Surfaces and supports keep experiments level

31 Oct 2003

Unwanted vibrations can misalign optics and cause images to blur. Jacqueline Hewett discovers how optical tables and vibration isolation systems can solve the problem.

From Opto & Laser Europe November 2003

All buildings vibrate, whether due to internal sources such as the movement of machinery, ventilation systems and human activity, or external sources such as traffic. Although these vibrations are usually small and go largely unnoticed from day to day, they can cause big problems if you are performing sensitive experiments such as interferometry or confocal microscopy.

A good way to deal with such noise is to use vibration isolation equipment. Manufacturers generally sell a range of systems, ensuring that something is available to suit all applications and budgets.

There are two key components in any vibration isolation system: an optical table (which counteracts high-frequency noise) and a set of legs to support and raise the table (which counteracts low-frequency noise). This month's Buyer's Guide takes a look at these components and the common sources of vibration found in laboratories.

1. Sources of noise Noise sources fall into one of three categories: ground-borne; acoustic; or experimentally-generated.

Ground-borne noise can be caused by any number of sources, including traffic, lift shafts, doors banging, people walking along corridors and construction work. It may also come from hidden sources, such as underground trains passing deep below the laboratory. Manufacturers divide this noise into vertical and horizontal components, as different mechanisms are used to counteract the vibrations in each direction.

Ground noise spectra are full of low frequencies, ranging from 1 to 100 Hz. It is the lower end of this spectrum that causes the most problems. There is an exponentially-decaying relationship between the frequency of the noise and the amplitude of the vibration. The cause is inertia - it is much harder to shake the ground at a high frequency than at a low one.

An important source of horizontal vibration is building sway, which comes in at around 1-10 Hz. The ideal location for an optics laboratory is in a basement, or on a vibration-isolated concrete floor, but more often than not laboratories are housed a few floors up - and the higher up the building the lab is, the greater the horizontal sway. The best place to site a sensitive experiment in this situation is near structural elements such as vertical pillars.

Acoustic noise is the result of the movement of air through the laboratory. Modern laboratories may be fitted with air- conditioning, which causes air currents that can disturb the optics. Currents can also be created when doors are opened or closed. By contrast, experimental noise - sometimes called locally-generated noise - comes from components on the optical table itself, such as the cooling fans in lasers or motorized equipment.

Acoustic and experimental noise has a higher frequency than ground-borne noise, generally in the 20-500 Hz range.

It is important to be aware of the sources of noise around your laboratory before talking to a sales engineer. Vendors usually offer vibration surveys, but most agree that these are not necessary unless an ultra-sensitive experiment is being performed.

2. Optical tables An optical table is essentially a flat surface with a grid of threaded holes drilled into the top for mounting and securing optics. The table is designed to counteract the effects of high-frequency acoustic and experimentally generated noise.

Before you select an optical table, it is important to decide how crucial its performance is to your experiment. It is also a good idea to think about the dimensions you require. Tables are supplied in a range of standard sizes, typically up to 1.5 x 4.25 m, and can also be joined end-on or in customized formations, such as an L- or E-shape. Where space is at a premium, smaller tables called breadboards can be used.

Optical tables require built-in damping because they have a natural frequency. This is typically around 90 Hz, falling within the range of common acoustic and experimentally-generated noise. A table will flex and bend in response to its natural frequency and harmonics and to minimize the vibrations, the design goal of any optical table is to be as stiff and light as possible.

According to vendors, optical tables must be stiff to ensure that the resonant frequencies are high, but light to ensure that the amplitude of vibration at the resonant frequencies is low. Table manufacturers have found that the best way to satisfy these criteria is to sandwich a honeycomb structure between two metallic plates.

The level of accuracy required from an experiment dictates the level of damping built in to the table. At the high-performance end of the scale, vendors offer "frequency-tuned damping." A damper is tuned to the natural frequency or a harmonic and embedded along the edge of the table. State-of-the-art tables contain many dampers, each tuned to a different frequency.

A damper is typically a sealed vessel filled with oil and containing a weight on a spring. The frequency of the spring is designed to match a resonant frequency of the table. If the table tries to vibrate at this frequency, the damper absorbs the energy of the vibration.

Makers of optical tables use epoxy to glue the honeycomb structure edge-on to the metallic top and bottom surfaces. Standard lower-performance tables use the energy-dissipating properties of the epoxy alone. Vendors tend to offer three or four grades of table with varying degrees of built-in damping.

The final factor that influences the performance of a table is its thickness. Standard optical tables are 8 inches thick, but stiffer optical tables could be 12-18 inches thick. The more sensitive the experiment, the thicker the table on which it is mounted should be. One rule of thumb states that the minimum table thickness should be between 6 and 10% of its length.

According to the vendors Opto & Laser Europe spoke to, a common pitfall for buyers is access. Optical tables may weigh in excess of a quarter of a tonne. It is important to consider how the table will get from the delivery van to the laboratory. If the only way is up a tight staircase or in a small goods lift, there could be a problem. Most vendors offer the option of a full installation service, which is well worth considering.

3. Vibration isolation support systems Vibration isolation support systems - essentially legs - are designed to raise the table to a comfortable working height and counteract low-frequency ground-borne vibrations. Support systems should be chosen while considering the performance you require for your experiment.

One vendor offered a simple equation for selecting support systems. The natural frequency of a building in Hz is equal to 46 divided by the height of the building in metres. For example, a 10 m-tall building will have a natural frequency of 4.6 Hz. This gives a guide to the frequency that has to be eliminated through the support system for a sensitive experiment to succeed.

Vibration isolation support systems, like optical tables, have their own resonant frequency. Noise at this frequency will be amplified, but noise at either side of it will be attenuated. According to one vendor, vibration isolation support systems are designed to have a low resonant frequency so that attenuation can commence at the lowest possible frequency.

High-performance vibration isolation systems counteract both the vertical and horizontal components of ground-based noise. The most advanced models use air springs to support load-carrying pistons as well as dual air chambers to eliminate vertical vibration, while damped pendulums reduce the horizontal component.

However, if your experiment does not require such high performance, an alternative might be to use a self-levelling table. These contain air valves that open and close to take the table back to its equilibrium position, and legs that can be pumped up with a foot pump. Rigid legs that essentially raise the table to a working height and offer no isolation are a more basic option.

Depending on the size of the table, either four or six legs will be required to support it. A four-leg system can typically support 2500 kg, which includes the weight of the table. It is important to have a balanced load spread evenly across the table.

All vendors agree that an optical table and support system should be viewed as an investment. With complete systems ranging in price from €3500 at the low-performance end to high-performance systems costing in excess of €7000, it is easy to see why vendors tell customers to think of their future as well as their current needs.