WAVEGUIDE MODE CONVERTER

Abstract

A SOI bent taper structure is used as a mode convertor. By tuning the widths of the bent taper and the bend angles, almost lossless mode conversion is realized between TE0 and TE1 in a silicon waveguide. The simulated loss is <0.05 dB across C-band. This bent taper can be combined with bi-layer TM0-TE1 rotator to reach very high efficient TM0-TE0 polarization rotator. An ultra-compact (9 m) bi-layer TM0-TE1 taper based on particle swarm optimization is demonstrated. The entire TM0-TE0 rotator has a loss <0.25 dB and polarization extinction ratio >25 dB, worst-case across the C-band.

Claims

1. A waveguide mode converter comprising: a first port; a second port; and a bilayer taper optically connecting the first port and the second port, the bilayer taper comprising a first layer and a second layer vertically coupled to the first layer; wherein the first layer increases in width in a first direction from the first port to the second port; and wherein the second layer comprises a first section that decreases in width in the first direction, and a second section that increases in width in the first direction.

2. The waveguide mode converter of claim 1 wherein the first section is disposed between the first port and the second section.

3. The waveguide mode converter of claim 1 wherein the bilayer taper comprises a length wherein the first layer increases in width in the first direction and the second layer decreases in width in the first direction.

4. The waveguide mode converter of claim 1 wherein the bilayer taper is configured to couple a TM mode at the first port to a TE mode at the second port.

5. The waveguide mode converter of claim 1 wherein the bilayer taper is configured to convert light propagating in a TM0 mode at the first port into a higher-order TE mode at the second port.

6. The waveguide mode converter of claim 5 wherein the higher-order TE mode comprises a TE1 mode.

7. The waveguide mode converter of claim 1 wherein the second layer is disposed over the first layer.

8. The waveguide mode converter of claim 1 wherein the first layer exceeds the second layer in width.

9. The waveguide mode converter of claim 7 wherein the first layer is disposed over a planar substrate.

10. The waveguide mode converter of claim 9 wherein the planar substrate comprises a semiconductor material.

11. The waveguide mode converter of claim 9 wherein the planar substrate comprises silicon.

12. A waveguide polarization rotator comprising: the waveguide mode converter of claim 5; and a TE mode converter optically coupled to the second port of the waveguide mode converter and configured to convert the higher-order TE mode into a TE 0 mode.

13. The waveguide polarization rotator of claim 12 wherein the TE mode converter comprises a bent waveguide taper.

14. The waveguide polarization rotator of claim 13 wherein the bent waveguide taper comprises a first end, a second end, and two or more bent waveguide segments optically connected in series between the first end and the second end.

15. The waveguide polarization rotator of claim 14 wherein the bent waveguide taper has a width that gradually increases from the first end toward the second end along at least a portion of a length of the bent waveguide taper.

16. The waveguide polarization rotator of claim 14 wherein the two or more bent waveguide segments are configured so that light that enters the bent waveguide taper from the second end in the higher order TE mode exits the second end in the TE0 mode.

17. The waveguide polarization rotator of claim 14 wherein the two or more bent waveguide segments comprise two oppositely-curved waveguide bends.

18. The waveguide polarization rotator of claim 12 wherein the waveguide mode converter and the TE mode converter are disposed on a same substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The objects and features of the invention can be better understood with reference to the drawings described below, and the claims. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views.

[0026] FIG. 1 is a schematic diagram of an embodiment of a bent taper constructed according to principles of the invention.

[0027] FIG. 2A is a graph showing the E-field of a bent taper TE0-TE1 convertor.

[0028] FIG. 2B is a graph of the simulated loss of the bent taper converter of FIG. 2A.

[0029] FIG. 3 is a schematic drawing of a TM0-TE1 bi-layer taper

[0030] FIG. 4A is a graph showing the simulated mode transition of a TM0-TE1 convertor.

[0031] FIG. 4B is a graph of the simulated loss of the converter of FIG. 4A.

[0032] FIG. 5A is a contour plot showing the magnitude of E-field for a TM0 input, TE0 output bent taper polarization rotator.

[0033] FIG. 5B is a contour plot showing the magnitude of E-field for a TE0 input bent taper polarization rotator.

[0034] FIG. 5C is a contour image of the field transitions of Ey component for the bent taper polarization rotator of FIG. 5A.

[0035] FIG. 5D is a contour image of the field transitions of Ey component for the bent taper polarization rotator of FIG. 5B.

[0036] FIG. 6A is a graph of the simulated performance has a function of wavelength for the TM0 to TE0 mode conversion of an embodiment of a bent taper converter.

[0037] FIG. 6B is a graph of the simulated performance has a function of wavelength for the TM0 to TM0 transmission of an embodiment of a bent taper converter.

[0038] FIG. 6C is a graph of the simulated performance has a function of wavelength for the TE0 to TM0 mode conversion of an embodiment of a bent taper converter.

[0039] FIG. 6D is a graph of the simulated performance has a function of wavelength for the TE0 to TE0 transmission of an embodiment of a bent taper converter.

[0040] FIG. 7 is an embodiment of an L shaped bent taper.

DETAILED DESCRIPTION

Acronyms

[0041] A list of acronyms and their usual meanings in the present document (unless otherwise explicitly stated to denote a different thing) are presented below. [0042] AMR Adiabatic Micro-Ring [0043] APD Avalanche Photodetector [0044] ARM Anti-Reflection Microstructure [0045] ASE Amplified Spontaneous Emission [0046] BER Bit Error Rate [0047] BOX Buried Oxide [0048] CMOS Complementary Metal-Oxide-Semiconductor [0049] CMP Chemical-Mechanical Planarization [0050] DBR Distributed Bragg Reflector [0051] DC (optics) Directional Coupler [0052] DC (electronics) Direct Current [0053] DCA Digital Communication Analyzer [0054] DRC Design Rule Checking [0055] DSP Digital Signal Processor [0056] DUT Device Under Test [0057] ECL External Cavity Laser [0058] E/O Electro-optical [0059] FDTD Finite Difference Time Domain [0060] FFE Feed-Forward Equalization [0061] FOM Figure of Merit [0062] FSR Free Spectral Range [0063] FWHM Full Width at Half Maximum [0064] GaAs Gallium Arsenide [0065] InP Indium Phosphide [0066] LiNO.sub.3 Lithium Niobate [0067] LIV Light intensity(L)-Current(I)-Voltage(V) [0068] MFD Mode Field Diameter [0069] MPW Multi Project Wafer [0070] NRZ Non-Return to Zero [0071] OOK On-Off Keying [0072] PIC Photonic Integrated Circuits [0073] PRBS Pseudo Random Bit Sequence [0074] PDFA Praseodymium-Doped-Fiber-Amplifier [0075] PSO Particle Swarm Optimization [0076] Q Quality factor

[00001]$Q=2.Math..Math.\frac{\mathrm{Energy}.Math..Math.\mathrm{Stored}}{\mathrm{Energy}.Math..Math.\mathrm{dissipated}.Math..Math.\mathrm{per}.Math..Math.\mathrm{cycle}}=2.Math..Math..Math.f_r\frac{\mathrm{Energy}.Math..Math.\mathrm{Stored}}{\mathrm{Power}.Math..Math.\mathrm{Loss}}.$ [0077] QD Quantum Dot [0078] RSOA Reflective Semiconductor Optical Amplifier [0079] SOI Silicon on Insulator [0080] SEM Scanning Electron Microscope [0081] SMSR Single-Mode Suppression Ratio [0082] TEC Thermal Electric Cooler [0083] WDM Wavelength Division Multiplexing

[0084] We describe a novel taper structure using a multimode bent taper.

[0085] The optical behavior of a multimode bend is defined by its geometry. By changing the bend width along with its angle, a bent taper can be formed.

[0086] In some embodiments, a bent taper is one that has one or more bent waveguide segments and that has a varying waveguide width at at least two different locations along a length of the waveguide. A bent taper can have a shape similar to the letter S, the letter L, or some other bent shape.

[0087] One can achieve more functions than a single mode bend by utilizing the multimode region if one can control the behavior of a multimode bend. One application of a bent taper constructed and operated according to the principles of the invention is use as a mode convertor that can be used in optical waveguides, which is an important category of structures in integrated optics to realize polarization diversified photonic integrated circuit (PIC). Another application of a bent taper constructed and operated according to the principles of the invention is use as a polarization rotator that can be used in optical waveguides. The bent tapers constructed and operated according to the principles of the invention can be used in applications such as mode division multiplexing (MDM), wavelength division multiplexing (WDM), polarization division multiplexing (PDM) or in combinations of multiplexing methods.

[0088] FIG. 1 is a schematic diagram of an embodiment of a bent taper constructed according to principles of the invention. The bent taper starts with an S-bend that has an optical input port 102 at one end and an optical output port 104 at the other end. The device is reciprocal so 102 can also serve as an output port and 104 an input port. The bent taper has a length measured as a distance from the input port to the output port. An S-bend waveguide can be defined by three factors: the center radius R.sub.0, the vertical offset dy measured as a lateral distance between the input and output ports, and the waveguide width. The waveguide width can vary at different angles and/or at different locations in the waveguide.

[0089] A highly efficient TE0-TE1 mode convertor is realized in one embodiment with a bent taper. In this design, we decompose the S-bend into 8 segments of equal angular measure, d.sub., and perform interpolation between each segment to make the transitions from one segment to the next smooth. In other embodiments, the angular measure may differ for different ones of the plurality of segments. The waveguide width of an S-bend is taken relative to the center radius R.sub.0. The center radius R.sub.0 divides the S-bend into the up side and the down side. In the embodiment illustrated, we choose asymmetric widths to increase the optimization freedom. Therefore, we have two sets of independent width parameters: {U1, U2, U3, . . . , U9} on the up side and {D1, D2, D3, . . . , D9} on the down side, as shown in in FIG. 1. The width of the taper at any specific location i is the sum of the respective Ui and Di at location i, where i is an ordinal number.

[0090] The TE0 mode is launched at the narrow end, or input port, e.g., the left side in FIG. 1 and TE1 mode emerges at the wide end, or output port, e.g., the right side in FIG. 1. Particle swarm optimization (PSO) is used during FDTD simulation to achieve an optimized geometry. Ultra-high efficiency was realized by only modifying the first two segments (U1, U2, D1, D2). The remaining six segments were maintained at the same widths as at the wide end. By fixing dy to 1.2 m, R.sub.0 is found to be 8.531 m after optimization. The length of the bend taper is calculated to be only 6.3 m.

[0091] FIG. 2A is a graph showing the E-field of a bent taper TE0-TE1 convertor. FIG. 2A clearly shows the field transition from TE1 at the left to TE0 at the right without notable scattering loss.

[0092] FIG. 2B is a graph of the simulated loss of the bent taper converter of FIG. 2A. The mode conversion loss is <0.05 dB (i.e., >98.8% efficiency) across the C-band.

[0093] The TE0-TE1 convertor can be combined with a TM0-TE1 convertor to realize polarization rotation in a SOI platform. A TM0-TE1 convertor can be realized by using a linear bi-layer taper, as been published. See D. Dai and J. E. Bowers, Opt. Express 19, 10940 (2011); and W. D. Sacher, T. Barwicz, B. J. F. Taylor, and J. K. S. Poon, Opt. Express 22, 3777 (2014). However, the mode conversion of those tapers is not efficient. In order to achieve high conversion efficiency (>95%), the length of the linear adiabatic taper is usually around a hundred micrometers or even longer.

[0094] FIG. 3 is a schematic drawing of a TM0-TE1 bi-layer taper. The taper has an input port 302, an output port 304, a first layer 310, and a second layer 320, an input port 302. We designed the widths w1, w2, w3, . . . w8, w9, of bi-layer taper by PSO to realize an ultra-short TM0-TE1 bi-layer taper. The taper length is only 9 m, which is at least an order of magnitude shorter than reported in the literature. Even with the short length, the loss remains small, <0.2 dB across the C-band, which is comparable or even better than those reported.

[0095] FIG. 4A is a graph showing the simulated mode transition of a TM0-TE1 convertor.

[0096] FIG. 4B is a graph of the simulated loss of the converter of FIG. 4A.

[0097] Finally, we can combine these two parts as a polarization rotator. The field transition is shown in FIG. 5A through FIG. 5D.

[0098] FIG. 5A is a contour plot showing the magnitude of E-field for a TM0 input, TE0 output bent taper polarization rotator.

[0099] FIG. 5B is a contour plot showing the magnitude of E-field for a TE0 input bent taper polarization rotator.

[0100] FIG. 5C is a contour image of the field transitions of Ey component for the bent taper polarization rotator of FIG. 5A.

[0101] FIG. 5D is a contour image of the field transitions of Ey component for the bent taper polarization rotator of FIG. 5B.

[0102] As clearly seen, when the bent taper polarization rotator receives an input signal with TM0 mode at left, the TM0 mode first transfers to TE1 mode at the bi-layer part and is then converted to TE0 mode at the narrow end of the bent taper. However, when the bent taper polarization rotator receives an input signal with TE0 mode, TE0 mode remains all the way across the bi-layer taper and gets scattered at the bent taper, as shown in the right half of FIG. 5B. The simulated performance versus wavelength is given in FIG. 6A through FIG. 6D.

[0103] FIG. 6A is a graph of the simulated performance has a function of wavelength for the TM0 to TE0 mode conversion of an embodiment of a bent taper converter.

[0104] As clearly shown in FIG. 6A, the total loss of the novel polarization rotator is <0.24 dB across C-band, peaked at around 1550 nm.

[0105] FIG. 6B is a graph of the simulated performance has a function of wavelength for the TM0 to TM0 transmission of an embodiment of a bent taper converter. Almost no TM0 information remained. As shown in FIG. 6B, the polarization extinction ratio (PER) for TM0 input is >35 dB across C-band.

[0106] FIG. 6C is a graph of the simulated performance has a function of wavelength for the TE0 to TM0 mode conversion of an embodiment of a bent taper converter. As in FIG. 6C, barely any polarization crosstalk (<40 dB) is observed. TE0>TM0 conversion is unwanted since it induces polarization crosstalk. FIG. 6D is a graph of the simulated performance has a function of wavelength for the TE0 to TE0 transmission of an embodiment of a bent taper converter. When launching the TE.sub.0 mode at the left side, significant power loss occurs due to scattering, resulting >25 dB loss across C-band (FIG. 6(d)).

[0107] FIG. 7 is a first embodiment of an L shaped bent taper 700. As illustrated in FIG. 7, the bent taper 700 has an input port 702 and an output port 704. A bend 710 is also present. The bent taper 700 has a plural number of widths that are different, but only two such widths, W.sub.701 and W.sub.702, are illustrated.

[0108] It is believed that apparatus constructed using principles of the invention and methods that operate according to principles of the invention can be used in the wavelength ranges described in Table I.

TABLE-US-00001 TABLE I Band Description Wavelength Range O band original 1260 to 1360 nm E band extended 1360 to 1460 nm S band short wavelengths 1460 to 1530 nm C band conventional (erbium window) 1530 to 1565 nm L band long wavelengths 1565 to 1625 nm U band ultralong wavelengths 1625 to 1675 nm

[0109] It is believed that in various embodiments, mode converters and polarization rotators can be fabricated that are able to operate at a wavelength within the range of a selected one of an O-Band, an E-band, a C-band, an L-Band, an S-Band and a U-band.

[0110] It is believed that apparatus constructed using principles of the invention and methods that operate according to principles of the invention can be fabricated using materials systems other than silicon or silicon on insulator. Examples of materials systems that can be used include materials such as compound semiconductors fabricated from elements in Groups III and V of the Periodic Table (e.g., compound semiconductors such as GaAs, AlAs, GaN, GaP, InP, and alloys and doped compositions thereof).

DESIGN AND FABRICATION

[0111] Methods of designing and fabricating devices having elements similar to those described herein are described in one or more of U.S. Pat. Nos. 7,200,308, 7,339,724, 7,424,192, 7,480,434, 7,643,714, 7,760,970, 7,894,696, 8,031,985, 8,067,724, 8,098,965, 8,203,115, 8,237,102, 8,258,476, 8,270,778, 8,280,211, 8,311,374, 8,340,486, 8,380,016, 8,390,922, 8,798,406, and 8,818,141, each of which documents is hereby incorporated by reference herein in its entirety.

DEFINITIONS

[0112] As used herein, the term optical communication channel is intended to denote a single optical channel, such as light that can carry information using a specific carrier wavelength in a wavelength division multiplexed (WDM) system.

[0113] As used herein, the term optical carrier is intended to denote a medium or a structure through which any number of optical signals including WDM signals can propagate, which by way of example can include gases such as air, a void such as a vacuum or extraterrestrial space, and structures such as optical fibers and optical waveguides.

THEORETICAL DISCUSSION

[0114] Although the theoretical description given herein is thought to be correct, the operation of the devices described and claimed herein does not depend upon the accuracy or validity of the theoretical description. That is, later theoretical developments that may explain the observed results on a basis different from the theory presented herein will not detract from the inventions described herein.

[0115] Any patent, patent application, patent application publication, journal article, book, published paper, or other publicly available material identified in the specification is hereby incorporated by reference herein in its entirety. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material explicitly set forth herein is only incorporated to the extent that no conflict arises between that incorporated material and the present disclosure material. In the event of a conflict, the conflict is to be resolved in favor of the present disclosure as the preferred disclosure.

[0116] While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be affected therein without departing from the spirit and scope of the invention as defined by the claims.