Laser Properties Of Nd³⁺ And Mg²⁺ Co-doped LiTaO₃/LiNbO₃ Crystals And Their Applications In Lased Based Display

Over the past decade, laser based display is a very active area of research. Compared with other display technologies, laser based display can get a large color gamut coverage over 160% National Television Standards Committee (NTSC) standard, has a lifetime of more than 20 000 hours. Most importantly, laser based display can get a high luminance which is far higher than the others.Laser is one of the core technologies for laser based display. The current research is focused on how to achieve a high electro-optical conversion efficiency, high reliability, low cost three primary colors (red, green and blue) lasers and how to suppress the coherent laser speckle effect. Currently, red and blue semiconductor laser technology has been commercialization. Now green laser is mainly obtained by mains of nonlinear optics, which convert the infrared lasers into green lasers using a nonlinear crystal (KTP, LBO or PPLN). This laser has high efficiency, high power, small size and other characteristics, but because of this laser has good coherence, obvious laser speckle was brought on the screen, which reduces the display image quality. In 2012, Nichia Corporation reported a 1.1 W green semiconductor laser, but still its price and reliability has not been acceptable. Getting green lasers with high efficiency, high brightness and low coherence has become a major issue to laser based display.Experimental study found that wide full wave at half maximum (FWHM), multi-wavelength laser can effectively suppress laser speckle. Our group discovered neodymium and magnesium co-doped lithium niobate and lithium tantalate has very excellent laser characteristics, recently. The output wavelength width is up to a few nanometers, and is multi-wavelength output. After frequency doubling and frequency summing, a wide width, multi-wavelength green laser could be obtained, which will have very important applications in laser based display.Our previous studies have found that neodymium and magnesium co-doped lithium niobate and lithium tantalate have wide fluorescence spectrum, large absorption cross section and large emission cross section, made the crystals fitting for high power, dual wavelength, broad band near-infrared laser output achieving. Further more, using periodically poled structure, Nd:Mg:LiNbO3 crystal can implement multiple cascaded nonlinear processes, which is expected to receive a green laser with linewidth FWHM greater than 5 nm. With the development of applied electric field polarization and growing fringe crystal growth technology in recent years, double-doped periodically poled Nd:Mg:LiNbO3 crystal self-frequency doubling green and red light frequency and optical parametric oscillation have been reported. Periodic prepared using external electric field polarize Nd:Mg:LiNbO3 crystal has the potential to achieve high efficiency green laser output.In order to provide stable and reliable green lasers for laser based display and reducing the laser speckle, this paper carried out the following work:1-. Periodically poled lithium niobate crystal was prepared using an applied electric field poling method. Complete periodic structure cannot be obtained using a single pulse voltage poling method. Domain merging occurred on the+ Z surface, which affecting the nonlinear frequency conversion efficient; after applying a reverse voltage, using the process of spontaneous reversal polarization, the domain merging on+Z surface can be effectively suppressed. Then a periodically change voltage was applied onto the crystal, PPLN with a period of 6.97 μm, polarization depth greater than 0.9 mm, and duty close to 50% was obtained, providing high quality nonlinear crystal based on quasi-phase matching of solid-state lasers.2、Single-pass frequency doubling was operated using PPLN crystal with period of 6.97 μm and size of 10 mm×5 mmx 1 mm (axbxc).The pumping laser is an acousto-optic Q-switched laser with 10 KHz and 84 ns.As pumping power increases, the optimal operating temperature of the crystal orientation toward low temperature offset, with a temperature bandwidth of approximately 2℃.A highest nonlinear conversion efficiency of 44.05% was received. PPLN crystal having three periods (28.5 μm,29μm, 29.5 μm) was used for optical parametric oscillation experiment, and lasers which have three mid-infrared (3822.05 nm,4001.46 nm,4159.95 nm) was obtained.3-, Optical properties of Nd:Mg:LiNbO3 crystal was measured. For σ polarization. the center of the absorption spectrum is at 809 nm, and the emission spectra have five separate peaks. For π polarization, the center of the absorption spectrum is at 813 run, although the emission spectra have three separate emission peaks, but emission peak at 1083.64 nm is significantly stronger than the other two, suggesting that crystals have different operating characteristics of the laser in different directions. The fluorescence lifetime is 107.64 μs, suitable for used as the laser host medium. LASCAD was used to analyze laser operation thermal, mechanical and other properties of the crystal. Using a c-cut Nd:Mg:LiNbO3 crystal as host medium,1.97 W dual-wavelength laser was obtained, the laser emits at 1077.5 nm and 1093.6 nm; using an a-cut Nd:Mg:LiNbO3 crystal,1.24 W of single wavelength laser was obtained. After nonlinear optics progress, multi-wavelength lasers emitting at visible light and infrared light regions can be generated, which have great applications in laser display, optoelectronic countermeasure, etc.4、Multi-wavelength, broad band green lasers was obtained by intracavity frequency doubling of Nd:Mg:LiTaO3 crystal using a multi-periods PPLN as nonlinear crystals. Using PPLN crystal has three periods (7.55 μm,7.38 μm and 7.22 μm), maximum output power of 62 mW three-wavelength green laser was obtained, the emitting peaks are at 538 nm,542 nm and 546 nm. The laser speckle contrast of 532 nm,546 nm,538 nm, and the three-wavelength green laser are 13.41%,5.16%,5.2% and 3.38%, respectively. The laser speckle of as obtained three-wavelength green laser is only 1/4 of 532 nm green, confirming this is an effective way to depress laser speckle.5-. Periodically poled Nd:Mg:LiNbO3 crystal was prepared. Its polarization voltage is a little higher than that of Mg:LiNbO3.+Z surface tends to form many chain domains nucleation sites. By applying a negative voltage, in the form of a multi-pulse train, a period of 7.41 μm with size of 16 mm×5 mm×1 mm (a×b×c) PPNdMgLN crystals was prepared. Using intracavity frequency doubling, self-frequency doubling green laser with 80 mW green laser was obtained.