Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2001-03-13
2002-12-17
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S010000, C385S036000, C385S037000, C359S199200, C359S199200
Reexamination Certificate
active
06496616
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to optical communication systems and components. More particularly, the invention relates to optical systems comprising micro- and nano-scale optical structures and components built from and upon monolithic substrates.
BACKGROUND OF THE INVENTION
In the field of telecommunication, it is recognized that optical communication components and systems, which use light waves and beams to carry information, offer many considerable advantages over conventional copper wire-based communication systems that carry information in the form of electrical signals. One advantage is the significantly greater amount of information that can be carried by a physical connection employing a single fiber optic strand as compared to a copper wire circuit.
To optimize the amount of information that can be transmitted along a single optical fiber, the technique known as Dense Wavelength Division Multiplexing (DWDM) is now being implemented. The use of DWDM is accelerating at a rapid pace, due to the development of essential network components such as optical fiber, infrared laser transmitters, fiber amplifiers, and the like. However, the rate of growth of DWDM networks is currently limited in part by the availability of low-cost mass produced components that provide acceptable reliability and resistance to environmental effects, such as, vibration, mechanical stresses, and temperature fluctuations.
In these optical telecommunications systems, information is transmitted in the form of infrared light signals that originate at laser sources. Each laser source is tuned to emit an infrared light beam comprising a narrow band of wavelengths (or, equivalently, frequencies) centered at a primary frequency. As used herein, the word “color” describes a characteristic band of wavelengths centered around a specific primary wavelength as emitted by a telecommunications laser. Information is encoded in each infrared beam by temporally modulating the laser power. Each primary frequency corresponds to a value specified by the standard International Telecommunication Union (ITU) grid. The ITU standard specifies transmission frequencies that are spaced at 100 GHz intervals, although a reduction to 50 or 25 GHz is anticipated as the technology evolves.
In DWDM, many distinct colors of infrared light may be transmitted simultaneously along a single optical fiber, each color carrying information that is distinct from information carried by other colors. Devices called multiplexers physically superimpose the light beams from several colors thereby creating multi-color light beams wherein each color carries its encoded information. The combined information is transmitted some distance. At a terminus of the transmission path, demultiplexers physically separate or disperse the multiple colors received from a single optical fiber onto multiple output fibers, each output fiber carrying a single color, thereby permitting the information carried by each color to be directed to its intended destination.
The ideal demultiplexer will direct all of the incoming light of each color onto its corresponding output optical fiber. However, in actual demultiplexer devices the color separation is generally imperfect—not all of the light of each color entering the demultiplexer is transmitted into each respective output beam, and a portion of the light from each color is transmitted into the paths of neighboring beams. This leakage causes undesirable performance effects such as crosstalk and insertion loss that must be limited in magnitude for the device to be practical.
At present, demultiplexing is often accomplished utilizing devices based upon either diffraction gratings or precision interference filters. Neither type of device is amenable to cost-effective mass production while maintaining acceptable performance.
For filter-based demultiplexers, such as those described in T. E. Stern, K. Bola, Multiwavelength Optical Networks, A Layered Approach, Addison Wesley, 1999, each filter must be manufactured separately from the others using multilayer vapor deposition techniques. The filters are then installed manually or robotically in an optical substrate and aligned to project light onto individual output optical fibers. Achieving and maintaining optical alignment in spite of thermal and mechanical stresses confounds attempts to reliably mass produce these devices, adding to the production cost and diminishing long-term reliability and resistance to environmental influences. Furthermore, the filters are frequently operated in a serial configuration, such that one color is transmitted through one filter while all other colors are reflected to the next, and so on. In this configuration, the insertion loss accumulates so that the last transmitted color has significantly higher loss than earlier colors.
Grating-based devices offer the advantage of being parallel rather than serial demultiplexers, and therefore have improved insertion loss uniformity. However, to achieve acceptable crosstalk, the optical components within grating-based devices must be several centimeters in size. Therefore, like filter-based devices, grating-based demultiplexers are difficult to align and maintain aligned. Low-cost mass production of reliable devices remains elusively difficult.
Recently, demultiplexers based on arrayed waveguide gratings (AWGs) have been introduced commercially. These small, thin monolithic devices, generally fabricated from silicon-based or InP-based wafer substrates, offer promise as low-cost components that can be mass produced. Nevertheless, despite more than a decade of intense development of AWG technology, and the emergence of several companies offering AWG products, the performance specifications achieved by production AWGs remain several orders of magnitude worse than theoretical predictions.
Monolithic demultiplexer devices based on waveguide gratings etched into wafers of semiconductor materials such as InP have been described in recent patent and technical literature. These devices also offer potential as low-cost mass-producible components but, like AWGs, have not yet achieved acceptable performance specifications. In particular, etched waveguide gratings demonstrated to date suffer from excessive crosstalk due to the small size and imperfections in fabrication of the grating structure.
SUMMARY OF THE INVENTION
The invention, in one embodiment, provides a miniature monolithic optical wavelength demultiplexer comprising an assembly of optical components built on a monolithic platform.
In one aspect, the invention relates to a miniature monolithic optical demultiplexer that includes a wavelength dispersing optical element that receives a multi-color light beam and separates the light spatially, providing a plurality of substantially single color optical beams, each substantially single color optical beam containing primarily a single color different from that of the other substantially single color optical beams. In some embodiments, a small portion of the light from other colors can remain in a substantially single-color optical beam after passing through the wavelength dispersing optical element. The demultiplexer also includes a wavelength filter array having at least one wavelength filter element. Each wavelength filter element receives one of the plurality of substantially single color optical beams and removes therefrom the portions of light from colors other than the primary color of that beam, thereby providing a purified single color output beam substantially free of light from other colors.
In some embodiments, one or more of the substrate, the wavelength dispersive optical element, and the wavelength filter array comprises at least one material selected from the group consisting of silicon, silicon monoxide, silicon dioxide, silicon-germanium alloys, silicon carbide, silicon nitride, and indium phosphide. In other embodiments, different materials can be employed. In one embodiment, the demultiplexer is preferably made from materials that are amenable to semiconductor fab
Davis Steven J.
Frish Michael B.
Keating Philip B.
Kessler William J.
Confluent Photonics, Inc.
Palmer Phan T. H.
Testa Hurwitz & Thibeault LLP
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