Optical waveguides – With optical coupler
Reexamination Certificate
2002-03-12
2003-12-02
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
C385S039000, C385S027000, C359S199200, C359S199200, C398S048000, C398S079000
Reexamination Certificate
active
06658172
ABSTRACT:
INTRODUCTION
This invention relates to improved optical communication systems incorporating certain optical interleavers for de-multiplexing or multiplexing closely spaced optical channels carried by the system. This invention also relates to methods of making and using such optical systems.
BACKGROUND
There is a growing demand for increasing capacity—or bandwidth—of optical communications systems, including WAN and LAN systems. The internet has greatly increased the amount of information transmitted over optical lines used in telecommunication systems. The use of other systems, such as microwave links, coaxial cables, and copper wires is not as desirable because propagation loss can be higher and channel capacity lower, and they are susceptible to electro-magnetic interference (EMI). Optical systems have the capacity to carry optical information at rates of several megabytes per second to tens of gigabytes per second and higher. Optical system designers and operators have used higher transmission rates to push information faster along optical fibers, and have used multiplexing, including dense wavelength division multiplexing (DWDM), to increase the number of channels carried by a single optical waveguide or signal carrier. In this way, optical systems are more efficiently used. At a receiver end, channels of different wavelengths are generally separated by narrow band filters and then detected or further processed. Optical systems have been designed, for example, to divide the C-band (approximately 1,530-1,565 nm) into 40 channels with 100 GHz spacings, or even 80 channels at 50 GHz spacings. New technology and components are needed to further increase and manage bandwidth in existing and future optical systems. Interleavers are used in some systems designed to employ dense channel spacings
An interleaver, or de-interleaver used in the opposite direction, can function essentially as (or as part of) an optical router or switch or add/drop or the like, to permit a system with individual channel passband filters designed for wider channel spacing to effectively isolate individual channels having narrower channel spacings. A system with individual channel passband filters designed to operate at 100 GHz spacings, for example, can be operated at 50 GHz spacings, thereby doubling the channel count. The interleaver combines (or in the case of the de-interleaver, separates) two sets of complementary (i.e., non-overlapping) channels into a more densely packed set of channels. Stated in another way, the interleaver is capable of either multiplexing or de-multiplexing optical signals. That is, the wavelength spectrum carried by an optical system typically is divided into multiple individual channels, each capable of carrying a signal substantially of any signal carried by the other channels carried by the system. Typically, each channel is assigned or pre-allocated a narrow passband straddling a center wavelength, with substantially uniform spacing form the center wavelength of one channel to the center wavelength of the adjacent channel(s). Multiplexing and de-multiplexing the individual channels can be performed with selectively transparent filters designed to reflect all channels except the one channel to be added or dropped. Suitable passband filters are known to those skilled in the art, such as a Fabry-Perot filter comprising a single or preferably multi-cavity thin film coating deposited, e.g. by sputtering, on a surface of a suitable “bulk optic” i.e., a silica glass or other optically transparent substrate. As channel spacings decrease, that is, as wavelength spacing between the assigned center wavelengths of adjacent channels decrease, the difficulty and cost of producing suitable passband filters increases. Thus, while desirable increase in system capacity is achieved at higher channel counts (i.e., at closer channel spacings), there is an undesirable cost increase associated with the correspondingly more narrow passband filters. Accordingly, there is a substantial need for avoiding or reducing such passband filter cost increase.
It has been suggested to use interleavers to partially multiplex or de-multiplex channels in an optical system that employs wavelength division multiplexing. Alternating channels of a multiplexed signal, e.g., a first, third, fifth, etc. channel, are passed by the interleaver as a semi-multiplexed signal (or semi-demultiplexed signal), depending on the direction of operation), while the second, forth, sixth, etc., channels are not passed, but rather reflected back by the interleaver. In current optical telecommunications systems and other demanding optical system applications, it is now found that interleavers having improved optical performance characteristics are needed, including low insertion loss, improved passband shape, etc. Interleavers employing multi-cavity Fabry-Perot thin film coatings, as opposed to single cavity interleavers, are found to provide higher levels of optical performance. Multi-etalon interleavers, i.e., two-cavity and preferably three or higher cavity designs are believed to be necessary to provide optical performance quality and characteristics needed for many optical systems.
It has long been a recognized problem in this industry, that producing interleavers having good optical performance characteristics can be difficult and expensive. In addition, there are industry-recognized problems associated with producing structurally robust interleavers comprising etalons having desired, precise optical properties. Prior known interleavers have employed etalons of various designs, such as the etalons used in the interferometric optical devices of U.S. Pat. No. 6,125,220 to Copner et al. In the interleaver/de-interleaver devices of Copner et al, two glass interferometric end plates are separated by a spacer region where the etalon is formed. The spacer region is an air gap having a predetermined dimension. In adjustable Fabry-Perot devices, such as those disclosed in U.S. Pat. No. 5,283,845 to Ip, tuning of the center wavelength of the spectral passband of an etalon is achieved by varying the effective cavity length (spacing) between two end plates carrying thin film reflectors. More specifically, in Ip a piezo actuator is used, extending between the two end plates. By varying the electric power applied to the piezo actuator, the axial length of the actuator can be varied, and thus the gap between the end plates varied. As alternatives to piezo-electric actuators, the tuning mechanism may employ liquid crystals, temperature, pressure, and other mechanisms. In U.S. Pat. No. 6,040,932 to Duck et al, a system and method are discussed for de-multiplexing closely spaced optical channels separated from one another by a distance “d”. A periodic multi-cavity Fabry-Perot etalon having a free spectral range of “2d” (or an integer multiple of 2d) is coupled to a circulator for launching an input beam. A first composite optical signal carrying channels
1
,
3
, . . . n is reflected from the input port of the etalon and a second composite optical signal carrying channels
2
,
4
, . . . n−1 is transmitted through the etalon. Duck et al do not disclose how a suitable multi-cavity etalon could be constructed. Only the adjustable gap etalon of the Ip patent is cited, with no suggestion as to how multiple such adjustable etalons could be optically coupled. The piezo actuators and associated hardware would seemingly prevent optical contact of adjacent etalons. It is also a disadvantage that adjustable etalons as in Ip involve considerable assembly complexity and cost. Also, maintaining strict parallelism between the end plates can present additional difficulties.
It is an object of the present invention to provide a system for separating closely spaced channels in a wavelength band and to methods of making and using them. Additional objects and aspects of the invention and/or of certain preferred embodiments of the invention will be apparent from the following disclosure and detailed description.
SUMMARY
This invention, in accordance with a
Derickson Dennis J.
Ghislain Lucien P.
Scobey Michael A.
Stokes Loren F.
Banner & Witcoff LTD
Cierra Photonics Inc.
Healy Brian
Lin Tina M
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