3R regeneration is necessary after an optical lightpath transmits for a long distance. This is because optical fiber transmission system is not ideal, affected by many factors such as dispersion, EDFA noise, non-linear effects, and crosstalk, etc, which may eventually degrade an optical signal to be unrecognizable at the receiver if the signal is not relayed.

3R regeneration relays or regenerates optical signals in three domains including power, shape and time. Optical-Electronic-Optical (OEO) conversion is so far the most popular and mature technique for this purpose. The fundamental principle of OEO regeneration is to convert an optical signal into electronic format first so that the time and shape are restored, and then use the electronic signal to modulate an optical laser to generate a new optical signal.

Besides the OEO technique, it is also possible to carry out 3R regeneration in all-optical domain without converting optical signal into electronic signal. The advantage of all-optical 3R regeneration is its bit-rate transparency, without a bottleneck from electronic modulation. However, currently the all-optical 3R regeneration technique is not mature and very expensive.