Programmable Phase Optics
Research
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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
Last update:
23-04-2009