| "Red,orange,yellow,green,blue and purple,who is holding the color and dancing?" The visible band laser has important demands in many fields related to the national economy and national security and has been applied.At present,the red and blue lasers have been matured based on the developed technology of semiconductor lasers and all-solid-state laser frequency doubling.In the green(green,blue)band,semiconductor lasers have been matured and have been occupied the commodity market in medium and small power laser.And all solid-state laser frequency doubling laser has been obtained practical application.Purple laser is mainly obtained by the triple frequency of all solid-state laser and 10 watts output power was achieved.However,the yellow(orange-yellow)laser,which is located at the visually sensitive wavelength band,still needs to be developed,and it is one of the research hotspots and difficulties in the field of lasers.At present,three main methods were put forward to generate yellow light lasers.Firstly,in linear optics,yellow laser has been realized using low phonon energy crystal(fluoride)doping Dy3+or Pr3+ions with blue laser diodes as the pump source.But the achievable yellow laser output power was constrained by available blue laser diodes pump power and the requirement of low phonon energy.The output power was very low and it is still in the stage of laboratory research.Secondly,in nonlinear optics,frequency doubling and summing are the two main techniques to generate yellow laser with at least a laser crystal and a nonlinear optical crystal.The frequency doubling yellow laser is mainly related to the stimulated Raman scattering at the wavelength band of 1000 nm generated by the stimulated radiation of Nd3+or Yb3+ions.The frequency summing yellow laser is mainly composed of the simultaneous emission of Nd3+ion at the wavelength of 1060 nm and 1340 nm.These two methods for generating yellow laser has a high laser threshold,a complex structure and optical path design,a low conversion efficiency and are not suitable for practicality.The development of yellow lasers urgently needs the introduction of new principles and the invention of new devices.The functional composite crystal possesses two or more functions in one single crystal,such as a self-frequency-doubled crystal(SFD)material,integrates the lasing and nonlinear frequency doubling into a single crystal,and its device has a compact and simple structure,a small volume,an easy adjustment,a good stability,and a high output power.It can meet the needs of modern information society for highly integrated optoelectronic devices.According to the exploration and research of scientists,a member of rare-earth calcium oxoborate(RECOB)crystals have attracted a great deal of research interest,because it has a wide transparency spectrum,a stable physical and chemical property.And it can be grown by a mature crystal growth method to obtain a large size and high quality single crystal.Based on the previous research of our group,this paper proposes that the laser band can be extended by electron-phonon coupling to achieve yellow light output.Starting from the Huang-Rhys factor,the relationship between crystal structure-doped ion-laser effect was studied in depth.It was found that Yb3+ ion possesses the strong electron-phonon couplirtg strength due to the lanthanide-contracted effect,and the Yb3+ ion doped calcium borate oxygen was selected according to Hume-Raeser's rule.A series of research work on Yb:YCOB crystal growth and self-frequency doubling yellow laser device design was carried out,involving crystal growth,basic performance characterization,crystal concentration and length optimization of self-frequency doubling yellow crystal.The watt-level output power of self-frequency doubling yellow laser was realized.The main works are as follows:1.Crystal growth and analysis of the basic properties of Yb:YCOB crystalThe different doping concentration Yb:YCOB crystals were grown by the Czochralski method.For the application of self-frequency doubling yellow laser,a series of crystals with doping concentration from 5 at.%to 30 at.%were grown,which breaks through the key problems of cracking,bubbles and composition segregation during crystal growth.Based on the optimized conditions,the growing crystal dimension is ?25 mm×40 mm and the average weight is 130 g.The quality of the Yb:YCOB crystal was characterized by X-ray powder diffraction and X-ray fluorescence spectrometer.The results indicate that the Yb:YCOB crystals exhibit good crystallization and homogeneity.Guided by the basic principles of crystal physics,a crystal sample was processed for realizing the thermal performance test of a monoclinic crystal.The thermal expansion,thermal diffusion,specific heat,density and thermal conductivity of Yb:YCOB crystals with different doping concentration were measured.The relationship between the doping concentration and the thermal properties was discussed with the results that the doping concentration has a slight impact on anisotropic thermal properties.It was concluded that the YCOB crystal can be doped with a high concentration of Yb3+ ion to improve the conversion efficiency in the laser process without deteriorative thermal problems.Then high power and high efficiency self-frequency-doubled yellow laser output can be realized.2.Characterization of optical properties and electron-phonon coupling of Yb:YCOB crystalThe phase matching condition is the basic requirement for realizing nonlinear frequency conversion,which depends on the birefringence and refractive index dispersion law of nonlinear optics crystals.YCOB crystal belongs to the monoclinic system with three different magnitudes principal refractive indices(nx,ny,nz)in the dielectric frame.The refractive index of different doping concentration Yb:YCOB crystals were measured by the minimum deviation method and the Sellmeier equations were fitted.It can be seen that the angles between the crystallographic axis and dielectric frame increases slightly as the doping concentration raises.Then the polarized absorption and emission cross sections were calculated with the results that the shape and peak position of absorption and emission spectra in different polarized directions and different doping concentrations are closer and the relative intensity of the peaks is anisotropic and the intensity of zero-phonon-line at the wavelength of 975 nm reduces as the doping concentration rises.The reason is that Yb3+ ions led to a strong electron-phonon coupling and intense vibronic transitions.Besides,for monoclinic crystal,the crystallographic axes do not coincide with the dielectric frame defined as the real part(refer to the refractive index)of the complex dielectric permittivity tensor is diagonal while the imaginary part(refer to the absorption or fluorescence cross sections)is not diagonal.The angular distribution surface of the real and imaginary parts of the complex permittivity was discussed and the high doping concentration makes a larger rotation angle.The lanthanide contraction makes the ionic radius of Yb3+ the smaller among rare earth ions.At the same time,it weakens the screening of the 4f shell by the filled 5s and 5p electron shells.The reduced screening enhances the electron-phonon interaction and extends spectrum to achieve vibronic transition.Thus,it is possible to obtain yellow laser with Yb:YCOB SFD laser crystal.The strength of the electron-phonon coupling is relevant to the ionicity of coordination polyhedron and the Huang-Rhys factor S.The calculated results show that the B-O bond ionicity is larger than other bonds.The Huang-Rhys factor S is related to the temperature and doping concentration,and as the temperature or doping concentration rises,the value of S increases.The relationship between relative intensity of the emission spectrum and phonon numbers was calculated.The emission spectrum has a high intensity when the phonon numbers equal to the value of S.The studied results indicate that it will achieve yellow laser with Yb:YCOB crystal.3.SFD yellow laser research of Yb:YCOB crystalThe self-frequency-doubled laser performance of Yb:YCOB crystal was studied.The SFD green laser was obtained by cavity design with a maximum output power of 710 mW at the wavelength of 523 nm in a Yb:YCOB crystal.And the conversion efficiency was 8.59%.By analyzing the spectral characteristics of this kind of crystal,it is found that the fundamental laser in the wavelength range of 1000 nm-1060 nm has a large gain,which needs to be suppressed by a suitable coating process or frequency selective technique to achieve light oscillation at the,wavelength of more than 1100 nm with a small gain.And then the SFD yellow laser was obtained.Guided by the calculated gain sections of the optimized phase matching direction crystal,the optimized phase-matching direction at the wavelength of 1140 nm was calculated by the fitting Sellmeier equations with different doping concentration Yb:YCOB crystals.It provides theoretical support for the realization of SFD yellow laser.Then the influence on the conversion efficiency by the doping concentration and crystal length was discussed and the experimental results show that the SFD yellow laser output powers of 1.868 W and 1.713 W were obtained using the Yb:YCOB crystals with the doping concentrations of 5 at.%and 30 at.%and the lengths of 8 mm and 4 mm,respectively.And the conversion efficiencies are 11.76%and 11.97%,respectively.According to the optimized results,the maximum yellow laser output power was 2.752 W and the conversion efficiency was 16.59%.The Yb:YCOB crystal doping concentration was 20 at.%and the crystal length was 6 mm.Besides,the coating technology of a narrower wavelength band was put forward and the stable SFD yellow laser at the wavelength of 570 nm was achieved.At present,the SFD yellow laser crystal and devices have been industrialized for the production of yellow-orange laser devices and laser sources with independent intellectual property rights in China. |
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