summary of Science material doctoral thesis: Research on dynamic properties and develop the solid-state ultraviolet laser system using CE3+ ion doped material

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The thesis is performed theoretically and experimentally. The theoretical studies on the dynamics of the single short pulse emission using transient cavity method and narrow linewidth and tunable wavelength generation using Littrow grating configuration were performed. On the other hand, the experimental investigations were carried out Experimental research was carried out based on the optimal results obtained by the theoretical studies.

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MINISTRY OF EDUCATION VIETNAM ACADEMY AND TRAINING OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY ----------------------------- PHAM VAN DUONG RESEARCH ON DYNAMIC PROPERTIES AND DEVELOP THE SOLID-STATE ULTRAVIOLET LASER SYSTEM USING Ce3+ ION DOPED MATERIAL Specialized: Optics Numerical code: 944 01 10 SUMMARY OF SCIENCE MATERIAL DOCTORAL THESIS HANOI – 2021
The thesis was completed at: Department Physics, Graduate University Science and Technology, VAST Center of Quantum Electronics, Institute of Physics, VAST Superviors: Assoc.Prof. Dr. Pham Hong Minh Prof. Drr. Nguyen Dai Hung Reviewer 1: .............................................................................................................................. .............................................................................................................................. Reviewer 2: .............................................................................................................................. .............................................................................................................................. Reviewer 3: ............................................................................................................................. .............................................................................................................................. The thesis will be defended at: .................................................................................................................................................... .................................................................................................................................................... Time: .......................................................................................................................................... The thesis could be found at:  National Library of VietNam  Library of Graduate University Science and Technology
LIST OF PUBLICATIONS 1. Pham Van Duong, Nguyen Xuan Tu, Nguyen Van Diep, Pham Hong Minh, Marilou Cadatal-Raduban, Sarukura Nobuhiko, “Development of a short pulse broadband and narrow linewidth ultraviolet laser using Ce:LiCAF crystal”, Communications in Physics, 29(3SI), (2019), pp. 341-349. 2. Minh Hong Pham, Marilou Cadatal-Raduban, Duong Van Pham, Tu Xuan Nguyen, Mui Viet Luong, Kohei Yamanoi, Toshihiko Shimizu, Nobuhiko Sarukura and Hung Dai Nguyen, “Tunable narrow linewidth picoseconds pulses from a single grating gain- switched Ce:LiCAF laser, Laser Phys. 28 (2018) 085802 (5pp). DOI: 10.1088/1555- 6611/aac369. 3. Marilou Cadatal-Raduban, Minh Hong Pham, Duong Van Pham, Duong Thi Thuy Bui,·Kohei Yamanoi, Kohei Takeda,·Melvin John F. Empizo, Luong Viet Mui, Toshihiko Shimizu, Hung Dai Nguyen, Nobuhiko Sarukura, Tsuguo Fukuda, “Total internal reflection-based side-pumping configuration for terawatt ultraviolet amplifier and laser oscillator development”, Applied Physics B, (2018,) 124:125, https://doi.org/10.1007/s00340-018-6995-9. 4. Arita Ren, Empizo Melvin John F, Minami Yuki, Luong Viet Mui, Taniguchi Takaya, Yamanoi Kohei, Shimizu Toshihiko, Sarukura Nobuhiko, Raduban Marilou Cadatal, Pham Duong Van, Nguyen Xuan Tu, Pham Hong Minh, “A High Q-Factor, Easy-To- Use, Broadband Ce:LiCAF Laser Resonator Based On Total Internal Reflection”, Communications in Physics, Vol. 26(3), (2016), pp. 245-249, DOI:10.15625/0868- 3166/26/3/8945. 5. Phạm Văn Dương, Nguyễn Xuân Tú, Bùi Thị Thúy Dương, Nguyễn Văn Điệp, Phạm Hồng Minh, “Nghiên cứu và thiết kế hệ laser Ce:LiCAF hoạt động trong vùng tử ngoại có độ phẩm chất khác nhau”, Kỷ yếu Hội nghị Vật lý Kỹ thuật Toàn quốc lần thứ V, ISBN: 978-604-913-232-2, (2018), trang 425-431. 6. Pham Van Duong, Nguyen Van Diep, Nguyen Xuan Loi, Hoang Tien Son, Nguyen Thi Minh Tam, Pham Hong Minh and Nobuhiko Sarukura, “Recent research and development of solid state ultraviolet lasers using Ce:LiCAF crystal”, Advances in Optics, Photonics, Spectroscopy & Applications IX, ISBN: 978-604-913-578-1, (2017), pp119-124. 7. Phạm Văn Dương, Nguyễn Văn Điệp, Nguyễn Xuân Tú, Phùng Việt Tiệp, Phạm Hồng Minh, Nghiên cứu và phát triển các hệ laser tử ngoại rắn sử dụng tinh thể Ce:LiCAF, Tuyển tập báo cáo Hội nghị Khoa học kỷ niệm 45 năm thành lập Viện Hàn lâm KH&CN Việt Nam, Tiểu ban CNTT-ĐT-TĐH- CNVT; ngày 14/10/2020, ISBN: 978-604-9985- 06-5, trang 81- 88.
INTRODUCTION Ultraviolet (UV) radiation was first detected by the German physicist J.W. Ritter (1801) when it was observed below the visible light and was commonly referred to as "oxidizing rays", "chemical rays", and "tithonic rays". In 1878, the effect of short- wavelength light on bacteria was discovered along with disinfection. UV radiation lies below 200 nm in air and was named vacuum ultraviolet radiation in 1893 by German physicist Victor Schumann. In 1903, UV radiation with a wavelength of 250 nm had an effect on bacteria, and it was not until 1960 that the effects of UV radiation on DNA molecules became known. The discovery of lasers with superior characteristics in 1960 brought a leap forward in the technology of UV radiation sources. Depending on the characteristics and wavelength region, different lasers have their own applications. UV lasers have many important applications spanning many fields of science and technology such as chemistry, physics, engineering, materials science, medicine, quantum information, optoelectronics, biology, and environmental science. These applications include cutting and drilling small holes in a variety of materials, such as transparent materials with visible light, and in lithography and semiconductor chip manufacturing. Pulsed and continuous UV sources can also be used to fabricate Bragg gratings and for eye and refractive disease surgery. UV radiation sources currently in use include molecular gas lasers (N2, H2), rare gas lasers (Ar+, Kr+, Ne+), laser excimers, dye lasers, semiconductor lasers, and those using frequency conversion. The common disadvantages of these are their complicated designs, toxicity, sizes, maintenance, performance, pulse duration, wavelength tunability and flexibility, and narrow linewidth which are not suitable for UV applications. For these reasons, the optimal solution to obtain an efficient, reliable, compact, and affordable UV radiation source is to develop solid-state laser sources. In environmental studies such as the study of ozone (O3) density and distribution, for example, UV laser sources will be used as excitation sources due to the absorption spectrum characteristics of ozone from 240 to 340 nm. Studying the environment can be done by examining the atmospheric layer through the absorption spectrum from 288 to 299 nm for O3 or from 299 to 305 nm for sulfur dioxide (SO2) gas. Research and development of solid-state lasers and laser materials is a current and global trend of science and technology because it not only involves basic science and technological development but also has practical significance and urgent applications. As a result, many research institutes and universities around the world have programs in the development of laser emitters and materials as well as the evaluation of the developed laser systems for various applications. In particular, the Photonics Research Center of the Macquarie Sydney University (Australia) is trying to develop a solid-state laser source that emits extremely short pulses in the UV range for laser spectroscopy application. Although the Research Center for Climate Change of the Taiwan Academy of Sciences does not develop lasers, they have an urgent need to use lasers in environmental monitoring through light detection and ranging (LIDAR). The Institute for Materials Research of Tohoku University (Japan) also grow application-oriented crystals as a laser emitting media. 1
Globally, UV laser materials science and technology are still being endeavored by scientists. Through theory and experiment, it has been proven that changing the pressure and temperature on the crystal in one or three dimensions will lead to the change in band gap of a cerium (Ce3+)-doped fluoride crystal, from which it can extend the emission spectrum of the laser. Recently, several studies on UV lasers emitting short pulses and wavelength corrections have also been performed. A 6-ps short-pulsed laser using a mode-locked cavity has been successfully developed. Furthermore, when using a birefringent filter, the short pulses and wavelength tunability were studied. Facing the revolutionary scientific and technological results of international scientists based on laser development and application, the scientific research, application, and training in the country – whether focused in physics, materials science, physicochemical, biomedical, information, and environmental – must be increased toward laser applications to enhance our capacity, quality, and level. In Vietnam, not many scientists and research institutes focus on materials science and technology, so laser application research is limited. Recently, at the Institute of Physics, Vietnam Academy of Science and Technology, the picosecond UV laser based on frequency conversion for LIDAR applications has been investigated and developed. Using the distribution feedback method, a Rhodamine 6G active medium was pumped by 4ω lasers Nd:YAG at 532 nm resulting in laser emissions of 565.8 nm and 572.6 nm. After using a BBO crystal for frequency conversion, UV laser wavelengths of 282.9 nm and 286.4 nm were developed. However, the output laser energy is small with only a few nanojoule. The wavelength conversion efficiency is very low (only a few %), and laser is not stable. In addition, the active medium is potentially harmful to the surrounding environment and the user. Therefore, the newly-developed picosecond UV laser has limited applications, even as part of the LIDAR system. In recent decades, active laser media using a rare earth ion-doped fluoride crystal has been developed. The fluoride crystals are doped with ions such as Ce3+ which has 4f-5d transitions resulting in the emission of UV radiation. Some of these crystals and their emissions include Ce:LiCaAlF6 (280-320 nm), Ce:LiSrAlF6 (280-320 nm), Ce:LuLiF4 (300- 340 nm), Ce:YLiF4 (300-340 nm), Ce:LaF3 (275-315 nm), and Ce:LuF3 (275-315 nm). The advantages of these fluoride materials as active laser medium are their broadband emission spectra, high laser efficiency, saturated pump power density, and large pumping threshold. Among the fluoride crystals, Ce:LiCaAlF6 or Ce:LiCAF is widely used due to its superior properties. A Ce:LiCAF crystal has a strong absorption at 266 nm that is suitable for the optical pump of Nd:YAG lasers, a wide tunable wavelength region from 280 to 320 nm, a large emission cross section (σe) of 6 × 10-18 cm2, a high saturation pump energy density of 115 mJ/cm2, a large destructive threshold of 5 J/cm2, and an efficiency of up to 46 %. These properties of Ce:LiCAF is conducive for the development of a solid-state ultraviolet laser emitting short pulses, narrow linewidth, tunable wavelength, and high power. From the above analysis, the research and development of solid ultraviolet laser sources with high power, short pulse, narrow linewidth, and tunable wavelength in Vietnam is not only urgent but also has scientific significance and high applicability. Stemming from these requirements, I chose the topic "Research on dynamic properties and develop the solid-state ultraviolet laser system using Ce3+ ion doped material" as my main research. 2
The objectives of the thesis include: - The dynamic study of the emission of solid-state, broadband, Ce:LiCAF UV lasers generating single short pulses (less than nanoseconds) by evaluating the effect of pump laser energy and cavity parameters on the output laser pulse duration; - The dynamic study of the narrow linewidth and tunable wavelength of a Ce:LiCAF UV laser with Littrow grating configuration by evaluating the effect of pump laser energy and cavity parameters on the emission linewidth and output laser pulse duration; - The development of a solid-state Ce:LiCAF UV laser system with Littrow grating configuration and pumped by a 266-nm (4ω) Q-switched Nd:YAG laser and generating a single short pulse, narrow linewidth, tunable wavelength emission; and - The research on other configurations for the Ce:LiCAF UV laser which is required for the development of high-powered amplifiers and under special experimental conditions. The thesis is performed theoretically and experimentally. The theoretical studies on the dynamics of the single short pulse emission using transient cavity method and narrow linewidth and tunable wavelength generation using Littrow grating configuration were performed. On the other hand, the experimental investigations were carried out Experimental research was carried out based on the optimal results obtained by the theoretical studies. In addition, the development of the laser system configuration using diamond cut crystals as well as the total internal reflection configuration to expand the emission capacity and application of the Ce:LiCAF UV laser was carried out. From these studies, the thesis is presented in 03 chapters: Chapter 1: Materials and ultraviolet lasers Ce:LiCAF. Chapter 2: The dynamics of ultraviolet laser emission using Ce:LiCAF crystal. Chapter 3: Research and development of solid ultraviolet laser system using Ce:LiCAF crystal. The thesis is performed at the Department of Physics, Academy of Science and Technology and Center of Quantum Electronics - Institute of Physics, Vietnam Academy of Science and Technology, under the scientific guidance of Assoc. Prof. Dr. Pham Hong Minh and Prof. Dr. Nguyen Dai Hung. CHAPTER I. MATERIALS AND ULTRAVIOLET LASERS Ce:LiCAF Chapter I presents some sources of directly emitting ultraviolet radiation currently being used in research and applications, especially the optical characteristics of active mediums doped ions Ce3+ ability to emit a laser in the ultraviolet wavelength region from 4f- 5d transition. In addition, we also present some short pulse and narrow linewidth emission method for ultraviolet laser using Ce:LiCAF crystal. The Ce:LiCAF laser actively medium The Ce:LiCAF laser medium was first published in 1993 by the M. A. Dubinskii. To date, this medium has been shown to be the most effective for the development of solid state UV laser systems. So far, Ce:LiCAF crystals cultivated by techniques such as Bridgeman- Stockbarger, Czochraski, micro pulling down, achieved outstanding results in ultraviolet radiation with the ability to tunable wavelengths in a wide range, up to 40 nm depending on 3
the concentration doped, while the quantum efficiency of Ce:LiCAF is above 90%, laser efficiency is up to 46%. The biggest advantage of the Ce:LiCAF crystal has an absorption range from 250 nm to 282 nm, the maximum at 266 nm (π-polarized) and 272 nm (σ- polarized), that is suitable for the optical pump of Nd:YAG lasers at 266 nm (4ω), Fig.1.10. Furthermore, the emission spectrum of the Ce:LiCAF crystal is a wide range, from 280 nm to 320 nm, so this Fig. 1.10. Spectra absorption and emission of crystalline medium is suitable for the Ce:LiCAF medium, corresponding to different development of laser sources for tunable polarization of pump source. wavelength and short pulse generation. In addition, the large emission cross-section of the Ce:LiCAF medium is an advantage to reduce the emission threshold of the laser. The Ce:LiCAF crystal has a large saturation energy density (115 mJ/cm2) and high destructive threshold (5 J/cm2), making it very suitable for the development of high power UV laser system. The Ce:LiCAF crystals have a long enough fluorescence lifetime (25÷30 ns), depending on the doping concentration of Ce3+ ions, which is suitable for multiple-pass amplifiers. Therefore, we choose Ce:LiCAF crystal as the active medium for the development of UV laser in our research. Following the goal of developing, broadband, narrow linewidth, tunable wavelength and generate short pulse of ultraviolet lasers for environmental research applications, the thesis presents some published research results on the technology. in ultraviolet lasers, including: short pulse, narrow linewidth, tunable wavelength regulation for the Ce:LiCAF ultraviolet laser. Summary of chapter I In chapter I, we have introduced and analyzed an overview of the development of ultraviolet lasers, especially lasers using Ce:Fluoride crystals, showing: i) Development of solid state UV lasers for scientific and technological applications is essential, especially solid ultraviolet laser sources using a rare earth ion doped active medium. ii) The advantages of Ce:LiCAF is favorable for the development of solid state UV lasers, able to emitting short pulses, narrow linewidth, tunable wavelength and high power in Vietnam. iii) The scientific and technological issues that need to be studied before are only limited to developing the experimental system, but there are no simulation studies to describe the emission mechanism of the UV Ce: LiCAF laser. Therefore, in Chapter 2 of the thesis, we choose the transient cavity method to generate short pulses below nanoseconds and configure using Littrow grating emits narrow linewidth laser, tunable wavelength for research and development of ultraviolet lasers using Ce:LiCAF crystal. The dynamics of emission for UV Ce:LiCAF lasers are based on a system of multi-wavelength rate equations for different configurations. 4
CHAPTER II. THE DYNAMICS OF ULTRAVIOLET LASER EMISSION USING Ce:LiCAF CRYSTAL In Chapter II, we present the dynamic of emission results of Ce:LiCAF laser for two configurations: i) A Fabry-Perot configuration emits broadband and short pulse generation; ii) Configure using Littrow grating for narrow linewidth, tunable wavelength and short pulse. In both configurations, we aim to generate short single pulses by the transient cavity method. The effects of pump energy and cavity parameters on dynamic of laser emission as well as single short pulse of the output laser were also assessed. 2.1. Theoretical model for dynamic of laser emission System of multi-wavelength rate equations describing dynamics of laser emission The dynamics of laser emission for UV lasers are performed based on a system of multi-wavelength rate equations with experimental parameters, which are built for two broad laser levels. The laser using Fabry-Perot configuration consists of amplified medium length l, two planar mirrors with reflectance R1, R2, length of cavity L. This calculation model assumes that the upper and lower laser levels is uniform expansion, ignoring the absorption effects at the pump wavelength and laser wavelength in the excited state and not including center of color. The crystal is optically pumping by Gaussian sharp pulse with a uniform internal distribution of the inside crystal. To represent broadband emissions with n different wavelengths, the system of rate equations is written as: ( ) ( ) [∑ ( )] ( ) [∑ ( ) ] ( ) (2.4) I i  t  I t    2  ei N1  t    ai N 0  t   l    i  Ai N1  t  (2.5) t t N=N0+N1 (2.6) Eq.(2.4) represents the cumulative variation of the laser level above; Equation (2.5) represents the variation in laser intensity in resonator. The quantity β - the mirror loss coefficient, with varying the value of according to the characteristics of each cavity, the system of rate equations can describe the laser emission mode, such as broadband and narrow linewidth. Depending on the cavity parameters and pump laser energy, laser emission characteristics can be assessed such as: emission threshold, laser intensity, spectrum linewidth, pulse duration, ... The loss in cavity is represented by a mirror loss and a loss due to the length of the resonator, characterized by a round-trip time of photon in the cavity. 2.2. Dynamics of Ce:LiCAF ultraviolet laser emission and short pulse by transient cavity method Dynamics of Ce:LiCAF laser for broadband emission In Fabry-Perot cavity using two planar mirrors, the mirror loss coefficient in a round trip, is determined as follows: β=-ln(R1R2); R1, R2 are mirror reflectivities; Dynamics of broadband emission for Ce:LiCAF laser with cavity parameters: L=20 mm, R1 =100%, R2=30% and variable pump energy is shown in Figure 2.2. The research results showed that the laser emission threshold at pump laser energy Ep=2.8 mJ. In region, which pumped energy is close to the threshold (about 2 times above with the laser emission threshold), the laser emits a single pulse (Fig.2.2a). Continuing to 5
increase the energy of the pumped laser energy, the Ce:LiCAF laser generates multiple pulses, the more energy it increases, the more laser pulses output (Figure 2.2b). When the pumped laser energy is too high compared to the emission threshold (over 8 times the laser emission threshold), the sharp of output laser pulse is saturated and almost repeating the sharp of pump pulse (Figure 2.2c). a. The dynamics of broadband emission with pumped laser energy ≤2 times above threshold. b. The dynamic of broadband emission with laser energy pumping 2 ÷ 8 times above the threshold. c. The dynamics of broadband emission with pumped laser energy above 8 times the threshold. Fig. 2.2. The dynamics of broadband emission for Ce:LiCAF laser with the pumped laser energy varies from the emission threshold to very high above the threshold. These results are interpreted on the basis of the buildup of cumulative stimulation. At small pump energy laser (pumping close to the threshold), the process to achieve accumulation inversion between the upper and lower laser levels takes longer, as re- accumulation is unlikely to reach the emission threshold after the process of first laser happened; This makes the timing of the laser pulse later and only one oscillation appears. When the pumped energy is high relative to the emission threshold, the ionic density at the excitation level can be re-accumulated to the threshold density after the laser emission, so the number of oscillations of the laser intensity increases. Particularly in the case the pumped energy is too high above the threshold, the accumulation and laser process can reach a stop state, so the laser pulse repeats the pump pulse. On the other hand, the greater the pumping energy, the higher the rate of accumulation of the excitation level, the earlier the timing of the laser pulse, as well as the narrower the distance between the vibrations. In order to meet the interaction requirement between the laser single pulse used as the excitation source and the object to be studied, therefore, the dynamic of laser emission for the UV laser use Ce:LiCAF crystal is concentrated under a single short pulse pumped near the threshold; evaluate the influence of parameters such as: pump laser energy, cavity length 6
and mirror reflection coefficient on the output laser pulse duration. From these results, we can find the optimal parameters to achieve the shortest laser output pulse duration. The effect of the pumped laser energy on the output laser pulse duration One of the important parameters influencing the pulse generator and output laser pulse duration is the pumped laser energy. Therefore, the effect of pumped laser energy on the output laser pulse duration was investigated. The results showed that, for a certain cavity, it is possible to optimize the pump laser energy to generate the shortest single pulse, which is the position corresponding to the pumped laser energy value before the laser generates multiple pulses. This is the transient point in the short single pulse by transient cavity method. The effect of the pumped laser energy on the output laser pulse duration is one of the conditions for single short pulse lasing by transient cavity method. The effect of output mirror reflectance on output laser pulse duration The reflection coefficient of the mirror (R1, R2) or the loss coefficient has a direct effect on the output laser pulse duration. The influence of mirror reflectance on output laser pulse duration when parameters such as pump laser energy, length of cavity and R1 are unchanged, reflectance coefficient R2 changes for Ce:LiCAF laser price. The results show that, under single-pulse condition, output laser pulse duration decreases while increasing mirror output reflectance, output laser pulse duration is minimum corresponding to R2 that before multi- pulse laser. In addition, from Equation (2.8), under condition of cavity laser, the output laser pulse duration depends on the value of the loss coefficient, so the shorter the reflection coefficient of the mirror to generate single pulses. is as small as possible. This is also one of the single short pulse generated conditions for transient cavity method. The effect of cavity length on output laser pulse duration The round trip time of the photon in cavity and the output laser pulse duration depend greatly on the length of cavity, so we examine the effect of the cavity length on the output laser pulse duration. The results show that, under single pulse emission condition, output laser pulse duration increases as cavity length increases, so to generate single short pulse laser requires short length of cavity. The Ce:LiCAF laser emits a short pulse by transient cavity method Through studying the effect of parameters such as pump energy, length and mirror reflection coefficient in cavity on the output laser pulse duration of Ce:LiCAF laser, the dependence of output laser pulses duration on different cavity parameters according to pump laser energy are shown, the results are summarized in Fig.2.7. Figure 2.7 shows that, for each cavity, the output laser pulse duration is a function of the pumped energy. At value of pump laser energy is close to the output laser pulse threshold is single pulse with large pulse duration, as the laser energy increases, the output pulse duration decreases gradually. 7
Line 4 represents the optimization according to the pump laser energy to obtain the shortest output laser pulse duration of cavity length L=20 mm, R1=30%; R2 =14%. The results show that, cavity using same mirror reflectivity, the shortest laser output pulse duration obtained with each cavity is shorter. As for each cavity of the same length, the shortest pulse duration received has a lower mirror reflectance. On that basis, we investigate the dynamics of short pulse emission below Fig. 2.7. The dependence of the shortest laser single pulse width nanoseconds by the L=20 mm transient cavity obtained with the varies cavity method with two pairs of mirrors: 1) R1=30% and R2 parameters according to the pumped = 14%; 2) R1 = 25% and R2 = 14%, the results are laser energy. shown in Fig.2.8. Fig. 2.8. Ce:LiCAF laser generated short pulse by transient cavity method with parameters: L=20 mm: a) R1=25%, R2 =14%, Ep=9.5 mJ; b) R1=25%, R2=14%, Ep=10.5 mJ. Output laser pulse duration are 292 ps and 267 ps, respectively. The results of the output laser pulse duration assessment using two pairs of mirrors showed that, after optimizing the pump laser energy, the shortest single pulse duration received was 267 ps for mirror pair (25%, 14%). The configuration using a pair of mirrors (R1 =30% and R2=14%), the shortest single pulse duration received is 292 ps with a pumped laser power of 9.5 mJ (1.7 times above threshold). For the configuration using this pair of mirrors, the photon lifetime and round-trip time in cavity are 48 ps and 150 ps, respectively. That means, the photon goes-about 3 times in cavity, after being amplified will emit the laser pulse. The study results of dynamic for the single short-pulse emission of Ce:LiCAF laser is used to guide the research and development of ultraviolet laser system that generates the picosecond pulses in the experiment. 2.3. Dynamics of Ce:LiCAF laser emission narrow linewidth, tunable wavelength using Littrow grating configuration 8
Below, the results of the study dynamic emission for Ce:LiCAF laser narrow linewidth, tunable wavelength and short pulse generation capability are presented using the Littrow grating configuration. Theoretical model for Ce:LiCAF laser emission narrow linewidth, tunable wavelength using Littrow grating configuration In the case of using the dynamic calculation model of narrow linewidth emission and tunable wavelength using Littrow grating configuration, the grating acts both as the end mirror in cavity as well as a selection spectral factor. wave. Therefore, to study narrow linewidth emission and tunable wavelength for Ce:LiCAF ultraviolet laser, we use a system of multi-wavelength rate equations with the effective diffraction coefficient of the grating replaced by reflection coefficient, described by a Gaussian function: ( ) ( ) ( ) [ ] (2.9) Rg(λi) is the calculated reflectance for the grating in the range of investigated wavelength λi ±∆λi; Ri(λi) is the reflection coefficient at the center wavelength λi. From there, the expression for the loss coefficient is transformed as follows: ( ) [ ( ) [ ]] (2.10) Solving the system of multi-wavelength rate equations (2.4), (2.5), (2.6) and (2.10) for Ce:LiCAF ultraviolet emits narrow linewidth, tunable wavelength laser using Littrow grating configuration was performed. The dynamic of emission narrow linewidth and tunable wavelength for Ce:LiCAF laser uses Littrow grating configuration Narrow linewidth emission of Ce: LiCAF ultraviolet laser using the Littrow grating configuration, the emission spectral width depends on parameters such as diffraction order, grating constant, laser spot size on grating surface, grating angle, number of spectral narrowing times and the wavelength tuning. The dynamic of narrow linewidth emission, tunable wavelength for Ce:LiCAF ultraviolet laser using Littrow grating has been studied, following parameters: L=20 mm, grating constant G = 2400 lines/mm, grating reflectivity at laser wavelength Rg=30%, output mirror reflectivity R2 =14%, laser spot radius in Fig.2.9. Spectra of narrow linewidth actively medium ω=0.5 mm (corresponding to emission of Ce:LiCAF laser, configured laser spot radius on the surface grating surface using Littrow grating ωg = 0.53 mm), pump laser energy Ep=9.5 mJ, grating angle βi=20.3o. In this calculation, due to 9
the low reflectivity of the grating at the output laser wavelength, only first order diffraction (m = 1) is calculated by the grating. Figure 2.9 shows the narrow linewidth emission spectrum of Ce:LiCAF ultraviolet laser using the Littrow grating configuration, peak emission spectrum at 288.5 nm, spectrum width is 30 pm, it shows that the grating is 3 times narrower when used as a wavelength selection. This also means that the photon needs to complete 3 time in cavity before the laser is emitted. The results of narrow linewidth emission spectral showed that the configuration using Littrow grating works effectively in selecting wavelengths and narrowing the spectrum for Ce:LiCAF ultraviolet lasers when compared with the emission spectrum of Fabry-Perot configuration (Fig. 2.8a) radiated in the wavelength range 286 nm to 289 nm, when both configurations were simulated under the same conditions. Laser ultraviolet Ce: LiCAF wavelength regulation The tunable wavelength region for Ce:LiCAF laser using the Littrow grating is done by changing the grating angle βi, the rotation angle corresponding to each tuning wavelength. The grating angle βi was varied from 18.9o to 23.3o (corresponding to the range of wavelengths from 270 nm to 330 nm), corrections for laser emitted wavelength were noted. The results for tunable wavelength region of Ce:LiCAF laser using the Littrow grating are shown in Figure 2.10. The tuned wavelength region is recorded from 278 nm to 302 nm, in order to generate the lateral wavelengths in the emission spectrum of the Ce:LiCAF active medium, it is necessary to increase the pumped laser energy. However, when increasing the pump laser Fig. 2.10. Tunable wavelength region of the Ce:LiCAF energy, it is necessary to pay laser using Littrow grating configuration. attention to the destruction threshold of the crystal, for the Ce:LiCAF medium, the destruction threshold is 40 mJ for the pump laser spot radius on the crystal surface is ωo = 0, 5 mm. It can be seen that the narrow linewidth emission with the wide wavelength regulation of the Ce:LiCAF ultraviolet laser uses the Littrow grating configuration, easy to tuning completely possible with the conditions at the laboratory in Vietnam. The wavelength regulation region depends not only on the rotation angle of the grating, the pumped laser energy, but also on the emission cross-section of the Ce:LiCAF medium at each regulated wavelength. The dynamic of laser emission single short pulse, narrow linewidth of the Ce:LiCAF laser uses Littrow grating In configuration using the Littrow grating, the grating has low reflectivity (Rg= 30%), able to generate short pulse by transient cavity method. Evaluating the influence of 10
parameters such as pumped laser energy, cavity length and output mirror reflectivity on the emission spectrum width as well as the output laser pulse duration can optimize these parameters to generate single short pulses for the narrow linewidth of Ce:LiCAF laser. The influence of pumped laser energy on the emission spectral width and the output laser pulse duration of Ce:LiCAF laser was investigated. The results showed that a Ce:LiCAF laser is capable of emitting narrow linewidth of a few tens of picometers, capable of tuning wavelength and simultaneously emitting a single short pulse below nanoseconds, the output laser pulse duration is 300 ps at pump laser energy Ep=9.5 mJ, which is 1.9 times above threshold. The effect of cavity length on pulse duration and narrow linewidth spectral The effect of cavity length on the dynamic of laser emission for narrow linewidth, single pulse of Ce:LiCAF lasers was investigated. The results showed that, as the length of cavity increases, the laser pulse duration increases, the timing of the displacement pulse towards the end of the pump pulse as has been explained previously. At the same time, as the length of cavity increases, the width of the narrow linewidth spectrum also increases, this is because, increasing the length of cavity leads to a decrease in the number of photons lifetime in cavity, which means that the time is correlated between photon on the grating decreases, so the spectral width of the emission increases. The influence of the output mirror reflectance on pulse duration and narrow linewidth spectrum The effect of the output mirror reflectance on the dynamic of narrow linewidth spectrum and pulse duration of Ce:LiCAF laser emission, was investigated. The results showed that, when the mirror reflectance increases, the laser pulse duration decreases while spectra width unchanged. Generation single short pulse of Ce:LiCAF laser using Littrow grating configuration by transient cavity method For the narrow band emission configuration for Ce: LiCAF using the Littrow grating, by BCH transient method with reflectance Rg = 30%, output mirror reflectivity R2 = 14%, length BCH L = 20 mm is capable of generating pulse of 299 ps with 7 ns pumped. Summary of chapter II Study of dynamic for Ce:LiCAF ultraviolet lasers emission have been performed systematically for two configurations, the results achieved include: The Fabry-Perot configuration, the broadband laser emission of Ce:LiCAF laser was studied, the results focused on the generation of single short pulses. The effects of cavity parameters and pumped laser energy on output laser pulse duration were evaluated. The Ce:LiCAF ultraviolet laser can generate pulses of 267 ps when pumped by a 7 ns pulse by transient cavity method. In configuration using Littrow grating, the obtained narrow-linewidth spectral of 30 pm at wavelength 288.5 nm, the tunable wavelength range from 278 nm to 302 nm, and a pulse of 299 ps received by transients cavity method. The effects of parameters (pumped laser energy, cavity length, output mirror reflectance) on spectral width and output laser pulse 11
duration have also been shown. This research results as a prerequisite to build experimental systems in Chapter III. CHAPTER III. RESEARCH AND DEVELOPMENT OF SOLID STATE LASER USING Ce:LiCAF CRYSTAL Chapter III presents research results in developing experimental systems of Ce:LiCAF solid-state ultraviolet laser. Using the Fabry-Perot configuration, the Ce:LiCAF laser system that emits broadband, single short pulse, has been successfully developed. The effects of parameters such as the pumped laser energy, the length of cavity and the mirror reflection coefficient on the output laser pulse duration were evaluated. Using the Littrow grating configuration as the end mirror has allowed the development of narrow linewidth ultraviolet laser system, tunable wavelength and short pulses generating. The research was also extended to the development of ultraviolet lasers using the Ce:LiCAF medium that was cut diamond and a total internal reflection configuration using two Pellin-Broca prisms. 3.1. Experimental system of Ce:LiCAF solid state ultraviolet laser Design for the experimental system of Ce:LiCAF solid ultraviolet lasers that are pumped by fourth harmonic of Nd:YAG Q-switched laser at wavelength of266 nm is shown in Fig. 3.3. Active medium: Ce:LiCAF crystals 1%, 5x5x10 mm. The cavity consists of two mirrors: end mirror R1 (96.7%) and output mirror R2 (30%, 25%, 14%). Pump source: Fourth harmonic of Nd:YAG Q- switched laser, 10 Hz, 7 ns, Fig. 3.3. Design for the experimental system of maximum pulse laser energy 55 mJ Ce:LiCAF laser. at 266 nm. Based on the development goals of the Ce:LiCAF ultraviolet laser for single short pulse of broadband or narrow linewidth, tunable wavelength laser emission, separate designs for the cavity are adopted, such as: i) Development Ce:LiCAF broadband laser emission of short pulses, Fabry-Perot configuration of two flat mirrors R1 (96.7%), R2 can be changed from 14% to 30%, as well as changes length of cavity and pump laser energy. ii) Development of narrow linewidth emission ultraviolet laser system, tunable wavelength and short pulse: end mirror R1 is replaced by Littrow grating G=2400 lines/mm, reflectance coefficient at first order diffraction Rg=30% for the laser wavelength range 280-320 nm. The correction of the emission wavelength of the Ce:LiCAF laser is obtained by rotating the grating. 3.2. Ultraviolet Ce:LiCAF laser emit broadband and single short pulse Characteristics of the Ce:LiCAF ultraviolet laser broadband Characteristics such as laser efficiency, emission spectrum of Ce:LiCAF were investigated. To evaluate the laser efficiency, the pumped laser energy was changed from 1 12
mJ to 18 mJ. The dependence of the output laser energy on the pumped laser energy absorbed in the crystal is shown in Figure 3.5a. The absorbed pumped energy is determined by the energy of the pumped laser beam behind the lens minus the portion of the pumped beam energy transmitted and reflected by the crystal. The laser efficiency (slope efficiency) is obtained up to 33%. Laser emission threshold at absorbing pump laser energy 3.6 mJ. The maximum output laser energy obtained is up to 3.4 mJ at the absorbed pump energy 14 mJ. Fig. 3.5. a) The efficiency and output laser pulse width of Ce: LiCAF broadband laser. Emission spectrum characteristics of Ce:LiCAF laser broadband following cavity parameters: length 20 mm, R1=96.7%, R2=30% at pump laser energy absorb 10 mJ, recorded by Spectrometer Thorlabs Compact spectrometer (resolution 1 nm) shown in Figure 3.5b. The laser spectral width of 2.2 nm (FWHM) in the range 287-293 nm at wavelength 288.5 nm, characterized by a dipole transition 5d - 4f of the Ce3+ ion. Study dynamic of Ce:LiCAF laser emission broadband As shown in the calculation, the dynamic of Ce:LiCAF ultraviolet laser emission including three mechanisms: single short pulse (near the threshold energy); multi-pulse and saturated single pulse generator. The results of dynamic laser emission for Ce:LiCAF ultraviolet laser depend on pumped laser energy are shown in Fig.3.6. a). b). c). d). 13
Fig. 3.6. Emission kinetics for UV Ce:LiCAF lasers broadband according to pump laser energy. In this configuration, the laser emission threshold is 3.6 mJ. The results shown that, in the energy range ≤8 mJ, laser emitting single pulse (Fig.3.6a). In range of pumped laser energy 8 mJ to 30 mJ (corresponding to 2 to 8 times of laser emission threshold), laser emitting multi-pulse laser (Figs.3.6b,c). The number of secondary pulses increases with increasing pumped laser energy. At pump energy is very high above the laser emission threshold (> 30 mJ), the output laser pulse is saturated and repeat the sharp of pump pulse with the output laser pulse width (6 ns) in Fig3.6d close to the pump laser pulse duration. This is completely consistent with the simulation results in Chapter II on broadband emission for the Ce:LiCAF ultraviolet laser. Further studies will be performed under the condition of single short pulse pumped near the threshold. As shown, the pulse duration of output laser depends on parameters such as the energy of the pump laser, the cavity length and the mirror reflectivity. Therefore, in order to obtain shortest of single pulse, we investigate the effect of these parameters on the output pulse duration by the transient cavity method. The influence of parameters on output pulse duration The effect of pump laser energy The dependence of the pulse duration of the output laser on the pump laser energy is shown in Fig. 3.7. At the pump energy ~ 4 mJ output laser pulse duration is 1.56 ns, as the pump energy increases, the pulse duration of output laser decreases. At pump energy is ~ 8 mJ (2 times above the threshold emission), the laser output pulse duration is minimum of 635 ps. As the pump energy continues to increase, the pulse duration increases due to the secondary pulse generation, the receiver only records the 1st order laser pulse, as the Ce:LLF laser has noted. Fig. 3.7. The effect of pumped laser energy on Ce:LiCAF output laser pulse duration 14
Thus, in the condition of short-pumped single pulse generator near threshold, at laser energy 2 times above threshold, output laser pulse width is shortest. This is also consistent with the simulation results in Chapter II for short single pulse BCH transient method. The effect of length cavity After finding the optimal pump laser energy, we continued to evaluate the effect of the cavity length on the output laser pulse duration, is shown in Figure 3.8 while parameters such as pump laser energy Ep=8 mJ, R1=96.7%, R2=30%; and length of cavity changing by 20 mm, 30 mm, and 40 mm, respectively. The results indicated that, each cavity lengths of 20 mm, 30 mm, and 40 mm, the laser output pulse durations were 553 ps, 663 ps and 811 ps, respectively. The received minimum pulse duration is 553 ps with the length L = 30 mm. Thus it can be confirmed that, under conditions of single short pulse generation, the shorter the cavity length is, the wider the output laser pulse short. The effect of the mirror reflection coefficient After the pumped laser energy, the length of cavity are optimized, the effect of output mirror reflection coefficient on the output laser pulse duration is performed, the results are shown in Figure 3.9 following parameters: Ep=8 mJ; L=20 mm; mirror R2 has reflectivity changes of 14%, 25% and 30% respectively, 15
Fig. 3.8. Effect of resonator length on laser Fig. 3.9. Effect of mirror reflectance on laser pulse width of UV Ce: LiCAF. pulse width of UV Ce: LiCAF The results indicated that, lasers cavity using mirror pairs had reflectance ratios (96.7% and 14%) emitting shortest pulses of 511 ps. It can be inferred that, under the conditions of single short pulse emission, pump laser energy and constant cavity length, the smaller the reflection coefficient of the mirror, the smallest output laser pulse duration is possible. Thus, parameters such as the pumped laser energy, the length of the resonance chamber and the reflection coefficient of the mirror have a visual influence on the output laser pulse width. The construction of a laser system with optimal parameters is capable of generating even shorter pulses. The Ce:LiCAF laser emits a single short pulse by transient cavity method From investigating the effect of the cavity parameters, the pumped laser energy on the output laser pulse duration, we construct optimal parameters to generate short ultraviolet laser pulses by transients cavity method with parameters such as length L=20 mm mirror pair R1=30% and R2=14%. Results of output laser pulse duration is shown in Fig.3.10. Ce:LiCAF laser emits single pulse of 447 ps. It can be seen that the pulse compression coefficient (the ratio of the length of the pump pulse to the laser pulse) is about 15 times. In this case, the output laser pulse energy is obtained 1.2 mJ with the pump laser pulse energy 8 mJ, corresponding to the pump energy 2 times above the laser emission threshold. It is clear that Fig. 3.10. The laser pulses 447 ps recorded by transient cavity method for Ce:LiCAF laser the shortest Ce:LiCAF ultraviolet laser broadband emission pulse is achieved after more than three round- trip in the cavity. Hence, it is possible to generate shorter laser pulses while optimizing the laser emission conditions. 16
3.3. Ultraviolet laser Ce:LiCAF emission narrow linewidth, tunable wavelength and single short pulse using Littrow grating configuration The research results of dynamics emission narrow linewidth, tunable wavelength for the Ce:LiCAF ultraviolet laser using the Littrow grating configuration show the possibility of developing an experimental system for the Ce:LiCAF ultraviolet laser for narrow linewidth, tunable wavelength and single short pulse of output laser. In this section, we present experimental development results for this laser system. The Ce:LiCAF ultraviolet laser system uses Littrow grating configuration In the Ce:LiCAF broadband configuration, the end mirror is replaced by the Littrow grating, we develop the laser system emitting narrow band, short pulse. Here, the grating acts both as the final mirror and as a wavelength selection and tuning wavelength factor. Experimental diagram of laser using Littrow grating is shown in Figure 3.11. In this cavity, length of 20 mm, output mirror has a reflectance of 14%, a grating of 2400 lines/mm. The grating's reflectance for the laser wavelength at primary diffraction is 30%. Fig. 3.11. The Ce:LiCAF laser using Littrow grating configuration. Ce:LiCAF laser emits narrow linewidth, tunable wavelength and single short pulse Ultraviolet Ce:LiCAF laser emits narrow linewidth using the Littrow grating The results recorded narrow linewidth emission spectrum of Ce:LiCAF ultraviolet laser at 290 nm, shown in Figure 3.12. Narrow linewidth of ultraviolet laser were recorded by Spectroscopy Instrument SP2500i spectrophotometer. The results showed that the spectral width recorded a change with the tunable wavelength, the narrowest laser spectral width obtained at 283 nm was 0.23 nm. Spectral width recorded at 290 nm is 0.27 nm, Fig. 3.12. Narrow linewidth emission correctly rotation of grating angle β=20.3o. It is at 290 nm of Ce:LiCAF ultraviolet explained that, at different wavelengths, the pump laser configured using Littrow grating. 17