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Abstract
A dual-wavelength pumping scheme at 976 nm and 808 nm is proposed to improve the performance of 3 µm Er:YAP laser. 976 nm and 808 nm correspond to the ground state absorption processes of 4I15/2→I11/2 and 4I15/2→I9/2, respectively. The experimental results indicate that the introduction of 808 nm pumping not only increases the total inversion population, but also can adjust the population distribution among the sublevels in the upper and lower manifold, thus supporting higher output power and multiple wavelengths emissions. Under the single-wavelength pumping, the maximum output powers of 1.192 W and 0.223 W are obtained for 976 nm and 808 nm pumping, respectively. With regard to the 976/808 nm dual-wavelength pumping, the achievable maximum output power is 1.398 W, increased by 17.3% compared to the case of single-wavelength pumping at 976 nm. The dual-wavelength pumped Er:YAP laser can also operate in a state of multi-wavelength emissions at 2.79 µm, 2.82 µm and 2.92 µm with different dual-wavelength pump power combinations. Considering the broadband absorption characteristics of ground state absorption and the convenience of obtaining near-infrared laser diodes pumping sources, the proposed dual-wavelength pump scheme shows great potential to realize high-power, high-efficiency 3 µm erbium-doped solid-state lasers with better cost-effectiveness and more compact structure.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
3 µm is located in the mid-infrared spectral region and has many attractive characteristics such as absorption peaks of liquids and non-metallic materials, fundamental absorption bands of gas molecules and atmospheric window. The development of efficient and cost-effective 3 µm mid-infrared laser sources is creating more application scenarios such as trace gases sensing, medical diagnosis and treatment, free space communication and material processing (such as semiconductors, polymers and plastics). Erbium-doped materials has strong emissions in 3 µm region induced by 4I11/2→I13/2 transition, and can be pumped by mature commercial laser diodes (LDs) due to the ground state absorption (GSA) in the near-infrared region. So LD pumped 3 µm erbium-doped laser can be characterized by the advantages such as compact structure, good robustness, and multiple operation mode (continuous-wave, Q-switching, mode locking), making it an important development direction for mid-infrared lasers [1–6].
The upper level lifetime of the erbium ion for 4I11/2→I13/2 transition is shorter than that of the lower level, which will result in the self-termination of 3 µm laser transition. Various methods have been adopted to overcome the population bottleneck effect. From the perspective of the laser medium, co-doping with sensitized ions of Yb3+ to populate Er3+:4I11/2 or deactivated ions of Pr3+ to depopulate Er3+:4I13/2 can be used to achieve population inversion [7,8]. However, the energy transfer efficiency of co-doped laser systems sensitively depends on the doping concentration of both doping ions and the host material [9]. With regard to the laser transition process, introduction of cascaded laser transition of 4I13/2→I15/2 at 1.6 µm can also reduce the population accumulated on the 4I13/2 level. However, for bulk erbium-doped materials, dual-wavelength cascaded laser operation needs to be carried out at cryogenic temperature [10,11].
Considering the rich energy level structure of Er3+ ions, using pumping sources with different wavelengths to excite different pump transition processes is the most direct means for establishing and sustaining population inversion. ∼970 nm is the most commonly used pumping wavelength for 3 µm erbium-doped lasers, corresponding to the GSA of 4I15/2→I11/2. To achieve efficient laser operation at this pumping wavelength, it is generally necessary to use high doping concentration erbium-doped laser materials to relieve the self-terminating behavior by utilizing the energy-transfer up-conversion (ETU) process of 4I13/2 + 4I13/2→4I9/2 + 4I15/2. The 1.5 µm upconversion pumping scheme can also achieve 3 µm erbium-doped laser operation. 1.5 µm corresponds to the GSA process of 4I15/2→4I13/2, which can first provide population at 4I13/2 level. Afterwards, the 4I11/2 level can be populated by the ETU process of 4I13/2 + 4I13/2 → 4I9/2 + 4I15/2. In 2017, Liu et al. reported a 1532 nm pumped Er:SrF2 laser. An output power of 203 mW at 2795 nm was obtained with a slope efficiency of 9.5% [12]. The 1.5 µm upconversion pumping scheme requires a two-step transition process and population inversion is mainly achieved by the ETU effect (requiring high doping concentration).
There have been reports on improving the slope efficiency of 3 µm erbium-doped laser using the ESA process of 4I13/2→4I9/2. In 2022, Zhang et al. demonstrated a 1.7 µm-pumped Er:ZBLAN fiber laser, where 1.7 µm corresponded to both GSA (4I15/2→4I13/2) and ESA (4I13/2→4I9/2) processes [13]. However, a balance between GSA and ESA should be further considered in this 1.7 µm pumping scheme. On the one hand, a high GSA rate at shorter wavelength can cause better pump absorption while the ESA rate will be significantly reduced. On the other hand, a high ESA rate at longer wavelength can cause better energy recycling while the significantly reduced GSA rate will limit the achieved output power. In 2022, Yao et al. proposed a dual-wavelength pumped 3 µm Er:YAP laser, where one pumping wavelength at 976 nm corresponding to GSA process (4I15/2→4I11/2) and the other pumping wavelength at 1.7 µm corresponding to ESA process (4I13/2→4I9/2) [14]. The ESA pump source used in this work is provided by a LD, and its broad output spectrum cannot provide high ESA rate. So a slight output power increase of 60 mW for the dual-wavelength pumping scheme was obtained compared with the 976 nm single-wavelength pumping scheme. It should be noted that the ESA spectrum usually have sharp absorption lines, which is contrary to the broadband absorption bands caused by GSA. Therefore, it is necessary to use a pumping wavelength that accurately corresponds to the ESA absorption peak to achieve better energy recycling efficiency. However, the ESA pump source with specific wavelength and narrow linewidth in 1.6-1.7 µm region undoubtedly increase the cost and complexity of 3 µm erbium-doped lasers.
In this paper, a dual-wavelength GSA pumping scheme is proposed to improve the performance of 3 µm erbium-doped solid-state laser, with corresponding energy levels depicted in Fig. 1. The ∼970 nm pump light corresponds to the conventional GSA transition (4I15/2→4I11/2), directly excites Er ions from the ground state to the 4I11/2 level, denoted as GSA1 in Fig. 1. It can be noted that the ∼970 nm pumping will also introduce an excited state absorption (ESA) of 4I11/2→2F7/2 transition, which is a disadvantage to obtain more inverted population. The second GSA pump light at ∼800 nm is further introduced, denoted as GSA2 in Fig. 1. This can produce two beneficial effects. Firstly, ∼800 nm corresponds to the GSA process of erbium-doped medium, which can excite Er ions from the ground state to the 4I9/2 level. Then a quick decay of excited Er ions to the 4I11/2 level is followed due to the short lifetime of 4I9/2 level. This will further populate the upper laser level of 4I11/2. Secondly, ∼800 nm pumping will also introduce an ESA2 process, which will excite the Er ions from the 4I13/2 level to a higher excited state of 2H11/2. This can reduce the population accumulated on the lower laser level. In addition, both ∼970 nm and ∼800 nm correspond to the GSA process characterized by broad absorption spectrum. So there are no strict restrictions on the linewidth of the pumping wavelengths. Therefore, the commercialized LDs at ∼970 nm and ∼800 nm can be used as the pumping sources, which shows great potential to develop 3 µm erbium-doped solid-state lasers with with better cost-effectiveness and more compact structure.
Fig. 1. Diagram of GSA dual-wavelength pumping scheme for 3 µm erbium-doped laser with the sublevel transitions between 4I11/2 and 4I13/2 also given [15]. GSA, ground state absorption; ESA, excited state absorption; NR, non-radiative relaxation.
A proof-of-principle experiment has been conducted in a Er:YAP laser with two pumping wavelengths at 976 nm and 808 nm. Detailed experimental study on the pump absorption behavior, oscillation threshold, output power and laser spectra under single-wavelength and dual-wavelength pumping are carried out. For the case of single-wavelength pumping, the maximum output powers of 1.192 W and 0.223 W are obtained with 976 nm and 808 nm pumping, respectively. With regard to the dual-wavelength pumping, the positive improvements in four aspects demonstrate the effectiveness of dual-wavelength pumping. These four aspects include an increase in the absorption efficiencies of the dual-wavelength pumping light, a decrease in the laser oscillation threshold, an increase in output power, and a wider range of multi-wavelength output state.
2. Spectral characteristics of Er:YAP and laser experimental setup
Er:YAP is attractive as a promising mid-IR solid-state laser material due to its high thermal conductivity (11.7 W/m·K, 10.0 W/m·K, and 13.3 W/m·K respectively along the a-, b-, and c-axes), low phonon energy (550 cm−1), excellent mechanical properties (similar to that of YAG crystal) and good commercial availability [16,17]. Figure 2 shows the room temperature absorption spectrum of the Er:YAP crystal, which is measured by a Lamdab 950 UV-VIS-FIR spectrophotometer. Er:YAP has five broad GSA bands ranging from visible to near-infrared region, respectively corresponding to the transitions of 4I15/2→4S3/2 + 2H11/2 (510-550 nm), 4I15/2→4F9/2 (640-670 nm), 4I15/2→4I9/2 (780-830 nm), 4I15/2→4I11/2 (950-1000 nm) and 4I15/2→4I13/2 (1450-1560 nm). It can be seen that the two GSA processes of 4I15/2→4I9/2 and 4I15/2→4I11/2 have a direct correlation with populating the upper laser level 4I11/2 for 3 µm laser transition. Further considering the convenience of available pumping wavelengths, 808 nm and 976 nm LDs are used as the pumping sources for the 3 µm Er:YAP laser.
Fig. 2. Room temperature absorption spectrum of the Er: YAP crystal with the inset showing the absorption bands at ∼800 and ∼970 nm in detail.
Figure 3 shows the experimental setup of 976/808 nm dual-wavelength pumped Er:YAP laser. A fiber-coupled 976 nm LD with a core diameter of 105 µm and numerical aperture of 0.22 is used as the GSA1 pump source. Its radiation is coupled into the laser crystal by a 1:3 focusing optical system, generating a pump spot with a diameter of 315 µm. A fiber-coupled 808 nm LD with a core diameter of 400 µm and numerical aperture of 0.22 is used as the second pump source denoted as GSA2. Its radiation is coupled into the laser crystal by a 1:0.8 focusing optical system, generating a pump spot with a diameter of 320 µm.
A dichroic mirror DM1 with high-transmission coatings at 976 nm and high-reflection coatings at 808 nm is used to configure a dual-wavelength single-end pumping scheme, which has the advantage of easy switching between single- and dual-wavelength pumping. The input mirror IM is a plane mirror with anti-reflection coatings at 800-1000 nm on one surface, high-reflection coatings at 2700-3000 nm (R > 99.8%), and high-transmission coatings at 800-1000 nm (T > 98%) on the other surface. The plane mirror OC is coated with T = 2% transmission at 2700-3000 nm. The dichroic mirror DM2 is highly-transmissive coated at 800-1000 nm and highly-reflective coated at 2700-3000 nm, which is used to realize the separation of output laser and residual pump light. A b-cut, Er:YAP crystal (HG Optronics., INC.) with 5 at.% doping concentration is used in the experiment, and its cross-section size is 3 mm × 4 mm and its length is 8 mm. It should be noted that the light-passing surfaces of the Er:YAP laser crystal are only optically polished, and the anti-reflection coatings for the pump and laser wavelength are not coated. The laser crystal is wrapped with indium foil and mounted in copper block cooled by water at a temperature of 16°C.
3. Experimental results and discussions
Firstly, the output characteristics of the Er:YAP laser under single-wavelength pumping are studied. Under the condition of laser operation, the pump absorption behavior at the two pump wavelengths is shown in the Fig. 4. The pump absorption efficiency for 976 nm pumping is increased from 39.5% to 68.8% with the incident pump power increased from 1 W to 26 W. For the 808 nm pumping, the absorption efficiency exhibit different behavior. As the incident 808 nm pump power increases, the absorption efficiency shows a slight increase. The pump absorption efficiency for 808 nm pumping is increased from 47.9% to 50.7% with the incident pump power increased from 1 W to 20 W.
Fig. 4. Dependence of pumping light absorption efficiencies on the incident pump power for the single- and dual-wavelength pumped Er:YAP laser.
Figure 5 shows the dependence of output power on the absorbed pump power for the single-wavelength pumping. It is found that the oscillation threshold for 808 pumping is significantly higher than that of 976 pumping. The absorbed threshold pump power are 1.92 W and 2.182 W for 976 nm and 808 nm single-wavelength pumping, respectively. With regard to the 976 nm pumping, a maximum output power of 1.192 W is achieved at an absorbed pump power of 11.35 W, corresponding to an optical-to-optical efficiency of 10.5% and a slope efficiency of 15%. With regard to the 808 nm pumping, a maximum output power of 0.223 W is achieved at an absorbed pump power of 5.75 W, corresponding to an optical-to-optical efficiency of 3.9% and a slope efficiency of 6.3%. Continuing to increase the pump power at 976 nm or 808 nm will result in a decrease in output power. Using the laser crystal coated with anti-reflection coatings at the laser wavelength is expected to further improve the conversion efficiency of the Er:YAP laser. The experimental results for the single-wavelength pumping show that 976 nm pumping is more conducive to achieving lower oscillation threshold and higher output power. Green upconversion luminescence was also observed in Er:YAP crystal at both pump wavelengths of 976 nm and 808 nm. According to Ref. [18], the green fluorescence corresponds to the Er ions transitions of 2H11/2→4I15/2 and 4S3/2→4I15/2. As shown in Fig. 1, the ESA1 process of 4I11/2 → 2F7/2 at 976 nm and ESA2 process of 4I13/2→2H11/2 at 808 nm can excite Er ions to the higher excited states of 2H11/2 and 4S3/2, which are important channels for the green upconversion fluorescence.
For the 808 nm pumping, the emitting wavelength is transited from 2796.6 nm to a multi-wavelength state of 2796.4 nm, 2798.2 nm, 2822.4 nm and 2920 nm, and ultimately reached a single wavelength output state of 2920.8 nm, as shown in Fig. 6. For the 976 nm pumping, the emitting wavelength underwent the following change of from 2796.8 nm to multi-wavelength of 2796.4 nm, 2823.3 nm and 2921.7 nm, and then to 2920 nm, as shown in Fig. 7. Although the output spectra achieved at the two pumping wavelengths exhibit similar change trend, the specific output wavelengths are different. It should also be noted that the absorbed pump power range corresponding to the multi-wavelength output at 976 nm pumping is only 2.5-3.31 W, which is less than that of 2.53-4.48 W for the 808 nm pumping. This indicates that sublevels populating behavior for the 4I11/2 and 4I13/2 level level is different for the two pumping wavelengths.
Fig. 6. Laser spectrum for the 808 nm single-wavelength pumped Er:YAP laser under different absorbed pump powers: (a) 2.27 W, (b) 2.55 W and (c) 5.75 W.
Fig. 7. Laser spectrum for the 976 nm single-wavelength pumped Er:YAP laserunder different absorbed pump powers: (a) 1.92 W, (b) 2.56 W and (c) 11.06 W.
According to Ref. [19], Er:YAP has a stimulated emission cross-section of 0.9 × 10−19 cm2 at 2796 nm, higher than that of 0.3 × 10−19 cm2 at 2920 nm (the stimulated emission cross-section at 2820 nm is between the above two values). So 2796 nm has a lower oscillation threshold than that of 2920 nm. The change in population inversion ratio of upper (4I11/2) and lower (4I13/2) energy level with increasing pump power can explain the shift in output wavelength. The gradually accumulated population on the 4I13/2 level can result in a reduction of population inversion ratio, which can introduce higher reabsorption loss at shorter wavelength. Then the laser output wavelength will be driven to the longer wavelengths although with a lower stimulated emission cross section than that of 2796 nm.
Next, a detailed study is conducted on the output characteristics of Er:YAP laser under dual-wavelength pumping. The pump absorption behavior for each wavelength is remeasured when the dual-wavelength pump lights are injected, with the results also given in Fig. 4. The results show that the absorption efficiency for the two pumping wavelengths are increased under dual-wavelength pumping, with the absorption efficiency at 976 nm increased from 41.8% to 70.1%, and the absorption efficiency at 808 nm increased from 50.4% to 52.9%. Dual-wavelength pumping can provide more Er ions for 4I11/2 and 4I13/2 levels. The increase in the absorption efficiency at 976 nm indicates that the ESA1 process (4I11/2→2F7/2) plays a greater role in comparison with the case of single-wavelength pumping. The ESA2 process of 4I13/2→2H11/2 also contribute to the increase in the absorption efficiency at 808 nm. Figure 8 shows the relationship between the oscillation threshold of the Er:YAP laser and the dual-wavelength absorbed pump powers. As the power of one pumping wavelength increases, the required pump power for the other wavelength decreases accordingly.
Fig. 8. The relationship between the oscillation threshold of the Er:YAP laser and the dual-wavelength absorbed pump power.
Figure 9 shows the dependence of Er: YAP laser output power on 808 nm absorbed pump power under different 976 nm given pump powers. As the absorbed pump power at 808 nm increases, the output power is also increased, indicating that introducing 808 nm pump light is indeed beneficial for increasing the output power. It is also evident that the applicable 808 nm pump power (corresponding to the maximum output power) decreases with an increase in the given 976 nm pump power.
Fig. 9. The dependence of Er:YAP laser output powers with 808 nm absorbed pump power under different 976 nm given pump powers.
Under single-wavelength pumping, when the absorbed pump power reaches a certain value, the output power reaches its maximum. Continuing to increase the pump power will lead to a decrease in output power. The enhanced thermal load within the Er:YAP crystal should be the reason for this decrease in the output power. However, if the pumping at another wavelength is introduced, a further increase in output power is achieved although with an increase in total pump power. Figure 5(a) compares the output powers under 976 nm single-wavelength and 976/808 nm dual-wavelength pumping (808 nm absorbed pump power is 1.906 W). Under the single-wavelength pumping, the maximum output power is 1.192 W when the absorbed pump power is 11.35 W. However, in the dual-wavelength pumping scheme, further increase in output power is achieved by introducing 808 nm pump light. When the absorbed pump power at 976 nm is 11.49 W and the absorbed pump power at 808 nm is 1.906 W, the achievable maximum output power is 1.398 W, increased by 17.3% compared to the case of single-wavelength pumping at 976 nm. This improvement can be understood as follows. The introduction of 808 nm pump will not only increase the population in the upper level, but also reduce the Er ions accumulated in the lower level by using the ESA2 process (4I13/2→2H11/2). This is equivalent to reducing the corresponding quantum loss, allowing for an increase in output power at higher pump powers (compared to single-wavelength pumping). As shown in Fig. 5(b), when the maximum output power is reached for single-wavelength pumping at 808 nm, the introduction of 976 nm pumping can greatly increases the output power from 0.223 W to 1.023 W, increased by 4.59 times. This is because 976 nm directly increases the Er ions in upper level, resulting in higher quantum conversion efficiency. The output power instability for the dual-GSA-wavelength pumped Er:YAP laser at the maximum output power of 1.398 W is measured to be less than 1.5% in 2 hours, with the results shown in Fig. 10.
The output spectrum under dual-wavelength pumping also exhibits interesting changes. Under single-wavelength pumping at 808 nm, when the absorbed pump power range is 2.53-4.48 W (corresponding threshold absorbed power is 2.18 W), the Er:YAP laser operates at multiple wavelengths of 2.79 µm, 2.82 µm and 2.92 µm. At 976 nm pumping, when the absorbed pump power range is 2.5-3.31 W (corresponding threshold absorbed power is 1.92 W), the Er:YAP laser enters a multi-wavelength operation state. However, under the dual-wavelength pumping, the Er:YAP laser can operate at multiple wavelengths of 2.79 µm, 2.82 µm and 2.92 µm when it reaches the threshold, and this multi-wavelength output state can be maintained at higher pump power (corresponding absorbed pump power range for 976 nm is 0.6-4.2 W). As shown in Fig. 11, the Er:YAP laser can operate in a state of multi-wavelength emission with different dual-wavelength pump powers combinations. When the absorbed pump power at 976 nm and 808 nm is respectively 1.97 W and 1.178 W, multiple wavelengths at 2796.8 nm, 2822.5 nm, and 2919.6 nm are emitted from the Er:YAP laser. When the absorbed pump power at 976 nm and 808 nm is respectively 2.01 W and 1.724 W, multiple wavelengths at 2796.2 nm, 2798.2 nm, 2823.2 nm, 2919.9 nm, 2921 nm, 2921.6 nm can be achieved. When the absorbed pump power at 976 nm and 808 nm is respectively 5.7 W and 1.746 W, a new wavelength of 2842 nm is further observed. It should be noted that this wavelength is not observed under single-wavelength pumping. When the absorbed pump power at 976 nm and 808 nm is respectively 11.49 W and 1.906 W, dual-wavelengths at 2918 nm and 2920.8 nm are emitted from the Er:YAP laser. By changing the dual-wavelength pump power ratio, the output wavelength is adjusted accordingly. As shown in Fig. 1, 2920 nm corresponds to the transition from the lowest sublevel of 4I11/2 level to the highest sublevel of 4I13/2 level. 2796 nm and 2821 nm corresponds to the transitions between the intermediate sublevels of 4I11/2 and 4I13/2 level. The introduction of dual-wavelength pump light can drive the redistribution of inverted Er ios between different sublevels in 4I11/2 and 4I13/2 level. So multi-wavelength laser operation are achieved, which can enable the 3 µm Er:YAP laser to find more applications in multi-component gas detection or multi-channel gas detection.
Fig. 11. Laser spectrum for the 808 nm and 976 nm dual-wavelength pumped Er:YAP laser under different dual-wavelength pump powers combinations: (a) 1.178 W for 808 nm and 1.97 W for 976 nm, (b) 1.724 W for 808 nm and 2.01 W for 976 nm, (c) 1.746 W for 808 nm and 5.7 W for 976 nm and (d) 1.906 W for 808 nm and 11.49 W for 976 nm.
4. Conclusions
A proof-of-principle of experiment for 976/808 nm dual-wavength pumped Er:YAP laser is realized. From the perspective of single-wavelength pumping, ∼800 nm pumping is not ideal for effective operation of 3 µm erbium-doped lasers. However, under the ∼970/800 nm dual-wavelength pump scheme, the introduction of ∼800 nm pumping not only increases the total inversion population, but also can adjust the population distribution among the sublevels in the upper and lower manifold, thus supporting higher output power and more abundant output wavelengths. Under the single-wavelength pumping, the maximum output powers are 1.192 W and 0.223 W for 976 nm- and 808 nm-pumping, respectively. Further increase in output power is achieved in the dual-wavelength pumping scheme. When the absorbed pump power at 976 nm is 11.49 W and the absorbed pump power at 808 nm is 1.906 W, the achievable maximum output power is 1.398 W, increased by 17.3% compared to the case of single-wavelength pumping at 976 nm. It is further found that the Er:YAP laser can operate in a state of multi-wavelength emissions with different dual-wavelength pump power combinations. In addition, compared with the single-wavelength pumping, the absorption efficiencies at 976 nm and 808 nm are increased by 1.3% and 2.2% in the dual-wavelength pumping scheme, respectively. The proposed 976/808 nm dual-wavelength pumping scheme demonstrates the effectiveness of improving 3 µm erbium-doped laser performance.
Funding
National Natural Science Foundation of China (61875077).
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.
References
1. T. Sanamyan, M. Kanskar, Y. Xiao, D. Kedlaya, and M. Dubinskii, “High power diode-pumped 2.7-µm Er3+:Y2O3 laser with nearly quantum defect-limited efficiency,” Opt. Express 19(S5), A1082–A1087 (2011). [CrossRef]
2. T. Li, K. Beil, C. Kränkel, and G. Huber, “Efficient high-power continuous wave Er:Lu2O3 laser at 2.85 µm,” Opt. Lett. 37(13), 2568–2570 (2012). [CrossRef]
3. Li Wang, Haitao Huang, Deyuan Shen, Jian Zhang, Hao Chen, Yong Wang, Xuan Liu, and Dingyuan Tang, “Room temperature continuous-wave laser performance of LD pumped Er:Lu2O3 and Er:Y2O3 ceramic at 2.7 µm,” Opt. Express 22(16), 19495–19503 (2014). [CrossRef]
4. H.T. Huang, L. Wang, D. Y. Shen, J. Zhang, and D. Y. Tang, “Self-Pulsed Nanosecond 2.7-µm Solid-State Erbium Laser by Cooperatively Enhanced Reabsorption,” IEEE Photonics J. 7(6), 1–7 (2015). [CrossRef]
5. Weichao Yao, Hiyori Uehara, Hiroki Kawase, Hengjun Chen, and Ryo Yasuhara, “Highly efficient Er:YAP laser with 6.9 W of output power at 2920 nm,” Opt. Express 28(13), 19000–19007 (2020). [CrossRef]
6. Chengjin Shi, Haitao Huang, Min Li, Yushuo Bao, and Zihan Li, “Passively Q-switched 3 µm erbium-doped solid state lasers using gold nanorods as broadband saturable absorber,” Opt. Laser Technol. 160, 109095 (2023). [CrossRef]
7. Dunlu Sun, Jianqiao Luo, Qingli Zhang, Jingzhong Xiao, Wenpeng Liu, Shangfei Wang, Haihe Jiang, and Shaotang Yin, “Growth and radiation resistant properties of 2.7–2.8µm Yb,Er:GSGG laser crystal,” J. Cryst. Growth 318(1), 669–673 (2011). [CrossRef]
8. Jiakang Chen, Dunlu Sun, Jianqiao Luo, Huili Zhang, Renqin Dou, Jingzhong Xiao, Qingli Zhang, and Shaotang Yin, “Spectroscopic properties and diode end-pumped 2.79 µm laser performance of Er,Pr:GYSGG crystal,” Opt. Express 21(20), 23425–23432 (2013). [CrossRef]
9. P. S. Golding, S. D. Jackson, T. A. King, and M. Pollnau, “Energy transfer processes in Er3+-doped and Er3+,Pr3+-codoped ZBLAN glasses,” Phys. Rev. B 62(2), 856–864 (2000). [CrossRef]
10. T. Sanamyan, “Diode pumped cascade Er:Y2O3 laser,” Laser Phys. Lett. 12(12), 125804 (2015). [CrossRef]
11. T. Sanamyan, “Efficient cryogenic mid-IR and eye-safe Er:YAG laser,” J. Opt. Soc. Am. B 33(11), D1–D6 (2016). [CrossRef]
12. Jingjing Liu, Jie Liu, Jimin Yang, Weiwei Ma, Qinghui Wu, and Liangbi Su, “Efficient mid-infrared laser under different excitation pump wavelengths,” Opt. Lett. 42(19), 3908–3911 (2017). [CrossRef]
13. Junxiang Zhang, Rui Wang, Shijie Fu, Quan Sheng, Wenxin Xia, Lu Zhang, Wei Shi, and Jianquan Yao, “Erbium-doped ZBLAN fiber laser pumped at 1.7 µm for emission at 2.8 µm,” Opt. Lett. 47(15), 3684–3687 (2022). [CrossRef]
14. Weichao Yao, Hiyori Uehara, Enhao Li, and Ryo Yasuhara, “Power-scalable two-wavelength pumped Er:YAP laser at 2.9 mu m,” Opt. Laser Technol. 152, 108073 (2022). [CrossRef]
15. M. Stalder and W. Luthy, “Spectroscopy of 3-µm laser transitions in YAlO3:Er,” J. Appl. Phys. 62(9), 3570–3572 (1987). [CrossRef]
16. R. L. Aggarwal, D. J. Ripin, J. R. Ochoa, and T. Y. Fan, “Measurement of thermo-optic properties of Y3Al5O12, Lu3Al5O12, YAlO3, LiYF4, LiLuF4, BaY2F8, KGd(WO4), and KY(WO4)2 laser crystals in the 80-300 K temperature range,” J. Appl. Phys. 98(10), 103514 (2005). [CrossRef]
17. M. Fibrich, H. Jelinkova, J. Sulc, K. Nejezchleb, and V. Skoda, “Diode-pumped Pr:YAP laser,” Laser Phys. Lett. 8(8), 559–568 (2011). [CrossRef]
18. M. Szachowicz, S. Tascu, M.-F. Joubert, P. Moretti, and M. Nikl, “Realization and infrared to green upconversion luminescence in Er3+:YAlO3 ion-implanted optical waveguides,” Opt. Mater. 28(1-2), 162–166 (2006). [CrossRef]
19. Hiroki Kawase and Ryo Yasuhara, “2.92-µm high-efficiency continuous-wave laser operation of diode-pumped Er:YAP crystal at room temperature,” Opt. Express 27(9), 12213–12220 (2019). [CrossRef]
