Fig. 1. Setup of 2D laser feedback frequency-shifted flow vector measurement. (Path 1: DFB–OC1–EDFA1–CIR1–Target–CIR1–OC2–AOMs–OC1–DFB; Path 2: DFB–OC1–AOMs–OC2–CIR2–EDFA2–CIR3–Target–CIR3–CIR2–OC2–AOMs–OC1–DFB.) PD, photodetector; WDM, wavelength-division multiplexer; OC1,2, optical coupler; EDFA1,2, erbium-doped fiber amplifier; AOMs, acoustic-optic modulator pairs; CIR1,2,3, circulator. The dashed box in the upper right corner shows the principle of the 2D measurement system.

Fig. 2. (a)–(d) Simulated spectral distribution for different directions of flow velocity measurements. (X-axis and Y-axis represent laser beam feedback direction; v represents the flow velocity; (−) and (+) represent negative and positive frequency shift, separately.)

Fig. 3. Typical spectrum of flow velocity in the 2D laser frequency-shifted feedback system. (Red background image: velocity signal in the X-dimension; blue background image: velocity signal in the Y-dimension.)

Fig. 4. Frequency spectra of laser output with different syringe pump speeds for two dimensions: (a) X-dimension and (b) Y-dimension. (The orange dotted line indicates the frequency shifting of the maximum Doppler velocity signal.) Maximum Doppler velocity signal extraction results of two dimensions: the corresponding frequency shift with different syringe pump speeds in the (c) X-dimension and (d) Y-dimension.

Fig. 5. Distribution of flow velocity inside the pipeline when the pump speed is 200 μL/min. (The inner diameter of the pipeline is 0.5 mm, vmx and vmy represent maximum velocity in the X- and Y-dimensions, and vm is maximum resultant velocity.)

Fig. 6. Doppler frequency shifts with different angles for two dimensions: (a) X-dimension; (b) Y-dimension. (c) The actual flow velocity in different angles by synthesizing the frequency shifts of the X-dimension and Y-dimension. (d) Measured angle versus the actual angle.

Fig. 7. (a) Principle of the 2D orthogonal measurement system. (b) Laminar flow distribution in a glass capillary.