Structured back focal plane interferometry (S-BFPI)

In optical micromanipulation, back focal plane interferometry (BFPI) has remained the most widely used method for tracking dielectric and metallic microparticles with nanometer resolution precision at speeds of up to few a MHz. Basic BFPI employs Gaussian beams which severely limits the detection range of the technique unless the focal parameters are tuned by increase the width of the beam, but this is usually concomitant with significant loss in detection sensitivity. A challenge in BFPI is to extend its linear range while maintaining this detection sensitivity along each axis. We constructed a system which utilizes a combination of structured beam shaping and structured detection (Annular Quadrant Detection), which we called Structured Back Focal Plane Interferometry (S-BFPI). A Gaussian beam is shaped by a spatial light modulator by imparting a conical wavefront, which increases the depth of focus while simultaneously maintaining the Gouy phase shift and hence the sensitivity of detection. In addition, an annular QPD is used for detection. Using S-BFPI, we were able to achieve a 200% axial range extension with only a 4.6% reduction in insensitivity, and a 167% lateral range extension with a 45% reduction in sensitivity. SBFPI can tailor its detection range and sensitivity over an intermediate range of displacement and sensitivity improvements at hand. We finally demonstrated its robustness against aberrations common to optical systems. S-BFPI presents itself as a flexible, tunable option for use as an optical measurement tool.