Diffraction Gratings Information
Diffraction gratings separate different wavelengths of light using a periodic pattern embedded in the grating. The diffraction pattern can affect the amplitude and/or phase of the incident light as well. Diffraction gratings are often used in monochromators for producing monochromatic light sources that can be tuned by adjusting the angle of the incident light on the diffraction grating.
Diffraction gratings are superimposed with a precise pattern of microscopic periodic structures. Usually these are in a pattern of corrugated surface grooves (a surface-relief grating), though some gratings are formed by the periodic variation of the refractive index inside the grating itself (a volume grating). Diffraction gratings can operate by either transmitting or reflecting.
Transmission gratings usually consist of parallel scratches whereas reflection gratings have grooves. Transmission gratings require relatively coarse groove spacings to maintain high efficiency. As the diffraction angles increase with the finer spacings, the refractive properties of the substrate materials used limit the transmission at the higher wavelengths and performance drops off. Reflection gratings avoid this issue by the nature of their operation.
Both transmission and reflection gratings produce multiple orders of diffracted beams for an incident light beam. The zero-order diffracted beam is the beam that passes straight through the grating in a transmission grating, or in the case of the reflection, grating is the reflected beam with an angle of reflection equal to the angle of incidence.
Gratings used to disperse ultraviolet (UV), visible, or near infrared (NIR) light usually contain between 100 and 3,000 grooves per millimeter, so the distance between adjacent grooves, or groove spacing, is on the order of a few hundred nanometers to a few microns. Groove spacing determines the wavelength range a grating covers.
The optimal wavelength of a ruled grating is called the blaze wavelength. As a rule, first order diffracted beam efficiency decreases by 50% at 0.66 and 1.5 times the blaze wavelength. No grating can diffract a wavelength greater than two times the groove period. By contrast, holographic gratings have a flatter spectral response and are usually defined in terms of their peak wavelength.
The resolving power of a grating is the product of the diffracted order in which it is used and the number of grooves intercepted by the incident light beam.
It can be expressed in terms of:
- grating width
- groove spacing
- diffracted angles
The resolving power of a grating depends on the accuracy of the ruling. In general, there is no difference in resolving power between holographic and ruled gratings with identical groove spacing.
Diffraction gratings may be either ruled or holographic, although there is a wider range of groupings within each style. Ruled diffraction gratings are produced by physically forming grooves into a reflective surface with a diamond mounted on a ruling engine. Ruled gratings have a relatively sharp peak around their blaze wavelength as a consequence of their "saw tooth" groove profile.
Ruled gratings tend to achieve higher efficiencies than holographic gratings due to their blaze angles. They are ideal for applications centered near the blaze wavelength. The distance between adjacent grooves and the angle the grooves form with respect to the substrate influence both the dispersion and efficiency of a grating. Diffraction gratings can be ruled on a variety of substrates; for example, glass, metal, and ceramic.
Holographic diffraction gratings are formed when a series of interference fringes, corresponding to the grooves of the desired grating, are recorded on a photosensitive layer, and the subsequent chemical treatment forms a modulated profile on the surface of the blank by selective dissolution. Holographic diffraction gratings include many configurations such as planar, curved (for example concave and toroidal), aberration-corrected, with uniform and non-uniform groove spacing.
Non-uniform spacing can provide superior focusing characteristics. They typically exhibit less stray light and "ghost" spectra than do classically ruled gratings, because they have fewer random and systematic imperfections. The low stray light of these gratings makes them ideal for applications where the signal to noise is critical. Holographic gratings can typically have a higher groove density than ruled gratings.