July 30, 2015, Thursday, 210

Low-loss Tunable Optical Filters


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Picture of experimental setup
Picture of experimental setup



Project Summary

In this project we create a polarization independent tunable optical filter with potential for low cost and compact implementation for applications including compactspectroscopy and hyperspectral imaging. The basic approach involves the use of tunable Polarization Gratings (PGs) in a stacked configuration analogous to Lyot or Solc Filters. The PGs are tuned by electrical control of their birefringence to create a passband filter without the use of polarizers, thereby allowing very high overall transmittance (~100%).

Motivation and Approach

General configuration of LCPG filter stack
General configuration of LCPG filter stack

The applications of tunable optical filters range from spectroscopy to optical communication networks to biomedical imaging and remote sensing. The most sucessful approaches include dispersive elements matched with aperture-stops (Czerny-Turner), mechanically tuned etalons (Fabry-Perot), and assemblies of stacked birefringent waveplates and polarizers (Lyot, Solc, and Evans). Despite their popularity, these designs are in many contexts costly to implement, may have high insertion losses, manifest strong polarization sensitivity, or are difficult to miniaturize into a physically small package. This project focuses on describing, analyzing, and demonstrating a tunable filter based on stacked Polarization Gratings (PGs) which exhibits complete polarization-independence, high peak transmittance (~90%), and potential for very low cost implementation.

Spectrum of filter prototype with four LCPGs
Spectrum of filter prototype with four LCPGs
Spectra depicting filter tuning with increased voltage
Spectra depicting filter tuning with increased voltage

The basic design involves layering at least three PGs of varying thicknesses and then collecting the zero-order transmittance. The overall transmittance of the stack can be described as a multiplication of the zero-th order efficiencies of each individual component. When properly tuned to a given design wavelength, this filter stack can produce a narrow pass band with very high transmittance. The key aspect of this approach is that we do not use any polarizers within the system.

Anticipated Benefits

This tunable optical filter will be completely independent of input polarization, which significantly improves maximum peak transmittance as compared to polarizer-based filters. We also show a potential for highly compact implementation for a variety of applications including compact spectrometry and imaging spectroscopy.

Project Publications

  • Polarization-independent tunable optical filters based on liquid crystal polarization gratings, Proceedings of the SPIE - Optics & Photonics Conference, vol. 6654, no. 4, pp. TBA, 2007.

Background References

  • R. Komanduri and M.J. Escuti, Elastic continuum analysis of the liquid crystal polarization grating, Physical Review E 76, 2007.
  • J. W. McMurdy et al., Monolithic microspectrometer using tunable ferroelectric liquid crystals, Applied Physics Letters 89, 2006.
  • L. Nikolova and T. Todorov, Diffraction efficiency and selectivity of plarization holographic recording, Optica Acta 31, pp. 579-588, 1984