In-depth Analysis of 400G SR8 Optical Transceiver

NADDOD Peter Optics Technician Aug 1, 2023

With the rapid development of technologies such as cloud computing, artificial intelligence, and the Internet of Things (IoT), the demand for data centers is increasing. In response to this demand, the 400G SR8 module has emerged.


This optical transceiver incorporates several key technologies, including VCSEL lasers, multi-mode lens PD design, and COB packaging design, to achieve high-speed and high-density data transmission.


In this article, we will delve into these key technologies and their principles, providing you with a comprehensive introduction to the features and advantages of the 400G SR8 optical transceiver.

Principle of 400G SR8 Optical Transceiver

The NADDOD 400G SR8 optical transceiver conforms to the IEEE 802.3cm (Draft) standard and meets the transmission link requirements of 70m OM3 and 100m OM4/OM5.


The block diagram of the 400G SR8 optical transceiver is shown in the figure below. The transceiver uses a standard 16-core multi-mode MPO connector and consists of six main components: the receiver, transmitter, VCSEL driver, transimpedance amplifier (TIA), PAM4 business chip (oDSP), controller, and others. The eight-channel optical and electrical components of the transmitter and receiver are compactly placed within the QSFP-DD package through engineering design.


400G SR8 Schematic Diagram

400G SR8 optical transceiver schematic diagram


The 400G SR8 optical transceiver consists of 8 channels, with each channel carrying a 53.125Gbps signal. It adopts PAM4 high-order modulation, and the internal oDSP of the module performs functions such as clock recovery, signal shaping, and conditioning.


Key Technologies of NADDOD 400G SR8

To meet the requirements of high performance, low cost, and deliverable 400G SR8 multi-mode short-reach optical transceivers for data center applications, NADDOD has employed several key technologies, including:


High-performance VCSEL Lasers

With the advent of the 400GE era, the modulation technology of optical transceivers has transitioned from 25.78125Gbaud NRZ signals to 26.5625Gbaud PAM4 signals.


To ensure the transmission performance of PAM4 services, NADDOD utilizes industry-leading high-bandwidth VCSEL lasers with a typical bandwidth of 19GHz.


The simplified diagram of the VCSEL structure is shown below. This laser achieves higher bandwidth (typical bandwidth of approximately 13-15GHz) compared to 25Gbaud NRZ devices by:


a. Reducing the oxide aperture to increase photon density.


b. Optimizing the design of the metal electrode to reduce parasitic parameters.


These improvements provide performance assurance for 400G service transmission.


 VCSEL Structural Diagram

VCSEL structural diagram

(Left) VCSEL Design for NRZ Application           (Right) VCSEL Design for PAM4 Application


Multi-mode Lens PD Design

The 400G SR8 optical transceiver utilizes high-performance multi-mode PD detector with a typical bandwidth of 20GHz. The chip incorporates a lens design. The schematic diagram below illustrates the structure. 


Compared to traditional surface-illuminated PDs, the lens PD exhibits focusing characteristics, resulting in stronger light reception and a 30% to 40% increase in coupling tolerance. This not only enhances the performance of the transceiver's receiving end but also reduces the precision requirements of the production process.


Schematic Diagram of Multimode Lens PD Structure