VIBROMET TM 500V: SINGLE POINT LASER
DOPPLER VIBROMETER
Abstract
This paper discusses a Single Beam LDV product
offered by MetroLaser; specifically it discusses the VibroMetTM
500V single point laser Doppler vibrometer (LDV). A discussion of
the single point LDV system is presented including its principles,
hardware, software, capabilities and specifications, applications
and examples. To avoid confusion the single point system is referred
to as the 500V but references to the 500 and an earlier VibroMetTM
100 are made throughout since they have been around longer and some
of the data presented here were collected with these earlier
versions. All of the single point VibroMetTM share
numerous similarities; a major difference is the visible pointer
laser included in the new VibroMetTM 500V
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Introduction
The VibroMetTM 500V (and its
predecessors the VibroMetTM 500 and the VibroMetTM
100) is a heterodyne laser Doppler vibrometer that employs
self-mixing1,2 to mix the object beam with the reference beam (also
known as local oscillator) on the face of the photodetector. The
system employs a diode laser at a wavelength of 780 nm to produce
both the object beam (to illuminate target) and the reference beam
(local oscillator) and a Bragg cell acousto-optic modulator (AOM)
to
impress a carrier frequency. The photodetector is an integral part
of the diode laser module commonly used to monitor laser power. The
LDV system is based on a patented electro-optical configuration3
and
is schematically illustrated in Figure 1. Most of the light
from the laser diode comes out of the laser and is collimated by
a custom lens. The collimated beam goes through the AOM where part
of
the beam is diffracted and frequency shifted (typically 35 to 40
MHz); this is the object beam. Part of the collimated beam (the
zero
order) goes through the Bragg cell undiffracted and it is blocked
by a beam stop. The object beam illuminates the target and part of
the
light scattered by the target goes back into the AOM where it is
diffracted and frequency shifted again (for a total now of 70 to
80
MHz). This effect is known as double frequency shift and has the
beneficial effect of creating a carrier frequency at double the
RF
drive thus helping to separate the signal from any possible RF
noise. The frequency shifted object beam enters the diode laser
where it experiences gain and it is mixed with the reference beam
(a small portion of the diode laser beam normally employed to monitor
laser power). An optional focusing lens is shown on Figure 1.
It is employed to create a small focused beam that may be needed for
some applications. The standard VibroMetTM 500V does not
include a focusing lens but instead uses a collimated object beam.
The collimated beam allows the VibroMetTM 500V to work at
variable distances from a few cm to several meters without the need
to adjust or align any optical components.
Fig 1:
Schematic Diagram of MetroLaser VibroMetTM
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