In a message dated 2007/07/03, gmvoeth@........... writes:

> Do you know if anyone has ever used a laser pointer
> in a seismic sensor ? Possibly using surfaced mirrors
> to provide mechanical amplification like a lever arm ?

Hi Geoff,

       Laser pointers are very noisy and drift in intensity with temperature. 
You could maybe use a multiple reflection optical lever system with surface 
silvered mirrors, but getting satisfactory results may be difficult. 
       The red light is too long a wavelength to use interference fringe 
counting at all easily. You have steps of ~300 nano metres between each fringe. 
Sensing to a fraction of a wavelength is possible using Moire fringes, but 
costly.

       Using large area differential Si or GaAs photocells and a stabilised 
voltage tungsten bulb work reasonably well. You can also use IR LEDs, but you 
need to stabilise the output - the photo output drifts quite strongly with 
temperature. IR and Superbright LEDs may be reasonably quiet, but ordinary LEDs 
can be quite noisy. 
       The % noise decreases with increasing total photo current. The signal 
is proportional to the number of photons N in any sample, but the noise is 
proportional to Sqrt N. The size of the detector needs to be several thousand 
wavelengths minimum. 

       You can also use IR LEDs in pulsed mode as the signal generator in a 
phase lock loop differential photo detector system, but the photo output needs 
to be stabilised.

       Photo detectors are well described at 
http://sales.hamamatsu.com/assets/applications/SSD/photodiode_technical_information.pdf
       
       Regards,

       Chris Chapman

   
In a me=
ssage dated 2007/07/03, gmvoeth@........... writes:



Do you know if anyone has ever=20=
used a laser pointer

in a seismic sensor ? Possibly using surfaced mirrors

to provide mechanical amplification like a lever arm ?




Hi Geoff,



       Laser pointers are very noisy and drift=
 in intensity with temperature. You could maybe use a multiple reflection op=
tical lever system with surface silvered mirrors, but getting satisfactory r=
esults may be difficult. 

       The red light is too long a wavelength=20=
to use interference fringe counting at all easily. You have steps of ~300 na=
no metres between each fringe. Sensing to a fraction of a wavelength is poss=
ible using Moire fringes, but costly.



       Using large area differential Si or GaA=
s photocells and a stabilised voltage tungsten bulb work reasonably well. Yo=
u can also use IR LEDs, but you need to stabilise the output - the photo out=
put drifts quite strongly with temperature. IR and Superbright LEDs may be r=
easonably quiet, but ordinary LEDs can be quite noisy. 

       The % noise decreases with increasing t=
otal photo current. The signal is proportional to the number of photons N in=
 any sample, but the noise is proportional to Sqrt N. The size of the detect=
or needs to be several thousand wavelengths minimum. 



       You can also use IR LEDs in pulsed mode=
 as the signal generator in a phase lock loop differential photo detector sy=
stem, but the photo output needs to be stabilised.



       Photo detectors are well described at h=
ttp://sales.hamamatsu.com/assets/applications/SSD/photodiode_technical_infor=
mation.pdf

       

       Regards,



       Chris Chapman