Beam steering

Beam steering is the controlled change of a beam's direction. It is done by tilting a mirror, by diffracting the beam from a grating whose period can be changed, or by setting the phase across an aperture so that the emitted wavefront tilts. Laser marking, scanning microscopy, lidar, free-space optical links, optical switching and laser displays all rely on it. The term has a second, unwanted meaning in semiconductor lasers: a sideways shift of the output beam with drive current, discussed at the end of this entry.

Figures of merit

The useful measure of a steerer is the number of distinct directions it can address, the number of resolvable spots: the total scan angle divided by the beam's own divergence. For a Gaussian beam of $1/e^2$ diameter $D$, the full divergence is $4\lambda/(\pi D)$, so

$$N \approx \frac{\Delta\theta\,\pi D}{4\lambda}.$$

A 10 mm beam at 1064 nm diverges by 0.135 mrad, and a mirror that sweeps it over 40° (0.698 rad) gives about 5100 spots. Scan angle can be traded for aperture with a telescope, but the product $\Delta\theta\,D$ is conserved, so $N$ is the quantity that compares devices. The other figures are speed (step settling time, or line rate for resonant scanners), pointing accuracy and repeatability, optical efficiency into the intended direction, and the wavelength range.

Methods

Galvanometer and fast steering mirrors. A galvo scanner rotates a mirror about ±10° to ±20° mechanically, twice that optically, and small steps settle in a few hundred microseconds. Piezoelectric tip-tilt mirrors cover smaller angles at higher bandwidth and are used for beam stabilization.

Polygon scanners. A spinning mirror polygon with $n$ facets sweeps $720°/n$ optically per facet, 90° for eight facets, in one direction at a constant rate. It suits printers, line-scan imaging and lidar.

MEMS mirrors. A MEMS mirror a few millimeters across tilts in one or two axes, often at resonance for fast raster scanning. The small mirror limits the aperture and therefore the spot count. Binary arrays such as the digital micromirror device switch between two fixed angles and are used for projection rather than continuous pointing.

Acousto-optic deflectors. An acousto-optic deflector steers the first diffracted order by changing the RF drive frequency, with $\Delta\theta \approx \lambda\,\Delta f/v_a$ in air. In the slow shear mode of TeO₂ ($v_a \approx 620$ m/s), a 50 MHz sweep at 1064 nm covers 4.9°, and a 5 mm aperture gives about 320 spots with an access time of 8 µs, the acoustic transit time across the beam. There are no moving parts, and the diffracted beam is frequency-shifted by the drive.

Liquid crystal and spatial light modulators. A spatial light modulator writes a blazed phase grating of period $\Lambda$, and the first order leaves at $\sin\theta = \lambda/\Lambda$. The shortest period is two pixels, so an 8 µm pixel at 1550 nm reaches at most 5.6°, at refresh rates of tens to hundreds of hertz. Liquid-crystal polarization gratings and wedge cells are related non-pixelated forms.

Optical phased arrays. An optical phased array sets the phase of each of many waveguide emitters. With a 2 µm emitter pitch at 1550 nm the unambiguous steering range, ±arcsin(λ/2d), is ±22.8°, and 512 emitters give a beam of 0.077°. Integrated arrays steer in one plane by phase and in the other by wavelength.

Refractive methods. Two rotating wedge prisms (a Risley pair) steer over a cone: two 10° N-BK7 wedges, each deviating about 5.2° in the thin-prism approximation, reach about 10.3° when aligned and zero when opposed.

Scan optics and pupils

In a scanning system the steering element sits at a pupil of the optics that follow it, so that changing the angle moves the focused spot without walking the beam off the next aperture (aperture stop and pupils). An f-theta scan lens maps angle to position linearly, $x = f\theta$: a 100 mm lens and a 10° deflection place the spot 17.5 mm off axis. For two-axis scanning the mirrors are either placed close together, accepting a small pupil error, or relayed onto each other with a lens pair. On an alignment bench, two adjustable mirrors in series give the four degrees of freedom (two positions, two angles) needed to place a beam on a target at a set angle.

Beam steering in laser diodes

In a single-mode ridge-waveguide laser, beam steering is the sideways shift of the far field, typically about a degree, that occurs when the first-order lateral mode reaches threshold and beats with the fundamental. Total power changes little, so an integrating-sphere light-current curve may show only a small kink, while the power coupled into a fiber drops or becomes noisy. It limits the usable power of fiber-coupled pump lasers.

Common questions

What is the fastest way to steer a laser beam?

Non-mechanical methods. Acousto-optic deflectors address any spot in microseconds, and electro-optic deflectors and integrated phased arrays can be faster, though with fewer resolvable spots or more loss. Mirrors are slower but cover large angles with large apertures and little loss.

What limits the angular resolution of a beam steerer?

Diffraction from its aperture. The beam cannot be pointed more finely than its own divergence in a useful sense, so a larger aperture gives more resolvable spots over the same scan range.

References: G. F. Marshall and G. E. Stutz (eds.), Handbook of Optical and Laser Scanning, 2nd ed. (CRC Press, 2012). P. F. McManamon et al., "Optical phased array technology," Proceedings of the IEEE 84, 268 (1996). A. Yariv and P. Yeh, Photonics: Optical Electronics in Modern Communications, 6th ed. (Oxford University Press, 2007). B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019).