| Fiber Bragg Grating(FBG) has many advantages of sharp reflection peak and Bragg wavelength sensitive to external factor. So, Fibre Optical Sensors based on FBG have been widely researched. The main sensing mechanism of them is wavelength demodulation. That is, employing external factor to modulate Bragg wavelength. This paper proposes for the first time the sensing mechanism of FBG power demodulation. Temperature, tilt, force, displacement and other physical variables have been successfully measured by employing the new sensing mechanism in this paper.The time of Plastic Optical Fiber(POF) being used in the sensing field is later than FBG. But the POF has more advantages than FBG, such as bigger fiber core and numerical aperture, higher mechanical strength, easy processing, soft, simple connection etc. Femtosecond laser processing can effectively avoid heat effect. The advantage is particularly important in POF processing, which is sensitive to heat.This paper employs femtosecond laser to make multi-hole in POF, and researches theory and experiment of force measurement employing multi-hole POF.Firstly, this paper introduces classification, application and research situation of Fibre Optical Sensors. From structure and measuring principle, FBG and POF are introduced in this paper, respectively.Secondly, this paper introduces theoretical and experimental study of Fiber Bragg Grating Power Demodulation Sensor. This paper employs a pair of FBGs which have same Bragg wavelengths. The reflective spectrum of one FBG enters the other one. Two peaks will appear on both sides of original Bragg wavelength in the transmission spectrum of the latter. That is, ‘left peak’ and ‘right peak’. When the external factor leads to Bragg wavelength of one FBG shifting, the power difference between right peak and left peak will change. This is mechanism of FBG power demodulation. We successfully measure temperature, tilt, force and displacement byemploying this mechanism. The segmental measurement of temperature around Bragg wavelength 1525 nm is achieved in the range of 25 °C–135 °C with an excellent sensitivity of 1.35±0.02 d Bm/°C. The Bragg wavelength shift is 9.12×10-3nm/°C under the same conditions. Using two pairs of FBGs successfully measure tilt angle, The sensor can distinguish the direction of tilt with an excellent sensitivity of403 d Bm/° and a highest achievable resolution of 2.481 × 10-5 °(that is, 0.08% of the measuring range). We choose two pairs of FBGs with different reflectivity of reflection peak to measure force. For the sensor with bigger reflectivity of reflection peak, the force sensitivity is(1.16 ± 0.06) × 103 d B N-1 with a measuring range of0-4.40 × 10-3 N. This paper employs two cascaded FBGs to measure displacement.There is only one peak in the transmission spectrum of the latter. Displacement measurement is achieved by monitoring power of the peak with sensitivity(3.80±0.11)×10-2 d Bm/μm and a measuring range 540 μm. Also, it is found that measuring range of mechanism of FBG power demodulation is proportional to the reflection peak linewidth, the sensitivity is also proportional to slope at the edge of the reflection peak.Finally, this paper employs multi-hole POF to measure force. The hole in POF is made by 800 nm femtosecond laser. Its diameter is around 150 μm. The effect of hole quantity and spacing on insertion loss is researched. The results show that, the insertion loss grows with hole quantity when spacing remains unchanged, the biggest insertion loss appears in spacing 1.25 mm when hole quantity remains unchanged.For the sensor with 13 holes and spacing 1.25 mm, the sensitivity is 0.24 d B/N with a measuring range of 0 N-65 N. The linearity and repeatability of the sensor is well.This paper also employs POF to monitor the physical properties of KNO3 solution.In the process of heating up KNO3 solution, the solubility and temperature are successfully measured with high sensitivity and linearity. |
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