Liquid refractive index (RI) is one of the important optical parameters closely related to density, concentration, and temperature. The measurement of liquid RI is widely used in chemistry, medicine, physics, and biology research. A Fabry- Perot interferometer-based liquid RI sensor by using PMMA polymer material is proposed and demonstrated experimentally. The influence of cavity length and reflectivity on the sensor sensitivity is theoretically investigated. A long cavity length gives rise to a small free space range. In the experiment, high reflectivity of F-P cavity is made by evaporating metal film (aluminum film). An open F-P interferometer is realized by punching a hole in the PMMA plate, which is convenient for liquid exchange and in situ measurement. Experimental results show that the FSR the F-P cavity is about 5 nm with a fringe contrast of 3 dB. The designed sensor is compact, low-cost, easy fabrication and flexible replacement. Keywords: Fabry-Perot interferometer, Refractive index sensor, polymethyl methacrylate.
KEYWORDS: Field programmable gate arrays, Temperature metrology, Temperature sensors, Control systems, Sensors, Digital signal processing, Analog electronics, Oceanography, Beam controllers
A laser frequency locking control circuit based on field-programmable-gate-array (FPGA) was developed to deploy a laser frequency locking system to meet the requirement of high-resolution temperature measurement. This system is based on the Pound-Drever-Hall (PDH) frequency stabilization technology. The FPGA is designed as the core control, combined with Direct Digital Synthesizer (DDS) technology to realize the function generator, ramp generator, Lock-in, infinite impulse response filter (IIR filter), proportion integration differentiation (PID), Relock and other functional modules. The function generator generates the modulation and demodulation signal to obtain the system error signal. According to the error signal sent to the PID, the output feedback voltage is adjusted by the PID to stay on the zero-cross point of the error signal so that the laser driver is controlled to stabilize the frequency of the laser to a resonance. This system has demonstrated frequency stability of 10-11 experimentally with a small size of 100mm×75mm.
The optical properties of seawater are of great significance to the monitoring of marine safety and marine environment by means of optical sensing. In order to further study the relationship between the refractive index (RI) of different salt solutions, temperature and salinity, two experiments are carried out. The effect of temperature and mass fraction on RI of single component solution, and the effect of type and quantity of ions on the RI of mixed solutions when the mass fraction is kept constant at 3.5% are studied. Experiment results show that mass fraction coefficient of NaCl, MgCl2 and MgSO4 are 1.74×10-4 , 2.54×10-4 and 1.94×10-4 , respectively. Mass fraction coefficient of NaCl is lower than that of MgCl2 and MgSO4. In the second experiment, it is found that the effect of valence state on RI is higher than that of ion number, and the RI of bivalent solution is higher than that of monovalent solution. Not only the temperature and salinity of seawater, but also the components of seawater salt ions have different effects on the RI. Since the intrinsic relationship between RI and salinity is not suitable for different sea areas, the study of the relationship between the RI of single solution or a binary mixed solution and temperature and salinity is of great significance for the modulation process of optical salinity sensing.
In this paper, a new method for measuring the refractive index of seawater based on the V-shaped cavity is proposed. MATLAB software is used to simulate the V-shaped cavity based on the optical thin film transfer matrix. By changing the refractive index of the liquid in the glassware placed in the cavity, the filtering characteristics of the V-shaped cavity are changed. The change of refractive index can be demodulated by monitoring the shift of transmission wavelength of V-shaped cavity. In this paper, the refractive index detection sensitivity is 217nm/RIU, and the refractive index high resolution measurement can be up to 10-8 RIU. In this paper, the empirical formula of the change of refractive index with the change of transmission wavelength shift is given. When the change of refractive index is 10-7 RIU, the accuracy of demodulation using the empirical formula is 0.03%. The outcomes indicate that the new method proposed in this paper has a strong application prospect in the field of high resolution ocean optical sensing.
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