At present, the main use EPON optical access network, or GPON, both up and down in a single wavelength division manner by each user for data transmission. This mechanism on a single wavelength for each user is assigned time slice, both limits the available bandwidth per user, but also a lot of waste fiber itself available bandwidth. The WDM PON technology is introduced into the system, that WDM-PON, the user access bandwidth can be increased even several times to meet the ultimate needs of users, WDM-PON therefore also considered to be the next generation access network solution .
In the WDM-PON system, a plurality of different wavelengths simultaneously, and therefore the most direct solution is WDM-PON OLT in a plurality of different wavelengths of light sources, each ONU using a specific wavelength of light source is also, according to various pre-designed connection point The configuration and operating wavelength. If the number of wavelengths, the more light sources are needed more serious storage problem, which is particularly prominent ONU. Due to serious storage problems ONU, a fixed light source used in commercial solutions is difficult WDM-PON system, the use of a colorless ONU has basically become the consensus of current research in the WDM-PON based technical solution is colorless WDM-PON ONU mainstream system. Colorless ONU implementation techniques based on the use of different devices can be divided into tunable lasers, broad spectrum light source and non-source categories.
Figure 1 ONU used in a wide spectrum of the light source WDM - PON System
Tunable laser using a wavelength tunable laser so that the ONU can work with different wavelengths, tunable lasers also work at a specific wavelength, but the wavelength can be tuned, such as electronic tuning, tuning and mechanical tuning temperature through the auxiliary means, so that The same system can be used to produce different laser operating wavelength. However, laser tunable laser used than conventional PON system more complex, the price is relatively high, and therefore in the present WDM-PON system is generally not used.
The second solution is placed in a wide spectrum of ONU light, the light from the ONU out, followed by a WDM equipment issue, such as a thin film filter or AWG, dividing the signal spectrum, allowing only certain wavelengths and transmitted in part by Located in the central office to the OLT. Thus each ONU with the same light source, but because they are connected to the different ports of the WDM multiplexer, thereby generating a separate signal for each wavelength channel. ONU used in a wide spectrum of the light source WDM-PON system shown in Figure 1. Broad spectrum light source can SLED, ASE-EDFA and ASE-RSOA like. The wide spectrum of the light source in WDM-PON system, the light emitted from the light source, only a very narrow part of the spectrum is used as an uplink carrier signal, and other large amount of energy is wasted, and therefore requires a light source to provide sufficient optical power. In addition, the spectrum split will cause a larger linear crosstalk limits the dynamic range of the system, you need to select the appropriate multiplexer and demultiplexer wide pass band spectrum and channel spacing.
Another solution is no light source in the ONU, the system of all light sources are placed at the OLT, through the AWG and be provided after a specific wavelength spectrum of the optical signal to the ONU is divided, and this ONU directly modulates the optical signal, to produce the uplink signal, as shown in Figure 2. Based on the path upstream optical signal, based on such programs are also known as the reflection of a colorless ONU implementation. In this implementation, the wide spectrum light emitted by a light source divided AWG different ONU as the upstream supply source, so there is no waste of the optical signal. Broad spectrum light source can still choose amplified spontaneous emission of SLED, EDFA and RSOA other is called the seed source. Depending on the devices used in reflection, there are a variety of technical solutions. No light modulators used in ONU, requiring inexpensive, able to work in the whole temperature range, from the polarization effects of large optical bandwidth, low insertion loss, low noise. Commonly used reflective modulator includes an injection-locked FP-LD, RSOA and EAM, M-ZSOA the like, they may be working spectral range is wide, i.e., the input device performance and substantially independent of the wavelength of the optical signal, which can be used in all the ONU the same device, realization of a colorless ONU.
WDM-PON PON can be regarded as the ultimate form, but it is still difficult in the near future large-scale applications, mainly including: the lack of international standards, equipment manufacturers invest less, a variety of devices such as chips, optical module and a broadband light source technology is not mature, can provide equipment manufacturers worldwide commercial WDM-PON systems are also numbered. WDM-PON system based on the use of the FP-LD injection locking technique, currently can achieve 32 wavelengths, each wavelength of 1.25Gb / s data transfer, so that unilateral available 20Gb / s bandwidth up. Japan's Fujitsu has also developed a dedicated WDM-PON technology to reduce the cost of short-term and long-term R & D projects at the ONU reflection modulation device, introduced transition technology between GPON and WDM-PON between. RSOA constructed using WDM-PON platform, can transmit 16 1.25Gb / s data.
Figure 2 ONU no source WDM- PON system
From the current situation, after the next-generation PON EPON and GPON technology will primarily toward 10GEPON and 10GGPON direction, and WDM-PON mainly by cost constraints, as well as the lack of appropriate business drivers, operators in the near future yet not large-scale deployment, but because of the nature of the technical aspects of WDM-PON, as it continues to mature, may become the technology of choice in the long term.
WDM-PON with its huge bandwidth provided is believed to be the next generation of optical networks. WDM-PON can be used for a variety of applications, such as FTTx, local convergence and possible transmission backhaul, to provide poor service differentiation for different users. However, due to the maturity of the current restrictions, equipment costs and standardization of terms and other devices, only a small number of commercial products deployed in experimental bureau, but with the further improvement and development of technology, WDM-PON will occupy in the future access network a place to play a greater role.