Device for coupling a plurality of different fibre modes

Abstract

An integrated optical coupler device comprising, on a substrate surface: an integrated optical coupling grating extending in lateral directions parallel to the substrate surface and which, by diffraction at its grating structures, either converts electromagnetic waves, guided parallel to the substrate surface, of at least two waveguide modes of integrated optical waveguides into fiber modes propagating perpendicularly to the substrate surface, or converts electromagnetic waves, propagating perpendicularly to the substrate surface, of a fiber mode into electromagnetic waves, propagating parallel to the substrate surface, of at least two waveguide modes, and a first conductor pair, connected to the coupling grating and formed by a first and a second integrated optical waveguide, through which, in mutually opposite first and second directions parallel to the substrate surface, electromagnetic waves of at least two waveguide modes can be conducted to the coupling grating or can be conducted away from the coupling grating.

Claims

1. An integrated optical coupler device, for use as a coupling interface between integrated optical waveguides and an optical multimode fiber, comprising, on a substrate surface: an integrated optical coupling grating extending in lateral directions parallel to the substrate surface, with side widths of more than 10 micrometers in each case and which, by diffraction at its grating structures, depending on the coupling direction, either converts electromagnetic waves of a mutual phase difference of 0 or 180, guided parallel to the substrate surface, of at least two waveguide modes of integrated optical waveguides into LP fiber modes propagating perpendicularly to the substrate surface, or converts electromagnetic waves of a mutual phase difference of 0 or 180, propagating perpendicularly to the substrate surface, of an LP fiber mode into electromagnetic waves, propagating parallel to the substrate surface, of at least two waveguide modes, and a first conductor pair, which is connected to the coupling grating at mutually opposite first and second sides of the coupling grating and which is formed by a first and a second integrated optical waveguide, through which, in mutually opposite first and second directions parallel to the substrate surface, electromagnetic waves of at least two waveguide modes can be conducted to the coupling grating or can be conducted away from the coupling grating.

2. The coupler device according to claim 1, in which the coupling grating is additionally connected at mutually opposite third and fourth sides to a second conductor pair, which is formed by a third and a fourth integrated optical waveguide, through which, in mutually opposite third and fourth directions parallel to the substrate surface and perpendicular to the first and second directions, electromagnetic waves of at least two waveguide modes can be conducted to the coupling grating or can be conducted away from the coupling grating, and in which the coupling grating is formed as a two-dimensional coupling grating which has a grating structure in two directions of the grating plane perpendicular to each other and to two of the directions of incidence.

3. The coupler device according to claim 2, comprising an integrated optical conditioning device which has more than two coupling ports assigned to each conductor pair, for coupling in and coupling out electromagnetic waves from or to the outside, wherein each of the coupling ports is connected to both waveguides of the conductor pair assigned to it, and wherein the conditioning device assigns a conditioning path to each coupling port by being adapted, depending on the coupling port used, a) either to convert between a fundamental waveguide mode which is present at the respective coupling port, and a higher waveguide mode, relative to the fundamental waveguide mode, which is present in the assigned conductor pair, or b) to shift the phase of electromagnetic waves of a waveguide mode by 180, or c) to combine the conditioning according to a) and b), in one waveguide mode, or d) to conduct the waveguide mode in unchanged form.

4. The coupler device according to claim 3, wherein the coupling ports each have a coupling grating for coupling in and coupling out electromagnetic waves.

5. The device according to claim 3, in which the conditioning device for converting between the fundamental waveguide mode and the higher waveguide mode has a mode conversion device which is either an asymmetric codirectional coupler, or forms a horn structure.

6. The coupler device according to claim 3, wherein the conditioning device for converting between the fundamental waveguide mode and the higher waveguide mode has a mode conversion device in the form of a Mach-Zehnder interferometer.

7. The coupler device according to claim 6, in which the Mach-Zehnder interferometer has a first multimode interference coupler which opens into two parallel waveguide branches on its side that faces the conductor pair, wherein a first waveguide branch of the two parallel waveguide branches contains a phase shifter adapted to shift the phase of supplied electromagnetic waves of a waveguide mode by 180 relative to a Phase in a second waveguide branch, and has a second multimode interference coupler facing the conductor pair, which is connected to a first and a second output branch such that the higher waveguide mode is present at the end of the second multimode interference coupler facing toward the conductor pair, and the fundamental waveguide mode is present at the end of the first multimode interference coupler facing away from the conductor pair.

8. The coupler device according to claim 7, wherein the coupling ports each have a coupling grating for coupling in and coupling out electromagnetic waves.

9. The coupler device according to claim 7, wherein the coupling grating has a translucent cover layer.

10. The coupler device according to claim 9, the coupler device being produced using silicon-on-insulator technology.

11. The coupler device according to claim 9, the waveguides thereof being less than 1 micrometer high in a direction perpendicular to the substrate surface.

12. An optoelectronic device comprising a light source, an integrated optical coupler device according to claim 1, an input line for electromagnetic waves emitted from the light source to the coupler device, and a conductor for electromagnetic waves outputted from the coupler device.

13. An optoelectronic device comprising an integrated optical coupler device according to claim 12, and comprising an optical multimode fiber which faces the coupling grating and is suitable for coupling in and coupling out the electromagnetic waves.

14. An optical arrangement comprising an integrated optical coupler device according to claim 1, and comprising an optical multimode fiber which faces the coupling grating and is suitable for coupling in and coupling out the electromagnetic waves.

15. The coupler device according to claim 1, comprising an integrated optical conditioning device which has more than two coupling ports assigned to each conductor pair, for coupling in and coupling out electromagnetic waves from or to the outside, wherein each of the coupling ports is connected to both waveguides of the conductor pair assigned to it, and wherein the conditioning device assigns a conditioning path to each coupling port by being adapted, depending on the coupling port used, a) either to convert between a fundamental waveguide mode which is present at the respective coupling port, and a higher waveguide mode, relative to the fundamental waveguide mode, which is present in the assigned conductor pair, or b) to shift the phase of electromagnetic waves of a waveguide mode by 180, or c) to combine the conditioning according to a) and b), in one waveguide mode, or d) to conduct the waveguide mode in unchanged form.

16. The device according to claim 15, in which the conditioning device for converting between the fundamental waveguide mode and the higher waveguide mode has a mode conversion device which is either an asymmetric codirectional coupler, or forms a horn structure.

17. The coupler device according to claim 15, wherein the conditioning device for converting between the fundamental waveguide mode and the higher waveguide mode has a mode conversion device in the form of a Mach-Zehnder interferometer.

18. The coupler device according to claim 17, in which the Mach-Zehnder interferometer has a first multimode interference coupler which opens into two parallel waveguide branches on its side that faces the conductor pair, wherein a first waveguide branch of the two parallel waveguide branches contains a phase shifter adapted to shift the phase of supplied electromagnetic waves of a waveguide mode by 180 relative to a phase in a second waveguide branch, and has a second multimode interference coupler facing the conductor pair, which is connected to a first and a second output branch such that the higher waveguide mode is present at the end of the second multimode interference coupler facing toward the conductor pair, and the fundamental waveguide mode is present at the end of the first multimode interference coupler facing away from the conductor pair.

19. The coupler device according to claim 1, wherein the coupling grating has a translucent cover layer.

20. The coupler device according to claim 1, the coupler device being produced using silicon-on-insulator technology.

21. The coupler device according to claim 1, the waveguides thereof being less than 1 micrometer high in a direction perpendicular to the substrate surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further embodiments shall now be explained with reference to the Figures, in which:

(2) FIG. 1 shows LP fiber modes (from left to right: LP.sub.01, LP.sub.11a, LP.sub.21a)

(3) FIG. 2 shows TE.sub.00 and TE.sub.10 waveguide modes

(4) FIG. 3 shows a sketch of the grating coupler with optical fiber and integrated optical waveguides

(5) FIG. 4 shows a schematic cross-sectional view of the coupling grating in FIG. 3.

(6) FIG. 5 shows the dependence of the effective refractive index (y-axis), shown on a linear scale, on the waveguide width (x-axis) in micrometers, shown here on a logarithmic scale.

(7) FIG. 6 shows a sketch of the grating coupler with optical fiber and integrated optical waveguides

(8) FIG. 7 shows a symbolic circuit diagram of an embodiment of the integrated optical device

(9) FIG. 8 shows a cross-sectional profile of the electric field with a TE.sub.00 mode and a phase angle of =0

(10) FIG. 9 shows a cross-sectional profile of the electric field with a TE.sub.00 mode and a phase angle of =180

(11) FIG. 10 shows the overlap between different grating areas and fiber modes

DETAILED DESCRIPTION

(12) The invention is based on the following realization: A structure is required which can generate a plurality of intensity maxima in an optical fiber, whose superposition results in a guided fiber mode. The greater the overlap between the superposition of the generated intensity maxima and the fiber modes, the greater the efficiency of the device.

(13) In the following, the LPrmodes in TE and TM polarization, typically found in the literature, are used as an example of fiber modes (without limiting the general validity of the solution), whereas the terms TExy and TMxy are used for modes in the integrated optical circuit. FIGS. 1 and 2 show examples of such known modes.

(14) One embodiment of a coupler device 300 according to the invention is shown in FIG. 3. FIG. 4 shows a schematic cross-sectional view of the coupling grating in FIG. 3, in a plane containing a line from input In1 to input In2. The following description refers to both these figures in parallel.

(15) An integrated one-dimensional coupling grating 302 of coupler device 300 is fed from two opposite sides with input In1 and In2, respectively, via integrated optical waveguides 304 and 306. The inputs are thus at an angle of 180 relative to each other. An optical fiber (not shown) to be coupled into is to be positioned at a distance above coupling grating 302. Waveguides 304 and 306 are adapted to guide a plurality of waveguide modes, and specifically the waveguide modes TE.sub.00 and TE.sub.10 in the present example. In the area adjoining coupling grating 302, waveguides 304 and 306 have portions 304.1 and 306.1 which widen like a funnel toward the coupling grating.

(16) Coupling grating 302 is formed in an SOI substrate comprising a silicon substrate 302.1, a silicon dioxide layer 302.2 and a silicon layer 302.3 disposed thereon, and contains in silicon layer 302.3 a structure of periodically repeated, strip-shaped ribs 302.4 and adjoining strip-shaped trenches 302.5. In the view shown in FIG. 4, the strips run in a direction perpendicular to the plane of the paper.

(17) If the fundamental TE.sub.00 modes of the integrated optical waveguides are guided without any phase shift (=0) into the two inputs In1 and In2, the two waves unite in coupling grating 302 and generate the fundamental LP.sub.01 fiber mode in TE polarization. If, however, the TE.sub.00 waves at inputs In1 and In2 have a phase difference of =180, the two waves do not unite to form the LP.sub.01 fiber mode, but are guided as separate intensity maxima to the fiber with a phase difference and thus generate one of the two possible LP.sub.11 fiber modes (often referred to as LP.sub.11,a). The other possible LP.sub.11 fiber mode (LP.sub.11,b) in TE polarization is formed by the superposition of TE.sub.10 modes at inputs In1 and In2. It should be noted in this regard that the fiber modes are non-fundamental modes in the integrated waveguides and each have two intensity maxima. If these two modes are superposed in the coupling grating without any phase difference (=0), respective pairs of intensity maxima (+90 and also 90 of the two inputs) merge such that the LP.sub.11,b fiber mode remains. If the phase difference between the two TE.sub.10 modes at the inputs is =180, all four intensity maxima are preserved on their way to the fiber and thus generate one of the possible LP.sub.21 fiber modes, also in TE polarization.

(18) FIG. 4 also shows electrical field distributions for the case where =0 (curve A, one maximum) and for the case where =180 (curve B, one maximum and one minimum). The field distribution is symmetric for =0 in relation to the middle of the coupling grating (vertical line M), and antisymmetric for =180. In both cases, therefore, the field which is coupled out propagates in a strictly vertical direction, that is, the radiant emission is in a direction perpendicular to the substrate surface S and shows only very slight spreading of the field profile. A suitable value for the grating period, i.e., for the distance between the middle positions of two adjacent ribs along the line connecting inputs In1 and In2, is =570 nm, for example. Other values for the grating period can be used. The value of the grating period influences the efficiency of the coupling grating, i.e., the spatial superposition between the electrical field at the grating and the fiber mode; cf. FIGS. 8-10.

(19) The integrated optical waveguides have to be at least partially multimodal, as they have to guide (in the present example) not only the TE.sub.00 modes, but also the TE.sub.10 modes, which requires insignificantly wider waveguides. In this regard, FIG. 5 shows the dependence of the effective refractive index (y-axis), shown on a linear scale, on the waveguide width (x-axis) in micrometers, shown here on a logarithmic scale from 0.4 to 11 micrometers. Three different curves, n1, n2 and n3, show the dependence of the refractive index on waveguide mode for the first three guided modes in an SOI nanoguide with a height of 220 nm, the wavelength of light being =1.55 m. The refractive index for the different modes TE.sub.00 (top curve n1), TM.sub.00/TE.sub.10 (middle curve n2) and TE.sub.10/TM.sub.00/TE.sub.20 (bottom curve n3) is identical for waveguide widths of about 10 micrometers, or more. It should be noted that the modes change position. In the example in FIG. 5, mode 2 starts as TM00 and changes after the kink to TE10.

(20) For that reason, the waveguides preferably have a width of at least 630 nm, approximately, in those portions where they have to guide a plurality of modes. That value must be adjusted accordingly for other wavelengths.

(21) This obviates the need to provide a separate coupling grating for higher modes.

(22) In another embodiment, vertical waveguide dimensions ranging, for example, between 500 and 900 nanometers are used.

(23) Part of another embodiment of a coupler device 600 according to the present invention is shown in a schematic perspective view in FIG. 6. An integrated optical two-dimensional coupling grating 602 is fed from four sides via waveguides 604, 606, 608 and 610, each having a respective input In2a-In2d. The inputs and the waveguides are arranged 90 apart from other about an axis perpendicular to the plane of the grating. Cross-sectional views of the coupling grating in the cross-sectional planes 90 apart from each other, i.e., perpendicular to a straight line connecting inputs In2a and In2b and perpendicular to a straight line connecting inputs In2c and In2d correspond respectively to the view shown in FIG. 4. Instead of strip-shaped ribs and trenches, as in preceding embodiments, cylindrical recesses are arranged in array-like manner in a silicon layer in coupling grating 602. The grating period in both directions is the same as that of the grating in FIG. 4. What is not shown here is an additional epitaxial layer with a thickness of 150 nanometers that is disposed on the grating and further enhances the directionality with which light is coupled out.

(24) An optical multimode fiber 612 into which light is to be coupled is positioned above coupling grating 602, with a gap therebetween. If the fundamental TE.sub.00 modes of the integrated optical waveguides are guided without any phase shift (=0) into input In2a and input In2b, the two waves unite in coupling grating 602 and generate the fundamental LP.sub.01 fiber mode in TE polarization. If, however, the TE.sub.00 waves at inputs In2a and In2b have a phase difference of =180, the two waves do not unite to form the LP.sub.01 fiber mode, but are guided as separate intensity maxima to the fiber with a phase difference and thus generate one of the two possible LP.sub.11 fiber modes (referred to as LP.sub.11,a), again in TE polarization. The other possible LP.sub.11 fiber mode (LP.sub.11,b) in TE polarization is formed by the superposition of TE.sub.10 modes at inputs In2a and In2b. It should be noted in this regard that these are non-fundamental modes in the integrated waveguides, each mode having two intensity maxima. If these two modes are superposed in the coupling grating without any phase difference (=0), respective pairs of intensity maxima (+90 from inputs In2a and In2b, and also 90 from inputs In2a and In2b) merge such that the LP.sub.11,b fiber mode remains. If the phase difference between the two TE.sub.10 modes at inputs In2a and In2b is =180, all four intensity maxima are preserved on their way to the fiber and thus generate one of the possible LP.sub.21 fiber modes, also in TE polarization. This functionality corresponds thus far to the example in FIG. 3.

(25) The same behavior is shown when inputs In2c and In2d are used instead of inputs In2a and In2b. In this case also, modes TE.sub.00 or TE.sub.10 are fed to the integrated waveguides and generate the respective fiber modes, but in TM polarization. Although the polarization in the integrated waveguides is still TE, the fact that inputs In2c and In2d are rotated by 90 relative to In2a and In2b means that the fields emitted by the two Input pairs are perpendicular to each other and therefore have orthogonal polarizations. The fiber modes of inputs In2a and In2b have been referred to hitherto as TE-polarized, from which it can be inferred that the fiber modes of inputs In2c and In2d are TM-polarized.

(26) The fiber modes of inputs In2a and In2b have been referred to hitherto as TE-polarized, from which it can be inferred that the fiber modes of inputs In2c and In2d are TM-polarized. All the fiber modes which can be generated in this way are listed again in the following Table, together with the switching that is required.

(27) TABLE-US-00001 TABLE 1 Possible fiber modes (ports according to FIG. 6, In1 and In2 according to FIG. 7) Port In1 In2 Mode Out Polarization I. a b TE.sub.00 0 LP.sub.01 TE II a b TE.sub.00 180 LP.sub.11,a TE III. c d TE.sub.00 0 LP.sub.01 TM IV c d TE.sub.00 180 LP.sub.11,a TM V a b TE.sub.10 0 LP.sub.11,b TE VI a b TE.sub.10 180 LP.sub.21,a TE VII c d TE.sub.10 0 LP.sub.11,b TM VIII c d TE.sub.10 180 LP.sub.21,b TM

(28) As can be seen from the Table, only TE-polarized modes are used in the integrated optical circuit, which is advantageous, firstly, in that the losses for TE-polarized waves are less than for TM-polarized waves in small waveguides (nanowires, nanoribs) and secondly because no polarization rotators are needed in order to detect both polarizations, or because there is no need for possible modulators to be integrated for both polarizations.

(29) In order to generate the higher modes necessary for this kind of coupling (in this case TE.sub.10) in the photonic integrated circuit, a further device for producing a higher mode from a fundamental mode (mostly TE.sub.00 in integrated circuits) is needed. There are various different approaches in that regard: An asymmetric codirectional coupler, in which the propagation constant of the fundamental mode of the one waveguide matches that of the desired higher mode in other waveguide, delivers the desired result, but is difficult to produce due to the low tolerance toward variations in the propagation constants of the modes in the waveguides. Another method is to convert a TM.sub.00 mode adiabatically into a TE.sub.10 mode by means of a horn structure (taper). This solution is very tolerant to produce, very efficient and broadband. However, the problem here is how to get TM-polarized light into the integrated optical circuit, given that coupling by means of gratings has turned out to be very inefficient in that respect and that inverse tapers entail confinement to the edge of the chip.

(30) One embodiment of the integrated optical coupler device according to the invention that applies a different solution is shown schematically in FIG. 7. Eight simple coupling gratings are used to couple light from standard single-mode fibers into the integrated optical circuit. Each of the eight inputs generates a different LP mode at the 2D grating (cf. Table 1).

(31) In this particular case, the device generates the TE.sub.10 mode with the aid of a Mach-Zehnder interferometer (MZI) composed of two multimode interference couplers (MMIs) and a phase shifter on one of the arms. The first MMI splits an inbound TE.sub.00 mode equally into two TE.sub.00 modes, one of which is phase shifted by the phase shifter by 180 relative to the other. In the second MMI, the two modes are then superposed with phase difference to form a TE.sub.10 mode. In other words, one 12 MMI splits the power into two waveguides, one of which induces a 180 phase delay of the field by means of a phase shifter, and one 21 MMI then superposes the field to form a TE.sub.10 modes at an output waveguide which is about twice as wide as the input waveguide. Further phase shifters or other MMI couplers are used, depending on the input branch of the device.

(32) FIG. 8 shows a cross-sectional profile of the electrical field (entered in arbitrary linear units on the y-axis) of a coupling grating excited from mutually opposite sides with a TE.sub.00 mode having with a phase difference of =0 (blue curve with three spikes), and for comparison a cross-sectional profile of the electrical field of an LP.sub.01 fiber mode (red curve with only one maximum), both expressed as a function of position relative to a middle position of the grating (entered on the x-axis in linear units of micrometers). A large overlap is shown, thus indicating that this concept is capable of implementation.

(33) As shown by FIG. 9, the same applies for the coupling in of higher modes. FIG. 9 shows a cross-sectional profile of the electric field (with arbitrary linear units entered on the y-axis) of a coupling grating excited from mutually opposite sides with a TE.sub.00 mode having a phase difference of =180 (the blue curve with spikes) and for comparison a cross-sectional profile of the electric field of an LP.sub.11,b fiber mode (red, smooth curve), both expressed as a function of position relative to a middle position of the grating (entered on the x-axis in linear units of micrometers).

(34) FIG. 10 shows the overlap between different grating areas and fiber modes for different cases. An overlap of at least 50% (for coupling into an LP.sub.21 fiber mode) and a maximum of over 80% (for coupling into an LP.sub.01 fiber mode) is achieved, in this case for a wavelength of 1.55 micrometers.