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.
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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