The Best Optical Transceiver Modules for 5G Fronthaul

Optical Interconnection Forum (OIF), domestic and international standardization organizations International Telecommunication Union (ITU-T), the Institute of Electrical and Electronics Engineers (IEEE), 4WDM and other multi-source agreement (MSA), the China Communications Standards Association (CCSA) and other 5G bearing-related optical module specification development, involving a wide variety of module types and interface characteristics, diverse. And the fronthaul transmission optical module mainly includes 25G Transceivers and 100G Transceivers two rate types, supporting hundreds of meters to 20 km of typical transmission distance, the specific technology status as shown in Table 1.



Based on application scenarios, the maturity of the technology, cost and other factors, NADDOD focuses on 25Gb/s dual-fiber bi-directional, 25Gb/s single-fiber bi-directional, 25Gb/s wavelength tunable, 100/200Gb/s single-fiber bi-directional and other key optical module technology solutions for front transmission to analyze and evaluate, and collaborate with the industry to focus and promote the positive and sound development of the 5G fronthaul transmission optical module industry.

25Gb/s Dual Fiber Bidirectional Gray Optical Module

Typical transmission distances for 25Gb/s dual-fiber bi-directional grey optical modules 25G BIDI SFP28 include 300m and 10km. 300m optical modules are typically used for tower-up and tower-down interconnections at base stations, while 10km optical modules are mainly used for direct fiber connection scenarios between AAUs and access rooms (sites) where the transmission distance is longer or the link loss is higher.

The IEEE 802.3cc has completed the 25GbE single-mode fiber interface specification, and CCSA initiated the development of the domestic industry standardization, to be completed in 2019 for approval.

The optical module can be implemented using 25G and 10G baud rate laser chips. 25G baud rate industrial grade laser chips have high reliability requirements and mass production process requirements, and the market supply channels are limited. 10G baud rate industrial grade laser chips can make full use of the mature supply chain, which can effectively reduce the cost of optical modules, and there are two main implementation options in the industry, namely overclocking and PAM4 high order modulation. The block diagrams are shown in Figure 2.

In the FP laser approach, the main factors affecting the transmission distance include link attenuation loss, inter-code interference (ISI) cost, mode assignment noise (MPN) cost, etc., which can theoretically support a transmission distance of more than 300 m. In the DFB laser approach, the central wavelength is closer to the zero dispersion point of the G.652 fiber, the width of the spectrum is narrower, and the mode assignment noise can be ignored, which can theoretically support a transmission distance of more than 10 km. The DFB laser approach can theoretically support transmission distances of more than 10km due to the central wavelength being closer to the zero dispersion point of the G.652 fiber, narrower spectral width, and negligible mode assignment noise.

The PAM4 solution uses a 10G baud rate industrial grade laser and photodetector, but the matching IC needs to be replaced with a more linear laser driver and TIA chip, and a DSP chip for 25Gb/s NRZ and 25Gb/s PAM4 interconversion. Currently 10~15km demonstration trials have been achieved, the complementary chip is still in the development stage and the comprehensive cost is to be further evaluated.

In summary, the 25Gb/s optical module using 10G baud rate industrial grade laser chip, 300m specification can be preferred to overclocking solution, 10km specification overclocking solution has certain technical challenges; PAM4 solution in 10km and longer transmission distance depends on the scale effect of supporting chip.

25Gb/s Single Fiber Bi-directional Module

BiDi optical module has the advantages of saving 50% of fiber resources, equal spacing between upstream and downstream can effectively ensure high-precision time synchronization, etc. Typical transmission distance 10km, 15km, 20km. 25G BiDi has two main technical solutions, one is to use different wavelengths of wavelength division multiplexing (WDM) to achieve, and the other is to use the same (or different) wavelengths combined with the way to achieve ringers, as shown in Figure 3.

The circulator solution is very sensitive to reflection crosstalk at the common end (the two ends in Figure 4b), and the outgoing fibers need to use a fiber-tilted end face interface with a high return loss index and high dust resistance requirements for practical engineering use. 25Gb/s BiDi optical modules are recommended to give preference to the WDM solution. In the wavelength pair selection industry mainly has 1270nm/1310nm and 1270nm/1330nm two options.

25Gb/s tunable colour optical module

In the early stage of 5G network construction, the front transmission will be mainly in the form of fiber direct drive, along with high-frequency networking and low-frequency incremental points and other deep cover, in order to make full use of existing fiber resources or solve the problem of fiber resource tension, WDM approach will become a useful supplement, of which the wavelength tunable (Tunable) optical module is its core unit. The ITU-T G.698.4 standard (G.Metro), drafted and released by China, has defined 10Gb/s access WDM networking and wavelength-independent, colorless implementation mechanism, and the industry is currently exploring the technical solution for 25Gb/s rate. 25Gb/s wavelength tunable optical module functional block diagram is shown in Figure 4.

Depending on the type of light source and the tuning method, there are various technical solutions for wavelength-tunable lasers, and a comparison of five typical solutions is shown in Table 2. The laser based on the sampling grating distribution Bragg reflector (SG-DBR) technology has the advantages of wide wavelength tunable range, fast tuning speed, high modulation rate and relatively low cost, which is the mainstream technology solution in the industry. At present, the domestic industry has the ability to industrialise DBR tunable lasers with a wavelength tuning range of 10nm, which can generally meet the application scenario of 20 channels @ 100GHz wavelength interval. In addition, external cavity lasers, MEMS VCSELs, DFB arrays and other solutions are still under further study due to cost, stability, operating bandwidth and tuning time limitations, and do not yet have the capacity for large-scale industrialisation.

100/200Gb/s Single Fiber Bi-directional Optical Module

The technical solution of 100/200Gb/s BiDi 10km optical module is in the research stage, typical implementation includes two types of ringers and WDM, the functional block diagram is shown in Figure 5.

The laser chip at the core of the 100/200Gb/s BiDi optical module is mainly provided by foreign manufacturers and can currently support either O-band CWDM (4-wavelength) or LWDM (4-wavelength) with a limited number of wavelengths. At this stage, the implementation of single-fiber bi-directional technology is recommended to give priority to miniaturised circulators (shown in Figure 8 a). Later, as PAM4 technology further matures, 2 x 50Gb/s or 1 x 100Gb/s may become the mainstream technology solution for the next generation of 100Gb/s optical modules, using WDM to achieve single-fiber bi-directional will be a more economical way (shown in Figure 5).