PDV (Photonic Doppler Velocimetry) system with a wide measure range from 1~6000 m/s (upper limit depends on the bandwidth of acquisition system) is employed to capture the transient velocity of the particle velocity of free surface during laser actions. The PDV employs a heterodyne technique, and the heterodyne beat signal is obtained by mixing a reference single frequency laser of f0 and its Doppler shifted reflection of fb(t)off a moving target surface. Beat frequency
relates the instantaneous surface velocity by
where λ0 is the wavelength of source laser. The source laser is a 1550 nm
DFB laser from JDS Uniphase Corporation, the output power is 40 mW, and the linewidth is 200 kHz. Heterodyne beat frequency is detected by using 12 GHz bandwidth InGaAs p–i–n photodiode detectors from New Focus, Inc., and is recorded by using an 8 GHz digital oscilloscope from Lecoy, Inc. The high speed oscilloscope can record four channels of data at a maximum sampling rate of 40 Gs/s on each channel.
Configuration of PDV diagnostic
LSP free surface velocity captured by PDV system (different processing method). (a) CWT, (b)STFT