At present, the medium power calibration system at the National Institute of Metrology (Thailand), which has a CO2 laser as a laser source, can be used to calibrate customers' laser power sensors from 100 mW-10 W. This power range of CO2 laser offers various applications, such as cutting of wood, paper, fabric, rubber, metals, and ceramics. CO2 lasers are also used widely as medical lasers for surgeries and cosmetic purposes such as removing mold and facial resurfacing. Originally, the calibration system of the CO2 laser has a measurement uncertainty of 4.0 % at a 95 % confidence level. This uncertainty level is considered high for medium laser measurement and sensors' accuracy. One of the reasons is the high power linearity error of the detector, which is taken from manufacturing specifications. Other reasons are overestimation of the uncertainty of the non-uniformity of the detector, and the large uncertainty from the calibration certificate of the standard. Several efforts have been made to reduce the total uncertainty of the system. First, by reducing the uniformity and positioning error of the standard detector and unit under the calibration detector, and second by reducing the power linearity error of the monitor detector of the calibration system. This paper discusses how to experimentally determine the uniformity and positioning error of the detector and the power linearity error of the detector. The new uncertainty of the system is then determined and adjusted from 4.0 % to 1.1 %.

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