Apparatus for generating a plurality of electromagnetic data signals

Abstract

An apparatus for generating a plurality of output electromagnetic, EM, signals using a plurality of electrical data signals. The plurality of output EM signals comprising at least a first output EM signal (4a), and a second output EM signal (4b). The plurality of electrical data signals comprising at least a first electrical data signal (6a) and a second electrical data signal (6b). The apparatus (1) comprising an MMI splitter (8), a set of one or more first data modulators (16) and a set of one or more second data modulators (18). The apparatus further comprising a set of one or more EM combiners (24) comprising a further MMI. The apparatus configured to output the second output EM signal (4b) as the conjugate of the first output EM signal (4a).

Description

[0001] Apparatus for generating a plurality of electromagnetic data signals

[0002] Field of the invention

[0003] The field of the present invention is in electromagnetic signals, in particular but not limited to data for communications using IQ. modulated optical signals.

[0004] Background

[0005] Optical communications signals propagating over optical fibres reduce in power the further they propagate and typically require amplification for longer fibre spans. Optical amplifiers are commonly used to amplify optical signals. Optical amplifiers may take different forms, for example, Raman amplifiers, rare-Earth-ion-doped optical fibres such as the Erbium-doped fibre amplifier (EDFA) and semiconductor optical amplifiers (SOA). Optical amplification can take place using different physical phenomena. One example of a phenomena is using Raman scattering. Another is using population inversions via an electrical or optical pump that allow for stimulated transitions between the material energy levels of the amplifier medium. Parametric nonlinearities can also be used for amplification. These provide an instantaneous response via the or nonlinearities of the amplifier medium, typically via three or four-wave mixing (FWM). Optical-fibre amplifiers based on such phenomena are commonly referred to as Fibre-Optic Parametric Amplifiers (FOPA). The spectral operational characteristics of parametric amplifiers are dictated by the dispersion properties of the amplification medium and the wavelength of the pump wave which in turn allow for amplification of photons of wavelengths other than the incoming signal wavelength.

[0006] Parametric amplifiers may act as Phase-Insensitive Amplifiers (PIA's) or Phase-Sensitive Amplifiers (PSA's) depending upon the input light. The gain for PSAs is dependent upon the optical phase of the incoming optical signal wave, whereas for PIAs it is not. The operation of PSA's where all the noise inputs to the amplifier carry useful signals can be termed 'noiseless' because the PSA redistributes quantum noise among eigenmodes without adding any excess noise, hence, in theory, not reducing the signal to noise ratio (SNR) of a shot-noise limited input signal. However, a PIA adds extra noise and reduces the SNR. This is due to the additional quantum noise present at the amplifier input, for example at the idler wavelength.

[0007] To operate in the PSA regime, prior art has used a set-up as shown in figure 1. Figure 1 is a set-up proposed in 'Gain characteristics of a frequency nondegenerate phase-sensitive fiber-optic parametric amplifier with phase self-stabilized input', by Tang, R et al., Optics Express, Volume 13, No.26, 26 December 2005, pp 10483-10493. This work was a continuation of the work presented by the same author in 'Noise-figure limit of fiber-optical parametric amplifiers and wavelength converters: experimental investigation', by Tang, R., et al., Optics Letters, Volume 29, Issue 20, 2004, pp. 2372-2374. Figure 1 of the present application shows the 'schematic of the phase-sensitive parametric amplification model' by Tang. Signal light 'Si' and pump light 'Pu' are coupled via a first optical coupler Cl' into a first length of dispersion shifted fibre DSF1' acting as a PIA. The left-hand inset in figure 1 shows the different light components input into the PIA wherein the horizontal axis denotes light frequency. The PIA amplifies the signal Si and generates a phase conjugate idler 'Id' which is then input into a length of single mode fibre SMF' used for transmission (or a free space link) before being input into a second optical coupler C2'. Second optical coupler C2' splits the light into a first path Mo for monitoring and a further path for inputting the light into a second length of dispersion shifted fibre DSF2' which acts as a PSA due to the presence of all three of the pump Pu, Signal Si and Idler Id at its input. The three signals entering DSF2' are shown in the right-hand inset of figure 1. In the output from the four-wave mixing process, the pump is symmetrically surrounded by the signal Si and idler Id waves. The PSA outputs amplified idler Id and signal Si light and any remaining pump light Pu along output path O'. Typically, in use, the residual pump at the output of DSF1 is attenuated to below the signal and idler. At Mo the residual pump would be used to determine the frequency of the local pump laser, and a monitor at O' used to determine its phase.

[0008] The locally regenerated pump then needs to be injected into DSF2.

[0009] The prior art in figure 1 therefore copies the signal onto a conjugate idler wave to enable the second amplifier (DSF2') to act as a PSA. If the original signal wave Si is modulated with data, then the idler wave will contain a phase conjugate copy of this data. Despite the prior art using a PSA set-up, it still generates unwanted noise by generating the conjugate idler Id in DSF1'.

[0010] A further publication discusses this type of prior art set-up: 'Fiber-based phase-sensitive optical amplifiers and their applications', by Andrekson, P., and Karlsson, M., Vol. 12., No. 2, June 2020, Advances in Optics and Photonics pp367-428. Andrekson discusses that in such four wave mixing processes total power is conserved between the signal and idler waves through the Manley-Rowe relation such that no signal light can be generated without an equal amount of idler light at the same time. Andrekson further discusses that in the limit of high gain, a PSA gives 6dB higher gain compared to a PIA for the same pump power. This is turn leads to a 6dB advantage in signal to noise ratio (SNR) for the phase sensitive case. The prior art therefore generates the idler in a manner which may degrade SNR. Using a PIA to generate the idler signal for inputting into a PSA also increases the footprint of the total amplification system which is undesirable and increases the power consumption. The components used to form the PIA may also be expensive and the PIA system may be costly in time to configure for use.

[0011] Other documents include the following.

[0012] The paper 'Demonstration of a 4 x 4-port universal linear circuit', by A. Ribeiro et al., Optica, Vol. 3, No. 12, Dec 2016 describes a silicon implementation of a 4 x 4-port universal linear optical circuit that can perform a linear operations between its four input ports and output ports. The circuit consists of a network of thermally tuneable symmetric Mach-Zehnder interferometers with phase and amplitude control, in-circuit optical power monitors, and local software-controlled feedback loops.

[0013] The paper 'Silicon Photonic Single-Sideband Generation with Dual-Parallel Mach-Zehnder Modulators for Atom Interferometry Applications', by A. Kodigala et al., describes, and shows in figure la and 2 of the paper, a silicon photonic carrier-suppressed single-sideband (CS-SSB) modulator with dual-parallel Mach-Zehnder modulators (DP-MZMs) operating near 1550 nm with a measured carrier suppression of 30 dB and a sideband suppression (SSR) of 47.8 dB at 1 GHz with peak conversion efficiency of -6.846 dB (20.7%).

[0014] The paper 'Analysis of Optical Single Sideband Modulators for Radio Over Fiber Link with and without Second Order Sidebands', by P. Goel and R. Kaushik, Optics and Optoelectronics, Vol., 19, No. 4, December 2021, describes systems based on hybrid coupler with distinct phase angle and Dual-Parallel Dual-Drive Mach-Zehnder Modulator (DP-DDMZM).

[0015] The paper 'Optical Sideband Modulation in Silicon Photonics Platform Using Mach-Zehnder Interferometers', by M. Fasih et al., ICACT2022 February 13-16, 2022, compares different sideband modulation methodologies, namely single dual-drive MZM (DD-MZM) dual-parallel single-drive MZM, dual-parallel, dual-drive MZM (DP DD-MZM) with 2 RF modulation signals and dual-parallel dual-drive MZM with 4 RF modulation signals.

[0016] The paper 'Photonic Techniques for Generating a Single RF Sideband With No Second Order Sidebands', by C. Huang and E. H. W. Chan, IEEE Photonics Journal, Vol. 14, No. 1, February 2022, describes two structures that can realise optical single sideband modulation without generating both second order upper and lower sidebands. They are based on a dual-parallel Mach Zehnder modulator (DP-MZM) with an optical filter and a dual-polarisation dual-parallel Mach Zehnder modulator (DPol-DPMZM) with a 90° hybrid coupler.

[0017] Prior documents focus on Mach-Zehnder interferometer (MZI) implementations.

[0018] US2023 / 0261755 describes an optical transmitter including an I component optical modulation unit, a Q component modulation unit and a 2x2 optical coupler. US20110229150 describes an optical modulation apparatus including a first modulator, a second modulator, a multiplexer, a calculator and an adjustor. US2022 / 0043320 describes a control processor performing in a start up sequence of an IQ optical modulator using a nested MZI. The article "Effectiveness of phase-conjugated twin waves on fiber nonlinearity in spatially multiplexed all-optical OFDM system" by J. Hmood et al., Optical Fiber Technology 30, 2016, 147-152 describes to investigate the effectiveness of using phase conjugate twin waves technique to mitigate fiber nonlinear impairments in spatially multiplexed all- optical orthogonal frequency division multiplexing systems.

[0019] Summary

[0020] In a first aspect there is presented an apparatus (1) for generating a plurality of output electromagnetic, EM, signals using a plurality of electrical data signals; the plurality of output EM signals comprising at least a first output EM signal (4a), and a second output EM signal (4b); the plurality of electrical data signals comprising at least a first electrical data signal (6a) and a second electrical data signal (6b); the apparatus (1) comprising: A) a set of components (3) comprising; a) an EM splitter (8); the EM splitter (8) configured to: receive an input EM wave (10); and, divide the input EM wave (10) into a plurality of EM waves; the plurality of EM waves comprising at least a first EM wave (12) and a second EM wave (14) wherein the first EM wave (12) propagates along a spatially separate path to the second EM wave (14); b) a set of one or more first data modulators (16) configured to: receive the first EM wave (12); receive the first electrical data signal (6a); modulate the first EM wave (12) in accordance with the first electrical data signal (6a); c) a set of one or more second data modulators (18) configured to: receive the second EM wave (14); receive the second electrical data signal (6b); modulate the second EM wave (14) in accordance with the second electrical data signal (6b); the set of components (3) configured to generate: first and second data modulated EM waves (EMai, EMbi) for the first output EM signal (4a) using a modulated EM wave (EMa, EMb) output from each of the first and second sets of data modulators (16, 18); and, first and second data modulated EM waves (EMaii, EMbii) for the second output EM data signal (ED2) using the modulated EM wave (EMa, EMb) output from each of the first and second sets of data modulators; B) a set of one or more EM combiners (24) configured to: a) receive and combine the first and second data modulated EM waves (EMai, EMbi) for the first output EM signal (4a); b) receive and combine the first and second data modulated EM waves (EMaii, EMbii) for the second output EM signal (4b); c) output the first output EM signal (4a) onto at least a first output path (20); d) output the second output EM signal (4b) onto a second output path (22); the second output path (22) being spatially separate from the first output path (20); wherein the set of components (3) imparts a different phase relationship between the first (EMai) and second (EMbi) data modulated EM waves for the first output EM signal (4a) to, the phase relationship between the first (EMaii) and second (EMbii) data modulated EM waves for the second output EM signal (4b), such that the second output EM signal (4b) is the conjugate of the first output EM signal (4a).

[0021] The first aspect may be adapted according to any teaching herein including but not limited to any one or more of the following options.

[0022] Optionally, the EM splitter comprises a Multimode Interference coupler (MMI).

[0023] Optionally, the apparatus may be configured such that the EM splitter comprises at least one input and at least four outputs wherein the EM splitter divides light between each of the outputs.

[0024] Optionally the outputs have equal powers or substantially equal powers. The EM splitter may comprise four outputs for outputting first, second, third and fourth EM waves along spatially separate output paths.

[0025] Optionally, each of the at least four outputs of the MMI, outputs a portion of the input EM wave, wherein at least two, preferably three of the MMI output EM waves comprise a different phase to each of the other said outputs EM waves of the MMI. Optionally, two of the MMI output EM waves have the same phase.

[0026] Optionally, the EM splitter MMI comprises a multimode coupling region comprising opposing first and second sidewalls; the input EM wave is input into the multimode coupling region proximal to the first sidewall and distal from the opposing second sidewall.

[0027] Optionally, the EM splitter MMI comprises at least four inputs into the multimode coupling region wherein each input is associated with a different input waveguide; each input waveguide is configured to input EM radiation into the multimode coupling region at a coupling interface; the input EM wave being guided by the input waveguide closest to the first sidewall at the coupling interface.

[0028] Optionally, the first and second modulators comprise phase modulators.

[0029] Optionally the first and second data modulators comprise electro-optic modulators. Optionally the first and second electrical data signals comprise a symbol rate between 100 Mbaud - 4 Tbaud.

[0030] Optionally the first and second electrical data signals comprise a symbol rate between 100 Mbaud to a symbol rate of 140 Gbaud. The rate of 140 Gbaud is equal to a maximum bandwidth of coaxial cables.

[0031] Optionally the first and second electrical data signals comprise a symbol rate between 25 Gbaud - 4 Tbaud.

[0032] Optionally the first and second electrical data signals comprise a symbol rate between 25 Gbaud- 200 Gbaud.

[0033] Optionally, the first and second electrical data signals comprise a highest frequency component between 1 GHz and 4 THz.

[0034] Optionally, the first and second electrical data signals comprise a highest frequency component between 1 and 500GHz.

[0035] Optionally, the first and second electrical data signals comprise a highest frequency component between 1 and 140GHz.

[0036] Optionally, the first electrical data signal is associated with the I component of IQ modulator data; the second electrical data signal is associated with the Q component of the IQ modulator data.

[0037] Optionally, the plurality of EM waves output by the EM splitter comprises the first EM wave, the second EM wave, a third EM wave and a fourth EM wave; the apparatus further comprises: a third data modulator configured to: receive the third EM wave; receive a third electrical data signal; modulate the third EM wave in accordance with the third electrical data signal; a fourth data modulator configured to: receive the fourth EM wave; receive a fourth electrical data signal; modulate the fourth EM wave in accordance with the fourth electrical data signal.

[0038] Optionally, the third electrical data signal is associated with the I component of IQ modulator data; the fourth electrical data signal is associated with the Q component of IQ modulator data.

[0039] Optionally: a) the first data modulator imparts a phase change to the first EM wave; the third data modulator imparts a phase change to the third EM wave that is a different phase change to the phase change to the first EM wave; b) the second data modulator imparts a phase change to the second EM wave; the fourth data modulator imparts a phase change to the fourth EM wave that is a different phase change to the phase change to the second EM wave.

[0040] Optionally, the set of one or more EM combiners comprises a further Multimode Interference coupler, MMI.

[0041] Optionally the MMI of the set of one or more EM combiners comprises first, second, third and fourth input waveguides and at least two output waveguides wherein one of the two output waveguides outputs the first EM output signal and the other of the two output waveguides output the second EM output signal.

[0042] Optionally, the apparatus is configured such that said first, second, third and fourth input waveguides of the further MMI each receive modulated EM radiation from a different one of the first, second, third and fourth data modulators.

[0043] Optionally, the apparatus is configured such that: I) the further MMI comprises a multimode coupling region comprising an input coupling interface; II) the first, second third and fourth input waveguides of the further MMI are respectively disposed sequentially along an input coupling interface.

[0044] Optionally, where the EM splitter: a) comprises an MMI for dividing the input EM wave: and, b) comprises the at least four outputs; the MMI for dividing the input EM wave further comprises: i) an output coupling interface; and, ii) first, second, third and fourth output waveguides respectively disposed sequentially along the output coupling interface.

[0045] Optionally, for when the EM splitter comprises an MMI comprising first, second, third and fourth output waveguides: I) the first output waveguide outputs the first EM wave towards the first data modulator; the second output waveguide outputs the second EM wave towards the second data modulator; the third output waveguide outputs the third EM wave towards the third data modulator; the fourth output waveguide outputs the fourth EM wave towards the fourth data modulator; II) the first input waveguide to the further MMI receives the data modulated first EM wave; the second input waveguide to the further MMI receives the data modulated second EM wave; the third input waveguide to the further MMI receives the data modulated third EM wave; the fourth input waveguide to the further MMI receives the data modulated fourth EM wave.

[0046] Optionally, for when the EM splitter comprises an MMI comprising first, second, third and fourth output waveguides: I) the first output waveguide outputs the first EM wave towards the first data modulator; the second output waveguide outputs the third EM wave towards the third data modulator; the third output waveguide outputs the second EM wave towards the second data modulator; the fourth output waveguide outputs the fourth EM wave towards the fourth data modulator; II) the first input waveguide to the further MMI receives the data modulated first EM wave; the second input waveguide to the further MMI receives the data modulated third EM wave; the third input waveguide to the further MMI receives the data modulated second EM wave; the fourth input waveguide to the further MMI receives the data modulated fourth EM wave.

[0047] Optionally, the first second, third and fourth modulators operate in a push-pull modulation scheme. Optionally, the first and second modulator operate together in a push-pull modulation scheme. Optionally, the third and fourth modulator operate together in a push-pull modulation scheme.

[0048] There is further presented a modulator system comprising the apparatus of the first aspect and optionally any of the optional adaptions to the said apparatus; the modulator system further comprising at least one or more of: I) one or more electrical controllers for outputting: i) the first electrical data signal and the second electrical data signal; and, ii) optionally, the third and fourth electrical data signals; II) one or more EM sources for generating the input EM wave.

[0049] In a second aspect there is presented a method for generating a plurality of output electromagnetic, EM, signals using a plurality of electrical data signals; the plurality of output EM signals comprising at least a first output EM signal (4a), and a second output EM signal (4b); the plurality of electrical data signals comprising at least a first electrical data signal (6a) and a second electrical data signal (6b); the method comprising: A) using: a) an EM splitter (8) to: receive an input EM wave (10); and, divide the input EM wave (10) into a plurality of EM waves; the plurality of EM waves comprising at least a first EM wave (12) and a second EM wave (14) wherein the first EM wave (12) propagates along a spatially separate path to the second EM wave (14); b) a set of one or more first data modulators (16) to: receive the first EM wave (12); receive the first electrical data signal (6a); modulate the first EM wave (12) in accordance with the first electrical data signal (6a); c) a set of one or more second data modulators (18) to: receive the second EM wave (14); receive the second electrical data signal (6b); modulate the second EM wave (14) in accordance with the second electrical data signal (6b); the set of components (3) configured to generate: first and second data modulated EM waves (EMai, EMbi) for the first output EM signal (4a) using a modulated EM wave (EMa, EMb) output from each of the first and second sets of data modulators (16, 18); and, first and second data modulated EM waves (EMaii, EMbii) for the second output EM data signal (ED2) using the modulated EM wave (EMa, EMb) output from each of the first and second sets of data modulators; the EM splitter, set of set of one or more first data modulators (16) and set of one or more second data modulators (16) forming at least part of a set of components (3); B) using a set of one or more EM combiners (24) to: a) receive and combine the first and second data modulated EM waves (EMai, EMbi) for the first output EM signal (4a); b) receive and combine the first and second data modulated EM waves (EMaii, EMbii) for the second output EM signal (4b); c) output the first output EM signal (4a) onto at least a first output path (20); d) output the second output EM signal (4b) onto a second output path (22); the second output path (22) being spatially separate from the first output path (20); wherein the set of components (3) imparts a different phase relationship between the first (EMai) and second (EMbi) data modulated EM waves for the first output EM signal (4a) to, the phase relationship between the first (EMaii) and second (EMbii) data modulated EM waves for the second output EM signal (4b), such that the second output EM signal (4b) is the conjugate of the first output EM signal (4a).

[0050] The second aspect may be adapted according to any teaching herein including, but not limited to including any of the optional features described above for the first aspect.

[0051] Brief list of figures

[0052] Embodiments of the present invention will now be described in detail with reference to the accompanying drawings, in which:

[0053] Figure 1 is a schematic example of a prior art system for generating a conjugate signal;

[0054] Figure 2a shows an example of IQ. modulation;

[0055] Figure 2b shows a further example of IQ modulation wherein the Q signal has reduced amplitude;

[0056] Figure 2c shows an example of a constellation diagram of 16-QAM;

[0057] Figures 3a and 3b shows schematic examples of the apparatus described herein;

[0058] Figure 4 shows an example of the apparatus using two MMI's;

[0059] Figure 5 shows a further example of the apparatus using two MMI's;

[0060] Figure 6 shows an example of the apparatus using MZIs and directional couplers;

[0061] Figure 7 shows a further example of an apparatus based on using EM polarisation.

[0062] Detailed description

[0063] There is presented examples of generating an electromagnetic (EM) data signal and its phase conjugate. The examples take the form of apparatus, methods and systems, wherein examples describing apparatus and / or systems may describe configurations and components useable in associated methods and vice versa.

[0064] The present application can apply to different EM wavelengths including, but not limited to, any of the ITU telecommunication bands. The telecommunication bands include any of, but not limited to: the O-band at 1260-1360nm; the E-band at 1360-1460nm; the S-band at 1460-1530nm; the C-band at 1530-1565nm; the L-band at 1565-1625nm; the U-band at 1625-1675nm. Other wavelengths such as 1064nm and / or bands centred around 1064 nm may be used.

[0065] For purposes of discussion, the EM waves and their amplification, modulation and transformation shall be referred to in any example herein as 'optical', however it should be appreciated that the systems, apparatus and methods described herein may be used non-optical EM waves.

[0066] If one treats the total amplification process as generating the conjugate signal and amplifying the data signal using a PSA, the examples in the present application: a) do not degrade the SNR to the same extent as the prior art described by Tang, and others, b) presents alternative ways of generating a conjugate signal. Other advantages of the examples presented herein are described elsewhere in this document.

[0067] The present application concerns EM data signals. The data signals are typically used for Quadrature Amplitude Modulation (QAM) systems using IQ modulation signals wherein 'I' refers to "in-phase" and Q refers to "quadrature". Despite the apparatus, methods and systems presented herein being preferably used for QAM, there may be other uses, for example obtaining a signal / conjugate pair for image processing.

[0068] Returning to IQ modulation, this modulation type typically comprises two sinusoids that have the same frequency but are 90° out of phase. The I signal is typically referred to as a cosine waveform, whilst the Q signal is typically referred to as a sine waveform. The I and Q signals are amplitude modulated such that the summed combination of the two amplitude-modulated waveforms gives a resultant waveform. Changing the amplitudes of the I and Q signals results in modulating the combined signal in amplitude and, in some circumstances, phase. If both I and Q signals are amplitude-modulated to the same amount, then the resulting amplitude of the combined signal changes but the phase of the combined signal does not change.

[0069] Figure 2a shows an example of an IQ modulated signal wherein the E-field amplitudes of an 'I' optical signal and a corresponding 'Q' optical signal are shown over time (t) and both I and Q are equal in amplitude. The combined signal 'l+Q' is shown to be a summation of the two I and Q signals where the 'l+Q' phase is such that the l+Q peak amplitude peaks midway between the peak amplitudes of the I and Q signals. The vertical axis E(Amp)(+ / -) is electrical field (E-field) amplitude of the EM signals which takes +ve and -ve values, rather than EM signal intensity which is proportional to the square of the E-field amplitude. Typical IQ. modulation uses I and Q signals of the same frequency; however, the apparatus is not limited to using I and Q signals of the same frequency.

[0070] Figure 2b shows a similar IQ modulated signal to figure 2a wherein the I signal has the same E-field amplitude as in figure 2a, however the Q signal is still at the same phase but has been amplitude modulated such that is has a reduction in E-field amplitude. The resultant l+Q signal not only has a corresponding reduction in amplitude, but also experiences a phase change. Figure 2b shows this phase change by the shift in l+Q peak amplitude from the l+Q peak amplitude M2a of figure 2a to the l+Q peak amplitude M2b of figure 2b.

[0071] Figure 2c shows an example of a QAM constellation diagram wherein the amplitude of the I signal is on the x-axis, shown in relative units upto a magnitude of 3 in both +ve and -ve E-field. The amplitude of the Q signal is on the y-axis, shown in relative units upto a magnitude of 3 in both +ve and -ve E-field. The amplitudes of each of the I and Q signals take any of the set {-3, -1, 1, 3} , which gives rise to sixteen possible points (labelled 'Pt' in figure 2c), otherwise known as 'symbols'. A given combined signal, for example represented by point Ptl is formed from an I signal with a relative amplitude of 1 to a Q-signal's relative amplitude of 3, hence a combined signal of [+11 and +3Q], A phasor Ph points from the origin to Ptl. The conjugate combined signal represented by point Ptl* is also shown in figure 2c which has a combined signal of [+11 and -3Q], In examples of the apparatus presented herein, that are for communications purposes, the conjugate signal may be equivalent to the idler signal propagated along with the signal, as depicted in the right-hand side inset of figure 1.

[0072] Figure 3a is a schematic example of an apparatus 2 for generating a plurality of electromagnetic, EM, data signals 4a, 4b using a plurality of electrical data signals 6a, 6b. The example in figure 3a is only intended to pictorially describe the apparatus 2 and does not physically represent the sizes and relative positions of features. In other examples herein the plurality of electromagnetic, EM, data signals 4a, 4b may be referred to as output EM signals or output EM data signals.

[0073] The plurality of EM data signals comprises at least a first EM data signal 4a and a second EM data signal 4b. The apparatus 2 is not limited to generating two EM data signals and may generate other EM signals.

[0074] The plurality of electrical data signals comprises at least a first electrical data signal 6a and a second electrical data signal 6b. The apparatus 2 is not limited to using two electrical data signals and may use other electrical data signals. The apparatus 2 comprises an EM splitter 8 configured to receive an input EM wave 10 and divide the input EM wave into a plurality of EM waves 12, 14. The input EM wave 10 is preferably a continuous wave (CW) waveform, however pulses of EM radiation may also be used. The input EM wave 10 is preferable a temporally coherent EM wave output from an EM source such as, but not limited to, a laser. The plurality of EM waves comprise at least a first EM wave 12 and a second EM wave 14. The first EM wave 12 propagates along a spatially separate path to the second EM wave 14.

[0075] The apparatus 2 further comprises a first data modulator 16 configured to receive the first EM wave 12 and the first electrical data signal 6a. The first data modulator 16 is configured to modulate the first EM wave 12 in accordance with the first electrical data signal 6a. For examples where the apparatus is used for IQ. modulation, the first electrical data signal 6a may comprise a plurality of electrical signals for modulating the first EM wave 12 according to one of the IQ signals, for example the first data modulator 16 may be used to generate I signals.

[0076] The apparatus 2 further comprises a second data modulator 18 configured to receive the second EM wave 14 and the second electrical data signal 6b. The second data modulator 18 is configured to modulate the second EM wave 14 in accordance with the second electrical data signal 6b. For examples where the apparatus 2 is used for IQ modulation, the second electrical data signal 6b may comprise a plurality of electrical signals for modulating the second EM wave 14 according to one of the IQ signals, for example the second data modulator 18 may be used to generate Q signals.

[0077] The apparatus 2 further comprises a set of one or more components. The set of one or more components comprises a plurality of output paths. The plurality of output paths comprises at least a first output path 20 and a second output path 22. The second output path 22 being spatially separate from the first output path 20.

[0078] The set of one or more components further comprises a set of one or more EM combiners 24. The said set of EM combiners 24 is configured to receive at least a portion of the data-modulated first EM wave 26. The said set of EM combiners 24 is further configured to receive at least a portion of the data-modulated second EM wave 28. The said set of EM combiners 24 is further configured to combine 30 the received data-modulated first EM wave 26 with the received data-modulated second EM wave 28. The said set of EM combiners 24 is further configured to output a first portion 32 of the combined data-modulated EM wave 30 onto at least the first output path 20 as the first EM data signal 4a. The said set of EM combiners 24 is further configured to output a second portion 34 of the combined data-modulated EM wave 30 onto at least the second output path 22 as the second EM data signal 4b. The apparatus 2 is further configured to impart a phase relationship c|) between the data-modulated first EM wave 26 and the data-modulated second EM wave 28 such that the second EM data signal 4b comprises a conjugate of the first EM data signal 4a.

[0079] Figure 3b shows a furthermore detailed example similar to figure 3a wherein like references represent like components and optional adaptions for figures 3a may apply to figure 3b and vice versa. Figure 3b may be adapted according to any teaching herein including, but not limited to, any of the features of: the input EM signal, the EM splitter, the EM combiner(s), the output EM signals, any of the EM waves generated or output between the EM splitter and EM combiners(s); the electrical signals.

[0080] In figure 3b the apparatus 1 comprises a set of components 3 that includes the EM splitter and the data modulators. In figure 3b there is set of one or more first data modulators (16) and a set of one or more second data modulators (18). Thus, each data modulator set may comprise a single modulator, two modulators or two or more modulators. Where multiple modulators are used in each data modulator set, an output data modulated EM wave may be output from each respective modulator.

[0081] The set of components 3 is configured to generate first and second data modulated EM waves EMai, EMbi for the first output EM signal 4a using a modulated EM wave EMa, Emb output from each of the first and second sets of data modulators 16, 18. Furthermore, the set of components 3 is configured to generate first and second data modulated EM waves EMaii, EMbii for the second output EM data signal ED2 using the modulated EM wave EMa, EMb output from each of the first and second sets of data modulators. The components in the set that generate EMai, EMbi, EMaii, and EMbii may differ. For example, the components in the set 3 may be the EM splitter together with four data modulators (see figures 4 and 5 as examples). Alternatively, in other example, the components in the set 3 may comprise an EM splitter, two or four data modulators, two further EM splitters and two phase shifters (see figure 6 for example).

[0082] The apparatus comprises a set of one or more EM combiners 24. The said combiners receive and combine the first and second data modulated EM waves EMai, EMbi for the first output EM signal 4a. The said combiners receive and combine the first and second data modulated EM waves EMaii, EMbii for the second output EM signal 4b. In some examples a single EM combiner may be used to do this, see for example figures 4 and 5. Alternatively in another example (see figure 6) multiple EM combiners may be used. The said combiners further output: the first output EM signal 4a onto at least a first output path 20; and, the second output EM signal 4b onto a second output path 22. The second output path 22 is spatially separate from the first output path 20. In some examples, the set of one or more combiners may also form the set of abovesaid set of components. For example, the EM splitter for receiving the input EM wave may also act as the EM combiner. In this alternative example the data modulators may be reflective in that they data modulate incoming EM waves but output the data modulated EM wave along the same input path that the incoming EM wave was received along. In some embodiments using reflective modulators, the outputs 104a and 104b shown in figures 4 and 5a would appear on ports 109b (for 104a) and either 109c (figure 4) or 109b (figure 5a). In other embodiments, the apparatus may use optical isolating elements such as circulators to separate out (onto a separate path for transmission to other apparatus) the input EM wave and the output EM signals present at 109a.

[0083] Turning back to the example shown in figure 3b, the set of components 3 imparts a different phase relationship between the first EMai and second EMbi data modulated EM waves for the first output EM signal 4a to, the phase relationship between the first EMaii and second EMbii data modulated EM waves for the second output EM signal 4b. By doing this the apparatus outputs the second output EM signal 4b as the conjugate of the first output EM signal 4a.

[0084] The present apparatus has a reduced power consumption compared to prior art, such as parametric amplifier-based copiers (Figure 1) or independent modulators for independent signal and conjugate generation. Furthermore, unlike prior art where conjugate signals are generated at the expense of increased noise and system footprint (at least due to multiple optical amplifiers as shown in figure 1 or additional electrical amplifiers), the present apparatus 2 is configurable to develop the conjugate signal, with a smaller footprint, without the additional noise and with the same set of components used to generate the (non-conjugate) data signal.

[0085] The apparatus 2 is configurable to output equal amounts of idler and signal amplitude compared to prior art where unequal amounts of signal and idler often need to be compensated for with further components and / or processes.

[0086] When compared to systems of the prior art that use relatively static phase shifters, such as thermooptic heaters, to configure a fixed optical transfer matrix, the present apparatus 2 operates at the rate of the data signal and provides electro-optic conversion.

[0087] Different example implementations of the apparatus are described underneath. The implementations may be optionally used for any of, but not limited to, the following applications optical data modulation; coherent optical systems; Quadrature amplitude modulation, IQ modulation, other applications such as image processing. In one example implementation, the apparatus takes advantage of utilising transfer functions of MMI couplers which impart different phases to the output light paths. Transfer functions have been examined for cascaded Mach-Zehnder Interferometer (MZI)- based structures and directional couplers such as in the following.

[0088] The paper 'Implementation of Optical Neural Network (ONN) based on Mach-Zehnder interferometer array', by D. Yanan, et al., IET Optoelectronics, 8thNovember 2022, describes the implementation methods of ONN matrix multiplication based on MZIs, the implementation methods of non-linear activation, and the on-chip training methods. The paper 'Optical Neural Network with Reduced Phase Shifters Using Multi-Plane Light Conversion', by R. Tanomura et al., OECC / PSC 2022, WC3-3, describes Optical Neural Network (ONN) architectures, using integrated optical unitary converters (OUCs) wherein "power-efficient" (ONN) are realised by using OUCs based on the multiplane light conversion (MPLC) mechanism. The paper 'Robust Integrated Optical Unitary Converter Using Multiport Directional Couplers', by R. Tanomura et al., Journal of Lightwave Technology, Vol. 38, No. 1, January 1, 2020, describes a silicon photonic 4x4 OUC based on a structure consists of cascaded multiport directional couplers and phase shifter arrays.

[0089] The above cascaded structures use either MZIs or multiport directional couplers.

[0090] Various works have been done in development of integrated optic Multimode Interference couplers (MMI's) including the paper 'Integrated Optical Unitary Converter based on Nonuniform Multimode Interference Coupler', by R. Tanomura et. al., 2021 IEEE Photonics Conference (IPC), which describes a multiple MMI-based structure. The paper describes the use of such a structure for 'all-optical multi-input and multi-output (MIMO) processing in the mode-division multiplexing (MDM) systems, the optical accelerator for deep neural networks, and task-specified quantum computing, such as Ising machines and boson sampling'. In the paper, a monolithic InP-based optical unitary converter (OUC) with nonuniform MMI couplers is used for reconfigurable 4-mode sorting. The structure consists of the N x MMI couplers and N +1 phase shifter arrays. A transfer matrix 'U' was described for the whole device wherein each MMI had a transfer matrix 'M' wherein the required condition of M was to provide dense fixed unitary conversion. This paper focussed on the reconfigurable output transmittance of the structure.

[0091] The paper 'Reconfigurable all-optical on-chip MIMO three-mode demultiplexing based on multiplane light conversion', by R. Tang et al., Optics letters, Vol. 43, no. 8, 15thApril 2018, describes a reconfigurable all-optical on-chip multi-input-multi-output three-mode demultiplexing based on multi-plane light conversion. The demultiplexer consists of cascaded phase shifter arrays and multimode interference couplers integrated on a compact silicon chip. By optimizing the phase shifters, reconfigurable three-mode demultiplexing is experimentally realized. The phase shifters are thermo-optic and ate therefore not used for data manipulation at data rates, but rather to configure different channels for different wavelengths. A related paper 'Reconfigurable Integrated MIMO Optical Mode Demultiplexer using MMI Couplers', by R. Tang et al., CLEO 2017, describes a series of NxN MMI couplers and phase shifter arrays that make a reconfigurable integrated MIMO optical mode demultiplexer. Another related paper 'Integrated Reconfigurable Unitary Optical Mode Converter Using MMI Couplers' by R. Tang et al., IEEE Photonics Technology Letters, Vol. 29, No. 12, June 15, 2017, describes a unitary optical mode converter for mode-division-multiplexed (MDM) optical communication, imaging, and quantum computation. The structure uses cascaded multimode interference couplers and phase shifter arrays wherein the optimal phase shifter condition was determined by the simulated annealing method.

[0092] Reviews with both MMI and MZI structures include the paper 'Photonic Integrated Unitary Processor based on Multi-Plane Light Conversion', by T. Tanemura et al., OFC 2022, M1I.6, which describes optical unitary processors (OUP) that can transform N input optical modes into another arbitrary set of N orthogonal modes in a reconfigurable manner. Another paper in this area is 'Photonic matrix multiplication lights up photonic accelerator and beyond', by H. Zhou et al., Light: Science & Applications ( 2022) 11:30.

[0093] In the above works using MMI's, the phase shifters used are (relatively) static compared to the data rate and are used for applications such as wavelength demultiplexing, Ising machines or Boson sampling.

[0094] Figure 4 shows an example of an apparatus 100 for generating a first EM data signal and a second EM data signal conjugate to the first EM data signal. The apparatus 100 comprises a plurality of MMI couplers.

[0095] Figure 4 is a schematic diagram of a planar light-wave circuit ('PLC') showing the components used wherein the absolute position and size of features in the figure does not indicate actual position and size of the features. Arrows between components should be interpreted as waveguides connecting the components and optically linking them. The components used for this example are preferably integrated-optic waveguides components, however other components may be used within the setup such as optical fibres and free space bulk optic components. Preferably the waveguides are integrated onto a single chip via monolithic or hybrid integration. The waveguides in this example, as in other examples, are 'optically linked' in that the light guided by the waveguides is guided by the light-wave circuit from one component to the next via the substantial continuation of a waveguide core material linked and physically connecting components, although it is understood that some components may include small breaks in waveguide core material along the direction of propagating of the light. Examples of break of linking waveguide core material is waveguide segmentation and the propagation of the guided light across a gap to go from one component to another where the components are formed in different fabrication processes and subsequently integrated. The waveguides are preferably formed of solid core and cladding materials, preferably dielectric or semiconductor materials, for example, but not limited to silica, polymer, silicon, GaAs, Lithium Niobate.

[0096] In figure 4, some of the reference labels for the broader example of figures 3a / 3b are also included for understanding purposes. This example assumes that the waveguides are nominally single mode waveguides carrying the fundamental TE and / or TM mode of the input light 310. References to 'light' in this and other examples herein may be interpreted to also refer to other electromagnetic radiation wavelengths. Any of the above details for the physical implementations of the apparatus of figures 3a / 3b may apply to other examples herein including the examples in figures 5 and 6.

[0097] The apparatus 100 comprises a first MMI 108 comprising four output waveguide channels 111a- llld. The apparatus also comprises a second MMI 124 comprising four input waveguides 121a-d, and four output waveguides 123a-d. The first MMI is also shown with four input waveguide channels 109a-d, however in some examples fewer input waveguides may be used, for example only one input waveguide 109a may be used.

[0098] For the first MMI 108, the output waveguides llla-d are continued in the PLC to physically, and optically, couple with spatially separate optical phase modulators 116, 117, 118 and 119 wherein each output waveguide couples light into a different one of the four phase modulators 116, 117, 118, 119. The phase modulators 116, 117, 118, 119 may be termed 'fast modulators' for purposes of describing the apparatus 100 herein. The optical phase modulators 116, 117, 118, 119 receive electrical control signals (not shown in figure 6) that are used by the fast modulators 116, 117, 118, 119 to impart a phase change to light propagating along the arm and through the phase modulators 116, 117, 118, 119.

[0099] Light output from the fast phase modulators 116, 117, 118, 119 is input into the four input channels 121a-d of the second MMI 124 such that light output from each fast phase modulator 116, 117, 118, 119 is input into a different one of the input channels 121a-d of the second MMI 124.

[0100] The MMI 108 comprises a multimode section 113, also referred to as the 'slab' or 'mode coupling region' or 'multimode coupling region' that supports a plurality of spatial modes of the same polarisation. Preferably the multimode section is rectangular in cross section, plan view and side view. The multimode section 113 comprises a first side wall 115a and an opposing second sidewall 115b that define the plan width of the section 113. Joining the first and second sidewalls 115a, 115b are front 115c and rear 115d wall portions that reside either side of the input and output waveguides 109a-d, llla-d. The input and output waveguides 109a-d, llla-d are preferably formed of the same core material as the slab. Preferably the core material is continuous between the slab and the adjoining input and output waveguides 109a-d. The MMI 108 comprises two peripheral input waveguides 109a and 109d. Peripheral input waveguide 109a is proximal to sidewall 115a and is the closest input waveguide to sidewall 115a. Peripheral input waveguide 109d is proximal to sidewall 115b and is the closest input waveguide to sidewall 115b. The MMI 108 comprises two peripheral output waveguides Illa and llld. Peripheral output waveguide Illa is proximal to sidewall 115a and is the closest output waveguide to sidewall 115a. Peripheral output waveguide 109d is proximal to sidewall 115b and is the closest output waveguide to sidewall 115b.

[0101] The first input waveguide 109a may be substantially opposite the first output waveguide Illa. The second input waveguide 109b may be substantially opposite the second output waveguide 111b. The third input waveguide 109c may be substantially opposite the third output waveguide 111c. The fourth input waveguide 109d may be substantially opposite the fourth output waveguide llld. The input waveguides 109a-d are sequentially disposed along the front of the MMI slab 113 and are spaced apart from each other by front wall portions 115c. Similarly, the output waveguides llla-d are sequentially disposed along the rear of the MMI slab 113 and are spaced apart from each other by rear wall portions 115d.

[0102] The structure, design and function of the second MMI 124 may be substantially similar to that described for MMI 108. Accordingly, the first input waveguide 121a may be substantially opposite the first output waveguide 123a. The second input waveguide 121b may be substantially opposite the second output waveguide 123b. The third input waveguide 121c may be substantially opposite the third output waveguide 123c. The fourth input waveguide 121d may be substantially opposite the fourth output waveguide 123d. The input waveguides 121a-d are sequentially disposed along the front of the MMI slab 135 and are spaced apart from each other by front wall portions 137c. Similarly, the output waveguides 123a-d are sequentially disposed along the rear of the MMI slab 135 and are spaced apart from each other by rear wall portions 137d.

[0103] The fast modulators 116, 117, 118, 119 in this example are monolithically or hybrid integrated into the PLC containing the MMI waveguides and associated connecting waveguides. In other examples, other means for coupling light to and from the fast modulators 116, 117, 118, 119 may be used such as optical fibres.

[0104] Light output from output waveguide Illa is input into fast modulator 116. Light output from fast modulator 116 is input into input waveguide 121a. Light output from output waveguide 111b is input into fast modulator 117. Light output from fast modulator 117 is input into input waveguide 121b. Light output from output waveguide 111c is input into fast modulator 118. Light output from fast modulator 118 is input into input waveguide 121c. Light output from output waveguide llld is input into fast modulator 119. Light output from fast modulator 119 is input into input waveguide 121d. The coupling between fast modulators 116, 117, 118, 119 and connecting waveguides that couple light to the MMIs 108, 124 is preferably end-on coupling.

[0105] Preferably the following optical paths are equal or substantially equal: a) the optical path light takes from the slab 113 of MMI 108 to the slab 135 of MMI 124, via fast modulator 116; b) the optical path light takes from the slab 113 of MMI 108 to the slab 135 of MMI 124 via fast modulator 117; c) the optical path light takes from the slab 113 of MMI 108 to the slab 135 of MMI 124 via fast modulator 118; d) the optical path light takes from the slab 113 of MMI 108 to the slab 135 of MMI 124 via fast modulator 119. In alternative examples the said optical paths may be unequal but the differences in phase resulting from the inequality of optical path length may be circumvented by extra phase delay imparted from one or more phase shifters along the paths between the two slabs 113, 135.

[0106] The MMIs 108 and 124 may have physical dimensions and material choices that accord to a particular MMI design. The design of the MMI may vary to change the intensity of light exiting each of the output waveguides given light incident upon the slab from a particular input waveguide. In figure 4, MMI 108 is a 4x4 port MMI coupler and MMI 124 is a 4x4 port MMI coupler. Other couplers may be used, including couplers having more ports. The apparatus 100 shown in Figure 4 and described herein may be adapted to use a single MMI on either the input side before the fast modulators 116-119 or on the output side after the said fast modulators 116-119 wherein the functions of splitting and or combining light provided by the MMIs 108, 124 may be replicated using other components such as: waveguide delay lines to provide requisite phase shifts along optical paths; other forms of couplers to split and combine light such as one or more directional or Y branch couplers or star couplers.

[0107] Figure 4 will now be described with reference to operation and transformation of light. In brief, input light 110 is shown being input into the peripheral input waveguide 109a, however in principle the light 110 may be coupled into peripheral waveguide 109d. The input light is split into four portions by the slab 113 wherein each portion is output from the slab 113 along a different one of the four output waveguides llla-d. Each portion of light comprises a particular phase such that the portions of light output along the output waveguides comprise a phase relationship with each other. Each of the fast phase modulators 116, 117, 118, 119 receive one of either the I or Q. electrical signals (not shown in the diagram). The electrical signals introduce a phase change to the light propagating along the respective phase modulator. The four portions of light, each being imparted a phase change according to the respective electrical signal, then enter the second MMI 124 wherein: a) the phase relationship of the four light portions entering the slab 135; and b) the modal evolution and coupling properties of the slab 135; entail that the light exiting the output waveguides 123a-d at least comprises an output waveguide 123b carrying the desired EM signal 104a and another different output waveguide 123d carrying the conjugate 4b of the signal 4a. The intensity changes and phase changes to input light of an MMI slab may be determined by an MMI transfer function associated with the slab. The transfer function may be dependent upon the position (along the front wall of the slab) the input light is incident.

[0108] There now follows a more detailed description of the operation of the apparatus 100 of figure 4 using MMI transfer functions. For this description: input waveguide 109a is referred to as input 1; input waveguide 109b is referred to as input 2; input waveguide 109c is referred to as input 3; input waveguide 109d is referred to as input 4; fast modulator 116 is referred to as dl; fast modulator 117 is referred to as d2; fast modulator 118 is referred to as d3; fast modulator 119 is referred to as d4.

[0109] A 4x4 port MMI coupler may have the following transfer function given in Equation 1 underneath:

[0110] [Equ. 1] where the relationships between a, b, and c in Equation 1 are provided by Equation 2 underneath. [Equ. 2]

[0111] Equations 1 and 2 represent a power symmetric 4x4 MMI coupler.

[0112] In Equation 2, represents the product of the product of the coupling coefficient between nonadjacent waveguides and the length of the slab 113. The length of the slab 113 is the dimension extending directly from a front wall portion (for example 115c) to the opposite backwall portion (for example 115d that runs parallel to the extent of the sidewalls (for example 115a, 115b).

[0113] When ( / >= TC / 8, Equations 1 and 2 yield the '90-degree hybrid', which may be used in coherent receivers wherein the phase difference between signals input to two adjacent input ports changes by 0, 7t / 2, 7t, and - n / 2 between each output port. The MMI's in figure 4 preferably use <f> = TT / 8.

[0114] The outputs from the 4x4 MMI, with a single EM input to an input port (nearest a wall), for example port 109a, are (n / 8, -5n / 8, -7K / 8, -5n / 8). The imparted phases listed are in order of the output port directly opposite the input port followed sequentially by the other output ports ending in the output port further away from the input port. For example, when using input port 109a, the output phase shifts described above are in order of port llla-llld. If all output EM signals are offset by K / 8, or the arbitrary 7t / 8 is ignored in all output ports, then the relative output phases are ( 0, -3 Pi / 4, -Pi, - 3Pi / 4).

[0115] For a given input in input waveguide 1 to the first MMI, the output field of the apparatus along output waveguides 123a-d of the second MMI is given by Equation 3:

[0116] [Equ. 3]

[0117] Where the phase imparted to light by modulators dl-4 are: dl = di; d2=dq+ TT / 2; d3 = -di; d4= TT / 2 - dq. The terms 'dq' and 'di' represent the phase terms introduced by the fast phase modulators 116, 117, 118, 119. The electrical signals provided to the fast phase modulators are configured to induce a phase change to the portion of light propagating along the respective modulator as listed below: a) for dl, the electrical signal provided represents the data signal for the I component of the IQ. signal b) for d2, the electrical signal provided represents the data signal for the Q component of the IQ signal, plus a phase of + K / 2 c) for d3, the electrical signal provided represents the data-bar signal for the I component of the IQ signal d) for d4, the electrical signal provided represents a phase of + K / 2 plus the data-bar signal (or + TT / 2 minus the data signal) for the Q component of the IQ signal.

[0118] The output EM field from waveguide 123b represents the usual output of an IQ modulator. The output EM field from waveguide 123d represents the conjugate to this usual output of an IQ modulator.

[0119] The configuration of figure 4 using compact MMIs optically linked in between by fast phase modulators minimises the overall spatial footprint of the apparatus, for example by minimising chip volume dedicated to the couplers. Figure 4 also provides reconfiguration of the apparatus to address multiple applications.

[0120] It is to be understood that this example may be adapted with components, configurations (and / or steps where a process or method is involved) and other features as presented in other examples herein. The adaption may be by addition or replacement. Existing features of this example may be removed. Correspondingly, it is to be understood that other examples herein may be adapted with components, configurations (and / or steps where a process or method is involved) and other features as presented in this example. Such features may include, but are not limited to, any one or more of: details about MMIs, the waveguides, the fast modulators, any of the electrical signals or EM radiation; any ranges or modes of operation.

[0121] In figure 5a there is shown a further example of an apparatus 200 for generating a first EM data signal 4q, 104a and a second EM data signal 4b, 104b that is conjugate to the first EM data signal 104a. The apparatus 200 is similar to the apparatus 100 wherein like numerals represent like components.

[0122] In this configuration the fast modulators 117 and 118 are swapped over for the purposes of equations 1 and 2 above. Light output from output waveguide 111b travels along a waveguide 139a, as shown in figure 5b to input light into fast phase modulator 117 (as shown by arrow 143). Light output from output waveguide 111c travels along a waveguide 139b, as shown in figure 5b to input light into fast phase modulator 118 (as shown by arrow 141). When implemented in a PLC the waveguides 139a, 139b cross as shown in figure 5b. The crossing of waveguides in figure 5b is preferably done so such that the direction of waveguide 139a is perpendicular or close to perpendicular, to the direction of 139b. Having waveguides oriented at such angles when crossing entails that: very little of the light propagating along path 143 couples into waveguide 139b; very little of the light propagating along path 141 couples into waveguide 139a. The conjugate signal 104b is output from waveguide 123c in this example.

[0123] An advantage of this configuration in figure 5a is that the fast phase modulators associated with the same signal component of the IQ. modulation (i.e., either the I component of the Q component) may be adjacent to one another. This allows a simpler electrical interface to the apparatus 200 for using the push-pull mode of operation.

[0124] It is to be understood that this example may be adapted with components, configurations (and / or steps where a process or method is involved) and other features as presented in other examples herein. The adaption may be by addition or replacement. Existing features of this example may be removed. Correspondingly, it is to be understood that other examples herein may be adapted with components, configurations (and / or steps where a process or method is involved) and other features as presented in this example. Such features may include, but are not limited to any one or more of: details about MMIs, the waveguides, the fast modulators, any of the electrical or EM radiation; any ranges or modes of operation.

[0125] In figure 6 there is shown another example of an apparatus 300 for generating a first EM data signal and a second EM data signal conjugate to the first EM data signal. Again, in this example, it is assumed than the apparatus 300 is being used for IQ modulation, however other modulation formats are applicable as well. As for other examples herein, this example may simultaneously generate the wanted signal on one port and its conjugate on another. For example, a data signal and its conjugate, a positively shifted subcarrier signal and the negatively shifted conjugate.

[0126] Figure 6 is a schematic diagram of a planar light-wave circuit ('PLC') showing the components used wherein the absolute position and size of features in the figure does not indicate actual position and size of the features. The components used for this example are preferably integrated-optic waveguides components, however other components may be used within the set-up such as optical fibres and free space bulk optic components. Preferably the waveguides are integrated onto a single chip via monolithic or hybrid integration. The waveguides in this example, as in other examples, are 'optically linked' in that the light guided by the waveguides is guided by the light-wave circuit from one component to the next via the substantial continuation of a waveguide core material linked and physically connecting components, although it is understood that some components may include small breaks in waveguide core material along the direction of propagating of the light. Examples of break of linking waveguide core material is waveguide segmentation and the propagation of the guided light across a gap to go from one component to another where the components are formed in different fabrication processes and subsequently integrated. The waveguides are preferably formed of solid core and cladding materials, preferably dielectric or semiconductor materials, for example, but not limited to silica, polymer, silicon, GaAs, Lithium Niobate.

[0127] In figure 6, some of the reference labels for the broader example of figures 3a / 3b are also included for understanding purposes. This example assumes that the waveguides are nominally single mode waveguides carrying the fundamental TE and / or TM mode of the input light 310. References to 'light' in this and other examples herein may be interpreted to also refer to other electromagnetic radiation wavelengths. Any of the above details for the physical implementations of the apparatus of figure 6 may apply to other examples herein including the MMI-based examples in figures 4 and 5.

[0128] The apparatus 300 comprises a Y-branch splitter 308 that has a single input waveguide for receiving continuous wave (CW) EM radiation 310. The Y-branch splitter 308 further comprises two output waveguides with waveguide core sections that are initially physically joined to each other and the input waveguide and then separate and branch away from each other to form two spatially separate output waveguide paths 303a and 303b. The Y branch 308 is designed such that each path 303a and 303b carries approximately or exactly 50% of the intensity of the input CW light 310. The Y branch splitter may be replaced by other splitter types including a 2 x 2 port directional coupler with a 50% splitting ratio at the wavelength of interest; or a 1 x 2 port MMI. The splitting ratio of the splitter 308 in this example may be another value other than 50 / 50 (50%), for example a 55 / 45 split or a 60 / 40 split. The term 'port' in this context meaning spatial separate output or input waveguide paths delivering EM radiation into / out-of the component.

[0129] Waveguide path 303a optically connects the Y branch 308 to a first Mach-Zehnder interferometer (MZI) 305a, which may also be referred to as a Mach-Zehnder Modulator, MZM. MZI 305a has a single input waveguide splitting light (preferably with a 50 / 50 intensity split or other splitting ratio) into two spatially separate arms which then, in turn, recombine to form a single output waveguide. In each arm is an optical phase modulator 316, 317. These optical phase modulators 316, 317 may be termed 'fast modulators' for purposes of describing the apparatus 300 herein. The optical phase modulators 316, 317 receive electrical control signals (not shown in figure 6) that are used by the modulators 316, 317 to impart a phase change to light propagating along the arm and through the phase modulators 316, 317. Each of the modulators 316, 317 form part of the arm path of the MZI such that light propagating along the arms end-couples into the modulator 316, 317, propagates along the length of modulator's core waveguide and is output back into a waveguide of the PLC that acts to deliver the light into an arm-combiner section. The arm combiner section in this example is another Y branch, however it may be another combiner comprising at least two input ports and at least one output port, for example a directional coupler or a 2x1 port MMI coupler.

[0130] The electrical signals input to the fast phase modulators 316, 317, have a base frequency that the electrical data is modulated at, for example 10GHz, 20GHz, 40GHz. Other ranges of data rate may optionally include: any of the lower limits 1GHz, 5GHz, 10GHz, 20 GHz, 30 GHZ, 40 GHz; any of the upper limits 5GHz, 10GHz, 20 GHz, 30 GHZ, 40 GHz; 80GHz, 100 GHz. This may be referred to as the 'data rate' or 'base data rate'. The electrical signals are for defining the I and Q. data. For data applications (or generally for any application) the electrical data signals input into the fast phase modulators may comprise a symbol rate between any of: 100 Mbaud - 4 Tbaud; 25Gbaud - 4 Tbaud; 25Gbaud - 200 Gbaud. Optionally, the electrical data signals may comprise a highest frequency component between any of: 1 GHz - 4 THz; 1 GHz - 500GHz. The phase modulators may be electrooptic based modulators or other types of phase modulators able to impart desired phase changes at the said data rate. In general, such fast modulators may be any free-carrier depletion-based phase modulator.

[0131] In this example, the MZI is a balanced MZI in that the optical path length of each interferometer arm is substantially the same or identical. In one arm of the MZI 305a there resides a controllable phase shifting element 307a. The phase shifting element 307a is configured to receive an input stimulus, such as electrical current, and impart a further phase shift on the light propagating along that arm. In figure 6, the phase shifting element is shown on the top-most arm. In this example the element is a electro-optic phase shifter. Optionally the element may be a thermo-optic heater that locally heats at least a portion of the waveguide core underneath it to introduce a phase change by changing the effective index of the waveguide mode. The surrounding cladding adjacent to the core is typically heated too. This element 307a may also be referred to herein as a 'heater' or 'heating element'. The heater may operate at a maximum rate (i.e., frequency at which the element changes from a first phase state to a second phase state) that is substantially less than the data rate. Preferably the heater is capable of imparting a phase change in the range 0-K; more preferably 0-2K or 0 to greater than 2K. One or both arms of MZI 305a may have one or more heaters (or other type of phase shifting element performing the function of the heater). In operation, the heater 307a is set to impart a K phase change so that the light travelling along the arms and combining at the second (output) Y branch destructively interfere when no further phase change is imparted by either of the fast modulators 316, 317. In the alternative to have the one or more heaters, or in addition, the MZI optical path lengths may be imbalanced by K (at the CW wavelength of operation).

[0132] The transmitted amplitude Atof the MZI 305a is given by the following equation:

[0133] [Equ. 4] Wherein A; is the original incident amplitude of the CW radiation to the MZI and 4> 1 / 2 are the phases imparted by each fast phase modulator.

[0134] The fast phase modulators 316, 317 in this example may optionally operate in a push-pull operation whereby the MZI 305a receives data signals to encode the 'I' signals of the IQ. modulation scheme. The incoming signal is split or otherwise replicated such that the I signal is input into each of the modulators 316, 317. In the push-pull operation the electrical signal for generating the same data bit is provided to both modulators simultaneously, however the apparatus is configured such that each modulator 316, 317 is driven in opposite phase to the other. This allows for a doubling of the effective phase shift between the portions of the light entering the modulators 316, 317. This furthermore reduces power consumption to 2 x V2rather than (2V)2. It also reduces chirp. Without push-pull, in response to the conventional drive signal the three "rings of constant amplitude" in Figure 2c would be rotated with respect to each other. This may be pre-compensated, but at the expense of complexity in the digital or electrical domains. The effective phase shift is doubled about a common mode level. To achieve this the modulators may be biased with a different DC bias. The application of the electrical I signal inducing opposite phase shifts to modulators 316 and 317, induces a phase change which results in a change in amplitude at the output of MZI 305a. Depending on the set-up of the modulators and the driving scheme the modulators 316 and 317 may have the same or a different DC voltage bias.

[0135] In other examples a push-pull mode of operating may not be required and a single fast modulator 316, 317 may be used in one of the arms wherein that single modulator induces the full phase shift required to produce the intensity modulation of the MZI 305a. VKis the voltage to provide a K phase shift in a phase modulator 316, 317. Each modulator 316, 317 is preferably drivable by at least upto 2V^. The MZM may be fabricated using various materials including but not limited to LiNbOa.

[0136] The light output from MZI 305a is input into a further Y branch splitter 309a. In general, the Y branch splitter may be any optical splitter comprising at least one input waveguide and at least two output waveguides wherein light propagating along one input waveguide is split into two of the output waveguides. Other examples include a 1 x 2 port MMI or a 2x2 port directional coupler wherein only one input waveguide is utilised. In this example the Y branch 309a has a 50% splitting ratio, although other ratios may be used. A first output waveguide 311a of the Y branch 309a outputs light into an input waveguide of a 2x2 port coupler 324b. A second output waveguide 313b of the Y branch 309a outputs light into an input waveguide of a further 2x2 port coupler 324a, wherein 2x2 port couplers 324a and 324b are different couplers that are spatially separate.

[0137] Returning back to the light output from the Y branch 308, the output waveguide paths 303b is input into a further MZI 305b that operates in a similar manner to MZI 305a wherein the fast modulators of MZI 305b are labelled 318 and 319 whilst the heater is labelled 307b. Fast modulators 318, 319 of MZI 305b receive data signals to encode the 'Q' signals of the IQ. modulation scheme, in a similar manner to the fast modulators 316, 317 of MZI 305a in that they receive electrical control signals (not shown in figure 6) that are used by the modulators 318, 319 to impart a phase change to light propagating along the arm and through the phase modulators 318, 319. Optional variations associated MZI 305a may equally apply to MZI 305b.

[0138] Light output from MZI 305b is input into Y branch 309b which splits the input light into two spatially separate waveguide paths 313a and 311b. Y branch 309b operates in a similar manner to Y branch 309a wherein optional variations associated Y branch 309a may equally apply to Y branch 309b.

[0139] A first output waveguide 313a of the Y branch 309b outputs light into an input waveguide of the 2x2 port coupler 324a. The input waveguide of 2x2 port coupler 324a that receives light from path 313a is different and spatially separate to the input waveguide of 2x2 port coupler 324a that receives light from path 313a.

[0140] A second output waveguide 311b of the Y branch 309b outputs light into an input waveguide of the 2x2 port coupler 324b. The input waveguide of 2x2 port coupler 324b that receives light from path 311b is different and spatially separate to the input waveguide of 2x2 port coupler 324b that receives light from path 311a.

[0141] A 'slow' phase modulator 315a, such as, but not limited to, a heater as described elsewhere herein, is incorporated along the path 313a. A further 'slow' phase modulator 315b, such as, but not limited to, a heater as described elsewhere herein, is incorporated along the path 311b. In an alternative arrangement the slow phase shifters can be placed along paths 311a and 313b.

[0142] Preferably, the electromagnetic path lengths (e.g., optical path lengths) that light takes from the point of splitting in Y branch 308 to the input of both the input waveguides of 2x2 port coupler 324a, are identical or substantially identical aside from any phase changes imparted by phase modulators. Therefore, light entering the input waveguides of coupler 324a may interfere.

[0143] Preferably, the electromagnetic path lengths (e.g., optical path lengths) that light takes from the point of splitting in Y branch 308 to the input of both the input waveguides of 2x2 port coupler 324b, are identical or substantially identical aside from any phase changes imparted by phase modulators. Therefore, light entering the input waveguides of coupler 324b may interfere.

[0144] The couplers 324a and 324b may be any coupler for interfering light from at least two spatially separate input paths; and outputting the interfered light into at least two spatially separate output paths. In figure 6 a 2x2 port directional coupler is shown, however this may also be any of, but not limited to: a 2x2 MMI coupler. The output waveguides of coupler 324a in figure 6 are link to output channels 323a and 323b of the apparatus 300. The output waveguides of coupler 324b in figure 6 are link to output channels 323c and 323d of the apparatus 300. Light output from channel 323a comprises a desired signal 304a. Light output from channel 323c comprises the desired conjugate signal 304b. This relationship is achieved when phase shifter 315a imparts a phase shift of K / 2 and 315b impart a phase shift of -K / 2 to the light propagating along the associated waveguide paths. In the above discussion of phase shifts for phase shifters 315a and 315b, the nominal path lengths of 311a and 313b are equal. Preferably the path lengths 311a, 311b, 313a, 313b are all equal when no phase shifts are imparted by phase shifters 315a and 315b. Preferably, assuming that the path lengths of 311a and 313b are identical, the phase shifts 315a and 315b may be set to give a phase difference of K. It is to be understood that this example may be adapted with components, configurations (and / or steps where a process or method is involved) and other features as presented in other examples herein. The adaption may be by addition or replacement. Existing features of this example may be removed. Correspondingly, it is to be understood that other examples herein may be adapted with components, configurations (and / or steps where a process or method is involved) and other features as presented in this example. Such features may include, but are not limited to, any one or more of: details about couplers / splitters, the MZI's, the waveguides, the fast modulators, any of the electrical or EM radiation; any ranges or modes of operation.

[0145] In another example, there is further presented an apparatus for generating a plurality of output electromagnetic, EM, signals using a plurality of electrical data signals; the plurality of output EM signals comprising at least a first output EM signal, and a second output EM signal. Figure 7 shows an example of such an apparatus in schematic form. The apparatus may be implemented using any of: integrated optic components, optical fibre components; bulk-optic components. EM radiation may be transmitted between different features of figure 7 using any of: free space propagation; propagation along one or more optical fibres; propagation along in integrated optic waveguide. Other details of apparatus implementation may be used from other examples herein such as using monolithic of hybrid integration.

[0146] The apparatus 400 comprises a plurality of modulators 402a, 402b. In figure 7, the apparatus 400 comprises a first data modulator 402a configured to receive the first EM wave 406a and the first electrical data signal 408a. In figure 7, The apparatus 400 further comprises a second data modulator 402b configured to receive the second EM wave 406b and the second electrical data signal 408b.

[0147] The modulators 402a, 402b are each data modulators that may be similar to modulators used in other examples herein, such as those described for figures 3a and 3b. The first data modulator 402a is configured to modulate the first EM wave 406a in accordance with the first electrical data signal 408a. For examples where the apparatus 400 is used for IQ. modulation, the first electrical data signal 408a may comprise a plurality of electrical signals for modulating the first EM wave 406A according to one of the IQ signals, for example the first data modulator 402a may be used to generate I signals. The second data modulator 402b is configured to modulate the second EM wave 406b in accordance with the second electrical data signal 408b. For examples where the apparatus 400 is used for IQ modulation, the second electrical data signal 408b may comprise a plurality of electrical signals for modulating the second EM wave 406b according to one of the IQ signals, for example the second data modulator 18 may be used to generate Q signals. The modulators 402a / b may be modulated according to an Amplitude Shift Keying protocol (ASK). The modulated EM waves 410a and 410b output from the respective first and second modulators 402a / b are then directed towards a polarisation combiner 412 to combine the different modulated EM waves 410a / b into a polarisation multiplexed EM output signal 414. Output signal 414 comprises the modulated EM wave 410a propagating with a first linear polarisation and the modulated EM wave 410b propagating with a second linear polarisation that is orthogonal to the first linear polarisation. This may be accomplished in different ways including any of: inputting different, orthogonal polarisations into the modulators; inputting non-orthogonal polarisations into the modulators and using one or more polarisation rotators on one or more of the output beams 410a / b to rotate the respective output beams 410a / b so they are orthogonal in polarization before they are input into combiner 412. The combiner 412 may be a polarisation beam splitter wherein the polarisation axes of the incoming EM beams 410a / b are aligned such that substantially all of the EM radiation is output along common optical path of output multiplexed beam 414. Furthermore splitter 404 may be a polarisation splitter (PBS) used to direct a different orthogonal polarisation to each of the modulators 402a / b. In this implementation, EM radiation 403 may have different polarisations or be linearly polarised and aligned with respect to the polarisation axis of the PBS to direct 50% of the light in a first polarisation towards modulator 402a and 50% of the light in a second orthogonal polarisation towards modulator 402b. Alternatively, splitter 404 may be a normal beam splitter or another amplitude splitting component, preferably with a 50 / 50 splitting ratio, and receives linearly polarised EM radiation 403, wherein a polarisation rotator is used to rotate the EM radiation of one channel to be orthogonal to the other channel.

[0148] The multiplexed output signal 414 is then input into a polarisation selective phase retarder 416 that delays one polarisation of EM radiation with respect to an orthogonal polarisation and outputs the EM radiation 418. Preferably the phase retarder 416 comprises a wave plate. Preferably the phase retarder delays one polarisation by K / 2 compared to the orthogonal polarisation. Preferably the retarder 416 is a quarter wave plate. The input EM radiation 414 is aligned with respect to the retarder 416 such that one of the polarisations associated with one of the modulators 402a / b is aligned along the fast axis of the retarder, whilst the other orthogonal polarisation (with respect to the other modulator 402a / b) is aligned along the slow axis. The output EM radiation 418 therefore has EM radiation from one modulator, in a first polarisation phase delayed with respect to the EM radiation modulated by the other modulator that has the second orthogonal polarisation. For purposes of this example, it will be assumed that phase retarder delays the linear polarisation signal associated with the first modulator 402a.

[0149] The EM radiation 418 is then input into a polarisation rotator 420 that rotates all polarisations by 45 degrees (K / 4) and outputs EM radiation 420. The actions of both 416 and 420 may be replicated by a single optical element, such as a polarisation controller that applies both the required relative phase retardation provided by 416 and the rotation provided by 420. Thus elements 416 and 420 may be replaced by a single polarisation controller.

[0150] The EM radiation of 422 therefore has: a first linear polarisation component, at a polarisation angle of 45 degrees, that has the modulation from one of the modulators; a second linear polarisation component, at a polarisation angle of -45 degrees, that has the modulation from the other one of the modulators.

[0151] Turning back to the implementation in figure 7, the polarisation rotator 420 then outputs radiation 422 into a further polarisation beam splitter 424.

[0152] The polarisation axis of the PBS 424 is set with respect to the incoming EM radiation 422 such that:

[0153] A) the linear polarisation of the EM radiation from the first modulator 402a has: a component that is transmitted by the PBS 424 to form part of the output radiation 426; and a component that is reflected by the PBS 424 to form part of the output radiation 428 that propagates along a separate path to radiation 426.

[0154] B) the linear polarisation of the EM radiation from the second modulator 402b has: a component that is transmitted by the PBS 424 to form part of the output radiation 426; and a component that is reflected by the PBS 424 to form part of the output radiation 428 that propagates along a separate path to radiation 426 and which acquires a Pi phase shift on reflection.

[0155] Preferably, the PBS splits each input linear polarisation (each corresponding to modulation from a different modulator 402a / b) to direct 50%, or substantially 50%, of that polarisation into the transmission channel and the corresponding remaining portion into the reflection channel 428.

[0156] In other words, the EM radiation 422 is incident upon PBS 424 such that: one output path 426 propagates 50% of the signal from modulator 402a and 50% of the signal from modulator 402b; whilst the other path 428 propagates 50% of the signal from modulator 402a and 50% of the signal from modulator 402b.

[0157] In the reflected EM radiation 428, one polarisation component of the input signals incurs a relative phase rotation by K by virtue of the reflecting action of the beam splitter 424. This combination of this rotation and the phase shift imposed by the QWP transforms the output (reflected) EM radiation 428 into the conjugate of the EM radiation 426 transmitted by the PBS 424 to within a fixed phase offset. Thus, 426 carries the QAM signal whilst 428 carries the QAM* signal.

[0158] The above apparatus described for figure 7 is an example of a third aspect for outputting a plurality of EM signals wherein the apparatus uses polarisation multiplexed EM signals. The third aspect is described underneath wherein features of the example in figure 7, and other examples herein, may be used with the third aspect described underneath, and vice versa. In particular, the third aspect may be adapted with any one or more of, but not limited to: the following features: details about physical parameters of EM components such as materials, devices, device types, EM transmission media, electronic devices and components for operating the EM components such as the EM modulators; parameters of the EM radiation such as polarisation, wavelength, frequencies, rates, phase changes; signal types such as QAM; parameters of electrical signals such as currents, voltages, signal types.

[0159] In a third aspect, there is presented an apparatus for generating a first output EM data signal and a second output EM data signal. Each of the first and second output EM signals are data signals. In other words, the first and second EM signals carry data. The apparatus comprises a polarisation controlling arrangement, PCA. The PCA is configured to receive a polarisation multiplexed EM signal (PMS) comprising first data on a first linear EM polarisation and second data on a second EM polarisation that is orthogonal to the first polarisation. The PCA may be configured to at least phase delay the first polarisation with respect to the second polarisation. The apparatus further comprises a further arrangement. The further arrangement comprising at least an EM splitter configured to receive the PMS and direct: a first portion of the PMS along a first path; and, a second portion of the PMS along a second path that is spatially separate to the first path. The first portion of the PMS comprises the first data and the second data. The second portion of the PMS comprises the first data and the second data. The PCA and the further arrangement are together configured such that: the first portion of the PMS is for outputting as the first output EM data signal; second portion of the PMS is for outputting as the second output EM data signal wherein the second output EM data signal is the conjugate of the first output EM data signal. The above example may optionally be adapted according to, but not limited to, any one or more of the following options.

[0160] The apparatus may comprise a PMS generating arrangement.

[0161] The PMS generating arrangement may comprise: a first EM modulator; a second EM modulator; an EM combiner. The first EM modulator may be configured to: receive a first electrical data signal carrying the first data; receive first EM radiation; modulate the first EM radiation with the first data; output the modulated first EM radiation towards the EM combiner. The second EM modulator may be configured to: receive a second electrical data signal carrying the second data; receive second EM radiation; modulate the second EM radiation with the second data; output the modulated second EM radiation towards the EM combiner.

[0162] The EM combiner may receive the modulated second EM radiation and the modulated first EM radiation and correspondingly output the PMS; the PMS comprising the modulated second EM radiation and the modulated first EM radiation.

[0163] The EM combiner may comprise a polarisation combiner. The EM combiner may comprise a polarisation beam splitter, PBS. The EM combiner may receive the modulated second EM radiation and modulated first EM radiation along different spatially separate paths. The modulated second EM radiation may be a linear polarisation.

[0164] The modulated second EM radiation incident upon the EM splitter may be an orthogonal polarisation to the modulated first EM radiation incident upon the EM splitter.

[0165] The first EM radiation may be linearly polarised. The second EM radiation may be linearly polarised. The first EM radiation may be orthogonally polarised to the second EM radiation.

[0166] The first data may comprise one of the I or Q data signals from an IQ. data signal. The second data may comprise the other one of the I or Q data signals from an IQ data signal.

[0167] The PMS generating arrangement may comprise an EM source for generation and outputting EM radiation for inputting to the EM splitter of the PMS generating arrangement. The EM source may comprise a laser. The EM source may output linearly polarised EM radiation. The PMS generating arrangement may comprise an EM splitter for receiving EM radiation from the EM source and directing: a first portion towards the first EM modulator; a second portion towards the second EM modulator. The said first and second portions may be orthogonally polarised.

[0168] The PCA may receive EM radiation from the PMS generating arrangement. The PCA may comprise a phase retarder. The phase retarder may comprise a waveplate. The wave plate may be a quarter waveplate. The phase retarder may comprise a fast polarisation axis and a slow polarisation axis wherein EM radiation propagates faster on the fast polarisation axis than the slow polarisation axis. The PCA may be configured such that the phase retarder receives: the first polarisation carrying the first data on one of the fast or slow axes; the second polarisation carrying the second data on the other one of the fast or slow axes.

[0169] The PCA may comprise a polarisation rotator. The polarisation rotator may receive EM radiation, in particular the first and second polarisations, output from the phase retarder. The polarisation rotator may rotate each of the first and second polarisations by 45 degrees. The polarisation rotator may output the polarisation rotated PMS towards the further arrangement. The polarisation rotator may output the polarisation rotated PMS towards the EM splitter of the further arrangement.

[0170] The EM splitter of the further arrangement may receive: a polarisation rotated first polarisation and a polarisation rotated second polarisation. A polarisation splitting axis of the EM splitter of the further arrangement may be oriented, with respect to the to the polarisation rotated first and second polarisations, to direct: a first portion of the polarisation rotated first polarisation along the first path; a first portion of the polarisation rotated second polarisation along the first path; a second portion of the polarisation rotated first polarisation along the second path; a second portion of the polarisation rotated second polarisation along the second path.

[0171] The EM splitter of the further arrangement may comprise a polarisation beam splitter, PBS. The PBS may transmit EM tradition for the first output EM data signal along the first path; reflect the second output EM data signal along the second path. The said PBS may apply a phase change to at least a portion of the reflected EM radiation.

Claims

Claims1. An apparatus (1) for generating a plurality of output electromagnetic, EM, signals using a plurality of electrical data signals; the plurality of output EM signals comprising at least a first output EM signal (4a), and a second output EM signal (4b); the plurality of electrical data signals comprising at least a first electrical data signal (6a) and a second electrical data signal (6b); the apparatus (1) comprising:A) a set of components (3) comprising; a. a Multimode Interference coupler, MMI, (8); the MMI (8) configured to: receive an input EM wave (10); and, divide the input EM wave (10) into a plurality of EM waves; the plurality of EM waves comprising at least a first EM wave (12) and a second EM wave (14) wherein the first EM wave (12) propagates along a spatially separate path to the second EM wave (14); b. a set of one or more first data modulators (16) configured to: receive the first EM wave (12); receive the first electrical data signal (6a); modulate the first EM wave (12) in accordance with the first electrical data signal (6a); c. a set of one or more second data modulators (18) configured to: receive the second EM wave (14); receive the second electrical data signal (6b); modulate the second EM wave (14) in accordance with the second electrical data signal (6b); the set of components (3) configured to generate: first and second data modulated EM waves (EMai, EMbi) for the first output EM signal (4a) using a modulated EM wave (EMa, EMb) output from each of the first and second sets of data modulators (16, 18); and,first and second data modulated EM waves (EMaii, EMbii) for the second output EM data signal (ED2) using the modulated EM wave (EMa, EMb) output from each of the first and second sets of data modulators;B) a set of one or more EM combiners (24) configured to: a) receive and combine the first and second data modulated EM waves (EMai, EMbi) for the first output EM signal (4a); b) receive and combine the first and second data modulated EM waves (EMaii, EMbii) for the second output EM signal (4b); c) output the first output EM signal (4a) onto at least a first output path (20); d) output the second output EM signal (4b) onto a second output path (22); the second output path (22) being spatially separate from the first output path (20); wherein the set of components (3) imparts a different phase relationship between the first (EMai) and second (EMbi) data modulated EM waves for the first output EM signal (4a) to, the phase relationship between the first (EMaii) and second (EMbii) data modulated EM waves for the second output EM signal (4b), such that the second output EM signal (4b) is the conjugate of the first output EM signal (4a) wherein the set of one or more EM combiners comprises a further MMI.

2. The apparatus of claim 1 wherein the MMI comprises at least one input and at least four outputs wherein the MMI divides light between each of the outputs.

3. The apparatus of claims 1 or 2 wherein the MMI comprises a multimode coupling region comprising opposing first and second sidewalls; the input EM wave is input into the multimode coupling region proximal to the first sidewall and distal from the opposing second sidewall.

4. The apparatus of any preceding claim wherein the MMI comprises at least four inputs into the multimode coupling region wherein each input is associated with a different input waveguide; each input waveguide is configured to input EM radiation into the multimode coupling region at acoupling interface; the input EM wave being guided by the input waveguide closest to the first sidewall at the coupling interface.

5. The apparatus of any preceding claim wherein, the first and second data modulators comprise phase modulators.

6. The apparatus of any preceding claim wherein the first and second data modulators comprise electro-optic modulators.

7. The apparatus of any preceding claim wherein the first and second electrical data signals comprise a symbol rate between 100 Mbaud - 4 Tbaud.

8. The apparatus of any preceding claim wherein the first and second electrical data signals comprise a symbol rate between 100 Mbaud to a symbol rate of 140 Gbaud.

9. The apparatus of any preceding claim wherein the first and second electrical data signals comprise a symbol rate between 25 Gbaud - 4 Tbaud.

10. The apparatus of any preceding claim wherein the first electrical data signal is associated with the I component of IQ. modulator data; the second electrical data signal is associated with the Q component of the IQ modulator data.

11. The apparatus of any preceding claim wherein the plurality of EM waves output by the MMI comprises the first EM wave, the second EM wave, a third EM wave and a fourth EM wave; the apparatus further comprises: a third data modulator configured to: receive the third EM wave; receive a third electrical data signal;modulate the third EM wave in accordance with the third electrical data signal; a fourth data modulator configured to: receive the fourth EM wave; receive a fourth electrical data signal; modulate the fourth EM wave in accordance with the fourth electrical data signal.

12. The apparatus of claim 11 wherein the third electrical data signal is associated with the I component of IQ. modulator data; the fourth electrical data signal is associated with the Q component of IQ modulator data.

13. The apparatus of claims 11 or 12 wherein: a) the first data modulator imparts a phase change to the first EM wave; the third data modulator imparts a phase change to the third EM wave that is a different phase change to the phase change to the first EM wave; b) the second data modulator imparts a phase change to the second EM wave; the fourth data modulator imparts a phase change to the fourth EM wave that is a different phase change to the phase change to the second EM wave.

14. The apparatus of any preceding claim wherein the further MMI comprises first, second, third and fourth input waveguides and at least two output waveguides wherein one of the two output waveguides outputs the first EM output signal and the other of the two output waveguides output the second EM output signal.

15. The apparatus of claim 14 wherein the apparatus is configured such that said first, second, third and fourth input waveguides of the further MMI each receive modulated EM radiation from a different one of the first, second, third and fourth data modulators.

16. The apparatus of claim 14 or 15 wherein the apparatus is configured such that:I) the further MMI comprises a multimode coupling region comprising an input coupling interface;II) the first, second third and fourth input waveguides of the further MMI are respectively disposed sequentially along an input coupling interface.

17. The apparatus of any preceding claim wherein where the MMI comprises the at least four outputs; the MMI for dividing the input EM wave further comprises: i) an output coupling interface; and, ii) first, second, third and fourth output waveguides respectively disposed sequentially along the output coupling interface.

18. The apparatus of claim 4 wherein the MMI comprises first, second, third and fourth output waveguides:I) the first output waveguide outputs the first EM wave towards the first data modulator; the second output waveguide outputs the second EM wave towards the second data modulator; the third output waveguide outputs the third EM wave towards the third data modulator; the fourth output waveguide outputs the fourth EM wave towards the fourth data modulator;II) the first input waveguide to the further MMI receives the data modulated first EM wave; the second input waveguide to the further MMI receives the data modulated second EM wave; the third input waveguide to the further MMI receives the data modulated third EM wave; the fourth input waveguide to the further MMI receives the data modulated fourth EM wave.

19. The apparatus of claim 4 wherein the MMI comprises first, second, third and fourth output waveguides:I) the first output waveguide outputs the first EM wave towards the first data modulator; the second output waveguide outputs the third EM wave towards the third data modulator; the third output waveguide outputs the second EM wave towards thesecond data modulator; the fourth output waveguide outputs the fourth EM wave towards the fourth data modulator;II) the first input waveguide to the further MMI receives the data modulated first EM wave; the second input waveguide to the further MMI receives the data modulated third EM wave; the third input waveguide to the further MMI receives the data modulated second EM wave; the fourth input waveguide to the further MMI receives the data modulated fourth EM wave.

20. The apparatus of any preceding claim wherein the first and second data modulator operate together in a push-pull modulation scheme.

21. The apparatus of claim 11 wherein the third and fourth data modulator operate together in a push-pull modulation scheme.

22. A modulator system comprising the apparatus of any preceding claim wherein the modulator system further comprises at least one or more of:I) one or more electrical controllers for outputting: i) the first electrical data signal and the second electrical data signal; and, ii) optionally, the third and fourth electrical data signals;II) one or more EM sources for generating the input EM wave.

23. A method for generating a plurality of output electromagnetic, EM, signals using a plurality of electrical data signals; the plurality of output EM signals comprising at least a first output EM signal (4a), and a second output EM signal (4b); the plurality of electrical data signals comprising at least a first electrical data signal (6a) and a second electrical data signal (6b); the method comprising:A) using: a. a Multimode Interference coupler, MMI (8) to:receive an input EM wave (10); and, divide the input EM wave (10) into a plurality of EM waves; the plurality of EM waves comprising at least a first EM wave (12) and a second EM wave (14) wherein the first EM wave (12) propagates along a spatially separate path to the second EM wave (14); b. a set of one or more first data modulators (16) to: receive the first EM wave (12); receive the first electrical data signal (6a); modulate the first EM wave (12) in accordance with the first electrical data signal (6a); c. a set of one or more second data modulators (18) to: receive the second EM wave (14); receive the second electrical data signal (6b); modulate the second EM wave (14) in accordance with the second electrical data signal (6b); the set of components (3) configured to generate: first and second data modulated EM waves (EMai, EMbi) for the first output EM signal (4a) using a modulated EM wave (EMa, EMb) output from each of the first and second sets of data modulators (16, 18); and, first and second data modulated EM waves (EMaii, EMbii) for the second output EM data signal (ED2) using the modulated EM wave (EMa, EMb) output from each of the first and second sets of data modulators; the MMI, set of set of one or more first data modulators (16) and a set of one or more second data modulators (16) forming at least part of a set of components (3);B) using a set of one or more EM combiners (24) to: a) receive and combine the first and second data modulated EM waves (EMai, EMbi) for the first output EM signal (4a);b) receive and combine the first and second data modulated EM waves (EMaii, EMbii) for the second output EM signal (4b); c) output the first output EM signal (4a) onto at least a first output path (20); d) output the second output EM signal (4b) onto a second output path (22); the second output path (22) being spatially separate from the first output path (20); wherein the set of components (3) imparts a different phase relationship between the first (EMai) and second (EMbi) data modulated EM waves for the first output EM signal (4a) to, the phase relationship between the first (EMaii) and second (EMbii) data modulated EM waves for the second output EM signal (4b), such that the second output EM signal (4b) is the conjugate of the first output EM signal (4a) wherein the set of one or more EM combiners comprises a further MMI.

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