Signal Processing Method Of Measuring The Refractive Index Of Air Based On Synthetic Wavelength Detection

The refractive index of air is an important parameter of laser interferometric lengthtechnology and it is a significant factor affecting the measurement accuracy especially for thelarge range measurement. Supported by the natural science foundation of Zhejiang province（Y1110495）, signal processing systems of measuring the refractive index of air based onsynthetic wavelength detection using TMS320F28335and ADLINK PCI-9820were designed.The principle of measuring the refractive index of air based on synthetic wavelengthdetection was introduced and two methods were put forward. One was to calculate the refractiveindex of air via counting the cycles of the signal which collected using ADLINK PCI₉820acquisition card through VB software and the other is first using zero-phase detection twicebased on filling pulse method to calculate the synthetic wavelength and then calculating thephase difference of two interference signals. Both were compared with PTF function methodseparately. Control interfaces were designed based on VB software for experimental operation,control, data display and save. The change of environmental parameters in experiment wasconsidered.The following experiments were conducted respectively:（1） Zero phase detectionexperiment: which was done in3different collecting points which were7000,1000and13000.The average positions were283388.8μm,285831.0μm and286533.2μm, standard deviationswere0.36μm,0.42μm and0.25μm.（2）Synthetic wavelength measuring experiment: the averagelength was137281.7μm, standard deviation was2.8μm.（3） Calibration experiment betweenXC-80intelligent sensor system and a high-precision sensor system: XC-80intelligent sensorsystem as the reference, the average of temperature, humidity, air pressure and air refractive errorof the high precision sensor were1.277℃,3.152%,1.202Pa and2.9×10^-6; error standarddeviations were0.679℃,2.322%,0.153Pa,0.62×10^-6.（4） Environmental monitoringexperiment: in407minutes the average of experimental ambient air temperature was25.91℃,the standard deviation was0.823℃; the average of material temperature was24.91℃, thestandard deviation was0.379℃; the average of difference between atmosphere and materialtemperature was1.01℃, the standard deviation was0.474℃; the average of air pressure was1019.909Pa, standard deviation was0.5515Pa; the average humidity was57.85%, standard deviation was1.759%.（5） The refractive index of air measurement experiment: phase differencemethod and cycle counting method were separately compared with the PTF formula method, theaverage errors were0.000374089and1.095×10^-5, standard deviations of error were0.000193197and0.126×10^-5.