Wally et all, I am a HLIT investor, SInce you people sell to HLIT,the lasers to build their products, I was wondering if you know who was producing these new YEDFA lasers?
WDM is the answer
Recent developments in wave division multiplexingÿ technologies and the availability of low-cost overlay transmitters will make possible low-cost, highly flexible broadband networks
JASON SHREERAM and DON SIPES
When looking for answers to their service deployment dilemmas, service providers ÿ take two roads.
For networks where services target small to moderate groups of users, 1550 nm transmission technology is used primarily for supertrunking. The targeted services are added at the radio frequency (RF) level and then distributed by relasing with 1310 nm distributed feedback (DFB) lasers.ÿ When the targeted service payload is heavy with multiple analog and digital channels, this method of 1550 nm supertrunking and 1310 relasing is still the preferred transmission method.
When primarily digital information is targeted to small numbers of nodes, though, a significant cost savings and increases in network flexibility can be realized through the use of an architecture that combines the low-cost, high-power 1550 nm broadcast with the targeted service directivity of 1310 nm DFB lasers through the use of wave division multiplexing (WDM) overlays.
In traditional 1310 nm distribution networks, targeted services are added at RF at the secondary hub site. In the 1310 nm overlay network, content broadcast over an area covered by a secondary hub site, such as advertising and near video on demand, is combined with the broadcast analog channels at the headend.
This payload is transmitted via a single 1550 nm transmitter with optical amplifier to the secondary hub site, where a high-power optical amplifier (+19 dBm to +25 dBm output power) boosts the signal. A high-count optical splitter network then divides the signal to provide the appropriate node count, link loss budget and received power at the node.
Content inserted at the secondary hub site--namely voice, interactive video and modem traffic--is delivered via a low-cost, low-power, 1310 nm transmitter optimized for WDM overlay applications. An optical -20 dB tap coupler provides a convenient test point for RF signal balancing and compensation for the differential optical loss. Both wavelengths are transmitted over the same fiber and combined on the same detector at the node.
A traditional hybrid fiber/coax (HFC) plant then delivers the forward traffic to residential users. An immediate result of transmitting analog channels at 1550 nm directly to the node is the gain in carrier-to-noise ratio and distortion performance that is normally lost in the 1550 nm to 1310 nm relasing process.
This performance improvement allows for the use of a single 1550 nm transmission link for the interconnection between the headend and the secondary hub site instead of the traditional high-performance, dual-tier or lower-performance "push pull" interconnect.
This approach offers additional savings in cost and network complexity. The additional benefit of increased network flexibility is also provided through the use of the 1550 nm broadcast/1310 nm WDM overlay.
A Catch-22
Most operators face a dilemma when trying to time their investments in new plant and upgrades to coincide with the actual availability of new services. Demand usually increases as new services become available--but there is much uncertainty about the availability of new technologies and new content, and the ever-changing legal, regulatory and competitive landscape.
If a service provider gambles and invests in the capacity to deliver new services before they are available or in demand,ÿ the carrier must be prepared to spend a lot of money upfront on technology that will not offer a return on investment for an undetermined period of time. On the other hand, those operators that delay this type of investment will be totally unprepared when new advanced services become available--and will find new revenue sources going to their more prepared competitors.
The 1550/1310 WDM overlay concept provides for a separation between the broadcast analog part of the spectrum and the digital new media portion. In this architecture, operators can use the lowest-cost method for delivering the broadcast portion of the spectra. This approach gives operatorsÿ flexibility to add new capacity to deliver new services consistent with their availability and with customer demand for these new services.
While the demand for the broadcast portion of the spectrum is fairly uniform, demand for interactive services such as Internet access will vary in different locales. The 1550 nm broadcast/1310 nm WDM overlay architecture can allow system investment to be more directly tailored to local demographics and their consequent demand for additional services.
Becoming competitive
The 1550 nm broadcast/1310 nm WDM overlay architecture has been hypothesized for several years. Only recently have the components become available to make this architecture competitive with more traditional architectures. The three primary components of this architecture--the high-powered fiber optic amplifier, the low-cost overlay transmitter and the splitter/WDM network--have seen dramatic improvements in performance, and equally dramatic reductions in price, during the past two years. And the prices continue to drop.
The development of high-powered, fiber optic amplifiers and their continued refinement have allowed for substantial improvements in the performance and reach of HFC networks. This is due to the nearly distortion-free, low-noise properties of these amplifiers.
Traditionally, erbium-doped fiber amplifiers (EDFAs) have been pumped by either 980 nm or 1480 nm laser diodes. High-power operation for low-cost broadcast architectures has been limited by the high cost and limited output power of the available pump lasers.
Today, the co-doped ytterbium-erbium fiber amplifier (YEDFA) pumped by diode pumped solid state lasers (DPSSLs) can be used to extend the output power of fiber optic amplifiers to over +27 dBm--for prices similar to existing 980 nm pumped fiber amplifier technology. This substantially lowers the dollar per mW cost of generating 1550 nm light.
The YEDFA optical amplifier system achieves this high power through the use of high-power DPSSLs. The pump source for the DPSSL is a high-power broad area AlGaAs (aluminum gallium arsenide) laser diode operating around 792 nm and emitting over 2 W of optical power from a 200 micron aperture.
Through a patented process, the high power from the laser diode is focused into a Nd:YLF (neodymium:yttrium lithium fluoride) crystalline laser and converted to 1047 nm with high efficiency, excellent spectral purity and with nearly diffraction-limited beam quality.
The output beam quality of the DPSSL is so good that more than 90% of the DPSSL light is focused into the fiber, compared to less than 60% for a 980 nm pump. Using the DPSSL source, almost 700 mW is available for pumping a fiber amplifier as compared to less than 150 mW for a 980 mW pump of similar cost.
The 1047 nm output is converted to 1550 nm gain through a patented phosphosilicate glass fiber containing both erbium and ytterbium as a sensitizer. High-power operation has been obtained through the use of cascaded 980 nm pumped EDFA gain stages, though at a much higher noise penalty than a single stage YEDFA.
The lower carrier-to-noise and distortion requirements of a purely digital tier of channels allow for significant savings in terms of the 1310 nm WDM overlay transmitter. A 1310 nm transmitter used in standard HFC networks must, along with the narrowcast tier of digital channels, carry the full analog and digital broadcast spectrum. This requirement places large demands on both the 1310 nm DFB and the electronics used to drive the laser.
The 1310 nm DFB used in traditional HFC networks is selected for high power, low noise and high linearity. It must be both temperature controlled and optically isolated.
The WDM overlay laser, on the other hand, has much lower requirements for power, noise and linearity. DFB lasers, such as those used in return-path applications, are suitable for this purpose. Sophisticated predistortion-type linearization electronics are employed in HFC 1310 nm transmitters, although they are not required for the WDM overlay transmitter. The relaxation of these requirements allows substantial savings in both transmitter cost and complexity.
The explosive worldwide growth of fiber optic transmission networks for both digital and analog applications has caused dramatic price reductions in the WDMs and other passive optical components used in these systems. Likewise, the high-count splitters and WDMs used in the 1550 nm broadcast/1310 nm WDM overlay architecture have also experienced recent large price reductions. The lower cost of these items enables optical WDM-based insertion of local content at various points in the network.
As 1550 technologies continue to mature, broadcast network costs will decrease even further and drive fiber deeper into the network. In addition, the use of local 1310 nm optical insertion via WDM will help network operators deploy targeted services to more efficiently generate new revenues.
Any comments, or leads? They have a huge contract with TCI,for a 8 DWDM system installation,and mentioned the price of lasers,and optical amplifiers have fallen dramatically.
Hiram |
