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Comment by madaxe | Hacker News Reader

> Why would Google (or anyone) link to them directly? with fiber no less!

"They have no direct access to our servers"

I wonder what a beam splitter consists of. Oh. A PRISM.

Beam splitters are, in general, not prisms. A prism, as traditionally referred-to (and in the NSA PRISM graphic) separates light of different wavelengths. In a signal tap, you want to split the intensity, not the wavelength. In simplest form, telecom signals are at a single wavelength; passing it through a 'Dark side of the moon' prism will only deflect the beam, not split it.

When one refers to a beamsplitter, it's usually a partially silvered mirror.

http://www.thorlabs.us/newgrouppage9.cfm?objectgroup_id=914

If it's fancy, it might use an evanescent wave to do the coupling, as in some cube beamsplitters.

Beamsplitters for optical fiber are more generally referred to as 'couplers' and involve bringing two fiber cores close enough for a long enough distance that the probability of coupling light from one to the other is the desired amount.

http://www.thorlabs.us/newgrouppage9.cfm?objectgroup_id=374

It is possible to split beams with a birefringent prism, but it is much less common.

http://www.thorlabs.us/newgrouppage9.cfm?objectgroup_id=745 http://www.thorlabs.us/newgrouppage9.cfm?objectgroup_id=917

Disclaimer for the following: I only work with optical fiber couplers occasionally, and not for telecom. Someone who works on telecom fibers daily will be more informed.

In summary, if someone wanted me to tap an optical fiber, I'd call up ThorLabs, get a matching coupler shipped overnight, cut the relevant fiber, slap APC ends on the fiber ends, and jack in. Splitting the beam in free space (outside of a fiber) with a prism is far more errorprone, unstable, and no more efficient. A fiber coupler has no moving parts, can't break, and won't take down a telecom's trunk line if someone breathes on it funny.

If they're actually using a prism, it's because of some sort of impedance/reflection minimization scheme; I can't conjure one that would work better than using simpler techniques though.

You can/do/might use actual prisms for a variety of reasons, however, such as if you're trying to get a frequency-multiplexed set of signals off a single fibre broken down as their constituent components - i.e. bulk data collection from a single tap on a mass fibre bridge.

Anyway, you're probably right, it's probably just bog standard parts, and PRISM was a buzzword for management.

Are couplers detectable via time domain reflectometry in practice?

Actually it's: "There is no free-for-all, no direct access, no indirect access, no back door, no drop box."

http://www.guardian.co.uk/technology/blog/2013/jun/19/google...

And I tend to believe him.

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