WO2007108941A1

WO2007108941A1 - Adding and dropping wavelength-channels

Info

Publication number: WO2007108941A1
Application number: PCT/US2007/005850
Authority: WO
Inventors: Mark Peter Earnshaw
Original assignee: Lucent Technologies Inc.
Priority date: 2006-03-23
Filing date: 2007-03-06
Publication date: 2007-09-27
Also published as: JP2009530970A; EP1997252A1; US7200299B1; CN101405975A; KR20090003257A

Abstract

In a first aspect, an apparatus includes a substrate having a planar surface, an optical demultiplexer located along the surface, and an optical cross-connect matrix located along the surface. The optical demultiplexer has N optical outputs. The optical cross-connect matrix has a first array of N optical input ports and a second array of N optical output ports. Each optical input port of the optical cross-connect matrix is optically connected to a corresponding one of the optical outputs of the optical demultiplexer. The output ports have a sequential ordering in the second array that is a nontrivial permutation of a sequential ordering of the corresponding input ports in the first array.

Description

ADDING AND DROPPING WAVELENGTH-CHANNELS BACKGROUND

Field of the Invention

The inventions relate generally to optical communications and, more particularly, to optical communications over multiple wavelength-channels. Discussion of the Related Art

This section introduces aspects that may be helpful to facilitating a better understanding of aspects of the invention. Accordingly, the statements of this section are to be read in this light. The statements of this section are not admissions about the scope of the prior art.

In a dense wavelength division multiplexed (DWDM) system, it is often desirable be able to add or drop any desired wavelength-channel at access points of the DWDM system. An optical component for adding or dropping any wavelength- channel at its optical ports is known as a colorless optical add/drop module. Figure 1 shows an exemplary optical add/drop module 10. When functioning to drop one or more wavelength-channels, the optical add/drop module 10 receives optical signals at external optical port 12 and routes the received optical signals towards eight external optical ports 14|, 14₂, 14₃, 14₄, 14s, 14β, 14_?, 14g. When functioning to add one or more wavelength-channels, the optical add/drop module 10 receives optical signals at the eight external optical ports 14j, 14₂, 14₃, 14₄, 14s, 14β, 14₇, 14₈ and routes the received optical signals to the. external optical port 12.

The optical add/drop module 10 includes an array 18 of seven 1x2 optical intensity splitters I6₁, I6₂, I6₃, I64, I6₅, 16_ό, 1O₇ and eight tunable optical band pass filters 2Oi , 2O₂, 2O₃, 2O₄, 2O₅, 2O₆, 2O₇, 2O₈. The array 18 has a tree-structure with three layers. In the first layer, an optical input (OI) of the optical intensity splitter 16) connects to the external optical port 12 of the optical add/drop module 10. In the second and third layers, OIs of the layers connect via optical waveguides (OWs) to optical outputs (OOs) of the previous layer. In the third layer, OOs of the optical amplitude splitters 16₄ - 16₇ connect via OWs to OIs of corresponding tunable optical band pass filters 2Oi - 2Og. The tunable optical band pass filters 2Oi - 2Og have OOs that function as the external optical ports 14i - 14s of the optical add/drop module 10. In the array 18, each 1x2 optical intensity splitter 16] - 16₇ directs about half of the light intensity received at its OI to each of its OOs. In particular, this splitting of received light intensities is performed in non-wavelength selective manner so that light of each wavelength-channel is directed to both OOs of the 1x2 optical intensity splitters 16i - I67. For that reason, the array 18 redirects only about an eighth of the received light on any wavelength-channel to the tunable optical band pass filters 20] — 2O₈.

Each tunable optical band pass filter 2Oi — 2O₈ has a tunable center band pass wavelength. In particular, the band passes of the individual optical band pass filters 2Oi - 2O₈ may be separately tuned to selectively pass any one of eight preselected adjacent wavelength-channels and to selectively stop the remaining seven of the preselected adjacent wavelength-channels. For that reason, the optical add/drop module 10 is able to function as a colorless optical module for dropping wavelength- channels to the external optical ports 14j — 14s. The optical add/drop module 10 is also a colorless add module when the external optical ports 14i — 14₈ function as optical input ports and the external optical port 12 functions as an optical output port.

Unfortunately, the 1x8 optical add/drop module 10 is wasteful with light received at external optical port 12. In particular, the optical add/drop module 10 only delivers about one eighth of such received light of any wavelength-channel to the external optical ports 14] - 14g. Furthermore, in going from the external optical port 12 to any of the external optical ports 14] - 14₈, light is transmitted through a long chain of optical components. In each such chain, optical losses can further reduce the intensity of the delivered light. BRIEF SUMlVIARY

Certain aspects commensurate in scope with the disclosed embodiments are set forth below. It should be understood that these aspects are presented merely as illustrative and that these aspects are not intended to limit the scope of the invention.

Various embodiments provide optical add/drop modules. Some such modules may be more power-efficient and/or simpler to manufacture than optical add/drop modules based on tree-structures of 1x2 optical intensity splitters. In particular, some of the embodiments for optical add/drop modules may pass a larger fraction of the received light intensity on wavelength-channels being dropped. Also, some of the embodiments for optical add/drop modules have less complex optical filters. Finally, some of the embodiments of optical add/drop modules may be easily manufactured as integrated planar optical components.

In a first aspect, an apparatus includes a substrate having a planar surface, an optical demultiplexer located along the surface, and an optical cross-connect matrix located along the surface. The optical demultiplexer has N optical outputs. The optical cross-connect matrix has a first array of N optical input ports and a second array of N optical output ports. Each optical input port of the optical cross-connect matrix is optically connected to a corresponding one of the optical outputs of the optical demultiplexer. The optical output ports have a sequential ordering in the second array that is a nontrivial permutation of the sequential ordering of the corresponding optical input ports in the first array. In another aspect, an apparatus includes an optical demultiplexer, an optical cross-connect matrix, and an array of optical filters. The optical demultiplexer has an array of optical outputs. The optical cross-connect matrix has a first array of N optical input ports and a second array of N optical output ports. Each optical input port of the optical cross-connect matrix is optically connected to a corresponding one of the optical outputs of the optical demultiplexer. Each optical filter has an optical input connected to a corresponding one of the optical output ports of the optical cross- connect matrix.

In another aspect, a method includes steps for routing optical signals that are received on a plurality of wavelength-channels. The method includes routing the received optical signals from an optical input port to an optical output of a first spatial array of N optical outputs such that each one of the optical outputs of the first spatial array receives those of the optical signals of a group of the wavelength-channels. The group of wavelength channels is in correspondence with the one of the optical outputs. The method includes routing the optical signals from each optical output of the first spatial array to a corresponding optical output of a second spatial array of optical outputs such that correspondences between the groups and the optical outputs in the second spatial array are a nontrivial permutation of the correspondences between the groups and the optical outputs in the first spatial array. The method includes filtering the optical signals received at least one of the optical outputs of the second spatial array to pass optical signals on only a proper subset of the wavelength- channels corresponding to the at least one of the optical outputs. BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments are described more fully by the Figures and Detailed Description of Illustrative Embodiments. Nevertheless, the inventions may be embodied in various forms and are not limited to embodiments described in the Figures and/or Detailed Description of Illustrative Embodiments. Figure 1 is a block diagram illustrating a colorless 1x8 optical add/drop module;

Figure 2 is a block diagram of one embodiment of an optical add/drop module;

Figure 3 is a block diagram of another embodiment of an optical add/drop module; Figures 4A — 4C are block diagrams for Mach-Zehnder interferometers that may be usable as optical filters of the optical add/drop module of Figure 3;

Figures 4D is a block diagram for an optical ring resonator device that may be usable in the optical filters of the optical add/drop module of Figure 3;

Figures 4E is a block diagram for hybrid Mach-Zehnder interferometer that includes optical ring resonators and may be usable in the optical filters of the optical add/drop module of Figure 3;

Figure 5 is a flow chart illustrating a method of operating an optical add/drop module, e.g., the optical add/drop modules of Figure 2 and 3;

Figure 6 is a block diagram for a multi-channel optical wavelength-converter that may include one of the optical add/drop modules of Figures 2 and 3;

Figure 7 is a block diagram illustrating a portion of an optical communication system that includes the multi-channel optical wavelength-converter of Figure 6; and

Figure 8 is a flow chart for an all-optical method of connecting one optical transmission fiber and another optical transmission fiber in a multi-channel optical communication system, e.g., the optical communication system of Figure 7. In the Figures and text, like reference numerals indicate elements with similar functions.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Figure 2 shows an exemplary optical add/drop module 22 that has an external optical port 12 and an array of N external optical ports 14], 142, • ■•. 14_N where N is three or more and may be eight of more. The optical adds/drop module 22 and external optical ports 12, 14i, 14 - 14_N are located along a surface of a planar substrate 25.

The optical add/drop module 22 may operate as an optical add module or an optical drop module. During operation as an optical drop module, the external optical port 12 receives optical signals and the optical add/drop module 22 routes the received optical signals to the external optical ports 14i - 14_N in a wavelength- selective manner. During operating as an optical add module, the external optical ports 14] - 14_N receive optical signals, and the optical add/drop module 22 routes the received optical signals to external optical port 12. The optical add/drop module 22 can add or drop a variety of wavelength-channels at each optical port 14i - 14_N and thus, may function as a quasi-colorless add/drop module.

The optical add/drop module 22 includes an optical demultiplexer (DEMUX) 24, an NxN optical cross-connect (OXC) matrix 26, and a set of N optical waveguides 281, 28₂, - .., 28_N. The optical DEMUX 24, NxN OXC matrix 26 and optical waveguides 28] - 28_N are located along the surface of the same substrate 25. Finally, the optical add/drop multiplexer 22 may include an electronic controller 30 that is located on or off the surface of the same substrate 25.

The optical DEMUX 24 routes optical signals between the external optical port 12 to its N optical outputs (OOs) in a wavelength-selective and cyclic manner. That is, the optical DEMUX 24 routes optical signals on the wavelength-channels 1, N+l, 2N+1, 3N + 1, etc. to its first OO, routes optical signals on the wavelength- channels 2, N+2, 2N+2, 3N + 2, etc. to its second OO, and more generally, routes optical signals on the wavelength-channels k, N+k, 2N+k, 3N + k, etc. to its k-th OO. The preselected set of adjacent wavelength-channels, i.e., 1, 2, 3, etc., have about equal-bandwidth. An exemplary planar structure for the optical DEMUX 24 may include first and second star couplers and an arrayed waveguide grating (AWG). The first star coupler optically connects the external optical port 12 to one end of the optical waveguides of the AWG. The second star coupler optically connects second ends of the optical waveguides of the AWG to the OOs of the optical DEMUX 24. Such AWG structures for optical DEMUXes and optical multiplexers (MUXes) are well- known to those of skill in the art. For example, U.S. Patents 5,002,350 and 5,136,671 describe optical MUXes and optical DEMUXes that may be suitable for apparatus described herein. These U.S. patents are incorporated by reference herein in their entirety.

The optical waveguides 28] - 28_N connect the OOs of the optical DEMUX 24 to corresponding optical inputs (OIs) of the NxN OXC matrix. 26.

The NxN OXC matrix 26 includes an array of N optical waveguides that optically connect each of its OIs to a corresponding one of its OOs. The optical waveguides produce substantially wavelength-independent optical connections.

These optical connections effectively produce a nontrivial permutation of the spatial sequence formed by the OIs on the substrate 25 into the spatial sequence formed by the OOs on the substrate 25. The permutation causes the k-th external optical port 14_k to output wavelength-channels P(k), P(k) + N, P(k) + 2N, .... Here, P(k) is the value of the N-object permutation on the k-th object.

The NxN OXC matrix 26 may produce fixed or adjustable permutation of the spatial sequence of its OIs into the spatial sequence of its OOs. In the embodiments that produce a fixed permutation, the OXC matrix 26 includes a pattern of crossing optical waveguides that connect its OIs to the corresponding OOs thereby producing the permutation. For embodiments that produce an adjustable permutation, the OXC matrix 26 includes an array of optical waveguides and optical switches that end-to- end connect the optical waveguides to form two-ended optical paths. The two-ended optical paths connect the OIs of the OXC matrix 26 to the corresponding OOs of the OXC matrix 26 thereby producing the permutation. In these embodiments, the states of the optical switches determine the permutation of the spatial sequence of OIs of the NxN OXC matrix 26 into the spatial sequence of OOs of the NxN OXC matrix 26. The states of the optical switches are changed to change the permutation of the OIs into the OOs.

In the NxN OXC matrix 26, the optical switches may be, e.g., thermal optical switches or electro-optical switches. The electronic controller 30 applies the control signals that operate the optical switches of the NxN OXC matrix 26. The electronic controller 30 applies thermal control signals via heaters in embodiments based on thermal-optic switches and applies voltage control signals via electrode pairs in embodiments based on electro-optical switches.

The optical add/drop module may be used to drop a number, M, of adjacent wavelength-channels, wherein M is less than, equal to, or greater than the number N of OOs of the optical DEMUX 24. If M < N, each external optical port 14i - 14_N receives optical signals from one or no wavelength-channel. If M > N, some or all of the external optical ports 14j - 14N receive optical signals from multiple wavelength- channels. In the later case, it may be desirable to modify the optical add/drop module 22 so that only one wavelength-channel is dropped at each external optical port 14) - 14N. Figure 3 provides an example of such a modified optical add/drop module 32. Referring to Figure 3, the modified optical add/drop module 32 includes the optical DEMUX 24, the NxN OXC matrix 26, and the optical waveguides 28i - 28_N as already described in Figure 2. The modified optical add/drop module 32 also includes optical filters 34], 34a,

..., 34N and optical waveguides 36i, 36₂, ..., 36N that connect the optical filters 34i — 34_N to corresponding OOs of the NxN OXC matrix 26. Each of the optical filters 34i - 34N is configured to block one or more of the selected wavelength-channels output thereto by the corresponding OO of the NxN OXC matrix 26. In some embodiments, each optical filter 34j — 34N may pass only one of the wavelength-channels output thereto by the corresponding OO of the NxN OXC matrix 26. For example, if the corresponding OO of the NxN OXC matrix 26 outputs the wavelength-channels p, p + N, p + 2N, and p + 3N, the optical filter 36_p may pass only wavelength-channel "p". The optical filters 34) - 34N may be tunable so that the selections of passed and blocked wavelength-channels may be varied as desired.

In some other embodiments (not shown), only some of the OOs of the OXC matrix 26 connect to an optical filer 34k as shown in Figure 3. In those embodiments, the remaining OOs of the OXC matrix 26 transmit optical signals on all received wavelength-channels to the corresponding external optical ports 14_q.

Some embodiments of the optical add/drop modules 22, 32 of Figures 2 and 3 may have advantages over the add/drop module 10 of Figure 1. First, since these optical add/drop modules 22, 32 do not have optical intensity splitters, they are able to route a larger percentage of light on the desired wavelength-channels between the external optical port 12 and the external optical ports 14i — 14_N- Second, the optical add/drop module 32 may have less complex optical filters 34i - 34N than the optical add/drop module 10. In particular, the optical filters 34] - 34N can have wider channel pass regions than the optical band pass filters 2Oi - 20s, because the optical filters 34] - 34N typically must block wavelength-channels that are more widely spaced from the desired wavelength-channels. For that same reason, the optical add/drop module 32 may be less complex, cheaper to construct, and/or more easily integrated on a single planar substrate than embodiments of the optical add/drop module 10 that have the same external optical port configurations.

A variety of planar designs are available for the optical filters 34] -34N of Figure 3. The planar designs are adapted to block one or more of the wavelength- channels that output to the individual optical filters 34i -34_N by the NxN OXC matrix 26. In the various embodiments, the optical filters 34j -34N may have notch transmission characteristics, band pass transmission characteristics, or transmission characteristics that combine both behaviors. Several planar designs for optical filters 34i - 34N or component optical filters thereof are shown in Figures 4A - 4E. Useful planar designs for some such optical filters may be described, e.g., in U.S. Patent Nos. 5,596,661 and 6,580,534. Both of these U.S. Patents are incorporated by reference herein in their entirety.

Figures 4A - 4C illustrate Mach-Zehnder interferometers 34A, 34B, 34C that may function as optical filters with band pass and/or notch spectral transmission characteristics. The Mach-Zehnder interferometers 34A, 34B, and 34C have one, three, and four stages, respectively. Each stage includes a pair of optical waveguides i.e., a relatively longer optical waveguide OW_L and a relatively shorter optical waveguide OWs- The pairs of optical waveguides OW_L, OWs in adjacent stages serially couple via 2x2 optical couplers (OCs). The 2x2 optical couplers bring the paired optical waveguides OW_L, OWs close together in a coupling region that is located between the adjacent stages. This configuration produces both a direct coupling between the paired optical waveguides OW_L, OW_S of the adjacent stages and a crossed coupling between the paired optical waveguides OW_L, OW₃ of the adjacent stages. The relative strengths of the direct and crossed couplings are determined by the length of the coupling region, refractive indexes in the coupling region, and the separation of the paired optical waveguides in coupling region. The paired optical waveguides OW_L, OW_S of the first stage connect to the optical outputs of an optical intensity splitter or a 2x2 optical coupler, OC, that determines their coupling strengths to the OO of the optical filter. The paired optical waveguides

OW_L, OW_S of the last stage connect to OIs of an optical intensity combiner or a 2x2 optical coupler, OC, that determines their coupling strengths to the OO of the optical filter.

In the Mach-Zehnder interferometers 34A - 34C, the relative phase differences that the paired optical waveguides OW_L, OWs of each stage produce between light propagating there through and the strengths of direct and crossed couplings in the 2x2 optical couplers, optical splitter and/or optical combiner, i.e., the OCs, substantially define the spectral transmission characteristics of the optical filters. Referring to Figure 4A, the single stage Mach-Zehnder interferometer 34A may be configured as an optical notch filter if the optical couplers produce equal strength direct and crossed couplings, and the optical waveguides OW_L, OW_S produce a relative phase difference of π between light signals propagating there through. The single stage Mach-Zehnder interferometer 34A can be configured to substantially block optical signals on a wavelength-channel that the Mach-Zehnder interferometer 34A receives from the corresponding OO of the NxN OXC matrix 26 of Figure 3. Referring to Figures 4B and 4C, the three and four stage Mach-Zehnder interferometers 34b, 34C have spectral transmission characteristics described by finite Fourier series, e.g., finite odd harmonic series. To obtain such spectral transmission characteristics, the optical waveguides OW_L, OW₈ of each stage should typically be constrained to generate special relative phase differences between the light propagating there through. In particular, ratios of the relative phase differences produced by the individual stages over to the relative phase difference produced by the stage that produces the smallest such relative phase difference should form a sequence of special values. For example, the sequence of special values may be 1, about 2, and about 4 in the three stage Mach-Zehnder interferometer 34B. Also, the sequence of these special values may be 1, about 2, about -2, and about -2 in the four- stage Mach-Zehnder interferometer 34C. The three and four stage Mach-Zehnder interferometers 34B, 34C whose stages satisfy the above constraints and whose 2x2 optical couplers, optical splitter and/or optical combiner generate suitable direct and crossed optical couplings can have wider spectral notch regions than the one-stage Mach-Zehnder interferometer 34A. In light of the description herein, suitable values of the direct and crossed optical couplings for the 2x2 optical couplers, optical splitter and/or optical combiner of the Mach-Zehnder interferometers 34B, 34C would also be easily determined by those of skill in the art and may be described, e.g., in U.S. Patent 5,596,661.

In some embodiments, the optical filters optical 34i - 34_N of Figure 3 are wavelength-tunable.

In such embodiments, the electronic controller 30 may also generate electrical signals that control the selection of the wavelength-channels passed by the optical filters 34i - 34_N. The electrical control signals either operate heaters that heat thermally active segments (Ss) of the optical waveguides OW_L, OW_S or operate paired electrodes that apply voltages across electro-optically active segments (Ss) of the optical waveguides OW_L, OW_S as shown in Figures 4A - 4C. The active segments, S, have refractive indexes that respond to temperature or electric field values therein. The resulting refractive index changes vary the relative phase differences that the paired optical waveguides OW_L, OW_S produce on light propagating there through.

In such embodiments, the electronic controller 30 may generate other electrical signals to control heaters or paired electrodes that regulate the temperature or electric field in the 2x2 optical couplers, optical splitter and/or optical combiner, i.e., the OCs, of the Mach-Zehnder interferometers 34B, 34C of Figures 4B - 4C. In particular, refractive indexes and refractive index contrasts fix the coupling between the closely spaced optical waveguide pairs in coupling regions of 2x2 optical couplers, optical splitters and/or optical combiners, i.e., the OCs. By controlling the Il application of heat to thermo-optically active portions of those coupling regions or the application of voltages across electro-optically active portions of those coupling regions, the electronic controller 30 may change relative strengths of the direct and crossed optical couplings between adjacent stages of the Mach-Zehnder interferometers 34B, 34C.

Some embodiments may configure the optical waveguides and couplers of the Mach-Zehnder interferometers 34A, 34B, 34C to respond to the above types of electrical control signals from the electronic controller 30. Such control signals can control wavelength ranges of band passes and/or band notches in the Mach-Zehnder interferometers 34A, 34B, 34C of Figures 4A - 4C. In light of the above-description, one of skill in the art would be able to implement wavelength-tunability in the Mach- Zehnder interferometers 34A, 34B, 34C without undue experimentation.

The optical filters 34_t -34_N of Figure 3 may include optical ring resonators (ORRs) as illustrated in optical filters 34D, 34E shown in Figures 4D and 4E. Referring to Figure 4D, the optical filter 34D includes an input optical waveguide (IOW), an output optical waveguide (OOW), and a sequence of coupled optical ring resonators. The sequence of coupled optical ring resonators forms an optical coupler between the input and output optical waveguides 1OW, OOW. In the sequence, the various optical ring resonators have close but different resonant wavelengths, i.e., their resonant wavelengths are verniered. Such a configuration of the resonant wavelengths can cause resonant wavelengths of the sequence to be more widely spaced than those of the individual optical ring resonators. Due to the wider spacing of these resonant wavelengths, the sequence of optical ring resonators may be configured to have wider band notch regions than an optical device in which input and output optical waveguides are coupled by a single optical ring resonator (not shown). The wider band notches can block more of the wavelength-channels outputted by the NxN OXC matrix 26 to the individual optical filters 34j - 34_N of Figure 3 thereby improving wavelength-selectivity.

While the illustrated optical band pass filter 34D includes a sequence of three optical ring resonators, other embodiments of such filters may have one, two, three, or more of the optical ring resonators in the sequence that couples the input and output optical waveguides IOW, OOW. Referring to Figure 4E, the optical filter 34E includes a hybrid Mach-Zehnder interferometer having multiple stages. Each stage includes a pair of optical waveguides OW_L, OW_S and one or more optical ring resonators that couple to the optical waveguides OW_L, OWs. In each stage, the paired optical waveguides OW_L, OWs produce a relative phase difference between optical signals propagating there through. Between different stages, the optical ring resonators may have close resonant wavelengths, i.e., the resonant wavelengths of the optical ring resonators of the various stages may have a vernier-like relationship. For that reason, the sequence of stages may provide a spectral transmission characteristic with wider optical notch regions as in the optical filter 34D of Figure 34D. Embodiments of such hybrid multistage Mach-Zehnder interferometers may be further described in U.S. Patent 6,580,534.

Some embodiments of the optical filters optical 34i - 34_N of Figure 3 include a wavelength-tunable version of the optical filter 34D, 34E of Figure 4D or 4E. In such embodiments, the electronic controller 30 is configured to operate one or more heaters that can apply heat to thermally active portions of the optical resonant rings or is configured to operate one or more electrode pairs that can apply voltages across electro-optically active portions of the optical ring resonators. In either configuration, the electronic controller 30 can vary refractive indexes in the active portions of the optical ring resonators thereby varying the resonant wavelengths of the optical ring resonators themselves. In embodiments that include the hybrid Mach-Zehnder interferometer 34E, the electronic control 30 may also apply control signals to heaters that can apply heat or electrode pairs that can apply voltages across active segments, S, of the pairs of optical waveguides OW_L, OW_S. In each such pair of optical waveguides OW_L, OWs, the active segments, S, have refractive indexes that respond to such control signals thereby changing the relative phase differences produced between the light that propagates through the pair. In such embodiments, the electronic control 30 may also be configured to apply control signals to heaters that can apply heat across active regions of optical couplers, OC, or may be configured to apply control signals to electrode pairs that can apply voltages across the active regions of the optical couplers, OC. Such control signals can vary the relative strengths of direct and crossed optical couplings between the optical waveguides OW_L, OW_S of the stages adjacent such optical couplers, OC. In light of the above- description, one of skill would be able to implement such control structures to make embodiments of the optical filters 13D, 13E that have wavelength-tunable band passes and/or band notches. In some embodiments, the individual optical filters 34i - 34_N of Figure 3 may include serial cascades of component optical filters. The component optical filters may have the designs illustrated in Figures 4A - 4E or may have wavelength-tunable versions of said designs as already described. In such cascades, different component optical filters can be configured to block different ones of the individual wavelength- channels output to the cascades by the corresponding OOs of the NxN OXC matrix 26. That is, such cascades can be configured to serially filter out the undesired wavelength-channels. For example, the optical filter 34^ may be a cascade of a first component optical filter having a transmission notch for wavelength-channel k, a second component optical filter having a transmission notch for wavelength-channel k + 2N, and a third component optical filter having a transmission notch for wavelength-channel k +3N. In such an embodiment, the optical filter 34_k would only pass optical signals on wavelength-channel k + N in response to receiving optical signals on wavelength-channels k, k +N, k + 2N, and k +3N from the NxN OXC matrix 26. Figure 5 illustrates a method 40 for operating an all-optical add/drop module that connects a first external optical port to N second external optical ports, e.g., the optical add/drop module 22 of Figure 2 or the optical add/drop module 32 of Figure 3. The method 40 includes routing received optical signals from the first external optical port into a first linear array of N OOs thereby spatially repartitioning the optical signals between the different OOs based on their associated wavelength- channel group (step 44). In particular, the routing step transmits the optical signals of each wavelength-channel group to a corresponding OO of the spatial sequence of OOs that is located on or along a surface of a substrate. Each wavelength-channel group may contain one, two, or more wavelength-channels. The optical DEMUX 24 may perform the routing step when each wavelength-channel group includes wavelength- channels that are spaced apart by N times the width of a single wavelength-channel. The method 40 also includes rerouting the optical signals from the first linear array of OOs to a second linear array of N OOs, which is also located on or along the surface of the substrate (step 44). The rerouting step nontrivially permutes the correspondences between the wavelength-groups and the individual OOs of the linear arrays. In particular, the permuting spatially reorders the assignments of the wavelength-channel groups to an array of OOs that is linearly ordered along the same surface of the substrate. For example, the NxN OXC matrix 26 may perform such a permuting action by connecting its N OIs into its N OOs with a crossing spatial pattern of two-ended optical paths. Optionally, the method 40 includes filtering those optical signals that are received at one or more of the individual OOs of the second linear array to remove the optical signals not belonging to proper subsets of the wavelength-channels of the group corresponding to the individual OOs (step 46). At the one or more OOs, the filtering step reduces the number of passed wavelength-channels. At each such OO, the filtering step may include blocking all optical signals except those on a single selected wavelength-channel corresponding to the OO even though optical signals on multiple wavelength-channels were delivered to the OO. The optical filters 34j — 34_N may perform, e.g., the filtering step so that each external port 14i - 14_N receives the optical signals on a smaller set of wavelength-channels, e.g., one such channel. Figure 6 shows a multi-channel optical wavelength converter 50 that includes a tunable optical add/drop module (OADM) 52 with N+l external optical ports, e.g., one of the optical add/drop modules 22, 32 shown in Figures 2 and 3. The multichannel optical wavelength converter 50 also includes N single-channel optical wavelength-converters 54i, 54₂, •■•» 54_N and an optical multiplexer (MUX) 56 with N OIs. Each single-channel optical wavelength-converter 54i - 54_N has an OI that connects to a corresponding one of the N external optical ports 14] — 14_N of the optical add/drop module 52 via an optical waveguide (OW). Each single-channel optical wavelength-converter 54i - 54_N has an OO that connects via another optical waveguide to a corresponding OI of the optical MUX 56. Each single-channel optical wavelength converter 54j - 54_N is pumped by a wavelength-tunable pump laser 58_1? 582, ..., 58N- In the multi-channel optical wavelength converter 50, the optical add/drop module 52 routes optical signals of only one wavelength-channel to each external optical port 14i - 14N- For that reason, each of the single-channel optical wavelength- converters 54i -54_N only wavelength-converts optical signals on a single wavelength- channel. The wavelength-conversions, e.g., permute the channel assignments for some of the independent data streams that are originally received at the external optical port 12.

In the multi-channel optical wavelength converter 50, the electronic controller 30 configures the optical add/drop module 52 to pre-compensate the spatial permutation of the wavelength-channels that will be caused by the wavelength- conversions in the single-channel optical wavelength-converters 54i -54_N. In particular, the pre-compensation ensures that the wavelength-converted optical signals will be transmitted to the proper OIs of the optical MUX 56 so that the optical MUX 56 routes the wavelength-converted optical signals to the external optical port 14. Indeed, the optical MUX 56 also functions. as an optical band pass filter for the non- wavelength-converted light passing through the single-channel optical wavelength- converters 54i — 54jM. Such non-wavelength-converted light is typically not routed to the external optical port 14 by the optical MUX 56.

In the multi-channel optical wavelength-converter 50, the single-channel optical wavelength-converters 54] - 54_N are pumped by the tunable lasers 58i - 58_N- In particular, the tunable lasers 58j - 58_N generate pump light at the converted- wavelength, and the input streams of optical signals modulate the pump light to produce the wavelength-converted optical signals. The pump lasers 58i — 58_N are wavelength tunable, and a second electronic controller 30' sets the output wavelengths of the pump lasers 58i - 58_N and also controls the electronic controller 30 of the tunable OADM 52 during operation.

In the multi-channel optical wavelength-converter 50, the single-channel optical wavelength-converters 54i - 54N may be, e.g., semiconductor optical amplifiers (SOAs). In a SOA, input optical signals amplitude modulate pump light at the converted-wavelength to produce the wavelength-converted optical signals. Such modulation is caused by nonlinear optical effects in the semiconductor of the SOA. The semiconductor may be a compound crystalline material that includes gallium (Ga), indium (In), arsenic (As), and/or phosphorus (P). For example, a SOA may be a layered InGaAsP structure that is fabricated on an InP substrate. The fabrication of SOAs for use as wavelength-converters is well-known to those of skill in the art.

The single-channel optical wavelength-converters 54i - 54_N may also be other types of single-channel optical wavelength converters known to those of skill in the art, e.g., lithium-niobate optical wavelength converters.

In the multi-channel optical wavelength converter 50, the optical MUX 56 may be, e.g., a planar AWG device. The construction of optical MUXes with AWGs is substantially similar to the construction of optical DEMUXes with planar AWGs and optical star couplers as has already been described. U.S. Patents 5,002,350 and 5,136,671 describe AWG-based optical MUXes that may be suitable for the optical MUX 56.

Hybrid integration techniques that combine dielectric and semiconductor planar substrates may be capable of producing integrated planar semiconductor ^" modules that include the optical DEMUXing, switching, filtering, wavelength converting, and MUXing components of the multi-channel optical wavelength- converter 50. Such hybrid integration techniques may produce modules with better overall performance while eliminating the need for costly optical connectors and fiber splices. The fabrication of the multi-channel optical wavelength-converter 50 may eventually be possible on a single planar InP substrate.

Figure 7 illustrates a portion 60 of a DWDM optical communication system that includes a node 62, an input optical transmission fiber 64, an output optical transmission fiber 66, and the multi-channel optical wavelength-converter 50 of Figure 6. In the node 62, the multi-channel optical wavelength-converter 50 end- connects the input optical transmission fiber 64 to the output optical transmission fiber 66. This all-optical connection can transfer to the output optical transmission fiber 66 the optical signals of the input optical transmission fiber 64 on a subset of the wavelength-channels used therein. In the output optical transmission fiber 66, the transferred optical signals can be transferred on the same or different wavelength- channels or on a combination of the same and different wavelength-channels as used in the input optical transmission fiber 64. At the node 62, the wavelength-conversion can change dynamically so that the above-described new wavelength-channel assignments are responsive to channel failures, channel congestion, and/or channel unavailabilities in the output optical transmission fiber 66. Finally, the cyclic operation of the optical add/drop module 52 of the multi-channel optical wavelength- converter 50 may enable optical signal transfers that use wavelength-conversions on many subsets of the set of wavelength-channels in the input optical transmission fiber 64.

Finally in the DWDM optical transmission system of Figure 7, the node 62 can also use a parallel combination of more than one of the multi-channel optical wavelength-converters 50 to transfer optical signals on more wavelength-channels from the input optical transmission fiber 64 to the output optical transmission fiber 66.

Figure 8 illustrates an all-optical method 70 for transferring optical signals between first and second optical transmission lines of a multi-channel optical communication system, e.g., between the transmission fibers 64 and 64 of the DWDM optical communication system of Figure 7. The method 70 includes routing optical signals received from an output end of the first optical transmission line to a first linear array of N OOs to repartition the received optical signals according to their associated wavelength-channel groups (step 72). Each wavelength-channel group may include one or more of the wavelength channels that transport optical signals on the first optical transmission fiber. The method 70 includes rerouting the optical signals from the first linear array of OOs to a second linear array of N OOs to nontrivially permute correspondences between the wavelength-channel groups and sequentially ordered individual OOs (step 74). For some or all of the OOs of the second linear array, the method 70 includes filtering the optical signals transmitted from each such OO to block all except one of the wavelength-channels (step 76). The filtering passes optical signals on a different wavelength-channel for each such OO. The method 70 includes wavelength-converting the optical signals from some of the OOs of the second linear array to permute the wavelength-channel of those optical signals (step 78). The method 70 also includes optically multiplexing the wavelength- converted optical signals to an external optical port that connects to one end of the second optical transmission fiber (80).

From the disclosure, drawings, and claims, other embodiments of the invention will be apparent to those skilled in the art.

Claims

What we claim is:

1. An apparatus, comprising: a substrate having a planar surface; an optical demultiplexer having N optical outputs and being located along the surface; an optical cross-connect matrix having a first array of N optical input ports and a second array of N optical output ports and being located along the surface, each optical input port of the optical cross-connect matrix being optically connected to a corresponding one of the optical outputs of the optical demultiplexer; and wherein the optical output ports have a sequential ordering in the second array that is a nontrivial permutation of a sequential ordering of the corresponding optical input ports in the first array.

2. The apparatus of claim 1, wherein the optical cross-connect matrix is tunable to route light from one of the optical input ports to different ones of the optical output ports therein.

3. The apparatus of claim 1, further comprising a controller configured to transmit a plurality of control signals to the optical cross-connect matrix, the optical cross-connect matrix capable of changing the permutation in response to each control signal.

4. The apparatus of claim 1, further comprising an array of optical filters, each of the optical filters being connected to receive light from a corresponding one of the optical output ports of the optical cross-connect matrix.

5. An apparatus, comprising: an optical demultiplexer having an array of optical outputs; an optical cross-connect matrix having a first array of N optical input ports and a second array of N optical output ports, each optical input port of the optical cross-connect matrix being optically connected to a corresponding one of the optical outputs of the optical demultiplexer; and an array of optical filters, each optical filter having an optical input connected to a corresponding one of the optical output ports of the optical cross-connect matrix.

6. The apparatus of claim 5, wherein some of the optical filters can be reconfigured to vary spectral pass regions thereof.

7. The apparatus of claim 5, wherein the optical cross-connect matrix is capable of changing optical routings between its optical input ports and its optical output ports.

8. A method of routing optical signals that are received on a plurality of wavelength-channels, comprising: routing the received optical signals from at an optical input port to an optical output port of a first spatial array of N optical outputs such that each one of the optical outputs of the first spatial array receives those of the optical signals on a group of the wavelength-channels, the group of wavelength channels being in correspondence with the one of the optical outputs; routing the optical signals from each optical output of the first spatial array to a corresponding optical output of a second spatial array of optical outputs such that correspondences between the groups and the optical outputs in the second spatial array are a nontrivial permutation of the correspondences between the groups and the optical outputs in the first spatial array; and filtering the optical signals received at least one of the optical outputs of the second spatial array to pass optical signals on only a proper subset of the wavelength- channels corresponding to the at least one of the optical outputs.

9. The method of claim 8, further comprising: wavelength-converting the optical signals from some of the optical outputs of the second spatial array to change wavelength-channels carrying optical signals received from the some of the optical outputs.

10. The method of claim 9, further comprising: optically multiplexing the wavelength-converted optical signals from the some of the optical outputs of the second spatial array to route said multiplexed optical signals to a single optical output port.