Patent Publication Number: US-2021175972-A1

Title: Device and Method for Processing an Optical Signal

Description:
TECHNICAL FIELD 
     The present disclosure relates to a device and method for processing an optical signal. In particular, the disclosure relates to an integrated optical device and a method for providing optical signal gain in an integrated optical device. 
     BACKGROUND 
     Telecommunication networks continue to evolve. 5 th  Generation (5G) networks are expected to bring about a major telecommunications transformation, providing an end-to-end infrastructure which is capable of providing a consistently high user quality of experience across heterogeneous network environments and for a wide range of use cases. Depending on the particular use case, performance demands may require high throughput, low latency, high reliability, high connectivity density, and/or high mobility range. To facilitate such a transformation, the underlying physical and logical telecommunications network infrastructure is subject to continuing development. 
     One part of the telecommunications network infrastructure is an optical transport network. An optical transport network includes one or more optical network elements which are typically interconnected by fiber optic cables and which are configured to provide specific functions, such as transport, multiplexing, switching, management, and resilience of optical channels carrying optical data signals. 
     One of the considerations associated with optical networks is the degradation of the optical signal during transmission, in particular from signal attenuation as the optical signal passes along a fiber optic cable or from insertion loss (loss of optical power) as an optical signal passes through, or is processed by, an optical network element. The performance and/or range of an optical network may be improved by providing optical amplification to the signal. 
     Another consideration is the ongoing drive to provide devices with higher throughput for a given footprint at a given cost. Photonic integrated circuits (PICs) may offer device miniaturization and performance capabilities consistent with this aim. A PIC may integrate into its circuit/sub-circuit multiple photonic functions or elements, including waveguides, filters, splitters, spot-size converters, polarization splitters/rotators, optical switches, variable optical attenuators, optical amplifiers, modulators, and photodiodes, among others. Waveguides, filters, splitters, spot-size converters, and polarization splitters/rotators, for example, may be considered to be passive elements; while optical amplifiers, modulators, and photodiodes, for example, may be considered to be active elements, having an intended dynamic interaction between the active element and light. 
     A PIC may be fabricated based on a range of different material platforms, including lithium niobate (LiNbO 3 ); silica on silicon (as planar lightwave circuits, or PLCs); silicon on insulator (SOI; also known as silicon photonics); and III-V semiconductors, such as gallium arsenide (GaAs) and indium phosphide (InP). Each material platform offers a respective set of advantages and constraints, so the selection of one material platform over another typically depends on the desired function of the PIC. Silicon photonics is of particular interest, however, as silicon photonic devices may be fabricated using complementary metal-oxide semiconductor (CMOS) manufacturing technology, which offers the possibility of high-volume and cost-effective manufacturing processes for SOI PICs. 
     A silicon-on-insulator wafer may be fabricated as follows. A single crystal ingot, or boule, of silicon is sliced into wafers, typically around 150-300 mm in diameter. Two wafer surfaces are oxidized, typically by dry or wet thermal oxidation techniques, to form a layer of silicon dioxide (SiO 2 ) on each wafer. One of the wafers is then implanted with hydrogen atoms to a specified depth. The wafer is placed on top of the other wafer to form a stack, with the oxide layers facing each other to bond the oxide layers together. The stack is baked in a vacuum, which results in the stack splitting or cracking at the hydrogen implant line and the oxide layers fully bonding together. The top surface is polished by chemical mechanical polishing, to provide a relatively thin layer of crystalline silicon (the device layer), on a layer of silicon dioxide (the buried oxide layer), on a bulk silicon support wafer (the handle layer). Other fabrication techniques may alternatively be used, including bonded and etchback processing or separation by implantation of oxygen. 
     Optical networks in telecommunications generally employ wavelengths of around 1.3 μm for datacom services and around 1.55 μm for telecom services, but other wavelengths, for example from the 1.26-1.68 μm range, may be used. Silicon is an indirect bandgap semiconductor, with a bandgap of 1.12 eV, so is transparent for light having wavelengths above 1.1 μm. Silicon is therefore generally not capable of generating or detecting light at telecommunications wavelengths, but is an attractive material for use as an optical waveguide, for switching, for spot-size conversion, for optical modulation, or for polarization splitting/rotation, among others. This is particularly so because of the relatively high refractive index contrast possible between silicon (n=3.48) and other materials, such as SiO 2  (n=1.45), which allows for strongly confining optical waveguide structures. However, silicon waveguide structures exhibit a large polarization dependent dispersion or loss for orthogonal, transverse electric (TE) and transverse magnetic (TM), components, which can increase complexity when coupling to a polarization-insensitive platform, such as optical fiber networks. 
     In order to provide light generation or optical amplification, then, other materials, such as group III-V materials, including InP or GaAs, may be integrated onto a silicon photonic chip. Providing optical amplification on a silicon photonic chip may compensate for or reduce the insertion loss for the chip, and allow the integration of multiple photonic circuits with many different functionalities on the chip, which may otherwise take up several optical processing cards of a telecommunications equipment shelf in existing implementations, such as line cards (for transmission), switching cards, or optical monitoring cards, among others. 
     A number of approaches for integrating active devices, such as optical amplification or gain devices, with silicon photonic devices is in principle possible, but there is as yet no clearly favored, stand-out approach. Monolithic integration techniques may include the use of epitaxially grown germanium as an optical gain medium; erbium-doped glass waveguides, such as aluminium oxide (Al 2 O 3 ), which require optical pumping; or epitaxially grown GaAs quantum dots. Wafer-to-wafer assembly techniques may include the bonding of a III-V gain region, such as layers of InP and indium gallium arsenide phosphide (InGaAsP), to a SOI wafer by oxide bonding, for example, using an Al 2 O 3  bonding interface; or by organic bonding, for example, using benzocyclobutene (BCB). Die-to-wafer assembly may include inserting a III-V die into a cavity in the SOI wafer and then patterning the waveguides. These techniques have the advantage that the full device may be integrated at the wafer level and tested before being diced out. The integration at wafer level may use standard fabrication equipment used in microelectronic packaging and does not require active alignment of dies with special flip-chip machines, which is not the case for die-to-die assembly. 
     Die-to-die assembly techniques may include butt coupling of a SOI die and a III-V die; or coupling a SOI die and a III-V die using a lens and a grating coupler. In the latter technique, flip-chip integration involves micro-packed semiconductor optical amplifier (SOA) assemblies being individually actively aligned, using an optical signal, with a respective silicon grating coupler by a flip-chip bonder. The alignment and flip-chip bonding steps complicate the fabrication process and add to the manufacturing costs. 
       FIG. 1  schematically shows an example of an integrated optical device which includes a SOI PIC in which InP SOAs are used to compensate for internal losses in the PIC. The integrated optical device is a reconfigurable optical add/drop multiplexer (ROADM)  100 . This device is discussed in lovanna et al.: “A Future Proof Optical Network Infrastructure for 5G Transport,” IEEE, OSA Journal of Optical Communications and Networking, vol. 8, issue 12, December 2016. 
     The device may be configured in a double ring network with one ring transmitting downlink signals from a hub to remote nodes, and the other ring transmitting upstream signals from the remote nodes to the hub, in an optical network based on wavelength division multiplexing (WDM), in particular dense wavelength division multiplexing (DWDM), technology. The ROADM  100  may be used in the front haul of a wireless communications network, for reconfigurable add-drop of selected local channels at a remote radio unit (RRU), or remote node, site. The ROADM  100  may also be used in the back haul of a wireless communications network. 
     The ROADM  100  is formed of two independent circuit structures: a drop circuit structure  102  for locally dropping, or removing or exiting, DWDM channels from the optical network; and an add circuit structure  104  for locally adding DWDM channels to the optical network. 
     Referring firstly to the drop circuit structure  102 , the structure is coupled at a first input/output end  106  to a first optical fiber  108  and is coupled at a second input/output end  110  to a second optical fiber  112 . Optical signals may be transmitted in a first direction from the first optical fiber  108  to the first input/output end  106 , through the drop circuit structure  102 , and pass out of the second input/output end  110  to the second optical fiber  112 . Alternatively, depending on configuration, network requirements, or operational status, optical signals may be transmitted in a second, reverse direction from the second optical fiber  112  to the second input/output end  110 , through the drop circuit structure  102 , and pass out of the first input/output end  106  to the first optical fiber  108 . The first input/output end  106  may be referred to as a first input/output port or a west line port, and the second input/output end  110  may be referred to as a second input/output port or an east line port, although it should be noted that no strict geographical correspondence is intended. 
     Because of the strong polarization sensitivity of silicon wire waveguides, a polarization diversity scheme is used to provide a dual polarization structure. The drop circuit structure  102  is divided into a first drop sub-circuit  114  and a second drop sub-circuit  116 . A first polarization splitter-rotator (PSR) in the form of a dual polarization grating coupler (DPGC)  118  is provided at the first input/output end  106 . The first DPGC  118  splits the unknown and random polarization of input optical signals from the first optical fiber  108  into two orthogonal polarization components, namely TE and TM components. The DPGC  118  then rotates one of the components by 90° so that both components have the same polarization at the output of the DPGC. One component is output to the first drop sub-circuit  114  and the other component is output to the second drop sub-circuit  116 , with the polarizations of both components now corresponding to the linear polarization mode of the silicon wire waveguide structures of the sub-circuits. From there, the two component optical signals may be processed identically, in parallel, along the first and second drop sub-circuits  114 ,  116 , respectively. At the second input/output end  110 , a second polarization splitter-rotator (PSR) in the form of a dual polarization grating coupler (DPGC)  120  is provided. The processed component optical signals are output from the first and second drop sub-circuits  114 ,  116  to the second DPGC  120 . The second DPGC  120  performs the reverse procedure on the component optical signals, rotating the polarization of one of the components and recombining the components into output optical signals to the second optical fiber  112 . The propagation delay through the parallel sub-circuits is fixed by design so that the two output component signals may be combined with constructive interference. 
     Referring to the first drop sub-circuit  114 , there is provided a first semiconductor optical amplifier  124 , an optical bus  126 , having provided along it a set of silicon micro-ring resonators (MRRs)  128 , and a second SOA  130 . The first SOA  124  is configured to amplify the DWDM comb of optical signals, either before the optical signals are processed by the MRRs  128  for transmission in the first direction, or after the optical signals are processed by the MRRs  128  for transmission in the second direction. Similarly, the second SOA  130  is configured to amplify the DWDM comb of optical signals, either after the optical signals are processed by the MRRs  128  for transmission in the first direction, or before the optical signals are processed by the MRRs  128  for transmission in the second direction. 
     The MRRs  128  are coupled to the optical bus  126  and act as optical switching elements to drop particular, respective wavelengths from the optical bus to local output ports  134 . Each of the MRR optical switching elements has two states: a first, off-resonance state, in which a respective incoming DWDM channel passes along the optical bus  126  without coupling into the MRR and being dropped; and a second, on-resonance state, in which a specific wavelength of the DWDM channel corresponding to the resonance wavelength of the MRR is coupled into the MRR and dropped from the optical bus  126 . Before transmission to a local output port  134 , a dropped wavelength signal is guided to a drop PSR, such as a drop DPGC  136 , for rotation and recombination processing with the corresponding wavelength polarization component from the second drop sub-circuit  116 . Thus, optical signals whose wavelength corresponds to a MRR  128  in an on-resonance state will be dropped from the optical bus  126  to a respective local output port  134 , while the remaining optical signals at different wavelengths, not corresponding to any on-resonance MRR, will by-pass or pass through the ROADM  100  to the second optical fiber  112 . 
     The first drop sub-circuit  114  has a respective micro-ring resonator  128  for each wavelength (λ 1 , λ 2 , λ 3 , . . . AN) to be dropped. The number of MRRs  128  and corresponding local output ports  134  to drop to may vary, depending on application. Typically, there may be up to 12 MRRs  128  and a corresponding number of local output ports  134 , although up to 24 MRRs and corresponding local output ports may be provided in some cases. 
     The second drop sub-circuit  116  is configured in a corresponding way, in parallel, to the first drop sub-circuit  114 . The second drop sub-circuit  116  is therefore configured to drop or pass second polarization component wavelengths in correspondence with the dropping or passing of the first polarization component wavelengths on the first drop sub-circuit  114 . 
     As noted above, the signal propagation direction through the drop circuit structure  102  may take place in a first direction, from the first input/output end  106  to the second input/output end  110 , or in a second direction, from the second input/output end  110  to the first input/output end  106 . While optical signals may be propagated in either direction, optical signals are not transmitted in both directions at the same time. The drop circuit structure  102  is configured for propagation in one direction at a time only. The bi-directional configuration of the drop circuit structure  102  allows for resilience protection of a ring network in which the drop circuit structure  102  operates. If a node of the ring should fail or an optical fiber in the network break, then the propagation direction may be reversed to allow optical signals still to be received and dropped by the drop circuit structure  102 . To allow for the MRRs  128  to couple optical signals propagating in the second direction, 1×2 switches  138  are integrated to receive optical signals coupling to second, or back-up, paths from the MRRs  128 , for transmission to the drop DPGCs  136  and local output ports  134 . 
     So that the reversible propagation direction configuration may be fully effective, it can be seen that corresponding functional elements are provided in the same order in the drop circuit structure  102  for the second direction as for the first direction. For example, in a drop circuit structure, incoming optical signals are typically amplified before being dropped or passed through. Providing first and second SOAs  124 ,  130  on either end of the first optical bus  126  allows for such optical amplification to take place, whether the propagation direction is in the first or second direction. 
     Referring now to the add circuit structure  104 , it will be appreciated that the configuration and operation of the add circuit structure is similar to that of the drop circuit structure  102 , except that optical signals at given wavelengths are added to the structure instead of being dropped from the structure. Local input ports  140  provide optical signals at given, respective wavelengths, which are passed via respective add DPCGs  142 , into the add circuit structure  104  for transmission to an outgoing optical fiber, third optical fiber  144  or fourth optical fiber  146 , depending on the propagation direction. 
     To provide optical amplification in the ROADM  100 , while allowing for adding or dropping in either propagation direction, it can be seen that eight semiconductor optical amplifiers are heterogeneously integrated with the silicon photonic device. 
     The techniques for integrating active devices, such as semiconductor optical amplifiers, with silicon photonic devices are generally subject to an ongoing desire to improve performance, yield, reliability, and/or cost, in particular to facilitate large-scale production of such devices. Alternative and/or improved integration implementations in view of the above background would therefore be of interest. 
     SUMMARY 
     According to a first aspect, there is provided a device for processing an optical signal. The device comprises a photonic device arranged between a first input/output and a second input/output. The photonic device is in optical communication with the first and second inputs/outputs by a signal path, the signal path for transmission of a first optical signal in a first direction from the first input/output to the second input/output or for transmission of a second optical signal in a second direction from the second input/output to the first input/output. The device further comprises an optical gain element for receiving the first or second optical signal and outputting an amplified first or second optical signal respectively. The device further comprises a path switching circuit. The path switching circuit comprises a first signal amplification path connectable between the first input/output and the photonic device for optically coupling the signal path to and from the optical gain element. The path switching circuit further comprises a second signal amplification path connectable between the photonic device and the second input/output for optically coupling the signal path to and from the optical gain element. The path switching circuit is arranged to selectively connect the first signal amplification path or the second signal amplification path into the signal path. 
     In this way, a single optical gain element may be configured for optically amplifying an optical signal, whether the optical signal is propagating in the first direction or the second direction. In particular, the single optical gain element may be configured to optically amplify an optical signal in a uniform or consistent manner, whether the optical signal is propagating in the first direction or the second direction. The functional stage along the signal path at which optical amplification is provided by the optical gain element may be configured to be the same for an optical signal propagating in either the first direction or second direction. This allows the device to operate on optical signals in a consistent way when the propagation direction of the optical signals is reversed. The device offers the above advantages while being provided with a single optical gain element, rather than a respective optical gain element on either side of the photonic device. As such, cost and fabrication complexity may be significantly reduced with this device, especially for embodiments in which the device is fabricated by hybrid integration of the optical gain element with the photonic device. 
     The path switching circuit may be configured to selectively connect, or introduce or add, the first signal amplification path into the signal path on a first side of the photonic device, between the first input/output and the photonic device. The path switching circuit may also be configured to selectively connect, or introduce or add, the second signal amplification path into the signal path on a second side of the photonic device, between the photonic device and the second input/output. 
     Optionally, the path switching circuit is arranged to selectively connect the first signal amplification path into the signal path, such that the first input/output is optically coupled to the optical gain element; the optical gain element is optically coupled to the photonic device; and the photonic device is optically coupled to the second input/output. The optical gain element may be selectively configurable to optically amplify the first optical signal upstream of the photonic device in the first direction. The optical gain element may also or alternatively be selectively configurable to optically amplify the second optical signal downstream of the photonic device in the second direction. 
     With the first signal amplification path included in the signal path, the signal path optically couples the first input/output to the optical gain element. The optical gain element is optically coupled to the photonic device. The photonic device is optically coupled to the second input/output. For propagation in the first direction, an optical signal is guided, or routed, along an active path from the first input/output to the optical gain element and is optically amplified to provide an amplified optical signal. The amplified optical signal is then routed to the photonic device for processing according to the particular optical function of the photonic device. The processed optical signal is then routed to the second input/output. For propagation in the second direction, an optical signal is guided, or routed, along an active path from the second input/output to the photonic device for processing according to the particular optical function of the photonic device. The processed optical signal is then routed to the optical gain element and is optically amplified to provide an amplified optical signal. The amplified optical signal is then routed to the first input/output. 
     Optionally, the path switching circuit is arranged to selectively connect the second signal amplification path into the signal path, such that the first input/output is optically coupled to the photonic device; the photonic device is optically coupled to the optical gain element; and the optical gain element is optically coupled to the second input/output. The optical gain element may be selectively configurable to optically amplify the first optical signal downstream of the photonic device in the first direction. The optical gain element may also or alternatively be selectively configurable to optically amplify the second optical signal upstream of the photonic device in the second direction. 
     With the second signal amplification path included in the signal path, the signal path optically couples the first input/output to the photonic device. The photonic device is optically coupled to the optical gain element. The optical gain element is optically coupled to the second input/output. For propagation in the first direction, an optical signal is guided, or routed, along an active path from the first input/output to the photonic device for processing according to the particular optical function of the photonic device. The processed optical signal is then routed to the optical gain element and is optically amplified to provide an amplified optical signal. The amplified optical signal is then routed to the second input/output. For propagation in the second direction, an optical signal is guided, or routed, along an active path from the second input/output to the optical gain element and is optically amplified to provide an amplified optical signal. The amplified optical signal is then routed to the photonic device for processing according to the particular optical function of the photonic device. The processed optical signal is then routed to the first input/output. 
     The photonic device may comprise a polarization-sensitive device. The signal path may comprise a polarization-sensitive signal path. In particular, the photonic device may be provided on a silicon photonics, or SOI, platform or substrate. 
     Optionally, the polarization-sensitive signal path comprises a first polarization component signal path for transmission of first polarization component signals and a second polarization component signal path for transmission of second polarization component signals. The optical gain element and the path switching circuit may be connected to the first polarization component signal path. The device may further comprise a second optical gain element for receiving the second polarization component signals and outputting amplified second polarization components. The device may also further comprise a second path switching circuit comprising a third signal amplification path connectable to the second polarization component signal path between the first input/output and the polarization-sensitive photonic device for optically coupling the second polarization component signal path to and from the second optical gain element; and a fourth signal amplification path connectable to the second polarization component signal path between the polarization-sensitive photonic device and the second input/output for optically coupling the second polarization component signal path to and from the second optical gain element. The second path switching circuit may be arranged to selectively connect the third signal amplification path or the fourth signal amplification path into the second polarization component signal path. In this way, a photonic device incorporating a polarization diversity scheme, or providing two polarization component paths, may be connected to a respective optical gain element and path switching circuit for each polarization component path. The number of optical gain elements provided with this arrangement may thereby be reduced. 
     Optionally, the photonic device comprises a plurality of signal paths, each signal path being connected to a respective path switching circuit and a respective optical gain element. In this way, rather than providing multiple optical gain elements on any particular signal path, a single optical gain element may instead be provided. 
     Optionally, the optical gain element is polarization-insensitive. In this way, the optical gain element may function outside of any polarization-sensitive portions of the device. In this specification, a component/element is considered to be polarization-insensitive, or polarization-independent, if it exhibits low, or no, polarization dependence. Optionally, the path switching circuit is polarization-insensitive. Optionally, the first signal amplification path is selectively connectable into a first portion of the signal path between the first input/output and the photonic device, the first portion not comprising the polarization-sensitive signal path. Optionally, the second signal amplification path is selectively connectable into a second portion of the signal path between the photonic device and the second input/output, the second portion not comprising the polarization-sensitive signal path. In this way, configurations of the path switching circuit outside of any polarization-sensitive portions of the device may be provided. 
     Optionally, the path switching circuit further comprises a first spot-size converter configured upstream of the polarization-insensitive optical gain element. The first spot-size converter may be configured to convert a first spot-size of the signal path upstream of the optical gain element to a second spot-size of the optical gain element. The path switching circuit may also optionally comprise a second spot-size converter configured downstream of the polarization-insensitive optical gain element, the second spot-size converter configured to convert the second spot-size of the optical gain element to a third spot-size of the signal path downstream of the optical gain element. The first spot-size and the third spot-size may be the same. In this way, coupling of optical signals at the spot-size, or mode size, of the signal path upstream of the optical gain element, for example, an optical fiber spot-size, into the optical gain element spot-size may be achieved. Similarly, coupling of optical signals at the spot-size of the optical gain element into the spot-size of the signal path downstream of the optical gain element may be achieved. 
     Optionally, the photonic device and the or each path switching circuit are monolithically integrated in a silicon photonics circuit. In this way, the device may be provided in a relatively small footprint and with the manufacturing and cost benefits associated with CMOS fabrication technologies, for example. Any incremental cost associated with the additional path switching circuit, including additional silicon waveguide structures and switches and possibly also splitter-combiners, is negligible compared with the cost saving by removing one or more optical amplifiers. 
     Optionally, the or each optical gain element comprises a semiconductor optical amplifier, SOA. Optionally still, the SOA may be formed from III-V material, such as InP. 
     Optionally, the or each SOA is hybrid integrated with the silicon photonics circuit. In this way, the device may be provided in a relatively small footprint, and, with the reduction in number of SOAs configured in the device, at a potentially reduced cost and manufacturing complexity. 
     Optionally, there is provided a reconfigurable optical add/drop multiplexer, ROADM, which may comprise the device. Compared with the ROADM of  FIG. 1 , a reduction in the number of optical gain elements configured in the ROADM may be achieved. 
     Optionally, the ROADM comprises first and second polarization component add signal paths and first and second polarization component drop signal paths, with each signal path being connected to a respective path switching circuit and a respective optical gain element. In this way, the number of optical gain elements configured in the ROADM may be reduced from eight to four, compared with the ROADM of  FIG. 1 , offering the advantage of significant savings in manufacturing complexity and cost. 
     Optionally, the ROADM comprises first and second polarization component add signal paths and first and second polarization component drop signal paths. The first and second polarization component add signal paths may be connected to a first polarization-insensitive path switching circuit and a first polarization-insensitive optical gain element. The first and second polarization component drop signal paths may be connected to a second polarization-insensitive path switching circuit and a second polarization-insensitive optical gain element. In this way, the number of optical gain elements configured in the ROADM may be reduced from eight to two, compared with the ROADM of  FIG. 1 , offering the advantage of significant savings in manufacturing complexity and cost. 
     According to a second aspect, there is provided a method for processing an optical signal. The method comprises receiving an optical signal at a first input/output or a second input/output. The method further comprises transmitting the optical signal either from the first input/output in a first direction along a signal path to a photonic device and from the photonic device to the second input/output, or from the second input/output in a second direction along the signal path to the photonic device and from the photonic device to the first input/output. The method further comprises amplifying the optical signal at an optical gain element by selectively directing the optical signal to either a first signal amplification path or a second signal amplification path. The first signal amplification path optically couples the signal path between the first input/output and the photonic device to and from the optical gain element, and the second signal amplification path optically couples the signal path between the photonic device and the second input/output to and from the optical gain element. 
     Other preferred features and advantages are set out in the description and in the dependent claims which are appended hereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be put into practice in a number of ways and some embodiments will now be described, by way of non-limiting example only, with reference to the following figures, in which: 
         FIG. 1  schematically shows a known reconfigurable optical add/drop multiplexer; 
         FIG. 2  schematically shows a device in accordance with a first embodiment; 
         FIG. 3  schematically shows the device of  FIG. 2  in a first operational mode; 
         FIG. 4  schematically shows the device of  FIG. 2  in a second operational mode; 
         FIG. 5  schematically shows the device of  FIG. 2  in a third operational mode; 
         FIG. 6  schematically shows the device of  FIG. 2  in a fourth operational mode; 
         FIG. 7  schematically shows a drop circuit of a ROADM in accordance with a further embodiment; 
         FIG. 8  schematically shows a drop circuit of a ROADM in accordance with a further embodiment; 
         FIG. 9  schematically shows a drop circuit of a ROADM in accordance with a further embodiment; 
         FIG. 10  schematically shows a polarization-insensitive amplification configuration in accordance with a further embodiment; 
         FIG. 11  schematically shows a polarization-insensitive amplification configuration in accordance with a further embodiment; 
         FIG. 12  schematically shows an add circuit of a ROADM in accordance with a further embodiment; 
         FIG. 13  schematically shows an add circuit of a ROADM in accordance with a further embodiment; 
         FIG. 14  schematically shows a polarization-insensitive amplification configuration in accordance with a further embodiment; 
         FIG. 15A  schematically shows a side view and  FIG. 15B  schematically shows a top view of a polarization-insensitive amplification configuration in accordance with a further embodiment; 
         FIG. 16A  schematically shows a side view and  FIG. 16B  schematically shows a top view of a spot-size converter configuration in accordance with a further embodiment; and 
         FIG. 17  shows a flowchart of a method for processing a signal in accordance with a further embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 2 , there is shown a device  200  for processing an optical signal in accordance with a first embodiment. The device  200  includes a photonic device  202  arranged between a first input/output  204  and a second input/output  206 . The photonic device  200  is in optical communication with the first and second inputs/outputs  204 , 206  by a signal path  208 . The signal path  208  may be configured for transmission of a first optical signal in a first direction from the first input/output  204  to the second input/output  206 . The signal path  208  may alternatively be configured for transmission of a second optical signal in a second direction from the second input/output  206  to the first input/output  204 . The device  200  also includes an optical gain element for receiving the first or second optical signal and outputting an amplified first or second optical signal, respectively. In this embodiment, the optical gain element is provided as a semiconductor optical amplifier  210 . 
     The device  200  also includes a path switching circuit  212 . The path switching circuit  212  provides a first signal amplification path  214  connectable between the first input/output  204  and the photonic device  202  for optically coupling the signal path  208  to and from the SOA  210 . The path switching circuit  212  further provides a second signal amplification path  216  connectable between the photonic device  202  and the second input/output  206  for optically coupling the signal path  208  to and from the SOA  210 . The path switching circuit  212  is arranged to selectively connect the first signal amplification path  214  or the second signal amplification path  216  into the signal path  208 . 
     In order to effect the path switching, the path switching circuit  212  is provided with a number of switching elements (SW). The switching elements may be provided by switches, or a combination of switches and splitter-combiners, as will be described more fully below. The switching elements (switches and/or splitter-combiners) may be provided as wavelength-independent, passive, optical elements. This allows for common operation on multiple-wavelength signals; for example, (D)WDM optical signals. 
     A switch may be configured as a 1×N switch, having a single port on one side and N (two or more) ports on the other side. An optical signal input to the single port may be selectively switched to be output from any one of the N ports. Alternatively, in the reverse configuration, an optical signal input to one of the N ports may be selectively output from the single port only when the switch is configured to that particular port of the N ports; otherwise, the signal is not passed onwards. A switch may alternatively be configured as an M×N switch (or cross-connect), having M ports on one side and N ports on the other side. In this case, any one of the M ports may be connected to any one of the N ports. Such a switch may be configured with the M ports as the input side, to allow an optical signal input to one of the M ports to be output to one of the N ports. At the same time, the switch may be configured to allow further optical signals input to any of the other M ports to be output to a respective one of the other N ports. Alternatively, the switch may be configured with the N ports as the input side, to allow an optical signal input to one of the N ports to be output to one of the M ports. At the same time, the switch may be configured to allow further optical signals input to any of the other N ports to be output to a respective one of the other M ports. In this way, switching may be provided by 1×2, 2×1, and/or 2×2 switches, among others. 
     A splitter-combiner may be configured as a splitter only; for example, when arranged such that an optical signal travels into a single input port and multiple optical signals leave via two (or more) output ports. A splitter-combiner may alternatively be configured as a combiner only; for example, when arranged such that multiple optical signals travel into two (or more) input ports and a combined optical signal leaves via a single output port. A splitter-combiner may alternatively still be configured as both a splitter and a combiner; for example, when arranged such that optical signals may be received as the input at either the single port side or the multi-port side and may leave as the output on the other side. 
     In this embodiment, the first signal amplification path  214  includes a first signal amplification sub-path  214   a  and a second signal amplification sub-path  214   b . The first signal amplification path  214  is optically coupled to a first switching element  218 , a second switching element  220 , and a third switching element  222 . The first switching element  218  is provided on the signal path  208  between the first input/output  204  and the photonic device  202 . The first switching element  218  may be configured to connect the signal path  208  to the first signal amplification sub-path  214   a . The first switching element  218  is coupled to the second switching element  220  by the first signal amplification sub-path  214   a . The second switching element  220  is coupled to the SOA  210 . The SOA  210  is coupled to the third switching element  222 . The third switching element  222  is coupled back to the first switching element  218  by the first signal amplification sub-path  214   b . In this way, the path switching circuit  212  may be configured to divert optical signals received from the signal path  208 , from either the first or second direction, away from the signal path  208  onto the first signal amplification path  214 , to the SOA  210  and then back to the signal path  208  from the SOA  210 , for onward transmission on the signal path  208 . In the first direction, the optical signals would then pass into the photonic device  202 , while, in the second direction, the optical signals would pass to the first input/output  204 . In this way, it may be generally seen that the path switching circuit  212  may be configured such that the first input/output  204  and the SOA  210  are optically coupled with each other, the SOA  210  and the photonic device  202  are optically coupled with each other, and the photonic device  202  and the second input/output  206  are optically coupled with each other. 
     In this embodiment, the second signal amplification path  216  includes a second signal amplification sub-path  216   a  and a second signal amplification sub-path  216   b . The second signal amplification path  216  is optically coupled to a fourth switching element  224 , the second switching element  220 , and the third switching element  222 . The fourth switching element  224  is provided on the signal path  208  between the second input/output  206  and the photonic device  202 . The fourth switching element  224  may be configured to connect the signal path  208  to the second signal amplification sub-path  216   a . The fourth switching element  224  is coupled to the second switching element  220  by the second signal amplification sub-path  216   a . The second switching element  220  is coupled to the SOA  210 . The SOA  210  is coupled to the third switching element  222 . The third switching element  222  is coupled back to the fourth switching element  224  by the second signal amplification sub-path  216   b . In this way, the path switching circuit  212  may be configured to divert optical signals received from the signal path  208 , from either the first or second direction, away from the signal path  208  onto the second signal amplification path  216 , to the SOA  210  and then back to the signal path  208  from the SOA  210 , for onward transmission on the signal path  208 . In the second direction, the optical signals would then pass into the photonic device  202 , while, in the first direction, the optical signals would pass to the second input/output  206 . In this way, it may be generally seen that the path switching circuit  212  may be configured such that the first input/output  204  and the photonic device  202  are optically coupled with each other, the photonic device  202  and the SOA  210  are optically coupled with each other, and the SOA  210  and the second input/output  206  are optically coupled with each other. 
     The first switching element  218  may be provided by a M×N optical switch. In this embodiment, the first switching element  218  may be provided by a 2×2 optical switch. Alternatively, the first switching element  218  may be provided by a combination of optical switches and splitter-combiners. The second switching element  220  may be provided by a 2×1 optical switch. Alternatively, the second switching element  220  may be provided by a combiner. The third switching element  222  may be provided by a 1×2 optical switch. Alternatively, the third switching element  222  may be provided by a splitter. The fourth switching element  224  may be provided by a M×N optical switch. In this embodiment, the fourth switching element  224  may be provided by a 2×2 optical switch. Alternatively, the fourth switching element  224  may be provided by a combination of optical switches and splitter-combiners. 
     The signal path  208  is shown in  FIG. 2  with a solid line from the first input/output  204  to the second input/output  206 , because the signal path  208  is active during use of the device  200 . This is also the case for the common sub-path coupling the second switching element  220  to the SOA  210  and the common sub-path coupling the SOA  210  to the third switching element  222 . The path switching circuit  212 , however, is selectively configurable to introduce either the first signal amplification path  214  or the second signal amplification path  216 . These paths  214 , 216  are therefore shown with dashed lines to indicate that one or other of the paths  214 , 216  may be inactive during use of the device  200 . 
       FIGS. 3-6  schematically show the device  200  in different operational modes for amplifying an optical signal. In these figures, solid lines with arrows indicate active sub-paths for signal transmission in the device  200 , while dashed lines indicate current inactive sub-paths. 
     In the embodiment of  FIG. 3 , the device  200  is configured to optically amplify a first optical signal from the first input/output  204  upstream of the photonic device  202  in the first direction. The first switching element  218  selectively connects the signal path  208  from the first input/output  204  to the first signal amplification sub-path  214   a . The second switching element  220  selectively connects the first signal amplification sub-path  214   a  to the SOA  210 . The SOA  210  is coupled between the second switching element  220  and the third switching element  222 . The third switching element  222  selectively connects the SOA  210  to the first signal amplification sub-path  214   b , back to the first switching element  218 . The first switching element  218  selectively connects the first signal amplification sub-path  214   b  to the signal path  208  towards the photonic device  202 . From the photonic device  202 , the signal path  208  passes to the second input/output  206  via the fourth switching element  224 . 
     In this way, the first optical signal may be received from the first input/output  204  and follow an active path onto the signal path  208 . The active path for the first optical signal is then directed onto the first signal amplification path  214  and the first optical signal is amplified by the SOA  210 . The amplified first optical signal is then passed back onto the signal path  208  for transmission to the photonic device  202 , for processing according to the particular function(s) the photonic device is configured to perform, and then transmitted along the active path to the second input/output  206 . 
     In the embodiment of  FIG. 4 , the device  200  is configured to optically amplify a first optical signal from the first input/output  204  downstream of the photonic device  202  in the first direction. The active path begins at signal path  208 , passing from the first input/output  204  via the first switching element  218  to the photonic device  202 . The fourth switching element  224  selectively connects the signal path  208  from the photonic device  202  to the second signal amplification sub-path  216   a . The second switching element  220  selectively connects the second signal amplification sub-path  216   a  to the SOA  210 . The SOA  210  is coupled between the second switching element  220  and the third switching element  222 . The third switching element  222  selectively connects the SOA  210  to the second signal amplification sub-path  216   b , back to the fourth switching element  224 . The fourth switching element  224  selectively connects the second signal amplification sub-path  216   b  to the signal path  208  towards the second input/output  206 . 
     In this way, the first optical signal may be received from the first input/output  204  and follow an active path onto the signal path  208 . The active path for the first optical signal continues to the photonic device  202 , for processing according to the particular function(s) the photonic device is configured to perform, and then continues further along the signal path  208 . The active path for the first optical signal is then directed onto the second signal amplification path  216  and the first optical signal is amplified by the SOA  210 . The amplified first optical signal is then passed back onto the signal path  208  for transmission along the active path to the second input/output  206 . 
     In the embodiment of  FIG. 5 , the device  200  is configured to optically amplify a second optical signal from the second input/output  206  upstream of the photonic device  202  in the second direction. The fourth switching element  224  selectively connects the signal path  208  from the second input/output  206  to the second signal amplification sub-path  216   a . The second switching element  220  selectively connects second signal amplification sub-path  216   a  to the SOA  210 . The SOA  210  is coupled between the second switching element  220  and the third switching element  222 . The third switching element  222  selectively connects the SOA  210  to the second signal amplification sub-path  216   b , back to the fourth switching element  224 . The fourth switching element  224  selectively connects the second signal amplification sub-path  216   b  to the signal path  208  towards the photonic device  202 . From the photonic device  202 , the signal path  208  passes to the first input/output  204  via the first switching element  218 . 
     In this way, the second optical signal may be received from the second input/output  206  and follow an active path onto the signal path  208 . The active path for the second optical signal is then directed onto the second signal amplification path  216  and the second optical signal is amplified by the SOA  210 . The amplified second optical signal is then passed back onto the signal path  208  for transmission to the photonic device  202 , for processing according to the particular function(s) the photonic device is configured to perform, and then transmitted along the active path to the first input/output  204 . In the embodiment of  FIG. 6 , the device  200  is configured to optically amplify a second optical signal from the second input/output  206  downstream of the photonic device  202  in the second direction. The active path begins at signal path  208 , passing from the second input/output  206  via the fourth switching element  224  to the photonic device  202 . The first switching element  218  selectively connects the signal path  208  from the photonic device  202  to the first signal amplification sub-path  214   a . The second switching element  220  selectively connects the first signal amplification sub-path  214   a  to the SOA  210 . The SOA  210  is coupled between the second switching element  220  and the third switching element  222 . The third switching element  222  selectively connects the SOA  210  to the first signal amplification sub-path  214   b , back to the first switching element  218 . The first switching element  218  selectively connects the first signal amplification sub-path  214   b  to the signal path  208  towards the first input/output  204 . 
     In this way, the second optical signal may be received from the second input/output  206  and follow an active path onto the signal path  208 . The active path for the second optical signal continues to the photonic device  202 , for processing according to the particular function(s) the photonic device is configured to perform, and then continues further along the signal path  208 . The active path for the second optical signal is then directed onto the first signal amplification path  214  and the second optical signal is amplified by the SOA  210 . The amplified second optical signal is then passed back onto the signal path  208  for transmission along the active path to the first input/output  204 . 
     It can be seen from the above embodiments that a single gain element, here in the form of the SOA, may be selectively configured to amplify an optical signal, either before or after processing by a photonic device, and whether the propagation direction is in a first or second direction in the device. In this way, amplification of an optical signal may be provided at the same functional stage through the device for propagation in the first direction or the second direction. The path switching circuit provides for reconfiguration of the active path in the device, such that a single gain element may be configured to provide optical amplification for any of the operational modes shown in  FIGS. 3-6 . Such an arrangement allows for a reduced number of gain elements to be provided in the device, thereby offering savings in cost and manufacturing complexity. 
     The photonic device in the above embodiments, and indeed in the following embodiments, may include a range of different devices configured to pass, bypass, process, and/or perform one or more photonic functions on an optical signal. The photonic device may include one or more photonic elements, including waveguides, micro-ring resonators, filters, splitters, spot-size converters, polarization splitters-rotators, optical switches, variable optical attenuators, isolators, optical amplifiers, modulators, and photodiodes, among others. For example, the photonic device may include a reconfigurable optical add multiplexer (ROAM). The photonic device may include a reconfigurable optical drop multiplexer (RODM). The photonic device may include a reconfigurable optical add/drop multiplexer (ROADM). 
     The device  200  may be provided as an integrated optical device. For example, the device  200  may be fabricated using silicon photonics or silicon-on-insulator platforms, taking advantage of CMOS fabrication techniques which may be employed for this. In particular, the photonic device  202 , the signal path  208 , and the path switching circuit  212  may be monolithically integrated on a silicon-on-insulator substrate. The gain element in the form of the SOA—for example, made from III-V group materials, such as InP or GaAs—may then be hybrid integrated with the silicon photonics circuit. While SOAs may be hybrid (or heterogeneously) integrated with silicon photonics circuits, monolithic integration techniques may alternatively be used to fabricate silicon photonics circuits and SOAs together. It is expected that such monolithic integration techniques will become more widespread as fabrication processes and costs improve. Germanium-based semiconductor optical amplifiers may also be used. A germanium-based SOA may be monolithically integrated with a silicon photonics circuit. 
     Providing the device at least in part using silicon photonics gives rise to considerations of the polarization-sensitivity of silicon optical waveguides. In view of this, embodiments may employ a polarization diversity scheme to provide a dual polarization structure in the device. The photonic device structure may be divided into a first polarization component structure and a second polarization component structure. A first polarization splitter-rotator (PSR)—in some embodiments in the form of a dual polarization grating coupler (DPGC)—may be provided at the first input/output end  204 . The first DPGC splits the unknown and random polarization of input optical signals into two orthogonal polarization components, namely TE and TM components. The DPGC then rotates one of the components by 90° so that both components have the same polarization at the output of the DPGC. One component may then be output to the first polarization component structure and the other component may be output to the second polarization component structure, with the polarizations of both components now corresponding to the linear polarization mode of the silicon optical waveguide structures of the polarization component structures. From there, the two component optical signals may be processed identically, in parallel, along the first and second polarization component structures, respectively. At the second input/output end  206 , a second polarization splitter-rotator (PSR)—in some embodiments in the form of a dual polarization grating coupler (DPGC)—may be provided. The processed component optical signals may be output from the first and second polarization component structures to the second DPGC. The second DPGC performs the reverse procedure on the component optical signals, rotating the polarization of one of the components and recombining the components into output optical signals for onward transmission at the second input/output  206 . 
     To provide optical amplification for the first and second polarization component signals, a respective gain element and path switching circuit may be provided for each of the first and second polarization component structures, respectively. 
       FIG. 7  shows an embodiment in which the photonic device is provided as a drop circuit structure  702  of a ROADM. The structure is polarization-sensitive and the drop circuit structure  702  is split into a first drop sub-circuit  714  for processing a first polarization component signal and a second drop sub-circuit  716  for processing a second polarization component signal. A path switching circuit  712  is connected to the first drop sub-circuit  714 . A second path switching circuit may also in some embodiments be connected to the second drop sub-circuit  716 . However, for simplicity of explanation, a second path switching circuit is not shown in  FIG. 7 . The description of the path switching circuit  712  for the first drop sub-circuit  714  will be understood to apply in an analogous manner to a second path switching circuit for the second drop sub-circuit  716 . 
     With a ROADM, a path switching circuit may be implemented as described above with respect to  FIGS. 2 to 6 . Alternatively, as shown in  FIG. 7 , the path switching circuit  712  is provided by a combination of optical switches and splitter-combiners. The switches in this embodiment may be provided by 1×2 optical switches. The path switching circuit  712  is configured for providing optical amplification at the input end, or upstream of the photonic device—in this embodiment, the drop structure—in either direction of propagation. 
     In the path switching circuit  712 , the signal path  708  of the first drop sub-circuit  714  is coupled from the first input/output  704  to a first optical switch  718 . The path is directed from there to a SOA  724  via a first combiner  722 . The SOA  724  is optically coupled to a second switch  726 , which directs the path to a second combiner  720 . From the second combiner  720 , the path rejoins the signal path  708  and passes through the drop structure. At the end of the drop structure, the signal path  708  is coupled to a splitter-combiner  730  and to a third switch  728 , which directs the path to the signal path  708  towards the second input/output  706 . 
     In this way, a first polarization component signal from the first input/output  704  on the signal path  708  will be directed to port a of the first switch  718 . The signal will leave port b of the first switch  718  and pass into the first combiner  722  to the SOA  724 . From the SOA  724 , the signal will pass into port e of the second switch  726  and leave at port f. The signal will then pass into the second combiner  720  and, from port d, back to the signal path  708  for processing in the drop structure. After being processed in the drop structure, the signal will be directed from the signal path  708  to port p of the third splitter-combiner  730 . The third splitter-combiner  730  passes the signal to port n of the third switch  728 . The signal leaves the third switch  728  from port m and returns to the signal path  708  towards the second input/output  706 . 
     For a signal propagating in the second direction, from the second input/output  706  to the first input/output  704 , the signal path  708  will be directed to port m of the third switch  728 . The signal will leave port h of the third switch  728  and pass into the first combiner  722  to the SOA  724 . From the SOA  724 , the signal will pass into port e of the second switch  726  and leave at port g. The signal will then pass into the third splitter-combiner  730  and, from port p, back to the signal path  708  for processing in the drop structure. After being processed in the drop structure, the signal will be redirected from the signal path  708  to port d of the first splitter-combiner  720 . The first splitter-combiner  720  passes the signal to port c of the first switch  718 . The signal leaves the first switch  718  from port a and returns to the signal path  708  towards the first input/output  704 . 
       FIG. 8  shows another embodiment, in which the photonic device is provided as a drop circuit structure  802  of a ROADM. The structure is polarization-sensitive and the drop circuit structure  802  is split into a first drop sub-circuit for processing a first polarization component signal and a second drop sub-circuit for processing a second polarization component signal. A path switching circuit  812  is connected to the first drop sub-circuit. A second path switching circuit may also in some embodiments be connected to the second drop sub-circuit. However, for simplicity of explanation, a second path switching circuit is not shown in  FIG. 8 . The description of the path switching circuit  812  for the first drop sub-circuit will be understood to apply in an analogous manner to a second path switching circuit for the second drop sub-circuit. 
     In this embodiment, the path switching circuit  812  is not provided by a combination of switches and splitter-combiners, but by switches only. This may be advantageous in reducing losses in the path switching circuit. A 2×2 optical switch typically incurs lower losses than a splitter-combiner, for example. In this embodiment, the first and fourth switches  818 , 824  are 2×2 optical switches and the second and third switches  820 , 822  are 1×2 optical switches, although other switch types and combinations may be employed. 
     In the path switching circuit  812 , the signal path  808  is coupled from the first input/output  804  to a first optical switch  818 . The path is directed from there to a SOA  810  via a second optical switch  820 . The SOA  810  is optically coupled to a third switch  822 , which directs the path back to the first switch  818 . From the first switch  818 , the path rejoins the signal path  808  and passes through the drop structure. At the end of the drop structure, the signal path  808  is coupled to a fourth switch  824 , which directs the path to the signal path  808  towards the second input/output  806 . 
     In this way, a first polarization component signal from the first input/output  804  on the signal path  808  will be directed to port a of the first switch  818 . The signal will leave port c of the first switch  818  and pass into port h of the second switch  820  and then to the SOA  810 . From the SOA  810 , the signal will pass to the third switch  822  and leave at port f. The signal will then pass back to the first switch  818  at port b and, from port d, pass back to the signal path  808  for processing in the drop structure. After being processed in the drop structure, the signal will be directed from the signal path  808  to port m of the fourth switch  824 . The signal will leave the fourth switch at port n and return to the signal path  808  towards the second input/output  806 . In this configuration, port a is connected to port c, and port b is connected to port d, in the first switch  818 . Port h is active and port i is inactive in the second switch  820 . Port f is active and port g is inactive in the third switch  822 . Port m is connected to port n, and port o is connected to port l, in the fourth switch  824 . 
     For a signal propagating in the second direction, from the second input/output  806  to the first input/output  804 , the signal path  808  will be directed to port n of the fourth switch  824 . The signal will leave port l of the fourth switch  824  and pass into port i of the second switch  820  and then to the SOA  810 . From the SOA  810 , the signal will pass to the third switch  822  and leave at port g. The signal will then pass back to the fourth switch  824  at port o and, from port m, pass back to the signal path  808  for processing in the drop structure. After being processed in the drop structure, the signal will be directed from the signal path  808  to port d of the first switch  818 . The signal will leave the first switch at port a and return to the signal path  808  towards the first input/output  804 . In this configuration, port a is connected to port d, and port b is connected to port c, in the first switch  818 . Port i is active and port h is inactive in the second switch  820 . Port g is active and port f is inactive in the third switch  822 . Port m is connected to port o, and port l is connected to port n, in the fourth switch  824 . 
     The device  800  has been described in configurations in which optical amplification is provided to the incoming signal, upstream of the photonic device. However, with appropriate configuration of the switching elements—for example, which ports are active or inactive and which ports couple to which ports in the switches—the device  800  may be configured to provide optical amplification to the outgoing signal, downstream of the photonic device. 
       FIG. 9  shows another embodiment, in which the photonic device is provided as a drop circuit structure  902  of a ROADM. The structure is polarization-sensitive and the drop circuit structure  902  is split into a first drop sub-circuit  914  for processing a first polarization component signal and a second drop sub-circuit for processing a second polarization component signal. A path switching circuit  912  is connected to the first drop sub-circuit  914 . A second path switching circuit may also in some embodiments be connected to the second drop sub-circuit. However, for simplicity of explanation, a second path switching circuit is not shown in  FIG. 9 . The description of the path switching circuit  912  for the first drop sub-circuit will be understood to apply in an analogous manner to a second path switching circuit for the second drop sub-circuit. 
     In this embodiment, the path switching circuit  912  is configured for providing optical amplification at the output end, or downstream of the photonic device—in this embodiment, the drop structure—in either direction of propagation. Whether optical amplification is provided upstream or downstream of the photonic device may depend on the function(s) to be provided by the photonic device. With a ROADM, it is generally desirable to provide optical amplification to the incoming signal to the drop structure and to provide optical amplification to the outgoing signal from the add structure. However, optical amplification may be provided in the reverse configuration in some implementations. It will be appreciated that, while  FIG. 9  is shown with a drop structure—in particular, with detail of a path switching circuit  912  connected to a first drop sub-circuit  914 —the path switching circuit  912  could be connected to either a first or second drop sub-circuit of a drop structure, or to either a first or second add sub-circuit of an add structure of a ROADM. Indeed, this is also the case the for path switching circuits  212 ,  712 ,  812  discussed above in relation to  FIGS. 2-8 . 
     In the path switching circuit  912 , the signal path  908  of the first drop sub-circuit  914  is coupled from the first input/output  904  to a first optical switch  918 . and the path is directed from there to a second optical switch  920 . From the second optical switch  920 , the path is directed back to the signal path  908  to pass through the drop structure. At the end of the drop structure, the path is coupled to a third optical switch  926  and then to a SOA  910  via a first combiner  922 . The SOA  910  is optically coupled to a second splitter-combiner  924 , which directs the path to a fourth optical switch  928 . From the fourth optical switch  928 , the path rejoins the signal path  908  and passes towards the second input/output  906 . 
     In this way, a first polarization component signal from the first input/output  904  on the signal path  908  will be directed to port a of the first switch  918 . The signal will leave the first switch  918  and pass into port b of the second switch  920 . The signal will leave the second switch  920  at port d and return to the signal path  908  for processing in the drop structure. After being processed in the drop structure, the signal will be directed from the signal path  908  to port n of the third switch  926 . The signal will leave the third switch  926  at port h and pass to the SOA  910  via the first combiner  922 . From the SOA  910 , the signal will pass into port e of the second splitter-combiner  924  and leave at port g. The signal will then pass to the fourth switch  928  and, from port p, back to the signal path  908  towards the second input/output  906 . 
     For a signal propagating in the second direction, from the second input/output  906  to the first input/output  904 , the signal path  908  will be redirected to port p of the fourth switch  928 . The signal will then pass to port m of the third switch  926  and leave the switch at port n. From there, the signal will return to the signal path  908  for processing in the drop structure. After being processed in the drop structure, the signal will be directed from the signal path  908  to port d of the second switch  920 . The signal will leave the second switch  920  at port c and pass to the SOA  910  via the first combiner  922 . From the SOA  910 , the signal will pass into port e of the second splitter-combiner  924  and leave at port f. The signal will then pass to the first switch  918  and, from port a, back to the signal path  908  towards the first input/output  904 . 
     It can be seen, then, that configuring a path switching circuit with an optical gain element as described may provide the advantage of reducing the number of optical gain elements in a device. When applied to a ROADM, having first and second polarization drop sub-circuits and first and second polarization add sub-circuits as shown in  FIG. 1 , with a respective path switching circuit and optical gain element for each add or drop sub-circuit, the number of SOAs integrated into the ROADM silicon photonic device may be reduced from eight down to four. This offers a significant saving on cost and manufacturing complexity. 
     The devices shown in  FIGS. 7-9  may be configured with a respective path switching circuit and optical gain element for each add or drop polarization sub-circuit. This arrangement would provide two path switching circuits and two optical gain elements per add or drop structure. The embodiment shown in  FIG. 10  provides an arrangement in which a single path switching circuit and a single optical gain element may be configured for amplification of optical signals passing to or from both polarization sub-circuits of either an add structure or a drop structure, as a whole. More generally, the path switching circuit and optical gain element are configured outside of the polarization-sensitive zone of the device. This arrangement provides for amplification of the optical signal with the optical signal having an unknown and/or random polarization state, before the optical signal is processed by a polarization splitter-rotator to separate the signal into polarization components. In this way, a single path switching circuit and optical gain element may be configured to provide amplification to a polarization-sensitive photonic device as a whole and not only on a single polarization component of the photonic device. When applied to a ROADM, having first and second polarization drop sub-circuits and first and second polarization add sub-circuits as shown in  FIG. 1 , with a respective polarization-insensitive path switching circuit and optical gain element for the add structure and the drop structure, the number of SOAs integrated with the ROADM silicon photonic device may be reduced from eight down to two. This offers a significant saving on cost and manufacturing complexity. 
     Referring to  FIG. 10 , there is shown a device  300  for processing an optical signal. The device  300  includes a polarization-sensitive photonic device  302  arranged between a first input/output  304  and a second input/output  306 . The photonic device  300  is in optical communication with the first and second inputs/outputs  304 , 306  by a signal path  308 . The signal path  308  may be configured for transmission of a first optical signal in a first direction from the first input/output  304  to the second input/output  306 . The signal path  308  may alternatively be configured for transmission of a second optical signal in a second direction from the second input/output  306  to the first input/output  304 . The polarization-sensitive photonic device  302  is configured between a first polarization splitter-rotator (PSR)  336  and a second polarization splitter-rotator (PSR)  338 . The region of the device  300  between the first and second PSRs  336 , 338 , including the photonic device  302 , may therefore be considered to be a polarization-sensitive region or zone. 
     The device  300  also includes a polarization-insensitive optical gain element for receiving the first or second optical signal and outputting an amplified first or second optical signal, respectively, outside of the polarization-sensitive region. In this embodiment, the polarization-insensitive optical gain element is provided as a polarization-insensitive semiconductor optical amplifier  310 . A polarization-insensitive SOA typically exhibits a low polarization-dependent gain. Such polarization-independent SOAs are known. 
     The device  300  also includes a polarization-insensitive path switching circuit  312 , also configured outside of the polarization-sensitive region. The path switching circuit  312  provides a first signal amplification path  314  connectable between the first input/output  304  and the first PSR  336  for optically coupling the signal path  308  to and from the SOA  310 . The path switching circuit  312  further provides a second signal amplification path  316  connectable between the second PSR  338  and the second input/output  306  for optically coupling the signal path  308  to and from the SOA  310 . The path switching circuit  312  is arranged to selectively connect the first signal amplification path  314  or the second signal amplification path  316  into the signal path  308 . 
     In this embodiment, the first signal amplification path  314  is includes a first signal amplification sub-path  314   a  and a second signal amplification sub-path  314   b . The first signal amplification path  314  is optically coupled to a first switching element  318 , a second switching element  320 , and a third switching element  322 . The first switching element  318  is provided on the signal path  308  between the first input/output  304  and the first PSR  336 . The first switching element  318  may be configured to connect the signal path  308  to the first signal amplification sub-path  314   a . The first switching element  318  is coupled to the second switching element  320  by the first signal amplification sub-path  314   a . The second switching element  320  is coupled to the SOA  310  via a first spot-size converter (SSC)  330 . The first spot-size converter  330  is a polarization-insensitive spot-size converter and is configured to convert a first propagation mode size of the optical signal from the first signal amplification sub-path  314   a  to a second mode size of the SOA  310 . The SOA  310  is coupled to the third switching element  322  via a second spot-size converter (SSC)  332 . The second spot-size converter  332  is a polarization-insensitive spot-size converter and is configured to convert the second propagation mode size of the optical signal from the SOA  310  to a third mode size of the first signal amplification sub-path  314   b . The first and third mode sizes may be configured to match, so that the first and second spot-size converters  330 , 332  may also match, but in opposite configurations. This can simplify fabrication processes. The first and third mode sizes may also be configured to correspond to a propagation mode size of incoming/outgoing optical signals from/to external optical fibers (not shown) to which the device  300  may be coupled. Alternatively, the signal path  308  and the first and second signal amplification paths  314 , 316  may have the same spot-size, so that the first and third spot sizes match, but do not correspond to that of an external optical fiber. In this case, a further spot-size converter (not shown) may be provided between an external optical fiber (not shown) and the signal path  308 . 
     The third switching element  322  is coupled back to the first switching element  318  by the first signal amplification sub-path  314   b . In this way, the path switching circuit  312  may be configured to divert optical signals received from the signal path  308 , from either the first or second direction, away from the signal path  308  onto the first signal amplification path  314 , to the SOA  310  and then back to the signal path  308  from the SOA  310 , for onward transmission on the signal path  308 . This may be performed in a polarization-insensitive manner. In the first direction, the optical signals would then pass into the polarization-sensitive region to the photonic device  302 , while, in the second direction, the optical signals would pass to the first input/output  304 . 
     For transmission of the optical signal from the first switching element  318  to the first PSR  336  and to the photonic device  302 , a third spot-size converter  334  is provided to convert the first or third propagation mode size to a mode size for coupling into the polarization-sensitive region. For example, the third spot-size converter  334  may be configured to convert the first or third propagation mode size to a fourth mode size of a silicon optical waveguide structure. 
     In this embodiment, the second signal amplification path  316  includes a second signal amplification sub-path  316   a  and a second signal amplification sub-path  316   b . The second signal amplification path  316  is optically coupled to a fourth switching element  324 , the second switching element  320 , and the third switching element  322 . The fourth switching element  324  is provided on the signal path  308  between the second input/output  306  and the second PSR  338 . The fourth switching element  324  may be configured to connect the signal path  308  to the second signal amplification sub-path  316   a . The fourth switching element  324  is coupled to the second switching element  320  by the second signal amplification sub-path  316   a . The second switching element  320  is coupled to the SOA  310  via the first spot-size converter  330 . The SOA  310  is coupled to the third switching element  322  via the second spot-size converter  332 . The third switching element  322  is coupled back to the fourth switching element  324  by the second signal amplification sub-path  316   b . In this way, the path switching circuit  312  may be configured to divert optical signals received from the signal path  308 , from either the first or second direction, away from the signal path  308  onto the second signal amplification path  316 , to the SOA  310  and then back to the signal path  308  from the SOA  310 , for onward transmission on the signal path  308 . This may be performed in a polarization-insensitive manner. In the second direction, the optical signals would then pass into the polarization-sensitive region to the photonic device  302 , while, in the first direction, the optical signals would pass to the second input/output  306 . 
     For transmission of the optical signal from the fourth switching element  324  to the second PSR  338  and to the photonic device  302 , a fourth spot-size converter  340  is provided to convert the first or third propagation mode size to a mode size for coupling into the polarization-sensitive region. For example, the fourth spot-size converter  340  may be configured to convert the first or third propagation mode size to a fourth mode size of a silicon optical waveguide structure. 
     The polarization-insensitive path switching circuit  312  and SSCs  330 , 332 , 334 , 340  may be formed from materials, or combinations of materials, including silicon dioxide (SiO 2 ), silicon oxide (SiOx), silicon oxynitride (SiON or SiO x N y ) and/or silicon nitride (Si 3 N 4 ), among others. Such materials are also compatible with CMOS fabrication processes, so the polarization-insensitive path switching circuit may be integrated on a silicon photonics substrate along with the polarization-sensitive photonic device. 
     Although the path switching circuit and optical gain element in this embodiment are polarization-insensitive and spot-size converters are additionally configured in the path switching circuit, the general discussion of operational modes for optical amplification provided in respect of  FIGS. 2-6  also applies to the device  300 , with the SSCs and PSRs performing their respective functions on optical signals as the optical signals pass through those elements in the relevant active path for a given operational mode, as discussed above. 
       FIG. 11  shows an embodiment of a polarization-insensitive path switching circuit  412  configured with a polarization-sensitive photonic device, which in this embodiment is provided as a drop structure  402  of a ROADM. The drop structure  402  is provided between a first PSR  436  and a second PSR  438 , representing a polarization-sensitive region of the device  400 . 
     The device  400  includes a polarization-insensitive optical gain element for receiving an optical signal and outputting an amplified optical signal, outside of the polarization-sensitive region. In this embodiment, the polarization-insensitive optical gain element is provided as a polarization-insensitive semiconductor optical amplifier  410 . 
     The device  400  also includes a polarization-insensitive path switching circuit  412 , also configured outside of the polarization-sensitive region. The path switching circuit  412  provides a first signal amplification path connectable between a first input/output  404  and the first PSR  436  for optically coupling the signal path to and from the SOA  410 . The path switching circuit  412  further provides a second signal amplification path connectable between the second PSR  438  and the second input/output  406  for optically coupling the signal path to and from the SOA  410 . The path switching circuit  412  is arranged to selectively connect the first signal amplification path or the second signal amplification path into the signal path. 
     In this embodiment, the path switching circuit  412  is provided by a combination of optical switches and splitter-combiners. The switches in this embodiment may be provided by 1×2 optical switches. The path switching circuit  412  is configured for providing optical amplification at the input end, or upstream of the polarization-sensitive zone—in this embodiment, the drop structure  402 —in either direction of propagation. 
     In the path switching circuit  412 , the signal path is coupled from the first input/output  404  to a first optical switch  418 . The path is directed from there to a SOA  410  via a first combiner  422  and a first spot-size converter (SSC)  430 . The SOA  410  is optically coupled to a second switch  426  via a second SSC  432 . The second switch  426  couples the path to a second combiner  420 . From the second combiner  420 , the path couples to a third SSC  434  for converting a propagation mode size to that of the polarization-sensitive region. From the third SSC  434 , the path passes to the first PSR  436  and into the polarization-sensitive region including the drop structure  402 . At the end of the drop structure  402 , the path passes to the second PSR  438  and to a fourth SSC  424  for converting the propagation mode size of the polarization-sensitive region to that of the polarization-insensitive region. 
     In this way, a first signal with a generic or random polarization from the first input/output  404  on the signal path will be directed to the first switch  418 . The signal will leave the first switch  418  and pass into the first combiner  422  to the SOA  410 , via the first SSC  430 . From the SOA  410 , the signal will pass via the second SSC  432 , to the second switch  426 . The signal will then pass into the second combiner  420  and back to the main signal path for processing in the polarization-sensitive drop structure  402 , via the first PSR  436 . After being processed in the drop structure  402 , the signal will pass to the second PSR  438 , and leave the polarization-sensitive region. The signal will be directed via the fourth SSC  424  and then to the third splitter-combiner  430 . The third splitter-combiner  430  passes the signal to the third switch  428 . The signal leaves the third switch  428  and returns to the signal path towards the second input/output  406 . 
     For a signal propagating in the second direction, from the second input/output  406  to the first input/output  404 , the signal path will be directed to the third switch  428 . The signal will leave the third switch  428  and pass into the first combiner  422  to the SOA  410 , via the first SSC  430 . From the SOA  410 , the signal will pass to the second switch  426 , via the second SSC  432 . The signal will then pass into the third splitter-combiner  430  and, via the fourth SSC  424 , back to the signal path for processing in the polarization-sensitive drop structure  402 , via the second PSR  438 . After being processed in the drop structure  402 , the signal will pass to the first PSR  436 , and leave the polarization-sensitive region. The signal will be directed via the third SSC  434  and then to the first splitter-combiner  420 . The first splitter-combiner  420  passes the signal to the first switch  418 . The signal leaves the first switch  418  and returns to the signal path towards the first input/output  404 . 
     As stated above, the path switching circuits of the above embodiments may be connected to either an add structure of a ROADM or to a drop structure of a ROADM. For embodiments in which the path switching is polarization-independent and takes place outside of the polarization-sensitive region of a ROADM, a path switching circuit may be coupled to the add structure; or a path switching circuit may be coupled to the drop structure; or one path switching circuit may be coupled to the add structure and another path switching circuit may be coupled to the drop structure. For embodiments in which the path switching takes place inside the polarization-sensitive region of a ROADM, a path switching circuit may be connected to either a first or second drop sub-circuit of a drop structure, or to a first or second add sub-circuit of an add structure of a ROADM. Alternatively, a first path switching circuit may be connected to a first drop sub-circuit of a drop structure and a second path switching circuit may be connected to a second drop sub-circuit of the drop structure. Alternatively still, a first path switching circuit may be connected to a first add sub-circuit of an add structure and a second path switching circuit may be connected to a second add sub-circuit of the add structure. Alternatively still, a respective path switching circuit may be connected to each of the first and second drop sub-circuits of the drop structure and to each of the first and second add sub-circuits of the add structure. It will be appreciated therefore that, while the embodiments shown in  FIGS. 7-9 and 11  include a photonic device in the form of a drop structure of a ROADM, the photonic device may alternatively be provided in the form of an add structure of a ROADM, with the details of the path switching circuits and optical gain elements remaining unchanged. Similarly, while the embodiment shown in  FIG. 14 , below, includes a photonic device in the form of an add structure of a ROADM, the photonic device may alternatively be provided in the form of a drop structure of a ROADM, with the details of the signal amplification paths and optical gain elements remaining unchanged. 
     By way of example,  FIG. 12  shows an embodiment of a device  850 , in which the photonic device is provided as an add circuit structure  852  of a ROADM. The structure is polarization-sensitive and the add circuit structure  852  is split into a first add sub-circuit for processing a first polarization component signal and a second add sub-circuit for processing a second polarization component signal. A path switching circuit  812  is connected to the first add sub-circuit. A second path switching circuit may also in some embodiments be connected to the second add sub-circuit. However, for simplicity of explanation, a second path switching circuit is not shown in  FIG. 12 . The description of the path switching circuit  812  for the first add sub-circuit will be understood to apply in an analogous manner to a second path switching circuit for the second add sub-circuit. 
     In this embodiment, the path switching circuit  812  is arranged and configured to function in exactly the same way as the path switching circuit  812  shown in the embodiment of  FIG. 8 , so a description of the layout and operation of the path switching circuit  812  is not repeated here. It will be understood that such a description of the embodiment of  FIG. 12  would differ from that provided in respect of  FIG. 8  in that references to the drop structure and the drop circuit structure  802  would be replaced with references to the add structure and the add circuit structure  852 , respectively. 
     By way of further example,  FIG. 13  shows an embodiment of a device  450 , in which a polarization-insensitive path switching circuit  412  is configured with a polarization-sensitive photonic device, which in this embodiment is provided as an add structure  452  of a ROADM. The add structure  452  is provided between a first PSR  436  and a second PSR  438 , representing a polarization-sensitive region of the device  450 . 
     In this embodiment, the polarization-insensitive path switching circuit  412  is arranged and configured to function in exactly the same way as the polarization-insensitive path switching circuit  412  shown in the embodiment of  FIG. 11 , so a description of the layout and operation of the polarization-insensitive path switching circuit  412  is not repeated here. It will be understood that such a description of the embodiment of  FIG. 13  would differ from that provided in respect of  FIG. 11  in that references to the drop structure  402  would be replaced with references to the add structure  452 . 
     In the above embodiments, a path switching circuit has been arranged to selectively engage or connect an optical gain element into the active path of a device, either upstream of a photonic device or downstream of the photonic device. The path switching circuit includes a first signal amplification path connectable between the first input/output and the photonic device and a second signal amplification path connectable between the photonic device and a second input/output. The path switching circuit is configured to selectively engage or connect the first or the second signal amplification path into the active path. Both the first and second signal amplification paths are optically coupled to the same optical gain element, so that whichever path is engaged the active path will pass through the optical gain element. In the embodiments shown in  FIGS. 10, 11, and 13 , the first and second signal amplification paths are provided outside of a polarization-sensitive region of the device; that is, outside of, or external to, a polarization-sensitive photonic device and a respective PSR either side of the photonic device. 
       FIG. 14  shows an embodiment of a device  600  having a polarization-insensitive amplification configuration arranged outside of a polarization-sensitive region of the device. In this embodiment, the polarization-sensitive region of the device  600  includes a polarization-sensitive photonic device  602 —here, in the form of an add structure  602  of a ROADM—optically coupled between a first PSR  608  and a second PSR  610 . A first polarization-insensitive optical gain element coupling device  612  is optically coupled between a first input/output  604  of the device and the first PSR  608 . A second polarization-insensitive optical gain element coupling device  614  is optically coupled between the second PSR  610  and a second input/output  606  of the device. 
     The first optical gain element coupling device  612  includes a first polarization-insensitive optical gain element, in this embodiment provided by a polarization-insensitive first SOA  618 . The first SOA  618  is optically coupled between a polarization-insensitive first SSC  616  and a polarization-insensitive second SSC  620 , for spot-size conversion of optical signals between the first input/output  604  and the first SOA  618 , and between the first SOA  618  and the first PSR  608 , respectively. The second optical gain element coupling device  614  includes a second polarization-insensitive optical gain element, in this embodiment provided by a polarization-insensitive second SOA  624 . The second SOA  624  is optically coupled between a polarization-insensitive third SSC  622  and a polarization-insensitive fourth SSC  626 , for spot-size conversion of optical signals between the second PSR  610  and the second SOA  624 , and between the second SOA  624  and the second input/output  606 , respectively. In some embodiments, the first and fourth SSCs  616 , 626  may be identical but arranged in opposite configurations. Similarly, the second and third SSCs  620 , 622  may be identical but arranged in opposite configurations. 
     The first optical gain element coupling device  612 —in particular, the first SOA  618 —is selectively engageable or configurable either to optically amplify optical signals passing in either direction between the first input/output  604  and the first PSR  608 , or to allow such optical signals to pass between the first input/output  604  and the first PSR  608  without actively amplifying the optical signals. When the first optical gain element coupling device  612  is selectively engaged to optically amplify optical signals, the first optical gain element coupling device  612  is considered to provide a first signal amplification path connected between the first input/output  604  and the first PSR  608 . When the first optical gain element coupling device  612  is selectively engaged to allow optical signals to pass without amplification, the first optical gain element coupling device  612  is considered simply to provide a portion of the signal path for the optical signals. 
     The second optical gain element coupling device  614 —in particular, the second SOA  624 —is selectively engageable or configurable either to optically amplify optical signals passing in either direction between the second PSR  610  and the second input/output  606 , or to allow such optical signals to pass between the second PSR  610  and the second input/output  606  without actively amplifying the optical signals. When the second optical gain element coupling device  614  is selectively engaged to optically amplify optical signals, the second optical gain element coupling device  614  is considered to provide a second signal amplification path connected between the second PSR  610  and the second input/output  606 . When the second optical gain element coupling device  614  is selectively engaged to allow optical signals to pass without amplification, the second optical gain element coupling device  614  is considered simply to provide a portion of the signal path for the optical signals. 
     In this way, for providing optical amplification to optical signals passing in either direction between the first input/output  604  and the first PSR  608 , the first optical gain element coupling device  612  may be selectively engaged to connect the first signal amplification path into the signal path of the device  600 , such that the first SOA  618  may provide optical amplification to the optical signals. The second optical gain element coupling device  614  may in this case be selectively configured to provide a portion of the signal path without optical amplification. For providing optical amplification to optical signals passing in either direction between the second PSR  610  and the second input/output  606 , the second optical gain element coupling device  614  may be selectively engaged to connect the second signal amplification path into the signal path of the device  600 , such that the second SOA  624  may provide optical amplification to the optical signals. The first optical gain element coupling device  612  may in this case be selectively configured to provide a portion of the signal path without optical amplification. Alternatively, in some embodiments, both the first and second optical gain element coupling devices  612 , 614  may be selectively engaged to connect the first and second signal amplification paths into the signal path of the device  600 , such that both the first and second SOAs  618 , 624  may provide optical amplification to the optical signals. 
     With the embodiment of  FIG. 14 , it can be seen that two optical gain elements may be configured to be selectively connected to either an add structure or a drop structure of a ROADM. Implementing the embodiment of  FIG. 14  with both an add structure and a drop structure of a ROADM would involve the provision of four optical gain elements, compared with the eight SOAs of the ROADM of  FIG. 1 . This embodiment therefore offers the advantage of significant savings in manufacturing complexity and cost. 
       FIG. 15A  shows a side view and  FIG. 15B  shows a top view of a further embodiment of a device  630  having a polarization-insensitive amplification configuration arranged outside of a polarization-sensitive region of the device. The arrangement shown in  FIGS. 15A ,B may be employed in the device  600  of  FIG. 14 , among others. 
     In this embodiment, the polarization-sensitive region of the device  630  includes a polarization-sensitive photonic device  632  optically coupled between a first PSR  636  and a second PSR  638 , provided on a SOI substrate  634 . In this embodiment, the first and second PSRs  636 , 638  are provided by a respective dual polarization grating coupler (DPGC). The polarization-sensitive photonic device  632  may therefore be implemented with a polarization diversity scheme, providing separate polarization component paths through the photonic device. 
     A first polarization-insensitive optical gain element coupling device  640  is optically coupled between a first input/output of the device and the first PSR  636 . A second polarization-insensitive optical gain element coupling device  642  is optically coupled between the second PSR  638  and a second input/output of the device. The first optical gain element coupling device  640  includes a first polarization-insensitive optical gain element, in this embodiment provided by a polarization-insensitive first SOA  646 . The first SOA  646  is optically coupled between a polarization-insensitive first SSC  644  and a polarization-insensitive second SSC  648 , for spot-size conversion of optical signals between the first input/output and the first SOA  646 , and between the first SOA  646  and the first PSR  636 , respectively. The second optical gain element coupling device  642  includes a second polarization-insensitive optical gain element, in this embodiment provided by a polarization-insensitive second SOA  654 . The second SOA  654  is optically coupled between a polarization-insensitive third SSC  652  and a polarization-insensitive fourth SSC  656 , for spot-size conversion of optical signals between the second PSR  638  and the second SOA  654 , and between the second SOA  654  and the second input/output, respectively. 
     Optical signals generally need to be coupled into a DPGC vertically. Accordingly, the first and second DPGCs  636 , 638  are provided at a level vertically separated from the first and second optical gain element coupling devices  640 , 642 . The second SSC  648  is provided with a mirror  650  configured provide horizontal-to-vertical coupling from the first optical gain element coupling device  640  to the DPGC  636 , and vice versa. Similarly, the third SSC  652  is provided with a mirror  658  configured provide vertical-to-horizontal coupling from the second DPGC  638  to the second optical gain element coupling device  642 , and vice versa. Of course, references to horizontal and vertical dimensions here are made for convenience of explanation, but do not indicate a specific alignment to any particular external frame of reference. 
     The first SSC  644  may be arranged to convert a first spot-size of, for example, an optical fiber input (for example of a diameter of 9 microns), to a second spot-size of the first SOA  646  (for example of a diameter of 3 microns). The second SSC  648  may be arranged to convert the second spot-size of the first SOA  646  to a third spot-size (for example of a diameter of 9 microns) for coupling into the photonic device  632  through the first DPGC  636 . The third SSC  652  may be arranged to convert the third spot-size (for example of a diameter of 9 microns) from the photonic device  632  through the second DPGC  638  back to the second spot-size of the second SOA  654  (for example of a diameter of 3 microns). The fourth SSC  656  may be arranged to convert the second spot-size of the second SOA  654  back to the first spot-size of, for example, an optical fiber output (for example of a diameter of 9 microns). 
     To facilitate the provision of suitable dimensions to the components of the device  630 , the materials used for the SSCs may be different from the materials used for the SOAs and different from the materials used for the polarization-sensitive photonic device. If the polarization-sensitive device is realized in silicon and the SOAs are realized in InP, a suitable material for the SSCs includes silicon nitride (Si 3 N 4 ), but other materials may alternatively be used. 
     With this arrangement, the SSCs, mirrors, DPGCs, and photonic device may be fabricated using CMOS processes, thereby offering a convenient and cost-effective manufacturing procedure. 
       FIG. 16A  shows a side view and  FIG. 16B  shows a top view of an embodiment of a polarization-insensitive optical gain element coupling device  660 . The optical gain element coupling device  660  may be implemented as the first optical gain element coupling device  640  of the embodiment of  FIGS. 15A ,B. With the arrangement reversed, the optical gain element coupling device  660  may also be implemented as the second optical gain element coupling device  642  of the embodiment of  FIGS. 15A ,B. 
     The optical gain element coupling device  660  is shown provided on a SOI substrate  634 . The SOI substrate  634  includes a DPGC; for example, the DPGC  636  of the embodiment of  FIGS. 15A ,B. 
     The optical gain element coupling device  660  includes a first polarization-insensitive optical gain element, in this embodiment provided by a polarization-insensitive first SOA  646 . The first SOA  646  is optically coupled between a polarization-insensitive first SSC  644  and a polarization-insensitive second SSC  648 , for spot-size conversion of optical signals between the first input/output and the first SOA  646 , and between the first SOA  646  and the DPGC  636 , respectively. 
     The polarization-insensitive first SSC  644  includes a first coupling element  662  and a second coupling element  664 . The polarization-insensitive second SSC  648  includes a third coupling element  666  and a fourth coupling element  668 . 
     In this embodiment, the first and second coupling elements  662 , 664  are configured as inverted tapers for spot-size conversion of light into the SOA  646 ; in particular, in applications where light is received into the device from an optical fiber input. The first coupling element  662  tapers inwardly in a direction towards the first SOA  646  and couples to the second coupling element  664  which tapers outwardly in the direction towards the first SOA  646 . At the coupling region between the first and second coupling elements  662 , 664 , the first coupling element  662  is provided on top of the second coupling element  664 . The mode size, or spot-size, is gradually contracted in the tapering top layer and at the same time optical power is conveyed along the gradually widening taper in the bottom layer. The coupling may be achieved by changing the respective waveguide cross-sections gradually along the propagation direction of the light, such that the optical power remains in a single eigenmode of the composite waveguide, while coupling to other modes is suppressed. In the narrowing taper of the first coupling element  662  in the top layer, it is desirable to prevent the mode going to cut-off. In the widening taper of the second coupling element  664  in the lower layer, it is desirable to prevent high-order modes from appearing. 
     In this embodiment, the first coupling element  662  may be formed from silicon oxynitride (SiON or SiO x N y ). The second coupling element  664  may be formed from silicon nitride (Si 3 N 4 ). The SOA  646  may be formed from indium phosphide (InP). Silicon nitride and silicon oxynitride are CMOS fabrication-compatible, like silicon, although they are generally formed as larger waveguides, with larger cross sections compared to silicon. This can be advantageous for providing optical coupling between an integrated device and an optical fiber. Using silicon nitride, it is relatively straightforward to realize a polarization-insensitive mode converter which may adapt the spot-size of an optical fiber to that of a SOA, which is smaller. Also, the SOA is configured to be polarization-insensitive, so the SOA may be arranged close to the interface with an optical fiber, thereby saving the number of active gain blocks, or optical gain elements, within the integrated device. 
     In this embodiment, between the first SOA  646  and the mirror  650  and the DPGC  636 , the third and fourth coupling elements  666 , 668  are configured as inverted tapers for spot-size conversion of light from the SOA  646  to that of the downstream photonic device. The third and fourth coupling elements  666 , 668  may be configured in the reverse formation of the first and second coupling elements  662 , 664 . In one embodiment, the dimensions of the third and fourth coupling elements  666 , 668  compared to those of the respective second and first coupling elements  664 , 662 , and the resulting spot-sizes, are arranged to correspond. It will be appreciated, however, that the dimensions of the third and fourth coupling elements  666 , 668  compared to those of the first and second coupling elements  662 , 664 , and the resulting spot-sizes, may not be identical for some applications. 
     The third coupling element  666  tapers outwardly in a direction away from the first SOA  646  and couples to the fourth coupling element  668  which tapers outwardly in a direction towards the mirror  650  and DPGC  636 . At the coupling region between the third and fourth coupling elements  666 , 668 , the fourth coupling element  668  is provided on top of the third coupling element  666 . The mode size, or spot-size, is gradually contracted in the tapering bottom layer and at the same time optical power is conveyed along the gradually widening taper in the top layer. 
     In this embodiment, the third coupling element  666  may be formed from silicon nitride (Si 3 N 4 ). The fourth coupling element  668  may be formed from silicon oxynitride (SiON or SiO x N y ). 
       FIG. 17  shows a method  500  for processing an optical signal in accordance with one embodiment. The method  500  begins at either step  510   a  or step  510   b , but not both at the same time. Beginning at step  510   a  includes receiving an optical signal at a first input/output. From step  510   a , the method  500  proceeds to step  520   a . Step  520   a  includes transmitting the optical signal from the first input/output in a first direction along a signal path to a photonic device and from the photonic device to a second input/output. Alternatively, beginning at step  510   b  includes receiving an optical signal at a second input/output. From step  510   b , the method  500  proceeds to step  520   b . Step  520   b  includes transmitting the optical signal from the second input/output in a second direction along a signal path to a photonic device and from the photonic device to a first input/output. 
     The optical signal may be received at either the first input/output or the second input/output, but optical signals may not be received as inputs at both the first and second inputs/outputs simultaneously. That is, when one of the first or second inputs/outputs functions as the optical signal input, the other of the first or second inputs/outputs functions as the optical signal output, and vice versa. 
     From either step  520   a  or step  520   b , the method proceeds to common step  530 . Step  530  includes amplifying the optical signal at an optical gain element by selectively directing the optical signal to either a first signal amplification path or a second signal amplification path, the first signal amplification path optically coupling the signal path between the first input/output and the photonic device to and from the optical gain element, and the second signal amplification path optically coupling the signal path between the photonic device and the second input/output to and from the optical gain element. 
     Other variations, modifications, and embodiments will be apparent to the skilled person and are intended to form part of the invention.