Folding optical modulator

Abstract

The invention relates to a folded optical modulator. An optical device includes: a substrate; at least first and second metal traces disposed on the substrate to define an electrical transmission line; and an optical waveguide disposed on the substrate along a serpentine path passing between the metal traces and including at least first and second electro-optical modulation segments arranged in series along the optical waveguide between the first and second metal traces and separated by a bend in the serpentine path. The device further includes a plurality of electrode pairs, each electrode pair including first and second electrodes connected to the first and second metal traces, respectively, and disposed proximate to each other on opposite sides of one of the electro-optical modulation segments, the plurality of electrode pairs include at least a first electrode pair disposed on an opposite side of the first electro-optical modulation segment and a second electrode pair disposed on an opposite side of the second electro-optical modulation segment.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 691,786, filed September 6, 2024, which is incorporated herein by reference. TECHNICAL FIELD

[0003] The present invention relates generally to optoelectronic devices, and in particular to optical modulators. BACKGROUND

[0004] In a guided-wave optical modulator configured as a Mach-Zehnder interferometer, a waveguide is split into two branches, with appropriately positioned electrodes adjacent to one or both of the branches. An electrical traveling microwave signal is coupled to the electrodes to modulate the relative optical phase between guided waves propagating through the respective branches by the electro-optic effect. When the two branches of the waveguide rejoin and the optical waves from the branches interfere with each other, the modulation of the relative optical phase is converted into modulation of the amplitude of the combined optical wave.

[0005] Guided-wave modulators, such as described above, are used in optical communication systems for transferring modulation from an electrical signal to an optical signal that can propagate at high speed and low propagation loss in an optical waveguide or optical fiber.

[0006] The terms “optical wave,” “optical guided wave,” and “guided wave” as used in this specification and claims refer generally to any and all radiation in the visible, infrared, and ultraviolet. SUMMARY

[0007] Embodiments of the invention described below provide improved designs for optical modulators.

[0008] Accordingly, in accordance with embodiments of the invention, there is provided an optical modulating device comprising: a substrate; at least first and second metal traces disposed on the substrate to define an electrical transmission line; and an optical waveguide disposed on the substrate along a serpentine path passing between the at least first and second metal traces. The optical waveguide comprises at least first and second electro-optic modulation segments arranged in series along the optical waveguide between the first and second metal traces and separated by a bend in the serpentine path. A plurality of electrode pairs are also provided, each electrode pair comprising first and second electrodes connected to the first and second metal traces, respectively, and disposed in close proximity to opposite sides of one of the electro-optic modulation segments. At least a first electrode pair is disposed on opposite sides of the first electro-optic modulation segment, and a second electrode pair is disposed on opposite sides of the second electro-optic modulation segment.

[0009] In disclosed embodiments, the first electro-optical modulation segment and the second electro-optical modulation segment are mutually parallel.

[0010] In other embodiments, the first electro-optical modulation segment and the second electro-optical modulation segment are separated by at least two bends in the serpentine path. In disclosed embodiments, light waves injected into the optical waveguide propagate through both the first electro-optical modulation segment and the second electro-optical modulation segment in the same direction relative to the electrical transmission line.

[0011] In another embodiment, the first pair of electrodes and the second pair of electrodes are interleaved along the electrical transmission line. Additionally or alternatively, the interleaved pairs of electrodes include at least four pairs of electrodes.

[0012] In disclosed embodiments, at least the first electro-optical modulation segment and the second electro-optical modulation segment include at least three electro-optical modulation segments between the first metal trace and the second metal trace.

[0013] In other embodiments, at least the first electro-optical modulation segment and the second electro-optical modulation segment include a silicon waveguide.

[0014] In another embodiment, the bends in the serpentine path include a silicon nitride waveguide.

[0015] According to embodiments of the invention, there is also provided an apparatus for optical modulation, the apparatus comprising an optical modulator device as described above and a reference waveguide. An optical splitter is coupled to split an input optical signal between the optical waveguide in the device and the reference waveguide, and an optical combiner is coupled to combine respective output optical signals from the optical waveguide in the device and the reference waveguide. An electrical drive circuit is coupled to apply a modulation signal to the electrical transmission line for modulating the input optical signal in the device.

[0016] Additionally, according to embodiments of the invention, there is provided a method for modulating an optical signal, the method comprising providing an optical waveguide on a substrate along a serpentine path between a first metal trace and a second metal trace, the first metal trace and the second metal trace defining an electrical transmission line. The optical waveguide includes at least a first electro-optical modulation segment and a second electro-optical modulation segment arranged in series along the optical waveguide between the first metal trace and the second metal trace and separated by a bend in the serpentine path. A plurality of pairs of electrodes are proximate the optical waveguide, each pair of electrodes including a first electrode and a second electrode connected to the first metal trace and the second metal trace, respectively, and disposed adjacent to one another on opposite sides of one of the electro-optical modulation segments. At least a first pair of electrodes is disposed on opposite sides of the first electro-optical modulation segment, and a second pair of electrodes is disposed on opposite sides of the second electro-optical modulation segment. A modulation signal is applied to the electrical transmission line to modulate an optical signal propagating through the optical waveguide.

[0017] The application will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is a schematic top view of an optical device including a folded optical modulator according to an embodiment of the application;

[0019] Figure 2 is a schematic detail view of a folded optical modulator according to an embodiment of the application; Figure 1 and

[0020] Figure 3 is a schematic detail view of a folded optical modulator according to another embodiment of the application. DETAILED DESCRIPTION

[0021] SUMMARY

[0022] Silicon-based guided-wave electro-optical modulators are capable of modulating bandwidths up to 60 GHz. The modulation bandwidth is limited by the loss of the travelling microwave signal in the electrical transmission line driving the modulator, which increases with frequency. There is a need for design modifications that increase the modulation bandwidth while maintaining high modulation efficiency and without increasing the optical insertion loss of the modulator.

[0023] Embodiments of the application described herein achieve increased modulation bandwidth by folding the modulator in a serpentine path into two or more modulation segments. These segments are coupled together in series and can be connected to each other by passive optical waveguides. Such folding allows the length of the electrical transmission line driving the modulator to be reduced relative to the length of the optical waveguide being modulated, which can be at least twice as long as the driving transmission line. This arrangement thus reduces the loss of the travelling microwave signal while permitting higher modulation frequencies, which increases the modulation bandwidth of the electro-optical modulator without significantly increasing optical loss or reducing modulation efficiency.

[0024] Thus, in the disclosed embodiments, an optical modulation device includes a substrate having at least two metal traces disposed on the substrate to define an electrical transmission line. An optical waveguide is disposed on the substrate along a serpentine path passing between the metal traces. The optical waveguide includes at least two electro-optical modulation segments (and possibly more than three electro-optical modulation segments) arranged in series along the optical waveguide between the metal traces and separated by bends in the serpentine path.

[0025] The term "serpentine" is used in its conventional sense in the specification and claims to refer to a path that meanders back and forth with turns that alternate right and left along the direction of propagation of the optical wave in the waveguide. In the disclosed embodiments, the electro-optical modulation segments are generally straight and parallel to one another, and are separated by at least two bends in the serpentine path, so that the optical wave injected into the optical waveguide propagates through the different electro-optical modulation segments in the same direction relative to the electrical transmission lines. Alternatively, other arrangements and interconnections of the electro-optical modulation segments between the metal traces can be used.

[0026] To transfer the modulation from the electrical drive signals in the transmission lines to the optical wave traveling through the waveguide, the device includes a plurality of pairs of electrodes in proximity to one another on opposite sides of each of the electro-optical modulation segments. In each pair of electrodes, one of the electrodes is connected to one of the metal traces of the transmission line (e.g., a signal line), and the other is connected to another metal trace (e.g., a ground line). The pairs of electrodes can advantageously be interleaved along the electrical transmission lines, with each pair of electrodes driving a different electro-optical modulation segment than the immediately adjacent pair of electrodes. Alternatively, other arrangements of the electrodes can be used.

[0027] In the embodiments described below, such electro-optical modulators are integrated into Mach-Zehnder type modulation devices. Alternatively, the various modulation schemes described herein can be integrated into other types of optical modulation devices with necessary modifications. All such alternative embodiments and implementations are considered to be within the scope of the present disclosure.

[0028] System Description

[0029] Figure 1 is a schematic top view of an optical device 100 according to an embodiment of the present invention. In this and subsequent figures, the same labels are used for similar or identical items.

[0030] The optical device 100 includes a substrate 101, such as a silicon-on-oxide (SOI) substrate. A folded optical modulator 102 and a reference waveguide 104 are formed on the substrate 101, for example, by thin film deposition and optical lithography processes known in the art. The folded optical modulator 102 includes an optical waveguide 122 composed of a plurality of segments, as described below. The optical waveguide 122 and the reference waveguide 104 are optically coupled at their respective input ends 106 and 108 by a beam splitter 110 to an input waveguide 112. They are further optically coupled at their respective output ends 114 and 116 by a beam combiner 118 to an output waveguide 120.

[0031] An optical waveguide 122 is formed on the substrate 101 along a serpentine path. The optical waveguide 122 includes two mutually parallel electro-optic modulation segments 124 and 126 separated by bends 125 and 127. In the illustrated example, the bends 125 and 127 are connected by a passive segment 128 (indicated by double dashed lines) of the waveguide 122. The electro-optic modulation segments 124 and 126 can include silicon, for example, while the bends 125 and 127, together with the passive segment 128, include a material with lower optical loss, such as silicon nitride. To match the optical phase in the modulation segments 124 and 126 from electrode pair 130c to electrode pair 132a, the total optical length of the bends 125 and 127, together with the passive segment 128, is preferably an integer multiple of the wavelength λ at which the optical device 100 operates; the total optical length is given by the product n x L, where n is the effective refractive index of the waveguide forming the bends 125 and 127 and the passive segment 128, and L is the total physical length of these parts.

[0032] A metal layer is deposited and etched on the substrate 101 to define an electrical transmission line 138 including a signal trace 134 and a ground trace 136. The transmission line 138 in the optical device 100 can be a differential transmission line, in which both traces 134 and 136 are signal lines differentially driven by a modulation signal, or a single-ended transmission line in which one of the traces is a signal trace and the other is grounded. The transmission line 138 can include one or more additional traces (not shown in the figure), such as an additional ground trace parallel to the signal trace 134 on the opposite side of the ground trace 136 (in a known ground-signal-ground configuration).

[0033] The serpentine path of the optical waveguide 122 is contained between the traces 134 and 136. Three electrode pairs 130a, 130b and 130c are formed on opposite sides of the electro-optic modulation segment 124, while two electrode pairs 132a and 132b are formed on opposite sides of the electro-optic modulation segment 126. One electrode in each of the electrode pairs 130a-c and 132a-b is connected to the signal trace 134, while the other electrode of each electrode pair is connected to the ground trace 136. The electrode pairs 130a-c and 132a-b are interleaved along the electrical transmission line 138. Since silicon nitride does not experience any electro-optic effects due to possible stray electric fields from the electrodes 130a-c and 132a-b, the position of the bends 125 and 127 and the passive segment 128 relative to the electrodes, both laterally and in depth, is not critical for the operation of the device 100.

[0034] The electrical drive circuit 140 applies a modulation signal between the signal trace 134 and the ground trace 136 in the electrical transmission line 138. This modulation signal generally generates a microwave frequency traveling signal in the transmission line 138. This traveling signal generates rapid voltage changes across the electrode pairs 130a-c and 132a-b, which induces a corresponding phase modulation of the optical signal propagating through the optical waveguide 122.

[0035] The Cartesian coordinates 142 are used to represent the orientation of features in subsequent figures relative to Figure 1 the Cartesian coordinates 142.

[0036] In some embodiments, as described above, the electro-optical modulation sections 124 and 126 comprise silicon (Si) waveguides, while the passive section 128 comprises a silicon nitride (SiN) waveguide, which has low loss and is insensitive to stray electric fields. In order to exploit the indirect electro-optical effect of Si in the modulation sections 124 and 126, their material can be appropriately doped to adjust the free carrier concentration for the refractive index and absorption coefficient of the modulating Si. In alternative embodiments, other appropriate materials can be used for both the modulation sections 124 and 126 and the passive section 128. The signal trace 134 and the ground trace 136, as well as the electrode pairs 130a-c and 132a-b, comprise metal films formed on the substrate 101.

[0037] The optical device 100 functions as a Mach-Zehnder interferometer. Thus, an unmodulated light guide wave 144 enters the device 100 through the input waveguide 112 and is split by the optical splitter 110 into a signal wave 146 propagating in the optical waveguide 122 and a reference wave 148 propagating in the reference waveguide 104. The signal wave 146 propagates in the same direction relative to the electrical transmission line 138 in the modulation sections 124 and 126.

[0038] The electrical drive circuit 140 applies a modulation signal to the electrical transmission line 138. As the signal wave 146 travels in the modulation sections 124 and 126 between the respective electrode pairs 130a-c and 132a-b, it accumulates an optical phase change due to the modulation signal and the electro-optical effect. The length of the waveguide 104 can be selected to have the same optical path length or a different optical path length than the modulator 102.

[0039] When the signal wave 146 and the reference wave 148 are recombined by the optical combiner 118 into an output light wave 150 in the output waveguide 120, the accumulated phase change in the signal wave 146 modulates the amplitude of the output light wave due to optical interference between the signal wave and the reference wave. Thus, the electrical modulation signal applied by the electrical driver 140 is converted into an amplitude modulation of the output light wave 150, which then propagates in the output waveguide 120.

[0040] In the disclosed embodiment, the combined length of the modulation segments 124 and 126 is approximately twice the length of the electrical transmission line 138. Thus, with a relatively short electrical transmission line 138 (which incidentally has a low loss for a traveling microwave signal even at high frequencies), a cumulative phase shift can be introduced to the signal wave 146 with high modulation efficiency. In the optical device 100, a microwave signal with a frequency of approximately 100 GHz can be used.

[0041] Folding modulator design

[0042] Figure 2 is a schematic detail view of a folding optical modulator 102 Figure 1 ) according to an embodiment of the invention. Figure 2 shows the folding optical modulator 102 rotated 90° in the counterclockwise direction relative to Figure 1 and includes labels for the dimensions of the items labeled in the figure. Figure 2 Also shown is an additional ground trace 202, which for simplicity is omitted in Figure 1 .

[0043] Typical dimensions for the folding optical modulator 102 are given in Table 1 below. These values are given by way of example and not limitation, and can be increased or decreased for optimal performance depending on constraints such as optical wavelength and modulation frequency. Furthermore, although in the illustrated example the modulator 102 includes five pairs of electrodes interleaved, in alternative embodiments the folding optical modulator can include a greater or lesser number of pairs of electrodes.

[0044] Table 1: Typical values for dimensions of the folding optical modulator 102.

[0045]

[0046]

[0047] Figure 3 is a schematic top view of a folding optical module 300 according to another embodiment of the invention.

[0048] The folding optical modulator 300 is similar to the folding optical modulator 102 Figure 1 and Figure 2 ), except that it includes three modulation segments instead of two. Similar to the optical modulator 102 in the optical device 100, the optical modulator 300 can be used as one arm of a Mach-Zehnder interferometer.

[0049] The optical modulator 300 includes an optical waveguide 302 disposed on a substrate (not shown) along a serpentine path. The optical waveguide 302 includes three mutually parallel electro-optic modulation segments 304, 306, and 308 separated by passive segments 310 and 312 (indicated by double dashed lines) that include appropriate bends. The electro-optic modulation segment 304 passes proximately between two electrode pairs 314a and 314b; the electro-optic modulation segment 306 passes proximately between two electrode pairs 316a and 316b; and the electro-optic modulation segment 308 passes proximately between two electrode pairs 318a and 318b. One electrode of each of the electrode pairs 314a-314b, 316a-316b, and 318a-318b is connected to a signal trace 320, while the other electrode of each electrode pair is connected to a ground trace 322, where the signal and ground traces form an electrical transmission line 324. The electrode pairs 314a-314b, 316a-316b, and 318a-318b are interleaved along the electrical transmission line 324.

[0050] The materials of the modulation segments 304, 306, and 308, the bends 310 and 312, the electrode pairs 314a-314b, 316a-316b, and 318a-318b, and the signal and ground traces 320 and 322 can be the same or similar to the respective entries in the optical modulator 102, respectively.

[0051] Similar to the modulator 102, the folded optical modulator 300 is driven by a microwave propagating signal coupled to the signal and ground traces 320 and 322, which introduces a modulated phase shift to the signal wave 324 propagating in the waveguide 302. Similar to the folded optical modulator 102, the signal wave 324 propagates in the same direction relative to the electrical transmission line 324 in the modulation segments 304, 306, and 308.

[0052] Having three modulation segments 304, 306, and 308 for a given length of the signal and ground traces 320 and 322 further increases the modulation efficiency of the optical wave within the two modulation segments of the optical modulator 102.

[0053] The waveguide in the folded optical modulator 300 can be "mirrored" about the X-axis so that the signal wave 324 enters on the right instead of on the left as in Figure 3

[0054] In alternative embodiments, the number of modulation segments can be increased to four or more, thus further increasing the modulation efficiency. Similarly, the modulator can include a larger number of electrode pairs.

[0055] ​It should be understood that the embodiments described above are cited by way of example, and that the present application is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present application includes both combinations and sub-combinations of the various features described above, as well as variations and modifications thereof that will occur to those skilled in the art upon reading the foregoing description and which fall within the scope of the application.

Claims

1. An optical modulation device, comprising: a substrate; at least first and second metal traces disposed on the substrate to define an electrical transmission line; an optical waveguide disposed on the substrate along a serpentine path passing between the first and second metal traces, and comprising at least first and second electro-optical modulation segments arranged in series along the optical waveguide between the first and second metal traces and separated by a bend in the serpentine path; and a plurality of electrode pairs each comprising a first electrode and a second electrode connected to the first and second metal traces, respectively, and disposed in proximity to one another on opposite sides of one of the electro-optical modulation segments, the plurality of electrode pairs comprising at least a first electrode pair disposed on opposite sides of the first electro-optical modulation segment and a second electrode pair disposed on opposite sides of the second electro-optical modulation segment.

2. The device of claim 1, wherein the first electro-optical modulation segment is parallel to the second electro-optical modulation segment.

3. The device of claim 2, wherein the first and second electro-optical modulation segments are separated by at least two bends in the serpentine path.

4. The device of claim 1, wherein the optical waveguide is configured such that an optical wave injected into the optical waveguide propagates through both the first and second electro-optical modulation segments in the same direction relative to the electrical transmission line.

5. The device of claim 1, wherein the first and second electrode pairs are interleaved along the electrical transmission line.

6. The device of claim 5, wherein the interleaved electrode pairs comprise at least four electrode pairs.

7. The device of claim 1, wherein the at least first and second electro-optical modulation segments comprise at least three electro-optical modulation segments between the first and second metal traces.

8. The device of claim 1, wherein the at least first and second electro-optical modulation segments comprise silicon waveguides.

9. The device of claim 8, wherein the bends in the serpentine path comprise silicon nitride waveguides.

10. An apparatus for optical modulation, comprising: an optical modulation device, the optical modulation device comprising: a substrate; at least first and second metal traces disposed on the substrate to define an electrical transmission line; an optical waveguide disposed on the substrate along a serpentine path passing between the first and second metal traces, and comprising at least first and second electro-optical modulation segments arranged in series along the optical waveguide between the first and second metal traces and separated by a bend in the serpentine path; and ​ a plurality of electrode pairs, each electrode pair including a first electrode and a second electrode connected to the first metal trace and the second metal trace, respectively, and disposed in proximity to one another on opposite sides of one of the electro-optical modulation segments, the plurality of electrode pairs including at least a first electrode pair disposed on opposite sides of the first electro-optical modulation segment and a second electrode pair disposed on opposite sides of the second electro-optical modulation segment; a reference waveguide; an optical splitter coupled to split an input optical signal between the optical waveguide and the reference waveguide in the device; an optical combiner coupled to combine respective output optical signals from the optical waveguide and the reference waveguide in the device; and an electrical drive circuit coupled to apply a modulation signal to the electrical transmission line for modulating the input optical signal in the device.

11. A method for modulating an optical signal, comprising: disposing an optical waveguide on a substrate along a serpentine path between a first metal trace and a second metal trace, the first metal trace and the second metal trace defining an electrical transmission line, the optical waveguide including at least first and second electro-optical modulation segments arranged in series along the optical waveguide between the first metal trace and the second metal trace and separated by a bend in the serpentine path; placing a plurality of electrode pairs in proximity to the optical waveguide, each electrode pair including a first electrode and a second electrode connected to the first metal trace and the second metal trace, respectively, and disposed in proximity to one another on opposite sides of one of the electro-optical modulation segments, the plurality of electrode pairs including at least a first electrode pair disposed on opposite sides of the first electro-optical modulation segment and a second electrode pair disposed on opposite sides of the second electro-optical modulation segment; and applying a modulation signal to the electrical transmission line to modulate an optical signal propagating through the optical waveguide.

12. The method of claim 11, wherein disposing the optical waveguide comprises: orienting the first electro-optical modulation segment and the second electro-optical modulation segment parallel to one another.

13. The method of claim 12, wherein disposing the optical waveguide comprises: separating the first electro-optical modulation segment and the second electro-optical modulation segment by at least two bends in the serpentine path.

14. The method of claim 11, wherein disposing the optical waveguide comprises: configuring the serpentine path such that an optical wave injected into the optical waveguide propagates through both the first electro-optical modulation segment and the second electro-optical modulation segment in a same direction relative to the electrical transmission line.

15. The method of claim 11, wherein placing the plurality of electrode pairs comprises: staggering the first electrode pair and the second electrode pair along the electrical transmission line.

16. The method of claim 15, wherein the staggered electrode pairs include at least four electrode pairs.

17. The method of claim 11, wherein disposing the optical waveguide comprises: providing at least three electro-optical modulation segments between the first metal trace and the second metal trace.

18. The method of claim 11, wherein the at least first and second electro-optical modulation segments include silicon waveguides.

19. The method of claim 18, wherein the bends in the serpentine path include silicon nitride waveguides.

20. The method of claim 11, and comprising splitting an input optical signal between the optical waveguide and a reference waveguide, and combining respective output optical signals from the optical waveguide and the reference waveguide to generate an amplitude-modulated optical signal.

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