Upper limit for angular compounding speckle reduction

Angular compounding is a technique for reducing speckle noise in optical coherence tomography that is claimed to significantly improve the signal-to-noise ratio (SNR) of images without impairing their spatial resolution. Here, we examine how focal point movements caused by optical aberrations in an angular compounding system may produce unintended spatial averaging and concomitant loss of spatial resolution. Experimentally, we accounted for such aberrations by aligning our system and measuring distortions in images and found that when the distortions were corrected, the speckle reduction by angular compounding was limited. Our theoretical analysis using Monte Carlo simulations indicates that "pure" angular compounding (i.e., with no spatial averaging) over our full numerical aperture (13° in air) can improve the SNR by not more than a factor of 1.3. Illuminating only a partial aperture cannot improve this factor compared to a spatial averaging system with equivalent loss of resolution. We conclude that speckle reduction using angular compounding is equivalent to spatial averaging. Nonetheless, angular compounding may be useful for improving images in applications where the depth of field is important. The distortions tend to be the greatest off the focal plane, and so angular compounding combined with our correction technique can reduce speckle with a minimal loss of resolution across a large depth of field.