Laser source
Wavelength and coherence determine signal quality, speckle behavior, and the usable measurement distance. HeNe and SWIR lasers are therefore suited to different surfaces.
5 min readLaser Doppler Vibrometry – Technology
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, particularly in the development of modern Laser Doppler vibrometers (LDVs) for a wide range of applications. Below, we take a closer look at two types of laser source and discuss their applications in vibrometry.
Helium-neon lasers (HeNe): the classic choice for vibrometry
Helium-neon laser sources (HeNe laser sources) have long been used in conventional Laser Doppler vibrometers and, thanks to their unique properties, are a valuable tool for non-contact vibration measurement. The helium-neon laser's distinctive characteristics and versatility make it indispensable in vibrometry. This gas laser uses a mixture of helium and neon and emits red light at a wavelength of 632.8 nm.
Commercial HeNe lasers are relatively compact devices, with cavity lengths typically ranging from 15 to 50 cm, though they can reach approximately 1 meter to achieve the highest power levels. Their optical output power ranges from 0.5 to 50 mW. The exact wavelength of red HeNe lasers is 632.991 nm in a vacuum and approximately 632.816 nm in air due to refraction.
The beam emitted by HeNe lasers is visible to the human eye and can be aligned without any aids. Its comparatively short wavelength allows the laser beam to be precisely focused on a very small measurement point, making the HeNe laser particularly suitable for measuring microstructures and fine details.
Vibrometers with HeNe lasers are commonly used when the measurement object scatters back a sufficiently strong signal, when the measurement surface has sufficient reflectivity, or when measurements need to be taken through water, as light with a wavelength of 633 nm penetrates water almost unhindered.
Infrared lasers: the versatile solution
Optomet uses the latest and most advanced short-wave infrared (SWIR) laser technology to ensure the best signal quality for demanding measurement tasks.
The SWIR laser emits invisible light at a wavelength of 1,550 nm with an output power of up to 10 mW. This is ten times the output power of a HeNe laser. Combined with photodetectors offering more than 98% efficiency, this results in a 20 dB higher signal-to-noise ratio (SNR) than conventional HeNe LDVs.
Despite its higher laser power, the SWIR laser is rated as laser safety class 1 in accordance with EN 60825-1 and is therefore eye-safe. The fluid in the eye absorbs the infrared light, thereby protecting the eye. The invisible measurement beam is positioned using a coaxially aligned green pilot laser (wavelength: approx. 532 nm). The pilot laser's power is limited to 1 mW, resulting in a laser safety class 2 rating in accordance with EN 60825-1. Consequently, the complete Optomet SWIR vibrometer system is always rated as laser safety class 2.
Today, aside from certain applications that require the unique properties of HeNe lasers, SWIR lasers are the optimal choice for Laser Doppler vibrometry, with Optomet leading the way in SWIR vibrometry. By combining higher output power with more sensitive photodetectors, SWIR vibrometers deliver a higher SNR and are therefore particularly well suited to demanding measurement tasks where vibrometers with conventional laser sources reach their limits.
Even when measuring on dark, rough, curved, or other low-reflectivity surfaces, infrared vibrometers eliminate the need for surface treatment. Vibration measurements over long distances are also the undisputed domain of SWIR LDVs, as are measurements on glowing hot surfaces and biological tissue. High temperature stability and a long service life provide further advantages. All these characteristics also apply to the new SMART series from Optomet.
Infrared lasers: the all-rounders
Except for certain applications that require the unique properties of HeNe lasers, SWIR lasers are now the optimal choice for laser Doppler vibrometry. Optomet is a leader in SWIR vibrometry. By combining higher output power with more sensitive photodetectors, SWIR vibrometers provide a higher SNR, making them particularly suitable for demanding measurement tasks where vibrometers with conventional laser sources reach their limits. Even on dark, rough, curved, or other low-reflectivity surfaces, infrared vibrometers eliminate the need to treat the measurement surface. Vibration measurements over long distances are also the undisputed domain of SWIR LDVs, as are measurements on glowing hot surfaces and biological tissue. High temperature stability and a long service life provide further advantages. All these characteristics also apply to the new SMART series from Optomet.
Despite its higher laser power, the SWIR laser meets the highest laser safety classification, Class 1, in accordance with EN 60825-1 and is therefore eye-safe. The fluid in the eye absorbs the infrared light, thereby protecting the eye. The invisible measurement beam is positioned using a coaxial green pilot laser (wavelength: approx. 532 nm). The pilot laser's power is limited to 1 mW, resulting in a Class 2 laser safety rating in accordance with EN 60825-1. Optomet SWIR vibrometers therefore always have a Class 2 laser safety rating for the overall system.
Real-world applications of laser Doppler vibrometers
Laser Doppler vibrometers (LDVs) are indispensable tools in many fields, including engineering, materials science, biology, medicine, and many others. They enable precise, non-contact measurements of oscillations and vibrations on the surfaces of various materials and structures.
From quality control in manufacturing to the analysis of biological tissue, the potential applications of LDVs are virtually unlimited. Their ability to deliver high-resolution data in real time makes LDVs indispensable tools in research, development, and quality control.
The future of vibrometry: innovations and advances
Vibrometry is a constantly evolving field driven by technological innovation and new research findings. Continuous advances in laser sources and LDV technologies enable more precise measurements and an even wider range of applications.
From classic HeNe lasers to modern infrared lasers, the range of vibrometers is more diverse than ever. At Optomet, we continuously strive to improve our products and services and provide our customers with innovative solutions for their measurement tasks.