LAUSR

Abstract We have fully investigated the integrated lensing effect that the beam suffers in a dual-fiber optical trap. A modified model for coupling of a beam to the opposing fiber has been built in consideration of the integrated lensing effect. We discussed the effect of z-axial position, radius and refractive index of the trapped sphere on the coupling efficiency using the modified coupling coefficient model. The measured and calculated coupling efficiency as a function of z-axial position have the same tendency in the allowable error. Comparison among above simulation and experiment has made clear that the magnitude relation between the smallest spot size and the mode field radius decide whether the spot size matching or wavefront phase matching dominate the coupling coefficient. What’s more, we point out that the dependence of relative coupling coefficient on the refractive index has knee point. That is an obstacle to the application of single live cell refractive index measurement method based on lensing effect. Nevertheless, this method is absorbing and promising because there is a linear range for the known experimental parameters in an optical trap, and it needs no additional accessory.