A grating is an optical element composed of a series of equidistant parallel grating lines. It is a dispersive element that uses the principle of light diffraction and interference to split light. It is widely used in biochemical instruments, spectrometers, spectrophotometers and other related products. In related fields.
There are many types of gratings and their classification methods are also different.
According to the shape of the working surface, it can be divided into the flat grating and concave grating. Concave gratings have both light splitting and focusing functions, and multiple detectors need to be spliced along the curved direction of the spectral surface to complete the full spectrum detection, and it will be accompanied by large aberrations, resulting in measurement errors in the results. Therefore, flat gratings are more commonly used.
According to different working methods, it can be divided into two types: transmission grating and reflection grating. Among them, the transmission grating has a low utilization rate of light, and the loss of light energy is relatively large.
According to different production processes, it can be divided into the holographic grating and ruled grating.
The scoring grating uses a grating scoring machine to process the material with a diamond cutter head. The scoring density is adjusted flexibly, but the scoring density that can be achieved is limited, and there may be periodic scoring errors, resulting in ghost line effects and interference The light splitting of the grating.
The holographic grating uses a laser to generate two beams of coherent light and produces a series of uniform interference fringes on a substrate coated with a photosensitive material coating so that the photosensitive material is exposed to light. Then use a special solvent to etch away the photosensitive part, that is, obtain a holographic image of interference fringes on the etched layer. Ion etching technology can also use ion beams to bombard the grooves, processing the shape of the grooves into a blazed grating, and enhancing the diffraction efficiency.
Spectroscopic performance and common parameters of the grating
- Grating equation
After collimation, the beam becomes parallel light, which illuminates the grating at a certain angle. Different wavelengths of light are emitted at different angles, and the equation that determines the positions of the main maximums at each level is called the grating equation. The most basic grating equation is:
d(sini ± sinθ)=mλ m=0,±1,±2，…
The equation shows that according to the value of mλ, parallel beams are incident obliquely on the grating with the slit spacing d at an incident angle i, and the light is deflected at a discrete angle θ, where m is the main maximum order. The angle of incidence i and θ are the angles between the incident light and the diffracted light and the grating normal, respectively. When examining the diffraction spectrum on the same side as the incident light, the above formula takes the positive sign; when examining the opposite side of the incident light When diffracting the spectrum, the above formula takes the minus sign. It can be seen that for a given order m, the light of different wavelengths will exit the grating at different angles.
- Diffraction efficiency
The diffraction efficiency of grating usually has two definitions, namely relative diffraction efficiency and absolute diffraction efficiency.
The relative diffraction efficiency is defined as the ratio of the diffracted luminous flux of the grating received by the detector to the reflected luminous flux of a standard mirror with the same aperture at a given wavelength and diffraction order;
Absolute efficiency refers to the ratio of monochromatic diffracted luminous flux to incident luminous flux in a given spectral order;
The grating diffraction efficiency curve refers to the relationship between the grating diffraction efficiency and the wavelength.
- Glare angle
Blazed grating is a special form of the reflective or transmissive diffraction grating. Its grooved surface is not parallel to the grating surface, and there is an angle γ between the two, which is called the blaze angle.
- Blaze wavelength
The blazed grating can produce the maximum grating efficiency at a specific diffraction order, that is, most of the optical power will be in the designed diffraction order, usually the first order, while reducing the power of other orders (especially the zero-order) as much as possible. Due to this design feature, the blazed grating will work at a certain wavelength, which is also called the blazed wavelength.
In the Littrow structure, that is, the incident light is perpendicular to the grating groove surface (called self-collimation incident in the spectrometer), the angle i of the incident light and the angle θ of the diffracted light is the same, i=θ=γ, at this time, 1 The blaze wavelength λB of the first-order spectral blazed grating is λB=2dsinγ.
Application and selection of grating
Users can combine the above table to determine the grating category according to their needs, and then select the grating that meets the needs based on key factors such as the band of use of the grating, the number of reticles, the blaze angle, the blaze wavelength, the size of the dispersion, the diffraction efficiency, and the size of the grating.