- Email: [email protected]
A fiber link equipment for the optical rotation measuring based on AC-DC modulation magneto-optical effect
Accepted Manuscript Title: A fiber link equipment for the optical rotation measuring based on AC-DC modulation magneto-optical effect Author: Fang Wang Hui-Jing Wei Yu-Fang Liu PII: DOI: Reference:
S0924-4247(16)30289-8 http://dx.doi.org/doi:10.1016/j.sna.2016.06.004 SNA 9705
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
31-1-2016 25-4-2016 6-6-2016
Please cite this article as: Fang Wang, Hui-Jing Wei, Yu-Fang Liu, A fiber link equipment for the optical rotation measuring based on AC-DC modulation magneto-optical effect, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2016.06.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A fiber link equipment for the optical rotation measuring based on AC-DC modulation magneto-optical effect Fang Wang a,b, Hui-Jing Wei a, Yu-Fang Liu b,c,* a
College of Electonic and Electrical Engineering, Henan Normal University, Xinxiang 453007, China
b
Infrared Optoelectronic Science and Technology Key Laboratory of Henan Province, Xinxiang 453007, China
c
College of Physics and Material Science, Henan Normal University, Xinxiang 453007, China *
Corresponding author. Tel.: +8613937348875; Email: [email protected]
1
Highlights
An AC-DC two-stage modulation equipment to measure the optical rotation is proposed.
Removing the error of the magnetic rotation angle brought by the AC modulation alone.
A linear correlation is found between the rotation angle and the concentration.
A Faraday coil is combined with the AC-DC signal generator.
The fiber link makes the equipment portable and compact.
Abstract Based on the magnetic-optical effect, an alternating current-direct current (AC-DC) two-stage modulation is applied in the newly developed equipment, which measures the concentration of solution through the optical rotation of optical active substance. A Faraday coil is combined with the AC-DC signal generator controlled by a pulse width modulation controller. To deduct the measuring error caused by the AC modulation or the DC modulation, a two-stage method based on the Superposition of DC component and AC component is demonstrated, which is characterized by removing the interference of the bigger magnetic rotation angle in the AC modulation alone. The tartaric acid, glucose, and sucrose sugar solutions are taken as the standard examples in testifying the equipment. A linear correlation is found between the measured optical rotation angle and the concentration of the solution, with the optical rotation angle being within a 1% margin of error and the maximum relative error smaller than 0.005 g/ml. The results show that the equipment has the advantage of high precision, high integration, and strong anti-interference capacity. The fiber link makes the equipment portable and compact. The AC-DC two-stage modulation equipment can potentially be used in the optical rotation measurement. Keywords: magnetic-optical effect; optical rotation; pulse-width modulation (PWM); AC-DC modulation; Fiber
2
1. Introduction As an instrument which measures the optical rotation of optical solution based on the magneto-optical effect, polarimeter is used in the measurements of the concentration, purity and content of substance [1-2]. Different methods have been proposed to measure the optical rotation. For examples, Wang et al [3] proposed that the Faraday rotation angle can be detected by analyzing the frequency spectrum based on the alternating current (AC) modulation, and found that the result is not affected by the high odd harmonics. Yeh [4] presented a precise optical metrology system with the aim to determine the concentration of glucose solution, in which the average refractive index of a liquid solution is analyzed. Liu [5] measured the electric quantity instead of the mechanical rotation, in which the Faraday effect is utilized to measure the light vector rotate automatically. By adopting a Zeeman laser, Chou et al [6] proposed a method using the polarized photon-pairs heterodyne polarimetry, and the structure of balanced detector is proposed. In 2009, Binu et al [7] sensed the variation of refractive index with concentration of glucose in distilled water by fiber optic probe, and concluded that the refractive index increases proportionately with the concentration or density of a solute. In 2011, based on the phase-locked extraction, Lin et al [8-10] developed a linear heterodyne interferometer to measure the low optical rotation angle. Recently, using the modified Drude’s equation with the optical-rotatory-dispersion-polarization technique, Soetedjo et al [11] measured the optical rotation of the sugar compounds. In all of these methods, the knowledge of the conventional optical elements is taken as the basis. Though the traditional polarimeter has superior resolution, it is expensive and complicated which requirs many devices. To improve the accuracy and reduce the cost, the design of polarimeter should be optimized. In this work, an equipment using the full fiber configurations is proposed for the first time, in which a two-stage modulation method is adopted to measure the optical rotation of solution. The design of the system is described in Sec. 2.1, and the principles for the measurements are described in Sec. 2.2. 2.
System design
2.1. Experimental Setup Fig. 1 shows the schematic diagram of the experimental setup to measure the optical rotation, which comprises a semiconductor laser (part 1), a polarizer (part 2), a sample tube, a Faraday coil [12], an analyzer, and a photo-electric detector. The light source is 650 nm, which is generated from the semiconductor laser. The KG-ELD tunable laser module is adopted, with low power consumption and a high extinction ratio. In part 2, the light is polarized by a polarizer to obtain the given polarization direction. The polarizer and the analyzer are the THORLABS LPVIS050-MP2 polarizers, which are made of the reflection and crystal birefringence. Part 3 is the place for a sample where the optical rotation happens, with the length of the sample is 5 mm for matching and convenient alignment. Part 4 is a Faraday coil to rotate the light vector automatically, with a turn number of 5000 to generate enough magnetic field. Part 5 is an analyzer which analyzes the original polarization angle of the optical rotation. Part 6 [Fig.1 (b)] is an intuit-doped fiber which magnifies the optical signal [13], and the length of the fiber is 1m. Finally, part 7 is a photodetector detecting the secondary trim angle. The KG-PR-200M-B light detection module is adopted and integrated with the PIN detector response and low noise amplifier. The photo detector has advantages of high gain, high sensitivity, and flat amplification factor.
In the system, when a polarized light passes, the light vector will be automatically rotated to the 3
set position by the Faraday coil, and the modulation current can be changed to control the angle of the magnetic rotation [14]. Because the magnetic rotation angle generated by the DC through the Faraday effect is opposite to the original optical rotation angle of the measured, the original optical rotation angle can be measured by the DC when the position of extinction is emerged. The polarized light is turned into a small optical rotation signal because of the error brought by the position of extinction. The Faraday coil is modulated with the AC used to measure the small optical rotation angle. Finally, the data are collected, processed by a signal processing controller (SPC). At the same time, the SPC is used to analyze the modulated signal output from the photo-electric detector and to obtain the optical rotation angle. From the correlation between the measured optical rotation angle and the concentration of the standard solution, the concentration of the known sample can be determined by the optical rotation angle. 2.2. Magnetic modulation optical rotation Principles The principle for the analysis of the optical rotation angle is shown in Fig. 2(a). The polarization plane of the linearly polarized light rotates when the light passes through the sample [15]. 1 is the original angle of the magnetic rotation generated by the DC and θ2 is the small optical rotation angle measured by AC modulated current. The sample optical rotation angle θ induced by the measured sample can be expressed as,
t LC ,
(1) in which α is a specific rotation, L is the distance of the light passes through the sample, λ is the wavelength of incident light, t is the ambient temperature, and C is the concentration of the sample. When the DC signal passes through the Faraday modulator [16], due to the Faraday magneto-optical effect, θ1 can be written as,
1 V B d , (2) where V is the Verdet constant of the material inside the Faraday modulator, d is the distance of the light passes through the optical crystal, B 0 nI M is the magnetic induction. Thus Eq. (2) can be written as, 1 V 0 ndI M , (3) in which 0 is the magnetic permeability of vacuum, n the number of solenoid turns, and I M is the DC modulation current. Since V, 0 , n, and d are constants, θ1 depends linearly on I M in theory. θ1 can be obtained by the output DC component. The AC modulation can be used to measure small angle [17, 18]. As shown in Fig. 2(B), the light intensity after the analyzer can be expressed as, I LO I Li sin 2 2 A sin t ,
(4) where I Li is the intensity of the incoming light, θ2 is the secondary trim angle measured by the AC modulation. If θ2 is small, I LO I Li 2 A sin t . (5) Eq. (5) indicates that the output optical signal includes the DC component, and the AC components of angular frequency and 2 . Defining the amplitude ratio A of angular frequency ω and 2ω, the effect of the change in the optical rotation angle δA can be ignored. The amplitude ratio could be expressed as, A2 2 A 8 2 A2 , (6) 2
4
If only using the DC modulation, the measurement depends on the intrinsic properties of certain substances, which might cause inaccuracy. A periodic calibration is thus needed to maintain the accuracy. While if only using the AC modulation, the measurement depends on the small optical rotation angle unless the error in Eq. (5) is obvious. The combination of the AC and DC modulation, which is called as the AC-DC two-stage method, is used in the system to avoid the error caused by the intrinsic properties and the approximation. 2.3. Experimental process The flow chart of the detecting system is shown in Fig. 3. When the polarized light pass through the system, the light vector is rotated by the Faraday coil, and is used to measure magnetic rotation angle θ1 which is generated by the DC modulation. The DC modulation current is adjusted to make the output to the extinction position, in which the current will be increased if the output of light intensity is decreased. When the output is larger than the previous one, the DC current is recorded and the angle θ1 of the magnetic rotation is obtained. Since θ1 is the angle for the over-modulation in the DC modulation process, the measured optical rotation angle θ should be smaller than θ1. An extra angle θ2 is defined as the remaining rotation angle, which is obtained by superimposing the AC signal on the basis of DC signal. The optical rotation angle can be deduced from the AC-DC two-stage method, which is the difference between θ1 and θ2,
1 2 .
(7)
According to the optical rotation angle in Eq. (7) and Eq. (1), the concentration of solution can be determined. 2.4. Design for PWM control of AC-DC signal generator The PWM control of the AC-DC signal generator includes two parts, with the first part being the DC voltage source, and the second part being the AC voltage source. As shown in Fig. 4, V1 is the DC power supply and V3 is the adjustable pulse signal. The AC/DC step-down PWM is based on the converter circuit consisting of the V1, Q1, V3, L1, D1 and C1. The value of the DC voltage is changed by adjusting the pulse signal V3. V2 is the AC signal applied to the load R1 via the coupling of the capacitor C2. The whole optical system must be carefully adjusted in alignment to ensure that the insertion loss of each optical element is minimized. Measurement error can be also reduced by using the magnetic-optical fiber with small birefringence. In addition, the system has a high mechanical stability to ensure the reliability of the optical path system. 3. Results and discussion 3.1. Analysis of intuit-doped fiber under DC modulation In order to analyze the relationship between the magnetic optical rotation of the intuit-doped fiber and DC modulation, the output signal of intuit-doped fiber under DC modulation through the Faraday Effect has been in first tested without a sample. In brief, we describe the measurement procedure [see Fig. 1(a)]. First, the light beam from a 650 nm semiconductor laser 1 is turned into a polarized one when it passes through the polarizer 2. Polarized light via intuit-doped fiber 6 is rotated. Then the analyzer 5 is adjusted to the extinction position. In the process of DC modulation without sample solution 3, the plane of the polarized light changes periodically with the increasing DC due to the Faraday effect caused by intuit-doped fiber. Finally, the power of the output optical signal is caught by the spectrometer when 5
DC is increased from 0A to 6A to the Faraday coil. According to the Marius's law [19] and Eq. (3), the theoretical output power is the square of the sine function when the DC modulation current is an independent variable. The relationship between the current and the output power is shown in Fig. 5. The theoretical distribution is in shape consists with the experimental one, which suggests that the intuit-doped fiber can be used to measure the optical rotation in the AC-DC modulation equipment. 3.2. AC modulation output signal detection In measuring the small optical rotation angle, the modulated signal output amplified by the photo detector is collected and processed by SPC. Fig. 6(a) shows the output waveform of the photo detector. And the transformed waveform by the Fast Fourier Transform (FFT) method, which will be used to analyze the output signal, is plotted in Fig. 6(b). The frequency of the current signal generator applied to the coil of the Faraday modulation is located at 50 Hz, and the voltage is 15V after the transformer. The intuit-doped fiber is provided for AC magnetic field through the coil. After the FFT, the amplitude ratio is observed at 50 Hz and 100 Hz. According to Eq. (6), A under AC modulation is 0.017o. 3.3. Test of PWM control of AC-DC source The voltage and current output are adjusted by changing the duty cycle of rectangular wave signal. The output waveform of PWM control of AC-DC source is shown in Fig.7. Modulated output waveform representation from DC output is superimposed by the AC output. The waveform earlier than 6s representation is from the DC modulation, while the back is output by the AC waveform representation. The frequency of the AC voltage source is 50 HZ. 3.4. Comparisons between A, θ2 and A under AC modulation As represented in Fig. 8, the correlation among A, θ2 and A under the AC modulation is simulated with theoretical calculation results. It is showned by the smaller graph in Fig. 8 that parameter A is influenced by two variables at the same time with previous algorithm. Comparatively, parameter A is only related to A through the improved algorithm showned by the larger graph in Fig. 8. The impact caused by the A is liminated. the measuring error and processing time of the optical rotation angle are obviously reduced. These theoretical analysis of process and data demonstrate that the system is simple in design, easy in operation and safe in use. 3.5. Experimental test of the system To testify the equipment, the tartaric acid，glucose and sucrose sugar solutions, with the concentrations range from 0.050 to 0.200 g/ml, have been adopted as the standard samples in the measurements. During the experiment, the temperature is kept constantly at 25 ± 0.01 °C by using a temperature controller. Repeatability features play an important role in the analysis of the measurements. The measurement in this work is conducted six times under the same condition and the reproducibility is within the expected error range according to results. In Fig. 9, the correlation between the optical rotation angle and the standard concentration of the solutions are plotted. The linear fittings between the correlation of the rotation angle and concentration for the tartaric acid yield y = -0.1005 + 12.5988x, for glucose y = -0.8399 + 55.2768x, and for sucrose sugar y = -1.2706 + 77.617x, with the adjusted R2 being 0.99878, 0.99912, and 0.99975, respectively. The implemented equipment yields high sensitivities of 77.617 ± 0.03g/ml. Within an error of optical 6
rotation smaller than 1%, the deviation between the measured and the standard concentration of the sample solution is no larger than 0.005 g/ml, indicating that the two-stage AC-DC method is more precise and resolution than the DC method [20]. At the same time, the AC-DC method potentially has a wider measurable range than the AC method. , 4. Summary In this study, based on the two-stage AC-DC modulation magneto-optical effect, an equipment is developed to measure the concentration of solution with high quality via measuring the optical rotation. The optical structure of the equipment is mainly composed of a semiconductor laser, a single mode fiber, an AC-DC two-stage power modulator, and a photo detector. In the first DC modulation stage, the angle correction is obtained. In the second AC modulation stage, the small angle is detected by the photo detector with high precision. A linear correlation between sample concentration and optical rotation is theoretically predicted, and confirmed in the measured results. A high optical rotation angle sensitivity of 77.617 ± 0.03g/ml is obtained for the concentration range between 0.010 and 0.200. A linear correlation is found between the measured optical rotation angle and the concentration. The margin of error in optical rotation angle is merely 1% and the maximum relative error is smaller than 0.005 g/ml. The measured results of the solutions confirm that the AC-DC method is convincing. The structure of this newly developed AC-DC system can improve the stability, and reduce the error caused by the conventional servo motor or other optical devices. Acknowledgment The work is supported by National Natural Science Foundation of China (No. 61127012) and College students' innovative entrepreneurial training project. Funding from the Henan Normal University in the project 201310476046 is acknowledged.
7
References [1] K. H. Chen, Y. C. Chu, J. H. Chen. Applying the phase difference property of polarization angle for measuring the concentration of solutions. Optics & Laser Technology, 44 (2012), pp. 251-254. [2] X. D. Wang, T. Y. Zhou, X. Chen, K. Y. Wong, X. R. Wang. An optical biosensor for the rapid determination of glucose in human serum. Sensors and Actuators B, 129 (2008), pp. 866–873. [3] X. Q. Wang, H. Z. Jia, Q. D. Cai. The measurement of Faraday rotation angle by the frequency spectrum analysis. Optik, 124 (2013), pp. 2877–2880. [4] Y. L. Yeh. Real-time measurement of glucose concentration and average refractive index using a laser interferometer. Optics and Lasers in Engineering, 46 (2008), pp. 666–670. [5] Z. Q. Liu. Using the light intensity distribution measuring specific rotation and concentration of sucrose solution. University physics, 29 (2010) ( in Chinese), pp.37-39. [6] C. Chou, K. H. Chiang, K. Y. Liao, Y. F. Chang, C.E. Lin. Polarized photon-pairs heterodyne polarimetry for ultrasensitive optical activity detection of a chiral medium. The Journal of Physical Chemistry B, 111 (2007), pp. 9919-9922. [7] S. Binu et al., Fibre optic glucose sensor. Materials Science and Engineering: C, 29 (2009),pp. 183-186. [8] J. Y. Lin, K. H. Chen, D. C. Su. Improved method for measuring small optical rotation angle of chiral medium. Optics Communications, 238 (2004), pp.113-118. [9] J. F. Lin, C. C. Chang, C. D. Syu, Y. L. Lo, S. Y. Lee. A new electro-optic modulated circular heterodyne interferometer for measuring the rotation angle in a chiral medium. Optics and Laser in Engineering, 47 (2009), pp. 39-44. [10] J. F. Lin, J. S. Wu, C. H. Huang, J. S. Jeng. An instrument for measuring low optical rotation angle. Optik, 122 (2011), pp. 733-738. [11] H. Soetedjo, J. Räty. Use of a modified Drude’s equation to investigate the optical rotation property of sugars. Optik, 125 (2014), pp.7162–7165. [12] Lin Li, Qun Han, Yaofei Chen, Tiegen Liu, and Rongxiang Zhang. An All-Fiber Optic Current Sensor Based on Ferrofluids and Multimode Interference. IEEE Sensors Journal, 14(2014),pp.1749-1753 [13]J. H. Liu, X. F. Zhang, H. Z. Li, Z. W. Bao. Measurement of Rotatory Optics Element in Tensor dielectric Matrix for Rotatory Optical Fiber. Transactions of Tianjin University, 11 (2005), pp.115-118. [14] Y. Y. Tang. Optical tutorial. Tsinghua University Press, Beijing (1979). [15] X. B. Liu, Y. Y. Wu, X. D. Zhang, J. L. Wang. Static automatic Polarimeter. Journal of Measurement, 30 (2009) (in Chinese), pp. 179-181. [16] F. Wang, C. Q. Wang, X. Wang, R. Y. Zhang, Y. F. Liu. The design based on the magnetic rotation effect of mass concentration meter optimization. Optoelectronic technology, 33(2013) (in Chinese), pp. 117-120. [17] S. B. Wang, C. X. Sun, L. Du, C. G. Yao, Y. Yang. Reciprocity of Faraday effect in ferrofluid: Comparison with magneto-optical glass. Optik, 123 (2012), pp. 553–558. [18] F. Wang, C. Q. Wang, M. Y. Wang, S. P. Xiu, Y. F. Liu. Optimization design based on filter rotation. Journal of Henan Normal University (Natural Science Edition), 41 (2013) (in Chinese), pp. 61-63. [19] Hamid Esmaeilzadeh, Ezatollah Arzi, Morteza Mozafari, Alireza Hassani, A broadband optical fiber based inline polarizer for telecom wavelength range. Sensors and Actuators A, 185(2012), pp. 59–65. [20] LIU Xiang-bin，WU Yue-yan，ZHANG Xu-dong， WANG Jia-li. A Static Automatic Poiarimeter. ACTA METROLOGICA SINICA, 30(2009),pp. 179-181
8
Biographies
Fang Wang received the M.A. in Measurement technology and instruments from the Dalian University of Technology, Dalian, China, in 2003. From 2008 and then, she was an Associate Professor in the College of Electonic and Electrical Engineering of Henan Normal University. Now She is now a PhD student of the Henan Normal University. Her mainly research is the photoelectric detection technology. At present, her interests are focused on the design and application research of the optical fiber sensor.
Hui-Jing Wei is a graduate student in the College of Electonic and Electrical Engineering of Henan Normal
University. Her research interests are optical rotation measuring, modulation magneto-optical, magneto optic fiber and its sensing applications. She has authored and co-authored several scientific papers.
Yu‐fang Liu received the Ph. D. degrees from Dalian University of Technology. He is the member of the National Natural Science Foundation Committee of the thirteenth expert review team. In 2006, he was a doctoral tutor and part‐time professor at Beijing Institute of Technology. He is currently a Full Professor with the Henan Normal University, Xinxiang, China. His current research interests include development of fiber‐optic sen‐sors and device,Infrared Physics and technology, novel sensor materials and principles, and optical measurement technologies. In the past five years, he has authored or coauthored more than 50 articles in the Phys. Chem. Chem. Phys J., Comput. Chem. Phys., Lett. Phys., Phys. Lett. Chem. and other important academic journals published in more than. He is a member of China Ordnance Industry Association Professional Committee of Optics and the member of Editorial Boards of journals such as the optical technique, Optical Instruments, and Process Automation Instrumentation. Prof.Liu was awarded as the outstanding youth fund of Henan Province in 2004. In 2008, he was awarded as the university outstanding scientific research talent innovation project of Henan Province. From 2006 to 2015, he was awarded by the National Natural Science Foundation of China for four times.
9
Fig. 1. (Color Online) (a) Schematic diagram of the experimental setup for the device: 1. Laser; 2. alignment device; 3. Polarizer; 4. Cuvette; 5. Faraday coil; 6. intuit-doped fiber; 7. Analyzer; and 8. photodetector (b) A picture of the splicing between the single-mode fiber and the magneto optical fiber.
Fig. 2. Schematic diagram for the angle rotation in the measurement.
10
Fig. 3. The flow chart of the detecting system.
Fig. 4. Circuit diagram of the AC-DC signal generator.
Fig. 5. Theoretical and experimental results for the output power response to current.
11
Fig. 6. (a) The output waveform of photo detector. detector output waveform
(b) The Fast Fourier Transform of the photo
Fig.7. Output waveform as the PWM control of the AC-DC source.
Fig. 8. (Color Online) Simulated correlation among A, θ2 and A under AC modulation.
12
Fig. 9. The correlation between the optical rotation and the concentration of the standard tartaric acid, glucose and sucrose sugar solution. The line denotes the linear fitting to the correlation
13
S0924-4247(16)30289-8 http://dx.doi.org/doi:10.1016/j.sna.2016.06.004 SNA 9705
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
31-1-2016 25-4-2016 6-6-2016
Please cite this article as: Fang Wang, Hui-Jing Wei, Yu-Fang Liu, A fiber link equipment for the optical rotation measuring based on AC-DC modulation magneto-optical effect, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2016.06.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A fiber link equipment for the optical rotation measuring based on AC-DC modulation magneto-optical effect Fang Wang a,b, Hui-Jing Wei a, Yu-Fang Liu b,c,* a
College of Electonic and Electrical Engineering, Henan Normal University, Xinxiang 453007, China
b
Infrared Optoelectronic Science and Technology Key Laboratory of Henan Province, Xinxiang 453007, China
c
College of Physics and Material Science, Henan Normal University, Xinxiang 453007, China *
Corresponding author. Tel.: +8613937348875; Email: [email protected]
1
Highlights
An AC-DC two-stage modulation equipment to measure the optical rotation is proposed.
Removing the error of the magnetic rotation angle brought by the AC modulation alone.
A linear correlation is found between the rotation angle and the concentration.
A Faraday coil is combined with the AC-DC signal generator.
The fiber link makes the equipment portable and compact.
Abstract Based on the magnetic-optical effect, an alternating current-direct current (AC-DC) two-stage modulation is applied in the newly developed equipment, which measures the concentration of solution through the optical rotation of optical active substance. A Faraday coil is combined with the AC-DC signal generator controlled by a pulse width modulation controller. To deduct the measuring error caused by the AC modulation or the DC modulation, a two-stage method based on the Superposition of DC component and AC component is demonstrated, which is characterized by removing the interference of the bigger magnetic rotation angle in the AC modulation alone. The tartaric acid, glucose, and sucrose sugar solutions are taken as the standard examples in testifying the equipment. A linear correlation is found between the measured optical rotation angle and the concentration of the solution, with the optical rotation angle being within a 1% margin of error and the maximum relative error smaller than 0.005 g/ml. The results show that the equipment has the advantage of high precision, high integration, and strong anti-interference capacity. The fiber link makes the equipment portable and compact. The AC-DC two-stage modulation equipment can potentially be used in the optical rotation measurement. Keywords: magnetic-optical effect; optical rotation; pulse-width modulation (PWM); AC-DC modulation; Fiber
2
1. Introduction As an instrument which measures the optical rotation of optical solution based on the magneto-optical effect, polarimeter is used in the measurements of the concentration, purity and content of substance [1-2]. Different methods have been proposed to measure the optical rotation. For examples, Wang et al [3] proposed that the Faraday rotation angle can be detected by analyzing the frequency spectrum based on the alternating current (AC) modulation, and found that the result is not affected by the high odd harmonics. Yeh [4] presented a precise optical metrology system with the aim to determine the concentration of glucose solution, in which the average refractive index of a liquid solution is analyzed. Liu [5] measured the electric quantity instead of the mechanical rotation, in which the Faraday effect is utilized to measure the light vector rotate automatically. By adopting a Zeeman laser, Chou et al [6] proposed a method using the polarized photon-pairs heterodyne polarimetry, and the structure of balanced detector is proposed. In 2009, Binu et al [7] sensed the variation of refractive index with concentration of glucose in distilled water by fiber optic probe, and concluded that the refractive index increases proportionately with the concentration or density of a solute. In 2011, based on the phase-locked extraction, Lin et al [8-10] developed a linear heterodyne interferometer to measure the low optical rotation angle. Recently, using the modified Drude’s equation with the optical-rotatory-dispersion-polarization technique, Soetedjo et al [11] measured the optical rotation of the sugar compounds. In all of these methods, the knowledge of the conventional optical elements is taken as the basis. Though the traditional polarimeter has superior resolution, it is expensive and complicated which requirs many devices. To improve the accuracy and reduce the cost, the design of polarimeter should be optimized. In this work, an equipment using the full fiber configurations is proposed for the first time, in which a two-stage modulation method is adopted to measure the optical rotation of solution. The design of the system is described in Sec. 2.1, and the principles for the measurements are described in Sec. 2.2. 2.
System design
2.1. Experimental Setup Fig. 1 shows the schematic diagram of the experimental setup to measure the optical rotation, which comprises a semiconductor laser (part 1), a polarizer (part 2), a sample tube, a Faraday coil [12], an analyzer, and a photo-electric detector. The light source is 650 nm, which is generated from the semiconductor laser. The KG-ELD tunable laser module is adopted, with low power consumption and a high extinction ratio. In part 2, the light is polarized by a polarizer to obtain the given polarization direction. The polarizer and the analyzer are the THORLABS LPVIS050-MP2 polarizers, which are made of the reflection and crystal birefringence. Part 3 is the place for a sample where the optical rotation happens, with the length of the sample is 5 mm for matching and convenient alignment. Part 4 is a Faraday coil to rotate the light vector automatically, with a turn number of 5000 to generate enough magnetic field. Part 5 is an analyzer which analyzes the original polarization angle of the optical rotation. Part 6 [Fig.1 (b)] is an intuit-doped fiber which magnifies the optical signal [13], and the length of the fiber is 1m. Finally, part 7 is a photodetector detecting the secondary trim angle. The KG-PR-200M-B light detection module is adopted and integrated with the PIN detector response and low noise amplifier. The photo detector has advantages of high gain, high sensitivity, and flat amplification factor.
In the system, when a polarized light passes, the light vector will be automatically rotated to the 3
set position by the Faraday coil, and the modulation current can be changed to control the angle of the magnetic rotation [14]. Because the magnetic rotation angle generated by the DC through the Faraday effect is opposite to the original optical rotation angle of the measured, the original optical rotation angle can be measured by the DC when the position of extinction is emerged. The polarized light is turned into a small optical rotation signal because of the error brought by the position of extinction. The Faraday coil is modulated with the AC used to measure the small optical rotation angle. Finally, the data are collected, processed by a signal processing controller (SPC). At the same time, the SPC is used to analyze the modulated signal output from the photo-electric detector and to obtain the optical rotation angle. From the correlation between the measured optical rotation angle and the concentration of the standard solution, the concentration of the known sample can be determined by the optical rotation angle. 2.2. Magnetic modulation optical rotation Principles The principle for the analysis of the optical rotation angle is shown in Fig. 2(a). The polarization plane of the linearly polarized light rotates when the light passes through the sample [15]. 1 is the original angle of the magnetic rotation generated by the DC and θ2 is the small optical rotation angle measured by AC modulated current. The sample optical rotation angle θ induced by the measured sample can be expressed as,
t LC ,
(1) in which α is a specific rotation, L is the distance of the light passes through the sample, λ is the wavelength of incident light, t is the ambient temperature, and C is the concentration of the sample. When the DC signal passes through the Faraday modulator [16], due to the Faraday magneto-optical effect, θ1 can be written as,
1 V B d , (2) where V is the Verdet constant of the material inside the Faraday modulator, d is the distance of the light passes through the optical crystal, B 0 nI M is the magnetic induction. Thus Eq. (2) can be written as, 1 V 0 ndI M , (3) in which 0 is the magnetic permeability of vacuum, n the number of solenoid turns, and I M is the DC modulation current. Since V, 0 , n, and d are constants, θ1 depends linearly on I M in theory. θ1 can be obtained by the output DC component. The AC modulation can be used to measure small angle [17, 18]. As shown in Fig. 2(B), the light intensity after the analyzer can be expressed as, I LO I Li sin 2 2 A sin t ,
(4) where I Li is the intensity of the incoming light, θ2 is the secondary trim angle measured by the AC modulation. If θ2 is small, I LO I Li 2 A sin t . (5) Eq. (5) indicates that the output optical signal includes the DC component, and the AC components of angular frequency and 2 . Defining the amplitude ratio A of angular frequency ω and 2ω, the effect of the change in the optical rotation angle δA can be ignored. The amplitude ratio could be expressed as, A2 2 A 8 2 A2 , (6) 2
4
If only using the DC modulation, the measurement depends on the intrinsic properties of certain substances, which might cause inaccuracy. A periodic calibration is thus needed to maintain the accuracy. While if only using the AC modulation, the measurement depends on the small optical rotation angle unless the error in Eq. (5) is obvious. The combination of the AC and DC modulation, which is called as the AC-DC two-stage method, is used in the system to avoid the error caused by the intrinsic properties and the approximation. 2.3. Experimental process The flow chart of the detecting system is shown in Fig. 3. When the polarized light pass through the system, the light vector is rotated by the Faraday coil, and is used to measure magnetic rotation angle θ1 which is generated by the DC modulation. The DC modulation current is adjusted to make the output to the extinction position, in which the current will be increased if the output of light intensity is decreased. When the output is larger than the previous one, the DC current is recorded and the angle θ1 of the magnetic rotation is obtained. Since θ1 is the angle for the over-modulation in the DC modulation process, the measured optical rotation angle θ should be smaller than θ1. An extra angle θ2 is defined as the remaining rotation angle, which is obtained by superimposing the AC signal on the basis of DC signal. The optical rotation angle can be deduced from the AC-DC two-stage method, which is the difference between θ1 and θ2,
1 2 .
(7)
According to the optical rotation angle in Eq. (7) and Eq. (1), the concentration of solution can be determined. 2.4. Design for PWM control of AC-DC signal generator The PWM control of the AC-DC signal generator includes two parts, with the first part being the DC voltage source, and the second part being the AC voltage source. As shown in Fig. 4, V1 is the DC power supply and V3 is the adjustable pulse signal. The AC/DC step-down PWM is based on the converter circuit consisting of the V1, Q1, V3, L1, D1 and C1. The value of the DC voltage is changed by adjusting the pulse signal V3. V2 is the AC signal applied to the load R1 via the coupling of the capacitor C2. The whole optical system must be carefully adjusted in alignment to ensure that the insertion loss of each optical element is minimized. Measurement error can be also reduced by using the magnetic-optical fiber with small birefringence. In addition, the system has a high mechanical stability to ensure the reliability of the optical path system. 3. Results and discussion 3.1. Analysis of intuit-doped fiber under DC modulation In order to analyze the relationship between the magnetic optical rotation of the intuit-doped fiber and DC modulation, the output signal of intuit-doped fiber under DC modulation through the Faraday Effect has been in first tested without a sample. In brief, we describe the measurement procedure [see Fig. 1(a)]. First, the light beam from a 650 nm semiconductor laser 1 is turned into a polarized one when it passes through the polarizer 2. Polarized light via intuit-doped fiber 6 is rotated. Then the analyzer 5 is adjusted to the extinction position. In the process of DC modulation without sample solution 3, the plane of the polarized light changes periodically with the increasing DC due to the Faraday effect caused by intuit-doped fiber. Finally, the power of the output optical signal is caught by the spectrometer when 5
DC is increased from 0A to 6A to the Faraday coil. According to the Marius's law [19] and Eq. (3), the theoretical output power is the square of the sine function when the DC modulation current is an independent variable. The relationship between the current and the output power is shown in Fig. 5. The theoretical distribution is in shape consists with the experimental one, which suggests that the intuit-doped fiber can be used to measure the optical rotation in the AC-DC modulation equipment. 3.2. AC modulation output signal detection In measuring the small optical rotation angle, the modulated signal output amplified by the photo detector is collected and processed by SPC. Fig. 6(a) shows the output waveform of the photo detector. And the transformed waveform by the Fast Fourier Transform (FFT) method, which will be used to analyze the output signal, is plotted in Fig. 6(b). The frequency of the current signal generator applied to the coil of the Faraday modulation is located at 50 Hz, and the voltage is 15V after the transformer. The intuit-doped fiber is provided for AC magnetic field through the coil. After the FFT, the amplitude ratio is observed at 50 Hz and 100 Hz. According to Eq. (6), A under AC modulation is 0.017o. 3.3. Test of PWM control of AC-DC source The voltage and current output are adjusted by changing the duty cycle of rectangular wave signal. The output waveform of PWM control of AC-DC source is shown in Fig.7. Modulated output waveform representation from DC output is superimposed by the AC output. The waveform earlier than 6s representation is from the DC modulation, while the back is output by the AC waveform representation. The frequency of the AC voltage source is 50 HZ. 3.4. Comparisons between A, θ2 and A under AC modulation As represented in Fig. 8, the correlation among A, θ2 and A under the AC modulation is simulated with theoretical calculation results. It is showned by the smaller graph in Fig. 8 that parameter A is influenced by two variables at the same time with previous algorithm. Comparatively, parameter A is only related to A through the improved algorithm showned by the larger graph in Fig. 8. The impact caused by the A is liminated. the measuring error and processing time of the optical rotation angle are obviously reduced. These theoretical analysis of process and data demonstrate that the system is simple in design, easy in operation and safe in use. 3.5. Experimental test of the system To testify the equipment, the tartaric acid，glucose and sucrose sugar solutions, with the concentrations range from 0.050 to 0.200 g/ml, have been adopted as the standard samples in the measurements. During the experiment, the temperature is kept constantly at 25 ± 0.01 °C by using a temperature controller. Repeatability features play an important role in the analysis of the measurements. The measurement in this work is conducted six times under the same condition and the reproducibility is within the expected error range according to results. In Fig. 9, the correlation between the optical rotation angle and the standard concentration of the solutions are plotted. The linear fittings between the correlation of the rotation angle and concentration for the tartaric acid yield y = -0.1005 + 12.5988x, for glucose y = -0.8399 + 55.2768x, and for sucrose sugar y = -1.2706 + 77.617x, with the adjusted R2 being 0.99878, 0.99912, and 0.99975, respectively. The implemented equipment yields high sensitivities of 77.617 ± 0.03g/ml. Within an error of optical 6
rotation smaller than 1%, the deviation between the measured and the standard concentration of the sample solution is no larger than 0.005 g/ml, indicating that the two-stage AC-DC method is more precise and resolution than the DC method [20]. At the same time, the AC-DC method potentially has a wider measurable range than the AC method. , 4. Summary In this study, based on the two-stage AC-DC modulation magneto-optical effect, an equipment is developed to measure the concentration of solution with high quality via measuring the optical rotation. The optical structure of the equipment is mainly composed of a semiconductor laser, a single mode fiber, an AC-DC two-stage power modulator, and a photo detector. In the first DC modulation stage, the angle correction is obtained. In the second AC modulation stage, the small angle is detected by the photo detector with high precision. A linear correlation between sample concentration and optical rotation is theoretically predicted, and confirmed in the measured results. A high optical rotation angle sensitivity of 77.617 ± 0.03g/ml is obtained for the concentration range between 0.010 and 0.200. A linear correlation is found between the measured optical rotation angle and the concentration. The margin of error in optical rotation angle is merely 1% and the maximum relative error is smaller than 0.005 g/ml. The measured results of the solutions confirm that the AC-DC method is convincing. The structure of this newly developed AC-DC system can improve the stability, and reduce the error caused by the conventional servo motor or other optical devices. Acknowledgment The work is supported by National Natural Science Foundation of China (No. 61127012) and College students' innovative entrepreneurial training project. Funding from the Henan Normal University in the project 201310476046 is acknowledged.
7
References [1] K. H. Chen, Y. C. Chu, J. H. Chen. Applying the phase difference property of polarization angle for measuring the concentration of solutions. Optics & Laser Technology, 44 (2012), pp. 251-254. [2] X. D. Wang, T. Y. Zhou, X. Chen, K. Y. Wong, X. R. Wang. An optical biosensor for the rapid determination of glucose in human serum. Sensors and Actuators B, 129 (2008), pp. 866–873. [3] X. Q. Wang, H. Z. Jia, Q. D. Cai. The measurement of Faraday rotation angle by the frequency spectrum analysis. Optik, 124 (2013), pp. 2877–2880. [4] Y. L. Yeh. Real-time measurement of glucose concentration and average refractive index using a laser interferometer. Optics and Lasers in Engineering, 46 (2008), pp. 666–670. [5] Z. Q. Liu. Using the light intensity distribution measuring specific rotation and concentration of sucrose solution. University physics, 29 (2010) ( in Chinese), pp.37-39. [6] C. Chou, K. H. Chiang, K. Y. Liao, Y. F. Chang, C.E. Lin. Polarized photon-pairs heterodyne polarimetry for ultrasensitive optical activity detection of a chiral medium. The Journal of Physical Chemistry B, 111 (2007), pp. 9919-9922. [7] S. Binu et al., Fibre optic glucose sensor. Materials Science and Engineering: C, 29 (2009),pp. 183-186. [8] J. Y. Lin, K. H. Chen, D. C. Su. Improved method for measuring small optical rotation angle of chiral medium. Optics Communications, 238 (2004), pp.113-118. [9] J. F. Lin, C. C. Chang, C. D. Syu, Y. L. Lo, S. Y. Lee. A new electro-optic modulated circular heterodyne interferometer for measuring the rotation angle in a chiral medium. Optics and Laser in Engineering, 47 (2009), pp. 39-44. [10] J. F. Lin, J. S. Wu, C. H. Huang, J. S. Jeng. An instrument for measuring low optical rotation angle. Optik, 122 (2011), pp. 733-738. [11] H. Soetedjo, J. Räty. Use of a modified Drude’s equation to investigate the optical rotation property of sugars. Optik, 125 (2014), pp.7162–7165. [12] Lin Li, Qun Han, Yaofei Chen, Tiegen Liu, and Rongxiang Zhang. An All-Fiber Optic Current Sensor Based on Ferrofluids and Multimode Interference. IEEE Sensors Journal, 14(2014),pp.1749-1753 [13]J. H. Liu, X. F. Zhang, H. Z. Li, Z. W. Bao. Measurement of Rotatory Optics Element in Tensor dielectric Matrix for Rotatory Optical Fiber. Transactions of Tianjin University, 11 (2005), pp.115-118. [14] Y. Y. Tang. Optical tutorial. Tsinghua University Press, Beijing (1979). [15] X. B. Liu, Y. Y. Wu, X. D. Zhang, J. L. Wang. Static automatic Polarimeter. Journal of Measurement, 30 (2009) (in Chinese), pp. 179-181. [16] F. Wang, C. Q. Wang, X. Wang, R. Y. Zhang, Y. F. Liu. The design based on the magnetic rotation effect of mass concentration meter optimization. Optoelectronic technology, 33(2013) (in Chinese), pp. 117-120. [17] S. B. Wang, C. X. Sun, L. Du, C. G. Yao, Y. Yang. Reciprocity of Faraday effect in ferrofluid: Comparison with magneto-optical glass. Optik, 123 (2012), pp. 553–558. [18] F. Wang, C. Q. Wang, M. Y. Wang, S. P. Xiu, Y. F. Liu. Optimization design based on filter rotation. Journal of Henan Normal University (Natural Science Edition), 41 (2013) (in Chinese), pp. 61-63. [19] Hamid Esmaeilzadeh, Ezatollah Arzi, Morteza Mozafari, Alireza Hassani, A broadband optical fiber based inline polarizer for telecom wavelength range. Sensors and Actuators A, 185(2012), pp. 59–65. [20] LIU Xiang-bin，WU Yue-yan，ZHANG Xu-dong， WANG Jia-li. A Static Automatic Poiarimeter. ACTA METROLOGICA SINICA, 30(2009),pp. 179-181
8
Biographies
Fang Wang received the M.A. in Measurement technology and instruments from the Dalian University of Technology, Dalian, China, in 2003. From 2008 and then, she was an Associate Professor in the College of Electonic and Electrical Engineering of Henan Normal University. Now She is now a PhD student of the Henan Normal University. Her mainly research is the photoelectric detection technology. At present, her interests are focused on the design and application research of the optical fiber sensor.
Hui-Jing Wei is a graduate student in the College of Electonic and Electrical Engineering of Henan Normal
University. Her research interests are optical rotation measuring, modulation magneto-optical, magneto optic fiber and its sensing applications. She has authored and co-authored several scientific papers.
Yu‐fang Liu received the Ph. D. degrees from Dalian University of Technology. He is the member of the National Natural Science Foundation Committee of the thirteenth expert review team. In 2006, he was a doctoral tutor and part‐time professor at Beijing Institute of Technology. He is currently a Full Professor with the Henan Normal University, Xinxiang, China. His current research interests include development of fiber‐optic sen‐sors and device,Infrared Physics and technology, novel sensor materials and principles, and optical measurement technologies. In the past five years, he has authored or coauthored more than 50 articles in the Phys. Chem. Chem. Phys J., Comput. Chem. Phys., Lett. Phys., Phys. Lett. Chem. and other important academic journals published in more than. He is a member of China Ordnance Industry Association Professional Committee of Optics and the member of Editorial Boards of journals such as the optical technique, Optical Instruments, and Process Automation Instrumentation. Prof.Liu was awarded as the outstanding youth fund of Henan Province in 2004. In 2008, he was awarded as the university outstanding scientific research talent innovation project of Henan Province. From 2006 to 2015, he was awarded by the National Natural Science Foundation of China for four times.
9
Fig. 1. (Color Online) (a) Schematic diagram of the experimental setup for the device: 1. Laser; 2. alignment device; 3. Polarizer; 4. Cuvette; 5. Faraday coil; 6. intuit-doped fiber; 7. Analyzer; and 8. photodetector (b) A picture of the splicing between the single-mode fiber and the magneto optical fiber.
Fig. 2. Schematic diagram for the angle rotation in the measurement.
10
Fig. 3. The flow chart of the detecting system.
Fig. 4. Circuit diagram of the AC-DC signal generator.
Fig. 5. Theoretical and experimental results for the output power response to current.
11
Fig. 6. (a) The output waveform of photo detector. detector output waveform
(b) The Fast Fourier Transform of the photo
Fig.7. Output waveform as the PWM control of the AC-DC source.
Fig. 8. (Color Online) Simulated correlation among A, θ2 and A under AC modulation.
12
Fig. 9. The correlation between the optical rotation and the concentration of the standard tartaric acid, glucose and sucrose sugar solution. The line denotes the linear fitting to the correlation
13