16 fibre pairs doesn't seem like very much? I wonder what the constraining factors are, as surely the material costs of having hundreds of pairs are negligible compared to the install costs? Is it amplification?
The amplification is obviously there for losses in the cable, but you also need to do clock recovery and thresholding because the light pulse gets spread out as it travels.
Higher data rate => More analog bandwidth (distribution of frequencies from the laser)
More analog bandwidth => Worse chromatic dispersion
You need to either:
* Increase the frequency of the laser (so BW/center frequency is smaller, hopefully reducing dispersion), which might not be possible with current tech or be very expensive
* Decrease the dispersion of the fiber (more expensive or lossy fiber)
* Have periodic signal conditioners
Due to the advent of coherent optical transmission and DSP, dispersion is no longer an issue for long distance optical communication. The limitation becomes nonlinear “crosstalk” between wavelengths and lower dispersion makes the nonlinear crosstalk worse as the phase matching condition for nonlinear processes can be reached more easily. To avoid this most new subsea cables use a high dispersion design as this helps mitigate nonlinear effects.
Additionally coherent systems have a higher spectral efficiency and are transmitting >2bits/symbol because the information is encoded in the electric field’s phase and not just intensity. Depending on the reach, the number of bits per symbol can be increased. The advent of probabilistic shaping, allows more bits per symbol to be transmitted in higher order constellations. Probabilistic shaping produces a more ‘random-like’ coding that gets us closer to the Shannon Limit and also reduces the optical intensity rms further reducing nonlinear penalties. In fact in optical communications there is something called the nonlinear Shannon limit that takes into account the additional “noise” produced by nonlinearities in the fiber. See:
It never occurred to me that fiber optic communication in glass actually used intensity to transmit bits. This sounds like AM radio albeit digital I guess the laser could be on or off. How long has it worked like that?
I always assumed it was like CDMA radio where like you say phase shifts correspond to a point in a constellation which represents more than 1 bit.
There is no perfect amount of dispersion. However allowing the dispersion to increase in modern cables also allows us to make the fiber core a little bigger. This reduces the intensity per unit area, reducing the fiber nonlinearity. Additionally there’s some benefit for highly dispersed signals as they look like noise.
For long distances we encode the information on the electric field phase and amplitude as us done in wireless. In actuality most of what’s done in optical from a modulation point of view and most of the theory comes from wireless originally. A difference is that we use a laser as a local oscillator to shift the data back down to baseband just like in wireless. We did intensity modulation for a long time because the signal speeds are very high and the semiconductor technology wasn’t there to do ADC and DACs at the rates we needed at low enough power and cost. This changed around 2008 when the first coherent systems became practical due to the availability of 45nm CMOS.
Now most everything above 25Gb/s and 80km is coherent. Below that we still do intensity modulation and inside data centers it’s all intensity modulation. This is due to cost and power reasons. Even in 7nm, coherent is still higher power on a bit/s basis. It’s also more expensive due to complexity of the transmitter and receiver as well as the need for better quality lasers.
https://www.popsci.com/google-fastest-internet-cable-atlanti...
Also keep in a mind that fiber itself is theoretically unlimited bandwidth wise. So as the transmission gear on each end that lights the fiber improves so does the available bandwidth each fiber pair can't provide. The other issue is that the cable plant itself has a limited life as well generally around 20 years although that number has gone up recently to 25 or so years. So anything that's fiber not lit for the majority of that 25 year life span is potentially money wasted.
You need one cable for each fiber, they aren't all in one bundle like fiber might come into your office.