The Design Of Optical Transmittance Measurement System For Coated Laser Elements With Low Transmittance

The coated elements with low transmittance (less than5%) required for the highpower facility have very stringent specifications about transmittance. In order tovalidate optic performance against specification, a special metrology system fortransmittance is proposed.A relative measurement method for measuring the coated elements with lowtransmittance is described. The system is composed of a laser source, an integratingsphere, a power meter and four uncoated wedged glasses. In this system, theincident beam is divided in two parts by a wedged uncoated element. The reflectedbeam is used as the reference beam, while the transmittance one which passes throughthe element under testing is as the measurement beam. By comparing the two beampowers, the transmittance of the tested element is obtained.Three features making the system unique and achieve high accuracy. First,uncoated wedged glasses instead of coated mirrors serve as the beam splitter andreflectors. As well as the avoidance of coating errors, the especial reflectors make thetwo beams suitably attenuated so that they are both within the ideal linear range ofdetector. Besides, since the surface reflectivity of uncoated glass can be calculatedaccurately by Fresnel formulas, it is used as the reliable reference to compare with thesample’s transmittance. Second, the powers of reference and measurement beam aredesigned close to each other, and one detector is used to detect the two beamsalternately, which avoids the system error caused by different performances includingnonlinear and nonuniform characters from two detectors. Third, the compensator inreference beam is designed to compensate the inner absorption of beam splitter inmeasurement beam. Besides, two surface reflections of compensator and beam splitterare equal and they can be eliminated when two beam powers are compared, thus theexistence of compensator simplifies the calculating formula of sample’s transmittance.Measurement results for a sample’s transmittance are presented. Experiment and analysis show that the relative standard uncertainty (σ_T/T) of measurement is0.424%，which is quite accordance with the results of theory analysis.