The Laser Source

The choice of the laser source has a major impact on the performance of a laser Doppler vibrometer. Eye-safety also is of great significance in a typical environment. OptoMET laser Doppler vibrometer make use both of classical HeNe laser sources and increasingly the modern SWIR technology.

Helium-Neon laser

The Helium-Neon (HeNe) laser is operating at a wavelength of 632.8 nm. The red measurement beam is visible to the naked eye and can be positioned without additional aid. It can be focused to a very small spot for the measurement of microstructures. Due to the negligible absorption it is possible to measure through water.

For the same reason the output power of the HeNe laser is limited to 1 mW, in order to guarantee eye-safety. HeNe-based LDV are therefore routinely used when a sufficiently reflective surface reflects enough light for sufficient signal quality.

The HeNe laser is emitting more than one longitudinal mode: Wave trains with close frequencies overlap and form a beat signal. As a consequence the intensity, and hence the signal quality, becomes dependent on the working distance.

Infrared vibrometry

OptoMET is relying on SWIR (Short Wave Infrared) laser technology for optimum signal quality in challenging measurement tasks. Positioning the invisible laser spot is done with the help of a green, coaxially guided pilot laser.

At a wavelength of 1,550 nm an output power of 10 mW is available for measurement - still belonging to the highest eye-safety class. Water in the human eye absorbes the laser power before it can cause any damage. In combination with the threefold sensitivity of SWIR photodetectors as compared to the visible regime, this creates the basis for sufficient signal quality in nearly all vibration measurement tasks, regardless of the working distance due to a single longitudinal mode.

Not only dark, non-smooth or other surfaces with poor reflectivity but also measurements over large distances are the uncontested domain of SWIR vibrometry. An execellent stability w.r.t. temperature and a long life time offer added value in using SWIR technology.

 

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