What Are the Key Components of Optical Transceiver Module?

The function of optical transceiver module is to perform photoelectric conversion, and its internal TOSA, ROSA and BOSA are the key components to realize the photoelectric conversion function. The optical device is composed of transmitter and receiver to complete the optical-electrical or electrical-optical conversion of optical signals.

The interior is composed of optical devices, functional circuits and optical interfaces. The optical device is the main component of the optical transceiver module.

The optical devices used for optical signal conversion are called TOSA and ROSA.

TOSA (Transmitting Optical Sub-Assembly) mainly completes the conversion of electrical signals into optical signals. With the light source (semiconductor light-emitting diode or laser diode) as the core, LD chip, monitor photodiode (MD) and other components are packaged in a TO coaxial package or butterfly package, which constitutes TOSA.

In TOSA, laser diode is the most commonly used semiconductor emitting device for optical transceiver modules. Threshold current (Ith) and slope efficiency (S) are the two main parameters. In order to make the LD work quickly, a DC bias current slightly greater than the threshold current must be provided to the LD, that is, the laser is emitted only when the forward current exceeds the threshold current.

ROSA (Resceiving Optical Sub-Assembly) optical receiving assembly, in the high data rate optical fiber module, PIN or APD photodiode and TIA are usually assembled in a sealed metal casing to form an optical receiving assembly.

The figure below is the schematic diagram of the optical module ROSA, which is composed of a photodetector (PIN/APD), a TIA pre-amplifier, and a limiting amplifier.

Photodetector, the main device of ROSA, is mainly used to convert optical signals into electronic signals through the photoelectric effect. The common photodetectors in optical communications are PIN photodiodes and avalanche photodiodes (APDs). APDs are high-sensitivity photodetectors that use the avalanche multiplication effect to double the photocurrent. Compared with PIN photodiode, the receiving sensitivity of APD can be improved by 6~10dB.

The weak signal current generated by the photodetector is converted into a signal voltage of sufficient magnitude by the preamplifier TIA, and then output. TIA is actually a voltage converter, which converts electro-optic current into voltage.

At this time, the voltage signal output by the TIA is still an analog signal, which needs to be converted into a digital signal before the signal processing circuit can recognize it. The function of the Poster Amplifier behind the TIA is to convert signals of different amplitudes into digital signals with the same amplitude.

After introducing TOSA and ROSA, let's take a look at what is BOSA?

With the development of process level technology, the modules can be made smaller. TOSA and ROSA integrate the transmission and reception of light (LD and PIN/APD) through the coaxial coupling process, plus splitters, optical fibers and other components, called BOSA (Bi-Directional Optical Sub-Assembly).

Nowadays, the high-speed optical transceiver module integrates high-performance DSP at the receiving end, and its performance in terms of dispersion and noise processing is really good.