Detection Of Aflatoxin B₁ In Food Based On Fiber Loop Ring Down Technique

Optic fiber sensing technology developed rapidly with the development of fiber optic communication technology in the 1970s.It has many advantages such as anti-electromagnetic interference,intrinsic safety and simple structure.However,it also has certain deficiencies,for example the detecting equipment is expensive and the sensitivity is affected by fluctuations of the light source.Therefore,the research on new optic fiber sensing system is of great significance.Fiber loop ring down(FLRD)technique is a new optical demodulation technique developed by cavity ring down(CRD)technique.In the FLRD system,the measured is sensed by the ring-down time of the fiber loop,which can be calculated from exponentially decayed light pulses sampled by an oscilloscope.Therefore,the technique has many advantages such as avoiding the effects of fluctuations of the light source,low cost,and high sensitivity.In the paper,the study of FLRD technique and its sensing applications have been proposed.The main contents are shown as following:1.The basic principles of FLRD technique are described and analyzed,the method of how to construct the FLRD experimental system is studied,and the processing method of the output light pulse signal of FLRD system is analyzed.2.A fiber curvature sensing experimental research based on single-mode fiber(SMF)taper structure in combination with fiber loop ring down(FLRD)technique was carried out.We sense the curvature by the ring-down time that is altered by the bending loss of the SMF taper with different curvatures.In the experiment,five SMF tapers with waist diameters from 44.1 to 65.4?m were tested,with the curvatures in the range of 0–8.33 m^-1.Experiments have shown that the thinner the cone of the SMF taper,the higher the sensitivity to curvature.The sensitivity of the SMF taper with 44.1?m and 65.4?m waist diameters were 0.725?s/m^-1and 0.328?s/m^-1respectively.Finally,the way of comparing the sensitivities measured by different devices with different units was supposed.3.A reflective optical fiber temperature sensing experimental research based on fiber Bragg grating(FBG)structure combined with FLRD technique was carried out.Temperature measurements were made in the temperature range of 30-38°C using an FBG with a center wavelength of 1547.33 nm.Experiments show that the average sensitivity is 1.003 ?s/°C,and the theoretical accuracy can reach 0.005 °C,which is 20 times the precision that the current FBG sensing demodulator can achieve,and the stability is good.4.A fiber sensing system combined with fiber loop ring down(FLRD)technique to detect aflatoxin B₁(AFB₁)in food is proposed.The single-mode fiber(SMF)taper was set as the sensor head,and the waist diameter of it was determined to be 17.1?m by the refractive index(RI)measurement experiments.When a light pulse enters and circulates in the fiber loop,it experiences a little loss each time as it passes through the sensor head.The ring-down time of the system would be altered by intensity loss near the sensor head,which is related to the concentration of the AFB₁solution.The experimental results showed that the sensitivity of AFB₁was 0.755?s/(?g/ml)in the concentration range of 0-5?g/ml with a precision of 0.016?g/ml.Compared with other AFB₁sensors,the system has the advantages of simple structure,low cost,high precision and insensitive to light source fluctuation.