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We present the design and characteristics of laser Doppler differential velocimeter using a dual-core photonic crystal fiber (DC-PCF). Transmission modes and mode interference characteristics of DC-PCF were investigated. Results show that higher order modes and cladding modes are excited, which can be cut off by bending the DC-PCF. The independent fundamental modes of two cores are clearly obtained. The mode interference near the distal end of DC-PCF was observed, and the interference process from independent faculae to the interaction of two core beams is presented. The velocimeter demonstrates a good working distance ($30 lm) and a small measuring volume ($273.4 lm 3 ), which is well suited for flow measurement in micro-scale fluidic channels. The velocity measurement is successfully demonstrated by observing the frequency modulation and the spacing of the fringe projected interference. The measurements are verified and in good agreement with the results obtained by the light gate sensor.

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