MEMS mirror

A MEMS mirror is a reflective plate etched from silicon, typically 0.5–5 mm across and tens of micrometers thick, held by torsion hinges and tilted by a microfabricated actuator. Single-axis devices tilt about one hinge pair; two-axis (tip-tilt) devices nest the mirror in a gimbal frame or suspend it on hinges in both directions. Mechanical tilts range from a fraction of a degree to about ±10°, and because reflection doubles angles, a ±10° tilt deflects the beam by ±20°. Unlike the digital micromirror device, whose mirrors snap between two fixed angles, an analog MEMS mirror can be held at any angle in its range or swept continuously, which makes it a compact alternative to the galvo scanner for beam steering.

Actuation and operating modes

Four actuation schemes are in common use. Electrostatic actuators, either parallel plates under the mirror or interdigitated comb fingers at its edge, draw almost no static current and are the standard choice for telecom switching arrays; parallel-plate designs are limited by pull-in, an instability beyond which the plate snaps to the electrode, so only part of the gap is usable. Electromagnetic actuators carry a coil on the mirror frame in the field of an external magnet and give larger angles at low voltage. Electrothermal bimorphs give large static tilts at the cost of power and millisecond response, and piezoelectric thin films give large force at moderate voltage.

A mirror can be driven quasi-statically, following an arbitrary waveform well below its first mechanical resonance, or at resonance, where the motion is sinusoidal and the angle is multiplied by the mechanical quality factor. Resonant operation gives the largest angles and highest speeds, but the mirror then traces a fixed sinusoid: one axis of a raster, or a Lissajous pattern when both axes resonate at different frequencies.

Resonant frequency and resolution

The torsional resonance of a mirror with moment of inertia $J$ on hinges of torsional stiffness $k$ is

$$f_0 = \frac{1}{2\pi}\sqrt{\frac{k}{J}}.$$

For a silicon disk of diameter 1 mm and thickness 50 µm, tilting about a diameter, $J = m r^2/4 = 5.7 \times 10^{-15}$ kg·m². Hinges of $9.0 \times 10^{-7}$ N·m/rad place the resonance at 2 kHz; reaching 20 kHz needs hinges 100 times stiffer, and so more drive torque for the same angle. Since $J$ grows as the fourth power of the radius, larger mirrors are much slower, and the trade among mirror size, angle and speed is the central design constraint.

A 20 kHz resonant mirror used for both sweep directions writes 40,000 lines per second, or 667 lines per frame at 60 frames per second. The number of resolvable spots along one axis is the optical scan angle divided by the beam's diffraction-limited divergence,

$$N = \frac{\theta_\text{opt}\,D}{1.22\,\lambda},$$

for a uniformly filled circular aperture of diameter $D$. A 1 mm mirror with ±10° mechanical tilt (40°, or 0.698 rad, optical) at 905 nm gives $N \approx 630$. The product $\theta_\text{opt} D$ is fixed by the mirror: a 4× beam expander after it gives a 4 mm beam but reduces the full scan to about 10°, so a wider field or finer spots at the target need a larger or faster-tilting mirror, a limit discussed further under diffraction limit.

Applications

• Optical switching. Arrays of two-axis electrostatic mirrors form the switching core of 3D optical circuit switches, with insertion loss around 2 dB and reconfiguration in milliseconds, and one-dimensional arrays steer individual wavelengths in the wavelength-selective switches of a ROADM.
• Fiber components. Variable optical attenuators, 1×N switches and external-cavity tunable lasers use a single MEMS mirror to change coupling or select a wavelength.
• Scanning and imaging. Two-axis or resonant mirrors raster a beam in compact lidar units, laser projectors and miniature endoscopic probes.

Pitfalls

A resonant mirror's frequency and amplitude drift with temperature and ambient pressure, so systems run closed-loop on an angle sensor or track the resonance in the drive. The sinusoidal motion dwells longer at the ends of the sweep, so pixel timing must be corrected and the edges are brighter unless the laser power is modulated. At high frequency the plate flexes under its own inertia (dynamic deformation), which degrades the reflected wavefront; thicker plates or stiffening ribs reduce it at the cost of inertia. Metal coatings add stress that curves thin mirrors, and absorbed power heats a plate whose only thermal path is through its hinges. As with galvos, datasheets quote mechanical or optical angle, and the two differ by a factor of two.

Common questions

What is the difference between a MEMS mirror and a galvo scanner?

A galvo moves a mirror of several millimeters to centimeters on a motor shaft with a closed-loop position sensor, giving larger apertures and arbitrary waveforms at a few hundred microseconds per small step. A MEMS mirror is smaller, lighter, cheaper in volume and much faster at resonance, but its aperture is small and resonant operation fixes the scan waveform.

How fast can a MEMS mirror scan?

Quasi-static mirrors follow waveforms up to a few hundred hertz to a few kilohertz, below their first resonance. Resonant mirrors of about 1 mm run at tens of kilohertz; larger mirrors resonate lower.

References: O. Solgaard, Photonic Microsystems: Micro and Nanotechnology Applied to Optical Devices and Systems (Springer, 2009); M. C. Wu, O. Solgaard and J. E. Ford, "Optical MEMS for lightwave communication," Journal of Lightwave Technology 24, 4433 (2006); G. F. Marshall and G. E. Stutz (eds.), Handbook of Optical and Laser Scanning, 2nd ed. (CRC Press, 2011).