Feature: Optical communications


in most of the transmitted energy never reaching the receiver. Tis is especially true in ground-to-satellite and satellite- to-satellite communications, where signals must travel thousands of miles to reach the intended target. To remedy this, the industry is embracing FSO communictions, a high-energy line- of-sight technology that eclipses the capabilities of RF alternatives.


FSO communication Free space optical communication technologies use infrared or optical lasers to encode vast amounts of information into high-frequency carrier waves. Also, lasers are inherently more focusable than traditional RF signals – meaning a greater proportion of the transmitted energy reaches the receiver – resulting in greater energy efficiency, bandwidth and data transfer speeds. Low laser divergence angles also make FSO communications harder to intercept than RF signals – requiring any rogue recipient to be located within the extremely narrow beam cross section – making it ideal for sensitive applications requiring additional security.


An issue of alignment Unfortunately, the benefits offered by FSO communications are somewhat offset by the need for perfect alignment of the transmitters and receivers, no mean feat considering the astronomical distances involved in ground-to-satellite and satellite-to-satellite communication. Another fly in the ointment is that any signal transmitted through air is susceptible to random phase and amplitude fluctuations from atmospheric turbulence, as well as severe signal attenuation caused by cloud cover, fog and dust storms. Furthermore, FSO in general is also extremely sensitive to beam driſt from building sway, vibration, thermal expansion of components and subpar tracking systems. Fast response tip/tilt mirror systems


– or fast steering mirrors (FSMs) – can compensate for these disturbances by keeping lasers stabilised and precisely on target. Piezo-based positioning


The industry is embracing free space optical communications, a high-energy line-of-sight technology that eclipses the capabilities of RF alternatives


technologies generally have faster response times than equivalent galvo- scanners and voice-coil driven systems, making them well suited to such highly- dynamic applications. Tese instruments have already been launched into space on the Solar Orbiter – a satellite space probe developed jointly by the European Space Agency and NASA – where they function at extreme temperatures in high vacuum. Electromagnetic drives – although


exhibiting slower response times than piezo-based alternatives – can also be used for FSO communications, and are perfect for applications requiring large steering angles. State-of-the- art multi-axis solutions, like those engineered by Physik Instrumente, can integrate pitch, roll and piston motion into one device with a single mirror – which is important in many precision-optics applications, as it allows laser polarisation direction to remain unchanged – with sub-millisecond response times, nano-radian range resolution and deflection angles of up to three degrees.


Moving the frontier Communications technologies like FSO will play a vital role in the exploration of the vast boundaries of our solar system, and beyond. In fact, NASA requires better bandwidth to support missions in space. When referring to the exploration of Mars, a NASA official said: “Future human and robotic expeditions into deep space must count on the fastest, most-efficient means of communicating with mission managers on Earth. A steady stream of high-definition imagery, live video feeds and real-time data


transmission across the gulf of space will be required to enable timely guidance and mission-critical updates during the long- duration journeys to the far reaches of the solar system.” Laser communications up to 100 times


faster than current rates are targeted by NASA, without increasing size or power consumption. However, even though lasers can carry significantly more information than RF signals, their propagation speed is still bound by the same physical laws, meaning that real- time communication to the edges of the universe is, in fact, a relative term. It is likely that onboard artificial intelligence will be required to make mission- critical decisions, complementing laser communications in discovering new frontiers.


A fibreless future for Earth? Fibre-optic communication networks will likely remain the go-to method for most ground-based applications for some time. However, FSO communication holds the power to enable fast, secure and efficient transfer of data between any two points in line-of-sight, whether in ground-to- ground, ground-to-satellite or satellite- to-satellite transmission. FSMs are essential to ensure accurate


laser positioning, with electromagnetic and piezo-driven systems capable of rapidly compensating for vibration, thermal drift and atmospheric disturbances. The importance of FSO is only likely to grow as the Internet of Things continues to gather steam, and the need for lightning-quick data transfer speeds will nudge us slowly but surely toward an increasingly fibreless future.


www.electronicsworld.co.uk July/August 2023 29


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