Cable-TV conference probes emerging fiber technologies
GEORGE LAWTON
Attended by more than 1000 cable-TV engineers, the 1996 Conference on Emerging Technologies held in San Francisco this past January explored several relevant fiber-optic technologies, such as semiconductor optical amplifiers, hybrid fiber/coaxial-cable, or HFC, networks and network power techniques.
Tony Werner, vice president of engineering at Tele-Communications Inc. in Englewood, CO, discussed network availability requirements and capabilities of HFC networks. TCI has performed extensive failure analysis on all HFC system components and over two years, has collected data from three systems.
Werner observed that because of the relatively short time during which data was collected, they were not able to obtain empirical data on fiber component failures. Consequently, they had to refer to warranty data supplied by fiber-optic component manufacturers.
He noted that these studies are important to determine the true causes of failure in different environments. A system evaluated in an urban area, for example, revealed that a large percentage of the outages were caused by gunfire from people shooting out cable-TV system equipment. This was not a problem in other systems.
HFC telephony networks
Andy Paff, president and chief executive of Integration Technologies in Englewood, CO, focused on the issues that need to be addressed to build reliable telephony networks with HFC. Integration Technologies is a new joint venture between Antec and Northern Telecom to provide systems integration for next-generation networks.
Paff said that when cable operators talk about reducing downtime to under 53 minutes a year, they are talking about providing lifeline telephony service. Operators should build networks with low incremental costs so that new services can be added. In addition, these networks should require nonintrusive maintenance and easy system modification. Paff noted, "The implications are extreme. If we plan it out, we will be able to build networks for 20 years rather than seven years. What will the financial impact of that be?"
Another issue that will affect network performance is digital video. For traditional cable-TV systems, low bit-rate communications for pay-per-view and other services is done using quadrature phase-shift keying, or QPSK, modulation. Most plans for delivering video to the home use technologies such as quadrature amplitude modulation and vestigial sideband modulation, both of which can transmit more bits per hert¥than can QPSK. For example, a QPSK transmission over a 6-megahert¥cable channel could carry 12 megabits per second, compared to 36 to 48 Mbits/sec for the other technologies.
To ensure that this signal is decoded properly, the laser source needs to maintain a higher carrier-to-noise ratio than that required for only video. If cable operators implement near-video-on-demand systems that use the entire 550- to 700-MH¥band for video-on-demand, the additional radio-frequency loading might have enormous consequences on the linearity requirements of the laser. He said that device yield and costs could be adversely affected.
In the field, a large number of return-path lasers for sending telephony and other information to the headend are expected to use QPSK. Paff said that there is little research on characteristics of lasers used to carry multiple QPSK signals. He noted that this is a key area in the performance equation because there will be so many lasers. Research is required to determine the saturation point of the current range of return lasers to identify RF loading in a fully utilized return path.
Richard Hockney, chief technical officer at Satcon Technology Corp. based in Cambridge, MA, discussed a flywheel power backup system that the company is developing for telecommunications networks. Flywheels offer lower cost and higher reliability than battery backup systems.
The flywheel is a spinning magnetic wheel in a sealed chamber. Energy can be added to the flywheel by spinning it faster. The flywheel can be connected to a generator when power is needed. According to company claims, its backup system can operate for seven years without maintenance and has an estimated life of 20 years, compared to lead acid batteries, which must be maintained every six months and last only a few years. In addition, flywheels can operate in extreme temperatures without performance degradation.
Satcon will begin the pilot production of flywheels for telecommunications networks in late 1996, with a full commercial rollout scheduled for early 1997. The initial model for telecommunications is projected to weigh 260 pounds and store 2 kilowatt-hours of energy.
Semiconductor optical amplifiers
L.F. Tiemeijer and V.G. Mutalik of Phillips Broadband Networks based in Manlius, NY, submitted a paper on the applications of semiconductor optical amplifiers in cable-TV networks. They surveyed applications best-suited for erbium-doped fiber amplifiers, or EDFAs, at 1550 nanometers.
Semiconductor optical amplifiers, however, are used with 1310-nm equipment and fiber. The control of these amplifiers is similar to that used for distributed feedback lasers, enabling them to be easily integrated into existing system architectures.
The use of semiconductor optical amplifiers for cable TV has been limited by their nonlinearity properties because of the short amplification recovery time. Because EDFAs have an amplification recovery time of 10 nanoseconds, they do not vary widely over instantaneous responses in signal. The amplifiers have a recovery time of only 0.2 nsec, so the optical gain follows instantaneous changes in the input signal level.
Recent research at Phillips suggested that this drawback can be overcome by using a gain-clamped multiquantum-well semiconductor optical amplifier. These types of amplifiers should be able to amplify signals to an output power of 15 decibel relative to milliwatt. The clamp is constructed by forming a Bragg grating in the device that causes lasing action at 1290 nm. When signal radiation from a nearby wavelength (that is, 1310 nm) passes through the system, it undergoes uniform amplification.
These linearized amplifiers promise a new age for 1310-nm fiber-optic systems. Cable operators are expected to be able to integrate them into existing systems, and they could be used for long-haul networks as line amplifiers to extend the network`s range. Furthermore, the semiconductor optical amplifier signals would not be affected by dispersion because most installed fiber possess zero dispersion at 1310 nm.
Mutalik says that "the chief advantages of 1310-nm semiconductor optical amplifier technology are its flexibility, scalability, dispersion immunity, compatibility with existing systems and low cost. Now for the first time, both power budget and dispersion are non-factors in transmitting optical signals." q
George Lawton writes from Brisbane, CA.