Subject: RE: Question on radiation resistance of optical fiber
From: "Risch Brian, US" 
Date: Fri, 21 Nov 2014 14:23:02 +0000
To: Jan Pirrong
CC: "hashemi@brandeis.edu"

It appears the wavelengths being used are outside the normal operational window of the fibers which are monitored during RIA measurements, and also outside wavelengths recommended for radiation exposure of even radiation hardened optical fibers.

It looks like 650nm and lower wavelengths are being used.  These lower wavelengths are impacted more by radiation than the higher ones, and there are transient effects of radiation on the fibers which also occur at these wavelengths.  A transient attenuation peak can be expected centered at about 700nm.  Actually the effects are so transient that, lower doses (lower times) can have a greater impact on attenuation than higher doses (higher times) which seems counter-intuitive.  The key is the mechanism which is associated with self-trapped holes which are unstable at room temperature.  The issue becomes less transient at cryogenic temperatures.   If the same patchcords which were irradiated are measured again, the attenuation should be much improved even after several hours. (Check this if you can.)  The question is what is wanted from the testing?  Do we want to simulate a higher dose over a longer time or is this the actual end use condition? At 700nm non-transient attenuation will still be >10x that observed at 1300nm, but the fibers should not be dark.  An illustration of the dramatic wavelength dependence on attenuation is shown below (1KGy dose).

Since we have a known distance of 0.45m I can do the calculations, extrapolating to the wavelengths in question.  We should have a non-transient attenuation of about 0.5-1dB for the length in question with Ge Doped Rad-Hard fibers.  The non-transient performance of F-doped SRH fibers is about 10x better, but transient behavior is also observed in these fibers.  I think the transient behavior is more attributed to the silica than dopants, so there may not a lot that can be done about that.

We also need to ensure that there is no other factor such as a broken fiber or bend, but I think there is a plausible radiation induced effect.

The jacket discoloration is hard to judge from pictures, but at such a low dose a noticeable jacket discoloration is surprising to me although this is a known phenomenon at higher doses.

For colored jackets, we would typically need a dose at least 10x this high to get discoloration which is perceptible to the human eye.

I’d double check your dosimetry equipment, but all of our experience has been with Co60 gamma radiation, so there may be a wavelength dependence (of irradiation photons).

I am available for additional discussion as this appears to be a fascinating application.  I’m hoping we can find some solution.

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