Window opens for PDFAs to benefit cable TV and WDM

Praseodymium-doped ZBLAN fiber, which provides the gain medium for 1310-nm amplification, is power-tested before being sealed in a hermetic enclosure. Excited-state absorption produces an orange and blue glow in the fiber, which is one of the major loss mechanisms in the amplification process.Click here to enlarge image

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Optical fiber amplifiers have become ubiquitous in modern telecommunications systems. Because rare-earth-doped fiber amplifiers amplify light inside a single-mode optical fiber, they allow system designers to compensate for the scattering and absorption losses that occur in every element within an optical network. Erbium-doped fiber amplifers (EDFAs) amplify light at wavelengths close to 1550 nm. Praseodymium-doped fiber amplifiers (PDFAs) amplify light with wavelengths close to 1310 nm, the second telecommunications window.

Unlike their 1550-nm cousins, 1310-nm amplifiers have remained as laboratory devices for nearly a decade and are only now available with sufficient reliability and at reasonable prices to be used in commercial systems. This progress has been driven by the need to make better use of the available optical bandwidth in currently installed single-mode fiber.

IGURE 1. In a PDFA, 1030-nm ytterbium fiber laser pumps the praseodymium-doped ZBLAN fiber, which amplifies at 1310 nm. Click here to enlarge imageIn a gain-flattened PDFA, the core of zirconium barium lanthanum aluminum sodium (ZBLAN) fiber contains approximately 0.1% praseodymium ions. The pump laser excites electrons from the ground state (³H₄) into the 1G4 orbital. A signal photon, with a wavelength near 1310 nm, de-excites the electron down to a ³H₅ orbital, producing a duplicate photon. These photons can in turn de-excite more electrons, further amplifying the signal. A total amplification of more than 40 dB is available (see Fig. 1).

Amplification process

This process is the same as the one that occurs in a laser with the exception that no mirrors are present; instead, as in EDFAs, the active fiber is shielded by fiber isolators. These one-way gates prevent feedback into the amplifier from back reflections within the optical network. At best, reflections add noise to the signal because signals reflected different numbers of times mix at the output. At worst, reflections can turn an amplifier into a laser.

Because the amplification process is less than 10% efficient, high-power 1030-nm pump lasers are required. Only recently have low-cost diode-pumped 1030-nm ytterbium fiber lasers become available, as well as reliable fused-tapered wavelength-division-multiplexing (WDM) couplers for combining the pump and signal light.

Historically, the development of 1310-nm amplifiers for both cable TV and telecommunications has required a great deal of materials research and, as a result, has been slower than many people hoped. Early ZBLAN fibers were hydroscopic, quickly becoming brittle and unusable. These problems were largely overcome with the development of strippable hermetic coating, which allowed the fiber to be handled and stored for long periods of time. However, to ensure a 25-year lifetime in hot, humid conditions, the fiber is now hermetically sealed in a laser-welded stainless-steel enclosure (see photo above).

Another significant problem has been the reliable joining of standard silica fiber to the ZBLAN fiber, which has a glass transition (softening) temperature of 260°C, hundreds of degrees below that of silica fiber. Joining the two by standard fusion splicing is not possible. Mechanical splicing has proved unreliable because blue light, produced by excited-state absorption, can over-cure the adhesive. However, splicing using an intermediate glass has proved a robust method of joining the fibers, leaving the splices able to withstand -40°C to +70°C cycling and power densities in excess of 3 MW/cm².

Market drivers

There are two major markets for PDFAs: cable TV and WDM tele communications. Integrating PDFAs into existing tele communications systems is both easy and difficult. Because 1310-nm and 1550-nm signals can be added and separated with cheap and reliable fused tapered components, adding 1310-nm wavelengths to a system is relatively straight forward. In addition, there is little risk of Raman-shifted signals affecting the 1550-nm transmission because the wavelength bands are well separated. Problems remain, however.

Modern long-haul telecommunications links often have amplification huts 80 km apart. At 1550 nm, standard good-quality fiber has a loss between 0.25 and 0.30 dB/km. This translates into a fiber loss of at least 20 dB for an 80-km link. At 1310 nm, a typical loss is around 0.35 to 0.4 dB/km, translating into a fiber loss of at least 28 dB. To regenerate the signal, the amplifier must supply considerably more gain. After losses in other components are included and some margin is allowed, an amplifier with between 35 and 40 dB gain is required.

FIGURE 2. Most current gain-flattened 1310-nm amplifiers have maximum gain around 35 dB, insufficient to comfortably span the required 80-km range. Click here to enlarge image

Existing gain-flattened 1310-nm amplifiers have maximum gain around 35 dB, insufficient to comfortably span the required 80-km range, although new systems under development are expected to draw close to the required gain (see Fig. 2). Other components are also needed. Until the fall of 1998, multiplexers and demultiplexers for combining and separating wavelengths and 1310-nm lasers for the International Telecommunications Union grid were difficult to find, and even now they are much less common than their 1550-nm equivalents.

Cable TV signals need a very different type of amplifier and can use PDFAs immediately. Many cable TV signals are already transmitted by a 1310-nm diode laser over single-mode fiber. Only over the last mile before the home is the optical signal likely to be converted to an electrical signal and transmitted through ordinary coaxial cable. Extending the reach of these lasers has been a goal of 1310-nm amplifier designers. However this must be done without significantly degrading the cable TV signal.

Cable TV systems are analog and are far more sensitive to both noise and distortion than digital telecommunications systems. Fortunately, distortion is not a serious problem in either erbium- or praseodymium-doped fiber amplifiers, but all optical (and electrical) amplifiers produce noise, and noise creates random dots or "snow" in the TV picture. The transmitted signal typically has a carrier-to-noise ratio (CNR) greater than 53 dB. The US Federal Communications Commission requires that the CNR at the customer`s set top be not less than 43 dB, a level that produces a visible, but small amount of snow in the picture. The amount of CNR degradation produced by a praseodymium-doped fiber amplifier is typically around 1.6 dB. At these levels, the signal can pass through several amplifiers without excessive degradation.

Alternative amplifiers

There are several methods of amplifying light for the 1310-nm window. Semiconductor, Raman, neodymium-doped fibers, and praseodymium-doped fibers have all been used in laboratory systems. Only semiconductor and praseodymium-doped fiber systems are commercially available.

Semiconductor-based amplifiers have the advantage of being cheap to manufacture and have potential use in single-channel systems, particularly in fiber-to-the-home applications. Noise and cross-talk between channels have limited their usefulness for dense-wavelength-division-multiplexing systems in the past. Raman systems require very high pump powers but have the advantage that they can be used at almost any wavelength. All three amplifiers are likely to find applications as multiple-wavelength systems become standard in most fiberoptic communications.

IAN G. CLARKE is project leader for 1310-nm products at IPG Photonics Corp., 660 Main St., Sturbridge, MA 01566; e-mail: [email protected].