Method and system for optical computing

Abstract

An optical data processing apparatus is disclosed, including a first interferometer arm having a first phase shifter for applying a first sequence of phase shifts to a first light signal to encode a first 5 data stream. A delay-repeater delays part of the first light signal and recombines the delayed part with the first light signal. The phase-shifted first light signal encodes the first data stream and comprises the delayed part of the first light signal. A second interferometer arm comprises a second phase shifter for applying a second sequence of phase shifts to a second light signal which encodes the second data stream. An optical splitter splits source light between the first and second interferometer arms for providing the first and second light signals. A light combiner combines the phase- shifted first and second light signals from the respective first and second interferometer arms to provide a combined light signal.

Description

[0001]METHOD AND SYSTEM FOR OPTICAL COMPUTING Technical Field The present invention relates to methods and apparatus for optical computing, and more particularly to methods and systems for optically combining streams of data. Background It is known to use optical systems instead of electronic systems for computation. In a conventional architecture, a Mach-Zehnder interferometer may be used to divide light into two arms and phase modulate the light according to an input data stream. By recombining the light, the combined light signal can be analysed to monitor the output power and infer characteristics of the input data stream. In this way, various logic gates can be implemented. It is an aim of the present invention to provide an improved optical computing system. Summary Embodiments of the present disclosure aim to provide high throughput data operations in an optical data processing system. In an aspect there is provided an optical data processing apparatus comprising: a first interferometer arm comprising: (i) a first phase shifter for applying a first sequence of phase shifts to a first light signal in the first interferometer arm to encode a first data stream; and (ii) a delay-repeater configured to delay part of the first light signal and to recombine the delayed part with the first light signal; whereby the first light signal provided from the first interferometer arm encodes the first data stream and comprises the delayed part of the first light signal; a second interferometer arm comprising a second phase shifter for applying a second sequence of phase a second light signal in the second interferometer arm whereby the second light signal provided from the second interferometer arm encodes the second an optical splitter configured to split source light between the first interferometer arm and the second interferometer arm for providing the first light signal and the second light signal; a light combiner connected to receive the first light signal from the first interferometer arm and the second light signal from the second interferometer arm and to combine the first light signal and the second light signal to provide a combined light signal. The apparatus may comprise a wavelength swept light source, such as a tunable laser configured to provide coherent wavelength-swept light to the optical splitter. Typically, however this light source is not included in the apparatus because it may be made and sold separately. The delay-repeater may be configured so that, when the light received from the optical splitter comprises a coherent wavelength-swept light signal, said recombining of the delayed part provides at least one beat frequency component in the first light signal. The delay-repeater may also be configured so that the delayed part comprises a plurality of delayed parts each delayed by a respective corresponding one of a plurality of delays. Each such delayed part may provide a corresponding beat frequency component- so the at least one beat frequency component comprises a plurality of beat frequency components. The delay-repeater may also be configured so that the delayed part with the longest delay provides more of the light signal as compared to the other delayed parts. For example, the delay-repeater may comprise a splitter configured so that the delayed part of the first light signal comprises a majority of the first light signal, for example more than 50% of the power in the first light signal may be delayed, for example more than 70%, for example more than 80%. The delay-repeater may precede the first phase shifter in the first interferometer arm. In some possibilities, the first phase shifter may precede the delay repeater in the first interferometer arm so as to provide multiple copies of the same phase shift. The first phase shifter may be integrated with the delay-repeater. For example, the delay-repeater may comprise a looped optical path, such as a looped optic fibre, a coupling may be provided to the first interferometer arm (such as by a beam splitter) to enable light to be input to the delay-repeater from the interferometer arm and provided back to the interferometer arm from the delay-repeater. The phase shifter may be provided between the looped path and the coupling to the interferometer arm. The first phase shifter and the second phase shifter may be synchronised with each other. The first data stream and the second data stream both comprise data symbols with the same unit interval. The first data stream and the second data stream may be provided to the first phase shifter and to the second phase shifter respectively in a synchronised manner, for example they may be provided according to a shared clock signal. In operation the first phase shifter may apply a first sequence of phase shifts to the first light signal, and the second phase shifter may apply a second sequence of phase shifts to the second light signal. The first sequence of phase shifts and the second sequence of phase shifts may be synchronised. The unit interval (e.g. the duration of each data symbol encoded by these phase shifts) may be identical in sequences. The apparatus may further comprise a wavelength swept light source configured to sweep the wavelength of the light periodically with a sweep period, the sweep period may comprise a first interval and a second interval. The apparatus may comprise a detector configured to superpose a first interval of the combined light signal with a second interval of the combined light signal to provide a superposed signal. Such function may be performed by a controller, which may be provided by the detector. The controller may be further configured to determine frequency data representing frequency components of the superposed signal. Each frequency component may indicate a combination of a data symbol of the first data stream with a data symbol of the second data stream. The amplitude of each frequency component may indicate combination. The controller may be configured to use the frequency components to perform data operations on the first data stream and the second data The first interval and the second interval may each comprise a corresponding part of a wavelength sweep of the coherent wavelength- swept light signal, for example the apparatus may further comprise a coherent light source configured to provide the coherent wavelength- swept signal, for example wherein the coherent light source comprises a tunable laser. The coherent light source may be configured to provide the wavelength sweep in a triangular waveform and the first interval and the second interval may each comprise half a period of the triangular waveform. The coherent light source may be configured to provide the wavelength sweep in a sawtooth waveform and the first interval and the second interval each comprise a period of the sawtooth waveform. The optical splitter may be a 50:50 splitter. At least one of the first phase shifter and the second phase shifter may comprise one of: a liquid crystal spatial light modulator, an acousto-optic modulator, and an electro-optic modulator. The first phase shifter may be configured so that the unit interval of the first data stream corresponds to the delay applied to the part of the light by the delay-repeater. The delay-repeater may comprise an optical path arrangement arranged to provide the light from the input to the output and to split the part from the light provided to the input. path arrangement may comprise a repeating path having an optical path length configured to provide said delay. The repeating path may comprise a loop, and the delay is provided by the optical path length around said loop. An amplifier may be provided in the loop to compensate for the signal loss due to light transmission through connectors. The optical path arrangement may comprise a coupling between the repeating path and an interferometer arm of the apparatus, such as a second optical splitter. For example, the second optical splitter may be positioned at a join in said loop and may provide an input to the delay-repeater. The unit interval of the data streams may be equal to the delay provided by one lap of the repeating path of the delay-repeater. The disclosure may also provide an optical data processing method comprising: splitting a coherent wavelength-swept light signal to provide a first light signal and a second light signal; applying a delay to a part of the first light signal and recombining the delayed part into the first light signal and applying a first sequence of phase shifts to the first light signal to encode, on the first light signal, a first data stream having a unit interval corresponding to the delay; applying a second sequence of phase shifts to the second light signal to encode, on the second light signal, a second data stream having the unit interval; interfering the first light signal with the second light signal to provide a combined light signal encoding a combination of the first data stream and the second data stream. It will be appreciated in the context of the present disclosure that the first light signal and the second light signal are generally mutually coherent at the splitter. The unit interval may be equal to the delay. The method may comprise combining a first interval of the combined light signal with a second interval of the combined light signal to provide a superposed signal. The first interval and the second may each comprise a corresponding part of a periodic wavelength sweep of the coherent wavelength-swept light signal. The wavelength sweep may comprise a triangular waveform and the first interval and the second interval each comprise half a period of the triangular waveform. The wavelength sweep may comprise a sawtooth waveform and the first interval and the second interval each comprise a period of the sawtooth waveform. The method may further comprise determining frequency data representing frequency components of the superposed signal and using said frequency components to determine combinations of respective corresponding symbols from the first data stream and the second data stream. It will be appreciated in the context of the present disclosure that, the optical data processing apparatus described and / or claimed herein provides a means to combine the first data stream with the second data stream and so may be referred to as an optical data combiner. The optical data combiner may be provided by the optical splitter, connected to the light combiner by the first interferometer arm and the second interferometer arm. This may also be referred to as a phase modulation assembly (see element 180 illustrated in Figure 1 and Figure 4). Embodiments of the disclosure also provide a system comprising at least two optical data combiners according to the present disclosure, coupled together to perform data processing operations. The plurality of optical data combiners may be arranged in series, so that an output of one data combiner is provided to an input of a subsequent optical data combiner in the series. For example, at least one output of a light combiner of a first optical data combiner may be connected to at least one input of a second optical data combiner. The at least one output may be either (a) the combined light signal or (b) a data stream carried by the combined light signal (e.g. a data stream which results from the phase modulation caused by the first data stream and the second data stream in the respective first and second arms of the first optical data combiner). The system may be configured so that the data stream carried by the combined light signal from the first optical data combiner is used to control a phase shifter of the subsequent optical data combiner. The system may be configured so that the combined light signal from the first optical data combiner is provided to the second optical data combiner (e.g. as the coherent light source for that second optical data combiner). For example, this combined light signal may be further modulated by further data stream(s) provided to either or both of the phase shifters of the subsequent optical data combiner. The system may comprise a plurality of optical data combiners arranged in parallel. A single (e.g. shared) coherent light source may be configured to provide light to a plurality of optical data combiners in parallel. In this arrangement, two or more pairs of data streams may be modulated onto the shared coherent light source in parallel. The system may be configured to recombine each combined light signal obtained from each of the plurality of optical data combiners. Accordingly, an aspect of the disclosure provides a system comprising a plurality of the optical data combiners arranged in parallel, in which the coherent light source of each of the plurality of optical data combiners is provided by a shared coherent light source. A first two data streams may be provided, one to each of the first and second phase shifters of a first one of the plurality of optical data combiners, and a second two data streams may be provided, one to each of the first and second phase shifters of a second one of the plurality of optical data combiners. The first two data streams may be different from the second two data streams. The system may be configured so that the combined light signal produced from the first one of the plurality of optical data combiners is optically combined with the combined light signal of the second one of the of the plurality of optical data combiners – for example the two combined light signals may be provided to a further light combiner to produce a further combined light signal, which encodes the four data streams in combination. It can therefore be seen that the apparatus of the present disclosure may provide a system configured to provide sequential and / or parallel data combination, for example by comprising a combination of sequential optical data combiners and / or parallel optical data combiners. Further embodiments are envisaged. Brief description of the drawings Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of an optical data processing apparatus according to embodiments of the disclosure; Fig. 2 shows two plots of frequency components of two different light signals in an apparatus such as hat described with reference to Figure 1; Fig. 3 shows two diagrams of different possible delay-repeaters for use in an apparatus such as that described with reference to Figure 1; Fig. 4 is a schematic diagram of a sequential optical computing system according to embodiments of the disclosure; Fig. 5 is a schematic diagram of a sequential optical computing system according to embodiments of the disclosure; and Fig. 6 is a schematic diagram of a parallel optical computing system according to embodiments of the disclosure. In the drawings like reference numerals are used to indicate like elements. Specific Description Embodiments of the disclosure relate to an optical data processing apparatus, which may provide an optical data combiner. The apparatus shares some structural features with a typical Mach- Zehnder interferometer, in that it includes an optical splitter, such as a beam splitter, to split the light into two paths, and an optical combiner to recombine the light from these two paths. A light source may also be provided. In examples of the disclosure, the light source is a coherent light source, such as a laser. The light source is configured to provide a wavelength swept signal. This means that at a given time, the light source emits light at a single frequency (and therefore a single wavelength), but the frequency of light emitted varies with time. For example, the frequency may vary periodically by sweeping up from a minimum frequency to a maximum frequency before sweeping back down to the minimum frequency and repeating the cycle. The optical data processing apparatus and the light source may be made and sold separately. The optical splitter splits the light signal so that it travels through a first arm of an interferometer and a second arm of the A delay-repeater may be provided in the first interferometer arm. As described below, this splits off part of the first light signal and delays it, e.g. by diverting it along an optical path, before recombining that delayed part of the light with the first light signal. This has the effect of providing that the first light signal includes the wavelength swept light from the source, and a delayed part of that same light. The length of the delay corresponds to the length of the optical path along which the part of the light is diverted. Because the source light has a swept wavelength, the result of combining the delayed part is to provide a beat frequency component in the recombined light, having a beat frequency corresponding to the length of the delay and the sweep rate of the source. This process of splitting part of the light and delaying it may be repeated, so that the first light signal includes a series of delayed parts, each successive part of this series being delayed relative to the next by the same delay. This can provide a plurality of beat frequency components. One way to achieve this is by the use of a loop of optical fibre, joined into the interferometer arm by a beam splitter. Each interferometer arm includes a respective phase shifter to apply phase shifts (e.g. a phase modulation) to the light signal in that interferometer arm. The phase shifts applied by the phase shifters may be controlled according to data streams which are provided to each phase shifter. In particular, the light signal in the first interferometer arm can be phase shifted to encode a first data stream and the light signal in the second interferometer arm can be phase shifted to encode a second data stream. In this way, the data from each data stream is encoded in phase shifts of the light signals in the first arm and the second arm. The phase shifter in the first interferometer arm may apply, to the first light signal, a sequence of phase shifts corresponding to the first data stream. These phase shifts may be applied to the first light signal including the delayed part(s) provided by the delay- repeater. It will be appreciated in the context of the present disclosure that a data stream comprises a series of successive data symbols, the time taken for each such data symbol may be referred to as a unit interval (the time allotted in the data stream for a single symbol). The unit interval with which the first data stream is encoded onto the first light signal generally matches the delay applied by the delay- repeater. Figure 1 shows an optical data processing apparatus 110. The optical data processing apparatus 110 comprises a splitter 102, a first interferometer arm 121, a second interferometer arm 122, and a light combiner 160. The two interferometer arms 121, 122, each comprise a respective path for light to travel from the splitter 102 to the light combiner 160, which are configured to enable the light from each of the two arms 121, 122 to interfere at the combiner. An optical path length difference may be provided between the first arm 121 and the second arm 122. The first interferometer arm 121 comprises delay-repeater 150 and a first phase shifter 131. The second interferometer arm 122 comprises a second phase shifter 132. The optical data processing apparatus is an arrangement of optical components which is configured to guide a light signal from the light source 101 to the light combiner 160. In the arrangement of Figure 1, the optical data processing apparatus 110 provides two paths for such a light signal, one through the first interferometer arm 121 and one through the second interferometer arm 122. A coherent light source 101 may also be provided. The coherent light source 101 is configured to provide temporally coherent light to the splitter 102. The coherent light source 101 may be a laser, such as a tuneable laser. The coherent light source 101 is configured to provide a frequency swept signal. In this respect, the coherent light source 101 provides coherent light at a frequency or wavelength that varies with time, for example in a periodic manner. The frequency swept signal may have a triangular profile, wherein the frequency increases linearly over a time period from a minimum frequency to a maximum frequency before decreasing to the minimum frequency, for example at the same linear rate. The splitter 102 is configured to split the frequency swept signal to provide a first light signal in the first arm 121 and a second light signal in the second arm 122. Typically, the splitter 102 is a beam splitter configured to receive the frequency swept signal from the coherent light source 101 and divide the signal between the first arm 121 and the second arm 122. The beam splitter may be a 50:50 beam splitter so as to provide an equal division of the signal power between the first arm 121 and the second arm 122. The delay-repeater 150 in the first interferometer arm 121 comprises an input connected to receive light from the splitter 102, and an output which, in the configuration illustrated in Figure 1, is connected to provide light to the first phase shifter 131. The delay- repeater comprises optically transmissive materials arranged so that, when provided with a wavelength-swept light input light, the delay- repeater provides output light comprising at least one beat frequency component corresponding to a path length difference of the delay- repeater and a sweep rate of the wavelength-swept input light. For example, the delay-repeater generally is configured to provide the light received at its input to its output and also to delay a part of the light received at its input. It is arranged to provide that delayed part back to the input. The delay causes a timing offset between the delayed part and the input light from which it was split and, because of this timing offset and the wavelength sweep of the input light, the result is that there is also a frequency offset (after combiner 160) between the delayed part and the rest of the light. This provides the light output from the delay-repeater with a frequency component corresponding to that frequency offset after interference at the combiner 160. Because the delay may be applied repeatedly, the output light comprises a number of such delayed parts, each delayed with respect to the next – so with an increasingly large delay with respect to the original light signal and hence (due to the wavelength sweep) a greater frequency offset from that light signal after interference. In operation the delay-repeater 150, when considered in time domain, the light output from the delay-repeater comprises a sequence of delay intervals. During the first delay interval, the output light may consist solely of the source light (i.e. the light which just passes straight through without any delay). After the first delay interval, (during the second delay interval), the output light may comprise both the source light and a part of the source light which has been delayed with respect to the source by the delay interval. During the third delay interval, the output light may comprise the source light, a part of the source light delayed by the delay interval, and a part of the source light delayed by twice the delay interval. This process of repeated delays of parts of the source light means that each delay interval of the light output from the delay-repeater may comprise a different beat frequency (or different set of beat frequencies) after interference at the combiner 160. It can therefore be seen that the delay-repeater may be configured to provide light from its input to its output, to delay part of the light provided in to that input, and to provide the delayed part back in to the input. The result, for a swept wavelength source, is that the frequency content of successive delay intervals of the light output from the delay-repeater after interference, comprises progressively higher beat frequencies after the combiner 160, corresponding to e.g., larger delays at the delay-repeater 150. As the total delay gets longer, the beat frequencies increase. In some embodiments of the disclosure, combiner 160 may be configured so that the beat frequencies decrease with increasing total delays. A straightforward way to implement such a delay-repeater 150 is with an optical fibre loop connected into the first interferometer arm with a circulator or beam splitter, but other structures such as optical cavities may also be used. The first phase shifter 131 is configured to modulate the phase of the light signal in the first arm 121. In this respect, the first phase shifter 131 is configured to apply phase shifts to the first light signal received from the delay-repeater 150 and to provide the phase modulated light to the light combiner. The first phase shifter 131 is further configured to control these phase shifts according to a first data stream 141, which may be provided to the first phase shift by a controller of the device or some other data stream source. The first phase shifter 131 is configured to apply the symbols of the data stream to the first light signal with a selected unit interval (e.g. the duration of each symbol). The first phase shifter 131 and / or the delay-repeater 150 may be configured so that the unit interval of the phase shifter 131 matches (e.g. is equal to) the delay interval provided by the delay-repeater 150. Accordingly, a plurality of different data symbols from the first data stream may be applied to a corresponding plurality of delay intervals of the output from the delay-repeater 150. The second phase shifter 132 is configured to modulate the phase of the light signal in the second arm 122. In this respect, the second phase shifter 132 is configured to control a phase modulation of the second light signal received from the splitter and direct the phase modulated light towards the light processing apparatus. The second phase shifter 132 is configured to control the phase modulation based on a second data stream 142. As such, the phase of the second light signal can be modulated based on data in the second data stream 142. The first phase shifter 131 and / or the second phase shifter 132 may comprise a controllable optical element such as a liquid crystal spatial light modulator, an acousto-optic modulator, and an electro- optic modulator, or any other device capable of performing suitable phase and / or amplitude modulation. The light combiner 160 is configured to combine the first light signal received from the first phase shifter 131 with the second light signal received from the second phase shifter 132 to produce a combined light signal. In particular, the light combiner 160 is configured to receive light from the first arm 121 and the second arm 122, and combine the received light into a combined light signal. The light combiner 160 may be provided by a beam splitter, such as a 50:50 beam splitter. The optical data processing apparatus 110 is arranged such that light is guided from the coherent light source 101 to the light combiner 160. This may be performed using a series of waveguides, which are illustrated in Figure 1 by the lines between the optical components, namely the coherent light source 101, the splitter 102, the first phase modulator 131, the second phase modulator 132, the delay component 150 and the light combiner 160. The coherent light source 101 is connected to the splitter 102. The splitter 102 is connected to the delay-repeater 150, which in turn is connected to the first phase shifter 131. The splitter 102 is also connected to the second phase shifter 132. As such, the splitter 102 is arranged between the coherent light source 101, delay-repeater 150, and the second phase shifter 132. The first phase shifter 131 is arranged in the first interferometer arm 121 and is connected between the delay-repeater 150 and the light combiner 160. The second phase shifter 132 is arranged in the second arm 122 and is connected between the splitter 102 and the light combiner 160. It will be appreciated that such connections are optical connections which facilitate light signals to travel between the optical components, for example along a waveguide. Optical fibres are one example of a waveguide, but other appropriate optical waveguide structures may be used as will be appreciated in the context of the present disclosure. Without wishing to be bound by theory, to illustrate operation of this apparatus and to explain its principle of operation, there follows a mathematical description of the light signals and certain examples of methods which the apparatus may perform.The wavelength swept source light, such as a laser, may be denoted( , ) = ( )exp [ ( , )], where ( ) = 2 / ( ) is the wavenumber, andis the initial phase. To keep the equations succinct, we will omit explicit time dependence in k (however, we keep in mind that k is- time-dependent). The laser wavelength is modulated (swept) over the tuning bandwidth, over time t according to a predefined pattern. denotes the discrete time between consecutive laser sweeps (in general is much longer than time needed to change the laser wavelength). This process is sketched in Fig. 2.As mentioned above, the laser source light ( , ) is split into twopaths at splitter 102, one for each interferometer arm, each of which contains a phase shifter. Signals at the end of those paths (e.g. atthe light combiner 160) may be denoted as: (, ) = (1 ) ( ) exp[ { ( , ) + ( ) + ( , )}],where: is the split ratio, and are the physical lengths of each path, (, ) and ( , ) – phase shifts induced by phase shifters 131,132 respectively. The interferometer arms 121, 122 may also be arranged to provide anon-zero optical path length mismatch = . This may ensure that( , ) and ( , ) are always a little bit out of phase at the combiner160, even if these signals would otherwise be in phase. may result of different optical path lengths (e.g. different lengths of optical fiber) for example, in the first interferometer arm 121 and the second interferometer arm 122, but may be implemented with any such delay component (not shown). The signals U and U are recombined at the beam combiner, and then detected by a light sensor. The light detector 170 determines the magnitude square of the combined signal, integrated over a certaintime interval. This provides a signal: where T is the detector integration time. The detector integration time here may be assumed to be one sweep period. After expanding theabove equation, one gets:( , ) = + 2 (1 ) ( ) {exp[ { + ( , )},where: term (offset)=( , ) = ( , ) ( , )For the sake of simplicity, we will assume that = 0.5 (50% split),and ( ) = 1 but other values of these parameters may be used. Underthese assumptions, ( , ) can be written as:( , ) = + 0.5 {exp[ { + ( , )},The DC offset term may be suppressed, so, that the ( ) is given( , ) = {exp[ { + ( , )} = cos[ + ( , )].For the fixed (=fixed fiber length on the two interferometer arms) the signal will be a single-frequency cosine wave. Its frequency depends on the path mismatch and the phase shift . The integration time of the detector may be longer than the sweep duration (T > sweep duration), and the signals are coherently added on the detector, leading to the effective (or combined signal ) signal of the form: Here, each corresponds to one sweep period and the total integration time T = N*sweep period, i.e. integration is over N symbols. Note that in this case, the phase can be adjusted for each symbol period and the light detector forms the sum of the signals over the number of N symbols. So that various outputs can be synthesized by controllingthe detector integration time and phase modulation. Specifically, for= 2, ( , ) = 0, ( , ) = , we get (trigonometric rule)( ) = cos[ ] + cos[ + ] = 0.( ) can be Fourier transformed and in general case, the amplitude of{ ( )} encodes the result of combining the two-phase shifts (twodata symbols) from the two data streams. This provides a way to perform an optical computation. To increase the number of computations which can be performed for each sweep period of the swept source, the delay-repeater can be introduced into one of the interferometer arms. As mentioned above, the delay- repeater may be provided by a loop of optical fibre of physical length . The loop is added to one of the interferometer arms, and is joined into that arm using an additional fiber splitter. This may be arranged so that some fraction of the light goes through the splitter, while the rest of the light recirculates in the loop and is fed back into its input – the part fed round the loop is therefore delayed with respect to the original part of the swept wavelength beam as described above. (, ) = cos[ + ( , )].The signal during the second delay interval (after the first pass ofthe loop) will be: where < 1 denotes the signal attenuation due to additional connectors in the loop and is the additional shift due to the loop. The path length of the loop may be selected so that the delay such that typically, an integer number of such loops corresponds to the sweep time of the source. After the second pass (during the third delay interval), the signalwill be: This can be continued until the interferometer sensing range is reached (the sensing range is a reciprocal of the instantaneous wavelength). Thus, the signal can be written as cos + + ( , ) . The above signal represents the coherent sum of M cosines, each ofwhich has a base frequency of + . The frequency increases withthe successively increasing delays of the delay-repeater (e.g. due to additional loops being traversed, i.e., increasing j). After takingthe Fourier transform of ( , ) we end up with a sequence of deltafunctions representing frequency components located at frequenciescorresponding to + (Fig. 3).Note that now depends on three parameters:– is the index denoting light pass through the loop, – instantaneous laser wavelength (as a function of time), – sweep index. The detector integration time may correspond to a number, N, of sweep times: Accordingly, the frequency components of the combined the detector each have a phase, corresponding to the combined phase shifts of the two data streams.The phase shifters ( , ) may be synchronised with each other andwith the timing of the delay-repeater (e.g. the timing of light passes through the loop) Thus, the phase shifter 131 of the first interferometer arm 121 may change the phase of each delay interval (each pass through the loop) separately (as denoted by the index j). This may be written: ). The phases of these frequency components of the combined light signal after combiner 160, and may therefore depend on both the phase of the first light signal and the phase of the second light signal – for example, the combination of the phase modulation caused to each signal by each of the two data streams 141, 142 is encoded in the combined light signal. Fig. 2. illustrates two plots of the frequency content of signals detected at the detector according to this disclosure. In the plot on the left-hand side is shown a plot which illustrates a first four of the frequency components of the combined light signal detected at the detector where no phase shift is applied to either signal by the two- phase shifters. It can be shown that these frequency components comprise: (i) a first frequency component having a frequency associated with the path difference, , of the light which has passed straight through the delay-repeater without any delayed part being added; (ii) a second frequency component having a frequency associated with the path difference, + , of the lightwhich has been delayed by one delay interval by the delay-repeater; (iii) a third frequency component having a frequency associated with the path difference, + 2 , of thelight which has been delayed by two delay intervals by the delay-repeater; and (iv) a fourth frequency component having a frequency associated with the path difference, + 3 , of thelight which has been delayed by three delay intervals by the delay-repeater. Depending on the degree of attenuation provided by the delay-repeater, the relative amplitudes of these peaks may decay to a greater or lesser extent for longer delays. However, it may been shown that the amplitude of the peaks also depends on the relative phase shift applied to the first light signal and the second light signals during the unit intervals of the data stream (delay intervals of the delay-repeater). The example shown in the plot on the left hand side of Figure 2 corresponds to a zero phase difference during each of the relevant intervals. The example illustrated in the plot shown in the right-hand side of Figure 2 illustrates amplitudes of frequency components measured by the detector 170 when a sequence of relative phase shifts (in thisexample), 0, and 5 are applied to the first, second, thirdand fourth frequency components respectively. This may correspond to different phase shifts being applied during the corresponding unit intervals of the data stream (delay intervals of the delay-repeater). It can therefore be seen that the controller 170 may be configured to use the amplitude (and / or phase depending on the detection method) of the respective frequency components of the output signals 161, 162 of the combiner 160 to infer the relative phase shifts of the processed first light signal and the processed second light signal during the sequence of unit intervals of the data stream (delay intervals of the delay-repeater). The sequence of unit intervals typically are synchronized with the sweep of the wavelength swept light source, so that each wavelength sweep comprises an integer number of unit intervals. Therefore, we can extend the processing bandwidth and utilize the entire sweep for data combining / processing. The phase and / or amplitude of the combined light signal may be used to perform data operations for the two data streams. These data operations may comprise elementary operations on pairwise elements of the two data streams – such as logic operations and arithmetic operations. Accordingly, the detector 170 may comprise an analysing and / or synthesizing capability for processing the combined light signal produced by the combiner 160. The detector 170 may comprise a controller and may comprise a quadrature detection capability so, in the arrangement shown, the controller receives a first output 161 and a second output 162 from the combiner 160 but these may be merely representative of signal components of a single light signal such as first and second quadrature signals e.g. obtained from phase sensitive detection of the combined light signal, or polarisation components or other such decompositions of a single signal. There may also be only one output from combiner 160, in accordance with various embodiments of the disclosure. To obtain the above signal I(k) the controller 170 can be configured to superpose a first interval of the combined light signal with a second interval of the combined light signal to produce a superposed signal. The first interval and the second interval may each comprise a sweep period of the wavelength swept light source, or a multiple of such periods. The superposition may be achieved by e.g., an integrating functionality over a certain time interval. In accordance with various embodiments, the superposition may be obtained with a photodetector or another form of light sensor. Thus, the controller 170 may be operable to convert an optical signal to a related electrical signal. In one example, the controller receives a combined light signal over a time interval T and separates the combined signal into a first interval of the combined light signal between 0 and T / 2 and a second interval of the combined light signal between T / 2 and T. The controller may superpose the first interval with the second interval to produce a superposed signal. For example, the first interval of the combined light signal may correspond to a first part of the frequency swept signal and the second interval of the combined light signal may correspond to a second part of the frequency swept signal. For example, assume that a sweep duration of the frequency swept signal lasts 1,000 ns. The first part of the signal in which the frequency linearly increases lasts, for example, 500 ns. This first part of the signal is split at the splitter 102 and proceeds to the first phase modulator 131 and the second phase modulator 132, where its phase is modulated based on the respective data streams. After the first part of the signal, the coherent light source 101 generates the second part of the signal, in which the frequency linearly decreases, and which also lasts 500 ns for example. The second part of the signal is split at the splitter and proceeds to the first phase modulator and the second phase modulator but no phase modulation takes place. Then, at the light combiner 160, the first part of the signal from the first arm is combined with the first part of the signal from the second arm. This forms the first interval of the combined signal, which lasts 500 ns (0 to T / 2) in this example. After, the second part of the signal from the first arm is combined with the second part of the signal from the second arm at the light combiner 160. This forms the second interval of the combined signal, which may last 500 ns (T / 2 to T) in this example. Similarly, the superposition time may be larger than a complete sweep period and may be multiple sweep periods, for example two sweep periods. Then, a first interval may be one complete sweep period and a second interval may be a complete sweep period. In the example of a frequency swept signal having a triangular profile and may have equal duration first and second intervals, the second interval of the combined signal can be reversed before being superposed with the first interval of the combined signal. This may be performed so that the two signals being superposed both relate to the same positive rate of frequency increase. In other words, if the first part of the frequency swept signal is an up-sweep and the second part is a down-sweep, then reversing one of the intervals of the combined signals will ensure that the time-varying nature of the frequency in each signal is matched before superposition. The described functionality can be used to construct logic gates, such as an AND gate, as described below. The following truth table provides an example of how the phase shifts can be set for each combination of input data. In the example, Z denotes the amplitude of the superposition. For inputs X=0, Y=0, a phase difference of 0 provides an output signal of 0. For inputs X=0, Y=1, a phase difference of provides an output signal of 1.25. For inputs X=1, Y=1, a phase difference of provides an output signal of 2. Therefore, by monitoring the signal at the first output port 161 to require that Z > 1.5 to generate 1, and 0 otherwise, an AND gate can be implemented. It will be appreciated that other logic gates can be implemented by modifying the phase shifts in phase modulators 131, 132 and the threshold for Z as required and / or using the second output of the combiner 160. Similarly, the superposition time T may be adjusted as desirable. As such, the detector 170 can be configured to implement one or more logic gates, for performing logic operations on the data X and Y, based on a threshold and the output of the combiner. In other examples, the detector 170 is configured to determine frequency data representing frequency components of the superposed signal, for example using a Fourier representation of the signals received. The controller may be further configured to compare the frequency data with a stored association, which may be stored in a memory of the controller. In this way, the controller can be configured to perform a logic operation and / or an elementary operation, such as an arithmetic operation. The arithmetic operation may be addition, subtraction, multiplication, or division, for example. The stored association may comprise a relation between the frequency data and a result of the operation. In some examples, the operation comprises a logic operation and the relation comprises a truth table. The logic operation may be a Boolean operation. The logic operation may be an arithmetic operation and the relation may comprise a mapping between frequency data and the arithmetic result of the combination. The controller may be configured to perform a frequency transformation of the output signal. For example, the analyser may be configured to perform a Fourier transformation on the output signal. The magnitude of the Fourier transform will vary based on the phase shift. As such, similarly to the method described above, specific thresholds can be applied to implement various logic gates. The controller can be configured to perform arithmetic operations for real numbers in a selected range, such as [-1,1]. It will be appreciated in the context of the present disclosure that the controller may be configured to perform additions and multiplications, for example by appropriately configuring the detector 170. It will be appreciated in the context of the present disclosure that the controller 170 can be configured to determine one or more frequency components of the combined signal (each of which may correspond to a different data symbol and different unit interval of the delayed first light signal), perform a frequency transformation (such as a Fourier transform) on the combined signal, and use a mapping (such as a lookup table or pattern recognition, for example), to obtain a result of the multiplication operation based on a component of the transformation, such as the amplitude. Other means of determining such data operations may also be used. In accordance with various embodiments of the disclosure, arithmetic operations may be implemented such that the amplitudes of e.g., the frequency components of the output 161 may directly correspond to the desired output, without the need for lookup tables. Fig. 3 shows two diagrams to illustrate different possible delay- repeaters for use in an apparatus such as that described with reference to Figure 1. The first example (labelled option 1) shows a very schematic representation of a data processing apparatus comprising a splitter 102 connected to split light between a first interferometer arm 121 and a second interferometer arm 122. The other ends of the first interferometer arm 121 and the second interferometer arm 122 are both connected to the light combiner 160. In “Option 1” - the first interferometer arm 121 comprises a delay- repeater 150 into which a first phase shifter 131 is integrated. Specifically the delay-repeater 150 comprises a second optical splitter 5002 connected to receive, at the input of the delay- repeater, the light from the first splitter 102 and to provide part of that light to the light combiner 160 and another part of that light to a delay path 5000, which maybe provided by a loop of optical fibre having an optical path length . The delay path 5000 provides a connection from the second optical splitter 5002 to a phase shifter 131, which is connected back to the input of the second optical splitter. Thus the phase shifter 131 may be integrated into the delay- repeater 150. The possibility illustrated in “Option 2” of Fig. 3 is identical to “Option 1” other than in that the phase shifter is not integrated into the delay-repeater 150. Instead, in “Option 2” - the first interferometer arm 121 comprises a delay-repeater 150 which precedes the first phase shifter 131. In this example, the delay-repeater 150 comprises a second optical splitter 5002 connected to receive, at the input of the delay-repeater, the light from the first splitter 102 and to provide part of that light to the light combiner 160 and another part of that light to a delay path 5000, which may be provided by a loop of optical fibre having an optical path length . The delay path 5000 provides a connection from the second optical splitter 5002 to the input of the second optical splitter 5002. The output of the second optical splitter is then connected to the phase shifter 131 and the output of the phase shifter is connected to the light combiner 160. Thus the phase shifter 131 may be provided in series after the delay-repeater 150 in the first interferometer Other configurations are possible. As mentioned above, the optical data processing apparatus of the present disclosure, such as that described with reference to Figure 1, provides an optical data combiner. These data combiners may be arranged in series or in parallel or otherwise combined in an assembly to perform more complex data operations. For example, a plurality of optical data combiners may be arranged in series, so that an output of one data combiner is provided to an input of a subsequent optical data combiner in the series. For example, at least one output of a light combiner of a first optical data combiner may be connected to at least one input of a second optical data combiner. The at least one output may be either (a) the combined light signal or (b) a data stream carried by the combined light signal (e.g. a data stream which results from the phase modulation caused by the first data stream and the second data stream in the respective first and second arms of the first optical data combiner). Figure 4 shows an example of the first of these two possibilities, in which the combined light signal from the first optical combiner is used as the light source for the second optical combiner. Figure 4 illustrates an apparatus comprising a first optical combiner 110 and or similar to the optical data combiner 110 described in relation to Figure 1. To simplify the drawing, Figure 4 shows the splitter (e.g. 102), the phase modulators (e.g. 131 and 132) and the light combiner (e.g. 160) of the first and second optical combiners as a single element 180 referred to as a first phase modulation assembly 180 and a second phase modulation assembly 180’, respectively. This is indicated by the dashed line shown in Figure 1. A difference from the arrangement shown in Figure 1 is that instead of the outputs 161, 162 of the light combiner 160 being detected by the analyser 170, the system is arranged such that an output 190 of the first optical combiner 110 is directed to the second optical ned light signal which, in Figure 1, is provided by the light combiner 160. This output 190 provides the light source for the second phase modulation assembly 180’. In this way, light encoding data from the first data stream 141 and the second data stream 142 can be provided to the second optical combiner 110’. The second optical combiner 110’ can then encode further data from a third data stream 141’ and a fourth data stream 142’ onto that same light signal. In this way, an output 190’ of the second optical combiner 110’ may encode data from all four data streams 141, 142, 141’, 142’. Figure 5 shows an example of the second of the two possibilities mentioned above, in which the data stream carried by the combined light signal is used to control a phase modulator of the second data combiner. A difference from the arrangement shown in Figure 4 is that light is provided to the second phase modulation assembly 180’ by a second light source 110’ and that the output 190 of the first optical combiner 110 is provided to the second optical combiner 110’ as a data input. In other words, the output 190 may be used to control a phase modulator (e.g. the first phase modulator 131, not shown in Figure 5) of the second optical combiner 110’. The apparatus can be configured such that the combined light signal provided by the light combiner 160 is received by an analyser 170. The analyser 170 is configured to demodulate the data. The apparatus can be configured to use the data from the combined light signal to control a phase modulation of one of the arms (e.g., the first arm 121 as shown in Figure 1). In the illustrated example, the output 190 can be considered to take the place of the first data stream 141’. In some arrangements, the second light source 101’ may originate from the first light source 110, for example as a branch (not shown) of the first light source 101. In either of the above examples illustrated in Figures 4 and 5, it will be appreciated that further optical combiners may be provided in series, such that the output of one optical combiner can be used as an input of a subsequent optical combiner, whether as a light source input or as a data input. The disclosure also encompasses a plurality of optical data combiners that can be parallelized to combine data in parallel. For example, light from the light source 101 can be split into multiple paths, and then directed to the plurality of optical combiners, wherein each optical combiner may be an optical data combiner 110 as described in relation to Figure 1. In this way, the initial data streams can be combined in parallel by the plurality of optical combiners. Figure 6 shows an example of such a parallelized arrangement comprising a first optical combiner 110 and a second optical combiner ame or similar, and may be configured in the same or a similar manner, to the corresponding components described in relation to Figure 1. A difference from the arrangement in Figure 1 is that in the illustrated example the coherent light source 101 is configured to provide coherent light to a primary splitter 101a, and the primary splitter 101a is configured to split the coherent light between the particular, the system is arranged such that part of the coherent light is directed to the first phase modulation assembly 180 and part of the coherent light is directed to the second phase modulation assembly 180’. In this way, two data streams 141, 142 may be encoded on a light signal to provide a first output 190, and another two data streams 141’, 142’ may be encoded on a light signal to provide a second output 190’, which will be coherent with the first output 190. The parallelized system (e.g. described in relation to Figure 6) can be combined with the sequential system (e.g. described in relation to Figures 4 and 5) to enable parallel sequential data combination. It will be appreciated from the discussion above that the embodiments shown in the Figures are merely exemplary, and include features which may be generalised, removed or replaced as described herein and as set out in the claims. With reference to the drawings in general, it will be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein. It will be appreciated however that the functionality need not be divided in this way, and should not be taken to imply any particular structure of hardware other than that described and claimed below. The function of one or more of the elements shown in the drawings may be further subdivided, and / or distributed throughout apparatus of the disclosure. In some embodiments the function of one or more elements shown in the drawings may be integrated into a single functional unit. In some examples the functionality of the controller described herein may be provided by a general purpose processor, which may be configured to perform a method according to any one of those described herein. In some examples the controller may comprise digital logic, such as field programmable gate arrays, FPGA, application specific integrated circuits, ASIC, a digital signal processor, DSP, or by any other appropriate hardware. In some examples, one or more memory elements can store data and / or program instructions used to implement the operations described herein. Embodiments of the disclosure provide tangible, non-transitory storage media comprising program instructions operable to program a processor to perform any one or more of the methods described and / or claimed herein and / or to provide data processing apparatus as described and / or claimed herein. The controller may comprise an analogue control circuit which provides at least a part of this control functionality. An embodiment provides an analogue control circuit configured to perform any one or more of the methods described herein. The above embodiments are to be understood as illustrative examples. Further embodiments are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

Claims:

1. An optical data processing apparatus comprising: a first interferometer arm comprising: a first phase shifter for applying a first sequence of phase shifts to a first light signal in the first interferometer arm to encode a first data stream; and a delay-repeater configured to delay part of the first light signal and to recombine the delayed part with the first light signal; whereby the phase-shifted first light signal provided from the first interferometer arm encodes the first data stream and comprises the delayed part of the first light signal; a second interferometer arm comprising a second phase shifter for applying a second sequence of phase shifts to a second light signal in the second interferometer arm whereby the phase-shifted second light signal provided from the second interferometer arm encodes the second data stream; an optical splitter configured to split source light between the first interferometer arm and the second interferometer arm for providing the first light signal and the second light signal; a light combiner connected to receive the phase-shifted first light signal from the first interferometer arm and the phase-shifted second light signal from the second interferometer arm and to combine the phase-shifted first light signal and the phase-shifted second light signal to provide a combined light signal.

2. The optical data processing apparatus of claim 1 wherein the delay-repeater is configured so that, when the light received from the optical splitter comprises a coherent wavelength-swept light signal, said recombining of the delayed part provides at least one beat frequency component in the first light signal.

3. The optical data processing apparatus of claim 2 wherein the delay-repeater is configured so that the delayed part comprises a plurality of delayed parts each delayed by a respective corresponding one of a plurality of delays.

4. The optical data processing apparatus of claim 1 wherein the delay-repeater precedes the first phase shifter in the first interferometer arm. The optical data processing apparatus1 wherein the first phase shifter is integrated with the delay-repeater.

6. The optical data processing apparatus of any preceding claim wherein the first phase shifter and the second phase shifter are synchronised and the first data stream and the second data stream both comprise data symbols with the same unit interval.

7. The optical data processing apparatus of claim 6 comprising a wavelength swept light source configured to sweep the wavelength of the light periodically with a sweep period, the sweep period comprising a first interval and a second interval.

8. The optical data processing apparatus of any preceding claim comprising a controller and configured to superpose a first interval of the combined light signal with a second interval of the combined light signal to provide a superposed signal.

9. The optical data processing apparatus of claim 8, wherein the controller is further configured to determine frequency data representing frequency components of the superposed signal.

10. The optical data processing apparatus of claim 9 wherein each frequency component indicates a combination of a data symbol of the first data stream with a data symbol of the second data stream.

11. The optical data processing apparatus of claim 10 wherein the amplitude of the each frequency component indicates the combination.

12. The optical data processing apparatus of claim 10 or 11 wherein the controller is configured to use the frequency components to perform data operations on the first data stream and the second data stream.

13. The optical data processing apparatus of any of claims 8 to 12 wherein the first interval and the second interval each comprise a corresponding part of a wavelength sweep of the coherent wavelength- swept light signal, for example wherein the apparatus further comprises a coherent light source configured to provide the coherent wavelength-swept signal, for example wherein the coherent light source comprises a tunable laser.

14. The optical data processing apparatus of claim 13 wherein the coherent light source is configured to provide the wavelength sweep in a triangular waveform and the first interval and the second interval each comprise half a period of the triangular waveform.processing apparatus of claim 13 wherein the coherent light source is configured to provide the wavelength sweep in a sawtooth waveform and the first interval and the second interval each comprise a period of the sawtooth waveform.

16. The optical data processing apparatus of any preceding claim, wherein the optical splitter a 50:50 splitter.

17. The optical data processing apparatus of any preceding claim, wherein at least one of the first phase shifter and the second phase shifter is one of: a liquid crystal spatial light modulator, an acousto-optic modulator, and an electro-optic modulator.

18. The optical data processing apparatus of any preceding claim wherein the first phase shifter is configured so that the unit interval of the first data stream corresponds to the delay applied to the part of the light by the delay-repeater.

19. The optical data processing apparatus of any preceding claim wherein the delay-repeater comprises an optical path arrangement arranged to provide the light from the input to the output and to split the part from the light provided to the input.

20. The optical data processing apparatus of claim 19 wherein the optical path arrangement comprises a repeating path having an optical path length configured to provide said delay.

21. The optical data processing apparatus of claim 17 wherein the repeating path is a loop, and the delay is provided by the optical path length around said loop.

22. The optical data processing apparatus of claim 19, 20, or 21 wherein the optical path arrangement comprises a second optical23. The optical data processing apparatus of claim 22 as dependent upon claim 21 wherein the second optical splitter is positioned at a in said loop and provides an input to the delay-repeater.

24. The optical data processing apparatus of any preceding claim wherein the unit intervalequal to the delay provided by the delay- repeater.

25. An optical data processing method comprising: splitting a coherent wavelength-swept light signal to provide a first light signal and a second light signal; applying a delay to a part of the first light signal and recombining the delayed part into the first light signal and applying a first sequence of phase shifts to the delayed first light signal to encode, on the phase-shifted delayed first light signal, a first data stream having a unit interval corresponding to the delay; applying a second sequence of phase shifts to the second light signal to encode, on the phase-shifted second light signal, a second data stream having the unit interval; interfering the phase-shifted first light signal with the phase- shifted second light signal to provide a combined light signal encoding a combination of the first data stream and the second data stream.

26. The method of claim 25 wherein the unit interval is equal to the delay.

27. The method of claim 25 or 26 comprising combining a first interval of the combined light signal with a second interval of the combined light signal to provide a superposed signal.

28. The method of claim 27 wherein the first interval and the second interval each comprise a corresponding part of a periodic wavelength sweep of the coherent wavelength-swept light signal.

29. The method ofwherein the wavelength sweep comprises a triangular waveform and the first interval and the second interval each comprise half a period of the triangular waveform.

30. The method of claim 28 wherein the wavelength sweep comprises a sawtooth waveform and the first interval and the second interval each comprise a period of the sawtooth waveform.

31. The method of claim 29 or 30 further comprising determining frequency data representing frequency components of the superposed signal and using said frequency components to determine combinations of respective corresponding symbols from thestream and the second data stream.

32. An optical computing system comprising at least one data processing apparatus according to any of claims 1 to 24.

33. The optical computing system of claim 32 configured to perform the method of any of claims 25 to 32.

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