Fast Steering Mirrors: Enabling Stable Laser Communication in Satellite Networks

The foundation of global communication began with the first successful undersea telegraph cable, laid across the Atlantic in 1858 between Ireland and Newfoundland. Although it failed after a few weeks, it enabled near-instant transatlantic communication and marked a major engineering milestone. Advances in materials and cable-laying techniques soon followed, paving the way for today’s global undersea fiber-optic networks that are the backbone of our current internet and telecom infrastructure.

The piezo-driven S-335 dual-axis fast steering mirror (FSM) combines large angular range, high bandwidth, and high-resolution motion in two degrees of freedom. A pushpin is shown for size comparison; beam paths are simulated. (Image: PI)

Cable-based information transport faces two major limitations: the high cost of laying cable to remote locations, and, as recent global conflicts have shown, the strategic vulnerability of undersea cables, which are both easy to disrupt and difficult to repair.

In recent years, space-based optical communication has attracted increased attention as a promising alternative to reduce reliance on physical cable infrastructure. This emerging technology addresses the limitations of traditional satellite communication via radio frequency (RF) signals and has been made feasible by advances in compact satellite design and laser communication systems.

Laser vs. RF: Why Optical Wins

Due to the limited bandwidth and poor focus of RF signals, laser-based optical communication systems have been developed to complement traditional satellite links. These systems offer significantly higher data throughput while consuming less power, a critical factor for satellites that rely solely on onboard energy sources. Additionally, optical signals provide enhanced security thanks to their narrow beam divergence, making them difficult to intercept. Their direct line-of-sight propagation also reduces the risk of interference, a common issue in the radio frequency spectrum.

Fast Steering Mirrors (FSMs)

Fast Steering Mirrors (FSMs) are high-performance opto-mechanical devices engineered for dynamic, sub-microradian control of beam direction. By enabling precise, rapid angular adjustments, FSMs play a critical role in directing optical energy and information in advanced photonics and optical systems.

A voice coil-driven, dual-axis fast steering mirror (FSM) featuring a differential, parallel-kinematic design. Compared to piezo-driven systems, voice coil FSMs offer larger tilt angles, at somewhat lower angular resolution. (Image: PI)

Originally developed for active and adaptive optics applications, such as in astronomy, where FSMs correct wave-front distortions in real time by sharpening telescope images and compensating for atmospheric turbulence, effectively “un-twinkling” starlight. FSMs enhance image sensor resolution through pixel sub-stepping, where the mirror introduces controlled micro-movements to shift the optical beam or sensor by fractions of a pixel, enabling super-resolution techniques without altering hardware.

Recently, space-based optical communication has become a new application field for FSMs. In this context, laser light is modulated to carry information without the use of optical fibers. All that is required is a clear line of sight between a transmitting station and receiving station. The advantage of this technique, known as free-space optical communication (FSO), lies in its flexibility and cost savings, as it eliminates the need for fiber-optic infrastructure. FSO is effective in both space and terrestrial applications, though its range on Earth is limited by environmental factors such as rain, fog, and dust, which can degrade performance and reliability over longer distances.

Space-Based Internet

The rapid expansion of low-Earth orbit (LEO) satellite networks — comprising thousands of nodes circling the planet — has driven demand for highly accurate, space-qualified FSMs. These precision components are critical for maintaining alignment between optical transmitters and receivers, ensuring stable, high-efficiency communication links as satellites travel at speeds of approximately 17,000 miles per hour.

FSM Operating Principle

Voice coil operating principle. In a voice coil, the generated force is proportional to the drive current. (Image: PI)

FSMs are available in single- and multi-axis configurations, with dual-axis designs being the most prevalent. A typical dual-axis FSM includes a reflective mirror, two actuators, a guiding mechanism, motion-tracking sensors for each axis, and a closed-loop controller. The controller continuously adjusts actuator voltages or currents based on real-time position sensor feedback to achieve the desired tip/tilt angles. It receives commands from a processing unit, which integrates data from optical sensors and the satellite’s attitude control system to ensure precise alignment of the optical transceiver.

Alignment and Tracking Algorithms

When an optical signal must be aligned with a target, efficient algorithms are essential — not only to locate the signal, but also to optimize it for maximum strength in conjunction with a high-speed mechanical system. This principle applies across scales, from aligning an optical fiber with a photonic chip just microns apart, to locking a laser beam onto a small receiver on a satellite hundreds of kilometers distant. Recent advancements in high-speed alignment and tracking algorithms, originally developed for fiber optic test and assembly automation, are now proving valuable for free-space optical communication as well.

Actuation Technologies Employed in FSMs

Piezo stack actuator operating principle. In a piezo actuator, the displacement is proportional to the drive voltage. (Image: PI)

To be viable for space applications, FSMs must meet the stringent SWaP-C² criteria — an acronym for size, weight, power, cost, and cooling. Most compact FSMs that meet these requirements are driven by either piezoelectric or voice coil actuators.

A voice coil actuator generates linear motion by using the Lorentz force — produced when electric current passes through a coil within a magnetic field — enabling smooth, precise, and frictionless movement. The term “voice coil” originates from its original use in loudspeakers, where a coil of wire moves within a magnetic field to produce sound by vibrating a diaphragm.

Piezoelectric-driven FSMs typically offer higher resolution and greater stiffness, enabling nanoradian-level precision without consuming power to hold position. In contrast, voice coil-driven FSMs support larger angular displacements and higher dynamic range, though they are inherently soft and require continuous power to maintain position.

The choice between piezo or voice coil actuators depends on the specific application requirements — whether high-resolution stability or wide-angle tracking is prioritized.

Recent innovations in voice coil FSMs have achieved closed-loop bandwidths up to 750 Hz and angular resolutions better than 0.1 μrad, making them increasingly competitive in both performance and responsiveness. For cost-effective implementation, microcontrollers can now be integrated at the chip or board level, further enhancing compactness and system efficiency.

Engineered for Endurance: Friction-Free, Maintenance-Free Designs

A compact, commercial off-the-shelf (COTS) fast steering mirror platform (Model S-331.2). With a mechanical resonant frequency of 10 kHz, it enables extremely fast scanning and step-and-settle performance. As early as 2020, COTS platforms, such as PI’s piezo-based S-331 FSM, demonstrated their suitability for space applications in simulation runs conducted according to NASA’s General Environmental Verification Standard (GEVS). (Image: PI)

In high-frequency applications involving millions of motion cycles, wear-free and maintenance-free designs are critical. Frictionless flexure-guided systems and wear-free actuators represent the state of the art in precision motion engineering. Flexures eliminate the need for lubrication, making them especially advantageous for use in vacuum and space environments, where traditional lubricants can outgas or degrade performance. Flexures are also robust, capable of withstanding significant shock loads.

Parallel-Kinematic Architectures with Differential Actuation

A key advantage of advanced dual-axis fast steering mirrors is their parallel-kinematics design, which employs coplanar rotational axes, and a single moving platform actuated by differential drives. This configuration not only preserves polarization orientation but also offers a significantly more compact and integrated solution compared to conventional setups using two serially mounted single-axis mirrors.

To achieve maximum angular stability regardless of temperature fluctuations, a differential actuator (push-pull) and sensor configuration is recommended. In this setup, thermal expansion or contraction of the actuators results in pure piston motion (phase shift), without altering the mirror’s angular orientation. This capability is especially vital in space environments, where extreme temperature variations are common and mechanical stability is mission critical.

A Brief Introduction to Piezo Technology

The term “piezo” originates from the Greek word for “pressure.” While the direct piezoelectric effect generates charge under stress as used in sensors and igniters, the inverse effect — shape change under electric fields — enables fast, precise, solid-state motion.

Piezoelectric actuators, with their sub-nanometer motion capabilities, are essential for advancements in semiconductor, photonics, and microscopy applications, to name a few. The piezo effect was first discovered in quartz by the Curie brothers in the 1880s, and piezoelectric materials have advanced significantly since then.

Modern, patented PICMA® multilayer piezo actuators feature ceramic encapsulation for exceptional durability and performance in extreme conditions — proven through 100 billion test cycles conducted by NASA for the Mars mission.

Beyond FSO: Expanding Applications of FSMs

A custom two-axis fast steering mirror mount developed for the Solar Orbiter satellite. (Image: PI)

Beyond FSO communication, FSMs are increasingly used in a wide range of high-speed, precision beam control applications. Their ability to rapidly and accurately direct laser beams makes them indispensable in fields where performance, responsiveness, and compact design are critical.

One emerging application is LiDAR, particularly in the autonomous vehicle sector, where FSMs are being explored for their speed, precision, and space-saving design.

In laser processing, FSMs and galvo scanners are core technologies for beam positioning. Unlike traditional two-mirror galvo systems, FSMs with a parallel-kinematic design use a single mirror for both axes — resulting in a more compact setup and preserving polarization, which is especially beneficial in high-precision industrial environments.

FSMs also play a vital role in defense systems, where laser-based solutions depend on high-speed, pinpoint accuracy to address threats that traditional systems cannot. The same active and adaptive optics principles used in scientific applications are employed here to maintain stability and performance under extreme conditions.

In the medical field, ophthalmology represents a growing use case, with FSMs precisely steering laser beams to reshape the cornea during corrective eye surgery, potentially reducing or eliminating the need for glasses or contact lenses.

Lastly, in microscopy, FSMs enable advanced techniques such as optical trapping, where highly focused laser beams manipulate microscopic particles without contact. Additionally, FSMs support adaptive optics, with piezo-driven mirrors actively correcting aberrations caused by refractive index variations in specimens — significantly enhancing microscope resolution, often by an order of magnitude.

As optical systems become more integrated into communication, sensing, and imaging technologies, FSMs will remain a key enabler of speed, precision, and stability in next-generation photonic applications.

This article was written by Stefan Vorndran, PI (Physik Instrumente) LP (Auburn, MA). For more information, visit here .