Optical Filter Solutions for Satcom Systems

The modern world today is characterized by lightning-fast access to information and ubiquitous connectivity. Today, in the “Communications Age,” we are connected virtually anytime, and almost everywhere. 

However, despite the massive strides that have been made in the past 50 years – right from hardline telephone to today’s ubiquitous connectivity via wireless “smart” devices – future communications technology is still evolving every day. This constant evolution naturally brings with it the need to extend the communications reach even further. 

While today there exists an unparalleled large physical infrastructure of wireline fiber-optic networks and wireless cellular base stations, future advances in communications such as 5G and machine-to-machine communications will require “help from above.”

The new networks will be needed to cover every remote corner of the earth with high speed, ultra-low latency networks that are, most importantly, secure networks. In other words, in the future, telecom will meet Satcom.

Over the last decade alone, internet traffic has grown by orders of magnitude¹. This rapid growth is only expected to continue over time and will be influenced by a massive increase in mobile internet and the Internet of Things (IoT). The latter will include the adoption of autonomous vehicles in the near future. Thus, all this growth will need to be supported by high speed, high bandwidth 5G networks. 

Today, there are 3x more devices tethered to the internet than there are humans on the planet. Moreover, this ratio is rapidly accelerating exponentially. What’s more, despite the developed world’s perception that everybody is “online,” there are still roughly 4 billion people in the developing world who do not have access to the internet. The lack of global connectivity can be due in part to the non-existence or inability to deploy the terrestrial fiber-optic backbone. Thus, the world’s communications infrastructure needs to evolve, both to support the increasing use and rising demand for low latency services such as video and machine-to-machine interactions and to provide access to commmunications in currently under or unserviced areas.

From the time they were first envisioned by Arthur C. Clarke in 1945 (in an article in Wireless World)² and then realized in 1962 by Telesat with Telstar1³, satellites have been an integral part of our communications networks, transforming the distribution of TV and some long haul telephony.  However, the high costs and low availability of satellites and launches, as well as the limitations (speed, security, and latency) of the space based communication technologies has precluded a reliance on a space-based communications infrastructure.

Many of these barriers have begun to disappear recently, which corresponds to the rising demand for such systems. Today, satellite launches by both space agencies and commercial launchers have become so commonplace that they do not even warrant a mention in the news.

To illustrate, in the first ten months of 2019, there were 43 launches, consisting of 218 unique satellites that only addressed the needs in communication and earth observation⁴. In the remainder of 2019, another 18 launches consisting of 300 satellites for communication and EO that were scheduled. These days, space is more accessible than ever before.

The provision of internet services from space was not previously feasible because most communications satellites were launched in geosynchronous orbits (GEO). This was done to provide broad global coverage using a limited number of satellites. 

Operating at over 35000 km above the earth’s surface provides broad geographical coverage and naturally results in a high ground-to-satellite latency – in this case, of hundreds of ms (theoretically no less than ~120 ms). 

In contrast, satellites that fly in low earth orbit (LEO) at ~1000 km above the earth’s surface can be expected to achieve a network latency of only tens of ms (Telesat LEO claims 30-50 ms; LeoSat claims direct ground-to-sat latency <20 ms). 

This minimal latency achievable by these LEO networks is far superior to even terrestrial fiber networks. For instance, LeoSat compares a terrestrial fiber latency from London to Singapore of <180 ms with <120 ms expected to be achieved by their eventual LEO satellite network.

However, to provide global coverage from LEO, it is necessary to have a large constellation of satellites connected in an orbital mesh network. The different network architectures can range from 60-100 satellites up to several thousand satellites.

SpaceX’s Starlink network alone has proposed 11000+ individual satellites^5,6. This number ultimately depends on the coverage and mandate of the networks planned.

When it comes to latency and redundancy, LEO systems provide an obvious advantage. However, these systems require a massive satellite-to-satellite communications infrastructure.

The need for infrastructure is due to the satellite’s ability to provide broad geographic coverage, which demands the large numbers of interconnected satellites that make up these constellations. Such a space-based infrastructure will need to be comprised of inter-satellite links (ISL), from a few hundred to a thousand. 

While the traditional satellite communication was based on microwave or radiofrequency (RF) bands, there exist a number of advantages for optical “laser-based” communication links operating at frequencies of ~10⁵ times that of RF^7,8.

These advantages are:

• Increased data rates owing to a larger modulation bandwidth (scales with frequency) – this can range from a maximum of hundreds of Mbps with RF, to many Gbps with WDM optical
• Narrow beam divergence (scales with wavelength) – which leads to a larger intensity at the receiver, smaller antennae/optics needed. In addition, there is less opportunity for inter-signal interference
• More bits per Watt/Kilogram (critical for the cost of launch) – resulting in increased data rates combined with smaller, lighter hardware
• More secure communications – narrow beam divergence makes data interception extremely difficult (because data travels point to point). Moreover, there is also the possibility for encryption using Quantum Key Distribution (QKD)
• Unregulated operating spectrum – this is due to the lack of interference between signals

Therefore, in a constellation that is connected by optical inter-satellite links (OISL), the data can be uplinked and routed around the globe literally at the speed of light. Moreover, this process can occur in a completely secure network before the data is downlinked to a terrestrial terminal at the intended final destination.

Such an exponential growth in extra-terrestrial communication constellations has laid the foundation for suppliers of systems and components to provide the specialized necessary “space-ready” hardware. 

While organizations and governmental agencies with a legacy of developing and deploying satellite-borne systems are taking an active role in this industry, there are also numerous new firms looking to participate in this new economy.

Globally, there are many conferences dedicated to addressing Satcom (and optical Satcom specifically) such as the International Communications Satellite Systems Conference (ICSSC).

More recently, the “Optical Satcom Consortium” (OSC) was formed, which brings together Canadian companies and academics, along with the National Research Council of Canada (NRC) and Satellite Canada Innovation Network (SatCan). This consortium was formed to develop next-gen Satcom photonics, in an effort to improve satellite-to-satellite and ground-to-satellite communications.

Optical filters are among the many optical components that will form a crucial element of OISL and ground-to-sat links. Like their terrestrial cousins, Satcom networks will function with multiple signal bands, each requiring wavelength-selective dichroic beam combiners and splitters. 

At the optical receivers, these optical filters will enable beam steering as well as provide “more signal, with less background.” However, there are still a few unique aspects to a satellite-based optical network, which will naturally require tailor-made filter functionality. These have driven the required improvements in the manufacture of optical filters.

In the case of Satcom, relatively large (i.e., one to two-inch) optics are required to catch and collect the free-space beams transmitted over relatively long distances, unlike in fiber-optic networks.

As previously described, the focused aspect of optical sat-to-sat communication makes it difficult to intercept the signal. However, this implies that the receiving optics need to be large enough to be forgiving for long-distance misalignment. 

In addition, solar rejection windows (SRW) are commonly used components, ranging to several inches in diameter, which allows only signal band wavelengths into the satellite while blocking the solar radiation to maximize the signal to noise ratio at the receiver and to keep everything inside the satellite's optical system relatively cool.

These large filters necessitate highly uniform coating performance. This is easily achieved for simple mirrors and anti-reflection coatings but is much more of a challenge when it comes to the precise wavelength-selective edge or band pass filters. 

Such uniformity is not only required to preserve spectral performance across the clear aperture but also to ensure minimized distortions in coating and substrate-induced contributions to the transmitted wave front.

The current “standard” filters specs such as ripple, transmission and rejection levels, and surface quality also need to be met to accommodate for all large Satcom filters. Further, occasionally there are some unusual requirements that need to be considered in filter design and manufacture⁹.

These include polarization preservation requirements associated with QKD encryption – a process that differs subtly, yet importantly, from polarization-dependent loss (PDL – a common spec for telecom filters).

Moreover, in addition to different functional requirements, the satellite-based optical filters are required to be designed and manufactured in such a way that they can withstand the demands of survival outside of the protective blanket of the earth’s atmosphere. 

In other words, the satellites need to survive radiation – i.e., solar, gamma, and proton – as well as vast variances in temperature – i.e., as a result of both sun-facing and complete darkness.

None of us, including optical designers of Satcom systems, know what is not known. Given the fact that space design has so far been risk-averse, the requirements for filter performance and demonstrated reliability are occasionally highly demanding, often even unnecessarily so.

Thus, this type of “spec creep” can potentially cause an undue increase in costs as well as lead-times in the design, manufacture, and testing of optical filters in order to meet these “space standards.”

Iridian Spectral Technologies is a company that is playing an active role in the provision of optical filter solutions for these Satcom systems. The company leverages its deep industry experience of over two decades in the development and manufacture of optical filters for terrestrial networks.

Additionally, the company has recently forayed into the new Satcom space by providing wavelength selective optical filters for use in space-based Earth observation (EO) imaging. 

It is important to note that whether the communication signals are carried wireline on earth or satellite-to-satellite in LEO, what remains the same are the fundamental needs of optical filters to optimize the signal-to-noise ratios by providing wavelength selectivity.

These are just altered slightly in form. As a result, it is necessary that the learnings and expertise from telecom can and should inform the development of Satcom optical filters.

References and Further Reading

1. https://en.wikipedia.org/wiki/Internet_traffic
2. https://web.archive.org/web/20071225093216/http://www.lsi.usp.br/~rbianchi/clarke/ACC.ETRelaysFull.html
3. https://www.telesat.com/about-us/why-satellite/brief-history
4. https://spaceflightnow.com/launch-log/
5. http://satellitemarkets.com/news-analysis/next-wave-low-earth-orbit-constellations
6. https://www.geekwire.com/2019/amazon-project-kuiper-broadband-satellite/
7. https://pdfs.semanticscholar.org/3187/3c8f71334857db18f7da61066ab272435ded.pdf
8. https://www.ijraset.com/fileserve.php?FID=7815)
9. https://www.nature.com/articles/s41534-017-0031-5

This information has been sourced, reviewed and adapted from materials provided by Iridian Spectral Technologies.

For more information on this source, please visit Iridian Spectral Technologies.