Optical waveguides – With optical coupler
Reexamination Certificate
1999-03-12
2001-07-17
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
C385S018000, C385S019000, C385S024000, C385S037000, C359S199200, C359S199200
Reexamination Certificate
active
06263123
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical components, particularly optical components for use in wavelength division multiplexed (WDM) optical networks.
BACKGROUND OF THE INVENTION
The growing importance of wavelength division multiplexing (WDM) has exerted a need for a variety of optical components for various functions important to the quality of such networks.
In particular, in WDM optical networks it may be necessary to provide equalizers to compensate for differences in signal power levels in the multiple channels of a WDM transmission system. Such differences may arise, for example, because of loss variations or non-linear effect in optical fibers or non-flat gain spectra in optical amplifiers. Moreover, in optical networks, signals originating from different modes or taking different paths may need to be combined at some point and so require adjustment or equalization in their power levels. An example of equalization of multiple channels of a WDM transmission system is described in U.S. Pat. No. 5,745,271 that issued on Apr. 28, 1998 to J. E. Ford, D. A. B. Miller, M. C. Nuss and J. A. Walker.
Equalizers can also be used at WDM transmitter locations to provide channel equalization, or pre-emphasis, before the signals are launched into the transmission system. WDM equalizers could also be useful in optical network components, such as a cross-connect where signal equalization might improve cross-talk and signal-to-noise ratios.
Also important in a WDM optical network is the optical monitoring function to monitor optical spectra, particularly as the number of channels increases, wavelength tolerances narrow, and systems evolve towards all-optical networks.
Of increasing interest for such functions has been the use of micromechanical optical devices because of potential greater ruggedness, longer life and lower costs that such devices promise. Such devices show great promise in applications where the action is not especially wavelength dependent, as for example in the control of the total power level of a multichannel signal rather than of individual components.
There currently are available optical components that have been developed for use in video and computer projections that comprise two-dimensional arrays in horizontal rows or vertical columns of single-axis tilting digitally (bistable) settable micromechanical mirrors, such as the Digital Micromirror Device (DMD). These have been commercialized for use in video and computer projection systems in large arrays (greater than 800×600 pixels). Each pixel in the array, for example, may comprise a 16×16 micron aluminum-coated mirror that is separately addressable and can be tilted, for example, plus or minus ten degrees electronically. The pixel spacing may be 17 microns, resulting in an overall fill factor of about ninety percent. Moreover, it appears likely that further improvements will be forthcoming that will permit smaller size with similarly tight packing.
SUMMARY OF THE INVENTION
The present invention is based on the use of a two-dimensional micromirror array, for example, of the kind described. In particular, for controlled attenuation of components of a multiwavelength optical signal, such as particular channels of a WDM optical signal, the multiwavelength signal is supplied to a dispersive element, such as a prism, diffraction grating, or arrayed waveguide grating router (AWGR) that separates spatially the different wavelength components and provides a unique direction for each. A micromirror array of the kind described is positioned in the paths of the separated wavelength components such that the components of different wavelengths are incident on different portions of the micromirror array. By adjusting appropriately the number of mirrors in each of said different portions whose tilt is such as to transmit or redirect its incident light in a selected direction, there is controlled the incident light of a particular channel that is directed in the selected direction. Advantageously, by appropriate adjustment of the tilt, all the light redirected in the selected direction can be combined in a single output signal by the same dispersive element initially used to separate the input signal into the different wavelength components. Alternatively, the redirected light can be combined in a single output signal by a separate dispersive element.
Advantageously, the mirrors in the panel are arrayed in essentially horizontal rows and vertical columns, and each component signal is directed to be incident on a selected different column (row), or group of columns, of the array and the attenuation provided is controlled by a number, and thus the area of mirrors in such selected column (row) or group whose tilt is such that the incident light is not transmitted or reflected in a direction to be successfully combined in the output signal. It should be apparent that the light can be dispersed either to separate horizontally or vertically by the choice of orientation of the dispersive element. To simplify the discussion, it will be assumed hereafter that the different wavelength components are displaced in a horizontal direction to be incident on different vertical columns of mircromirrors with the understanding that by a ninety degree rotation a column becomes a row. It would also be advantageous if each component is spread a uniform amount in the vertical direction to utilize a large number of pixels in each vertical column so that the amount of light reflected in the selected direction is primarily dependent on the number, and thus the area of mirrors appropriately tilted in the column on which the light is incident.
It can be appreciated that because there can be available hundreds of columns, a large number of wavelength channels can be accommodated. In particular, in many cases, there will be sufficient columns to permit each channel to use a plurality of adjacent columns to reduce the waste of signal power. Moreover, because there also can be hundreds of rows, there can be hundreds of pixels in each column that are available for use so that the attenuation level introduced can be controlled with a continuous fine grain structure, even though each pixel is only digitally settable. Moreover, because of the large number of mirrors that can be assigned to each channel, the failure of a few mirrors would have little affect on the operation, and so be tolerable.
Additionally the alignment of the mirror columns and rows can be arranged to compensate for aberrations in the dispersive element or included optics that distort the shape of the beam that is incident on the panel.
In a different application, a micromirror array of the kind described can be used to form an optical monitor for use in a WDM optical network acting as spectrum analyzer to measure the optical power spectrum of the WDM, channel by channel, allowing determination of signal power, signal bandwidth noise spectrum, and signal-to-noise-ratio (SNR).
For such application, a multiwavelength optical signal, such as a WDM optical signal, is again supplied to a dispersive element for separating spatially the various channels and the dispersed components are made incident on a micromirror array of the kind described, so that the optical spectrum is dispersed across the plane formed by the micromirror array. In this application, the mirrors are operated so that at any one instant, only mirrors in a single column, or single unique group of adjacent columns corresponding to one channel of the array, are tilted to reflect the light in a chosen direction and this reflected light of each channel is collected in turn to provide an electrical signal whose amplitude can be recorded or displayed as a measure of the power level of the channel or wavelength accessed at the corresponding time. Because of the large number of possible columns available, fine grain resolution can be achieved. In some instances, it may be desirable to direct for detection only a known fraction of the signal to permit the use of the undetected portion for useful signal transmis
Bishop David J.
Giles Clinton R.
Lucent Technologies
Pak Sung
Sanghavi Hemang
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