pumped Pr3+-lasers with this transition, e.g. Pr:LiYF4 (53%, 720 nm [2]), Pr:SRA (59%, 724.4 nm [10]), and Pr:ASL (37%, 725 nm [24]). The plane-parallel resonators are stabilized by thermal lensing effect, which can be quantified by the effective focal length [28]:
where ${A_\textrm{p}}$ is the pump beam area, ${P_{\textrm{ph}}}$ is the fraction of the absorbed pump power that results in heating, and ${K_\textrm{c}}$ and $dn/dT$ are the thermal conductivity and thermo-optic coefficient of the gain medium, respectively. The effective focal lengths at the maximum output power were calculated to be 24 mm and 19 mm for a-axis and c-axis, respectively, which are longer than the cavity length of 9 mm and suggest a stable resonance. Moreover, a mode radius of around 49–55 µm inside the gain medium could be estimated from the effective focal length, which is comparable to the pump beam radius. Despite the suitable effective focal length, significant thermal rollover of the laser efficiency was observed in low ${T_{\textrm{OC}}}$ cases. The thermal effect was eliminated when applying higher output transmittance of 5.1%, although the slope efficiency dropped to 54%. The dependent behavior of thermal effect on ${T_{\textrm{OC}}}$ can be simply attributed to the fact that plano-plano cavities with low ${T_{\textrm{OC}}}$ are more sensitive to thermally induced misalignment. Due to the insufficient data points, which did not allow a Findlay-Clay analysis, we estimated the maximum round-trip internal losses using the following equation:2.4 Laser emission spectra
The typical spectra of these free-running Pr:YAP lasers as well as the polarized fluorescence spectra are presented in Fig. 7. The polarizations of the laser emissions correspond to those of the emission lines that yield the highest transition cross-sections. We did not observe any secondary oscillations ($\lambda < 1.1\textrm{ }{\mu m}$) or changes of the lasing wavelengths with the variation of input power or cavity type. The spectral FWHM of the lasers at 621.5 nm, 662.2 nm, and 747.1 nm are found to be 0.6 nm, 0.6 nm, and 1.3 nm, respectively. Laser operations in the green spectral region should be plausible as well viewed from the spontaneous emission spectra, while no lasing was observed by using a planar output coupler with 1% transmittance from 520 nm to 560 nm. In fact, LD-pumped Pr:YAP lasing at 547 nm was only realized at cryogenic temperatures lower than 140 K [19].
Fig. 7. Polarized fluorescence spectra and laser emission spectra of the b-cut Pr:YAP crystal. The emission cross-sections were taken from [17].
3. Conclusion
In conclusion, we reported the visible laser properties of a b-cut Pr:YAP using a 2ω-OPSL pump emitting at 488.0 nm. The absorption efficiency of the crystal was found to be around 22%. Cw laser oscillations were fulfilled with the 3P0→3H6 (621.5 nm), 3P0→3F2 (662.2 nm), and 3P0→3F4 (747.1 nm) transitions and the highest slope efficiencies were found to be 37% (${T_{\textrm{OC}}} = 7.0\%$, planar), 35% (${T_{\textrm{OC}}} = 1.2\%$, planar), and 59% (${T_{\textrm{OC}}} = 1.8\%$, planar), respectively. These 2ω-OPSL pumped Pr:YAP lasers have lower laser threshold powers and larger slope efficiencies than those using LD or Ar+ laser pumping. Although the optical-to-optical efficiencies in terms of incident pump power in our laser demonstrations were limited, we believe that the absorption efficiency can be further enhanced by employing a more suitable pump wavelength in the future to achieve high output power. Our results indicate that efficient q-switched, mode-locked, or frequency conversion operations of Pr:YAP can be further realized. Moreover, the first demonstration of the 622-nm laser at room temperature in a plano-plano cavity points toward the possibility of a microchip resonator at this wavelength.
Funding
Japan Society for the Promotion of Science (15KK0245, 18H01204); National Institute for Fusion Science (ULHH040, URSX204).
Disclosures
The authors declare no conflicts of interest.
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