As optical amplifiers evolve, new passive components are required. Developments in design and manufacturing of fiber-based components using fused couplers, tapered-fiber filters, or fiber Bragg gratings, offer advantages that will help manufacturers resolve issues.
When considering fiberoptic transmission systems, there is much more than just pumping 980- or 1480-nm light in erbium-doped fiber and using the ITU grid conventional C-band transmission window. Optical amplifiers continue to be the major technology driver of the evolution in fiberoptic transmission networks. Erbium-doped fiber amplifiers (EDFAs) were the enabling technology of the mid-1990s, but EDFAs have limitations with regard to noise, nonlinearities and gain window. The recent introduction of Raman amplifiers overcomes these limitations and opens up possibilities for optical amplification. New achievements in transmission capacities reach multiterabit levels, achieved by combining several solutions.
Currently, system design engineers choose between narrower channel spacing, from 200 GHz down to 100, 50, or even to 25 GHz (in the conventional wavelength C band between1525 and 1565 nm) or expanding toward the L band (1570 to 1620 nm). Even the short-wavelength band (S band, approximately 1440 to 1520 nm) is drawing attention. In fact, in the near future, the entire transmission window of silica optical fiber will most likely be utilized as new fibers are being developed—from fibers without the water absorption peak, at 1380 nm, to nonzero dispersion-shifted fibers (NZDSF, with zero dispersion at 1440 nm). Yet another way to increase the overall transmission capacity is by migrating to higher transmission speeds per channels, from 2.5 Gbit/s to 10 or 40 Gbit/s. Overall, next-generation optical devices will amplify more transmission signals at higher transmission speeds, using narrower spacing between channels, across longer distances, without regeneration.
Initially, the growth in WDM system capacities led to EDFA designs that used higher power 980- or 1480-nm pump lasers. Later on, amplifier designers used both types of pump lasers to benefit from combined advantages. Today's situation is similar—rather than displacing incumbent EDFA technology altogether, Raman amplifiers can be used in combination with EDFAs to amplify signals with the highest gain and lowest noise figures across a broad spectral window.
DESIGN GAINS, MANUFACTURING CHALLENGES
Gone are the days when single-pump 980- or 1480-nm EDFAs amplified 8- or 16-channel C-band WDM systems (see Fig. 1). To reach higher levels of performance, EDFAs and distributed Raman amplifiers now combine polarization and wavelength pumping schemes, resulting in complex multistage optical amplifiers (see Fig. 2 and Fig. 3). Consequently, the number of optical components required to manufacture optical amplifiers has increased dramatically. Furthermore, the new breed of optical amplifiers calls for increased requirements for optical components in terms of performance, power handling, and reliability.
Tradition has it that optical amplifiers are expensive, and their rapid cost reduction is being held off by a few factors: the first is the increasing complexity of amplifier designs requiring a large number of components. The second factor is the ever-increasing power-level requirements of pump-laser modules. A third important constraint is the lack of standardization for optical-amplifier designs. Amplifier designs are system specific and will most likely continue to be, particularly because of network configurations, types of transmission fiber used, and overall network capacity.
The first amplifier manufacturers' efforts were focused on the availability of pump-laser modules: 980 and 1480 nm for EDFAs. Pump-laser module manufacturers also achieved great progress in providing high-power pump-laser modules in mass production. Another important development was the availability of a wide range of 14xx-nm (ranging from 1400 to 1500 nm) pump laser modules, essential to the production of Raman amplifiers.
The amplifier manufacturers' focus is now on the availability of a range of passive optical components—pump stabilizers, polarization-pump combiners, wavelength-pump multiplexers, pump/signal couplers, gain-flattening filters, and tap couplers—now required to build multistage amplifiers. Most of these components must be available in scores of different configurations to satisfy wavelength and spacing specifications in 980- and 1480-nm EDFAs, and in Raman amplifiers for C-band, L-band, and S-band systems.
In this industry, characterized by a high level of customization and strict performance requirements, every tenth of a dB gain is valuable. Amplifier manufacturers are confronted with issues involving procuring, qualifying, and assembling all the components from numerous vendors, often resulting in design and performance compromises, as well as long lead-time nightmares. The key to solving these issues is to use a technology that lends itself to design and manufacturing flexibility, rapid prototyping, and large-scale low-cost production, while meeting the performance requirements for low insertion loss and high-power handling characteristics.
Integrated optical devices were long envisioned as the most promising solutions, with potential for high-volume manufacturability and component size reductions. But integrated optics, mostly using a combination of micro-optics and planar lightwave circuits, must cope with several major disadvantages, including costly and long design cycles, high insertion losses, and limited power handling characteristics.
The latest developments in design and manufacturing of fiber-based components (see "Three structures of fiber-based component technology," p. 72 ) using fused couplers, tapered-fiber filters, or fiber Bragg gratings, offer advantages that will help amplifier manufacturers resolve many issues. Fused-couplers are already the de facto choice for wideband multiplexing applications such as 980/1550-nm pump signal couplers, and power-splitting applications such as 980-, 1480-, or 1550-nm tap couplers, mainly because of their low loss and low cost.
Considerable improvement in the fusing and tapering process enables the production of narrowband fused couplers with spacing as small as 2 nm and with very precise center-wavelength alignment. Wavelength-pump multiplexers, built using these new narrowband couplers, allow for multiplexing from two to eight pump-laser modules, significantly increasing an amplifier's total available power. Channel spacing and center wavelengths are easily adjustable in any EDFA or Raman amplification bands. These wavelength pump multiplexers are characterized by very low insertion loss, typically 0.15 dB per coupler.
Another key optical component allowing for high-power multiplexing schemes in optical amplifiers is the polarization-pump combiner. Initially, these products were built using micro-optics, typically characterized by high insertion loss (0.5 to 0.7 dB) and limited power handling. A recent development in component technology was the introduction of fiber-based polarization-pump combiners. These components exhibit the same performance benefits as fiber-based wavelength pump multiplexers (with insertion loss of 0.25 dB) and precisely match the pump center wavelength (see Fig. 4). Fiber-based polarization-pump combiners and wavelength-pump multiplexers can be effectively used together and optimized to offer the highest power multiplexing with the lowest overall loss.
Fiber-Bragg-grating pump stabilizers are regularly utilized to ensure higher stability of the pump-laser power and a higher accuracy of its emission wavelength. Gain-flattening filters are built using either fiber Bragg gratings or tapered-fiber filters, depending on overall specification requirements. These gain flattening filters can match the most complex gain-curve specifications, with excellent optical characteristics (low insertion loss and error function) and temperature stability across a wide bandwidth, in any transmission bands.
Fiber-based components, built with fused couplers, tapered-fiber filters, or fiber Bragg gratings, complement each other and can, in fact, constitute a continuous fiber path from the pump-laser diode to the transmission fiber. Furthermore, all these components can be easily tuned or adjusted to any different set of wavelengths across the optical spectrum, thus guaranteeing the lowest insertion loss possible for maximum power transfer to the transmission signals.
Fiber-based end-to-end solutions are easily customized and can be designed and manufactured in very short time frames compared to other component technologies. New fabrication and packaging techniques have been developed to produce passive temperature-insensitive, high-performance, reliable fiber-based optical components.
Temperature stability is an extremely important parameter in high-performance optical components. Special passive temperature compensation and packaging techniques have been developed, making the fiber-based components temperature insensitive over a broad operating range, from -5°C to +70°C. Furthermore, with proper packaging, fiber-based components are highly reliable in hostile environments, meeting strict reliability tests such as Telcordia GR-1209 and GR-1221. In addition, high-power tests have been conducted to demonstrate fiber-based components' long-term power handling characteristics (see Fig. 5).
Due to their simple optical structure and packaging, fiber-based components offer relatively low cost per function compared to products manufactured using other technologies, and are projected to be among the most cost-effective passive components as production volumes are increased. Fiber-based component manufacturing processes are highly automated and computer-controlled with manufacturing yields much higher than traditional processes.
A flexible, manufacture-to-order production model enables custom requirements to be met with minimal inventory requirements and minimal disruption to the manufacturing process. Moreover, manufacturing capacities can be rapidly scaled-up because the manufacturing processes are faced with less labor, raw material and manufacturing equipment constraints compared with other component technologies.
Stéphane Bourgeois is the director of marketing—amplification products, ITF Optical Technologies, 45 Montpellier Blvd., Ville St-Laurent, Québec, H4N 2G3 Canada. He can be reached at 514-744-1044.
Three structures of fiber-based component technology
Fiber-based components are made by transforming the properties of the optical fiber itself, either by heating and reshaping the fiber or by modifying its photosensitivity. Because the light never leaves the fiber, with no discontinuities in the optical path, the integrity of the signal is preserved without degradation. Consequently, these fiber-based components offer the advantages of very low excess loss, near-zero polarization-mode dispersion and chromatic dispersion, and high-power handling characteristics.
Three basic fiber-based component structures can be made by modifying the fiber: fused couplers, tapered-fiber filters, and fiber Bragg gratings. By precisely controlling process parameters, components can be made that allow certain signals to pass while filtering unwanted signals.
Fused Couplers. Fiber-based couplers are made by pulling two laterally fused fibers, thus creating a two-mode fiber interferometer with a sinusoidal wavelength response. The length of the interferometer determines the phase difference between the modes: a large phase difference gives a small wavelength period. The resulting sinusoidal response can be used to multiplex two wavelengths. Fused couplers can also be used to combine two signals at the same wavelength but in different polarization states.
Because of their low loss (typically less than 0.15 dB), and their ability to handle large optical power (several Watts), fused couplers are ideal for wavelength or polarization-pump laser multiplexing in high-output EDFA or Raman amplifiers. Fused couplers are also used to manufacture ultralow-loss pump/signal couplers and signal and power taps.
Tapered-fiber filters. Tapered-fiber filters are made by nonadiabatically tapering a single fiber, thus creating a multimode cladding structure in which several modes interfere. Like the fused coupler, if the phase difference is large, the wavelength response will be narrow with the difference that the device preserves the circular symmetry of the fiber and is thus almost insensitive to polarization. The sine response is highly customizable in both amplitude and wavelength periods. Combining several filters can generate complex nonsinusoidal wavelength responses. Such assemblies are used to build highly precise wideband gain-flattening filters.
Fiber Bragg gratings. Fiber Bragg gratings are created by modifying the fiber refractive index along its length through exposure to ultraviolet light. Fiber Bragg gratings thus reflect certain wavelengths while transmitting others. The wavelength reflected and its intensity depends on the nature of the grating and the control of the manufacturing process. Fiber Bragg gratings are ideal to build pump-laser-diode stabilizers and gain-flattening filters.