Direct Detection and Coherent: what is the difference between these technologies? What are their benefits and limitations? In the following video, we give a short explanation about these two technologies. Firstly and foremost, Direct Detect and Coherent use different levels of information. Direct Detection works by changing the amplitude of the light to transmit information. In this case, the achievable transmission distance depends on the speed of the data signal: at lower data rates, the transmission distance is more than 100 km, but as the speed increase, it exponentially gets shorter. Coherent Optical Transmission uses three different properties of light: amplitude, phase, and polarization. This way, it is possible to increase the speed of the data signal, without compromising the transmission distance. With Coherent, it is possible to transmit information across long distances with very high data rates enabling operators to upgrade their networks without replacing the physical fiber infrastructure in the ground.

1. Coherent vs DD– Logo intro

2. Coherent vs DD – Title card
Direct Detection or Coherent?

3. Video joost full screen

4. Room for PIP
Coherent vs DD – Scene 1
Direct Detection 1G 120km

5. Direct Detection 10G 80km
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6. Direct Detection 25G 15km
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7. Direct Detection 56G 1km
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8. Coherent 400G 400+km
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9. Coherent 400G 120km
Higher Data Rates
1
Longer distances
2
Simplified network upgrades
3
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Coherent vs DD – Scene 1

10. The laser converts electrical to optical signals
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11. The photo diode converts the light back into electrical signals
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12. Light passing through the fiber gets dispersed
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13. As speed goes up, pulses get closer together ..
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14. … and start overlapping
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15. To overcome dispersion effects and increase transmission speeds, an
alternative approach is needed.
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Coherent vs DD – Scene 3

16. Apart from using just the amplitude
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17. .. we can also use the phase
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18. .. and the polarization
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19. .. and polarization
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20. .. and polarization
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21. Amplitude
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22. Phase
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23. Polarization
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24. Incoming signal combing amplitude, phase and polarization
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25. Local oscillator reference signal
Incoming signal combing amplitude, phase and polarization
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26. By mixing the two signals coherently...
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27. By mixing the two signals coherently...
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28. … we can reconstruct the information
contained in the signal
Amplitude
Phase
Amplitude
Phase
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29. The Digital Signal Processor compensates for transmission impairments
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30. Direct Detection or Coherent?
Higher Data Rates
1
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Coherent vs DD – Scene 4

31. Direct Detection or Coherent?
Higher Data Rates
1
Longer distances
2
Room for PIP
Coherent vs DD – Scene 4

32. Direct Detection or Coherent?
Higher Data Rates
1
2
Simplified network upgrades
3
Longer distances
Room for PIP
Coherent vs DD – Scene 4

33. Coherent vs DD– Logo outro