Distributed traveling wave photoelectric detector

Abstract

The embodiment of the invention relates to a distributed traveling wave photodetector. An optical communication receiver includes: an optical input configured to receive, from a communication link, a modulated light wave that transmits data over the communication link; and a multimode waveguide coupled to receive the modulated light wave from the optical input and having a width selected such that the modulated light wave forms a plurality of interference maxima over a region of the multimode waveguide. The optical communication receiver further includes: a plurality of optical detectors disposed over the multimode waveguide in alignment with respective ones of the interference maxima, and configured to output an electrical signal in response to optical energy absorbed by the optical detectors from the multimode waveguide; and signal processing circuitry coupled to process and demodulate the electrical signal to extract and output the data.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 627,775, filed January 31, 2024, which is incorporated herein by reference. Technical Field

[0003] The present disclosure relates generally to optical communications and, in particular, to photodetector design. Background Art

[0004] Photodetectors are fundamental building blocks of optical communication devices and other optoelectronic systems. Some photodetectors integrated in silicon-on-insulator (SOI) waveguide optical circuits use germanium as the light-absorbing layer. For example, silicon-germanium (Si-Ge) photodetectors are described by Ito and Ishikawa in “Waveguide-Integrated Vertical PIN Photodiodes of Ge Fabricated on p+ and n+ Si-on-Insulator Layers,” Extended Abstracts of the 2016 International Conference on Solid State Devices and Materials, Tsukuba, Japan, 2016; and by Hu et al. in “High-speed and high-power germanium photodetector with alateral silicon nitride waveguide,” Photonics Research, Vol. 9, No. 5, May 2021. Summary of the Invention

[0005] Embodiments of the present invention described below provide improved designs for integrated photodetectors.

[0006] Therefore, according to an embodiment of the present invention, an optical communication receiver is provided, comprising: an optical input configured to receive a modulated lightwave from a communication link, transmitting data over the communication link; and a multimode waveguide coupled to receive the modulated lightwave from the optical input, the multimode waveguide having a width selected such that the modulated lightwave forms a plurality of interference maxima over a region of the multimode waveguide. The optical communication receiver also includes: a plurality of optical detectors positioned over the multimode waveguide aligned with respective ones of the interference maxima and configured to output electrical signals in response to optical energy absorbed by the optical detectors from the multimode waveguide; and signal processing circuitry coupled to process and demodulate the electrical signals to extract and output data.

[0007] In disclosed embodiments, the optical input includes an optical waveguide having a first width, wherein the width of the multimode waveguide is a second width greater than the first width.

[0008] In some embodiments, the interference maximum and the optical detector are positioned along the length of the multimode waveguide, and the receiver includes an electrical transmission line coupled to the optical detector and configured to transmit the electrical traveling wave in response to an electrical signal output by the optical detector. In one embodiment, the electrical transmission line has an impedance selected to match the velocity of the electrical traveling wave to the modulated optical wave propagating in the multimode waveguide. Additionally or alternatively, the electrical transmission line includes a signal electrode connected in series to the optical detector along the length of the multimode waveguide, and at least one ground electrode extending parallel to the signal electrode along the length of the multimode waveguide.

[0009] In yet another embodiment, a receiver includes a semiconductor substrate, wherein a multimode waveguide is deposited on the semiconductor substrate, an optical detector is deposited over the multimode waveguide, and a signal electrode is deposited over the optical detector.

[0010] In a disclosed embodiment, the multimode waveguide comprises silicon (Si). Additionally or alternatively, the plurality of optical detectors comprises germanium (Ge).

[0011] In disclosed embodiments, the electrode comprises suicide.

[0012] According to an embodiment of the present invention, a method for communication is also provided, the method comprising: receiving a modulated light wave from a communication link that transmits data through the communication link; coupling the modulated light wave into a multimode waveguide having a width selected so that the light wave forms a plurality of interference maxima over a region of the multimode waveguide; and sensing the modulated light wave using a plurality of optical detectors, the plurality of optical detectors being arranged over the multimode waveguide aligned with respective ones of the interference maxima and configured to output electrical signals in response to optical energy absorbed by the optical detectors from the multimode waveguide.

[0013] Through with Figure 1The present invention will be more fully understood from the following detailed description of embodiments of the present invention, in which: BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a block diagram schematically illustrating an optical communication receiver according to an embodiment of the present invention;

[0015] Figure 2A and Figure 2B According to an embodiment of the present invention Figure 1 a schematic top view of a photodetector in a communications receiver of FIG.

[0016] Figure 3 According to an embodiment of the present invention Figure 2A and Figure 2B Schematic cross-sectional view of a photodetector. DETAILED DESCRIPTION

[0017] Photodiodes are used in optical communication receivers to convert modulated light waves propagating in silicon-on-insulator waveguides into electrical signals. The bandwidth of a photodiode is limited by the photodiode's resistor-capacitor (RC) bandwidth, fRC, and the carrier transmission bandwidth, ftr, which combine to form a total bandwidth, f, given by 1 / f² = 1 / fRC² + 1 / ftr². The RC bandwidth, fRC, is determined by the RC time constant of the photodiode's series resistance and its junction capacitance. The transmission bandwidth, ftr, is limited by the thickness of the photodiode's PIN junction (i.e., the intrinsic thickness of the absorber), which determines the carrier transmission time across the junction. Reducing the junction thickness to increase ftr has the disadvantage of increasing the junction capacitance and reducing fRC, thereby reducing the total bandwidth, f. Reducing the detector area to reduce the junction capacitance also reduces the detector's responsivity, i.e., the efficiency of converting light waves into photocurrent.

[0018] Embodiments of the invention described herein address these issues by providing a distributed traveling-wave photodetector architecture that offers high bandwidth with minimal impact on the detector's responsivity. A modulated lightwave propagating in a waveguide (such as the input waveguide of an optical receiver) is coupled into a wider multimode waveguide that acts as a multimode interferometer (MMI). The lightwaves propagating within the multimode waveguide reflect from its sidewalls, where the reflections interfere with each other and form multiple interference maxima of high intensity within the multimode waveguide. These maxima have small lateral dimensions (in the plane of the semiconductor substrate forming the MMI), for example on the order of 1 μm to 2 μm.

[0019] Optical detectors (such as Ge detectors with lateral dimensions comparable to the interference maxima) are positioned above the locations of the corresponding maxima. The detectors typically output electrical signals in response to the optical energy they absorb from the multimode waveguide via evanescent coupling. Signal processing circuitry processes and demodulates the electrical signals to extract and output the data conveyed by the modulated light waves.

[0020] In some embodiments, to couple electrical signals from the optical detectors, a signal electrode is deposited above the Ge detectors, coupling the detectors in series. One or more ground electrodes, parallel to the signal electrodes, are deposited on the substrate and connected to the substrate via metal-filled vias. The signal and ground electrodes form an electrical transmission line, whose impedance is selected to match the speed of the electrical traveling wave with the speed of the modulated light wave propagating in the multimode waveguide. This transmission line couples the detector output to signal processing circuitry.

[0021] The small area of the Ge detectors in this embodiment allows for a reduction in detector thickness, increasing ftr without excessively reducing frc. Because each Ge detector is aligned with the interference maximum of the light wave, typically absorbing 30% of the light energy within that maximum, this ensures a high overall receiver responsivity. Due to the small size of the interference maximum, the electrode vias can be spaced sufficiently far from the interference maximum to minimize their impact on the detector's absorption of light energy from the interference maximum.

[0022] Figure 1 is a block diagram schematically illustrating an optical communication receiver 100 according to an embodiment of the present invention.

[0023] Communications receiver 100 includes an optical device 104 serving as a photodetector (PD) and signal processing circuitry 105, which includes a transimpedance amplifier (TIA) 108, an analog-to-digital converter (ADC) 110, and a digital signal processor (DSP) 112 coupled in series. Communications receiver 100 receives a modulated lightwave 102 from a communications link 113, conveying data over the communications link. Modulated lightwave 102 enters device 104, which absorbs optical energy from the modulated lightwave and converts it into an electrical output signal, such as a photocurrent signal 106. Signal processing circuitry 105 processes and demodulates signal 106 to extract the data and output it into an output data signal 114. Specifically, TIA 108 converts the photocurrent into an analog voltage signal 109. Analog signal 109 is coupled to ADC 110, which converts it into a digital signal 111. The digital signal 111 is coupled to a DSP 112 which demodulates the signal to extract the data and outputs the data as an output data signal 114 of the communication receiver 100 .

[0024] Figure 2A and Figure 2B is a device 104 ( Figure 1 ). For clarity, Figure 2A The device 104 is shown without superimposed electrodes, and then the device 104 is Figure 2B Added superimposed electrodes.

[0025] Figure 2A A device 104 is shown, which is deposited on a semiconductor substrate 200 (such as an SOI substrate) and includes an input waveguide 202 with a width w1 and a multimode waveguide 204 with a width w2, where w2>w1. A modulated lightwave 102 enters the input waveguide 202 and is coupled into the multimode waveguide 204. The width w2 of the multimode waveguide 204 is selected so that the lightwave forms a plurality of interference maxima 206 over a region of the multimode waveguide. For a symmetric multimode waveguide 204, the interference maxima 206 form along the length of the multimode waveguide, i.e., on its symmetry axis 207. The creation of interference maxima 206 along axis 207 is schematically illustrated by arrows 209, which represent local maxima in the multimode interference pattern. However, arrows 209 do not accurately reflect the entire mode pattern of the lightwave within the multimode waveguide 204; they are added for visualization purposes only.

[0026] The device 104 also includes a plurality of optical detectors 208 disposed above the multimode waveguide 204 and aligned with respective interference maxima 206. The detectors 208 output electrical signals in response to optical energy absorbed by the optical detectors from the multimode waveguides. The optical detectors 208 comprise a semiconductor material, such as Ge, that absorbs optical energy from the interference maxima 206 through evanescent coupling between the multimode waveguide 204 and the detectors. The lateral dimensions of the detectors 208 are comparable to the lateral dimensions of the interference maxima 206, typically 1 μm to 2 μm. Because most of the energy in the guided optical waves within the multimode waveguide 204 is concentrated in the interference maxima 206, even a small detector 208 absorbs a significant portion (typically 30% at each maximum 206) of the optical energy and converts it into an electrical signal.

[0027] Figure 2B The device 104 is shown to further include a signal electrode 210 and two ground electrodes 212 deposited on the device, wherein the electrodes comprise a conductive material, such as a suitable silicide. The signal electrode 210 is connected in series to the optical detector 208 along the length of the multimode waveguide 204, and the ground electrode 212 extends parallel to the signal electrode along the length of the multimode waveguide. The signal electrode 210 and the ground electrode 212 together form an electrical transmission line 213, the impedance Z of which is selected to match the velocity velectrical of the traveling electrical wave in the transmission line to the velocity voptical of the modulated optical wave propagating in the multimode waveguide 204. The output electrical signal 106 ( Figure 1 ) is coupled to the TIA 108 and is also coupled to other components of the receiver 100, as described above. Figure 1 shown.

[0028] Figure 3 The device 104 according to an embodiment of the present invention is along Figure 2B 3. Cross-sectional view 300 along line 214 in FIG.

[0029] Multimode waveguide 204( Figures 2A to 2B ) includes an n-doped or p-doped Si layer 302. The multimode waveguide 204 and the optical waveguide 202 are deposited and etched on a semiconductor substrate 200, such as an SOI substrate, and include a silicon dioxide (SiO2) layer 304 and a Si layer 306. As previously described, the optically guided waves propagating in the layer 302 form interference maxima 206 in the multimode waveguide 204, one of which is shown in the cross-sectional view of the present figure. Optical detectors 208 are deposited and etched above the layer 302, aligned with the interference maxima 206. Each detector 208 includes an intrinsic Ge layer 308 and a p-doped or n-doped Ge layer 310, wherein the doping is selected to be opposite to the doping of the layer 302, deposited and etched on the multimode waveguide 204. An insulating layer 312, for example, comprising SiO2, is deposited above the layer 302, encapsulating the detectors 208.

[0030] The waveguides 202 and 204, SiO2 layer 304, Si layer 306, and Ge layers 308 and 310 are deposited, etched, and doped as necessary using semiconductor fabrication processes known in the art.

[0031] The signal electrode 210 and the ground electrode 212 are deposited on the insulating layer 312. The signal electrode 210 is connected to the detector 208 using one or more vias 314 filled with a conductive material, such as metal. The ground electrode 212 is similarly coupled to the Si layer 302 using metal-filled vias 316. The vias 314 are sufficiently far from the interference maximum 206 so as not to interfere with the absorption of light energy from the interference maximum by the detector.

[0032] It should be noted that the above embodiments are cited by way of example, and the present invention is not limited to what is particularly shown and described above. On the contrary, the scope of the present invention includes combinations and sub-combinations of the various features described above, as well as variations and modifications that occur to those skilled in the art after reading the foregoing description and that are not disclosed in the prior art.

Claims

1. An optical communication receiver, comprising: an optical input configured to receive, from a communication link, a modulated light wave conveying data via the communication link; a multimode waveguide coupled to receive the modulated lightwave from the optical input and having a width selected such that the modulated lightwave forms a plurality of interference maxima over a region of the multimode waveguide; a plurality of optical detectors disposed over the multimode waveguide in alignment with respective ones of the interference maxima and configured to output electrical signals in response to optical energy absorbed by the optical detectors from the multimode waveguide; as well as Signal processing circuitry is coupled to process and demodulate the electrical signal to extract and output the data.

2. The receiver of claim 1, wherein the optical input comprises an optical waveguide having a first width, and wherein the width of the multimode waveguide is a second width greater than the first width.

3. The receiver of claim 1 , wherein the interference maximum and the optical detector are arranged along the length of the multimode waveguide, and The receiver includes an electrical transmission line coupled to the optical detector and configured to transmit an electrical traveling wave in response to the electrical signal output by the optical detector.

4. The receiver of claim 3, wherein the electrical transmission line has an impedance selected to match the velocity of the electrical traveling wave to the modulated optical wave propagating in the multimode waveguide.

5. The receiver of claim 3 , wherein the electrical transmission line comprises: a signal electrode connected in series to the optical detector along the length of the multimode waveguide; as well as At least one ground electrode extends along the length of the multimode waveguide parallel to the signal electrode.

6. The receiver of claim 5, further comprising a semiconductor substrate, wherein the multimode waveguide is deposited on the semiconductor substrate, the optical detector is deposited above the multimode waveguide, and the signal electrode is deposited above the optical detector. The receiver of claim 6 , wherein the multimode waveguide comprises silicon (Si). The receiver of claim 7 , wherein the plurality of optical detectors comprises germanium (Ge).

9. The receiver of claim 5, wherein the electrode comprises silicide.

10. A method for communication, comprising: receiving, from a communication link, a modulated light wave conveying data via said communication link; coupling the modulated lightwave into a multimode waveguide having a width selected such that the lightwave forms a plurality of interference maxima over a region of the multimode waveguide; as well as The modulated light wave is sensed using a plurality of optical detectors disposed over the multimode waveguide aligned with respective ones of the interference maxima and configured to output electrical signals in response to optical energy absorbed by the optical detectors from the multimode waveguide.

11. The method of claim 10, wherein receiving the modulated light wave comprises inputting the modulated light wave into the multimode waveguide through an input waveguide having a first width, and wherein the width of the multimode waveguide is a second width greater than the first width.

12. The method of claim 10, wherein the interference maximum and the optical detector are arranged along the length of the multimode waveguide, and Wherein sensing the modulated light wave comprises coupling an electrical transmission line to the optical detector, wherein the electrical transmission line is configured to transmit an electrical traveling wave in response to the electrical signal output by the optical detector.

13. The method of claim 12, wherein coupling the electrical transmission lines comprises: The impedance of the electrical transmission line is set to match the speed of the electrical traveling wave to the modulated optical wave propagating in the multimode waveguide.

14. The method of claim 12, wherein coupling the electrical transmission line comprises: connecting a signal electrode in series to the optical detector along the length of the multimode waveguide; and At least one ground electrode is extended along the length of the multimode waveguide parallel to the signal electrode.

15. The method according to claim 14, further comprising: providing a semiconductor substrate; depositing the multimode waveguide on the semiconductor substrate; depositing the optical detector on the multimode waveguide; as well as The signal electrode is deposited over the optical detector.

16. The method of claim 15, wherein depositing the multimode waveguide comprises depositing and etching a silicon (Si) layer on the semiconductor substrate. The method of claim 16 , wherein depositing the plurality of optical detectors comprises depositing and etching a germanium (Ge) layer over the Si layer.

18. The method of claim 14, wherein connecting the signal electrode comprises depositing a silicide electrode over the optical detector.

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