From time to time, the needs for more bandwidth in fiber access networks have never decreased. And these demands such as file-sharing, TV services and high-definition videos have just increased the desire for broadband access network based on fiber. People may ask what will be the next fiber access technology. There is one technology which stands out for fiber access networks: WDM-PON. This publication mainly presents WDM-PON technical issues and corresponding solutions.
Wavelength Division Multiplexing-Passive Optical Network (WDM-PON) is the next generation in development of fiber access networks. It uses multiple different wavelengths over a physical point-to-multipoint fiber infrastructure. These different wavelengths allow for traffic separation within the same physical fiber, thus high bandwidth is delivered to subscribers over long distances.
To implement WDM-PON, there are two ways. One is to use wavelength routers and the other is to use power splitters in the optical distribution networks (ODNs). The table below shows the applications and issues of two types of WDM-PON.
Just as the above table shows, both types of WDM-PON face the common technical issue that is the need for colorless ONU. Besides, the long-reach WDM-PON type encounters protection issue. These issues need to be addressed. The following passages provide the corresponding solutions for the above-mentioned technical issues.
The ONUs should be colorless (in other words, no ONU is wavelength specific) to reduce the costs of operation, administration, and maintenance (OA&M) functions, as well as the production cost, since mass production becomes possible with just one specification. Local emission is proposed to solve this problem.
Two local emission approaches exist: wavelength tuning and spectrum slicing. The ONU of the wavelength tuning approach consists of a tunable laser diode (TLD) as a transmitter (Tx), an optical receiver (Rx) with wavelength selector (WS), and a WDM coupler that divides or combines the upstream and downstream signals. The configuration of the ONU in the spectrum slicing approach is similar to that in the wavelength tuning approach except that a broadband light source (BLS) with WS is used instead of the TLD. The following picture shows the ONU configuration of local emission approaches.
In the long-reach WDM-PON, the failure of the feeder fiber carrying the WDM signals may cause the loss of data for all users. Thus, a protection function for the feeder fiber is required. One solution has been demonstrated to achieve the protection. That is 3-dB optical couplers are used to split or combine the path of WDM signals to or from both the working and protected fibers in the OLT (Optical Line Terminal) or in the wavelength router. Note that the OLT and the wavelength router are typically located in the central office (CO) and in the access node (AN) respectively.
Unfortunately, this protection scheme has a low loss budget because of the use of the 3-dB optical couplers. To address this issue, a wavelength-shifted protection scheme has been proposed, which is deploying the cyclic property of the 2×N athermal arrayed-waveguide grating (AWG) and two wavelength allocations for working and protection. In this case, 3-dB optical couplers are not needed.
Although a large scale deployment of WDM-PON is not possible within the next 3 years, WDM-PON is poised to gain more and more popularity in bandwidth-hungry applications, serving as a next generation for fiber access networks. Fiberstore supplies those WDM-PON solution components, such as ONUs and OLTs. You can visit Fiberstore for more information about WDM-PON solutions.