Introduction and application of 25G SFP28 CWDM optical module



2020 is the first year that 5G will be commercialized. At present, 25G optical modules have also been used in the construction of 5G signal base stations. Among them, 25G CWDM optical modules have a high market prospect in 5G fronthaul. Compared with other 25G optical modules, 25G CWDM optical module can save more fiber resources, thus effectively saving the total cost for operators.


As early as 2019, China Telecom published the technical requirements for passive color light coarse wavelength division multiplexing equipment for 5G fronthaul. Among them, the central wavelengths range of the 25G coarse wavelength division multiplexing module on the AUU side selected by China Telecom are to be 1271, 1291, 1311nm, and the ring color code is half-color. The center wavelengths range of the 25G coarse wavelength splitting module on the DU side are 1331, 1351, 1371nm, and the ring color code is full color. Because the working environment of 5G fronthaul is relatively changeable, 25G SFP28 CWDM needs to meet the temperature range of industrial grade and extended grade.


The 25G SFP28 CWDM optical module developed by China Telecom has a transmission distance of 10km and 15km, but the most common one on the market is 25G SFP28 CWDM with 10km.


25G CWDM SFP28 uses 1270nm-1370nm (interval 20nm) CWDM DML laser and PIN receiver, or 1470nm-1570nm (interval 20nm) CWDM EML laser and APD receiver, its receiving and transmitting channel built-in CDR circuit, the maximum rate is up to 25.78 Gbps.



The core optical components of the 25G SFP28 CWDM optical module are TOSA and ROSA, and the core electrical chip is the CDR chip. In the design, the Driver chip and the CDR chip use the same chip. MCU mainly realizes the configuration of the chip and the monitoring of the working status of the module through the I2C bus. MCU adjusts the size of the bias current through software design and adjusts the output light size by querying the temperature compensation table.


MCU acts as a host and a slave respectively, and realizes bus communication through the I2C serial port. Use MCU internal algorithm plus loopback control circuit to adjust the digital quantity to control the average optical power. When the output light becomes stronger, the backlight current detected by the PD will become larger. It is compared with another preset input current initial value through the internal comparator to reduce the drive current, thereby reducing the average optical power; otherwise, If the light emission becomes weaker and the backlight current becomes smaller, the driving current will increase after passing through the comparator, increasing the average optical power. In this way, the average optical power output by the laser remains stable. The optical modules provided by OPTCOM can support the digital diagnosis function (DDM), which effectively helps users monitor various parameters of the optical module in real time, such as temperature, received optical power, and transmitted optical power etc.


In 2020, China's 5G base station construction volume will reach 600-800,000 stations, and the demand for 25G fronthaul optical modules will reach 7.2 million-9.6 million. The optical modules used in the fronthaul of 5G have been improved in transmission rate and the number of users. With the increase in CRAN deployment requirements of operators, the demand for wavelength division multiplexing optical modules has increased, and 25G CWDM optical modules will be favored.