Fig. 9.5 — Interferometric position detection set-up

(a) Schematic of a forward interferometric position detection set-up and (b) corresponding implementation in our optical tweezers using an InGaAs quadrant photodetector (QPD) with a small area to increase signal sensitivity at λ₀ = 1064 nm and to decrease the detector noise level. (c) QPDs work by measuring the intensity difference between the light impinging on the left (top) and right (bottom) sides of the detection plane [Eq. (9.11)]. (d) Position sensing detectors (PSDs) measure the position of the centroid of the impinging light intensity distribution [Eq. (9.39)].