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Application boom drives vibrometry explosion

24 Sep 2002

It might not be the most glamorous application in the optical technology sector, but laser Doppler vibrometry could claim to be one of the most versatile. Sales of the systems are booming, and the niche market is enjoying double-digit growth, writes Michael Hatcher.

From Opto & Laser Europe October 2002

There can't be a great deal of things that the former power plant at Chernobyl has in common with a 14th-century fresco in Italy, but the use of laser Doppler vibrometry (LDV) is one of them.

In fact, it seems that LDV has been used just about everywhere. The method is commonly used to analyse anything from car engines, brakes and tyres, to washing machines and eardrums. Also used at disaster sites to assist in rescue operations - it was employed at the World Trade Center after last year's attacks - the robustness and versatility of LDV systems is clear.

LDV is a non-destructive, non-contact technique that measures the vibrations of a target object (see box). The movement of a light-reflecting surface causes a Doppler shift in laser light, which can be detected and analysed using interferometry.

Automotive applications Behind the scenes, LDV has been making serious headway in the automotive sector in recent years. After initial applications in research and development, the technique is now making inroads into the industry's entire supply chain.

One example of this is the use of LDV by brake systems manufacturers. Brake squeal is a complex problem that arises from vibrations caused by contact between brake discs and pads. Using LDV, engineers can quickly identify the cause of the squeal and repair it. Other uses include the identification of low-vibration materials to improve soundproofing in cars.

Applications are not restricted to mass-manufactured cars - motorsports companies BMW, McLaren and Cosworth have all used a high-speed laser vibrometer to optimize their high-performance engines.

Alexander Brozeit is product manager for Polytec's LDV systems in Germany. He says the sector is enjoying double-digit growth - a boast that few can make in today's depressed photonics industry. Brozeit says Polytec is the current market leader outside Japan, with some 15 products on the market. The other significant player in the European market is Ometron, a subsidiary of the UK technology group Sira.

Brozeit is reluctant to reveal the size of the market, but he says that Polytec is targeting LDV as a high-growth area and is shipping thousands of systems every year.

"The real benefit of LDV is that it provides continuous, real-time information in the spectral domain," said Brozeit. A major contributor to the recent success of LDV is its simplicity. Brozeit told OLE: "A vibrometry system can be set up easily, and after just one day's training anybody can use it."

The heart of an LDV system is typically a low-power helium-neon or argon-ion laser (the narrow linewidth of these gas sources is crucial, as the Doppler shift of scattered light is very small). The optics inside an LDV system are well established - recent advances have instead involved signal capture and processing and shrinking the size of the vibrometer. "We can measure displacements of just 3 pm, and our signal is shot-noise limited, so the optics is pretty much at the limit," Brozeit said.

Polytec's latest development is a hand-held LDV featuring digital signal processing. Analogue signal processing tends to suffer from aging, thermal drift and non-linear effects - problems that should not arise with a digitized Doppler signal. According to Polytec, its digital system ensures high calibration accuracy and sub-nanometre displacement resolution - key factors for the testing and characterization of microelectromechanical system (MEMS) devices (figure 1). The only drawback with this system is that it cannot cope with high-frequency vibrations of more than 2 MHz.

Clinical use The heavy machinery of car manufacturing is a far cry from the delicate structure of the human eardrum. But LDV has found a use here too - enabling non-contact analysis of the eardrum's response to sound.

Although it is not yet in clinical use, LDV can differentiate between a number of middle-ear diseases and has been endorsed in a recent study by a clinical team from the University of Zurich, Switzerland. The team concluded that: "Laser Doppler interferometry provides a valuable addition to routine audiological investigations."

LDV has also been used to study the vibration characteristics of materials that might be used to reconstruct or even replace a malfunctioning eardrum.

Ear examinations could be just the start of a wider field of applications in biomedicine: for example, Lorenzo Scalise and colleagues at the University of Ancona, Italy, have used LDV to measure the tensile properties of an Achilles tendon. "There is not, to our knowledge, a non-invasive technique allowing measurement of the tendon's biomechanical properties," said Scalise.

Currently, the health of this tendon can only be determined from the sensations of the patient and an expert medical opinion. Using LDV could provide an objective measurement of the tendon's health.

Italian expertise Scalise and his colleagues are nearing the end of a series of in vitro animal studies at the moment, with in vivo follow-up studies planned for the future.

Under the leadership of Enrico Tomasini, the world-leading Ancona group hosts the Italian Association of Laser Velocimetry (AIVELA) conference every two years. A burgeoning interest in the medical applications of LDV is also much in evidence at the annual Laser Florence meeting, where Tomasini regularly holds a course on LDV in medicine, aimed at doctors and dentists.

The Ancona group is at the forefront of many application areas. In cooperation with Ometron, Paolo Castellini and colleagues have used the technique to assess the condition of precious works of art under the EU project Laserart. The project, which included tests on a number of icons and 14th-century frescoes at Orvieto cathedral in Italy, led to the development of Ometron's Duoscan vibrometer.

Using acoustic excitation to induce a measurable vibration, the LDV measurements showed up several damaged areas of the fresco that had not previously been identified by the restorers, or that they had considered insignificant. The areas showing intense "contours" of vibration, highlighted in red and yellow in the fresco image (figure 2), indicate the places where paint has become detached from the surface.

Another Ometron LDV system has been used to optimize the design of loudspeakers. Instead of the usual subjective optimization process, which involves listening to the speaker output after each design "tweak", US speaker-design firm Mackie Designs used vibrometry to get an objective picture of the deformations produced at the speaker's surface as it vibrates.

To the design team's surprise, the LDV analysis revealed that a hard, metallic speaker dome produced a higher-quality treble (high-frequency sound) than a dome made from a soft fabric. Company founder Greg Mackie was impressed by the LDV, saying: "It produces accurate, instant images of the vibrations that occur in a transducer dome or cone at any given frequency."

Mackie even suggested that the LDV could "do for speaker design what the oscilloscope did for electronic design".

Polytec's Georg Siegmund, who helped develop the digital LDV, believes that the test and characterization of MEMS structures is a rapidly expanding field that LDV can exploit. Polytec has now developed specialized vibrometer accessories, which scan the MEMS surface with a microscope.

With such a range of application areas demanding non-contact, non-destructive motion analysis, continued growth of this niche sector would appear assured.

Back to basics: how laser Doppler vibrometry worksLaser Doppler vibrometry is based on detecting and analysing the laser light that is scattered on impact with an object. If the object is in motion, the scattered light will be affected in two ways. First, the motion will induce a Doppler shift in the frequency of the scattered light: to the blue if the object is moving towards the light source, and to the red if it is moving away. By measuring this change in frequency, the object's velocity can be calculated.

Second, the phase of the scattered light will be affected, owing to the object changing its position. A heterodyne interferometer detection system measures the subtle frequency and phase change caused by the moving target. Inside the interferometer, the laser beam is divided into reference and signal paths. The signal beam is fired off towards the vibrating structure before the scattered light is recombined with the reference on a photodetector.

Polytec LDV systems use a helium-neon source, emitting less than 1 mW. LDV can typically measure displacement frequencies of up to 20 MHz, and stand-off distances from anything between 4 cm and more than 100 m can be accommodated.

According to Polytec's LDV product manager Alexander Brozeit, LDV has a number of advantages over similar techniques such as electronic speckle-pattern interferometry (ESPI). For example, using ESPI often means that the investigated surface needs to be coated to enhance the amount of light it reflects, whereas this is not necessary with LDV. ESPI also requires a high-power laser and a laboratory-style optical bench with vibration isolation, whereas some LDV systems come packaged in a hand-held module. LDV also gives the user a continuous flow of information on the object's vibration frequency. Finally, the displacement resolution of LDV is approximately 1000 times better than that of ESPI.

"To do ESPI, you need a physicist, but anybody can use an LDV system," said Brozeit. Of course, there are some drawbacks to LDV: ESPI is useful for obtaining lots of information about a surface in one shot, whereas LDV can only measure vibration frequencies up to approximately 20 MHz - making high-frequency applications, such as GHz filters in GSM phone handsets, all but impossible.

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