Non-Contact Optical Vibration Measurement with Subnanometer Resolution
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Non-Contact Optical Vibration Measurement with Subnanometer Resolution

Non-Contact Optical Vibration Measurement with Subnanometer Resolution

:: 17 July, 2008

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The new LSV laser-interferometric vibrometer is the ideal instrument for accurate, non-contact determination of temporal changes in the positions of objects or surfaces of arbitrary roughness. The laser-based vibrometer can detect mechanical vibrations at frequencies ranging from 0 to 500 kHz, with subnanometer resolution. These instruments have been designed around the industry proven SP-S Series of laser-interferometric vibrometers

The working distance of the LSV-Series vibrometer can be adjusted continuously in a wide range by a zoom objective with a working distance of between 240mm and 2.5m.

The complete system consists of a compact fibreoptic-coupled sensor head, a modular electronics unit incorporating a HeNe laser, and various interfaces.

The fibre-coupled interferometer converts motions along the optical axes into interference fringes that are transmitted to fast, high-resolution, demodulation electronics for processing.

The operation and display of results employs a PC running specialised data-analysis software.

The modular nature of the system and the separate measurement head ensures that accurate and highly resolving measurements can be performed even in aggressive environments.

Rob Roach, Armstrong Optical Sales Director said: "The great strength of the system is its ability to make measurements over large distances, on vibrating surfaces with low reflectivity, making it ideal for a variety of tasks in many industrial sectors".

Note for Helium-Neon Laser
A helium-neon laser, usually called a HeNe laser, is a type of small gas laser. HeNe lasers have many industrial and scientific uses, and are often used in laboratory demonstrations of optics. Its usual operation wavelength is 632.8 nm, in the red portion of the visible spectrum.
The gain medium of the laser, as suggested by its name, is a mixture of helium and neon gases, in a 5:1 to 20:1 ratio, contained at low pressure (an average 50 Pa per cm of cavity length) in a glass envelope. The energy or pump source of the laser is provided by an electrical discharge of around 1000 volts through an anode and cathode at each end of the glass tube. A current of 5 to 100 mA is typical for CW operation. The optical cavity of the laser typically consists of a plane, high-reflecting mirror at one end of the laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end.

HeNe lasers are typically small, with cavity lengths of around 15 cm up to 0.5 m, and optical output powers ranging from 1 mW to 100 mW.

The red HeNe laser wavelength is usually reported as 632nm. However, the true wavelength in air is 632.816 nm, so 633nm is actually closer to the true value. For the purposes of calculating the photon energy, the vacuum wavelength of 632.991 nm should be used. The precise operating wavelength lies within about 0.002 nm of this value, and fluctuates within this range due to thermal expansion of the cavity. Frequency stabilized versions enable the wavelength to be maintained within about 2 parts in 1012 for months and years of continuous operation.

The laser process in a HeNe laser starts with collision of electrons from the electrical discharge with the helium atoms in the gas. This excites helium from the ground state to the 23S1 and 21S0 long-lived, metastable excited states. Collision of the excited helium atoms with the ground-state neon atoms results in transfer of energy to the neon atoms, exciting neon electrons into the 3s2 level. This is due to a coincidence of energy levels between the helium and neon atoms.

Note for Interferometry
Interferometry is the technique of using the pattern of interference created by the superposition of two or more waves to diagnose the properties of the aforementioned waves. The instrument used to interfere the waves together is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, quantum mechanics and plasma physics.

Interferometry makes use of the principle of superposition to combine separate waves together in a way that will cause the result of their combination to have some meaningful property that is diagnostic of the original state of the waves. This works because when two waves with the same frequency combine the resulting pattern is determined by the phase difference between the two waves -- waves that are in phase will undergo constructive inference while waves that are out of phase will undergo destructive interference. Most interferometers use light or some other form of electromagnetic wave.

Typically a single incoming beam of light will be split into two identical beams by a grating or a partial mirror. Each of these beams will travel a different route, called a path, before they are recombined at a detector. The path difference, the difference in the distance travelled by each beam, creates a phase difference between them. It is this introduced phase difference that creates the interference pattern between the initially identical waves. If a single beam has been split along two paths then the phase difference is diagnostic of anything that changes the phase along the paths. This could be a physical change in the path length itself or a change in the refractive index along the path.

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