Galvo scanner

A galvo scanner, short for galvanometer scanner, is a small mirror fixed to the shaft of a moving-magnet motor that rotates over a limited range, with an optical or capacitive position detector on the same shaft. A servo amplifier compares the detector signal with the commanded angle and drives the coil, so the mirror follows an arbitrary waveform: a raster, a vector path for marking, or a jump to a point and a hold. Because reflection doubles angles, a mechanical rotation of ±10° deflects the beam by ±20°. Two galvos with orthogonal axes make an XY scan head, the standard way to steer a laser across a field in marking, cutting, microscopy and display systems. Mirrors range from a few millimetres to a few centimetres, and larger mirrors are slower because the moment of inertia grows steeply with size.

Scan field and spot size

After the scan head, a scan lens focuses the beam onto the work plane. An ordinary lens places the spot at $f\tan\theta$ from the axis; scan lenses are designed with controlled distortion so that position is linear in angle,

$$x = f\,\theta,$$

which makes a constant angular velocity give a constant spot speed; these are called f-theta lenses. With focal length $f = 100$ mm and an optical half-angle of 20° (0.349 rad), the field is ±34.9 mm, about 70 mm across; a plain lens would put the edge at 36.4 mm. The focused spot diameter for a Gaussian beam of diameter $D$ is about

$$2w_0 = \frac{4\lambda f}{\pi D},$$

13.5 µm at 1064 nm for $D = 10$ mm and $f = 100$ mm, so the field holds about 5,000 spots across. A larger beam gives a smaller spot but needs larger, slower mirrors, and a beam expander before the head sets this trade-off.

Speed, accuracy and resonant scanners

Galvo performance is specified by small-step response (the time to move and settle for a step of a fraction of a degree), large-angle slew time, repeatability and drift. Small-step times of a few hundred microseconds are typical for mirrors near 10 mm; repeatability is typically a few microradians, and thermal drift of the position detector over warm-up is usually the larger error. Position accuracy across the field is improved by a calibration map measured by marking a grid and measuring it.

When only a fast repetitive line is needed, a resonant scanner oscillates a mirror sinusoidally at its mechanical resonance. An 8 kHz resonant scanner used bidirectionally gives 16,000 lines per second, or about 31 frames per second at 512 lines, which is why it is common in fast laser-scanning and two-photon microscopes, paired with a conventional galvo for the slow axis. Polygon mirrors, acousto-optic deflectors and optical phased arrays are the alternatives when higher speed or no moving parts matter more than range or efficiency.

Where galvo scanners are used

• Laser marking, engraving, welding and laser machining, usually with an f-theta lens and sometimes a third axis for dynamic focus.
• Laser-scanning confocal microscopy, where a scan lens and tube lens relay the galvo mirrors to the objective's back aperture so that tilting the mirror moves the focus without moving the beam on the aperture.
• Laser light shows, lidar prototypes, ophthalmic imaging and additive manufacturing by laser powder-bed fusion.

Pitfalls

In an XY head the two mirrors are separated along the beam, so the pivot points differ and the field shows pincushion distortion unless it is corrected in software or optics. The beam footprint moves across the second mirror as the first scans, so the second mirror is larger. Where a pupil must stay fixed, as in microscopy, both mirrors cannot sit in the same conjugate plane without relay optics between them, and an unrelayed pair causes some beam walk on the objective. The servo tuning trades speed against overshoot, and aggressive tuning can excite mirror resonances that show up as ripple in scan lines. For high-power lasers, mirror coatings must suit the wavelength and power, and mirror heating adds to the drift. The achievable spot size is set by the diffraction limit of the scan lens aperture and by the lens's field-dependent aberrations, which grow toward the edge of the field.

Common questions

Why is it called a galvanometer?

The motor descends from the moving-coil galvanometer used to measure current, in which a coil rotates in a magnetic field against a spring. Modern scanners use a moving magnet and closed-loop feedback, but the name remained.

What is the difference between mechanical and optical scan angle?

The optical angle is the change in beam direction, twice the mechanical rotation of the mirror. Specifications should state which is quoted; ±20° optical and ±10° mechanical describe the same scanner.

References: G. F. Marshall and G. E. Stutz (eds.), Handbook of Optical and Laser Scanning, 2nd ed. (CRC Press, 2011); J. B. Pawley (ed.), Handbook of Biological Confocal Microscopy, 3rd ed. (Springer, 2006); B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019).