Laser Interferometer

Laser sources for precise vibration measurements

An introduction to the world of lasers and vibrometers

The type of laser source plays a crucial role in precise non-contact vibration measurement, especially in the development of advanced laser Doppler vibrometers (LDVs) for various applications. In the following, we will take a closer look at two types of laser sources and discuss their applications in vibrometry.

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

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 gas laser consists of a gas mixture of helium and neon and emits red light with a wavelength of 633 nm. This measuring beam is visible to the human eye and can be aligned without aids. In addition, the 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 if the measuring surface has sufficient reflectivity or is to be measured through water, as light with a wavelength of 633 nm passes through water almost unhindered.

The output power of the HeNe laser is limited to 1 mW in order to protect the human eye. HeNe-based laser Doppler vibrometers (LDVs) from Optomet are classified in laser protection class 2 in accordance with EN 60825-1 and are safe for brief 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. 

The HeNe laser has excellent coherence with very long coherence lengths and is therefore ideally suited for use in interferometry.
It is important to note that the HeNe laser has more than one longitudinal mode. Wave trains with closely spaced frequencies 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 is noticeable over their service life in a decreasing laser power up to complete failure. 

Overall, the helium-neon laser is a proven and reliable laser in vibrometry, which 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 here 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 photodetectors 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. Optomet SWIR vibrometers therefore always have laser protection class 2 in relation to the overall system.

Due to the highest SNR, SWIR vibrometers are 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, there is no need to prepare the measuring surface with the infrared vibrometer. The SWIR vibrometer is also the best choice for measurements over long distances, on glowing surfaces and on biological tissue. High temperature stability and a long service life offer additional advantages.

Applications in practice: Laser Doppler vibrometers

Laser Doppler vibrometers (LDVs) are indispensable tools in many fields, including engineering, materials science, biology, medicine and many more. They enable non-contact, precise measurements of oscillations and vibrations on surfaces of various materials and structures.

From quality control in manufacturing to the examination of biological tissue, the applications of LDVs are almost limitless. Thanks to their ability to provide high-resolution data in real time, LDVs are an indispensable tool in research, development and quality control.

The future of vibrometry: innovations and advances

Vibrometry is a constantly evolving field, driven by technological innovations and new research findings. Continuous improvements in laser sources and LDV technologies are enabling more precise measurements and an even wider range of applications.

From classic HeNe lasers to modern infrared lasers, the choice of vibrometers is more diverse than ever before. At Optomet, we are constantly working to improve our products and services and offer our customers innovative solutions for their measurement tasks.

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