To accommodate the increasing demands for various multimedia services, it is desirable to move optical fibers closer to the subscribers. Passive optical network (PON) is attractive for this purpose as it could substantially reduce the maintenance and operation costs. In addition, with wavelength-division-multiplexing (WDM) technology, PON could provide large capacity, network security, and upgradability. However, the access networks are known to be extremely cost-sensitive. Thus, for the practical implementation of WDM PON′s, it is essential to develop low-cost WDM sources and new techniques enabling the efficient use of optical fibers, multiplexers, and demultiplexers.
In this dissertation, we propose and demonstrate a new WDM PON architecture that uses NxN WGR′s in the central office (CO) and remote node (RN) for simultaneous multiplexing and demultiplexing of N-1 channels in each direction. In addition, the proposed network utilizes a spectrum-sliced fiber amplifier light source and LED′s for downstream and upstream traffics, respectively. The spectrum-sliced fiber amplifier light source consisted of a two-stage erbium-doped fiber amplifier (EDFA) pumped in the counter-propagating direction by using a pump laser, a bandpass filter placed between the stages, and a WGR for spectrum slicing. The bandpass filter was used to limit the spectral width of the amplified spontaneous emission (ASE) to be same as the free-spectral range (FSR) of the WGR.
We have first demonstrated a WDM PON based on the spectrum-slicing technique. The spectrum-sliced fiber amplifier light source was used to transmit 15 downstream channels operating at 500 Mb/s. The 155-Mb/s upstream channels used 1.5-㎛ LED′s. The spectral width of each LED was reduced to suppress the effect of dispersion by using a bandpass filter. In addition, an EDFA was used at the CO to compensate for the slicing losses of low-power LED′s. The 16×16 WGR′s placed in the CO and RN were fully utilized for both multiplexing 15 downstream channels and demultiplexing 15 upstream channels and vice versa. The crosstalk, caused by using WGR′s for both multiplexing and demultiplexing channels, was suppressed to a negligible level by using two types of bandpass filters centered at two distinct wavelengths. There was no significant degradation in the receiver sensitivity caused by this crosstalk. We have also demonstrated this network using bidirectional transmission over a single strand of optical fiber. For bidirectional transmission, a circulator and a 3-dB coupler were used at the CO and each customer premise, respectively. The measured results showed that there was no additional power penalty caused by bidirectional transmission over a single strand of fiber.
To demonstrate the cost-effectiveness of the proposed network, we transmitted the digital broadcast video signals in addition to baseband WDM signals without using additional light sources (i.e., the downstream channels were modulated with both the baseband and digital video signals using a $LiNbO_3$ modulator). The results showed that we could transmit up to 30 digital video signals in addition to 155-Mb/s baseband signal in the proposed network. The upstream channels also operated at 155 Mb/s.
In WDM PON′s, the spectral characteristics of the WGR located at the unpowered RN are susceptible to the ambient temperature at the outside plant. This would result in the spectral misalignment between the WGR′s at the CO and RN, and cause significant power loss and crosstalk. We demonstrated a simple wavelength-tracking technique to align the wavelength combs of the WGR′s used at the RN and CO in the proposed network. This technique could monitor the spectral misalignment simply by comparing the total optical power of the multiplexed upstream channels with the optical power of the demultiplexed upstream channel at the CO. By using the proposed technique, the optical powers of the spectrum-sliced channels were maintain within 0.2 dB while the outside-plant temperature was varied from 15℃ to 55℃, as rapidly as 0.5℃/min.
