Patent Application
20060275038
This application is related to U.S. patent application Ser. No. entitled “METHODS AND APPARATUS FOR MULTIPLE SIGNAL AMPLIFICATION” filed contemporaneously and which is hereby incorporated in its entirety by reference.
1. Field of the Invention
The present invention relates generally to a passive optical network, and more specifically, to a passive optical network module for selective signal amplification.
2. Technical Background
Passive optical networks (PONs) are used to provide high- bandwidth information to and from an end user or subscriber of the metropolitan area network. Typically, PONs are fiber-based, tree architecture networks with one or two levels of passive splitting providing a total split ratio up to thirty-two, which provides for some cost sharing of the expensive broadcast and digital downstream equipment. Existing PONs have about a 20 kilometer (km) reach with dedicated fiber drops to every subscriber premises, shared TDMA (timed division multiple access) upstream at a different wavelength, and, by definition, no electrical power in the outside fiber plant. While PONs are in use, they have not been widely commercially deployed because of a high cost per subscriber and the low rate of return for the telecommunications service provider or carrier. Based on increases in demand for high bandwidth and interactive services over bi-directional links, there is a renewed interest in PONs from telecommunications service providers for multiple reasons. First, new applications such as file sharing and software downloads require much higher connection speeds than the current digital subscriber line (DSL) technology can provide. Second, there is strong competition for services from cable television (CATV) companies, which already have a majority of the broadcast TV market and offer similar quality internet connections and telephony services. For telecommunications service providers to remain competitive, it would be desirable to provide a technology and a network that can surpass the bandwidth of CATV's hybrid fiber-coax to provide a subscriber with all desired services, some of which include TV, POTS (plain old telephone service) and internet connection.
Fiber-to-the-home (FTTH), fiber-to-the-business (FTTB) and fiber-to-the-premises (FTTP), referred to generically as FTTx, is just such a technology. Telecommunication service providers are attempting to standardize a FTTx PON solution to drive equipment prices to levels that offer an acceptable return on investment. Current subscriber equipment costs are in the thousands of dollars even with the 32-way sharing from single or two-stage splitting. The present invention addresses the sharing of costs issue by considering outside-plant amplification without outside-plant electrical powering. Amplification improves the total optical power loss in a system, referred to as the “loss budget”, and allows additional splitting and/or increased transmission distance, thereby distributing more of the infrastructure costs, especially the head end electronics and optics, over more subscribers. With the existing distribution of central offices (COs), increased distance may not be required, but consolidating several COs into one increases the typical transmission distance while improving equipment utilization especially important at low penetration rates.
Various modes of amplification in PONs have previously been proposed and are known in the literature. However, a mode of amplification is needed that lowers the amplification costs to improve the cost savings from additional equipment sharing.
One aspect of the invention is a module employed in a passive optical network (PON), wherein the module includes a single coil of erbium and a passive optical splitter whereby multiple signals pass through the module from the CO/head end and the subscriber, while only the analog broadcast signal passes through the erbium coil contained within the module.
In another aspect, the present invention provides a FTTx PON including a CO/head end including one or more high power pumps, a wavelength division multiplexer/demultiplexer (WDM) system for combining multiple signals, a module including a single coil of erbium and passive optical splitters whereby multiple signals pass through the module while only the analog broadcast signal passes through the erbium coil, a local convergence point (LCP) and one or more network access points (NAP) for providing fiber drops to a plurality of subscriber locations. In alternative embodiments, the module may include an optional isolator for protecting the erbium coil from reflections from the subscriber premise equipment. In preferred embodiments, the FTTx PON including the module provides 1×32 splitting at the LCP and 1×4 splitting at the NAPs, thus providing 128 splits and about a 20 km network reach. In contrast, conventional PONs provide 1×4, 1×8 or 1×16 splitting at the LCP and 1×4 splitting at the NAPs for a maximum of up to 32 splits and up to a 20 km reach, but not both together. In preferred embodiments, the erbium coil is positioned medially between the CO/head end and the LCP, in more preferred embodiments, the module is positioned immediately before the LCP splitter.
In yet another aspect, the present invention provides an amplifier/splitter package adjacent to an LCP in a passive optical network. In a preferred embodiment, the package includes a single coil of erbium and a passive optical splitter whereby multiple signals pass through the package from the CO/head end and the subscriber, while only the analog broadcast signal passes through the erbium coil. Within the package, the digital downstream signal is separated from and recombined with the analog signal by way of a plurality of WDMs. The analog and digital signals are preferably split 1×32 at their first split point at the LCP. In further embodiments, for additional splitting or longer network reach, additional gain may be achieved by semiconductor optical amplifiers (SOAs) in the CO/head end. The SOAs operate as a booster amplifier for the digital downstream and as a pre-amplifier for the digital upstream. Various embodiments with respect to specific pump sharing configurations, optimum range of noise figures, coil length, pump power, gain and gain flatness are provided.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description present exemplary embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the detailed description, serve to explain the principles and operations thereof. Additionally, the drawings and descriptions are meant to be illustrative and not limiting.
Reference will now be made in detail to exemplary embodiments of the invention, and examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. A module employed in a PON wherein the module includes a coil of erbium and a passive optical splitter whereby multiple signals pass through the module while only a portion of the signals pass through the erbium coil for signal amplification is shown in
Throughout the detailed description, the current full service access network standard specifies analog downstream between 1550 and 1560 nanometers (nm), digital downstream between 1480 and 1490 nm and digital upstream between 1260 and 1360 nm. The present invention retains these wavelength designations. Referring now to
The module 20 architecture includes a first optical branch 28 and a second optical branch 30, wherein the first optical branch 28 includes the single coil of erbium 22. All multiple signals, including analog downstream, digital downstream and digital upstream, pass through the module 20 from the CO/head end 32 and the subscriber 34 (
The first optical branch 28 operates in the 1550 to 1560 nm band of the erbium gain spectrum. The erbium coil 22 acts as an amplifier by exciting weakened analog signals that enter the coil 22. The erbium coil 22 increases the analog signal to compensate for the loss budget of the analog signal that is approximately 3 dB worse than that of the digital signals. In some embodiments, the loss budget for the analog signal is equalized to that of the digital signals by gain provided by the erbium coil 22. In other words, an erbium gain of approximately 3 dB will equalize the loss budget over all three signals.
The downstream digital signals are separated from the downstream analog signal at the first WDM 36, and are later recombined after the erbium coil 22 by the second WDM 38. Upstream digital signals are separated from and later recombined in the reverse order. As stated above, both the first and the second WDMs 36, 38 may be simple low pass filters. Prior to the first module WDM 36, the multiple signals are carried by various wavelengths of light through a single optical fiber 40 through a conventional WDM system 42. The conventional WDM system 42 may include a transmission system that includes a series of transmitters each coupled to a multiplexer. The multiplexer provides an output that is coupled to the optical fiber 40. Although not shown, at the receiving end is a system that includes a demultiplexer and a series of receivers. The optical fiber 40 is also coupled to an input of the demultiplexer of the receiving system. The WDMs transmit the light signals at the appropriate wavelengths and combines the signals for transmission along the optical fiber 40.
The optical fiber may be a SMF-28®, HI 980 or HI 1060 single mode optical fiber available from Corning Cable Systems of Hickory, N.C., which exhibit consistently low splice loss when coupled with an erbium-doped fiber. Gain-flattening filters (GFF) are not needed due to the fact that the 1550 to 1560 nm bandwidth is sufficiently flat. An optional isolator 44 may be contained within the module 20 in the first optical branch 28 to protect the erbium coil 22 from reflections from the customer premise equipment.
As stated above, splitting may occur within the module 20, but typically occurs within an LCP after amplification. The LCP is the first splitter from the CO/head end 32 and is typically located between the CO/HE 32 and the one or more network access points (NAPs) (
Referring now to
Referring to
Referring to
In a further embodiment, the present invention provides an amplifier/splitter package at an LCP 52 in a passive optical network. Preferably, the package includes a single coil of erbium 22 and a passive optical splitter whereby multiple signals pass through the package from the CO/head end 32 and the subscriber, while only the analog broadcast signal passes through the erbium coil. As in the previous embodiments, within the package the digital downstream signal is separated from and recombined with the analog signal by way of a plurality of WDMs. The analog and digital signals are preferably split 1×32 at their first split point at the LCP 52. In this particular embodiment, for additional splitting or longer network reach, additional gain may be achieved by semiconductor optical amplifiers (SOAs) in the CO/head end 32. The SOAs provide a broad gain bandwidth tuned to the digital wavelengths, while the fiber-based erbium amplifier is ideal for the high output power of the analog signal. The SOAs operate as a booster amplifier for the digital downstream and as a pre-amplifier for the digital upstream. An embodiment including SOAs is shown in
Referring to
Referring to Tables 1(a) and (b), the effect of the amplifier noise figure on digital optical signal to noise ratio (OSNR) is shown, wherein Table 1(a) is the digital downstream and Table 1(b) is the digital upstream. An OSNR of 2.1 dB corresponding to a Q of 8.5 dB is sufficient at 622 Mb/s data rate. The erbium gain bandwidth is between 1550 and 1560 nm. This is a flat part of the emission spectrum so no additional gain flattening filter is needed. The SOAs have a broader bandwidth than erbium, which enables the use of uncooled Fabry-Perot (FP) laser diodes
With respect to SOA noise figures, assuming only one amplifier and a low data rate, the noise figure does not set a tight requirement on digital transmission performance. The relatively high noise figure of typical SOAs is sufficient. However, it is preferred that the analog noise figure stay below 6.7 dB for less than 0.5 dB carrier-to-noise ratio degradation. Erbium fiber amplifiers may achieve this if pump power is properly specified. Thus, the hybrid amplifier configuration described above provides sufficient performance for access networks and stays within the designated FSAN wavelengths. The SOAs provide a broad gain bandwidth tuned to the digital wavelengths while fiber-based erbium amplifiers are ideal for the high output power of the analog signal. Typical noise figures will not impair the transmission. For future cost reduction, the SOAs may be integrated into the transmitter or receiver.
One benefit of amplifying the analog signal with an erbium coil prior to the LCP is boosting signal strength for increased splitting, which leads to cost reduction. The remote amplification in the outside fiber plant of a PON avoids stimulated Brillouin scattering of the analog signal while spreading expensive shared equipment costs over more subscribers. Today, a nearly SBS-limited signal power is launched at the CO/head end to maximize the analog signal power budget. Remote amplification allows the signal power to drop in the fiber plant before gain is added, just before the splitter. The gain adds to the power budget allowing increased loss from more optical splitting, more fiber or more connectors. Given today's architectures and the cost of components, increased optical splitting provides the greatest costs savings, increasing to at least 1×128 splitting from 1×16 for subscribers out to 8 km from the CO/head end, and 1×32 for subscribers between about 8 to 20 km from the CO/head end. The required additional gain, about 7 dB, is cost effectively achieved with a remotely pumped erbium coil. Any future upgraded transmitter speed increase could be accommodated by an upgrade of the amplifier gain.
It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
