Balanced differential modulation method for a silicon photonic modulator

Abstract

The present invention relates to a photonic differential modulator, and more particularly to different electrode configurations for a differential drive scheme to reduce the drive voltage for a given phase shift and device footprint. This embodiment includes two reverse-biased pn junctions driven in a push-pull configuration from signal (S) to signal bar (S bar). The first embodiment shares the S bar electrode between the two pn junctions to reduce the device footprint at the expense of impedance matching difficulties. The second embodiment does not include any shared electrode and adds an additional electrode to facilitate impedance matching. In particular, the second embodiment separates the two pn junctions with their respective transmission lines to facilitate impedance matching. The third embodiment shares both the S and S bar electrodes with an interleaved electrode design to reduce the device footprint and facilitates impedance matching through the slow wave effect on the transmission lines.

Description

[Technical field]

[0001] The present invention relates to the field of silicon photonics and various modulators, and more particularly to a differential modulation scheme for a balanced silicon photonic modulator. [Background technology]

[0002] Power consumption in communication networks increases as data rates increase. One way to reduce power consumption is to reduce the driving voltage of electro-optic modulators used in optical communication networks. In particular, driving silicon photonic modulators with high modulation efficiency (i.e., low driving voltage) in a compact footprint is an ongoing effort.

[0003] US Patent No. 5,999,363 provides a compact structure that effectively shortens the device length by meandering the waveguide shape. However, in this device shape, it is difficult to match the group velocities of the optical field and the modulated microwave field, and therefore the electro-optical bandwidth of the modulator is limited.

[0004] Patent Document 2 proposes a differential modulation scheme for driving silicon-based Mach-Zehnder modulators (MZMs), whose operation relies on the modulation of optical interference by optical phase change induced by the free carrier plasma dispersion effect. However, the proposed driving scheme requires many electrodes. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] US Patent Application Publication No. 2019 / 0162987 [Patent Document 2] U.S. Patent No. 9,507,237 Summary of the Invention [Problem to be solved by the invention]

[0006] An ideal MZM should have high electro-optic bandwidth, high efficiency (i.e., low drive voltage), low insertion loss, compact footprint, and operational stability. However, the prior art still lacks a drive scheme for electro-optic modulators that realizes modulation with small drive voltages in a compact footprint. [Means for solving the problem]

[0007] In one embodiment, the present invention relates to a photonic differential modulator, which includes two pn junction diodes connected to at least one signal electrode (S) and at least one signal bar electrode (S), where the signal bar electrode refers to the signal electrode configured in a push-pull driving scheme and driven by a complementary inverted signal. In one of these junctions, the p-doped and n-doped sides are connected to the S and S-bar electrodes, respectively. In the other junction, the p-doped and n-doped sides are connected to the S-bar and S-electrodes, respectively. These two junctions share an S-bar electrode at each part. Both pn junction diodes are reverse biased externally via on-chip bias or bias tee. The DC bias configuration is not shown here for simplicity and beyond the scope of the present embodiment.

[0008] In one embodiment of the photonic differential modulator, the modulator further comprises two or more ground electrodes (G) that can be used for electromagnetic isolation of the driving RF electromagnetic field.

[0009] In one embodiment of the photonic differential modulator, the modulator further comprises a GSS_SG electrode structure.

[0010] In one embodiment of the photonic differential modulator, the drive voltage for a given phase shift is halved by driving each pn junction from S to S-bar and vice versa in a push-pull configuration compared to a conventional differential drive scheme where each pn junction is driven from S or S-bar to ground.

[0011] In another embodiment, the photonic differential modulator comprises two pn junction diodes connected to at least one signal electrode (S) and at least one signal bar electrode (S bar) configured in a push-pull driving scheme. In one of these junctions, the p-doped and n-doped sides of the pn junction are connected to the S and S bar electrodes, respectively. In the other junction, the p-doped and n-doped sides of the pn junction are connected to the S bar and S electrodes, respectively. Compared to the previous embodiment, these two junctions do not share the S bar electrode. Each has its own independent S and S bar electrodes, with a ground electrode in between. In this way, the two pn junction diodes are isolated from each other from their respective transmission lines by the additional S, S bar, and ground electrodes to facilitate impedance matching at the expense of an increased footprint. Similarly, both pn junctions are reverse biased externally via an on-chip bias or a bias tee.

[0012] In one embodiment of the photonic differential modulator, the modulator further comprises three or more ground electrodes (G) that can be used for electromagnetic isolation of the driving RF electromagnetic field.

[0013] In one embodiment of the photonic differential modulator, the modulator comprises a GSS GSS G electrode structure.

[0014] In one embodiment of the photonic differential modulator, the drive voltage for a given phase shift is halved by driving each pn junction from S to S-bar and vice versa in a push-pull configuration compared to a conventional differential drive scheme where each pn junction is driven from S or S-bar to ground.

[0015] In another embodiment, the photonic differential modulator described above comprises two pn junction diodes connected to at least one signal electrode and at least one signal bar electrode configured in a push-pull drive scheme. As in the previous embodiment, the p-doped and n-doped sides of one of these junctions are connected to S and S-bar electrodes, respectively. In the other junction, the p-doped and n-doped sides are connected to S-bar and S-electrodes, respectively. Since these two junctions share both S and S-bar electrodes in their respective parts through an interleaved electrode structure, this design reduces the need for additional electrodes and saves on the device footprint. Similarly, both pn junctions are reverse biased via on-chip biases or externally via bias tees.

[0016] In another embodiment, the present invention relates to a photonic differential modulator having an interleaved electrode design to provide modulation with low drive voltages in a compact footprint.

[0017] In one embodiment of the photonic differential modulator, the modulator further comprises two or more ground electrodes (G) that can be used for electromagnetic isolation of the driving RF electromagnetic field.

[0018] In one embodiment of the photonic differential modulator, the modulator comprises a GSSG electrode structure.

[0019] In one embodiment of the photonic differential modulator, the drive voltage for a given phase shift is halved by driving each pn junction from S to S-bar and vice versa in a push-pull configuration compared to a conventional differential drive scheme where each pn junction is driven from S or S-bar to ground.

[0020] In one embodiment, the present invention relates to the photonic differential modulator as described above, wherein the material is selected from silicon, lithium niobate (LN), barium titanate (BTO), III-V materials (e.g., InP, GaAs, InGaAs, InGaAsP), and EO polymers. [Brief description of the drawings]

[0021] Other objects, features and advantages of the present embodiments will become apparent from the following description when read in conjunction with the accompanying drawings, in which like reference numerals indicate corresponding parts throughout the several views. The figures are for illustrative purposes only and are therefore not limiting of the disclosure. [Figure 1a] FIG. 1a illustrates a cross-sectional view of a first photonic differential modulator design 100 according to an embodiment of the present disclosure. [Figure 1b] FIG. 1b illustrates a top view of a first photonic differential modulator design 100 according to an embodiment of the present disclosure. [Figure 2a] FIG. 2a illustrates a cross-sectional view of a second photonic differential modulator design 200 according to an embodiment of the present disclosure. [Figure 2b] FIG. 2b illustrates a top view of a second photonic differential modulator design 200 according to an embodiment of the present disclosure. [Figure 3a] FIG. 3a illustrates a cross-sectional view of a third photonic differential modulator design 300 according to an embodiment herein. [Figure 3b] FIG. 3b illustrates a top view of a third photonic differential modulator design 300 according to an embodiment herein.

[0022] To facilitate understanding, like reference numbers have been used whenever possible to designate like elements common to the figures. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The embodiments of the present specification and various features and advantageous details thereof will be more fully described with reference to the non-limiting embodiments illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as not to unnecessarily obscure the embodiments of the present specification. The embodiments used in the present specification are merely intended to facilitate the understanding of how the embodiments of the present specification can be implemented and to further enable those skilled in the art to implement the embodiments of the present specification. Therefore, the examples should not be interpreted as limiting the scope of the embodiments of the present specification.

[0024] Throughout the prior art, there remains a need for advanced differential modulation schemes that can achieve low drive voltages in a compact footprint.

[0025] The present invention provides a photonic differential modulator comprising two pn junction diodes connected to at least one signal electrode (S) and at least one signal bar electrode (S bar). The pn junction diodes are reverse biased. The S and S bar electrodes are connected to the junctions such that the two junctions operate in a push-pull configuration. The p-doped and n-doped sides of one pn junction are connected to the S and S bar electrodes, respectively. The p-doped and n-doped sides of the other pn junction are connected to the S bar and S electrodes, respectively. The S bar electrode is shared between the two junctions and is reverse biased externally via an on-chip bias or a bias tee.

[0026] In one embodiment, the modulation principle is based on a Mach-Zehnder Interferometer Modulator (MZM). MZMs are used to control the amplitude of light waves, and their working principle is as follows: First, the input light is split into two optical paths. Then, a phase shift is induced between the light waves propagating in these two optical paths. Finally, these two beams are recombined together such that the relative phase shift is converted into amplitude modulation due to wave interference.

[0027] 1a and 1b, a first photonic differential modulator design 100 according to an embodiment of the present invention is provided with a GSS - SG electrode design.

[0028] The first photonic differential modulator design 100 includes a first ground electrode 102, a first signal electrode S104, a signal bar electrode (S-bar) 106, a second signal electrode S108, a second ground electrode 110, a reverse biased pn junction diode 112, and another reverse biased pn junction diode 114.

[0029] The first photonic differential modulator design 100 includes a first S electrode 104 connected to a p-doped side of a p-n junction diode 112. The n-doped side of the p-n junction diode 112 is further connected to an S-bar electrode 106. The S-bar electrode 106 is further connected to a p-doped side of another p-n junction diode 114. The n-doped side of the p-n junction diode 114 is further connected to a second S electrode 108. The ground electrodes 102 and 110 are optional and are used to improve electromagnetic isolation of the driving RF electromagnetic field.

[0030] 2a and 2b, a second photonic differential modulator design 200 according to an embodiment of the present invention is provided.

[0031] In one embodiment, to overcome the challenge of matching three signal lines (104, 106, and 108) to a practical characteristic impedance (eg, 100 ohms), a circuit 200 with GSS GSS G is described.

[0032] The second photonic differential modulator design 200 includes a first ground electrode 202, a first signal electrode 204, a first signal bar electrode (S-bar) 206, a second ground electrode 208, a second signal electrode 210, a second signal bar electrode (S-bar) 212, a third ground electrode 214, a reverse biased pn junction diode 216, and another reverse biased pn junction diode 218.

[0033] In the second photonic differential modulator design 200, the first S-electrode 204 is connected to a p-doped side of a p-n junction diode 216, whose n-doped side is connected to the first S-bar electrode 206. The n-doped side of a p-n junction diode 218 is connected to a second S-electrode 210, whose p-doped side is connected to a second S-bar electrode 212. The ground electrodes 202, 208, and 214 are optional and are used to improve electromagnetic isolation of the RF electromagnetic fields.

[0034] The S and S-bar electrodes are traveling wave electrodes, whose characteristic impedance needs to be matched to the impedance of the driver and RF termination. Otherwise, impedance mismatch may cause reflection of the modulated RF electromagnetic field, ultimately leading to inter-symbol interference (ISI) and degradation of the electro-optical bandwidth of the modulator. Impedance matching of the S and S-bar electrodes in the first embodiment 100 to a practical characteristic impedance value (e.g., 100 ohms) is difficult because the shared S-bar electrode 106 limits the freedom of adjusting the design parameters (e.g., electrode width, separation, and thickness) to achieve impedance matching. The second embodiment 200, however, separates these two pn junctions from each other by omitting the shared S-bar electrode in between and adding an additional electrode instead. This provides the advantage of facilitating impedance matching of the traveling wave electrodes, but at the expense of an increased footprint.

[0035] 3a and 3b, a third differential modulator design 300 according to an embodiment of the present invention is provided in a GSS G design.

[0036] The third photonic differential modulator design 300 includes a first ground electrode 302 , a signal electrode 304 , a signal bar electrode (S-bar) 306 , a second ground electrode 308 , a reverse biased pn junction diode 310 , and another reverse biased pn junction diode 312 .

[0037] The third photonic differential modulator design 300 includes a first ground electrode 302 and a second ground electrode 308. The S electrode 304 is connected to the n-doped side of a pn junction diode 310. The S-bar electrode 306 is connected to the p-doped side of the pn junction diode 310. The signal electrode 304 is further connected to the p-doped side of a pn junction diode 312. The S-bar electrode 306 is also connected to the n-doped side of the pn junction diode 312. The ground electrodes 302 and 308 are optional and are used to improve the electromagnetic isolation of the RF electromagnetic field. Different from the previous embodiment, here the S electrode 304 and the S-bar electrode 306 are both interleaved and shared by these two pn junctions 310 and 312.

[0038] In one embodiment, the interleaved electrode structure shown in Figures 3a and 3b is utilized to achieve the differential drive scheme introduced in the previous embodiment, but in a much more compact footprint while maintaining the ease of impedance matching, where the interleaved electrode configuration alleviates the need to accommodate a larger number of electrodes, as in embodiment 200.

[0039] The term "interleaved" in the present invention refers to the layout of the S electrodes 304 and S-bar electrodes 306, and not to the connection to the pn junctions underneath. The interleaved electrode structure facilitates impedance matching of the transmission lines due to the slow wave effect, compared to the first photonic differential modulator 100. The interleaved electrode structure reduces the number of electrodes required for the differential driving scheme, compared to the photonic differential modulator 200.

[0040] In another embodiment, the third photonic differential modulator design 300 reduces the footprint as the electrode configuration is changed from GSS GSS G to GSS G electrode scheme. The footprint of the device is roughly proportional to the number of electrodes, so the GS electrode configuration occupies about half the area of ​​the electrodes.

[0041] In one embodiment, materials for fabricating the photonic differential modulator are selected from silicon, lithium niobate (LN), barium titanate (BTO), III-V materials (e.g., InP, GaAs, InGaAs, InGaAsP), and EO polymers.

[0042] Characteristic impedance Z of a lossless transmission line 0 can be written as follows: Z 0 =√(L / C) Here, L and C represent the impedance and capacitance of the transmission line. Microwave index n μ can be written as follows: n μ =c 0 √(LC) where c 0 is the speed of light in a vacuum. In one embodiment, the interleaved electrode structure increases the inductance L of the transmission line and the characteristic impedance Z 0 However, the microwave index n μ increases, resulting in the microwave group velocity being slower than the optical group velocity, reducing the electro-optical bandwidth of the modulator.

[0043] In all embodiments, the reverse biased pn junction is driven from S to S-bar, which improves modulation efficiency by a factor of two over a conventional differential drive scheme driving from S or S-bar to ground, and by a factor of four over a single-ended drive scheme driving from S to ground.

[0044] A comparative table of designs and performance metrics of a conventional serial push-pull (SPP) modulator, a conventional differential modulator, and embodiments 100, 200, 300 of the present invention is provided below. [Table 1]

[0045] From Table 1, it can be seen that the driving voltages of Examples #1 (100), #2 (200), and #3 (300) are lower than the conventional driving scheme. Each of these examples provides unique design and performance tradeoffs to meet different application requirements.

[0046] As will be readily apparent to those skilled in the art, this embodiment may be readily produced in other specific forms without departing from its essential characteristics. The present embodiment is therefore to be considered merely as illustrative and not limiting, the scope of which is indicated by the claims rather than the foregoing description, and all modifications contained therein are therefore intended to be embraced therein.

Claims

1. Two pn junction diodes connected to at least one signal electrode (S) and at least one signal bar electrode (S bar) configured in a push-pull driving manner; The p-doped and n-doped sides of one pn junction are connected to the S and S-bar electrodes, respectively. The p-doped and n-doped sides of the other pn junction are connected to the S-bar and S-electrodes, respectively. Photonic differential modulator.

2. The S-bar electrode is shared between the two pn junction diodes.

10. The photonic differential modulator of claim 1.

3. The two pn junction diodes are reverse biased.

10. The photonic differential modulator of claim 1.

4. Further, the device is provided with two or more ground electrodes (G). A photonic differential modulator according to any one of claims 1 to 3.

5. The modulator comprises a GSS-SG electrode structure; A photonic differential modulator according to any one of claims 1 to 4.

6. By driving each pn junction from S to S-bar and vice versa in a push-pull configuration, the drive voltage for a given phase shift is halved; A photonic differential modulator according to any one of claims 1 to 5.

7. Two pn junction diodes connected to at least one signal electrode (S) and at least one signal bar electrode (S bar) configured in a push-pull driving manner; The p-doped and n-doped sides of one pn junction are connected to the S and S-bar electrodes, respectively. The p-doped and n-doped sides of the other pn junction are connected to the S-bar and S-electrodes, respectively; The two pn junction diodes are isolated from each other by the additional S, S bar and ground electrodes from their respective transmission lines to facilitate impedance matching at the expense of increased footprint. Photonic differential modulator.

8. The two pn junction diodes are reverse biased.

8. The photonic differential modulator of claim 7.

9. Further, the antenna is provided with three or more ground electrodes (G).

8. The photonic differential modulator of claim 7.

10. The modulator comprises a GSS GSS G electrode structure; A photonic differential modulator according to any one of claims 7 to 9.

11. By driving each pn junction from S to S-bar and vice versa in a push-pull configuration, the drive voltage for a given phase shift is halved; A photonic differential modulator according to any one of claims 7 to 10.

12. Two pn junction diodes connected to at least one signal electrode (S) and at least one signal bar electrode (S bar) configured in a push-pull driving manner; The p-doped and n-doped sides of one pn junction are connected to the S and S-bar electrodes, respectively. The p-doped and n-doped sides of the other pn junction are connected to the S-bar and S-electrodes, respectively; The S and S-bar electrodes are shared between two pn junction diodes, The S and S bar electrodes have an interleaved electrode structure. Photonic differential modulator.

13. The two pn junction diodes are reverse biased.

13. The photonic differential modulator of claim 12.

14. Further, the device is provided with two or more ground electrodes (G).

13. The photonic differential modulator of claim 12.

15. The electrode structure is GSS G; A photonic differential modulator according to any one of claims 12 to 14.

16. By driving each pn junction from S to S-bar and vice versa in a push-pull configuration, the drive voltage for a given phase shift is halved; A photonic differential modulator according to any one of claims 12 to 15.

17. The photonic differential modulator is fabricated from a material selected from silicon, lithium niobate (LN), barium titanate (BTO), III-V materials (e.g., InP, GaAs, InGaAs, InGaAsP), and EO polymers; A photonic differential modulator according to any preceding claim.

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