2-D polarisation encoding using phase-only spatial light modulators
We propose a method for spatially encoding the state of polarisation in a two-dimensional wavefront with elliptically polarised light controlling both the ellipticity and rotation angle of the major axis of the
ellipses (see further reading). We have constructed a demonstration system using two optically addressable phase-only spatial light modulators, where it is possible to control the relative phase of each modulation cell in the spatial light modulator. This gives a full two-dimensional control over the state of polarisation.The encoding system is shown schematically on the figure below.
Figure 1. An optical system for converting incident polarised light into an arbitrary state of elliptically polarised light with the major axis of the elliptically polarised light rotated an arbitrary angle. The lines denote the extraordinary axis of the SLMs, the quarter wave plates
(l/4) and the polarisation direction of the linear polariser.
The system shown in Figure 1 can be thought of as consisting of two sub systems, 1) an elliptical generator and 2) an elliptical rotator. The system is capable of converting an input state of polarisation (SOP) into an arbitrary output SOP.
In order to illustrate the capabilities of the 2-D polarisation encoder,
examples of generated polarisation states are tabulated in the figure below.
Figure 2. A graphical representation of the output polarisation ellipticity and orientation as a function of the phase shift implemented in corresponding regions of SLM-1
(f1) and SLM-2 (f1)
We have made several experiments in order to test the 2-D encoding system. We have used two SLMs divided into four quadrants. Each quadrant (cell) of SLM-1 is imaged on to a corresponding cell on SLM-2 and imaged and visualised using a polarisator and a CCD camera. The phase modulation of each cell is controlled from a PC-interface connected to the SLMs.
Figure 3 demonstrates examples of parallel polarisation encoding.
Figure 3. Experimental encoding results for the rotation of a distribution of linearly polarised light. The upper half of the figure shows the polarisation state (indicated by arrows) and phase modulation for the quadrants (1 to 4) in each SLM and the resulting encoded information. The lower half of the figure shows the corresponding experimental results where the polarisation distribution for each SLM and the encoded information are recorded with a polariser and a CCD-camera.
Eriksen, R.L.; Rodrigo, P.J.; Daria, V.R.; Glückstad, J;
Spatial light modulator controlled alignment and spinning of birefingent
particles optically trapped in an array. Applied Optics (2003) 42, 5107-5111
Eriksen, R.L.; Mogensen, P.C.; Glückstad, J., Elliptical polarisation
encoding in two dimensions using phase-only spatial light modulators. Opt.
Commun. (2001) 187, 325-336