| The development of high energy laser systems is limited by the quality of the optical thin films on the surfaces of mirrors, so measuring the absorption coefficients of optical thin films effectively has become the work need to be done in the construction of high power laser systems. Although large combustion-driven lasers can provide high power output, they can only be used as the laser sources to measure the absorption coefficients of large mirrors in actual application, but not as commonly used laser sources to study the absorption coefficients of small size mirrors by experiments, because of their high operation costs. In this thesis, a DC-discharge driven CW DF chemical laser is investigated and developed as an appropriate laser source to measure the absorption coefficients of optical thin films under high power density with a Closed-Cavity.The structure and function of each part of the DC-discharge driven CW DF chemical laser was and introduced and described in detail in this thesis. A preliminary experiment was carried out under the condition of power supply of 3.2k V, 110 m A and output mirror transmittance of 20%. The stable output power of 65 W shows that the laser can operate well.By the way of "running by time sequences", the operation method and procedure of the laser have been simplified, and the output performance of experiments has been improved. By proper selection of the electric parameters, gas parameters and optical parameters, the basic operation rules of the laser have been obtained, and the output power of the laser has been improved. When the power supply increased to 3.9k V, 200 m A, the output power of the laser reached 169 W, which shows that the increase of power supply can increase the output power effectively. But in order to protect the power supply, the maximum output current is limited to 150 m A in the experiments. The best optical axis is located at about 15 mm downstream of the exit plane of the nozzle array, by comparing the maximum output power when the axises were different.By changing parameters, the flow rate of each experimental gas was optimized, and the optimal flow rate of Hep, NF3, D2, Hes and N2(gas curtain) was obtained as 0.353g/s, 0.928 g/s, 0.202 g/s, 0.291 g/s and 0.879 g/s respectively, with the power supply of 4.3k V, 150 m A, and transmittance of 20%. In view of the actual demand of the Closed-Cavity, the optical axis of the laser was adjusted to about 15 mm downstream of the exit plane of the nozzle array, and power supply of 3.2k V, 110 m A, output mirror transmittance of 3% were set in the experiment where 126 W stable output was obtained when the gas flow rates were optimized. The power density on the surfaces of mirrors inside the cavity was about 3.68 k W/cm2, which meets the demand of the Closed-Cavity.The measurement of the absorption coefficient of optical thin films under high power density has been realized with the Closed-Cavity, using the optimized DC-discharge driven CW DF chemical laser as the laser source. The structure and testing principle of the Closed-Cavity were introduced, and the power density of about 3.09 k W/cm2 on the surfaces of mirrors inside the Closed-Cavity was obtained in the performance test, indicating that the Closed-Cavity can be used to measure the absorption coefficients of optical thin films under high power density.The absorption coefficients of 4 mirrors were measured using this Closed-Cavity, and the absorption coefficients of about 62.7ppm, 98 ppm, 133 ppm and 86 ppm were obtained when the power density on the surfaces of mirrors under test is about 3.05 k W/cm2. The Closed-Cavity platform can measure the absorption coefficients of 4 mirrors simultaneously, with low gas consumption and simple operation. The experimental results shows that the optimized DC-discharge driven CW DF chemical laser is an applicable laser source of the Closed-Cavity, having solved the practical problem of the measurement of absorption coefficients of small size mirrors commonly and frequently. |
PDF Full Text Request