Novel optical Measurement System for Laser Transmittance. and Reflectance of Materials*

Transcription

1 Novel optical Measurement System for Laser Transmittance and Reflectance of Materials* Shuhai Jia 1, 2, Yang Jia 1 1. Department of Optical Information Science and Technology, School of Science, Xi an Jiaotong University, Xi an , China En Hong 2 2. NSF Engineering Research Center for Reconfigurable Manufacturing Systems, Department of Mechanical Engineering, College of Engineering, University of Michigan Ann Arbor, MI , USA ABSTRACT More and more science and industry application require that the transmittance or reflectance characteristics of materials be measured conveniently and accurately. Furthermore, as the laser technology is increasingly playing a vital role in military and civil fields. The measurement of the optical character of material becomes more important. A new measurement system for testing the laser transmittance and reflectance of materials that uses a dual-beam relative method is researched. This method largely eliminates the influence of instability of light source and thus provides better measurement accuracy. The measuring system built in this paper adopts a high-performance light source and a proper photodetector to improve the whole performance of the system. A preamplifier fitting the principle of low noise design and a frequency selecting filter are used in the detection circuit to inhibit the inner noise. To validate measurement deviation and accuracy of the measurement system, some standard materials were practically tested. The experiment results match the theoretical character very well. The maximum relative error is less than 2%. This system has the advantages of high accuracy, strong noise immunity and high measurement speed. The cost of this measurement system is low. Key words: Low noise design, Laser transmittance and reflectance, dual-beam relative method, noise matching, frequency selecting filter. 1. INTRODUCTION Nowadays, More and more science and industry application require that the transmittance or reflectance characteristics of materials are measured conveniently and accurately [1-6]. Furthermore, as the laser technology is increasingly playing a vital role in military and civil fields [7, 8], people always have a strong desire that the transmittance and reflectance characteristics of material should be measured when materials are irradiated by laser. However, it is complicated to get them precisely in real work circumstance. They are determined not only by inner characteristics of materials but also by physical conditions of surfaces, density, temperature [9, 10], and reflection and incidence angle. All these unstable factors make it difficult to measure them. In the literature, H. Furuse used the free-electron laser to measure the Midinfrared optical absorption in germanium [11]. S. Ding used Brewster s angle method to measure the absorption coefficient of high-resistivity silicon based on CW THz laser[12]. But the structure of their measurement system is complex and expensive. A dual-beam relative method which can effectively eliminate the error created by the instability of light

2 source is presented in this paper. The instability of light source always causes notable measurement error in the single-beam method. The measuring system proposed in this paper adopts a high-performance light source and a proper photodetector to improve the whole performance of the system. A low-noise preamplifier is used to improve the weak signal detection and measuring sensitivity [13, 14]. A frequency selecting filter is used to improve the noise immunity [15] and a proper data collector is adopted to guarantee the measuring rapidity. Therefore, this measuring system has the features of high sensitivity, strong noise immunity, and high measuring speed. Some standard materials were practically tested to validate the measurement deviation and accuracy of the measurement system, and the experiment results match the theoretical analysis very well. The optical and electronic components are cheap. So the cost of this measurement system is low. 2. MEASUREMENT RINCILE 2.1 Configuration of the Measurement system The configuration of the measurement system is shown in Fig. 1. The modulated light from the laser diode (LD) passes through a beam splitter (BS) and is divided into two beams. LM is the Light Modulator. One beam is directly detected by a photodetector D1 as a reference signal 1 of the output power of laser diode, while the other one irradiates the object to be measured (M) and then is detected by another photodetector D2 as the transmitted or reflected signal 2. These two signals are processed by a low-noise amplifier (LNA), a frequency selecting filter (FSF) and a peak-value detector (VD) respectively and transferred into a computer through an A/D converter. Before measurement, the whole measurement system needs to be calibrated. A calibration object is then put on the experimental board. The reflectance of the beam splitter is supposed as K, the output power of laser diode is 0 and the transmittance of the calibration object is α. Then, 1 and 2 are given by Fig. 1 Configuration of measuring system: LD, laser diode; LM, laser diode modulator; BS, beam splitter; M, the object needed to be measured; D, photo detector; LNA, low-noise amplifier; FSF, frequency selecting filter; and VD, peak-value detector. 1 = K 0 (1) 2 K) = ( α (2) which lead to the result of the calibration K 1 = K α. (3) In the course of measurement, the object needed to be measured is put on the experimental board. Suppose the output power of laser diode is 01 and the transmittance of the object is β, then 1 and 2 are changed into 11 and 12 given by The result is 11 = K 01 (4) 21 K) = ( β. (5) K = K 01 1 β Finally, from the equations (3) and (6), the following equation can be obtained. β (6) 21 1 = α (7) 112

3 Fig. 2 Specific circuits of measurement system If the calibration object is air whose α =1, the transmittance β will be obtained by the operation of 1, 2, 11, and 21. Similarly, when α is replaced by the reflectance of the calibration object, β will be the reflectance of the measured object. This measurement process is called the dual-beam relative method. Many other measurement methods that use only one beam could hardly have any higher accuracy than this method even if more sophisticated processing circuits are used because the instability of the light intensity of LD can never be eliminated, and which has the most significant influence on the measurement accuracy. On the contrary, the method adopted in this paper has a real-time detection for the light intensity and from the equation (7), it can be seen that the fluctuation of light source has no effect on the measurement results. Therefore, the problem of measuring instability is solved. And the measuring accuracy is improved. 2.2 Specific parts of measurement system Fig. 2 shows the specific circuits of our measurement system. The light modulation circuit is a simple audion amplifier. The frequency of the input signal controls the modulation frequency of the laser. D1 and D2 are both commonly used photodiodes. The reference light signal 1 received by D1 is large enough relative to the noise, so D1 just works in a zero biased mode with an operational amplifier. The light signal 2 received by D2 is much weaker than 1 and noise becomes a great part of it, so D2 works in a reverse biased mode. A low-noise JFET (Junction Field Effect Transistor) is then used as a preamplifier to amplify 2 and improve SNR (Signal Noise Ratio) in terms of noise matching principle. In addition, a second-order active band pass filter is used as a frequency selecting filter to eliminate noise and detect useful signals. The peak-value detector is composed of a diode and an RC filter. ADC0809 is used as an A/D converter. 1 and 2 are finally processed in a computer according to equation (7). This system is composed of common electric components, The optical and electronic components are cheap in this system. So the cost of this measurement system is low. The noise matching method helps inhibit the inner noise and ensures a good performance of noise immunity. ADC0809 and a

4 calculation software provide a high measurement speed. The software is programmed in Vc EXERIMENTS To validate the measurement deviation and accuracy of the measurement system, some standard materials were practically tested. First, some film materials whose transmittance changes regularly with the increase of number of thin films was measured. Fig. 3(a) is the measured relationship between transmittance and the number of thin films. The dots are the measured actual transmittance values of thin films. The solid line is the fitting curve. Theoretically, the transmittance changes exponentially with the increase of the number of films. The measured result in Fig. 3(a) matches the theoretical analysis well. Meanwhile, from multiple measured results of one single film, it can be known that the maximum relative error is less than 1.51%. Next, the transmittance of a linearly attenuating plate was measured. Fig. 3(b) is the relationship between the transmittance and displacement of the linearly attenuating plate. Theoretically, the transmittance decreases linearly with the increase of displacement of the plate. From the Fig. 3(b), it can be seen that the measured result matches the linear characteristic. The maximum relative error of each of the tested places is less than 2%. In addition, the reflectance of a reflection membrane and a piece of black paper was measured. The maximum relative error is also less than 2%. These results show that the measurement system has largely eliminated the influence of instability of light source and has high accuracy and low noise interference. It is effective for practical materials. 4. CONCLUSIONS (a) Relationship between transmittance and number of thin films (b) Relationship between transmittance and displacement of a linearly attenuating plate Fig. 3 Relationship between transmittance and change of standard materials A measurement system for laser transmittance and reflectance of materials that uses a dual-beam relative method is developed in this paper. This method largely eliminates the influence of instability of light source and thus has a strong ability of anti-interference. The measuring system adopts a high-performance light source and a proper photodetector to improve the whole performance of the system. A preamplifier fitting the principle of noise matching and a frequency selecting filter are used in the detection circuit to inhibit the inner noise. Some experiments are completed to validate the measurement deviation and accuracy of the measurement system by using standard materials. The experiment results match the theoretical analysis very well. The maximum relative error is less than 2%. This system has advantages of high accuracy, strong noise immunity and high measuring speed. The cost of this measurement system is low.

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