Comparison of Moxtek Proflux® UVD Series Spectrophotometer Analyzers with Glan-Taylor and Glan-Thompson Polarizer Designs

The Moxtek Proflux^® UVD260 and UVD240 analyzers deliver unprecedented broadband performance in spectroscopy applications ranging from 3.3µm in the mid-wavelength infrared to 240nm in the Deep UV. The Moxtek Proflux^® designs yield the finest-pitch commercial wire grid polarizers available on the market. These deeply sub-wavelength optics provide the benefits of minimal performance variation with wavelength, an unprecedented wide field acceptance angle, conserved space, without the adverse short-wavelength infrared disturbances found in Glan-Taylor (GT) and Glan-Thompson (GTh) polarizers.

UV Performance of Moxtek Proflux^® UVD Series

The light path for a simplified spectrophotometer equipped with a polarizing analyzer is shown in Figure 1. This setup is suitable for analyzing diffraction gratings, dichroic and birefringent samples, or characterizing samples in reflectance. This analyzer application uses GT and GTh polarizers, which typically use Calcite prisms. However, these polarizers have limited UV transmittance and signal to noise ratio due to scattering from impurities and inclusions and inherent absorption in calcite.

Figure 1. Simplified spectrophotometer with analyzer.

Conversely, the Moxtek Proflux^® UVD Series polarizers exhibit significantly improved Deep UV transparency to achieve superior signal to noise ratio performance. Figure 2a provides the comparison of the UV passing state transmittance between the Moxtek UVD260 and a standard calcite GTh analyzer, whereas Figure 2b evaluates their performance during the measurement of the blocking state transmittance of a reference polarizer.

Figure 2. Performance comparison of ProFlux^® UVD260 and Glan-Thompson polarizers in an analyzer application.

Infrared Performance of Moxtek Proflux^® UVD Series

The GT and GTh polarizer designs feature two birefringent prisms positioned with their faces either filled with optical cement (GTh), or isolated with a small air gap (GT). The isolation of an inward beam into orthogonal polarization states depends upon total internal reflection, thereby imparting stringent requirements on entrance angle and beam collimation.

However, as refractive index usually declines in the infrared, the allowed deviation from normal incidence decreases for the GT and GTh polarizer designs. This causes considerable leakage of the unwanted polarization state in spectroscopic applications, which typically require far from ideal beam collimation. Conversely, the wire grid polarizer performance is relatively insensitive to angle and wavelength, thus showing virtually no IR leakage of the unwanted polarization state. The ProFlux^® UVD designs is capable of accommodating ±20° deviations from normal incidence with least performance variation, thus improving light utilization when employing poorly collimated sources and alleviating alignment problems.

Figure 3a illustrates the superior infrared performance of the ProFlux^® UVD260 over a GTh polarizer when examining the same part. The UVD260 analyzer does not show such infrared leakage or resonance peaks, thanks to its fused silica substrate and sub-wavelength grating design. Figure 3b depicts the transmittance of the GTh and UVD260 polarizers in the passing state, showing the superiority of the UVD260 in terms of enhanced short- wavelength infrared throughput and signal to noise.

Figure 3. Infrared performance comparison of UVD260 and calcite Glan-Thompson analyzers

Figure 4 shows the contrast ratio between passing and blocking state transmittances (inverse of extinction ratio) for UVD260 and GTh polarizers, delineating the dramatic effect of the GTh infrared leakage on the contrast. Hence, the wire grid design is ideally suited for challenging IR spectroscopic applications.

Figure 4. Contrast ratio between passing and blocking state transmittances for ProFlux^® UVD260 (—) and Glan- Thompson (—) polarizers.

Environmental and Form Factor Concerns

The UVD260 uses the same materials used in Moxtek’s standard visible spectrum wire grid polarizer products, which have proven their ability in high temperature and high humidity projection display applications. The buried nanowire design of the UVD series facilitates protecting from environmental contamination and handling damage. Furthermore, the UVD Series wire grid polarizers are made on non-hygroscopic fused silica, thus demonstrating sustained performance in humid environments as opposed to calcite GT and GTh polarizers that show performance degradation due to their moisture uptake nature.

The large critical angle and total internal reflection operating principle make the GT and GTh polarizers to have clear aperture dimension and large aspect ratios between their lengths. The increase in aperture size forces the prism-based designs to occupy more space in an optical system. Conversely, the planar wire grid polarizer configuration has a fixed thickness (typically 2.1mm) set by the spacer thickness and substrate choice, and maintains a fixed physical space that is needed along the beam propagation direction, irrespective of the selection of aperture size.

Moreover, the prism length needs to be increased for larger aperture size GT and GTh designs and this requirement results in performance degradation in the UV and short-wave IR regions owing to scattering and absorption. ProFlux^® UVD Series polarizers can cover the whole spectral range of most spectrometers, from the shortwave IR through the Deep UV, utilizing the same part and without discarding light from the larger field angles.

For GT and GTh polarizer designs, the prism sides are highly polished and placed in an absorptive case or applied with an absorptive coating to dump the internally reflected beam. Nevertheless, Fresnel reflections can still take place, causing leakage of the unwanted polarization state via the polarizer’s exit face. Figure 5 depicts typical ProFlux^® UVD series broadband performance plots for passing state transmittance (Tp) and contrast ratio (passing state / blocking state transmittance).

Figure 5. Typical ProFlux^® UVD series broadband performance plots for passing state transmittance (T_p) and contrast ratio (passing state / blocking state transmittance).

By contrast, ProFlux^® UVD Series polarizers apply an anisotropic absorption and reflection mechanism for beam separation at the wire grid surface and are unaffected by birefringence and total internal reflection. This eliminates the unwanted IR performance deviation with wavelength and the long optical path length and absorptive surface/casing requirements innate in GT and GTh products. The key features of ProFlux^® UVD Series polarizers and GT and GTh polarizers are compared in Table 1.

Table 1. Comparative Summary

Conclusion

The aforementioned facts clearly demonstrate the superior broadband performance of the ProFlux^® UVD Series polarizers over GT and GTh designs, in polarization sensitive spectroscopic applications.

This information has been sourced, reviewed and adapted from materials provided by Moxtek, Inc.

For more information on this source, please visit Moxtek, Inc.