7 ms·

> A 99/1 splitter (i.e. 99% one way, 1% the other) would be pretty hard to detect in a fiber optic line that is subject to fluctuating loss caused by bending, t

by ceph_ 13y ago

> A 99/1 splitter (i.e. 99% one way, 1% the other) would be pretty hard to detect in a fiber optic line that is subject to fluctuating loss caused by bending, temperature change, etc.. Please reference an example of a 99/1 splitter existing and usable on an optical line system. You can't get a usable signal from 1% of the light. The levels you would need to split off would be entirely noticeable. > So how do you turn a piece of fiber into a beam-splitter without cutting and splicing? Well, that's surprisingly easy. One way to create a fiber beam splitter is to take two fibers and fusion-combine them. This is not possible to do on a live system without it being service affecting.

beloch 13y ago
>Please reference an example of a 99/1 splitter existing and usable on an optical line system. http://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1718 http://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=171... >You can't get a usable signal from 1% of the light. The levels you would need to split off would be entirely noticeable. http://www.perkinelmer.com/CMSResources/Images/44-12462DTS_SPCM%20AQRH.pdf http://www.perkinelmer.com/CMSResources/Images/44-12462DTS_S... Obviously, you'd need something faster. However, 1% of a microwatt is more than detectors like this need to get damned good signal. In fact, it might be enough to burn it out. If you have good control over the fusing process, splitting off much less than 1% would probably be preferable. >This is not possible to do on a live system without it being service affecting. Why not? The only impact on the fiber being tapped would be a brief period of heating and a temporary change in refractive index over, perhaps, a couple cm of fiber.
- fnordfnordfnord 13y ago
  >If you have good control over the fusing process, splitting off much less than 1% would probably be preferable. Nah, get the best split/splice you can without affecting the target fiber (without causing detection). You could use a power meter to determine the resultant signal strength, but if you've planned/done this more than a couple of times, you know what it is going to say. You could use an attenuator if needed to protect the detector, but since the detector current is dependent upon the gain of its amplifier, it would be simpler to simply adjust the gain. In fact using an auto-gain-circuit (AGC) here would be preferred, since it will automatically compensate for any differences in splice quality (within range). Also, it would be clumsy to design an amp that could easily burn out your detector, but sometimes you do crazy things when pushing performance to the limits. IMO you couldn't use a silicon detector like the one from Perkin Elmer. It is fairly large (170um x 170um), and therefore has a large Capacitance, and therefore is as you say, slow. However, such detectors exist. They would probably use InGaAs if they used off the shelf stuff. Here is an example http://www.thorlabs.com/Thorcat/10700/APD310-Manual.pdf http://www.thorlabs.com/Thorcat/10700/APD310-Manual.pdf There are almost certainly better choices than InGaAs if they forget about cost and don't need to produce enough to supply the whole world. Given that NSA has its own chip-fab capabilities (it does), this seems like a possibility. Being on the ocean floor here is great because it is cool and dark, and therefore low-thermal-noise (also not a lot of RF, I bet). >>This is not possible to do on a live system without it being service affecting. >Why not? The only impact on the fiber being tapped would be a brief period of heating and a temporary change in refractive index over, perhaps, a couple cm of fiber. I also see no reason that this or some other method could not be employed on a live cable, and go unnoticed. My question is how the NSA gets the data back to where they want it. They have to lay cables of their own, probably have remote stations to reject some of the data; or maybe NSA has a secret undersea fiber network that rivals the normal one. They certainly have the money, I just never considered it.

by ceph_ 13y ago

beloch 13y ago

>Please reference an example of a 99/1 splitter existing and usable on an optical line system. http://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1718 http://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=171... >You can't get a usable signal from 1% of the light. The levels you would need to split off would be entirely noticeable. http://www.perkinelmer.com/CMSResources/Images/44-12462DTS_SPCM%20AQRH.pdf http://www.perkinelmer.com/CMSResources/Images/44-12462DTS_S... Obviously, you'd need something faster. However, 1% of a microwatt is more than detectors like this need to get damned good signal. In fact, it might be enough to burn it out. If you have good control over the fusing process, splitting off much less than 1% would probably be preferable. >This is not possible to do on a live system without it being service affecting. Why not? The only impact on the fiber being tapped would be a brief period of heating and a temporary change in refractive index over, perhaps, a couple cm of fiber.

fnordfnordfnord 13y ago
>If you have good control over the fusing process, splitting off much less than 1% would probably be preferable. Nah, get the best split/splice you can without affecting the target fiber (without causing detection). You could use a power meter to determine the resultant signal strength, but if you've planned/done this more than a couple of times, you know what it is going to say. You could use an attenuator if needed to protect the detector, but since the detector current is dependent upon the gain of its amplifier, it would be simpler to simply adjust the gain. In fact using an auto-gain-circuit (AGC) here would be preferred, since it will automatically compensate for any differences in splice quality (within range). Also, it would be clumsy to design an amp that could easily burn out your detector, but sometimes you do crazy things when pushing performance to the limits. IMO you couldn't use a silicon detector like the one from Perkin Elmer. It is fairly large (170um x 170um), and therefore has a large Capacitance, and therefore is as you say, slow. However, such detectors exist. They would probably use InGaAs if they used off the shelf stuff. Here is an example http://www.thorlabs.com/Thorcat/10700/APD310-Manual.pdf http://www.thorlabs.com/Thorcat/10700/APD310-Manual.pdf There are almost certainly better choices than InGaAs if they forget about cost and don't need to produce enough to supply the whole world. Given that NSA has its own chip-fab capabilities (it does), this seems like a possibility. Being on the ocean floor here is great because it is cool and dark, and therefore low-thermal-noise (also not a lot of RF, I bet). >>This is not possible to do on a live system without it being service affecting. >Why not? The only impact on the fiber being tapped would be a brief period of heating and a temporary change in refractive index over, perhaps, a couple cm of fiber. I also see no reason that this or some other method could not be employed on a live cable, and go unnoticed. My question is how the NSA gets the data back to where they want it. They have to lay cables of their own, probably have remote stations to reject some of the data; or maybe NSA has a secret undersea fiber network that rivals the normal one. They certainly have the money, I just never considered it.