Currently, 25G optical transceivers are widely used in the 5G front-haul network. In order to save fiber resources, 25G WDM modules are adopted for 5G front-haul transmission. However, 25G WDM transceivers are much more expensive than other 25G optical modules, like SFP28 SR module, SFP28 LR transceiver, 25G SFP28 BiDi modules and so on. 25G SFP28 LWDM transceiver can also achieve the multi-channel signals transmission on one single fiber and it has a lower cost than 25G WDM transceivers. Due to its flexibility, stability, and cost-controllability, 25G SFP28 LWDM transceiver tends to be widely promoted in 5G front-haul network.
25G SFP28 LWDM transceiver is fully compliant with the IEEE802.3ba standard to provide a flexible way for the 25G Ethernet network to smoothly evolve to single-wavelength 50G or 100G in the future. The LWDM represents the LAN WDM and the 25G SFP28 LWDM module has twelve wavelengths, which belong to the O band. Eight of these wavelengths are standard wavelengths with 800Hz band interval, and the rest four wavelengths of 25G LWDM transceiver multiplex those of 25G SFP28 CWDM transceivers’ (1269.23nm, 1332.41nm, 1313.73nm, 1291.10nm). Like other 25G WDM modules, 25G SFP28 LWDM transceiver can also utilize the WDM technology to multiplex multiple wavelengths onto a pair of fiber to save a bunch of fiber resources.
Figure 1: 25G SFP28 LWDM & CWDM Transceiver Wavelength
We can see from the figure above, the 25G LWDM module adopts wavelength channels within 4nm which is narrower than that of 25G SFP28 CWDM’s, the large wavelength shift will appear. In general, the working temperature of 25G LWDM is -40℃ to 85℃. If the temperature changes by 1℃, an offset of about 0.1nm will be produced. And the SFP28 LWDM transceiver might produce a wavelength shift of 12.5 nm. In order to keep a stable transmission, the TEC (Temperature Control) control is very essential for 25G LWDM optical transceiver. It can generate current through the movement of charge carriers in the conductor. When DC(Direct Current) passes through two different conductor materials, heat absorption or exotherm will occur on the contact end to keep a steady wavelength. In this way, the SFP28 LWDM optical module can ensure stable and accurate data transmission in an environment with demanding transmission requirements.
The following table shows some common specifications of the 25G SFP28 LWDM transceiver.
|Features||25G SFP28 LWDM|
|Wavelength||1295.56nm, 1300.05nm, 1304.58nm, 1309.14nm; 1273.54nm, 1277.89nm, 1282.26nm, 1282.66nm; 1269.23nm, 1332.41.nm, 1313.73nm, 1291.10nm|
|Max Cable Distance||30km without FEC/40km with FEC|
|Operating Temperature||-40°C to +85°C|
|Protocols||IEEE802.3ba; RoHS Compliant;|
|Applications||25GBASE-ER Ethernet Data Center Networks; CPRI/eCPRI 5G Fronthaul Networks|
Like 25G CWDM/DWDM transceivers, the 25G SFP28 LWDM modules can also multiplex multiple wavelengths onto a single fiber, but they are implemented at a lower budget. In the design of the receiver, since SFP28 25G CWDM transceiver has a larger dispersion which needs a more expensive APD receiver to ensure the stability of the transmission. But the wavelengths of SFP28 LWDM module are located at the low dispersion areas with PIN receivers at the receiving end to satisfy the transmission requirements (no need for APD receivers, which saves the transmission budgets). As for the lasers, compared with SFP28 DWDM module, 25G SFP28 LWDM transceivers doesn’t have to use the expensive EML lasers, but they can be realized by the relatively cheap DML lasers to keep a stable transmission and save cost for the construction, making them an ideal and efficient solution in 5G front-haul transmission.
The connection of 25G SFP28 LWDM transceivers is similar to that of CWDM/DWDM modules, which need to be used with LWDM Mux/Demux. Just plug four 25G SFP28 LWDM modules into two 25G switches respectively. Then four single mode fiber optic cables are used to connect the SFP28 LWDM modules and LWDM Mux/Demux respectively. The connectivity between the two LWDM Muxes/Demuxes can be achieved by one single mode fiber optic cable. The following figure shows a fundamental point-to-point direct connection for 25G SFP28 LWDM transceivers by using LAN-WDM Muxes/ Demuxes.
Figure 2: 25G SFP28 LWDM Transceiver Application
25G SFP28 LWDM transceiver can support multi-channel signals transmission on a single fiber. It makes up for the defects of large dispersion of 25G CWDM module and high laser costs of 25G DWDM transceiver, which delivers a stable transmission and saves a lot of fiber resources in 5G front-haul transmission. That is why it might be a stable and cost-effective solution in 5G front-haul network and it is still promising in the future development of 5G front-haul transmission.
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