As well as the increasing demand for bandwidth, network owners face more and more challenges, including how to send 100 Gbps transmission over distances?
There are, of course, DWDM systems DWDM – what it is and why you need it? which can solve this problem effectively, both over short ("QSFP28 DWDM") and long distances. 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 for 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 kilometre of fibre, it is hard for a transceiver to reach larger distances. Another difficulty is that the current standard clearly specifies that the maximum power consumption of the transceiver shall be 4.5W. So, how to send a single 100 Gbps transmission over a longer distance? Can we somehow amplify the signal?
Semiconductor Optical Amplifier (SOA) is the answer to the last question. This device operates similarly to a laser. It consists of two boards that have been custom designed which are 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 light reflection inside. In the case of an amplifier the focus is on receiving the signal, amplifying it and sending it on. Another difference is that with a laser we want to emit the light a single specified wavelength, while in the case of an amplifier we want to amplify the largest possible range of wavelengths. This is because when we want to amplify the signal coming from a source, such as a QSFP28 LR4 transceiver, we need to amplify 4 wavelengths simultaneously.
To sum up, we already have a transceiver that generates 4 wavelengths of approximately 1310 nm and we have an amplifier that can amplify them all simultaneously, so… let’s get started!