Light ring modulator

Abstract

A ring modulator used as a PAM-N modulator includes: a first optical waveguide forming a bus waveguide; and a ring waveguide optically coupled to the bus waveguide; wherein the ring waveguide includes: a first electrode region having a first pn junction or a first MOScap configured to generate a first phase shift when a given voltage is applied across the first pn junction or the first MOScap; and a second electrode region having a second pn junction or a second MOScap configured to generate a second phase shift when the given voltage is applied across the second pn junction or the second MOScap, wherein the second phase shift is smaller than the first phase shift.

Description

Technical Field

[0001] One or more aspects of embodiments of the present invention relate to a ring modulator, and more specifically to a ring modulator comprising at least first and second electrode regions. Background Technology

[0002] Silicon microring modulators are used in transmitters with advanced modulation formats, particularly PAM-4 modulation. Many silicon microring modulators use a single drive electrode. With a single drive electrode, the modulator is inherently nonlinear, which requires a complex, high-power driver for compensation.

[0003] Dual-drive versions are known (see, for example, US 9,784,995), in which the ring modulator comprises two electrodes of different lengths, such that the length of one electrode is approximately twice the length of the first electrode. However, the requirement for different electrode sizes becomes more complex when the modulator is scaled up from PAM-4 to other PAM-N systems, as the ring resonator eventually runs out of space. Summary of the Invention

[0004] Therefore, the present invention aims to solve the above-mentioned problems by providing a ring modulator for use as a PAM-N modulator according to a first aspect, the ring modulator comprising:

[0005] The first optical waveguide forms a bus waveguide;

[0006] A ring waveguide optically coupled to the bus waveguide;

[0007] The ring waveguide includes:

[0008] A first electrode region having a first pn junction, the first pn junction being configured to generate a first phase shift when a given voltage is applied across the pn junction; and

[0009] A second electrode region having a second pn junction, the second pn junction being configured to generate a second phase shift when the given voltage is applied across the second pn junction, wherein the second phase shift is smaller than the first phase shift.

[0010] Optional features of the invention will now be described. These may be applied individually or in any combination with any aspect of the invention.

[0011] Optionally, the pn junction in the first electrode region contains a higher dopant concentration than the pn junction in the second electrode region. Thus, the first electrode region is the "most significant bit" (MSB), and the second electrode region is the "least significant bit" (LSB).

[0012] Optionally, the pn junction in the first electrode region forms a narrower junction than the pn junction in the second electrode region. Thus, the LSB physically has a wider junction, either as in a PIN junction or effectively by reducing the doping level and thus providing a wider depletion region.

[0013] According to a second aspect of the present invention, a ring modulator is provided for use as a PAM-N modulator, the ring modulator comprising:

[0014] The first optical waveguide forms a bus waveguide;

[0015] A ring waveguide optically coupled to the bus waveguide;

[0016] The ring waveguide includes:

[0017] A first electrode region having a first metal-oxide-semiconductor capacitor (Moscap), the first Moscap being configured to generate a first phase shift when a given voltage is applied across the Moscap; and

[0018] A second electrode region having a second Moscap, the second Moscap being configured to generate a second phase shift when the given voltage is applied across the second Moscap, wherein the second phase shift is smaller than the first phase shift.

[0019] In such an implementation, the transfer function (and therefore the phase shift experienced by the optical signal through the relevant segment of the ring waveguide) is controlled by the thickness of the oxide within the ring waveguide and the doping density of the p and n regions constituting the moscap. The moscap may include an n-doped semiconductor, an insulator, and a p-doped semiconductor. The insulator may be at least 1 nm thick and no more than 100 nm thick (i.e., the n-doped semiconductor and the p-doped semiconductor span a distance of at least 1 nm and no more than 100 nm). The junction formed by the n-doped semiconductor, the insulator, and the p-doped semiconductor may be horizontal or vertical.

[0020] Each Moscap can be formed as a region of a silicon ring waveguide, with one side doped with p-type dopant and the other side doped with n-type dopant. The insulator gap is formed by an insulating oxide layer between the p-doped and n-doped portions, so that when a current is applied between the p-doped and n-doped portions, charge will accumulate on the insulator gap.

[0021] In some implementations, the p-doped portion is located on the outside of the ring waveguide, while the n-doped portion is located on the inside of the ring waveguide (i.e., within the ring of the ring waveguide).

[0022] Optionally, the Moscap of the first electrode region contains a higher dopant concentration than the Moscap junction of the second electrode region. Thus, the first electrode region forms the most significant bit (MSB), while the second electrode region forms the least significant bit (LSB).

[0023] Optionally, the moscap of the first electrode region comprises a first oxide layer having a first thickness, and the moscap of the second electrode region comprises a second oxide layer having a second thickness greater than the first thickness. Similarly, the first electrode region thus forms the most significant bit (MSB), while the second electrode region forms the least significant bit (LSB).

[0024] Optionally, the magnitude of the second phase shift is half the magnitude of the first phase shift. "...half" should be understood as the second phase shift causing the signal to deflect by a certain amount (e.g., in radians), which is half or approximately half the amount by which the first phase shift causes the signal to deflect.

[0025] Optionally, the modulator is used as a PAM-4 modulator. For a PAM-4 modulator, only the first and second electrode regions are needed. If two different voltages can be applied to each electrode region, a total of four different signal configurations will be generated. In the example, one of the two different voltages can be zero volts, while the other is a voltage greater than zero. Alternatively, both different voltages can be non-zero voltages. Additional electrodes will generate additional signals. For example, in some embodiments, the ring modulator may also include a third electrode region configured to generate a third phase shift when a given voltage is applied. The third phase shift can be less than the first phase shift and less than the second phase shift. Thus, assuming the third electrode is also capable of operating in two different configurations, the ring modulator will take the form of a PAM-8 modulator with eight possible overall signals.

[0026] In some implementations, the arc length of the first electrode region along the ring waveguide is equal to the arc length of the second electrode region. Thus, the segment of the ring modulator covered by the first electrode is equal to the segment of the ring modulator covered by the second electrode. "Equal" means that the arc lengths of the first and second electrodes are the same or approximately the same. An important feature is that the response of the electrode region is not determined by the arc length of the electrode region along the ring waveguide. Instead, when a voltage is applied across the electrode region, parameters designed into the junction itself (such as dopant level and junction width) determine the phase shift experienced by the light within the ring modulator.

[0027] Other optional features of the invention are described below. Attached Figure Description

[0028] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, wherein:

[0029] Figure 1A This is a schematic diagram of a ring modulator according to a first embodiment of the present invention, and Figure 1B An example of refractive index versus voltage with respect to the most significant bit (MSB) relative to the least significant bit (LSB) is depicted; and

[0030] Figure 2A This is a schematic diagram of a ring modulator according to a second embodiment of the present invention, and Figure 2B An example of refractive index versus voltage for the most significant bit (MSB) relative to the least significant bit (LSB) is depicted. Detailed Implementation

[0031] The detailed description set forth below with reference to the accompanying drawings is intended as a description of an exemplary embodiment of the optical ring modulator provided according to the present invention, and is not intended to represent the only form in which the invention can be constructed or utilized.

[0032] The following is for reference. Figure 1A and Figure 1B A first embodiment of the optical ring modulator 1 is described. The optical ring modulator comprises a ring waveguide 2, which is typically a silicon waveguide, such as a ribbed waveguide fabricated on a silicon-on-insulator platform. Light is coupled into and / or out of the ring waveguide via a bus waveguide 3, which is also typically a straight waveguide fabricated on a silicon-on-insulator platform. In an alternative example not shown, the bus waveguide 3 may include separate input and output waveguides coupled to the ring waveguide. That is, the bus waveguide 3 may be discontinuous due to interruptions along its length.

[0033] A first electrode region 4 is positioned in a segment of the ring waveguide 2. The first electrode region is formed of a first pn junction, which is configured to generate a first phase shift when a given voltage is applied across the pn junction. Electrodes (not shown) facilitate the application of a given voltage bias across the junction. The first pn junction is formed by a p+ doped region on one side of the ring waveguide and an n+ doped portion on the other side of the ring waveguide. In the illustrated embodiment, the p+ doped portion is located on the outer side of the ring waveguide, while the n+ doped portion is located on the inner side of the ring waveguide.

[0034] The second electrode region 5 is located in a segment of the ring waveguide that differs from the first electrode region. The second electrode region 5 includes a second pn junction configured to generate a second phase shift when a given voltage is applied across the second pn junction, wherein the second phase shift is smaller than the first phase shift. In the illustrated embodiment, the pn junction of the second electrode region is formed by a p-doped region on the outer side of the ring waveguide and an n-doped region on the inner side of the ring waveguide. Compared to the n+ and p+ regions of the first electrode region, the n-doped and p-doped regions have lower dopant concentrations. The dopant used to provide the n / n+ doped regions can be any suitable dopant, such as phosphorus or arsenic. The dopant used to provide the p / p+ doped regions can be any suitable dopant, such as boron or aluminum. The n-doped and p-doped regions can be doped to at least 1 × 10⁻⁶. 16 cm -3 Up to 1×10 19 cm -3 The levels of these regions can be determined by any suitable method known to a person skilled in the art. Thus, the first electrode region 4 forms the most significant bit (MSB) of the ring modulator 1, while the second electrode region 5 forms the least significant bit (LSB) of the ring modulator 1. By applying two different voltages to either of the electrode regions, two different phase shifts can be generated. Since there are two electrode regions, this results in a total of four different modulation states. That is, the modulator can be used as a PAM-4 modulator. If additional electrodes are added and the electrodes operate in a similar manner to the first and second electrode regions, the ring modulator can be used as a PAM-8 modulator. The applied voltage can be in the range of not less than -5 V and not greater than 5 V. The electrodes can be provided by any suitable metallization process known to a person skilled in the art.

[0035] Besides the dopant density of the two electrode regions, the second electrode region differs from the first electrode region in that it has a wider pn junction. This results in a larger depletion width, which also affects the transfer function of the control electrode region. The depletion width can be at least 10 nm and no more than 1000 nm. The combination of the difference in dopant concentration and the difference in depletion width leads to a phase shift caused by a given voltage applied across the pn junction of the second electrode region, which differs from the phase shift caused by the same magnitude voltage applied across the pn junction of the first electrode region.

[0036] exist Figure 1A In the illustrated embodiment, the first electrode region extends over a segment equal to or approximately equal to the segment along which the second electrode region 5 extends. Therefore, since the two regions are of the same size, any difference between the phase shift response of the second electrode region and the phase shift response of the first electrode region is caused only by differences in pn junction characteristics, particularly dopant concentration and depletion width. Figure 1BAn example illustrating the difference is shown where the effective refractive index is plotted for the applied voltage bias across the junction for each of the two electrode regions. It can be seen that the refractive index change experienced by the first electrode region (MSB) is approximately twice that experienced by the second electrode region (LSB).

[0037] The following is for reference. Figure 2A and Figure 2B A second example of the ring modulator 10 is described, where similar numbers are used to represent the figures already mentioned above. Figure 1A and Figure 1B The characteristics of the ring modulator are described in detail.

[0038] Figure 2A The ring modulator 10 and Figure 1A The difference between this ring modulator and others is that the first electrode region 14 and the second electrode region 15 are formed by metal-oxide-semiconductor capacitors (MOScaps) instead of pn junctions. Therefore, they typically operate in reverse-biased mode. In some examples, they can operate under reverse bias.

[0039] and Figure 1A The implementation plan is the same. Figure 2A The ring modulator includes first and second electrode regions that are the same size in terms of the arc length of the segment they occupy along the circumference of the ring waveguide 2. Figure 2B It can be seen that, when any given voltage is applied, the refractive index change exhibited by the first electrode region 15 (MSB) is at least twice that exhibited by the second electrode region (LSB). In this case, the fabrication parameters of the Moscap of the respective electrode regions determine the relative phase shift experienced by light traveling through each electrode region when a given voltage is applied. Specifically, the Moscap of the first electrode region 14 includes an oxide region 17 that is thinner than the oxide region 18 within the Moscap of the second electrode region 15. The thickness of the oxide 19 should be understood as corresponding to its width along a direction perpendicular to the direction of light travel around the ring waveguide. The oxide or other insulator located between the doped regions may have a thickness of at least 1 nm and no more than 100 nm. An important feature is the width of the oxide layer measured between the p-doped and n-doped regions, as this width is the size of the insulating gap in the capacitor.

[0040] In addition to the thickness of the oxide layer, the dopant concentrations of the first and second electrode regions are also selected to influence the phase shift experienced by light traveling through the respective electrode regions when a given voltage is applied. Figure 2AIt can be seen that, compared with the relatively unconcentrated n and p doped regions of the second electrode region 15 forming the LSB of the ring modulator, the first electrode region 14 forming the MSB is composed of more concentrated n+ and p+ doped portions.

[0041] The first electrode region 14 extends above a segment that is equal to or approximately equal to the segment along which the second electrode region 15 extends.

[0042] For all embodiments described herein, the ring modulator is fabricated to have a diameter that can be considered associated with a micro-ring modulator. For example, the diameter of the ring modulator can be at least 5 μm and no more than 5000 μm. Figure 1A and Figure 2A In the illustrated embodiment, the electrode has a 90° arc angle. However, other angles can also be used.

[0043] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when this disclosure is given. Therefore, the exemplary embodiments of the invention set forth above are to be considered illustrative rather than restrictive. Various changes may be made to the embodiments without departing from the spirit and scope of the invention.

[0044] All references mentioned above are incorporated herein by reference.

Claims

1. A ring modulator used as a PAM-N modulator, the ring modulator comprising: The first optical waveguide forms a bus waveguide; A ring waveguide optically coupled to the bus waveguide; The ring waveguide includes: A first electrode region having a first pn junction, the first pn junction being configured to generate a first phase shift when a given voltage is applied across the pn junction; and A second electrode region having a second pn junction, the second pn junction being configured to generate a second phase shift when the given voltage is applied across the second pn junction, wherein the second phase shift is smaller than the first phase shift; The pn junction in the first electrode region contains a higher dopant concentration than the pn junction in the second electrode region.

2. The optical ring modulator according to claim 1, wherein the pn junction in the first electrode region forms a narrower junction than the pn junction in the second electrode region.

3. The optical ring modulator according to claim 1, wherein the second phase shift is half of the first phase shift.

4. The optical ring modulator according to claim 2, wherein the second phase shift is half of the first phase shift.

5. The halo modulator according to any one of claims 1 and 4, wherein the modulator is used as a PAM-4 modulator.

6. The ring modulator according to claim 2, wherein the modulator is used as a PAM-4 modulator.

7. The ring modulator according to claim 3, wherein the modulator is used as a PAM-4 modulator.

8. The optical ring modulator according to any one of claims 1, 4, 6 and 7, further comprising a third electrode region configured to generate a third phase shift when the given voltage is applied, wherein the third phase shift is less than the first phase shift and less than the second phase shift.

9. The optical ring modulator of claim 2, further comprising a third electrode region configured to generate a third phase shift when the given voltage is applied, wherein the third phase shift is less than the first phase shift and less than the second phase shift.

10. The optical ring modulator of claim 3, further comprising a third electrode region configured to generate a third phase shift when the given voltage is applied, wherein the third phase shift is less than the first phase shift and less than the second phase shift.

11. The optical ring modulator of claim 5, further comprising a third electrode region configured to generate a third phase shift when the given voltage is applied, wherein the third phase shift is less than the first phase shift and less than the second phase shift.

12. The optical ring modulator according to any one of claims 1, 4, 6, 7 and 9-11, wherein the arc length of the first electrode region along the ring waveguide is equal to the arc length of the second electrode region.

13. The optical ring modulator according to claim 2, wherein the arc length of the first electrode region along the annular waveguide is equal to the arc length of the second electrode region.

14. The optical ring modulator according to claim 3, wherein the arc length of the first electrode region along the ring waveguide is equal to the arc length of the second electrode region.

15. The optical ring modulator according to claim 5, wherein the arc length of the first electrode region along the annular waveguide is equal to the arc length of the second electrode region.

16. The optical ring modulator of claim 8, wherein the arc length of the first electrode region along the annular waveguide is equal to the arc length of the second electrode region.

17. A ring modulator used as a PAM-N modulator, the ring modulator comprising: The first optical waveguide forms a bus waveguide; A ring waveguide optically coupled to the bus waveguide; The ring waveguide includes: A first electrode region having a first metal-oxide-semiconductor capacitor (Moscap), the first Moscap being configured to generate a first phase shift when a given voltage is applied across the Moscap; and A second electrode region having a second Moscap, the second Moscap being configured to generate a second phase shift when the given voltage is applied across the second Moscap, wherein the second phase shift is smaller than the first phase shift; The Moscap in the first electrode region contains a higher dopant concentration than the Moscap junction in the second electrode region.

18. The halo modulator of claim 17, wherein the Moscap of the first electrode region comprises a first oxide layer having a first thickness, and the Moscap of the second electrode region comprises a second oxide layer having a second thickness greater than the first thickness.

19. The optical ring modulator of claim 17, wherein the second phase shift is half of the first phase shift.

20. The optical ring modulator of claim 18, wherein the second phase shift is half of the first phase shift.

21. The halo modulator according to any one of claims 17 and 20, wherein the modulator is used as a PAM-4 modulator.

22. The ring modulator of claim 18, wherein the modulator is used as a PAM-4 modulator.

23. The ring modulator of claim 19, wherein the modulator is used as a PAM-4 modulator.

24. The optical ring modulator according to any one of claims 17, 20, and 22-23, further comprising a third electrode region configured to generate a third phase shift when the given voltage is applied, wherein the third phase shift is less than the first phase shift and less than the second phase shift.

25. The optical ring modulator of claim 18, further comprising a third electrode region configured to generate a third phase shift when the given voltage is applied, wherein the third phase shift is less than the first phase shift and less than the second phase shift.

26. The optical ring modulator of claim 19, further comprising a third electrode region configured to generate a third phase shift when the given voltage is applied, wherein the third phase shift is less than the first phase shift and less than the second phase shift.

27. The optical ring modulator of claim 21, further comprising a third electrode region configured to generate a third phase shift when the given voltage is applied, wherein the third phase shift is less than the first phase shift and less than the second phase shift.

28. The optical ring modulator according to any one of claims 17, 20, 22-23 and 25-27, wherein the arc length of the first electrode region along the ring waveguide is equal to the arc length of the second electrode region.

29. The optical ring modulator of claim 18, wherein the arc length of the first electrode region along the ring waveguide is equal to the arc length of the second electrode region.

30. The optical ring modulator of claim 19, wherein the arc length of the first electrode region along the annular waveguide is equal to the arc length of the second electrode region.

31. The optical ring modulator of claim 21, wherein the arc length of the first electrode region along the annular waveguide is equal to the arc length of the second electrode region.

32. The optical ring modulator of claim 24, wherein the arc length of the first electrode region along the ring waveguide is equal to the arc length of the second electrode region.

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