MEMS-tunable optical ring resonator

A microelectromechanical systems (MEMS)-tunable optical ring resonator is described herein. The ring resonator includes a resonator ring and a tuner ring, along with one or more springs. The springs may be internal or external, i.e., either within or outside the areal footprint of the resonator ring and the tuner ring. The one or more springs are configured to displace the tuner ring from the resonator ring by a desired gap based upon a desired resonant wavelength of the resonator ring. Tuning is implemented by applying a voltage to the ring resonator, with motion of the tuner ring causing a corresponding change in the effective index of the resonator ring. As the ring resonator is essentially a capacitive device, it draws very little power once tuning is achieved.

TECHNICAL FIELD

The present invention relates to a compact, low power, tunable optical ring resonator based on microelectromechanical system (MEMS) technology.

BACKGROUND

An optical ring resonator generally includes a segment of optical waveguide arranged as a ring-shaped, closed loop. Optical ring resonators have a variety of uses in photonic systems. For example, an optical ring resonator can be included in a photonic filter, can be included in a photonic frequency multiplexer or demultiplexer, can be included as an optical switch, can be used to adjust performance of a photonic filter, etc.

Conventionally, an optical ring resonator can be tunable, such that the wavelength of light that optically couples into the optical waveguide can be altered. Optical ring resonators have historically been tuned by altering the temperature of the optical waveguide of the optical ring resonator. Altering the temperature of the optical waveguide alters the refractive index of the material of the optical waveguide, which in turn alters the resonant wavelength of the optical waveguide. Hence, the wavelength of light that optically couples into the optical waveguide and resonates within the optical waveguide can be altered by altering the temperature of the optical waveguide. In another example, the index of refraction and resonant wavelength of the optical waveguide has conventionally been tuned by injecting current into the optical waveguide.

Tunable optical ring resonators, however, tend to require a relatively large amount of power to be tuned, and further require a relatively large amount of power to maintain a desired resonant wavelength. Other conventional tunable optical ring resonators have a relatively large areal footprint and/or require a relatively high voltage to be applied to a tuning electrode on the optical ring resonator.

SUMMARY

Described herein is a tunable optical ring resonator that has a relatively small areal footprint on-chip (particularly compared to conventional microelectromechanical systems (MEMS) tunable optical ring resonators). Additionally, the tunable optical ring resonator requires less power to tune the optical ring resonator to a desired wavelength and to maintain a desired resonance wavelength when compared to conventional optical ring resonators.

The optical ring resonator described herein is a MEMS tunable optical ring resonator, wherein the index of refraction of material of the resonator ring of the optical ring resonator and resonant wavelength of the resonator ring are altered by changing the position of a tuner ring relative to the resonator ring. More specifically, the optical ring resonator includes a tuner ring and a resonator ring, wherein the tuner ring and the resonator ring have a substantially similar radius, and further wherein the tuner ring is positioned directly above the resonator ring. One or more springs, which may be internal or external, i.e., either within or outside the areal footprint of the resonator ring and the tuner ring, are configured to displace a top electrode from a bottom electrode of the ring resonator until a desired gap exists between such electrodes. The gap between the electrodes and the tuning gap between the tuner ring and the resonator ring control the effective index of refraction of the material of the resonator ring, and thus the resonant wavelength of the resonator ring.

In at least one embodiment of the present invention, a microelectromechanical systems (MEMS)-tunable optical ring resonator comprises a resonator ring (the resonator ring having a minimum free spectral range of 1 nm), a tuner ring that is positioned directly above the resonator ring (the tuner ring having a minimum free spectral range of 1 nm, the tuner ring moving in a direction normal to a plane of the resonator ring), a top electrode that is mechanically coupled to the tuner ring, a bottom electrode that is mechanically coupled to the resonator ring, and one or more springs each of which is mechanically and electrically coupled to the top electrode, a resonant wavelength of the resonator ring being tunable by applying a voltage between the top electrode and the bottom electrode.

In various embodiments, each of the one or more springs is one of an external folded spring, an external linear segment spring, an external multi-linear segment spring, an external cantilever spring, an internal folded spring, an internal linear segment spring, an internal multi-linear segment spring, or an internal cantilever spring; each of the one or more springs is an internal folded spring; the resonant wavelength of the resonator ring being tunable over a range of at least a difference between two adjacent operating wavelengths; and an effective index of the resonator ring is tunable by approximately 1% due to motion of the tuner ring.

In other embodiments, the resonator ring and the tuner ring each have an inner radius of between approximately 5 μm and approximately 15 μm; the resonator ring includes one of Si, Si3N4, Al2O3, LiNbO3, or Ta2O5and the tuner ring includes one of SiO2or Si3N4; and each of the resonator ring and the tuner ring has a height of between approximately 100 nm and approximately 500 nm, the resonator ring has a width of between approximately 0.3 μm and approximately 2.0 μm, and the tuner ring has a width of between approximately 0.3 μm and approximately 5.0 μm.

In still other embodiments, the resonator further comprises a plurality of dimples adjacent one of the top electrode or the bottom electrode; the resonator further comprises an anchor/via that is mechanically and electrically coupled to at least one of the one or more springs (the anchor/via functions as an anchor for the tuner ring and as an electrical via for the top electrode); the resonator further comprises a topside electrical contact electrically coupled to at least one of the one or more springs (the topside electrical contact functions as an electrical via for the top electrode); and the topside electrical contact includes an airbridge; the resonator further comprises at least one driver (the at least one driver changing a position of the tuner ring relative to the resonator ring).

In yet other embodiments, a spring constant of the one or more springs provides a minimum restoring force of at least 1 μN; each of the one or more springs has a thickness of between approximately 100 nm and approximately 700 nm and each of the one or more springs has a width of between approximately 0.5 μm and approximately 5 μm; the resonator further comprises an input waveguide (the input waveguide adjacent the resonator ring, the input waveguide being approximately critically optically coupled to the resonator ring, and the input waveguide including one of Si, Si3N4, Al2O3, LiNbO3, or Ta2O5); the resonator further comprises a first photodiode (the first photodiode outputting a first signal indicative of a magnitude of light not coupled from the input waveguide to the resonator ring); the resonator further comprises a drop waveguide (the drop waveguide adjacent the resonator ring, the drop waveguide being approximately critically optically coupled to the resonator ring, and the drop waveguide including one of Si, Si3N4, Al2O3, LiNbO3, or Ta2O5); the resonator further comprises a second photodiode (the second photodiode outputting a second signal indicative of a magnitude of light coupled from the resonator ring to the drop waveguide); and the ring resonator is a portion of one of an optical filter, a wavelength division multiplexer, or a wavelength division demultiplexer.

DETAILED DESCRIPTION

Various technologies pertaining to a photonics transceiver with relatively high areal bandwidth density and relatively low power consumption are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more aspects. It may be evident, however, that such aspect(s) may be practiced without these specific details. In other instances, structures and devices are shown in block diagram form in order to facilitate describing one or more aspects.

Described herein is a microelectromechanical systems (MEMS) tunable optical ring resonator that exhibits various advantages over conventional optical ring resonators. The optical ring resonator described herein requires less energy to tune the optical ring resonator to a desired resonant wavelength when compared to conventional non-MEMS tunable optical ring resonators. Further, the optical ring resonator described herein has a smaller areal footprint when compared to conventional MEMS tunable optical ring resonators.

Now referring toFIG.1, a schematic depicting an exemplary MEMS tuner100for an optical ring resonator is illustrated. The MEMS tuner100can be viewed as a capacitor102with a movable top electrode104and a fixed electrode106, forming a gap therebetween. The gap between the two electrodes104and106(and between a tuner ring and a resonator ring, described in detail below) is voltage-controlled, and the movable electrode104is clamped to an anchor108that is mechanically coupled to a spring110that has a designable spring constant k. Electrostatic pull-in must be accounted for, which occurs when displacement x of the top electrode104is ⅓ of the original gap go. Accordingly, the electrode (air) gap of the MEMS tuner100must be at least three times the desired displacement range. In an exemplary embodiment, the initial electrode gap can be 3·300 nm+25% (1.125 μm). The voltage V required to achieve a desired displacement, x, is given by:

V=1ϵox⁢2⁢kx⁡(ϵair⁢dox+ϵox(dair-x))2ϵair⁢A,(Eq.1)
where A is the overlap area of the two electrodes104and106, ϵairand ϵoxare the permittivities of air and the oxide between the electrodes104and106, respectively, and dairand doxare the thicknesses of the air gap and the oxide between the electrodes104and106, respectively.

A design consideration involves permanent and destructive pull-in due to stiction. A rough rule of thumb is that the spring of restoring force at pull-in should be at least 1 μN to avoid permanent stiction (assuming small area dimple contact structures without electro-migration). Stiction therefore places a lower limit on the spring stiffness.

FIG.2Adepicts a pair of springs200, for example, an exemplary instantiation of the spring110, wherein the springs200have a folded spring design that provides strain relief due to film stress with relatively low curling. The springs200are called external folded springs as the springs200are located outside a tuner ring202. Note that while the illustrated folded springs200employ linear segments, other instantiations (not illustrated) with folded springs may be formed of arced segments. While the springs200illustrated inFIG.2Aare formed of linear segments folded back on themselves, in other instantiations (not illustrated), the springs may be a single linear segment or may include two or more linear segments with an angle therebetween, i.e., a “multi-linear segment spring.”FIG.2Bdepicts a spring210, for example, another exemplary instantiation of the spring110, wherein the spring210has a cantilever spring design that provides various benefits. The benefits of this cantilever design include a lower spring constant, and thus a lower operating voltage for the same spring area, as well as eliminating one of the MEMS-waveguide crossings. Similar to the external folded springs200, the cantilever spring210is located outside of a tuner ring212, though in other instantiations (not illustrated) the cantilever spring may be located within the tuner ring212.FIG.2Cdepicts a pair of springs220, for example, yet another exemplary instantiation of the spring110, wherein the springs220have a folded spring design that provides strain relief due to film stress with relatively low curling. The utilization of the springs220results in a reduction in the areal footprint required for a MEMS-tunable optical ring resonator. The springs220are called internal folded springs as the springs220are located within a tuner ring222(FIGS.5and6illustrate similar internal folded springs520). While the springs220illustrated inFIG.2Care formed of arced segments folded back on themselves, in other instantiations (not illustrated), the springs220may be a single linear segment spring or a multi-linear segment spring.

The MEMS-tunable optical ring resonator described herein is capable of being operated over a wide range of temperatures (such as between 4 K and 373 K). This is an advantage over conventional ring resonators that require heating of the resonator ring for tuning, as such ring resonators are not practical in cryogenic systems due to the need for continuous heating. In contrast, the MEMS-tunable optical ring resonator described herein does not require heating, rendering it useable at cryogenic temperatures. The MEMS-tunable optical ring resonator can be included in a photonic filter, can be included in a photonic frequency multiplexer or demultiplexer, can be an optical switch, can be used to adjust performance of a photonic filter, etc. The ring resonator described herein is capable of accepting data of at least 32 Gbps, which implies an approximately 0.2 nm or larger linewidth and a quality factor Q of 5000 or less at 980 nm in waveguides with an effective index neffof approximately 1.56. When the MEMS-tunable optical ring resonator is included in a wavelength division multiplexer, the multiplexer can comprise eight MEMS-tunable optical ring resonators and is therefore capable of multiplexing eight wavelength channels having a channel separation of 1 nm onto a single waveguide. This necessitates an 8 nm or larger free-spectral range (FSR) to avoid simultaneous resonance with two channels. In an example, the MEMS-tunable optical ring resonator described herein can have a FSR of 10 nm, and thus can have a radius of

10⁢μm=980⁢nm22⁢π⁢n·FSR
or less for both the tuner ring and the resonator ring. For full flexibility, the wavelength division multiplexer should tune the resonant wavelength by an entire 10 nm FSR, which is equivalent to approximately 1% of the operating wavelength. This approximately 1% range in wavelength tuning in turn leads to a requirement of approximately 1% effective index tunability. Similarly, when the MEMS-tunable optical ring resonator is included in a wavelength division demultiplexer, the demultiplexer can comprise eight MEMS-tunable optical ring resonators and is therefore capable of demultiplexing eight wavelength channels having a channel separation of 1 nm from a single waveguide (which again necessitates 8 nm or larger FSR to avoid simultaneous resonance with two channels). As will be appreciated by those of ordinary skill, the tuning range is also a function of the overall system architecture, including, for example, the number of operating wavelengths and the wavelength spacing between the operating wavelengths. Thus, with other system architectures, the desired or required tuning range may be less than the FSR of the MEMS-tunable optical ring resonator. As will also be appreciated by those of skill in the art, the desired minimum FSR will likewise be a function of the overall system architecture. For example, the FSR for both the tuner ring and the resonator ring may preferably be at least as great as the difference between the longest and shortest operating wavelengths. In a system with a single operating wavelength, a minimum FSR of 1 nm is preferred to provide the desired degree of tunability and tuning finesse.

Further, in another exemplary embodiment, the MEMS-tunable optical ring resonator can employ vertical-actuation, with the effective index of a fixed resonator ring (e.g., a Si3N4ring) being controlled by varying a vertical tuning gap between a (moveable) tuner ring (e.g., a SiO2ring) and the Si3N4ring.FIG.3is a plot300that depicts a simulation that illustrates the effective index tuning of the waveguide's transverse magnetic (TM) mode assuming an initial air gap of 300 nm, where the air gap can be varied from 40 to 300 nm. As illustrated, the effective index can be tuned by more than 1% over the air gap range of 40 nm to 300 nm (e.g., a 260 nm displacement).

FIG.4is a plot400that depicts the voltage required to reach the 260 nm displacement, i.e., a tunable range corresponding to the full FSR of the optical ring resonator, as a function of the tuner spring constant based upon a model of the MEMS-tunable ring resonator. The plot400includes a vertical line402that indicates the necessary spring constant to achieve a 10 μN restoring force at the 260 nm displacement. Curve404represents the voltage required for a first exemplary design of the MEMS-tunable ring resonator, in which the entire inner area of the MEMS-tunable ring resonator is used as an electrode with the springs being external to the ring, such as the external folded springs200illustrated inFIG.2A. Curve406represents the voltage required for a second exemplary design of the MEMS-tunable ring resonator, in which the springs and the capacitive electrodes are enclosed within the ring (for a more compact areal footprint). With respect to the first exemplary design, a tuning voltage of approximately 20V is required, while with respect to the second exemplary design, a tuning voltage of approximately 40V is required. With respect to the second exemplary design, the required 42 N/m spring constant can be realized by 400 nm thick Au (100 nm)/SiO2(300 nm) folded springs with a length, Lspringof 8 μm and a width of 2.5 μm. While a spring constant of 42 N/m was required for this instantiation, other instantiations may employ a spring constant between approximately 1 N/m and 100 N/m. Further, the required tuning voltage can be reduced through designs set forth in greater detail below. In other exemplary designs requiring different spring constants, the springs may have a thickness in the range of approximately 100 to 700 nm, a length of 6 to 10 μm, and a width of 0.5 to 5 μm. In certain instantiations, the optical ring resonator may not require tunability over a full FSR. Some system architectures may require tuning over just a fraction of the FSR, which may, for example, depend upon the difference between the longest and shortest operating wavelengths or upon the difference between two adjacent operating wavelengths, with the difference between two adjacent operating wavelengths preferably being the minimum tuning range.

FIG.5depicts a top down view of an exemplary MEMS-tunable optical ring resonator500, andFIG.6depicts a cross-sectional view of the optical ring resonator500. In an exemplary embodiment, the ring resonator500can be tunable over a range of 10 nm, for example, between approximately 980 nm and approximately 990 nm. In another example, the ring resonator500can be tunable over a range of 15 nm, for example, between approximately 975 nm and 990 nm. The ring resonator500includes a SiO2tuner ring502and a Si3N4resonator ring504that is positioned directly below the tuner ring502, wherein heights of both the tuner ring502and the resonator ring504are between approximately 100 nm and approximately 500 nm. In other examples, the resonator ring504may include Si, Al2O3, LiNbO3, or Ta2O5, while the tuner ring may include Si3N4. In an example, the widths for the resonator ring504and the tuner ring502are approximately 0.3 μm to approximately 2.0 μm and approximately 0.3 μm to approximately 5.0 μm, respectively. In an example, the tuner ring502and the resonator ring504can have an inner ring radius of between 5 μm and 20 μm. In another example, the tuner ring502and the resonator ring504can have an inner ring radius of between 8 μm and 15 μm. In a more specific example, the tuner ring502and the resonator ring504can have an inner ring radius of approximately 10 μm. In an example, the tuner ring502should have an inner ring radius approximately the same as, or slightly smaller than, the inner ring radius of the resonator ring504. In an example, the tuner ring502should have an outer ring radius greater than the outer ring radius of the resonator ring504.

An input waveguide506is positioned adjacent the resonator ring504, wherein the input waveguide506can be a Si3N4waveguide, which acts as a pulley coupler and preferably approximately critically optically couples to the resonator ring504. In other examples, the input waveguide506may include Si, Al2O3, LiNbO3, or Ta2O5. A drop waveguide508(which can also be a Si3N4, Si, Al2O3, LiNbO3, or Ta2O5waveguide) is positioned adjacent the resonator ring504and on an opposite side of the resonator ring504as the input waveguide506. The drop waveguide508is also preferably approximately critically optically coupled to the resonator ring504.

The MEMS-tunable optical ring resonator500further includes a top electrode510and a bottom electrode512. While the instantiation illustrated inFIG.5includes a buried bottom electrode512, other instantiations may include a doped Si substrate with a backside metal contact; this combination forming an alternative bottom electrode. The ring resonator500additionally includes a layer514of polysilicon positioned directly beneath the top electrode510and a layer516of SiO2, wherein a portion of the layer516of SiO2is positioned directly above the bottom electrode512, and further wherein an air gap exists between the layer514of polysilicon and the layer516of SiO2. The layer514of polysilicon has several dimples518(from etching) that extend approximately 40 nm from the bottom of the layer514of polysilicon towards the layer516of SiO2. While the dimples518illustrated inFIGS.5and6are adjacent the top electrode510, in other examples the dimples are adjacent the bottom electrode512. A gap between the top electrode510and the bottom electrode512can initially be set to approximately 1.125 μm, with an initial gap between the dimples518in the layer514of polysilicon and the layer516of SiO2being approximately 260 nm. In an example, the size of the gap between the top electrode510and the bottom electrode512can be between 1 μm and 1.125 μm depending upon a voltage applied between the top electrode510and the bottom electrode512. The desired size of the initial gap between the top electrode510and the bottom electrode512can be tailored based upon the thickness of the tuner ring502, tuning voltage, and dielectric constant of non-air material (e.g., polysilicon and SiO2) between the top electrode510and the bottom electrode512.

The MEMS-tunable optical ring resonator500further comprises a pair of internal folded springs520(e.g., such as the springs220illustrated inFIG.2C), which are designed to have a desired spring constant (e.g., 42 N/m). The springs520, in an exemplary embodiment, can have a width Wspringof approximately 2.5 m and length Lspringof approximately 8 μm. As shown inFIG.6, the springs520are mechanically and electrically coupled to the top electrode514and anchors/vias522and524, wherein the anchors/vias522and524extend through the layer516of SiO2. The anchors/vias522and524can be filled with doped polysilicon, wherein the doped polysilicon can serve as an anchor for the tuner ring502, an electrical via, and a sacrificial layer for release of the ring resonator500under dry etching. The anchor/via functionality is maintained by protecting the anchors/vias522and524with SiO2and top-side Au prior to release. Electrical accesses to the ring resonator500are by way of the anchors/vias522and524, and further by way of TSVs526and528, with an implant doped Si interconnect530between the anchors/vias522and524and the TSV526. The dimples518are included in the ring resonator500to prevent stiction in the event that the springs520pull-in. The ring resonator500can optionally include a low stiction contact material, such as sputtered Ruthenium, deposited on and/or below the dimples518.

In other exemplary embodiments, topside electrical contacts (not illustrated), as opposed to TSVs526and528, are employed and provide electrical access to the top electrode510and the bottom electrode512of the MEMS-tunable optical ring resonator500. These topside electrical contacts may include air bridges over one or more features, for example, the tuner ring502, the input waveguide506, and/or the drop waveguide508. In yet other exemplary embodiments, the ring resonator500includes both TSVs526and528and the topside electrical contacts for maximum design flexibility.

In an exemplary embodiment, a first photodiode532can optionally be optically coupled to the input waveguide506and a second photodiode534can optionally be optically coupled to the drop waveguide508, the MEMS-tunable optical ring resonator500can be tuned based upon the output(s) of the first photodiode532and/or the second photodiode534.

In systems where the MEMS-tunable ring resonator500may be employed (e.g., a filter, a multiplexer, a demultiplexer, etc.), the ring resonator500is configured to set the resonance wavelength to a fixed wavelength or track a slowly varying wavelength (e.g., locking to a wavelength of a laser that is thermally drifting at millisecond timescales or slower). Conventional optical ring resonators use thermal phase shifters for this task, where thermal phase shifters constantly draw power to fix the resonance wavelength—both during the initial active lock process and to maintain the temperature required for lock. In like manner, carrier injection phase shifters constantly draw power due to the current injection required to fix the resonance wavelength—both during the initial active lock process and to maintain lock. Put differently, both thermal phase shifters and carrier injection phase shifters are effectively resistors. The ring resonator500is capacitive in nature, and thus the ring resonator500only draws power when the capacitor is charged/discharged to change the location of the tuner ring502during wavelength locking. Once the resonance of the resonator ring504is at the desired wavelength, no power need be drawn to maintain the resonance wavelength, and it therefore can be said that the quiescent power draw is zero (or near zero) for the ring resonator500. This is particularly advantageous over conventional ring resonators in environments where temperatures vary slowly relative to Gb/s data transmission rates, since the required power draw is fairly low overhead. In addition, the ring resonator500is particularly advantageous over conventional ring resonators in cryogenic environments where it is undesirable to add any additional heat to the chamber. The ring resonator500also exhibits advantages over conventional capacitive tunable optical ring resonators, which typically are optically lossy, and reverse biased p-n junction tunable optical ring resonators, which typically can tune over just a small fraction of the FSR.

Exemplary operation of the MEMS-tunable optical ring resonator500is now set forth. The input waveguide506carries light from a light source towards the ring resonator500, wherein the light has an operating wavelength, and further wherein the light is desirably optically coupled into the resonator ring504of the ring resonator. A gap of an initial size exists between the top electrode510and the bottom electrode512, wherein the initial size of the gap corresponds to an initial index of refraction (and therefore an initial resonant wavelength) of the resonator ring504. Light of the initial resonant wavelength optically couples into the resonator ring504and subsequently optically couples into the drop waveguide508. The first photodiode532receives light on the input waveguide506that has not optically coupled into the resonator ring504and outputs a first electrical signal that is indicative of a magnitude of the light that has not optically coupled into the resonator ring504. The second photodiode534receives light on the drop waveguide508that was optically coupled into the resonator ring504and outputs a second electrical signal that is indicative of a magnitude of the light that was optically coupled into the resonator ring504.

Based upon the first electrical signal or the second electrical signal, the MEMS-tunable ring resonator500can be tuned by altering the size of the gap between the top electrode510and the bottom electrode512. By altering the size of the gap between the top electrode510and the bottom electrode512, the index of refraction of the material of the resonator ring504is likewise altered. This altering of the index of refraction of the material of the resonator ring504can be used to alter the resonant wavelength of the resonator ring504to match the operating wavelength of the light from the light source. A voltage is applied between the top and bottom electrodes510and512, thereby pulling the tuner ring502towards the resonator ring504or pushing the tuner ring502away from the resonator ring504. Thus, the ring resonator500can be tuned to the operating wavelength.

In an exemplary embodiment, the tuning voltage for the MEMS-tunable ring resonator500may be approximately 40 V. In another exemplary embodiment, the tuning voltage may be approximately 20 V or less. For example, a MEMS-tunable ring resonator employing a cantilever design in accordance withFIG.2Bwith a 10 μm radius demonstrated tunability over more than its FSR with a voltage of less than 15 V. In yet other embodiments, the tuning voltage can be between 3 V and 6 V. This relatively low tuning voltage can be accomplished by, for example, using various alternative configurations. For example, an exemplary embodiment uses vertical comb drives, which employ control voltages in the 6 V range. Another exemplary embodiment uses relatively low spring constant structures enabled by low stiction dimples and contact materials. Yet another exemplary embodiment uses resonant switches that take advantage of the switch quality factor and DC bias amplification. A further exemplary embodiment uses a less confined, lower index contrast structures such as relatively thin waveguides and ridge-waveguides. A still further exemplary embodiment uses a relatively small resonator ring504radius to increase optical field overlap with the tuner ring502. In contrast to previous demonstrations of optical MEMS tuning of integrated optics, the ring resonator500depicted inFIGS.5and6leads to better scalability to larger aggregate bit rates, while the low insertion loss, cross-talk, and low power are advantages over non-MEMS approaches.

The use of a tuner ring502that moves in a direction normal to the plane formed by the resonator ring504, especially one in which the springs520are directly coupled to the tuner ring502, contrasts with various prior art MEMS-tunable ring resonators. For example, Haffner, et al., discloses a MEMS-tunable ring resonator that employs a gold disk-shaped membrane suspended over a fixed resonator ring via a fixed pedestal located in the middle of the resonator ring. Haffner, et al., “Nano-opto-electro-mechanical switches operated at CMOS-level voltages,” Science, Vol. 366, November 2019, pages 860-864, the entirety of which is incorporated herein by reference. Nielson, et al., discloses a MEMS-tunable ring resonator that employs an aluminum rectangular-shaped membrane suspended over a fixed resonator ring via fixed risers at both long ends of the rectangular-shaped membrane, with the risers being located outside of the resonator ring. Nielson, et al., “Integrated Wavelength-Selective Optical MEMS Switching Using Ring Resonator Filters,” IEEE Photonics Technology Letters, Vol. 17, No. 6, June 2005, pages 1190-1192, the entirety of which is incorporated herein by reference. Abdullah, et al., discloses a MEMS-tunable ring resonator that employs a gold coated dielectric rectangular-shaped membrane suspended over a fixed resonator ring via a fixed riser at one of the long ends of the rectangular-shaped membrane, with the riser being located outside of the resonator ring. Abdulla, et al., “Tuning a racetrack ring resonator by an integrated dielectric MEMS cantilever,” Optics Express, Vol. 19, No. 17, August 2011, pages 15864-15878, the entirety of which is incorporated herein by reference.

FIG.7is a schematic of an exemplary circuit700that can be employed to control the MEMS-tunable ring resonator500. The circuit700can be configured to lock the resonant wavelength of the ring resonator500to a desired wavelength (e.g., light output by a laser702) by altering the gap between the tuner ring502and the resonator ring504. The photodetector532detects light that was not coupled to the ring resonator500. The circuit700comprises a receive amplifier704that converts a photocurrent output by the photodetector532to a voltage. In an exemplary embodiment, the receive amplifier704can be a transimpedance amplifier or a CMOS charge pump, depending upon the desired power consumption, sensitivity, and bandwidth. An analog-to-digital converter (ADC)706is electrically coupled to the receive amplifier704, and a digital feedback controller708is electrically coupled to the output of the ADC706. The controller708can be a microcontroller, a field programmable gate array (FPGA), etc. The controller708is programmed to determine a wavelength offset and output a necessary voltage to a digital-to-analog converter (DAC)710. The circuit700further includes a voltage amplifier712that is electrically coupled to the output of the DAC710, wherein the voltage amplifier712is configured to amplify voltage output by the DAC710and direct the voltage to the control terminal of the MEMS-tunable ring resonator500.

The digital feedback controller708can include a look-up table that is usable to identify a new voltage based upon the voltage output by the ADC706and a previous voltage. Further, the controller708can include a proportional, integral, and derivative gain (PID) control algorithm or a PID chip. It can be noted that when maximizing coupling to the resonator ring504is desired, wavelength locking can be obtained by minimizing the signal from the photodetector532. In another exemplary embodiment, the circuit700can include the photodetector534in place of the photodetector532. In such an embodiment, wavelength locking is obtained by maximizing the signal from the photodetector534.

The tuning algorithm can include a simple lookup table, dither and lock technique, Pound-Drever-Hall locking, or a Sigma-Delta locking technique for precise control. Since the detected photocurrent is the same both slightly above and below resonance, the slop of the detected signal versus time are used in addition to amplitude.FIG.8depicts an exemplary circuit800that includes locking electronics for a dither and lock technique. In the exemplary circuit800, the control voltage applied to the control terminal of the ring resonator500includes a rapidly varying dither between a high and a low voltage, plus a slowly varying tuning voltage offset. By dithering the tuning voltage, the slope of the received signal can be determined in addition to the magnitude. The dither plus the tuning voltage can be multiplied (by a multiplier802) with the received photocurrent to generate an error signal, which is on average zero exactly on-resonance and non-zero off-resonance. The digital controller708determines the average and determines a new tuning setpoint using a PID circuit within the controller708. To control multiple MEMS-tunable ring resonators, analog multiplexers can be utilized preceding the ADC706and an analog demultiplexer can follow the DAC710. It can be noted existing chip manufacturers manufacture a sample and hold array that combines the functionality of an analog demultiplexer and voltage amplifier. Li, et al., “A 25 Gb/s, 4.4 V-Swing, AC-Coupled Ring Modulator-Based WDM Transmitter With Wavelength Stabilization in 65 nm CMOS,” IEEE Journal of Solid-State Circuits, Vol. 50, No. 12, December, 2015, pages 3145-3159 makes reference to exemplary technologies for controlling voltage applied to ring modulators, wherein such techniques can be employed in connection with controlling the MEMS-tunable ring resonator500. The entirety of such paper is incorporated herein by reference.

FIG.9depicts a MEMS-tunable mirror900, wherein the MEMS-tunable mirror900includes an exemplary single-layer vertical comb drive902that is configured to actuate a spring904. A vertical comb drive that is similar to the vertical comb drive902depicted inFIG.9can be employed to actuate the springs520such that the tuner ring502is positioned as desired relative to the resonator ring504in a MEMS-tunable ring resonator500.FIG.10is a diagram of another MEMS-tunable mirror1000, wherein the MEMS-tunable mirror1000includes an exemplary two-layer vertical comb drive1002that is configured to actuate a spring1004. A vertical comb drive that is similar to the vertical comb drive1002depicted inFIG.10can be employed to actuate the springs520such that the tuner ring502is positioned as desired relative to the resonator ring504in a ring resonator500. The vertical comb drives902and1002are described in Hah, et al., “Low-Voltage, Large-Scan Angle MEMS Analog Micromirror Arrays with Hidden Vertical Comb-Drive Actuators,” Journal of Microelectromechanical Systems, Vol. 13, No. 2, pages 279-289, April 2004, the entirety of which is incorporated herein by reference.

FIG.11depicts optical micrographs illustrating exemplary dimple1100/low stiction contact1102that can be employed in connection with actuating the tuner ring502. The exemplary dimple1100/low stiction contact1102are described in Czaplewski, et al., “RF MEMS Switches With RuO2—Au Contacts Cycled to 10 Billion Cycles,” Journal of Microelectromechanical Systems, Vol. 22, No. 3, pages 655-661, June 2013, the entirety of which is incorporated herein by reference.

Nielson, et al., “Dynamic Pull-In of Parallel Plate and Torsional Electrostatic MEMS Actuators,” Journal of Microelectromechanical Systems, Vol. 15, No. 4, pages 811-821, August 2006, discloses an analysis of the dynamic characteristics of pull-in on parallel-plate and torsional electrostatic actuators, which can be employed to reduce the voltage required to alter position of the tuner ring502relative to the resonator ring504in a MEMS-tunable ring resonator500. The entirety of such paper is incorporated herein by reference. Siddiqui et al., “Waveform Optimization for Resonantly Driven MEMS Switches Electrostatically Biased Near Pull-In,” 2018 IEEE Micro Electro Mechanical Systems (MEMS), Belfast, pages 795-800, 2018, describes a resonant switch that can be employed to reduce the voltage required to alter position of the tuner ring502relative to the resonator ring504in a MEMS-tunable ring resonator500. The entirety of such paper is incorporated herein by reference.