Detecting AC signal on top of a DC with a CCD (non-intensified)?

[Sutherland, Scott]

Posted to CCD-world:
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Hello all:

I have somewhat of a dilemma I am trying to resolve.
I have an optical sensor application in which I am trying
to detect relatively low light levels of signal in the 850-1100
nm range.  However, the system must be run outside
any enclosure (using a fiber optic probe) and must
discriminate ambient light (fluorescent, incandescent,
and sunlight) from the signal.  Most solutions to this kind
of problem involve using a pulsed source and either gating
for an integrating detector (CCD, PDA) or demodulation
of a CW signal (e.g. a photomultiplier tube of photodiode).

The ICCD is not an option for me.  Is there any way, using
an integrating detector like a CCD, to detect a modulated
signal over a DC (or slowly varying) background that
includes a continuum (e.g. incandescent) and discrete
line (e.g. fluorescent) sources?  Keep in mind that this
is a 16 bit CCD application and the detectors I am using
are operated in binned mode.  The current system bins
128 pixels and has a total readout maximum speed
(including binning) of 6 msec.  If necessary, I could
go to a 14 bit or even 12 bit (with reservations) system.

The operating temperature of the CCD will be ~ -10 deg
C (less than that draws too much power for the battery
powered application, and LN2 is not practical), so there
is significant dark current (~ 50 electrons per pixel per
second), read noise is ~ 15 electrons (in the current
module, which could be changed), and a typical maximum
signal level (best case) is ~ 100,000 electrons per pixel
per second (fully binned pixel, so no more than 900 electrons
per individual pixel per second (thus low light), and as low
as 10-20 e/p/s).  Thus, I am dark current limited.

Any pointers to technology, literature references, etc. would
be appreciated.

Thanks for any suggestions.

Dr. Scott Sutherland
Senior Scientist
GAMMA-METRICS

BTW, I know Perkin Elmer tried to address this problem in 
a UV/Vis spectrometer with a PDA detector and ended up
with a single channel, scanning PMT based system and a 
very bright pulsed light source to eliminate ambient light
problems.  Not too encouraging a start, is it?  Unfortunately,
such an option is not viable for us.


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