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WDM-PON Networking Solution

Date: 2021-12-01 14:04:18 From: C-light Onclick:

At present, optical fiber access networks mainly use EPON or GPON, which work in a single wavelength upstream and downstream, and each user carries out data transmission through time division. This mechanism of allocating time slices to each user on a single wavelength, WDM technology is introduced into the PON system, namely WDM-PON, which can increase the user access bandwidth several times or even tens of times to meet the ultimate needs of users. Therefore, WDM-PON is also considered as the solution of the next generation access network. 


Technical solution


In a WDM-PON system, multiple different wavelengths work at the same time, so the most direct WDM-PON scheme is to have multiple light sources of different wavelengths in the OLT, and each ONU also uses a light source of a specific wavelength. Each point-to-point connection is configured and operated at a pre-designed wavelength and as the number of wavelengths increases, the variety of light sources required increases, causing serious storage problems, especially for ONUs. Because of the presence of severe ONUs storage problems, fixed light source solution is difficult to be applied to commercial WDM-PON system, so using colorless ONU has basically become the consensus of current WDM-PON related research. Colorless ONU based technical solution is the mainstream. Colorless ONU realization technology can be divided into adjustable lasers, broad spectrum light source, no light source according to the different devices used.  


The tunable laser with a wide spectrum light source in ONU is a wavelength tunable laser that enables the ONU to work at different wavelengths. The tunable laser also works at specific wavelengths, but the wavelength can be tuned by auxiliary means, such as electrical tuning Temperature tuning and mechanical tuning, so that the same laser can be used in the system to produce different working wavelengths, but the tunable laser is more complex than the laser used in the traditional PON system, the price is also relatively high, so in the current WDM-PON system it is generally not used. 


The second solution is to place a broad spectrum light source in the ONU, and after the light comes out of the ONU, a WDM device, such as a thin film filter or AWG, divides the signal spectrum, allowing only the specific wavelength part to pass through and transmit to the OLT located in the central office In this way, each ONU has the same light source, but since they are connected to different ports of THE WDM combination, separate wavelength signals can be generated for each channel, as shown in Figure 1, the WDM-PON system with wide spectrum light source can adopt SLED, ASE-EDFA and ASE-RSOA, etc. In a WDM-PON system with a wide spectrum light source, only a very narrow part of the spectrum line of the light source is used to carry the uplink signal, while the rest of the energy is wasted, so the light source is required to provide enough optical power. In addition, spectrum segmentation will lead to the large linear crosstalk, the dynamic range of the system is limited, the passband width and channel interval of the multiplexer and demultiplexer should be appropriately selected. 


 Figure 1   WDM-PON system with a wide spectrum light source in ONU


In the other scheme, there is no light source at ONU, and all light sources in the system are placed at OLT. After spectral segmentation by AWG, light signals of specific wavelength are provided to ONU, and ONU directly modulates the light signals to generate uplink signals, as shown in FIG. 2. According to the path of the ascending light signal, this kind of scheme is also called colorless ONU implementation scheme based on reflection. In this implementation scheme, the light emitted by the wide spectrum light source is provided to different ONU as the ascending light source after the AWG split wave. Therefore no wasted Light signal. The wide spectrum light source can still choose the amplified spontaneous radiation SLED, EDFA and RSOA, which are called seed light sources, and there are a variety of technical solutions depending on the reflector used. Modulator used in no light source ONU requires low cost, operating over the entire temperature range, not affected by polarization, large light bandwidth, low insertion loss, and low noise. Commonly used reflection modulators such as injection locked FP-LD, RSOA, and EAM, M-ZSOA, which can work in a wide spectral range, that is, the device performance is basically independent of the wavelength of the input optical signal, so that the same device can be used in all ONU, to achieve the colorless ONU. 



FIG. 2  WDM-PON system with a lightless ONU 


Multi-push and independent


On the operator side, Korea Telecom (KT) has been the world's strongest supporter of WDM-PON technology, partnering with LG-Nortel for deployment and NoveraOptics for equipment, starting in Gwangju in 2005 for 50,000 units. Although only KT has deployed a relatively large-scale WDM-PON network for operation, other operators are also showing strong interest in WDM-PON and have plans to select WDM-PON as the candidate technical solution of the next generation access network, including NTT Verizon and several European operators, UNET of the Netherlands and INS of Norway, are experimenting with WDM-PON based on LG-Nortel's technology. UNET is using WDM-PON to deploy FTTB pilot commercial networks to provide better performance for high-end business users. 


As things stand, the next generation of PON technologies after EPON and GPON will mainly be developed in the direction of 10GEPON and 10GGPON, while WDM-PON is mainly subject to cost. Operators will not deploy WDM-PON on a large scale in the near future. However, due to the advanced technology of WDM-PON, WDM-PON may become a medium and long term technology with its continuous maturity.


WDM-PON is considered as the next generation optical access network technology with its huge bandwidth. WDM-PON can be used in a variety of applications, such as FTTx, local convergence transmission and possible base station back transmission, etc., to provide differentiated services for different users. However, due to the current device maturity, equipment cost and standardization degree and other limitations, only a small number of commercial products are deployed in the laboratory, but with the further improvement of technology And development, WDM-PON will play a bigger role in the future access network.