A reconfigurable optical add-drop multiplexer comprises a first waveguide layer having formed therein a first multiplexer-demultiplexer, a second multiplexer-demultiplexer, and a plurality of optical switches. The reconfigurable optical add-drop multiplexer further comprises a second waveguide layer optically coupled to the first waveguide and having a second effective index of refraction, said second waveguide layer having an optical amplifier formed therein. An input signal is amplified by the optical amplifier and communicated to the first optical multiplexer-demultiplexer where the signal is demultiplexed into a plurality individual wavelength signals. The second optical multiplexer-demultiplexer is adapted to receive a multiplexed add signal and to demultiplex the add signal into component wavelength signals. The individual wavelength signals are received at the optical switches and selectively routed to either an optical detector or toward the first multiplexer-demultiplexer. The individual wavelength signals received at the first multiplexer-demultiplexer are multiplexed into an output signal.

FIELD OF THE APPLICATION

The present application is related to the field of optical communication devices, and more particularly to reconfigurable optical add-drop multiplexers.

BACKGROUND

Reconfigurable optical add-drop multiplexers (ROADMs) have a multitude of uses and have shown great promise for use in optical systems. For example, one promising area of application for ROADMs is in the field of wavelength division multiplexed (WDM) light wave systems. ROADMs may be used for the selective broadcasting, dropping, and monitoring of discrete wavelengths.

In the field of optical systems, photonic integrated circuits (PICs) provide an integrated technology platform increasingly used to form complex optical systems. This technology allows multiple optical devices to be integrated on a single substrate. For example, PICs may comprise integrated amplifiers, receivers, waveguides, detectors, and other active and passive optical devices arranged in various configurations.

Asymmetric twin waveguide (ATG) technology has proven to be a promising method for optoelectronic integration and offers a relatively simple fabrication process for even the most complex PIC design. The ATG design significantly reduces modal interference by substantially confining different modes of light to propagation in different waveguides. Modal confinement is accomplished by designing waveguides such that the mode of light that propagates in a waveguide has a different effective index of refraction than the mode of light that propagates in the adjacent waveguide. This feature substantially isolates the light propagating in each waveguide, which lends itself to the specialization of functions performed by the waveguides. Transfer of light between the waveguides is facilitated by lithographically defined taper couplers. The minimal modal interference and efficient coupling result in high-performance lasers, p-i-n and avalanche photodiodes, SOAs, and integrated combinations of these fundamental photonic functionalities. U.S. Pat. Nos. 6,381,380, 6,330,387, 6,483,863, 6,795,622, and 6,819,814, the contents of which are hereby incorporated herein by reference in their entirety, provide a description of ATG and various embodiments of ATG.

SUMMARY

Applicants disclose herein new and improved ROADMs. According to an aspect of an illustrative embodiment, a ROADM may be formed in a monolithic structure having asymmetric waveguide layers.

An illustrative ROADM comprises a first optical multiplexer-demultiplexer, a second optical multiplexer-demultiplexer, a plurality of switches, and at least one active optical device such as, for example, an optical amplifier, a laser, a detector, or an electrically activated optical switch. In an illustrative embodiment, an input signal is amplified by an optical amplifier and communicated to the first optical multiplexer-demultiplexer. The first optical multiplexer-demultiplexer receives the amplified input signal and demultiplexes the input signal into a plurality individual wavelength signals. The second optical multiplexer-demultiplexer is adapted to receive an add signal comprising a plurality of individual wavelength signals and to demultiplex the add signal into the component individual wavelength signals. The individual wavelength signals comprised in the input signal and the add signal are selectively dropped at the plurality of switches. After the individual wavelength signals have been added and/or dropped, the first optical multiplexer-demultiplexer is further adapted to receive the resulting plurality of individual wavelength signals, and to multiplex these plurality of individual wavelength signals into an output signal.

In an illustrative embodiment, the first optical multiplexer-demultiplexer, a second optical multiplexer-demultiplexer, and the plurality of switches are formed in a first waveguide layer. The optical amplifier, and any other active optical components, are formed in the second waveguide layer. The first waveguide layer has a first effective index of refraction and the second waveguide layer has a second effective index of refraction such that a first mode of light propagates primarily in the first waveguide layer and a second mode of light propagates primarily in a second waveguide layer in the area where the first waveguide and the second waveguide overlap.

These and other illustrative embodiments, including additional elements and features, are described below.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Monolithic integrated ROADMs may be used to drop individual wavelength signals from a multiplexed signal, as well as to add individual wavelength signals to the multiplexed signal.

FIG. 1provides a layout view of an illustrative monolithic integrated ROADM110. The illustrative ROADM comprises an input waveguide or port112for receiving an optical input signal. The input signal may be, for example, a signal comprising a plurality of individual wavelength signals, which may be separately added and dropped via ROADM110. As described below in connection withFIG. 2, ROADM110may be constructed using asymmetric waveguide layers comprising a passive waveguide layer218and an active waveguide layer216. In an illustrative embodiment, input port112is formed in passive layer218of the structure.

Illustrative ROADM110further comprises an optical amplifier114. Optical amplifier114is adapted to receive the input signal from input port112and amplify the signal. In an embodiment constructed using asymmetric waveguide layers comprising a passive waveguide layer and active waveguide layer as shown inFIG. 5, optical amplifier114is formed in active layer216of the structure. In the illustrative embodiment ofFIG. 5, optical amplifier114is positioned in the circuit after input port112. Optical amplifier114could alternatively be positioned anywhere in the ROADM circuit110. Further, while only one optical amplifier114is depicted, a ROADM circuit110may comprise a plurality of optical amplifiers located throughout the circuit. Moreover, in other illustrative embodiments, one or more other optical components such as, for example, a laser, a modulator, an active ring resonator, and/or optical attenuator may appear in the circuit in the place of optical amplifier114.

After being amplified by optical amplifier114, the amplified input signal propagates along waveguide116, which is formed in passive waveguide layer218ofFIG. 5, and then on to optical coupler118. Coupler118is adapted to couple light from waveguide116into a first multiplexer-demultiplexer120. Coupler118is further adapted to couple light from a plurality of waveguides122for guiding individual wavelength signals into multiplexer-demultiplexer120. Still further, coupler118is adapted to couple light from one or more of a plurality of add ports124into a second multiplexer-demultiplexer126. Coupler118may be any type of optical coupler suitable for moving light from waveguides116,122, and124into multiplexer-demultiplexers120,126. In an illustrative embodiment, coupler118may be, for example, an overlap star coupler. Coupler118and add ports124may be formed in the passive waveguide layer218ofFIG. 5.

First optical multiplexer-demultiplexer120is adapted to demultiplex the input signal which is received via coupler118from waveguide116. Input signal may comprise a plurality of individual wavelength signals, i.e. discrete signals carried on different wavelengths of light. The number of individual wavelength signals comprised in the input signal may be one, two, three, four, or any number sufficient to carry the individual wavelength signals comprised in the input signal. Optical multiplexer-demultiplexer120operates to demultiplex the input signal into the separate individual wavelength signals. The individual wavelength signals are received at optical coupler130where each of the individual wavelength signals is coupled into one of the plurality of waveguides122. (In the illustrative embodiment ofFIG. 1, waveguides extending from the lower left side of coupler130carry signals from multiplexer-demultiplexer120.) Optical multiplexer-demultiplexer120may be any type of device operable to multiplex and demultiplex signals as described herein, such as, for example, an array waveguide grating. In an illustrative embodiment, first optical multiplexer-demultiplexer120and waveguides122are formed in the passive waveguide layer218ofFIG. 5.

Optical coupler118is also adapted to receive an add signal from add ports124. Add signal may be a multiplexed signal comprising one or more individual wavelength signals that are meant to be multiplexed with signals comprised in the input signal. Optical coupler118is adapted to couple the add signal into second multiplexer-demultiplexer126. Second multiplexer-demultiplexer126is adapted to demultiplex the add signal into the individual wavelength signals that may be comprised in the add signal. The demultiplexed individual wavelength signals are received at coupler130, where they are coupled into the plurality of waveguides122. (In the illustrative embodiment ofFIG. 1, waveguides extending from the upper left side of coupler130carry signals from multiplexer-demultiplexer126.) Optical multiplexer-demultiplexer126may be any type of device operable to multiplex and demultiplex signals as described herein, such as, for example, an array waveguide grating. In an illustrative embodiment, second optical multiplexer-demultiplexer126is formed in the passive waveguide layer218ofFIG. 5.

Coupler130may be any type of optical coupler suitable for moving light from waveguides multiplexer-demultiplexers120and126into waveguides122and132. In an illustrative embodiment, coupler130may be, for example, an overlap star coupler. Coupler may be formed in the passive layer218of the multi-layer asymmetric structure described in connection withFIG. 5.

Each of waveguides122carry an individual waveguide signal from multiplexer-demultiplexers120,126. In the embodiment shown inFIG. 1, there are four waveguides122shown (labeled channels3,4,5, and6) extending from each of multiplexer-demultiplexers120and126. It should be appreciated that while four waveguides are illustrated, any number of waveguides122may be employed sufficient to carry the individual waveguide signals that are multiplexed in the input signal. Each waveguide122is adapted to provide a propagation path for an individual waveguide signal.

Each of waveguides122leading from first multiplexer-demultiplexer120joins with a corresponding waveguide122leading from second multiplexer-demultiplexer126at waveguide junctions134. For each pair of waveguides122that is joined at a junction134, the individual waveguide signals propagating in the respective waveguides leading from first multiplexer-demultiplexer120and second multiplexer-demultiplexer126continue to propagate as separate signals in the joined waveguide. Thus, after junction134, a waveguide122may have two separate signals propagating therein—one originating from the input signal and multiplexer-demultiplexer120, and another originating from add signal and multiplexer-demultiplexer126. Each of waveguides122may have an optical amplifier formed therein which may be used for various purposes including, for example, channel equalization.

Each of plurality of waveguides122is communicatively coupled with an optical switch140. Each of optical switches140is adapted to selectively either allow the particular individual waveguide signal in the particular waveguide122to continue to propagate in the waveguide where it will eventually propagate to first multiplexer-demultiplexer120to be combined in the output signal, or to switch the individual wavelength signal for further processing, which, in the illustrative example, is processing by optical detector150. In an illustrative embodiment, when optical switch140is switched to one state, e.g., the “off-state,” a signal propagating in waveguide122that originated from multiplexer-demultiplexer120continues to propagate in the circuit toward multiplexer-demultiplexer120, and a signal propagating in the circuit that originated from multiplexer-demultiplexer126is routed by switch140to detector150. Also, in an illustrative embodiment, when optical switch140is switched to another state, e.g., the “on-state,” a signal propagating in waveguide122that originated from multiplexer-demultiplexer120is routed by switch140to detector150, and a signal propagating in the circuit that originated from multiplexer-demultiplexer126continues to propagate in the circuit toward multiplexer-demultiplexer120.

Thus, optical switches140may be any type of optical switch that is adapted to selectively switch an optical signal from one waveguide to one of either of two propagation paths, e.g., to detector150or to multiplexer-demultiplexer120. An individual wavelength signal may be rerouted for any number of reasons such as, for example, for monitoring, processing, or simply to drop the signal from the ultimate output signal. Optical switches140may be any devices that is operable to provide the described functionally, and may be, for example, a switch such as that disclosed in the article titled “Compact Polarization-Insensitive InGaAsP—InP 2×2 Optical Switch,” 17IEEE Photonics Technology Letters1 (January 2005), by Agashe, Shiu, and Forrest, the contents of which are hereby incorporated by reference in their entirety. In an illustrative embodiment, a portion of optical switch140is formed in the passive waveguide layer218as depicted inFIG. 5. In an embodiment, the optical switch may further comprise a portion that is formed in active waveguide layer216as well. For example, optical switches140may be an electrically activated optical switch or an optically activated optical switch which may be formed at least in part in active waveguide layer216as well.

Each of optical detectors150is adapted to detect an optical signal that has been routed to it by the corresponding optical switch140. The individual wavelength signals that are routed to detectors150may be further routed for further processing. In an embodiment constructed using asymmetric waveguide layers comprising a passive waveguide layer and active waveguide layer, such as that shown inFIG. 5, detectors150are formed in an active layer216of the structure. The optical detectors150may be any type suitable for detecting an optical signal. The optical detector150may be, for example, avalanche photodetector or a uni-travelling carrier photodetector.

The individual wavelength signals that are not switched to detectors150continue to propagate in waveguides122. The individual wavelength signals propagate to coupler118, and eventually into first multiplexer-detector120. First optical multiplexer-demultiplexer120is adapted to multiplex the individual wavelength signals that are received via coupler118from waveguides122. First optical multiplexer-demultiplexer120multiplexes the individual wavelength signals into a single signal that may be referred to as an output signal. The multiplexed output signal is received at optical coupler130and coupled into output waveguide or port132. The output waveguide132is formed in passive waveguide layer218of the monolithic structure.

As described above, ROADM110comprises a loop-back design surrounding first multiplexer-demultiplexer120. The multiplexed input signal enters multiplexer-demultiplexer120and is demultiplexed into the individual wavelength signals. The individual wavelength signals propagate on waveguides122and may be added to and/or dropped, with the resulting signals ultimately propagating back to multiplexer-demultiplexer120. Multiplexer-demultiplexer120then multiplexes the received signals into an output signal. Thus, in a disclosed embodiment, a single multiplexer-demultiplexer120may be used to perform both multiplexing and demultiplexing functions. This design reduces the number of optical devices and allows for a compact design.FIG. 6is a block diagram depicting the looping structure surrounding multiplexer-demultiplexer120.

Illustrative ROADM110comprises active devices—semiconductor optical amplifier114and detectors150—and passive devices—waveguides122, couplers118,130, multiplexer-demultiplexers120,126. Devices such as switches140may in some instances comprise active components. An asymmetric waveguide design may be used to integrate these devices into a monolithic integrated package while also providing for isolation between devices.FIG. 2provides an illustration of semiconductor optical amplifier114that may be suitable for an illustrative asymmetric waveguide embodiment. As shown, optical amplifier114comprises a waveguide formed in passive waveguide layer218ofFIG. 5, which may be a passive waveguide such as a fiber guide, and a second waveguide formed in active waveguide layer216formed on top of passive waveguide layer218. Passive waveguide layer218is formed on a substrate (not shown).

In illustrative embodiment of ROADM110, a first mode of light and a second mode of light are divided unequally between passive waveguide layer218and active waveguide layer216. Waveguide layers218and216have differing indices of refraction, resulting in the uneven division of light in the regions where the waveguide layers218and216overlap. In an exemplary embodiment, active waveguide layer216has a higher refractive index than passive waveguide layer218. A first mode of light is confined primarily to passive waveguide layer218, while a second mode of light is confined primarily to active waveguide layer216in the area where the two waveguides overlap. Because the second mode of light, as compared to the first mode, is confined primarily to active waveguide layer216, the second mode of light is primarily affected by amplifier114formed in active waveguide layer216.

In illustrative structure, amplifier114comprises a portion of active waveguide layer216. The portion of waveguide layer216comprised in amplifier114may have tapers213formed therein for facilitating the transfer of light energy into and out of waveguide layer216. Tapers213may be lateral tapers with exponential or polynomial shapes, but may comprise any geometry, shape, or configuration operable to move light between waveguides. Ridge215is formed on waveguide layer216.

In an illustrative embodiment, active waveguide layer216comprises a bulk material of bandgap that both emits and detects light in a wavelength band of interest. Active waveguide216may include a plurality of quantum wells separated by a plurality of barrier materials. An intermixed region may be formed in the taper areas213wherein the plurality of quantum wells are intermixed with the plurality of barrier materials. In an alternative embodiment, non-intermixed quantum wells may be employed. In an illustrative embodiment, five quantum wells may be embodied in active waveguide layer216. Amplifier112further comprises signal contacts217formed thereon for applying a forward bias to the portion of waveguide216formed in amplifier112. Applying the forward bias to active waveguide layer216causes light propagating in waveguide216to be amplified.

FIG. 3provides an illustration of detector150formed in the layer structure ofFIG. 5that may be suitable for an illustrative asymmetric waveguide embodiment. As shown inFIG. 3, detector150comprises a waveguide formed in active waveguide layer216. The portion of waveguide layer216comprised in detector150may have a taper213formed therein for facilitating movement of light energy into waveguide216. Ridge219is formed on top of waveguide layer216. As previously mentioned, in an illustrative embodiment, active waveguide layer216may comprise a plurality of quantum wells separated by a plurality of barrier materials. Detector150further comprises signal contacts221for applying a reverse bias to the portion of waveguide layer216formed in detector140. Applying a reverse bias to active waveguide216allows for detecting light that is propagating in waveguide216.

In an illustrative embodiment, amplifier114and detector150, are formed in the same active waveguide layer216. Forming amplifier114and detector150in a single active layer216in an ATG structure eliminates re-growth steps and thereby simplifies the overall fabrication process of integrated photonic devices.

FIGS. 4A and 4Billustrates the movement of light energy between waveguide layers in illustrative ROADM110in the areas of optical amplifier114and detectors150, respectively. Referring toFIG. 4A, in connection with amplifier114, light energy, which is depicted using broken lines, enters passive waveguide layer218(shown at the left ofFIG. 4A) and is coupled into the portion of active waveguide layer216that forms amplifier114. The coupling may be facilitated by a taper213formed in waveguide layer216. While propagating in the portion of waveguide216incorporated in amplifier114, the light energy is amplified as a result of the forward bias applied to active waveguide layer216. The amplified light energy is coupled back into passive waveguide layer218.

Referring toFIG. 4B, light energy propagates in passive waveguide layer218toward detector150. The amplified light propagates in passive waveguide layer218and is coupled into active waveguide layer216beginning in the area where the two overlap. While propagating in the portion of waveguide layer216corresponding to detector150, light energy is absorbed as part of the operation of detector150and as a result of the reverse bias applied to active waveguide layer216. Thus, in the illustrative embodiment, the same waveguide epitaxial structure or layer, active waveguide layer216, is used for both amplification and detection of light.

FIG. 5is a diagram depicting material layers comprised in an illustrative embodiment of ROADM structure110. As shown, structure110may comprise cladding and contact layer234, active waveguide layer216, passive waveguide layer218, and insulating or conducting substrate220.

Active waveguide layer216may comprise, in an exemplary embodiment, five quantum wells separated by barriers, wherein the quantum well-barrier structure is sandwiched between two 0.12 μm-thick Q1.2 layers that are substantially transparent to the movement of light into and out of the waveguide216. Quantum-well intermixing (QWI), which relies on the mixing of quantum wells and barrier materials of an active region, may be used to reduce absorption loss in the active layers in couplers employing ATG technology. A 0.15 μm-thick InP layer cladding layer is formed between active layer216and cladding and contact layer234. Active devices, i.e. devices that convert electrical energy to optical energy or vice verse, such as optical amplifiers, optical detectors, lasers, an optically activated optical switch, etc., are formed at least in part in the active waveguide layer216. Often the active devices are those that emit/amplify light and/or absorb light.

In the illustrative embodiment depicted inFIGS. 2 and 3, the forward-biased, i.e., amplifier, and reverse-biased, i.e., detector, regions of active waveguide layer216are electrically isolated by the imposition of a gap between the two regions. While the two regions are electrically insulated, light is communicated between the two regions via passive waveguide218. In another embodiment, electrical insulation between the forward-biased and reverse-biased regions may be provided through means other than a gap such as, for example, by forming an insulating region in active waveguide layer216between the forward-bias and reverse-bias. The electrical insulation may be formed, for example, by proton bombardment and/or by ion implantation.

FIG. 7is a block diagram of another illustrative ROADM710. As shown, ROADM710comprises an input waveguide712for receiving a multiplexed input signal that may comprise a plurality of individual wavelength signals. One, two, three, or any number of individual wavelength signals may be comprised in the input signal. An optical amplifier714is communicatively formed with input waveguide712and operates to amplify the input signal. A waveguide716guides the amplified input signal to multiplexer-demultiplexer720, which may be, for example, an arrayed waveguide grating. Waveguide716may be coupled to multiplexer-demultiplexer720using an optical coupler such as, for example, a star coupler.

Multiplexer-demultiplexer720is adapted to demultiplex the input signal into the individual wavelength signals that are comprised in the input signal. An optical coupler is employed to route the plurality of individual wavelength signals into a plurality of waveguides722. Each of the plurality of individual waveguides722is adapted to guide one of the plurality of individual wavelength signals from the multiplexed input signal. While only three waveguides722are depicted inFIG. 7, any number of waveguides may be employed to handle the plurality of individual waveguide signals that are comprised in the input signal.

Each of waveguides722is adapted to guide one of the individual waveguide signals to an optical switch740. Each of optical switches740is adapted to selectively either allow the particular individual waveguide signal in the particular waveguide722to continue to propagate in the waveguide where it will eventually propagate to first multiplexer-demultiplexer720to be combined in the output signal, or to switch the individual wavelength signal for further processing such as by optical detector750. Thus, each of optical switches740operates to selectively control whether to redirect a particular individual wavelength signal. An individual wavelength signal may be rerouted for any number of reasons such as, for example, monitoring, processing, or otherwise dropping the signal from the ultimate output signal.

Optical switches740may be any type of optical switch that is adapted to selectively switch an optical signal to either of two propagation paths. For example, optical switches may be a switch such as that disclosed in the article titled “Compact Polarization-Insensitive InGaAsP—InP 2×2 Optical Switch,” 17IEEE Photonics Technology Letters1 (January 2005), the contents of which are hereby incorporated by reference in their entirety.

Each of optical detectors750is adapted to detect an optical signal that has been routed to it by the corresponding optical switch740. The individual wavelength signals that routed to detectors750may be further routed for additional processing.

The individual wavelength signals that are not switched to detectors750continue to propagate in waveguides722. The individual wavelength signals propagate to an optical coupler, and eventually into multiplexer-detector720. First optical multiplexer-demultiplexer720multiplexes the individual wavelength signals into a single signal that may be referred to as an output signal. The output signal is coupled onto output waveguide732.

ROADM710further comprises an add signal waveguide or port724. Add signal may be a multiplexed signal comprising one or more individual wavelength signals that are meant to be combined with signals comprised in the input signal. Add signal waveguide724and output waveguide732are communicatively coupled to optical multiplexer/coupler760. Optical multiplexer/coupler760operates to multiplex the individual wavelength signals comprised in the add signal with the signals comprised in the output signal received from multiplexer-demultiplexer720.

ROADM710may be formed in a structure comprising asymmetric waveguide layers similar to that described above in connection withFIG. 5. Thus, active devices—optical amplifier714and detector740—may be formed in an active waveguide layer, while the passive devices—input waveguide716, multiplexer-demultiplexer720, waveguides722, output waveguide732, and multiplexer/coupler760—may be formed in a passive waveguide layer. Devices such as switches740may be formed in both the active and passive waveguide layers. The active and passive waveguide layers may be formed to have effective indices of refraction so that a first mode of light and a second mode of light are divided unevenly between the layers. Tapers may be used to move light energy between waveguides. The asymmetry between waveguide layers allows for optically isolating the active and passive waveguide components from each other.

Thus, illustrative ROADM structures have been disclosed. The ROADM structures have novel designs and arrangements of components. Furthermore, the illustrative ROADMs may be formed in monolithic integrated structures comprising asymmetric waveguides.

It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the potential embodiments and applications. While the concepts have been described with reference to various embodiments, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation and that other embodiments are considered suitable. For example, while a particular waveguide layer structure is described above in connection with illustrative embodiments, other structures might also be used. Further, although the novel concepts have been described herein with reference to particular means, materials, and embodiments, the concepts are not intended to be limited to the particulars disclosed herein; rather the concepts extend to all functionally equivalent structures, methods, and uses, such as are within the scope of the appended claims.