In this last, (but technology is ever developing, it is last just at the time we are writing this blog post), coherent detection, the transmitted optical signal is combined with an optical signal created by optical receiver.
After that, receiver analyzes combined signal received from both sources – including the recovered signal from transmitter.
Another thing to mention which is important at Step 9 – it is the number of electrical signal bits transmitted by 1 optical pulse. Naturally, the bigger number it is, the more information you can send by. Up to Step 8, this number was so, that five optical bits translated to four electrical bits. But after Stage 8, new system was introduced so, that EACH optical bit carried more than one electrical bit.
With such kind of modulation it came possible to for every optical bit carry up to 8 electrical data bits. Using his method it was also possible to reduce optical pulse rate by number of bits transmitted by single optical pulse.
Such reduction reduced the optical dispersion. Such way, this method can compensate signal distortion that appeared because the dispersion.
Just for an example to illustrate this concept – coherent 100 GBit/s link with each optical pulse transmitting 8 electrical bits can operate at the optical pulse rate at 12.5 Gbit/s. Thus, we can allow all optical transmission path have far greater dispersion values, that it could be at an optical bit rate of 100 Gbit/s. And – bigger tolerance to dispersion, it also results in higher transmission distances.
In Step 9, intensity of optical pulse is not modulated, but instead of that modulation now is applied to polarization and phase of optical pulse. Recovery of such modulates electrical signals requires high speed digital processing, and finally, even a single wavelength can carry 100 Gbit/s. So, system transmitting several wavelengths can extend capacity of single fiber to more than 1 terabit per second.