Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
2002-08-02
2004-10-05
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
C398S084000
Reexamination Certificate
active
06801310
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical communications using separation techniques to control individual wavelengths of dense wavelength division multiplexed beams, and more particularly to systems and methods which use liquid crystal spatial light modulation to efficiently modify individual wavelength components that have arbitrary states of polarization.
BACKGROUND OF THE INVENTION
Next generation optical networks will focus on economically exploiting the inherent bandwidth of optical fiber. A primary technique now used for that purpose is dense wavelength division multiplexing (DWDM), which propagates a number of wavelength signals or channels each separated in accordance with standardized protocols to provide 25, 50, 100 GHz, etc., channel separation at specified data rates, in consequence of which the bandwidth of optical fiber is more effectively utilized. Optical routing, agile wavelength provisioning, and precise wavelength management are key characteristics needed for these next generation communication systems. These systems must provide the end user with the ability to perform attenuation, blocking and switching in a channel independent fashion.
Optical communication systems, however, impose a number of particular and demanding requirements on units that are to modify, block, split, equalize or add individual wavelengths. These requirements involve among other things the need for low chromatic dispersion, low polarization mode dispersion, low polarization dependent frequency and loss, and other factors which constitute, at various levels, unacceptable aberrations in optical signals. Such factors are becoming increasingly stringent as data rates are increased and wavelength separations are reduced. For example, even at 50 GHz separation, prior art units have difficulty in meeting existing performance needs on a per wavelength basis. Optical systems with these dense channel separations are even less tolerant of optical signal aberrations which compromise insertion loss and channel isolation. Additional performance factors, such as linear attenuation within a wide dynamic range and high extinction ratio, can also be of crucial importance, depending on the system application.
Various techniques are known for beam control along a pixel array, but the leading approach in the field of optical communications is to utilize liquid crystal spatial light modulators (LC-SLMs). LC-SLMs consist of an array of individual pixels or cells that can be individually controlled electrically and suitably miniaturized. Extensive development and production work has been done on these devices for large scale projection and flat panel television systems. The properties needed for optical communication cells and the special needs of data transmission, however, impose different demands. For optical communications, the cells can be used in analog fashion to function as attenuators, limiters, or equalizers, or they can also or separately be used in a digital fashion, in which case they function to control optical beams between essentially full transmission and full extinction (>40 dB) states. In display applications, the modulation of the optical properties of the cells can accommodate a degree of unwanted variations, because a display will still appear consistent or flicker-free to the viewer. Fiber optic applications, however, require a much more precise and uniform optical response and freedom from subtle aberrations, and therefore demand new and unique designs to meet optical systems requirements.
Operative cells in LC-SLM arrays can be disposed within existing optical modules, provided that they meet specific performance specifications, because they are sized and spaced for accepting spatially separated wavelength signals and have low power demands. Therefore they can conveniently be used in systems such as routers, multiplexers, demultiplexers and dynamic equalizers for multiple channels in optical networks. Systems with LC-SLM arrays can in turn be incorporated into broader system designs, so as to provide new system and method capabilities. For example, a new multichannel optical filter system is now known which disposes a complementary combination of Fourier optics and diffraction gratings in a very compact configuration employing multiple three dimensional folding at small angles of two dimensional beam patterns. The system generates, from an input wavelength multiplexed signal, a plurality of parallel, spatially and spectrally distributed but closely adjacent beams. Individual wavelength components can be modified statically, dynamically, or in a predetermined and preset fashion, and then can be recombined by three dimensional folding to provide a reconfigured WDM output. The introduction of a dynamic multi-cell control array to modulate the individual demultiplexed beams enables generation of a wide range of possibilities for new optical communications networks.
Liquid crystals for switch and/or attenuator applications have been described generically in numerous papers and patents. For example, Ranalli et al. (U.S. Pat. No. 6,285,500) describe a wavelength selective switch utilizing diffraction gratings, a twisted nematic LC cell, an LC-SLM linear array, and a complex four-port configuration with polarization independence achieved by splitting the s and p polarizations at the input into separated paths which propagate in parallel through the filter, to be recombined at the outputs by polarization beam splitters. Liquid crystal SLM arrays have been used in numerous designs of all-optical crossconnects. The techniques and processes to fabricate LC-SLM components are clearly quite advanced and are based on years of design and manufacturing development derived from flat panel displays. Patel (U.S. Pat. No. 6,252,644) describes a wedge shaped liquid crystal cell which provides high extinction by introducing a gradient in thickness of the twisted nematic liquid crystal cell in the lateral direction. The application of this type of cell to a WDM switch is described, for example, in the article “Liquid crystal and grating based multiple wavelength cross connect switch,” IEEE Photonics Technology Letters, vol. 7, no. 5, May 1995, pp. 514-516.
Wu et al. (U.S. Pat. Nos. 6,175,432 and 6,134,358) describe a multi-wavelength crossconnect utilizing polarization beam splitters, birefringent crystal filter elements and liquid crystal polarization rotators. Liu et al. (U.S. Pat. No. 6,208,442) describe a programmable optical multiplexer based on similar concepts. Wong et al. (U.S. Pat. No. 6,201,593) disclose a multiple wavelength, twisted nematic (TN) liquid crystal switch with an extinction ratio in excess of 25 dB, using a twisted nematic cell with specially angled LC alignment layers.
Pan (U.S. Pat. No. 6,181,846) describes a fiberoptic liquid crystal on-off switch and variable attenuator which operate in reflection mode. This device has a single input fiber and a single output fiber, and selectively directs all or a part of input beams into the output port.
The liquid crystal devices described in the prior art are highly specialized devices tailored to particular applications. Apart from these applications, they do not address the general needs of optical communications networks, or confront the specific problems faced where LC-SLMs are to be used with diffractive Fourier optical systems. The potential that these systems have for very flat passbands, sharp spectral roll-off, low polarization dependence, low adjacent channel crosstalk and low chromatic dispersion should not be diminished to any meaningful extent by the modulation techniques that are used. However, diffraction gratings do not distribute wavelength components uniformly in space, only inputs of arbitrary states of polarization are accepted, and the systems are so physically compact and beam paths so dense that minute variations in alignment or angle can alter operating performance in a negative manner. Furthermore, the liquid crystal cells themselves have unique characteristics which must be taken into account. Th
Kewitsch Anthony S.
Leyva Victor
Rakuljic George
Arroyo Optics Inc.
Evans F. L.
Geisel Kara
Jones Tullar & Cooper P.C.
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