OPTICAL DEVICE CONFIGURED FOR STRESS MITIGATION

An electro-optic device is described. The electro-optic device includes at least one optical material having an electro-optic effect. Further, the optical material(s) include lithium. The optical material(s) have a slab and a ridge waveguide. The slab has a top surface. The slab includes free surfaces. Each of the free surfaces is at a nonzero angle from the top surface of the slab and mitigates stress in the slab.

BACKGROUND OF THE INVENTION

Electro-optic devices (also termed optical devices herein) frequently include waveguides and electrodes in proximity to portions of the waveguides. The waveguide carries an optical signal and includes an electro-optic material. An electro-optic material exhibits the electro-optic effect and has its index of refraction modulated by an electric field. The electrodes are used to generate an electric field at or near the waveguide. This electric field causes a change in the index of refraction of the waveguide, which results in the optical signal being modulated. The desired modulation of the optical signal may be achieved by driving the appropriate electrode signal through electrodes.

Although electro-optic devices function, their performance may be limited by various factors. Bulk lithium niobate (LN), for example, may be desired to be used in electro-optic devices because of its large variation in refractive index for a given applied external electric field. However, bulk LN, as well as other technologies, suffer from significant drawbacks. Fabrication of LN optical devices having desired performance characteristics is challenging. For example, LN waveguides may have higher optical losses than desired. In some cases, scattering losses (e.g. from sidewalls) and absorption losses (e.g. from characteristics of the material itself) may be larger than desired. Consequently, techniques for improving the performance of electro-optic devices are desired.

DETAILED DESCRIPTION

The basic elements of electro-optic devices (also termed optical devices), such as electro-optic modulators, include waveguides and electrodes around the waveguides. The waveguide carries an optical signal and includes an electro-optic material. An electro-optic material exhibits the electro-optic effect and has its index of refraction modulated by an electric field. The electrodes are used to generate an electric field, or voltage difference, at or near the waveguide. This electric field causes a change in the index of refraction of the waveguide, which results in the optical signal being modulated. For example, an electrode signal (e.g. a microwave signal) may be applied to the electrodes. Thus, the electrodes act as transmission lines. The electrode signal travels in the same direction as the optical signal propagating through the waveguide. The electrode signal generates a corresponding electric field at the waveguide, modulating the index of refraction of the waveguide. Therefore, the optical signal is modulated as the optical signal travels through the waveguide. Thus, the desired modulation of the optical signal may be achieved by driving the appropriate electrode signal through electrodes.

Although electro-optic devices function, their performance may be limited by a number of factors. Many technologies have been proposed to improve the optical devices. These technologies include waveguides utilizing semiconductors (e.g. silicon and/or indium phosphide), bulk lithium niobate (LN), barium titanate (BTO), and/or plasmonics. However, these and other technologies suffer significant drawbacks in one or more of the characteristics mentioned above. For example, LN is desired to be used in electro-optical devices. The desirability of LN is due at least in part to variation in the refractive index of LN with an applied external electric field. However, fabrication of LN optical devices having desired performance characteristics is challenging. For example, LN waveguides may have higher optical losses than desired. In some cases, scattering losses (e.g. from sidewalls) and absorption losses (e.g. from characteristics of the material itself) may be larger than desired.

An electro-optic device is described. The electro-optic device includes at least one optical material having an electro-optic effect. Further, the optical material(s) include lithium. The optical material(s) have a slab and a ridge waveguide. The slab has a top surface. The slab includes free surfaces. Each of the free surfaces is at a nonzero angle from the top surface of the slab and mitigates stress in the slab. The optical material(s) may include or consist of lithium niobate and/or lithium tantalate. Further, the optical material(s) may be thin films. Thus, the optical material(s) may have a thickness of not more than ten micrometers, not more than five micrometers, not more than three micrometers, not more than one micrometer, not more than seven hundred nanometers, not more than four hundred nanometers, and/or at least one hundred nanometers.

In some embodiments, the electro-optic device also includes an electrode. A portion of the slab is between the ridge waveguide and the electrode. In such embodiments, at least one of the free surfaces is further from the ridge waveguide than the electrode is. Thus, the electrode is on (e.g. directly on or above) the top surface of the slab. In some other embodiments, the electro-optic device also includes an electrode. At least a portion of the slab is between the ridge waveguide and the electrode. In such embodiments, at least one of the free surfaces is closer to the ridge waveguide than the electrode is. In some such embodiments, the electro-optic device also includes a cladding layer. At least a portion of the electrode is on the cladding layer. In some embodiments, the first edge and the second edge are substantially parallel to at least a portion of the ridge waveguide.

The slab may have an edge. At least one of the free surfaces is between the edge of the slab and the ridge waveguide. In some embodiments, the slab resides on a substrate. At least one of the free surfaces extends from the top surface of the slab to the substrate. The slab may reside on a substrate. The slab has a thickness. At least one of the free surfaces extends through the slab a distance less than the thickness. In some embodiments, the free surfaces define at least one aperture in the slab. The optical material(s) may include an additional ridge waveguide. Thus, multiple waveguides may be formed on or in by portions of the optical material. The slab may include a trench therein. The trench has a sidewall and is parallel to at least a portion of the ridge waveguide. A free surface of the plurality of free surfaces being the sidewall.

An electro-optic device including optical material(s0, electrodes and a substrate is described. The optical material(s) exhibit an electro-optic effect and include lithium, The optical material(s) having a slab and a ridge waveguide. The slab has a top surface, a first edge, and a second edge. The first edge and the second edge are substantially parallel to a portion of the ridge waveguide. The ridge waveguide is between a first electrode and a second electrode of the plurality of electrodes. The substrate has substrate edges. The first edge of the slab is between the ridge waveguide and a first substrate edge. The second edge of the slab is between the ridge waveguide and a second substrate edge. The first edge and the second edge form free surfaces at a nonzero angle from the top surface of the slab. In some embodiments, the first edge of the slab is between the first electrode and the ridge waveguide/The second edge of the slab is between the second electrode and the ridge waveguide.

A method provides an electro-optic device. The method includes providing a ridge waveguide from optical material(s) having an electro-optic effect and including lithium. The optical material(s) have a slab and the ridge waveguide. The slab has a top surface. The method also includes providing, for the slab, a plurality of free surfaces. Each of the free surfaces is at a nonzero angle from the top surface of the slab and mitigating stress in the slab. In some embodiments, the optical material(s) include at least one of lithium niobate or lithium tantalate. In some embodiments, the method further includes annealing at least one anneal temperature greater than 300 degrees Celsius. In some such embodiments, the anneal temperature(s) are greater than one thousand degrees Celsius.

FIGS.1A-1Ddepict embodiments of electro-optic devices100and100′.FIGS.1A and1Bdepict perspective and cross-sectional views of an embodiment of electro-optic device100utilizing optical material110exhibiting the electro-optic effect. The optical material may also include lithium. For example, the optical material110may include or consist of one or more of lithium niobate (LN), lithium tantalate (LT), barium titanate (BTO), and/or plasmonics. Thus, although described in singular terms, optical material110may include multiple constituents. In some embodiments, the electro-optic effect includes a change in index of refraction in an applied electric field (e.g. due to the Pockels effect). Thus, in some embodiments, optical materials possessing the electro-optic effect in one or more the ranges described herein or consistent with the material(s) described are considered nonlinear optical materials regardless of whether the effect is linearly or nonlinearly dependent on the applied electric field. For example, a nonlinear optical material may exhibit the electro-optic effect of at least (e.g. greater than or equal to) 5 picometer/volt. In some embodiments, the nonlinear optical material has an effect that is at least 10 picometer/volt. In some such embodiments nonlinear optical material has an effect of at least 20 picometer/volt. The nonlinear optical material experiences a change in index of refraction in response to an applied electric field. In some embodiments, the nonlinear optical material is ferroelectric. The nonlinear optical material may be a non-centrosymmetric material. Therefore, the nonlinear optical material may be piezoelectric.

Electro-optic material110may also be a thin film. In some embodiments, electro-optic material110is not more than ten micrometers in thickness as-deposited. In some embodiments, electro-optic material110may be not more than three micrometers thick as-deposited. In some embodiments, electro-optic material110may be not more than one micrometer in thickness as-deposited. In some embodiments, the thickness of electro-optic material110as-deposited may be not more than seven hundred nanometers. In some such embodiments, this thickness may be not more than four hundred nanometers. In some embodiments, the thickness may be at least one hundred micrometers as-deposited. Other thicknesses are possible. Further

The optical material110is on a substrate101. In some embodiments, the substrate101includes an oxide layer104(e.g. SiO2) and an underlying wafer102(e.g. silicon). In some embodiments, oxide layer104is at least three micrometers thick. In some embodiments, oxide layer104is at least five micrometers thick. Other thicknesses are possible. In some embodiments, oxide layer104may be omitted. For example, for an underlying sapphire substrate102, no oxide layer may be present. Also shown is cladding130, which has an index of refraction that differs from that of ridge waveguide112. For example, cladding130may include or consist of silicon dioxide. For simplicity, cladding130is not shown inFIG.1A.

The optical material110has ridge waveguide112and slab114formed therefrom. In some embodiments, the thickness of ridge waveguide112is the thickness of optical material110as-deposited. For example, ridge waveguide112may have a height (or maximum height if there is a variation in height of ridge waveguide112) of four hundred nanometers, which may be the as-deposited thickness of optical material110. In such embodiments, slab114may have a height of two hundred nanometers. Ridge waveguide112may have a height of less than the thickness of optical material110in some embodiments. Slab114has a top surface and free surfaces120. For clarity, only some free surfaces120are labeled inFIGS.1A-1B. In the embodiment shown inFIGS.1A-1B, free surfaces120are formed by apertures122in optical material110. For clarity, not all apertures122are labeled. In some embodiments, apertures122extend through slab114. In some embodiments, one or more depressions in slab114are used in lieu of some or all of apertures122. Although indicated as terminating at oxide layer104, in some embodiments, apertures122may extend into or through oxide104(e.g. to or into the underlying substrate102). In some embodiments, apertures122may have another configuration. For example, apertures122may have other shape(s) (e.g. triangles, circles, hexagons, squares), be separated by other distance(s), and/or be distributed across slab114in another manner (e.g. a close-packed distribution and/or a non-rectangular array).

Free surfaces120are at a nonzero angle from the top surface of slab114. In the embodiment shown, free surfaces120are substantially perpendicular (e.g. within ten degrees of perpendicular) to the top surface of slab114and/or the top surface of substrate101. In other embodiments, free surfaces120may be at another nonzero angle with respect to the top surface of slab114and/or substrate101. For example, free surfaces120may be at least fifty degrees and up to ninety degrees from horizontal surfaces (e.g. the top surface of substrate101). Free surfaces120may mitigate stress (e.g. in-plane stress) in slab114. For example, stress due to annealing and/or other processing may be reduced by free surfaces120.

Optical structures, such as ridge waveguide112, formed from optical material110may have improved performance. Such optical structures may be formed using UV and/or DUV lithography and other processing that allows for improved surface roughness. In some embodiments, the short-range root mean square (RMS) surface roughness is the RMS surface roughness for lengths (e.g. along direction the axis of ridge waveguide112) of not more than two hundred nanometers. The short-range RMS surface roughness of sidewalls of waveguide112in optical device100is less than ten nanometers. In some embodiments, the short-range RMS surface roughness is not more than five nanometers. The short-range RMS surface roughness of the sidewalls of ridge waveguide112do not exceed two nanometers in some embodiments. Further, the short-range RMS roughness of the top surfaces of ridge waveguide112is not more than one nanometer in some embodiments. In some embodiments, the long range (lengths greater than two hundred nanometers through two hundred micrometers) RMS surface roughness of the sidewalls of ridge waveguide112may differ from the short-range RMS surface roughness.

Further, optical material110may undergo higher temperature annealing. In some embodiments, optical material110is annealed at anneal temperatures greater than 300 degrees Celsius. Optical material110may be annealed at anneal temperatures greater than 400 degrees Celsius. In some embodiments, optical material110is annealed at anneal temperatures greater than 500 degrees Celsius. Optical material110may be annealed at anneal temperatures greater than 600 degrees Celsius. In some embodiments, optical material110is annealed at anneal temperatures greater than 700 degrees Celsius. Optical material110may be annealed at anneal temperatures greater than 800 degrees Celsius. In some embodiments, optical material110is annealed at anneal temperatures greater than 900 degrees Celsius. In some embodiments, optical material110is annealed at anneal temperatures greater than 1000 degrees Celsius. High temperature annealing may improve the crystal structure of the optical material (e.g. the structure of LN and/or LT). For example, losses due to absorption in optical material110may be reduced.

FIGS.1C and1Ddepict perspective and cross-sectional views of an embodiment of electro-optic device100′ that is analogous to electro-optic device100. For example, the optical material110may include or consist of one or more of LN, LT, BTO, and/or plasmonics. Further, optical material110includes ridge waveguide112and slab114that are analogous to corresponding structures shown inFIGS.1A-1B. The optical material110is on substrate101including oxide layer104, and underlying wafer102that are analogous to that are analogous to corresponding structures shown inFIGS.1A-1B. In some embodiments, oxide layer104may be omitted. Also shown is cladding130, which is analogous to cladding130shown inFIG.1B. For clarity, cladding130is not shown inFIG.1C. Also depicted are apertures122and free surfaces120in slab114. In some embodiments, some or all of apertures122and free surfaces120may be omitted. Thus, slab114may not include free surfaces120in some embodiments.

Also shown inFIGS.1C and1Dis depression132having free surface130. In contrast to apertures122, depression132does not extend through slab114. Free surface130may have a similar function as free surfaces120. In some embodiments, the ridge on which free surface130is present has a different height than ridge waveguide112. In other embodiments, the ridge on which free surface130is present has the same height as ridge waveguide112. In some embodiments, the portion of optical materials110below depression132has the same height as the remainder of slab114. In some embodiments, shown inFIGS.1C-1D, the portion of optical materials110under depression132has a different height than the remainder of slab114. In the embodiment shown, free surface132is on one side of ridge waveguide112. In other embodiments, depressions132and free surfaces130may be on both sides of ridge waveguide112.

Thus, electro-optic device(s)100and/or100′ may have improved performance. Because optical material includes materials such as LN and/or LT, the modulation of the index of refraction of waveguide112and slab114by a given applied electric field may be increased. Because of the fabrication using UV or DUV lithography resulting in reduced surface roughness, optical losses (e.g. due to scattering) may be reduced. Further, annealing at optical material110at higher temperatures may further reduce optical losses. For example, optical losses due to absorption may be reduced. The presence of free surfaces120and/or130can mitigate stress in optical material110(e.g. slab114and ridge waveguide112) that might otherwise build up due to annealing. Consequently, optical material110may be less likely to undergo delamination or other stress-induced damage. Further, formation of free surface130may be accomplished with less etching of optical material110than for free surfaces120. Fabrication of electro-optic device110′ may thus be facilitated. Thus, performance and reliability of electro-optic device(s)100and/or100′ may be improved.

FIGS.2A-2Ddepict embodiments of electro-optic devices200and200′.FIGS.2A and2Bdepict perspective and cross-sectional views of an embodiment of electro-optic device200utilizing optical material210exhibiting the electro-optic effect. Electro-optic device200and optical material210are analogous to electro-optic device100and optical material110, respectively. For example, the optical material210may include or consist of one or more of LN, LT, BTO, and/or plasmonics. Further, optical material210includes ridge waveguide212and slab214that are analogous to ridge waveguide112and slab114. The optical material210is on substrate201including oxide layer204, and underlying wafer202that are analogous to substrate101, oxide layer104, and underlying wafer102. In some embodiments, oxide layer204may be omitted. Also shown is cladding230, which is analogous to cladding130.

Slab214has a top surface and includes free surfaces220. For clarity, only some free surfaces220are labeled inFIGS.2A-2B. In the embodiment shown inFIGS.2A-2B, free surfaces220are formed by trenches222in optical material210. In some embodiments, trenches222extend through slab214. In some embodiments, one or more depressions in slab214(i.e. trenches that do not extend through slab114) are used in lieu of some or all of trenches222. Although indicating as terminating at oxide layer204, in some embodiments, trenches222may extend into or through oxide204(e.g. to or into the underlying substrate202). In some embodiments, trenches222may have another configuration. For example, trenches222may have other shape(s) (e.g. the width, length, and/or depth of the trench may vary along the trench), be separated by other distance(s), and/or be distributed across slab214in another manner (e.g. may not run parallel to ridge waveguide212). In some embodiments, trenches222may extend further than (i.e. are wider than) shown. For example, trenches222may extend to the edge of the slab214. Stated differently, optical material210, and thus slab214, may terminate at the edge of trenches222closes to ridge212.

Free surfaces220are at a nonzero angle from the top surface of slab214. In the embodiment shown, free surfaces220are substantially perpendicular (e.g. within ten degrees of perpendicular) to the top surface of slab214and/or the top surface of substrate201. In other embodiments, free surfaces220may be at another nonzero angle with respect to the top surface of slab214and/or substrate201. For example, free surfaces220may be at least fifty degrees and up to ninety degrees from horizontal surfaces (e.g. the top surface of substrate201). In some embodiments, slab214and ridge waveguide212may thus be considered to form a double trapezoid (e.g. ridge waveguide212is a trapezoid on a portion of slab214that is also trapezoidal in cross section). Free surfaces220may mitigate stress (e.g. in-plane stress) in slab214. For example, stress due to annealing and/or other processing may be reduced by free surfaces220.

Optical structures212and214(i.e. ridge waveguide212and slab214) are analogous to optical structures112and114and may be formed using analogous processes. As a result, optical structures212and214and electro-optic device200may have improved performance. Optical structures212and214may be formed using UV and/or DUV lithography and other processing that allows for improved surface roughness. In some embodiments, the short-range RMS surface roughness of sidewalls of waveguide212is in the ranges described for waveguide112. Further, the short-range RMS roughness of the top surfaces of ridge waveguide212may be in the same range as described for ridge waveguide112. In some embodiments, the long range (lengths greater than two hundred nanometers through two hundred micrometers) RMS surface roughness of the sidewalls of ridge waveguide212may differ from the short-range RMS surface roughness. The presence of trenches222may also improve optical confinement by ridge waveguide212. In some cases, the sidewalls of ridge waveguide212may be desired to be shallow (further from perpendicular to the top surface of slab214) to provide more efficient modulation. However, for sidewalls that are shallow, confinement of optical mode213may be reduced. Stated differently, optical mode213may extend laterally further than desired. The presence of trenches222and free surface220closest to ridge waveguide212enhances lateral confinement of optical mode213. Thus, modulation may be made more efficient through the use of shallower sidewalls of ridge waveguide212, while optical mode213confinement may be enhanced by the presence of trenches222. Thus, performance of optical device200may be improved.

Further, optical material210may undergo higher temperature annealing. In some embodiments, optical material210is annealed at anneal temperatures described for optical material110. High temperature annealing may improve the crystal structure of the optical material (e.g. the structure of LN and/or LT). For example, losses due to absorption in optical material210may be reduced.

FIGS.2C and2Ddepict perspective and cross-sectional views of an embodiment of electro-optic device200′ that is analogous to electro-optic device200. For example, the optical material210may include or consist of one or more of LN, LT, BTO, and/or plasmonics. Further, optical material210includes ridge waveguide212and slab214that are analogous to corresponding structures shown inFIGS.2A-2B. The optical material210is on substrate201including oxide layer204, and underlying wafer202that are analogous to that are analogous to corresponding structures shown inFIGS.2A-2B. In some embodiments, oxide layer204may be omitted. Also shown is cladding230, which is analogous to cladding shown inFIG.2B. Also depicted are trench222and free surface220in slab214. In some embodiments, some or all of trench222and free surface220may be omitted. Thus, slab214may not have free surfaces220therein.

Also shown inFIGS.2C and2Dis depression232having free surface230. In the embodiment shown, the portion of optical materials210below depression232has the same height as the remainder of slab214. In some embodiments, the portion of optical materials110under depression132has a different height than the remainder of slab114. Although one depression232is shown, multiple depressions may be present in ridge waveguide212. Free surface230may have a similar function as free surfaces220. In some embodiments, the ridge on which free surface230is present has a different height than ridge waveguide212. In other embodiments, the ridge on which free surface230is present has the same height as ridge waveguide212. In the embodiment shown, free surface232is on one side of ridge waveguide212. In other embodiments, depressions232and free surfaces230may be on both sides of ridge waveguide212.

Thus, electro-optic device(s)200and/or200′ may share the benefits of electro-optic device100. Electro-optic material(s) such as LN and/or LT may be used, allowing for a larger modulation of the index of refraction for a given applied electric field. Because of the fabrication using UV or DUV lithography resulting in reduced surface roughness, optical losses (e.g. due to scattering) may be reduced. Further, annealing at optical material210at higher temperatures may further reduce optical losses (e.g. due to absorption). The presence of free surfaces220can mitigate stress in optical material210(e.g. slab214) that might otherwise build up due to annealing. Consequently, optical material210may be less likely to undergo stress-induced damage. Trenches220may also enhance confinement of optical mode213and improve efficiency of devices200and/or200′ as optical modulators. Further, formation of free surface230may be accomplished with less etching of optical material210than for free surfaces220. Fabrication of electro-optic device210′ may thus be facilitated. Thus, performance and reliability of electro-optic device(s)200and/or200′ may be improved.

Use of optical structures having reduced surface roughness and higher anneal temperatures may improve performance of a variety of electro-optic devices. For example,FIGS.3A and3Bdepict top and perspective views of an embodiment of electro-optic device300utilizing optical material310exhibiting the electro-optic effect. Electro-optic device300is an optical modulator. Electro-optic device300and optical material310are analogous to electro-optic device100and optical material110, respectively. For example, the optical material310may include one or more of LN, LT, BTO, and/or plasmonics. Optical material310includes slab314and ridge waveguides312and316. Waveguides312and316are analogous to ridge waveguide112. Slab314is analogous to slab114. Also shown are electrodes340,350,360, and370. Although four electrodes340,350,360, and370are shown, in some embodiments, another number and/or configuration of electrodes may be used. Cladding330and substrate301including oxide304and underlying wafer302are analogous to cladding130, substrate101, oxide104, and wafer102. In some embodiments, oxide304is sufficiently thick to reduce or prevent the intersection of a microwave mode due to an electrode signal carried by one or more of electrodes340,350,360, and/or370with silicon wafer302. In some embodiments, oxide layer304may be omitted. Although shown has having a particular size, distance from slab314and separation, in some embodiments, electrodes340,350,360, and/or370may be configured differently. For example, electrodes340,350,360, and370may be further from slab314and/or closer to waveguide312or316. In other embodiments, electrodes340,350,360, and/or370may be set into slab314.

Slab314has a top surface and includes free surfaces320that are analogous to free surfaces120. Thus, slab314has a distribution of apertures322. For clarity, only some free surfaces320and apertures322are labeled inFIGS.3A-3B. Some embodiments (e.g. the embodiment shown inFIGS.3A-3B), apertures322extend through slab314. In some embodiments, one or more depressions in slab314are used in lieu of some or all of apertures322. Although indicating as terminating at oxide layer304, in some embodiments, apertures322may extend into or through oxide304(e.g. to or into the underlying substrate302). In the embodiment shown, the portion of slab314between electrodes340and350and waveguide316and the portion of slab314between electrodes360and370and waveguide312are free from apertures322. In some embodiments, one or more apertures may exist in one or both of these regions. Thus, one or more free surfaces320may be between the electrodes360and370and waveguide312. In some embodiments, no apertures322are between electrodes340,350,360, and370and the underlying substrate301(e.g. none are aligned with and directly under electrodes340,350,360, and/or370). In some embodiments, apertures may exist in these regions. Thus, one or more free surfaces320may be between the electrodes360and370and underlying substrate301. Although shown with a particular size, shape, and distribution, these and other characteristics of apertures322may be varied. In some embodiments, trenches analogous to trenches222may be used in lieu of or in addition to apertures322. Although not shown inFIGS.3A-3B, in some embodiments, depression(s) analogous to depression132and/or232may be present in addition to or in lieu of apertures322.

Optical modulator300may have improved performance. As discussed with respect to optical device100, optical properties of optical material310may be improved. For example, the sidewall roughnesses of waveguides312and316may be in the ranges described for waveguide112. Further, optical material310may be annealed. Thus, optical losses may be reduced. Further, waveguides312and316cross in the embodiment shown. Because of the improved surface roughness and anneal, waveguides312and316may cross (as shown inFIG.3A) while maintaining lower optical losses. Electrodes340,350,360, and370may also be formed without crossings. Because materials such as LN and/or LT may be used for optical material310, electrodes340,350,360, and370may induce a larger change in the indices of refraction for waveguides312and316. As indicated inFIG.3A, both waveguides312and316and electrodes340,350,360, and370have turns. As a result, the velocities of the optical signals in waveguides312and316may be matched with the velocities of the microwave signals in electrodes340,350,360, and/or370. Further, the optical losses for such turns may be reduced due to the improved surface roughnesses of waveguides312and316. Thus, performance of optical modulator300may be improved.

In another example,FIGS.4A and4Bdepict top and perspective views of an embodiment of electro-optic device400utilizing optical material410exhibiting the electro-optic effect. Electro-optic device400is an optical modulator. Electro-optic device400and optical material410are analogous to electro-optic device200and optical material210, respectively. For example, the optical material410may include one or more of LN, LT, BTO, and/or plasmonics. Optical material410includes slab414and ridge waveguides412and416. Waveguides412and416are analogous to ridge waveguide212. Slab414is analogous to slab214. Also shown are electrodes440,450,460, and470. Although four electrodes440,450,460, and470are shown, in some embodiments, another number and/or configuration of electrodes may be used. Cladding430and substrate401including oxide404and underlying wafer402are analogous to cladding230, substrate201, oxide204, and wafer202. In some embodiments, oxide204is sufficiently thick to reduce or prevent the intersection of a microwave mode due to an electrode signal carried by one or more of electrodes440,450,460, and/or470with silicon wafer402. In some embodiments, oxide layer404may be omitted.

Slab414has a top surface and includes free surfaces420that are analogous to free surfaces420. Thus, slab414has a distribution of trenches422. For clarity, only some free surfaces420and trenches422are labeled inFIGS.4A-4B. Some embodiments (e.g. the embodiment shown inFIGS.4A-4B), trenches422extend through slab414. In some embodiments, one or more depressions in slab414are used in lieu of some or all of trenches422. Although indicating as terminating at oxide layer404, in some embodiments, trenches422may extend into or through oxide404(e.g. to or into the underlying substrate402). In the embodiment shown, the portion of slab414between electrodes440and450and waveguide416and the portion of slab414between electrodes460and470and waveguide412are free from trenches422. In some embodiments, one or more trenches may exist in one or both of these regions. Thus, one or more free surfaces420may be between the electrodes460and470and waveguide412. In some embodiments, no trenches422are between electrodes440,450,460, and470and the underlying substrate401(e.g. none are aligned with and directly under electrodes440,450,460, and/or470). Thus, one or more free surfaces420may be between the electrodes460and470and underlying substrate401. In some embodiments, trenches may exist in these regions. Although shown as extending through slab414to the edge of device400, in some embodiments, trenches422may extend over a smaller region. For example, trenches422may only be in the region of electrodes462and472. Although shown with a particular size, shape, and distribution, these and other characteristics of trenches422may be varied. In some embodiments, trenches422may extend further than (i.e. are wider than) shown. For example, trenches422may extend to the edge of the slab414. Stated differently, optical material410may terminate at the edge of trenches422closes to ridge412. In some embodiments, apertures analogous to apertures122may be used in lieu of or in addition to trenches422. Although not shown inFIGS.4A-4B, in some embodiments, depression(s) analogous to depression132and/or232may be present in addition to or in lieu of trenches422.

Electrodes440,450,460, and470includes extensions channel regions and extensions. For clarity, channel regions462and472and extensions464and474are labeled only inFIG.4B. In the embodiment shown, extensions464and474include a connecting portion464A and474A, respectively, and a retrograde portion464B and474B, respectively. In some embodiments, extensions464and474may have a different shape. For example, extensions464and/or474may have an “L”-shape, may omit the retrograde portion, may be rectangular, trapezoidal, parallelogram-shaped, may partially or fully wrap around a portion of waveguide412, and/or have another shape. Similarly, channel regions462and/or472, which are shown as having a rectangular cross-section, may have another shape. Further, extensions464and/or474may have different sizes. Although all extensions464and474are shown as the same distance from ridge412, some of extensions464and/or some of extensions474may be different distances from ridge412. In some embodiments, extensions464and474are desired to have a length that corresponds to a frequency less than the Bragg frequency of the signal for electrodes460and470. Thus, the length of extensions464and/or474may be desired to be not more than the microwave wavelength of the electrode signal divided by π at the highest frequency of operation for electrodes460and470. In some embodiments, the length of extensions464and/or474is desired to be less than the microwave wavelength divided by twelve. For example, if the maximum operation frequency is 300 GHz, which corresponds to a microwave wavelength of 440 micrometers in the substrate, extensions464and474are desired to be smaller than approximately 37 micrometers. Individual extensions464and/or474may be irregularly spaced or may be periodic. Periodic extensions have a constant pitch. In some embodiments, the pitch is desired to be a distance corresponding to a frequency that is less than the Bragg frequency. Thus, the pitch for extensions464and474may be desired to be not more than the microwave wavelength of the electrode signal divided by π at the highest frequency of operation for electrodes460and470. In some embodiments, the pitch is desired to be less than the microwave wavelength divided by twelve. In some embodiments, the pitch is desired to be less than the microwave wavelength divided by seventy-two, allowing for a low ripple in group velocity. Although shown has having a particular size, distance from slab414and separation, in some embodiments, electrodes440,450,460, and/or470may be configured differently. For example, electrodes440,450,460, and470may be further from slab414such that portions of the extensions are over waveguide412or416. In other words, the separation between the extension may be less than the width of ridge waveguide410. In other embodiments, electrodes440,450,460, and/or470may be set into slab414.

Optical modulator400may have improved performance. As discussed with respect to optical device200, optical properties of optical material410may be improved. For example, the sidewall roughnesses of waveguides412and416may be in the ranges described for waveguide212. Further, optical material410may be annealed. Thus, optical losses may be reduced. In addition, confinement of the optical mode (not shown inFIGS.4A-4B) may be improved by trenches422. In some embodiments, sidewalls of ridge waveguide412may be shallower to enhance modulation while maintaining optical mode confinement using trenches222. Thus, flexibility of optical device400may be improved. Further, waveguides412and416cross in the embodiment shown. Because of the improved surface roughness and anneal, waveguides412and416may cross (as shown inFIG.4A) while maintaining lower optical losses. Electrodes440,450,460, and470may also be formed without crossings. Because materials such as LN and/or LT may be used for optical material410, electrodes440,450,460, and470may induce a larger change in the indices of refraction for waveguides412and416. As indicated inFIG.4A, both waveguides412and416and electrodes440,450,460, and470have turns. As a result, the velocities of the optical signals in waveguides412and416may be matched with the velocities of the microwave signals in electrodes440,450,460, and/or470. Further, the optical losses for such turns may be reduced due to the improved surface roughnesses of waveguides412and416. Moreover, use of electrodes440,450,460, and/or470having extensions may further improve modulation, reduce microwave losses, and allow for enhanced velocity matching. Extensions, such as extensions464and474, allow for the electric field to be enhanced at the waveguide412(because extensions are closer to waveguide412), while allowing the microwave signal to be carried by channels462and472. Thus, a higher optical modulation may be obtained while reducing the microwave losses. Moreover, extensions in combination with engineering of substrate401may improve velocity matching between the optical and microwave signals. Thus, performance of optical modulator400may be improved.

In another example,FIG.5depicts a perspective view of a portion of an embodiment of electro-optic device500utilizing optical material510exhibiting the electro-optic effect and including lithium. Electro-optic device500is an optical modulator. Electro-optic device500and optical material510are analogous to electro-optic device400and optical material410, respectively. For example, the optical material510may include one or more of LN, LT, BTO, and/or plasmonics. Optical material510includes slab514and ridge waveguide512. Waveguide512is analogous to ridge waveguide412. Slab514is analogous to slab414. Thu, trenches522and free surfaces520are analogous to trenches420and free surfaces422. Also shown are electrodes560and570that are analogous to electrodes460and470. Thus, extensions564and574having connecting portions564A and574A and retrograde portions564B and574B are analogous to extensions464and474. For simplicity, only one waveguide412and two electrodes460and470are shown. However, typically multiple waveguides and more pairs of electrodes (e.g. as inFIGS.4A and4B) are utilized. Although two electrodes560and570are shown, in some embodiments, another number and/or configuration of electrodes may be used. Cladding530and substrate501including oxide504and underlying wafer502are analogous to cladding430, substrate401, oxide404, and wafer402. In some embodiments, oxide504is sufficiently thick to reduce or prevent the intersection of a microwave mode due to an electrode signal carried by one or more of electrodes560and/or570with silicon wafer502. In some embodiments, oxide layer504may be omitted.

In the embodiment shown, trenches522extend to the region between ridge waveguide512and extensions564and574. In some embodiments, trenches422extend to retrograde portions564B and574B. In such embodiments, slab514extends from ridge waveguide512to retrograde portions564B and574B. In some embodiments, trenches422extend to the region between retrograde portions564and574and channels562and572. Thus, slab514extends from ridge waveguide512to retrograde portions564B and574B. In some embodiments, trenches422extend to the channels562and572. Thus, slab514extends from ridge waveguide512to channels560and570. Thus, in some embodiments, slab514need not and does not extend past electrodes560and570. Electro-optic device500shares some or all of the benefits of electro-optical device400, though is configured somewhat differently.

Electro-optic devices100,100′,200,200′,300,400, and500have been described. Various feature(s) of devices100100′,200,200′,300,400, and/or500may be combined in manners not explicitly described herein.

FIG.6depicts an embodiment of method600for providing an electro-optic device, such as one or more devices100,200,300, and/or400. Method600is described in the context of processes that may have sub-processes. Although described in a particular order, another order not inconsistent with the description herein may be utilized. Although fabrication of a single device is described, multiple devices are typically fabricated together. Method600starts after an electro-optic material, such as an LN and/or LT layer has been provided on a substrate. In some embodiments, the LN and/or LT layer may be thin, for example, not more than ten micrometers in thickness. In some embodiments, the LN and/or LT layer may be not more than three micrometers thick. In some embodiments, the LN layer may be not more than one micrometer in thickness. In some embodiments, the thickness of the LN layer may be not more than seven hundred nanometers. In some such embodiments, the thickness may be not more than four hundred nanometers. In some embodiments, the thickness may be at least one hundred micrometers. Other thicknesses are possible. Underlayers, such as silicon dioxide, may exist between the LN layer and a carrier wafer. In some embodiments, the carrier wafer may include silicon, quartz, silica, LN, sapphire and/or another material. For example, the LN layer may reside on a silicon dioxide underlayer having a thickness of nominally at least two and not more than five micrometers. Other thicknesses, additional layers and/or other layers may be present. Method600may also be used in connection with one or more of the techniques described in the above-identified co-pending applications.

Optical and stress relief components are provided for the electro-optic device at602. In some embodiments, these components are formed from the electro-optic material. For example,602may include utilizing the methods described in the above-identified patent applications to form ridge and/or channel waveguide(s) as well as stress relief components such as free surfaces. Other optical components, such as mode converter(s) and polarization beam rotator(s), may also be formed.

Electrical components are formed, at604. In some embodiments,604may include forming electrodes for an optical modulator. Other electrical components, such as CMOS or other components, may also be formed at604.

Using method600, electro-optic devices such as devices100,200,300and/or400may be formed. Thus, the benefits described herein, including but not limited to stress management, may be achieved.

FIG.7depicts an embodiment of method700for providing an electro-optic device, such as one or more devices100,200,300, and/or400. Method700is described in the context of processes that may have sub-processes. Although described in a particular order, another order not inconsistent with the description herein may be utilized. Although fabrication of a single device is described, multiple devices are typically fabricated together. Method700starts after an electro-optic material, such as an LN and/or LT layer has been provided on a substrate. In some embodiments, the LN and/or LT layer may be thin, for example, not more than ten micrometers in thickness. In some embodiments, the LN layer may be not more than one micrometer in thickness. In some embodiments, the thickness of the LN layer may be not more than seven hundred nanometers. In some such embodiments, the thickness may be not more than four hundred nanometers. Other thicknesses are possible. Underlayers, such as silicon dioxide, may exist between the LN layer and a carrier wafer. In some embodiments, the carrier wafer may include silicon, quartz, silica, LN, sapphire and/or another material. For example, the LN layer may reside on a silicon dioxide underlayer having a thickness of nominally at least two and not more than five micrometers. Other thicknesses, additional layers and/or other layers may be present. Method700may also be used in connection with one or more of the techniques described in the above-identified co-pending applications.

A ridge waveguide is provided from optical material(s) having an electro-optic effect, at702. In some embodiments, one or more depressions analogous to depressions130and/or230may be formed as part of702. Thus,702may include utilizing lithography and etch(es) to pattern one or more electro-optic materials. Such processing may be performed using techniques analogous to those described in the above-identified co-pending applications. Consequently, the electro-optic material has been formed into at least the ridge waveguide and slab. In some embodiments, multiple ridge waveguides and/or additional structures such as mode converters are also formed at702.

Free surfaces are formed in the slab, at704. In some embodiments,704includes forming depressions, trenches and/or apertures in the slab. The free surfaces may be formed using lithography and etch(es) of the electro-optic materials. Such processing may be performed using techniques analogous to those described in the above-identified co-pending applications. In some embodiments,702and704may be performed together. In some embodiments,704is performed prior to702. In other embodiments,704is performed after702. As indicated above, the free surfaces are at nonzero angle(s) from the top surface of the slab and mitigate stress in the slab.

The device being fabricated is annealed at anneal temperature(s) greater than 300 degrees Celsius, at706. In some embodiments, optical material110is annealed at anneal temperatures greater than 400 degrees Celsius. Optical material110may be annealed at anneal temperatures greater than 500 degrees Celsius. In some embodiments, the anneal temperature(s) are greater than 600 degrees Celsius. In some embodiments, the anneal temperature(s) are greater than 700 degrees Celsius. In some embodiments, the anneal temperature(s) are greater than 800 degrees Celsius. In some embodiments, the anneal temperature(s) are greater than 900 degrees Celsius. In some embodiments, the anneal temperature(s) are greater than 1000 degrees Celsius. In some embodiments,706includes performing multiple anneals, at least one of which is at the anneal temperature(s) described herein. The anneal performed at706may be performed after704has been completed. In some embodiments, the anneal is performed after702and704are performed.

Fabrication of the electro-optic device is completed, at708. For example, other optical structures may be formed and electrical components fabricated. The individual electro-optic device may also be separated from the wafer (or array of devices being fabricated) at708.

Using method700, electro-optic devices such as devices100,200,300and/or400may be formed. Thus, the benefits described herein, including but not limited to stress management, may be achieved.