Of Litv Laser Power / Energy Meter Preliminary Study

Based on the fact that CMR thin films grown on SrTiO₃ or LaAlO₃ vicinal-cut substrates display the laser-induced thermoelectric voltage (LITV) effect, and the linear relationship between peak of LITV and laser energy/power, we have fabricated a laser energy/power-meter, which consists of laser-response unit, that is, the films, and data-processing unit. The power-meter is designed for directly displaying the energy of pulsed laser. In comparison with commercial laser energy/power meter, the device has advantages of response to broad wavelength, faster response-time and low cost. According to the results on the excimer laser of 248nm and 308nm, the displayed value shows that a linear dependence of the measured laser energy. However, it can not accurately indicate the exact laser energy, due to its big fluctuation. The reason lies in the response speed of this instrument to signal is too low to accurately catch the peak of LITV each time. In order to make up for this disadvantage, we posed some ideas: one is to improve the signal-processing unit of circuit or replace the current method, which is directly to catch the peak of LITV, by integrating for LITV signal; the other (already realized) is to use the data-collecting card of tens of or hundreds of MHz assisted by computer, which can easily find peak-value of LITV, therefore, accurately measure laser energy.Further, the stability of laser-response unit is studied. It shows, the unit is safe when directly exposed in a laser beam till the energy of the laser beam is higher than 150mJ or so, therefore, we must add an attenuator in front of it in order to measure higher energy, which is made by metal-wire meshwork. The attenuators of different attenuation ratio can be: an attenuator with multi-layered metal-wire meshwork or with metal-wire meshwork of different mesh dimension, or a combination of several attenuators. This kind of attenuator is our innovation. Furthermore, the attenuator is not restricted by laser wavelength, therefore, is suitable for many kind of lasers. For the sake of protecting the film (e. g., LCMO thin film, etc.) inside the laser-response unit,LCMO thin film is covered by MgF2 thin film using vacuum evaporation method. When shot by laser beam of 200mJ per pulse, the film is still kept undamaged. It clearly demonstrates that MgF2 enhances the endurance of laser shooting of film. The experimental results show that the film covered with MgF2 has a betterlinear relationship between LITV peak-value and laser energy.We also studied the properties of LCSMO thin film. Through measuring the LITV signal, we discovered both the rising-time and the signal width are smaller than those of LCMO thin film. So LCSMO film can be used to fabricate more faster-response light detector. In addition, its resistance dependence of temperature (R-T) is also measured. The metal-insulator (M-I) transition temperature is 313K, the R-T curve is very sharp nearby the M-I transition regime. The biggest temperature coefficient of resistance(TCR) is 1.6%. The TCR of LSCMO film still can be increased. Therefore, LCSMO thin film is expected to be used in Bolometer device.