Easy characterization of SLMs' phase deformation

[D. Marco et al., Opt. Lett., 45 (2020)]

Technical papers are important for the scientific community, it helps in particular to reproduce experimental setups. They are unfortunately not valued enough by scientific journals. I want today to highlight such a paper. Liquid crystals phase modulators - and indeed any kind of spatial light modulator (SLM) - are not free of imperfections. One effect that appears is a phase distortion of the reflected field due to spatial non-uniformities that occur during the fabrications. In practice, if you illuminate an SLM with a plane wave and you display a uniform mask, one does not end up with a plane wave, but an aberrated wavefront. In the present paper, the authors use a quite easy to implement technique to retrieve the phase distortion introduced by the SLM.

Aberration introduced by SLMs can be very dramatic. Calibration files can be provided by some manufacturers, but even so, it is usually safer to characterize it in the lab. We published, for instance, a way to characterize such effects for DMDs. This characterization requires a specific setup and an optimization, other approaches would require an interferometric setup with a separated reference arm. In this paper, the authors present a technique that takes advantage of the protective glass window that is present in virtually all modulation devices. The idea is quite simple, one illuminates the modulator with a plane wave at a wavelength different than the one it is designed for, in order to be outside the range of the anti-reflection coating usually present on the glass window. Then, we observe the interference pattern between the specular reflection on the front surface of the glass window and the multiple reflections inside the glass window that are affected by the phase modulation. By changing the global relative phase on the SLM modulation, one can derive the aberration profile introduced by the modulator. The experimental setup is shown in Fig. 1.

Figure 1. Experimental setup for the characterization of phase distortion. Image from [D. Marco et al., Opt. Lett., 45 (2020)].

At a working wavelength outside the working range of the anti-reflection coating, the glass window acts as Gires-Tournois interferometer: it consists of a transparent plate with two reflecting surfaces, one of which has very high reflectivity, the backplane of the SLM, and one with a lows reflectivity, the front surface of the glass window. The particularity here is that the phase of the reflected light on the highly reflective surface can be tuned by changing the pixel value globally on the SLM. 

Figure 2. Measured interferograms for uniform modulation on the SLM with different grayscale values set (global phase modulation). Image from [D. Marco et al., Opt. Lett., 45 (2020)].

Interferences between the multiple reflections in the glass plate and the specular reflection are observed and shown in Fig. 2 for different pixel values \(g\). Dark fringes correspond to contours where the field has the same phase, and they can be moved by tuning the pixel value \(g\). By localizing these fringes, one can retrieve the surface distortion, shown in Fig 3., that can be used for any working wavelength.

Figure 3. Reconstructed phase distortion.  Image from [D. Marco et al., Opt. Lett., 45 (2020)].

The main advantage of the technique is that, while it requires using a laser source at a different wavelength than the working one, it does not require a complicated setup nor good stability as some interferometric approaches do.