Optical Multiplexer

Abstract

An optical multiplexer. The optical multiplexer comprises: a plurality of input waveguides, each of which comprises an input slab portion and an input ridge portion; an output waveguide, the output waveguide comprising an output slab portion and an output ridge portion; and a wavelength multiplexer element coupled to each of the input waveguides and the output waveguide, the wavelength multiplexer element comprising a slab waveguide, the slab waveguide comprising a grating configured to multiplex signals of different wavelengths received from the input waveguides into a multiplexed signal, and to provide the multiplexed signal to the output waveguide. The input ridge portion of one or more of the input waveguides is tapered so as to reduce the width in a direction toward the slab waveguide of the wavelength multiplexer element.

Description

Technical Field

[0001] The present invention relates to an optical multiplexer. Background Art

[0002] An important issue in the design of multiplexers, especially echelle grating multiplexers, is the desire to have wider channel bandwidth. This will reduce the losses caused by alignment errors and temperature drift, because channel crosstalk is not significant. However, the traditional method of widening the channel bandwidth comes at the expense of reducing transmission.

[0003] For example, in an echelle grating multiplexer with optimal transmission, the input and output waveguides are strip waveguides. Figure 1 The strip waveguide has an optical mode center that is physically centered in the vertical direction. This maximizes coupling to the slab area of ​​the echelle grating, whose optical mode center is also physically centered in the vertical direction.

[0004] There are two common methods to broaden the channel bandwidth: (1) making the input waveguide width different from the output waveguide width; and (2) using a ridge waveguide so that the mode extends into the surrounding ridge-etched region. FIG. 2A to FIG. 2C The problem with the first approach is that by making the input pattern width different from the output pattern width, the mismatch reduces the peak transmission and there is no overall advantage in performance (e.g. Figure 3 The problem with the second approach is that the ridge waveguide has a mode center that is not physically centered in the vertical direction, and this also reduces the peak transmission, so no overall benefit in performance is seen (as shown in Figure 4 shown).

[0005] Therefore, it is desirable to increase the channel bandwidth of a multiplexer without incurring additional losses. Summary of the invention

[0006] Thus, in a first aspect, embodiments of the present invention provide an optical multiplexer comprising:

[0007] a plurality of input waveguides, each of the plurality of input waveguides comprising an input slab portion and an input ridge portion;

[0008] an output waveguide comprising an output slab portion and an output ridge portion; and

[0009] a wavelength multiplexer element coupled to each input waveguide and the output waveguide, the wavelength multiplexer element comprising a slab waveguide including a grating configured to multiplex signals of different wavelengths received from the input waveguides into a multiplexed signal and provide the multiplexed signal to the output waveguide;

[0010] Wherein the input ridge portion of one or more of the input waveguides is tapered so as to reduce in width in a direction towards the slab waveguide of the wavelength multiplexer element.

[0011] The tapered width of the input ridge section significantly broadens the optical mode width, so the multiplexer channels are correspondingly wider in the spectral domain.

[0012] The optical multiplexer may have any one of the following optional features or any combination of the following optional features to the extent that the following optional features are compatible.

[0013] All input waveguides may be tapered, ie all their input ridge portions may be tapered so as to reduce the width in the direction towards the slab waveguide of the wavelength multiplier element.

[0014] As will be readily appreciated, a multiplexer can be used as a demultiplexer since the device is optically symmetric (according to the Helmholtz reciprocity principle).

[0015] The output waveguide may be tapered, for example the output ridge portion may be tapered so as to reduce in width in a direction towards the slab waveguide of the waveguide multiplexer element.The output waveguide may have a similar geometry to the or each input waveguide.

[0016] The or each input waveguide may be tapered, i.e. the or each input ridge portion may be tapered such that an optical mode within the or each input waveguide is confined to the slab portion of the input waveguide at the point where the input waveguide couples to the slab waveguide of the wavelength multiplexer element.

[0017] The height of the input slab portion of the input waveguide and / or the output slab portion of the output waveguide may be substantially equal to the height of the slab waveguide in the wavelength multiplexer element.

[0018] The wavelength multiplexer element may be a grating wavelength multiplexer element. The wavelength multiplexer element may be an echelle grating multiplexer element. The wavelength multiplexer element may be an arrayed waveguide grating multiplexer element.

[0019] The optical mode of each input waveguide may at least partially overlap the optical mode of an adjacent input waveguide.

[0020] The wavelength multiplexer element may be configured to perform coarse wavelength division multiplexing.

[0021] The optical multiplexer may operate over a wavelength range of at least 1.2 μm.

[0022] The optical multiplexer may form part of a spectrometer (e.g. a Raman spectrometer) or other imaging device. The spectrometer or imaging device may be located on a photonic integrated chip. For example, it may be located in a silicon-on-insulator platform. The multiplexer may be downstream of a laser (e.g. a pump distributed Bragg reflector laser) and upstream of one or more lenses.

[0023] The present invention includes any combination of the described aspects and preferred features unless such a combination is expressly impermissible or explicitly avoided. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

[0025] Figure 1 An echelle grating based multiplexer is shown;

[0026] FIG. 2A to FIG. 2C They are shown respectively Figure 1 Optical modes in the echelle region;

[0027] Figure 3 Among them Figure 1 A graph of wavelength versus transmission for a multiplexer in which an input waveguide differs in width from an output waveguide;

[0028] Figure 4 It is among Figure 1 A graph of wavelength versus transmission of a ridge waveguide in a multiplexer of

[0029] Figure 5 is a top view of an input waveguide according to an embodiment of the present invention;

[0030] FIG. 6A to FIG. 6D Shown along Figure 5 A cross-sectional view of a point on the input waveguide;

[0031] Figure 7 Shown along Figure 5 Simulation of the electric field density in the cross section of the input waveguide;

[0032] FIG. 8A to FIG. 8D Shown along Figure 5 The electric field density of the cross-section of the input waveguide;

[0033] Fig.9A An optical multiplexer is shown;

[0034] Fig. 9B Shows Fig.9A A cross-sectional view of a portion of a grating multiplexer;

[0035] Fig. 10A An optical multiplexer is shown;

[0036] Fig. 10B Shows Fig. 10A A cross-sectional view of a portion of an optical multiplexer;

[0037] Fig.11A and Fig. 11B shows the optical modes within the tapered waveguide input, and a graph of the corresponding wavelength versus transmission;

[0038] Fig. 12A and Fig. 12B showing an optical mode within a ridge waveguide input, and a graph of the corresponding wavelength versus transmission; and

[0039] Fig.13A and Fig. 13B The optical modes within the strip waveguide input are shown, along with a graph of the corresponding wavelength versus transmission. DETAILED DESCRIPTION

[0040] Various aspects and embodiments of the present invention will now be discussed with reference to the accompanying drawings. Other aspects and embodiments will be apparent to those skilled in the art.

[0041] Figure 5 5 is a top view of an input waveguide 500 used in an optical multiplexer according to an embodiment of the present invention. The input waveguide 500 has an input face 502 and is connected to the reference Fig.9A The input waveguide 500 includes an input ridge portion 506 and an input slab portion 508, the ridge portion extending vertically from the slab portion ( Figure 5 504, where the y-axis extends along the guiding direction of the waveguide. The input ridge portion 506 of the input waveguide is tapered so that the width (measured on the x-axis) decreases in the direction toward the slab waveguide 504 of the wavelength multiplexer element. As shown by the dashed lines 510a-510d, the optical mode of light passing through the input waveguide 500 widens as the light passes from the input facet 502 to the slab waveguide 504. As the input ridge portion 506 narrows, the optical mode is pushed into the input slab portion 508, which causes the optical mode to expand.

[0042] FIG. 6A to FIG. 6D They are shown along Figure 5 506. It can be seen that as the input ridge portion 506 becomes narrower, the portion of the optical modes 510a-510c located within the input ridge portion 506 is squeezed in the x-direction (generally perpendicular to the guiding direction y of the waveguide). Ultimately, the entire optical mode 510d is contained within the input slab region 508 and is smaller than Fig. 6A The initial optical mode 510a shown in FIG. 5 is broader.

[0043] Although Figure 5 and FIG. 6A to FIG. 6D An input waveguide is shown where light is received in the input facet and provided to a slab waveguide 504 of the wavelength multiplexer element, but the output waveguide in some examples of the multiplexer has substantially the same geometry as the input waveguide. In such examples, light is coupled from the slab waveguide 504 into the output ridge and slab portion before exiting the output waveguide at the output facet. As a result, the optical mode of the light within the output waveguide is pushed up into the output ridge portion and is therefore narrowed compared to the mode within the slab waveguide 504.

[0044] Figure 7 Shown along Figure 5 Simulation of the electric field density in a cross section of the input waveguide. It can be seen that as the light propagates from left to right, it is pushed downward into the lower input slab section. FIG. 8A to FIG. 8D The lines A-A', B-B', C-C' and D-D' are shown respectively. Figure 5 Electric field density of the cross section of the input waveguide.

[0045] Fig.9A An optical multiplexer 900 according to an embodiment of the present invention is shown. The optical multiplexer 900 comprises a wavelength multiplexer element 902 provided as an echelon grating having a plurality of teeth. The wavelength multiplexer element 902 is coupled to a Figure 5 9 and output waveguides 908 of the type shown in FIG. 8 , which in this example have the same geometry as the input waveguides.

[0046] In use, a plurality of optical signals enter the optical multiplexer via input waveguides 906. These signals are separated in wavelength and multiplexed via wavelength multiplexer element 902. The multiplexed signal, now comprising a combination of the separated signals, is provided for transmission via output waveguides 908. Of course, it will be appreciated that the system is symmetrical and therefore the multiplexer can be used as a demultiplexer when the multiplexed signal is coupled from output waveguides 908 into the slab waveguides 904 of the wavelength multiplexer element.

[0047] Fig. 9B Shows Fig.9A 5. A cross-sectional view of the optical multiplexer 900 along line AA' is shown. It can be seen that at this point, the input ridge portion 506 has been sufficiently tapered to substantially push the optical mode 510c of light contained therein into the input slab portion 508. Conveniently, due in part to the proximity of the input ridge portion 506, a large amount of overlap can be seen between the modes.

[0048] Fig. 10A An optical multiplexer 1000 according to a variant embodiment of the present invention is shown. The optical multiplexer 1000 includes a wavelength multiplexer element 1002 provided as an arrayed waveguide grating (AWG). The AWG 1002 is coupled to the slab waveguides 1004a and 1004b via the slab waveguides 1004a and 1004b. Figure 5 006 and output waveguides 10008 of the type shown in FIG. 1 , which in this example have the same geometry as the input waveguides.

[0049] Likewise, in use, a plurality of optical signals enter the optical multiplexer 1000 via the input waveguide 1006. These signals are separated in wavelength and multiplexed via the arrayed waveguide grating 1002. The multiplexed signal, now comprising a combination of the separated signals, is provided for transmission via the output waveguide 1008. Of course, it will be appreciated that the system is symmetrical and therefore the multiplexer can be used as a demultiplexer 1002 when the multiplexed signal is coupled from the output waveguide 1008 into the slab waveguide 1004b of the wavelength multiplexer element.

[0050] Fig. 10B Shows Fig. 10A FIG. 5 is a cross-sectional view of the optical multiplexer 1000 along line AA'. It can be seen that at this point, the input ridge portion 506 has been sufficiently tapered to substantially push the optical mode 510c of light contained therein into the input slab portion 508. Conveniently, due in part to the proximity of the input ridge portion 506, a large amount of overlap can be seen between the modes.

[0051] Fig.11A and Fig. 11B The simulated optical modes within the tapered waveguide input are shown, along with a plot of the corresponding wavelength versus transmission. Fig. 11B As can be seen in Figure 2, a wide channel is achieved while maintaining high transmission (approximately -1dB). In contrast, Fig. 12A and Fig. 12B shows a simulated optical mode within a ridge waveguide input and a graph of the corresponding wavelength versus transmission, and Fig.13A and Fig. 13B The simulated optical modes within the strip waveguide input and the corresponding wavelength versus transmission plot are shown. The ridge waveguide input also exhibits wider channels, but with reduced transmission (approximately -2dB). The strip waveguide maintains a higher transmission level (approximately -1dB), but has narrower channels without any substantial degree of overlap.

[0052] The features disclosed in the description or in the accompanying claims or in the drawings, expressed in a specific form or in terms of means for performing the disclosed function or a method or process for obtaining the disclosed results (as the case may be), may be used alone or in any combination of said features to realize the invention in its various forms.

[0053] Although the present invention has been described in conjunction with the above exemplary embodiments, those skilled in the art will understand many equivalent modifications and variations when considering the present disclosure. Therefore, the exemplary embodiments of the present invention set forth above should be considered to be illustrative rather than restrictive. Various changes may be made to the described embodiments without departing from the spirit and scope of the present invention.

[0054] To avoid any doubt, any theoretical explanations provided herein are provided for the purpose of enhancing the reader's understanding. The inventors do not wish to be bound by any of these theoretical explanations.

[0055] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0056] Throughout this specification, including the appended claims, unless the context requires otherwise, the words "comprise" and "include" and variations such as "comprises / comprising" and "including" will be understood to imply the inclusion of stated integers or steps or groups of integers or steps but not the exclusion of any other integers or steps or groups of integers or steps.

[0057] It must be noted that the singular forms "a", "an", and "the" used in the specification and the appended claims include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from "about" one particular value and / or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when a value is expressed as an approximation by using the antecedent "about", it will be understood that the particular value forms another embodiment. The term "about" in relation to a numerical value is optional and means, for example, + / - 10%.

Claims

1. An optical multiplexer comprising: a plurality of input waveguides, each of the plurality of input waveguides comprising an input slab portion and an input ridge portion; An output waveguide, the output waveguide comprising an output slab portion and an output ridge portion v and a wavelength multiplexer element coupled to each input waveguide and the output waveguide, the wavelength multiplexer element comprising a slab waveguide including a grating configured to multiplex signals of different wavelengths received from the input waveguides into a multiplexed signal and provide the multiplexed signal to the output waveguide; wherein the input ridge portion of one or more of the input waveguides is tapered so as to reduce in width in a direction towards the slab waveguide of the wavelength multiplexer element, wherein the wavelength multiplexer element is an echelon grating multiplexer element having a plurality of teeth, wherein the optical multiplexer forms part of an imaging device located in a silicon-on-insulator platform, and the ridge portion extends vertically from the plate portion, wherein each input waveguide is tapered so that an optical mode within the or each input waveguide is confined to the slab portion of the input waveguide and expanded at the point where the input waveguide couples to the slab waveguide of the wavelength multiplexer element, wherein the optical mode of each input waveguide at least partially overlaps the optical mode of an adjacent input waveguide due in part to the proximity of the input waveguides, And wherein the height of the input slab portion of the input waveguide and / or the output slab portion of the output waveguide is substantially equal to the height of the slab waveguide in the wavelength multiplexer element.

2. An optical multiplexer as claimed in claim 1, wherein all of said input waveguides are tapered so as to decrease in width in a direction towards said slab waveguide of said wavelength multiplexer element.

3. An optical multiplexer as claimed in claim 1 or claim 2, wherein the output waveguide is tapered so as to reduce in width in a direction towards the slab waveguide of the wavelength multiplexer element.

4. An optical multiplexer as claimed in any preceding claim, wherein the wavelength multiplexer element is configured to perform coarse wavelength division multiplexing.

5. An optical multiplexer as claimed in any preceding claim, capable of operating over a wavelength range of at least 1.2 μm.

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