GB2594450A

GB2594450A - Optical apparatus

Info

Publication number: GB2594450A
Application number: GB2005827.7A
Authority: GB
Inventors: Paesani Stefano; Borghi Massimo; Pavesi Lorenzo; Signorini Stefano
Original assignee: University of Bristol; Universita degli Studi di Trento
Current assignee: University of Bristol; Universita degli Studi di Trento
Priority date: 2020-04-21
Filing date: 2020-04-21
Publication date: 2021-11-03
Also published as: GB202005827D0; WO2021214161A1

Abstract

An optical apparatus that comprises a multimode waveguide 4 which guides at least a first mode 6 and a second mode 8, the first mode is a different order transverse mode to the second mode, and which generates and guides the signal 10 and idler 12 pair by annihilating 14 a photon in each of the first mode and the second mode. The apparatus further comprises an optical arrangement 16 to input 18, 20 a first optical pulse into the waveguide to propagate in the first mode, and input a second optical pulse to the waveguide. The second optical pulse is input into the waveguide to propagate in the second mode, and such that the second mode propagates along the waveguide ahead in time of the first mode. The first mode has a higher group velocity than the second mode.

Description

Optical apparatus

Field of the invention

The field of the present invention is optical apparatus for generating photons, including, but not limited to, generating signal and idler photon pairs using a waveguide such as an integrated waveguide or optical fibre.

Background

Different optical applications use quantum information, such as quantum computing, quantum communication and quantum networks. These applications may require the use of single photons and typically require the output single photons to have high degrees of spectral purity.

The generation of such photons has previously been demonstrated using different systems and physical effects such as spontaneous parametric down conversion (SPDC) which uses the second-order (x(2)) nonlinearity and spontaneous four-wave mixing (SFWM) which use the third-order (x(3)) nonlinearity. For SPDC photon sources, one pump light photon is used to generate a signal and idler photon pair. This means that the pump light has a wavelength that is well spaced from the wavelength of the signal and idler photons created from the pump photon. Such spacing allows for simpler filtering of the pump light. In contrast, for most known FWM demonstrations, all of the photon fields in the process have similar wavelengths and therefore the pump light is difficult to filter out and the newly created wavelengths are limited in how far away their wavelengths are compared to the pump wavelength.

One such system that utilised FWM and reported greater wavelength spacing between pump and generated signal/idler photons is described in the research Article Intermodal four-wave mixing in silicon waveguides' by Stefano Signorini et al., Photonics research, Vol. 6, no. 8, August 2018. This article describes an experimental demonstration of intermodal spontaneous as well as stimulated four-wave mixing in silicon waveguides. In the experimental set-up of this article, a tapered lensed optical fibre was used to inject pump light into a waveguide on a silicon-on-insulator (S01) chip. The 501 chip had several multimode waveguides with different waveguide widths. The input tapered lensed optical fibre was positioned to excite multiple pump modes in a particular waveguide. The light transmitted by the waveguide was collected at the output by another tapered lensed fibre.

The authors of this paper reported measuring both spontaneous and stimulated FWM via intermodal phase matching between the multiple pump modes.

Another system is that described in "Silicon photonics chip for inter-modal Four Wave Mixing on a broad wavelength range'' by Signorini, Stefano, et al., Frontiers in Physics 7 (2019): 128. This paper described intermodal FWM generation on a chip using asymmetrical directional couplers.

Despite the results of the above intermodal FWM approaches, some photonic applications may require a spectral purity better than that achieved in the above papers.

Summary

According to a first aspect of the present invention there is presented an optical apparatus comprising: I) a multimode waveguide capable of: a)guiding at least a first mode and a second mode; the first mode being a different order transverse mode to the second mode; b) generating and guiding a signal and idler photon pair by annihilating a photon in each of the first mode and second mode; II) an optical arrangement configured to: c) input a first optical pulse into the multimode waveguide to propagate in the first mode; d) input a second optical pulse into the multimode waveguide: i) to propagate in the second mode; ii) such that the second mode propagates along the multimode waveguide ahead in time of the first mode; wherein the first mode has a higher group velocity than the second mode.

The first aspect may be modified according to any feature or configuration described elsewhere herein including, but not limited to, any one or more of the following.

The optical arrangement may be configured to: I) split an optical pulse from an optical source into the first optical pulse and second optical pulse wherein: the first optical pulse propagates along a first optical path and the second optical pulse propagates along a second optical path spatially separated from the first optical path; II) input the first optical pulse from the first optical path into the multimode waveguide to propagate as the first mode; III) input the second optical pulse from the second optical path into the multimode waveguide to propagate as the second mode such that the second mode propagates along the multimode waveguide ahead in time of the first mode.

The optical apparatus may comprise: I) an optical splitter for splitting light from the optical source into the first and second spatially separated optical paths; II) an optical delay for delaying the first optical path with respect to the second optical path such.

The optical apparatus may be configured such that: the first and second input optical pulses propagating in the first and second modes each have a wavelength from a first set of one or more wavelengths of light; the signal and idler photon pair have different wavelengths to the first set of one or more wavelengths of light.

The optical apparatus may comprise a further optical arrangement configured to split the signal and idler photons into spatially separate output optical paths.

The optical apparatus may be configured such that the signal photon propagates in the multimode waveguide in a transverse mode order that is different to the transverse mode order of the idler photon.

The optical apparatus may be configured such that the further optical arrangement comprises a mode converter for receiving light from the multimode waveguide and outputting the signal and idler photons in spatially separate optical waveguide output paths; each of the signal and idler photons propagating in the respective fundamental modes of the respective optical waveguide output paths.

The optical apparatus may further comprise one or more optical filters for separating at least one of the signal and idler photons from light from the first set of wavelengths.

The optical apparatus may comprise: a first optical filter for receiving light from a first of the spatially separate optical waveguide output paths; a second optical filter for receiving light from a second of the spatially separate optical waveguide output paths.

The optical apparatus may be configured such that the multimode waveguide comprises a length such that the first mode propagates past the second mode as both modes are propagate in the multimode waveguide.

The optical apparatus may be configured such that the multimode waveguide comprises an integrated optic waveguide.

The optical apparatus may be configured such that the multimode waveguide and optical arrangement are integrated together.

There is further presented an integrated optic device comprising the optical apparatus. The integrated optic device may further comprise one or more optical sources for generating the first and second optical pulses.

The optical apparatus may be configured such that the multimode waveguide comprises an optical fibre.

The optical apparatus may be configured such that the optical arrangement comprises a phase mask.

The optical apparatus may be configured such that the optical arrangement comprises a variable optical delay. The further optical arrangement may comprise a further phase mask.

According to a second aspect of the present invention there is presented an optical system comprising: I) the integrated optical device as described above; or, II) the optical apparatus as described above or elsewhere in this application; and one or more optical detectors for receiving the signal and/or idler photons.

Brief description of the drawings

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an example of an optical apparatus; Figure 2 is a diagram of pump pulse overlap in a multimode waveguide; Figure 3 is a schematic diagram similar to figure 1 with a further optical arrangement; Figure 4 is a diagram of an optical apparatus with a spiralled multimode waveguide; Figure 5 is a diagram showing an example of a mode converter for use with the optical apparatus; Figure 6 is a further example of an optical apparatus using optical fibres.

Detailed description

There is presented an optical apparatus for generating a signal and idler photon pair. A schematic diagram showing an example of this optical apparatus is shown in figure 1. The optical apparatus 2 comprises a multimode waveguide 4 capable of: a) guiding at least a first mode 6 and a second mode 8; the first mode being a different order transverse mode to the second mode; b) generating and guiding the signal 10 and idler 12 photon pair by annihilating 14 a photon in each of the first mode 6 and second mode 8. The optical apparatus 2 further comprises an optical arrangement 16 configured to: a) input 18 a first optical pulse into the multimode waveguide 4 to propagate in the first mode 6; b) input 20 a second optical pulse into the multimode waveguide 4. The second optical pulse is input into the multimode waveguide 4: i) to propagate in the second mode 8; and, ii) such that the second mode 8 propagates along the multimode waveguide 4 ahead in time 22 of the first mode 6. The first mode 6 has a higher group velocity than the second mode 8.

Photon sources such as integrated silicon sources may be used for their nonlinear effects to create signal 10/idler 12 photon pairs. By using nonlinear effects such as SFWM, where, if phase-matching (momentum conservation) and energy conservation conditions are satisfied, light from a pump laser can be converted into signal 10/idler 12 photon pairs. For example, in standard SFWM in single-mode waveguides, near-zero dispersion produces broad phase-matching bands around the pump wavelength. However, this process is dominated by energy conservation conditions, inducing undesired strong spectral anticorrelations between the emitted photons. In the present application, such correlations are suppressed by adopting an inter-modal approach in a multimode waveguide by using, for example, SFWM. By propagating two pump pulses in different transverse modes, signal / idler 12 photon pairs are generated by inter-modal phase-matching and may propagate in the same or different transverse modes of the multimode waveguide 4.

The dispersion relations between the two pump modes (for example the TMO and TM1 modes) may be such that a discrete narrow phase-matching band appears.

By tailoring the waveguide geometry, the modal dispersion can be accurately engineered to design the phase-matching band with a bandwidth similar to the pump bandwidth (related to energy conservation). This suppresses the frequency anticorrelations imposed by energy conservation, and enhances the spectral purity of the emitted photons. Moreover, the tailoring of the cross-section allows to precisely control the central wavelengths of the signal/idler emitted pairs. Increasing the spectral separation with respect to the intense pump laser may be beneficial for filtering out the background noise of the pump.

Intermodal nonlinear processes such as intermodal FWM has the advantage of not requiring anomalous group velocity dispersion (GVD), which is usually considered for intramodal FWM to achieve phase matching. This results in an easier handling of the phase-matching condition. Intermodal FWM exhibits higher flexibility, larger spectral conversion, and easier phase matching.

To further suppress residual correlations in the joint spectrum, the apparatus further provides means to gradually increase and decrease the temporal overlap between the pump pulses. This is described in more detail as follows. The combination of using both an intermodal scheme and by gradually overlapping the pump pulses means that the optical apparatus may obtain an improved spectral purity that is closer to near-unity than other known systems.

The first 6 and second 8 input pulses may hereinafter be referred to as the 'pump pulses', which may originate from a pump laser. When the pump pulses temporally overlap in the waveguide, a photon in each of the two transverse modes annihilates via a nonlinear optical process, generating the signal 10 and idler 12 photons. The non-linear effect described throughout the examples in the application is four-wave mixing using the waveguide's third-order (x(3)) nonlinearity material, however other nonlinear effects may also be utilised such as SPDC and higher order nonlinear effects. The optical apparatus 2 allows the pump pulses 6, 8 to be input into the multimode waveguide 4 and allow one pulse to slowly overlap with the other pump pulse in the multimode waveguide 4.

An example of this overlap is shown in figure 2 whereby the first mode 6 is the fundamental TM mode (TMO) of the multimode waveguide 4 and the second mode 8 is the lst order TM (TM1) mode. The generated signal 10 and idler 12 photon pair are not shown in this figure however it should be appreciated that the photon pair 10, 12 may be generated at any point during the gradual temporal overlap and separation of the pump pulse modes 6,8. The dimensions and lengths of the waveguide and modes in this figure are not intended to be to scale. The multimode waveguide 4 in figure 2 is continuous but is shown in the figure as divided into two sections with arrows showing how the waveguide continues from one section to another. At time Ti the TM1 pulse in the second mode 8 starts ahead of time in the multimode waveguide 4 to the first pulse TMO in the first mode 6. The slow overlap of TMO and TM1 occurs because the first mode 6 has a higher group velocity than the second mode 8. As both pulses propagate along the multimode waveguide the first mode 6 catches up with the second mode Sand allows for an adiabatic switching of the nonlinear interaction of the two modes as they propagate along the multimode waveguide 4. This is shown in figure 2 at sequential times T2, T3, T4 and T5 where the TMO mode slowly catches up with the TM1 mode, overlaps it and then over takes it, travelling further ahead in time along the multimode waveguide 4 than the TM1 mode. In this example the TMO and TM1 modes both adiabatically begin to overlap and then they adiabatically pull away from each other. Such adiabatic overlap suppresses spurious spectral correlations and thus improves spectral purity of the generated signal 10 and idler 12 photons.

In principle, the signal 10 and idler 12 photons may be produced as heralded single photons in pure quantum states or as correlated photons. The degree of purity or correlation depending upon the pump light and optical components used in the optical apparatus 2. The purity of the signal 10 and idler 12 photons may be any purity, including any of the following purity ranges: above 90%, above 95%, above 99%. Photons with high levels of purity, towards 100%, are typically desirable in applications such as quantum computing. Alternatively, the apparatus 2 may be configured to generate strongly correlated photons (near 0% purity), for example, the signal 10 and idler photons may have a purity below 10%.

In the example of this figure the TMO first mode 6 is separated from the TM1 second mode such that the mode overlap is negligible, however in principle, the modes may be input into the multimode waveguide 4 with a significant overlap (for example, the modes may be input into the multimode waveguide 4 at the position shown at T2). Furthermore, this example shows the multimode waveguide being of a length to allow both pump modes TMO 6 and TM1 8 to adiabatically overlap and also adiabatically separate fully to a negligible overlap, however the optical apparatus 2 may be designed to separate the TMO 6 and TM1 8 modes of the pump pulses at any point along the journey from overlapping to separating. For example, the optical apparatus 2 may separate out the pump pulses 6,8 into separate waveguides at time T2, T3, T4, T5 after some pump pulse overlap has occurred.

The input first 6 and second light pulses 8 and the generated signal 10 and idler 12 photons may in principle have any wavelength. The wavelength ranges of operation of the apparatus 2 typically correspond to the transparency window of the material medium through which the light propagates, in particular the material of the multimode waveguide and/or any of the materials of the optical arrangement that inputs the first 6 and second 8 pulses into the multimode waveguide 4.

Wavelengths of the pump pulses 6,8 may be the same, which gives rise to degenerate four-wave mixing or different which gives rise to non-degenerate four-wave mixing. As such, the first 6 and second 8 input optical pulses propagating in the first and second modes may each have a wavelength from a first set of one or more wavelengths of light. The signal 10 and idler 12 photon pair may have different wavelengths to the first set clone or more wavelengths of light.

Optionally, a further input light source may be input into the multimode waveguide 4 to seed the FWM so that the optical apparatus 2 operates stimulated FWM.

The multimode waveguide 4 may guide a first set of optical modes comprising at least a first mode and a second mode and optionally other higher order transverse modes. The modes may be in any of the Transverse Electric (TE) or Transverse Magnetic (TM) polarisations. For example, the first mode, having the higher group velocity than the second mode, may be the fundamental TM mode 'TMO' whilst the second mode may be the TM first order mode 'TM1'.

Multimode waveguides have an advantage over single mode waveguides in that modes travelling in multimode waveguides typically experience lower propagation losses than equivalent order modes propagating in single mode waveguides. This is typically due to the multimode waveguides being wider than the single mode waveguides, therefore having higher confinement of the modes in the waveguide core.

The multimode waveguide 4 also supports the modes of the generated signal 10 and idler 12 photons. These modes may also be in any of the Transverse Electric (TE) or Transverse Magnetic (TM) polarisations and/or may be of different transverse orders. For example, the idler 12 may be a TMO mode and the signal 10 may be a TM1 mode or vice versa. Another example may be the signal 10 and idler 12 both being in the TMO mode of the multimode waveguide 4, but having different wavelengths. Other modes combinations of the signal 10, idler 12 and pump pulses 6, 8 may be designed for.

The waveguide 4 may have any suitable cross section that, together with the core/cladding material system forming the waveguide 4, supports at least two transverse modes at the wavelength of operation of the pump photons.

The multimode waveguide 4 may be of any suitable structure including any of: an optical fibre, a channel waveguide, a buried waveguide, a rib or ridge waveguide. The multimode waveguide 4 typically comprises a core section and at least one adjoining cladding section. At least part of the core section may be air-clad. Furthermore, any of the optical components described herein that optically couple to the multimode waveguide may be any of the above types of waveguide.

The core material may be a semiconductor material (being doped or undoped) or a dielectric. The core material may be any of: third order or second order nonlinear materials.

The one or more cladding materials may be similar materials as the core (but with a different refractive index) or they may be different. For example, the core material of the multimode waveguide may be silicon but the cladding may be silica. Third order nonlinear materials may be used for the core such as, but not limited to, silicon, silica (Si02), silicon nitride (Si3N4), and silicon oxy-nitride (SiOxNy). Second order nonlinear (or x(2)) materials such as, but not limited to, lithium niobate, strained silicon, polymer and semiconductors may also be used. While possessing both a x(2) and x(3), these materials are commonly referred to as 'x(2)' materials since the second-order response x(2) dominates the x(3) response. semiconductors such as AlGaAs, GaAs, AIN, InGaAs, InAs or InP variants may be used. These materials exhibit a x(2) response and are direct bandgap semiconductors. Other alternatives are hybrid heterostructures where x(2) materials are interfaced with waveguides.

Other waveguides forming the optical apparatus 2 may be formed of similar material systems as the multimode waveguide 4. For example, any of the waveguides forming any one or more of: A) the optical arrangement 16 for inputting pump pulses into the multimode waveguide; B) any optical waveguide circuits that receive any of the pump pulses or signal 10/idler 12 photons from the multimode waveguide 4; such as, but not limited to: optical filters, optical splitters; optical couplers; optical combiners; optical mode converters; optical tapers; C) any active optical components that are optically in communication with the multimode waveguide 4 such as, but not limited to: any one or more optical sources for producing pump light; any one or more optical modulators for gating or creating pump pulses; any one or more optical receivers for receiving signal 10 or idler 12 photons.

The optical apparatus 2 may be an integrated optic apparatus. Any optical components described herein that may form the optical apparatus may be integrated together. This integration may take the form of monolithic integration, hybrid integration or both. For example, certain components of the optical apparatus 2 may be monolithically integrated together wherein the monolithic device may be further hybrid integrated with other portions of the optical apparatus 2.

The processes used to create and assemble different portions of the optical apparatus 2 may use those processes that are standard with integrated optic device manufacture and assembly including material deposition, etching and packaging processes. These processes may be similar to those used for CMOS.

The optical apparatus 2 may include one or more means to tune one or more physical characteristics of the multimode waveguide. Such physical characteristics may be the cross-sectional dimensions of the multimode waveguide, for example the waveguide width or height. For example, the multimode waveguide may be a graded index waveguide (GRIN) with materials that can be tuned to change the effective cross section of the waveguide.

Another characteristic may be the refractive index of the core and/or cladding waveguide material. Tuning the refractive index in turn changes the effective index of the waveguide. This tuning element may be, for example, a thermo-optic heating element in thermal connections with the multimode waveguide (for example a metallic layer deposited over the portion of the apparatus 2 requiring tuning). The tuning means may be activated by inputting one or more electrical signals from an electrical power source. The tuning means may be able to tune the whole or part of the multimode waveguide 4. Changing the physical characteristics of the waveguide 4 allows for different phase matching conditions and different group velocities of different transverse modes of the waveguide 4. This tuning may be used to controllably alter the adiabaticity of overlap between the first and second pump pulse modes propagating in the multimode waveguide. Additionally, or alternatively, the tuning may be used to change the phase matching conditions for generating the signal and idler 12 photons, hence providing the ability to controllably alter the wavelengths of the signal 10 and idler 12 photons.

The optical apparatus 2 may further comprise a further optical arrangement 24 configured to split the signal 10 and idler 12 photons into spatially separate output optical paths. A schematic example of this is shown in figure 3. The elements shown in figure 3 are similar to those shown in figure 1 with like numerals representing like elements. Figure 3 shows a further optical arrangement 24 that receives the signal and idler photon 10, 12 and sends them in different directions along spatially separate optical paths. The arrangement utilises any appropriate optical component including, but not limited to, any one or more of: free space optical path ways; bulk optical components and optical pathways; integrated optic components such as waveguides and integrated optic pathways.

The splitting of the signal 10 and idler 12 photons may utilise any optical property, including but not limited to: a transverse mode splitter that splits different order transverse modes into different optical paths; a filter that splits the signal 10 and idler 12 photon based on their different wavelengths. For example, a filter may be an integrated optic unbalanced Mach-Zehnder filter or a thin film filter. The transverse order mode splitter may be similar to the mode converter MC' described below for the optical arrangement 16 for introducing the pump photons into the multimode waveguide 4 (albeit the photons travel in the opposite direction).

The further optical arrangement 24 may include a portion of the multimode waveguide 4. For example, a mode converter optical circuit may have a waveguide that is a continuous extension of the multimode waveguide 4.

The optical arrangement 16 that introduces pump pulses 18, 20 into the multimode waveguide 4 may utilise any appropriate optical component including, but not limited to, any one or more of: free space optical path ways; bulk optical components and optical pathways; integrated optic components such as waveguides and integrated optic pathways.

The inputting of the pump pulses 6, 8 may utilise any optical property including, but not limited to: a transverse mode combiner that combines different order transverse modes into a single waveguide; a filter that combines the pump photons based on their different wavelengths (assuming the pump pulses are of different wavelengths). For example, a filter may be an integrated optic unbalanced Mach-Zehnder filter or a thin film filter. The transverse order mode combiner may be similar in design to the mode converter 'MC' described below with respect to figure 4.

As described above, the pump pulses 6, 8 may be non-degenerate and have different wavelengths. In such an apparatus design the pulses 6, 8 may come from different optical sources or the same source. If originating from the same source, the different wavelength pump pulses may be carved out from a single broadband pump pulse or be output by fast tuning the source between the two pump pulse wavelengths using optical delays as required to take account of tuning times and ensure the pump pulses are input into the multimode waveguide 4 at the correct times with respect to each other.

If the pump pulses 6,8 are degenerate or non-degenerate then the pulses may come from different optical sources or the same source. In a preferred example the pump pulses 6,8 originate from the same source. In such an example the optical arrangement 16 may be configured to: split an optical pulse from an optical source into the first pulse 6 and second pulse 8. The first pulse propagates along a first optical path and the second pulse propagates along a second optical path spatially separated from the first optical path. The optical arrangement 16 inputs the first pulse 6 from the first optical path into the multimode waveguide 4 to propagate as the first mode 6. The optical arrangement 16 inputs the second pulse 8 from the second optical path into the multimode waveguide to propagate as the second mode 8 such that the second mode propagates along the multimode waveguide ahead in time of the first mode.

Figure 4 shows an example of an optical apparatus 102. Any of the features and configurations of this example may be adapted according to any teaching described herein including any of the optional modifications described above. For example, at least any of the following elements may be adapted with alternative or additional features/configurations: the pump pulses 106, 108, the signal and idler photons 110, 112; the optical arrangement 116; the multimode waveguide 104; the further optical arrangement 124. Furthermore, any of the features and configurations of this example may be used with other examples of an optical apparatus 2 described herein.

The multimode waveguide 104 has, sequentially, a first straight portion 104a connected to a spiral race track-type portion 104b that is then connected to a third straight portion 104c. The first, second and third portions of the multimode waveguide 104 form a continuous waveguide 104. In this example the multimode waveguide 104, first optical arrangement 116 and second optical arrangement 124 are integrated and formed on the same monolithic substrate and may be referred herein as the 'chip'.

The first portion 104a forms part of the optical arrangement 116, which shall be referred to as the first optical arrangement 116 for purposes of this discussion. A light pulse 126 from an optical source (not shown) is input to a first input waveguide of a 2x2 port optical splitter 128. The optical source in this example outputs pulses at a wavelength centred upon 1550 nm with a 4.5 nm bandwidth that initially propagate in the TMO mode of the first input waveguide of the splitter 128. The splitter 128 in this example is an MMI having two input waveguides and two output waveguides. The input and output waveguides of the splitter 128 are single mode waveguides. The splitter 128 amplitude-splits the incoming pulse 126 from the source into the two output waveguides of the splitter at a ratio of 50/50. Each of the output waveguides of the splitter 128 continue to form the respective first and second spatially separate waveguide arms 130 and 132 respectively. Each of these arms 130 and 132 are input into and continue to form the two waveguides of a mode converter 'MC' 134 waveguide circuit. The first waveguide 130 arm extends into and adjoins the multimode waveguide 104 such that the multimode waveguide is a continuous extension of the first waveguide 130. A tapered section is used to transform the first waveguide 130 into the multimode waveguide 104, similar to that described below in figure 5. The taper may be any suitable taper.

The second waveguide arm 132 is spatially separate from the first waveguide arm such that the modes propagating along the arms 130, 132 do not significantly overlap. The portion of the first waveguide arm 130 from the end of the splitter 128 to the wide (multimode) end of the taper 104d has a longer optical path to the interaction region L (see figure 5) of the MC 134 than the equivalent path in the second waveguide arm 132. The equivalent path being the length of the second waveguide arm 132 from the end of the splitter 128 to the start of the interaction region L (see figure 5) of the MC 134. In figure 4 this is shown be the first waveguide arm following a sinusoidal path before entering the tapered region. The difference in optical path length between the longer first waveguide arm 130 and the shorter second waveguide arm 132 gives rise to the delay of the first pump pulse 106 with respect to the second pump pulse 108 in the multimode waveguide 104. The delay in this example is 1.46ps however any delay may be used.

The first waveguide arm 130 is a single mode waveguide from its beginning as the output of the splitter 128 to the narrow end (start of) the taper section 104a. The first pump pulse propagates through the first waveguide arm 130 as the fundamental mode. As the same mode propagates through the taper from the narrow end to the wide, multimode, end, it stays in its fundamental mode and becomes the fundamental mode of the multimode waveguide 104. The energy from the first pump pulse has negligible conversion into the higher order modes of the multimode waveguide 104 along the taper. This is aided by using a symmetrical taper and launching the mode along the central longitudinal axis of the multimode waveguide 104.

The multimode waveguide 104 then extends along as a straight section 104a until it then turns into a spiral geometry 104b and then exits the spiral geometry to become another straight section parallel to the beginning section 104a. The spiral section 104b of the multimode waveguide 104 is formed to create a large length of the multimode waveguide 104 in a relatively low area of the chip. Along the length of this waveguide 104 the first pump pulse 106 adiabatically catches up, overlap and pulls away from the second pump pulse 108, in a similar manner to that described above for figure 2. Area 136 in figure 4 shows where the first and second pump pulses temporally overlap to the greatest extent. In this area of overlap, the creation of a signal 110 and idler 112 photon pair is most likely to OCCUr.

The idler photon 112 gets generated in the fundamental order mode of the multimode waveguide 104 whilst the signal photon 110 gets generated in the first order mode of the multimode waveguide. The signal photon 110 has a wavelength around 1588 nm in the TM1 mode and the idler 112 photon has a wavelength around 1516 nm in the TMO mode, however the signal 110 and idler 112 photons may have different orders, be TE or TM or have other wavelengths as described elsewhere herein. For example, any of the signal and idler photons propagating in the multimode waveguide 104 may be in the TMO, TM1, TEO, TEl modes.

Assuming a signal/idler photon pair 110, 112 get generated, all 4 modes (which includes the two residual pump pulse modes) propagate along the multimode waveguide 104 and enter the second MC 138 of the second optical arrangement 124. This second MC is similar to the first MC 134 and is used to separate the signal 110 and idler 112 modes. The signal 110 and idler 112 modes in this example have different wavelengths to each other and each has a different wavelength to the pump pulses. The design of the second MC may therefore be different to that of the first MC to ensure that as close to 100% coupling of the signal mode 110 occurs between the multimode waveguide 104 and the single mode waveguide 140 of the second optical arrangement 124. The idler mode 112 does not couple into the output single mode waveguide 140 of the second MC 138, but stays in the multimode waveguide 104 and enters a taper that narrows the multimode waveguide 104 into a single mode waveguide 142 in a manner similar to that described for figure 5 but in reverse. The signal and idler modes 110, 112 are therefore output on two different spatially separated waveguides 140, 142. It is envisaged that the residual light of the pump pulses 106 108 (that was not used to convert to the signal/idler photons 110, 112) are also output by this apparatus 102 as shown in figure 4. The majority of the second pump pulse 108 is coupled into the output waveguide 140 and the majority of the first pump pulse stays in the fundamental mode of the multimode waveguide and propagates through the taper of the second optical arrangement into the output waveguide 142.

The input of light into the optical apparatus 102 and the output of light from the apparatus 102 may be facilitated by optically coupling the optical apparatus 102 to one or more optical fibres or other optical components such as bulk optics (e.g. bulk optic lenses). These further components may be physically coupled to the optical apparatus 102 and optionally may be integrated with the optical apparatus 102.

One or more optical filters (not shown) may then be used to filter out the pump pulses 106, 108 from the signal/idler photons 110, 112 so that the output from the apparatus 102 are two spatially separate optical paths, one carrying the signal photon 110, one carrying the idler photon 112. For example, signal 110 and idler 112 photons may be out-coupled to optical fibres, where the pump wavelength light is filtered out via broad-band fibre Bragg-gratings.

The output single photons (signal, idler photons) may be detected with one or more optical detectors such as, but not limited to superconducting-nanowire single photon detectors (SNSPDs). Other filters, filtering arrangements, detectors and detecting arrangements may be used instead or in addition.

The waveguides of the example apparatus 102 have silicon cores fabricated using CMOScompatible UV-lithography processes. These waveguides have a height of 220 nm (0.22 Rm) and are located atop a 2 pm layer of silica. The waveguides have a silica over cladding of 3 Rm. Other waveguide materials and geometries may be used.

The multimode waveguide 104 has a waveguide width of 2 Rm and a height of 0.22 Rm. To clarify, the width of the waveguide is that measured about the cross section of the waveguide in a plane parallel to the plane of the wafer the waveguide is formed upon or parallel to the major surfaces of the other layers in the chip such as the under-cladding layer that the core is formed upon. The height is the dimension perpendicular to this. The multimode waveguide 104 has a length of 11 mm. The initial temporal delay between the TMO and TM1 pump pulse modes 106, 108 is 1:46 ps. Modal cross-talk in the spiral is kept below -25 dB extinction by adopting 90 Euler bends of radius 45 pm. The plan footprint of the optical apparatus 102 is approximately 200m x 900pm. The TMO-TM1 mode converters 134, 138 used to inject the pump pulses into the multimode waveguide 104 and separate the signal 110 and idler 112 photons at the output have < -30 dB characterised modal crosstalk, and >95% conversion efficiency.

It should be understood that the above configurations and details of the waveguides, filters and general chip make-up may be different and that in general, the above apparatus 102 may optionally be adapted according to any suitable feature or configuration described elsewhere herein.

Any one or more of: the multimode waveguide 104; first 116 and second 124 optical arrangements, may be formed as separate elements and then optically coupled together or may be formed integrally with other elements of the apparatus 102. For example, the multimode waveguide may be formed as an optical fibre whilst the mode converters may be formed as integrated optic components.

The wavelength of operation of the apparatus 102 (hence the wavelengths of the pump 106, 108, signal 110 and idler 112 pulses) may be any suitable wavelength range, including, but not limited to, any one or more of the following: the visible range (380-750nm), the near infrared range (750nm-2500nm), the mid-infrared range (2.5 -10pm), the far-infrared range (10pm -lrnm).

The wavelength range used typically depends upon the materials and configurations of the optical apparatus 2. For example, if a 501 platform is used the preferred wavelength range may be 1500-1600 nm, however if a SIN platform is used the preferred wavelength range may be 750-1600 nm.

The wavelength of operation of the apparatus 102 (hence the wavelengths of the pump 106, 108, signal 110 and idler 112 pulses) may be between 700-1625 nm. For telecommunications and other applications this may be in any one or more of the following bands: the first window TLC (700-900 nm), the 0-band (original band: 1260-1360 nm); the C-band (conventional band: 1530-1565 nm), the L-band (long-wavelength band: 1565-1625 nm); the S-band (short-wavelength band: 1460-1530 nm); the E-band (extended-wavelength band: 1360-1460 nm).

The pump pulse wavelengths may be any of: between 300-5000 nm for degenerate pumping; between 300-5000 nm for non-degenerate pumping. The signal 110/idler 112 wavelengths may be any of: between 300-15000 nm for degenerate pumping; between 300- 5000nm for non-degenerate pumping. The pump pulses may have any of: a repetition rate of between 0-1000 GHz; a bandwidth of between 0.001m -100 nm.

The optical source for generating the pump pulses 126 may be separate to or part of the optical apparatus 102. The pump source may be integrated with the other components of the apparatus 102. An example of a pump source that may be integrated with the optical apparatus 102 may be an optical source formed from an electrically pumped III-V laser material on a SOI wafer that is formed by hetero-integration.

For example, an optical source may be used such as one disclosed in "Integrated heterogeneous silicon/iii-v mode-locked lasers," Photonics Research 6, 468-478 (2018), by Michael L Davenport, Songtao Liu, and John F Bowers. The entire contents of this reference are incorporated herein by reference. This source is a hetero-integrated mode locked laser having a 20 GHz repetition rate, around 98 mW of peak power and a sech2 shaped pulse width of 900 fs. Monolithic interconnection between the optical apparatus 102 and such a source may feed the pump laser the optical apparatus 102 directly on-chip. This may improve the scaling towards large-scale devices.

The splitter 128 may alternatively be optical splitter having at least one input and at least two outputs, hence at least a 1x2 port splitter, for example, a Y-branch or a 1x2 Multimode Interference (MMI) coupler.

The multimode waveguide 104 may have a different length to that given above. For example, the length of the multimode waveguide may be between 0.01-1000mm.

The mode converter 'MC' 134 used in the example in figure 4 is shown in an expanded view in figure 5. In principle, any type of mode converter may be used for any of the optical apparatus 2 examples described herein. The MC may be designed similarly to that described in: 'Dimensional variation tolerant mode converter/ multiplexer fabricated in 501 technology for two-mode transmission at 1550 nm' by David Garcia-Rodriguez et al, Optics letters, Vol. 42, No. 7, 1st April 2017, the entire contents of which are incorporated herein by reference. A summary of the operation of the coupler in this paper is provided below. The modes described in this coupler are TE, however TM mode designs are also achievable.

The operation of directional couplers can be described as being governed by coupled mode theory where the following equations [1] and [2] describe the optical fields 'E' in each waveguide (A, B): [1, 2] z, t) = A(z)itnn(x, y)g661-g-'0'

B B

E (x, y, z, = B(z)En (x, y)eMPBqz-"), Where: x and y are the horizontal and vertical axis in the cross section of the waveguide, z is the propagation direction; co is optical frequency, t is time; mnfrf Y t( ,)/) are the modal profiles of the modes under consideration in each waveguide ('mn' in the waveguide A and 'pq' in the waveguide B); A(z), B(z) are the complex envelopes along the direction of propagation 13Amn and (3Apq are the propagation constants for both modes, respectively.

Optical power is periodically exchanged between both waveguides appearing at the first maximum in the coupled output for a distance 'Lc' governed by equations [3] and [4]: [3] L, [4] fic = VKabKba ± 62 where wab and Kba are the mutual coupling coefficients of the waveguide B to A and vice versa; 8 is the phase mismatch.

The mode converter 134 in figure 5 is constructed as an asymmetrical directional coupler (ADC), where the waveguide widths are not equal. The waveguides in the ADC are separated a distance, d, with a certain coupling length, L, and height, h (not shown in the figure).

The ADC has a single-mode waveguide 132 propagating the TED mode and a multimode waveguide 104a (in this example two-mode 104a waveguide) propagating both TEO and TEl modes. TED enters the single-mode waveguide 132, and it is converted to TEl in the two-mode waveguide 104a. In order for mode conversion the effective index of TED in the single-mode waveguide must match the TEl in the two-mode waveguide which can be accomplished by increasing the width of the two-mode waveguide. The operation principle of the MC is regulated by the phase-matching condition, where the effective index of the TED mode in the waveguide 132 matches the effective index of the TEl mode on the multimode waveguide 104a to enable 100% power transfer.

In the example of figure 5, the first waveguide arm 130 starts as a single mode waveguide and then is linearly tapered 104d to a width that supports multiple modes, hence becoming the first part 104a of the multimode waveguide 104. The second waveguide arm 132 approaches the first part of the multimode waveguide 104a, then changes its direction to run parallel to the multimode waveguide 104a at a distance d, for a length L before it then turns in a direction away from the multimode waveguide 104a. Over the length L, the majority of the mode coupling occurs such that the optical energy in waveguide 132, carried as a fundamental mode TEO couples across and into the first order TEl mode in the multimode waveguide TE1. Conversely there may be some mode coupling of the fundamental mode TED propagating in the multimode waveguide 104a into the fundamental mode TED of the single mode waveguide 132. This however is negligible because the fundamental mode of the multimode waveguide 104a is well confined in the wide core and so has weak coupling to the single mode waveguide 132. This means that the length Lc required to couple energy from the multimode waveguide 104a to waveguide 132 is much longer than the interaction length L. Furthermore, the effective indices of the two modes are very different so that power transfer from the multimode waveguide 104a to the single mode waveguide 132 is far below 100%.

Figure 6 shows another example of an optical apparatus 2. This example is an alternative to the apparatus 102 shown in figure 4 and is a fibre-based optical apparatus 200. This example may be adapted according to any suitable feature or configuration described herein. Furthermore, any feature or configuration described in this example may be used in other examples described herein.

A pulse 201 from an optical source (not shown) is input to a first single-mode optical fibre 202. The single mode fibre 202 continues as the input to a 2x2 or 1x2 fibre optical splitter 203, where the input pump pulse 201 is split via into two output single-mode fibres of the splitter at a ratio of 50/50 (although other ratios may be used). Each of the output waveguides of the splitter 203 continue to form the respective first and second spatially separate single-mode fibres 204a and 204b respectively.

Two separate polarisation controllers 206a and 206b receive light from the two output fibres 204a, 204b and are used to control the polarisation in the single-mode fibres 204a and 204b, respectively before the light from the fibres is input into a first phase mask (PM) 207a. Each of the optical fibre paths 204a, 204b extending between the output from the splitter 203 to the input to the phase mask 207a may be formed of one or more optical fibres. The polarisation controllers may be formed integrally with the output fibres from the splitter 203 or may be a separate component optically coupled to the said output fibres.

The first phase mask (PM) 207a is used to receive light from the optical fibre paths 204b, 204a (after the light exits the polarisation controllers 206a, 206b and input light into a multimode optical fibre 208 (that supports multiple transverse modes). The first PM 207a converts the light pump pulse from the first single-mode fibre 204a into the fundamental transverse mode of a multimode optical fibre 208. The light pump pulse from the second single-mode fibre 204b is instead converted by the first PM 207a into the first order transverse mode of the same multimode optical fibre 208. The polarization controllers 206a and 206b are used to adjust the polarisation of the two input pulses to improve the overlap between the two pump pulses propagating through the multimode fibre 208 in order to optimise the spontaneous photon generation non-linear process.

Variable Optical Delay Lines (VODL's) 210 are shown in the optical fibre paths 204b/ 204b.

These are optional elements wherein one or both optical paths, including the extra fibre delay length 205, may have one or more VODL's. The VODLs are variable optical delays wherein the delays are controllable by a user either manually or via an electronic control signal. VODL's may be used to finely tune the delay between the pump pulses, hence optimizing their overlap in the multimode waveguide 208. The VODL's may be useful at least for the following reasons. Firstly, it allows the system to be adjusted to account for the any random temporal impairments which may be introduced from the separate fibres which carry the pump pulses. Secondly the VODLs provide for control the degree of spectral entanglement of the photon pairs and the brightness of the apparatus 200.

The end of the multimode optical fibre 208 (that is distal from the end optically coupled to the phase mask) is connected to a second PM 207b. The second PM 207b receives light from the multimode fibre 208 and converts the first order and the fundamental transverse modes of the multimode fibre 208 into the two separated output single-mode fibres 209a and 209b, respectively.

The portion of the first single-mode fibre 204a from the end of the splitter 203 to the start of the first PM 207a has a longer optical path than the path of the second single-mode fibre 204b from the end of the splitter 203 to the start of the first PM 207a. In Figure 6 this is shown as an additional spool of fibre 205. This difference between the optical paths of the two fibres 204a and 204b gives rise to the delay between the pump pulses in different transverse modes injected in the multimode fibre. The functioning principles of the delay and the overlap between the different transverse modes are similar to that described above for the other examples.

Signal/idler photon pairs can be generated inside the multimode fibre 208 due to the inter-modal spontaneous non-linear process pumped by the two pump pulses propagating through the multimode fibre 208 in the fundamental and first order transverse modes. Assuming a signal/idler photon pair get generated, both photons propagate with the pump pulses along the multimode waveguide 208 and enter the second PM 207b. The signal photon, emitted in the first order mode, is then routed by the second PM 207 into the output single-mode fibre 209a. The idler photon, emitted in the fundamental mode, is instead routed by the second PM 207 into the other output single-mode fibre 209b. The signal and idler modes are thus output on two different spatially separated fibres. Remaining pump light can also be converted by the second PM 207 and be present in each of these output fibres 209a and 209b. One or more optical fibre filters (not shown) may then be used to filter the pump pulses from the signal idler photons so that the output from the apparatus 200 are two spatially separate optical fibres, one (209a) carrying the signal photon, one (209b) carrying the idler photon.

It is appreciated that any of the components described herein that require an electrical signal to operate, such as a laser or detector, may be electrically coupled to appropriate driving electronics including electrical lines and electrical apparatus outputting electrical signals via the electrical lines to the said components.

The optical apparatus 2 and any one or more components associated with the apparatus 2 may be part of an optical system. Any one or more components of the optical system may be integrated into an integrated optical device. Such associated components may be, but not limited to: the filters, the pump pulse sources, one or more optical detectors to detect any of the light output from the apparatus; one or more optical fibres to input light into the apparatus or receive light output from the apparatus; any electrical apparatus to drive any of the components requiring or outputting electrical signals.

Claims

Claims 1. An optical apparatus (2) comprising: I) a multimode waveguide (4) capable of: a) guiding at least a first mode (6) and a second mode (8); the first mode being a different order transverse mode to the second mode; b) generating and guiding a signal (10) and idler (12) photon pair by annihilating (14) a photon in each of the first mode (6) and second mode (8); II) an optical arrangement (16) configured to: c) input (18) a first optical pulse into the multimode waveguide (4) to propagate in the first mode (6); d) input (20) a second optical pulse into the multimode waveguide (4): i) to propagate in the second mode (8); ii) such that the second mode (8) propagates along the multimode waveguide (4) ahead in time (22) of the first mode (6); wherein the first mode (6) has a higher group velocity than the second mode (8).
2. The optical apparatus as claimed in claim 1 wherein the optical arrangement is configured to: I) split an optical pulse from an optical source into the first optical pulse and second optical pulse wherein: the first optical pulse propagates along a first optical path and the second optical pulse propagates along a second optical path spatially separated from the first optical path; II) input the first optical pulse from the first optical path into the multimode waveguide to propagate as the first mode; III) input the second optical pulse from the second optical path into the multimode waveguide to propagate as the second mode such that the second mode propagates along the multimode waveguide ahead in time of the first mode.
3. An optical apparatus as claimed in claim 2 comprising: I) an optical splitter for splitting light from the optical source into the first and second spatially separated optical paths; I) an optical delay for delaying the first optical path with respect to the second optical path such.
4. An optical apparatus as claimed in any preceding claim wherein: I) the first and second input optical pulses propagating in the first and second modes each have a wavelength from a first set of one or more wavelengths of light; II) the signal and idler photon pair have different wavelengths to the first set of one or more wavelengths of light.
5. An optical apparatus as claimed in any preceding claim comprising a further optical arrangement configured to split the signal and idler photons into spatially separate output optical paths.
6. An optical apparatus as claimed in any preceding claim wherein the signal photon propagates in the multimode waveguide in a transverse mode order that is different to the transverse mode order of the idler photon.
7. An optical apparatus as claimed in claim 6 wherein the further optical arrangement comprises a mode converter for receiving light from the multimode waveguide and outputting the signal and idler photons in spatially separate optical waveguide output paths; each of the signal and idler photons propagating in the respective fundamental modes of the respective optical waveguide output paths.
8. An optical apparatus as claimed in any of claims 4-7 further comprising one or more optical filters for separating at least one of the signal and idler photons from light from the first set of wavelengths.
9. An optical apparatus as claimed in claim 8 comprising: I) a first optical filter for receiving light from a first of the spatially separate optical waveguide output paths; II) a second optical filter for receiving light from a second of the spatially separate optical waveguide output paths.
10. An optical apparatus as claimed in any preceding claim wherein the multimode waveguide comprises a length such that the first mode propagates past the second mode as both modes are propagate in the multimode waveguide.
11. An optical apparatus as claimed in any preceding claim wherein the multimode waveguide comprises an integrated optic waveguide.
12. An optical apparatus as claimed in claim 11 wherein the multimode waveguide and optical arrangement are integrated together.
13. An integrated optic device comprising the optical apparatus as claimed in any preceding claim.
14. An integrated optic device as claimed in claim 13 further comprising one or more optical sources for generating the first and second optical pulses.
15. An optical apparatus as claimed in any of claims 1-10 wherein the multimode waveguide comprises an optical fibre.
16. An optical apparatus as claimed in claim 15 wherein the optical arrangement comprises a phase mask.
17. An optical apparatus as claimed in any of claims 15-16 wherein the optical arrangement comprises a variable optical delay.
18. An optical system comprising: I) the integrated optical device as claimed in claims 13 or 14; or, II) the optical apparatus as claimed in any of claims 1-12; or 15-17; and one or more optical detectors for receiving the signal and/or idler photon.