Fused-fiber technology poised for growth

By ROBERT MANDRA and KEVIN SLOCUM

We've addressed issues related to the manufacture of dense wavelength-division multiplexing (DWDM) components on several occasions, highlighting the current supply shortage in high-performance components needed to build a communications infrastructure capable of meeting projected bandwidth demands. The design, manufacture, and packaging of cost-effective photonic components that can enable next-generation optical networks presents a number of significant technical challenges. Today's passive DWDM photonic components suffer from high manufacturing costs, inflexible and manually intensive manufacturing methods, and performance shortcomings such as high insertion losses, dispersion, temperature sensitivity, limited power-handling capabilities, and limited unit and channel scalability.

The component supply shortage, combined with the need for components with increasing levels of performance, has created a window of opportunity for new technologies to enter into this market. Technologies that address both manufacturing and performance issues will find consumers willing to explore new solutions. To this end, we remain enthusiastic about the future of components based upon fused-fiber technology. We've briefly mentioned this technology in a previous column but felt that it deserved a more thorough discussion due the impact we feel it is likely to have on the marketplace in the near future.

Fused-fiber technology is a means by which certain optical performance characteristics, such as wavelength discrimination and filtering, can be achieved through the appropriate splicing, stretching, and fusing of optical fibers. Fiber Bragg gratings can be used to further extend the functionality of such devices.

The use of fused and tapered fibers to provide wavelength filtering and power-splitting functions has historically been limited to use in broadband power splitting and multiplexing applications. However, new fabrication and packaging methods now make these fibers ideal candidates for emerging high-performance applications such as high-power amplification, narrow-spaced multiwavelength transmission, highly precise complex-gain flattening, and switching applications such as optical add/drop multiplexing. Enhancements to the traditional fiber-based devices have produced very-high-performance components that are gaining market acceptance. For example, fused-fiber devices have become popular for use in amplifier pump combining.

A wide variety of products can be manufactured using this technology, including DWDM multiplexers and demultiplexers, couplers, spectral filters, laser-pump combiners, slicers/interleavers, and optical add/drop multiplexers. Advantages of fused-fiber technology over current technology in the marketplace include:

• Low insertion losses. By processing the light in the fiber itself, fused-fiber components avoid many of the discontinuities competing technologies introduce into the optical path, thereby significantly reducing losses and backreflection.
• Low dispersion. For many applications, fused-fiber components can exhibit virtually negligible dispersion.
• High-power handling. Fused-fiber components can accommodate in excess of 1 W of optical power without reduction in performance.
• High spectral resolution. The structures employed to construct optical slicers or interleavers can multiplex signals separated by as little as 25 GHz; even lower channel separations have been achieved in laboratories.

Additionally, there are a number of advantages related to the manufacturing model this technology enables. The performance features of fused-fiber components are derived from the splicing and fusing of and writing of gratings in the fibers, resulting in:

• Cost-effectiveness. The use of optical fiber as the constituent medium should result in reduced materials supply and inventory costs.
• Scalability. Much of the process used to manufacture fused-fiber components can be automated, thereby utilizing minimal amounts of manual labor and enabling efficient scaling of manufacture.
• Flexibility. Since the performance features of the components are determined by the manufacturing process, components can be manufactured to custom specifications with minimal requirements on inventory and little disruption to work flow.

Several young companies are now developing DWDM products using fused-fiber technology. ITF Optical Technologies (Montreal, Canada) has developed a full line of components using its All-Fiber technology, which encompasses proprietary processes in the implementation of fused-fiber and Bragg-grating technologies. ITF's product line includes laser-pump combiners, multiplexer/demultiplexer components, optical slicers/interleavers, and gain-flattening filters. ITF (which stands for "in the fiber") has focused its energy completely on the development of components using its All-Fiber technology. The strength of ITF's focus is evident in its optical slicer/ interleaver. It is the only such interleaver which can separate DWDM channels separated by as little as 50 GHz and maintain temperature insensitivity over an operating temperature range from -5°C to +70°C.

"Since our router is completely passive, it is significantly less costly to implement than devices which re quire active temperature stabilization," states ITF CEO Eric Geoffrion. ITF has developed proprietary manufacturing tools and processes for All-Fiber technology that the company says will enable ITF to meet the volume needs of customers while maintaining the flexibility needed to meet individual customers needs.

Wavesplitter Technologies (Fremont, CA) is combining fused-fiber technology with more traditional technologies to deliver components with higher overall levels of performance. In this regard, Wavesplitter disclosed at OFC that along with marketing a line of fused-fiber components, it will also manufacture arrayed waveguide gratings (AWGs).

Dr. Cheau Chen, CEO of Wavesplitter, notes that, for large-channel-count systems, producing components based entirely on fused fiber becomes impractical since the size of the components become too large. "For high-channel-count applications," he reports, "Wavesplitter will offer hybrid solutions which will combine the high-resolution capabilities of fused-fiber components with the large-channel-count capabilities of AWGs." For example, a fused-fiber optical slicer can be used to separate 50-GHz DWDM channels into two streams of 100-GHz channels, at which point AWGs can be used to separate the individual channels.

Arroyo Optics (Santa Monica, CA) introduced a number of products at OFC. In addition to the benefits afforded by fused-fiber technology, Arroyo's products are designed to take advantage of the company's patented Fused Fiber Bragg Grating (FFBG) technology, which enables the company to write Bragg gratings into the junctions of fused fibers. Arroyo is the only manufacturer of fused-fiber devices we know that has this capability.

This technology is used in Arroyo's multiplexer/demultiplexer devices as well as both passive and active optical add/drop multiplexers. Arroyo's Switch ingFilter system is capable of multiplexing, adding, and dropping up to 40 DWDM chan nels with rela tively low loss. Switch ing times of 10 to 15 msec are achieved through the use of electromagnetic tuning of the Bragg gratings.

Kevin Slocum is a managing director and communications research analyst for Wit SoundView (Stamford, CT). He has more than 18 years of financial industry experience, including equity research, sales, and analysis. He can be reached at (203) 462-7219 or kslocum@witsoundviewcom.