There are three types of beam splitters for optical research applications. These include plate beam splitters, coated plate beam splitters, and cube beam splitters. This article describes the fundamental design characteristics of all three beam splitters in detail.
Plate Beam Splitters
Figure 1. Primary, secondary, and tertiary reflections
Figure 2. Multiple reflections from a beam splitter at 45°
In any type of plate beam splitter, primary, secondary and tertiary beams are produced from multiple reflections (Figure 1). Here, only incoherent beams are considered. For many applications, only the primary beams are important, whilst the other beams typically have low intensity. Figure 2 demonstrates the relative power in the subsidiary beams for a plate beam splitter utilized at 45°. Here, the beam splitter surfaces exhibit reflectances and transmittances of r1 r2 and t1 t2 for the polarization state, wavelength, and angle of incidence of the beam of power P. The calculated, transmitted, and reflected beam powers assume no interference effects, and no absorption in the substrate or coatings.
Using Uncoated Plate for Beam Sampling
Figure 3. How the transmittance of the main beam, for s, p and unpolarized light incident on a fused silica beam splitter varies with angle of incidence
This setup is easy to tap off a small part of a beam for intensity monitoring. Figure 5 shows how the transmittance, via a fused silica plate, differs with polarization and angle of incidence. For least sensitivity to polarization, angle, and beam, an angle of incidence of less than 20° should be used. The transmittance and reflectance of a fused silica plate beam splitter utilized at 45° is sensitive to beam polarization and angle of incidence, especially for p polarized beams. Figure 3 also demonstrates that an unpolarized beam becomes partially polarized on traveling via an inclined beam splitter.
Wedged/Plane Plate Beam Splitter
As we move away from the beam splitter, the reflected beams from a wedged beam splitter separate and, thus, enable us to utilize the reflection from a single surface. It is important to prevent the overlap of reflected beams when using a narrowband monochromatic source or laser. This can be done by eliminating the second reflection, or by utilizing a wedged beam splitter. The wedge should be able to prevent spatial overlap of the reflected beams. The preferred wedge angle depends on the angle of incidence and beam diameter. In case there is overlap of beams and the beam’s coherence length is longer than the path difference for the two reflected beams, the interference will alter the transmittance for the overlapping parts of the beam. In such a case, the beam splitter serves as an etalon; the transmittance depends on the angle of incidence and wavelength.
Coated Plate Beam Splitters
A thin film coating applied on the front surface of a beam splitter alters the reflectance from that surface. The reflectance from most dielectric thin film coatings depends on the angle of incidence, polarization, and wavelength and, hence, dielectric beam splitters are usually designed for a particular application. Oriel Instruments’ inconel coated beam splitters are broadband, and not sensitive to beam polarization. Although Polka Dot beam splitters are broadband and efficient, they show some degree of sensitivity due to Fresnel reflectance.
Pellicle Beam Splitters
Figure 4. Reflectance for a HeNe laser from an uncoated pellicle against angle of incidence. (a) a 1.8 µm thick pellicle; (b) a 5 µm thick pellicle. The curves show one effect of interference on the reflectance from a thin substrate
Pellicle beam splitters are a type of membrane beam splitters that are so thin that the two reflected subsidiary beams overlap completely and appear as if there is only one transmitted beam. Since the membrane is very thin, and there is overlap of beams, interference will always be present between the beams. Figure 4 shows one result of interference. Here, it is not possible to use the Fresnel formulae to find the reflected intensity.
Pellicle beam splitters can be utilized in diverging or converging beam paths, and induce negligible focal shifts or optical aberrations. While a traditional inclined plate beam splitter causes focal shift and astigmatism, the thin pellicle membrane can be easily damaged and is sensitive to vibrations.
Cube Beam Splitters
Figure 5. Beam splitter cube
A typical cube beam splitter is shown in Figure 5. A coated interface rotates part of the beam through 90°. Efficient cube polarizers utilize dielectric coatings that are polarization and wavelength specific. The output and input faces are antireflection coated, in order to prevent subsidiary beams. Cube beam splitters create no beam displacement and exhibit the orthogonal beam splitting geometry.
A number of techniques can be used to prevent subsidiary beams. For instance, an anti-reflection coating can be used on the rear surface; a cube beam splitter can be used instead of a plate beam splitter; or a pellicle beam splitter can be utilized, because they are so thin that the two reflected beams are overlapped and appear as one.
About Oriel Instruments
Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.
Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.
This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.
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