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From: tell@rudolph.cs.unc.edu (Stephen Tell)
Newsgroups: sci.electronics
Subject: Re: tapping into a fiber optic cable
Message-ID: <14414@thorin.cs.unc.edu>
Date: 1 Jun 90 03:57:59 GMT
Article-I.D.: thorin.14414
References: <46249@iuvax.cs.indiana.edu> <151@coplex.UUCP>
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Reply-To: tell@rudolph.cs.unc.edu (Stephen Tell)
Organization: University Of North Carolina, Chapel Hill
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In article <151@coplex.UUCP> dannie@coplex.UUCP (Dannie Gregoire) writes:
>beckman@iuvax.cs.indiana.edu (Peter Beckman) writes:
>>Can a fiber optic cable be "tapped" into without disturbing the data
>>being transmitted?  I would like to run some fiber optic cable through
>If I recall correctly, it can be tapped without distubing transmitted data.
>I remeber reading an article in Info World about a year ago about just such
>a method.  It seems that if you bend a FO cable at more than  90 degrees
>that about 20% of the signal will "bleed" out of the cable at the bend. 
>An opto-receiver can be installed there with an amplifier to read the signal
>without disturbing any data.
>\\  Dannie Gregoire  \\\\  (dannie@coplex)  \\

some minor items of information:
"tapping" this way is detectable in two ways:
The ammount of light recieved at the other end will be reduced; although
most recievers will be designed & selected to have some tolerance of
reduced levels before failing to recieve reliably.  This is to account
for the reduction in output level of LEDs and laser diodes as they age.

It can also be detected with a time-domain reflectometer, as noted by
someone else in this forum.

I believe that this tapping technique is sometimes used to verify the
quality of a splice or connector without having to go down to the
other end of the cable.  Even if that was convenient, you might not be
able to tell which end you had to re-terminate.   I'm not sure how common
this practice is, or if the equipment used could be adapted for tapping
of active lines, but it is concievable that the necessary equipment to
tap a data fiber could be more common than you would like.

--------------------------------------------------------------------
Steve Tell					tell@wsmail.cs.unc.edu
CS Grad Student, UNC Chapel Hill.