Wavelength Division Multiplexing (WDM) optical fiber technology

Brief Introduction

  WDM (Wavelength Division Multiplexing) is a unique technology in optical fiber transmission. It is a method of sending signals of different wavelengths together at the transmitting end into one optical fiber for transmission, and then separating the signals of each wavelength at the receiving end. The technology, which makes full use of the characteristics of different wavelengths (frequencies) of light that do not interfere with each other in the transmission process. As wavelength-division multiplexing technology matures, device and equipment costs are getting lower and lower. WDM-PON introduces wavelength division multiplexing in the PON. It has the following two aspects.
  1. The use of wavelength division multiplexing to achieve multi-user access: through wavelength division multiplexing technology for each ONU to provide independent wavelength channel, so that a number of ONU and OLT communication becomes simple, easily and effectively solve the problem of uplink channel contention.
  2. The use of wavelength division multiplexing to achieve multi-service access: through the wavelength division multiplexing technology allows data services and CATV video services in a single optical fiber with different wavelengths, effectively solve the integrated service access problem.

The key technology of WDM-PON

  As a new generation of PON technology, WDM-PON is being studied. At present, many schemes have been proposed. In most network architectures of a scheme, a wavelength router replaces the optical splitter in a traditional PON network. Thus each OLT-ONU pair can be assigned a dedicated and permanent wavelength, while two pairs of transmitters/receivers are needed to form a point-to-point link. The following figure is a typical WDM-PON  network architecture with 32 ONUs.

Network structure of WDM-PON

  Wavelength routers are typically implemented using Arrayed Waveguide Gratings (AWG) or Thin Film Filters (TEF).
  It should be noted that due to the production of quartz, the main material of the AWG, its refractive index changes with the ambient temperature, which will have an impact on the wavelength of channels that are multiplexed in the AWG. However, there are some methods to enhance the ambient temperature of the AWG to increase the stability of work.
  The AWG optical power loss is approximately 5dB, which is 12dB less than a typical 1*32 splitter. The AWG-based WDM-PON reduces the link budget from 28 dB (class B+) to 21 dB, allowing the use of inexpensive WDM light sources.

A point-to-point WDM-PON architecture

  Upgrading a prior PON to the above P2P WDM-PON requires replacing the existing splitter with an AWG router. This upgrade needs to be performed on outdoor equipment and disrupts existing user services. It is relatively easy to upgrade the centralized optical split PON (star structure). For a distributed optical PON (tree-like structure), this upgrade is difficult to operate.
  Another disadvantage of this method is that the outdoor equipment loses its transparency. There is an architecture that can avoid this problem and can reuse the existing PON and retain the optical splitter. The ONU performs wavelength selection with an additional bandpass filter (1dB loss) and the OLT selects either AWG or TEF. The class B+ link budget thus increases from 28dB to 34dB.
  The 7dB link budget that the first architecture above (the wavelength router placed on the remote node) can be used to attach a 1:4 splitter behind the router. In this way, each WDM channel provides one GPON/EPON, and each GPON/EPON user is reduced to four, and each user’s bandwidth is increased by eight times.

WDM light source

  DWDM (dense WDM) lasers emit a single or adjustable wavelength, which is generally used in long-distance metropolitan area networks. It is too expensive for WDM-PON. Especially for ONUs, there is a need for a way to achieve ONU wavelength settings or adapt automatically. Currently, colorless ONUs can be used to achieve this goal. In this solution, the WDM-PON only requires one type of ONU, thus avoiding the trouble of separately configuring a separate DWDM transmitter. The colorless ONU currently has the following three specific implementation methods.
  •   The first method is spectral division. The working principle is: WDM-PON uses a broadband light source as the light source of the ONU. After the light is transmitted through the AWG, the spectrum of the output signal is a part of the original broadband signal. The wavelength depends on the multiplexer port connected to the ONU, and the output signal is multiplexed. On a fiber, it reaches the destination receiver through the demultiplexer on the OLT. The main disadvantage of spectral segmentation is that the spectral splitting results in a large loss of optical power (18dB), resulting in a tight power budget.
  •   The second method uses wavelength-locked FP lasers as signal transmitters for OLTs and ONUs. The working principle is: the erbium-doped fiber amplifier generates spectrum amplified spontaneous emission (ASE) signal. The ASE signal reaches the AWG through the OLT and is spectrally split by the AWG to generate multiple narrowband signals. These signals are injected into the same type of FP laser of different ONUs. This forces the FP laser to produce a single-wavelength mode that suppresses the generation of multiple wavelength modes.
  •   The third method is that the WDM WDM laser sends an unmodulated signal to the ONU, and the ONU modulates and reflects the modulated light back to the OLT. Reflective semiconductor optical amplifiers (RSOAs) are used in ONUs for modulation, amplification, and emission. This solution also has some problems with reflections and backscatter, thus limiting the link budget to 16 dB.

  One of the major drawbacks of WDM-PON over TDM-PON (eg, EPON and GPON) is that the central office (CO) requires multiple optical ports and therefore requires the use of a highly integrated multi-channel transmitter and receiver array.

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