Scanning Optics
Scanning optics are the optical components and subsystems that steer and focus a laser beam across a line or a two-dimensional field, rather than holding it fixed on one point. In lasers and photonics they convert a collimated or focused beam into a controlled “flying spot” used for marking, imaging, sensing, or materials processing.
The term covers both the deflectors (mirrors or other elements that change beam angle) and the specialized lenses that keep the focused spot sharp and uniformly sized over a flat work plane.
How it works:
A typical two-axis system uses two orthogonal galvanometer (galvo) mirrors. The first mirror deflects the beam in one axis; the second deflects it in the perpendicular axis. Because reflection doubles the mechanical angle, a modest mirror rotation produces a useful optical scan angle (often ±10–25° mechanical, corresponding to a larger optical field).
Two common architectures exist:
Pre-objective scanning — mirrors sit before the focusing lens. The lens must accept an incoming beam whose angle is changing. This is the most common industrial layout.
Post-objective scanning — the beam is focused first, then steered. Field size is limited and the focus surface is naturally curved, so a dynamic focus (Z) axis is often added.
An ordinary focusing lens maps scan angle θ to spot position as y = ftanθ. That relationship is nonlinear: constant mirror angular speed produces faster motion at the edges, uneven energy delivery, and a curved focal surface.
F-theta (f-θ) scan lenses introduce controlled barrel distortion so that:
y ≈ f⋅θ
(with θ in radians). Spot position then scales linearly with angle, scan speed is nearly constant, and the field is flattened. Telecentric f-theta designs keep the beam nearly perpendicular to the work surface across the field, which is valuable for drilling and inspection.
Supporting optics usually include a beam expander (to set the input diameter that determines focused spot size) and high-reflectivity, high-damage-threshold scan mirrors coated for the laser wavelength (fiber 1064 nm, CO₂ 10.6 µm, UV 355 nm, etc.).
Common scanning technologies:
Galvanometer scanners — closed-loop moving-magnet or moving-coil motors with position feedback (analog photodetector or digital encoder). Flexible vector or raster paths; typical step times of hundreds of microseconds. Dominant in marking, welding, and 3-D printing.
Polygon scanners — a multi-faceted rotating mirror. Very high linear scan speeds (tens to hundreds of m/s) but a fixed, repetitive line scan. Used in laser printers, some LiDAR, and large-area surface treatment.
MEMS mirrors — compact, low-power, often two-axis on a single chip. Common in portable or automotive LiDAR and displays.
Non-mechanical methods — acousto-optic deflectors, electro-optic scanners, Risley (rotating wedge) prisms, and optical phased arrays. These trade field size or efficiency for speed or solid-state reliability.
Key design trades include aperture size versus speed (larger mirrors have more inertia), number of resolvable spots, scan velocity versus laser pulse repetition rate, and thermal/optical stability of the mirrors and lens.
Applications:
Materials processing: laser marking, engraving, cutting, welding, drilling, additive manufacturing, and surface texturing. F-theta + galvo systems are the workhorse of industrial fiber- and CO₂-laser machines.
Imaging and metrology: confocal microscopy, optical coherence tomography (OCT), scanning laser ophthalmoscopy, semiconductor inspection.
Sensing and ranging: LiDAR (automotive, mapping, industrial), barcode readers.
Displays and entertainment: laser projectors and light shows.
Printing and lithography: high-speed raster systems (historically polygon-based).
Scanning optics take the spatial coherence and focusability of a laser and turn them into a programmable, high-speed tool that can address thousands to millions of locations per second across a plane.