Laser Interferometer

The Laser Source - HeNe & SWIR

The choice of the laser source has a major impact on the performance of a laser Doppler vibrometer (LDV). Eye-safety is of great importance in a typical working environment, too. Optomet laser Doppler vibrometers utilize cutting-edge Short-Wavelength-Infrared technology or traditional Helium-Neon laser sources. Conclusively, a laser vibrometer by Optomet guarantees highest health safety standards while maintaining the best achievable signal quality.

Helium-Neon laser vibrometers

Laser doppler vibrometers with Helium-Neon (HeNe) lasers have been the traditional choice for many years. The laser operates at a wavelength of 632.8 nm. The red measurement beam is visible to the naked eye and can be positioned without additional aid. While modern laser vibrometry focusses on SWIR laser sources, HeNe systems are still highly relevant in specific use cases.

  • Helium-Neon lasers operate in the visible regime. Visible light is only very weakly absorbed in water allowing HeNe lasers to measure through it. Thus, they are irreplaceable in all cases when the laser has to traverse a thick layer of water to hit the vibrating object.

  • The theoretical limit of the smallest laser-spot size is directly proportional to the wavelength of the laser.  HeNe lasers have a shorter wavelength than SWIR lasers, which also leads to a smaller laser-spot size. HeNe lasers can be focused to a spot size in the scale of a few um and are the first choice for measurements of very small microstructures. 

In order to achieve laser safety class 2 (eye-safe), the output power of the HeNe laser is limited to 1 mW. HeNe-based LDV are therefore routinely used when the surface of the measurement object reflects enough light to produce sufficient signal quality.

The HeNe laser emits 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 laser vibrometers

Optomet pioneered Short-Wavelength-Infrared (SWIR) laser technology for optimum signal quality in the most challenging measurement tasks. Aligning the invisible laser spot is carried out with the help of a green, coaxially guided targeting laser. Some of the main benefits of infrared laser vibrometry are:

  • The SWIR laser operates at a wavelength of 1,550 nm with a laser power of10 mW, ten times the power of a HeNe system, while maintaining full eye safety at laser safety class 1.

  • Photodetectors in this infrared range have an efficiency of more than 98%. Compared to a HeNe-Vibrometer the increased laser power and higher photo-efficiency lead to a 20 dB higher SNR.

  • The SWIR laser vibrometer has a single stabilized laser mode which guarantees stability of the measurement with respect to changes in the working distance or ambient temperature.

These factors create the basis for superior signal quality in nearly all vibration measurement tasks, regardless of the working distance or the suface properties.

Not only dark, non-smooth or other surfaces with poor reflectivity but also measurements over large distances are the uncontested domain of SWIR vibrometry. High signal levels on all surfaces, excellent temperature stability, and a long laser life time of more than a decade offer a manifold magnified value in using SWIR technology in laser doppler vibrometers.
 

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