紫外チャープパルス増幅レーザーシステムに用いられる増幅媒質の利得評価法の開発 Development of gain-media evaluation method for ultraviolet chirped-pulse-amplification laser systems
Development of gain-media evaluation method for ultraviolet chirped-pulse-amplification laser systems
High-intensity light sources in the UV region are in high demand in varios applications such as basic research, high-intensity physics, spectroscopy, material processing, and pump sources for XUV (extremely ultraviolet) and VUV (vacuum ultraviolet) generation. In this thesis, a novel optical evaluation method for new laser materials for high-intensity solid-state lasers in the UV region is described. By using this method, it is found that Ce3+ ion-doped fluoride crystals have potential for the development of the lasers because of their broad-band gain spectra and peak gain at a wavelength of around 290nm. A high-peak power laser in the UV region is demonstrated by CPA (chirped-pulse amplification) utilizing Ce3+:LiCaA1F6 (Ce:LiCaF) crystal as a gain medium for the amplifier.<br /> In chapter 1, the motivation and the background of our research are addressed. In chapter 2, the newly developed method for the evaluation of gain media is described. In chapter 3, ultrashort pulse generation in the UV region using a solid-state gain medium is described. Finally, in chapter 4, the conclusion is presented.<br /> High-peak-power lasers using Ti:sapphire crystals as laser media opened up new fields such as material processing, XUV and VUV generation, high-field physics, and ultrafast spectroscopy. They have been limited in the infrared (IR) region because of the properties of the gain media. Recently the development of high-peak-power lasers in a shorter wavelength region has been strongly required for advanced research. In order to generate output ultrashort pulses in the UV region, a wavelength conversion from IR to UV was performed. However, the conversion was not sufficient because of low efficiency. A CPA laser system utilizing KrF excimer as gain medium has been demonstrated at a wavelength 248nm. Excimer lasers have been widely used as coherent light sources in the UV region. However, their drawbacks include the narrow gain spectrum, bulkiness and high-cost operation. Thus, solid-state-laser media that have a high gain in the UV region for high efficiency, a small-sized system for easy handling and low cost operation are suitable.<br /> UV-high-peak-power laser systems use solid-state-gain media have been researched extensively. However a new solid-state laser medium applicable in the UV region for high-peak-power lasers requires further research. Although we have successfully demonstrated laser oscillation experiments in the UV region using Ce3+ ion-doped fluoride-crystal laser media, such as Ce:LiCAF, Ce3+:LiSrA1F6 (Ce:LiSAF), and Ce3+:LuLiF4 (Ce:LLF), not all solid-state laser media in the UV region have been researched. Because laser oscillation trails for new materials sometimes yield misleading information due to either restricted optical qualities of new materials or imperfect laser cavity alignment, laser gain evaluation methods are required to estimate the potential of the laser media.<br /> A pump and probe measurement has been one of the methods for evaluating both the unknown gain and the loss properties of a potential laser medium. This method requires a light source for a probe for at least one sample medium. However, preparing probe light at the proper wavelength throughout the required time scale has been the primary technical difficulty in this method for a transient gain or loss evaluation. Because a light source with a broad spectrum is required in the measurement of a medium with a broad gain property at unknown wavelength, the development of a new method that can solve this problem is strongly reuqired in the research for a broad-band gain medium in the UV region for generation of ultrashort pulses.<br /> We developed a pump and probe scheme which is capable of evaluating the gain properties of potential laser materials without using lasers as the probe light source. The scheme employs two samples for measurement of a laser medium. Both media are simultaneously excited. Fluorescence emission from one of the media is collected and focused onto the other laser medium as probe light with a pair of lenses. Then a sample medium amplifies or absorbs the probe light while the probe light propagates in the sample medium. The amplified probe light is finally collected with a lens and acquired with a streak camera through a spectrograph as an image showing wavelength and a time axis. The image includes fluorescence intensity from the sample medium that amplifies the probe light, which is required to subtract the fluorescence-intensity image acquired with no probe light incident from the image of amplified probe fluorescence. An increase (decrease) from the subtracted image and the probe fluorescence image light means amplification (absorption), i.e., gain (loss) of the sample medium. The subtracted image includes information about the spectrum and time. Inserting half-wave plates and polarizers between the two sample media is also useful for obtaining information about gain polarization dependence.<br /> Using this pump and probe method, we obtained the gain properties of new ultraviolet (UV) laser crystals, Ce:LiCAF, Ce:LiSAF and Ce3+:LiSr0.8Ca0.2A1F6 (Ce:LiSCAF), to evaluate their bandwidth and to select which a gain medium has the most potential for CPA laser systems in the UV region. As a result, we found that they have broad-band gain in the UV region and polarization dependence properties. Because of its broad-band gain spectrum, the resistance to UV irradiations and its large size with high quality, the Ce:LiCAF crystal was most the suitable.<br /> We have successfully demonstrated a CPA laser system in the UV region for the first time using the Ce:LiCAF crystal. The four-pass amplifier had a 370-time gain and delivered 6mJ pulses at 290nm wavelength. After dispersion compensation, output pulses were compressed down to 115fs. Future scaling of this Ce:LiCAF CPA laser system will open up a new category of high-peak-power lasers in the UV region.<br /> In conclusion, we developed a new pump and probe scheme. We have applied this newly developed, two-crystal, pump and probe scheme to evaluate the gain properties of new laser media in the UV region, Ce3+ ion-doped fluoride crystals. This method indicated that the crystals have broad gain spectra which can be attractive for ultrashort pulse amplification in the UV region. The first CPA using the Ce:LiCAF crystal in the UV region was continued on the basis of the result of the gain evaluation. This method is also attractive for a gain spectrum measurement in various wavelength regions; it is not necessary to prepare a laser oscillator as the probe light source and made possible acquiring a gain spectrum and a decay curve with simple measurements. This method can be applied to other prospective laser materials to evaluate gain properties.