TECHNICAL
To increase the bandwidth, an advanced amplitude and phase modulation is used, giving the possibility to increase the number of bits per symbol. The
modulation is called QPSK.
used, giving the possibility to increase the number of bits per symbol. The modulation is called QPSK.
DSP (Digital Signal Processor) for high-speed ANALOGUE-to-digital conversion (ADC) The DSP is an important component of the coherent application as it runs different corrections and optimisations during the optical to electrical conversion: n Chromatic dispersion compensation n Polarisation mode dispersion compensation n Intradyne carrier recovery The Coherent technology enables long reach link at high bitrate but requires a pluggable form factor with enough space and enough power. To perform long-haul links, a transport platform is required with short-distance client transceivers (SFP+ 10G, QSFP28) and CFP/CFP2 ports. The Coherent technology is today available on different pluggable form factors known as CFP and CFP2. CFP Form Factor DCO (Digital Coherent Optics) - DSP included in transceiver n CFP DCO 100G ZR (up to 80km without amplification) n CFP DCO 100G DWDM (metro – up to 1200km with amplification). n CFP DCO 100G DWDM (long-haul – up to 2500km with amplification).
Polarisation multiplexing
Because light is used for transmission, it is possible to multiplex the signal with two channels by using two polarisations. By combining the QPSK modulation and the polarisation multiplexing, the final solution is called PM-QPSK (Polarised-multiplexed + Quadruple phase-shift keying).
Key information’s about Coherent pluggable tranceivers: There is no interoperability between DSP from different vendors. A complete optical link must include two Coherent transceivers from the same vendor for CFP-DCO and CFP2-DCO. Because the DSP is not included in CFP2-ACO, it’s the host-board which must be from the same vendor on both sides of the link. A CFP-DCO and CFP2-DCO can be plugged in any CFP/CFP2 ports if these are MSA compliant and support CAUI for 100Gbits interface. The CFP2-ACO can only be plugged in a dedicated CFP2 Coherent port as the DSP is on the host-board. The CFP is only DCO because it has enough space and power to include the DSP. On CFP2 form factor, an ACO version was initially released by moving the DSP on the host-board as it was too challenging to include it in the form factor.
Fig 2: Vertical and horizontal polarisation
Coherent detection
The IM-DD technology (regular 1G/10G/25G) is using direct detection with linear or limiting output, proportional to the light intensity. For Coherent application, a local oscillator (LO) laser is added on the receiver side to extract the two phases channels.
MARCH 2025 Volume 47 No.1
87
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