Polarization controller and method

Abstract

In an embodiment there is provided a polarization controller (100, 1000) which comprises a first polarization adjustment module (105, 1100, 1400) coupled to a second polarization adjustment module (110, 1500); and a monitoring module (115, 1300) configured to determine a ratio of a first polarization component of a received optical signal (E(t), 1005) and an orthogonal second polarization component of the received optical signal. The polarization controller (100, 1000) is configured to control the second polarization adjustment module (110, 1500) dependent on the ratio of the first and second polarization components of the received optical signal.

Description

[0001] POLARIZATION CONTROLLER AND METHOD

[0002] Technical Field

[0003] Embodiments disclosed herein relate to methods and apparatus for controlling polarization in optical signals.

[0004] Photonic chips are a promising technology for next generation transceivers, being capable of handling a larger bandwidth at a cost per bit that becomes increasingly smaller as the data throughput is increased. Future applications see Photonic Integrated Chips (PIC) replacing part of the electrical connections between single electrical integrated chips (EIC) inside large multi-chip modules.

[0005] PICs comprise a network of waveguides, routing the light inside the photonic chip, and several optical components along the guide that perform different functions, such as modulators, Bragg reflectors, switches, etc. Some optical components in PICs are structurally birefringent due to the contrast index and typically exhibit a polarizationdependent behavior. Modulators such as Electro-Optical Modulators (EOM) are extremely sensitive to polarization since the driving field determines an absorption in the active material that is dependent on the polarization of incoming light. Other elements, such as ring resonators and couplers, behave differently depending on the mode that is propagated.

[0006] Conventional waveguides in photonic chips have a rectangular section and are typically 450-41 Onm wide and 220 nm high. This aspect corresponds to supporting a single mode, with the electric field E mainly in the transversal direction, i.e. along the same direction as the longest side of the waveguide section. This mode is commonly designated as TE (transverse electric) mode. The mode orthogonal to this, is designated as TM (transverse magnetic). In such optical systems, light is fed into the transceiver PIC by optical fibers where the light can suffer polarization rotations so that polarization changes continuously along the path. Standard single-mode fiber (SSMF) does not preserve the polarization due to bending and imperfections so that the performances and reliability of the PIC are compromised.

[0007] In order to achieve a performance that is reasonably polarization independent, (e.g. a power transmission penalty below 2dB) different polarization diversity schemes have been implemented in photonic devices. Generally, these schemes comprise polarization splitters and rotators integrated in PIC devices. Polarization splitters separate two orthogonal polarizations and send each of them to a different path in the photonic chip. The separation is typically operated near the input zone of the transceiver, where the PIC couples with a light carrying element, e.g. the fiber, that provides a radiation having random polarization. In the polarization diversity schemes used near edge coupled interfaces, the two polarizations are typically split via adiabatic structures. Then one polarization path is rotated and merged with the other branch via directional couplers. In the case of vertical coupling instead, the splitting is operated by grating couplers 2D that are designed in a way that the polarization of light in the fiber is projected on two different directions in the PIC plane. These directions are perpendicular and correspond each to a waveguide input of the PIC so that in both branches the polarization corresponds to the main mode of the waveguide and does not need to be rotated. Then the two branches can be coupled into a single output via directional couplers.

[0008] Optical polarization controllers are devices for dynamic transformation of the state of polarization (SOP) of the incoming light. The ideal polarization controller should be endless: capable to follow the state of polarization of the input light without the need to reset the operative conditions and eventually unwind toward feasible rotational conversions. This goal is difficult to achieve due to the limited range of variability of the rotational conversion controllers (e.g. based on Faraday polarization rotators). Some previous approaches have addressed this by using sequences of rotators and complex control algorithms. An example is described in Zeqin Lu, Minglei Ma, Han Yun, Yun Wang, Nicolas A. F. Jaeger, and Lukas Chrostowski “Silicon Photonic Polarization Beam-splitter and Rotator for On-chip Polarization Control”, IEEE Group IV Photonics Conference, ThD1. The described arrangement performs any arbitrary state of polarization conversion with an algorithm based on rotational conversions.

[0009] In WO2018145753A1 , the rotational conversions are realized through two phase shifting devices which are so arranged as to be mutually orthogonal. Each phase shifter is driven by tuning voltages in a range of few Volts, capable to apply a phase difference in a limited range (i.e. 0, 2TT). If one of these reaches the limit of the range of variability (e.g. reaches the maximum phase difference), that device can no longer increase the phase difference further. An analytical model is defined to set the optimal point of operation unwinding the polarization rotators with in-range control values, thus avoiding the saturation of the operative system.

[0010] There have been alternative approaches for achieving endless like performance, by using more than two phase shifting devices (i.e. to realize rotational conversions) driven with a higher precision and accuracy; for example as described in Christopher R. Doerr and Long Chen, Monolithic PDM-DQPSK receiver in silicon, IEEE, Optical Communication (ECOC), (2010). For this approach to achieve quasi-endless tracking it is recommended to have at least four phase shifting devices: three for arbitrary conversion and one to compensate for unwanted variations due to the limited range of the control parameters (e.g. typically voltages and currents). The design of such polarization controllers is complex, since driving more than two phase shifting devices with high accuracy is not trivial, and synchronous and complex phase variation matching should be applied as well.

[0011] It would be desirable to have solutions that simplify the management of polarization control, as well as the related costs and consumption.

[0012] According to certain embodiments described herein there is provided a polarization controller comprising a first polarization adjustment module coupled to a second polarization adjustment module, and a monitoring module configured to determine a ratio of a first polarization component of a received optical signal and an orthogonal second polarization component of the received optical signal. The polarization controller is configured to control the second polarization adjustment module dependent on the ratio of the first and second polarization components of the received optical signal.

[0013] This provides a simpler algorithm which exploits as input signal the intensity ratio of the two polarizations of the injected light which can be measured by two monitoring PDs at the first phase shifter. The ratio of intensities of the TE and TM polarizations at the two PDs then determines the control signal at the second phase shifter.

[0014] According to certain embodiments described herein there is provided a method of controlling a polarization controller comprising a first polarization adjustment module coupled to a second polarization adjustment module. The method comprises determining a ratio of a first polarization component of a received optical signal and an orthogonal second polarization component of the received optical signal, and controlling the second polarization adjustment module dependent on the ratio of the first and second polarization components of the received optical signal.

[0015] Certain embodiments also provide corresponding computer programs and computer program products.

[0016] Brief Description of Drawings

[0017] For a better understanding of the embodiments of the present disclosure, and to show how it may be put into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0018] Figure 1 is a schematic diagram illustrating a polarization controller according to an embodiments;

[0019] Figure 2 is a flow diagram illustrating a method of controlling polarization of an optical signal according to an embodiment;

[0020] Figure 3 is a schematic diagram illustrating a polarization controller according to an embodiment; Figure 4 is a flow diagram illustrating a method of controlling polarization of an optical signal according to an embodiment;

[0021] Figure 5 is a flow diagram illustrating a method of controlling polarization of an optical signal according to an embodiment;

[0022] Figure 6 is a schematic diagram illustrating a controller for controlling a polarization controller according to an embodiment; and

[0023] Figure 7 illustrates thermos-optical coefficients for TEO and TMO modes at 1550nm.

[0024] Detailed Description

[0025] Generally, all terms used herein are to be interpreted according to their ordinary meaning in the relevant technical field, unless a different meaning is clearly given and / or is implied from the context in which it is used. All references to a / an / the element, apparatus, component, means, step, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any methods disclosed herein do not have to be performed in the exact order disclosed, unless a step is explicitly described as following or preceding another step and / or where it is implicit that a step must follow or precede another step. Any feature of any of the embodiments disclosed herein may be applied to any other embodiment, wherever appropriate. Likewise, any advantage of any of the embodiments may apply to any other embodiments, and vice versa. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following description.

[0026] The following sets forth specific details, such as particular embodiments or examples for purposes of explanation and not limitation. It will be appreciated by one skilled in the art that other examples may be employed apart from these specific details. In some instances, detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as not obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or more nodes using hardware circuitry (e.g., analog and / or discrete logic gates interconnected to perform a specialized function, ASICs, PLAs, etc; and photonics integration) and / or using software programs and data in conjunction with one or more digital microprocessors or general purpose computers. Nodes that communicate using the air interface also have suitable radio communications circuitry. Moreover, where appropriate the technology can additionally be considered to be embodied entirely within any form of computer-readable memory, such as solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein.

[0027] Hardware implementation may include or encompass, without limitation, digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analogue) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and / or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions. Memory may be employed to storing temporary variables, holding and transfer of data between processes, non-volatile configuration settings, standard messaging formats and the like. Any suitable form of volatile memory and non-volatile storage may be employed including Random Access Memory (RAM) implemented as Metal Oxide Semiconductors (MOS) or Integrated Circuits (IC), and storage implemented as hard disk drives and flash memory. Photonic integration may include photonics circuits or modules utilising, without limitation, Silicon on Insulator (SOI) and / or Silicon Nitride (SiN) platforms.

[0028] Embodiments described herein relate to polarization control of optical signals. Such signals may be received over long optical fibres by polarization sensitive equipment such as photonic integrated chips (PIC). These signals may have unpredictable and / or varying polarization. Embodiments provide for controlling or adjusting this polarization to improve downstream processing. In some embodiments, a polarization controller having first and second polarization adjustment modules, such as phase shifters, uses the ratio of orthogonal components of a received optical signal to control the downstream or second polarization adjustment module. The upstream or first polarization adjustment module may additionally be used and controlled based on an output optical signal. This provides a simple control strategy for improving the polarization of received optical signals for processing by PIC and other polarization sensitive optical components.

[0029] Figure 1 illustrates a polarization controller according to an embodiment. The polarization controller 100 comprises a first polarization adjustment module 105 which receives an input optical signal E(t) and is coupled to a second polarization adjustment module 110 which outputs a modified optical signal Eout(t). A controller or control module 115 receives information about the input and output optical signals E(t), Eout(t) and is configured to optimize the output of a target or wanted polarization by effecting various mechanisms such as polarization rotation, phase shifting and interference. An optimized configuration may be achieved by applying first and second polarization adjustment parameters (p1, <p2to the respective first and second modules 105, 110.

[0030] The embodiment determines and utilises a ratio of orthogonal optical polarization components such as TEO and TM0 in order to control one of the polarization adjustment parameters (<p2). The performance of the polarization controller, for example the level of wanted (or unwanted) optical polarization components is then used to optimize the other polarization adjustment parameter This simplifies overall control of the polarization controller 100.

[0031] In some embodiments, the polarization adjustment parameters may be control signals to adjust phase shifts applied within the modules, however other types of polarization adjusting mechanisms and control signals could alternatively be applied. For example retardance waveplates or liquid crystals could be used to implement phase shifting.

[0032] The polarization controller 100 provides a target polarization for an output optical signal Eout(t), from an input optical signal E(t) with any arbitrary State of Polarization (SOP). The input light can be described as: where Exand Eyare the horizontal and vertical electrical components of the plane light wave, axand ayare the amplitudes, 6Xand 6yare the initial phases, respectively. The SOP of the light can be characterized by the polarization extinction a ratio (PER) 101og10(^2) and the phase difference 60= 6y- 6X. ay

[0033] Then the input light can be described by the Jones vector:

[0034] When light with an arbitrary SOP is launched into an optical device such as a PIC, the TE0and TM0modes in the silicon photonic waveguide are excited, respectively.

[0035] The ratio of these two modes can be used to set the second polarization adjustment module 110. The setting for the first polarization adjustment module 105 may then be adjusted iteratively to find a more optimum operating setting for the polarisation controller 100.

[0036] Figure 2 illustrates a method 200 of controlling the polarization of an optical signal E(t), which may be implemented using the polarization controller of Figure 1.

[0037] At 205, the method determines a ratio of TEO to TM0 modes or optical polarization components of the incoming optical signal E(t). In an example, this may be implemented using a polarization beam splitter to split a small portion of the optical signal into TEO and TM0 components which are applied to respective photodiodes. The respective two generated electrical signals can then be used to determine the ratio.

[0038] At 210, the method adjusts or sets a second polarization adjustment parameter (p2based on the ratio. In an example, this may be implemented by applying a phase difference between different arms of a Mach-Zehnder Interferometer (MZI) in which different polarization components of the optical signal are passing. By appropriate configuration of the implementation components and the applied phase difference, the proportion of the output signal Eout(t) with the target polarization can be improved. This second polarization adjustment may be said to provide a coarse tuning of the polarization. At 215, the method iterates the first polarization adjustment p}to further optimize the polarization of the output signal Eout(t). In an example, this may be implemented by making small changes to this first polarization adjustment <p and monitoring the output signal Eout(t) to determine whether this increases or reduces the target polarization. Improvements may result in a further small change in the same direction whilst deterioration may result in a small change in the opposite direction. This first polarization adjustment may be said to provide a fine tuning of the polarization.

[0039] At 220, the method determines when the wanted or target polarization component (e.g. TEO) associated with Eout(t) is below a threshold compared; in other words the adjustments are no longer able to sufficiently optimize the polarization of the optical signal. This may occur because the polarization of the incoming optical signal E(t) may vary over time and some adjustments available to the equipment used are exceeded. For example, the first polarization adjustment <p1provided by the first polarization adjustment module 105 may have reached a limit so that it is no longer capable of providing any further polarisation adjustment. In this case the second polarization adjustment <p2is reset by returning to 205. The first polarization adjustment <p may then iterate about this new set point; until the threshold is again exceeded and / or the limit of this adjustment is reached.

[0040] Figure 3 is an example implementation of a photonic platform based polarization controller 1000, which provides a target polarization of an input optical signal 1005 having TE0and TM0polarization components. The polarization controller 1000 includes a phase shifter 1100 for providing optical phase delay between the TE0and TM0polarizations based on a first control signal CS1. Since the thermo-optic coefficients of the TE0and TM0polarizations are different, it is achievable to obtain a desired phase difference between the TE0and TM0polarizations in a received optical signal, referred to here as optical beam 1105. The polarization controller 1000 includes a directional coupler (DC1) 1200 as an optical tap to tap of a portion of the optical beam 1205, e.g. a small fraction such as 5% of the optical power, 1205. This is directed to a polarization monitoring module 1300. The polarization monitoring module 1300 consists of a polarization beam splitter (PBS) 1310, which splits the input optical beam 1205’s TE0and TM0portions to the two monitoring photodiodes 1320 and 1330, respectively. The electrical signals from these photodiodes (PD1 , PD2) are used to generate a second control signal CS2.

[0041] The optical beam 1210 of the other port of the DC 1200 is fed into a polarization rotator (PR) 1400, which converts the TM0component into a TE and allows the TE0component to pass through. The output of the PR is coupled to a first Mach-Zehnder interferometer (MZI) stage 1500 which includes a Dual mode power splitter (DM PS) 1510, which evenly splits the TE0and TE components into two ports 1515 and 1520: TE0component remains in TE0, and the TE polarization is converted into a TE0component. The first MZI stage comprises a second phase shifter (PS2) 1525 for providing an optical phase delay between the first and second feeds 1515 and 1520 based on the second control signal CS2. The first MZI stage also comprises a first mixer 1530, e.g., 2x2 multi-mode interference (MMI) coupler, for mixing the first and second feeds having the second optical phase delay therebetween, to provide third and fourth feeds 1535 and 1540.

[0042] The polarization controller 1000 may also include a second MZI 1600 coupled to the first MZI stage 1500 and comprising a third phase shifter (PS3) 1605 for providing a third optical phase delay between third and fourth feeds 1535 and 1540 based on a third control signal (CS3). The third and fourth feeds 1520 and 1530 are then mixed by a second mixer 1610, which provides fifth and sixth feeds 1615 and 1620. The polarization controller 1000 may also include a third MZI 1700 coupled to the second MZI stage 1600 and comprising a fourth phase shifter (PS4) 1705 for providing a fourth optical phase delay between fifth and sixth feeds 1615 and 1620 based on a fourth control signal (CS4), and a fourth mixer 1710 for mixing the fifth and sixth feeds 1615 and 1620 having the fourth optical phase delay therebetween.

[0043] The two outputs from each MZI stage provide optical signals having orthogonal polarizations. The power of one or more of these can be used to determine the CS1 control signal. In this example, the optical beam 1715 from the third MZI stage is fed to a third photodiode (PD3) 1725, this optical beam 1715 being orthogonal to the optical beam 1720 having the fixed target polarization. In another example, a small tap of optical beam 1720 may be used. These measurements may be used to provide a performance metric for the polarization component, that is whether it is minimizing unwanted polarization components and / or whether it is maximizing wanted or target polarization components.

[0044] The mixers 1530, 1610, 1710 may be 2x2 multi-mode interference (MMI) couplers or 2x2 direction couplers, or any other suitable couplers.

[0045] Although only the PS1 1100, PR 1400 and the first MZI stage 1500 of the polarization controller 1000 are sufficient to transform any polarization state into a TFo-polarized output, the second and the third stage MZIs 1600, 1700 are employed to provide redundant control and enable the endless operation, in case the PS1 1100 and first MZI stage 1500 reach the ends of their adjustment ranges and need a reset.

[0046] The phase shifters PS2’, PS3’, and PS4’ of the bottom arm of the polarization controller 1000 are shown in dashed lines as being optional. Advantageously, these phase shifters will introduce the same insertion loss as their counterpart in the same MZIs. Hence the insertion losses in the two arms of the MZI would be balanced, which may be preferable for broadband operation. The positive phase shifts introduced by PS2’, PS3’, and PS4’ are equivalent to negative phase shift introduced by PS2, PS3, and PS4. However, thermal phase shifters cannot introduce negative phase shift to the propagating light due to the limit of the working principle. Therefore, the optional phase shifters PS2’, PS3’, and PS4’ double the tuning range.

[0047] The input light of beam or optical signal 1005 can be described as:

[0048] {EY= aYelSx

[0049] Ey= aye‘By ™ where Exand Eyare the horizontal and vertical electrical components of the plane light wave, axand ayare the amplitudes, 6Xand 6yare the initial phases, respectively. The SOP of the light can be characterized by the polarization extinction a ratio (PER) 101og10(^2) and the phase difference 60= 6y- 6X. ay

[0050] The input light can be described by the Jones vector:

[0051] When light with an arbitrary SOP is launched into the polarization controller 1000, the TE0and TM0modes in the input photonic waveguide are excited, respectively.

[0052] After the thermal tuning of the first phase shifter (PS1), a phase different of (p1will be applied between the TE0and TM0modes due to the different thermo-optical coefficients (TOCs) of the two modes. psi = (oe^i) )

[0053] In an example, using thermal tuning of a phase shifter, this phase difference is due to the different thermo-optical coefficients (TOCs) of the two modes.

[0054] CPS1 = (oei° i) (3)

[0055] Figure 7 is an example, showing the TOC difference of the TE0and TM0modes at the wavelength of 1550 nm.

[0056] The directional coupler 1 (DC1) is employed to tap a certain fraction of the light and won’t change the SOP of the light fed into the polarization rotator (PR). Therefore, this is not considered further in understanding the transfer function.

[0057] The TE0component or polarization passes through the polarization rotator (PR) losslessly in theory, while the TM0mode or component is efficiently converted to the TE1mode by the polarization rotator. Meanwhile, there will be a phase difference 8 introduced between the TE0and TE modes, depending on the waveguide birefringence.

[0058] CPR = (oe°s) (4)

[0059] The first MZI (MZI1) consists of a dual-mode power splitter (DMPS), two phase shifters (PS2 and PS2’), and a mixer. The TE0mode is uniformly split into two TE0modes with identical phases, while the TE mode is uniformly split into two TE0modes with a phase difference of TT. PS2 and PS2’ are used to achieve the desired phase difference <p2between the two TE0modes propagating in the two arms of the MZI1 . Then these two TE0modes in two arms interfere at the Mixer (1510). The whole transfer matrix of the MZI1 is given as:

[0060] In which CMMI, CPS2, CDMPSare the transmission matrices of MMI (1510), PS2 (or PS2’), and the dual-mode power splitter (DMPS).

[0061] After the MZI1 , the output light in the two output branches (1535 and 1540) can be expressed as:

[0062] Aoutlis a transfer matrix with one input port and two output ports, instead of a Jones vector of the output light of the MZ11 , since there is no TM0component in the output light

[0063] The objective of the polarization controller is maximizing the output of the upper branch (1535) and minimizing the output of the bottom branch (1540). The absolute phase shift term in front of the matrix can be ignored, as it is determined by the initial input and does not affect the SOP of the output. The output of the bottom branch of the MZI1 should be zero:

[0064] For any combination of general solutions are: 277171 (8) + 2nn where m and n are integers.

[0065] With the (p1and (p2in Equation (8) properly applied to the input light, the output of the upper branch of the MZI1 is (iay- ax)eiSx, and the norm of the output equals to the norm of the input light: This means that input light with any arbitrary SOP can be fully converted to output light with pure TE0mode at the upper branch of the MZI1 .

[0066] This polarization controller can be operated in a semi-deterministic way by knowing the value of <p2. This makes controlling the polarization controller easier as once the value of <p2is determined, then only needs to be optimized.

[0067] In the polarization monitoring module 1300, the TE0and TM0modes are split by the polarization beam splitter (PBS) and fed into the photodetectors PD1 and PD2, respectively. The measurements of the PD1 and PD2 are the intensity of the TE0and TM0modes, i.e. , ITEoand ITMo, respectively. Then we have:

[0068] Then the output of MZI1 becomes:

[0069] The optimal value of <p2can be determined by the measurements of PD1 and PD2 for the input light with any specific SOP. The only variable we need to optimize by the algorithm during the operation is (p1, since the initial phase difference 60is unknown.

[0070] In general, the polarization controller discussed above are capable of converting the light with any arbitrary SOP into the one target polarization component TE0.

[0071] The second MZI 1600 and the third MZI 1700 are adopted to provide redundant controlling freedom to realize the endless operation. Given that each phase shifter has a finite operational range, constrained by the limitations of the voltage source, a reset of retardance is an inevitable aspect of the continuous tracking process. The swift decrease in the phase shifter's retardance by 2TT can momentarily disrupt the tracking of the incoming State of Polarization (SOP). To address this, a phase shifter reset algorithm can been implemented to facilitate what is referred to as 'endless polarization control'. Detailed information about this algorithm can be found in “Endless polarization control systems for coherent optics”; R. Noe, H. Heidrich, D. Hoffmann; Journal of Lightwave Technology, 1988.

[0072] Figure 4 illustrates a method of operating a polarization controller of Figure 3. This method may also be applied to other polarization controllers. The method 2000 adjusts two polarisation adjustment parameters based on monitoring characteristics of light received by the polarisation controller as well as light output by the polarization controller. In this example, the polarization adjustment parameters are phase shifts.

[0073] For simplicity, only two polarization adjustment parameters are considered, the phase shift of PS1 1100 which is controlled by CS1 and the phase shift of PS2 1525 which is controlled by CS2. The cascaded second and third MZIs, 1600 and 1700 respectively, may initially set to the bar state. This is achieved by setting (p3= 2n and (p4= n / 2.

[0074] At 2005, the method monitors the photocurrent I3 by PD3, which is then compared to a threshold current, I3th. This threshold serves as a criterion to determine whether optimization should commence. If I3 is greater than I3th, this implies that a significant portion of the incoming light is not routed to the output port 1720 but is lost through the monitoring port 1715, suggesting inadequate tracking of the SOP of the incoming optical signal. If this threshold is exceeded, the method moves to 2010. Otherwise, the method keeps monitoring I3 until it exceeds the I3th.

[0075] As noted previously, photodiode PD3 receives light from output port 1715 of the final mixer 1710 of the polarization controller 1000. The level or intensity of light received by PD3 provides a measure of the level of unwanted polarization components orthogonal to the wanted or target polarization components output from port 1720. If a high intensity or power of these unwanted components exceeds a threshold corresponding to I3th this indicates that the current settings (the current polarization adjustment parameters) of the polarization controller are not sufficiently optimised. The value of 13 may therefore be said to provides a performance metric for the polarization controller 1000, and if this is below a suitable threshold i3th, the method simply continues monitoring until the performance metric exceeds the threshold (i.e. i3 < i3th).

[0076] At 2010, the method measures 11 and I2 using PD1 and PD2. As noted previously, PD1 and PD2 measure the intensity of a first polarization component (e.g. TEO) and an orthogonal second polarization component (e.g. TM0) respectively of the received optical signal 1005. The ratio of 11 and I2 can then be used to calculate the ratio of the first and second components of the received optical signal.

[0077] At 2015, the method calculates a control signal CS2 from this ratio. CS2 is used to control or set the phase shift of phase shifter PS2 within the first MZI stage 1500. According to Equations (8) and (10), the phase shift to be introduced by PS2 is:

[0078] 2nn shows the periodic nature of the phase shifter, however this can be ignored when limiting consideration to within 2n.

[0079] Substitute Equation (11 ) into Equation (12), and regarding CS2 as the power to be applied to PS2:

[0080] For dtp = <p2'.

[0081] This is the only the case for a thermal phase shifter. As will be appreciated, the calculation will be different if using a p-n junction phase shifter or an electro-optical phase shifter.

[0082] CS2 corresponds to the power P that needs to be applied to PS2:

[0083] P = dTG (11)

[0084] Where G is the thermal conductance between the heated waveguide and the heat sink, which is determined by the material platform and the geometry of the phase shifter. dT is the temperature change of the phase shifter needed to introduce a certain phase shift d<p

[0085] Where — is the thermal optical coefficient of the mode passing through the waveguide, dT which is determined by the material and cross-section geometry of the waveguide, and k0is the wave vector and determined by wavelength, L is the length of the phase shifter.

[0086] At 2020, the method determines whether the calculated CS2 exceeds a threshold CS2th, which corresponds to a physical limit of the available power supply. Once this limit is reached it is no longer possible to further increase / decrease the phase shift applied at PS2. In this situation the polarization controller needs to be reset in order to further optimize it’s performance. In this case the method moves to 2025. If CS2 is within its operational limits, the method moves to 2030.

[0087] At 2025, the method resets CS2 using CS3 and CS4. Because phase shift is circular, once the limit of the phase shift in one direction is reached (e.g. TT), the phase shifter PS2 may be reset to the limit in the opposite direction (e.g. -TT). This allows the phase shift to continue to move in the same direction. The second and third MZI stages may be controlled by CS3 and CS4 as noted above in order to allow the polarization controller to smoothly transition from one limit of CS2 to the other limit. The method returns to 2005 where 13 is again monitored.

[0088] To realize the phase shift required if the voltage cannot be further increased, the voltage applied on CS1 or CS2 is reduced and the voltages applied on CS3 and CS4 are increased to compensate for the lost track of the SOP caused by reducing CS1 or CS2. This can be achieved because CS3 and CS4 provide redundant control of freedom, and for each injected SOP, there are different combinations of CS1 , CS2, CS3, CS4 to keep track, instead of the sole solution using only CS1 and CS2.

[0089] The setting of CS2 or the second polarization adjustment parameter provides an initial or coarse optimization point for the polarization controller which is based on the received optical signal, rather than an output. This allows the polarization controller to quickly adopt a good starting operating point for any arbitrary SOP of an incoming optical signal, and simplifies ongoing optimization control. This also allows the polarization controller to quickly adapt to changes in the SOP of the incoming optical signal. This compares with other optimization approaches which require randomly searching for optimal operating points for at least two variables, which is complicated and may result in settling in sub-optimal local maxima, rather than a global maxima.

[0090] At 2030, the method optimizes CS1 based on I3. In other words, the power applied to the first phase shifter PS1 is derived from the intensity of light from the monitoring port 1715. This may be achieved by adjusting CS1 and monitoring whether this improves (I3 reduces) or deteriorates (I3 increases) the performance of the polarization controller 1000. An example method for implementing this is described below with respect to Figure 5. Optimization of CS1 may be considered a fine tuning for the optimization controller. This may be performed for a predetermined time or number of iterations before the method returns to 2005 to determine whether the performance of the polarization controller is within a required threshold and if not to perform a resetting of the coarse optimization setting CS2.

[0091] Figure 5 illustrates a method of optimising CS1 in the polarization controller 1000 of Figure 3. This method 3000 may be used to implement step 2030 of the method of Figure 4.

[0092] Point A corresponds to the start of step 2030. At 3005, the method initialises the iteration or loop counter i to i=0. At 3010, the method adds a small change value (increase or decrease) ACS 1 to the current CS1 value (i.e. CS1 i+1 = CS1 i + ACS1).

[0093] At 3015, the method determines whether or not this improves the performance metric of the polarization controller by checking whether I3i+1 < I3i. If there is no improvement, this implies that the optimization direction is wrong and the method moves to 3020, otherwise the method moves to 3025.

[0094] At 3020, the method resets the change value to the opposite sign in order to change the optimization direction. This will be applied at 3010 in the next iteration. At 3025, the method checks if the current iteration count i is above a maximum threshold, loopmin value, and if not continues optimization by moving to 3040 where the iteration count is incremented (i.e. i = i + 1). If the iteration or loop count is above loopmin, then the method moves to 3030.

[0095] At 3030, the method calculates the change in I3 (i.e. AI3 = I3i+1 - I3i) following the applied change in CS1 (i.e. + ACS1). At 3035, the method determines whether this change in I3 exceeds a threshold (i.e. AI3 > Al 3th). This implies that the SOP of the incoming optical signal has been changed a lot by changes in CS1 and that CS2 should now be reset in order to keep track of the incoming SOP. In this case the method moves to point B, which is the output from step 2030 where method 2000 moves to check I3 at 2005 and reset CS2 at 2015.

[0096] If the change in I3 is below the threshold (i.e. Al 3 < AI3th) , the method moves to 3040, where the iteration or loop counter i is incremented. The method then moves to 3045 to check whether the current value of CS1 exceeds a threshold CS1th (i.e. CS1 > CS1th). CS1th is the physical limit of the power supply for CS1 meaning that CS1 needs to be reset. In this case the method moves to 3050, otherwise the method returns to 3010 and a new optimization iteration.

[0097] At 3050, the method resets CS1. This may be achieved by changing CS1 from its current limit to the opposite limit (e.g. from TT to -TT). This may be implemented with the assistance of CS3, CS4 of corresponding MZI stages 1600, 1700, as previously described. The method then moves to point B, which is the output from step 2030 where method 2000 moves to check I3 at 2005 and reset CS2 at 2015.

[0098] This method 3000 in combination with the method 2000 of Figure 4 provides for optimising the two phase shifts PS1 , PS2 in the polarization controller 1000, in order optimise the targeted polarization output at 1720.

[0099] More generally, two polarization adjustment parameters ((p1CS1 and <p2CS2) are controlled for respective polarization adjustment modules (105, 1100 and 110, 1525). Using the ratio of orthogonal polarisation components in the incoming optical signal in order to set the second polarization parameter (<p2CS2) simplifies overall control of the polarization controller, as the first polarization adjustment parameter ((p1CS1) may then be separately optimized using search or iteration for each setting of the second polarization adjustment parameter (<p2CS2).

[0100] In an embodiment the orthogonal polarization components may be horizontal and vertical electrical components if the received optical signal.

[0101] Whilst embodiments have employed MZI, other types of interferometers may alternatively be used; and different numbers of such interferometers may be cascaded, including just using a single interferometer. The interferometer(s) provide for destructive and / or constructive interference of optical components on different arms or paths, and adjusting the phase shift between these paths enables a wanted configuration for blocking unwanted components and passing wanted components.

[0102] Similarly, whilst a particular arrangement of optical components has been described with respect to Figure 3, it will be appreciated that the described adjustment of first and second polarization adjustment parameters may be applied to alternative optical circuit architectures.

[0103] The embodiments provide a number of advantages. Some embodiments provide a simplified control algorithm which exploits as input signal the intensity ratio of the two polarizations of the injected light which can be measured by two monitoring PDs at the first phase shifter. The ratio of intensities of the TE and TM polarizations at the two PDs then determines the control signal at the second phase shifter. This reduces the complexity of the polarization converting algorithm from O(n2) to O(n).

[0104] Some embodiments provide a compact size of polarization controller. For example, a polarization rotator (PR) and dual-mode power splitter (DM PS) is used to convert light with any arbitrary SOP into two branches of TE0. This compares with other approaches using a conventional Polarization Beam Rotator Splitter (PBRS) which has a large footprint. A simple implementation of the DMPS can be a Y-branch. Compared to some PBRS designs, this design strategy avoids a long adiabatic mode evolution coupler used to separate the TE0and TE1modes and convert the TE±to TE0mode. The DMPS takes the advantage of the symmetry of the TE0to split this evenly with the same phase, and takes the advantage of the asymmetry of the TE to directly split this evenly with a TT phase difference.

[0105] Embodiments also enable semi-deterministic operation by utilizing a polarization beam splitter (PBS) and two monitoring PD to obtain the intensities of the TE and TM polarizations. The ratio of intensities of the TE and TM polarizations is then used to determine the phase shift to be introduced by the second phase shifter. The control signal can be simply calculated and applied to the second phase shifter. This may reduce the complexity of the polarization converting algorithm from O(n2) to O(n).

[0106] Some embodiments simplify designs where at least two MZI are required to realize polarization control (and four MZI to realize endless operation). Embodiments replace one of these MZI with a phase shifter, and taking advantage of the discrepancy of thermos-optic coefficients of the TE and TM polarizations. Thus compared with some know four MZI implemented polarization controllers, some embodiments enable the first stage MZI to be replaced with a phase shifter, with only three MZIs then being required for endless operation, simplifying the design.

[0107] Figure 6 illustrates a controller or control module for controlling a polarization controller according to an embodiment. The polarization controller which is controlled by the controller may be that of Figures 1 or 3, or any other suitable optical circuit.

[0108] The controller 4000 comprises a processor 4005 and memory 4010 comprising processor readable instructions 4015. The controller receives information about the optical or polarization components of an incoming optical signal as well as information about the polarization components of the output from the polarization controller. As described, in some embodiments this may be implemented using photodetectors to measure the intensity of orthogonal components (e.g. TEO and TM0) in the input optical signal as well as the intensity of these components in the output. The controller 4000 outputs two (or more) control signals - first and second polarization adjustment parameters - which are used to control respective controllable polarization adjustment modules of the polarization controller. In some embodiments, these may be implemented using phase shifters.

[0109] When executed by the processor 4005, the executable instructions 4015 cause the controller to perform a method of controlling the polarization of an incoming optical signal, in order to optimise a wanted polarisation. The executable instructions cause the processor to perform the following steps.

[0110] At 4050, determine a ratio of a first polarization component and an orthogonal second polarization component of a received optical signal having any arbitrary SOP.

[0111] At 4055, control (using a second polarization adjustment parameter- PAP2) a second polarization adjustment module dependent on the ratio.

[0112] Additional instructions may be used to optimise the first polarization adjustment module as previously described.

[0113] Modifications and other variants of the described embodiment(s) will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the embodiment(s) is / are not limited to the specific examples disclosed and that modifications and other variants are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

[0114] It should be noted that the above-mentioned examples illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative examples without departing from the scope of the appended statements. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the statements below. Where the terms, “first”, “second” etc. are used they are to be understood merely as labels for the convenient identification of a particular feature. In particular, they are not to be interpreted as describing the first or the second feature of a plurality of such features (i.e. the first or second of such features to occur in time or space) unless explicitly stated otherwise. Steps in the methods disclosed herein may be carried out in any order unless expressly otherwise stated. Any reference signs in the statements shall not be construed to limit their scope.

Claims

CLAIMS1 . A polarization controller comprising: a first polarization adjustment module coupled to a second polarization adjustment module; a monitoring module configured to determine a ratio of a first polarization component of a received optical signal and an orthogonal second polarization component of the received optical signal; wherein the polarization controller is configured to control the second polarization adjustment module dependent on the ratio of the first and second polarization components of the received optical signal.

2. The polarization controller of claim 1 , wherein the first and second polarization components are horizontal and vertical electrical components of the received optical signal.

3. The polarization controller of claim 1 or 2, wherein the second polarization adjustment module comprises a phase shifter which is configured to introduce a phase shift between orthogonal polarization components of an optical signal passing therethrough, the phase shift dependent on the ratio of the first and second polarization components of the received optical signal.

4. The polarization controller of claim 3, wherein the secondary polarization adjustment module comprises Mach-Zehnder Interferometer, MZI, having the phase shifter coupled into an arm of the MZI.

5. The polarization controller of claim 3, wherein the MZI comprises a dual mode power splitter and a multi-mode interference coupler.

6. The polarization controller of any one of claims 3 to 5, configured to reset the phase shifter in response to the phase shift exceeding a phase shift threshold.

7. The polarization controller of any one of claims 1 to 6, configured to control the first polarization adjustment module dependent on a performance metric of the polarization controller.

8. The polarization controller of claim 7, wherein the first polarization adjustment module comprises an input phase shifter coupled to a polarization rotator, the input phase shifter configured to introduce a phase shift between orthogonal polarization components of an optical signal passing therethrough dependent on the performance metric.

9. The polarization controller of claim 8, configured to adjust the phase shift of the input phase shifter until an input phase shift threshold is reached10. The polarization controller of claim 8 or 9, configured to adjust the phase shift of the input phase shifter until a corresponding reduction in the performance metric exceeds a performance metric threshold.

11. The polarization controller of any one of claims 7 to 10, wherein the performance metric is dependent on an intensity of an optical signal output from the polarization controller.

12. The polarization controller of any one of claims 1 to 11 , comprising a third polarization adjustment module coupled to the second polarization adjustment module and a fourth polarization adjustment module coupled to the third polarization adjustment module.

13. A method of controlling a polarization controller comprising a first polarization adjustment module coupled to a second polarization adjustment module; the method comprising: determining a ratio of a first polarization component of a received optical signal and an orthogonal second polarization component of the received optical signal; controlling the second polarization adjustment module dependent on the ratio of the first and second polarization components of the received optical signal.

14. The method of claim 13, wherein the first and second polarization components are horizontal and vertical electrical components of the received optical signal.

15. The method of claim 13 or 14, wherein the second polarization adjustment module comprises a phase shifter in a Mach-Zehnder Interferometer, MZI, and the method comprising: controlling the phase shifter to adjust a phase shift between optical signals in respective arms of the MZI.

16. The method of claim 15, comprising resetting the phase shifter in response to the phase shift exceeding a phase shift threshold.

17. The method of claim 15 or 16, comprising adjusting the phase shift responsive to a performance metric of the polarization controller being below a minimum performance metric threshold.

18. The method of any one of claims 13 to 17, comprising controlling the first polarization adjustment module dependent on a performance metric of the polarization controller.

19. The method of claim 18, wherein the first polarization adjustment module comprises an input phase shifter, the method comprising: controlling the input phase shifter to adjust a phase shift between orthogonal polarization components of an optical signal passing through the input phase shifter dependent on the performance metric.

20. The method of claim 19, comprising adjusting the phase shift of the input phase shifter until an input phase shift threshold is reached.21 . The method of claim 19 or 20, comprising adjusting the phase shift of the input phase shifter until a corresponding reduction in the performance metric exceeds a performance metric threshold.

22. The method of any one of claims 19 to 21 , wherein the performance metric isdependent on an intensity of an optical signal output from the polarization controller.

23. A computer program product comprising instructions which, when executed on a processor, cause the processor to carry out the method of any one of claims 13 to 22.

24. A computer program product comprising non-transitory computer readable media having stored thereon a computer program according to claim 23.

25. A transceiver comprising a polarization controller according to any one of claims 1 to 12.

26. A network node comprising the transceiver of claim 25.

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