With regards to exactly directing mild in two dimensions, optical system designers wield a number of well-established applied sciences.
These embody galvanometer mirror scanners (galvo scanners), piezoelectric swift steering mirrors, voice coil-driven steering mirrors, and kinematic mirror mounts and gimbal mounts. Every of those applied sciences was crafted for particular purposes, and this text will define a few of their strengths and limitations.
Picture Credit score: PI (Physik Instrumente) LP
2-Axis Galvo Scanners – 2 Particular person Mirrors
These optical scanners comprise two separate, perpendicular planar mirrors that rotate independently. They’re positioned in a way whereby laser mild bouncing off the primary mirror (axis 1) redirects to the second mirror (axis 2) and subsequently reaches the goal.
Galvo scanners are ceaselessly utilized in laser materials processing, benefitting from their excessive manufacturing quantity, permitting for comparatively cheap prices.
Due to their light-weight mirrors, they permit extremely dynamic movement in closed-loop operation, attaining step response instances within the sub-millisecond vary. One other benefit lies of their expansive scanning subject, sometimes round 20 levels for each axes.
Nonetheless, the downside of the galvo scanner’s twin single-axis mirror design is the polarization rotation impact it induces.
This impact can pose challenges in purposes reliant on a selected polarization state, corresponding to sure forms of interferometry, the more and more common optical coherence tomography in dermatology, and ellipsometry. Ellipsometry is a method for figuring out movie properties and thickness optically.
A excessive precision laser processing demonstrator, primarily based on a Scanlab 2-axis galvo scanner and 2-axis XY linear stage for just about infinite subject of view operation. Combining the linear movement offered by the XY-stage with the high-bandwith angular deflection of the galvo scanner improves each the element precision and throughput. The ACS movement controller permits for steady operation over giant areas, avoiding idle time and eliminating stitching errors. Picture Credit score: PI (Physik Instrumente) LP
Single Mirror, 2-Axis Scanners with Mounted Pivot Level for Each Axes
A) Piezoelectric 2-Axis Steering Mirrors
Quick steering mirrors utilizing piezoelectric know-how can shift a single mirror alongside two axes round a set pivot level. This design retains mild polarization, shrinks the general dimension, and boosts sturdiness.
Frictionless flexures information the mirror’s motion, splendid for small angles and excessive accelerations achievable with piezo drives. These flexures don’t want lubrication, are maintenance-free, work in excessive vacuum, and deal with broad temperature ranges.
Piezoelectric actuators swiftly reply to adjustments in voltage. Their velocity is determined by the piezoceramic materials’s velocity of sound. For small parts, microsecond response instances are attainable.
Whereas conventional direct-drive piezo mirrors had been restricted to a couple milliradians in angular vary, new designs utilizing flexure movement amplifiers can deflect mild as much as 70 mrad (4°).
Pressure gauges are constructed into these mirrors for high-bandwidth, closed-loop operation, and linearization. Utilizing a differential drive design with two actuator pairs working in a push-pull mechanism tremendously enhances angular stability throughout varied temperatures in comparison with a single actuator/axis strategy.
The S-335 built-in, single-mirror, 2-axis piezo scanner for quick beam steering mirror employs flexure movement amplifiers to increase its angular vary. 2 angular ranges can be found – shorter journey permits for increased frequencies. Picture Credit score: PI (Physik Instrumente) LP
Open loop step efficiency of an S-335.2SH, 2-axis piezo steering mirror mount: unloaded, ½” and 1” mirror. The chosen step dimension is 50% of the max vary: 17.5mrad ~1 diploma. Picture Credit score: PI (Physik Instrumente) LP
Flexure-guided piezo mirror scanners are maintenance-free, work in ultra-high-vacuum environments, and could be tailor-made for area operations.
Fundamental design of a quick steering mirror with differential piezo (PZT) actuation (push/pull). One of many benefits is angular insensitivity to temperature adjustments. Picture Credit score: PI (Physik Instrumente) LP
An area-qualified built-in 2-axis piezo steering mirror mount used on the secondary mirror of the PHI telescope on the solar-observing satellite tv for pc. This compact unit with diameter of a 45mm is barely 44mm tall and weighs 255g. The mirror mounted on the tip/tilt platform weighs 71g and is product of Zerodur. Picture Credit score: PI (Physik Instrumente) LP
Benefits of piezo-driven steerable mirrors over galvo scanners embody:
- Parallel kinematics (coplanar multi-axis movement with out polarization rotation)
- The quadra-pod differential drive design (in S-335 and S-330 mannequin households) runs two orthogonal axes with a set pivot level primarily based on two pairs of actuators working in push/pull mode.
- Channels/Axes could be operated concurrently or independently, because of the paired piezo actuators within the design.
- Piezoelectric solid-state properties present for quick response (sub-millisecond) and excessive decision (right down to the nanoradian vary).
The reliability of piezo flexure-guided steering mirrors is excessive because of the lack of any carrying components. Assessments demonstrated lifetimes of over 2 billion cycles with out failures or degradation. NASA life-tested the actuators built-in into the steering mirror for 100 billion cycles for the Mars mission.
Theta-X, Theta-Y, and Path-Size Modulation
One other parallel-kinematic steering mirror resolution makes use of a tripod piezo stack design. Together with Theta-X and Theta-Y rotation, this design modulates path size with sub-nanometer precision (part modulation).
B) Voice Coil-Pushed, Flexure Guided 2-axis Steering Mirrors
An alternate strategy for a single-mirror, 2-axis scanner makes use of voice-coil motors and flexure guides.
Because of the prolonged journey vary of voice-coil motors, bigger angular spans could be achieved (beam deflection angles approaching 10 levels in each Theta-X and Theta-Y). Using a quadra-pod differential drive design allows wonderful temperature stability.
Variations between Piezo Drives and Voice Coil Drives
Though piezo actuators are inherently inflexible and may preserve a place with minimal power consumption and warmth loss, voice coils lack inherent holding power when de-energized and require energy to keep up a place.
Compact, built-in voice-coil-driven 2-axis quick steering mirror module with single transferring mirror platform. Picture Credit score: PI (Physik Instrumente) LP
Simulation of mirror floor deformation / flatness. Picture Credit score: PI (Physik Instrumente) LP
1µrad steps carried out with a V-931 voice-coil pushed two-axis steering mirror. Picture Credit score: PI (Physik Instrumente) LP
Bode plot of the voice coil 2-axis steering mirror. Picture Credit score: PI (Physik Instrumente) LP
2-Axis Motorized Kinematic Mirror Mounts
Designed for Set-and-Neglect Optical Alignment Functions with Excessive Stability
Most commercially out there kinematic mirror mounts permit energetic changes in two axes, usually using micrometers or fine-pitch screws. Kinematic mounts are ceaselessly employed in laser programs to exactly alter the beam path after which maintain the optimum place with very excessive stability.
Instance of a motorized optical kinematic mirror mount. This unit supplies as much as 16 levels of rotation in Theta-X and Theta-Y. Excessive precision piezo motors drive the tremendous adjustment screws and supply angular step decision to 0.3µrad. Picture Credit score: PI (Physik Instrumente) LP
A vacuum appropriate motorized kinematic mirror mount. Picture Credit score: PI (Physik Instrumente) LP
Motorized variations allow distant management, robotically compensating for variations in system efficiency or returning a fancy optical system post-shipping. Because the axis of rotation of a kinematic optical mirror mount is located behind the mirror, the changes of each axes should not fully impartial.
A slight linear translation of the beam happens with each angular adjustment, inflicting the axes to deviate from good orthogonality to the optical axis. Sometimes, kinematic mirror mounts provide bigger angular ranges in comparison with high-bandwidth piezo and voice coil steering mirrors however normally lower than 20 levels.
Gimbal Mounts
Customized, 2-axis gimbal mounts with ultra-high precision piezo linear motors. Picture Credit score: PI (Physik Instrumente) LP
Check outcomes exhibiting 2µrad and 5µrad steps, measured with Zygo ZMI2000 laser interferometer. Picture Credit score: PI (Physik Instrumente) LP
Essentially the most substantial angular deflections come from gimbal mount designs. Gimbals can present rotation as much as 360 levels in 1, 2, or 3 axes.
The axes of a 2-axis optical gimbal mount intersect on the entrance and middle of the hooked up optic, remaining completely orthogonal. Gimbals, designed for giant mirror diameters and geared up with direct-drive motors, are sometimes employed in high-precision monitoring purposes.
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