To upgrade GPON to XGPON1 or XGSPON, the mainstream solution in the market is the external combiner solution. This program increases XGPON1/XGSPON boards, and then through the external combiner, the signals of GPON and XGPON1/XGSPON are combined in the form of WDM, and transmitted to the remote users. The principle of this program is relatively simple, but there are many drawbacks such as long upgrading and opening cycle, high cost, more supporting equipment, large space occupation, etc. Moreover, the introduction of external combiner will certainly bring additional optical power loss, and there is a risk of affecting the service of existing customers.

Combo PON is an amalgamation of GPON and XGPON1/XGSPON, the principle of which is to realize the independent transceiver of GPON and XGPON1/XGSPON optical signals in one optical module at the same time and to combine and divide the four different carrier wavelengths through the built-in WDM device, and to provide one optical fiber interface externally, which is compatible with the existing GPON network services and the XGPON1 or XGSPON services simultaneously. XGPON1 or XGSPON services at the same time, avoiding the need to change the existing network structure and increase the space of additional equipment room, thus realizing a seamless and efficient GPON upgrade.

Combo-PON System Diagram

Quad-OSA for Combo PON OLT integrates two sets of transceiver devices in one optical device: 2.5G 1490nm DFB laser and 1310nm APD for traditional GPON and 10G 1577nm EML laser and 1270nm APD for XGPON, and adopts WDM merge coupling to the output optical port. to the output optical port.

The Combo PON OLT protocol involves two sets of protocols, GPON and XGPON, and multiple specifications. For the link budget, there are N1, N2a, D1, D2 and other specifications. Simply put, in order to correspond to the 28dB and 32dB budgets of traditional GPON's B+ and C+ links, XG(S)-PON corresponds to the same link budgets, which is the most favorable for the smooth upgrade from GPON to NGPON. These are the D1 and D2 specifications for Combo PON.

One of the main reasons for the relatively slow deployment of hybrid integrated Combo PON networks in the past few years is the maturity of the supply chain of single-fiber bi-directional optical devices at the OLT end of Combo PON, especially the maturity of high link budget optical devices. Optical device companies have been facing huge challenges in terms of yielding high-specification Quad-OSAs. 

In order to promote the adoption of Combo PON, all parties in the industry chain compromised and accepted the transition program of lowering the specification to 31dB N2a. However, for most of the optical device companies developing PON products, the yield of N2a-specification Quad-OSA products is still under pressure, and the yield of 32 dB D2-specification optical device products is even lower.

On the other hand, at the application level, operators have been looking forward to the maturity of high link budget optical devices/optical modules as soon as possible. At the 2018 China Optical Network/FTTH Forum held in Beijing in June 2023, operators such as China Mobile and China Telecom still explicitly require D2-specification optical devices/optical modules to form a more mature supply chain to support the full-coverage application of Combo PON.