Patent ID: 12228808

SUMMARY

In an implementation, an optical modulator includes: a semiconductor substrate; an optical waveguide portion disposed on the semiconductor substrate, the optical waveguide portion configured to provide an optical path for light that is to be modulated; a first P-N junction disposed on the semiconductor substrate along the optical path, the first P-N junction defining a border between an N-doped portion disposed on the semiconductor substrate and a P-doped portion disposed on the semiconductor substrate; and a second P-N junction disposed on a portion of the semiconductor substrate alongside the optical path and spaced apart from the first P-N junction.

In an implementation, a method of optical modulation includes: optically coupling light to be modulated onto an optical path provided by an optical waveguide portion disposed on a semiconductor substrate; and electrically coupling a modulation signal along an electrical path by: electrically coupling across a first P-N junction disposed on the semiconductor substrate along the optical path and that defines a border between a N-doped portion disposed on the semiconductor substrate and a P-doped portion disposed on the semiconductor substrate; and electrically coupling across a second P-N junction disposed on the semiconductor substrate alongside the optical path and spaced apart from the first P-N junction.

In an implementation, a method of manufacturing an optical modulator includes: providing a semiconductor substrate; fabricating an optical waveguide portion disposed on the semiconductor substrate, the optical waveguide portion configured to provide an optical path for light that is to be modulated; fabricating a first P-N junction disposed on the semiconductor substrate along the optical path, the first P-N junction defining a border between an N-doped portion disposed on the semiconductor substrate and a P-doped portion disposed on the semiconductor substrate; and fabricating a second P-N junction disposed on a portion of the semiconductor substrate alongside the optical path and spaced apart from the first P-N junction.

DETAILED DESCRIPTION

In various contexts, optical modulators encode electro-magnetic modulation signals onto light signals. In some cases, one or more radio-frequency (RF) signals, e.g., potentially aggregated from multiple sources, provide the to-be encoded modulation signals. An optical modulator receives the RF signals and unencoded light as inputs and produces light with the RF signal encoded thereon as an output. An example of a type of optical modulator is an optical modulator that splits coherent source light in two portions, phase shifts at least one of the portions in accordance with the RF signal to be encoded, and then recombines the portions to create an interference effect that encodes the RF signal into the intensity profile of the output light. Ring modulators, which impart the phase shift in a ring coupled to an input/output channel, are one example. Another example is a Mach-Zehnder modulator (MZM) that splits input light into two arms and then recombines the light after phase shifting light passing through (at least) one of the arms.

In various example semiconductor-based systems (using both ring and MZM formats), the phase shift is imparted by passing light through an optical waveguide coincident with a P-N junction that extends down the optical path of the optical waveguide. The RF signal is electrically coupled across the P-N junction, e.g., from an input contact to a DC reference point, to impart the phase shift on the passing light. In some cases, the bandwidth capacity of such semiconductor-based modulators is dependent on the electrical performance of the semiconductor device, e.g., the ability of the device to handle high frequency, 1-100 gigahertz or higher frequency, RF signals. In some cases, factors such as resistivity and/or capacitance over an electrical path cause frequency-dependent losses over the electrical path. In some cases, those losses increase with the frequency of the signal.

In an illustrative example semiconductor-based optical modulator with a single P-N junction (e.g., the P-N junction coincident with the optical waveguide) on the electrical path between the input contact to the DC reference point, the single P-N junction defines a border between one N-doped portion and one P-doped portion. In other words, a P-N junction defines a border between opposingly-doped portions. In this illustrative example, to avoid the comparatively high resistance of un-doped semiconductor substrate, the entire electrical path is either P-doped or N-doped. Thus, in this illustrative example, without additional P-N junctions, the one N-doped region and one P-doped region is the most the example optical modulator supports.

Due to their structure, P-N junctions contribute capacitance. Further, because the loss contributions due to capacitance grow quadratically with RF signal frequency, conventional wisdom has held that only P-N junctions used to effect an optical phase shift should be included in the electrical path and that other P-N junctions are to be avoided. Nevertheless, the resistivity of P-doped regions is higher than that of N-doped regions with similar dopant levels. Accordingly, although contrary to conventional wisdom, inventors have found that an optical modulator with an electrical path including multiple P-N junctions has the potential to reduce overall RF signal losses. Moreover, by increasing the N-doped path length relative to P-doped path length, resistivity can be reduced silicon as to overcome additional contribution to RF-signal loss from capacitance.

Referring now toFIG.1, a cross-sectional view of an example semiconductor based optical modulator100is shown. The example optical modulator100includes a semiconductor substrate102. An optical waveguide104is disposed on the substrate102. The optical waveguide104is structured to guide light in one or more optical modes106on an optical path (going into the page). A first P-N junction (e.g., an “on-path” P-N junction)108is coincident with the optical waveguide104. In other words, the on-path P-N junction is on the optical path. The on-path P-N junction108defines a border between the P-doped portion112on the substrate102and a first N-doped portion114on the substrate.

When exposed to a change in electrical potential, the P-doped portion112and the first N-doped portion114undergo different changes in their respective refractive indices. In other words, a change in electrical potential results in relative change in refractive index between the P-doped portion112and the first N-doped portion114. Accordingly, when exposed to an RF signal, the relative refractive index of the P-doped portion112and the first N-doped portion114is dependent on the changing E-field of the RF-signal. Because the one or more optical modes106straddle the on-path P-N junction108, any relative index difference of the causes the phase of the optical modes106to process. Refractive index is the inverse of phase velocity. Therefore the relative index causes the portion of the optical mode106overlapping the P-doped portion112to have a different phase velocity than the portion of the optical mode overlapping the first N-doped portion114. Further, because the relative index is controlled by the input RF-signal, the resultant phase shift for the optical mode106also is dependent on the RF signal. When recombined and interfered with the other split coherent light, the time-dependent intensity of the interfered modulated output light depends on the time-dependent intensity of the E-field of the RF-signal.

In various implementations, the relative refractive index shifts of P-doped and N-doped materials arises out of free-carrier effects such as plasma dispersion effects and photon-absorption effects.

A second P-N junction (e.g., an “off-path” P-N junction)110is spaced apart from the optical waveguide104and the on-path P-N junction108. In other words, the off-path P-N junction110is off the optical path. The off-path P-N junction110defines a border between the P-doped portion112and a second N-doped portion116disposed on the substrate102. Together, the on-path P-N junction108and the off-path P-N junction110form an “NPN” modulator, where a P-doped portion is positioned between two N-doped portions.

In the example, the on-path P-N junction108and off-path P-N junction110are laterally spaced apart and have the same spatial orientation with respect to the semiconductor substrate102. However, in other implementations, the on-path P-N junction108and off-path P-N junction110are horizontally spaced apart and/or spaced apart both horizontally and laterally. In some implementations, the on-path P-N junction108and off-path P-N junction110have different orientations with respect to the semiconductor substrate102. For example, in some cases, the off-path P-N junction110is horizontal with respect to the substrate while the on path P-N junction108vertical with respect to the substrate.

The on-path P-N junction108and off-path P-N junction110are spaced apart such that at least two non-contiguous same-doped portions are created within the example semiconductor based optical modulator100. Thus, regardless of the relative orientations of the on-path P-N junction108and off-path P-N junction110, the P-N junctions108,110create at least two non-contiguous same-doped portions that do not touch one another. Rather these two non-contiguous same-doped portions are separated by at least one opposingly-doped portion which, in some cases, provides electrical coupling between the two non-contiguous same-doped portions. In the example semiconductor based optical modulator100, two non-contiguous N-doped portions114,116are present. The two non-contiguous N-doped portions114,116are separated by the P-doped portion112. The P-doped portion provides electrical coupling between the two non-contiguous N-doped portions114,116.

In the example, the spacing140between the on path108and off path110P-N junctions is selected to avoid interference between the optical modes106and the off-path P-N junction110. In an example, the spacing140is selected based on the size of the optical mode106. In an example, the spacing140is selected to balance a ratio of electrical losses associated with comparatively increased device resistance associated with a longer P-doped portion112to optical losses associated with optical absorption (e.g., free carrier absorption effects, or other absorption effects) caused by the off-path P-N junction110.

In various implementations, the size of the optical mode106is characterized using various measures of the optical intensity profile of the optical mode. For example, in some cases, the optical mode106is approximated as circular and characterized by a radius. For example, in some cases, the optical mode106is approximated as elliptical and characterized by a minor and major axis. For example, in some cases, the optical mode106is simulated using one of various electro-magnetic field simulation software packages.

The loss contribution of the off-path P-N junction110depends on the E-field distribution of the optical mode and the overlap between the E-field distribution and the off-path P-N junction110. In some implementations relying on simulation, the loss effects of the off-path P-N junction110are simulated in addition to the optical mode106simulation. In various implementations, the loss contributions of the off-path P-N junction110are characterized empirically, by testing systems with varying spacings140. Various implementations, are characterized using a combination of mode distribution estimation, mode distribution simulation, and/or empirical characterization of the mode distribution.

The example optical modulator100includes a signal contact122and a DC (direct current) reference contact124. In the example optical modulator100, the contacts122,124are metal contacts. However, other conductive materials are used in other implementations. The signal contact122is used to supply an RF signal input that traverses an electrical path120across the cross-section to the DC reference contact124. The DC reference contact124is used as a DC reference point (e.g., set at ground, a selected bias level, or other DC reference level).

In the example optical modulator100, the electrical path120traverses the comparatively longer combined length of the two N-doped portions114,116, the comparatively shorter length of P-doped portion112, the on-path P-N junction108, and the off-path P-N junction110. In view of an inherent property of silicon being that i) an increase in the length of P-doped portions causes increased resistance along the electrical path120, and ii) that P-N junctions contribute to added capacitance, a technical implementation design goal as disclosed herein is to sufficiently reduce resistivity by reducing P-doped portion electrical path length to outweigh additional contributions to capacitance resulting from inclusion of the off-path P-N junction110.

As an unexpected result, the comparative RF bandwidth performance of various optical modulators including such off-path P-N junctions improves beyond the expected gains from the reduction in resistivity from reduced P-doped electrical path length. In some cases, the bandwidth performance gains from the inclusion of off-path P-N junctions is large enough to render resistivity a secondary concern. Thus, various example modulators using the off-path P-N junction to include two P-doped portions along electrical path length to form a “PNP” modulator.

FIG.2shows a plot200of loss versus RF frequency for example semiconductor-based optical modulators. The electro-optical loss202measured for the example single P-N junction modulator becomes increasingly pronounced relative to the electro-optical losses204,206measured for two example multiple P-N junction modulators as frequency is increased. Accordingly, the example modulators with multiple P-N junctions have a greater operational bandwidth than the example single P-N junction modulator. An optical modulator with a wider operating bandwidth supports systems that handle greater levels of data traffic with the same number of modulators (and similar material costs) as lower performing systems.

Referring again toFIG.1, the observed results are explained through analysis of the interrelation of the on-path P-N junction108and the off-path P-N junction110. Rather than independently contributing to loss in RF signals traversing the electrical path120, the on-path P-N junction108and the off-path P-N junction110behave in the aggregate as series capacitors. Accordingly, a relevant measure of capacitance for the purposes of RF signal loss becomes the series capacitance of the on-path P-N junction108and the off-path P-N junction110. The series capacitance (CNPN, e.g., the capacitance of the whole NPN structure) of the on-path P-N junction108and the off-path P-N junction110is lower than the individual capacitance of either one in isolation (e.g., Con-pathor Coff-path).

Because the off-path P-N junction110makes the opposite transition (e.g., from P-doped to N-doped going from left to right) of the on-path P-N junction (e.g., from N-doped to P-doped going from left to right), the off-path P-N junction110is modelled as a capacitive diode111that is opposingly oriented to and in series with the capacitive diode109of the on-path P-N junction108. The series capacitance, CNPN, is:

CNPN=Con-path⁢Coff-pathCon-path+Coff-patheq.1

In various other implementations, two or more off-path P-N junctions are included in the electrical path.

Applying higher values of reverse bias on a P-N junction increases depletion width for that P-N junction. This results in a reduction of in the capacitance contribution of that P-N junction. However, because successive P-N junctions make opposing transitions, a reverse bias for one P-N junction serves as a forward bias for any neighboring P-N junctions. In some cases, the total number of P-N junctions in the electrical path is selected based on a ratio of gain in modulator bandwidth due to capacitance reductions to electrical losses due to the presence of forward biased P-N junctions.

In some implementations with one or more off-path P-N junctions, no bias is applied to the electrical path to avoid creation of forward biased P-N junctions.

In some examples, a forward-biased on-path P-N junction reduces the signal voltage level used to achieve a 180 degree relative phase shift between split portions the input light. Accordingly, use of a forward-biased on-path P-N junction increases operational efficiency. However, as discussed above, the presence of forward-biased P-N junctions reduces operational bandwidth. Accordingly, some implementations balance the use of a forward-biased on-path P-N junction for electrical efficiency with the operational bandwidth loss associated with such usage. In some cases, the operational bandwidth gains from the inclusion of one or more off-path P-N junctions are used to offset bandwidth losses associated with usage of forward-biased P-N junctions. Thus, the benefits of off-path P-N junctions are realized (at least in part) in increased power consumption efficiency instead of increased operational bandwidth.

In various implementations, semiconductor based optical modulators with one or more off-path P-N junctions are implemented using II-IV semiconductor platforms. For example, in some cases, the semiconductor based optical modulator is implemented on a polysilicon chip.

In various implementations, semiconductor based optical modulators with one or more off-path P-N junctions are implemented using III-V semiconductor platforms. For example, in some cases, the semiconductor based optical modulator is implemented using Gallium Arsenide, Gallium Nitride, and/or other III-V semiconductor materials.

FIG.3shows an example cross-sectional view of a second example semiconductor based optical modulator300. In the second example semiconductor based optical modulator300, the N-doped portions314,316are divided into sub-portions342,344,346,362,364,366with different dopant levels. In the example, the differing dopant levels are ordered such that dopant level increases with distance from the optical waveguide104and optical mode106. Increased dopant levels increase free carrier levels (for a given level of carrier injection/depletion level). Free carriers contribute to linear optical absorption. Accordingly, in the example, sub-portions with higher dopant levels have higher light absorption. In the example, the sub-portions with the highest dopant levels346,366are farthest from the optical waveguide104. In the example, sub-portions with higher dopant levels have lower electrical resistance.

In various other implementations, P-doped portions, additionally or alternatively, are divided into sub-portions with different dopant levels to reduce the overall resistance of the P-doped portions.

In various other implementations, each portion has its sub-portions ordered separately from those of other portions. Accordingly, the sub-portions with the highest dopant levels within a given portion do not necessarily correspond to the highest dopant levels of sub-portions in other portions. Thus, in some cases, a sub-portion with a higher dopant level than another sub-portion in a different portion is closer to the optical waveguide104than that other sub-portion.

FIG.4shows a positioning401of the example cross-section ofFIG.1or3as implemented in an example semiconductor based ring modulator400. In the example, an input/output waveguide402is optically coupled to a resonator410forming a ring. The cross-sections ofFIG.1or3correspond to a radial cross-section of the resonator410, e.g., at position401. N-doped portions414,416, a P-doped portion412, and an optical waveguide404form (at least portions of) concentric annular regions making up the resonator410. An electrical path420travels radially across the resonator410from a signal input contact422on an outer portion of the resonator410to a reference contact424at an inner portion of the resonator. Light circulating in the optical waveguide404is phase-shifted in accord with an RF signal applied at the signal input contact422. The light circulating in the optical waveguide interferes with light traveling in the input/output waveguide402because, in the example, the coupling is bi-directional.

FIG.5shows a positioning501of the example cross-section ofFIG.1or3as implemented in an example semiconductor based MZM500. In the example semiconductor based MZM500, light is guided into the MZM500from an input waveguide502. The light is split into two arms510,540for interference and recombination at an output waveguide572. The cross-sections ofFIG.1or3correspond to cross sections of one or both of the arms510,540of the example MZM500(e.g., at position501). N-doped portions514,544,516,546, P-doped portions512,542, and optical waveguides504,554extend along the arms510,540of the MZM500between contacts522,524. For example, light on the first arm510undergoes a phase shift and then interferes with un-shifted light of the second arm540resulting in intensity modulation of recombined light at the output waveguide572. In an example, light traveling on both arms510,540is imparted with opposite phase shifts by applying the modulation signal additional across contacts562,564(e.g., mirroring the physical layout of the arms510,540such that the relative index shift is reversed with respect to the direction of travel of the optical mode). Accordingly, the degree of relative phase shift in the arms is doubled (e.g., a 90 degree phase shift on one arm corresponds to a 180 degree relative phase shift between the light in the respective the arms).

FIG.6shows a cross-sectional view of an example push-pull semiconductor based optical modulator600. The example push-pull semiconductor based optical modulator600includes two optical waveguides604,654. Accordingly, the push-pull semiconductor based optical modulator600is implemented, in various examples, as two arms of an MZM, e.g., the example MZM500ofFIG.5or another suitable MZM.

The example push-pull semiconductor based optical modulator600includes, for each of the optical waveguides604,654, a respective on-path P-N junction608,658and a respective off-path P-N junction610,660. The P-N junctions divide the push-pull semiconductor based optical modulator600into three N-doped portions614,616,664and two P-doped portions612,662disposed on the semiconductor substrate602.

A first electrical path620runs from a first signal contact622across a first optical waveguide604to the DC reference point624. A second electrical path670runs from a second signal contact672across a second optical waveguide654to the DC reference point624. In this example, the DC reference point includes a fixed DC contact674coupled to the N-doped portion616via an inductor676. Accordingly, the first and second electrical paths620,670are allowed a level of electrical crosstalk that is defined by the inductance of inductor676. When complementary RF signals (e.g., in phase or 180 degrees out of phase depending on operation) are provided simultaneously to the first622and second672contacts, the electrical crosstalk provides constructive interference leading to increased efficiency in operation (e.g., a reduction in the signal voltage level used to obtain a 180 degree relative phase shift between the light in the two waveguides604,654) of the push-pull semiconductor based optical modulator600relative to no-crosstalk-allowed operation.

FIG.7shows an example method of operation700for an example semiconductor based optical modulator. The method of operation700is implemented by any of the example optical modulators discussed above with reference toFIGS.1-6, or another suitable optical modulator.

Light to be modulated is optically coupled into an optical waveguide (702). In various implementations, an illumination source, such as a continuous wave laser tuned to a communication band or coherent illumination source, provides the light to be modulated. In various implementations, the coupling of the light into the optical waveguide includes splitting the provided coherent illumination (e.g., by coupling into a resonator (such as the resonator410of the example ring modulator400ofFIG.4) or an arm of an MZM (such as the arm510or the arm540of the MZM500ofFIG.5).

While the light traverses the optical waveguide, a modulation signal is electrically coupled across an electrical path including an on-path P-N junction coincident with the optical waveguide and an off-path P-N junction space apart from the optical waveguide (704). The modulation signal (such as an RF signal) causes a relative refractive index shift between the P-doped and N-doped portions that meet at the on-path P-N junction. The relative refractive index shift causes a phase shift in the light as the light traverses the optical waveguide. In various implementations, electrically coupling the modulation signal includes electrically coupling the modulation signal to one or more contacts at the ends of the electrical path.

After traversing the optical waveguide, the split portions of the light are recombined (706) to cause interference that results in the output light having intensity profile that is dependent on the modulation signal.

FIG.8shows an example method of manufacture800for an example semiconductor based optical modulator. The method of manufacture800is implemented to manufacture any of the example optical modulators discussed above with reference toFIGS.1-6, or another suitable optical modulator. In other examples, any of the example optical modulators discussed above with reference toFIGS.1-6are manufactured according to another suitable method of manufacture different than the method of manufacture800.

An optical waveguide is fabricated on a semiconductor substrate (802). In various implementations, the optical waveguide includes a region of high refractive index surrounded by lower refractive index material. For example, the waveguide includes a raised channel or ridge of comparatively higher index material (such as silicon, doped silicon, or other comparatively higher index material) surrounded on one or more sides by a material of comparatively lower index material (e.g., such as air, silicon dioxide, or other comparatively lower index material).

An on-path P-N junction is fabricated on the semiconductor substrate (804) and an off-path P-N junction is fabricated on the semiconductor substrate (806). In various implementations, the fabrication of the P-N junctions includes fabrication of P-doped and N-doped portions on the substrate with the P-N junctions defining the borders between the P- and N-doped portions.

In some cases, the P-doped portions and the N-doped portions are fabricated prior to fabrication of the waveguide. For example, in some cases, the P- and N-doped portions are fabricated by applying a mask, diffusing a first one of the P-dopants and the N-dopants into the masked substrate, removing the mask and applying a compliment mask, and diffusing the other of the P-dopants and the N-dopants into the complimentarily masked substrate, removing the compliment mask, applying a mask over the optical waveguide, and etching the substrate around the waveguide to form a raised ridge. However, other implementations use other suitable fabrication techniques. For example, in some cases, either or both of the P-dopants and the N-dopants are diffused in multiple cycles of masking and diffusion to create sub-portions with differing dopant levels.

Electrical contacts are fabricated on the substrate (808). In various implementations, metal contacts are added to support application of the modulation signal across the electrical path created by the P-doped and the N-doped portions on the substrate. In some cases, the metal contacts are applied in multiple layers. For example, in some cases, vias are formed in contact with the substrate which is then covered in an intermediate layer (such as an oxide layer). The vias extend through the intermediate layer to connect with metal contacts fabricated on top of the intermediate layer.

FIG.9shows another example MZM900. The example MZM includes example RF signal supply circuitry980, optical waveguides904,954, an optical input901, an optical delay903, an optical output905, N-doped regions914,916,964,966, P-doped regions912,914, on-path P-N junctions908,958, and off-path P-N junctions910,960. The example RF signal supply circuitry980supplies RF signals across the electrical paths920,970from the signal supply contacts922,972to the bias contacts924,974. The example RF signal supply circuitry980includes RF sources982,984, input resistance986, termination resistance988, and inductance990and capacitance992for application of the bias (e.g., a DC reference). The supply of the RF signals across the electrical paths causes opposing phase shifts in the optical waveguides904,954leading to interference upon recombination at the optical output905. The optical delay903is selected to control the relative phase (before shifting) of the two arms. For example, in some cases, optical delay903is selected such that light in the arms is initially 180 degrees out of phase resulting in destructive interference in the absence of relative phase shifting in the optical waveguides904,954. In some cases, optical delay903is selected such that light in the arms is initially in phase resulting in constructive interference in the absence of relative phase shifting in the optical waveguides904,954.

In various implementations, the example RF signal supply circuitry980is used with the example modulators of any ofFIGS.1and3-6. However, in various other implementations, including those using the structures of any ofFIGS.1and3-6, other suitable signal supply circuitry is used.

For the example MZM900, the method700ofFIG.7is used to operate the MZM900. However, in various other implementations including those using the structure of the example MZM900, other suitable methods of operation are used.

For the example MZM900, the method800ofFIG.8is used to fabricate the MZM900. However, in various other implementations including those using the structure of the example MZM900, other suitable methods of fabrication are used.

Various example implementations have been included for illustration. Other implementations are possible.

Table 2 includes various examples.

TABLE 2ExamplesE1. An optical modulator, including:a semiconductor substrate;an optical waveguide portion disposed on the semiconductor substrate, the opticalwaveguide portion configured to provide an optical path for light that is to bemodulated;a first P-N junction disposed on the semiconductor substrate along the optical path,the first P-N junction defining a border between an N-doped portion disposed on thesemiconductor substrate and a P-doped portion disposed on the semiconductorsubstrate; anda second P-N junction disposed on a portion of the semiconductor substratealongside the optical path and spaced apart from the first P-N junction, wherein:optionally, the optical modulator is in accord and/or implemented in accord with anyother example in this table.E2. The optical modulator of any other example in this table, wherein the first P-Njunction and the second P-N junction are electrically coupled in series along anelectrical path transverse to the optical path.E3. The optical modulator of any other example in this table, wherein the N-dopedportion and the P-doped portion are configured to undergo a shift in refractive indexrelative to one another while an electrical potential is applied across the first P-Njunction.E4. The optical modulator of any other example in this table, wherein:the optical modulator is structured as a ring modulator including a resonator portionforming a ring.; andthe optical waveguide portion, N-doped portion, and P-doped portion include at leastparts of concentric annular regions of the resonator portion.E5. The optical modulator of any other example in this table, wherein:the optical waveguide portion, the first P-N junction, and the second P-N junction arecomponents of a Mach-Zehnder modulator; andthe optical waveguide forms an arm of the Mach-Zehnder modulator.E6. The optical modulator of any other example in this table, wherein the secondP-N junction defines a border between either the N-doped portion or the P-dopedportion and an opposingly-doped portion disposed on the semiconductor substrate.E7. The optical modulator of example E6 or any other example in this table,wherein the N-doped portion, the P-doped portion and/or the opposingly-dopedportion include sub-portions with differing dopant levels.E8. The optical modulator of example E7 or any other example in this table,wherein, separately in each of the N-doped, the P-doped and the opposingly-dopedportions, any sub-portions are ordered such that sub-portion dopant level increaseswith increasing distance from the optical path.E9. The optical modulator of example E6 or any other example in this table,wherein:the opposingly-doped portion includes a second P-doped portion; andthe second P-N junction defines a border between the N-doped portion and thesecond P-doped portion.E10. The optical modulator of example E6 or any other example in this table,wherein:the opposingly-doped portion includes a second N-doped portion; andthe second P-N junction defines a border between the P-doped portion and thesecond N-doped portion.E11. The optical modulator of any other example in this table, wherein a distancebetween the first and second P-N junctions is selected based on a size of an opticalmode the optical waveguide portion is configured to guide.E12. The optical modulator of example E11 or any other example in this table,wherein the distance is further selected based on a ratio between an expectedoptical loss from interference with the optical mode by the second P-N junction andexpected electrical loss due to resistivity of a portion of the semiconductor substratebetween the first and second P-N junctions.E13. The optical modulator of any other example in this table, further including:a first metal contact disposed on the semiconductor substrate alongside the first P-Njunction, the first metal contact configured to receive, via electrical coupling, amodulation signal; anda reference point disposed on the semiconductor substrate alongside the first P-Njunction, the reference point electrically coupled to the first metal contact in seriesthrough the first and second P-N junctions, the reference point contact configured tobe held at, via electrical coupling, a DC reference potential.E14. The optical modulator of example E13 or any other example in this table,wherein the reference point includes a second metal contact disposed on thesemiconductor substrate alongside the first P-N junction.E15. The optical modulator of any other example in this table, further including:a second optical waveguide portion disposed on the semiconductor substrate, thesecond optical waveguide portion configured to provide a second optical path forlight that is to be modulated;a third P-N junction disposed on the semiconductor substrate along the secondoptical path; anda fourth P-N junction disposed on a portion of the semiconductor substrate alongsidethe second optical path and spaced apart from the third P-N junction; anda refence point disposed on the semiconductor substrate between the two opticalwaveguide portions, refence point configured to be held at, via electrical coupling, aDC reference potential.E16. The optical modulator of any other example in this table, wherein the first P-Njunction is disposed on the semiconductor substrate to bisect the optical waveguideportion.E17. A method of optical modulation, including:optically coupling light to be modulated onto an optical path provided by an opticalwaveguide portion disposed on a semiconductor substrate; andelectrically coupling a modulation signal along an electrical path by:electrically coupling across a first P-N junction disposed on the semiconductorsubstrate along the optical path and that defines a border between a N-dopedportion disposed on the semiconductor substrate and a P-doped portion disposedon the semiconductor substrate; andelectrically coupling across a second P-N junction disposed on the semiconductorsubstrate alongside the optical path and spaced apart from the first P-N junction,wherein:optionally, the method is implemented using an optical modulator in accord with anyother example in this table.E18. The method of optical modulation of any example in this table, whereinelectrically coupling the modulation signal along the electrical path includes:electrically coupling the modulation signal from a first metal contact to a second metalcontact in series through the first and second P-N junctions; andholding the second metal contact at a DC reference potential.E19. The method of optical modulation of any example in this table, further includingmodulating the light by causing the N-doped and P-doped portions to shift inrefractive index relative to one another responsive to an electrical potential of themodulation signal.E20. The method of optical modulation of any example in this table, whereinelectrically coupling across the second P-N junction includes electrically couplingthe modulation signal across a border between either the N-doped portion or the P-doped portion and an opposingly-doped portion disposed on the semiconductorsubstrate.E21. A method of manufacturing an optical modulator, including:providing a semiconductor substrate;fabricating an optical waveguide portion disposed on the semiconductor substrate, theoptical waveguide portion configured to provide an optical path for light that is to bemodulated;fabricating a first P-N junction disposed on the semiconductor substrate along theoptical path, the first P-N junction defining a border between an N-doped portiondisposed on the semiconductor substrate and a P-doped portion disposed on thesemiconductor substrate; andfabricating a second P-N junction disposed on a portion of the semiconductorsubstrate alongside the optical path and spaced apart from the first P-N junction,wherein:optionally, the method is used to fabricate an optical modulator in accord with anyother example in this table.E22. The method of fabrication of an optical modulator of any example in this table,further including fabricating metal contacts on the semiconductor substrate to definean electrical path across the first and second P-N junctions.E23. The method of fabrication of an optical modulator of example E22 or any otherexample in this table, wherein fabricating the N-doped portion, the P-doped portionand/or the opposingly-doped portion includes fabricating sub-portions with differingdopant levels.E24. The method of fabrication of an optical modulator of any example in this table,where:fabricating the optical modulator includes fabricating a ring modulator including aresonator forming a ring; andfabricating the resonator includes fabricating concentric annular regions forming theoptical waveguide portion, the N-doped portion, and the P-doped portion.E25. The method of fabrication of an optical modulator of any example in this table,where:fabricating the optical modulator includes fabricating a Mach-Zehnder modulator; andfabricating an arm of the Mach-Zehnder modulator includes fabricating the opticalwaveguide portion, the N-doped portion, and the P-doped portion.E26. A method of optical modulation including implementing the optical modulator ofany example in this table.E27. A method of manufacture including fabricating the optical modulator of anyexample in this table.E28. The optical modulator of any other example in this table, where the first P-Njunction and the second P-N junction are laterally spaced apart.E29. The optical modulator of any other example in this table, where the first P-Njunction and the second P-N junction are spatially oriented in the same directionwith respect to the semiconductor substrate.E30. The optical modulator of example E6 or any other example in this table, where:either the N-doped portion or the P-doped portion separate the other of the N-dopedportion or the P-doped portion and the opposingly-doped portion; andthe other of the N-doped portion or the P-doped portion and the opposingly-dopedportion are non-contiguous, whereoptionally, the either of the N-doped portion or the P-doped portion provides electricalcoupling between other of the N-doped portion or the P-doped portion and theopposingly-doped portion.

The present disclosure has been described with reference to specific examples that are intended to be illustrative only and not to be limiting of the disclosure. Changes, additions and/or deletions may be made to the examples without departing from the spirit and scope of the disclosure.

The foregoing description is given for clearness of understanding only.