The Research Of All-fiber Laser Heterodyne Detection For Velocity Measurement

All-fiber laser heterodyne detection for high-precision velocity measurement is presented. The laser Doppler shift is detected with heterodyne technology. The use of all-fiber components permits a small and robust system. All-fiber laser heterodyne detection for velocity measurement will be used widely in wind shear detection and vehicle speed measurement and positioning etc.A 1.55μm all-fiber coherent laser demo system for high-precision velocity measurement is presented. A 1.55μm fiber laser source plus an erbium-doped fiber amplifier are used as the transmitter. A fiber optical circulator and a telescope are used as the optical antenna. PIN photodetector is used to detect the heterodyne signal. All the optical devices are connected by single mode fiber. The signal and local oscillator (LO) filed are well matched in this system. Fast Fourier transform (FFT) is used to derive the signal Doppler shift.With this demo system, the high-precision velocity measurement of hard targets and medium-precision velocity measurement of water vapor are well conducted. The Aluminum sheet, sand paper belt and sand paper disc are selected as the hard target for velocity measurement. The velocity range is 0.4 m/s-17 m/s. The comparative error of the velocity measurement is less than 1%. Because of the motor speed error, the velocity measurement error is～mm/s when motor at low speed and～cm/s when motor at high speed. At the same time, the velocity of water vapor is measured, the result show a good velocity detection capability of aerosol.Finally, the wind velocity measurement is simulated. The signal to noise ratio of Coherent Doppler Lidar (CDL) for wind detection is analyzed under different pulse energy, weather & local optical power conditions. Also, the improvements for demo system are proposed in order to make a basis for system engineering.