The Need for Vibration Isolated Rigid Support Structures in Optics Research

Optical tables provide a rigid platform for high-precision optical experiments and systems. They are designed to eliminate errors caused by relative motion between optical components in the beam path. Rigidity is the primary consideration in optical table design. Table rigidity can be quantified in terms of static or dynamic rigidity.

• Static rigidity describes the optical table's ability to resist deflection when the static or quasi-static load distribution is changed. It also defines the table' performance when stages move across the table, or when equipment is relocated, added or removed.
• Dynamic rigidity describes the optical table's ability to resist deflection in response to mechanical excitation. It also defines the table's performance in response to floor vibration, acoustic noise and tabletop mechanical sources.

Honeycomb is commonly used to produce very low weight, highly rigid structures. Reduced weight dramatically improves the dynamic rigidity of the structure by moving structural resonance modes to higher, less detrimental frequencies. The structural resonance modes are the frequencies at which the platform deflects, therefore causing relative motion across the optical mounting surface. For a given input vibration force, the deflection is reduced as the mode frequency increases. This is the primary reason why steel honeycomb has replaced granite in most high-end optical applications - since granite is relatively heavy, the resonance modes occur at lower frequencies and therefore produce higher amplitudes of surface deflection.

Trussed Core Design - Newport innovated the trussed core design to maximize the table's rigidity-to-weight ratio. The trussed core design uses an additional steel member to bridge across the center of the honeycomb cell. This extra mechanical component significantly stiffens the cell, with very little increase in weight. The truss improves both the static and dynamic rigidity and increases the structure's point load carrying ability. Many manufacturers use a traditional open cell construction, which is not as statically rigid, when compared to a Newport table.

Vertically Bonded Core - All Newport tables are vertically bonded along the height of the honeycomb core. Most other manufacturers forego this extra step because of added expense and time required to apply adhesive to each individual core member. However, vertical bonding is the most important step to maximize the table's rigidity-to-weight ratio. An additional benefit is the introduction of constrained layer damping effects into the core construction.

Triple Core Interface - The trussed core design enables a triple core interface at each honeycomb cell. Whereas open core designs only have two structural members at each interface, the Newport table has three. Since most manufacturers use 0.010 thick steel sheet, their core interface has a thickness of 0.020 inches. The Newport design offers a 50% increase in local rigidity by using a 0.030 inch support member. Additionally, the three sheets are fully bonded for the full table height producing greater stiffness and constrained layer damping.

Custom Cores - Newport offers the greatest experience and core selection available. Newport commonly builds trussed cores in several sizes, open aluminum cores for very lightweight structures and perforated cores for high vacuum or cleanroom applications. Other constructions include egg crate and tubular structure designs. This allows extending Newport's capabilities beyond optical tables to build honeycomb towers for drawing optical fibers, vertical systems for large laser beam lines, pedestal flooring systems for semiconductor manufacturing equipment and a large variety of instrument platforms, gantries and bridges.

Beam path stability is a primary requirement for many optical experiments and processes. Newport meets the challenges of leading edge nano-technology by offering the best surface flatness and thermal stability available in a honeycomb structure. Whereas other manufacturers seek to make tables more quickly, Newport takes the extra steps to focus on making the best.

Precision Bonding Platens - Metrology quality steel assembly platens are used to ensure the superior flatness of our optical tables. The platen supports the top surface of the table during the assembly and bonding process and imprints the top surface with the same flatness characteristics. Routinely verified with optical interferometers, these platens are the largest and flattest in the industry. These platens allow Newport to build the world's largest optical tables.

Gravity Pressure Bonding - Applying constant pressure across the optical table during the bonding process ensures that the top surface conforms to the platen flatness. Newport uses gravity pressure bonding to control the load across the table surfaces and to ensure process consistency from table to table. Other manufacturers use hydraulic presses to speed up the assembly process. However, the hydraulic pressure must be monitored for consistency and may apply uneven forces across the table top.

Low Thermal Stress Bonding - Slow curing of the table bonds at room temperature is critical both to maintain table flatness and ensure long term stability of the table structure. Fast curing adhesives heat the core and lock thermal stresses into the table. Some manufacturers apply heat to the table to improve their production cycle times. Temperature induces stress causing the table to bow when it is returned to room temperature. Temperature curing can also induce long term bowing, and in extreme cases delamination of the core, top or bottom surface. Newport's slow cure method takes longer but allows production of the flattest tables available. Newport also provides a lifetime guarantee against delamination with each table.

Super Invar and other Advanced Materials - Newport has more experience with advanced materials than anyone else in the optical table industry. Newport's long relationship with the Aerospace industry and advanced labs all over the world has given the Newport Vibration Control team unsurpassed materials capabilities. Newport is the premier supplier of Super Invar tables for the absolute leading edge of optical research. Super Invar exhibits almost no thermal expansion near room temperature. Newport also has extensive experience with very light, highly stable carbon structures. For the ultimate in flatness, Newport can also provide granite platforms, granite/-epoxy composites or granite/honeycomb composites.

Experimental or process disturbances caused by relative motion between optical components generally occur at the structural dominant bending or torsional modes. In addition to pushing these natural modes to higher, less detrimental frequencies, a major advantage of honeycomb over granite is the high level of damping present in the honeycomb structure. Damping attenuates the amplitude of the natural modes and reduces the relative motion across the table surface. In very precise applications such as optical interferometry, maximizing the table's damping properties is absolutely critical.

Three types of damping are used for optical tables: Narrow Band Tuned Damping, Broadband Tuned Damping and Constrained Layer Broadband Damping. Newport is the only company that offers all three forms of damping in a single optical table.

• Narrow Band Tuned Damping Using Vibration Absorbers - Newport is the only manufacturer to build narrow band vibration absorbers into an optical table. These vibration absorbers allow tuned damping of specific modes. Narrow band tuned damping is the most effective means for eliminating structural resonances. These dampers selectively cancel vibration modes to minimize the Dynamic Deflection Coefficient and make the table behave more like an ideal rigid structure. For very large structures, such as optical tables over 15 feet in length, that exhibit structural resonances below 150Hz, narrow band techniques are the only effective means of damping.

Narrow band tuned dampers can only be found in Newport RS Series tables. Newport carries in stock over 200 individual dampers plus damp modes from 20–480 Hz. A trained vibration engineer tests each table to determine the natural modes of the structure.

Narrow band vibration absorbers are then selected to eliminate these modes and installed in the highest amplitude position (usually the corners). Different grades are available to offer varying levels of damping. In applications where heavy payloads can significantly effect the structural modes of the table, Newport can simulate the load and optimize the tuned damping. When damping doubler systems, where two or more tables are rigidly connected, the tables are assembled and tuned as a monolithic structure.

• Broadband Damping: Tuned Vibration Absorbers - Broadband damping is less effective than narrow band techniques. However, Newport tables offer a variety of broadband damping mechanisms that further improve table performance. The dash pot system contained in Newport's tuned vibration absorbers offers higher frequency broadband damping. This broadband damping extends over several octaves above the tuned frequency of the vibration absorber.
• Broadband Damping: Constrained Layer Core Interface - Constrained layer damping structures usually consist of two or more metallic sheets separated by a compliant material. The Newport vertically bonded trussed honeycomb core consists of three metallic sheets separated by an adhesive. Although the adhesive is rigid, its damping factor is much higher than that of steel and introduces substantial damping into the core. This construction produces substantial constrained layer damping at each core interface.
• Damped Working Surface - The top of a Newport table is bonded to a polymeric material that serves to seal the honeycomb core and damp the working surface. The polymer experiences the same bending and shear stresses as the stainless steel top. Since the damping factor of this material is much higher than that of steel, considerable damping is introduced into the work surface. This design eliminates the skin resonance problems found with undamped table to designs.
• Damped Table Sides - The sides of a laboratory grade Newport table are made of a highly damped, epoxy sealed wood composite. Similar to the panels used in high end audio speakers, composite wood materials are acoustically "dead". Compared to the metal sides used by most other table manufacturers, the composite wood sides offer significant damping to the structure and eliminate another source of resonance. Knock on the side of a Newport table to feel the difference. Newport offers steel sides on cleanroom and vacuum compatible products.

Optical laboratories and high technology industries require high levels of product cleanliness.

• Pre-tapped and Washed Worksurfaces - All Newport worksurfaces are precision tapped and ground flat with a non-glazed finish before bonding to the sealed honeycomb core. They are then cleaned with automated industrial washing systems to ensure that all cutting fluids and metal particles are removed before bonding. Cleanroom products undergo a patented process to further protect the work surface before it is bonded to the honeycomb structure. Other manufacturers tap the grid on their optical tops after assembly and introduce contamination into the core or sealed hole cups. Even after extensive cleaning of "post tapped" tables, substantial residues still remain.
• Countersunk Mounting Holes - All tapped holes in a Newport table are countersunk and deburred. This step ensures easy thread engagement and eliminates a possible contamination problem. Tables without countersunk holes can shed metal particles as mounting bolts strip burrs from the thread. Non-countersunk holes are also more prone to cross threading and damage.
• Non-Corrosive Individually Sealed Holes - Each tapped hole in a Newport optical table is registered to the core and individually sealed with a non-corrosive conical cup. The cup seals the honeycomb cell to prevent laser dyes, cooling fluids or the occasional mislaid cup of coffee from contaminating the table interior. Sealing each hole individually eases cleanup by eliminating places for liquids to hide. The conical design of the sealing cup aids in removing any remaining residues. A polymeric cup material is used to prevent particle shedding and breach of the sealing system due to corrosion. The cup is impervious to acids, bases and common laboratory solvents.
• Newport cups are built into a complete sheet and vacuum bonded to the top surface to eliminate all air contaminants. This step eliminates the leakage problems found in competitive cup techniques.

• Cleanroom Compatible Bonding Adhesives - Aerospace grade adhesives are used to bond the core of all Newport optical tables. These adhesives meet NASA low outgassing requirements and are approved for high vacuum use. They are cleanroom compatible and do not shed or cause particulate contamination. Newport uses automated measuring and mixing systems to ensure adhesive batch consistency.
• Hermetically Sealed Vibration Absorbers - Tuned damping vibration absorbers are hermetically sealed and fully qualified for cleanroom and high vacuum use.
• Perforated Core Designs - Perforated cores are available for high vacuum or cleanroom use. Each honeycomb cell of these special designs is ported to allow a high vacuum to be drawn. Laminar flow designs for cleanroom floor structures (FabFloor™) combine the perforated core with a table airflow system.

Feature	Benefit
Trussed core design	Better rigidity-to-weight ratio Better static rigidity Better dynamic rigidity Increased point load carrying ability
Vertically bonded core	Better rigidity-to-weight ratio Better static rigidity Better dynamic rigidity Increased point load carrying ability
Triple core interface	Better rigidity-to-weight ratio Better static rigidity Better dynamic rigidity Increased point load carrying ability
Low thermal stress bonding	Improves table flatness Eliminates long term table bowing Guarantees non-delamination
Narrow band tuned damping	Most effective method of eliminating specific vibration modes Much more effective than broadband at low resonance frequencies
Broadband damping	Reduces the amplitude of vibration modes across broader frequency ranges More effective than constrained layer over specific frequency bands
Damped top skin	Reduces ringing of work surface
Individually sealed holes	Prevents liquid spill from moving to other honeycomb cell locations or contaminating the table interior
Non-corrosive sealed holes	Prevents long-term table degradation from corrosive liquid spills

Pneumatic isolators filter vibration before the mechanical noise can reach the optical bench work surface. Improved vibration isolation reduces errors caused by relative motion between optical components in the beam path. Pneumatic isolators combine with the optical table and payload to form a mass/spring/damper system. Pneumatic systems are used instead of mechanical springs since they offer self-leveling and minimize the effect of varying mass on isolation. The performance of the isolator is defined primarily by its natural frequency and damping characteristics:

• Natural Frequency (Natural Mode, Resonance) - The pneumatic isolator is essentially a simple harmonic oscillator that uses the "fast roll-off" at higher frequencies to act as a low pass mechanical filter. Below the natural frequency of the harmonic oscillator the isolator is essentially rigid and vibration is passed directly to the platform. At the natural frequency vibration is actually amplified. Therefore a primary goal is to lower the natural frequency since this improves low frequency isolation and overall isolation bandwidth.
• Damping - Another primary goal is to damp the harmonic oscillator amplitude at resonance. This lowers the magnification of vibration at low frequencies and improves system stability. Unfortunately there is a compromise between the natural frequency and damping as damping is increased, the isolator natural frequency moves slightly higher, and higher frequency isolation is decreased.

Conventional isolators use a compliance chamber to act as an air spring and a damping chamber to increase system stability. The two chambers are connected through a thin tube or orifice.

• Compliance Chamber - The compliance chamber is sealed with a flexible diaphragm to form a piston and support the optical table on compressed air. If the piston is pushed further into the compliance chamber, the pressure of the gas increases and provides a restoring force - somewhat like a soft spring. The isolation performance is primarily related to the volume of the compliance chamber. As the compliance chamber volume is increased, the natural frequency is decreased.
• Damping Chamber - After the compliance chamber, air is pumped to the damping chamber through a flow restricter - usually a thin tube or orifice. The restricter dissipates energy in the air and essentially damps the system. The design of both chambers and the restricter must be optimized to minimize the natural frequency/damping trade-off. When added together the volume of these chambers is relatively large and unresponsive due to the amount of air that must pass between them.

Newport's proprietary Stabilizer™ design offers a unique and greatly improved approach to isolator design. The Stabilizer™ provides significantly more damping of the natural frequency amplitude due to the closer coupling of the piston and damping element. The result is improved isolation and better stability when compared to conventional isolators.

• Hybrid Chamber - The primary innovation of the Stabilizer isolator is the Hybrid chamber system. The hybrid chamber configuration enhances damping efficiency by minimizing air volume between the piston and the damping elements. Rather than the conventional design that features two equivalently sized chambers, where the first volume is the compliance chamber and the second the damping chamber, the hybrid chamber uses the entire isolator volume for compliance. The initial chamber is reduced dramatically in size. This forces the isolator to use the second chamber as part of the compliance volume. The hybrid chamber offers much larger compliance volume for a given package size. The larger compliance volume lowers the frequency of the natural mode and improves isolation.

The natural frequency of a pneumatic isolator decreases as the compliance volume increases, thereby improving the isolation performance. The hybrid chamber design maximizes the compliance volume; Ultra-soft diaphragms reduce the stiffness limitation.

• Laminar Flow Damping - Connecting the hybrid chamber volume is a laminar flow element. This element is made of porous metal and adds resistance to the airflow as it is forced between the hybrid chamber volumes. The resistance added to the airflow significantly damps the amplitude of the natural mode with minimal effect of the isolation bandwidth. The laminar flow element also improves isolator settling time and responsiveness. The airflow volume can be reduced in the factory to add higher levels of damping.

• Ultra-Soft Diaphragms - One of the limitations on pneumatic systems is the diaphragm stiffness. The stiffness of the diaphragm limits the isolation improvements when using a larger compliance volume. Newport uses custom molded ultra-soft diaphragms to maximize the benefits of the hybrid chamber design.

Pneumatic isolators commonly use valves to stabilize and relevel the optical table after disturbance. Three two-way valves are supplied with each Newport system to control the platform in three degrees of freedom (vertical, pitch and roll). The valve is mounted to the isolator and senses the floating platform height with a lever arm. As the table is disturbed the lever arm moves and actuates the leveling valve to add or exhaust air in the pneumatic isolators as required to maintain platform height. Air is only supplied when the system is releveling.

Newport manufacturers its own custom valve designed specifically to meet the needs of optical table users. Other manufacturers use less than optimal off-the-shelf valves and cannot offer high accuracy releveling. Only 3 leveling valves are required regardless of the number of isolators in the system.

• Airflow Control Valves - Newport provides needle valves to control the airflow from the leveling valve to the hybrid chamber. This system offers a much higher level of user control and allows the system response to be optimized for any specific optical table system.
• EAR Lever Arm Connection - Newport uses a specially designed EAR damping material to connect the lever arm to the optical table. This material prevents high frequency vibrations from traveling through the arm to the table. Other manufacturers use common foam for this purpose, however, the foam increases releveling errors and degrades over time.
• Pressure Gauges - Pressure gauges are included on the leveling valves to monitor the pressure inside the isolator.

The pneumatic isolator offers isolation in the vertical direction; horizontal isolation is achieved through a mechanical filter system. Many manufacturers use some form of a bearing surface to provide a pivot or rolling surface for vibration filtering. However these contacting systems always have mechanical defects and surface roughness between the bearing surfaces. The bearing surface introduces frictional noise and limits the isolation of very low level vibrations that commonly disturb high accuracy systems.

Trifilar Pendulum - Newport uses a frictionless trifilar pendulum system for horizontal vibration isolation. The natural mode of the pendulum is very low in frequency, typically 1 to 2 Hz depending on the length of the pendulum. Above the natural frequency the pendulum isolates vibration. Since the system operates by flexing, it offers no frictional resistance to motion and can isolate very low amplitude mechanical vibration.

Viscous Damping - The natural frequency of the pendulum is damped using low vapor pressure oil. This system offers a significant decrease in the amplitude of the natural mode without adding higher frequency frictional noise.

Newport has refined our laboratory grade pneumatic isolator for many years and offers a variety of features not found in imitative products.

• Self-Centering (US Patent 5,071,108) - Only Newport offers a self-centering device to ensure unrestricted piston motion. This patented feature eliminates problems caused by shorting out the vibration isolator during table set-up.
• Height Adjustment - Laboratory grade pneumatic isolators include a mechanical height adjustment system to allow improved table leveling, height control and compensation for non-level floors.
• Modular Support Bases - All LabLeg™ support systems are built on modular support bases. These support bases offer seven standard heights to choose from. Each system can be modified to a new height by simply ordering new support bases. Rigid legs can be upgraded to pneumatic systems by simply ordering the top assembly.

• Freestanding Design - Newport legs come standard without any mechanical connection between them. Freestanding legs are preferred for unlevel floors and produce better decoupling from vibrations. Tie-bar systems can introduce significant structural resonances in the 25–100 Hz frequency range. Newport only recommends tie-bar systems if the table will be moved frequently and requires a caster wheel system.

• SafeLock™ mounting brackets offer a safe and reliable connection of the isolator to the optical table. The brackets use a slotted connection to make installation easier. Numerous SafeLock™ secured systems have survived earthquakes throughout California and Japan. Earthquake restraints are available as a table accessory.

Feature	Benefit
Hybrid chamber design	Lowest natural frequency offers maximized isolation bandwidth Optimized damping capability Compact design is 30% smaller than conventional isolators Improved isolator responsiveness Improved high center of mass stability
	Less amplification at resonance Improved setting time Very little effect on isolation bandwidth Adjustable damping for special applications
Ultra-soft diaphragms	Improved low frequency isolation Improved isolation of low amplitude vibrations
High accuracy leveling valves	More accurate leveling of table Improved repositioning of table after disturbance Allows table-to-table beam pointing applications
Airflow control valves	Allows optimization of pneumatic control
EAR gain arm interface	Decouples sensor arm vibration from table Better leveling accuracy Does not degrade like foam pads
Trifilar pendulum horizontal isolation	Zero-friction design enables isolation to much lower vibration levels Improved re-positioning after table disturbances
Self centering	Ensures isolator alignment Eliminates horizontal shorting
Height adjustment	Adaptable to uneven floors Allows alignment of separate tables
Modular support bases	Allows system height to be field modified Allows field integration of tie-bar caster system
Free standing design	Better isolation performance Eliminates structural resonances found on tie-bar systems Adaptable to uneven floors