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.
Both types of transceiver have the same architecture. Among the most important parts are:
The Tx CDR & DRV transmitter module that 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 role of a CDR unit is to recover an electrical signal which enters the transceiver in two dimensions. The horizontal axis (time), represents the correction of signal period deviation (jitter) and the vertical axis represents the value of the signal. A CDR system has to correctly decode the signal amplitude on which distortions, or noise, may overlap. Additionally, in the transmitter tract, Finite Impulse Response (FIR) filters are used to improve the characteristics of signal frequency, by minimising the noise generated by the transmission tract. The CDR system operates without a reference clock signal and decodes both phase and frequency from the input signal.
The Rx CDR is 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, however, in the receiver different filters are used. As a fibre-optic line can be compared, for simplicity, to a low-pass filter, 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, in the receiver tract there is a Decision Feedback Equalization (DFE) filter, which is a logical unit in the receivers loopback which aids eliminate errors while decoding this value.
The Transmitter Optical Sub-Assembly (TOSA) consists of a transmitter that uses a DML type laser (most often in single rate transceivers) or EML-type laser (most often 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: 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, it can damage the transceiver. While commissioning a circuit with such a transceiver, it is of prime importance 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 which manages the module’s systems, EPROM memory which includes a module configuration, and communication with a host that is provided by Inter-Integrated Circuit (I2C).
To be continued...