Integrated waveguide optics can be based on inorganic or organic materials of various types. Silica-on-silicon planar lightguide circuits offer versatility, easy manufacture, and high performance.
Bob Shine, Jerry Bautista, and Kevin Sullivan
While optical amplifiers and dense wavelength-division-multiplexing (DWDM) have enabled vast increases in optical-network transmission capacity, service providers must find ways to further reduce transmission costs. Integrated optics such as planar lightguide circuits (PLCs) are a promising technology to provide lower cost through high-volume wafer-scale processing, as well as through integration of multiple functions on a single substrate. Products made from PLCs can include passive splitters, arrayed waveguide gratings, variable optical attenuators, optical monitoring circuits, and even waveguide amplifiers. As might be expected from such a wide range of products, there are a number of technology choices. Planar-lightguide circuit components have been fabricated using silica, polymer, silicon-oxynitride, and silicon waveguides. The relative merits of these approaches can be analyzed by looking closely at the requirements for different products.
Designs and requirements
One of the simplest waveguide structures is a splitter. While fused fiber works very well for 1 x N splitters when N is relatively small, waveguide structures are ideal for multipath applications and can easily be formed into high-count (1 x 16 and 1 x 32) splitters. For this relatively simple component, the main parameters of interest are insertion loss (consisting of interface and waveguide-propagation losses) and cost.
Another very important PLC component is the arrayed waveguide grating (AWG). In an AWG, multibeam interference causes the simultaneous filtering of a large number of narrowly spaced DWDM channels. When the device is used as a demultiplexer, each wavelength is coupled into a separate output waveguide due to the different phase tilts created in the arrayed waveguide section. Key factors for this device include insertion loss, crosstalk, and passband shape; parameters dependent upon the waveguide designs. Since a large number of waveguides are required in an AWG, an extremely uniform refractive-index profile is required. Additionally, since this device is used in high-capacity DWDM systems in which gigabits and potentially terabits per second of information can be transmitted, reliability of the device is critical.
Active devices are possible
Active functions also can be added to PLCs by incorporating thermo-optic control of the waveguide refractive index. Devices such as variable optical attenuators and dynamic gain controllers are then possible. Attenuation is typically achieved by heating one arm of an interferometer to create a controlled amount of interference at the output. While the insertion loss is still of concern, other critical factors include the thermo-optic response time of the material and the power required to create a significant level of attenuation.
Another class of integrated components is achieved by integrating photodetectors with passive waveguide structures. In one example, a detector array is integrated with an arrayed waveguide grating to create an optical channel monitor. Fabrication of the device requires the ability to mount and wire-bond the detector array to the waveguide substrate.
A final product example based on PLC technology is the waveguide amplifier. In this device, the waveguide structure is doped with erbium atoms using an ion-exchange process. Waveguide amplification can be combined with the other products. An example of such combination is a fully loss-compensated splitter in which amplification is combined with a power splitter to compensate for losses introduced by the splitter. Here, the ability to incorporate erbium ions into the waveguide at sufficient dopant levels is required. The finished device must meet cost targets for the relevant metropolitan application.
A concern common to all the products mentioned above is the overall size of the device: even when all other parameters have been met, physical size will limit attempts to lower cost as a result of device yield per wafer. The device size is limited by the bend radius allowed by the refractive-index difference between the core and cladding layers. The table illustrates the waveguide-propagation parameters such as interface-coupling loss and bend radius achievable with three different refractive-index differences (D).1 The medium D in the table closely approximates the core-cladding difference for standard single-mode fiber; it has the lowest coupling loss, but requires the largest bend radius.
Waveguide technology choices
The silica (or silica-on-silicon) waveguide is by far the most-common material choice for PLCs, due to its refractive-index match to silica-based optical fiber. Two major types of deposition processes are widely used today: chemical-vapor deposition (CVD), and flame hydrolysis (FHD). The CVD approach, which is a modification of standard semiconductor-processing techniques, is compatible both with clean-room processes and high-volume wafer production. As an example, more than 100 wafers are loaded at a time into deposition chambers. A raised refractive index is created in the core-guiding region by adding phosphorus, germanium, or both during the deposition process (see Fig. 1).
The FHD deposition process is markedly different from CVD. Glass precursor chemicals are introduced into a hydrogen/oxygen torch in which a gentle flame hydrolyzes the chemicals to form the appropriate glass composition. The glass particles (roughly 0.1 mm in diameter) are deposited on the substrate—silica or a silicon wafer—in a thick, porous, and fluffy layer. Finally, this fragile structure is placed in a furnace and heated to consolidate the porous layer into a solid, clear glass layer free of bubbles or other defects.
From a product standpoint, the use of silica waveguides has a number of significant advantages. An index of refraction roughly equal to that of optical fiber minimizes losses at the fiber-chip interface. The CVD process is also very mature and allows low-cost volume production using wafer-processing technology similar to that developed for the semiconductor industry. The rather large dimensions of AWG devices—roughly 1 x 3 cm—makes it imperative that clean-room conditions prevail to avoid incorporation of particulates (a factor in favor of CVD processing). Use of a silicon substrate allows active components to be added to make products integrating active and passive components. One disadvantage of silica waveguides is the limited range of refractive-index differences that can be achieved, ultimately limiting the size reduction of individual devices.
Alternatives to silica
A waveguide can also be based on polymer. In this type of structure, a polymer layer is deposited on a substrate and patterned to create a waveguide. One advantage of polymer waveguides is that the chemistry of the waveguide can be almost continuously varied to control desired properties such as refractive index, thermal response, or dopant levels. Polymer waveguides have been used for switches because of their faster thermal response times than silica. In addition, a dopant can be added to a polymer waveguide in much higher concentrations that would be possible in a crystal structure, a property of interest for waveguide amplifiers.
Typically, polymer is deposited using spin coating. For many polymers, this process creates a preferential orientation of the polymer chains, in turn creating significant birefringence in the waveguide—a serious limitation of the device. Also, polymer waveguides tend to be very sensitive to moisture, leading to problems of reliability or requiring hermetic sealing. Finally, both the interface and waveguide losses are higher in polymer waveguides than in silica waveguides.
Yet another material for waveguides is silicon-oxynitride (SiON). In the fabrication process for SiON, silica is exposed to ammonia to create silicon-oxynitride; much of the original work for this process is being done by IBM (East Fishkill, NY). With the use of this material, the index-difference limitation of silica waveguides is overcome, making very high-contrast waveguides possible. High-contrast waveguides allow very tight bend-radii and hence the possibility of very small devices. However, as the index difference increases, the waveguide size decreases, making it harder to couple fiber to the waveguide. A large refractive-index difference also can produce higher waveguide propagation losses due to scattering. Propagation losses inherent in the SiON processing are also high. Typically, a silica overcoat is added to reduce the refractive-index difference at the waveguide boundaries.
Silicon is another waveguide material. Silicon allows for a high index-contrast, reducing device size but increasing fiber-to-waveguide interface loss. A key benefit of silicon is the process technology developed for the semiconductor industry. Automated assembly techniques can be used to integrate active components such as lasers or photodetectors, creating low-cost integrated devices. However, as with silicon-oxynitride, interface and propagation losses can be a challenge for products based on silicon.
Planar lightguide circuits are critical to the evolution of fiberoptic devices from discrete micro-optic components to integrated optical components. As a technology platform, PLCs offer a very wide range of capabilities, including integration of active and passive functions. While there are a number of different waveguide technologies, each having its advantages and drawbacks, silica-on-silicon waveguides offer a very good overall combination of optical performance, product range, and manufacturing processing capability.
REFERENCE
- K. Okamoto, Fundamentals of Optical Waveguides, Academic Press, New York, NY (2000).
BOB SHINE is director of marketing, JERRY BAUTISTA is chief technical officer and senior vice president of technology, and KEVIN SULLIVAN is vice president of engineering at WaveSplitter Technologies Inc., 46430 Fremont Blvd., Fremont, CA 94538; e-mail: [email protected].