What Are Fiber Optical Transceivers?

More and more people in the low voltage industry have the same question: What Are Fiber Optical Transceivers? Today let me try to answer this question and provide the most comprehensive overview from an insider's perspective. 

1. The Definition and function of Fiber optical transceivers.

 

The definition and function of fiber optical transceivers is to perform electro-optical and photoelectric conversion. At the transmitting end, the electrical signal is converted into an optical signal, which is then transmitted through an optical fiber. At the receiving end, the optical signal is converted back into an electrical signal.

Composition of fiber optical transceivers

 

Here's a block diagram showing how fiber optical transceivers works.

On the left side of the diagram is a device that uses fiber optical transceivers, such as a switch. The device board signals input into fiber optical transceivers, which converts the electrical signal into an optical signal and sends it out. The receiving process is the reverse of transmission.

 

2. The main components of Fiber optical transceivers include:

 

CDR (Clock and Data Recovery):

The role of the clock and data recovery chip is to extract the clock signal from the input signal and establish the phase relationship between the clock signal and the data. Basically, it recovers the clock signal. Additionally, the CDR can compensate for signal losses in the wiring and connectors.

 

LDD (Laser Diode Driver):

It converts the output signal from the CDR into a corresponding modulation signal to drive the laser to emit light. Different types of lasers require different types of LDD chips. In short-haul multimode fiber optical transceivers (e.g., 100G SR4), the CDR and LDD are generally integrated into the same chip.

 

TOSA:

It performs the electro-optical conversion and includes components such as the laser, MPD, TEC, isolator, Mux, and coupling lens. It comes in various packaging forms like TO-CAN, Gold-BOX, COC (chip on chip), and COB (chip on board). For fiber optical transceivers used in data centers, to save costs, TEC, MPD, and isolators are not required. The Mux is only used in fiber optical transceivers that require wavelength division multiplexing. Additionally, some fiber optical transceivers have the LDD also packaged within the TOSA.

Here's an image of the CWDM4 AWG Mux/Demux components. The CWDM4 Mux/DeMux AWG component is an array waveguide grating product based on PLC planar lightwave circuit technology, which serves to multiplex and demultiplex in fiber optical transceivers. It is mainly used in high-speed fiber optical transceivers, such as 100G QSFP28 FR4. The center wavelengths are 1271, 1291, 1311, and 1331 nm.

Here's an image of the LWDM4 Demux components. It is an array waveguide grating product based on PLC planar lightwave circuit technology, with quartz and silicon-based chips. It serves to multiplex and demultiplex in fiber optical transceivers and is mainly used in high-speed fiber optical transceivers, such as 100G LR4 and 200G LR4. 

ROSA:

It performs the opto-electronic conversion and includes components such as PD/APD, DeMux, and coupling assembly. The packaging types are generally the same as those of the TOSA. PD is used in short- and medium-distance fiber optical transceivers, while APD is mainly used in long-distance fiber optical transceivers.

 

TIA (Transimpedance Amplifier):

It is used in conjunction with a detector. The detector converts the optical signal into a current signal, and the TIA processes the current signal into a voltage signal of a certain amplitude. You can think of it as a big resistor.

 

LA (Limiting Amplifier):

The output amplitude of the TIA changes with the received optical power. The role of the LA is to process the varying output amplitude into a constant amplitude electrical signal, providing a stable voltage signal to the CDR and decision circuitry. In high-speed modules, the LA is usually integrated with the TIA or CDR.

 

MCU:

It handles the underlying software operations, monitoring fiber optical transceivers' DDM functions, and performing some specific functions. DDM monitoring mainly implements real-time monitoring of signals such as temperature, Vcc voltage, Bias current, Rx power, and Tx power. These parameters help assess the fiber optical transceivers' operational status, facilitating the maintenance of the optical communication link.