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The 10/1.25 Gbit/s wavelength division multiplexing-passive optical network (WDM-PON) service is presented with uni-lambda bidirectional transmission over 30 km of singlemode fibre. With low-cost wavelength tracking methods, power variations of the upstream were achieved within ±0.33 dB under temperature fluctuation in the passive remote node.
Introduction: Bidirectional transmission in the wavelength division multiplexing-passive optical network (WDM-PON) is preferred because it can reduce the number of fibres to be installed. Such bidirectional service can be easily implemented using passive optical devices such as a wavelength grating router (WGR). Likewise, the use of the same wavelengths (i.e. uni-lambda service) in both downstream and upstream links decreases the number of ports in such devices and consequently reduces system cost further. Nonetheless, wavelength drift in such passive optical devices induces misalignment owing to the temperature fluctuation among the wavelengths in the optical line terminal (OLT), the optical network unit (ONU), and the passive optical devices as the remote node (RN) [1, 2]. Various approaches to minimise this undesirable effect have been tried, including performance maintenance through wavelength tracking in the conventional WGR temperature-controlled equipment [3], spectrum slicing method to split each channel using passbands of the WGR [4], and loopback method for monitoring and tuning the channel wavelengths to a free spectral range (FSR) of the WGR [5]. Nevertheless, such approaches are mostly confined to the stabilisation of the passive devices in the RN, requiring power feeding for active control of the devices or additional electrical functions. Therefore, they are still a far cry from the low-cost solutions in WDM-PON service.
In this Letter we present a method with uni-lambda service that can reduce the maintenance cost in the RN while maintaining the system performance. No active devices are desired in outside plant. This technique was based on two steps: 1. in the OLT, the amount of wavelength drift in the RN was monitored and the wavelengths of the optical sources adjusted; 2. the wavelength of each ONU was adjusted with the downstream channel wavelength.
Experiments: Fig. 1 shows the system configuration. The OLT consisted of a 32-channel DFB-LD array for downstream transmission and a tracking part for controlling the wavelengths of...