Spectroscopic Properties And All-solid-state Laser Performances Of Pr³⁺:GdLiF₄Crystal

With the characteristics of good monochromaticity, strong directivity, high brightness, and stabilized coherence, lasers have widely applications in many fields such as military, medicine, communication, scientific research, and gradually become an integral part of our daily life. As a kind of lasers, visible lasers have some unique applications in fields of large-screen display, underwater communication, metal processing, biomedicine, quantum information, and visible lasers are the potential fundamental sources for the generation of violet and even ultraviolet lasers by direct frequency doubling. All-solid-state visible lasers have become the hot research because of their advantages in miniaturization, compaction, efficiency and so on.Laser diode radiation, nonlinear frequency conversion, and stimulated emission of doped rare earth ions are three effective and common methods to obtain solid visible lasers. Laser diodes (LDs) have advantages of small appearance, low-power dissipation, and simple drive. Meanwhile, LDs exist disadvantages of imperfect beam quality, sensitivity to temperature and static electricity, and absence of some wavelength. Currently, the most common way to generate visible lasers is nonlinear frequency conversion of infrared lasers with the assistance of nonlinear crystals. This method requires at least two completely different crystals of laser crystals and nonlinear crystals. Moreover, it needs two different theories of laser principle and nonlinear frequency conversion. All of these may lead to a complicated structure and difficult design of frequency conversion lasers. With the development of LDs, blue LDs become mature and commercialized gradually. It becomes possible to generate visible lasers by directly pumping rare earth ions doped gain medium with blue LD as the pump source. This method does not need any nonlinear crystals and is beneficial to the development of miniaturized diode-pumped all-solid-state visible lasers. The energy level structure of trivalent praseodymium ions (Pr3+) determines that Pr3+ ions can generate blue light (479 run), green light (522 nm,545 nm), orange light (605 nm, 607 nm), red light (639 nm,670 nm), deep red light (697 nm,720 nm), and other wavelengths. In recent years, Pr3+ doped gain materials have gotten the attention of researchers in the aspect of directly generating visible lasers. Fluoride crystals have low phonon energies that are beneficial to reduce non-radiative transition probabilities caused by multi-phonon relaxation and improve emitting efficiency. Synthesizing the advantages of Pr3+ ions and fluoride crystals, Pr3+ doped fluoride crystals have become the optimized material for directly generating visible lasers.Constrained by the development of visible optical modulators, the researches of directly generating visible pulse lasers (including Q-switching and mode-locking) are rare up to now, which may hinder the applications of high peak power, large pulse energy, and high repetition rate visible pulse lasers. In this paper, Pr3+:GdLiF4 crystal has been studied systematically and comprehensively from spectroscopic properties to all-solid-state laser performances. We find that the fluorescence lifetime and emission cross-sections of Pr3+:GdLiF4 crystal change with temperature. By optimizing laser resonant cavity, efficient continuous-wave (CW) visible lasers have been obtained. Based on the analysis of 2D material band structure, it has been validated that MoS2 can be a new visible optical modulator and Q-switched visible pulse lasers have been generated. We also promote the development of self-mode-locking (SML), and multi-GHz SML visible pulse lasers have been realized. The specific research work are as follows:1. Spectroscopic properties of Pr3+:GdLiF4 crystalThe absorption spectrum of Pr3+:GdLiF4 crystal is measured. The absorption peak is located at 443 nm, which is consistent to the wavelength of blue LD. Fluorescence lifetimes and polarized emission spectra in the visible range are measured from 77 K to 300 K. We find that fluorescence lifetime and emission cross sections decrease with the increase of temperature, except 522 nm. These phenomena have been explained in the paper.2. CW laser performances of Pr3+:GdLiF4 crystalWith laser theory as the theoretical guidance, a 442 nm blue LD is used as the pump source and Pr3+:GdLiF4 crystal is employed as the gain material, cw lasers at the wavelengths of 522 nm,605 nm,639 nm, and 720 nm have been generated. The maximum output power is 615 mW with the slope efficiency of 46% at the wavelength of 639 nm.3. Q-switched laser performances of Pr3+:GdLiF4 crystalBased on the analysis of MoS2 band structure, MoS2 sample is prepared and measured in the properties of morphology and absorption in visible range. By employing MoS2 sample as the optical modulator,522 nm,605 nm,639 nm, and 720 nm Q-switched pulse lasers have been generated for the first time, the repetition rates are about 200 kHz and the pulse width are about 200 ns. These results experimentally verify that MoS2 material is a potential optical modulator in visible range and the conclusion enriches the kind of visible optical modulator. It may open a new way to the study of visible optical modulators.4. Self-mode-locked laser performances of Pr3+:GdLiF4 crystalBased on the research status of SML and the principle of KML, multi-GHz SML pulse lasers have been generated by using a simple plane concave cavity at the wavelengths of 522 nm,607 nm,639 nm, and 720 nm for the first time. The repetition rates are about 3 GHz and the pulse width are about 50-70 ps. This study has extended the SML technology to the visible range and has a certain reference value in the generation of high repetition rate visible pulse laser.Through the above research, the potential applications of Pr3+:GdLiF4 crystal using as laser gain medium in visible range have been proved. Spectroscopic properties of Pr3+:GdLiF4 crystal have been measured with the change of temperature from 77 K to 300 K. The feasibility of MoS2 material as visible optical modulator has been verified and visible Q-switched pulse lasers have been generated. The technology of SML has been developed and visible SML pulse lasers have been obtained.