| As a hybrid gas phase/solid state laser, diode pumped alkali vapor lasers(DPALs)have great potential for high power scaling due to its many advantages, for example the high quantum efficiency, convenient thermal management by convective cooling of the gaseous medium, all-electrical operation by diode pumping, light weight and compact configuration, and good atmosphere transmittance for infrared atomic emission lines. Since the concept of DPAL being proposed in 2003, it has attached great interest and fast development by many countries, especially the USA and Russia. In 2012, DPAL with average power of a kilowatt has been realized with high efficiency. Till now, most research focused on power scaling and basic physical problems of DPALs. Little work on diagnostic tests of DPAL parameters has been carried out, for example the alkali concentration or the temperature rise of pumped gain medium. Because precise measurement of these parameters are important for laser performance analysis and laser design, it is necessary to carry out such studies in time. In this paper, we have measured and analyzed the critical parameters of DPALs including alkali concentration and gain medium’s temperature rise. The main contents are presented as follows:1. Several detection methods of alkali concentration are reviewed and compared, and the method used in this paper is identified. According to the interaction theory of light and atoms, there are three usual methods: absorption spectrometry by detecting the light’s amplitude, faraday rotation measurement by detecting the polarization state and interferometry by detecting the phase, and then the alkali concentration could be obtained indirectly. For DPAL operation condition, the faraday rotation measurement produce an extremely small angle change which is difficult to detect; for interferometry, the heating of alkali cell as well as the heat induced deformation of windows could affect the detection precision. As a comparison, absorption spectroscopy processes advantages of simple operation, convenient for system integration, and high measurement accuracy. Therefore, we use absorption spectroscopy to measure the alkali concentration of gain medium in the paper.2. The atom concentration of a rubidium DPAL is measured. First, by considering of the hyperfine energy level structure, isotopic abundance and the collision broadening and frequency shift of alkali D1 lines by buffer gases, we calculate the absorption cross-section of rubidium D1 line basing on Voigt line shape. Based on the cross-section, the transmittance spectrum could be obtained theoretically for different alkali concentrations. In the experiment, we use the DBR single-frequency diode laser with center wavelength 795 mm and mode-hop free tuning range 23 GHz as the probe laser to conduct the atom concentration measurement. For the first time, we proposed a new spectral joint method to increase the mode-lop free tuning range by neatly adjusting the current and temperature. As a result, we achieve a mode-hop free tuning range of 100 GHz, which could scan the complete absorption spectrum of rubidium D1 line with 1atm buffer gas. By comparing the measured transmission spectrum with theoretical result, the rubidium concentration could be obtained. These results match well with the results calculated by using saturated vapor pressure formula. It certifies the correctness of this method, and also the effectiveness when applying this method in cases of high power pumping and flowing medium, in which the saturated vapor pressure formula could not be used.3. The experimental detection method mentioned above is integrated by LabVIEW virtual instruments. The system could also be used to measure the temperature distribution of a DPAL gain medium. It improves the operability, real-time and visual effect of the measurement process by integrated with LabVIEW, and with modular packing the system is easy to be transplanted with good repeatability. The temperature rise of gain medium is obvious even under weak pump. We observe a significant temperature gradient under the average pump power of 4W. Temperature of the edge of pump area increase 60 degrees as compared to the heated oven edge. On one hand, the experiment demonstrates that high pump intensity could induce high thermal power density and temperature rise although DPAL have low quantum defect; on the other hand, it establishes the experiment foundation for the temperature distribution detection of DPALs under high-power pump and flowing medium. |
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