Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1998-12-29
2001-03-06
Ngo, Hung N. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S047000, C385S034000
Reexamination Certificate
active
06198857
ABSTRACT:
The present invention is directed to an optical multiplexing device. More particularly, the invention is directed to an optical multiplexing device suitable for removing a single channel or other selected wavelength sub-range from the multiplexed signal of a system employing wavelength division multiplexing, and to re-inject a new signal at the same channel or within the same wavelength sub-range.
BACKGROUND
Wavelength division multiplexing of optical signals is finding widespread use in various fields, including especially for data transmission and other telecommunication applications. The use of wavelength division multiplexing in fiber-optic systems has gained interest as a feasible method of increasing data transfer capacity of a fiber-optic line and/or other waveguide. In particular, wavelength division multiplexing can increase capacity of a fiber-optic trunk line at substantially lower cost than the laying of additional new fiber-optic lines. Wavelength division multiplexing allows multiple signals at different wavelengths to be carried simultaneously by a fiber-optic line or other waveguide.
The increase in carrying capacity of a fiber-optic line can be approximately linearly proportional to the number of multiplexed channels. That is, for example, a fiber-optic system employing 16 channel wavelength division multiplexing has approximately sixteen times the carrying capacity or throughput at a given bit transfer rate as the same system not employing wavelength division multiplexing. Presently preferred wavelength bands for fiber-optic transmission media include those centered at 1.3 m and 1.55 m. The latter is especially preferred because of its minimal absorption and the commercial availability of erbium doped fiber amplifiers. The useful bandwidth is approximately 10 to 40 nm, depending on application. Wavelength division multiplexing can separate this bandwidth into multiple channels. Ideally, the 1.55 m wavelength band, for example, would be divided into multiple discreet channels, such as 4, 8, 16 or even as many as 32 or more channels, through a technique referred to as dense channel wavelength division multiplexing, as a low cost method of substantially increasing a waveguide's signal carrying capacity, such as long-haul telecommunication capacity over existing fiber-optic transmission lines. The International Telephony Union (ITU) Grid provides standard center wavelengths for channels in the 1.55 m wavelength band, at 100 Ghz spacing (approximately 0.8 nm). Wavelength division multiplexing may be used to supply telephony and data transmission and, more and more in the future, such services as video-on-demand and other existing or planned multimedia, interactive services. Techniques and devices are required, however, for multiplexing the different discreet carrier wavelengths. That is, the individual optic signals must be combined onto a common fiber-optical waveguide and then later separated again into the individual signals or channels at the opposite end of the fiber-optic cable. Thus, the ability to effectively combine and then separate individual channels (or wavelength bands) on a fiber-optic trunk line or other optical signal source is of growing importance to fiber-optic telecommunications and other fields.
Known devices for this purpose have employed, for example, diffraction gratings, prisms and various types of fixed or tunable filters. Gratings and prisms typically require complicated and bulky alignment systems and have been found to provide poor efficiency and poor stability under changing ambient conditions. Fixed wavelength filters, such as interference coatings, can be made substantially more stable. In this regard, quality interference coatings of metal oxide materials, such as niobia and silica, can be produced by commercially known plasma deposition techniques, such as ion assisted electron beam evaporation, ion beam sputtering, and reactive magnetron sputtering, e.g., as disclosed in U.S. Pat. No. 4,851,095 to Scobey et al and U.S. Pat. No. 5,525,199 to Scobey. Such coating methods can produce interference cavity filters formed of stacked dielectric optical coatings which are advantageously dense and stable, with low film scatter and low absorption, as well as low sensitivity to temperature changes and ambient humidity.
Optical multiplexing devices are known for combining the multiple channel signals at one end of a trunk line and for separating out the individual signals at the opposite end of the trunk line. That is, multiplexing here refers to adding channels, removing channels or both. For simplicity of explanation, only the demultiplexing functionality is described here in detail, since those skilled in the art will readily understand the correlative multiplexing functionality. That is, those skilled in the art will recognize how the same device can be employed in reverse. The term “multiplexing” will be used here to refer to both the combining and separating of channels. The term “trunk line” is used here to refer to any fiber-optic or other waveguide carrying a multi-channel optical signal, that is, a signal comprising multiple wavelength sub-ranges multiplexed together on the trunk line. It is known to optically couple a trunk line carrying multiple channels to a common port of a wavelength division multiplexer (“WDM”—this term is used here to mean devices which combine signals, separate signals or both). Such WDM common port is, in turn, optically coupled within the WDM to multiple channel ports. Associated with each channel port is an interference filter or the like which is substantially transparent to the wavelength band of that particular channel. Thus, signals having the wavelength assigned to a particular channel are passed by the WDM through its respective channel port to and/or from the individual waveguide for that channel.
Interference filters of the Fabry-Perot type, which are preferred in various multiplexing applications, typically transmit only a single wavelength or range of wavelengths. Multiple filter units can be used together in a WDM, e.g., on a common parallelogram prism or other optical block. Multiple optical filters are joined together, for example, in the multiplexing device of U.K. patent application GB 2,014,752A to separate light of different wavelengths transmitted down a common optical waveguide. At least two transmission filters, each of which transmits light of a different predetermined wavelength and reflects light of other wavelengths, are attached adjacent each other to a transparent substrate. The optical filters are arranged so that an optical beam is partially transmitted and partially reflected by each optical filter in turn, producing a zigzag light path. Light of a particular wavelength is subtracted or added at each filter. Similarly, in the multiplexing device of European patent application No. 85102054.5 to Oki Electric Industry Co., Ltd., a so-called hybrid optical wavelength division multiplexer-demultiplexer is suggested, wherein multiple separate interference filters of different transmissivities are applied to the side surfaces of a glass block. A somewhat related approach is suggested in U.S. Pat. No. 5,005,935 to Kunikani et al, wherein a wavelength division multiplexing optical transmission system for use in bi-directional optical fiber communications between a central telephone exchange and a remote subscriber employs multiple separately located multiplexers each having separate filter elements applied to various surfaces of a parallelogram prism.
In addition to multiplexing signals at opposite ends of a trunk line, systems employing wavelength division multiplexing have been evolving more complicated architectures, employing, for example, add/drop optical multiplexing devices for removing and/or injecting a single channel at any point along a trunk line. Filter devices for multiplexing a single wavelength subrange, and the use of a series of such devices for multiplexing multiple wavelength subranges in sequence, are shown for example, in U.S. Pat. No. 4,768,849 to Hicks, J
Grasis Michael E.
Lafreniere Robert W.
Scobey Michael A.
Spock Derek E.
Alden Philip G.
Corning Oca Corporation
Ngo Hung N.
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