Coherent technology can give free-space optical systems greater reach and capacity, enabling these new applications in space and terrestrial links.

Coherent Free Space Optics for Ground and Space Applications

Coherent Free Space Optics for Ground and Space Applications

In a previous article, we described how free-space optics (FSO) could impact mobile fronthaul and enterprise links. They can deliver a wireless access solution that can be deployed quickly, with more bandwidth capacity, security features, and less power consumption than traditional point-to-point microwave links.

Figure 1: Free space optical system diagram

However, there’s potential to do even more. There are network applications on the ground that require very high bandwidths in the range of 100 Gbps and space applications that need powerful transceivers to deliver messages across vast distances. Microwaves are struggling to deliver all the requirements for these use cases.

By merging the coherent technology in fiber optical communications with FSO systems, they can achieve greater reach and capacity than before, enabling these new applications in space and terrestrial links.

Reaching 100G on the Ground for Access Networks

Thanks to advances in adaptive optics, fast steering mirrors, and digital signal processors, FSO links can now handle Gbps-capacity links over several kilometers. For example, a collaboration between Dutch FSO startup Aircision and research organization TNO demonstrated in 2021 that their FSO systems could reliably transmit 10 Gbps over 2.5 km.

However, new communication technologies emerge daily, and our digital society keeps evolving and demanding more data. This need for progress has motivated more research and development into increasing the capacity of FSO links to 100 Gbps, providing a new short-reach solution for access networks.

One such initiative came from the collaboration of Norweigan optical network solutions provider Smartoptics, Swedish research institute RISE Acreo, and optical wireless link provider Norwegian Polewall. In a trial set-up at Acreo’s research facilities, Smartoptics’ 100G transponder was used with CFP transceivers to create a 100 Gbps DWDM signal transmitted through the air using Polewall’s optical wireless technology. Their system is estimated to reach 250 meters in the worst possible weather conditions.

Fredrik Larsson, the Optical Transmission Specialist at Smartoptics, explains the importance of this trial:

“Smartoptics is generally recognized as offering a very flexible platform for optical networking, with applications for all types of scenarios. 100Gbps connectivity through the air has not been demonstrated before this trial, at least not with commercially available products. We are proud to be part of that milestone together with Acreo and Polewall,”

Meanwhile, Aircision aims to develop a 100 Gbps coherent FSO system capable of transmitting up to 10km. To achieve this, they have partnered up with EFFECT Photonics, who will take charge of developing coherent modules that can go into Aircision’s future 100G system.

In many ways, the basic technologies to build these coherent FSO systems have been available for some time. However, they included high-power 100G lasers and transceivers originally intended for premium long-reach applications. The high price, footprint, and power consumption of these devices prevented the development of more affordable and lighter FSO systems for the larger access network market.

However, the advances in integration and miniaturization of coherent technology have opened up new possibilities for FSO links. For example, 100ZR transceiver standards enable a new generation of low-cost, low-power coherent pluggables that can be easily integrated into FSO systems. Meanwhile, companies like Aircision are working hard in using technologies such as adaptive optics and fast-steering mirrors to extend the reach of these 100G FSO systems into the kilometer range.

Coherent Optical Technology in Space

Currently, most space missions use radio frequency communications to send data to and from spacecraft. While radio waves have a proven track record of success in space missions, generating and collecting more mission data requires enhanced communications capabilities.

Coherent optical communications can increase link capacities to spacecraft and satellites by 10 to 100 times that of radio frequency systems. Additionally, optical transceivers can lower the size, weight, and power (SWAP) specifications of satellite communication systems. Less weight and size means a less expensive launch or perhaps room for more scientific instruments. Less power consumption means less drain on the spacecraft’s energy sources.

Space military satellite, a weapon in space shoots a laser against the background of the earth. Attack, technology, space war. mixed medium, copy space. image furnished by NASA
Figure 2: Space satellite with a laser against the background of the earth.

For example, the Laser Communications Relay Demonstration (LCRD) from NASA, launched in December 2021, aims to showcase the unique capabilities of optical communications. Future missions in space will send data to the LCRD, which then relays the data down to ground stations on Earth. The LCRD will forward this data at rates of 1.2 Gigabits per second over optical links, allowing more high-resolution experiment data to be transmitted back to Earth. LCRD is a technology demonstration expected to pave the way for more widespread use of optical communications in space.

Making Coherent Technology Live in Space

Integrated photonics can boost space communications by lowering the payload. Still, it must overcome the obstacles of a harsh space environment, with radiation hardness, an extreme operational temperature range, and vacuum conditions.

The James Webb Telescope. Space Observatory for the Study of the Universe and exploration of deep space. Elements of this image furnished by NASA
Figure 3: The James Webb Telescope. Space Observatory for the Study of the Universe and exploration of deep space

For example, the Laser Communications Relay Demonstration (LCRD) from NASA, launched in December 2021, aims to showcase the unique capabilities of optical communications. Future missions in space will send data to the LCRD, which then relays the data down to ground stations on Earth. The LCRD will forward this data at rates of 1.2 Gigabits per second over optical links, allowing more high-resolution experiment data to be transmitted back to Earth. LCRD is a technology demonstration expected to pave the way for more widespread use of optical communications in space.

Mission TypeConditions
Pressurized Module+18.3 ºC to 26.7 °C
Low-Earth Orbit (LEO)-65 ºC to +125°C
Geosynchronous Equatorial Orbit (GEO)-196 ºC to +128 °C
Trans-Atmospheric Vehicle-200 ºC to +260 ºC
Lunar Surface-171 ºC to +111 ºC
Martian Surface-143 ºC to +27 ºC
Table 1.  Temperature Extremes by Mission Type. Source: NASA

The values in Table 1 are unmanaged environmental temperatures and would decrease significantly for electronics and optics systems in a temperature-managed area, perhaps by as much as half.

A substantial body of knowledge exists to make integrated photonics compatible with space environments. After all, photonic integrated circuits (PICs) use similar materials to their electronic counterparts, which have already been space qualified in many implementations.

Much research has gone into overcoming the challenges of packaging PICs with electronics and optical fibers for these space environments, which must include hermetic seals and avoid epoxies.  Commercial solutions, such as those offered by PHIX Photonics Assembly, Technobis IPS, and the PIXAPP Photonic Packaging Pilot Line, are now available.

Takeaways

Whenever you want to send data from point A to B, photonics is usually the most efficient way of doing it, be it over a fiber or free space.

This is why EFFECT Photonics sees future opportunities in the free-space optical (FSO) communications sectors. In mobile access networks or satellite link applications, FSO can provide solutions with more bandwidth capacity, security features, and less power consumption than traditional point-to-point microwave links.

These FSO systems can be further boosted by using coherent optical transmission similar to the one used in fiber optics. Offering these systems in a small package that can resist the required environmental conditions will significantly benefit the access network and space sectors.

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