The Research On Nonlinear Optical Properties Of A Class Of Calcium Fluoride Crystals At 800 Nm

In the past few decades since the laser was born, calcium fluoride?CaF₂? crystals have been widely studied and applied as laser media. CaF₂ is the first synthetically practical laser crystal with the advantage of a wide transmission range, a low refractive index and nonlinear coefficient, and the ability of large scale growth. They can be used as window materials in optical instruments, optical lenses and gain media. With the development of laser technology, especially the invention of ultrafast laser and excimer laser, CaF₂ crystals, as known as “old materials”, now are facing up to new challenges. Doping with rare element ion is an effective way to obtain new types of crystals. However, the Nd³⁺ doped fluoride crystals such as Nd: CaF₂ always have a very detrimental concentration quenching effect resulting from the clustering of Nd³⁺ ions, which undoubtedly will seriously affect the optical properties of the laser crystals. The scientists have recognized that Y³⁺ co-doping with Nd: CaF₂ can adjust the local coordination structure of Nd³⁺ ions. And it is considered as an effective way to reduce the quenching effect. Laser operations with many different types of co-doped crystals have been achieved up to now. But no measurement of the nonlinear optical properties of these materials has been carried out, especially the third-order nonlinearity. Considering all the problems above, we investigated the third-order nonlinear optical properties of CaF₂ crystals and SrF₂ crystals with different doping concentrations of Nd³⁺ and co-doping concentrations Y³⁺ by Z-scan measurement system?center wavelength: 800 nm, pulse duration: 100 fs, 3 d B spectral width: 15 nm, and repetition rate: 1 k Hz?. The data have been analyzed and an explanation at theoretical level has been given. The main results of our works are as follows:Based on Coherent Libra Ti: sapphire Amplifier System?center wavelength of 800 nm, pulse width of 100 fs, 3 d B spectral width of 15 nm, a repetition rate of 1 k Hz?, combined with the Newport ILS Series linear stage and using Newport's OD3 IR laser Energy Probe and a type-2936 power meter, we built up the Z-scan measurement platform. And the system of stage movement and synchronously data acquisition was controlled by computer software programmed by Lab VIEW and MATLAB.Third-order nonlinear optical properties of the standard sample, carbon disulfide, have been measured under our Z-scan system, and we figured out that the third-order nonlinear refractive index equals to 2.9×10^-19 m²/W, which is quite closed to the reported value. No significant nonlinear absorption was found. It suggests the reliability and precision of our Z-scan measurement system. It was also a verification of the Z-scan measurement technique.CaF₂ crystals and SrF₂ crystals with different doping concentrations of Nd and co-doping concentrations of Y had been characterized with an 800 nm, 100 fs, 1 k Hz, Z-scan system. Significant self-focusing effect, but no obviously nonlinear absorption effect, was observed. We also calculated the value of third-order nonlinear refractive index. At the same time, we measured a pure CaF₂ crystal as a contrast, too. By comparison and analysis, we found that the Nd and Y co-doping do not have great influences on the third-order nonlinear index for these two kinds of fluoride crystal. Some samples even have lower values of ? and not any negative effects at least. This thesis provides some guideline to find an effective dope type in terms of obtaining required suitable optical materials in laser systems.