Optical waveguides – Integrated optical circuit
Reexamination Certificate
1999-07-12
2001-08-14
Healy, Brian (Department: 2874)
Optical waveguides
Integrated optical circuit
C385S015000, C385S011000, C385S031000, C385S037000, C385S001000, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06275623
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to dynamically controlling spectral power distributions within optical networks and particularly to controlling spectral power distributions between channels of wavelength division multiplexing systems.
2. Technical Background
Wavelength division multiplexing (WDM) systems, which convey a number of different wavelength channels, undergo various changes that unequally affect the transmission of the different channels. Wavelength-dependent propagation losses and repeater gains, system aging, environmental influences, and the addition or substitution of new wavelength-dependent components can all affect channel power distributions.
Such wavelength-dependent variations accumulating along WDM optical systems, sometimes referred to as “ripple”, lead to dissimilar received signal power among the channels and a worsening signal-to-noise ratio (SNR).
Since the weakest signal (channel) must be received with a reasonable SNR, system ripple constrains the maximum system reach.
Permanent spectral power adjustments are often incorporated into optical transmission systems. For example, some optical amplifiers are packaged with gain flattening filters to compensate for uneven gain profiles of the amplifiers. Maintenance adjustments, referred to as “trimming”, are sometimes made in established systems to compensate for planned or incidental changes to system profiles. We have recognized that optimized system performance can require continuous or periodic adjustments that compensate for less predictable or temporary fluctuations in the spectral response.
Tunable filters, particularly tunable fiber Bragg gratings, are available with spectral responses that can be shifted along the spectrum. Filter gratings are tuned by varying their periodicity under the control of an external force such as compression or stress. However, the system spectral transmission characteristics that vary over time are not easily counteracted by the shifting of narrow attenuation bands. Especially with respect to closely spaced signals along the spectrum, shifting attenuation bands can disturb adjacent signals.
SUMMARY OF THE INVENTION
Our invention in one or more of its various embodiments dynamically controls spectral power distributions among channels of wavelength division multiplexing (WDM) systems. Individual wavelength channels are separately attenuated in accordance with a desired power distribution among the channels. Real-time or other regular monitoring can be used to oversee the ongoing attenuations, and a control system can be used to further modify the individual attenuations based on any differences between the actual and desired spectral power distributions.
One implementation of our dynamically configurable spectral filter includes a wavelength dispersing system that receives an input beam incorporating a plurality of different wavelength channels and spatially separates the different channels according to their wavelengths. A spatial light modulator differentially affects the channels of the input beam depending on their spatial positions. A spectral monitor distinguishes optical power among the channels. The wavelength dispersing system also realigns the differentially affected channels into a common output beam. However, before doing so, a controller that receives the optical power information from the spectral monitor adjusts the spatial light modulator to achieve a predetermined power distribution among the channels in the output beam.
The controller preferably compares a monitored optical power distribution among the channels to a desired power distribution and adjusts the spatial light modulator to minimize differences between the monitored and desired power distributions. The spatial light modulator, the spectral monitor, and the controller are all preferably arranged in a feedback loop to iteratively reduce the differences between the monitored and desired power distributions among the channels. In addition, the spatial light modulator can also be controlled to attenuate wavelengths between the channels to improve signal-to-noise (SNR) ratios. The wavelength dispersing system preferably includes a diffraction grating that not only disperses the different wavelengths through one diffraction order for controlling the spatial separation between the channels but also disperses portions of the differentially affected channels through another diffraction order for directing the portions to the spectral monitor.
A polarizing system can be used to avoid the effects of polarization sensitivities throughout the filter. The input beam is divided into two polarizations. One of the two polarizations is rotated into alignment with the other, and the parallel polarizations propagate along similar optical paths through the spatial light modulator to reduce polarization-dependent losses. Preferably, the parallel polarizations follow similar optical paths through the wavelength dispersing system to further reduce polarization-dependent losses.
The spatial light modulator can function in a variety of ways, such as by directly attenuating amplitudes or by varying phases or polarities in combination with a directional multiplexing device that converts the phase or polarity variations into amplitude attenuations. A phase modulator can also be used in combination with a polarization dispersive element for attenuating amplitudes of the spatially dispersed wavelengths.
Another implementation of our dynamically configurable spectral filter includes a spatial light modulator that receives a plurality of spatially separated wavelength channels and modulates polarization directions of the channels depending on their relative spatial positions. A polarization-sensitive optic that exhibits different transmission efficiencies as a function of polarization direction aligns the separated channels into a common output beam at relative efficiencies corresponding to the polarization directions of the individual channels. A control system converts a monitored optical power distribution among the channels into a feedback adjustment of the spatial light modulator to achieve a desired power distribution among the channels in the output beam.
The polarization-sensitive optic is preferably a diffractive optic whose diffraction efficiency varies with the polarization direction. Several other functions can also be carried out by the same diffractive optic. For example, the diffractive optic preferably aligns major portions of the separated channels through one order of diffraction for constructing the common output beam and diverts remaining portions of the separated channels through another order of diffraction for carrying out the feedback adjustment. Operating in a retro-mode that retraces a path toward the input, the same diffractive optic can be used both to spatially separate the channels in advance of the spatial light modulator and to realign the separated channels returning from the spatial light modulator.
A polarization manager is preferably used to linearly polarize the channels before first encountering the polarization-sensitive diffractive optic. Mixed polarizations of the channels are converted into pairs of pure polarization states. The spatial light modulator can be arranged as a phase or polarization modulator that converts the linear polarizations of the channels into elliptical polarizations. The division of light between the orthogonal polarization axes of the channels affects the efficiency by which the wavelengths are further diffracted into realignment.
Optical paths through the new filter are preferably formed in a planar waveguide. The different wavelength channels are conveyed along an optical path that extends (a) past a wavelength disperser that spatially separates the different channels, (b) past a spatial light modulator that at least indirectly modulates individual amplitudes of the spatially separated channels, and (c) through a common output. A control loop includes another optical path that extends from the spa
Brophy Christopher P.
Liu Yongqian
Wigley Peter G.
Corning Incorporated
Eugene Stephens & Associates
Healy Brian
Ryan Thomas B.
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