20kHz to 2MHz Laser Vibrometer

Dear Friends,
I am currently looking for a cheap Laser Vibrometer to measure
ultrasonic vibrations upto ideally 2MHz. I know Polytech have a line of
laser vibrometers but unfortunately its out of my budget. Currently i
am using AE (acoustic emission sensor) to measure vibrations upto 2MHz
but due to the size of the sensor i am unable to attach it directly to
the test specimen which in my case is a steel rod embeded in concrete
and hence i am using an attachment fixed to the rod and the sensor
placed on this attachment.

Does anyone have any suggestion on any type of noncontact measurement
which operates in a wideband (upto 2MHz or more) such as noncontact
ultrasonic transducers or magnetostrictive sensors or EMATs? My budget
is approximately $2000.

Is there any Laser measurement system or something that can be
fabricated to measure these high and wideband frequencies using lasers?
I read in previous posts where someone mentioned that its quite easy to
fabricate a laser based vibration measurement system...but i am quite
sceptical on the accuracy.

Thanking you in advance for your time and any comments are much
appreciated,

best regards,
Ravi

 

I use the Ono Sokki. I think it's a 1700 series. I am away from the
clean area right now. But I don't think you can even buy a case for it
at $2000

 

Do you have a instrument capable of a fourier transform to say 4 Mhz
and 10 bit resolution laying around? If so the optical part is
difficult but maybe doable in a 2K budget, depending on your skills.

Steve Roberts

 

Hi Steve,
I have a card which is capable of fourier transform up to 20MHz with a
12 bit resolution. Do you know if i could outsource the fabrication as
i am not very skillful at this. Do you know where i can get info on
fabricating such a device? Is it really doable?

Thanking you in advance for your time,

regards,
Ravi

 

take a look at the following for some examples, its not too much
different from a laser doppler velocimeter

Sun Yusheng and Jin Jundong
Optics and Laser technology 20 (100) 1988

High resolution self-mixing laser-Doppler vibrometry with a
laser-diode-pumped microchip LiNdP4O12 laser.
Otsuka, Kenju; Fukazawa, Takeki; Sekita, Nobuatsu; Higashihara,
Shin-ichi.
Japanese Journal of Applied Physics, Part 2: Letters (2000),
39(7B), L763-L765. Publisher: Japan Society of Applied Physics, in
English.

Multipoint optical fiber vibrometer Raffaella Di Sante Lorenzo Scalise
Rev. Sci. Instrum. 73, 1321 (2002)


If you do it with fiber optics, it would not seem to be a problem,
again depends on what you need.
really take a look at:

Applied Physics Letters Volume 76 , Number 23, 5 June 2000.

minimum parts required:

1 hene laser or stable 1520 nm laser (telcom surplus, very cheap)
1 4 port fiber optic 50/50 coupler (~ 525$ for visible, much
cheaper for IR)
1 small length of fiber as needed maybe 15$
1 beaker of index matching fluid again maybe 50$
1 high speed photo detector depends a lot on specs.
4 couplers for the fiber optics,
1 microscope objective
1 aluminum plate to mount it on.

thats less then 1000$ in parts, leaving you 1000$ for a fast detector
and some positioning.

FFT board that you already have.

adding direction sensitivity or scanning to that is complex but
doable.


Steve Roberts

 

Could the optics from a dvd or cd device be used? I guess it could launch
into fiber with some effort. Would it be stable enough?

Tom

 

There may be no need to attach anything to the rod, or at minimum a
small reflective "patch".
Then one might use simple interferometry and a "$10" laser..

Or magnetize the rod or attach a small magnet (RS 64-1895 Rare Earth
Magnet), and use a simple coil pickup to an appropiate receiver.

Sound pickup to 2MHz i think would be a problem, but at least check
out the possibility.

 

Thank you so much for all the input. Steve, thanks alot for the listed
papers and your great suggestions, i will start looking at them.

Robert, thanks for your suggestions. Would a simple interferometry and
'$10' laser be accurate enough for high frequency measurement or would
it be better looking at fibre optic hydrophone type sensors (i have
included a website below)?

Just to give you a bit of detail on my application, I am using a novel
ultrasonic generator which is capable of vibrating any structure at
different natural vibration modes simultaneously using a self adaptive
feedback system. I will be using a pattern recognition software
(wavelet based analysis) to discern differences in vibration pattern to
infer the condition of the structure after some pre-processing of the
time domain signal (de-noising and SNR improvements using another
wavelet based analysis). In order to transform the time domain signal
to frequency domain i will be using a novel Hilbert transform which is
capable of frequency,phase and amplitude estimation with a relatively
high degree of acuracy. I came across this website
http://www.rp-acoustics.de/english/FOPH2000_e.htm
The Fiber Optic Probe Hydrophone and PVDF hydrophone are used to
measure liquid pressure but do you think a similar concept using
fibre-optics can be adapted for my application where i am measuring
vibrations in solids?

Once again, thank you so much for all your input, i really appreciate
it.

best regards,
Ravi

 

Almost sounds like you might want interference (2D/3D object) holograms.

 

Look at thorlabs, they stock a 2X2 fiber coupler, this has 4 fibers
coming out of it. One goes to the source laser, one goes to the
detector , one goes to a batch of index matching liquid in a beaker at
act as a dummy load for one port. The forth port is aimed at your
vibrating rod. This builds a sensitive fiber optic interferometer.
Adding a microscope objective increases the amount of light you could
couple from the pump diode to the system. adding another one lets you
look at a very tiny point on the rod.

SEE Applied Physics Letters Volume 76 , Number 23, 5 June 2000. for
a drawing.

The hene or red diode (a pointer would do) 2X2 coupler is 525$ ,
the IR coupler is a cheaper 190$ or so.

Steve Roberts

 

Hi Steve,
Thank you so much for your help. Based on what you mentioned i have the
following items to purchase

1. 2X2 fiber coupler
http://www.thorlabs.com/ProductDetail.cfm?DID=6&ObjectGroup_ID=374&Product_ID=26499
($525)
2. 1 or 2 microscope objective
http://www.thorlabs.com/ProductDetail.cfm?DID=6&ObjectGroup_ID=1044&Product_ID=31752
($422)
3. High speed Photodetector. Which would you reccommend?
http://www.thorlabs.com/NewGroupPage9.cfm?Category_ID=77&ObjectGroup_ID=1295
(DET210 - $129) or
http://www.thorlabs.com/NewGroupPage9.cfm?Guide=10&Category_ID=77&ObjectGroup_ID=4
(PDA 400 - $389)
4. 1 small length of fiber as needed maybe 15$
5. 1 beaker of index matching fluid again maybe 50$ (where can i
purchase this?)
6. 4 couplers for the fiber optics,
7. 1 aluminum plate to mount it on.

Lastly would it be possible to obtain a schematic drawing of such a
system? I am unable to obtain the Applied Physics Letters Volume 76 ,
Number 23, 5 June 2000. Any help on this matter is greatly
appreciated. looking forward to your kind response.

with best regards,
Ravi

 

A schematic is shown in the first picture at

http://www.dipmec.unian.it/misure/projects/PhD/disante/disante_en.htm

you only need about half the parts, his shows a dual vibrometer. This
means he has built two of them and is running them both off another 2x2
splitter

Steve