100G LR4 QSFP28 single & dual rate transceivers | components

The aim of these four articles about optical transceivers is to familiarise potential users with the technology used in the modules, the potential solutions, functional differences, areas of application and possible integration with other GBC Photonics products.

Transceivers’ architecture

The following block diagram summarises the components of 100G single and dual rate transceivers. Both types of transceivers have the same architecture. Some of the most important components are the following.

The TX CDR & DRV transmitter module, which processes four input electrical channels. It includes a Clock Data Recovery (CDR) unit and laser driver. The electrical channels work with transmission speeds of 25.78 Gb/s — a total 103.125 Gb/s in the case of single rate transceivers — or handle two speeds, 25.78 Gb/s and 27.95 Gb/s — a total 103.125 Gb/s or 111.81 Gb/s in the case of dual rate transceivers.

The RX CDR module, a receiver module that handles four electrical signals generated by receiver diodes and includes the CDR system for the received signal. The CDR system operates in the same way as in the transmitter tract, but in the receiver different filters are used.

For simplicity, we can compare a fibre-optic line to a low-pass filter, meaning that the signal that passes over such a tract is exposed to the activity of Continuous-Time linear Equalization (CTE) filters, whose role is to equate the characteristics of the signal received by the module. Additionally, the receiver tract contains a Decision Feedback Equalization (DFE) filter, which is a logical unit in the receiver’s loopback which helps eliminate errors while decoding this value.

The influence of a CDR system on the quality of a signal is illustrated by the eye diagrams below. One includes a properly working CDR system and the other does not.

The Transmitter Optical Sub-Assembly (TOSA) consists of a transmitter that uses a DML type laser (especially in single rate transceivers) or EML type laser (especially in dual rate transceivers). 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 fibre as LAN WDM.

A Receiver Optical Sub-Assembly (ROSA) is a receiver consisting of receiver diodes, which are PIN or APD in the case of transceivers for longer distances. Its role is to convert an optical signal into electrical signals that are subsequently processed in the receiver tract. In the case of Avalanche Photo Diodes (APD) it is of critical importance to provide an adequate low level input signal, as given in catalogue cards. If the level of the signal is too high, this can damage the transceiver. While commissioning a circuit with such a transceiver, it is critical that the amplitude of the signal received lies in the range specified in a catalogue card. If a fibre-optic tract is very short, it might be necessary to use attenuators.

The controller is a microprocessor that manages the module’s systems, the EPROM memory, including a module configuration, and communication with a host that is provided by Inter-Integrated Circuit (I2C).

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