Electro-optic modulator devices

Abstract

Electro-optic modulators comprising cascaded apodized Bragg gratings for use as slow- light modulators. At least one waveguide arm of the modulator includes a first section comprising an outward apodized grating and a second section connected in series comprising an inward apodized grating. Some embodiments include one or more heaters for tuning the central operational wavelength of the modulator.

Description

ELECTRO-OPTIC MODULATOR DEVICESCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 614,106 filed December 22, 2023 and U.S. Provisional Patent Application No. 63 / 688,493 filed August 29, 2024, the contents of both of which are incorporated by reference as if disclosed herein in their entireties.BACKGROUND

[0002] On-chip silicon Mach-Zehnder modulators (“MZMs”) are important elements for converting electrical signals to the optical domain for use in a wide range of applications including high speed communication, optical computing, and sensing. Many silicon MZMs use the free carrier dispersion effect to induce modulation. A p-n junction is used and as a reverse bias or a forward bias is applied, the refractive indices change as the p, n. and depletion region geometries change. This change in refractive index is used in an interferometer configuration to realize modulation in output optical power intensity.

[0003] Slow-light MZMs are those which incorporate resonant structures such as Bragg gratings to help increase the light-matter interaction in MZMs. Slow-light MZMs can achieve increased modulation efficiency, reduced modulator phase shifter length, and reduced energy consumption. Apodization of a Bragg grating waveguide is applied to reduce side-band oscillation. The optical spectrum that displays slow-light effect only occurs near the stop band edges. When an outwards apodization is used, the apodization smooths the side band oscillation in the right side of the stop band, as shown in Fig. 5A. When an inward apodization is used, the slow-light spectrum with reduced side band oscillation will be at the left side of the photonic stop band, as shown in Fig. 5B.

[0004] In many cases, the typical optical bandwidth with substantial slow-light effect only occupies a spectrum width of approximately 2-3 nm. Fabrication offset from targeted waveguide / grating dimensions may cause the slow-light spectrum to move to a different wavelength from the design. Variation in the dimensions across wafer and batch-to-batch is also difficult to control. During operation, environmental changes, particularly temperature change will cause the stop band edge to shift as well. The slow-light modulator spectrum shift poses a challenge in field deployment to align the laser output wavelength to the operation regime of the slow-light modulator. Further, in some cases, while phase shifted Bragg grating can expand the slow-light bandwidth in MZMs, the transmitted optical power in the extended slow-light pass band can be reduced substantially — i.e., there is a high insertion loss.

[0005] What are needed, therefore, electro-optic modulators that permit tuning of the operational spectrum to account for issues in fabrication and operational environmental conditions.SUMMARY

[0006] Aspects of the present technology are directed to electro-optic modulator devices, including slow-light Mach-Zehnder modulators. In some embodiments, a first waveguide arm includes a first section with an outward apodized grating and a second section with an inward apodized grating. In some embodiments, the sequence of outward and inward apodized gratings is switched. A second waveguide arm, or reference arm, is positioned substantially parallel to the first waveguide arm. In some embodiments, the reference arm is a straight waveguide without apodization. In other embodiments, the reference arm is identical to the apodized arm. Reference arm design can depend on the driving scheme of the electro-optic modulator.

[0007] In some embodiments, a first heater is positioned adjacent to the first section for applying heat to the outward apodized grating. In some embodiments, a second heater is positioned adjacent to the second section for applying heat to the inward apodized grating.

[0008] In some embodiments, the first waveguide arm comprises a p-n junction. In some embodiments, the level of p and n doping increases with increasing distance from the p-n junction toward vias. In some embodiments, the level of p and n doping increases in sections of material each having a different level of doping.

[0009] In some embodiments, the combination of both gratings in the first waveguide arm allows for an increase in the optical slow-light bandwidth of the modulator on the scale of multiple nanometers. Meanwhile, in some embodiments, the apodization allows for removal of group delay ripples otherwise present in uniform gratings in the vicinity of the operational wavelength. In some embodiments, the use of heaters near the gratings, permits tuning of the central operational wavelength.

[0010] Additional aspects and features of the present technology will be apparent to those of skill in the art upon consideration of the following description and the attached drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The drawings show embodiments of the disclosed subject matter for the purpose of illustrating the technology. However, it should be understood that the present application is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein:

[0012] FIG. 1A shows a schematic top view of a modulator according to an embodiment of the present technology;

[0013] FIG. IB shows a schematic cross-section view of the modulator according to an embodiment of the present technology;

[0014] FIG. 2A shows a schematic view of y-splitter for use in some embodiments of the present technology;

[0015] FIG. 2B shows a schematic view of y-combiner for use in some embodiments of the present technology;

[0016] FIG. 3A shows a transmission spectrum associated with an embodiment of the present technology;

[0017] FIG. 3B shows a transmission spectrum associated with the embodiment of FIG. 3 A but with a 40° C temperature increase applied over the concave grating;

[0018] FIG. 4A shows a transmission spectrum associated with an embodiment of the present technology;

[0019] FIG. 4B shows a transmission spectrum associated with the embodiment of FIG. 4A but with a 40° C temperature increase applied over the convex grating;

[0020] FIG. 5 A shows a chart illustrating temperature effects on the right-hand side band edge for an outward apodized Bragg grating;

[0021] FIG. 5B shows a chart illustrating temperature effects on the left-hand side band edge for an inward apodized Bragg grating;DETAILED DESCRIPTION

[0022] The following discussion relates to various embodiments of electro-optic modulator devices. It will be understood that the herein described versions are examples that embody certain inventive concepts as detailed herein. To that end, other variations and modifications will be readily apparent to those of sufficient skill.

[0023] In some embodiments of the present technology, a spectrum-tunable slow-light modulator is provided to at least partially address the above challenges related to manufacturing and operation. In some embodiments, the device comprises a Mach-Zehnder modulator.

[0024] Referring now to FIG. 1, a first embodiment of an electro-optic modulator device 100 is shown schematically. The device 100 comprises a first waveguide arm 101 and a second waveguide arm 102 positioned substantially parallel to the first optical waveguide arm 101. In some embodiments, the first waveguide arm 101 comprises a first section 103 comprising an outward apodized grating 104 and a second section 105 connected in series with the first section103 and comprising an inward apodized grating 106. Thus, in some embodiments, the first waveguide arm comprises cascaded concave (outward apodized) and convex (inward apodized) gratings to form a slow-light MZM. In some embodiments, such as the embodiment shown in FIG. 1, the outward apodized grating is adjacent the input end 107 of the first waveguide arm 101 and the inward apodized grating is adjacent the output end 108. In other embodiments, the inward apodized grating is adjacent the input end 107, and the outward apodized grating is adjacent the output end 108.

[0025] In the embodiment shown in FIGs. 1A and IB, the device 100 is an “unbalanced” MZM, in that the second waveguide arm 102 (i.e., the “reference” arm) is a regular waveguide. In other embodiments, the reference arm includes phase shifter features, including slow-light photonic structures such as Bragg gratings or photonic crystal waveguides to form balanced or other types of unbalanced MZM configuration. Different grating designs are used to form other unbalanced configurations. In some embodiments, a cascaded apodized grating structure is used in the second waveguide arm similar to the design shown in FIG. 1A for the first waveguide arm. In the embodiment shown, the second waveguide arm 102 includes an input end 112 and an output end 113.

[0026] In some embodiments, the device 100 further comprises a heater positioned adjacent to either the first section or the second section. Some embodiments comprise a first heater 109 positioned adjacent to the first section 103 for applying heat to the outward apodized grating 104. In still other embodiments, the device 100 further comprises a second heater 110 positioned adjacent to the second section 105 for applying heat to the inward apodized grating 106. In some embodiments, one or both of the first and second heaters are used to tune the spectrum of the device 100. In some embodiments, tuning by applying heat permits adjustment of the central operational wavelength. In some embodiments, one or both of the heaters comprise a doped silicon portion over which a voltage is applied. In some embodiments, one or both of the heaters comprise a metallic heating element. In some embodiments, one or both heaters are positioned on the first waveguide arm in the first and / or second sections.

[0027] In some embodiments, the one or more heaters over the grating segments can be used to tune the gratings to account for any fabrication offsets present in the parameters of one or both of the gratings. By applying a heater, the band edge of the heated structure will shift to the right. The tunability allows one to correctly align the region of maximum slow-light resulting from the concave and convex gratings with the operational wavelength of the laser light source being utilized in the overall system of which the device is a part. By correctly designing the convex and concave gratings and tuning to maximum slow light, the increased light-matter interactionresulting from the slow-light can be used to improve the MZM performance. In some embodiments, only a single heater is provided over both of the first and second sections, and over one or the other of the first and second sections in other embodiments. In embodiments that include two heaters, the heaters can be controlled individually to allow separate control of the temperature of each of the outward and inward apodized gratings.

[0028] In some embodiments, the device 100 further comprises a spacer waveguide 111 positioned between the first and second sections 103, 105. The spacer waveguide 111 helps to avoid unwanted thermal crosstalk between the first and second sections 103, 105 of the first waveguide arm. The length of the spacer waveguide can be selected and adjusted according to particular design requirements. The length of the spacer is indicated as “S” in FIG. 1 A.

[0029] FIG. IB shows a schematic cross-section view of an embodiment of a device 100. As shown, the first waveguide arm comprises a p-n junction 114. In this embodiment, the p-n junction 114 extends from the input end 107 to an output end 108 of the first waveguide arm 101. In some embodiments, such as the embodiment shown in FIG. IB, the second waveguide arm comprises a p-n junction 115 extending from an input end 112 to an output end 113 of the second waveguide arm 102. The extent of the p-n junctions in the first and second waveguide arms is shown in FIG. 1A.

[0030] As shown in FIG. IB, some embodiments include waveguide materials with different levels of doping. In some embodiments, the area of the p-n junction 114 in the first waveguide arm comprises a set of p and n doped materials 118, 119 having a first level of doping. In some embodiments, the same is true about the second waveguide arm 102: it comprises p and n doped materials 120, 121 having a first level of doping. In some embodiments, the material used for the waveguides is doped silicon. In some embodiments, a doping level of around 1 x 10Al 8 cm’3is used. In some embodiments, a doping level of around 1 x 10Al 9 cm’3. The doping level can be varied to use different concentrations for different foundry considerations. In some embodiments, the doping level can be varied for different designs within one order higher or lower than 1 x 10A18 cm’3and 1 x 10Al 9 cm’3.

[0031] In the embodiment shown, the waveguides connect to the vias 122, 123 through additional doped materials. In some embodiments, the device comprises at least one second set of p and n doped materials having a second level of doping that connect the waveguides to the vias 122, 123. In some embodiments, the second set of materials has a higher level of doping than the first set. In the embodiment shown in FIG. IB, for example, the device comprises n++ doped materials 124, 125 coupled to the vias 122, 123, which receive control signals from the conductors 126, 127, and p++ material 128 coupled to the via 129 that is connected to ground 130. In someembodiments, therefore, the second set of materials is disposed between the first set of materials and the conductors and ground that act as device control electrodes. In some embodiments, the second set of materials comprises a lower doping level, e.g., n+ and / ?+, that is still higher than the doping level of the first set of materials.

[0032] Also in the embodiment shown in FIG. IB, the device includes an intermediate region of n doped materials 131, 132 associated with the first and second waveguide arms 101, 102, respectively. In the embodiment shown, these regions comprise n+ doped material that is a doping level between the doping level of the first n doped material 118, 120 and the second n doped material 124, 125.

[0033] Although FIG. IB shows the p-type doping on the ground side of the p-n junction and the n-type doping on the control electrode side, other embodiments include the p-type doping on the control electrode side of the p-n junction and the n-type doping on the ground side. In other embodiments, other doping profiles are used, such as interleaved doping junctions, vertical junctions, and / or PiN junctions.

[0034] In some embodiments, the waveguide length that will allow a it phase difference in the arms to accumulate is preferred, and while the gratings themselves may be slightly different lengths, the band edges they cause are should be similarly sloped. A shorter than the it phase length will result in utilizing only a portion of the sinusoidal transfer function, i.e. reduced extinction ratio.

[0035] FIG. 1A shows a signal splitter 116 coupled to the input ends 107, 112 of the first and second waveguide arms 101, 102 and a signal combiner 117 coupled to output ends 108, 113 of the first and second waveguide arms. In the embodiment shown, the signal splitter 116 comprises a 2x2 directional coupler, or optical power splitter. In other embodiments, a y-splitter is used to split an input signal and provide it to each of the first and second waveguide arms 101, 102 at their respective input ends 107, 112. A schematic view of a y-splitter is shown in FIG. 2A. Similarly, in the embodiment shown, the signal combiner 117 comprises a 2x2 optical power combiner. In other embodiments, a y-combiner is used to combine the signals from the first and second waveguide arms 101, 102 departing their respective output ends 108, 113. A schematic view of a y-combiner is shown in FIG. 2B.

[0036] According to one embodiment, a design of a cascaded silicon Bragg grating waveguide is provided with an outward apodized (concave) grating having length 30 pm and an inward apodized (convex) grating having length 29 pm. The period of the gratings in the concave gratings is 290 nm and in the convex gratings is 300 nm. The duty cycle, i.e., the grating frequency over theperiod, is 50% in the concave gratings and 56% in the convex gratings. The maximum grating width Wmax is 800 nm and the minimum grating width is 400 nm. This design includes aheater over the concave grating.

[0037] In some embodiments, the apodization parameters for the grating design are alpha = 0.39 and n = 8. For outward apodized gratings, Wmin or Wmax values are often determined by the waveguide core and cladding materials and thickness. For silicon waveguides with SiCh core and Si thickness of 220nm, a typical range of Wmin is 400 to 500 nm for outward grating while a typical range of Wmax is 700nm to 1.5um. For the inward grating, a typical range of Wmin is 250nm to 400nm while typical range of Wmax is ~ 500nm to match or have similar value as the Wmin of the outward grating. In the embodiment shown in FIG. IB, Hf = 220 nm, Hs= 110 nm, Ws= 2.45 pm, and Ws+ = 1.25 pm.

[0038] By using the convex Bragg grating alone, the left edge of stop band appears at 1546 nm. By using the concave Bragg grating alone, the right edge of the stop band appears at 1552 nm. The spectrum of a joint concave Bragg grating followed by a convex grating is plotted in FIG. 3 A. By applying, via a heater, a temperature increase of 40°C in the concave grating segment, an operational spectrum bandwidth of about 4 nm centered around 1550 nm is obtained as shown in FIG. 3B. In an un-cascaded design (i.e., a design with only a single apodized grating), there will only be about 2 nm of usable slow light near the band edge due to either losses getting too large due to approaching the stop band or not having a strong enough slow light effect due to going further into the pass band. By cascading the two types of grating and overlapping the smoothed- out band edges of the respective apodized concave and convex gratings, a larger region of slow light effect is achieved, which in the case of FIG. 3 A and 3B, is about 4 nm. The introduction of the heater permits accounting for fabrication offsets, allowing a shift of the band edges to get the desired overlap while shifting this larger region of slow light towards the desired wavelength, 1550 nm. The results shown in Fig. 3A and 3B show the 7 nm wide operational bandwidth centered around 1550 nm.

[0039] According to another embodiment, a design of a cascaded silicon Bragg grating waveguide is provided with an outward apodized (concave) grating having length 29.2 pm and an inward apodized (convex) grating having length 29.8 pm. The period of the gratings in the concave gratings is 292 nm and in the convex gratings is 298 nm. The duty cycle, i.e., the grating frequency over the period, is 50% in the concave gratings and 57% in the convex gratings. The maximum grating width Wmax is 800 nm and the minimum grating width Wmin is 400 nm. This design includes a heater over the convex grating.

[0040] By using the convex Bragg grating of this design alone, the left band edge appears at 1538 nm while the concave Bragg grating alone will have a right band edge at 1558 nm. Cascading the structures results in the transmission spectrum shown in FIG. 4A and shows a 1 nm wide operational bandwidth centered around 1549 nm. By increasing the temperature of the convex grating segment by 40°C, an operational spectrum bandwidth of about 4 nm around 1550 nm as shown in FIG. 4B is obtained.

[0041] To encode the input optical signal with a matrix value, voltages are applied through the vias 631.

[0042] Although the technology has been described and illustrated with respect to embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without parting from the spirit and scope of the present technology.

Claims

CLAIMSWhat is claimed is:

1. An electro-optic modulator device, comprising: a first waveguide arm comprising: a first section comprising an outward apodized grating; and a second section connected in series with the first section and comprising an inward apodized grating; a second waveguide arm positioned substantially parallel to the first waveguide arm.

2. The device of claim 1, further comprising a first heater positioned adjacent to the first section for applying heat to the outward apodized grating.

3. The device of claim 1 or claim 2, further comprising a second heater positioned adjacent to the second section for applying heat to the inward apodized grating.

4. The device of claim 2, further comprising a spacer waveguide positioned between the first and second sections.

5. The device of claim 3, further comprising a spacer waveguide positioned between the first and second sections.

6. The device of claim 1, further comprising that the first waveguide arm comprises a p-n junction extending from an input end to an output end of the first waveguide arm.

7. The device of claim 1 or 6, further comprising that the second waveguide arm comprises a p-n junction extending from an input end to an output end of the second waveguide arm.

8. The device of claim 1, further comprising a signal splitter coupled to input ends of the first and second waveguide arms and a signal combiner coupled to output ends of the first and second waveguide arms.

9. The device of claim 8, wherein the signal splitter comprises a 2x2 directional coupler.

10. The device of claim 8, wherein the signal splitter comprises a Y-splitter.

11. The device of claim 8, wherein the signal combiner comprises a 2x2 optical power combiner.

12. The device of claim 8, wherein the signal combiner comprises a Y-combiner.

13. An electro-optic modulator device, comprising: a first waveguide arm comprising: a first section comprising an outward apodized grating; and a second section connected in series with the first section and comprising an inward apodized grating; a second waveguide arm positioned substantially parallel to the first waveguide arm; and a heater positioned adjacent to either the first section or the second section.

14. The device of claim 13, wherein the heater is a first heater positioned adjacent to the first section for applying heat to the outward apodized grating and further comprising a second heater positioned adjacent to the second section for applying heat to the inward apodized grating.

15. The device of claim 13, further comprising a spacer waveguide positioned between the first and second sections.

16. The device of claim 13, further comprising that the first and second waveguide arms each comprise a p-n junction extending from an input end to an output end of each of the first and second waveguide arms.

17. An electro-optic modulator device, comprising: a first waveguide arm comprising: a first section comprising an outward apodized grating and a first p-n junction extending from an input end of the first section to an output end of the first section; and a second section connected in series with the first section and comprising an inward apodized grating; and a p-n junction extending from an input end to an output end of the first waveguide arm; and a second waveguide arm positioned substantially parallel to the first waveguide arm.

18. The device of claim 17, further comprising that the first waveguide arm comprises a first set of p and n doped materials that comprise the p-n junction coupled to a second set of p and n doped materials having a higher level of doping than the first set, wherein the second set of materials is disposed between the first set of materials and device control electrodes.

19. The device of claim 18, further comprising a region of / / -doped material having an intermediate level of doping between the / / -doped material of the first set and the / / -doped material of the second set.

20. The device of claim 17, further comprising that the second waveguide arm comprises a p- n junction extending from an input end to an output end of the second waveguide arm.

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