Advances in component technology aid fiber amplifier cost and reliability gains
Improvements in manufacturing, assembly, and packaging for optical components installed in optical-fiber amplifiers have increased device performance and lowered overall cost
Optical-fiber amplifiers offer improved network operations by amplifying optical signals without first converting them to electronic form. While their importance has been recognized for enhancing optical communications over long-transmission distances, fiber amplifiers
also serve as important components in applications such as cable-TV and other local-loop networks. Amplifiers are generally operated at three network locations, as needed: at the transmitter to increase output power, in midspan to replace regenerative repeaters, and at the receiver to increase sensitivity by acting as a preamplifier.
Typical fiber amplifiers use erbium-doped fiber, 1550-nm optical signals, and a pump laser operating at either 980 or 1480 nm. To amplify an optical signal, the pump laser first excites the erbium ions. Then, when a 1550-nm photon collides with an erbium ion, the erbium returns to a relaxed state. As a result, a 1550-nm photon is emitted that helps boost the optical signal.
Similar technologies allow amplification in other transmission windows, although these technologies are not as mature. For example, in the 1310-nm window, which has a large installed base, praseodymium-doped silica fiber-based optical amplifiers could be used.
In only a few years, fiber amplifiers have moved from the proving stage to become commercialized, off-the-shelf devices. As with other fiber technologies, the next logical market step is to reduce costs and increase reliability--important considerations for bringing broadband communications to the local loop. As costs drop, fiber is expected to move closer to the end-user. To that end, fiber amplifier technology is a critical part of local-loop networking because it allows flexible, expandable topologies to be created.
The passive components used in optical-fiber amplifiers markedly affect device cost and reliability. Fortunately, recent advancements in component manufacturing and assembly techniques have yielded significant improvements in amplifier device performance.
Two common optical-fiber amplifier configurations are designated as single- pump and dual-pump designs (see figure). The single-pump amplifier employs a single pump laser on the upstream side of the doped fiber. The dual-pump version uses a pump laser on both ends. Typical amplifier gains are +17 and +35 dB for single- and dual-pump amplifiers, respectively.
A typical 1550-nm transmitter used in cable-TV applications has an output power of -5 dBm for digital signals and +5 to +7 dBm for analog signals. Amplified signals can run as high as +30 dBm for digital applications and +42 dBm for analog applications. These high levels allow optical energy to be tapped off several times in branching applications, thereby making fiber-to-the-curb or fiber-to-the-home networks more practical.
Considerable work is being done to achieve a flatter gain response over the 1530- to 1560-nm bandpass in amplifiers. Flat gain is an important characteristic in multichannel transmission systems that use several closely spaced wavelengths. The International Telecommunication Union, for example, recommends a channel spacing of 100 GHz, or 0.8 nm, for dense wavelength-division multiplexing (dwdm) systems of up to 32 channels.
An erbium-doped silica fiber exhibits about an 8-dB difference between the minimum and the maximum output levels over the 1530- to 1560-nm band. The use of a fluoride-erbium fiber can reduce this difference to approximately 2 dB. Although a flat output response is an important performance goal in networks where capacity gains are economically achieved through multichannel transmission using dwdm, it is a less critical factor in cable-TV and local-loop applications. In the latter, signal multiplexing is not important.
An optical-fiber amplifier consists of a doped fiber, a pump laser, a wavelength-division multiplexer (WDM), an isolator, and a tap coupler. The WDM is used first to combine the 1550-nm signal and the 980- or 1480-nm pump energy and then to couple the resultant signal into the fiber amplifier. The isolator prevents the amplified signal from reflecting back upstream, where it would increase noise and decrease efficiency. The tap coupler allows a small amount of the signal to be used for monitoring and feedback purposes.
Typical isolators employ bulk magnetics and optics in their construction to propagate the optical signals in one direction only. All bulk optic subcomponent surfaces in an isolator are treated with an antireflective coating to reduce insertion loss. In addition, all surfaces are angled to reduce light reflection (see Table 1).
In operation, light enters the input fiber and passes through a graded-index lens, which collimates the light rays. The light then passes through an isolating element consisting of a polarizer prism, a Faraday rotator, and an analyzer prism, all in series. Dual-stage isolators include a second isolating element. Following the isolating element, an output graded-index lens refocuses the light into the output fiber. Light traveling upstream is prevented from being refocused into the input fiber.
The crystals used in an isolator are birefringent; that is, the materials possess a different refractive index for each of the two polarization states. They, therefore, cause the two polarization states to travel along different paths. The Faraday rotator turns the optical axis of both polarization states by 45°, affecting the rotation relative to the direction of the magnetic field and not to the light`s direction of propagation. In the reverse direction, however, the Faraday rotator causes the polarized light to change its polarization state another 45° and become out of phase. The light, therefore, is not refocused into the incoming fiber.
Important parameters in amplifier applications are polarization-dependent loss (PDL) and polarization-mode dispersion (PMD). PDL measures the change in insertion loss as a result of a change in the polarization of light. PMD defines the transmission delay of the two orthogonal propagation modes of light through a device. It has become an important consideration in high-speed singlemode fiber systems. For example, a 2-Gbit/sec signal can tolerate only about 2 psec of PMD before the bit-error rate increases. Obviously, optical-fiber amplifier components must minimize PMD and PDL values. An isolator, for example, offers a PDL of 0.1 dB or less, and a PMD of less than 0.05 psec.
WDMs used for optical-fiber amplifiers are 2-channel, wavelength-sensitive couplers for multiplexing either the 980- and 1550-nm or the 1480- and 1550-nm wavelengths (see Table 2). They typically use a fused biconic tapered (FBT) process. Low insertion loss, high directivity and low reflectance obtained with these devices are important parameters in fiber-amplifier applications to ensure low-noise operation.
Tap couplers are wavelength-insensitive. They also use the FBT process to split the optical signal. For optical-fiber-amplifier applications, tap couplers typically use a splitting ratio of 99:1, 99:2, or 95:5. They are often used on both sides of the amplifier to let the incoming unamplified signal be compared to the outgoing amplified signal.
Passive components are generally rugged and compact. Fused tapered couplers and WDMs are inherently rugged because they are essentially all-fiber devices. Although isolators contain discrete subcomponents, they are manufactured in more-rugged packages than earlier versions. For example, AMP Inc. uses an advanced soldering technology on bulk optical components to achieve an epoxy-free optical path and a ruggedized package. The resulting isolator package, which is cylindrical, measures 8 ¥ 40 mm for the standard package size and 5.5 ¥ 35 mm for the compact version. The tap couplers, WDMs, and isolators are designed to meet the Bellcore GR-1209 and GR-1221 specifications, which cover the environmental and mechanical performance and reliability assurance requirements for fiber-optic branching devices.
Value-added solutions can be provided from these optical components. Products in this category include the passive portion of the amplifiers in a module. These modules comprise a tap coupler, a WDM, and an isolator in a customer-defined enclosure. Other products contain several closely integrated components in a single package for ease in designing and building small, highly reliable optical-fiber amplifiers. Such integrated devices include isolator/WDMs and isolator/ WDM/tap couplers in a single package.
Equally important are new manufacturing and packaging methods that make passive components considerably easier to make and less expensive to buy. Cost reduction is a major issue in the economics of deploying local-loop networks. In these networks, broadband services to the home must be affordable to attract end-users. Hybrid fiber-optic/coaxial-cable (HFC) networks are favored by cable-TV providers because of their reasonable costs in amplifying and tapping optical signals in the local loop. Installed in HFC networks, low-cost, reliable optical-fiber amplifiers are expected to drive fiber closer to subscribers.
For optical-component fabrication and packaging, silica waveguide and silicon waferboard technologies can offer enhanced manufacturing efficiencies such as batch processing. In addition, they allow semiconductor-style processing, such as photolithography, wet and dry etching, and laser micromachining, which produces precision geometries. This precision, in turn, helps to simplify component alignment and automated assembly. Active alignment of subcomponents in contemporary isolators and other optical devices is labor-intensive. This major cost factor warrants attention at every level of component design and assembly. These enhanced manufacturing techniques can also combine active and passive products more efficiently to further reduce the design complexity of optical-fiber amplifiers. Lower component count means higher reliability; fewer splices lead to lower losses; and compact packages result in smaller end-devices. u
Sanjay Sudeora is director of engineering for the AMP Global Passive Products Div. in Palo Alto, CA.