Abstract
In this paper, defective microwave photonic crystals (MPCs) are designed to sense the salinity of aqueous solutions. The defective MPC sensors are constructed by two kinds of microwave dielectric layers and one defective salt solution layer. Transfer matrix method (TMM) for lossy medium is developed to calculate the transmittance spectra of the sensors. It is found that the peak transmittance of both the defective resonance within the microwave band gap (MBG) and transmitting modes outside the MBG monotonously decrease with the increase of salinity, while the resonant and transmitting mode frequencies remain unchanged. By comparing the four MPC sensor structures, the first transmitting mode in the upper frequency band outside the MBG of the 15-layer MPC sensor has the largest salinity sensing range from 0 to 40‰ with relative stable detecting sensitivity. The sensing principle is based on the fact that the dielectric loss factor of saline solution is much more sensitive to salinity than the dielectric constant in the microwave frequency band. The sensitivity, quality factor, and salinity detection range of the MPC sensors are calculated and compared. The reported defective MPC sensors are suitable to be used for non-contact salinity detection.
Highlights
Salt plays an important role in human life as it has been broadly used in food preparation since prehistoric times [1]
Considering the reported defective microwave photonic crystals (MPCs) sensors work at room temperature (20 ◦C), the complex permittivity of the saline solution is plotted in Figure 2 by using the regression equations provided in the Klein and Swift model [40]
When the defective layer is configured with saline solution under salinity sensing, the frequencies of the defective resonance and transmitting modes remain almost unchanged with the variation of salinity, but the resonant transmittance monotonously decreases with the increase of salinity
Summary
Salt plays an important role in human life as it has been broadly used in food preparation since prehistoric times [1]. As a result, it is regarded as the most important food flavoring additive. The intake of excessive salt from foods and beverages may lead to some serious health problems, such as hypertension, cardiovascular disease, stomach cancer, osteoporosis, kidney disease, and obesity [2,3]. It is important to determine the salt contents in foods and beverages during both their manufacturing processes and shelf-life, so as to ensure the products’ quality. Salinity sensing is widely applied to agriculture [5], marine environment monitoring [6], and oilfield water injection engineering [7]
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