Study On High Sensitivity Optical Fiber Anemometers

As a commonly used measuring instrument,anemometer plays an essential role in many fields such as wind power generation,industrial and agricultural production,meteorological monitoring,and medical apparatus.Recently,Optical fiber thermal anemometers have become a research hotspot due to their compact structure,high sensitivity,anti-electromagnetic interference,and long-distance measurement capabilities.At present,optical fiber thermal anemometers have been realized based on various classical optical fiber sensing structures,such as fiber gratings and optical fiber interferometers.Although the performance of optical fiber thermal anemometers is constantly improving,there are still some critical technical problems that need to be solved.First,most designs measure wind speed by reading wavelength,which requires the use of a large and expensive optical spectrometer to demodulate wavelength,limiting the practical application;Second,silica fiber gratings with low-temperature sensitivity are widely used as sensing elements,which limits the improvement of the sensitivity;Third,according to Newton’s law of cooling,the sensitivity of the optical fiber thermal anemometer is greatly reduced at high wind speeds,affecting measurement performance.To address the abovementioned problems,this thesis proposed an intensity interrogated anemometer based on the chirp effect of the fiber Bragg grating（FBG）and a high-sensitivity fiber thermal anemometer based on a polymer Fabry–Pérot interferometer.The main contents are as follows:（1）The theory of temperature-based FBG bandwidth tuning is theoretically described.The reflection spectral characteristics,reflection bandwidth,and reflected optical power of the chirped FBG are simulated and studied using the transfer matrix method and Matlab programming.（2）An intensity-interrogated thermal anemometer based on the chirp effect of FBG is proposed.The 15 mm FBG is non-uniformly heated by a pump laser and produces a strong chirp effect.Since the reflected optical power changes with different chirp ratios,it successfully enables wind speed measurement with optical power demodulation.The experimental results show that the anemometer has a bandwidth broadening ratio of 1.087pm/m W.In the measurement range of 0 to 11 m/s,the reflected power of the anemometer decreases by 19.77μW in total.High sensitivity up to-28.60μW/(m·s^-1)is achieved at wind speed of 0.1 m/s.The average response times of the anemometers are 0.3 s（wind speed from0.3 to 7 m/s）and 0.9 s（wind speed from 7 to 0.3 m/s）,respectively.（3）A light source-heated optical fiber anemometer integrated on single-mode fiber（SMF）tip is proposed.A Fabry–Pérot interferometer（FPI）is fabricated on the tip of SMF as a sensing probe using ultraviolet curing glue that absorbs broadband light source.After the light from the broadband light source enters the sensing probe,part of the light is absorbed and converted into heat to increase the temperature of the FPI,and part of the light is reflected to the optical spectrometer for wind speed measurement.The interference fringe of the FPI shifts with wind speed because airflow not only reduces the temperature of the FPI but also introduces strain due to wind force.In the measurement range of 0 to 7 m/s,the dip of the interference fringe shifts by a total of-3.42 nm.High sensitivity up to-3.13nm/(m·s^-1)is achieved at wind speed of 0.1 m/s.The average response and recovery time are250 and 580 ms,respectively.Since the polymer FPI is also affected by the wind,a sensitivity of-0.26 nm/(m·s^-1)can maintain at wind speed of 7 m/s.