NASA’s Laser Communications Relay Demonstration (LCRD) will send data to and from ground stations and, eventually, in-space user missions over laser links.

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While industry is focused on in-space optical communications to support terrestrial users, NASA is demonstrating direct-to-Earth capability from geosynchronous orbit to increase communications capabilities for future missions. With laser communications onboard, missions will be able to communicate more data in a single transmission than they could with traditional radio frequency communications.

An ideal BBARTo create a coating that operates over the broadest possible wavelength band, some simple design principles apply. If there were a substrate of index 4.0 such as germanium in the infrared spectral band, the square root equation indicates that the best QWOT AR coating layer would have an index of 2.0. And if two layers are used, their indices and thickness can be further optimized to give a broader AR band (see Fig. 3). This procedure can be extended to three, four, and five or more layers by following a step-down-index-vs.-thickness refractive index profile.

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While LCRD’s experimenter program will allow NASA and industry to test and refine techniques, both the agency and the commercial sector have been demonstrating and using laser communications for the past few decades.

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This high and low index combination is commonly referred to as a "V-Coat" because it has only a narrow bandwidth near 0% reflectance, with the curve approximating a "V" shape. Another option for more broadband antireflection is a three-layer broadband AR (BBAR) coating on an NBK7 substrate. This coating has a QWOT of medium index (1.65), two QWOTs of high index (2.1), and one QWOT of 1.38, and is sometimes referred to as a QHQ or MHL design.

For a very broadband AR coating, the ideal profile of the index-of-refraction-vs.-thickness from a substrate to the media (usually air or vacuum) is a Gaussian-like curve starting from the index of the substrate and going to the index of the medium over a thickness/distance of approximately one half-wave optical thickness at the longest wavelength in the antireflection band. This can be approximated by simpler coatings of homogeneous index for narrower AR bands, and by lithographically etched substrates for some materials.

Basically, the reflectance will be zero at the wavelength at which the path delay between the front and rear reflections in the coating is 180° (or a multiple of that), representing one quarter-wave optical thickness (QWOT) at such wavelengths. At 510 nm, for example, this condition is satisfied on a substrate with refractive index 1.52 by a coating with a refractive index of 1.233, which is the square root of 1.52 (see Fig. 1).

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“Free-space” refers to the absence of the insulated, fiber optic cables that enable the terrestrial internet. Free-space laser communications flow freely through the vacuum of space, however atmosphere poses unique challenges to communications engineers.

Generally, commercial efforts have been focused on developing space-to-space laser systems for use in low-Earth orbit. Companies are investing in satellite constellations which leverage laser communications to provide global broadband coverage. Proposed constellations have several hundred to thousands of satellites, creating an extensive in-space laser communications network. Commercial constellations still largely rely on radio frequency links to send data back to Earth. Whereas LCRD uses lasers for both in-space and direct-to-Earth communications.

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Ron Willey has more than 50 years experience as individual contributor and director of optical system and optical coating development, design, and production. He eaches short courses internationally, and is author of many technical papers and five books. Mr. Willey graduated from the Massachusetts Institute of Technology, having majored in optical instrumentation. He also earned a masters degree in computer science from the Florida Institute of Technology. He has taught optics at the Florida Institute of Technology, the University of Wisconsin, and Martin Marietta Corporation. He has taught optical thin film design, development, and production at SPIE, the Society of Vacuum Coaters, Florida Institute of Technology, and the University of Wisconsin.

LCRD is led by Goddard and in partnership with JPL and the MIT Lincoln Laboratory. LCRD is funded through NASA’s Technology Demonstration Missions program, part of the Space Technology Mission Directorate, and the Space Communications and Navigation (SCaN) program at NASA Headquarters in Washington, D.C.

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An uncoated surface of crown glass, such as Schott's NBK7 with index of refraction around 1.52 at 550 nm, will reflect about 4.26% of incident light across the visible spectrum. In camera and microscope lenses, this reflection will unfortunately cause ghost images and lost transmittance of the wanted image flux. To prevent such reflections, the concept of the single-layer AR coating was patented in the 1930s by Smakula, and John Strong reported a single-layer AR (SLAR) coating in 1936.1, 2

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It is practical to overcome the index-of-refraction limitations for a laser AR coating at one wavelength (over a narrow bandwidth) by using two materials of high and low index. In essence, a thin layer (non-QWOT) of appropriate thickness of high index (such as 2.3) material is first deposited on the substrate, which makes the combination of the substrate with the new layer act more like the 1.9 index glass substrate above so that a layer of index 1.38 will then be more nearly the ideal AR index for that combination (at the design wavelength).

With these two different yet complementary goals, NASA also is partnering with industry to further refine laser communications hardware. In fact, LCRD includes commercially designed and built components alongside custom, NASA-developed systems. The optical module, which sends lasers to and from the payload, was designed by the Massachusetts Institute of Technology Lincoln Laboratory and built at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. However, multiple parts of LCRD came from companies like L3Harris Technologies, SEAKR Engineering, Moog Inc., and the Sierra Nevada Corporation. These include the telescope system, controller electronics system, and space switching unit – all critical to LCRD operations.

LCRD’s mission will be spent proving out the technology by testing laser communications capabilities with experiments from NASA, other government agencies, academia, and – in particular – the commercial aerospace community. Industry-developed experiments will allow companies to test their own technologies, software, and capabilities. NASA is providing these opportunities to grow the body of knowledge surrounding laser communications and promote its operational use.

3. R. R. Willey, Practical Design of Optical Thin Films, Fourth Edition, Sec. 2.10, Willey Optical, Consultants, Charlevoix, MI (2014).

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The microscope will shine excitation light on the sample, but the objective will filter the light going into the eyepiece to only pass through ...

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Katherine Schauer is a writer for the Space Communications and Navigation (SCaN) program office and covers emerging technologies, commercialization efforts, exploration activities, and more.

Finally, in some special cases where the AR band required is for only one wavelength (as in a laser), single- or double-layer AR coatings can work quite well.

With a refractive index of approximately 1.38, magnesium fluoride (MgF2) results in a reflection of around 1.26% (not 0%) for the previously mentioned NBK7 substrate. Another case producing 0% reflection at the design (QWOT) wavelength is where the substrate is of index 1.9 and the coating is of index 1.38, like MgF2. The reflection of this 1.9 index glass when uncoated would be 9.63%, but the SLAR coating of MgF2 reduces the reflection to 0% at the design wavelength of 550 nm in this case.

NASA and commercial aerospace companies are applying similar technologies to space communications, bringing optical speeds to the final frontier. Free-space optical communications leverages recent advancements in telecommunications to allow spacecraft to send high-resolution images and videos over laser links.

NASA is committed to fostering the growing space economy and the commercial aerospace community to encourage technological innovation/ bring down costs for all users. This dedication extends to new technologies and capabilities like laser communications systems.

Banner image: Illustration of STPSat-6 with the Laser Communications Relay Demonstration (LCRD) payload communicating data over infrared links. Credit: NASA’s Goddard Space Flight Center

In the extreme case of an infinite number of layers, the index profile would have a Gaussian shape. The overall thickness of this ideal AR coating is slightly over two QWOTs at the longest wavelength or lowest frequency in the pass band (300 cm-1). The AR band of this ideal AR coating would extend from that longest wavelength to ALL shorter wavelengths!

Antireflection (AR) coatings, the most common optical coatings used in the world, range from single-wavelength operation (for narrowband lasers) to coatings functioning over very broad spectral bands such as 380–1550 nm or from 3 to 12 μm, for example.

This MgF2 SLAR coating could be an adequate AR coating for a laser at wavelength 550 nm, or the thickness of the layer could be adjusted to be a QWOT at some other laser wavelength as needed.

1. O. Smakula, "Verfahren zur Erhoehung der Lichtdurchlaessigkeit optischer Teile durch Erniedrigungdes Brechungsexponenten an den Grenzflaechen dieser optischen Teile," German patent DE685767 (C) (1935).

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Whether fabricated with a single layer or even tens of layers, the basic characteristics of ideal AR coatings and their real-world approximations can be described in terms of reflectance and transmittance vs. wavelength. Reflectance vs. thickness and index of refraction vs. thickness are just some of the presentations that help to illustrate both the possibilities and limitations of modern-day AR coating technology.

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With the rapid advance of lithographic techniques and the materials that can be etched, it is now possible to make mechanical surfaces whose textures/shapes create the index-vs.-thickness profiles on a microscopic scale that approximate those needed for very BBAR coatings. The individual features of such an etched surface must be less in height and width than the shortest wavelength in the desired AR band.

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“The Earth’s atmosphere distorts laser beams due to turbulence. Understanding these challenges are critical to enabling operational optical communications relay capability,” said Jason Mitchell, Director of the Advanced Communications and Navigation Technology division in the Space Communications and Navigation (SCaN) program at NASA Headquarters.

The primary underlying principle of AR coatings is that the reflection of light from the outside surface of a single coating layer interferes with the reflection from the interface between the coating layer and the substrate. The reflection can be eliminated (made to be zero) if the refractive index of the coating material (n2) is equal to the square root (SQRT) of the product of the indices of the substrate (n3) and the surrounding medium (n1, air or vacuum). That is, n2 = SQRT(n1*n3).

The U.S. Department of Defense’s Space Test Program Satellite-6 (STPSat-6), part of the Space Test Program 3 (STP-3) mission, will launch on a United Launch Alliance Atlas V 551 rocket from the Cape Canaveral Space Force Station in Florida no earlier than December 5, 2021. STP is managed by the United States Space Force’s Space Systems Command.

Additionally, NASA’s Low-Cost Optical Terminal (LCOT) will use commercial off-the-shelf or slightly modified hardware to reduce expense and speed implementation. NASA depends on a robust domestic space industry and hopes these investments will allow the U.S. government to purchase future laser communications hardware and services. This will decrease costs while enabling more missions to use laser communications.