Noise minimization in optical detection of small particle

The ultimate sensitivity of optical detection is limited by the signal-to-noise ratio (SNR). The first part of the paper shows that coherence plays an important role in the noise analysis. Although interference between an auxiliary wave and a signal wave makes the photo detector response to the signal stronger, the coherent noise also enhances. This makes insignificant the gain in the SNR. Pulsed-excitation gated-detection (PEGD) is described and analyzed in the second part to show that 1) a high brightness of detected particles is not a prerequisite for a high SNR, 2) optimized parameters of the PEGD protocol demonstrate interesting bifurcation making a sudden jump from an effectively continuous regime to PEGD, and 3) photo-physical properties of NV centers in nano crystals of diamond approach those ideal for PEGD.