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Advances in optical prisms are being used to help researchers gain a greater understanding of space and astronomy

Prisms are an established part of numerous photonic technologies, having been in existence for a very long time. 

One of the first documented uses of these solid glass optics dates back to the 1660s, when Sir Isaac Newton conducted a series of experiments with sunlight, in which he identified the ROYGBIV colours (red, orange, yellow, green, blue, indigo and violet) that make up the visible spectrum of clear white light. 

Types of prisms include dispersive prisms, which are used to break up light into its constituent spectral colours; reflective prisms, which are used to reflect light, in order to flip, invert, rotate, deviate or displace it; and beam splitting prisms, in which thin-film optical layers are deposited on the hypotenuse of one right-angled prism, and cemented to another to form a beam-splitter cube. Use cases for these types of prisms include imaging applications in the fields of spectroscopy, telecommunications, surveillance and astronomy, to name just a few. 

It is the latter category in which advances in optical prisms have been used to provide a greater understanding of space. A team of researchers from the Xi’an Institute of Optics and Precision Mechanics (XIOPM) of the Chinese Academy of Sciences recently designed a rigid-flexible, dual-mode coupling support structure for space-based, rectangular curved prisms (SRCPs). It is hoped this will help to obtain stable and reliable optical components for space remote sensors. The results of the work were published in Applied Optics. 

Ahead with the curve 

The new structure of the curved prism includes a mirror frame, two rigid adhesive structures, two flexible adhesive structures, four fixing clamps, four positioning pads, the adjustment pad and a precision measuring mirror. 

The team performed in-depth studies of the support principle and engineering realisation of the SRCPs and optimisation of the flexible adhesive structure. Static and dynamic simulations were conducted on the mirror subassembly using finite element analysis. The tests revealed that the surface shape error of the mirror subassembly after mechanical testing was 0.021", the displacement of the mirror body was 0.008mm, the inclination angle was equal to 0.8", the mass of the mirror subassembly was 4.79kg, the fundamental frequency was 283Hz and the maximum amplification of the total rms acceleration was 4.37. 

The team said that indexes were superior to those of the design requirements, and so bonding tests and mechanical tests of a rectangular curved prism reflector, a rectangular curved prism, and a rectangular plane reflector employing this proposed support structure were continued. The test results verified the reliability, stability and universal applicability of the proposed rigid–flexible, dual-mode peripheral bonding support structure. Researchers believe the structure can withstand the mechanical environment of vibration and shock when the satellite platform is launched. 

Meanwhile, in the US, researchers at the Rensselaer Polytechnic Institute have used prisms to help develop holographic lenses that render visible and infrared starlight into either a focused image or a spectrum. The method could be used to create a large yet lightweight flexible lens that could be rolled for launch and unfurled in space. The research recently appeared in Nature Scientific Reports. 

Telescopes that must be launched into space can be limited by the weight and bulk of glass mirrors used to focus light, which can realistically span only a few metres in diameter. A lightweight flexible holographic lens, known as a holographic optical element, used to focus light could be dozens of metres across. An instrument like this could be used to directly observe an exoplanet in a leap over current methods that detect exoplanets based on their effect on light coming from the star they orbit, according to Heidi Jo Newberg, a Rensselaer professor of physics, applied physics and astronomy. ‘To find Earth 2.0, we really want to see exoplanets by direct imaging – we need to be able to look at the star and see the planet separate from the star. And for that, we need high resolution and a really big telescope,’ she said. 

Ringing true 

The holographic optical element is a refined version of a Fresnel lens, a category of lenses that use concentric rings of prisms arrayed in a flat plane to mimic the focusing ability of a curved lens without the bulk. The concept of the Fresnel lens was developed for use in lighthouses and dates back to the 19th century.

Modern-day Fresnel lenses of glass or plastic can be found in automobile lamps, micro-optics and camera screens. 

But while Fresnel holographic optical elements, created by exposing a light-sensitive plastic film to two sources of light at different distances from the film, are relatively common, existing methods have been limited to lenses that could only focus light, rather than separating it into its constituent colours. The new method allows the designers to either focus light onto a single point, or disperse it into its constituent colours, producing a spectrum of pure colours. It uses two sources of light, positioned very close to one another, which create concentric waves of light that, as they travel towards the film, either build or cancel each other out. This pattern of convergence or interference can be tuned based on the formulas developed by Mei-Li Hsieh, a visiting researcher at Rensselaer Polytechnic Institute. It is printed, or ‘recorded’ onto the film as a holographic image and, depending on how the image is structured, light passing through the holographic optical element is either focused or stretched. 

Shawn-Yu Lin, another member of the Rensselaer team and an expert in photonic crystals and nano-photonics, said: ‘We wanted to stretch the light, so that we could separate it into different wavelengths. Any Fresnel lens will stretch the light a little, but not enough. With our method, we can have super resolution on one end, or super sensitive, with each colour separated. When the light is stretched like that, the colour is very good, as pure and as vivid as you can get.’

Further information 

Xin-Yin Jia, Fei-Cheng Wang, Li-Bo Li, Zhao-Hui Zhang, Jia Liu, and Bing-Liang Hu, “Design analysis and test verification of a rigid–flexible, dual-mode coupling support structure for space-based rectangular curved prisms,” Appl. Opt. 60, 7563-7573 (2021) https://www.osapublishing.org/ao/abstract.cfm?URI=ao-60-25-7563 

Hsieh, ML., Ditto, T.D., Lee, YW. et al. Experimental realization of a Fresnel hologram as a super spectral resolution optical element. Sci Rep 11, 20764 (2021). https://doi.org/10.1038/ s41598-021-99955-w

Commercial products

Vendors that offer optical prism solutions include Edmund Optics, which offers a variety of designs, substrates and coating options. Designs include right-angle, amici, penta, schmidt, wedge, anamorphic, equilateral, dove, or rhomboid prisms, in addition to corner cube retroreflectors or light pipe homogenising rods. Antireflection coatings include MgF2, UV-VIS, UV-AR, VIS 0°, VIS-NIR, or multiple laser line options. 

Prisms available from Taiyo Optics include right-angle, penta, beamsplitter penta, corner cube reflector, porro, dove, rhomboid, brewster prism, and many more, including customised options. 

Precision Optical’s core product is the ultra-precise prism and the company manufactures numerous shapes, from standard prism shapes to more intricate custom geometries. It can produce sizes from 3 to 300mm, with or without custom thin film coatings, as individual components or as multicomponent prism assemblies. 

CeNing manufactures a range of precision prisms, including right-angle prisms, wedge prisms, dove prisms, penta prisms, corner retroreflectors and rhomboid prisms. These are also offered in custom design. 

Available from Sydor Optics are custom optical wedges that can be used in a variety of applications. Flat optics with a wedge angle are typically used in laser applications for beam steering and beam displacement, or for preventing stray-back reflections, and can also be used in optical components to change the direction of light. A. Optical supplies a range of inverting, rotating, deviating, displacing or dispersing prisms. 

FOCtek provides a range of high-precision prisms, including penta prism, beamsplitter penta prism, right-angle prism, and corner cube. The micro penta prism and right-angle prism are most widely used in optical communications, such as optical switches. 

Kingsview Optical can manufacture optical prisms for a range of requirements, from 0.8mm diameter micro prisms up to 100mm periscope prisms.

Featured product: CeNing Optics

CeNing, established in 2004, manufactures a wide range of precision prisms, including right-angle prisms, wedge prisms, dove prisms, penta prisms, corner retroreflectors and rhomboid prisms. These prisms are offered in custom design with reasonable cost. 

CeNing’s prisms are able to achieve very high flatness and angular accuracy: L/20 flatness and 2 arcsec tolerance. The surface quality can be up to 10-5 S/D. We have an advanced test device to guarantee the specifications. Additional coatings, like anti-reflective and high reflective coatings, can be applied upon request. CeNing offers precision optical prisms, from prototype to volume production, with short lead times. In addition to prisms, CeNing manufactures a wide range of optics, including lenses, mirrors, filters, beamsplitters and waveplates. 

Further information www.cn-optics.com/products/Prisms.asp

Featured product: Manx Precision Optics

 

Laser systems are growing increasingly complex and laser intensities are becoming higher than ever before. With this, conventional quartz waveplates are beginning to approach their limits. This is because the waveplates are limited by the LIDT of the intrinsic material and the physical size of naturally occurring quartz. There are also limits to waveplate polishing, leading to imperfections on the surface, causing inhomogeneities in the phase across the diameter of the beam. 

Hybrid dielectric-metal coatings specifically designed for polarisation control provide an alternative coating approach. Advantages of this method include not being limited to the size of naturally occurring material and high LIDT due to the reflection from dielectric layers. Along with this, the coating can be specifically tailored to the application through the coating design process, also allowing the manufacturing of broadband phase shifting mirrors. 

Manx Precision Optics offers a wide range of optical components for ultrafast and high-power lasers. Its phase shifting mirrors are capable of good performance over broad bandwidths, with broadband reflectivity over 99 per cent, low group delay dispersion and broad characteristic phase delay. 

Further information www.mpo.im/products 

 

Robert Smith is a professor at the University of Alberta and historian to JWST since 2002

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