**Acknowledgements**

shift in the position of the Bragg peak increases as the spatial frequency decreases and the

In some applications, it may be necessary to obtain a diffraction efficiency increase in one grat‐ ing selected from a number of gratings angularly multiplexed in the same region of the pho‐ topolymer layer, without affecting its neighbours. The selectivity demonstrated in **Figure 14**

In order to investigate if it would be possible to selectively boost one grating from a series, a layer of photopolymer 135 µm in thickness was used and five seed gratings of equal strength were angularly multiplexed into it. One of the gratings was then illuminated at its Bragg angle in order to increase its efficiency. For separations up to 1.8°, illumination caused a sig‐ nificant increase in the diffraction efficiency of gratings on either side of the intended grating. **Figure 17** shows the result for gratings 2° apart. Although there are only five seed gratings in these examples, the principle is demonstrated that one of a set of angularly multiplexed gratings, separated by 2°, can be enhanced by illuminating it at the appropriate angle with a single beam of light. Working with higher spatial frequency and thickness would be likely to

High efficiency diffractive optical elements have been recorded holographically with low spa‐ tial frequency. Three slanted gratings were successfully stacked using lamination and shown to increase significantly the range of angles from which light could be collected. However, positioning gratings in the path of the detector/cell meant that although light incident at large angles was coupled into the detector, an equivalent amount of light was deflected away from the detector at lower angles. When arranged off‐axis, however, they increased the total light collected. The HOEs were then tested with a solar simulator and shown to improve the

**Figure 17.** Variation of diffraction efficiency with the reading beam angle of incidence for a series of gratings, from which one individual grating has been enhanced by illuminating with a single on‐Bragg beam of light. The angular separation

energy collected at a Si solar cell by up to 60% when used off‐axis in pairs.

magnitude of the effect increases.

94 Holographic Materials and Optical Systems

implies that this should be possible.

**6. Conclusion**

between neighbouring gratings is 2°.

*5.2.4. Angular multiplexing of data using a one beam system*

allow smaller angular separations between neighbouring gratings.

The authors would like to acknowledge the IEO Centre's funding from Enterprise Ireland through its Commercialization Fund (which is co‐funded by the European Union through the European Regional Development fund), the Dublin Institute of Technology's Fiosraigh Scholarship Fund, and the Irish Department of Education's Programme for Third Level Research Institutes Strand 1 scholarships.
