Reconfigurable optical add-drop multiplexers with servo control and dynamic spectral power management capabilities

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 then 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 present invention may be further equipped with servo-control and spectral power-management capabilities, thereby maintaining the coupling efficiencies of the spectral channels into the output ports at desired values. The WSR apparatus of the present invention can be used to construct a novel class of dynamically reconfigurable optical add-drop multiplexers (OADMs) for WDM optical networking applications.

Claims

.[. 1. An optical add-drop apparatus comprising an input port for an input multi-wavelength optical signal having first spectral channels; one or more other ports for second spectral channels; an output port for an output multi-wavelength optical signal; a wavelength-selective device for spatially separating said spectral channels; a spatial array of beam-deflecting elements positioned such that each element receives a corresponding one of said spectral channels, each of said elements being individually and continuously controllable .Iadd.in two dimensions .Iaddend.to reflect its corresponding spectral channel to a selected one of said ports .Iadd.and to control the power of the spectral channel reflected to said selected port..Iaddend. .].

.[. 2. The optical add-drop apparatus of claim 1 further comprising a control unit for controlling each of said beam-deflecting elements. .].

.[. 3. The optical add-drop apparatus of claim 2, wherein the control unit further comprises a servo-control assembly, including a spectral monitor for monitoring power levels of selected ones of said spectral channels, and a processing unit responsive to said power levels for controlling said beam-deflecting elements. .].

.[. 4. The optical add-drop apparatus of claim 3, wherein said servo-control assembly maintains said power levels at predetermined values. .].

.[. 5. The optical add-drop apparatus of claim 2, wherein the control unit controls said beam-deflecting elements to direct selected ones of said first spectral channels to one or more of said second ports to be dropped as second spectral channels from said output multi-wavelength optical signal. .].

.[. 6. The optical add-drop apparatus of claim 2, wherein the control unit controls said beam-deflecting elements to direct selected ones of said second spectral channels to said output port to be added to said output multi-wavelength optical signal. .].

.[.7. The optical add-drop apparatus of claim 1 further comprising alignment mirrors for adjusting alignment of said input and output multi-wavelength optical signals and said second spectral channels with said wavelength-selective device..].

.[.8. The optical add-drop apparatus of claim 7 further comprising collimators associated with said alignment mirrors, and imaging lenses in a telecentric arrangement with said alignment mirrors and said collimators..].

.[. 9. The optical add-drop apparatus of claim 1, wherein said wavelength selective device further combines selected ones of said spectral channels reflected from said beam-deflecting elements to form said output multi-wavelength optical signal. .].

.[. 10. The optical add-drop apparatus of claim 1, wherein said one or more other ports comprise an add port and a drop port for respectively adding second and dropping first spectral channels. .].

.[. 11. The optical add-drop apparatus of claim 1 further comprising a beam-focuser for focusing said separated spectral channels onto said beam deflecting elements. .].

.[. 12. The optical add-drop apparatus of claim 1, wherein said wavelength-selective device comprises a device selected from the group consisting of ruled diffraction gratings, holographic diffraction gratings, echelle gratings, curved diffraction gratings, and dispersing prisms. .].

.[. 13. The optical add-drop apparatus of claim 1, wherein said beam-deflecting elements comprise micromachined mirrors. .].

.[.14. The optical add-drop apparatus of claim 1, wherein said beam-deflecting elements comprise reflective membranes..].

.[. 15. An optical add-drop apparatus, comprising an input port for an input multi-wavelength optical signal having multiple spectral channels; an output port for an output multi-wavelength optical signal; one or more drop ports for selected spectral channels dropped from said multi-wavelength optical signal; a wavelength-selective device for spatially separating said multiple spectral channels; and a spatial array of beam-deflecting elements positioned such that each element receives a corresponding one of said spectral channels, each of said elements being individually and continuously controllable .Iadd.in two dimensions .Iaddend.to reflect its corresponding spectral channel to a selected one of said ports .Iadd.and to control the power of the spectral channel reflected to said selected port, .Iaddend.whereby a subset of said spectral channels is directed to said drop ports. .].

.[. 16. An optical add-drop apparatus, comprising an input port for an input multi-wavelength optical signal having multiple spectral channels; an output port for an output multi-wavelength optical signal; one or more add ports for selected spectral channels to be added to said output multi-wavelength optical signal; a wavelength-selective device for reflecting said multiple and said selected spectral channels; and a spatial array of beam-deflecting elements positioned such that each element receives a corresponding one of said spectral channels, each of said elements being individually and continuously controllable .Iadd.in two dimensions .Iaddend.to reflect its corresponding spectral channel to a selected one of said ports .Iadd.and to control the power of the spectral channel reflected to said selected port, .Iaddend.whereby said spectral channels from said add ports are selectively provided to said output port. .].

.[. 17. A method of performing dynamic add and drop in a WDM optical network, comprising: separating an input multi-wavelength optical signal into spectral channels; imaging each of said spectral channels onto a corresponding beam-deflecting element; and controlling dynamically and continuously said beam-deflecting elements .Iadd.in two dimensions .Iaddend.so as to combine selected ones of said spectral channels into an output multi-wavelength optical signal .Iadd.and to control the power of the spectral channels combined into said output multi.Iaddend.-.Iadd.wavelength optical signal..Iaddend. .].

.[. 18. The method of claim 17, wherein said selected ones of said spectral channels comprises a subset of said spectral channels, such that other non-selected ones of said spectral channels are dropped from said output multi-wavelength optical signal. .].

.[. 19. The method of claim 18, wherein said controlling comprises reflecting said non-selected ones of said spectral channels to one or more drop ports. .].

.[. 20. The method of claim 17 further comprising imaging other spectral channels onto other corresponding beam-deflecting elements, and controlling dynamically and continuously said other beam-deflecting elements so as to combine said other spectral channels with said selected ones of said spectral channels into said output multi-wavelength optical signal. .].

.[. 21. The method of claim 17, wherein said imaging comprises focusing said spectral channels onto said beam-deflecting elements. .].

.[. 22. The method of claim 17 further comprising monitoring a power level in one or more of said selected ones of said spectral channels, and controlling an alignment between said input multi-wavelength optical signal and corresponding beam-deflecting elements in response to said monitoring. .].

.Iadd.23. An optical add-drop apparatus comprising an output port and fiber collimators serving as an input port and one or more other ports, the apparatus comprising: the fiber collimator input port for an input multi-wavelength optical signal having first spectral channels; the fiber collimator one or more other ports for second spectral channels; the output port for an output multi-wavelength optical signal; a wavelength-selective device for spatially separating said spectral channels; a spatial array of beam-deflecting elements positioned such that each element receives a corresponding one of said spectral channels, each of said elements being individually and continuously controllable in two dimensions to reflect its corresponding spectral channel to a selected one of said output port or the fiber collimator ports and to control the power of the spectral channel reflected to said output port or the fiber collimator selected port..Iaddend.

.Iadd.24. The optical add-drop apparatus of claim 23 further comprising a control unit for controlling each of said beam-deflecting elements..Iaddend.

.Iadd.25. The optical add-drop apparatus of claim 24, wherein the control unit further comprises a servo-control assembly, including a spectral monitor for monitoring power levels of selected ones of said spectral channels, and a processing unit responsive to said power levels for controlling said beam-deflecting elements..Iaddend.

.Iadd.26. The optical add-drop apparatus of claim 25, wherein said servo-control assembly maintains said power levels at predetermined values..Iaddend.

.Iadd.27. The optical add-drop apparatus of claim 24, wherein the control unit controls said beam-deflecting elements to direct selected ones of said first spectral channels to one or more of said fiber collimator other ports to be dropped as second spectral channels from said output multi-wavelength optical signal..Iaddend.

.Iadd.28. The optical add-drop apparatus of claim 24, wherein the control unit controls said beam-deflecting elements to direct selected ones of said second spectral channels to said output port to be added to said output multi-wavelength optical signal..Iaddend.

.Iadd.29. The optical add-drop apparatus of claim 23 further comprising alignment mirrors for adjusting alignment of said input and output multi-wavelength optical signals and said second spectral channels with said wavelength-selective device..Iaddend.

.Iadd.30. The optical add-drop apparatus of claim 29 further comprising collimators associated with said alignment mirrors, and imaging lenses in a telecentric arrangement with said alignment mirrors and said collimators..Iaddend.

.Iadd.31. The optical add-drop apparatus of claim 23, wherein said wavelength selective device further combines selected ones of said spectral channels reflected from said beam-deflecting elements to form said output multi-wavelength optical signal..Iaddend.

.Iadd.32. The optical add-drop apparatus of claim 23, wherein said fiber collimator one or more other ports comprise a fiber collimator add port and a fiber collimator drop port for respectively adding second and dropping first spectral channels..Iaddend.

.Iadd.33. The optical add-drop apparatus of claim 23 further comprising a beam-focuser for focusing said separated spectral channels onto said beam deflecting elements..Iaddend.

.Iadd.34. The optical add-drop apparatus of claim 23, wherein said wavelength-selective device comprises a device selected from the group consisting of ruled diffraction gratings, holographic diffraction gratings, echelle gratings, curved diffraction gratings, and dispersing prisms..Iaddend.

.Iadd.35. The optical add-drop apparatus of claim 23, wherein said beam-deflecting elements comprise micromachined mirrors..Iaddend.

.Iadd.36. The optical add-drop apparatus of claim 23, wherein said beam-deflecting elements comprise reflective membranes..Iaddend.

.Iadd.37. The optical add-drop apparatus of claim 23: wherein none of the multi-wavelength optical signal, the second spectral channels, or the output multi-wavelength optical signal are transmitted through a circulator..Iaddend.

.Iadd.38. The optical add-drop apparatus of claim 23, wherein each of the fiber collimators is a GRIN lens..Iaddend.

.Iadd.39. The optical add-drop apparatus of claim 23, wherein the fiber collimator input port, the one or more other fiber collimator ports, and the output port are arranged in a one dimensional array..Iaddend.

.Iadd.40. The optical add-drop apparatus of claim 23, wherein the fiber collimator input port, the one or more other fiber collimator ports, and the output port are arranged in a two dimensional array..Iaddend.

.Iadd.41. The optical add-drop apparatus of claim 23, wherein each of the beam-deflecting elements is configured to be capable of reflecting its corresponding spectral channel to each and every one of the fiber collimators ports..Iaddend.

.Iadd.42. The optical add-drop apparatus of claim 23, wherein the output port is separate from the fiber collimator input port..Iaddend.

.Iadd.43. The optical add-drop apparatus of claim 23, wherein the output port is three or more output ports that are separate from the fiber collimator input port..Iaddend.

.Iadd.44. The optical add-drop apparatus of claim 23, further comprising a power-management system configured to manage power levels of at least one of the first spectral channels and the second spectral channels..Iaddend.

.Iadd.45. The optical add-drop apparatus of claim 44, wherein the power-management system is further configured to control coupling efficiency of one of the first and second spectral channel to at least one port..Iaddend.

.Iadd.46. The optical add-drop apparatus of claim 23, wherein the beam-deflecting elements are micromirrors..Iaddend.

.Iadd.47. An optical add-drop apparatus, comprising: a fiber collimator serving as an input port for an input multi-wavelength optical signal having multiple spectral channels; an output port for an output multi-wavelength optical signal; one or more fiber collimators serving as one or more drop ports for selected spectral channels dropped from said multi-wavelength optical signal; a wavelength-selective device for spatially separating said multiple spectral channels; and a spatial array of beam-deflecting elements positioned such that each element receives a corresponding one of said spectral channels, each of said elements being individually and continuously controllable in two dimensions to reflect its corresponding spectral channel to a selected one of said output port or the fiber collimators serving as said ports and to control the power of the spectral channel reflected to said output port of the fiber collimator serving as said selected port, whereby a subset of said spectral channels is directed to said fiber collimator serving as said drop ports..Iaddend.

.Iadd.48. The optical add-drop apparatus of claim 47, wherein none of the input or output multi-wavelength optical signal or the spectral channels are transmitted through a circulator..Iaddend.

.Iadd.49. An optical add-drop apparatus, comprising: a fiber collimator serving as an input port for an input multi-wavelength optical signal having multiple spectral channels; an output port for an output multi-wavelength optical signal; one or more fiber collimators serving as one or more add ports for selected spectral channels to be added to said output multi-wavelength optical signal; a wavelength-selective device for reflecting said multiple and said selected spectral channels; and a spatial array of beam-deflecting elements positioned such that each element receives a corresponding one of said spectral channels, each of said elements being individually and continuously controllable in two dimensions to reflect its corresponding spectral channel to a selected one of said output port or the fiber collimator serving as said ports and to control the power of the spectral channel reflected to said output port or said fiber collimator serving as said selected port, whereby said spectral channels from said fiber collimators serving as said add ports are selectively provided to said output port..Iaddend.

.Iadd.50. The optical add-drop apparatus of claim 49, wherein none of the input or output multi-wavelength optical signal or the spectral channels are transmitted through a circulator..Iaddend.

.Iadd.51. A method of performing dynamic add and drop in a WDM optical network, comprising: separating an input multi-wavelength optical signal into spectral channels; imaging each of said spectral channels onto a corresponding beam-deflecting element; controlling dynamically and continuously said beam-deflecting elements in two dimensions so as to combine selected ones of said spectral channels into an output multi-wavelength optical signal and to control the power of the spectral channels combined into said output multi-wavelength optical signal; receiving the output multi-wavelength optical signal at an output port that transmits the output multi-wavelength optical signal to an optical fiber; and wherein said selected ones of said spectral channels comprises a subset of said spectral channels, such that other non-selected ones of said spectral channels are dropped from said output multi-wavelength optical signal; and said controlling comprises reflecting said non-selected ones of said spectral channels to one or more fiber collimator serving as drop ports..Iaddend.

.Iadd.52. The method of claim 51 further comprising imaging other spectral channels onto other corresponding beam-deflecting elements, and controlling dynamically and continuously said other beam-deflecting elements so as to combine said other spectral channels with said selected ones of said spectral channels into said output multi-wavelength optical signal..Iaddend.

.Iadd.53. The method of claim 51, wherein said imaging comprises focusing said spectral channels onto said beam-deflecting elements..Iaddend.

.Iadd.54. The method of claim 51 further comprising monitoring a power level in one or more of said selected ones of said spectral channels, and controlling an alignment between said input multi-wavelength optical signal and corresponding beam-deflecting elements in response to said monitoring..Iaddend.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIGS. 1A-1D show a first embodiment of a wavelength-separating-routing (WSR) apparatus according to the present invention, and the modeling results demonstrating the performance of the WSR apparatus;

(2) FIGS. 2A-2C depict second and third embodiments of a WSR apparatus according to the present invention;

(3) FIG. 3 shows a fourth embodiment of a WSR apparatus according to the present invention;

(4) FIGS. 4A-4B show schematic illustrations of two embodiments of a WSR-S apparatus comprising a WSR apparatus and a servo-control assembly, according to the present invention;

(5) FIG. 5 depicts an exemplary embodiment of an optical add-drop multiplexer (OADM) according to the present invention; and

(6) FIG. 6 shows an alternative embodiment of an OADM according to the present invention.

DETAILED DESCRIPTION

(7) In this specification and appending claims, a spectral channel is characterized by a distinct center wavelength and associated bandwidth. Each spectral channel may carry a unique information signal, as in WDM optical networking applications.

(8) FIG. 1A depicts a first embodiment of a wavelength-separating-routing (WSR) apparatus according to the present invention. By way of example to illustrate the general principles and the topological structure of a wavelength-separating-routing (WSR) apparatus of the present invention, the WSR apparatus 100 comprises multiple input/output ports which may be in the form of an array of fiber collimators 110, providing an input port 110-1 and a plurality of output ports 110-2 through 110-N (N3); a wavelength-separator which in one form may be a diffraction grating 101; a beam-focuser in the form of a focusing lens 102; and an array of channel micromirrors 103.

(9) In operation, a multi-wavelength optical signal emerges from the input port 110-1. The diffraction grating 101 angularly separates the multi-wavelength optical signal into multiple spectral channels, which are in turn focused by the focusing lens 102 into a spatial array of distinct spectral spots (not shown in FIG. 1A) in a one-to-one correspondence. The channel micromirrors 103 are positioned in accordance with the spatial array formed by the spectral spots, such that each channel micromirror receives one of the spectral channels. The channel micromirrors 103 are individually controllable and movable, e.g., pivotable (or rotatable) under analog (or continuous) control, such that, upon reflection, the spectral channels are directed into selected ones of the output ports 110-2 through 110-N by way of the focusing lens 102 and the diffraction grating 101. As such, each channel micromirror is assigned to a specific spectral channel, hence the name channel micromirror. Each output port may receive any number of the reflected spectral channels.

(10) For purposes of illustration and clarity, only a selective few (e.g., three) of the spectral channels, along with the input multi-wavelength optical signal, are graphically illustrated in FIG. 1A and the following figures. It should be noted, however, that there can be any number of the spectral channels in a WSR apparatus of the present invention (so long as the number of spectral channels does not exceed the number of channel mirrors employed in the system). It should also be noted that the optical beams representing the spectral channels shown in FIG. 1A and the following figures are provided for illustrative purpose only. That is, their sizes and shapes may not be drawn according to scale. For instance, the input beam and the corresponding diffracted beams generally have different cross-sectional shapes, so long as the angle of incidence upon the diffraction grating is not equal to the angle of diffraction, as is known to those skilled in the art.

(11) In the embodiment of FIG. 1A, it is preferable that the diffraction grating 101 and the channel micromirrors 103 are placed respectively at the first and second (i.e., the front and back) focal points (on the opposing sides) of the focusing lens 102. Such a telecentric arrangement allows the chief rays of the focused beams to be parallel to each other and generally parallel to the optical axis. In this application, the telecentric configuration further allows the reflected spectral channels to be efficiently coupled into the respective output ports, thereby minimizing various translational walk-off effects that may otherwise arise. Moreover, the input multi-wavelength optical signal is preferably collimated and circular in cross-section. The corresponding spectral channels diffracted from the diffraction grating 101 are generally elliptical in cross-section; they may be of the same size as the input beam in one dimension and elongated in the other dimension.

(12) It is known that the diffraction efficiency of a diffraction grating is generally polarization-dependent. That is, the diffraction efficiency of a grating in a standard mounting configuration may be considerably higher for P-polarization that is perpendicular to the groove lines on the grating than for S-polarization that is orthogonal to P-polarization, especially as the number of groove lines (per unit length) increases. To mitigate such polarization-sensitive effects, a quarter-wave plate 104 may be optically interposed between the diffraction grating 101 and the channel micromirrors 103, and preferably placed between the diffraction grating 101 and the focusing lens 102 as is shown in FIG. 1A. In this way, each spectral channel experiences a total of approximately 90-degree rotation in polarization upon traversing the quarter-wave plate 104 twice. (That is, if a beam of light has P-polarization when first encountering the diffraction grating, it would have predominantly (if not all) S-polarization upon the second encountering, and vice versa.) This ensures that all the spectral channels incur nearly the same amount of round-trip polarization dependent loss.

(13) In the WSR apparatus 100 of FIG. 1A, the diffraction grating 101, by way of example, is oriented such that the focused spots of the spectral channels fall onto the channel micromirrors 103 in a horizontal array, as illustrated in FIG. 1B.

(14) Depicted in FIG. 1B is a close-up view of the channel micromirrors 103 shown in the embodiment of FIG. 1A. By way of example, the channel micromirrors 103 are arranged in a one-dimensional array along the x-axis (i.e., the horizontal direction in the figure), so as to receive the focused spots of the spatially separated spectral channels in a one-to-one correspondence. (As in the case of FIG. 1A, only three spectral channels are illustrated, each represented by a converging beam.) Let the reflective surface of each channel micromirror lie in the x-y plane as defined in the figure and be movable, e.g., pivotable (or deflectable) about the x-axis in an analog (or continuous) manner. Each spectral channel, upon reflection, is deflected in the y-direction (e.g., downward) relative to its incident direction, so to be directed into one of the output ports 110-2 through 110-N shown in FIG. 1A.

(15) As described above, a unique feature of the present invention is that the motion of each channel micromirror is individually and continuously controllable, such that its position, e.g., pivoting angle, can be continuously adjusted. This enables each channel micromirror to scan its corresponding spectral channel across all possible output ports and thereby direct the spectral channel to any desired output port. To illustrate this capability, FIG. 1C shows a plot of coupling efficiency as a function of a channel micromirror's pivoting angle , provided by a ray-tracing model of a WSR apparatus in the embodiment of FIG. 1A. As used herein, the coupling efficiency for a spectral channel is defined as the ratio of the amount of optical power coupled into the fiber cone in an output port to the total amount of optical power incident upon the entrance surface of the fiber (associated with the fiber collimator serving as the output port). In the ray-tracing model, the input optical signal is incident upon a diffraction grating with 700 lines per millimeter at a grazing angle of 85 degrees, where the grating is blazed to optimize the diffraction efficiency for the 1 order. The focusing lens has a focal length of 100 mm. Each output port is provided by a quarter-pitch GRIN lens (2 mm in diameter) coupled to an optical fiber (see FIG. 1D). As displayed in FIG. 1C, the coupling efficiency varies with the pivoting angle , and it requires about a 0.2-degree change in for the coupling efficiency to become practically negligible in this exemplary case. As such, each spectral channel may practically acquire any coupling efficiency value by way of controlling the pivoting angle of its corresponding channel micromirror. This is also to say that variable optical attenuation at the granularity of a single wavelength can be obtained in a WSR apparatus of the present invention. FIG. 1D provides ray-tracing illustrations of two extreme points on the coupling efficiency vs. curve of FIG. 1C; on-axis coupling corresponding to =0, where the coupling efficiency is maximum; and off-axis coupling corresponding to =0.2 degrees, where the representative collimated beam (representing an exemplary spectral channel) undergoes a significant translational walk-off and renders the coupling efficiency practically negligible. All in all, the exemplary modeling results thus described demonstrate the unique capabilities of the WSR apparatus of the present invention.

(16) FIG. 1A provides one of many embodiments of a WSR apparatus according to the present invention. In general, the wavelength-separator is a wavelength-separating means that may be a ruled diffraction grating, a holographic diffraction grating, an echelle grating, a dispersing prism, or other types of spectral-separating means known in the art. The beam-focuser may be a focusing lens, an assembly of lenses, or other beam-focusing means known in the art. The focusing function may also be accomplished by using a curved diffraction grating as the wavelength-separator. The channel micromirrors may be provided by silicon micromachined mirrors, reflective ribbons (or membranes), or other types of beam-deflecting elements known in the art. And each micromirror may be pivoted about one or two axes. What is important is that the pivoting (or rotational) motion of each channel micromirror be individually controllable in an analog manner, whereby the pivoting angle can be continuously adjusted so as to enable the channel micromirror to scan a spectral channel across all possible output ports. The underlying fabrication techniques for micromachined mirrors and associated actuation mechanisms are well documented in the art, see U.S. Pat. No. 5,629,790 for example. Moreover, a fiber collimator is typically in the form of a collimating lens (such as a GRIN lens) and a ferrule-mounted fiber packaged together in a mechanically rigid stainless steel (or glass) tube. The fiber collimators serving as the input and output ports may be arranged in a one-dimensional array, a two-dimensional array, or other desired spatial pattern. For instance, they may be conveniently mounted in a linear array along a V-groove fabricated on a substrate made of silicon, plastic, or ceramic, as commonly practiced in the art. It should be noted, however, that the input port and the output ports need not necessarily be in close spatial proximity with each other, such as in an array configuration (although a close packing would reduce the rotational range required for each channel micromirror). Those skilled in the art will know how to design a WSR apparatus according to the present invention, to best suit a given application.

(17) A WSR apparatus of the present invention may further comprise an array of collimator-alignment mirrors, for adjusting the alignment of the input multi-wavelength optical signal and facilitating the coupling of the spectral channels into the respective output ports, as shown in FIGS. 2A-2B and 3.

(18) Depicted in FIG. 2A is a second embodiment of a WSR apparatus according to the present invention. By way of example, WSR apparatus 200 is built upon and hence shares a number of the elements used in the embodiment of FIG. 1A, as identified by those labeled with identical numerals. Moreover, a one-dimensional array 220 of collimator-alignment mirrors 220-1 through 220-N is optically interposed between the diffraction grating 101 and the fiber collimator array 110. The collimator-alignment mirror 220-1 is designed to correspond with the input port 110-1, for adjusting the alignment of the input multi-wavelength optical signal and therefore ensuring that the spectral channels impinge onto the corresponding channel micromirrors. The collimator-alignment mirrors 220-2 through 220-N are designated to the output ports 110-2 through 110-N in a one-to-one correspondence, serving to provide angular control of the collimated beams of the reflected spectral channels and thereby facilitating the coupling of the spectral channels into the respective output ports according to desired coupling efficiencies. Each collimator-alignment mirror may be rotatable about one axis, or two axes.

(19) The embodiment of FIG. 2A is attractive in applications where the fiber collimators (serving as the input and output ports) are desired to be placed in close proximity to the collimator-alignment mirror array 220. To best facilitate the coupling of the spectral channels into the output ports, arrays of imaging lenses may be implemented between the collimator-alignment mirror array 220 and the fiber collimator array 110, as depicted in FIG. 2B. By way of example, WSR apparatus 250 of FIG. 2B is built upon and hence shares many of the elements used in the embodiment of FIG. 2A, as identified by those labeled with identical numerals. Additionally, first and second arrays 260, 270 of imaging lenses are placed in a 4-f telecentric arrangement with respect to the collimator-alignment mirror array 220 and the fiber collimator array 110. The dashed box 280 shown FIG. 2C provides a top view of such a telecentric arrangement. In this case, the imaging lenses in the first and second arrays 260, 270 all have the same focal length f. The collimator-alignment mirrors 220-1 through 220-N are placed at the respective first (or front) focal points of the imaging lenses in the first array 260. Likewise, the fiber collimators 110-1 through 110-N are placed at the respective second (or back) focal points of the imaging lenses in the second array 270. And the separation between the first and second arrays 260, 270 of imaging lenses is 2f. In this way, the collimator-alignment mirrors 220-1 through 220-N are effectively imaged onto the respective entrance surfaces (i.e., the front focal planes) of the GRIN lenses in the corresponding fiber collimators 110-1 through 110-N. Such a telecentric imaging system substantially eliminates translational walk-off of the collimated beams at the output ports that may otherwise occur as the mirror angles change.

(20) FIG. 3 shows a fourth embodiment of a WSR apparatus according to the present invention. By way of example, WSR apparatus 300 is built upon and hence shares a number of the elements used in the embodiment of FIG. 2B, as identified by those labeled with identical numerals. In this case, the one-dimensional fiber collimator array 110 of FIG. 2B is replaced by a two-dimensional array 350 of fiber collimators, providing for an input-port and a plurality of output ports. Accordingly, the one-dimensional collimator-alignment mirror array 220 of FIG. 2B is replaced by a two-dimensional array 320 of collimator-alignment mirrors, and first and second one-dimensional arrays 260, 270 of imaging lenses of FIG. 2B are likewise replaced by first and second two-dimensional arrays 360, 370 of imagining lenses respectively. As in the case of the embodiment of FIG. 2B, the first and second two-dimensional arrays 360, 370 of imaging lenses are placed in a 4-f telecentric arrangement with respect to the two-dimensional collimator-alignment mirror array 320 and the two-dimensional fiber collimator array 350. The channel micromirrors 103 must be pivotable biaxially in this case (in order to direct its corresponding spectral channel to any one of the output ports). As such, the WSR apparatus 300 is equipped to support a greater number of the output ports.

(21) In addition to facilitating the coupling of the spectral channels into the respective output ports as described above, the collimator-alignment mirrors in the above embodiments also serve to compensate for misalignment (e.g., due to fabrication and assembly errors) in the fiber collimators that provide for the input and output ports. For instance, relative misalignment between the fiber cores and their respective collimating lenses in the fiber collimators can lead to pointing errors in the collimated beams, which may be corrected for by the collimator-alignment mirrors. For these reasons, the collimator-alignment mirrors are preferably rotatable about two axes. They may be silicon micromachined mirrors, for fast rotational speeds. They may also be other types of mirrors or beam-deflecting elements known in the art.

(22) To optimize the coupling of the spectral channels into the output ports and further maintain the optimal optical alignment against environmental effects such as temperature variations and mechanical instabilities over the course of operation, a WSR apparatus of the present invention may incorporate a servo-control assembly, for providing dynamic control of the coupling of the spectral channels into the respective output ports on a channel-by-channel basis. A WSR apparatus incorporating a servo-control assembly is termed a WSR-S apparatus, thereinafter in this specification.

(23) FIG. 4A depicts a schematic illustration of a first embodiment of a WSR-S apparatus according to the present invention. The WSR-S apparatus 400 comprises a WSR apparatus 410 and a servo-control assembly 440. The WSR 410 may be in the embodiment of FIG. 1A, or any other embodiment in accordance with the present invention. The servo-control assembly 440 includes a spectral monitor 460, for monitoring the power levels of the spectral channels coupled into the output ports 420-1 through 420-N of the WSR apparatus 410. By way of example, the spectral monitor 460 is coupled to the output ports 420-1 through 420-N by way of fiber-optic couplers 420-1-C through 420-N-C, wherein each fiber-optic coupler serves to tap off a predetermined fraction of the optical signal in the corresponding output port. The servo-control assembly 440 further includes a processing unit 470, in communication with the spectral monitor 460 and the channel micromirrors 430 of the WSR apparatus 410. The processing unit 470 uses the power measurements from the spectral monitor 460 to provide feedback control of the channel micromirrors 430 on an individual basis, so as to maintain a desired coupling efficiency for each spectral channel into a selected output port. As such, the servo-control assembly 440 provides dynamic control of the coupling of the spectral channels into the respective output ports on a channel-by-channel basis and thereby manages the power levels of the spectral channels coupled into the output ports. The power levels of the spectral channels in the output ports may be dynamically managed according to demand, or maintained at desired values (e.g., equalized at a predetermined value) in the present invention. Such a spectral power-management capability is essential in WDM optical networking applications, as discussed above.

(24) FIG. 4B depicts a schematic illustration of a second embodiment of a WSR-S apparatus according to the present invention. The WSR-S apparatus 450 comprises a WSR apparatus 480 and a servo-control assembly 490. In addition to the channel micromirrors 430 (and other elements identified by the same numerals as those used in FIG. 4A), the WSR apparatus 480 further includes a plurality of collimator-alignment mirrors 485, and may be configured according to the embodiment of FIGS. 2A, 2B, 3, or any other embodiment in accordance with the present invention. By way of example, the servo-control assembly 490 includes the spectral monitor 460 as described in the embodiment of FIG. 4A, and a processing unit 495. In this case, the processing unit 495 is in communication with the channel micromirrors 430 and the collimator-alignment mirrors 485 of the WSR apparatus 480, as well as the spectral monitor 460. The processing unit 495 uses the power measurements from the spectral monitor 460 to provide dynamic control of the channel micromirrors 430 along with the collimator-alignment mirrors 485, so to maintain the coupling efficiencies of the spectral channels into the output ports at desired values.

(25) In the embodiment of FIG. 4A or 4B, the spectral monitor 460 may be one of spectral power monitoring devices known in the art that is capable of detecting the power levels of spectral components in a multi-wavelength optical signal. Such devices are typically in the form of a wavelength-separating means (e.g., a diffraction grating) that spatially separates a multi-wavelength optical signal by wavelength into constituent spectral components, and one or more optical sensors (e.g., an array of photodiodes) that are configured such to detect the power levels of these spectral components. The processing unit 470 in FIG. 4A (or the processing unit 495 in FIG. 4B) typically includes electrical circuits and signal processing programs for processing the power measurements received from the spectral monitor 460 and generating appropriate control signals to be applied to the channel micromirrors 430 (and the collimator-alignment mirrors 485 in the case of FIG. 4B), so to maintain the coupling efficiencies of the spectral channels into the output ports at desired values. The electronic circuitry and the associated signal processing algorithm/software for such processing unit in a servo-control system are known in the art. A skilled artisan will know how to implement a suitable spectral monitor along with an appropriate processing unit to provide a servo-control assembly in a WSP-S apparatus according to the present invention, for a given application.

(26) The incorporation of a servo-control assembly provides additional advantages of effectively relaxing the requisite fabrication tolerances and the precision of optical alignment during initial assembly of a WSR apparatus of the present invention, and further enabling the system to correct for shift in the alignment over the course of operation. By maintaining an optimal optical alignment, the optical losses incurred by the spectral channels are also significantly reduced. As such, the WSR-S apparatus thus constructed is simpler and more adaptable in structure, more robust in performance, and lower in cost and optical loss. Accordingly, the WSR-S (or WSR) apparatus of the present invention may be used to construct a variety of optical devices and utilized in many applications.

(27) For instance, by directing the spectral channels into the output ports in a one-channel-per-port fashion and coupling the output ports of a WSR-S (or WSR) apparatus to an array of optical sensors (e.g., photodiodes), or a single optical sensor that is capable of scanning across the output ports, a dynamic and versatile spectral power monitor (or channel analyzer) is provided, which would be highly desired in WDM optical networking applications. Moreover, a novel class of optical add-drop multiplexers (OADMs) may be built upon the WSR-S (or WSR) apparatus of the present invention, as exemplified in the following embodiments.

(28) FIG. 5 depicts an exemplary embodiment of an optical add-drop multiplexer (OADM) according to the present invention. By way of example, OADM 500 comprises a WSR-S (or WSR) apparatus 510 and an optical combiner 550. An input port 520 of the WSR-S apparatus 510 transmits a multi-wavelength optical signal. The constituent spectral channels are subsequently separated and routed into a plurality of output ports, including a pass-through port 530 and one or more drop ports 540-1 through 540-N (N1). The pass-through port 530 may receive any number of the spectral channels (i.e., the pass-through spectral channels). Each drop port may also receive any number of the spectral channels (i.e., the drop spectral channels). The pass-through port 530 is optically coupled to the optical combiner 550, which serves to combine the pass-through spectral channels with one or more add spectral channels provided by one or more add ports 560-1 through 560-M (M1). The combined optical signal is then routed into an existing port 570, providing an output multi-wavelength optical signal.

(29) In the above embodiment, the optical combiner 550 may be a K1 (K2) broadband fiber-optic coupler, wherein there are K input-ends and one output-end. The pass-through spectral channels and the add spectral channels are fed into the K input ends (e.g., in a one-to-one correspondence) and the combined optical signal exits from the output-end of the K1 fiber-optic coupler as the output multi-wavelength optical signal of the system. Such a multiple-input coupler also serves the purpose of multiplexing a multiplicity of add spectral channels to be coupled into the OADM 500. If the power levels of the spectral channels in the output multi-wavelength optical signal are desired to be actively managed, such as being equalized at a predetermined value, two spectral monitors may be utilized. As a way of example, the first spectral monitor may receive optical signals tapped off from the pass-through port 530 and the drop ports 540-1 through 540-N (e.g., by way of fiber-optic couplers as depicted in FIG. 4A or 4B). The second spectral monitor receives optical signals tapped off from the exiting port 570. A servo-control system may be constructed accordingly for monitoring and controlling the pass-through, drop and add spectral channels. As such, the embodiment of FIG. 5 provides a versatile optical add-drop multiplexer in a simple and low-cost assembly, while providing multiple physically separate drop/add ports in a dynamically reconfigurable fashion.

(30) FIG. 6 depicts an alternative embodiment of an optical add-drop multiplexer (OADM) according to the present invention. By way of example, OADM 600 comprises a first WSR-S apparatus 610 optically coupled to a second WSR-S apparatus 650. Each WSR-S apparatus may be in the embodiment of FIG. 4A or 4B. (A WSR apparatus of the embodiment of FIG. 1A, 2A, 2B, or 3 may be alternatively implemented.) The first WSR-S apparatus 610 includes an input port 620, a pass-through port 630, and one or more drop ports 640-1 through 640-N (N1). The pass-through spectral channels from the pass-through port 630 are further coupled to the second WSR-S apparatus 650, along with one or more add spectral channels emerging from add ports 660-1 through 660-M (M1). In this exemplary case, the pass-through port 630 and the add ports 660-1 through 660-M constitute the input ports for the second WSR-S apparatus 650. By way of its constituent wavelength-separator (e.g., a diffraction grating) and channel micromirrors (not shown in FIG. 6), the second WSR-S apparatus 650 serves to multiplex the pass-through spectral channels and the add spectral channels, and route the multiplexed optical signal into an exiting port 770 to provide an output signal of the system.

(31) In the embodiment of FIG. 6, one WSR-S apparatus (e.g., the first WSR-S apparatus 610) effectively performs dynamic drop function, whereas the other WSR-S apparatus (e.g., the second WSR-S apparatus 650) carries out dynamic add function. And there are essentially no fundamental restrictions on the wavelengths that can be added or dropped (other than those imposed by the overall communication system). Moreover, the underlying OADM architecture thus presented is intrinsically scalable and can be readily extended to any number of cascaded WSR-S (or WSR) systems, if so desired for performing intricate add and drop functions. Additionally, the OADM of FIG. 6 may be operated in reverse direction, by using the input ports as the output ports, the drop ports as the add ports, and vice versa.

(32) Those skilled in the art will recognize that the aforementioned embodiments provide only two of many embodiments of a dynamically reconfigurable OADM according to the present invention. Those skilled in the art will also appreciate that various changes, substitutions, and alternations can be made herein without departing from the principles and the scope of the invention as defined in the appended claims. Accordingly, a skilled artisan can design an OADM in accordance with the principles of the present invention, to best suit a given application.

(33) Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the principles and the scope of the invention. Accordingly, the scope of the present invention should be determined by the following claims and their legal equivalents.