What code type is used in fiber optic communication

Fiber optic communication primarily uses Non-Return-to-Zero (NRZ) and its variants, along with advanced modulation schemes like NRZI and PAM4, to encode digital data into light signals.Basic Encoding Techniques

Non-Return-to-Zero (NRZ) is the most widely used line code in fiber optics. In NRZ, a bit of '1' is represented by the presence of light, and a bit of '0' is represented by the absence of light. This simple scheme is cost-effective and easy to implement, but long sequences of identical bits can cause timing synchronization issues at the receiver . On-Off Keying (OOK), also known as Amplitude Shift Keying (ASK), is conceptually similar to NRZ, where the presence or absence of light represents binary data. OOK is suitable for low-cost, short-reach applications but is less efficient for high-speed transmission due to susceptibility to noise and clock recovery challenges .

Advanced Variations

NRZ-Inverted (NRZI) improves upon NRZ by representing a '1' bit as a transition in the signal (light-to-dark or dark-to-light) and a '0' bit as no transition. This ensures frequent transitions even during long sequences of identical bits, enhancing clock recovery at the receiver . Return-to-Zero (RZ) coding represents a '1' bit with a pulse that returns to zero within the bit period, guaranteeing a transition for every '1' bit. This simplifies timing extraction but requires more bandwidth than NRZ . Pulse-Amplitude Modulation (PAM), particularly PAM4, is increasingly used for high-speed fiber links. PAM4 encodes two bits per symbol by using four distinct amplitude levels, effectively doubling the data rate without increasing the symbol rate .

Block and Bit Insertion Codes

For long-haul or high-speed optical systems, block codes (e.g., 3B4B, 5B6B, 7B8B) and bit insertion codes (mBIC, DmBIM) are employed. These codes insert extra symbols to reduce long sequences of identical bits, improve timing recovery, and maintain signal integrity over the fiber .

Summary

In fiber optic communication, binary line codes like NRZ and NRZI dominate due to their simplicity and reliability. For higher data rates and long-distance transmission, advanced modulation schemes like PAM4 and block coding techniques are used to enhance efficiency, reduce errors, and facilitate accurate timing recovery .