24. February 2022

Components of an Optical Module

Components of optical transceivers

Transceivers can be divided into two essential components: optoelectronic components and the PCBA.


Optoelectronic components

Optoelectronic components are considered to be the core components of transceivers. Different types of optoelectronic components are used for different optical transceiver applications.

The two most widely used types of optoelectronic devices are Transmitter Optical Sub Assembly (TOSA) and Receiver Optical Sub Assembly (ROSA). A combination of these devices is used in general purpose transceivers. In the following you will learn about the functions of TOSA and ROSA as well as the third type especially for BiDi transceivers.



TOSA is the component within the transceiver that is responsible for converting the electrical signal into an optical signal and then transmitting it over the connected optical fiber strand. It consists of a light source (laser diode or light-emitting diode), a monitoring photodiode, and an optical and electrical interface. Depending on the application of the optical module or the transmission distance, additional components such as a filter element can also be used with the TOSA.

Nowadays, optical modules prefer to use a laser diode as a light source. Compared to LEDs, laser diodes consume less power but provide higher output with higher coupling efficiency at the same time. However, due to their long lifetime and low cost, LEDs are still used today for short distance and low speed transmission systems.



ROSA is the opposite component in the transceiver that is responsible for receiving the optical signal sent by the TOSA of the opposite side transceiver and converting it back into an electrical signal. This consists of a photodiode and an optical and electrical interface.

As with TOSA, additional filters or amplifiers can be used in conjunction with ROSA depending on the transmission requirements. There are two types of amplifiers: pre-amplifier and post-amplifier. The preamplifier converts the current signal into a voltage signal and amplifies it. The post-amplifier, on the other hand, receives the amplified signal and outputs a signal suitable for the downstream digital circuits.

A combination of TOSA and ROSA is used in an optical transceiver and represents its core elements that enable it to transmit and receive data.


BOSA in BiDi transceivers

TOSA and ROSA are the core components in unidirectional transceivers that transmit on one fiber strand and receive on the other fiber strand. In bidirectional transceivers, the so-called Bidirectional Optical Sub Assembly (BOSA) is used. This is a combination of TOSA, ROSA and an additional WDM filter.

The WDM filter enables the bidirectional transceiver to split the wavelengths into two separate wavelengths. Then the transmit and receive functions can be performed on a single fiber strand.



While the term PCB describes the printed circuit board itself, PCBA (Printed Circuit Board Assembly), in contrast, refers to the assembly of the printed circuit board. In the assembly process, various electronic components (assemblies) are mounted on the PCB. These can be resistors, ICs (Integrated Circuits), capacitors, transformers, etc. The assembly is done by SMT, THT or Chip-on-Board (CoB). For the mechanical connection on the PCB, the PCB with the assembled components usually goes through a reflow soldering process or solder paste printing.

For optical transceivers, the PCB is assembled with the components described above.



Optical transceivers consist of various components. The outer jacket protects the sensitive electronic and optoelectronic components. Unidirectional transceivers are equipped with a TOSA - to convert an electrical signal into an optical signal - and a ROSA - to convert an optical signal into an electrical signal. In BiDi transceivers, a BOSA is used. This involves soldering all the components of an optical transceiver onto a PCBA.

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