Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
2002-10-25
2004-10-12
Mai, Huy (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By partial reflection at beam splitting or combining surface
C359S637000, C359S638000, C356S450000, C356S491000
Reexamination Certificate
active
06804063
ABSTRACT:
FIELD OF INVENTION
This invention relates generally to optical interferometry. In particular, the invention relates to methods, devices and device components employing interferometers and optical interference filters for processing optical signals. Optical interleavers having parallel phase control elements are described, which are particularly useful for wavelength division multiplexing and demultiplexing applications.
BACKGROUND OF INVENTION
Optical telecommunication systems are capable of efficient and accurate signaling at extremely high rates ranging several mega-bits per second to several tens of giga-bits per second. In addition, optical signaling techniques have significant advantages over non-optical communication methods, such as coaxial cable, copper wire and microwave transmission techniques, which include lower propagation loss, higher channeling capacity and insusceptibility to electromagnetic interference. As a result of these benefits, optical communication systems are prevalent in nearly all existing telecommunication networks and a great deal of research has been directed at developing purely optical telecommunications systems.
As worldwide telecommunications usage continues to expand, the need for greater data-carrying capacity has made potential gains in channeling capacity via optical telecommunications methods especially attractive. To provide additional data-carrying capacity without requiring new optical fiber transmission lines, coarse wavelength division multiplexing and dense wavelength division multiplexing techniques have developed over the last decade. Wavelength division multiplexing is used to increase the transmission capacity of fiber optic communication systems by allowing multiple wavelengths to be transmitted and received over a single optical fiber. In wavelength division multiplexing, a plurality of optical signals of different wavelength are multiplexed by coupling each signal to a common transmission line. The multiplexed transmission signal is then propagated over a single optical medium to a variety of receivers. When received, the multiplexed transmission signal is demultiplexed into discrete channels corresponding to individual wavelengths and detected by a receiver. Typically, signal demultiplexing is achieved by a variety of wavelength selective optical filtering devices including optical interference filters, birefringent filters, cutoff filters, prisms, diffraction gratings and fiber optic devices. Although wavelength division multiplexing provides a simple, effective and inexpensive way of increasing transmission capacity, the number of channels employable over a given wavelength domain is limited by cross talk between transmission channels. Cross talk refers to incomplete separation of selected and non-selected optical channels such that light corresponding to one or more non-selected optical channels remains in combination with a selected channel and is detected. As understood by those skilled in the art, cross talk degrades the overall efficiency and accuracy of an optical communication system and substantially limits the narrowest channel spacing achievable. Accordingly, the feasibility of wavelength division multiplexing technology is dependent on the development of high resolution, high throughput optical filters.
Adoption of universal standard transmission channels for fiber optic transmission promotes efficient application of wavelength division multiplexing. The International Telecommunication Union (ITU) has adopted a standard channel definition providing a 45 channel system over a wavelength range of 1520 nm to 1565 nm with a uniform channel spacing of 100 GHz (approximately 0.8 nm). The universal standard of telecommunication transmitting frequencies ensures intercompatibility of optical telecommunications systems and promotes realization of the full benefits of wavelength division multiplexing. As conventional thin film dielectric filters are not capable of efficiently and accurately separating the closely spaced transmission channels of the ITU frequency standard, an immediate need exists for more precise demultiplexing optical devices capable of high resolution, high throughout optical filtering.
Improvements in wavelength division multiplexing technology have focused on development of (1) optical devices capable of combining multiple optical signals corresponding to different transmission wavelengths or optical channels into a single fiber and (2) optical devices capable of separating multiplexed optical signals comprising of a plurality of data streams into discrete optical signals corresponding to selected transmission wavelengths or optical channels. In addition, these efforts have focused on developing optical signaling technology capable of supporting the use of more closely spaced transmission channels. One method of achieving these goals involves the development of optical interleavers suitable for multiplexing and demultiplexing optical signals. Interleavers provide multiplexer functionality by combining two or more streams of optical signals into a single, plural optical signal stream and provide demultiplexer functionality by separating a plural optical signal stream into individual optical signal stream components, typically corresponding to odd and even transmission channels. Four primary types of interleaver devices have emerged over the last several years: (1) interferometric optical interleavers, (2) dielectric thin film and birefringent filters, (3) planar wave guides and (4) fiber-based devices. Interferometric optical interleavers are especially promising for wavelength division multiplexing applications because of their low cost, wide free spectral range and fiber compatibility.
Interferometric optical interleavers are devices that replace at least one of the reflecting mirrors of a dual beam interferometer with a Gires-Toumois etalon (GT etalon). Over the last several years, interferometric optical interleavers have proven very useful in a variety of multiplexing and demultiplexing applications. Interferometric optical interleavers and deinterleavers operate by multiple-beam optical interference generated by the separation of an incident light beam into two sub-components that separately undergo phase modification, are coherently recombined and undergo constructive or destructive optical interference.
FIG. 1
illustrates an interferometric optical interleaver (
10
) of the prior art comprising a cube-type beam splitter (
15
) in optical communication with a GT etalon (
20
) and an air gap phase control element (
25
), which are positioned along orthogonal axes with respect to each other. During operation, an incident beam (
27
) is directed onto the beam splitter (
15
), which separates the incident beam into first beam component (
30
) and second beam components (
35
), propagating on axes that are orthogonal to one another. The first component is directed through an air gap phase control element (
25
) and is reflected back toward the cube-type beam splitter by an external reflector. The second component is directed onto a GT etalon (
20
) wherein it is further separated into a plurality of sub-beams by a partially reflective internal reflector and a highly reflective external reflector. First and second beam components are coherently combined at the cube-type beam splitter and undergo optical interference. The nature and extent of the optical interference experienced depends on the optical paths of each beam component and the reflectivities of the reflectors comprising the GT etalon and air gap phase control element. As a result of optical interference, only certain frequencies of light are transmitted through the interleaver as output beams (
40
) corresponding to the transmission bands of the optical filter. By selection of the appropriate optical path length difference for reflected and transmitted beam components, interferometric interleavers of the prior art are capable of providing transmission spectra comprising periodic, substantially square-wave transmission bands.
U.S. Pat. No.
Greenlee Winner and Sullivan P.C.
Mai Huy
Research Electro-Optics, Inc.
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