emission of photons. With

hardtofindcurtainUrban and Civil

Nov 16, 2013 (3 years and 11 months ago)

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2

The

basic

physical

principle

of

a

laser

(light

amplification

by

stimulated

emission

of

radiation)

is

the

induced

emission

of

photons
.

With

induced

emission

the

emitted

photons

have

identical

properties

and

thus

produce

coherent

light

of

the

same

wavelength
.

A laser consists of an optical cavity which contains the lasing material
with a mirror placed at each end. The light, which is repeatedly reflected
between the two mirrors, is amplified. As one of the mirrors is only
partially reflecting, a small laser beam emanates from the cavity. To
keep the process going, energy is supplied in order to excite the atoms
in the lasing material.

3

The

wave

along

the

laser

cavity

is

a

standing

wave

and

the

cavity

of

length

L

only

resonates

when

there

is

an

integral

number

n

of

half

wavelengths

l

between

the

mirrors
:

n

=

2
L

/

l

The

frequency

W

is

given

by


W

=

nc
/

2
L

where

c

is

the

velocity

of

the

light
.

The

separation

of

the

multiple

wavelengths

(longitudinal

modes)

is

DW

=

c

/

2
L
.

However,

not

all

possible

modes

in

a

laser

cavity

will

be

excited
.

Only

those

that

are

within

the

gain

profile

of

the

lasing

medium

will

generate

an

emission
.


coherence length


The maximum distance over which the phase relationship will exist is called coherence length.
The relation between the coherence length and the bandwidth is c/Dl

The laser used for laser Doppler
vibrometers

is a helium neon (He
-
Ne) laser. This laser produces
a visible red beam (l = 0.6328 µm). This gas laser is an extremely low
-
noise light source and
therefore ideally suited for this application.

Such a laser can be stabilized so that only a single mode is excited. The line width is then a
couple of MHz which yields coherence lengths of about 200 to 300m.

Laser
vibrometers

are usually operated with multimode lasers where the lasers oscillate at 2
-
3
modes at the same time. The coherence length is only 10
-
20 cm because of the interference of
the beat frequencies of these different modes. However, this beat frequency generates a cos2
dependence of the visibility as the distance to the object is varied. The signal has a maxima at
distances of 2mL and minima at (2m
-
1)L where m is an integer. It is therefore possible to make
measurements at very long distances with such a laser. The measuring distance should be
adjusted to a visibility peak for maximum signal strength.



A

laser

Doppler

vibrometer

is

based

on

the

principle

of

the

detection

of

the

Doppler

shift

of

coherent

laser

light

that

is

scattered

from

a

small

area

of

a

test

object
.

The

object

scatters

or

reflects

light

from

the

laser

beam,

and

the

Doppler

frequency

shift

is

used

to

measure

the

component

of

velocity

which

lies

along

the

axis

of

the

laser

beam
.



As

the

laser

light

has

a

very

high

frequency


W

(approx
.

4
.
74

x
10
14

Hz),

a

direct

demodulation

of

the

light

is

not

possible
.

An

optical

interferometer

is

therefore

used

to

mix

the

scattered

light

coherently

with

a

reference

beam
.

The

photo
-
detector

measures

the

intensity

of

the

mixed

light

whose

beat

frequency

is

equal

to

the

difference

frequency

between

the

reference

and

the

measurement

beam
.



Heterodyne

and

Homodyne

detection



In

homodyne

detection,

for

a

given

relative

phase

shift

the

output

is

a

constant

(DC)

signal

level
.

This

level

is

indirectly

related

to

the

phase

shift
.



In

heterodyne

detection

one

modulates,

usually

by

a

frequency

shift,

one

of

two

beams

prior

to

detection
.

Optical

heterodyne

detection

detects

the

interference

at

the

beat

frequency
.

The

AC

signal

now

oscillates

between

the

minimum

and

maximum

levels

every

cycle

of

the

beat

frequency
.

Since

the

modulation

is

known,

the

relative

phase

of

the

measured

beat

frequency

can

be

measured

very

precisely

even

if

the

intensity

levels

of

the

beams

are

(slowly)

drifting
.

This

phase

is

identical

in

value

to

the

phase

one

measures

in

the

homodyne

case
.

There

are

many

additional

benefits

of

Optical

heterodyne

detection

including

improved

signal

to

noise

when

one

of

the

beams

is

weak
.



4

5

The

test

beam

is

directed

to

the

target,

and

scattered

light

from

the

target

is

collected

and

interfered

with

the

reference

beam

on

a

photodetector
,

typically

a

photodiode
.

Most

commercial

vibrometers

work

in

a

heterodyne

regime

by

adding

a

known

frequency

shift

(typically

30
-
40

MHz)

to

one

of

the

beams
.

This

frequency

shift

is

usually

generated

by

a

Bragg

Cell
,

or

Acousto
-
Optic

Modulator
.

6

One

difference

from

Bragg

diffraction

is

that

the

light

is

scattering

from

moving

planes
.

A

consequence

of

this

is

the

frequency

of

the

diffracted

beam

f

in

order

m

will

be

Doppler
-
shifted

by

an

amount

equal

to

the

frequency

of

the

sound

wave

F
.



A

photodiode

is

a

type

of

photodetector

capable

of

converting

light

into

either

current

or

voltage
,

depending

upon

the

mode

of

operation
.

Photovoltaic mode

a solar cell is just an array of large area
photodiodes.

Photoconductive mode

In this mode the diode is often
reverse
biased
. This increases the width of the
depletion layer, which decreases the
junction's
capacitance

resulting in faster
response times.



In
radio

and
signal processing
,
heterodyning

is the generation of new frequencies by mixing, or multiplying, two
oscillating

waveforms. It is useful for
modulation

and
demodulation

of signals, or placing information of interest
into a useful
frequency range
. This operation may be accomplished by a
vacuum tube
,
transistor
, or other
signal
processing

device. Mixing two frequencies creates two new frequencies, according to the
properties of the sine
function
: one at the sum of the two frequencies mixed, and the other at their difference. Typically only one of
these frequencies is desired

the higher one after modulation and the lower one after demodulation. The other
signal is either not passed by the tuned circuitry that follows, or may be filtered out.





Optical heterodyne detection


Since optical frequencies are far beyond any feasible electronic circuit bandwidth, all photon detectors are
inherently energy detectors not oscillating electric field detectors. However since energy detection is inherently
"square
-
law" detection, it intrinsically mixes any optical frequencies present on the detector. Thus sensitive
detection of specific optical frequencies is possible by
Optical heterodyne detection

when two different (close
-
by) wavelengths of light illuminate the detector so that the oscillating electrical output corresponds to their
difference frequency. This allows extremely narrow band detection (much narrower band than any possible color
filter can achieve) as well as precision measurements of phase and frequency of a signal light relative to a
reference light source.



The heterodyne detection of the
vibrometer

signal


The

scattered

or

reflected

light

has

a

frequency

equal

to

f
o

+

f
b

+

f
d
.

This

scattered

light

is

combined

with

the

reference

beam

at

the

photo
-
detector
.

The

initial

frequency

f
o

of

the

laser

is

very

high

(>

10
14

Hz),

which

is

higher

than

the

response

of

the

detector
.

The

resulted

beat

frequency

between

the

two

beams,

which

is

at

f
b

+

f
d

(typically

in

the

tens

of

MHz

range)
.


The

output

of

the

photodetector

is

a

standard

frequency

modulated

(FM)

signal,

with

the

Bragg

Cell

frequency

as

the

carrier

frequency,

and

the

Doppler

shift

as

the

modulation

frequency
.

This

signal

can

be

demodulated

by

demodulator

in

the

instrument

controller

to

derive

the

velocity

vs
.

time

of

the

vibrating

target
.



7

8


An

excitation

(shaker,

loudspeaker
,

hammer

etc
.
)

cause

the

object

under

investigation

to

vibrate
.




The

heterodyne

detection

signal

is

demodulated

by

the

decoder

in

the

controller
.


An

output

of

which

is

proportional

to

the

velocity

of

the

vibration

parallel

to

the

measurement

beam

is

achieved

at

the

vibrometer

channel
.



The

voltage

data

on

the

vibrometer

channel

is

sampled

by

DAQ

system

in

the

industrial

PC
.

Final

data

is

presented

by

PSV

software

system
.

The
excitation

type



Periodic

i.e.
with

a
repeating

signal (
sinsusiodal
,
periodic

chirp
,
periodic

random
,
etc
)


Transient i.e.
with

a pulse (
e.g
.
rectangular

pulse, hammer
blow
, etc.)


Stochastic

(
random
) i.e.
with

noise

(
noise

generator,
self

excited
)

9

Specular

surfaces, i.e.
highly reflecting
surfaces, obey the
law: angle of
incidence = angle of
reflection. When
making
measurements from
such surfaces, the
optics of the LDV
need to be aligned

so
that the reflected
light returns within
the aperture of the
collecting optics.

Diffuse surfaces scatter the
incident light over a large
angular area. The intensity
of the scattered light power
per unit solid angle follows
Lambert's cosine law. It can
vary greatly

between shiny
surfaces and dull black
surfaces that absorb most
of the light.

Speckle patterns are
always produced when a
coherent light source is
focused onto a rough
surface. It is caused by
interference effects
between the beams
originating at the different
scattering centers on the
surface. If the focused
spot is very small, the
number of scattering
centers is small and the
angular dependence of the
path length differences in
a given direction is also
small. This leads to a large
angle over which the
interference condition is
reasonably constant and
thus a large solid angle
for the speckle.

10


Aerospace
-

LDVs are being used as tools in non
-
destructive
inspection of aircraft components.


Acoustic
-

LDVs are standard tools for speaker design, and have
also been used to diagnose the performance of musical instruments.


Automotive
-

LDVs have been used extensively in many automotive
applications, such as structural dynamics, brake diagnostics, and
quantification of Noise, Vibration,
and_Harshness

(NVH).


Biological
-

LDVs have been used for diverse applications such as
eardrum diagnostics

and insect communication.


Calibration
-

Since LDVs measure motion that can be calibrated
directed to the wavelength of the light, there are frequently used to
calibrate other types of transducers.


Hard Disk Drive Diagnostics
-

LDVs have been used extensively in
the analysis of hard disk drives, specifically in the area of head
positioning.


Polytec

offers a comprehensive line of scanning laser Doppler
vibrometers

(SLDV).
Scanning offers all the advantages of a laser
vibrometer

together with speed, ease
of use, laser positioning accuracy and comprehensive data processing in a single
automated, turnkey package.


Users get a very quick, easily understood and accurate visualization of a
structure's
vibrational

characteristics without the inconvenience of attaching and
interpreting data from an array of transducers.


11


Working distance

> 0.4 m

(shorter distances accessible by using close
-
up unit)


Laser wavelength


633 nm, visible beam


Laser protection class


Class II He
-
Ne laser, < 1
mW
, eye
-
safe


Sample size


Several mm² up to m² range


Scan grid


Multiple grid densities and coordinate systems (polar,
cartesian

and
hexagonal) each with up to 512 x 512 points


12

13

14


Position the PSV


The PSV system
measures

velocity

component

parallel to the laser
beam
. Position
scanning head and
object

such

that
:

1.

The laser
beam

can

cover the scanning
area

2.

The
longitudinal

axis

of the scanning head is
positioned

perpendicular

to the
area

to
be

scanned

3.
Optimize

the signal
level



Observe

the signal
level

indicator

at the back side of the scanning head. The
higher

the
value
, the
better

signal
-
to
-
noise

ratio
will

be
.



The
surface

of the
area

can

be

treated
.
Highly

reflective

and transparent
surfaced

usually

do not
scatter

enought

laser light back to the scanning head.
Evenly

matt

surfaces

are

ideal.
Water
-
slouble

white

wall

paint

can

be

used

for the
surface

treatment
.



15


Enable

laser and
beam

shutter


Adjust

the
camera

view

(
Autofocus
)


Laser
autofocus

(Signal
Intensity
)


Alignment

(Scanning
mirror
-
object

surface
)


Defined

scanning
area


Select

the scanning parameter (
excitation
,
decoder
,
frequency
, etc.)


Perform

the scan


Post
processing

the data


16

17

Alignment

18

Define

scanning
patterns

19

Scanning

20

Post
p
rocessing



http://en.wikipedia.org/wiki/Vibrometer


Polytec

Manual


21

Thanks
,

22