Improved interferometric tracking of trapped particles using two frequency-detuned beams

For most optical tweezer applications, precise and reliable tracking of the trapped particle is an important requirement. Backfocal-plane interferometry is the fastest and most accurate tracking technique if the particle displacements are limited to half of the focal width. Especially for positive axial displacements, the nonlinear detector response can lead to incorrect tracking results. Here we show how the linear detection range around the trap center can be extended by a factor of 2 to 4 in the axial direction using a second frequency-detuned tracking focus that is generated by the same laser as the optical trap. Additionally, we show how the noise in the axial signal can be decreased significantly using a second detector.