From Liquid Crystals: Frontiers In Biomedical Applications

3.2 Diffraction Gratings and Tunable Filter Spectrometers

A light source can be decomposed into its spectral constituents by either dividing the spectrum over space in a dispersive configuration or dividing the spectrum over time in a nondispersive configuration. Each method presents distinct advantages and disadvantages, depending on the specific application. Implementation of a variety of liquid crystal technologies in spectrometers can lead to performance benefits and reduction of device size and cost.

3.2.1 Dispersive devices

The most direct and commercially embraced methods of UV/Vis/NIR absorption spectroscopy are spatially dispersive techniques utilizing wavelength dispersive prisms or diffraction gratings. Dispersive methods of spectroscopy are advantageous in that these devices are easy to fabricate and simple to operate. Additionally, with technological improvements in both grating fabrication and detectors, these devices have steadily become more inexpensive while maintaining a high performance benchmark. In a static diffraction grating configuration, operating in either transmission or reflection mode, the spectrum of interest is broken down into its relative frequency components by angular separation. The intensity of each spatially discriminated frequency is recorded simultaneously using a focal plane array such as a CCD or photodiode array, as is shown in Figure 3.1.


Fig. 3.1: A static grating spectrometer with all spectral components detected simultaneously by a focal plane array.

Using a diffraction grating, the angular separation as a function of wavelength, ? m( ?) , is determined from the basic grating relation:


where d is the grating period or pitch, ? i

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Products & Services
Optical Spectrum Analyzers
Optical spectrum analyzers (OSA) can divide a lightwave signal into its constituent wavelengths. This means that it is possible to see the spectral profile of the signal over a certain wavelength range.
Monochromators
Monochromators are optical subassemblies used to isolate narrow portions of a light spectrum. They accept polychromatic input from a lamp or laser, and outputs monochromatic light.
Diffraction Gratings
A diffraction grating uses a substrate with parallel grooves to disperse light into its spectra. Eschelles are included in this area.
UV and Visible Spectrometers
UV and visible spectrometers measure the amount of ultraviolet (UV) and visible light transmitted or absorbed by a sample placed in the spectrometer.

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