【正文】 /s and the minimum detectable velocity of m/s. Furthermore, Karman vortex shedding frequency was also detected and indicated to have a linear relationship with the bulk velocity. This can be also utilized to measure the flow velocity, and the flow measurement system is expected to be reliable by bining two measurement techniques. The flowmeter presented in this paper has many advantages including passive nature, explosionprotection, EMI immunity and capability of remote sensing. We try to apply this flowmeter to geophysical use because the advantages are leveraged in the application. By bining the flowmeter with FBG pressure and temperature sensors, FBG sensor system for pressure, temperature and flow measurement in a borehole may be constructed. References [1] G. Meltz, Overview of fiber gratingbased sensors, in: Proceedings of the SPIE’s Symposium on Smart Structures and Materials, vol. 2838, 1996, pp. 2–22. [2] . Rao, Infiber Bragg grating sensors, Meas. Sci. Technol. 8 (1997) 355–375. [3] . Kersey, . Davis, . Patrick, M. LeBlanc, . Koo, . Askins, . Putnam, . Friebele, Fiber grating sensors, J. Lightwave Technol. 15 (8) (1997) 1442–1463. [4] . Davis, . Kersey, Matchedfilter interrogation technique for fibre Bragg grating arrays, Electron. Lett. 31 (10) (1995) 822–823. [5] K. Wood, T. Brown, R. Rogowski, B. Jensen, Fiber optic sensors for health monitoring of morphing airframes. I. Bragg grating strain and temperature sensor, Smart Mater. Struct. 9 (2021) 163–169. 使用雙光纖布拉格光柵傳感器和互相關技術的水流量計 摘要 本文 對 使用光纖光柵 (FBG)傳感器 的互相關流量計的 工作原理和實驗結果 進行了介紹。10V measurement range used in the system yields VLSB = 10?4 V. . Intensity noise of the optical source The fluctuation in the intensity of the ASE output should be considered. The intensity noise arise as a fluctuation in the voltage Vin. In the ideal situation, it is found from Eqs. (6) and (7) that the intensity noise is deleted. But in the practical situation, the intensity noise arises due to the existence of the abovementioned noises. We assumed that the intensity noise was white noise whose RMS was 1% of the output power of the ASE. . Noise estimation of the FBG sensor Three uncorrelated white noises were given for noise estimation of the FBG sensor. The output voltages Vest,m (m = 1, 2, 3) and the sensor signal θMZI are then written as: )(, mmn o isemme st VVVV ?? （ 13） ,)(3tan1,3,2,3,2,1e s te s te s te s te s tNM Z I VVV VV ?? ??? ??? (14) where Vnoise,m(Vm) is white noise whose RMS is ? ?2122 m, QN VV ? and θN is the fluctuation of the signal. The noise spectral density is calculated from the RMS of θN divided by 21WB and then converted to the noise in the wavelength domain using Eq. (8). Fig. 4 shows the relationship between FWHM _λB of the FBG in the wavelength domain and the noise spectral density for sufficient optical power. It is found that the noise spectral density is mainly determined by the noise of the photodetector. The increasing noise spectral density with the broad _λB is due to decrease in the visibility γ. At _Λb above nm, the noise spectral density for L = is much higher than that for L = mm. To the contrary, at _λB below nm, the noise spectral density for L = indicates lower level than that for L = mm. This is because the optical path difference L has influences on both the wavelengthtophase sensitivity (=?2πL/λ2B) and the visibility γ. The Mach–Zehnder interferometer for L = is expected to yield the lower noise density than that for L = because FBGs in the system normally have _λB of nm. The noise spectral densities of the FBG sensor with the interferometric detection were estimated to be 210?4 and 10?4 pm/(Hz)1/2 for L = and mm, respectively. These values can be converted to 10?1 and 10?2 nε/(Hz)1/2, respectively in consideration of the strain sensitivity of the FBG ( pm/_ε). 4. Laboratory experiments of the crosscorrelation flowmeter and discussion Fig. 5 illustrates an experimental configuration of the FBG sensors and the bluff body in the flow measurement section. The size dw of the bluff body is 3 mm. From the principle of the crosscorrelation function RSCOT(τ), the coherent signal with broad bandwidth is desirable, and we tested several configurations of the sensors and the bluff body for broad bandwidth of signal. The parameters of the configurations are db, ds and dc, which are the distance between the bluff body and the upstream sensor, the distance between two sensors and the distance between the bluff body and the center of the PVC pipe, respectively. . Performances of the FBG sensor Fig. 6 shows a parison of waveforms detected with a reference strain sensor (KYOWA, KFG20120C111) and the FBG sensor. The sensors were attached on a metal cantilever in this experiment. The waveforms of both the FBG sensor and the reference sensor are similar as shown in this figure. The difference between signal amplitudes of two sensors is due to the difference of coupling of the sensors. Fig. 7 shows the noise spectral density of the FBG sensor as a function of _λB. This figure indicates that the experimental noise spectral density is higher than the estimate at narrow _λB. The reason for this difference is considered to be due to the difference of characteristics of the photodetectors and quality (the split ratio) of the couplers used in the Mach–Zehnder interferometers. On one hand, at _λB of nm, the experimental noise spectral density is lower than the estimate. This difference is due to assumption of the spectral profile of the FBG. The minimum noise density of 4 10?4 pm/(Hz)1/2 was achieved for both L = and at _λB of nm. This value corresponds to nε/(Hz)1/2 and is sufficient low to detect vortices in the fluid.