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Wavelength detection using narrow-band
reflectance filters

For a wide range of spectroscopic applications, it is required to provide the spatially resolved spectrum of an incident optical beam. Typically, diffraction gratings operating in the Raman-Nath regime and prisms can provide such resolution through multi-order diffracted waves and dispersion effects respectively. However, the need to develop cost effective, portable solutions that can provide improved resolution and efficiency has driven the search for alternative solutions. We have developed a highly cost-effective and compact system for the spectral resolution of an incident optical beam by the use of the guided-mode resonance effect. Such a system is shown to have high resolution and is comprised of only two working elements, namely a graded-wavelength guided-mode resonance filter (hereafter referred to as a graded-wavelength filter or GWF) and a line camera based on linear charge-coupled device (CCD) sensor array

The filter provides a spatially resolved transmission minimum, the position of which is controlled by the wavelength of the incoming light. The sensor array collects the spatially resolved transmitted intensity and the wavelength is determined by recording the position of the transmission minimum along its length. Using this technique, wavelength changes as small as 0.011 nm can be detected.

Figure 1: Position of the transmission minimum in pixel number (as determined by curve fitting) versus the incident wavelength. The solid line represents the line fit to the data and shows a high degree of linearity. The equation describes the line and the sensitivity characteristics of the system. (Inset): Lorentzian fit for the data point corresponding to λ = 849.00 nm.

Figure 2: Time resolved dispersion of the pixel for which minimum intensity was detected by the system for set wavelengths of λ = 849.00 nm (boxes) and λ = 849.02 nm

References:

1. Fabrication of a graded-wavelength guided-mode resonance filter photonic crystal
   Appl. Phys. Lett. 89, 123113 (2006); DOI:10.1063/1.2356695

2. Compact wavelength detection system incorporating a guided-mode resonance filter
   Appl. Phys. Lett. 90, 081103 (2007); DOI:10.1063/1.2591342