PSN-L Email List Message

Subject: Re: Publication on the Web
From: ChrisAtUpw@.......
Date: Sat, 17 Nov 2001 19:48:43 EST


In a message dated 17/11/2001, jimesler@.......... writes:

> Does anyone have any experiences in building an inferometer for measuring
> displacement in seismographs?
> Jim Esler

Dear Jim Esler,

       The problem with using methods which depend on the wavelength of light 
is that you are ultimately limited by this dimension, which is about 0.6 
micron. When observing seismic signals, you will probably want a resolution 
of better than 1/20 of this. While optical sensors which measure to small 
fractions of a wavelength can be bought, they are neither simple nor cheap. 

       The wavelength limitation may be largely avoided by using a fairly 
intense and  constant light source to illuminate a pair of silicon 
photodiodes through a moving shutter attached to the seismometer arm. The 
difference in the photocurrents is measured, filtered and amplified. This 
'photon counting' method is also simpler in principle than interferometer 
techniques. Hope that this helps.

       Regards,

       Chris Chapman            
In a message dated 17/11/2001, jimesler@.......... writes:


Does a nyone have any experiences in building an inferometer for measuring
displacement in seismographs?
Jim Esler


Dear Jim Esler,

      The problem with using methods which depend on the wavelength of light is that you are ultimately limited by this dimension, which is about 0.6 micron. When observing seismic signals, you will probably want a reso lution of better than 1/20 of this. While optical sensors which measure to small fractions of a wavelength can be bought, they are neither simple nor cheap.

      The wavelength limitation may be largely avoided by using a fairly intense and  c onstant light source to illuminate a pair of silicon photodiodes through a moving shutter attached to the seismometer arm. The difference in the photocurrents is measured, filtered and amplified. This 'photon counting' method is also simpler in principle than interferometer techniques. Hope that this helps.

      Regards,

      Chris Chapman            

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