A grating is an important optical device that contains periodic structures capable of causing changes in the spatial distribution of light. These periodic structures can be etched onto glass or metal sheets.

A grating can decompose polychromatic light (light containing multiple wavelengths) into monochromatic light, forming a spectrum. This occurs because different wavelengths of light, when passing through the grating, due to diffraction and interference effects, produce maximum light intensity at different angles, forming distinct spectral lines. The ruling density of the grating (i.e., the number of rulings per unit length) is very high, reaching thousands of lines per millimeter, thus possessing high spatial resolution and enabling precise control over the direction of light propagation and phase.

Gratings include transmission gratings, reflection gratings, blazed gratings, fiber Bragg gratings, volume Bragg gratings, and others. The reflection grating is a relatively common type, and its schematic diagram is shown in Figure 1. The output angle can be calculated using the following formula, where d represents the slit spacing, λ represents the wavelength of the beam, and m represents the order (integer):

From the formula, we can deduce that when a single-frequency light beam is input into a grating, multiple reflected light beams can emerge, classified as 0th-order, 1st-order, 2nd-order light, etc., based on different values of m. Additionally, different wavelength components of the input light will have different reflection angles due to variations in λ, making gratings widely used as dispersion devices.

A blazed grating is a special type of reflection grating designed to enhance the diffraction efficiency for a specific wavelength, known as the blaze wavelength. It achieves this by incorporating a tilted "blaze angle" on the grating's rulings. This allows, at a specific wavelength and incident angle, the grating's diffracted light power to concentrate on a particular diffraction order, typically the first-order diffraction. When light strikes the blazed grating at a certain angle, each ruling acts like a small mirror reflecting the light. Due to the tilt of the rulings, the reflected light beams are concentrated in a specific direction rather than being dispersed in various directions as in a conventional grating. This specific direction is known as the blaze direction, and the corresponding wavelength is the blaze wavelength. At the blaze wavelength and blaze angle, the grating's diffraction efficiency can reach very high levels, approaching or even exceeding 80%-90%.

In addition, there are Bragg gratings, commonly including volume Bragg gratings and fiber Bragg gratings. A Bragg grating is a type of reflection grating based on the principle of Bragg diffraction. The principle of Bragg diffraction refers to the diffraction of light within a crystal when a light beam illuminates its surface, due to the periodic arrangement of atoms inside the crystal. When the incident angle is the Bragg angle, the diffracted light beams will interfere constructively, forming a series of bright and dark fringes, which is known as Bragg diffraction. A Bragg grating consists of a series of parallel rulings, with each ruling's width and spacing being integer multiples of the lattice constant. When the incident angle is the Bragg angle, the light beam will be reflected back, forming a series of bright and dark fringes. These fringes can be used to separate light of different wavelengths, thereby enabling spectral analysis.