Application of spatial light modulators to composite vortex light

Background

Vortex phenomena are seen in life, such as bathtub vortices that occur when draining water, wake vortices that detach from ships as they travel, tornadoes, typhoons, and ocean circulation. Vortex light (carrying orbital angular momentum, OAM) was first discovered and applied mainly in the field of optics, i.e., the generation of vortex photons and vortex beams, and the concept of vortex beams was first proposed by Coullet et al. in 1989. In 1922, L. Allen et al. theoretically proved the existence of OAM in vortex beams, which pushed the field into the world's forefront.

Compared with the traditional single-ring vortex light, composite vortex light (COV) is a composite light field combined by multiple vortex lights, and thus has more complex and diverse properties, which allows for more diversified application potential in different fields.

For example, in particle manipulation, composite vortex light can generate light beams with different orbital angular momentum, allowing for more complex manipulation of particles; in optical communication, composite vortex light can transmit more information in the same optical path, which is of great significance for the continued expansion of the capacity of optical communication.

As one of the mainstream ways to modulate the optical field, spatial light modulators are widely used in various fields due to their ease of operation and ability to produce good imaging effects, and the use of spatial light modulators to generate vortex light has a broad application prospect in both optical communication and particle manipulation.

Abstract

An optical vortex is a beam with a helical phase wavefront with orbital angular momentum (OAM) that can carry different topological charge numbers. Recent advances in vortex beam research have revolutionized beam applications such as advanced optical manipulation, high-capacity optical communications, and super-resolution imaging. There is no doubt that vortex beam generation and detection methods are crucial for vortex beam applications.

Generation principle

The generation methods based on spatial light mainly include spiral phase plate method, spatial light modulator method, holographic grating method, and column lens method. Among them, a spatial light modulator is an optoelectronic device that can spatially modulate some or all of the physical information such as the amplitude, phase, and polarization state of a light wave.

Using the electro-optical effect of liquid crystals, the spatial light modulator can be realized to modulate the amplitude and phase of the incident light wave, so that the light wave realizes the wavefront transformation. The spatial light modulator can be utilized to either load a hologram to form a vortex light, or optionally to input phase information from a helical phase plate.

Vortex beams corresponding to different topological charge numbers (image from in-house measurements)

Experimental realization

In this experiment, the laser chosen to be used is a He-Ne laser with a wavelength of 632.8 nm. the experimental optical path is shown in the figure below. The laser first passes through a collimated beam spreading system to form a large-format near-flat-top light field, and then passes through a polarizer before reaching the spatial light modulator, where a light shield is placed immediately in front of the spatial light modulator.

The laser light reaches the spatial light modulator after modulation of the light intensity by the mask. After phase modulation, the laser is reflected to another polarizer, and the experimental results can be observed after this polarizer. Here, because the imaging area of the spatial light modulator is 15.36mm×8.64mm, which is much larger than the image receiving area of the CCD, the image is received by the CCD through the 4f system.

Experimental device

The spatial light modulator used in this experiment is our FSLM-2K70-P02, and its main parameters are as follows:

Results

(a)-(d) show the light intensity distributions produced when the topological charges are 2,5, -5,10, respectively.

Figures (a)(b) show two annular spiral phase plate (ASPP) shades with the same ring width but different radii with opaque centers. The r1, r2 of the two are 1.2mm,2.4mm; 2.4mm,3.6mm, respectively.

Figures (c)(d) show the images obtained by setting the topological charge of the spatial light modulator to 2 and placing different light masks in front of the spatial light modulator. Figures (e) (f) show the images observed after setting the topological charge number to 10.

Fig. (a) shows the shades of the composite vortex light generation device.

Figure (b) shows the light intensity distribution generated when the inner and outer topological charges are 1 and 3, respectively.

Fig. (c) shows the light intensity distribution when the topological charge number is constant and the opaque strip is removed.

Figure (d) shows the light intensity distribution after the topological charges are kept constant and the strips are replaced with transparent ones.

Figures (e) and (f) show the light intensity distributions generated when the internal and external topological charges are 5,1; 20,1, respectively.

Conclusion

The modulation of light intensity and phase is realized by combining a light mask and a phase-type spatial light modulator, and the ability of the composite vortex light generation device to generate COVs and its characteristics are experimentally verified, as well as the characteristics of the spiral phase plate (SPP), and the annular spiral phase plate (ASPP) in generating vortex light.

The composite vortex light generation device designed in this paper can form any number of concentric rings according to the usage requirements. This composite vortex light will have a wide range of applications in the fields of optical communications and particle manipulation.