Frequently Asked Questions about Laser Doppler Vibrometer

The LV-0121A digital displacement meter (fringe count) is an optional component for displacement measurement that utilizes an interferometer built into the sensor head, and is used in combination with Laser Doppler Vibrometer. It counts the brightness and darkness of light (interference fringes: fringes) caused by the phase difference between the reference light and the reflected light from the object being measured, and outputs the displacement as both a digital and analog signal.

The digital signal offers a maximum sampling rate of 1 MHz and a maximum resolution of 0.155 nm ^(*), while the analog displacement signal allows for minute displacement measurements including a DC offset of DC to 100 kHz (-3 dB). (*Requires displacement analysis software LV-0930.)

The LV-0121A digital displacement meter can measure transient displacements (rectangular) from a stationary state (DC) of an object, as well as displacement amplitudes including DC offset. By using the displacement output of the digital displacement meter for low-frequency amplitudes and displacements including offset, and the velocity output of the Doppler vibrometer for high-frequency amplitudes, it enables wideband amplitude measurement.

principle

The LV-0121A digital displacement meter operates on the principle that when a laser beam is shone onto an object and the reflected laser beam is superimposed with the original incident light, the intensity of the light is maximized if the phases of the light (the peaks and troughs of the wave) match, and cancel each other out and become dark if they are completely opposite.

In the diagram below, when _L1 = _L2, the phases of the reflected light from the reference mirror and the reflected light from the measuring mirror are aligned, resulting in a brighter image. Next, when the measuring mirror moves and the optical path difference becomes λ/2 (λ = the wavelength of the laser's oscillation), the phases become opposite (shifted by 180 degrees), and the image becomes dimmer.

In this way, as the measuring mirror (object being measured) moves, interference fringes of light and dark are generated at intervals of λ/2 in the optical path difference between _L1 and _L2. By counting these interference fringes, the amount of displacement can be measured.

Note: The oscillation wavelength (λ) of the He-Ne laser used in the LV series is approximately 632.8 nm. The LV-0121A digital displacement meter measures displacement based on this wavelength.

Measurement example

Rectangular displacement value of the cantilever using the LV-0121A digital displacement meter.