In areas of high lightning, the flash from the thunderbolt has been observed to induce optical impulses in fiber-optic cable, potentially limiting reliable transmission of analog and digital signals. This paper discusses an active pulse cancellation technique which mitigates this effect. Any architecture which uses fiber, especially passive optical networks and fiber-to-the curb, must use these or similar measures.
Cable television systems have embraced the use of fiber optics since its introduction in AM applications in the late 1980s. Fiber backbone and Cable Area Network architectures have given way to Fiber-to-the-Serving Area, and node sizes are now typically 500 homes or smaller. Passive optical networks have been installed, pushing fiber even deeper. However, there is a growing fear that at some point this is too much of a good thing, and coax tree-and-branch is just being traded in for optical tree-and-branch. Studies by CableLabs have shown that reverse path noise is limited by electrical impulse noise. Use of more fiber cuts down amplifier cascades and normally reduces reverse noise.
Impulse ingress, however, is relatively unrelated to node size. This demands a closer look at the benefits of more fiber.
Anyone who has been caught in a thunderstorm can attest to the brightness of lightning bolts. John Walsh (Time Warner, Orlando) once arranged a lightning storm for my benefit that has left me impressed to this day! Lightning strikes the earth 100 times a second; more than 8 million times a day!
The spectrum emitted by lightning is broad, covering the entire visible range from 410 nm to 710 nm, and continuing into the infrared. This is plotted in Figure 1. The strong infrared content should come as no surprise because the air surrounding the lightning bolt is heated to 2,500 degrees. The expansion and collapse of the air molecules due to this heat is what causes thunder.
It should not come as a surprise, therefore, that the strong signal at both 1310 nm and 1550 nm has been observed to cause optical impulses-light leaks into the fiber and interferes with the signal content. Raynet has demonstrated a passive optical tap for years, which injects and extracts light from an intact fiber. Also, anyone who has used a fusion splicer with local injection/detection measurement of the completed splice knows that light can leak into an intact fiber.
At first, people said, “Go fly a kite” and so to further examine this effect, the experimental set-up shown in Figure 2 was assembled. Unfortunately, the fiber used was leftover Corning Titan fiber donated to the cause, and the titanium coating conducted electricity and blew up the test equipment! The inescapable conclusion is that in areas such as Jacksonville and Orlando, there is a deadly combination of lots of fiber and lots of infrared light. It is speculated that this is the primary reason that the BellSouth Fiber-to-the-Home trials in Heathrow and Hunter’s Creek, Fla. have never been heard from again.
Once the impairment root cause was identified, and disbelief suspended, the mitigation technique became obvious. A dark fiber, running parallel to the signal fiber, acts as the collector of a pure ingress signal. As shown in Figure 3, the signal fiber carries the signal S, and the optical ingress signal e. The dark fiber only picks up the error signal e. Error cancellation is thus possible. If the cancellation is done electrically, the optical impulse will overload the signal receiver, because of the much higher level of the optical impulse, and reliable cancellation does not take place.
Therefore, it is preferable to have the cancellation take place in the optical domain. This is accomplished as shown in the rest of Figure 3. The error signal is received by a PIN diode (properly biased so as not to overload), and the signal is inverted by an amplifier. For cancellation to take place in the optical domain, anti-photons need to be generated to cancel the photons of the error signal in the signal fiber. Because photons have no mass, the annihilation does not generate the side-effects that normal matter-antimatter collisions do. The anti-photons are generated by a dark-emitting diode (DED).
The DED is shown in Figure 4. A normal laser diode is doped with the rare-earth metal erbium for operation at 1550 nm, and with either praseodymium or neodymium for operation at 1310 nm. Photons are emitted at the front facet, which is the normal facet for signal collection. Anti-photons are emitted out the rear facet, and for the purposes of impulse control, this is the facet that is connected to the fiber. This fiber is combined with the signal fiber, and the resulting cancellation is fed to a normal fiber receiver.
Note: for those who are more comfortable treating light as a wave instead of a particle, the anti-photon is the equivalent of a wave 180 degrees out of phase with the error signal, resulting in cancellation between the two.
Heretofore, system architectures were measured using the (Tom) Elliot factor, which is the weight of sand used per customer (the sand in the fiber, plus the sand in the silicon chips). Now it is apparent that there is a need to add the Fellows factor, which incorporates the transparency of the sand used. The good news for the cable industry is that the phenomenon described herein is the Achilles’ heel for Fiber-to-the-Curb, or, heaven forbid, Fiber-to-the-Home.
Some in the cable TV industry may argue that by introducing a method of correcting the optical ingress phenomenon, this paper represents the savior to these approaches, but one would have to be an idiot to try these techniques. Without an intellectual property license, I mean.
Besides, I work for an RBOC now. I call upon CableLabs to extend this body of work and tackle two important issues: control of ingress because of Fourth of July fireworks, and determining why anti-photons only exist on April first.
|About the author|
|David Fellows is senior vice president of engineering and technology, Continental Cablevision. Fellows also serves on CED’s editorial advisory board. In addition, he has a great sense of humor.|