WDM receiver and method of operating the same

Abstract

A wavelength division multiplexing (WDM) receiver comprises: an input waveguide; and a demultiplexer connected to the input waveguide. The demultiplexer is configured to: demultiplex a signal received from the input waveguide into a plurality of separate signals, one or more of the separate signals having a plurality of optical modes, and output each of the plurality of separate signals into a corresponding output waveguide connected to a corresponding output port of the demultiplexer. At least one output waveguide configured to carry one of the separate signals having a plurality of optical modes is connected to a corresponding mode rotator, the mode rotator or each mode rotator being configured to rotate the plurality of optical modes of the corresponding separate signal received therein. The mode rotator or each mode rotator is connected to a corresponding waveguide photodiode, the corresponding waveguide photodiode being configured to generate a photocurrent according to the separate signal received from the corresponding mode rotator.

Description

[0001] The present invention relates to a WDM receiver and a method of operating the same.

[0002] An optical receiver supporting wavelength division multiplexing (WDM) contains a wavelength demultiplexer (DeMux) and a series of detectors to convert each wavelength signal into an electrical signal. Typical wavelength demultiplexers are based on arrayed waveguide gratings (AWGs), echelle gratings, angled multimode interference (MMI) waveguides, and / or Mach-Zehnder interferometers, and are usually designed with single-mode waveguides.

[0003] The detector in such a receiver is typically a photodiode (PD) and can be implanted as a surface illumination device or a waveguide device. For higher speed applications, waveguide photodiodes are preferred because they offer lower capacitance than surface illumination devices. In addition, for integrated receivers where the light from the demultiplexer is already in a waveguide, waveguide photodiodes are a more common solution. When implemented in silicon, waveguide photodiodes typically have a horizontal pin junction, by horizontal it is meant that the p and n doped layers are vertical and extend up along the sidewalls of the junction.

[0004] In some applications with wide channel spacing and wide channel width, like coarse wavelength division multiplexing (CWDM), there is a need for wide / flat passbands, because the output channels require low insertion loss within any given channel, but require high rejection from adjacent channels (and therefore require low crosstalk). Several approaches have been proposed for this, such as double Gaussian filtering, or tracking of the MZI at the input to the demultiplexer. However, these increase insertion loss, have high complexity, and low manufacturing yields.

[0005] Previous implementations using surface illuminated photodiodes have used multimode (MM) waveguides for each of the output waveguides of the demultiplexer. This is suitable for demultiplexers based on scanning the beam across the output waveguides (e.g. AWG, mid-step, angled MMI) as the wavelength changes, resulting in wavelength-dependent coupling efficiency to the output waveguides. When changing the output waveguide from single mode (SM) to MM, the waveguide is wider, and therefore the moving spot will couple to the MM waveguide for a larger wavelength range, which has a flatter coupling efficiency, but will still have a sharp drop in rejection when the spot passes the edge of the waveguide.

[0006] Those demultiplexers with MM output waveguides work well as long as all optical circuits after the demultiplexer enforce the requirement for multiple modes to propagate in the waveguide at any time, for example by implementing surface illuminated photodiodes which typically have a wide enough active area to absorb all optical modes.

[0007] As a multiplexer scans the spot with wavelength scanning in the horizontal direction, the generated high-order modes are called horizontal high-order modes. These can have multiple peaks in the horizontal axis (in-plane relative to the photonic integrated circuit (PIC)) and be single-peaked in the vertical axis (perpendicular to the PIC).

[0008] For waveguide photodiodes with horizontal junctions, the horizontal edges of the waveguide experience large optical losses due to the presence of a thin layer of doping at the edge to create the pin junction. This loss does not produce any usable photocurrent. Since higher order modes have a larger portion of their power near the waveguide edge, they will generate less photocurrent, and thus the photodiode will have a responsivity profile that is strongly dependent on wavelength. This negates the benefit of the flattening of the wavelength response provided by the MM waveguide.

[0009] Therefore, the best silicon based photodiodes (e.g., with horizontal junction waveguides) will not work well with the flat wavelength response provided by a demultiplexer with one or more MM output waveguides. The photodiode will need to be redesigned, and this will result in a loss of performance.

[0010] Thus, in a first aspect, embodiments of the present invention provide a wavelength division multiplexing (WDM) receiver comprising:

[0011] Input waveguide;

[0012] A demultiplexer is connected to the input waveguide and is configured to:

[0013] demultiplexing a signal received from an input waveguide into a plurality of separate signals, one or more of the separate signals having a plurality of optical modes, and

[0014] outputting each of the plurality of separated signals into a corresponding output waveguide connected to a corresponding output port of the demultiplexer;

[0015] wherein at least one output waveguide configured to carry one of the split signals having a plurality of optical modes is connected to a respective mode rotator, the or each mode rotator being configured to rotate the plurality of optical modes of the respective split signal received therein; and

[0016] Wherein the or each mode rotator is connected to a respective waveguide photodiode, the respective waveguide photodiode being configured to generate a photocurrent in dependence on a split signal received from the respective mode rotator.

[0017] Such WDM receivers show enhanced performance, and flatter wavelength response.

[0018] The WDM receiver may have any one of the following optional features or any combination thereof under the condition that they are compatible.

[0019] At least one of the plurality of split signals may have a plurality of horizontal optical modes, the or each mode rotator may be configured to rotate the plurality of horizontal modes into a plurality of vertical modes, and the or each waveguide photodiode may comprise a horizontal semiconductor junction.

[0020] At least one of the plurality of split signals may have a plurality of vertical modes, the or each mode rotator may be configured to rotate the plurality of vertical optical modes into a plurality of horizontal optical modes, and the or each waveguide photodiode may comprise a vertical semiconductor junction.

[0021] The WDM receiver may further include an intermediate waveguide located between at least one mode rotator and the corresponding waveguide photodiode, and the intermediate waveguide may have a width measured in a direction perpendicular to the guiding direction of the intermediate waveguide, which narrows along a length parallel to the guiding direction of the intermediate waveguide. The WDM receiver may further include an intermediate waveguide located between the mode rotator or each of the mode rotators and the corresponding waveguide photodiode, and each intermediate waveguide may have a width measured in a direction perpendicular to the guiding direction of the corresponding intermediate waveguide, which narrows along a length parallel to the guiding direction of the corresponding intermediate waveguide so as to reduce the spot size of multiple optical modes transmitted therethrough.

[0022] Each waveguide photodiode may be disposed on a silicon-on-insulator wafer and include a rib or ridge waveguide including one or more doped regions. Each rib or ridge waveguide may be formed of germanium. Each waveguide photodiode may have a first doped region and a second doped region separated by an intrinsic region. The first doped region of each waveguide photodiode may include a lower doped region and an upper doped region, and the lower doped region may contain a dopant having a higher concentration than the upper doped region. Each waveguide photodiode may include a first electrode electrically contacting the first doped region, and a second electrode electrically contacting the second doped region. The first electrode may be electrically contacted with the lower doped region of the first doped region, and the second electrode may be electrically contacted with the lower doped region of the second doped region. The lower doped region having a higher doping concentration than the upper doped region is spatially located below the upper doped region.

[0023] The demultiplexer may be any of the following: an arrayed waveguide grating, an echelle grating, an angled multimode interference demultiplexer, or a Mach-Zehnder interferometer.

[0024] All separated signals may have multiple optical modes, all output waveguides may be connected to corresponding rotators, and all mode rotators may be connected to corresponding waveguide photodiodes.

[0025] In a second aspect, an embodiment of the present invention provides a method of operating a wavelength division multiplexing (WDM) receiver, the method comprising the following steps:

[0026] receiving a signal at an input waveguide;

[0027] providing the received signal to a demultiplexer;

[0028] demultiplexing the received signal into a plurality of separate signals by a demultiplexer, one or more separate signals having a plurality of optical modes;

[0029] outputting each of the plurality of split signals into a corresponding output waveguide connected to a corresponding output port of the demultiplexer;

[0030] in at least one output waveguide carrying one of the separated signals having a plurality of optical modes, using a mode rotator to rotate the plurality of optical modes of the corresponding signal received therein; and

[0031] A corresponding signal of the rotation is provided to a waveguide photodiode, which generates a photocurrent therefrom.

[0032] Such an approach results in a larger degree of photocurrent being generated and the receiver having a flatter wavelength response.

[0033] The method may have any one of the following optional features or any combination thereof under conditions where they are compatible.

[0034] At least one of the plurality of split signals may have a plurality of horizontal optical modes, the or each mode rotator may rotate the plurality of horizontal modes into a plurality of vertical optical modes, and the or each waveguide photodiode may comprise a horizontal semiconductor junction.

[0035] At least one of the plurality of split signals may have a plurality of vertical modes, the or each mode rotator may be configured to rotate the plurality of vertical modes into a plurality of horizontal modes, and the or each waveguide photodiode may comprise a vertical semiconductor junction.

[0036] The receiver may also include an intermediate waveguide located between at least one mode rotator and the corresponding waveguide photodiode, the intermediate waveguide may have a width as measured in a direction perpendicular to the guiding direction of the intermediate waveguide, which narrows along a length parallel to the guiding direction of the intermediate waveguide so as to reduce the spot size of multiple optical modes transmitted therethrough.

[0037] Each waveguide photodiode may be disposed on a silicon-on-insulator wafer and include a rib or ridge waveguide including one or more doped regions. Each rib or ridge may be formed of germanium. Each waveguide photodiode may have a first doped region and a second doped region separated by an intrinsic region. The first doped region of each waveguide photodiode may include a lower doped region and an upper doped region, and the lower doped region may contain a dopant having a higher concentration than the upper doped region. The second doped region of each waveguide photodiode may include a lower doped region and an upper doped region, and the lower doped region may contain a dopant having a higher concentration than the upper doped region. Each waveguide photodiode may also include a first electrode in contact with the first doped region, and a second electrode in electrical contact with the second doped region. The first electrode may be in electrical contact with the lower doped region of the first doped region, and the second electrode may be in electrical contact with the lower doped region of the second doped region.

[0038] The demultiplexer may be any of the following: an arrayed waveguide grating, an echelle grating, an angled multimode interference demultiplexer, or a Mach-Zehnder interferometer.

[0039] All split signals may have multiple optical modes, all output waveguides may be connected to corresponding mode rotators, and all mode rotators may be connected to corresponding waveguide photodiodes.

[0040] Other aspects of the present invention provide: a computer program comprising code that, when executed on a computer, causes the computer to perform the method of the second aspect; a computer-readable medium storing a computer program comprising code that, when executed on a computer, causes the computer to perform the method of the second aspect; and a computer system programmed to perform the method of the second aspect.

[0041] Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

[0042] Figure 1 A schematic diagram showing a WDM receiver;

[0043] Figure 2 A graph showing transmission (dB) versus wavelength for the output of a demultiplexer in a WDM receiver;

[0044] Figure 3A and3B Light in a higher-order horizontal mode and a higher-order vertical mode are shown, respectively;

[0045] Figure 4 shows the structure of a waveguide photodiode; and

[0046] Figure 5 A detailed schematic diagram of a WDM receiver is shown.

[0047] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying drawings. Additional aspects and embodiments will be apparent to those skilled in the art.

[0048] Figure 1 FIG. 1 is a schematic diagram of a WDM receiver 100. In this example, wavelength multiplexed signals: λ1, λ2, . . . , λ n The multiplexed signal is provided to the demultiplexer 102 through the single-mode input waveguide 101. The demultiplexer demultiplexes the multiplexed signal and provides the separated signals to n output waveguides 103a-103n. Each output waveguide is configured to carry the wavelength-dependent part of the multiplexed signal. One or more of the output waveguides are configured to carry multiple optical modes. Preferably, all output waveguides are configured to carry multiple optical modes. The demultiplexer can be, for example, an echelle grating, an arrayed waveguide grating, an MZI, or an angled MMI.

[0049] Figure 2 20 is a graph showing the conductance (dB) versus wavelength of the output of a demultiplexer in a WDM receiver. The solid line 201 illustrates the conductance profile for a single-mode output waveguide, which is Gaussian in shape and includes a maximum. The dashed line 202 illustrates the conductance profile for a multimode output waveguide, which has a wavelength response that is broader and flatter than the single-mode output waveguide.

[0050] Figure 3A and 3B 302a-302d are supported by the waveguide ridge 303. The horizontal mode can be considered as an array of horizontally spaced peaks in amplitude. The ridge is formed by an upright region of the silicon layer 304 and is defined on the lower side by a buried oxide layer 305 (formed by SiO2 in this example). The buried oxide layer is sandwiched between the silicon layer 304 and the silicon substrate 306.

[0051] As discussed previously, the optical signal is then provided to a mode rotator and the optical mode is rotated 90°. Figure 3BAs shown in , the optical signal now comprises a high order vertical mode comprising a plurality of peaks: 307a - 307d. This vertical mode can be considered as an array of vertically spaced peaks in amplitude.

[0052] In another embodiment not shown, the operation is reversed and thus the light is rotated from multiple vertical mode states to multiple horizontal mode states.

[0053] Figure 4 The structure of a waveguide photodiode 400 of the type used in WDM receivers is shown. In general, the photodiode includes a waveguide ridge 401 protruding from germanium slabs 403 and 404 and a silicon slab 402. The silicon slab sits atop a buried oxide layer 405 (SiO2 in this example), which is sandwiched between the silicon slab and a silicon substrate 406. The fabrication process generally includes: (i) providing a silicon-on-insulator wafer; (ii) thinning the silicon device layer; (iii) depositing bulk germanium with uniform thickness; (iv) forming the waveguide ridge by etching adjacent portions, leaving the slab; and (v) providing one or more electrode contacts.

[0054] In this example, the ridge 401 is formed of germanium. The ridge includes doped sidewalls 407 and 408 separated by an intrinsic germanium region 409. This forms a horizontal pin junction of the type mentioned previously. The photodiode also includes a heavily n+ doped region 403, and a heavily p+ doped region 404. These regions are connected to electrodes, respectively, and the higher doping concentration reduces the series resistance of the photodiode.

[0055] The ridge is preferably relatively narrow, for example at least 0.5 μm and not more than 1.5 μm. By providing such a ridge, the transmission time is kept short and the resulting photodiode can therefore operate at a higher speed. Therefore, by providing an optical signal having multiple vertical modes, the ridge waveguide can support a greater number of modes than if the optical signal were in multiple horizontal modes. Accordingly, as has been discussed previously, the wavelength conduction distribution is flatter. In addition, the majority of each optical signal is located within the central portion of the ridge. This eliminates the effect that the doped regions have on the optical signal as a whole, since no particular mode is primarily located in any one of the doped regions.

[0056] In an alternative example not shown, the photodiode structure has a vertical junction. Instead of doped sidewalls, the ridge has upper and lower doped regions that extend laterally across the ridge and are separated by intrinsic regions. In this example, the optical signal preferably includes multiple horizontal modes, each extending from the upper portion of the ridge to the lower portion. Again, this ensures that no one optical mode dominates at any one of the doped regions, and therefore the optical losses due to these doped regions are spread over the optical signal as a whole.

[0057] Figure 5 Detailed schematic diagram of a WDM receiver is shown. As previously described, input waveguide 101 provides a wavelength division multiplexed signal to demultiplexer 102. After demultiplexing, multiple separated signals are provided to corresponding output waveguides 103a-103n. Along each output waveguide is a mode rotator 501a-501n, which performs the above-mentioned mode rotation.

[0058] After rotation, each output waveguide tapers from a first width to a second width (the second width is less than the first width) in a respective tapered region 502-502n. After the tapered region, the output waveguides are coupled to respective waveguide photodiodes 400a-400n of the type discussed previously.

[0059] Although the present invention has been described in conjunction with the above exemplary embodiments, many equivalent modifications and variations will be apparent to those skilled in the art when this disclosure is given. Therefore, the exemplary embodiments of the present invention set forth above are 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.

Claims

1. A wavelength division multiplexing (WDM) receiver, comprising: Input waveguide; a demultiplexer connected to the input waveguide and configured to: demultiplexing a signal received from the input waveguide into a plurality of separate signals, one or more of the separate signals having a plurality of optical modes, wherein the plurality of optical modes include a horizontal optical mode or a vertical optical mode, and outputting each of the plurality of separated signals into a corresponding output waveguide connected to a corresponding output port of the demultiplexer; wherein at least one output waveguide configured to carry one of the split signals having a plurality of optical modes is connected to a respective mode rotator, the or each mode rotator being configured to rotate the plurality of optical modes of the respective split signal received therein by 90°; and Wherein the or each mode rotator is connected to a respective waveguide photodiode, the respective waveguide photodiode being configured to generate a photocurrent in dependence on the split signal received from the respective mode rotator.

2. A WDM receiver according to claim 1, wherein at least one of the plurality of split signals has a plurality of horizontal optical modes, the mode rotator or each mode rotator is configured to rotate the plurality of horizontal optical modes into a plurality of vertical modes, and wherein the waveguide photodiode or each waveguide photodiode comprises a horizontal semiconductor junction.

3. A WDM receiver according to claim 1 or claim 2, wherein at least one of the plurality of split signals has a plurality of vertical optical modes, the mode rotator or each mode rotator is configured to rotate the plurality of vertical optical modes into a plurality of horizontal optical modes, and wherein the waveguide photodiode or each waveguide photodiode comprises a vertical semiconductor junction.

4. The WDM receiver of claim 1 , further comprising an intermediate waveguide between at least one mode rotator and a corresponding waveguide photodiode, wherein the intermediate waveguide has a width measured in a direction perpendicular to a guiding direction of the intermediate waveguide, the width narrowing along a length parallel to the guiding direction of the intermediate waveguide.

5. The WDM receiver according to claim 1 , further comprising an intermediate waveguide located between the mode rotator or each of the mode rotators and the corresponding waveguide photodiode, wherein each intermediate waveguide has a width measured in a direction perpendicular to a guiding direction of the corresponding intermediate waveguide, the width narrowing along a length parallel to the guiding direction of the corresponding intermediate waveguide so as to reduce a spot size of the plurality of optical modes transmitted therethrough.

6. The WDM receiver of claim 1, wherein each waveguide photodiode is disposed on a silicon-on-insulator wafer and comprises a rib or ridge waveguide including one or more doped regions.

7. A WDM receiver according to claim 6, wherein each rib or ridge waveguide is formed of germanium.

8. The WDM receiver of claim 6, wherein each waveguide photodiode has a first doped region and a second doped region separated by an intrinsic region.

9. The WDM receiver of claim 8, wherein the first doped region of each waveguide photodiode comprises a lower doped region and an upper doped region, and wherein the lower doped region contains a dopant having a higher concentration than the upper doped region.

10. The WDM receiver of claim 9, wherein the second doped region of each waveguide photodiode comprises a lower doped region and an upper doped region, and wherein the lower doped region contains a dopant having a higher concentration than the upper doped region.

11. The WDM receiver of claim 10, wherein each waveguide photodiode further comprises a first electrode in electrical contact with the first doped region, and a second electrode in electrical contact with the second doped region. 12 . The WDM receiver of claim 11 , wherein the first electrode is in electrical contact with the lower doped region of the first doped region, and the second electrode is in electrical contact with the lower doped region of the second doped region.

13. The WDM receiver of claim 1, wherein the demultiplexer is any one of: an arrayed waveguide grating, an echelle grating, an angled multimode interference demultiplexer, or a Mach-Zehnder interferometer.

14. The WDM receiver of claim 1, wherein all of the split signals have multiple optical modes, all of the output waveguides are connected to corresponding mode rotators, and all of the mode rotators are connected to corresponding waveguide photodiodes.

15. A method of operating a wavelength division multiplexing (WDM) receiver, comprising the steps of: receiving a signal at an input waveguide; providing the received signal to a demultiplexer; demultiplexing, by the demultiplexer, the received signal into a plurality of separate signals, one or more of the separate signals having a plurality of optical modes, wherein the plurality of optical modes include a horizontal optical mode or a vertical optical mode; outputting each of the plurality of separated signals into a corresponding output waveguide connected to a corresponding output port of the demultiplexer; In at least one output waveguide carrying one of the separated signals having a plurality of optical modes, the plurality of optical modes of a corresponding signal received therein are rotated by 90° using a mode rotator; and A corresponding signal of the rotation is provided to a waveguide photodiode, which generates a photocurrent therefrom.

16. A method according to claim 15, wherein at least one of the plurality of split signals has a plurality of horizontal optical modes, the mode rotator or each mode rotator rotates the plurality of horizontal optical modes into a plurality of vertical optical modes, and wherein the waveguide photodiode or each waveguide photodiode comprises a horizontal semiconductor junction.

17. A method according to claim 15 or 16, wherein at least one of the plurality of split signals has a plurality of vertical optical modes, the mode rotator or each mode rotator is configured to rotate the plurality of vertical optical modes into a plurality of horizontal optical modes, and wherein the waveguide photodiode or each waveguide photodiode comprises a vertical semiconductor junction.

18. The method of claim 15, wherein the receiver further comprises an intermediate waveguide between at least one mode rotator and a corresponding waveguide photodiode, wherein the intermediate waveguide has a width measured in a direction perpendicular to a guiding direction of the intermediate waveguide, the width narrowing along a length parallel to the guiding direction of the intermediate waveguide so as to reduce a spot size of the plurality of optical modes transmitted therethrough.

19. The method of claim 15, wherein each waveguide photodiode is disposed on a silicon-on-insulator wafer and comprises a rib or ridge waveguide including one or more doped regions.

20. The method of claim 19, wherein each rib or ridge waveguide is formed of germanium.

21. The method of claim 19, wherein each waveguide photodiode has a first doped region and a second doped region separated by an intrinsic region.

22. The method of claim 21, wherein the first doped region of each waveguide photodiode comprises a lower doped region and an upper doped region, and wherein the lower doped region contains a dopant having a higher concentration than the upper doped region.

23. The method of claim 22, wherein the second doped region of each waveguide photodiode comprises a lower doped region and an upper doped region, and wherein the lower doped region contains a dopant having a higher concentration than the upper doped region.

24. The method of claim 23, wherein each waveguide photodiode further comprises a first electrode in contact with the first doped region, and a second electrode in electrical contact with the second doped region. 25 . The method of claim 24 , wherein the first electrode is in electrical contact with the lower doped region of the first doped region, and the second electrode is in electrical contact with the lower doped region of the second doped region.

26. The method of claim 15, wherein the demultiplexer is any one of: an arrayed waveguide grating, an echelle grating, an angled multimode interference demultiplexer, or a Mach-Zehnder interferometer.

27. The method of claim 15, wherein all of the split signals have multiple optical modes, all of the output waveguides are connected to corresponding mode rotators, and all of the mode rotators are connected to corresponding waveguide photodiodes.

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