Cable-TV laser powers 32 nodes
A fiber-optic transmitter has the potential to allow affordable Internet access through cable-TV networks, as well as enabling video-on- demand and other services. The transmitter, from Fiber Optic Network Solutions Corp., or FONS, in Northborough, MA, uses two solid-state lasers to provide continuous light to a number of external modulators. Because the device uses external radio frequency, or RF, modulation after the light is split, the same light source can provide many different signals. This splitting process permits narrowcasting, or the transmission of large amounts of information to select groups of subscribers, so providers can offer different services over one fiber. The cost of narrowcasting with existing technologies has proven prohibitive.
In most current hybrid fiber/coaxial-cable networks, lasers modulate the light, and the laser output is split and sent along several fibers. Rich Mack, a vice president for Kessler Marketing Intelligence Corp. in Newport, RI, explains: "FONS has done something different: Its transmitter outputs, splits and then modulates the light." He notes that the transmitter`s diode-pumped neodymium yttrium aluminum garnet, or Nd:YAG, lasers provide higher powers than distributed feedback, or DFB, lasers, which allows them to feed more nodes per laser than is possible with a DFB laser transmitter.
Hybrid networks that use fiber from the headend to a node (from which coaxial-cable buses deliver the signal to a group of customers) send analog signals using DFB diode lasers. In these lasers, the signal is modulated onto the light beam inside the laser. Expensive DFB lasers are used, rather than less-expensive diode lasers, because the amplitude modulation, or AM, signal transmission must be highly linear with low distortion. An AM signal, rather than a digital signal, is used to satisfy television set reception. To spread the costs of expensive DFB lasers, higher-power versions have been developed that can be shared by many customers.
A single DFB laser can be the broadcast source for the same analog signal to three to five nodes. Splitting the signal works fine for broadcasting but wastes bandwidth for services to just a few subscribers.
Unlike today`s DFB lasers, solid-state lasers can be modulated outside the laser, after the signal has been split from four to 32 times. To modulate the beams, FONS has developed a low-cost lithium niobate modulator. Most commercial lithium niobate modulators are expensive, high-performance devices. According to FONS President Mike Noonan, the company can quarter the price of these modulators because system requirements are much easier to meet. He notes, "Most of these modulators are made for [synchronous optical network] applications, which transmit at 9.6 gigabits per second. The FONS transmitter is not even operating at 1 Gbit/sec."
The higher powers available from solid-state lasers make it possible to split the power more ways. Although this is not a large advantage in today`s typical broadcast market, the trend among cable-TV providers is to offer two-way services and allocate a part of the bandwidth to digital signals.
According to Kessler`s Mack, "For customized services, the solid-state approach has potential. However, there are alternatives to using solid-state lasers. One method is to use a 1550-nanometer diode laser and an amplifier. Another method, being worked on by Ortel Corp. in Alhambra, CA, is to provide lower-cost DFB lasers, which could make them cost-effective against this system. Or, if the fiber-to-the-curb advocates get their way, then all-digital transmissions could be used, which require digital-to-analog converters close to the customer end, but would loosen the requirements on the transmitters, allowing the use of less-expensive diode lasers."
Another viewpoint comes from Donald Gall, a senior project engineer at Time Warner in Englewood, CO. Gall has been encouraging the fiber optics communications industry to develop technology like the FONS transmitter. "To be able to provide one-on-one transport for video-on-demand or telephony," he says, "the coaxial-cable buses need to be divided into small groupings."
With current DFB lasers, the cost per transmitter rises as the buses get smaller. "This external modulation scheme allows us to do that fairly efficiently," says Gall. At the moment, Time Warner is exploring narrowcasting using external modulators, cheaper DFBs and a two-laser system.
FONS`s Noonan says that systems using the company`s transmitter allow providers to offer different services over one fiber, keeping the complexity of the system at the headend of the network. With this system, he says, the type of information is transparent to the cabling plant.
In the opinion of John Browse, director of network development at Jones Intercable, in Englewood, CO, "The fiber-optic network needs one optical transmitter per node to manage spectrum allocation for two-way services." He points out that it does not make sense to have one telephony signal--used by one subscriber--transmitted to all subscribers.
Although Jones Intercable could spread the cost of DFBs by using different transmission products for customized services and broadcasting, this adds to the complexity of the network in the field. The company faced a problem, says Browse: "How can we build plant that holds down the skill set of the technicians?" The solution: "We integrate everything in the headend, thus requiring technicians to know only one transmission scheme. The company already uses multiple DFBs and RF allocation at the headend," he says.
Browse continues, "If the FONS transmitter performs as advertised and the cost is where we think it needs to be," then it would be useful for the company`s migration strategy to two-way services.
The FONS transmitter has several features to provide redundancy. Both Nd:YAG lasers work at 50% power during normal operation, and either can switch to full power if the other laser fails. The transmitter also includes two power supplies for the lasers. Reliability, however, hasn`t been a problem with DFB lasers. The largest risk, according to Browse, is losing the fiber link. Network redundancy depends, for the most part, on the number of paths per node.
Jones Intercable uses diverse routings and switchable optical splitters from Optivideo in Boulder, CO. In regular service, the signal is split 90/10. The 10% signal, sent along the backup route, is enough to detect power through the fiber, confirming that the backup route is working. If the main line goes down, all the optical power is switched to the backup route.
In general, Browse adds, providing redundancy has been left to the network owners rather than built into products. The FONS transmitter includes a feature that makes providing redundancy easier, however: Each optical modulator encodes the RF signal onto the light beam and provides two identical outputs. These can be routed to the customer through different paths, so that if one path fails, a backup is available.
One advantage of this architecture compared to the more common tree-and-branch schemes, says Noonan, is that network maintenance can be done during the day because it doesn`t disrupt service. The alternative requires waiting until the early morning hours when the fewest customers will notice the disruption.
Mack observes that if services go to two-way soon, then FONS is in a good position with this product. Browse, who is already dedicated to the idea of two-way capability, agrees: "If the price is [right]. . . I think it`s a wonderful step."q
Yvonne Carts-Powell writes from Belmont, MA.