SINGLE-PHOTON OPTICAL DEVICE

Abstract

This disclosure relates to an optical device comprising: a first filter waveguide section having an input for receiving a pump signal, the first filter waveguide section further having an output; an emitter waveguide section having an input coupled to the output of the first filter waveguide section to receive a transmitted pump signal therefrom, the emitter waveguide section supporting at least a first guided lower-order optical mode and a second guided higher-order optical mode, the emitter waveguide section comprising a photon emitter coupled to the first guided mode to emit radiation into the first guided mode and coupled to the second guided mode to allow optical pumping of the photon emitter by pump signal power carried in the second guided mode, the emitter waveguide section further having an output for outputting radiation emitted from the photon emitter; a second filter waveguide section having an input coupled to the output of the emitter waveguide section and having an output, the second filter waveguide section being configured to transmit radiation emitted into the first guided mode with lower loss than radiation emitted into modes other than the first guided mode; the first filter waveguide section being configured to couple pump signal power predominantly into the second guided mode of the emitter section.

Claims

1.-15. (canceled)

16. An optical device comprising: a first filter waveguide section having an input for receiving a pump signal, the first filter waveguide section further having an output, an emitter waveguide section having an input coupled to the output of the first filter waveguide section to receive a transmitted pump signal therefrom, the emitter waveguide section supporting at least a first guided lower-order optical mode and a second guided higher-order optical mode, the emitter waveguide section further comprising a photon emitter coupled to the first guided mode to emit radiation into the first guided mode and coupled to the second guided mode to allow optical pumping of the photon emitter by pump signal power carried in the second guided mode, the emitter waveguide section further having an output for outputting radiation emitted from the photon emitter, a second filter waveguide section having an input coupled to the output of the emitter waveguide section and having an output, the second filter waveguide section being configured to transmit radiation emitted into the first guided mode with lower loss than radiation emitted into modes other than the first guided mode, the first filter waveguide section being configured to couple pump signal power predominantly into the second guided mode of the emitter section.

17. An optical device in accordance with claim 16, wherein the first guided mode is a fundamental mode of the emitter waveguide section and the second guided mode is a first-order mode or a second-order mode of the emitter waveguide section.

18. An optical device in accordance with claim 16, wherein the first filter waveguide section is configured to suppress transmission of pump signal power into the first guided mode of the emitter waveguide section while allowing transmission of pump signal power into the second guided mode of the emitter waveguide section.

19. An optical device in accordance with claim 16, wherein the first filter waveguide section comprises a photonic crystal structure configured to suppress transmission of pump signal power into the first guided mode of the emitter waveguide section while allowing transmission of pump signal power into the second guided mode of the emitter waveguide section.

20. An optical device in accordance with claim 16, wherein the second filter waveguide section comprises: one or more tapers configured to cause power carried in modes other than the first guided mode, but not power carried in the first guided mode, to leak; and/or one or more bends configured to cause power carried in modes other than the first guided mode, but not power carried in the first guided mode, to leak.

21. An optical device in accordance with claim 16, wherein the photon emitter is a quantum-confined structure, such as a quantum dot.

22. An optical device in accordance with claim 16, wherein the photon emitter is situated away from a symmetry axis of the emitter waveguide section.

23. An optical device in accordance with claim 16, wherein the input of the first filter waveguide section is coupled to a grating coupler for receiving the pump signal, and the second filter waveguide section output is coupled to a grating coupler for coupling radiation emitted by the photon emitter out of the optical device.

24. An optical device in accordance with claim 16, further comprising: an input fibre for receiving the pump signal and for coupling the received pump signal into the first filter section, and/or an output fibre for coupling single photons generated in the emitter waveguide section out of the optical device.

25. An optical device in accordance with claim 16, wherein the first filter waveguide section, the emitter waveguide section, and at least an initial part of the second filter waveguide section are monolithically integrated.

26. An optical device in accordance with claim 16, wherein one or more of the waveguide sections are based on one or more III-V semiconductor materials, such as In and/or Ga and/or Al and/or As, or other group III or group V material.

27. An optical device in accordance with claim 16, wherein the photon emitter is based on In and/or Ga and/or Al and/or As, such as InGaAs, or other group III or group V material.

28. An optical system comprising an optical device according to claim 16 and an optical pump signal source for providing the pump signal.

29. An optical device in accordance with claim 16 or an optical pump signal source for providing the pump signal, wherein the optical device or optical system is configured to provide single photons at a wavelength in the range 400 nm-1600 nm.

30. An optical device or optical system in accordance with claim 16, wherein the pump signal is configured to resonantly excite the photon emitter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0066] The invention is explained in detail below with reference to the drawings.

[0067] FIG. 1 illustrates schematically a single-photon optical device in accordance with an embodiment of the invention.

[0068] FIG. 2A illustrates a single-photon optical device in accordance with an embodiment of the invention.

[0069] FIG. 2B illustrates a first filter waveguide section in more detail.

[0070] FIG. 3 shows a fabricated single-photon optical device based on a layout similar to that shown in FIG. 2A.

[0071] FIG. 4 shows a transmission spectrum of a first filter waveguide section comprising a photonic crystal structure.

[0072] FIG. 5 shows a transmission spectrum illustrating the influence of having a first filter waveguide section that includes a photonic crystal structure.

[0073] FIG. 6 shows a ratio between the number of residual laser photons per single photon emitted as a function of a position of the photon emitter.

[0074] FIG. 7 shows schematically guided modes supported by an illustrative waveguide as a function of the width of the waveguide.

DETAILED DESCRIPTION OF SELECTED EMBODIMENTS

[0075] The invention is described in the following with reference to the drawings. The drawings are not necessarily to scale, except if otherwise indicated.

[0076] FIG. 1 schematically illustrates an optical device 100 for generating single photons 160 from a pump signal 140 (pulsed). The optical device 100 comprises a first filter waveguide section 101 having an input 111 and an output 121, an emitter waveguide section 102 having an input 112 and an output 122, and a second filter waveguide section 103 having an input 113 and an output 123. In the schematic illustration in FIG. 1, the first filter waveguide section 101, the emitter waveguide section 102, and the second filter waveguide section 103 are arranged in series, the output 121 of the first filter waveguide section 101 being coupled to the input 112 of the emitter waveguide section 102, the output 122 of the emitter waveguide section 102 being coupled to the input 113 of the second filter waveguide section 103. The pump signal 140 enters the first filtering waveguide section via input 111 of the first filter waveguide section, and the single photons are provided at the output 123 of the second filter waveguide section. The pump signal 140 and the single photons 160 are only schematically illustrated. Nothing shall be inferred from the drawing about the properties of the pump signal or the single photons, including any temporal width, amplitude, frequency, or otherwise.

[0077] The person skilled in the art will recognize that the “input” and “output” of the different sections 101, 102, 103 may be distinguishable or not. If the sections 101, 102, 103 are monolithically integrated, the inputs and outputs, in particular inputs 112 and 113, and the outputs, in particular outputs 121 and 122, may be virtual only, being defined by the function of the particular sections and not necessarily by a distinguishable structural property, such as an air gap or taper in the waveguide geometry.

[0078] The emitter waveguide section 102 comprises a photon emitter 150, such as a quantum dot.

[0079] The first filter waveguide section 101, the emitter waveguide section 102, and the second filter waveguide section 103 have waveguiding properties and preferably each supports one or more guided modes. At the quantum dot 150, the emitter section supports at least two modes, in particular at least a lower-order mode, such as a fundamental mode E.sub.1, and a higher-order mode, such as a first-order mode O.sub.1. The mode E.sub.1 may be even or odd. It is preferable that E.sub.1 is an even mode, preferably a fundamental mode of the emitter waveguide section. Similarly, mode O.sub.1 may be even or odd. Preferably, it is a first-order mode of the emitter waveguide section. In the present example, the first filter waveguide section and the emitter waveguide section are shown as having a uniform and identical widths, which means that the emitter waveguide section supports virtually the same modes at the emitter waveguide section input 112, at the position of the quantum dot 150, and at the emitter waveguide section output 122. The emitter waveguide section may instead have a non-uniform width, in which case the mode profiles change along the waveguide.

[0080] In the present example, the quantum dot 150 is located off the centreline 130 of the (in this case symmetric) emitter waveguide section. Both guided modes E.sub.1 and O.sub.1 overlap with the quantum dot at this position. Radiation in the first-order mode O.sub.1 can therefore excite the quantum dot 150, which in response emits one or more photons. The coupling of the first-order mode O.sub.1 to the quantum dot is preferably relatively small and the coupling to the fundamental mode E.sub.1 relatively high. This increases the degree to which photons are preferentially emitted into the mode E.sub.1. Since the mode O.sub.1 is used to excite the quantum dot, emission into the first-order mode from the quantum dot is unavoidable.

[0081] The modes in the different section 101, 102, 103 are schematically illustrated below the illustration of the optical device 100. As described above, the emitter waveguide section supports modes E.sub.1 and O.sub.1. In accordance with embodiments of the invention, the first filter waveguide section suppresses the mode E.sub.1. Thus, below section 101, the mode E.sub.1 is shown with a dashed line, indicating that it is not supported or at least experiences a high loss compared to mode O.sub.1.

[0082] The second filter waveguide section, on the other hand, does not support O.sub.1, but supports a mode F.sub.1, which for the purpose of this example is the mode at the output 123 of the second filter waveguide section. F.sub.1 is illustrated as having a different shape from that of the mode E.sub.1. This may or may not be the case and is a matter of design, for instance dictated by requirements of the intended use of the device.

[0083] As described above, single photons are emitted into both mode E.sub.1 and mode O.sub.1, and they are mixed with residual pump signal power, carried mostly in mode O.sub.1. To isolate the emitted single photons, the radiation from the emitter waveguide section is filtered so that only photons emitted by the photon emitter remain. It is important to suppress the coupling of pump signal from the first filter waveguide section into mode E.sub.1 in the emitter waveguide section as much as possible, since any background pump signal in the mode E.sub.1 affects the purity of the generated single photons.

[0084] Due to the mode structure in the second filter waveguide section described above and illustrated below the second filter waveguide section 103 in FIG. 1, residual pump signal and single-photon power emitted into mode O.sub.1 in the emitter waveguide section is filtered out, while single-photon power emitted into the mode E.sub.1 is transmitted in the second filter waveguide section.

[0085] In this way, the optical device 100 can create virtually pure single photons.

[0086] FIG. 2A illustrates an optical device 200 for generating single photons. It comprises a first filter waveguide section 201, an emitter waveguide section 202, and a second filter waveguide section 203, similar to the schematic device 100 in FIG. 1, but with a geometry suitable for generating indistinguishable single photons. The layout 200 furthermore comprises a grating coupler 221 for receiving a pump signal, and a waveguide 222 for carrying the pump signal to the first filter waveguide section 201.

[0087] FIG. 2A furthermore shows a fundamental mode E.sub.1 and a first-order mode O.sub.1 for a nanobeam waveguide having a thickness of 170 nm and a width of 450 nm, as illustrated in the mode image for mode E.sub.1. As shown, emitter waveguide section 202 supports both E.sub.1 and O.sub.1, whereas the photonic crystal part of first filter waveguide section 201 supports only O.sub.1.

[0088] As discussed above, the division into sections can be performed in many ways without departing from the invention. For instance, the first filter waveguide section 201 in FIG. 2A could be “lengthened” to include a part that does not include holes, for instance a part of the emitter waveguide section 202 (but not the photon emitter), and accordingly the emitter waveguide section would be correspondingly shorter (as the input of the emitter waveguide section is coupled to the output of the first filter waveguide section). A part of the first filter waveguide section would then support both E.sub.1 and O.sub.1. However, the first filter waveguide section would still provide filtering away of E.sub.1 as is the purpose of the first filter waveguide section. Thus, such an alternative division into sections would still be an embodiment of the invention. The important thing is that the device comprises sections capable of performing the respective functions.

[0089] In FIG. 2A, the first filter waveguide section 201 comprises a photonic crystal structure 210, which supports only mode O.sub.1 due to an array of holes. The first filter waveguide section is shown in more detail in FIG. 2B. The holes are made in a beam (nanobeam) waveguide having a width of 450 nm and a thickness of 170 nm, the same dimensions as the first filter waveguide section in FIGS. 2A and 3. Each of the 40 holes has a radius of 70 nm, and the hole-to-hole distance is 210 nm.

[0090] To produce single photons, a pump signal is provided at grating coupler 221. Between the grating coupler 221 and the first filter waveguide section 201, the pump signal is prepared in modes E.sub.1 and O.sub.1. Thus, after the waveguide 222, and just before the first filter waveguide section 201, pump signal power is carried in exactly those two guided modes. At the first filter waveguide section 201, the power carried in fundamental mode E.sub.1 is filtered out (reflected, in fact), leaving only pump signal power carried in the first-order mode O.sub.1.

[0091] In emitter waveguide section 202, a quantum dot (not visible in FIG. 2A) is pumped by the pump signal transmitted through the first filter waveguide section 201 in the first-order mode O.sub.1. The pumping occurs via the coupling between the quantum dot and the first-order mode. In response, a photon is emitted, partly and mostly into the fundamental mode E.sub.1, and partly into the first-order mode O.sub.1. After the quantum dot, the first-order mode O.sub.1 carries both residual pump power and single-photon radiation emitted into the first-order mode O.sub.1. This radiation affects the purity of the emitted photon. Therefore, second filter waveguide section 203 is designed to remove power carried in the first-order mode O.sub.1 in the emitter waveguide section. This is achieved in this example by including bends 204, 205 that cause the first-order mode O.sub.1 to leak out (much more than the fundamental mode E.sub.1). Tapering and widening sections as shown in FIG. 2A also cause the first-order mode O.sub.1 power to be removed.

[0092] This leaves a single photon with high purity at the end of the second filter waveguide section 203, where it is shaped as illustrated by mode F.sub.1. Here, the waveguide is a nanobeam having a width of 200 nm (as indicated in FIG. 2A) and a thickness of 170 nm.

[0093] Finally, a grating coupler 231 couples the single photon out of the optical device 200.

[0094] FIG. 2A also illustrates a further grating coupler 241, coupled to the first filter waveguide section 201 by a waveguide 242. This grating coupler, which can be employed generally in embodiments of the invention, not just the particular embodiment in FIG. 2A, is used to align a pump signal source (not shown) with the grating coupler 221. The pump signal is reflected by the photonic crystal structure in the first filter waveguide section 201, and the reflected signal is split by the Y-coupler. The coupling of pump signal power into the device is optimized by aligning the pump signal source such that the signal coupled out at grating coupler 241 is maximized.

[0095] FIG. 3 illustrates an actual device 300 fabricated in accordance with a layout very similar to the optical device 200 shown in FIG. 2A. The black parts and the lighter areas surrounding the black parts are grooves created by etching. The lighter-shaded parts between the black grooves are suspended nanobeams. The grooves defining the grating coupler are also clearly visible.

[0096] In the following, the principle of the first filter waveguide section is described in more detail. As an example, a first filter waveguide section comprising a photonic crystal is designed using finite-element numerical calculations. It is designed to support two the two modes E.sub.1 (fundamental mode) and O.sub.1 (first-order mode). The first filter waveguide section in this example has a photonic crystal with 20 holes (whereas the optical device in FIG. 2A, the illustration of the first filter waveguide section 201 in FIG. 2B, and the device in FIG. 3 have holes).

[0097] The holes are made in a beam (nanobeam) waveguide having a width of 450 nm and a thickness of 170 nm, the same dimensions as the first filter waveguide section in FIGS. 2 and 3. Each of the 20 holes has a radius of 70 nm, and the hole-to-hole distance is 210 nm. FIG. 4 illustrates the transmission of the two modes across this photonic crystal. It can be seen that the photonic crystal provides a broad stop band with a >40-dB suppression of the fundamental mode E.sub.1 in the range 920-960 nm. This high degree of suppression of the mode E.sub.1 compared to the mode O.sub.1 by the photonic crystal filter provides a way to prepare the pump signal selectively in the O.sub.1 mode for coupling into the emitter waveguide section. This signal is subsequently used to pump the photon emitter in the emitter waveguide section. As described above, filtering out E.sub.1 is very important, since there may not be a way to filter out pump signal power in mode E.sub.1 once it reaches the emitter waveguide section, especially in embodiments where the pump signal is resonant with the photon emitter. In that case, the pump signal cannot be filtered out for instance using a spectral filter.

[0098] To improve the suppression of the fundamental mode even more, additional holes can be added, as illustrated in the device 200 in FIG. 2A and device 300 in FIG. 3.

[0099] In a simulation, the transmission of the first-order mode O.sub.1 after the emitter section across the taper and the subsequent waveguide bends in the device 300 in FIG. 3 is estimated to be around 10.sup.−6-10.sup.−7.

[0100] The measured transmission Ti of the pump laser across the actual device 300 (see FIG. 3) is shown in FIG. 5. The transmission is normalized to a device without the photonic crystal in the first filter waveguide section, i.e. where the fundamental mode is not suppressed before the emitter waveguide section. It can be seen that the transmission fluctuates between 10.sup.−3 and 10.sup.−5 over the design range. Regions of minimum transmission where T.sub.l˜2.Math.10.sup.−5 are routinely observed across fabricated devices. Since the transmission levels match quite well the simulated transmission of E.sub.1, it can be seen that the residual laser signal is caused entirely by leakage across the photonic crystal section. It is therefore a matter of design to suppress E.sub.1.

[0101] An essential figure-of-merit of a resonantly excited quantum dot is the intensity of the residual pump signal relative to the intensity of emitted single photons. Here, this ratio is denoted E.sub.QD and indicates the number of laser photons per single photon. The single-photon purity at the collection grating is related to this quantity by

g.sup.(2)(0)=2ϵ.sub.QD−ϵ.sub.QD.sup.2

where g.sup.(2)(τ) is the second-order coherence function of the signal. The background laser intensity at the collection grating is given by I.sub.lT.sub.l, where I.sub.l is the input laser intensity and T.sub.l is the measured transmission shown in FIG. 5. Under pulsed-resonant excitation, the single-photon intensity at the collection grating is express as I.sub.sp=(I.sub.lT.sub.inβ.sub.in)β.sub.out, which holds under weak pumping of the quantum dot and when omitting any effect of dephasing. β.sub.in and β.sub.out are the photon β-factors that express the probability that the quantum dot absorbs a pump photon and emit a single photon into the waveguide, respectively. T.sub.in is the transmission efficiency of the O.sub.1 mode through the photonic crystal waveguide (see FIG. 4), so that I.sub.lT.sub.in represents the intensity of the pump at the quantum dot. Consequently,

[00001]${ϵ}_{\mathrm{QD}}=\frac{I_l{T}_l}{I_{\mathrm{sp}}}=\frac{1}{r_{\mathrm{QD}}{\beta }_{\mathrm{in}}{\beta }_{\mathrm{out}}}$

where r.sub.QD=T.sub.in/T.sub.l is the extinction ratio of the collection and the excitation modes that should be maximized for the optimum performance of the device.

[0102] The quantum dot position affects the value of ϵ.sub.QD, minimizing it whenever β.sub.in≃β.sub.out. Yet, if the device extinction ratio r.sub.QD is sufficiently large, β.sub.out can be increased. An example of a calculated figure of merit ϵ.sub.QD is shown in FIG. 6, where T.sub.l=2.Math.10.sup.−5 and T.sub.in=5.Math.10.sup.−1 (average value from a simulation). For a quasi-centered quantum dot with β.sub.out ≃0.9, a value ϵ.sub.QD ≃5.Math.10.sup.−4 is obtained, indicating that a purity g.sup.(2)(τ)≃1.Math.10.sup.−3 is achieved. The extinction ratio r.sub.QD can be further enhanced, e.g. by improving the photonic crystal fabrication as such. This will lead to even better single-photon purity.

[0103] FIG. 7 illustrates guided modes in terms of effective refractive index as a function of waveguide width for the emitter waveguide section 202. As described in relation to FIG. 2A, the waveguide at the output of the second filter waveguide section 203 has a width of 200 nm and a thickness of 170 nm. As shown in FIG. 7, this waveguide shape supports only one mode, which is the fundamental (0.sup.th-order) mode F.sub.1 described above. Here, a 1.sup.st order mode appears. With widening waveguide width, the fundamental mode becomes more tightly bound, and eventually an additional, 2.sup.nd order mode, appears. At waveguide width 450 nm, the waveguide supports the fundamental mode E.sub.1 shown in FIG. 2A, the first-order mode O.sub.1 shown in FIG. 2A, and a very weakly bound 2.sup.nd order mode. The waveguide in FIGS. 2A and 3 have a width of 450 nm in order to achieve a low loss for the fundamental and first-order modes. The second-order mode supported at a width of 450 nm has only a very small overlap with the photon emitter and thus will not affect the performance.

[0104] Various embodiments are defined in the following items:

[0105] 1. An optical device (100, 200, 300) comprising: [0106] a first filter waveguide section (101, 201) having an input (111) for receiving a pump signal (140), the first filter waveguide section further having an output (121), [0107] an emitter waveguide section (102, 202) having an input (112) coupled to the output (121) of the first filter waveguide section (101, 201) to receive a transmitted pump signal therefrom, the emitter waveguide section supporting at least a first guided lower-order optical mode and a second guided higher-order optical mode, the emitter waveguide section further comprising a photon emitter (150) coupled to the first guided mode to emit radiation into the first guided mode and coupled to the second guided mode to allow optical pumping of the photon emitter by pump signal power carried in the second guided mode, the emitter waveguide section further having an output (122) for outputting radiation emitted from the photon emitter, [0108] a second filter waveguide section (103, 203) having an input (113) coupled to the output (122) of the emitter waveguide section (102, 202) and having an output (123), the second filter waveguide section being configured to transmit radiation emitted into the first guided mode with lower loss than radiation emitted into modes other than the first guided mode, the first filter waveguide section (101, 201) being configured to couple pump signal power predominantly into the second guided mode of the emitter section (102, 202).

[0109] 2. An optical device in accordance with item 1, wherein the first guided mode is a fundamental mode of the emitter waveguide section and the second guided mode is a first-order mode or a second-order mode of the emitter waveguide section.

[0110] 3. An optical device in accordance with item 1 or 2, wherein the first filter waveguide section is configured to suppress transmission of pump signal power into the first guided mode of the emitter waveguide section while allowing transmission of pump signal power into the second guided mode of the emitter waveguide section.

[0111] 4. An optical device in accordance with any of the preceding items, wherein the first filter waveguide section (201) comprises a photonic crystal structure (210) configured to suppress transmission of pump signal power into the first guided mode of the emitter waveguide section while allowing transmission of pump signal power into the second guided mode of the emitter waveguide section.

[0112] 5. An optical device in accordance with any of the preceding items, wherein the second filter waveguide section comprises: [0113] one or more tapers configured to cause power carried in modes other than the first guided mode, but not power carried in the first guided mode, to leak; and/or [0114] one or more bends configured to cause power carried in modes other than the first guided mode, but not power carried in the first guided mode, to leak.

[0115] 6. An optical device in accordance with any of the preceding items, wherein the photon emitter (150) is a quantum-confined structure, such as a quantum dot.

[0116] 7. An optical device in accordance with any of the preceding items, wherein the photon emitter is situated away from a symmetry axis (130) of the emitter waveguide section.

[0117] 8. An optical device in accordance with any of the preceding items, wherein the input of the first filter waveguide section is coupled to a grating coupler (221) for receiving the pump signal, and the second filter waveguide section output is coupled to a grating coupler (231) for coupling radiation emitted by the photon emitter out of the optical device.

[0118] 9. An optical device in accordance with any of the preceding items, further comprising: [0119] an input fibre for receiving the pump signal and for coupling the received pump signal into the first filter section, and/or [0120] an output fibre for coupling single photons generated in the emitter waveguide section out of the optical device.

[0121] 10. An optical device in accordance with any of the preceding items, wherein the first filter waveguide section, the emitter waveguide section, and at least an initial part of the second filter waveguide section are monolithically integrated.

[0122] 11. An optical device in accordance with any of the preceding items, wherein one or more of the waveguide sections are based on one or more III-V semiconductor materials, such as In and/or Ga and/or Al and/or As, or other group III or group V material.

[0123] 12. An optical device in accordance with any of the preceding items, wherein the photon emitter is based on In and/or Ga and/or Al and/or As, such as InGaAs, or other group III or group V material.

[0124] 13. An optical system comprising an optical device according to any of the preceding items and an optical pump signal source for providing the pump signal.

[0125] 14. An optical device in accordance with any of items 1-12 or optical system in accordance with item 13, wherein the optical device or optical system is configured to provide single photons at a wavelength in the range 400 nm-1600 nm.

[0126] 15. An optical device or optical system in accordance with any of items 1-14, wherein the pump signal is configured to resonantly excite the photon emitter.

LIST OF REFERENCE NUMERALS

[0127]

TABLE-US-00001 100 Optical device 101, 201 First filter waveguide section 102, 202 Emitter waveguide section 103, 203 Second filter waveguide section 111, 112, 113 Inputs 121, 122, 123 Outputs 130 Waveguide centreline 140 Pump signal 150 Photon emitter 160 Generated single photons 200 Optical device 204, 205 Bends 210 Photonic crystal structure 221, 231, 241 Grating couplers 222, 242 Grating coupler waveguides 300 Fabricated optical device E.sub.1, O.sub.1, F.sub.1 Guided modes