The Laser Source – Helium-Neon & Infrared
The laser source is a critical factor in determining the performance and eye safety of the laser Doppler vibrometer. In addition to the traditional helium-neon (HeNe) laser based vibrometers, Optomet pioneered and offers short wavelength infrared (SWIR) laser based vibrometers.
The Helium-Neon Laser
The HeNe laser source operates at a wavelength of 632nm. The red measurement beam is visible to the naked eye and can be positioned without the need for a targeting laser. With the comparatively short wavelength, the laser beam can be focused to a very small spot diameter. This makes the HeNe laser vibrometer highly suitable for measuring microstructures. Due to the negligible absorption, the HeNe laser can also measure through water.
The output power of the HeNe laser is limited to 1 mW for eye safety considerations. HeNe-based vibrometers are therefore regularly used on highly reflective surfaces when sufficient signal is scattered back from the measurement object.
The HeNe laser has more than a longitudinal mode: wave trains with frequencies close to each other overlap to create beat frequencies, and as a result the intensity of the measuring beam, and thus the signal quality, depends on the working distance.
Infrared Vibrometry
For best signal and data quality on poorly reflective surfaces or on objects far away, Optomet developed the SWIR (Short Wavelength Infrared) laser vibrometer. The invisible measurement beam is positioned using a coaxial green targeting laser.
With a wavelength of 1,550 nm and up to 10 mW of laser power, ten times the output power is available compared to a HeNe laser – while having the highest eye safety class (Class 1). In conjunction with the three-fold higher efficiency of the photodetectors, infrared vibrometry creates the conditions for sufficient signal quality for almost all vibration measurement tasks – regardless of the measurement distance, thanks to a single longitudinal laser mode. High temperature stability and a long service life offer additional added value.
Even on dark, rough or surfaces with low reflectivity, there is no need to treat the measurement surface when using the Optomet SWIR Laser vibrometer. Measurements over long distances, on glowing surfaces or even on biological tissue are the types of challenging applications where the Optomet SWIR vibration measurement technology truly shines.
