We demonstrate a laser Doppler velocimeter (LDV) based on self-mixing effect in a single longitudinal mode VCSEL
for velocity measurement of flowing milk using Polymer Optical Fiber (POF). We find that self-mixing signal in
frequency domain was broadened using POF compare with space coupled. The flow velocity was measured when POF
outside and inside the milk. Their self-mixing signals of power spectrum can be fitted with Gaussian and exponential
decay function, respectively. The differences of self-mixing signal in frequency domain when POF outside and inside
milk are discussed. Due to POF have the advantages of flexibility and compatible with biology, this velocimeter can also
be used for the measurement of blood flow in vivo.