Achromatic wave plate
Wave plates are generally made of birefringent materials, and the resulting phase retardation is：
Among them, n0 and ne are the refractive indices of o light and e light respectively. Since optical materials have different refractive indices for colored lights of different wavelengths, that is, both n0 and ne are functions of wavelength, so the thickness of the wave plate d is also a function of wavelength. According to the above theory, the commonly used quarter-wave plates and 1/2-wave plates have a certain thickness, and when used only for a single wavelength, errors will occur to other wavelengths, or even can not be used at all. This causes non-monochromatic spectroscopy work. difficult.
Achromatic wave plate can effectively reduce the influence of wavelength on phase retardation, realize the same wave plate has the same retardation in multiple bands, and achieve uniform phase retardation in a wide wavelength range.
Introduction to the principle of the achromatic wave plate
The concept of achromatic aberration is too familiar to practitioners in the field of geometric optics. Chromatic aberration is essentially aberration caused by the different refractive indices (ie, dispersion) of optical materials for different wavelengths of colored light, as shown in Figure 2 ( As shown in a), the light of different colors of the same aperture has different intersections with the optical axis after passing through the optical system. At any image surface position, the object point looks like a colored diffuse spot. Figure 2(b) is a schematic diagram of a typical achromatic lens, which compensates for chromatic aberration by using a combination of two different refractive index glasses.
The “chromatic aberration” of the “achromatic wave plate” has a different meaning from the imaging chromatic aberration described in geometric aberration. It refers to the influence of wavelength on phase retardation.
The retardation of a single birefringent crystal wave plate can be simplified as:
Among them, μ is the birefringence of the material, d is the thickness of the crystal, and λ is the wavelength of the incident light. If you can find a crystalline material whose birefringence changes linearly with wavelength, then its retardation will no longer change with wavelength. But in reality, it is difficult to find birefringent crystals that can meet this requirement.
Similar to an achromatic lens, the birefringence of different crystals varies with wavelength. Using this principle, two crystals of different materials can be used to form an achromatic wave plate. The phase retardation of light of different wavelengths is as follows:
When μ1, μ′1, μ2, μ′2, λ1, and λ2 are known, the thickness of d1 and d2 can be obtained to ensure that the same phase retardation is obtained at the two wavelengths of λ1 and λ2, so as to achieve the purpose of achromatic aberration.
The commonly used achromatic wave plate is composed of quartz crystal and magnesium fluoride (MgF2). By aligning the fast axis of the multi-stage quartz wave plate with the slow axis of the magnesium fluoride wave plate, a zero-order achromatic wave plate can be obtained. The optical path difference of the two wave plates is λ/4 and λ/2, and λ/2 and λ/4 achromatic waveplates are obtained respectively.
Achromatic wave plate application
Achromatic waveplates are often used in some complex physical optical instruments, such as spectral ellipsometers, birefringent filters, solar magnetic field telescopes, etc.;
Achromatic waveplates are also often used in the field of infrared lasers for spectral shaping, laser tuning and optical communications, etc.;