Fiber Bragg gratings are optical components with a periodic structure that separate light into beams that propagate in predictable directions based on wavelength. Gratings serve as the core dispersive element of many modern spectroscopic instruments. They provide the critical function of selecting the wavelength of light required to perform the analysis at hand. Selecting the best grating for an application is not difficult, but it usually requires a degree of decision making when prioritizing the key parameters of the application.

Any spectroscopic application has at least two fundamental system requirements: it must be able to analyze materials over the desired spectral range of interest, and it must be able to provide a spectral bandwidth small enough to resolve the features of interest. These two key requirements form the basis for grating selection. Other grating characteristics are then selected to optimize performance within these fundamental constraints.

The two most common groove profiles are known as ruled and holographic, which is related to the method used to make the master grating. Ruled gratings can be produced using a scribing tool, where grooves are physically formed in a reflective surface with a diamond tool. Ruled grating groove profiles are very controllable and easy to optimize for a given application, and in most cases will provide the best diffraction efficiency due to this degree of freedom.

Dispersion, Resolution, and Resolving Power

The primary function of a diffraction grating in a spectroscopic instrument is to angularly separate a broadband source into a spectrum with each wavelength having a known direction. This property is called dispersion,and the equation that indicates the relationship between wavelength and angle is often called the grating equation:

n λ = d (sin θ + sin θ’)

Resolution is a system property, not a grating property. A spectroscopic instrument must provide a spectral bandwidth narrow enough to distinguish features of interest. This is achieved by a combination of the angular dispersion of the grating and the focal length of the system, and by limiting the width of the aperture. Spectral bandwidth at the detector plane can be achieved just as well with a low-dispersion grating and a long focal length as with a high-dispersion grating and a shorter focal length. In systems with a single-element detector, such as a scanning monochromator, the limiting aperture is usually a physical slit of known width. In a fixed-grating spectrometer, the limiting aperture is usually an array element or camera pixel.