Helium-neon (HeNe) laser: the classic in vibrometry

Helium-Neon laser sources (HeNe laser sources) have been used for conventional laser Doppler vibrometers for many years and, thanks to their unique properties, are a useful tool for non-contact vibration measurement. The helium-neon laser is an indispensable tool in vibrometry thanks to its unique properties and versatility. The gas laser consists of a gas mixture of helium and neon and emits red light with a wavelength of 632.8 nm in the red spectrum.

Commercial HeNe lasers are relatively small devices with cavity lengths usually between 15 and 50 cm, but sometimes up to about 1 meter to achieve the highest powers, and optical output powers between 0.5 and 50 mW. The exact wavelength of red He-Ne lasers in vacuum is 632.991 nm and is refracted to about 632.816 nm in air.

This measuring beam of the HeNe laser is visible to the human eye and can be aligned without any aids. This comparatively short wavelength makes it possible to focus the laser beam precisely on a very small measuring point, which makes the HeNe laser particularly suitable for measuring microstructures and fine details. Vibrometers with HeNe lasers are therefore regularly used if a sufficient signal is scattered back from the measurement object, the measurement surface has sufficient reflectivity or measurements are to be taken through water, as light with a wavelength of 633 nm penetrates water almost unhindered.

HeNe laser sources have been used for conventional laser Doppler vibrometers for many years and are a useful tool for non-contact vibration measurement thanks to their unique properties. The helium-neon laser is an indispensable tool in vibrometry thanks to its unique properties and versatile applications. The gas laser consists of a gas mixture of helium and neon and emits red light with a wavelength of 632.8 nm in the red spectrum.

Commercial He-Ne lasers are relatively small devices with cavity lengths usually between 15 and 50 cm, but sometimes up to about 1 meter to achieve the highest powers, and optical output powers between 0.5 and 50 mW. The exact wavelength of red He-Ne lasers in vacuum is 632.991 nm and is refracted to about 632.816 nm in air.

This measuring beam of the HeNe laser is visible to the human eye and can be aligned without aids. This comparatively short wavelength allows the laser beam to be precisely focused on a very small measuring point, which makes the HeNe laser particularly suitable for measuring microstructures and fine details. Vibrometers with HeNe lasers are therefore regularly used when a sufficient signal is scattered back from the measurement object, the measurement surface has sufficient reflectivity or is to be measured through water, because . Light with a wavelength of 633 nm passes through water almost unhindered.

Another advantage of the HeNe laser is its safety. The output power of the HeNe laser is limited to 1 mW in order to protect the human eye, especially as it consists mainly of water. HeNe-based laser Doppler vibrometers (LDVs), such as the single-point vibrometer of the Vector series, are classified in laser protection class 2 according to EN 60825-1 and are safe for short-term exposure of the eye, as natural averting reactions (e.g. eyelid closure reflex) are effective. Staring into the laser suppresses the protective reflex and can damage the eye. are therefore used regularly if a sufficient signal is scattered back from the measurement object.

The HeNe laser has excellent coherence with very long coherence lengths and is therefore ideally suited for use in interferometry.

The coherence length of a helium-neon (HeNe) laser is usually a few centimetres to several meters, depending on the specific properties of the laser. This parameter determines the length of the interference range of the laser and is important for certain applications, particularly when used in interferometry or holographic techniques.

It is important to note that the HeNe laser has more than one longitudinal mode. Wave trains with frequencies close to each other overlap to form a beat, which means that the intensity of the measurement beam and therefore the signal quality is dependent on the working distance.

However, HeNe lasers are subject to a strong ageing process, which becomes noticeable over their service life in a reduction in laser power and even complete failure.

Overall, the helium-neon laser is a proven and reliable laser in vibrometry that is widely used in both research and industry due to its unique properties and versatile applications.

Infrared laser: the all-rounder

Optomet relies on the latest and most advanced short wavelength infrared (SWIR) laser technology for the best signal quality for demanding measurement tasks.

The SWIR laser used emits non-visible light at a wavelength of 1,550 nm with an output power of up to 10 mW. This provides ten times the output power of a HeNe laser. In combination with FPhotodetectors with an efficiency of more than 98%, this results in a 20 dB higher signal-to-noise ratio (SNR).

Despite the higher laser power, the SWIR laser has the highest laser protection class 1 according to EN 60825-1 and is therefore eye-safe. The liquid in the eye absorbs the infrared light and thus protects the eye. The non-visible measuring beam is positioned by means of a coaxially guided green pilot laser (wavelength: approx. 532 nm). The power of the pilot laser is limited to 1 mW, resulting in classification to laser protection class 2 in accordance with EN 60825-1. Therefore, theOptomet SWIR vibrometers always have laser protection class 2, based on  for the overall system.

For special applications where a greater range or higher signal quality is required, we at Optomet also rely on the advanced Short Wavelength Infrared (SWIR) technology. Due to the highest SNR, SWIR vibrometers offer a higher output power and are therefore particularly suitable for demanding measurement tasks where vibrometers with conventional laser sources reach their limits. Even on dark, rough, curved  or other surfaces with low reflectivity, the infrared vibrometer eliminates the need to treat the measuring surface. The SWIR vibrometer is also the undisputed domain of SWIR vibration measurement for measurements over long distances, on glowing surfaces and on biological tissue. High temperature stability and a long service life offer additional advantages.