Designer fibers perform photonic functions

Jocelyn Lauzon

André Croteau

Karine LeFoulgoc

Michel Bégin

Low-loss single-mode and multimode optical fibers are an enabling technology for today`s optical communications systems. Fibers with characteristics other than simply that of a low-loss waveguide-those with photonic functions-are called specialty fibers and are fundamental to functions such as amplification, attenuation, filtering, and dispersion compensation. The erbium-doped fiber, for example, is the basis of erbium-doped fiber amplifiers (EDFAs), which are one of the technologies underlying wavelength division multiplexed (WDM) systems. Other emerging specialty fibers will further increase the capacity of these systems.

Because specialty fibers have a photonic function, they are generally used to make optical communication components rather than fiberoptic cables and are used in much shorter lengths than their "standard" fiber counterparts (see Fig. 1).

Rare-earth doped fibers are the "mother of all" specialty fibers. Because some rare-earth elements have transition bands that correspond to the optical spectrum (mainly near-infrared) they can absorb light when inserted in a glass matrix, thereby storing energy that can subsequently be used via photon emission to create a laser or an amplifier. This is the same principle underlying flashlamp-pumped solid-state lasers such as Nd:YAG systems-with the difference that rare-earth elements are in an amorphous glass waveguide rather than in a bulk crystal rod (see Fig. 2).1

These specialty fibers have two fundamental characteristics that have been critical to the development of optical communications. First, the fluorescence lifetime of the 1550-nm excited state of an erbium-doped fiber is on the order of 11 ms, which means that erbium-doped fiber amplifiers are transparent to bit rates above the kilohertz level; the available energy will not be depleted by individual digital data at these rates. In this case, the continuous wave (CW) gain of the fiber amplifier is the same as the average gain on a high-speed digital signal. The amplifier is, therefore, transparent to protocol, bit rate, and bit format.

The second fundamental characteristic is that, because a glass medium is amorphous (rather than crystalline), it offers a large gain spectrum when doped with a rare-earth element. This spectrum can be shared, nearly homogeneously, between multiple wavelength channels, which paves the way to a wavelength-division multiplexed optical communications system.

Rare-earth doped fibers

The characteristics of specialty fibers can be adjusted by optimizing the glass mixture or the waveguide geometry to get the performance required. Reducing the background losses of fiber has a positive impact on any application, but for some specific applications a particular adjustment is necessary. Adding aluminum to the glass mixture, for example, both improves the solubility of the rare-earth dopant in the silica-glass host, and broadens the gain spectrum of fiber amplifiers. The geometry of the fiber-core diameter and dopant confinement-also has a strong influence on the overlap between pump and signal, and consequently the conversion efficiency of the amplifier. All these parameters are interrelated and fiber design optimization always means finding the optimal trade-off between them.

An erbium-doped fiber designed for a pre-amplifier application is different from one intended for in-line amplification. For erbium-fiber laser applications, the fiber design is also very different from those in amplifiers. Opening the L-band window (1565 to 1625 nm) also required development of erbium-doped fibers with particular specifications.

The next step in broadening the optical communication transmission band (so that more WDM channels are available) is opening up the S-band (1440 to 1530 nm). Erbium-doped fibers cannot work in this window, so a different rare-earth doped fiber is required. In fact, fluoride-glass hosts doped with thulium allow efficient amplification in the S-band.2 Nonetheless, fluoride-glass technology is not as mature as its silica counterpart. The fabrication of fluoride-glass fiber must evolve before they become robust and reliable commodity products. This is a good example of how a specialty fiber can open up a new area of WDM communications.

When a fiber amplifier is used to amplify multiple channels simultaneously, as is the case with WDM applications, the available energy in the fiber is shared between all the channels. This means less amplification per channel, due to amplifier saturation. A higher pump power raises the saturation level of the amplifier so development of high-power pump lasers coupled into single-mode fibers is also crucial to WDM communications. There is, however, an alternative way to get higher pump power into a rare-earth doped fiber-broad-area laser diodes or arrays emit very high pump power. Although it is impossible to couple the output from these devices into a single-mode fiber, it is possible to do so in a multimode fiber. But the signal to be amplified must be single mode for propagation and amplification stability reasons. So the multimode pump and the single-mode signal must be combined into one fiber-a problem solved by using a double-clad rare-earth doped fiber (see Fig. 3).

The single-mode rare-earth doped core is inside the multimode pump region. If the pump signal overlaps with the doped core, it will end up being absorbed over a certain section of fiber. To optimize this overlap, the circular symmetry of the fiber has to be broken using either an off-center core or a rectangular, polygonial, or D-shaped pump region. The absorption in the doped core has to be made very strong in order to reach bleaching power (gain) with relatively low pump power densities. One way of achieving this is by co-doping-a first rare-earth dopant acts as an absorber that will then transmit its energy to the (second) dopant that provides gain in the needed band. This is exactly what happens when ytterbium is used in combination with erbium to make fiber boosters or double-clad fiber amplifiers.

Raman amplifiers also are gaining prominence. They offer gain in any transmission window, as long as the proper pump laser is chosen. Although exactly how these components will be used in the field remains to be seen, we believe they will most probably be used for complementary amplification between rare-earth doped amplifier nodes. Even if Raman amplification itself does not make use of specialty optical fibers, the Raman pump lasers probably will. Fibers designed to have high transmission power density favor Raman amplification. A Raman pump laser would benefit from such a specialty fiber. These fibers also can be used for functions involving optical nonlinear phenomena-wavelength conversion through cross-phase modulation, for example.

Photosensitive fibers

Fiber Bragg gratings have many applications in WDM systems. These include channel filters, add/drop filters, and fiber amplifier gain equalizers. The gratings are permanently written into an optical fiber by proper illumination along a specific section. And the fiber must be photosensitive to ultraviolet (UV) light in order for an efficient reflection grating to be imprinted in the fiber-the UV light creates a permanent refractive index change profile in the fiber.

Efficient fiber Bragg gratings can be written into a "standard" fiber if it has been previously treated with hydrogen. It is also possible to make specialty fibers that are photosensitive enough that the hydrogenation process can be omitted. Specialty optical fibers also can produce fiber Bragg gratings with improved spectral responses (see Fig. 4). In the case of hydrogen-treated "standard" fibers, writing a strong grating means having a larger FWHM (full-width at half-maximum) response due to saturation of the index change. It also creates undesirable side-lobes at shorter wavelengths due to coupling of radiation modes. Using a specialty fiber, these side-lobes can be reduced to less than 0.1dB for a grating stronger than 30 dB written in a few minutes. Such fibers become indispensable if fiber Bragg gratings are to be used as WDM filters without having to demultiplex the transmission bandwidth into sub-segments.

Dispersion compensation

These same photosensitive fibers also can be used to write chirped fiber Bragg grating dispersion compensators. These types of dispersion-compensating modules have been around for a while, but concern about their delay curve has slowed their development. Recently, it has been demonstrated that the delay curve of such grating-based dispersion compensators can be designed to be nearly perfectly linear. So now it seems likely that this type of module will ultimately replace dispersion-compensating fibers as the primary dispersion-compensating elements. The fiber Bragg grating dispersion compensators can accommodate one or more WDM channels. The intrinsic polarization-mode dispersion (PMD) of fiber Bragg grating dispersion compensators easily can be overcome by coupling them with the appropriate PMD compensator.

Dispersion compensation is becoming a hot topic with the advent of fiber amplifiers and the consequent growth of the optical transmission link over which dispersion accumulates. The current deployment of 10-Gbit/s high-bit rate channels, and the prospect of 40-Gbit/s channel rates are more reasons for dispersion compensators to be sought after.

Dispersion-compensating fibers also are considered specialty fibers. Their design can likely be improved so they can compete with the emerging fiber Bragg grating dispersion compensation technology. To improve their performance, the waveguide loss and nonlinear coefficient must be reduced without reducing their negative dispersion slope. The market for dispersion compensation is becoming so vast that other alternative technologies probably will be developed. No doubt these alternatives also will rely somewhat on specialty optical fibers with specific designs and unique characteristics.

High-attenuation fibers

Fibers with large attenuation that is flat over a certain spectral region are called high-attenuation fibers. They also are specialty fibers. It is perhaps curious that high-attenuation fibers are of interest when so much effort has previously gone into reducing fiber attenuation. In a context in which optical power must be controlled by the optical communication system user, however, optical attenuators are as important as optical amplifiers.

High-attenuation fibers are used to make fixed connector or patchcord type attenuators. Attenuators made with high-attenuation fibers have proven to be more robust and reliable than air-gap fixed-attenuators, for example. The flat spectral attenuation allows these types of connectors to be used in association with WDM systems and components. They have many applications, the most obvious of which is testing and measurement of optical components, systems, and sub-systems. Integrating these attenuators into optical fiber components is also occurring in some applications.

Future developments

These various types of specialty fibers are just the precursors of others to come. Already, fibers with optical lens applications are being commercially introduced.3 These fibers are not used as longitudinal waveguides, but sideways as laser diode collimators. The specialty optical fiber fabrication techniques may also prove to be an economical way of making GRIN lenses. New types of glass, such as fluoride or chalcogenide, might allow the opening of new WDM transmission windows. Photonic crystal or holey fibers are currently a hot topic in the fiber research arena. Their most promising application currently appears to be dispersion compensation.

REFERENCES:

1. E. Snitzer, Phys. Rev. Lett., 7, 444 (1961).

2. T. Komukai et al., Electron. Lett., 28(9), 830 (1992).

3. S. Doric, US Patent #5,607,492, March 4, 1997.

FIGURE 4. Comparison of two fiber Bragg gratings of similar strength written in "standard" and photosensitive optical fibers shows the superior spectral response of the specialty fiber grating.