Currently, high-reflectivity optical elements are widely used in optical fields, such as ring laser gyroscopes, inertial confinement fusion systems, and gravitational wave detection. Their reflectivity values have a decisive influence on the performance improvement of these systems; therefore, accurate measurement of their reflectivity values is necessary to optimize the system performance. At present, there are many measurement methods, among which spectrophotometry and cavity ring-down (CRD) technology are the most widely used. However, these two methods have certain shortcomings. Spectrophotometry is associated with low accuracy, and its measurement limit is usually lower than 99.9%; especially when measuring high reflectivity (>99%), its accuracy is reduced. Devices using CRD technology for high reflectivity measurement are relatively complex and expensive, and the measurement conditions, such as adjusting the incident angle and polarization state, are inflexible. Moreover, according to the measurement principle, the lower the reflectivity of the sample to be measured, the lower is the measurement accuracy. Therefore, for samples with the reflectivity of 99.9%-99.99%, existing testing methods have some limitations. Based on the described situation, it is necessary to study a high-precision reflectivity measuring device that can measure the reflectivity of 99.9%-99.99% accurately. It is necessary to develop a reflectivity measuring device which has the advantages of simple device, convenient adjustment and good measurement stability.
Based on traditional spectrophotometry, we study a more accurate reflectivity measurement method. The main improvement in this method is measuring the difference signal instead of the reflected light and incident light signal. In this study, a reflectance measuring device based on spectrophotometry is developed, which adopts a double-path measurement. First, we measure the signal difference between the reference optical path and the initial optical path without samples, and the reference optical path signal (Fig. 2). We then measure the signal difference between the reference optical path and the test optical path after placing the sample (Fig. 3). The reflectivity of the sample is calculated according to the measured reference and differential signals. The reflectivity is calculated by measuring the difference between the reference signal and the initial signal and the difference between the reference signal and the test signal. Compared with the reference signal, initial signal, and test signal with larger absolute values, the signal difference itself is relatively small; therefore, the sensitivity of the lock-in amplifier can be fully utilized to improve the reflectivity measurement accuracy. In addition, we use a quick fixing mechanism to shorten the measurement time and reduce the influence of unstable factors such as the light source and environment on the experimental results. In the experiment, there are some differences in the responses of the detectors' photosensitive devices at different positions; therefore, mobile platforms are installed at the three positions where the two detectors are placed. Before the formal measurement, the position of the detector is adjusted to ensure that the beam irradiates the position where the conversion efficiency is the highest on the receiving surface of the detector .
We use this device and the CRD method to test the highly reflective mirror samples. The wavelength in the two methods is 1064 nm, and the incident angle is 45°. The high-reflection wavelength of the two high-reflection mirror samples is 1064 nm, the use angle is 45°, the lens diameter is 50 mm, and the thickness is 5 mm. The two samples are measured using S-polarized light and P-polarized light, respectively, and the highest reflectivity is 99.986% (Table 1). In this study, the error is calculated using an uncertainty transfer formula. When only one significant digit is retained, the error is of the order of 10-5,so the reflectivity calculation result is also of the same order, which meets the reflectivity measurement requirements of a high reflector. The accuracy of the measurement method introduced in this study reaches 0.01%. Compared with the CRD method, the measurement error is less than 0.009%, which shows that this method can achieve a higher measurement accuracy with a simpler device. The advantages of this device are as follows: 1) By using a lock-in amplifier to measure the signal with high precision and a small range and shortening the measurement time, higher accuracy can be achieved; 2) The sample fixture is installed on the rotating platform, and the adjustable position range of the detector of the test optical path is large, so the adjustable incident angle range of the device is also large; 3) The device is simple and easy to operate.
In this study, traditional spectrophotometry is improved, the measurement accuracy is increased by measuring differential signals, and the high-precision range of the lock-in amplifier is fully utilized. In addition, in the calibration of the device, the measurement time is shortened by means of a quick fixing mechanism, and the influence of light source fluctuation is reduced, which ensures the accuracy of the measurement results. The measurement accuracy can reach 0.01% stably, which meets the measurement requirements of the existing high-reflector elements, compensates for the shortcomings of the traditional measuring methods, greatly increases the application range of spectrophotometry, and fills the measuring gap between commercial spectrophotometry and CRD technology in the reflectivity range of 99.9%-99.99%.
