Embedded Files
In order to create the LG beams, we use a Spatial Light Modulator (SLM). This is one of a few techniques to give a beam of light OAM. SLMs are ideal for this project because they can be reprogrammed quickly to create many different LG beams, and can store approximately 8000 different screens at once.
The SLM used in this research is an electrically addressed liquid-crystal (LC) SLM (Fig. 1). It consists of a top-covering layer of glass and a LC layer that sits on a silicon backplane that contains reflective mirrors. These mirrors serve to make the SLM optically flat, which is important for using coherent light, as well as serving as the electrodes through which the SLM is controlled. Each electrode corresponds with a fully controllable and independent pixel. When a voltage is applied across the electrodes, it changes the refractive index of the LC layer so that as light enters this layer, it shifts the phase of the light.
Figure 1: Spatial Light Modulator (a) diagram and (b) device
Figure 2: SLM screen for Laguerre-Gaussian beam using Eq. 6, a linear phase grating of 8 pixels and SLM parameters at z = 0 [4]
When different voltages are applied across the electrodes that correspond with each pixel, the liquid-crystal’s index of refraction independently changes for each pixel. By applying voltages in an engineered manner, it is possible to shift the phase of a coherent beam so that the reflected beam takes the form of a Laguerre-Gaussian beam, therefore carrying OAM and propagating with a helical waveform [1-3]. Fig. 2 shows an example of an SLM screen that will create the beam U_0_1.
It is important to note that the number of pixels and their physical size, in a given SLM, directly limits the resolution of the phase changes that can be imparted on the beam and therefore constrains the largest topological charge and Laguerre polynomial used in the beam design.
References
[1] S. Avramov-Zamurovic, C. Nelson, and J. Esposito. “Classification of beams carrying orbital angular momentum propagating through underwater turbulence,” SPIE Proceedings, September, 2020. PowerPoint Presentation. https://drive.google.com/file/d/18mANF7FNjBBDmGrqnLtgBJNaWXw89YBB/view
[2] https://www2.ph.ed.ac.uk/~wjh/teaching/mo/slides/slms/slm.pdf
[3] https://www.laserfocusworld.com/optics/article/16546728/spatial-light-modulators-liquidcrystal-slms-benefit-the-study-of-atmospheric-turbulence
[4] S. Avramov-Zamurovic, A. T. Watnik, J. R. Lindle, and K. Peter Judd, “Designing laser beams carrying OAM for a high-performance underwater communication system,” Journal of the Optical Society of America. (2020).
Page updated
Report abuse