With the large-scale application and development of new businesses such as cloud computing, ultra clear film&television and the Internet of things, the demand for traffic and bandwidth in the data center has increased sharply, which also forces the optical module to upgrade to higher speed and lower power consumption, and this trend is still rising. With the rapid development of 400Gbit/s communication technology in recent years, the data center optical communication network has gradually begun to transition from 100G to 400G interconnection.
Therefore, 400Gbit/s network system will eventually become a new generation of high-speed transmission technology deployed in the data center. As an optical device to realize optical/electrical and electrical/optical conversion functions, optical module is an indispensable part of data center optical communication network system.
This paper describes the relevant standards of 400G optical module and PAM4 modulation technology, and then takes 400G FR4 optical module as an example to test and analyze the performance of the module.
- Introduction to 400G optical transceiver standards
Nowadays, 400Gbit/s Ethernet interface has been gradually applied to the data centers of major Internet companies. IEEE has started the research on the standardization of 400Gbit/s Ethernet (400GE) interface since March 2013, and officially approved and released IEEE 802.3bs 400GE (including 200GE) protocol standard in December 2017 and IEEE 802.3cu 400GE protocol standard in April 2020, Among them, several physical layer technical schemes mainly adopted by 400GE optical module are shown in the table below:
In order to reduce the cost and support 400G Ethernet transmission in a more economical way, the industry generally adopts the mode of optical port rate of 4x100Gbit/s, that is, four single channel PAM4 encoded optical components with a rate of 100Gbit/s are used, and then the speed of single fiber 400Gbit/s is achieved through 4x100Gbit/s CWDM (coarse wave length division multiplexing) technology. The 100G lambda MSA defines 4x100Gbit/s CWDM4 optical interface. Through this transmission mode of 4x100Gbit/s, the number of optical devices can be doubled, so as to reduce the power consumption and cost of 400G optical transceiver. Therefore, The 100G lambda MSA not only provides the technical platform required for the development of growing bandwidth demand, but also lays a solid foundation for the development of economical 400G.
- Core technology of 400G transceiver: PAM4 modulation
400G FR4 optical module is mainly composed of PAM4 rate conversion, receiving, transmitting and monitoring units. Its basic principle block diagram is shown in the figure below:
The rate conversion unit mainly completes the conversion between 8 channels of PAM4 electrical signals with 53.125Gbit/s rate and 4 channels of PAM4 electrical signals with 106.250Gbit/s rate. The transmitting unit uses a 4-channel laser driving chip to drive 4 groups of Tosa to realize electric/optical conversion. After conversion, the 4-channel optical signals with central wavelengths of 1271, 1291, 1311 and 1331nm are coupled into an SMF by a MUX.
The receiving unit uses a DEMUX to divide the received optical signal into four optical signals with different wavelengths, and then completes optical/electrical conversion and current voltage conversion through four groups of Rosa. The four electrical signals obtained after conversion are output after amplification and differential driving. In the monitoring unit of the module, MCU (micro controller unit) completes the reading and writing operation of each electric chip register in the optical module and data communication with the host computer through I2C (inter integrated circuit), so as to realize the digital diagnosis and monitoring function of the module.
In data transmission, using PAM4 modulation format reduces the number of parallel channels, which not only saves the cost of optical module, but also can effectively alleviate the requirements for the bandwidth of optical devices in optical module, so that the optical module has smaller package size and power consumption. Nowadays, PAM4 technology has become the inevitable trend of high-speed signal interconnection in the next generation data center.
- How to test and analyse 400G transceiver
Taking the 400G FR4 optical module as an example, the test contents mainly include the optical eye pattern test at the transmitting end and the sensitivity test at the receiving end. The specific test methods and results are as follows:
·Transmitting Unit
According to the regulations of 400G FR4 technical specification in 100G Lambda MSA, when the single channel rate is 106.250Gbit/s, the optical eye diagram of each channel at the transmitting end shall meet the following conditions:
-The average optical power is -3.3 ~ 3.5dBm
-OMA (outer optical modulation amplitude) is -0.3 ~ 3.7dBm
-Extinction ratio > 3.5dB
-TDECQ (transmitter and dispersion eye closure quarterly) < 3.4dB
The module has four channels with different central wavelengths in the emission direction, and a single channel can be selected one by one through the optical switch for testing.
The following figure shows the emission eye diagram of a single channel when the module temperature is 0°C, 40°C and 70°C under the test conditions at 106.250Gbit/s and the Short Stress Pattern Random Quaternary (SSPRQ) is shown below, with the TDECQ-balanced eye diagram on the right.
The table below shows the performance parameters of the 400G FR4 optical module for the emitted optical eye diagram at three different temperatures.
According to the test results, in the whole temperature range, all index parameters of the optical eye diagram at the transmitting end are within the specified range of the agreement, and the fluctuation of each parameter value with the change of temperature is small.
·Receiving Unit
According to the provisions of 400G FR4 technical specification in 100G lambda MSA, when the single channel rate is 106.250Gbit/s, the sensitivity of each channel at the receiving end shall be < -4.6dBm.
The reception sensitivity of each of the four channels of the module was tested at 106.250Gbit/s, PRBS31Q (Pseudo-Random Binary Sequence 31Q) code type and 2.4×10^-4 BER at 0°C, 40°C and 70 °C. The test results are shown in the table below.
It can be seen from the above test results that within the full temperature range, the receiving sensitivity of the four channels meets the specified range of the protocol, and its fluctuation is small with the change of temperature.
PS: The 400G FR4 optical module proposed in this paper is suitable for the two mainstream 400G packaging forms: QSFP-DD module (Quad Small Form Factor Pluggable-Double Density) and OSFP (Octal Small Form Factor Pluggable Module). Module).
To summary, 400G optical transceiver is one of the core components of the physical layer in the construction of 400G optical communication network in the data center, and has great advantages in low cost and low power consumption. It is believed that with the maturity of optical device technology and various communication technologies, it will have broader development prospects in the high-speed optical communication network of the data center.
