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CWDM- spectral multiplexing of optical channels

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Introduction

I work for a small, by the standards of our city, Internet company. In the last 5 years, our optical network has grown by leaps and bounds. Initially the network topology was chosen incorrectly, namely bus topology. As a consequence, if a failure occurred at an intermediate communication node (long power outage, equipment hang-up, etc.), it affected all communication nodes located further away. The obvious solution to this problem was to rebuild the network according to the logical star topology. To do this, it was necessary to connect each communication node with a separate fiber. But here popped up another problem, the number of free fibers was catastrophically low. Laying new fiber optic cables from the main communication nodes was financially expensive. The way out of this situation was the introduction of CWDM.

CWDM

CWDM (Coarse Wavelength Division Multiplexing) – Sparse Wavelength Division Multiplexing. In other words, it is a technology that allows multiple information channels to be transmitted simultaneously over the same optical fiber at different carrier frequencies. CWDM wavelength grid is in the range of 1271 nm to 1611 nm in 20 nm increments.
The CWDM operating principle is simple. Each transceiver module generates a signal at a specific frequency. Before reaching the optical fiber, the signal from the modules is combined by a multiplexer and transmitted to the fiber. At the receiving end, the signal is separated by a demultiplexer. In order to convert the optical network from a bus topology to a star topology, the demultiplexer must not only receive the signal at a given length, but also continue to pass the signal without changing it. For this we used OADM.

OADM

OADM (Optical AddDropMultiplexor) – is an optical I/O multiplexer of a CWDM system that extracts a signal at a given wavelength from an optical line, and passes all other radiation through unchanged.
The OADM module has four interfaces :
Com – receives a signal from the multiplexer side
Express – Passes the signal on
Add – incoming line at a specific wavelength
Drop – Outgoing line at a specific wavelength

SFP

Transceivers SFP (Small Form Factor Pluggable ) is an industry standard for modular compact transceivers used for data transmission. Each SFP CWDM transceiver operates over two fibers, at two different wavelengths – a receiver at one wavelength and a transmitter at the other.

Practical Implementation

In practice, we used a multiplexer (MUX) with 8 channels, SFP transceivers, and OADM modules. The wavelengths used are shown in the table.
CWDM- spectral multiplexing of optical channels
Below is an implemented diagram.
CWDM- spectral multiplexing of optical channels

Conclusion

Problems

In implementing CWDM, we encountered some problems. At each communication node, the optical cross-connects are terminated with SC connectors. SFP modules have LC connectors. When ordering OADM modules, a mistake was made, OADM modules were ordered with LC connectors. When implementing CWDM, we had to use a bunch of patch cords, LC and SC sockets, which gave not a weak attenuation in the optical line, resulting in SFP-modules at remote sites refused to work. Plus OADM modules introduce attenuation from 0.8 to 1.2 dB. The solution was to abandon the transition patchcords, and sockets. Optical cross-connectors were welded to LC connectors.

Benefits

  • The CWDM system is power independent. Power is only needed for the active equipment. That’s what we wanted to achieve in our case. If a communication node goes down, the other nodes work
  • Increase of transmitted traffic up to 8 times per fiber
  • Ability to install OADM modules in a variety of locations (cross-connects, muffs, etc.)
  • CWDM implementation is less expensive than installing new optical lines over long distances

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