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Optical table advances quiet vibrations in highly sensitive applications STEVE RYAN New techniques allow air isolator supports to be stacked on active vibration-cancellation hard mounts, creating stable optical tables for sensitive multiphoton imaging and single-molecule studies. The backbone of every photonics lab is an optical table. Complex optoelectronic systems require the special properties of optical tables that include stiffness, damping, flatness, cleanliness, tapped hole arrays, and uniform coefficient of thermal expansion. But the most important characteristic is that they provide an extremely quiet and stable work surface. Optical tables isolate floor vibration by combining an extremely stiff, structurally damped steel honeycomb top with a low-frequency pneumatic vibration isolation support stand. This creates a six degree-of-freedom (DOF) mass-spring-damper system, with the honeycomb top acting as the idealized, infinitely stiff mass and the vibration isolation support stand acting as the damped spring. A mass-spring-damper has a characteristic resonant frequency (fn) at which it amplifies vibration. Above approximately 1.4 × fn, it begins to isolate and isolation roll-off improves with increasing frequency. The amplification at resonance and the isolation slope are a function of the isolator’s damping coefficient. The typical support stand starts to isolate floor vibration above 3–4 Hz. An effective top is very stiff with its first bending mode above 100 Hz and its modes effectively damped by structural dampers mounted inside the honeycomb structure. With resonant frequencies above 100 Hz, vibration reaching the top through the isolators or other paths is not amplified in the critical frequency range where optoelectronic devices are most sensitive—that is, from 0.5 to 30 Hz. However, vibration damping is not the problem—instead, new optical table designs with stacked isolation supports improve table stability in this critical region by isolating these low-frequency vibrations, allowing even the most sensitive multiphoton imaging and single-molecule biophysics studies to take place. structure and the vibration isolation support stand. Energy reaching the isolated structure must be dissipated (converted to heat). Mass-spring-dampers built into the table-top structure provide damping for flexing of the top at and above the first bending modes of the top (above 100 Hz). When the optical table system is disturbed, the air isolators are excited at their resonant frequency (1–3 Hz). This motion is dissipated as orifices, dashpots, and other devices built into the air isolators convert this energy to heat. Isolation, on the other hand, is the reduction of floor vibration reaching the payload achieved by mechanisms in the isolator support stand. Active damping of the top’s structure by electromechanical devices built into the top should be considered damping and not isolation, as it does not prevent vibration from reaching the top, but rather damps it. Historically, improvements to optical Active systems Damping vs. isolation Vibration damping and isolation are different characteristics that are often incorrectly used interchangeably. Damping is the conversion of mechanical energy to heat, which applies to both the table-top Serial type Mass (Optical table) FIGURE 1. Parallel- and serial-type active vibration control systems are shown in which the support spring and cancellation actuator are either in parallel or in series. Mass (Optical table) Spring (k1) Parallel type Floor platform or frame (b2) Sensor V Inner mass Force (piezo) Reprinted with revisions to format, from the September 2017 edition of LASER FOCUS WORLD Copyright 2017 by PennWell C

table vibration performance have focused on increasing the structural damping of the top—the goal being to combine extremely high stiffness-to-weight ratios achieved with steel honeycomb technology with high structural damping and little amplification at resonance. In general, this has been successful, with the best tops now achieving critical damping at their lowest resonant frequencies. To the extent that this has had good success, further improvements now provide diminishing returns. Until recently, there have been fewer advances in the isolation systems that support the tops from floor...

an ultrastiff inner mass that supports the payload through a stiff, 15–20 Hz spring. The actuator supports the inner mass to ground (see Fig. 1). With this approach, linear motors and other conventional actuators are not feasible because the actuator in a serial-type configuration must support the static weight of the top. But, developments in piezoelectric actuator technology make piezos the ideal choice for serial-type configurations, as they can now be designed to support a large static mass and have excellent response characteristics to very low displacements. In this embodiment, floor...