Spatial light modulator-based reconfigurable optical add-drop multiplexer and method of adding an optical channel using the same

A reconfigurable optical add-drop multiplexer (ROADM) and a method of passing at least one optical channel through the multiplexer. In one embodiment, the multiplexer includes: (1) a main input port, (2) a main output port, (3) an add input port, (4) a drop output port, (5) dispersive optics configured spatially to spread and recombine optical spectra containing optical channels and (6) a spatial light modulator having an integral, lateral-gradient volume Bragg grating and configured to assume a bar state in which at least one of the optical channels is passed from the main input port to the main output port and at least another of the optical channels is passed from the add input port to the drop output port and a cross state in which the integral, lateral-gradient volume Bragg grating is transmissive with respect to the channels.

CROSS REFERENCE RELATED APPLICATION

Application Ser. No. 12/210,860, filed Sep. 15, 2008, entitled “The Use of an Angle-Selective Retro-Reflector to Recapture Off State Energy” (now U.S. Pat. No. 7,826,121) discloses related subject matter and is incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The invention is directed, in general, to optical networking and, more specifically, to a spatial light modulator (SLM)-based reconfigurable optical add-drop multiplexer (ROADM) and a method of passing at least one optical channel through the same.

BACKGROUND OF THE INVENTION

The ROADM is a key component for today's dense-wavelength-division-multiplexing (DWDM) optical communication networks. It provides the ability selectively to drop a channel (i.e., wavelength) from within a band of communication channels as well as provide the introduction of a new information-carrying channel at the same wavelength without interrupting the adjoining channels.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, one aspect of the invention provides a ROADM. In one embodiment, the ROADM includes: (1) a main input port configured to receive at least one main input optical channel from an optical network, (2) a main output port configured to provide at least one main output optical channel to the optical network, (3) an add input port configured to receive at least one add input optical channel, (4) a drop output port configured to provide at least one drop output optical channel, (5) dispersive optics coupled to the main input port, the main output port, the add input port and the drop output port and configured spatially to spread and recombine optical spectra containing at least one of the optical channels and (6) an SLM associated with the dispersive optics, having an integral, lateral-gradient volume Bragg grating and configured to assume a bar state in which the at least one of the optical channels is passed from the main input port to the main output port and at least another of the optical channels is passed from the add input port to the drop output port and a cross state in which the integral, lateral-gradient volume Bragg grating is transmissive with respect to the channels.

In another embodiment, the ROADM includes: (1) a main input port configured to receive at least one main input optical channel from an optical network and a main output port configured to provide at least one main output optical channel to the optical network, (2) an add input port configured to receive at least one add input optical channel and a drop output port configured to provide at least one drop output optical channel, (3) dispersive optics coupled to the main input port, the main output port, the add input port and the drop output port and configured spatially to spread and recombine optical spectra containing at least one of the optical channels and (4) a spatial light modulator associated with the dispersive optics and having an integral, lateral-gradient volume Bragg grating located in or on a window covering thereof, the lateral-gradient volume Bragg grating having a grating pitch gradient predetermined to correspond with wavelengths of the optical spectra incident on the spatial light modulator from the add input port that are to be routed to the drop output port, the spatial light modulator acting as a blazed grating configured to produce a diffracted light beam in a Littrow configuration for at least one of the optical channels traveling from the main input port to the main output port and a non-Littrow blazed configuration for at least one of the optical channels traveling from the add input port to the drop output port.

Another aspect of the invention provides a method of passing at least one optical channel through a ROADM. In one embodiment, the method includes: (1) receiving the at least one optical channel into an add input port of the ROADM, (2) spatially spreading optical spectra containing the at least one optical channel with dispersive optics of the ROADM, (3) configuring an SLM associated with the dispersive optics and having an integral, lateral-gradient volume Bragg grating to assume a bar state in which the at least one optical channel retro-reflects off the integral, lateral-gradient volume Bragg grating, the integral, lateral-gradient volume Bragg grating configured to be transmissive with respect to the at least one optical channel in a cross state, (4) spatially recombining the optical spectra with the dispersive optics and (5) providing the at least one optical channel at a drop output port of the ROADM.

DETAILED DESCRIPTION

Most of the above-described ROADM architectures employ a single pixel to switch a given wavelength (channel). Unfortunately, this does not provide fault tolerance. In fiber optic communication and networking applications, some ROADM switches may remain in one state for more than 100,000 hours. If the pixel fails, one-wavelength-per-pixel ROADM architectures can result into a catastrophic loss of that particular channel. For this reason, a macro-pixel architecture that uses multiple pixels per wavelength is desirable.

That a spatial light modulator (SLM) (e.g., a DLP™ digital micromirror device (DMD) available from Texas Instruments, Dallas, Tex.) could be used to form a high channel-capacity DWDM add-drop filter using a 2-D micromirror array was introduced in Riza, et al., “Fault-Tolerant Dense Multiwavelength Add Drop Filter with a Two-Dimensional Digital Micromirror Device,” Appl. Opt., vol. 37, no. 27, pp. 6355-6361, September 1998, Riza, et al., “Small-Tilt Micromirror-Device-Based Multiwavelength Three-Dimensional 2×2 Fiber Optic Switch Structures,” Opt. Eng., vol. 39, no. 2, pp. 379-386, February 2000, U.S. Pat. No. 6,222,954, which issued on Apr. 24, 2001, to Riza, “Fault-Tolerant Fiber-Optical Beam Control Modules,” and Khan, et al., “Demonstration of the MEMS Digital Micromirror Device-Based Broadband Reconfigurable Optical Add-Drop Filter for Dense Wavelength-Division-Multiplexing Systems,” J. Lightw. Technol. 25, 520-526 (2007), all incorporated herein by reference. Although they operate well, the ROADMs therein described require bulk retro-reflection optical elements to render them fully reversible in both bar and cross states. The elements take the form of a curved mirror or a collimating lens and flat mirror and potentially add cost, complexity, size and weight to the ROADMs and potentially increase light losses therein. Although such a bulk optics approach nominally provide the desired add-to-drop routing functionality, they are nevertheless space consuming and undesirable for fiber-optics applications, where most of the networking equipment takes the form of line-cards that should conform to a compact form-factor. A ROADM that does not require those additional bulk elements would be advantageous and therefore desirable.

A ROADM is a 2×2 wavelength-selective switch, meaning that it has two input ports and two output ports. A certain wavelength from one of the two input ports can be routed to any one of the two output ports. Reconfigurability, as the name suggests, means that any of the wavelengths or channels from any of the two input ports can be selectively routed to any of the two output ports at will and dynamically. Reversibility means that any of the ports functionality can be changed dynamically at will meaning that a port can be configured to be used as either an input or an output but the number of input ports remains equal to the number of output ports.

Before describing various embodiments of a ROADM constructed according to the principles of the invention, the problem of reversibility will be described.FIGS. 1A and 1Bare high-level schematic diagrams of an SLM-based ROADM100in respective cross and bar states and will be used to describe the operation of the ROADM100in the context of a DWDM optical network (not shown). The ROADM100has a main input port110coupled to one backbone segment of the optical network and a main output port120coupled to another backbone segment of the optical network. The backbone segments are part of a backbone of the optical network, e.g., a Fiber Distributed Data Interface, or FDDI, ring. An add input port130allows one or more channels (wavelengths) to be inserted into (added to) the backbone (by way of the main output port120). A drop output port140allows one or more channels (wavelengths) received by way of the main input port110to be extracted (dropped) from the backbone. Naming convention and terminology may vary depending upon the reference, e.g., the “main input port” port is sometimes called the “input” port, while the “main-output” port is sometimes called the “express” port, etc.

The ROADM100is controllably switchable between two different states: a cross state and a bar state.FIG. 1Ashows the ROADM100in the cross state (signified by an “X” label in the ROADM100).FIG. 1Bshows the ROADM100in the bar state (signified by an “=” label in the ROADM100). Since the optical network is a WDM network, the main input port110is assumed to receive plural channels. However,FIGS. 1A and 1Bshow only one channel150for clarity's sake. The add input port130may receive one or more channels, but only one channel160is shown, again for clarity's sake. It is assumed that the wavelength of the channel150is the same as that of the channel160but different from that of any other channels received by the main input port110or the add input port130.

InFIG. 1A(illustrating the cross state), the main output port120provides the channel160, and the drop output port140provides the channel150. The net result is that the channel150has been dropped from the backbone, and the channel160has been added to the backbone, effecting a substitution of the channel160for the channel150. InFIG. 1B(illustrating the bar state), the main output port120continues to provide the channel150(no add or drop has taken place). However, the drop output port140does not provide the channel160; the channel160did not carry over from the add input port130. Thus, the channel160is interrupted, the ROADM100is not a fully functional 2×2 switch in its bar state.

To understand why the SLM-based ROADM100is not a fully functional 2×2 switch in its bar state, an SLM-based ROADM will now be described. Various embodiments of a ROADM constructed according to the principles of the invention will then be described.FIG. 2is a diagram of one embodiment of an SLM-based ROADM.

A first fiber-optic circulator C1receives plural channels via the main input port110and provides plural channels via the main output port120. C1routes the received plural channels to a first gradient index of refraction (GRIN) fiber for lens) collimator210, which collimates the optical input signals (channels) from the fiber into freespace optical signals (channels) incident on a volume Bragg grating (VBG), VBG1. VBG1angularly separates the optical channel spectra. A first collimating (e.g., spherical) lens S1steers the channel spectra onto the face of an SLM220forming a spot pattern that forms a line. A 2-D grid of micromirrors (not shown) in the SLM220is then controlled to reflect each spot independently either toward a second collimating lens S2to effect a cross state for channels to be dropped or back toward the first collimating lens S1to effect a bar state for channels to be passed through the ROADM100. In the latter case, the SLM220may act as a blazed grating with the diffracted light beam being in the high-efficiency Littrow configuration, resulting in a low insertion loss for channels being conveyed from the main input port110to the main output port120.

S2focuses the spots representing the channels to be dropped on a second VBG, VBG2, which recombines and steers them toward a second GRIN fiber collimator230. The second GRIN collimator230routes these channels towards a second fiber-optic circulator C2, which routes them to the drop output port140.

C2receives one or more channels to be added via the add input port130, routing them to the second GRIN collimator230. The second GRIN collimator230collimates the optical signals (channels) from the add input port130into freespace optical signals (channels) incident on VBG2. VBG2angularly separates the optical channel spectra. S2steers the add input port spectra into a spot pattern formed onto the face of the SLM220, where the relevant micromirrors are already oriented to reflect the added channel(s) toward S1. S1focuses the spots originally reflected back from the SLM220(those to be passed through the ROADM100) and the line(s) representing the added channel(s) on VBG1, which recombines and steers them toward the first GRIN fiber collimator210. The first GRIN collimator210focuses the channels into C1, which routes the channels to the main output port120.

In the illustrated embodiment, the distances between the first and second GRIN lens collimators210,230and their corresponding first and second VBGs, VBG1and VBG2, are chosen to be half-self imaging distances such that the Gaussian light beams emerging from the GRIN lenses210,230form beam waists at the location of the corresponding VBG (see, e.g., Buren, et al., “Foundations for low-loss Fiber gradient-index lens pair coupling with the self-imaging mechanism,” Appl. Opt.-LP, vol. 42, no. 3, pp. 550-565, January 2003). VBG1and VBG2are placed at their Bragg angle θBraggwith respect to the input light beam so that the input channel spectrum spreads in the first order by an angle2Δθ along the x-dimension, where:
2Δθ=θmax−θmin,
θmax=sin1[(λmax/L)−sin θBragg], and
θmin=sin−1[(λmin/L)−sin θBragg],
and λmaxand λminrespectively correspond to the maximum and minimum channel wavelengths. The spatial extent of any wavelength λ is defined by the VBG resolution:
δλ=Lλc/2W|m|,
where m is the grating order number, L is the grating period, λcis the grating center wavelength, and 2 W is the 1/e2beam diameter incident on the grating.

In the illustrated embodiment, S1and S2spread their input source spectra spatially onto the SLM220such that the spectrum size is X=2F tan(Δθ) in the x-dimension, where Fnis the focal length of the first collimating lens Snwhere n represents 1, 2 or both 1 and 2. In the illustrated embodiment, S1and S2spread their input source spectra such that the depth of focus of the input source spectra is at least about twice a distance between the SLM220and the lateral-gradient VBG that is integral with the SLM220. The input light beam with 1/e2beam waist of wnat the VBG1location is transformed at the SLM location into a waist wn+1which is given by:
wn+1=Fλ/πwn.
Thus, the input optical spectrum to the ROADM100forms a generally rectangular-shaped beam that is X units wide and 2 wn+1units high in the plane of the SLM220. In effect, this allows independent control of N=X/2wn+1channels within the Δλ=λmax−λminsource spectrum. Thus, the wavelength-control resolution of the illustrated embodiment of the ROADM100is Δλ/N, with the VBG resolution δλ being the fundamental limiting resolution.

Turning briefly toFIGS. 3A and 3B, geometrically illustrated are the operation of an incomplete 2×2 switch-based SLM-based ROADM in respective cross and bar states. The problem with the ROADM identified above is that added channels are not passed to the drop output port in the bar state.FIGS. 3A and 3Billustrate why.FIGS. 3A and 3Billustrate a single micromirror310of the 2-D grid of micromirrors in the SLM220ofFIG. 2. The micromirror310may be controllably oriented at an angle of −θ or +θ relative to a plane320of the face of the SLM220.FIG. 3Ashows the micromirror310oriented at −θ to effect a cross state. Channels received via the main input port110ofFIGS. 1A and 1Bare properly routed to the drop output port140ofFIGS. 1A and 1Bas lines330,340represent. Likewise, channels received via the add input port130ofFIGS. 1A and 1Bare properly routed to the main output port120ofFIGS. 1A and 1Bas lines350,360represent.FIG. 3Bshows the micromirror310oriented at +θ to effect a bar state. Channels received via the main input port110are properly routed to the main output port120as the lines330,340represent. Unfortunately, channels received via the add input port130, represented by the line350, are not routed to the drop output port140. Instead, they are steered in a direction indicated by the line360. What is needed is a structure for retro-reflecting the channels back along the line360such that they return along the line350and are properly routed to the drop output port140. The references set forth in the first paragraph of the Detailed Description above employ bulk retro-reflection optical elements to render their ROADMs fully reversible in both the bar and cross states.

Returning now toFIG. 2, a novel and advantageous SLM will be described that eliminates the need for such optical elements. The SLM220ofFIG. 2has an integral, laterally gradient VBG (not shown, but illustrated in detail in subsequent FIGs.) located over its micromirrors. The VBG is designed such that, in the bar state, channels received via the add input port130are retro-reflected, by this integral laterally gradient VBG, back towards the drop output port140. Accordingly, C2receives one or more channels via the add input port130, routing it or them to the second GRIN collimator230. The second GRIN collimator230spreads the channel(s) over VBG2. VBG2angularly separates the optical channel spectrum or spectra. S2steers the channel(s) spectrum or spectra onto the face of the SLM220in the form of spot(s) that form a line, where the relevant micromirrors are already oriented to reflect them such that the laterally gradient VBG that is integral to the SLM220retro-reflects them back to the same micromirrors and back toward S2. S2focuses the spots originally reflected back from the SLM220(those to be passed through the ROADM100to the drop output port140) and the spots representing any dropped channels on VBG2, which recombines and steers them toward the second GRIN fiber collimator230. The second GRIN collimator230focuses the channels into C2, which routes the channels to the drop output port140, as is desired.

FIGS. 4A and 4Bgeometrically illustrate the operation of an SLM-based ROADM, in which the SLM has an integral, lateral-gradient volume Bragg grating, in respective bar and cross states.FIG. 4Ashows the bar state and is taken along lines4-4ofFIG. 6. As those skilled in the pertinent art are aware, an SLM such as a Texas Instruments DLP™ digital micromirror device (DMD) has a substrate410to which micromirrors420are hingedly mounted. In the embodiment ofFIG. 4A, the micromirrors420are substantially square and hinged along the diagonals thereof such that they can be oriented between −θ and +θ angles as shown inFIGS. 3A and 3Bunder control of MEMS actuators (not shown) associated with the micromirrors420. The micromirrors420may be arranged in a Cartesian or diamond 2-D grid. For simplicity's sake,FIG. 4Ashows only one line (row or column) of micromirrors420.

The SLM220is not conventional, however. The SLM220has an integral, lateral-gradient VBG430that lies over (e.g., on the surface of) the micromirrors420. “Lateral gradient” indicates that pitch of the VBG varies such that the Bragg wavelength, λBragg, shifts from one end of the grating to the other in a desired manner to match the wavelength spread incident on the micromirrors for a particular ROADM application. The pitch may vary smoothly (e.g., linearly) or step-wise across the integral, lateral-gradient VBG430. The integral, lateral-gradient VBG430is therefore designed such that light of certain wavelengths incident at a particular incidence angle upon the integral, lateral-gradient VBG430is reflected in a Littrow configuration in the bar-state of the 2×2 ROADM switch. Other wavelengths and incident angles are transmitted un-perturbed. In the embodiment ofFIG. 4A, the certain wavelengths correlate with the various rays representing various channels projected onto the face of the SLM220within the ROADM100ofFIG. 2. In the embodiment ofFIG. 4A, the Bragg angle is based on the angle at which the light is incident on the face of the SLM220and the orientation angle, θ, that the micromirrors420have with respect to the plane of the SLM220when the micromirrors are oriented to achieve a bar state. A line normal to the orientation of the micromirrors in their bar state is referenced as440.

As shown inFIG. 4A, a representative beam of light of a first wavelength (bearing a first channel)450-1is incident on the face of the SLM220at an angle substantially normal to one of the micromirrors, referenced as micromirror460. The light beam450-1may correspond, for example, to a channel that is to pass from the main input port110to the main output port120of the ROADM100ofFIG. 1. The light beam450-1passes through the integral, lateral-gradient VBG430and is reflected off the micromirror460back in the opposite direction, passing through the integral, lateral-gradient VBG430once again. However, since the light beam450-1is not incident on the integral, lateral-gradient VBG430at the Bragg angle required for Bragg reflection to take place, it is transmitted substantially through the integral, lateral-gradient VBG430in both directions asFIG. 4Ashows.

A representative beam of light of the same, first wavelength (bearing a first channel)450-2is also incident on the face of the SLM220. The light beam450-2may correspond, for example, to a channel that is to pass from the add input port130to the drop output port140of the ROADM100ofFIG. 2. However, the angle at which the light beam450-2is incident on the micromirror460differs from normal. The light beam450-2passes through the integral, lateral-gradient VBG430and is reflected off the micromirror460. However, the light beam450-2is substantially equal to the Bragg wavelength and reflects off the micromirror460at an angle that is substantially equal to the Bragg angle for Bragg reflection off the integral, lateral-gradient VBG430. Consequently, it is retro-reflected off the integral, lateral-gradient VBG430, back to the micromirror460and back in the direction from which it originally came, passing through the integral, lateral-gradient VBG430once again.

Beams460-1,460-2, are processed in substantially the same way as the light beams450-1,450-2, except that the light beams460-1,460-2differ in wavelength from the light beams450-1,450-2and correspond in wavelength to the localized portion of the integral, lateral-gradient VBG430at which they are incident. Likewise, light beams470-1,470-2are processed in substantially the same way as the light beams450-1,450-2,460-1,460-2except that the light beams470-1,470-2differ in wavelength from the light beams450-1,450-2and the light beams460-1,460-2and correspond in wavelength to the localized portion of the integral, lateral-gradient VBG430at which they are incident.

FIG. 4Bgeometrically illustrate the operation of the SLM-based ROADM in the cross state. Reference numerals are as they were inFIG. 4A. In the cross state, the integral, lateral-gradient VBG430is transmissive with respect to the channels. The SLM220acts as a blazed grating configured to produce diffracted light beams for optical channels traveling from the main input port to the drop output port and from the add input port to main output ports, as shown. For example, a light beam450-1from the main input port is as incident upon the SLM220. After reflection from the SLM220, the light beam, now450-2, is routed to the drop output port. Although it is difficult to illustrate inFIG. 4B, another light beam450-1′ of the same wavelength is incident on the SLM220from the add input port, and the SLM220routes this light beam, now450-2′, to the main output port. These two collinear and spatially overlapping light beams are of the same wavelength, but are independent and bear separate channels. Other information-carrying wavelengths/channels undergo the same process and are hence labeled as they are inFIG. 4B.

FIGS. 5A-5Care elevational views, taken along lines5-5ofFIG. 6, of respective first, second and third embodiments of an SLM220having an integral, lateral-gradient VBG430and constructed according to the principles of the invention.FIG. 5Ashows the integral, lateral-gradient VBG430as being located within a window covering510located over the micromirrors420. In this embodiment, the window covering510may consist of or include a photosensitive glass material in which the integral, lateral-gradient VBG430is formed. Those skilled in the pertinent art are familiar with the manner in which an integral, lateral-gradient VBG may be formed in a photosensitive material such as glass.FIG. 5Bshows the integral, lateral-gradient VBG430as being located on a lower surface of the window covering510. The integral, lateral-gradient VBG430may be gradient volume hologram. Those skilled in the pertinent art are familiar with the manner in which gradient volume holograms are made and used.FIG. 5Cshows the integral, lateral-gradient VBG430as being located on an upper surface of the window covering510. Again, the integral, lateral-gradient VBG430may be gradient volume hologram. The lateral gradient can be either a continuous linear gradient or a step-wise linear gradient.

FIG. 6is a plan view, taken along lines6-6ofFIGS. 5A-5Cof one embodiment of an SLM220having an integral, lateral-gradient VBG and constructed according to the principles of the invention.FIG. 6is presented primarily for the purpose of showing how a spectrum processing zone610contains spots620-ncorresponding to channels to be steered by the micromirrors420. The spots620-nare illustrated inFIG. 7as being larger than the micromirrors420such that multiple micromirrors420are used to reflect each of the spots (channels)620-n. In other embodiments the spots620-nformed by the illuminating wavelength channels with dispersive optics on the SLM220may illuminate one or more micromirrors and may be circular, rectangular or any other shape based upon the optics used for forming these spots on the SLM220. The array of micromirrors630-nassociated with a certain channel spot size620-nmay be arranged in an appropriate array shape and size to optimally conform to the illuminating spot while causing minimal interference with adjoining channel spots. One such arrangement of array shape is shown as630-ninFIG. 7. Note that different colors (values of n) inFIG. 7represent different wavelengths/channels (λ).

Since the micromirrors420are independently steerable, the micromirrors420are able to steer the spots (channels)620-nindependently, such that the ROADM100ofFIG. 2may be in a bar state for one or some channels and in a cross state for any remaining channels. In the illustrated embodiment, each of the spots620-nimpinges on 13 of the micromirrors420, the invention is not limited to a particular number of micromirrors420. The SLM220has a periphery (not shown); the 2-D grid of micromirrors420lies within the periphery.

FIG. 7is a flow diagram of one embodiment of a method of passing at least one optical channel through a ROADM based on an SLM having an integral Bragg grating carried out according to the principles of the invention. The method begins in a start step710. In a step720, the at least one optical channel is received into an add input port of the ROADM. In a step730, optical spectra containing the at least one optical channel are spatially spread with dispersive optics of the ROADM. In a step740, an SLM associated with the dispersive optics and having an integral, lateral-gradient volume Bragg grating is configured to assume a bar state. In the bar state, the optical channel retro-reflects off the integral, lateral-gradient volume Bragg grating. In a step750, the optical spectra are spatially recombined with the dispersive optics. In a step760, the at least one optical channel is provided at a drop output port of the ROADM. The method ends in an end step770.

Those skilled in the art to which the invention relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments without departing from the scope of the invention.