Apparatus for outputting a pulse of electromagnetic radiation

Abstract

An apparatus with a modulator that imparts a first phase to received EM radiation and then immediately imparts different phase. An interferometer 106 with arms having different paths, receives the mod

Description

Field The field of the present invention is in apparatus for generating one or more pulses of ElectroMagnetic (EM) radiation. Background Lithium niobate is used for electro-optic modulators (EOMs) for rapid phase and intensity modulation of optical pulses due to its high electro-optic coefficient. Intensity modulation is usually performed by interfering two beams or photonic circuit paths and applying a relative phase change so that the light interferes constructively or destructively, depending on the phase difference, allowing for modulation in intensity. For example, this can be done with a Mach Zehnder modulator or a ring resonator modulator. However, the phase of a path in lithium niobate is known to “drift” when a voltage is applied for a long period of time, meaning that there is a gradual change in the phase or the intensity of the light passed through a lithium niobate modulator. This can cause problems, especially for applications where high extinction ratios are needed. The system must be continuously monitored within a feedback look to ensure that the modulator minimises transmission in its off state. Usually, modulators are designed to allow for arbitrary pulses or pulse sequences, most commonly for telecoms / datacoms / communications applications. However, for quantum applications, optical pulses are frequently used to manipulate qubits, for example for performing single qubit gates such as spin rotations. Here, there is a requirement for a well-controlled pulse in both power and duration to ensure high-fidelity gates. Drift, and the consequential leakage / transmission of light beyond what is intended for the control pulse results in additional spin / state rotation and consequently lower fidelity operations and errors. Here, the requirement is generally for few, well-controlled pulses, ratherthan arbitrary sequences-i.e. the modulator spends most of its time in the off state. Summary There is presented, an apparatus for: receiving Electromagnetic, EM, radiation; and, outputting an EM radiation pulse from the received EM radiation; the apparatus comprising; A) a modulator arrangement comprising a phase modulator; the modulator arrangement configured to: a) receive the EM radiation; b) phase modulate the received EM radiation; c) output phase modulated EM radiation; and, B) an interferometer configured to: d) receive the EM radiation; e) direct a first portion of the received EM radiation along a first interferometer arm path; f) direct a second portion of the received EM radiation along a second interferometer arm path that is a different path to the first interferometer arm path; g) interfere the first and second portions propagated along the first and second interferometer arm paths to output the EM radiation pulse; wherein: I) the modulator arrangement is configured to: i) impart a first phase to the EM radiation at a first time; ii) impart a set of one or more second phases to the EM radiation, beginning at a second time immediately after the first time; the set of one or more second phases continuingfor a continuous time period; the one or more second phases being different to the first phase; II) the interferometer is configured to interfere the first portion and second portion such that: iii) a first time-section of the first portion, comprising the first phase, interferes with a first time-section of the second portion, comprising the first phase, to form a first output time-section; iv) a second time-section of the first portion, comprising the set of one or more second phases, interferes with a second time-section of the second portion, comprising the first phase, to form a second output time-section; v) a third time section of the first portion comprising the set of one or more second phases interferes with a first time section of the second portion comprisingthe set of one or more second phases to form a third output time section; the second output time section comprising the EM radiation pulse. Optionally: the third time-section of the first portion is adjacent in time to and after the second time-section of the first portion; the second time-section of the first portion is adjacent in time to and after the first time-section of the first portion; the third time-section of the second portion is adjacent in time to and after the second time-section of the second portion; the second time-section of the second portion is adjacent in time to and after the first time-section of the second portion. Optionally the interferometer is configured to output the interfered EM radiation along at least one of two output paths. Optionally, the at least two output paths are spatially separate to each other. Optionally, the at least two output paths are spatially separate to input paths of the interferometer. Optionally, the apparatus is configured to output: the first and third output time sections along a first of the at least two output paths; the second output time section along a second of the at least two output paths wherein the first output path is spatially separate to the second output path. Optionally, the interferometer comprises an unbalanced Mach Zehnder interferometer. Optionally, the apparatus is configured to output the EM pulse comprising a time duration determined by a path length difference between the first and second interferometer arm paths. Optionally, the first interferometer arm path is spatially separate from second interferometer arm path. Optionally, at least one of the first or second interferometer arm paths comprises a further phase modulator. Optionally, both of the first or second interferometer arm paths comprises a further phase modulator. Optionally, the further phase modulator comprises a thermo-optic phase modulator. Optionally, the phase modulator is configured to operate faster than the further phase modulator. Optionally, the phase modulator comprises a switching time that is faster than the further phase modulator. Optionally, the interferometer comprises a Sagnac interferometer. Optionally, the interferometer comprises: a loop; and, an EM component for: directing the first portion around the loop in a first direction; directing the second portion around the loop in a second direction that is different to the first direction; interfering the first and second portions. Optionally, the loop comprises the phase modulator located asymmetrically within the loop. Optionally, the phase modulator comprises a free-carrier-based phase modulator. Optionally, the phase modulator is an electro-optic phase modulator. Optionally, the modulator arrangement comprises an electric signal generator for outputting an electrical data signal for inputtingto the phase modulator. Optionally, the electrical data signal comprises: a first portion for driving the phase modulator to impart the first phase; a second portion for driving the phase modulator to impart the set of one or more second phases; the second portion being input to the phase modulator after the first portion. Optionally, the second portion of the electrical data signal comprises a time duration for drivingthe phase modulator longer than the length of the output EM pulse. Optionally, a path length difference between the first interferometer arm path and the second interferometer arm path corresponds to a time duration less than the time duration of the second portion of the electrical data signal. Optionally, the interferometer comprises: A) a first EM coupler for receiving the EM radiation and directing the EM radiation along the first and second arm path; B) a second coupler for receiving and interfering the EM radiation from the first and second interferometer arm paths. Optionally, the first coupler comprises a tuneable coupler for controllably outputting different amounts of EM radiation, at different times, into the same interferometer arms paths. Optionally the first coupler comprises a Mach Zehnder interferometer. Optionally, the Mach Zehnder interferometer is a balanced Mach Zehnder interferometer. Optionally, the set of one or more second phases comprises a plurality of phases. Optionally, the plurality of phases corresponds to a change in an electrical signal input to the phase modulator. Optionally, the change in the electrical signal comprises a change in voltage. Optionally, the change in voltage is less than a voltage difference between: the voltage of the electrical signal associated with the first phase and the voltage of the electrical signal associated with any of the phases of the set of one or more phases. In other words, the set of one or more second phases corresponds to a slow and small voltage drift over time in the electrical signal whereas the voltage change in electrical signal to change the imparted phase from the first phase to the second phases is large, preferably much larger. For example, any of, but not limited to: two or more times larger, three or more times larger, four or more times larger, five or more times larger, six or more times larger, seven or more times larger, eight or more times larger, nine or more times larger, ten or more times larger. Optionally, the apparatus is an integrated optic apparatus. Optionally, the apparatus comprises a hybrid integrated device comprising the phase modulator and the interferometer. Optionally, the apparatus comprises an integrated optic waveguide for transporting EM radiation from the phase modulator to the interferometer. Optionally, the phase modulator comprises an integrated optic waveguide. Optionally, the phase modulator in an integrated optic phase modulator. Optionally, the apparatus comprises an integrated optic waveguide circuit comprising the interferometer. Optionally, any of the integrated optic waveguides comprises any of: a buried channel integrated optic waveguide; a slot waveguide, a rib waveguide, a ridge waveguide. Optionally, any of the integrated optic waveguides comprises any of: a dielectric waveguide core; a silicon nitride waveguide core; a silica waveguide core; a semiconductor waveguide core; a silicon waveguide core. There is further presented a system comprising the apparatus and an electromagnetic, EM, source for generating and outputting the EM radiation. There is further presented a qubit control system comprising the apparatus or the system. Brief description of the figures Figure 1 shows a schematic example of an apparatus for outputting an EM pulse. Figure 2 shows a schematic example of an apparatus for outputting an EM pulse, using an unbalanced MZI. Figures 3a, 3b and 3c show an example of operation of an apparatus similarto that shown in figure 2. Figure 4 shows a further example of an apparatus for outputting an EM pulse. Figure 5 shows diagrams illustrating an example of the active operation of an example of an apparatus for outputting an EM pulse. Detailed description There is presented an apparatus for outputting an EM pulse. The EM pulse is created from EM radiation input to the apparatus. Various ways of implementing the apparatus are described wherein details of one example implementation may be utilised in other examples. In particular, details throughout concerning features and configurations maybe used in different example implementations by virtue of auditing, removal or replacement of features from that said example. The features and configurations may include any one or more of, but not limited to: details of EM paths for carrying EM radiation including details of waveguides; details about the phase modulator and modulator arrangement; details about the electrical data signals; details about the interferometer; details about input or output paths; details about time periods; details about phases of EM radiation. Figure 1 shows a schematic example of the apparatus 2. The apparatus 2 comprises a phase modulator4. In this example the phase modulator4 comprises an integrated optic electro-optic phase modulator, however other modulators may be used. The apparatus 2 also comprises an interferometer 6. An EM source 8 (which optionally may or may not form part of the apparatus 2) generates and outputs EM radiation 10 that is input to the apparatus 2. The apparatus 2 uses the EM radiation to generate an EM pulse 12 that is output from the apparatus 2. The EM radiation 10 is continuous wave (CW) radiation in this example, however the EM radiation 10 may be a pulse longer than the output pulse 12 emitted from the apparatus 2. The EM radiation 10 in this example comprises a central wavelength in the visible region of the EM spectrum, however EM wavelengths in the Ultraviolet (UV), visible and infrared spectrums (IR) may be used depending upon the intended use of the output pulse 12. The phase modulator 4 forms part of a modulator arrangement that may include other components such as an electrical signal generator (not shown in figure 1) for driving the phase modulator and / or any electrical signal carrying components such as electrical wires or transmission lines. The apparatus 2 may be configured in different ways to provide the output pulse 12. In some examples the phase modulator 4 is located within one or both arms of the interferometer. In some examples the EM radiation is received first by the phase modulator 4, phase modulated and then output towards the interferometer 6. Other components may be used in the apparatus 2 such as, but not limited to: polarisation controllers; optical couplers; optical fibres or bulk optic components to directing the EM radiation to / from the apparatus and potentially between components in the apparatus 2; electrical components such as electrical power units for outputting voltage or current signals to components; other phase modulators. Figure 2 shows a schematic example 100 of the apparatus 102 wherein the phase modulator 104 is not within the interferometer 106. The EM source 108 in this example is an external laser 108 outputting continuous wave EM radiation 110 at a central wavelength of 400 nm. The interferometer outputs an EM output pulse 112 carved out from the input EM radiation 110. The interferometer 106 is an integrated optic unbalanced Mach-Zender interferometer 106 comprising a first coupler 114, a second combining coupler 116 having two output integrated optic waveguides 117a, 117b. The apparatus may be operated such that the pulses are triggered by a rapid phase change of an EOM and with a length determined by a delay arm. This means that light may only transmitted when there is a rapid (of the order of the length of the delay arm) change in phase, ratherthan being determined by the instantaneous phase introduced by the EOM. This means that slow EOM phase drift is cancelled out, and does not result in unintended light transmission. This provides a low background and therefore a high extinction ratio. Feedback and monitoring to ensure a low light leakage can then be done using phase tuning mechanisms which suffer less from drift. In prior art, a high extinction ratio has typically been addressed using active monitoring and feedback. Some light will be tapped off the modulator output and sent to a detector to monitor the transmission, and the DC voltage applied to the EOM is adjusted to minimise the transmission in the off state. This may be possible during operation, but if high extinction ratios are required, even a small amount of transmission to be detectable when tapped off to the detector may be enough to introduce errors in the operations, meaning that the system needs to be repeatedly taken “offline” to be recalibrated. In the apparatus presented herein, the problem of EOM phase drift is removed (at the cost of reduced flexibility in pulse pattern generation), meaning that the extinction ratio / phase minimisation can be done using materials or techniques which suffer less from drift or suffer from drift at longer timescales. Active feedback may still be required, but likely at longer timescales, depending on the drift or phase stability of the delay / interference arms of the interferometer. (This design is intended to reduce the impact of drift from an EOM, it is not immune to thermal drift.) Advantages of the apparatus may potentially include any one or more of the following: EOM phase drift problem is removed DC (direct current) control of EOM is not needed because there is no need to set a particular phase / phase difference usingthe EOM. DC for power matching and extinction ratio is separated from rapidly varying voltages for modulator control Enables multiple materials to be used to make use of their optical properties -e.g. lithium niobate for EOM, SiN for interferometers The EOM can be off-chip (or done using a hybrid chip architecture), and early versions can be demonstrated with SiN chips which are easily available from high-quality processes. Turning back to the apparatus of figure 2. The first coupler is an integrated optic coupler comprising an input waveguide and two output waveguides that respectively form two interferometer arms 118,120. The first arm 118 carries a first portion 146 of the EM radiation. The second arm 120 carries a second portion 148 of the EM radiation. First arm 118 is shorter in length, hence shorter optical path length) between the first coupler and second coupler Y branch coupler than the second arm 120. The input waveguide 119 to the first coupler is shown as only a single input waveguide, however multiple waveguides may be input to the first coupler wherein one of the inputs carries the EM radiation origination from the EM source 108. Types of couplers usable as the input coupler include a one input waveguide to two output waveguide Y-branch (referred to as a 1x2 Y branch). Other examples are: 1x2 MMI coupler; 2x2 MMI coupler; 2x2 directional coupler. The second coupler 116 comprises two input waveguides and two output waveguides, however the input and output may have more than two waveguides, in principle. The second coupler 116 may be, for example, a 2x2 MMI coupler or 2x2 directional coupler. Other types of couplers and numbers of input / output waveguides may be used. The first and second couplers are nominally 50 / 50 couplers that receive input EM radiation and amplitude-split that EM radiation in a 50 / 50 ratio into the two output waveguides of the same coupler. Other coupling ratios otherthan 50 / 50 may be used. Each of the interferometer arms 118,120 incorporate a region 122,124 for controllably imparting a phase change to EM radiation propagating along that arm. The control may be implemented by receiving a current from an electrical power source (not shown). Such regions may be thermo-optic phase shifters or other phase shifting modulators such as modulators using the electro-optic effect. The thermo-optic phase shifters 122, 124 may be used to set the balance of the interferometer such that EM radiation nominally exits via one of the two output waveguides 117a, 117b. Such phase tuning of the interferometer arms may be set to provide a nominally high extinction ratio of EM intensity between the two output waveguides 117a, 117b. Other balances of output EM power between the two output waveguides 117a, 117b may be implemented. The interferometer in this example is hosted on a chip 142 wherein the interferometer 106, including its input and output waveguides 119,117a / b are part of a planar light-wave circuit (PLC). In this example EM radiation propagates between components, hence the components are optically linked as follows. EM radiation 110 emitted by EM source 108 is input into an opticalfibre 126 (preferably a single mode optical fibre). The EM radiation is output from the optical fibre and input into the phase modulator 104. The phase modulator 104 comprises a waveguide comprising an input and an output end wherein the output end couples EM radiation 144 from the phase modulator 104 to an input waveguide 119 of the first coupler 114. After exitingthe interferometer 106, the EM radiation either: a) exits via waveguide 117a which couplers EM radiation into optical fibre 130; or b) exits via waveguide 117b which couplers EM radiation into optical fibre 132. An electrical power source 134 is shown that provides electrical power to the EM source 108 via electrical wire connection 136. An electrical signal generator 138 is shown that provides one or more electrical data signals to the phase modulator 104 via electrical wire connection 140. The phase modulator 104 and the electrical signal generator 138 may form a modulator arrangement, optionally together with the electrical connection 140. In operation, the apparatus may work as follows and is exemplified in figure 3a-3c which show zoomed-in schematic versions of an interferometer similar to figure 2. The sequence of the figures from figure 3a - 3c shows the time evolution of the apparatus when the phase modulator imparts a different phase upon the EM radiation. EM radiation is shown in figures 3a-c by the references ¢1 or ¢2 in successive time bins (indicated by an ellipse surroundingthe phase symbol). For the different phases, ¢1 or ¢2, the number 1 indicates a first phase and the number 2 indicates a second phase. It is assumed for this explanation that each of ¢1 and ¢2 are constant for the time periods shown in figures 3a-3c, however as discussed elsewhere herein, phase shifts may undesirably change over time, typically from voltage drift of the electrical signal being input into the phase modulator. Thus, at least the period of time the phase modulator is set to impart ¢2 upon the EM radiation, there may be changes to the value of ¢2 during the course of the time period. The same drift may occur for other periods of time of operation of the apparatus, such as any of the first and third time-sections described in examples herein. In the figures 3a-3c, when EM radiation indicated by the time bins gets split into the two arms, the (lower) first (shorter) arm shows time bins having phases ¢1 ’ or (|)2’ whereas the (upper) second (longer) arm shows time bins having phases ¢1 ” or (|)2”. When the time bins exit the interferometer 106 from the second coupler 116, the EM radiation from both arms combine (indicated by a'+’ symbol). Where the phases interfered by the second coupler 116 are the same i.e. either ¢1 ’ + ¢1 ”, or (|)2’ + (|)2”, the combined interfered EM radiation exits via the first (lower) output waveguide 117b. Where the phases interfered by the second coupler 116 are different i.e. (|)2’ + ¢1 ”, the combined interfered EM radiation exits via second (upper) output waveguide 117a. This 100% outputting of EM radiation into either the first output waveguide 117b or the second output waveguide 117a typically occurs when the interferometer is set to nominally have 100% constructive interference in output waveguide 117b given the same input EM radiation (hence same phase) at the input to the first coupler 114. If the phase modulator 104 imposes a phase lag or lead to the EM radiation ¢2 compared to the EM radiation ¢1 that has come before, then EM radiation may be output partially of fully from the other output waveguide. If the phase shifted EM radiation ¢2 is 180 degrees (k) out of phase with ¢1, then, these portions of interfered light that are out of phase with each other combine constructively at the upper output waveguide 117b. Such constructive interference and ideal high extinction ratio further assume the amplitudes of the interfered EM radiation ports are the same as well. Other examples herein provide devices and methods to help provide these ideal or near ideal conditions for high extinction ratio switching of EM radiation from one output waveguide to another. In particular, amplitude differences of the two interfered EM radiation portions from the two different arms may arise through EM propagation losses wherein the portion of EM radiation propagated along the longer arm suffers higher propagation losses than the shorter arm. A variable, and controllable, EM intensity splitting ratio coupler may be used as the first coupler 114. An example of this is provided else wherein herein includingfigure 4. Such a first coupler may be a Mach-Zehnder interferometer (MZI) comprisingtwo output ports and at least one phase shifting element in one of its arms. Preferably, this is an integrated optic MZI. Preferably this is a balanced MZI (BMZI). Preferably each arm comprises a phase shifter. Preferably the phase shifter is thermo-optic and comprises a layer of metal disposed proximal to a portion of the waveguide core of the respective arm. Turning back to figures 3a-3c. In more detail, looking at figure 3a, atT1, and at times before that, the phase modulator 104 was outputting EM radiation with the first phase ¢1. Then immediately afterTI, atT2, the phase modulator outputs EM radiation with the second phase ¢2. EM radiation at the second phase ¢2 is then continually output from T2 onwards, at least for a time period longer than the optical path length time difference between the shorter and longer arms of the interferometer 106. The ‘phase’ of these two time-sections of EM radiation is an arbitrary phase relative to each other. One can see from figures 3a-3c that because the portion of EM radiation carrying the first phase ¢1 that travels along the longer arm 120 takes longer to arrive at the second coupler 116, a time section of EM radiation with phase ¢2 propagated along the shorter arm 118 interferes with a time section of EM radiation with phase ¢1 propagated along the longer arm 120. Because in this time section the phases are different, EM radiation constructively interferes in the upper output waveguide 117a. Then, when the EM radiation with second phase ¢2 arrives at the second coupler, it interferes with other second phase ¢2 EM radiation propagated alongthe shorter arm 118. Because (|)2’ and (|)2” are the same phase, they constructively interfere at the output waveguide 117b. Tus for a time period set by the optical path length difference between the short 118 and long 120 arms, the apparatus can output an EM pulse 112 from the output waveguide 117a given only a single phase change provided by the phase modulator 104. One can describe this as the EM radiation having different time sections that travel alongthe different arms 120,118, wherein the EM radiation propagated alongthe long arm 120 is the first portion of EM radiation whilst the EM radiation propagated along the long arm 120 is the second portion of EM radiation. The interferometer is therefore configured to interfere the first portion and second portion to provide at least three time-sections. A first time-section of the first portion, comprising the first phase ¢1 interferes with a first time-section of the second portion, comprising the first phase ¢1, to form a first output time-section output from the output waveguide 117b. A second time-section of the first portion, comprising the second phase ¢2, interferes with a second time-section of the second portion, comprising the first phase ¢1, to form a second output time-section from the output waveguide 117a. The second output time section comprises the EM radiation pulse output from the apparatus 102. Athird time section of the first portion comprising the second phase ¢2 interferes with a first time section of the second portion comprising the second phase ¢1 to form a third output time section output from the output waveguide 117b. The third time-section of the first portion is adjacent in time to and after the second time-section of the first portion. The second time-section of the first portion is adjacent in time to and after the first timesection of the first portion. The third time-section of the second portion is adjacent in time to and after the second time-section of the second portion; the second timesection of the second portion is adjacent in time to and after the first time-section of the second portion. Further examples There now follows further examples of the apparatus. The examples described underneath and elsewhere herein may be adapted with removal of features, replacement of features or addition of further features. The features and configurations described in the following examples underneath may optionally be combined with other examples described herein. Furthermore, the features and configurations in examples presented above may be combined with examples described underneath. Features and associated configurations may include any of, but not limited to: details of EM paths for carrying EM radiation including details of waveguides; details about the phase modulator and modulator arrangement; details about the electrical data signals; details about the interferometer; details about input or output paths; details about time periods; details about phases of EM radiation. There is presented a circuit for generating controlled pulses with a high extinction ratio by removing the impact of modulator phase drift. Figure 4 shows an example of such a circuit. In the figure it should be understood that phase modulation does not have to be EOM or heaters. This circuit separates the fast electro-optic (or similar) modulation (EOM) and thermally tuned components so that: Drift due to a voltage applied to the EOM is cancelled out as does not allow for transmission in the off state Balancing the phase and intensity of the interferometer arms can be done thermally, where typically less drift is seen if a stable temperature is maintained. This solution is ideally suited to photonics integrated circuits due to the phase stability offered. The separation of the EOM and interferometer sections allows hybrid material solutions, such as using lithium niobate for the EOM to enable fast switching and silicon nitride for the interference and thermal tuning to the low loss and low thermo-optic coefficient giving stability. (It would be possible to increase this using slot waveguides to give a near zero or negative thermo-optic coefficient to counter background temperature drift.) The setup of the example in figure 4 is as follows. EOM: Light enters from the left of the diagram above and passes through the EOM, which modulates only phase. The EOM is held at a constant voltage Power Balancing: The light then passes through a thermally-tuned balanced Mach Zehnder interferometer which is used to balance the power of the light exiting each arm of the next section, the delay and phase matching. The balanced Mach Zehnder interferometer for power balancing could be tuned in one or a combination of ways such as phase change materials, strain, and electro-optic effect in place of or alongside temperature tuning. The power balancing balanced Mach Zehnder interferometer could also be positioned after the delay and phase matching (unbalanced) interferometer. The power balancing could also be done using any method of varying light transmission including using a tuneable ring resonator, variable absorbers, phase change materials, in the input and / or output arm / arms of the unbalanced delay and phase matching interferometer. Delay and phase matching: This is an unbalanced Mach Zehnder interferometer. One interferometer arm contains a delay relative to the other arm which will set the pulse length enabled by the circuit (in this example 1 ns), and one or both arms contain heaters to ensure phase balancing and that minimal light is transmitted to the output. Optimisation and feedback of the heaters would be required to correctly set the powers and phases such that a high extinction ratio is achieved and maintained. This phase matching could also be one using one or a combination of any method of phase tuning such as the electro-optic effect or strain tuning. Active operation. Figure 5 shows diagrams describing an example of the active operation of the apparatus. In figure 5, it is assumed that voltage and phase switching is instantaneous - in reality, this may not be the case and the lines would be smoother, but figure 5 shows this for illustration purposes. In Figure 5: A) The EOM is held at a positive (or higher) voltage. It is then switched to a negative (or lower) voltage at t=1 ns and held at a negative voltage. This means that for the duration of the delay arm, the light passing through the delay arm has experienced a different EOM phase to the light passing through the shorter arm. Therefore, for the duration of the delay arm, after the EOM voltage change, there is constructive (or incomplete destructive) interference at the output, and a pulse determined by the length of the delay line (1 ns) is emitted. B) The EOM is held at a negative (or lower) voltage. At t=1 ns, it is switched to a positive (or higher) voltage. Via the same mechanism as A, a 1 ns pulse is emitted. If the pulse phases are tracked, this pulse will have a different phase to the pulse in A. C) The EOM is held at a constant voltage. The thermal section of the circuit is held constant so there is destructive interference at the output and no light is transmitted. D) The EOM is held at a constant voltage, but now a gradual phase drift occurs over a timescale exceeding 10s of seconds. For drift on timescales longer than 1ns (determined by the delay arm), itapplies the same phase to light travelling through the upper and lower arm, meaning that they still interfere destructively at the output and no light is transmitted. Slow EOM phase drift does not result in transmission. The apparatus in the examples described in figures 2, 3a-c, 4, 5 may be implemented in alternative ways as follows. This could be done with any combination of integrated photonics (a range of possible platforms), fibre optics, or bulk / free-space optics. It could be done using a Sagnac interferometer with the EOM at one end of the spiral arm. The modulation could be done via some other effect such as plasma dispersion rather than the electro-optic effect Phase tuning (shown here as thermal tuning), could be done via any phase tuning mechanism - electro-optic effect, stress / strain, free space delays, fibre stretching etc. The delay arm could be made into a tuneable delay (using bulk optics, mirror position, MZI switching between different delay lines.)

Claims

1. An apparatus for:receiving Electromagnetic, EM, radiation; and,outputting an EM radiation pulse from the received EM radiation;the apparatus comprising;A) a modulator arrangement comprising a phase modulator; the modulator arrangement configured to:a) receive the EM radiation;b) phase modulate the received EM radiation;c) output phase modulated EM radiation;and,B) an interferometer configured to:d) receive the EM radiation;e) direct a first portion of the received EM radiation along a first interferometer arm path;f) direct a second portion of the received EM radiation along a second interferometer arm path that is a different path to the first interferometer arm path;g) interfere the first and second portions propagated along the first and second interferometer arm paths to output the EM radiation pulse;wherein:I) the modulator arrangement is configured to:i) impart a first phase to the EM radiation at a first time;ii) impart a set of one or more second phases to the EM radiation, beginningat a second time immediately after the first time; the set of one or more second phases continuingfor a continuous time period; the one or more second phases being different to the first phase;II) the interferometer is configured to interfere the first portion and second portion such that:iii) a first time-section of the first portion, comprising the first phase, interferes with a first time-section of the second portion, comprising the first phase, to form a first output time-section;iv) a second time-section of the first portion, comprising the set of one or more second phases, interferes with a second time-section of the second portion, comprising the first phase, to form a second output time-section;v) a third time section of the first portion comprising the set of one or more second phases interferes with a first time section of the second portion comprisingthe set of one or more second phases to form a third output time section; the second output time section comprising the EM radiation pulse.

2. The apparatus as claimed in Claim 1 wherein:the third time-section of the first portion is adjacent in time to and after the second time-section of the first portion; the second time-section of the first portion is adjacent in time to and after the first time-section of the first portion;the third time-section of the second portion is adjacent in time to and after the second time-section of the second portion; the second time-section of the second portion is adjacent in time to and after the first time-section of the second portion.

3. The apparatus as claimed in any preceding Claim wherein:the interferometer is configured to output the interfered EM radiation along at least one of two output paths.

4. The apparatus as claimed in Claim 3 wherein the at least two output paths are: spatially separate to each other; and, spatially separate to input paths of the interferometer.

5. The apparatus as claimed in Claim 3 wherein the apparatus is configured to output:the first and third output time sections along a first of the at least two output paths;the second output time section along a second of the at least two output paths wherein the first output path is spatially separate to the second output path.

6. The apparatus as claimed in any preceding Claim wherein the interferometer comprises an unbalanced Mach Zehnder interferometer.

7. The apparatus as claimed in any preceding Claim wherein the apparatus is configured to output the EM pulse comprising a time duration determined by a path length difference between the first and second interferometer arm paths.

8. The apparatus as claimed in any preceding Claim wherein at least one of the first or second interferometer arm paths comprises a further phase modulator.

9. The apparatus as claimed in Claim 8 wherein the further phase modulator comprises a thermo-optic phase modulator.

10. The apparatus as claimed in Claim 8 or9wherein the phase modulator comprises a switching time that is faster than the further phase modulator.

11. The apparatus as claimed in any preceding Claim wherein the phase modulator comprises a free-carrier-based phase modulator.

12. The apparatus as claimed in any preceding Claim wherein the interferometer comprises:A) a first EM coupler for receiving the EM radiation and directing the EM radiation along the first and second arm path;B) a second coupler for receiving and interfering the EM radiation from the first and second interferometer arm paths.

13. The apparatus as claimed in Claim 12 wherein the first coupler comprises a tuneable coupler for controllably outputting different amounts of EM radiation, at different times, into the same interferometer arms paths.

14. The apparatus as claimed in Claims 12 or 13 wherein the first coupler comprises a Mach Zehnder interferometer.

15. The apparatus as claimed in any preceding Claim wherein the set of one or more second phases comprises a plurality of phases.

16. The apparatus as claimed in Claim 15 wherein the plurality of phases corresponds to a change in an electrical signal input to the phase modulator.

17. The apparatus as claimed in Claim 16 wherein the change in the electrical signal comprises a change in voltage less than a voltage difference between: the voltage of the electrical signal associated with the first phase and the voltage of the electrical signal associated with any of the phases of the set of one or more phases.

18. The apparatus as claimed in any preceding Claim wherein the apparatus is an integrated optic apparatus.

19. The apparatus as claimed in any preceding Claim wherein the phase modulator comprises an integrated optic waveguide.

20. The apparatus as claimed in Claim 1 wherein the interferometer comprises a Sagnac interferometer.

21. The apparatus as claimed in Claim 1 or 20 wherein the interferometer comprises:a loop; and,an EM component for:directing the first portion around the loop in a first direction;directing the second portion around the loop in a second direction that is different to the first direction;interfering the first and second portions.

22. The apparatus as claimed in Claim 21 wherein the loop comprises the phase modulator located asymmetrically within the loop.T +44(0)30 0300 2000A