A compact continuous-wave intracavity frequency doubling Pr:YLF ultraviolet laser at 360 nm

A compact continuous-wave intracavity frequency doubling Pr:YLF ultraviolet laser at 360 nm | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article A compact continuous-wave intracavity frequency doubling Pr:YLF ultraviolet laser at 360 nm Jianjian Ruan, Zichen Zhang, Dong Sun, Hongyi Lin This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3923623/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract We demonstrated a continuous-wave (CW) ultraviolet laser (UV) at 360 nm generated by intracavity frequency doubling of Pr:YLF laser at 720 nm. It was more compact and simpler than the reported UV laser, because a straight cavity was adopted. The entire length of the laser is 75 mm, and the resonant cavity length is only 25 mm. By using an InGaN laser diode (LD) emitting at 444 nm with a maximum incident power of 3.1W, we achieved the CW UV laser with an output power of 252 mW. The optical to optical conversion efficiency was up to 8.1%, and the output power stability was better than 2% in 2 hours. It can be applied in spectral analysis, material analysis, bioengineering, optoelectronic detection, medical treatment, etc. Pr:YLF Ultraviolet laser Intracavity frequency doubling 360 nm 720 nm Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction UV laser has lots of advantages such as short wavelength, easy focusing, high resolution, and large energy, can be widely used in many fields, including spectroscopy, biological analysis, medicine, precision manufacturing, optical data storage, high-resolution printing and lithography [ 1 – 6 ]. There are several approaches to obtain UV lasers, the most broadly applied methods is to use nonlinear crystal frequency tripling Nd 3+ :YAG lasers in the cavity. However, this method requires two frequency transformations, significantly reduces the conversion efficiency of the laser and increases the complexity of the structure [ 7 – 9 ]. Trivalent praseodymium ion (Pr 3+ ) has attracted much attention as a rare earth element ion that can directly achieve laser output in visible spectral regions, including deep red (697 and 720 nm), red (638 nm), orange (604 nm), green (522 nm), and blue (480 nm) [ 10 – 13 ]. This makes it is possible to obtain UV laser through a single frequency transformation in cavity, thereby increasing the conversion efficiency and simplifying the structure [ 14 ]. Among them, praseodymium doped yttrium lithium fluoride (Pr:YLF) crystal is considered the most promising one [ 15 ]. 720 nm is an important spectral line of Pr:YLF crystal, furthermore the second harmonic generation (SHG) of this line is 360 nm which is close to the wavelength of the third harmonic generation of Nd 3+ :YAG lasers. It has potential application in the areas of DNA sequencing, flow cytometry, cell sorting, spectrum analysis, optical instrument, biotechnology, medical treatment, Raman spectroscopy and fluorescence analysis, etc [ 16 – 18 ]. In 2007, Ostroumov et al. achieved UV laser at 360 nm with the output power of 1.3 W pumped by 5.3 W optically pumper semiconductor (OPS) laser at 479 nm [ 19 ]. The optical to optical conversion efficiency was as high as 24.5%. However, using OPS laser as the pump source requires fiber coupling, which results in poor polarization and complex structure of the laser, moreover OPS is expensive. Due to the development of InGaN LD at 444 nm, it has become easier to realize more compact and affordable Pr:YLF lasers. In 2014, Zhang el al. demonstrated the output power of 460 mW at 360 nm pumped by using an InGaN LD with a maximum incident power of 5 W [ 20 ]. It adopted a three-mirror folded cavity, beam shaping system, two plano-convex lenses, which also made the structure of the laser complex. In 2019, Dou el al. reported a combined dual-wavelength laser diode beam end-pumped single longitudinal mode Pr 3+ :LiYF 4 all-solid-state UV laser at 360 nm, with the output power of 112 mW [ 21 ]. A V-folded cavity structure was employed. By using a combined beam of 1.4 W 444 nm LD and 1.5 W 469 nm LD as the pump source also complicated the structure of the laser. In this paper, we reported a compact CW UV laser at 360 nm, which was generated by intracavity frequency doubling of the Pr:YLF laser at 720 nm. It adopted a simple straight cavity structure with the cavity length of only 25 mm. By using an anamorphic prism pair with the length of 20 mm for beam shaping and then focusing through a plano-convex lens, the structure of the laser can be effectively simplified. The entire length of the laser is only 75 mm. The output power is 252 mW when the pump power is 3.1 W. 2. Laser design and experimental setup The schematic of the intracavity frequency doubling Pr:YLF UV laser at 360 nm is shown in Fig. 1 . An α- cut Pr:YLF crystal was used in the experiment. The dopant concentration of Pr 3+ is 0.5%, with the size of 3×3×10 mm. The both surface of the crystal is coated a film with the high transmittance (T > 99%) at 300–720 nm. The fluorescence spectrum of Pr:YLF crystal was measured when the pump power of the InGaN LD was 0.85 W as shown in Fig. 2 . It contains an absorption spectrum at 440 nm and multiple emission spectra in the visible regions, with wavelengths of 480, 522, 545, 604, 607, 638, 697, and 720 nm, respectively. It could be found that the spectral line at 480, 604, 607 and 638 nm has stronger intensity than the spectral line at 720 nm. In order to obtain a single fundamental frequency light at 720 nm, the coating of the input and output mirrors of the cavity must be elaborately devised to ensure enough gain at 720 nm and retain strong loss at 480, 604, 607, 638 nm simultaneously. The lithium triborate (LBO) crystal is a frequency doubling crystal with dimensions 2×2×10mm, cutting for critical type I phase matching (θ = 90°, φ = 40.7°) and antireflection coated at 720&360 nm (T > 99%) on both sides to reduce the intracavity loss of the lase. Pr: YLF and LBO crystals were respectively wrapped with indium foil and mounted at copper blocks, which were precisely controlled through a thermal electronic cooled (TEC). The temperature was maintained at 25°C. In order to simplify the structure, the simplest straight cavity structure was adopted with a resonant cavity length of 25 mm. According to the analysis of crystal properties above, M 1 (Φ6×2 mm) is a planar mirror with high transmission (T > 95%) at 444 nm and high reflection (R > 99.8%) at 360&720 nm. The output mirror M 2 (Φ6×2 mm) is a plane-concave mirror with a curvature radius of 50 mm, which is coated on the concave surface. It has high transmission (T > 95%) at 360 nm and high reflection (R > 99.8%) at 720 nm. In addition, the cavity mirror has high transmittance at 480, 604, 607 and 638 nm, with a reflectivity of less than 5%. Due to the short length of the resonant cavity, the spot radius ( ω s ( z )) of the oscillating beam in various parts of the laser crystal is approximately the beam waist size ( ω 0s ). The beam waist size of a plane-concave cavity is represented by Ref. [ 22 ]. $${\omega _{0s}}=[{{{\lambda ^2}L(R - L)} \mathord{\left/ {\vphantom {{{\lambda ^2}L(R - L)} {{\pi ^2}{]^{{1 \mathord{\left/ {\vphantom {1 4}} \right. \kern-0pt} 4}}}}}} \right. \kern-0pt} {{\pi ^2}{]^{{1 \mathord{\left/ {\vphantom {1 4}} \right. \kern-0pt} 4}}}}}$$ 1 Where λ is the wavelength of the oscillating beam; L is the length of the resonant cavity; R is the curvature radius of the concave. It can be calculated that ω 0s equal to 75.7 µm. This is an important basis for selecting the size of pumping light, in order to satisfy the mode match requirement of the laser cavity and pumping beams. The pump source is a 444 nm InGaN LD (Φ8×10 mm) which can output blue laser (439–445 nm) with a maximum power of 3.1 W. It also has polarization emission characteristics to meet the polarization absorption property of Pr: YLF crystals, further more they are compact and inexpensive. The emission wavelength of the LD was changed with the pump power and the temperature, as shown in Fig. 3 . From the linear fit, it can be found that the increase rate of the wavelength with the pump power and temperature are 1.49 nm/W and 0.077 nm/°C, respectively. Therefore, in order to match the optimal absorption of the laser crystal, we must precisely adjust the emission wavelength in the experiment. Pump beam coming out from the InGaN LD is elliptical with 1820 µm in the horizontal direction and 420 µm in the vertical direction. Therefore, it is necessary to shape and focus it to satisfy the mode match requirement. We used an anamorphic prism pair (20×15×11 mm) as the beam shaping system, which can transform the elliptical beam into a 432×416 µm nearly circular beam by compress the horizontal direction to approximately equal the vertical direction. An average throughput of 95% can be achieved if the prisms are oriented such that the incident light enters the prism pair at Brewster's angle and each surface has the appropriate AR coating for the wavelength of the incident light. Then with a help of a plano-convex lens (Φ8×3 mm; f = 15 mm), the beam was focused down to a spot size of 68×65 µm in diameter, which can match the beam waist size of the plane-concave cavity as calculated above (75.7 µm). A filter (Φ8×1 mm) with high transmission (T > 95%) at 360 nm and high reflection (T < 1%) at 420–720 nm was used to remove the stray light after the output, in order to achieve UV laser at 360nm. The length of the entire structure is approximately 75 mm. 3. Results and discussion We measured the output power of the Pr:YLF laser in Fig. 4 . The lasing threshold was only about 0.61 W, and the maximum output power was 252 mW when the pump power was 3.1 W. The pump threshold ratio was about 5, and the optical to optical conversion efficiency was 8.1%. The output power presented a linear relationship with the pump power, and the slope efficiency was 10%. We also measured the stability of the output power was better than 2% in 2 hours. With the pump power of 0.85 W, which was close to the threshold, the output spectra lines of the Pr:YLF laser was measured in Fig. 5 . The spectral lines at 480, 604, 607 and 638 nm were completely suppressed, while the single wavelength line at 720 nm was achieved. The spectral line at 360 nm was obtained through by intracavity frequency doubling of 720 nm. The pump threshold ( P th ) of the Pr:YLF laser is represented by Ref. [ 23 ]. $${P_{th}} \propto \frac{{\ln (1+r)+l}}{{\eta \sigma }}$$ 2 Where r and l are the reflectivity and the loss of the cavity, respectively; η and σ are the quantum efficiency and the emission cross section of the Pr:YLF crystal. The emission cross section at 480, 604, 607, 638 and 720 nm is 2.0, 1.4, 1.0, 2.2 and 0.9×10 − 19 cm 2 , respectively. The loss is 5‰. According to the formula, when the reflectivity of the cavity at 720 nm is greater than 99.8%, and the reflectivity at other wavelengths is less than 89.4% (at 480 nm), 94.2% (at 604 nm), 95.9% (at 607 nm) and 91.2% (at 638 nm), the threshold of 480, 604, 607 and 638 nm will exceed 5 times of 720 nm, which is greater than the pump threshold ratio. This will result in easy emission of the spectral line at 720 nm, and complete suppression of the spectral line at 480, 604, 607 and 638 nm. In the experiment, we can achieve a UV laser output at 360 nm by using the filter. 4. Conclusion Based on the Pr:YLF and LBO crystals, we investigated a compact intracavity frequency doubling UV laser at 360 nm. We employed a straight cavity structure and designed an appropriate coating of the mirrors by analyzing the fluorescence spectrum of Pr:YLF crystals. The pumping beam was shaped and focused by an anamorphic prism pair and a plano-convex lens, in order to match the beam waist size of the cavity. When the pump power of the InGaN LD was 3.1 W, we obtained a CW UV laser at 360 nm with the output power of 252 mW, the optical to optical conversion efficiency can reach to 8.1%. The CW Pr:YLF laser at 360 nm could be further improved by optimizing the coating qualities of the output coupler and getting the higher pump power which can be achieved through combined InGaN LD arrays into the crystal. Declarations Author Contribution J.R. conducted the experimental design, data collection, analysis and wrote the first draft of the manuscript. Z.Z. assisted in the preparation of the experiment. D.S. did the work on the manuscript comments and revisions. H. L. was mainly responsible for providing the overall guidance of the experiment and the manuscript. All authors reviewed the manuscript. Acknowledgements This work was supported by Natural Science Foundation of Fujian Province (2021J011217); Science Technology Innovation Plans of Xiamen (2022CXY0412 and 2022CXY0430); Technique Plans of Longyan City (2022LYF9017). References A.J. Winchester, T. J. Anderson, J. K. Hite, R. E. Elmquist, S. Pookpanratana. Ultramicroscopy. 253, 113819 (2023). X. Li, Z. Zhang, S. Ji, R. Fang, B. Xiao, H. Xu, Z. Cai. High Power Laser Sci. 11(1), 44–51 (2023). R. O. Lawal, L. T. Richardson, C. Dong, F. Donnarumma, T. Solouki, K. K. Murray. Anal. Chim. Acta. 1184(0), 339021 (2021). Y. Zhang, J. Zou, W. Zheng, K. Feng, B. Xu, Z. Yu. Chin. Opt. Lett. 19(9), 091406 (2021). M. Chen, Z. Wang, B. Wang, F. Yang, G. Zhang, S. Zhang, F. Zhang, X. Zhang, N. Zong, Z. Wang. J. Lumin. 172(15), 254–257 (2016). G. K. Simon, P. B. Steffen, H. F. Kim, N. K. Peter. ACS Omega. 5(14), 7962–7968 (2020). S. Xu, S. Zhai, B. Li, S. Gao. Infrared Phys. Techn. 128 (2023). V. Ezhilmaran, L. Vijayaraghavan, N.J. Vasa. Procedia Manufacturing. 26, 712–719 (2018). A. I. Lyashenko1, E. M. Volodina1, Yu. A. Goldin, B. A. Gureev. J Commun. Technol. EL+. 67(12), 1475–1478 (2022). H. P. Labaki, F. H. Borges, F. J. Caixeta, R. R. Goncalves. J. Lumin. 236(0), 118073 (2021). F. Li, J. Chen, Y. Weng. AIP Adv. 13(8), 085323 (2023). T. Hiroki, K. Sascha, B. Moritz, D. Maxim, K. V. Nikolai, K. Fumihiko, K. Christian. Prog. Quant. Electron. 84, 100411 (2022). Y. Ma, A. Valles, J. Tung, Y. Chen, K. Miyamoto, T. Omatsu. Opt. Express. 27(13), 18190–18200 (2019). W. Dou, S. Pu, D. Qu, Z. Zheng, K. Wang, Q. Zheng. Appl. Phys. B-Lasers O. 129(2), 1–5 (2023). N. Niu, D. Qu, W. Dou, G. Ren, Y Zhou, L. Xia, M. Lu, Q. Zheng. Chin. J. Lasers. 45(12), 19–23 (2018). D. Yun, B. Shin, J. Park, Y. Ma, C. Gwak, D. You, B. Kim. J. nanosci. nanotechno. 20(1), 128–134 (2020). B. Scott. Laser Focus World. 57(7), 33–35 (2021). A. D. Shutov, G. V. Petrov, D. W. Wang, M. O. Scully, V. V. Yakovlev. Opt. Lett. 44(23), 5760–5763 (2019). V. Ostroumov, W. Seelert, L. Hunziker, C. lhli, A. Richter, E. Heumann, G. Huber. Proceedings of SPIE. 6451, 1–6 (2008). C. Zhang, W. Yu, C. Zhang, Y. Yao, P. Zhu, P. Song, L. Bai. Opt. Spectrosc+. 118(6), 998–1001 (2015). W. Dou, S. Pu, N. Niu, D. Qu, X. Meng, L. Zhao, Q. Zheng. Acta Phys. Sin. 68(5), 054202 (2019). L. Shang. Acta Phys. Sin. 52(10), 2476–2480 (2003). H. Lin, H. Liu, X. Huang, N. Copner, D. Sun. J. Mod. Optics. 65(4), 423–426 (2018). Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-3923623","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":271237175,"identity":"128519f4-aabd-424c-a494-773fae96481a","order_by":0,"name":"Jianjian Ruan","email":"","orcid":"","institution":"Xiamen University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Jianjian","middleName":"","lastName":"Ruan","suffix":""},{"id":271237176,"identity":"27e68d8e-09ed-46b3-83d7-8df0611b9d48","order_by":1,"name":"Zichen Zhang","email":"","orcid":"","institution":"Xiamen University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Zichen","middleName":"","lastName":"Zhang","suffix":""},{"id":271237177,"identity":"b18fc93a-ed55-4a8a-80ea-1c4c2cfe3bce","order_by":2,"name":"Dong Sun","email":"","orcid":"","institution":"Xiamen University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Dong","middleName":"","lastName":"Sun","suffix":""},{"id":271237178,"identity":"a02d9ae2-d00b-4e0c-99d3-9291334dad25","order_by":3,"name":"Hongyi Lin","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAx0lEQVRIiWNgGAWjYBACPmYgkWDwn4exvbHx4QditLCBtVQwyzD3HG42liBKC5g8w2zDPiO9TYCHKC3svMckHrax8fDOfNjGIMFgJ6fbQNBhfGkSiW08PJKzE9seFDAkG5sdIKiFxwyoRYLHcHZiu4EEw4HEbURqMeCxv3kQqJFoLQlnEngYZzASr8XYIqHiAA9jTyIwkA2I8As//xnDmz8MDtgzth9/+PBDhZ0cQS1AwIIUgQaElYMAM1HJZBSMglEwCkYwAACyJDnDFZuZhgAAAABJRU5ErkJggg==","orcid":"","institution":"Xiamen University of Technology","correspondingAuthor":true,"prefix":"","firstName":"Hongyi","middleName":"","lastName":"Lin","suffix":""}],"badges":[],"createdAt":"2024-02-03 10:44:23","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-3923623/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3923623/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":50725126,"identity":"97e6b74b-038c-4a11-8378-645e4a3cd98f","added_by":"auto","created_at":"2024-02-06 10:59:33","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":57883,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of the intracavity frequency doubling Pr:YLF UV laser at 360 nm\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-3923623/v1/edd11230cb3bd03e13cb1f95.png"},{"id":50725130,"identity":"284bf2d5-f98f-4618-a3f4-e96d85495973","added_by":"auto","created_at":"2024-02-06 10:59:33","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1067153,"visible":true,"origin":"","legend":"\u003cp\u003eThe fluorescence spectrum of Pr:YLF crystal\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-3923623/v1/0cb3997dc0c638309884f6ca.png"},{"id":50725129,"identity":"4313ec5e-20b9-4123-a441-347b2f3a2bed","added_by":"auto","created_at":"2024-02-06 10:59:33","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":305582,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The relationship between the emission wavelength and the pump power.\u003c/p\u003e\n\u003cp\u003e(b) The relationship between the emission wavelength and the temperature.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-3923623/v1/e529d790519a16c8e26ac330.jpeg"},{"id":50725585,"identity":"94cd2813-356b-4ddd-bdfb-91d879a54929","added_by":"auto","created_at":"2024-02-06 11:07:33","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1388479,"visible":true,"origin":"","legend":"\u003cp\u003eThe output power of the Pr:YLF laser.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-3923623/v1/39d9a38e26127c417609ec75.jpeg"},{"id":50725127,"identity":"9a49b7a3-562c-4483-b5c4-3e61a7650cd8","added_by":"auto","created_at":"2024-02-06 10:59:33","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":887951,"visible":true,"origin":"","legend":"\u003cp\u003eThe output spectra of the Pr:YLF laser.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-3923623/v1/01a8274b916a4fd84122ab3a.png"},{"id":51860926,"identity":"f2584a47-378a-44c5-b860-68241d60335b","added_by":"auto","created_at":"2024-03-01 13:17:35","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1808934,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3923623/v1/aee74838-346b-4e74-9d3e-c01968d4d3e8.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"A compact continuous-wave intracavity frequency doubling Pr:YLF ultraviolet laser at 360 nm","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eUV laser has lots of advantages such as short wavelength, easy focusing, high resolution, and large energy, can be widely used in many fields, including spectroscopy, biological analysis, medicine, precision manufacturing, optical data storage, high-resolution printing and lithography [\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. There are several approaches to obtain UV lasers, the most broadly applied methods is to use nonlinear crystal frequency tripling Nd\u003csup\u003e3+\u003c/sup\u003e:YAG lasers in the cavity. However, this method requires two frequency transformations, significantly reduces the conversion efficiency of the laser and increases the complexity of the structure [\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTrivalent praseodymium ion (Pr\u003csup\u003e3+\u003c/sup\u003e) has attracted much attention as a rare earth element ion that can directly achieve laser output in visible spectral regions, including deep red (697 and 720 nm), red (638 nm), orange (604 nm), green (522 nm), and blue (480 nm) [\u003cspan additionalcitationids=\"CR11 CR12\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. This makes it is possible to obtain UV laser through a single frequency transformation in cavity, thereby increasing the conversion efficiency and simplifying the structure [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Among them, praseodymium doped yttrium lithium fluoride (Pr:YLF) crystal is considered the most promising one [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e720 nm is an important spectral line of Pr:YLF crystal, furthermore the second harmonic generation (SHG) of this line is 360 nm which is close to the wavelength of the third harmonic generation of Nd\u003csup\u003e3+\u003c/sup\u003e:YAG lasers. It has potential application in the areas of DNA sequencing, flow cytometry, cell sorting, spectrum analysis, optical instrument, biotechnology, medical treatment, Raman spectroscopy and fluorescence analysis, etc [\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. In 2007, Ostroumov et al. achieved UV laser at 360 nm with the output power of 1.3 W pumped by 5.3 W optically pumper semiconductor (OPS) laser at 479 nm [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. The optical to optical conversion efficiency was as high as 24.5%. However, using OPS laser as the pump source requires fiber coupling, which results in poor polarization and complex structure of the laser, moreover OPS is expensive. Due to the development of InGaN LD at 444 nm, it has become easier to realize more compact and affordable Pr:YLF lasers. In 2014, Zhang el al. demonstrated the output power of 460 mW at 360 nm pumped by using an InGaN LD with a maximum incident power of 5 W [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. It adopted a three-mirror folded cavity, beam shaping system, two plano-convex lenses, which also made the structure of the laser complex. In 2019, Dou el al. reported a combined dual-wavelength laser diode beam end-pumped single longitudinal mode Pr\u003csup\u003e3+\u003c/sup\u003e:LiYF\u003csub\u003e4\u003c/sub\u003e all-solid-state UV laser at 360 nm, with the output power of 112 mW [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. A V-folded cavity structure was employed. By using a combined beam of 1.4 W 444 nm LD and 1.5 W 469 nm LD as the pump source also complicated the structure of the laser.\u003c/p\u003e \u003cp\u003eIn this paper, we reported a compact CW UV laser at 360 nm, which was generated by intracavity frequency doubling of the Pr:YLF laser at 720 nm. It adopted a simple straight cavity structure with the cavity length of only 25 mm. By using an anamorphic prism pair with the length of 20 mm for beam shaping and then focusing through a plano-convex lens, the structure of the laser can be effectively simplified. The entire length of the laser is only 75 mm. The output power is 252 mW when the pump power is 3.1 W.\u003c/p\u003e"},{"header":"2. Laser design and experimental setup","content":"\u003cp\u003e \u003c/p\u003e \u003cp\u003eThe schematic of the intracavity frequency doubling Pr:YLF UV laser at 360 nm is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. An α- cut Pr:YLF crystal was used in the experiment. The dopant concentration of Pr\u003csup\u003e3+\u003c/sup\u003e is 0.5%, with the size of 3\u0026times;3\u0026times;10 mm. The both surface of the crystal is coated a film with the high transmittance (T\u0026thinsp;\u0026gt;\u0026thinsp;99%) at 300\u0026ndash;720 nm. The fluorescence spectrum of Pr:YLF crystal was measured when the pump power of the InGaN LD was 0.85 W as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. It contains an absorption spectrum at 440 nm and multiple emission spectra in the visible regions, with wavelengths of 480, 522, 545, 604, 607, 638, 697, and 720 nm, respectively. It could be found that the spectral line at 480, 604, 607 and 638 nm has stronger intensity than the spectral line at 720 nm. In order to obtain a single fundamental frequency light at 720 nm, the coating of the input and output mirrors of the cavity must be elaborately devised to ensure enough gain at 720 nm and retain strong loss at 480, 604, 607, 638 nm simultaneously.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe lithium triborate (LBO) crystal is a frequency doubling crystal with dimensions 2\u0026times;2\u0026times;10mm, cutting for critical type I phase matching (θ\u0026thinsp;=\u0026thinsp;90\u0026deg;, φ\u0026thinsp;=\u0026thinsp;40.7\u0026deg;) and antireflection coated at 720\u0026amp;360 nm (T\u0026thinsp;\u0026gt;\u0026thinsp;99%) on both sides to reduce the intracavity loss of the lase. Pr: YLF and LBO crystals were respectively wrapped with indium foil and mounted at copper blocks, which were precisely controlled through a thermal electronic cooled (TEC). The temperature was maintained at 25\u0026deg;C.\u003c/p\u003e \u003cp\u003eIn order to simplify the structure, the simplest straight cavity structure was adopted with a resonant cavity length of 25 mm. According to the analysis of crystal properties above, M\u003csub\u003e1\u003c/sub\u003e (Φ6\u0026times;2 mm) is a planar mirror with high transmission (T\u0026thinsp;\u0026gt;\u0026thinsp;95%) at 444 nm and high reflection (R\u0026thinsp;\u0026gt;\u0026thinsp;99.8%) at 360\u0026amp;720 nm. The output mirror M\u003csub\u003e2\u003c/sub\u003e (Φ6\u0026times;2 mm) is a plane-concave mirror with a curvature radius of 50 mm, which is coated on the concave surface. It has high transmission (T\u0026thinsp;\u0026gt;\u0026thinsp;95%) at 360 nm and high reflection (R\u0026thinsp;\u0026gt;\u0026thinsp;99.8%) at 720 nm. In addition, the cavity mirror has high transmittance at 480, 604, 607 and 638 nm, with a reflectivity of less than 5%.\u003c/p\u003e \u003cp\u003eDue to the short length of the resonant cavity, the spot radius (\u003cem\u003eω\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e(\u003cem\u003ez\u003c/em\u003e)) of the oscillating beam in various parts of the laser crystal is approximately the beam waist size (\u003cem\u003eω\u003c/em\u003e\u003csub\u003e\u003cem\u003e0s\u003c/em\u003e\u003c/sub\u003e). The beam waist size of a plane-concave cavity is represented by Ref. [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$${\\omega _{0s}}=[{{{\\lambda ^2}L(R - L)} \\mathord{\\left/ {\\vphantom {{{\\lambda ^2}L(R - L)} {{\\pi ^2}{]^{{1 \\mathord{\\left/ {\\vphantom {1 4}} \\right. \\kern-0pt} 4}}}}}} \\right. \\kern-0pt} {{\\pi ^2}{]^{{1 \\mathord{\\left/ {\\vphantom {1 4}} \\right. \\kern-0pt} 4}}}}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere \u003cem\u003eλ\u003c/em\u003e is the wavelength of the oscillating beam; \u003cem\u003eL\u003c/em\u003e is the length of the resonant cavity; \u003cem\u003eR\u003c/em\u003e is the curvature radius of the concave. It can be calculated that ω\u003csub\u003e0s\u003c/sub\u003e equal to 75.7 \u0026micro;m. This is an important basis for selecting the size of pumping light, in order to satisfy the mode match requirement of the laser cavity and pumping beams.\u003c/p\u003e \u003cp\u003eThe pump source is a 444 nm InGaN LD (Φ8\u0026times;10 mm) which can output blue laser (439\u0026ndash;445 nm) with a maximum power of 3.1 W. It also has polarization emission characteristics to meet the polarization absorption property of Pr: YLF crystals, further more they are compact and inexpensive. The emission wavelength of the LD was changed with the pump power and the temperature, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. From the linear fit, it can be found that the increase rate of the wavelength with the pump power and temperature are 1.49 nm/W and 0.077 nm/\u0026deg;C, respectively. Therefore, in order to match the optimal absorption of the laser crystal, we must precisely adjust the emission wavelength in the experiment.\u003c/p\u003e \u003cp\u003ePump beam coming out from the InGaN LD is elliptical with 1820 \u0026micro;m in the horizontal direction and 420 \u0026micro;m in the vertical direction. Therefore, it is necessary to shape and focus it to satisfy the mode match requirement. We used an anamorphic prism pair (20\u0026times;15\u0026times;11 mm) as the beam shaping system, which can transform the elliptical beam into a 432\u0026times;416 \u0026micro;m nearly circular beam by compress the horizontal direction to approximately equal the vertical direction. An average throughput of 95% can be achieved if the prisms are oriented such that the incident light enters the prism pair at Brewster's angle and each surface has the appropriate AR coating for the wavelength of the incident light. Then with a help of a plano-convex lens (Φ8\u0026times;3 mm; f\u0026thinsp;=\u0026thinsp;15 mm), the beam was focused down to a spot size of 68\u0026times;65 \u0026micro;m in diameter, which can match the beam waist size of the plane-concave cavity as calculated above (75.7 \u0026micro;m).\u003c/p\u003e \u003cp\u003eA filter (Φ8\u0026times;1 mm) with high transmission (T\u0026thinsp;\u0026gt;\u0026thinsp;95%) at 360 nm and high reflection (T\u0026thinsp;\u0026lt;\u0026thinsp;1%) at 420\u0026ndash;720 nm was used to remove the stray light after the output, in order to achieve UV laser at 360nm. The length of the entire structure is approximately 75 mm.\u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eWe measured the output power of the Pr:YLF laser in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The lasing threshold was only about 0.61 W, and the maximum output power was 252 mW when the pump power was 3.1 W. The pump threshold ratio was about 5, and the optical to optical conversion efficiency was 8.1%. The output power presented a linear relationship with the pump power, and the slope efficiency was 10%. We also measured the stability of the output power was better than 2% in 2 hours.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWith the pump power of 0.85 W, which was close to the threshold, the output spectra lines of the Pr:YLF laser was measured in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The spectral lines at 480, 604, 607 and 638 nm were completely suppressed, while the single wavelength line at 720 nm was achieved. The spectral line at 360 nm was obtained through by intracavity frequency doubling of 720 nm.\u003c/p\u003e \u003cp\u003eThe pump threshold (\u003cem\u003eP\u003c/em\u003e\u003csub\u003e\u003cem\u003eth\u003c/em\u003e\u003c/sub\u003e) of the Pr:YLF laser is represented by Ref. [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$${P_{th}} \\propto \\frac{{\\ln (1+r)+l}}{{\\eta \\sigma }}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere \u003cem\u003er\u003c/em\u003e and \u003cem\u003el\u003c/em\u003e are the reflectivity and the loss of the cavity, respectively; \u003cem\u003eη\u003c/em\u003e and \u003cem\u003eσ\u003c/em\u003e are the quantum efficiency and the emission cross section of the Pr:YLF crystal. The emission cross section at 480, 604, 607, 638 and 720 nm is 2.0, 1.4, 1.0, 2.2 and 0.9\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;19\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e, respectively. The loss is 5\u0026permil;. According to the formula, when the reflectivity of the cavity at 720 nm is greater than 99.8%, and the reflectivity at other wavelengths is less than 89.4% (at 480 nm), 94.2% (at 604 nm), 95.9% (at 607 nm) and 91.2% (at 638 nm), the threshold of 480, 604, 607 and 638 nm will exceed 5 times of 720 nm, which is greater than the pump threshold ratio. This will result in easy emission of the spectral line at 720 nm, and complete suppression of the spectral line at 480, 604, 607 and 638 nm. In the experiment, we can achieve a UV laser output at 360 nm by using the filter.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eBased on the Pr:YLF and LBO crystals, we investigated a compact intracavity frequency doubling UV laser at 360 nm. We employed a straight cavity structure and designed an appropriate coating of the mirrors by analyzing the fluorescence spectrum of Pr:YLF crystals. The pumping beam was shaped and focused by an anamorphic prism pair and a plano-convex lens, in order to match the beam waist size of the cavity. When the pump power of the InGaN LD was 3.1 W, we obtained a CW UV laser at 360 nm with the output power of 252 mW, the optical to optical conversion efficiency can reach to 8.1%. The CW Pr:YLF laser at 360 nm could be further improved by optimizing the coating qualities of the output coupler and getting the higher pump power which can be achieved through combined InGaN LD arrays into the crystal.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eJ.R. conducted the experimental design, data collection, analysis and wrote the first draft of the manuscript. Z.Z. assisted in the preparation of the experiment. D.S. did the work on the manuscript comments and revisions. H. L. was mainly responsible for providing the overall guidance of the experiment and the manuscript. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis work was supported by Natural Science Foundation of Fujian Province (2021J011217); Science Technology Innovation Plans of Xiamen (2022CXY0412 and 2022CXY0430); Technique Plans of Longyan City (2022LYF9017).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eA.J. Winchester, T. J. Anderson, J. K. Hite, R. E. Elmquist, S. Pookpanratana. Ultramicroscopy. 253, 113819 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eX. Li, Z. Zhang, S. Ji, R. Fang, B. Xiao, H. Xu, Z. Cai. High Power Laser Sci. 11(1), 44\u0026ndash;51 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eR. O. Lawal, L. T. Richardson, C. Dong, F. Donnarumma, T. Solouki, K. K. Murray. Anal. Chim. Acta. 1184(0), 339021 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY. Zhang, J. Zou, W. Zheng, K. Feng, B. Xu, Z. Yu. Chin. Opt. Lett. 19(9), 091406 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. Chen, Z. Wang, B. Wang, F. Yang, G. Zhang, S. Zhang, F. Zhang, X. Zhang, N. Zong, Z. Wang. J. Lumin. 172(15), 254\u0026ndash;257 (2016).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eG. K. Simon, P. B. Steffen, H. F. Kim, N. K. Peter. ACS Omega. 5(14), 7962\u0026ndash;7968 (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eS. Xu, S. Zhai, B. Li, S. Gao. Infrared Phys. Techn. 128 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eV. Ezhilmaran, L. Vijayaraghavan, N.J. Vasa. Procedia Manufacturing. 26, 712\u0026ndash;719 (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eA. I. Lyashenko1, E. M. Volodina1, Yu. A. Goldin, B. A. Gureev. J Commun. Technol. EL+. 67(12), 1475\u0026ndash;1478 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH. P. Labaki, F. H. Borges, F. J. Caixeta, R. R. 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Sin. 52(10), 2476\u0026ndash;2480 (2003).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH. Lin, H. Liu, X. Huang, N. Copner, D. Sun. J. Mod. Optics. 65(4), 423\u0026ndash;426 (2018).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Pr:YLF, Ultraviolet laser, Intracavity frequency doubling, 360 nm, 720 nm","lastPublishedDoi":"10.21203/rs.3.rs-3923623/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3923623/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWe demonstrated a continuous-wave (CW) ultraviolet laser (UV) at 360 nm generated by intracavity frequency doubling of Pr:YLF laser at 720 nm. It was more compact and simpler than the reported UV laser, because a straight cavity was adopted. The entire length of the laser is 75 mm, and the resonant cavity length is only 25 mm. By using an InGaN laser diode (LD) emitting at 444 nm with a maximum incident power of 3.1W, we achieved the CW UV laser with an output power of 252 mW. The optical to optical conversion efficiency was up to 8.1%, and the output power stability was better than 2% in 2 hours. It can be applied in spectral analysis, material analysis, bioengineering, optoelectronic detection, medical treatment, etc.\u003c/p\u003e","manuscriptTitle":"A compact continuous-wave intracavity frequency doubling Pr:YLF ultraviolet laser at 360 nm","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-02-06 10:59:28","doi":"10.21203/rs.3.rs-3923623/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a396d915-04f7-4938-bc25-d055fe749644","owner":[],"postedDate":"February 6th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-03-01T13:16:57+00:00","versionOfRecord":[],"versionCreatedAt":"2024-02-06 10:59:28","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-3923623","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3923623","identity":"rs-3923623","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}