Spatial filtering of structured light

Abstract

Spatial filtering is a commonly deployed technique to improve the quality of laser beams by optically filtering the noise. In the “textbook” example, the noise is usually assumed to be high frequency and the laser beam, Gaussian. In this case, the filtering is achieved by a simple pinhole placed at the common focal plane of two lenses. Here, we explain how to generalize the concept of spatial filtering to arbitrary beam profiles: spatial filtering of structured light. We show how to construct the spatial filters using a range of structured light examples and highlight under what conditions spatial filtering works. In the process, we address some misconceptions in the community as to how and when spatial filters can be applied, extend the concept of spatial filtering to arbitrary beam types, and provide a theoretical and experimental framework for further study at both the undergraduate and graduate level.

High-resolution digital spatial control of a highly multimode laser

“…After free space propagating 𝑧 = 12.5 mm, the intensity distribution is significantly distorted where each spot broadens, as expected for fully incoherent light. Figure 3 (b) shows the results for laser light with higher coherence, obtained by inserting a small pinhole aperture in the far-field (Fourier) plane of the DCL [13,14]. As evident, the intensity distribution at 𝑧 = 0 mm is similar to that in Fig.…”

“…The SLM at the near-field serves as a mirror. Its reflectivity and phase distributions are both locally controlled by manipulating the phase difference between 2 × 2 adjacent pixels so as to form a super-pixel scattering light (inset at SLM) and then filtering the relevant light with the far-field aperture [13,18,22,23]. The control of the reflectivity distribution of the SLM allows control of the intensity distribution of the laser beam in the near-field, whereas control of the phase distribution of the SLM allows control of the frequency distribution.…”

“…The separate control of the spatial coherence was done rapidly and efficiently by means of a degenerate cavity laser (DCL) [12] and an intra-cavity spatial filter aperture [13], demonstrating that the number of independent lasing modes can range from 1 to 320,000 with less than a 50% change in output power [14]. It was exploited for wide field speckle-free illumination and imaging [15].…”

Spatial filtering of structured light

“…In the context of structured light, we ask: how can we remove additive noise from arbitrary incoming light to achieve a desired, noise-free field? Relying heavily on concepts from Fourier optics, we present a tutorial-style method on spatially removing unwanted noise from structured light [2].…”

Terahertz Spiral Spatial Filtering Imaging

“…In addition, spatial filtering imaging is an effective method for optimizing image quality, which is implemented by inserting a spatial filter in the Fournier plane. Various optical approaches of spatial filtering imaging have been developed to improve image resolution, image contrast, and achieve edge enhancement [25][26][27][28][29]. Spiral spatial filtering (SSF) imaging, as an optical implementation of the edge-enhancing transformation, offers a vital, convenient tool for edge detection in image processing.…”