Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
1999-07-26
2003-11-11
Pascal, Leslie (Department: 2633)
Optical communications
Multiplex
Wavelength division or frequency division
C398S087000
Reexamination Certificate
active
06647209
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to an optic device, more particularly to an optical multiplexer and demultiplexer for dense wavelength division multiplexed (“DWDM”) fiber optic communication systems, and more particularly still to a method and apparatus for adding and dropping signals at intermediate nodes along a network line in a DWDM system.
BACKGROUND
The impact of advances in photonics technology in the area of communication systems has been dramatic. By way of example, new communication system architectures have been proposed based on such photonics technology. These communication architectures take advantage of the ability of optical fibers to carry very large amounts of information—with very little marginal cost once the optical fiber is in place.
Photonics communication system architectures based on optical wavelength division multiplexing (WDM) or optical frequency division multiplexing (coherent techniques) to increase the information carrying potential of the optical fiber systems are being developed. For WDM systems, a plurality of lasers are used with each laser emitting a different wavelength. In these types of systems, devices for multiplexing and demultiplexing the optical signals into or out of a single optical fiber are required. Early WDM systems used a wide wavelength spacing between channels. For example, the bandwidth of a &lgr;=1310 nm link was increased by adding a 1550 nm channel. Fiber optic directional coupler technology was used to multiplex such widely spaced wavelength channels. Since optical fiber system performance is best when optimized for use at a single wavelength window, optimum WDM systems use several closely spaced wavelengths within a particular wavelength window. Currently, the telecommunications industry is working towards the deployment of dense wavelength division multiplexed (DWDM) systems with up to 32 channels in the 1530 to 1565 nm wavelength window—with adjacent channels separated in wavelength by 8 angstroms (100 GHz optical frequency spacing). Future developments envision channel wavelength separations of 4 angstroms (50 GHz optical frequency spacing).
Several technologies are being developed to provide for DWDM. These include micro-optical devices, integrated optic devices, and fiber optic devices. Micro-optical devices use optical interference filters and diffraction gratings to combine and separate different wavelengths. Integrated optic devices utilize optical waveguides of different lengths to introduce phase differences so that optical interference effects can be used to spatially separate different wavelengths. Fiber optic devices utilize Bragg gratings fabricated within the light guiding regions of the fiber to reflect narrow wavelength bands.
Micro-optical devices utilizing diffraction devices have been proposed in the literature (See, e.g., W. J. Tomlinson, Applied Optics, vol. 16, no. 8, pp. 2180-2194, 1977; J. P. Laude,
Technical Digest of the Third Integrated Optics and Optical Fiber Communication Conference
, San Francisco, 1981, pp. 66-67; R. Watanabe et. al., Electronics Letters, vol.16, no. 3, pp. 106-107, 1980; Y. Fujii et. al., Applied Optics, vol. 22, no. 7, pp. 974-978, 1983). These references describe generally how diffraction gratings can be used for WDM. However, to meet the needs of DWDM fiber optic communication systems, high performance is required with respect to parameters such as polarization dependent loss, cross talk, return loss, and insertion loss. In order to meet the specifications for these DWDM performance parameters, the incorporation of additional optical elements to effectively use the wavelength multiplexing and demultiplexing capabilities possible with diffraction gratings is required.
Initial development of wavelength division multiplexed fiber systems were primarily point to point systems. However, there is increasing interest in being able to add and drop signals at intermediate nodes along the network main line. Also of interest is the ability to adjust the network configuration to satisfy changing traffic demands and service requirements.
Therefore, there arises a need for a high performance optical apparatus and method for use in a DWDM system. The present invention directly addresses and overcomes the shortcomings of the prior art by providing DWDM with low polarization dependent loss (<0.5 dB), low insertion loss with single mode fiber optic systems (<5 dB), low cross talk between wavelength channels (<35 dB for 100 GHz channel separation and <30 dB for 50 GHz channel separation), and low return loss (<55 dB). There is also provided the ability to accomplish network reconfiguration by switching any channel between add/drop and passthrough status.
SUMMARY
The present invention provides for an optical multiplexer and demultiplexer for dense wavelength division multiplexed (“DWDM”) fiber optic communication systems. In one preferred embodiment of the present invention, a device may be constructed in accordance with the principles of the present invention as a multiplexer. This device functions to spatially combine the optical signals from several laser sources (each of which is a different wavelength) and launch the spatially combined laser beams into a single optical fiber. In a second preferred embodiment of the present invention, a device may be constructed in accordance with the principles of the present invention as a demultiplexer. Here the device functions to spatially separate the different wavelengths of a wavelength division multiplexed optical link and launch each of the different wavelengths into a different optical fiber.
In the preferred embodiments described herein, the device includes both bulk optic and integrated optic components. The spatial separation or spatial combination of laser beams of different wavelength is achieved with the use of bulk diffraction gratings. Also, bulk optical components are used to collimate and shape (or steer) the free space propagating laser beams to enable efficient coupling of light into single mode optical fibers, or integrated optic waveguides, and to reduce optical cross talk. Polarizing beamsplitters orient the polarization direction of the light to enable maximum diffraction efficiency by the gratings and to reduce the polarization dependent loss.
Another feature of the present invention is that the end faces of optical fibers and integrated optic waveguides are angle polished to reduce back reflection and thereby reduce noise caused by feedback to the laser source. Preferably, the diffraction grating and focusing optics are specified to permit multiplexing and demultiplexing of laser wavelengths separated by 0.4 nanometers (nm) in the 1550 nm wavelength band. The preferred field of view of the optics permit multiplexing and demultiplexing of up to 32-48 wavelength channels separated by 0.4 nanometers in the 1550 nm wavelength band. Although examples of performance are provided for the 1550 nm optical wavelength band, the device components can be designed for use at other wavelength bands, e.g., the optical fiber low absorption loss band at &lgr;~1310 nm.
Therefore, according to one aspect of the invention, there is provided a bidirectional optical apparatus, of the type which is used in connection with optical signals generated by a plurality of laser sources and which is carried by optical fibers, the apparatus comprising: an optical fiber; multiplexer means for spatially combining the optical signals from several laser sources, each of which is a different wavelength, and launching the spatially combined optical signals into a single optical fiber to form a wavelength division multiplexed optical signal; and demultiplexer means for spatially separating the different wavelengths from a single optical fiber carrying a wavelength division multiplexed optical signal and launching each of the different wavelengths into a separate optical fiber.
According to another aspect of the invention, there is provided a bi-directional optical apparatus, comprising: means for collimating th
Boord Warren Timothy
Jain Anil K.
APA Optics, Inc.
Pascal Leslie
Singh Dalzid
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