Reflection Grating

A Reflection Grating is a core optical diffraction component. Periodic microstructures are fabricated on a high-reflectivity surface. By leveraging the reflection, diffraction and interference effects of light, it disperses (separates) polychromatic light into spectra according to wavelength.

Compared with transmission gratings, reflection gratings deliver higher energy utilization efficiency and cover a broader spectral range (especially ultraviolet and infrared bands). They serve as critical components for modern spectrometers, lasers and astronomical equipment.

Plane Ruled Reflection Grating is 12.7x 12.7 mm and 6 mm thick. The grating has 1200 grooves per mm a nominal wavelength at 500 nm, and a nominal blaze angle of 17.5°. The diffraction grating is coated with an aluminum coating on glass substrate and designed for first order Littrow use with high efficiency in the spectral region around the blaze wavelength.

In general, for ruled diffraction gratings the groove spacing determines the diffraction angles, and the groove depth and blaze angle determines how diffracted energy is distributed between diffraction orders.

Plane Ruled Diffraction Gratings are most efficient when used near the design wavelength in the Littrow configuration, that is aligned so that the diffraction angle of the dominant diffraction order is coincident with the input beam, effectively behaving as a retroreflector at a specific wavelength. For blazed gratings, maximum efficiency occurs for wavelengths that the Littrow condition at the angle normal to the blazed grating facets. As ruled blaze gratings are asymmetric, correct orientation is indicated with an arrow marking on the size of the substrate. The arrow is on the side of the substrate perpendicular to the ruled grooves, and points toward the steeper edge of the triangular groove profile. Equivalently, the arrow points away from the grating normal toward the facet normal. The arrow should point toward the incident (and diffracted) beam.

The basic grating equation determines the discrete directions into which monochromatic light of wavelength λ is diffracted. The equation is shown below:

mλ = dG (sinα + sinβm)

The above figure illustrates this diffraction. Light of wavelength λ is incident at an angle α and diffracted by the grating (with a groove spacing dG) along a set of angles βm. These angles are measured from the grating normal, which is shown as the dashed line perpendicular to the grating surface at its center. If βm is on the opposite side of the grating normal from α, its sign is opposite. In the equation, m is the order of diffraction, which is an integer. For the zeroth order (m = 0), α and β0 are equal and opposite, resulting in the light simply being reflected, i.e., no diffraction. The sign convention for m requires that it is positive if the diffracted ray lies to the left (counter-clockwise side) of the zeroth order and negative if it lies to the right (the clockwise side). When a beam of monochromatic light is incident on a grating, the light is simply diffracted from the grating in directions corresponding to m = -2, -1, 0, 1, 2, 3, etc. When a beam of polychromatic light is incident on a grating, then the light is dispersed so that each wavelength satisfies the grating equation as shown in the figure. Usually only the first order, positive or negative, is desired and so higher order wavelengths may need to be blocked. In many monochromators and spectrographs, a constant-deviation mount is used where the wavelength is changed by rotating the grating around an axis while the angle between the incident and diffracted light (or deviation angle) remains unchanged.

Core Performance Parameters

Groove Density

Unit: grooves per millimeter (gr/mm). Common specifications include 300, 600, 1200, 1800 and 2400 gr/mm. This parameter directly determines spectral resolution and dispersion capability.

Diffraction Efficiency

Defined as the ratio of reflected light energy to incident light energy at a specific wavelength and diffraction order; it is the core metric for evaluating grating performance. Conventional holographic gratings achieve diffraction efficiency above 80%, while Volume Phase Holographic (VPH) gratings can reach up to 95%.

Stray Light Level

Reflection holographic gratings feature extremely low stray light (typically less than 10⁻⁵), as they are free from ghost lines and periodic errors caused by mechanical ruling. They are the preferred choice for weak-signal detection such as Raman spectroscopy and fluorescence measurement.

Wavelength Coverage

Determined by substrate material and reflective coating layers. They can cover the full spectrum: ultraviolet (200 nm), visible light (400–700 nm), near-infrared (1100 nm), mid-infrared (5 μm), and more.

Diffraction Order

The 1st diffraction order (m=1) is normally selected to balance diffraction efficiency and resolution. Optical design is required to suppress interference from higher orders (2nd order and above).