BACKGROUND

Field

The present disclosure relates to optical communication equipment and, more specifically but not exclusively, to optical switches that can be used in an optical-network node.

Description of the Related Art

This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.

An optical cross-connect (OXC) switch can be used to perform remote

(re)configuration of a network node. An OXC switch can be software-provisionable, which enables the network operator to remotely specify which of the carrier wavelengths are to be added, dropped, and/or passed through at the network node. OXC switches can be used, e.g., in network nodes of regional, metro, and long-haul optical networks. Some OXC switches can be used to implement reconfigurable optical add/drop multiplexers.

An OXC engineer sometimes needs to accommodate conflicting OXC-design requirements, such as low insertion losses and high degree of optical isolation between wavelength channels. The latter two requirements conflict with each other, e.g., because the high degree of optical isolation typically requires tighter optical filtering which normally leads to higher insertion losses. An OXC architecture that provides flexibility for achieving an acceptable compromise between these and possibly other conflicting design requirements is therefore desirable.

SUMMARY OF SOME SPECIFIC EMBODIMENTS

Disclosed herein are various embodiments of an optical cross-connect switch having ingress and egress wavelength-selective switches (WSSes) interconnected such that at least some ingress/egress WSS pairs are connected by way of two different respective optical paths originating at two different output ports of the ingress WSS. One of the two optical paths may include an optical splitter, different optical outputs of which are connected to different respective egress WSSes. Some embodiments may include 2x 1 optical couplers, each configured to join the two respective optical paths prior to connecting them to an input port of the egress WSS. In some embodiments, the ingress WSS may have two sets of wavelength channels characterized by two different slot widths and be configurable to direct optical signals corresponding to one set through one of the two optical paths while directing optical signals corresponding to the other set through the other one of the two optical paths.

Some embodiments can advantageously be used to satisfy different filtering and insertion-loss requirements for different sets of wavelength channels.

According to an example embodiment, provided is an apparatus comprising: a plurality of ingress wavelength-selective switches, each having a respective input port and a respective set of output ports; a plurality of egress wavelength-selective switches, each having a respective output port and a respective set of input ports; and an interconnect device configured to optically connect the output ports of the ingress wavelength-selective switches and the input ports of the egress wavelength-selective switches using optical waveguides; and wherein, for at least some pairs of the ingress and egress wavelength- selective switches optically connected through the optical waveguides, the interconnect device is configured to: optically connect one of the output ports of a respective ingress wavelength-selective switch to one of the input ports of a respective egress wavelength- selective switch; and optically connect a different one of the output ports of the respective ingress wavelength-selective switch to said one of the input ports or to a different one of the input ports of the respective egress wavelength-selective switch.

In some embodiments of the above apparatus, the interconnect device comprises an optical splitter having an optical input and a plurality of optical outputs, the optical input being optically connected to said different one of the output ports of the respective ingress wavelength-selective switch, one of the optical outputs being optically connected to said one of the input ports or to said different one of the input ports of the respective egress wavelength-selective switch.

In some embodiments of any of the above apparatus, another one of the optical outputs of the optical splitter is optically connected to an input port of another egress wavelength-selective switch. In some embodiments of any of the above apparatus, each of the optical outputs of the optical splitter is optically connected to an input port of a different respective egress wavelength-selective switch.

In some embodiments of any of the above apparatus, the interconnect device further comprises an optical coupler having first and second optical inputs and an optical output, the first optical input being optically connected to said one of the output ports of the respective ingress wavelength-selective switch, the second optical input being optically connected to said one of the outputs of the optical splitter, the optical output of the optical coupler being optically connected to said one of the input ports of the respective egress wavelength-selective switch.

In some embodiments of any of the above apparatus, the apparatus further comprises a degree-N optical cross-connect switch, where N is an integer greater than two; and wherein the optical splitter has N-l optical outputs.

In some embodiments of any of the above apparatus, the interconnect device comprises N optical splitters, each having N-l optical outputs, each of the N-l optical outputs being optically connected to an input port of a different respective egress wavelength-selective switch.

In some embodiments of any of the above apparatus, the interconnect device comprises an optical coupler having first and second optical inputs and an optical output, the first optical input being optically connected to said one of the output ports of the respective ingress wavelength-selective switch, the second optical input being optically connected to said different one of the output ports of the respective ingress wavelength- selective switch, the optical output of the optical coupler being optically connected to said one of the input ports of the respective egress wavelength-selective switch.

In some embodiments of any of the above apparatus, the apparatus further comprises a degree-N optical cross-connect switch, where N is an integer greater than two; and wherein the interconnect device comprises N(N l) 2x 1 optical couplers connected to the optical waveguides.

In some embodiments of any of the above apparatus, the interconnect device comprises N optical splitters, each having N-l optical outputs, each of the N-l optical outputs being optically connected to an input port of a different respective egress wavelength-selective switch by way of a respective one of the N(N l) 2x 1 optical couplers. In some embodiments of any of the above apparatus, the interconnect device comprises: a first optical coupler having first and second optical inputs and an optical output, the optical output of the first optical coupler being optically connected to one of the input ports of a corresponding egress wavelength-selective switch, the first optical input of the first optical coupler being optically connected to one of the output ports of a first respective ingress wavelength-selective switch, the second optical input of the first optical coupler being optically connected to one of the output ports of a second respective ingress wavelength-selective switch; and a second optical coupler having first and second optical inputs and an optical output, the optical output of the second optical coupler being optically connected to another one of the input ports of the corresponding egress wavelength-selective switch, the first optical input of the second optical coupler being optically connected to another one of the output ports of the first respective ingress wavelength-selective switch, the second optical input of the second optical coupler being optically connected to another one of the output ports of the second respective ingress wavelength-selective switch.

In some embodiments of any of the above apparatus, the interconnect device is configured to optically connect said different one of the output ports of the respective ingress wavelength-selective switch to said different one of the input ports of the respective egress wavelength-selective switch.

In some embodiments of any of the above apparatus, the apparatus further comprises a wavelength blocker coupled between said different one of the output ports of the respective ingress wavelength-selective switch and the interconnect device.

In some embodiments of any of the above apparatus, the apparatus further comprises an optical amplifier serially connected with wavelength blocker and coupled between said different one of the output ports of the respective ingress wavelength- selective switch and the interconnect device.

In some embodiments of any of the above apparatus, each of the ingress wavelength-selective switches has K output ports; and each of the egress wavelength- selective switches has L input ports, where L>K.

In some embodiments of any of the above apparatus, the respective ingress wavelength-selective switch has a first set of wavelength channels, each having a first slot width, and a second set of wavelength channels, each having a second slot width different from the first slot width. In some embodiments of any of the above apparatus, the respective ingress wavelength-selective switch is configurable to: route optical signals of the first set of wavelength channels to said one of the output ports thereof; and route optical signals of the second set of wavelength channels to said different one of the output ports thereof

In some embodiments of any of the above apparatus, the respective ingress wavelength-selective switch is configured to: have the first set of wavelength channels arranged within a first spectral band; and have the second set of wavelength channels arranged within a second spectral band, the second spectral band being distinct from the first spectral band.

In some embodiments of any of the above apparatus, the apparatus further comprises a plurality of optical add/drop blocks; and the interconnect device is further configured to: optically connect the output ports of the ingress wavelength-selective switches to at least some of the optical add/drop blocks; and optically connect the input ports of the egress wavelength-selective switches to at least some of the optical add/drop blocks.

In some embodiments of any of the above apparatus, the plurality of egress wavelength-selective switches has N egress wavelength-selective switches, where N is an integer greater than one; the plurality of ingress wavelength-selective switches has N ingress wavelength-selective switches; the plurality of optical add/drop blocks has M optical add/drop blocks, where M is an integer greater than one; each of the ingress wavelength-selective switches has K output ports, where K>N+M; and each of the egress wavelength-selective switches has L input ports, where L>2N+M-2.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and benefits of various disclosed embodiments will become more fully apparent, by way of example, from the following detailed description and the accompanying drawings, in which:

FIG. 1 shows a block diagram of an OXC switch according to an embodiment;

FIG. 2 schematically shows some of the optical paths in the OXC switch of FIG. 1 according to an embodiment;

FIG. 3 schematically shows some of the optical paths in the OXC switch of FIG. 1 according to another embodiment; FIG. 4 shows a block diagram of an optional optical module that can be used in the OXC switch of FIG. 1 according to an embodiment;

FIGs. 5A-5D graphically show example spectra of WDM signals that can be transmitted through the OXC switch of FIG. 1 according to an embodiment; and

FIG. 6 schematically shows some of the optical paths in the OXC switch of FIG. 1 according to yet another embodiment.

DETAILED DESCRIPTION

A network node or OXC switch is often described in reference to its number N of “degrees.” Each degree represents a respective switching direction that is typically associated with a duplex fiber pair. For example, a degree-2 OXC switch operates to switch optical signals in two directions, typically referred to as“East” and“West.” A degree-4 OXC switch operates to switch optical signals in four directions, typically referred to as“North,”“South,”“East,” and“West,” and so on. The number of degrees supported by modem network nodes and OXC switches typically falls in the range between N=2 and N=20. OXC devices corresponding to N>20 are also used, albeit less frequently.

FIG. 1 shows a block diagram of an OXC switch 100 according to an embodiment. OXC switch 100 is a degree-N OXC switch, where N is an integer greater than one. Although FIG. 1 illustrates an embodiment of OXC switch 100 corresponding to N>2, embodiments in which N=2 are also contemplated. Various embodiments of OXC switch 100 can be used, e.g., to implement various network nodes.

OXC switch 100 comprises optical-line modules 110 _I -110 _N , each connected between a respective one of duplex fiber pairs (102I/104I)-(102N/104N) and a fiber- interconnect device 130. Each of duplex fiber pairs (102I/104I)-(102N/104N) is configured to support a duplex optical connection associated with the respective degree of OXC switch 100. A fiber 102 _n (where n=l, 2, ..., N) is configured to apply incoming optical signals to optical-line module 110 _n . A fiber 104 _n is configured to transmit outgoing optical signals outputted by optical-line module 110 _n . Fiber-interconnect device 130 optically interconnects optical-line modules 110I-110N and optical add/drop blocks 150I-150M as described in more detail below, where M is an integer greater than one. In some embodiments, M>N. In an example embodiment, an optical-line module 110 _n comprises optical amplifiers 112 _n and 114 _n and a twin wavelength-selective switch (WSS) 116 _n . One or more of optical amplifiers 112 _n and 114 _n may be optional. When present, optical amplifier 112„ is connected to fiber 102 _n and operates to amplify the input optical signals received by optical-line module 110 _n . When present, optical amplifier 114 _n is connected to fiber 104 _n and operates to amplify output optical signals transmitted by optical-line module 110 _n . Twin WSS 116 _n includes an ingress WSS and an egress WSS (not explicitly shown in FIG. 1, see FIGs. 2-3). (Herein, a twin WSS is a pair of WSSes. Each member of such a pair communicates light with one optical fiber of an input/output pair at the same degree of an OXC.) The ingress WSS of twin WSS 116 _n is configured to appropriately de multiplex an input wavelength-division-multiplexed (WDM) signal received through an input port 113 _n connected to fiber 102 _n and apply the resulting de-multiplexed optical signals to a corresponding subset of optical fibers 122 _n that connect the twin WSS and fiber-interconnect device 130. The egress WSS of twin WSS 116 _n is configured to multiplex optical signals received through a remaining subset of optical fibers 122 _n from fiber-interconnect device 130 and apply the resulting output WDM signal to an output port 115 _n connected to fiber 104 _n .

In an example embodiment, each set of optical fibers 122 _n may include (K+L) individual optical fibers, each configured to provide a simplex connection, where K is the number of output ports in the ingress WSS of twin WSS 116 _n , and L is the number of input ports in the egress WSS of twin WSS 116 _n . The numbers K and L depend, inter alia, on the numbers N and M, and other pertinent characteristics of the particular embodiment of OXC switch 100. As an example, in some embodiments, each set of optical fibers 122 _n may include up to sixty-four or up to eighty optical fibers. In some embodiments, K¹L, e.g., as explained below in reference to FIGs. 2-3.

In an example embodiment, an add/drop block 150 _m (where m=l, 2, ..., M) includes an add sub-block and a drop sub-block (not explicitly shown in FIG. 1). The add sub-block of add/drop block 150 _m operates to appropriately multiplex the optical signals received through a subset of optical fibers 152 _m from the corresponding external optical transmitters (not explicitly shown in FIG. 1) and apply the one or more resulting WDM signals to the corresponding subset of optical fibers 148 _m that connect add/drop block 150 _m and fiber-interconnect device 130. The drop sub-block of add/drop block 150 _m operates to appropriately de-multiplex the WDM signals received through another subset of optical fibers 148 _m and direct the resulting de-multiplexed optical signals, through another subset of optical fibers 152 _m , to the corresponding external optical receivers (not explicitly shown in FIG. 1).

In an example embodiment, each or some of the add and drop sub-blocks of add/drop block 150 _m can be implemented using a respective optical multicast switch. A person of ordinary skill in the art will understand that the sizes of these optical multicast switches may depend on the numbers of external optical transmitters and receivers connected, by way of optical fibers 152 _m , to each particular add/drop block 150 _m .

Fiber-interconnect device 130 is a passive optical device that uses optical waveguides (e.g., planar optical waveguides, optical fibers, and/or fiber-optic cables) and/or free-space optics to interconnect the sets of optical fibers 122 _n and 148 _m in a manner that provides the following optical connections: (i) between the ingress WSS of optical-line module 110i and the egress WSS of optical-line module 110 _j , where i¹j, i=l, 2, ..., N, and j=l, 2, ..., N; (ii) between the ingress WSS of optical-line module 110 _n and the drop sub-block of add/drop block 150 _m , where n=l, 2, ..., N and m=l, 2, ..., M; and (iii) between the add sub-block of add/drop block 150 _m and the egress WSS of optical-line module 110 _n , where n=l, 2, ..., N and m=l, 2, ..., M. As used herein, the term“passive” should be interpreted to mean that fiber-interconnect device 130 has a fixed optical signal routing configuration and does not generate any new light. The signal-routing

configuration is“fixed” in the sense that the topology of the optical paths through fiber- interconnect device 130 does not change (is constant) during operation of OXC switch 100. In some (e.g., monolithic fiber-shuffle) embodiments, fiber-interconnect device 130 can be such that the topology of the optical paths therethrough cannot physically be changed without damaging the device in some manner.

In an example embodiment, fiber-interconnect device 130 may connect the set of optical fibers 122i to the sets of optical fibers 122 ₂ -122 _N and to the sets of optical fibers 148 _I -148 _M to provide at least (N+M-l) duplex connections for optical-line module 110i, wherein at least (N l) duplex connections connect optical-line module 110i to each of optical line modules 110 ₂ -IIO _N , and M duplex connections connect optical-line module 110i to each of optical add/drop blocks 150 _I -150 _M . Fiber-interconnect device 130 may also connect the set of optical fibers 122 ₂ to the sets of optical fibers 122i, 122^-122 _\ and to the sets of optical fibers 148 _I -148 _M to provide at least (N+M-l) duplex connections for optical-line module 110 ₂ , wherein at least (N l) duplex connections connect optical-line module IIO2 to each of optical line modules 110i, IIO3-IIO _N , and M duplex connections connect optical-line module IIO2 to each of optical add/drop blocks 150 _I -150 _M , and so on.

In addition, for some or all optical-line modules 110 _I -110 _N , fiber-interconnect device 130 may connect the ingress WSS of optical-line module 110i to the egress WSS of optical-line module 110 _j by way of two different optical paths therethrough, where i¹j.

The first of the two optical paths may connect one output port of the ingress WSS of optical-line module 110i to an input port of the egress WSS of optical-line module 110 _j . The second of the two optical paths may connect another output port of the ingress WSS of optical-line module 110i to the same input port of the egress WSS of optical-line module 110 _j as the first path or to another input port of the egress WSS of that optical-line module 110 _j . Examples of such different optical paths are described in more detail below in reference to FIGs. 2-3.

In some alternative embodiments, fiber-interconnect device 130 may only support reduced (with respect to the above-described) connectivity for one or more of twin WSSes 116I-116N and/or one or more of optical add/drop blocks 150I-150M.

In an example embodiment, the sets of optical fibers 122 _n and 148 _m can be implemented using fiber-optic cables terminated by suitable multi- fiber connectors. In such an embodiment, each of the fiber-optic cables that implements optical fibers 122 _n has a first connector that is mated with a matching connector located on optical-line module 11 On, and a second connector that is mated with a matching connector located on fiber- interconnect device 130. Each of the fiber-optic cables that implement optical fibers 148 _m similarly has a first connector that is mated with a matching connector located on fiber- interconnect device 130, and a second connector that is mated with a matching connector located on add/drop block 150 _m . In some embodiments, the connectors used in the fiber optic cables that implement the sets of optical fibers 122 _n and 148 _m can be selected from the assortment of multi- fiber connectors defined in the following standards: (i) IEC- 61754-7,“Fibre optic interconnecting devices and passive components— Fibre optic connector interfaces— Part 7: Type MPO connector family” and (ii) TIA-604-5-D,“Fiber Optic Connector Intermateability Standard, Type MPO,” both of which standards are incorporated herein by reference in their entirety. These standards refer to the multi- fiber connectors defined therein as Multi- fiber Push On (MPO) connectors. In the

corresponding embodiments, optical-line modules 110, fiber-interconnect device 130, and add/drop blocks 150 have the matching MPO connectors that can be properly mated with the MPO connectors of the fiber-optic cables.

In some embodiments, OXC switch 100 can be modified in a relatively

straightforward manner to remove the add and/or drop functionality. The modification may include removing or disconnecting optical add/drop blocks 150I-150M and optionally changing the topology of fiber-interconnect device 130 by removing some or all of the optical waveguides connected to the sets of optical fibers 148I-148M.

FIG. 2 schematically shows some of the optical paths in OXC switch 100 according to an embodiment. More specifically, FIG. 2 shows an ingress WSS 210i and an egress WSS 220 _j connected through fiber-interconnect device 130 as indicated in the figure. Ingress WSS 210i is a part of twin WSS 116i and is connected through input port 113i to fiber 102i and optical amplifier 112i. Egress WSS 220 _j is a part of twin WSS 116 _j and is connected through output port 115 _j to fiber 104 _j and optical amplifier 114 _j . A person of ordinary skill in the art will understand that the ingress WSS 210i and the egress WSS 220 _j shown in FIG. 2 represent any pair of a plurality of such ingress/egress WSS pairs optically connected in the indicated manner.

In an example embodiment, some or all ingress WSSes 210i (where i=l, 2, ..., N) and/or some or all egress WSSes 220 _j (where j=l , 2, ... , N) can be implemented using optical devices in which beam-steering elements are implemented using the Liquid Crystal on Silicon (LCoS) technology or the micro-electro-mechanical-system (MEMS) mirror- array technology, or a combination of both.

In different embodiments, fiber-interconnect device 130 may be configured to provide two different optical paths illustrated in FIG. 2 for each of different WSS pairs, each pair including a respective ingress WSS 210i and a respective egress 220 _j .

For example, in one embodiment, fiber-interconnect device 130 may be configured to provide two different optical paths for each pair of WSSes 210i and 220 _j defined by the following possible values of the indices“i” and“j”: i¹j ; i=l, 2, ..., N; and j=l, 2, ..., N.

In this embodiment, fiber-interconnect device 130 comprises N optical splitters 250i, where i= 1 , 2, ... , N. Each optical splitter 250i is a 1 xP splitter having (i) an input port 248i connected to an output port of ingress WSS 210i using an optical waveguide 240i and (ii) P=N-l output ports 252i _j , each connected by way of a respective optical waveguide 260i _j to an input port of a respective egress WSSes 220 _j , where j¹i and j=l, 2, ..., N. Fiber-interconnect device 130 also connects an output port of ingress WSS 210i and an input port of egress WSS 220 _j by way of a respective waveguide 230i _j .

For clarity of depiction, FIG. 2 explicitly shows only one of the connections for the P output ports 252i _j of optical splitter 250i, which connection is by way of waveguide 260i _j in fiber-interconnect device 130. A person of ordinary skill in the art will understand that different output ports 252i _j of optical splitter 250i are connected to different respective egress WSSes 220 _j by way of different respective waveguides 260i _j . These connections enable the optical signal applied by ingress WSS 210i to waveguide 240i to be broadcast to the corresponding plurality of egress WSSes 220 _j connected to output ports 252i _j of optical splitter 250i. To support the shown connections, the number K of output ports in an ingress WSS 210i may satisfy the following inequality: K>N+M. The number L of input ports in an egress WSS 220 _j may satisfy the following inequality: L>2N+M-2.

However, in the above-mentioned alternative embodiments, wherein fiber- interconnect device 130 supports reduced connectivity for one or more of twin WSSes 116 _I -116 _N and/or one or more of optical add/drop blocks 150 _I -150 _M , one or both of the numbers K and L may be smaller than indicated in the preceding paragraph.

As indicated in FIG. 2, the first of the two different optical paths between ingress WSS 210i and egress WSS 220 _j includes waveguide 230i _j . The second of the two different optical paths between ingress WSS 210i and egress WSS 220 _j includes waveguide 240i, optical splitter 250i, and waveguide 260i _j . The first of the two different optical paths connects one output port of the ingress WSS 210i to an input port of the egress WSS 220 _j . The second of the two different optical paths connects another output port of the ingress WSS 210i to another input port of the egress WSS 220 _j .

Each of ingress WSSes 210i is connected to fiber-interconnect device 130 using a corresponding set of optical fibers 212i. Each of egress WSSes 220 _j is similarly connected to fiber-interconnect device 130 using a corresponding set of optical fibers 218 _j . The set of optical fibers 212i is a subset of optical fibers 122i (see FIG. 1) and has K optical fibers. The set of optical fibers 218 _j is a subset of optical fibers 122 _j (also see FIG. 1) and has L optical fibers.

In some alternative embodiments, fiber-interconnect device 130 may have fewer than N optical splitters 250i. In such embodiments, fiber-interconnect device 130 is configured to provide two different optical paths only for some pairs of WSSes 210i and 220 _j . In some of such embodiments, some or all of the optical splitters 250i may have fewer than N-l output ports 252i _j . In an example embodiment, P>2.

In some embodiments, some of ingress WSSes 210i may have different numbers K, of output ports connected to fiber-interconnect device 130.

In some embodiments, some of egress WSSes 220 _j may have different numbers L _j of input ports connected to fiber-interconnect device 130.

FIG. 3 schematically shows some of the optical paths in OXC switch 100 according to another embodiment. Although the embodiment of FIG. 3 is functionally similar to the embodiment of FIG. 2, the embodiment of FIG. 3 differs from the embodiment of FIG. 2 in that fiber-interconnect device 130 further comprises up to N(N l) 2x 1 optical couplers 360i _j . For example, each of egress WSSes 220 _j may have up to (N l) 2x 1 optical couplers 360i _j connected to a corresponding subset of the input ports thereof.

Each of optical couplers 360i _j is configured to join the two corresponding different optical paths between WSSes 210i and 220 _j prior to connecting both paths to the egress WSS 220 _j . For example, the optical coupler 360i _j shown in FIG. 3 connects both optical waveguides 230i _j and 260i _j to an optical waveguide 370i _j , which is connected to the corresponding input port of egress WSS 220 _j . As a result, the first of the two different optical paths between WSSes 210i and 220 _j includes waveguide 230i _j , optical coupler 360i _j , and waveguide 370i _j . The second of the two different optical paths between WSSes 210i and 220 _j includes waveguide 240i, optical splitter 250i, waveguide 260i _j , optical coupler 360i _j , and waveguide 370i _j . These two different optical paths connect two different respective output ports of the ingress WSS 210i to the same input port of the egress WSS 220 _j .

A person of ordinary skill in the art will understand that the embodiment of FIG. 3 potentially enables some or all of the egress WSSes 220 _j to have fewer input ports than in the embodiment of FIG. 2. For example, the number L of input ports in egress WSS 220 _j may now satisfy the following inequality: L>N+M-l .

FIG. 4 shows a block diagram of an optional optical module 400 that can be used in OXC switch 100 according to an embodiment. Module 400 can be inserted, e.g., at location A indicated in FIGs. 2 and 3. Some embodiments of OXC switch 100 may have up to N-l modules 400.

Module 400 comprises an optical amplifier (OA) 410 and a wavelength blocker 420. If present, optical amplifier 410 can be used, e.g., to pre-compensate optical losses induced by the corresponding downstream optical splitter 250i (also see FIGs. 2-3).

Wavelength blocker 420 is a configurable band- stop filter that can pass through a selected set of wavelength channels while stopping (blocking) all other wavelength channels.

Different instances (nominal copies) of wavelength blocker 420 can be configured to pass through different respective sets of wavelength channels. Wavelength blocker 420 can be implemented using any suitable wavelength-selective device. In some embodiments, wavelength blocker 420 can be implemented using a suitable WSS.

An example frequency grid that is used in fiber-optic communication systems is defined by the ITU-T G.694.1 Recommendation, which is incorporated herein by reference in its entirety. This frequency grid can be used, e.g., in the frequency range from about 186 THz to about 201 THz, with a 100, 75, 50, 25, or l2.5-GHz spacing of the channels therein. While defined in frequency units, this grid can equivalently be expressed in wavelength units. For example, in the wavelength range from about 1528.8 nm to about 1563.9 nm, the lOO-GHz spacing between the centers of neighboring wavelength channels is equivalent to approximately 0.8 nm spacing. Other frequency grids are also used in fiber-optic communication systems.

The following terms are typically used to refer to certain characteristics of a frequency grid and the corresponding optical signals.

Frequency grid: A reference set of frequencies used to denote nominal central frequencies that may be used for defining specifications and applications.

Frequency slot: The frequency range allocated to a slot and substantially unavailable to other slots within a frequency grid. A frequency slot is defined by its nominal central frequency and its slot width. A frequency slot may also be referred to as a wavelength channel.

Slot width: The full width of a frequency slot in a frequency grid.

Channel spacing: The frequency difference between the nominal central frequencies of two adjacent frequency slots (wavelength channels).

For illustration purposes and without any implied limitations, example

embodiments are described below in reference to a flexible frequency grid having a 50- GHz slot width in one spectral band and a 75 -GHz slot width in another spectral band. However, embodiments are not limited to this particular grid or these slot widths. From the provided description, a person of ordinary skill in the art will be able to practice various embodiments while using other suitable frequency grids and/or slot widths. FIGs. 5A-5D graphically show example spectra of WDM signals that can be transmitted through OXC switch 100 according to an embodiment. More specifically,

FIG. 5A shows a spectrum 502 of a WDM signal received by ingress WSS 210i through fiber 102i. FIG. 5B shows a spectrum 504 of a WDM signal transmitted by egress WSS 220 _j through fiber 104 _j . FIG. 5C shows a spectrum 506 of a WDM signal transmitted through waveguide 230i _j . FIG. 5D shows a spectrum 508 of a WDM signal transmitted through waveguide 260i _j . A person of ordinary skill in the art will understand that spectra 502-508 shown in FIGs. 5A-5D are approximate spectra that are presented herein solely to illustrate and/or explain certain technical features of OXC switch 100, and that some embodiments may be configured to operate on optical signals whose spectra differ from the shown spectra.

The abscissa, which has the same scale in all of FIGs. 5A-5D, indicates the optical frequency. The ordinate in each of FIGs. 5A-5D indicates light intensity (I). The dashed vertical lines are visual guides spaced by 12.5 GHz. A frequency band 510 is divided into 50-GHz frequency slots in accordance with the employed frequency grid. A frequency band 520 is similarly divided into 75-GHz frequency slots in accordance with the employed frequency grid. In some embodiments, neighboring frequency bands 510 and 520, which have single channel slots of different respective widths, may be separated by an optional guard band 516, e.g., as indicated in FIG. 5A. In the shown example, optional guard band 516 is spectrally located between the slots that carry WDM components 512n and 522i.

Each of the independently modulated (e.g., data-carrying) optical signals (WDM components) is schematically shown in FIGs. 5A-5D using an example hat-shaped spectral envelope. A person of ordinary skill in the art will understand that each of the modulated optical signals typically has a carrier center band and modulation sidebands, which are not explicitly shown in FIGs. 5A-5D. In general the shapes of individual spectral envelopes depend, inter alia, on the type of modulation and the type of filtering applied to the specific WDM component.

Frequency band 510 is illustratively shown as having WDM components 512i- 512n, each generated using a modulation rate of 33 GBd. Each of WDM components 512i-512n occupies a respective 50-GHz frequency slot. Frequency band 520 is illustratively shown as having WDM components 522 _I -522 ₃ , each generated using a modulation rate of 64 GBd. Each of WDM components 522 _I -522 ₃ occupies a respective 75 -GHz frequency slot.

In an example embodiment corresponding to FIGs. 5A-5D, WSSes 210 and 220 have spectral pass bands that enable these WSSes to route WDM components 512i-512n and 522 _I -522 ₃ , in a reconfigurable manner, between appropriate output ports of ingress WSSes 210 and input ports of egress WSSes 220. FIGs. 5A-5D graphically illustrate one example of such routing.

Referring to FIG. 5A, spectrum 502 has WDM components 512i-512n and 522i- 522 ₃ and represents an input WDM signal received by ingress WSS 210i through fiber 102i (also see FIG. 2). OXC switch 100 operates to route WDM components 512i-512n and 522 _I -522 ₃ in accordance with their intended destinations using appropriate configurations of the ingress WSS 210i and the corresponding egress WSSes 220 _j .

Referring to FIG. 5B, spectrum 504 has WDM components 512i, 512 ₂ , 512 ₄ -512e, 512s, 512n, 522i, and 522 ₃ and represents an output WDM signal applied by a particular egress WSS 220 _j to fiber 104 _j (also see FIG. 2). These WDM components have been routed through fiber-interconnect device 130 to the egress WSS 220 _j as indicated in FIGs. 5C-5D. WDM components 512 ₃ , 512 ₇ , 512 ₉ , 512io, and 522 ₂ , which are missing in spectrum 504, may have been routed through fiber-interconnect device 130 to one or more egress WSSes 220 _k (where k¹j) in a manner that is analogous to that indicated in FIGs. 5C-5D or dropped through optical add/drop blocks 150 _I -150 _M .

Referring to FIG. 5C, spectrum 506 has WDM components 512i, 512 ₂ , 512 ₄ -512e, 512s, and 512n and represents an output WDM signal applied by the ingress WSS 210i to the output port thereof connected to waveguide 230i _j (also see FIG. 2). The pass bands of the ingress WSS 210i corresponding to the latter output port have 50-GHz bandwidths and match the frequency slots of band 510. The ingress WSS 210i may be configured to direct WDM components 512i, 512 ₂ , 512 ₄ -512e, 512s, and 512n through waveguide 230i _j , e.g., because it may be beneficial for these WDM components to go through an optical path characterized by relatively good optical isolation stemming from the double band-pass filtering caused by the corresponding 50-GHz pass bands of WSSes 210i and 220 _j .

Referring to FIG. 5D, spectrum 508 has WDM components 522i and 522 ₃ and represents an input WDM signal received by egress WSS 220 _j through the input port thereof connected to waveguide 260i _j (also see FIG. 2). The pass bands of the egress WSS 220 _j corresponding to the latter input port have 75-GHz bandwidths and match the frequency slots of band 520. Ingress WSS 210i may be configured to direct WDM components 522i and 522 ₃ through waveguide 240i connected to waveguide 260i _j , e.g., because it may be beneficial for these WDM components to go through an optical path characterized by relatively low amounts of narrow band-pass filtering enabled by the corresponding 75-GHz pass bands of WSSes 210i and 220 _j .

Some embodiments of OXC switch 100 described above in reference to FIGs. 1-5 may enable the following engineering considerations and/or concerns to be addressed in a satisfactory manner.

As indicated above, fiber-interconnect device 130 may provide two different optical paths between an ingress WSS 210i and an egress WSS 220 _j , e.g., as shown in FIG. 2. In some embodiments, to further reduce the amounts of narrow band-pass filtering at ingress WSS 210i for the WDM components directed through waveguide 240i and optical spliter 250i (FIG. 2), the ingress WSS may be configured to have additional guard bands for the 75-GHz channels. However, these additional guard bands may disadvantageously decrease the overall spectral efficiency. To mitigate or avoid this potential drawback, some embodiments may be designed to use different distinct, contiguous spectral bands for the 75-GHz and 50-GHz channels, respectively. In some embodiments, such spectral bands may be analogous to bands 510 and 520 (FIG. 5A). In such embodiments, the

WDM components corresponding to the 75-GHz channels may not undergo any penalizing narrow-band filtering when passing through ingress WSS 210i because the entire band of those channels, e.g., analogous to band 520 (FIG. 5 A), can be routed as a single entity without being subjected to any extra filtering at the edges of the individual channels. The resulting filtering impact of ingress WSS 210i on the 75-GHz channels routed in this manner may be minimal.

To mitigate the additional optical losses imposed by optical spliters 250i on the channels routed therethrough, one optical amplifier 410 (FIG. 4) per ingress WSS 210i can be inserted in some embodiments, e.g., at locations A indicated in FIGs. 2 and 3. Such an optical amplifier can be packaged on the same pack as the corresponding ingress WSS 210i at a reasonable cost, e.g., because the total output power of the amplifier might not need to exceed 15 dBm. The cost can further be lowered if the percentage of wavelength channels directed through waveguides 240i and optical spliters 250i is limited, for instance, to less than 50%. Wavelength blockers 420 can be used to mitigate potential OSNR degradation due to the wide-band noise of optical amplifiers 410. It may be advantageous to design wavelength blockers 420 such that the signals passing therethrough are not subjected to tight band-pass filtering.

Optical couplers 360i _j can be asymmetric. For example, a 30/70 coupler 360i _j , the 70% port of which is connected to waveguide 230i _j , will add only a ~2 dB insertion loss for the corresponding optical path. Even though the 30/70 coupler 360i _j will add a ~5 dB insertion loss for the optical path connected to the 30% port, this loss can be easily compensated by the corresponding optical amplifier 410. Additional wavelength blockers 420 may then be used to protect the signals going through waveguide 230i _j from potential OSNR degradation due to the wide-band noise of optical amplifiers 410 in embodiments employing optical couplers 360i _j . Ingress WSSes 210 may also need to be configured such that the optical isolation between the wavelength channels therein is relatively strong, e.g., better than 35 dB, to reduce detrimental in-band crosstalk for the channels recombined by optical couplers 360i _j .

In general, the choice of the configuration of the OXC switch and of the optical paths therethrough may be based on any number of pertinent criteria. For example, some sets of criteria may primarily be based on comparing the relative sensitivities to filtering and optical crosstalk, with some additional criteria being directed at saving as much of the spectral resource as possible (e.g., by reducing spectral guard bands). These and other pertinent characteristics may not depend exclusively on the channel bandwidths. For example, the quality of the clock recovery applied to the channel when it is detected and the constellation size and type may also meaningfully influence the routing choices for each individual channel going through the OXC switch. As a result, in some embodiments, some of WDM components 512 may be routed through waveguide 240i, whereas some of WDM components 522 may be routed through waveguide 230i _j .

Some embodiments can be constructed to leverage recent advances in the FCoS technology, based on which a corresponding WSS can be designed to have at least some of the following features: (i) be programmable, e.g., to implement a desired channel configuration, including customizable frequency slots, grids, guard bands, etc., and (ii) have a relatively high spatial resolution to enable implementations of sharper (at the edges) and/or flatter (in the middle) filtering functions than those enabled by the previous FCoS generation (e.g., commercially produced before the year 2014). FIG. 6 schematically shows some of the optical paths in OXC switch 100 according to yet another embodiment. Although the embodiment of FIG. 6 is functionally similar to the embodiment of FIG. 3, the embodiment of FIG. 6 differs from the embodiment of FIG. 3 in the manner in which the 2x 1 optical couplers are connected between the

corresponding ingress and egress WSSes. More specifically, in the embodiment of FIG. 6, fiber-interconnect device 130 comprises a plurality of 2x 1 optical couplers 660i _kj and a plurality of 2x 1 optical couplers 662i _kj connected as explained below. In an example embodiment, each of optical couplers 660 and 662 can be similar to the above-described optical coupler 360 (FIG. 3).

Each of optical couplers 660i _kj is configured to join one of the two optical paths between WSSes 210i and 220 _j and one of the two optical paths between WSSes 210 _k and 220 _j prior to connecting both paths to the egress WSS 220 _j . Each of optical couplers 662i _kj is similarly configured to join the other one of the two optical paths between WSSes 210i and 220 _j and the other one of the two optical paths between WSSes 210 _k and 220 _j prior to connecting both paths to the egress WSS 220 _j .

Optical coupler 660i _kj connects both optical waveguides 230i _j and 230 _kj to an optical waveguide 670i _kj , which is connected to the corresponding input port of egress WSS 220 _j . As a result, the first of the two different optical paths between WSSes 210i and 220 _j includes waveguide 230i _j , optical coupler 660i _kj , and waveguide 670i _kj . The first of the two different optical paths between WSSes 210 _k and 220 _j includes waveguide 230 _kj , optical coupler 660ik _j , and waveguide 670ik _j . Waveguide 670ik _j connects the two optical paths joined thereby to the same respective input port of the egress WSS 220 _j .

Optical coupler 662i _kj connects both optical waveguides 260i _j and 260 _kj to an optical waveguide 672i _kj , which is connected to another corresponding input port of egress WSS 220 _j . As a result, the second of the two different optical paths between WSSes 210i and 220 _j includes waveguide 240i, optical splitter 250i, waveguide 260i _j , optical coupler 662i _kj , and waveguide 672i _kj . The second of the two different optical paths between WSSes 210 _k and 220 _j includes waveguide 240 _k , optical splitter 250 _k waveguide 260 _kj , optical coupler 662i _kj , and waveguide 672i _kj . Waveguide 672i _kj connects the two optical paths joined thereby to the same respective input port of the egress WSS 220 _j .

In some embodiments, fiber-interconnect device 130 shown in FIG. 6 can be modified to swap the connections of waveguides 230i _j and 260i _j such that waveguide 230i _j is connected to optical coupler 662i _kj while waveguide 260i _j is connected to optical coupler

660i _kj .

Some embodiments may be constructed such that an ingress WSS 210 and one corresponding wavelength blocker 420 are integrated into the same module. This integration can be done in a relatively straightforward manner, e.g., because most of the commercially available flexible-grid WSSes and wavelength blockers feature the same LCoS technology and the same integration technology. In fact, in some embodiment, wavelength blocker 420 can be implemented as a l x l WSS.

Some embodiments can potentially be leveraged to develop novel wavelength routing approaches. For example, in cases of 300 Gb/s PDM-8QAM and 400 Gb/s PDM- 16QAM, the adverse effects of tight filtering and optical crosstalk may be comparable or not predictable a priori. In this context, the two different optical paths between an ingress WSS 210 and an egress WSS 220 may be tested, e.g., during the execution of the resource-reservation protocol, to determine which of the optical performs better for the signal in question. The better-performing optical path can then be selected for transmitting the corresponding payload signals during normal operation of the OXC switch.

Additional flexibility in the ways the above-described OXC-switch architecture can be leveraged to meet the specifications of a particular application is provided by embodiments of OXC switch 100 having fewer than the above-indicated maximum number of optical splitters 250i. In such embodiments, some pairs of WSSes 210 and 220 are connected by a single respective optical path while some other pairs of WSSes 210 and 220 are connected by two respective optical paths. As indicated above, the number of optical splitters 250i also has an impact on the sizes of WSSes 210 and 220 used in the OXC switch. Depending on the particular application, one can decide whether to implement a connection between two particular degrees of the OXC switch using a single respective optical path or two respective optical paths and then design fiber-interconnect device 130 and select the sizes of WSSes 210 and 220 accordingly.

According to an example embodiment described above, e.g., in the summary section and/or in reference to any one or any combination of some or all of FIGs. 1-6, provided is an apparatus (e.g., 100, FIG. 1) comprising: a plurality of ingress wavelength- selective switches (e.g., 210, FIG. 2), each having a respective input port (e.g., 113, FIG.

1) and a respective set of output ports (e.g., at 212, FIG. 2); a plurality of egress wavelength-selective switches (e.g., 220, FIG. 2), each having a respective output port (e.g., 115, FIG. 1) and a respective set of input ports (e.g., at 218, FIG. 2); and an interconnect device (e.g., 130, FIG. 1) configured to optically connect the output ports of the ingress wavelength-selective switches and the input ports of the egress wavelength- selective switches using optical waveguides (e.g., 230, 240, 260, 370, FIGs. 2, 3); and wherein, for at least some pairs of the ingress and egress wavelength-selective switches optically connected by the optical waveguides, the interconnect device is configured to: optically connect (e.g., through 230i _j , FIGs. 2, 3) one of the output ports of a respective ingress wavelength-selective switch (e.g., 210i, FIGs. 2, 3) to one of the input ports of a respective egress wavelength-selective switch (e.g., 220 _j , FIG. 2); and optically connect (e.g., through 260i _j , FIGs. 2, 3) a different one of the output ports of the respective ingress wavelength-selective switch to said one of the input ports (e.g., as in FIG. 3) or to a different one of the input ports (e.g., as in FIG. 2) of the respective egress wavelength- selective switch.

In some embodiments of the above apparatus, the interconnect device comprises an optical splitter (e.g., 250i, FIGs. 2, 3) having an optical input (e.g., 248i, FIGs. 2, 3) and a plurality of optical outputs (e.g., 252i _j , FIGs. 2, 3), the optical input being optically connected (e.g., using 240i, FIGs. 2, 3) to said different one of the output ports of the respective ingress wavelength-selective switch, one of the optical outputs being optically connected (e.g., using 260i _j , FIGs. 2, 3) to said one of the input ports or to said different one of the input ports of the respective egress wavelength-selective switch.

In some embodiments of any of the above apparatus, another one of the optical outputs of the optical splitter is optically connected to an input port of another egress wavelength-selective switch.

In some embodiments of any of the above apparatus, each of the optical outputs of the optical splitter is optically connected to an input port of a different respective egress wavelength-selective switch.

In some embodiments of any of the above apparatus, the interconnect device further comprises an optical coupler (e.g., 360i _j , FIG. 3) having first and second optical inputs and an optical output, the first optical input being optically connected (e.g., through 230i _j , FIG. 3) to said one of the output ports of the respective ingress wavelength-selective switch, the second optical input being optically connected (e.g., through 260i _j , FIG. 3) to said one of the outputs of the optical splitter, the optical output of the optical coupler being optically connected (e.g., through 370i _j , FIG. 3) to said one of the input ports of the respective egress wavelength-selective switch.

In some embodiments of any of the above apparatus, the apparatus further comprises a degree-N optical cross-connect switch, where N is an integer greater than two; and wherein the optical splitter has N-l optical outputs.

In some embodiments of any of the above apparatus, the interconnect device comprises N optical splitters, each having N-l optical outputs, each of the N-l optical outputs being optically connected to an input port of a different respective egress wavelength-selective switch.

In some embodiments of any of the above apparatus, the interconnect device comprises an optical coupler (e.g., 360i _j , FIG. 3) having first and second optical inputs and an optical output, the first optical input being optically connected (e.g., through 230i _j , FIG. 3) to said one of the output ports of the respective ingress wavelength-selective switch, the second optical input being optically connected (e.g., through 260i _j , 250i, 240i, FIG. 3) to said different one of the output ports of the respective ingress wavelength-selective switch, the optical output of the optical coupler being optically connected (e.g., through 370i _j , FIG. 3) to said one of the input ports of the respective egress wavelength-selective switch.

In some embodiments of any of the above apparatus, the apparatus further comprises a degree-N optical cross-connect switch, where N is an integer greater than two; and wherein the interconnect device comprises N(N l) 2x1 optical couplers (e.g., 360i _j , FIG. 3) connected to the optical waveguides.

In some embodiments of any of the above apparatus, the interconnect device comprises N optical splitters, each having N-l optical outputs, each of the N-l optical outputs being optically connected to an input port of a different respective egress wavelength-selective switch by way of a respective one of the N(N l) 2x1 optical couplers.

In some embodiments of any of the above apparatus, the interconnect device comprises: a first optical coupler (e.g., 660i _kj , FIG. 6) having first and second optical inputs and an optical output, the optical output of the first optical coupler being optically connected (e.g., through 670i _kj , FIG. 6) to one of the input ports of a corresponding egress wavelength-selective switch, the first optical input of the first optical coupler being optically connected (e.g., through 230i _j , FIG. 6) to one of the output ports of a first respective ingress wavelength-selective switch (e.g., 210i, FIG. 6), the second optical input of the first optical coupler being optically connected (e.g., through 230 _kj , FIG. 6) to one of the output ports of a second respective ingress wavelength-selective switch (e.g., 210 _k , FIG. 6); and a second optical coupler (e.g., 662i _kj , FIG. 6) having first and second optical inputs and an optical output, the optical output of the second optical coupler being optically connected (e.g., through 672i _kj , FIG. 6) to another one of the input ports of the corresponding egress wavelength-selective switch, the first optical input of the second optical coupler being optically connected (e.g., through 260i _j , 250i, and 240i, FIG. 6) to another one of the output ports of the first respective ingress wavelength-selective switch, the second optical input of the second optical coupler being optically connected (e.g., through 260 _kj , 250 _k , and 240 _k , FIG. 6) to another one of the output ports of the second respective ingress wavelength-selective switch.

In some embodiments of any of the above apparatus, the interconnect device is configured to optically connect (e.g., through 260i _j , 250i, 240i, FIG. 2) said different one of the output ports of the respective ingress wavelength-selective switch to said different one of the input ports of the respective egress wavelength-selective switch.

In some embodiments of any of the above apparatus, the apparatus further comprises a wavelength blocker (e.g., 420, FIG. 1) coupled between said different one of the output ports of the respective ingress wavelength-selective switch and the interconnect device.

In some embodiments of any of the above apparatus, the apparatus further comprises an optical amplifier (e.g., 410, FIG. 1) serially connected with wavelength blocker and coupled between said different one of the output ports of the respective ingress wavelength-selective switch and the interconnect device.

In some embodiments of any of the above apparatus, each of the ingress wavelength-selective switches has K output ports; and each of the egress wavelength- selective switches has L input ports, where L>K.

In some embodiments of any of the above apparatus, the respective ingress wavelength-selective switch has a first set of wavelength channels (e.g., 512, FIG. 5A), each having a first slot width, and a second set of wavelength channels (e.g., 522, FIG.

5A), each having a second slot width different from the first slot width.

In some embodiments of any of the above apparatus, the respective ingress wavelength-selective switch is configurable to: route optical signals of the first set of wavelength channels to said one of the output ports thereof; and route optical signals of the second set of wavelength channels to said different one of the output ports thereof

In some embodiments of any of the above apparatus, the respective ingress wavelength-selective switch is configured to: have the first set of wavelength channels arranged within a first spectral band (e.g., 510, FIG. 5A); and have the second set of wavelength channels arranged within a second spectral band (e.g., 520, FIG. 5A), the second spectral band being distinct from (e.g., having no common wavelengths with) the first spectral band.

In some embodiments of any of the above apparatus, the apparatus further comprises a plurality of optical add/drop blocks (e.g., 150, FIG. 1); and wherein the interconnect device is further configured to: optically connect the output ports of the ingress wavelength-selective switches to at least some of the optical add/drop blocks; and optically connect the input ports of the egress wavelength-selective switches to at least some of the optical add/drop blocks.

In some embodiments of any of the above apparatus, the plurality of egress wavelength-selective switches has N egress wavelength-selective switches, where N is an integer greater than one; the plurality of ingress wavelength-selective switches has N ingress wavelength-selective switches; the plurality of optical add/drop blocks has M optical add/drop blocks, where M is an integer greater than one; each of the ingress wavelength-selective switches has K output ports, where K>N+M; and each of the egress wavelength-selective switches has L input ports, where L>2N+M-2.

While this disclosure includes references to illustrative embodiments, this specification is not intended to be construed in a limiting sense. Various modifications of the described embodiments, as well as other embodiments within the scope of the disclosure, which are apparent to persons skilled in the art to which the disclosure pertains are deemed to lie within the principle and scope of the disclosure, e.g., as expressed in the following claims.

Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word“about” or“approximately” preceded the value or range.

It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this disclosure may be made by those skilled in the art without departing from the scope of the disclosure, e.g., as expressed in the following claims.

Reference herein to“one embodiment” or“an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the disclosure. The appearances of the phrase“in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term“implementation.”

Unless otherwise specified herein, the use of the ordinal adjectives“first,”

“second,”“third,” etc., to refer to an object of a plurality of like objects merely indicates that different instances of such like objects are being referred to, and is not intended to imply that the like objects so referred-to have to be in a corresponding order or sequence, either temporally, spatially, in ranking, or in any other manner.

Also for purposes of this description, the terms“couple,”“coupling,”“coupled,” “connect,”“connecting,” or“connected” refer to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms“directly coupled,”“directly connected,” etc., imply the absence of such additional elements.

The described embodiments are to be considered in all respects as only illustrative and not restrictive. In particular, the scope of the disclosure is indicated by the appended claims rather than by the description and figures herein. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.

The description and drawings merely illustrate the principles of the disclosure. It will thus be appreciated that those of ordinary skill in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the disclosure and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the disclosure and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass equivalents thereof.