Technical University of Denmark

Programmable Phase Optics









The Generalised Phase Contrast method Advanced optical micro-manipulation Phase-only optical encryption and decryption Spatial phase-only modulation by the reverse phase contrast method GPC implemented in plana-integrated micro-optics Complex field coupling to advanced optical fibers 2D polarization encoding using Spatial light modulators

Reverse phase contrast for generating spatial phase modulation

The generation of a well-controlled phase distribution has a number of applications in contemporary applied optics and there currently exist a number of techniques for producing such phase modulation of an optical wavefront. These include for example electrically or optically addressed phase-only spatial light modulators (SLMs) which modulate the phase of a wavefront by a spatial variation in the optical path length of transmitted or reflected light. Accurate phase-only SLMs are rather complicated and expensive items, and we are thus interested in a simple practical technique for producing reconfigurable two-dimensional phase modulation.

In this section we propose a technique for the conversion of a given amplitude pattern to a phase distribution by a technique which we refer to as the Reverse Phase Contrast (RPC) method (see further reading). In this method, a high contrast amplitude mask is used to generate a phase-encoded version of the amplitude pattern using the basic 4-f filtering setup as shown in Fig. 1. The spatial filter determines the resultant phase shift between the elements of the output wavefront. Using a liquid crystal based phase-contrast filter the dynamic range of the phase modulation can be adjusted arbitrarily within the interval . Thus combining an amplitude modulator with a tuneable phase filter would result in a high performance phase-only SLM in which the spatial light modulation and phase shift are effectively decoupled.

Figure 1. Generic RPC based phase-only spatial light modulator

Reconfigurable spatial phase modulation of a light field is required in a number of areas in optics, including phase modulation for holographic multiplexing, storage and encoding, phase-only encryption and decryption and the testing of focus in optical apparatus. In addition, the RPC technique can be used with a binary amplitude mask acting as the input information to create interchangeable but static phase distributions. In the case of a fixed phase distribution, a major advantage of the use of amplitude masks to define the required phase pattern is the relative simplicity with which they can be manufactured when compared to phase-only elements. The use of standard chrome on glass mask technology would make it possible to achieve high resolution phase patterns, the phase shift of which would be controlled by the filtering system. In fact, it is possible to tune the output phase shift via the contrast ratio of the mask or by tuning the filter parameters. If a dynamic phase modulator is required, then an amplitude modulator, in the form of a commercially available liquid crystal display (LCD) projector element, or possibly a MEMS (Micro Electronic Mechanical System) type device can be used.

We have undertaken experiments to characterise the performance of the RPC technique using both fixed amplitude masks and spatial light modulators for the input amplitude modulation. We used a Hamamatsu parallel-aligned liquid crystal modulator, in conjunction with polarisors to generate a binary on/off modulation of the amplitude of the input wavefront. In general, such an SLM will have a lower contrast than a fixed mask and the resolution of the resulting phase distribution will be limited to that of the modulator.

Figure 2: Experimental results for the generation of phase modulation using an SLM operating as the input amplitude modulator. These show (a) an image of the input amplitude distribution without the filter in place and (b) interference fringe measurement of the output phase modulation.

In Fig 2(a), we show the input image without the Fourier plane filter in place. The image consists of a number of circular and ellipsoidal dark regions on a light background. The 4mm iris is slightly out of focus due to an axial displacement between the SLM and iris and some slight interference fringes are visible due to stray light scattered off the beam-splitter placed in front of the SLM. The interferometric measurement of the phase is shown in Fig. 2(b) and it can be clearly seen that there is a binary phase modulation imposed on a uniform amplitude wavefront. The fringe spacing indicates that we have a phase shift of approximately pi in the output modulation and have thus successfully converted our input amplitude distribution into a spatially identical phase distribution. The fringes in the region outside the aperture are due to the small fraction of light scattered by the filtering operation.

Further reading

Glückstad, J.; Daria, V.R.; Rodrigo, P.J.; Decrypting binary phase patterns by amplitude. Opt. Engineering. (in press) 

Mogensen, P.C.; Glückstad, J., Reverse phase contrast: An experimental demonstration. Appl. Opt. (2002) 41, 2103-2110

Glückstad, J.; Mogensen, P.C., Reverse phase contrast for the generation of phase-only spatial light modulation. Opt. Commun. (2001) 197 , 261-266

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Last update: 23-04-2009