Transverse Doppler Velocimetry of Optically Propelled Microparticles in Anti-resonant Hollow-Core Fiber

Laser Doppler velocimetry is a powerful tool for monitoring the instantaneous velocities of moving objects [1], [2]. Doppler velocimetry has been exploited to measure the axial velocities of single and multiple optical propelled particles over anti-resonant hollow-core fiber (AR-HCF) for the purpose of particle delivery and distributed sensing [3]. Here we report the observation of angle-dependent transverse Doppler frequency shifts of optically trapped microparticles in AR-HCF. The experiment setup is sketch in Fig. 1(a). The trapping beam is coupled into the fundamental core mode from one end of the fiber. An objective lens with numerical aperture of 0.8 is placed at the side of the fiber to measure the transverse Doppler frequency. Two pinholes (diameter 1 mm) with different spacings <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(D)$</tex> are placed in front of the objective lens so that side-scattered light from determined scattering angles <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\alpha$</tex> is collected by PD2.