Together with the increasing demand for bandwidth, network owners face more and more challenges. How to send 100 Gbps transmission at long distances?
There are, of course, DWDM systems ("What it is DWDM") which can solve this problem effectively both at short ("QSFP28 DWDM") and long distances ("Understanding coherent transmission"), but what if you need to send only one 100 Gbps transmission and you don’t want to invest in building a DWDM system?
In order to set up 100 Gbps transmission at a distance of a couple or a dozen of kilometres, you can use 100G LR4 transceivers. The most common are the transceivers in QSFP28 form factor. On the optical side, the transceiver sends a signal on 4 different wavelengths: 1295.56, 1300.05, 1304.58 and 1309.14 nm. Inside the transceiver, there is an appropriate multiplexer embedded on the transmitter side and a demultiplexer on the receiver side.
The power budget of QSFP28 LR4 transceivers is usually a couple of decibels. As the transmission takes places in the second optical window where insertion loss is 0.35 dB per each 1 km of fibre, it is hard for a transceiver to reach larger distances. Another difficulty is the fact that the current standard clearly specifies that the maximum value of power consumption of the transceiver shall be 4.5 W. So, how to send a single 100 Gbps transmission at a longer distance? Can we somehow amplify the signal?
The answer to the last question is: Semiconductor Optical Amplifier (SOA). This device operates similarly to a laser. It consists of two custom-designed boards made of semiconductor material and a layer of a different type of material that forms an active layer. The primary difference between SOA and a laser is the lack of inside light reflection. In the case of an amplifier the focus is on directing the signal inside, amplifying it and sending it outside. Another difference is that in the case of a laser we want to emit the light of only one specified wavelength, while in the case of an amplifier we want to amplify the largest possible range of wavelengths. It happens so because when we want to amplify the signal coming from e.g. a QSFP28 LR4 transceiver, we need to amplify 4 wavelengths simultaneously. To sum up, we already have a transceiver that generates 4 wavelengths of approx. 1310 nm, we have an amplifier that can amplify them all simultaneously, so… let’s get started!