The telecommunications industry is experiencing growth in demand for its services, sparked by the proliferation of voice, fax, data, cellular, and Internet usage. As a result, capacity in long-haul systems needs to be expanded. There are at least three ways to meet this demand: improve the infrastructure by laying more cable, increase the flow by sending the information at higher transmission rates, or increase the volume by putting more information through the existing cables. Each of these options is being pursued, but, according to Bob Steele, president of Strategies Unlimited (Mountain View, CA), in a recent presentation to the Optical Society of America, the real driver in the telecommunications market is upgrading systems to meet the growth.
Upgrading, by increasing the volume of information carried, is of paramount interest to laser manufacturers. Multiplexing—using multiple laser wavelengths that each carry a bundle of information—requires both carrier lasers and a pump laser for the fiber amplifier, as well as many specialized optics and accessories.
This Product Focus surveys the recent developments in diode lasers for the telecommunications industry, looking at dense wavelength division multiplexing (DWDM) and the application of existing technologies for this market segment (see Fig. 1). It also touches briefly on some of the developments in optics and accessories that support DWDM. The table that follows on p. 174 is a partial listing of vendors of diode lasers. For a more complete listing, see the 1997 Laser Focus World Buyers Guide, p. 408.
Wavelength division multiplexing
The WDM concept depends upon the theory that a discrete light frequency can carry its own unique package of information. If two separate frequencies are combined and transmitted along the same fiber, the information carried by each can be separated and reclaimed at the end of the conduit as long as the frequencies can be optically separated. On this basis, the first WDM systems combined and separated 1310 and 1550 nm.
This choice of wavelengths effectively doubled the capacity of fiber systems but could not be used for long-haul routes because amplification, necessary over long distances, was only becoming available at one of the wavelengths—1550 nm. Technical limitations are still hampering commercialization of an amplifier for 1310 nm. The specialized amplifiers for long-haul lines—erbium-doped fiber amplifiers (EDFAs)—have been developed and commercialized in the past five years. One feature of these amplifiers is that they have a 25-nm effective bandwidth, centered around 1550 nm. This operating bandwidth can provide room for several laser wavelengths. As such, the EDFA is an enabling technology for DWDM transmission systems.
An EDFA is a broadband optical amplifier pumped by a diode laser; it is capable of amplifying either a single carrier signal or a number of closely spaced laser wavelengths or optical channels in parallel. It can be set up in either single-stage or dual-stage operation. In single-stage operation, the pump source stimulates emission of additional photons of the same wavelength as the signal already in the fiber. In dual-stage operation, the initial pump laser is usually a low-output-power diode laser, at either 980 nm for a silicon-based fiber amplifier or at 1480 nm for a fluoride-based device. In both cases, the goal is to produce a low noise figure at the end of the first stage. The second stage is then used to amplify the power. In multiple-wavelength systems, usually the incoming light is demultiplexed, each wavelength is individually amplified, and then the light is recombined.
Diode lasers are not only used to amplify the light already in the fiber, they are also used to provide the original light source. The traditional diode laser used in a fiberoptic telecommunications system operates at a minimum of 100 mW of output power with a spectral emission range of up to 15 nm on either side of the central wavelength. The distribution of the out put power around the central wavelength is spiky and un predictable (see Fig. 2 top).
In a DWDM system, however, this power-distribution curve would not be effective and cannot be used. Instead, system installers use a distributed-feedback (DFB) diode laser. In this laser, the grating is in the resonator, and the laser oscillates in the single longitudinal mode. The single peak wavelength can be determined very accurately, usually to within ۫ nm, and the center wavelengths of multiple devices for the same system can be specified in increments of 100 GHz (approximately 1.6 nm; see Fig. 2 bottom). Narrow linewidths are important as is side-mode suppression. In both cases, this is to prevent crosstalk between adjacent channels. In other words, one conversation should not interfere with the other.
The standard for channel separation has been established by the International Telecommunications Union (Geneva, Switzerland), which created a grid based upon multiples of 100 GHz. (Note the influence of the electronics, determining the measurement in frequency rather than wavelength.) With this standard, an eight-channel, commercially available system requires a total minimum operating bandwidth from the EDFA of 12 nm. However, the system installer could specify 200-GHz spacing, which is acceptable according to the standard. Then the system would need a 25-nm operating bandwidth. This spacing is workable for either a four- or eight-channel system; 16- or 32-channel systems, both of which are ready for commercialization, require tighter spacing.
Another important piece of the DWDM puzzle is the optical components that combine and separate the wavelengths. Multiplexer/demultiplexer technology is key to the ultimate implementation of these system improvements, and the jury is still out on the best way to accomplish the feat of distinguishing four, eight, or 16 wavelengths, while minimizing insertion losses. No matter which technology is used, each of the testing setups will also require a diode laser at each operating wavelength to test the optical components.
Future developments
What do the efforts with WDM technology, including the DFB diode laser, provide for the customer? Ultimately, there could be an eight- or 16-fold increase in capacity on certain trunk lines without having to lay new cable routes. Right-of-ways are difficult and expensive to obtain, and laying new cable in existing right-of-ways is still very costly. Upgrading existing systems already in the ground will help quite significantly toward meeting the increasing demand. The telecommunications industry has an installed base of fiber working at two different wavelengths—1310 and 1550 nm. Either wavelength would be acceptable for multiplexing; currently all long-haul multiplexed installations are being done at 1550 nm because of the EDFAs available at this wavelength region.
Development efforts continue on most fronts. Work is being done at 1310 nm on the commercialization of a praseodymium-doped amplifier (one commercial device was shown at CLEO ‘97), but that development is still several years behind the EDFA. It is possible that multiplexed systems can be implemented on the 1310-nm systems, in which there is no need for amplification, such as short distances used in cable-TV systems. However, there is less need and therefore less demand for additional bandwidth in the cable-TV transmission systems. Development of DWDM systems will proceed more slowly at 1310 nm.
Efforts with EDFAs are also focusing on flattening and broadening the gain shape of the amplifier so that the operating bandwidth can be increased and more channels be accommodated. Materials work continues, looking at more responsive fibers for the amplifier and dispersion-shifted fibers for the system. Development efforts with DFB diode lasers continue to further narrow the output band, with better suppression on the side lobes, in an on-going effort to minimize crosstalk. As is almost always the case with lasers, the manufacturers continue to improve the output-power levels so that longer distances can be covered with fewer amplifiers.
Diana Zankowsky | Contributing Editor
Diana Zankowsky was a contributing editor for Laser Focus World.