Basic principles of WDM PON

WDM PON is a point-to-point passive optical network using wavelength division multiplexing technology. That is, in the same fiber, the number of wavelengths used in both directions is more than 3, and the use of wavelength division multiplexing technology to achieve uplink access can provide greater working bandwidth at a lower cost, which is an important development direction for the future Optical fiber access. A typical WDM PON system consists of three parts: Optical Line Terminal (OLT), Optical Wavelength Distribution Network (OWDN) and Optical Network Unit (ONU: Optical Network Unit), as shown in Figure 1. OLT is the central office equipment, including optical wavelength division multiplexer/demultiplexer (OM/OD). Generally, it has functions such as control, exchange, and management. The OM/OD of the central office can be physically separated from the OLT equipment. OWDN refers to the optical network located between the OLT and the ONU, and realizes the wavelength distribution from the OLT to the ONU or from the ONU to the OLT. The physical link includes feeder fiber and passive remote node (PRN: Passive Remote Node). PRN mainly includes thermally insensitive arrayed waveguide grating (AAWG: Athermal Arrayed Waveguide Grating). AAWG is a wavelength-sensitive passive optical device that performs optical wavelength multiplexing and demultiplexing functions. The ONU is placed at the user terminal and is an optical terminal device on the user side.

  In the downstream direction, multiple different wavelengths ld1…ldn are transmitted to the OWDN after the OM/OD multiplexing of the central office, and are allocated to each ONU according to different wavelengths. In the upstream direction, different user ONUs emit different optical wavelengths lu1…lun to the OWDN, multiplex at the PRN of the OWDN, and then transmit to the OLT. Complete the upstream and downstream transmission of optical signals. Among them, the downstream wavelength ldn and the upstream wavelength lun can work in the same waveband or different wavebands.