WDM Technology in Transceivers: Principles, Components, and Application Progress
01.17.2025 | SZETERN | Etern

In the field of high-speed optical communication, with the explosive growth of data traffic, Transceiver, as the core components of optical communication systems, are crucial for performance improvement. Wavelength Division Multiplexing (WDM) technology plays a key role in Transceiver and provides an effective solution to meet the increasing bandwidth requirements.

 

I. Approaches to Bandwidth Enhancement of Transceiver and the Principle of WDM Technology

There are two main ways to increase the bandwidth of Transceiver. One is to increase the bit rate of each channel, such as directly increasing the baud rate or using complex modulation and demodulation methods like PAM4 while keeping the baud rate constant. The other is to increase the number of channels, such as increasing the number of parallel fibers or adopting WDM technology (CWDM, DWDM). The core of WDM technology lies in its ability to enable a single fiber to transmit multiple wavelengths of signals simultaneously, which significantly improves the transmission capacity of the fiber and has been widely used in medium and long-distance optical communication transmission and data center interconnection.

 

Its working principle is that at the transmitting end, optical signals of different wavelengths are combined into a single fiber for transmission through a multiplexer (MUX), and at the receiving end, the mixed optical signals are separated into individual original wavelength signals by a demultiplexer (DEMUX), thus realizing efficient transmission of multi-channel data.

 

II. Components for Implementing WDM Technology in Transceiver

(A) TFF (Thin Film Filter) Technology

In Transceiver, the TFF technology usually adopts the Z-block method for implementation. It is based on free space optics design and combined with collimators, using four CWDM wavelength filters to perform multiplexing and demultiplexing operations in a micro-optical manner, enabling the transmission of signals with wavelengths of 1271nm, 1291nm, 1311nm, and 1331nm in a single fiber.

 

The Z-block, as the core device of the TFF wavelength multiplexing/demultiplexing component, has a processed rhombic prism (parallelogram glass substrate) in the middle. A high-reflection film is coated on part of the back of the rhombic prism, and WDM filters of different wavelengths are attached on the other side. Each filter only allows the optical signal of the current channel wavelength to pass through and reflects the wavelengths of other channels. In the multiplexing and transmitting optical path, the optical signals emitted from the four collimators on the right side pass through the corresponding filters and after different numbers of reflections, reach the collimator at the left common end and are finally coupled into the output fiber. In the wavelength demultiplexing and receiving optical path, the optical signal at the common end is input from the left collimator, and the optical signals of each channel are filtered and reflected, and then focused onto the corresponding units of the photodetector array by a microlens. Since the active area of the photodetector is usually small (usually Φ50 μm) and the diameter of the collimated beam is large, a microlens is required for focusing and fine adjustment to align with the active area, which also increases the complexity of the assembly process. Limited by optical performance and assembly yield, the currently commonly used number of channels of the Z-block is 4 channels.

(B) AWG (Arrayed Waveguide Grating) Technology

AWG is a wavelength multiplexing component based on PLC (Planar Lightwave Circuit). The CWDM4-AWG chip has matured and is widely used in 100Gbps CWDM4 QSFP28 products. In the early stage, the input/output ports of the CWDM4 AWG chip were located at both ends. To facilitate fiber winding and integration into fiber transceiver modules, chips with single-sided input/output have been developed. The input port is wound to the output end through a curved waveguide, simplifying the coupling process between the waveguide and the fiber array. However, due to the limited width of the chip, a waveguide bending radius of less than 1mm will introduce a certain amount of bending loss.

 

In a CWDM4 fiber transceiver module, two CWDM4 AWG chips are required. The transmitting-end chip usually adopts a single-sided input/output structure, and the receiving-end chip usually adopts a two-sided input/output structure because the demultiplexed optical signals need to directly incident on the photodetector array. The output port is a multimode optical waveguide and the end face is polished into a 45° bevel to achieve a 90-degree turn of the beam and incident on the photodetector array mounted on the PCB board.

 

III. Application Status and Trends of WDM Technology in Transceiver

 

In the current 800G Transceiver (such as FR8, LR8, etc.), the 8×100G scheme is widely used, and the Z-block technology scheme is more commonly applied. Although the 800G schemes of different manufacturers vary, the Z-block has the advantages of low loss and good channel quality and can support the transmission of signals at a rate of 100G or higher for 10 kilometers or more. AWG is more commonly used in the receiving end of traditional Transceiver because of its significant cost and packaging advantages.

 

In the future, with the continuous development of optical communication technology, WDM technology is expected to continuously make breakthroughs in improving the integration of Transceiver, reducing costs, increasing the number of channels, and increasing the transmission rate, further meeting the needs of data centers, 5G networks, and other applications for high-speed and large-capacity optical transmission and promoting the optical communication industry to a new stage of development.