I am interested in using optical fibers with sharpened tips for detection in biochemical systems. We are coating the tips with metallic films or nanoparticles. I've noticed that groups that make these types of sensors always detect the transmitted light from the tip of the fiber. I am curious whether or not the reflected light could be harnessed to do the same sorts of measurements.
I know the relationship between the light transmittance and reflectance of a metallic-coated tapered optical fiber is not trivial, but is there any reason to believe the light reflectance would not be usable in measuring environmental changes?
Any info on the subject is tremendously helpful, even if it does not directly resolve my question.
Thanks.
Dear Adam,
I not 100% sure what you are aiming at but lets put it like this: In reflection through the fiber you will either detect elastic (i) or inelastic light scattering (ii).
For case (i) no matter how you will design your sensing principle what you will receive back is a large reflected signal from the fiber termination that will dominate the signal. Since this reflected signal will inevitably contribute to the same single fiber mode, any sensor effects will be small changes on top of this signal. This is inherently unfavourable since the background creates a large shot noise floor. However, no problem for large changes, e.g. induced by a global index change or so.
For case (ii) you will be looking for some red-shifted signal, be it fluorescence or Raman scattering. In any case if you look for a weak signal (sensitive sensing...) you will have to deal with a strong background of inelastic scattering that is created inside the fiber (proportional to fiber length). If the signal you want to detect is large enough then this background will not bother you and you can measure in reflection through a fiber no problem. The background can be overcome though by making use of characteristics of the signal to be detected (people have detected single molecules in reflection through a finer) or by making use of hollow-core fibers. Hope this helps!
Bert
Thanks Bert. You wouldn't remember offhand the name of any papers where people detected single molecules through reflection sensing, would you?
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