GBC Photonics 100G Optical Modules - Internal Design

Flowchart of 100G Single Rate and Dual Rate Modules:

Both types of modules have the same architecture. The most important blocks are:

Tx CDR & DRV — a transmitter module supporting four input electrical channels, comprising CDR circuits (Clock and Data Recovery) and laser drivers. Electrical channels can operate at 25.78 Gbps (103.125 Gbps total for single rate modules) or support two speeds: 25.78 Gbps and 27.95 Gbps (total 103.125 Gbps or 111.81 Gbps for dual rate modules).

CDR chip — on the horizontal axis is time, representing the deviation of the signal period (jitter), and on the vertical axis is the signal value; the CDR is meant to provide correct decoding of the signal amplitude, on which noise can be superimposed.

In addition, in the transmitter track, FIR filters (finite impulse response) are used. The CDR chip works without a reference clock signal and decodes the phase and frequency from the input signal.

Rx CDR — a receiver module that handles four electrical signals generated by the receiver diodes together with the CDR of the received signal. A CDR works similarly to the transmitting track, but different filters are used in the receiver track.

As a fiber optic line has, in simple terms, the characteristics of a low-pass filter, the signal passing through such a path undergoes CTLE filtering (continuous-time linear equalization) designed to equalize the characteristics of the signal received by the module.

In addition, in the receiver track, it is used DFE filter (decision feedback equalization), that is, a logical system in the feedback loop of the decoding system of the logical value of the signal, which helps to eliminate decoding errors of this value.

The effect of CDR operation on signal quality is illustrated by the comparison of the eye diagrams below. Correspondingly, with a working CDR and without such a system.

TOSA - a transmitter optical sub-assembly, i.e. a transmitter assembly built on the basis of previously described lasers of the DML type (most often in single rate modules) or EML (most often in dual rate modules). The four electrical signals are converted into four wavelengths: 1295.56 nm, 1300.05 nm, 1304.58 nm and 1309.14 nm, and then multiplexed into a single fiber as a WDM LAN. ‍

ROSA - optical sub-assembly of the receiver, i.e. receiver assembly composed of PIN or APD receiving diodes in the case of modules of greater ranges. In the case of APD (avalanche photodiode), it is critically important to ensure a sufficiently low input level consistent with the catalog data.

A signal level that is too high can lead to physical damage to the module. With shorter fiber paths, it may also be necessary to use compatible attenuators.

Controller — a microprocessor chip managing the operation of the module chips, including EEPROM memory for module configuration and communication with the host via I2C bus

Where to use single rate and dual rate modules?