Overcoming the challenge of going beyond 10km on 100G links.
The most frequently used transceivers on the market today are typically sourced in volume. For many access applications, the cost per gigabit and per transceiver unit is key, as these components must be purchased to the scale of the project size. These transceivers are capable of bitrates today clocking in at around 1Gbits or 10Gbits per line, while the jump to 100Gbits calls for a higher 25Gbits baud rate (4x25G baud to achieve 100G).
The technology between the 1/10Gbit and 25Gbit baud rates is similar, however, the modulation schema used is called “intensity modulation” (transmitter side) with “direct detection” (receiver side): IM-DD. This technology is also widely addressed in the industry as “on/off keying” (OOK) or “non-return to zero” (NRZ). This modulation schema has been widely adopted by the market and today is used in network infrastructures around the globe. Unfortunately, IM-DD technology has a crucial limitation: higher is the bitrate, shorter is the reach.