# Non-Classic Atmospheric Optical Turbulence: Review
## 4 SLM/DMD Benchtop Simulations  
Computer simulations of atmospheric optical turbulence and its effects on light beams have been popular since the 1990s (see Reference [78] and references wherein). They allowed to make predictions about the trends in optical beam evolution through a variety of turbulent regimes, both classic and non-classic. An interesting alternative to purely digital simulations are the laboratory benchtop systems, using real light as a source and a spatial light modulator (SLM) or a digital mirror device (DMD) as a simulator of a thin phase screen properly tuned to mimic an extended turbulence path [79]. The non-classic (anisotropic) turbulent thin phase screens were developed and studied in depth, e.g., in [80]. Unlike in computer simulations, having the possibility of using a large set of screens distributed along the propagation path, the SLM/DMD based simulations are limited to one, or possibly two to three screens because of the substantial power loss and the need for synchronization. The fundamental difference between the two devices is their spatial modulation quality and the refresh rate. A quality SLM (with a high fill factor), being a relatively slow device (several hundred Hz), can provide a fine spatial profile, while a DMD, attaining the kHz rates, produces a crude spatial modulation. In addition, at very high refresh rates, the DMD can only operate in a binary mode. In order to explore the effects of non-Kolmogorov, isotropic and anisotropic turbulence along the vertical paths the laboratory experiment with a liquid crystal, nematic SLM was carried out [81]. The wave optics simulation (WOS) [82] was adopted to the SLM dimensions and the propagation path between the SLM and the camera. It was revealed that for a larger ratio of anisotropic factors, the effect of turbulence on the intensity fluctuations diminishes, i.e., anisotropy acts as a factor to power the spectrum strength. At the same rate, the largest effects on the scintillation index were observed for power law \(\alpha\) in the range from 3.1 to 3.2, depending on the applied anisotropy. We remark that the scintillation index shows a peak in the mentioned range if the chosen length unit is the meter [83]. The same numerical procedure and the laboratory arrangement was also used for illustration of the effects of anisotropic turbulence in the horizontal scenario [84]. The anisotropic turbulence was shown to stretch the initially circular beam profile to an elliptical shape oriented along the vertical direction. This is implied by the fact that the weaker/stronger refractive index fluctuations in the horizontal/vertical directions lead to smaller/larger turbulence-induced diffraction: for the fixed ratio of anisotropic factors, \(\mu_{x}\) and \(\mu_{y}\) with the largest eccentricity corresponding to \(\alpha\approx 3.2\) (for fixed ratio of anisotropic factors). Typical phase screens used in these measurement campaigns are given in Figure 11.  
Figure 11: Turbulent phase screens of non-classic turbulence with \(2\pi\) wrapped phase for application on SLM/DMD. Vertical turbulence with \(\alpha=3.2\) (**a**) \(\mu_{z}=1\), (**b**) \(\mu_{z}=5\). Horizontal turbulence with \(\mu_{x}=3\), \(\mu_{y}=1\), (**c**) \(\alpha=3.2\), (**d**) \(\alpha=3.67\). Reprinted with permission from References [81; 84]. The single SLM arrangement used in References [81; 84] can be also modified to include the second SLM which pre-modulates laser light before sending it through the SLM with the turbulent screens [85]. In this case, a sufficient distance between the two devices must be set to send the pre-modulated light into the far-zone. The iris before the second SLM is used to pass the first-order of the first SLM. The optical setup is controlled from three personal computers: for the two (not synchronized) SLMs and for the camera.