Reflection Gratings

Reflection gratings are precision diffractive optical elements operating in reflective mode. Relying on periodic micro-nano groove structures on their surfaces, they realize polychromatic light dispersion, wavelength filtering and beam steering via light diffraction and reflection. Capable of dispersion without a transmission light path, they feature compact structures and flexible optical layout, serving as fundamental core dispersive components widely adopted in spectrometers, laser systems and high-precision optoelectronic equipment.

Plane Ruled Reflection Grating is 12.7x 12.7 mm and 6 mm thick. The grating has 600 grooves per mm a nominal wavelength at 500 nm, and a nominal blaze angle of 8.6°. 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.

A reflection grating mainly consists of an optical substrate, periodic diffractive grooves and a high-reflection coating layer. Common substrate materials include K9 optical glass and fused silica; the surface is coated with aluminum film, gold film or dielectric reflective film to greatly boost reflectivity. Tightly bonded structural layers and precisely distributed grooves collectively deliver outstanding diffractive dispersion and reflective optical performance.

Reflection gratings boast superior properties: high diffraction efficiency, ultra-low stray light and high spectral resolution, paired with uniform linear dispersion, minor polarization dependence and slight wavefront distortion. They exhibit excellent physical and chemical stability, barely affected by temperature and humidity fluctuations, and maintain stable dispersion accuracy during long-term continuous operation, making them ideal for high-precision measurement and scientific research experiments.

Reflection gratings cover spectral bands ranging from ultraviolet, visible light, near-infrared to mid- and far-infrared. Multiple standard groove density specifications are available, while custom groove counts, dimensions and coating types can be provided on demand. They can accommodate various optical path wavelengths and precision requirements, and fit dispersion measurement and laser modulation applications across diverse industries.

Non-standard customization is supported for the size, profile, thickness and groove parameters of reflection gratings. Miniature small-size variants are available for portable instruments, and large-format industrial models for large optical path equipment. With high customization flexibility, they satisfy non-standard assembly demands in scientific research, medical treatment, industrial inspection and other fields.