Optical amplifiers gain in numbers
Buoyed by recent deployments in commercial long-distance telecommunications networks, optical amplifier products from more than a dozen companies attracted widespread attention at the 1995 Optical Fiber Communications conference. Substantial technological progress was noted in optical amplifier gain, linearity and integration characteristics.
Market acceptance was reflected by increased sales and deliveries, lower prices and tighter specifications, all across the product portfolio--from such small devices as pum¥lasers to large equipment, such as fully integrated amplifiers.
The integrated optical amplifier systems are primarily being used to replace $50,000 electrical regenerators used in long-distance networks. For example, Pirelli Cable Corp. in Lexington, SC, appears to be the market leader in deploying fully integrated erbium-doped fiber amplifiers. The company claimed that by the end of 1994, more than 1000 of its T30 line of optical amplifier systems had been installed worldwide.
The company has been exclusively building these amplifiers in Pirelli`s facilities in northern France. However, because the U.S. demand for optical amplifiers is so great, Pirelli is building a new optical amplifier manufacturing facility in North Carolina.
The company is developing an optical amplifier for the wavelength-division multiplexing of four 2.4-gigabit-per-second optical signals onto a single fiber. BellSouth plans to use them to increase the capacity of some busy networks. Basil Garabet, North American marketing manager at Pirelli, says the company is developing an amplifier that could support 16 different optical wavelengths on one fiber; it is expected to be ready by the end of this year.
A key limitation in using optical amplifiers in long-distance networks is that they do not compensate for fiber dispersion. This deficiency occurs because most of the installed fiber base has minimum dispersion at 1310 nanometers, but commercially deployable optical amplifiers operate in the 1550-nm range where the dispersion is higher. Over long distances, dispersion garbles the message unless compensation techniques are implemented.
At OFC, Bosch Telecom in Gaithersburg, MD, and Corning Inc. in Corning, NY, displayed new or improved dispersion-compensating modules. The Bosch Telecom DCM-060 module plugs into the integrated optical amplifier system rack. It produces a negative dispersion of 1020 pico seconds/nm, with less than 7 decibels of attenuation. The company claims that when the DCM-060 is used with an optical amplifier, the module can increase a network`s transmission distance by 60 kilometers.
At the low end, the Corning DCM-20 module provides a negative dispersion of 340 ps/nm with a loss of less than 3 dB. At the high end, the DCM-80 module generates a negative dispersion of 1360 ps/nm with a loss of less than 13.6 dB; this module can compensate for 80 km of dispersion.
Most of the optical amplifiers that were shown at OFC can be integrated into a standalone optical amplifier for laboratory applications or directly into a telecommunications network.
The lowest-cost module was presented by a joint venture between IRE Polus, a Russian company, and German laser manufacturer IPG Laser GmbH.
Wilfred Bauer, managing director at IPG Laser in Berlin, says IRE Polus moved amplifier production from Russia to Germany to improve product quality. The venture is offering a 20-decibel-relative-to-milliwatts optical gain module for $15,000 in small quantities.
Litton Systems Inc. in Woodland Hills, CA, showed a line of optical amplifier modules that use a single 1480-nm pum¥laser. The module generates an output of approximately 15 dBm and costs approximately $20,000 in small quantities. The company is also developing a high-powered booster module for the cable-TV market as part of a $250,000 technology reinvestment project with the state of California. Product samples should be available by next month.
Mitsubishi Electronic America Inc. unveiled a $30,000 erbium-doped fiber amplifier module based on the company`s 1480-nm pum¥lasers. Fujitsu Network Transmission Systems Inc. exhibited a similar optical amplifier module. However, a Fujitsu representative said the company was holding off on volume production until a low-noise 980-nm pum¥laser is available.
The 1310-nm goal
Optical amplifiers that operate efficiently at 1310 nm have been pursued vigorously in telecommunications because most of the installed equipment and fiber is optimized for that wavelength. However, only two dopants have been identified that could be used for optical amplifiers--praseodymium and neodymium. But 1310-nm amplifiers fabricated with these dopants have proved weak and expensive for practical applications.
However, Nippon Telegraph and Telephone Corp. in Japan proclaimed a line of praseodymium-doped fiber amplifiers optimized for 1310-nm communications. The $80,000 low-end version has a gain of 17 dB; the $130,000 high-end version supplies a gain of 35 dB. With a peak gain centered at approximately 1300 nm, both amplifiers respond between 1280 and 1312 nm. Noise for both amplifiers is less than 5 dB. An NTT spokesperson states the amplifiers will be delivered in sample quantities by the third quarter of this year.
A major problem with optical amplifiers is they do not provide an even gain level across their window of operation. This characteristic can be a problem when the amplifiers multiplex multiple optical signals onto a single fiber. In addition, linear signals used for carrying analog video traffic can be distorted.
Photonic Technologies in Pagewood, Australia, has developed a filter product that flattens the gain of an optical amplifier across its entire window of operation with no polarization sensitivity. The 5ٸٴ.5-centimeter module sells for $4000 to $5000. However, according to Simon Poole, sales and marketing manager, the company plans to compress its amplifier technology into a 1-cm cube at reduced cost when volume production begins soon.
A time-consuming and expensive process in building optical amplifiers involves the pigtailing of singlemode fibers onto all the interior components; for example, pum¥lasers, couplers and isolators. q
George Lawton is a freelance writer based in Brisbane, CA.