3 ms·

At long distance, inter-symbol interference caused by dispersion are the main causes of bit-errors. I saw a Clariphy whitepaper about a very good digital disper

by aortega 11y ago

At long distance, inter-symbol interference caused by dispersion are the main causes of bit-errors. I saw a Clariphy whitepaper about a very good digital dispersion compensation engine, based on a custom implementation of the Viterbi algorithm. No a lot of experience in the subject but I believe that, unlike RF, if you need more OSNR you can always inject a lot of power in the fiber (I've heard about 5-10 watts lasers for long-haul data links). Also, if you need to go farther away, you can always increase the FEC overhead.

mrjoeblack 11y ago
Close, but dispersion compensation is now very easy to do digitally, and has very little impact on system performance now. As for power, it is actually the opposite, optical systems are ultimately limited in reach by nonlinearity, and so you can not just increase the transmit power like you might do in a wireless system, even if your amplifiers were very linear.
mrjoeblack 11y ago
Close, but dispersion compensation is now very easy to do digitally, and has very little impact on system performance now. As for power, it is actually the opposite, optical systems are ultimately limited in reach by nonlinearity, and so you can not just increase the transmit power like you might do in a wireless system, even if your amplifiers were very linear.
mmmBacon 11y ago
Unfortunately you can't just increase the power into the fiber. It's complicated but I'll try to clarify (pun not intended) why you are OSNR challenged here. First, fiber and just about any optical material have a nonlinear index. So as you increase the power into the fiber, the light modifies the local index. This creates a kind of noise. A wavelength channel can do this to itself even. When you have lots of channels on the fiber, there is a lot of modulation of the refractive index by all those channels. This is call cross-phase modulation (XPM). You can think of XPM as a phase noise. As you increase the power in the fiber, there's a point where the XPM "noise" increases faster than the improvement in OSNR. So you have to keep the launch power low to keep the nonlinear penalty low. This limits our launch power and ultimately the OSNR we can achieve. Generally we try to operate near the peak of this curve.

by aortega 11y ago

mrjoeblack 11y ago

Close, but dispersion compensation is now very easy to do digitally, and has very little impact on system performance now. As for power, it is actually the opposite, optical systems are ultimately limited in reach by nonlinearity, and so you can not just increase the transmit power like you might do in a wireless system, even if your amplifiers were very linear.

mrjoeblack 11y ago

mmmBacon 11y ago

Unfortunately you can't just increase the power into the fiber. It's complicated but I'll try to clarify (pun not intended) why you are OSNR challenged here. First, fiber and just about any optical material have a nonlinear index. So as you increase the power into the fiber, the light modifies the local index. This creates a kind of noise. A wavelength channel can do this to itself even. When you have lots of channels on the fiber, there is a lot of modulation of the refractive index by all those channels. This is call cross-phase modulation (XPM). You can think of XPM as a phase noise. As you increase the power in the fiber, there's a point where the XPM "noise" increases faster than the improvement in OSNR. So you have to keep the launch power low to keep the nonlinear penalty low. This limits our launch power and ultimately the OSNR we can achieve. Generally we try to operate near the peak of this curve.