For transmission distance exceeding 10km in a 100G network, there are mainly three solutions: 100G ER4 (40km), 100G ZR4 (80km) and 100G DCO (2000km).
100G ER4 can reach 40km.
[With a semiconductor amplifier (SOA) inside the transceiver, the transmitting end can work on four LAN WDM bands of 1295nm, 1300nm, 1305nm, and 1310nm. The optical signals on the four bands transmitted on a single-mode fiber through a duplex LC connector after multiplexed by WDM multiplexer. At the receiving end, the SOA can amplify the signal then decomposes it into a single channel.]
It used to packed in CFP and CFP2. But 100G QSFP28 is more popular nowdays due to the high-density, low power consumption and maintenance costs.
100G ZR4 can reach 80km.
[The four-channel serial data (NRZ) at the transmitting end is recovered by CDR and transmitted to four laser drivers. These drivers control four lasers with center wavelengths of 1296nm, 1300nm, 1305nm and 1309nm. The optical signal is multiplexed to the single-mode fiber through a standard LC connector. At the receiving end, the optical signals of the four channels are amplified through SOA, and then the optical signals are demultiplexed through the integrated optical demultiplexer. Each optical signal is recovered by the PIN detector, and then transmitted to the CAUI-4 compliant output driver through the transimpedance amplifier and CDR.
Same as above, QSFP28 package is more popular.
100G DCO can reach 2000km. And there are two packages, CFP and CFP2. CFP2 DCO provides greater flexibility compared with CFP DCO, it supports higher panel density. In addition, CFP2 DCO adopts QP-QPSK modulation and built-in DSP, which automatically has the electronic dispersion compensation function instead needing a separate dispersion compensation module (DCM), thus it can transmit a longer distance between the amplification sites.