Principle of spectroscopic element
A beam splitter is an optical element that divides a beam of light into two beams according to a certain reflection and transmission ratio. It is mostly used in laser optical systems, illumination optical systems, and spectrometer optical systems. The beam splitter is made of optical glass coating. Usually, a beam splitting film is plated on the beam splitting surface. The beam is divided into reflected light and transmitted light according to certain requirements and methods. Commonly used beam splitting films mainly include light intensity beam splitting film, polarization beam splitting film, and Several types of the polarization beam splitting film. An anti-reflection coating is plated on the exit surface to increase the light transmission of the optical surface and reduce the interference effect between the beams and ghost images.
Classification of spectroscopic film
- Light intensity splitting film:
According to a certain light intensity ratio, the incident light is divided into two parts, which are used for ordinary light splitting plates and light splitting prisms. Among them, only considering a certain wavelength is called a monochromatic spectroscopic film; considering a spectral region is called a broadband spectroscopic film. According to the bandwidth of the spectroscopic film plated by the spectroscope, it can be divided into a narrowband spectroscope and a broadband spectroscope.
- Polarization splitting film:
It is made by using the polarization effect of the film when the light is incident obliquely, that is, when the light obliquely enters through each layer of the optical film, the boundary conditions require that the tangential components of the electric and magnetic fields are always continuous, so that the p component of the light The and s components exhibit different effective refractive indexes, causing polarization separation. It can be divided into prism-type polarizing film and flat-type polarizing film.
Take the polarization beam splitter prism as an example, which is composed of two 45° right-angled triangular prisms, and the polarized beam splitter film is coated on the inclined surface of the triangular prism. When light is incident at Brewster’s angle, the reflection of p-polarized light is zero, while s-polarized light is partially reflected. In order to increase the reflectivity of s-polarized light and keep the transmittance of p-polarized light close to 1, two thin-film materials can be alternately deposited to form a multilayer film. When the film layers are selected appropriately and enough, 50/50 polarization can be achieved Spectroscopy.
- Depolarization splitting film:
All kinds of optical films used for oblique incidence will have a polarization effect. For some optical systems, polarization separation brings about the deterioration of optical system performance, which must be eliminated or reduced. The depolarization splitting film is designed to ensure that the s-light and plight are split according to a certain reflection and transmittance ratio within the specified wavelength range while ensuring that the polarization state of the s-polarization state and the p-polarization state does not change after passing through the prism.
- Anti-reflection coating:
When the thickness of the film is appropriate, the optical path of the light reflected on the two surfaces of the film is exactly equal to half the wavelength, so they cancel each other out. This greatly reduces the reflection loss of light, enhances the intensity of the transmitted light, and reduces or eliminates the stray light of the system. Usually, a single-layer film only provides anti-reflection to a certain wavelength of electromagnetic waves. In order to achieve anti-reflection in a larger range and more wavelengths, multi-layer coatings are often used to achieve the anti-reflection.
Selection of spectroscopic components
Spectroscopes have a wide range of uses, and the needs of users are also different. United Optics can provide K9 broadband beam splitter, elliptical broadband beam splitter, aluminum-plated lattice beam splitter, UV fused silica aluminum-plated lattice beam splitter, and ultraviolet beam splitter. Wedge, calcium fluoride (CaF2) infrared broadband splitter wedge, zinc selenide (ZnSe) infrared broadband splitter wedge, standard cubic beam splitter, polarized cubic beam splitter (PBS), depolarized beam splitter prism and other standard beam splitters.
How to choose the right spectroscopic element?
1. According to the type of light source, select the applicable waveband and bandwidth of the spectroscopic element. Choose light-splitting elements suitable for different wavelengths of visible light, ultraviolet, and infrared, and choose narrow-band light-splitting elements or broadband light-splitting elements.
2. According to the size of the optical path space and the requirements for the influence of the beam shift, the splitting flat plate and the splitting prism are selected. When the splitting flat plate is selected, the reflected beam and the transmitted beam will have unequal optical paths, and the transmitted beam will shift. For the design of the imaging optical path, the introduction of the beam splitter will affect the aberration correction of the optical path.
3. According to the polarization state requirements of the reflected light and transmitted light in the optical path, select the polarization beam splitter and the depolarized beam splitter.
4. After determining the type of spectroscopic element, make a comprehensive selection of specific parameters such as the size, splitting ratio, and wavelength of the spectroscopic element.