Hot-wire Anemometer Based on Fiber Grating Fabry-Perot Interferometer
Airflow velocity measurement is important in many fields such as meteorological monitoring and wind power generation.Currently,it is common to utilize ultrasonic,differential pressure,eddy current,or heat transfer methods for measuring.However,in some extreme environments,these measurements may be encounter interference,resulting in less accurate or non-functional results.Fiber optic anemometers have attracted much attention because of their advantages of electrically passive operation,resistance to electromagnetic interference,high sensitivity,integrated structure,and the ability to measure over long distances.In order to enhance the performance of fiber optic anemometers,this paper proposes a hot-wire anemometer based on fiber grating Fabry-Perot Interferometer(FPI)structure.Fiber grating FPI anemometers are prepared by sandwiching a section of cobalt-doped fiber with a length of 500 μm in the middle of two Fiber Gratings(FBGs).The cobalt-doped fiber in the interference cavity is heated with a 1 480 nm laser to form a hot wire.Due to the effect of thermal expansion and thermo-optic,the optical range difference of the FPI cavity length changes with the temperature of the cobalt-doped fiber,which brings about the wavelength redshift of the interference spectrum.As air flows through the FPI anemometer,the heat dissipation from the FPI is accelerated.The temperature of the FPI decreases,causing a blue shift in the interference spectrum,and the airflow velocity value can be calculated by measuring the change in the interference spectrum.We first performed temperature calibration experiments on the FPI anemometer probe by placing the anemometer probe in a temperature-controlled box.The output power of the broadband light source was set to 5 mW,and the output of the 1 480 nm laser was turned off to avoid the heat generated by the cobalt-doped fiber absorbing the laser.The temperature of the temperature control box was adjusted to gradually increase from-20 ℃ to 110℃ in 10 ℃ steps,and the interference spectral drift of the anemometer in this temperature range was measured.Thus,the wavelength response of the FPI anemometer to temperature is obtained.The temperature sensitivity obtained by the linear fitting is 11.8 pm/℃,and the linear R2 is 0.997.To explore the impact of laser power on the initial temperature of the anemometer,we recorded the interference spectra of the FPI when the laser power was increased from 0 mW to 500 mW at a step of 10 mW.The experimental results show that the peak wavelength of the FPI interference spectrum shifts by 1.71 nm when the laser power is increased to 500 mW,which corresponds to an increase of 144.92 ℃ in the initial temperature of the anemometer.Finally,we carried out airflow velocity measurement experiment at different laser powers of 150,330 and 500 mW.The airflow velocity was changed from 0 to 9 m/s.The experimental results indicate that the sensitivity of the FPI anemometer increases with laser power but decreases with airflow velocity.It reached-1 053.86 pm·m-1·s at a airflow velocity of 0.1 m/s when laser power was 500 mW.It was decreased to-746.47 and-15.15 pm·m-1·s for airflow velocity of 0.5 and 5 m/s,respectively.The sensitivity of an anemometer exhibits an exponential decay trend due to Newton's law of cooling,which states that the rate of temperature decrease of a hot wire is proportional to the temperature difference between the wire and the surrounding environment.In low airflow velocity conditions,the temperature difference between the cobalt-doped optical fiber and the airflow is significant,resulting in a high rate of temperature decrease.However,as the airflow velocity increases,the temperature difference between the cobalt-doped optical fiber and the airflow sharply decreases,leading to a reduction in the sensitivity of the anemometer.The fiber grating FPI anemometer proposed in this paper has the advantages of high sensitivity,simple and compact structure,easy fabrication and low cost when compared with the previously reported optical fiber hot-wire anemometers.It is expected to be widely used for airflow velocity measurement in the related fields.
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