Reconfigurable optical add-drop multiplexers employing polarization diversity

Abstract

This invention provides a novel wavelength-separating-routing (WSR) apparatus that uses a diffraction grating to separate a multi-wavelength optical signal by wavelength into multiple spectral channels, which are focused onto an array of corresponding channel micromirrors. The channel micromirrors are individually controllable and continuously pivotable to reflect the spectral channels into selected output ports. As such, the inventive WSR apparatus is capable of routing the spectral channels on a channel-by-channel basis and coupling any spectral channel into any one of the output ports. The WSR apparatus of the invention may further employ a polarization diversity scheme, whereby polarization-sensitive effects become inconsequential and insertion loss is minimized. The WSR apparatus of the invention may additionally be equipped with servo-control and channel equalization capabilities. The WSR apparatus of the invention can be used to construct a novel class of dynamically reconfigurable optical add-drop multiplexers (OADMs) for WDM optical networking applications.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application is a continuation-in-part of U.S. patent application Ser. No. 09 / 938,426, filed on Aug. 23, 2001, <?insert-start id="INS-S-00001" date="20070313" ?>now U.S. Pat. No. 6,625,346 <?insert-end id="INS-S-00001" ?>and which claims priority from U.S. Provisional Patent Application Ser. No. 60 / 277,217, filed on Mar. 19, 2001.BACKGROUND[0002]This invention relates generally to optical communication systems. More specifically, it relates to a novel class of dynamically reconfigurable optical add-drop multiplexers (OADMs) for wavelength division multiplexed optical networking applications.[0003]As fiber-optic communication networks rapidly spread into every walk of modern life, there is a growing demand for optical components and subsystems that enable the fiber-optic communications networks to be increasingly scalable, versatile, robust, and cost-effective.[0004]Contemporary fiber-optic communications networks commonly...

AI Technical Summary

Benefits of technology

[0017]The invention provides a polarization diversity wavelength-separating-routing (WSR) apparatus and method which minimizes insertion loss and polarization-dependent loss (PDL).

[0024]In another aspect, the WSR apparatus of the invention may include a servo-control assembly, in communication with the channel micromirrors and the output ports. The servo-control assembly serves to monitor the optical power levels of the spectral channels coupled into the output ports and further provide control of the channel micromirrors on an individual basis, so as to maintain a predetermined coupling efficiency of each spectral channel into one of the output ports. As such, the servo-control assembly provides dynamic control of the coupling of the spectral channels into the respective output ports and actively manages the optical power levels of the spectral channels coupled into the output ports. (If the WSR apparatus includes an array of collimator-alignment mirrors as described above, the servo-control assembly may additionally provide dynamic control of the collimator-alignment mirrors.) Moreover, the utilization of such a servo-control assembly effectively relaxes the requisite fabrication tolerances and the precision of optical alignment during assembly of a SR apparatus of the invention, and further enables the system to correct for shift in optical alignment over the course of operation. A WSR apparatus incorporating a servo-control assembly thus described is termed a WSR-S apparatus, in the following discussion.

[0028]3) The coupling of the spectral channels into the output ports is dynamically controlled by a servo-control assembly, rendering the OADM less susceptible to environmental effects (such as thermal and mechanical disturbances) and therefore more robust in performance. By maintaining an optimal optical alignment, the optical losses incurred by the spectral channels are also significantly reduced.

[0030]5) The use of free-space optics provides a simple, low loss, and cost-effective construction. Moreover, the utilization of the servo-control assembly effectively relaxes the requisite fabrication tolerances and the precision of optical alignment during initial assembly, enabling the OADM to be simpler and more adaptable in structure, and lower in cost and optical loss.

[0031]6) The use of a polarization diversity scheme renders the polarization-sensitive effects inconsequential in the OADM. This enables the OADM to minimize the insertion loss; and enhance spectral resolution in a simple and cost-effective construction (e.g., by making use of high-dispersion diffraction grating commonly available in the art). The polarization diversity scheme further allows the overall optical paths of the two polarization components for each spectral channel to be substantially equalized, thereby minimizing the polarization-dependent loss. Such attributes would be particularly desirable in WDM optical networking applications.

Problems solved by technology

Irrespective of the underlying architecture, the OADMs currently in the art are characteristically high in cost, and prone to significant optical loss accumulation.

Moreover, the designs of these OADMs are such that it is inherently difficult to reconfigure them in a dynamic fashion.

Although the aforementioned OADM disclosed by Askyuk et al. has the advantage of performing wavelength separating and routing in free space and thereby incurring less optical loss, it suffers a number of limitations.

This additional multiplexing / demultiplexing requirement adds more cost and complexity that can restrict the versatility of the OADM thus-constructed.

Second, the optical circulators implemented in this OADM for various routing purposes introduce additional optical losses, which can accumulate to a substantial amount.

There are, however, no provisions provided for maintaining the requisite alignment; and no mechanisms implemented for overcoming degradation in the alignment owing to environmental effects such as thermal and mechanical disturbances over the course of operation.

Moreover, as in the case of Askyuk et al., there are no provisions provided for maintaining requisite optical alignment in the system, and no mechanisms implemented for combating degradation in the alignment due to environmental effects over the course of operation.

As such, the prevailing drawbacks suffered by the OADMs currently in the art are summarized as follows:1) The wavelength routing is intrinsically static, rendering it difficult to dynamically reconfigure these OADMs.2) Add and / or drop channels often need to be multiplexed and / or demultiplexed, thereby imposing additional complexity and cost.3) Stringent fabrication tolerance and painstaking optical alignment are required.

Moreover, the optical alignment is not actively maintained, rendering it susceptible to environmental effects such as thermal and mechanical disturbances over the course of operation.4) In an optical communication network, OADMs are typically in a ring or cascaded configuration.

There lacks, however, a systematic and dynamic management of the optical power levels of various spectral channels in these OADMs.5) The inherent high cost and optical loss further impede the wide application of these OADMs.

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