Description:F O R M 2

THE PATENTS ACT, 1970
(39 of 1970)

COMPLETE SPECIFICATION
(See section 10 and rule 13)

TITLE
SYSTEM FOR OPTICAL BEAM STEERING USING VERTICALLY ACTUATED MEMS GRATING COUPLER
INVENTORS:
NAIR, Viswas S., Indian Citizen
Tharayil House, Kummanam P. O.
Kottayam, Kerala 686 005

K N, Jishnu, Indian Citizen
Sivakripa (H), Anand Nagar, Puthurkkara, Ayyanthole (P.O.)
Thrissur, Kerala 680 003
APPLICANT
AMRITA VISHWA VIDYAPEETHAM
Amritapuri, Clappana PO
Kollam 690 525

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED:

SYSTEM FOR OPTICAL BEAM STEERING USING VERTICALLY ACTUATED MEMS GRATING COUPLER
CROSS-REFERENCES TO RELATED APPLICATION
None.
FIELD OF INVENTION
The present disclosure relates to opticaldevices and in particular to beam steering devices used in 3D mapping and optical communication.
DESCRIPTION OF THE RELATED ART
The ability to control the direction of optical beams is crucial in various emerging technologies such as optical communication and imaging. For instance, LiDAR systems are vital for creating 3D maps of objects and surroundings, which is essential for autonomous vehicles and scientific applications. Additionally, free space optical communication systems are gaining significance due to their high data transfer speeds and low latency, especially in the visible and infrared regions of the electromagnetic spectrum. Enhancements in beam steering technologies in these wavelengths would significantly improve 3D mapping capabilities and wireless communication systems.
Optical beam steering is conventionally achieved using liquid crystal devices and bulk mechanical systems. The bulk mechanical systems have large size and hence have low speed. The liquid crystal devices also have response time in the order of milliseconds. Applications like LiDAR demand smaller, faster and more efficient ways to steer beams. Optical phased arrays (OPA) provide the solution to the above limitations, and are thus a common technique used for 2D beam steering. Optical phased arrays can form directed beams and its beam can be steered electronically. General OPA uses a 2D array of antenna elements (like gratings) fed via dedicated phase shifters. The phases of the individual elements are adjusted so that they interfere with each other to form a beam in a desired direction in the far field. However, these methods necessitate an N × N array of phase shifters and antennas. Each of these N2 phase shifters consume power and chip area, and need fast and complex driving circuitry. Additionally, achieving N2optical interconnects between modulators and antennas can lead to a very complex structure requiring 3D opticalrouting or crossovers.Larger aperture areas providehigher resolution and is thus necessary for high precision beam steering applicationslike LiDAR. In the case of wavelength tuning, the angular dispersion of the grating coupler is used to achieve angle tuning with the help of a tunable laser as source.
This requireslarge change in wavelengths to bring about significant change in dispersion angle. It isalso highly susceptible to noise since the broadband detectors used in such systems will collect noise from multiplesources.
Combining 1D phased array and some other 1D beam steering technique, can help bring down the number of phaseshifters required from N2 to N and thus increase the aperture area for a given complexity.CN113885126A proposes a system consisting of a bus waveguide on a substrate, connected through a spring, a reaction electrode positioned near the optical antenna, and an actuation electrode that applies force to the optical antenna.It combines multiple gratings with a lens to achieve beam rotation.Here, the systemis a laterally actuated MEMS with vertically actuated or rotated grating for ON-OFF switch functionality.
WO2018132795A1 introduces optical phased arrays (OPAs) that manipulate one or more output optical beams in either one or two dimensions. These systems utilize co-planar diffractive elements to diffract light into beamlets, offering control over the shape and direction of the output signal by adjusting the relative phases of these diffracted beamlets. This specific configuration involves a laterally actuated array of fixed grating couplers with a fixed radiation angle, where the combined effect achieved by the array facilitates beam steering. In this setup, the MEMS component primarily provides phase shifts to individual gratings rather than tuning the angle of individual grating.
Moving a dielectric slab near an optical waveguide significantly changes the waveguide’s effective refractive index without increasing the loss significantly. Moving metallic parts into the evanescent field can increase the loss of the waveguide and cause narrow bandwidths due to plasmonic resonance. Moving a structure near the sides, top or below the waveguide core gives only small tuning since the evanescent field in these regions are weak. The proposed structure places the grating in the centre of the waveguide core with parts of the core on either side of the grating. This allows the grating to interact strongly with the propagating field. Additionally, the split and the relative movement is also taking place at the centre of the core. This results in large change in the effective index for a given movement. The use of such split structure in dielectric waveguides allow longer low loss waveguides and hence large apertures required for narrow beam widths.
SUMMARY OF THE INVENTION
The invention proposes systems and devices for optical beam steering using MEMS tunable grating coupler.
Inone embodiment of the present subject matter, is a system for optical beam steering that uses MEMS grating coupler. The device feeds signal to a pair of photodiodes. The device comprises a vertically actuated split waveguide core with internal gratings. The waveguide core is split to form a top segment and a bottom segment. The top segment is suspended above the bottom segment using springs. The two segments are separated by an air, fluid,or vacuum gap. The internal grating may be etched underneath the top segment or on the bottom segmentor both.
In some embodiments, the internal grating is positioned at the center of the waveguide core. The waveguide core is extended on either side of the internal grating.
In some embodiments the split separation between the top segment and the bottom segment is changed using MEMS actuation.
In certain embodiments there is a layer of cladding above and below the waveguide core. The waveguide core is made of a material which has high refractive index compared to the material used for cladding.
In certain embodiments there arecontact materialspositioned above the top cladding and below the bottom claddingof the waveguide core. In certain embodiments at least one of the contact materialsisa transparent material liketransparent conducting oxide (TCO).
In certain embodiments, the device comprises a semiconducting/dielectric waveguideplaced between the photo detector, LED or laser and the grating, designed to selectively absorb unintentional visible or near-infrared wavelengths while guiding the short-wave infrared wavelengths.
In certain other embodiments, a device for 2D beam steering may be designed using a phased array of 1D optical beam steering devices The device for 2D beam steering comprises an optical phased array of vertically actuated MEMS tunable waveguide grating coupler. The array elements are placed at a distance less than half of the wavelength of the light. The integrated optical phase shifters in the optical phased array induce progressive phase difference for angle tuning in transverse direction. And the 1D MEMS grating coupler system is utilized to steer the beam in longitudinal direction by varying the effective mode index of the MEMS grating coupler system.
In certain embodiments, the optical phased array of vertically actuated MEMS tunable waveguide grating coupler, comprising the split wave guide core, is clad on its top and bottom with a material having refractive index lower than that of the waveguide core.
In some embodiments the MEMS actuation is achieved through electrostatic means.

BRIEF DESCRIPTION OF THE DRAWINGS
The invention has other advantages and features, which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:
FIG.1 is a cross- sectional view of the vertically actuated MEMS grating coupler.
FIG. 2 is a 3D cut section of the 2D beam steering device using phased array of MEMS grating coupler.
FIG. 3 shows the relationship between the effective mode index of the split waveguideon both the etched region and unetched regions, with their separation for MEMS grating coupler.
FIG. 4 plots the angle of incidence vs. coupling (S21) for different separations of MEMS grating coupler.
FIG. 5 shows the radiation pattern in a 2-D Cartesian plot for different values.
FIG. 6 shows the radiation pattern in a 2-D Cartesian plot for a given s=120 nm in~100 µm to ~400 µm long gratings in steps of ~100 µm.
FIG. 7 shows the 3D polar plot of the gain (dB) of the 2D beam steering device for different values of phase difference (ß) and separation (s).
Referring to the figures, like numbers indicate like parts throughout the views.
DETAILED DESCRIPTION OF THE EMBODIMENTS
While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.
Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of “a”, “an”, and “the” include plural references. The meaning of “in” includes “in” and “on.” Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.
The present subject matter describes a system for optical beam steering using vertically actuated MEMS grating coupler and internally positioned grating. The internal placement of the grating within the waveguide core increases the coupling and the ability to change the effective index.The proposed MEMS-based grating coupler uses a small relative vertical motion between the two halves of a dielectric waveguide grating. This relative motion changes its effective index and, thus, its listening angle at a given wavelength. Vertical electrostatic actuation for MEMS implementation increases its power efficiency, and out-ofplanemotion does not require additional chip area.
In various embodiments the present invention features a tunable grating coupler100 which acts as a dichroic beam splitter as shown in FIG. 1. The device consists of a transparent waveguide core110comprising a top segment111suspended over a bottom segment 112. The waveguide core has an internal grating113either beneath the top segment111or on the bottom segment112. The internal grating 113is placed in the center of the waveguide core with parts of the core on either side of the figure as shown in FIG. 1. The top segment and bottom segment are separated by an air, fluidor vacuum gap. Cladding layers130are placed above and below the waveguide core110. The refractive index of the cladding material is lower than that of the waveguide core material. There is a contact material layer140above the top cladding and below the bottom claddingeach, at least one of which is transparent.Electrostatic actuation uses the conducting contacts provided above and below the cladding. The top layer of the waveguide core111, the cladding 130 above it, and the contact layer140above the cladding forms the top suspended part. Springs 120are used to hold the top suspended part in place. The springs extend out of the plane and are shown by the dotted lines.
In certain embodiments the waveguide core 110may be made of TiO2 and the cladding material130may be made of SiO2. The top half111and the bottom half112of the waveguide core may each be 250nm thick. The internal grating 113may be 100nm in depth. In certain embodiments, the SiO2 cladding layers may be500 nm thick for the layerabove and 1000 nm thick for the below layer. The transparent contact layers140positioned above and/or below the top and bottom cladding130may be transparent conducting oxide (TCO).
In certain embodiments, the proposed device may be embodied as a receiver. In this case, the tunable grating coupler functions as a dichroic beam splitter feeding a pair of waveguide-coupled photodetectors150.The tunable grating coupler allows only selected incident angles for a given wavelength to the photodetector150. This directional reception helps to reduce IR noise from other sources from reaching the detector. An additional semi-insulating Si waveguide 160between the detector and the grating absorbs anyunintentional visible or near IR wavelength.
In certain embodiments, the proposed device may be embodied as a source or emitter wherein a laser or LED is utilized instead of photodetector150.
In certain embodiments, the visible and near infrared transparent materials used in the design allow its integration on the front side of a solar cell170for simultaneous energygeneration and communication.The photodetector150is biased independent of the solar cell and hence can operate in reverse biased mode with low capacitance. The detector may have a small junction area even though the light with the designed wavelength falling on the entire grating area can be collected. The optical power in the remaining wavelengths is passed to the solar cell beneath for energy generation. An implementation using TiO2 waveguide core and SiO2 cladding supported by four springs provided a beam width of ~0.95° for a 100 µm long grating, and it reduces to ~0.55° for a 300 µm long grating. Keeping the tunable grating coupler in the shadowed back side of the solar panel may eliminate the estimated 15% reduction in the solar cell efficiency caused due to front-side integration. The actuation may be using electrostatic attraction, requiring no steady state current flow, and is thus ideal for low power, mobile applications.
In various embodiments, the proposed system uses a parallel plate electrostatic actuator140, shown in FIG. 1, to achieve the required mechanical motion. The two parallel electrodes separated by a distance d makes it a capacitor with capacitance ?? = ????/??= ????/(??0-??). Here, ?? is thepermittivity of the medium between the two plates, A is the areaof the parallel plate, ??0 is the initial separation, and x is thedisplacement of the mobile electrode from its initial position. Apotential difference (V) applied between the plates results in thepotential energy ?? =1/2 C??2. The electrostatic force on theplates due to the stored potential energy is given by ???? =1/2??2dC/dx=??????2/2 (??0-??)2. The restoring force (???? = -????) provided bythe supporting beams (net spring constant k) balances Fe.Equating the forces give the potential difference required forthe desired displacement as
?? = (?2kx(d_0- x)?^2/Ae)^(1/2) (1)
The proposed structure shown in FIG. 1 may require an effectiveseparation T between the plates instead of d0to account for the multiple dielectric layers. An instability onset on theMEMS beyond a critical value for displacement called pull-indisplacement. The value of pull-in displacement is given by
?????? = T/3 (2)
where?? =e_0 ?_i¦t_i/e_i considering the dielectric stack. Here, t_iis the thickness, and ???? is the permittivity of the ith layer,respectively. Thus, stable operation of MEMS is limited to thedisplacement 80% transmission in both visible and 1550 nmwavelengths.
The waveguide core of the grating coupler is split into two, separated by air or vacuum gap. A change in the separation s of the split waveguide can change the effective index neffof the grating coupler and can be varied using a MEMS. Equation (4) gives the grating period (?) after fixing a value for theacceptance angle at the initial separation s0.
Coupling of the incident signal wavelength onto the split waveguide is achieved using the grating etched inside the waveguide, thus making it a split grating coupler. Grating is etched only on one half of the split waveguide so that one half provides the periodic perturbation while the other provides mode matching to the output waveguide. Placing the grating at the centre of the core region increases its interaction with the field and hence the coupling. The split is also at the centre and helps increase the change in neff. The sensitivity of neffon s isat a maximum when the two halves are of the same thickness.
In various embodiments, a device for 2D beam steering 200may be implemented using a hybrid of 1D optical phased array 201of 1D MEMS tunable grating coupler100 systems as shown in FIG. 2. 1D optical phased array achieves angle tuning in a transverse direction by introducing progressive phase difference in the grating coupler waveguides. The required phase difference can be created using integrated optical phase shifters. The tunable grating coupler achieves angle tuning by varying its effective mode index using MEMS actuation in the longitudinal direction. The structure places the grating in the centre of the waveguide core with parts of the core on either side of the grating as shown in FIG. 2. This allows the grating to interact strongly with the propagating field. Additionally, the split and the relative movement is also taking place at the centre of the core. This results in large change in the effective index for a given movement. The use of such split structure in dielectric waveguides allow longer low loss waveguides and hence large apertures required for narrow beam widths.Vertical electrostatic actuation of the MEMS implementation for out-of-plane motion doesnot require any additional chip area and is power efficient.
Considering the phased array, the net field of the array is the vector sum of the fields of the individual elements. These individual fields constructively interfere in a particular direction and destructively in other directions, resulting in a desired beam direction and pattern.The net field of an array (??????????l) is the field due to a single antenna (?????? ) multiplied by a factor called the arrayfactor (AF).
????????????=?????????????????????? × ???? (5)
An array of identical elements, all excited by sources with the same excitation field magnitude, but with a progressive phase difference (??) is called a uniform array. Such an array’s array factor depends on the geometry of the arrayand the excitation phase difference, and is given by
???? =?_(n=1)^N¦e^(i(n-1)(kd cos ?_m + ß)) (6)
where?? is the number of radiating elements present in the array, ?? (=2??/??) is the free space wavevector, ?? is theseparation between adjacent elements and ???? is the angle for which the array factor is being found.
The array factor of a particular wavelength in a particular direction may be varied by changing the separation d between the individual elements and/or the difference in phase, (??) between them. Change in the distance between the individual elements may require a lateral actuator and uniformly stretchable membrane. Instead, a progressive phase difference between the inputs to each grating coupler is used here to change the direction of the beam. Theprogressive phase difference, ?? required for maximum radiation in a particular direction can be found from equation (6) as,
?? = -???? cos ????(7)
The phase matching condition for 2D grating coupler mode is given as
????????k = ????k sin ??i + 2??/?? (8)
where ???????? is the average effective index of the grating coupler mode, ?? =2p/?is the freespace wavevector, ?? is thewavelength of light, ???? is the refractive index of the cladding, ??i is the angle of incidence of the light and ?? is thegrating period.It can be observed from the equation, that the incident angle required to couple an incident light on to a waveguidemode can be varied by varying any of the other parameters of the grating coupler. So, the incident angle required tocouple light onto the grating coupler (called the coupling angle or acceptance angle) can be changed by changing theeffective index of the mode itself.
In various embodiments, the distance between the elements of the array201, d may be less than that of wavelength of light, to prevent the unwanted side lobes. Dielectric waveguides cannot be placed very close as there will be evanescent field coupling between the waveguides. So, a waveguide spacing which will not couple significant power to the adjacent waveguide is required. The theoretical values of ß required for a desired direction is calculated from equation (7). Increasing the ß, beyondsome value would lead to poor beam profile or creation of unwanted lobes and thus limit the sweep range of theOPA.
In various implementations, the waveguide-to-waveguide separation, dof the optical phased array may be kept at 500nm, as the coupling between the adjacent waveguides may be negligible at that separation. The dimensions of the aperture region may be 100 µm × 100 µm, and thus, there may be 100 radiating waveguide elements in the array. Each waveguide has a width of 500 nm. The lower half of the waveguide in the grating region has a thickness of 120 nm and upper half has a thickness of 120 nm in the unetched regions, and the etch depth may be 10 nm. The lower and upper SiO2 claddings are 1000 nm and 500 nm thick, respectively. The optimized device used serpentine springs202. The width of the individual beams in the spring is 2 µm while the width at the turns is 2.5 µm. The turns are connected to the beams by smooth transitions and there are two full periods in the optimized spring structure. A beam steering of 35° may be possible along transverse direction by varying ß from -70° to 70°, while it was ~19° along longitudinal direction by applying a potential difference of ~ 2.4 volts to the MEMS tunable grating couplers. Thus, the field of view (FOV) of the device is ~35° × ~19°. The beam width of the device with a mechanically and optically designed aperture of 100 µm × 100 µm is ~ 1.75° × 0.82°. The beam width further reduces to ~ 0.15° × 0.5° when the aperture is increased to 1 mm × 0.2 mm. The structure with 100 µm × 100 µm aperture supports a frequency of operation up to ~ 50 kHz for the MEMS, which is comparable to the available state of the art devices using other technologies like wave length tuning and full 2D phased arrays. The maximum voltage required by the MEMS is 2.4 volts, and thus needs no additional electronic circuitry to step up the voltage. Also, since the actuation is achieved using electrostatic means, there is no current flow, and is thus ideal for low power, mobile applications.
The invention has multiple advantages as set forth here. Moving a dielectric slab near an optical waveguide significantly changes the waveguide’s effective refractive index.So, the incident angle required to couple light onto the grating coupler can be changed by changing the effective index of the mode itself. The present invention places the internal grating in the centre of the waveguide core with parts of the core on either side of the grating. This allows the grating to interact strongly with the propagating field. This electrostatically produced relative motion changes the effective index of the grating coupler core and thus changes its listening angle.
Combining 1D phased array with this MEMS tunable grating coupler can enable 2D beams steering. This can help bring down the number of phase shifters required from N2 to N, when compared to 2D optical phased arrays, and thus increase the aperture area for a given complexity.
Both these structures can be fabricated on top of any optical device, if visible and near-infrared transparent materials are used in the design. Fabricating the proposed structure on top of solar panels can help utilize this area for other applications like communication or LiDAR. The proposed system due to its small dimensions will help to reduce the equipment's mass and volume, which is crucial in space and portable, self-powered applications. This system can co-exist with other radio-based communication transceivers and provide additional bandwidth, redundancy, or LiDAR imaging capability, without adding significant weight or consuming more area.
Although the detailed description contains many specifics, these should not be construed as limiting the scope of the invention but merely as illustrating different examples and aspects of the invention. While the invention has been disclosed with reference to certain embodiments and specific illustrative examples, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope, which should be as detailed in the claims appended herewith.
EXAMPLES
Example 1: Simulationsetup and results for 1D MEMS grating coupler
For the MEMS,the mobile section requires four springs of appropriatespring constant and internal stress to support it. The serpentine springs with spring constant value 7N/m are chosen for the proposed device. The effective index and acceptance angle for each s was obtained by varying s in the simulation. FIG.3 shows the variation in effective mode indices in the etched and unetched regions at different gaps. The effective index reduces with the increasing gap as the volume fraction of air/vacuum increases in the waveguide core. The field interaction between the upper and lower halves reduces exponentially for large gaps, reducing the sensitivity of the effective index on the separation. The trench width spans 50% of the grating’s period (fill factor=0.5). Hence, the net effective refractive index will be close to the average of the two effective mode indices. The effective mode index of the unetched region changes by ~ 0.15, and that of the etched region changes by ~ 0.05 when the gap changes from 320 nm to 40 nm.
FIG.4 shows the change in coupling to the photodiode with a change in the effective index. It is an S-parameter (S21) plot with port 1 providing the incident excitation at different angles while measuring the output at port 2 (photodiode). The peak coupling varies between -2 to -4.7 dB for all the values of s. The incident angle for maximum coupling increases monotonously for decreasing s. FIG.5 shows the plot of normalized far-field expressed in dB.FIG.5 also shows the angular sweep with changing s. There was no significant variation in the beamwidth for different s.
The coupling (FIG.4) and the maximum gain (FIG.5) reduce for very large and very small values of s, with a peak around s = ~ 280 nm. The fall in the coupling/gain at low/high values of s can be due to the excitation of higher order modes, Fresnel reflections due to the effective index change, and poor coupling due to large separation. The thickness of the grating structure is designed to prefer a single mode for a range of s. However, some energy may get scattered to higher order modes for small values of s, resulting in a mode mismatch with the output Si waveguide. Additionally, the effective index of the grating will further deviate from the effective index of the output Si waveguide for large gaps, resulting in higher reflections at the interface between them. The coupling may also decrease for very large separations since the top half with the grating moves farther from the waveguiding bottom half. The effect of Fresnel reflections and poor coupling dominates at higher gaps, while the scattering to higher order modes dominates at lower gaps. These competing mechanisms are proposed to result in the peak at s = ~ 280 nm.
An angular sweep of around 7° is visible from FIG.4 and FIG. 5 when the gap is changed from 320 nm to 40 nm, with a maximum actuation voltage of 5 V. This tuning range compares positively with other tunable grating-based systems giving 22.8 nm wavelength tuning, i.e., ~ 2° (estimated using equation 4) and 5.6° angular tuning in stretching MEMS.
The resolution of pointing, alignment, and scanning systems depends on the major radiation lobe's half-power beam width (HPBW). The in-plane scalability of the proposed planar MEMS structures may allow large collection apertures and hence narrower HPBW. Increasing the grating region's length reduces the HPBW, as shown in FIG.6. The waveguide power decays exponentially along the grating length, and henceincreasing the grating length has diminishing returns in terms of the reduction in beam width. The grating length of 300 µm provides a beamwidth of 0.55°, beyond which there is no significant reduction. Further increasing the length while reducing the beam width will require reduced grating contrast.
Example 2:Simulation setup and results for 2D beam steering using phased array of MEMS tunable grating coupler
The waveguide-to-waveguide separation, d of the optical phased array is kept at 500nm, as the coupling between the adjacent waveguides is found to be negligible at that separation. The dimensions of the aperture region are taken as 100 µm × 100 µm, and thus, there will be 100 radiating waveguide elements in the array. Each waveguide has a width of 500 nm. The lower half of the waveguide in the grating region has a thickness of 120 nm and upper half has a thickness of 120 nm in the unetched regions, and the etch depth is taken as 10 nm. The lower and upper SiO2 claddings are 1000 nm and 500 nm thick, respectively. The equation (7) provides the theoretical estimate of phase difference ß required for sweeping the beam along transverse direction to ????. Table 1 shows the calculated value ofß for each ????.
Table 1: The calculated value of phase difference ß for steering the beam in transverse direction to ????
Transverse angle????(°) Phase difference, ß (°)
-45 164.2
-30 116.1
-15 60.1
0 0
15 -60.1
30 -116.1
45 -164.2
Equation (6) gives the equation of AF and is theoretically calculated for ????=0 to 360° by plugging in ß=0°, 45°, 90°, 125° and 135°. It isbe observed that ????increases from 90° when ß is increased from 0°. But, beyond ß=125°, the AF pattern starts to get additional peaks. The angular tuning predicted by varying ß has been obtained from the polar plot of the gain of the device and limited by the 3 dB gain at ß= ±70° to????= ±17.3°.
The half power beam width (HPBW) determines the resolution of pointing and scanning systems. HPBW is defined as the angular separation at which the power of a beam becomes half its peak value. The transverse HPBW for number of elements N = 100 (total array width = 100 µm), when ß=0 and separation between the top and bottom halves of the grating coupler s = 80 nm, is 1.75°. The transverse beam width can be further reduced by increasing N. The HPBW reduces to just 0.15° for N = 1000 (total array width = 1 mm).
The effective mode index of the split waveguide grating coupler is different in the etched and unetched regions. The effective mode index in both these regions changes when the separations between the two halves is varied.The effective mode index changes by about 0.26 in the etched region and by 0.28 in the unetched region. The net change in theeffective mode index will be around 0.27 as thenet change in theeffective index is the average of the two when the fill factor ff= 50%.This change in the effective index changes the coupling angle as given in Equation (8). The efficiency and directionality of a radiating element in the far field can be obtained from its radiation pattern.
The longitudinal beam steering achievable for this structure is from ~ 15.6° to ~ 34.8°. This is achieved when the separation between the split waveguides is varied from 180 nm to 40 nm. Thus, a longitudinal beam steering of 19.2° is achieved using the MEMS tuning of the grating coupler. The peak value of gain is obtained at a separation of 80 nm and falls for both the lower and higher values of separation. The fall in gain can be due to various factors like the Fresnel reflections due to the effective index change, higher-order mode excitations, and poor coupling due to large separation. The grating coupler is designed for 80 nm separation, and hence, the changes in separation from this optimum value will result in aslight mode mismatch with the output waveguide and reflections at the interface where the grating coupler ends and the output waveguide begins. The thickness of the split waveguide is designed to prefer only a single mode for a range of separation values, but some power may get scattered to higher-order modes for lower separation values, and mode mismatch with the output waveguide occurs as a result.At higher values of separation, poor coupling to the grating coupler can also occur due to the weak interaction betweenthe two split waveguides, where only one half has agrating on it and the other half acts only as a waveguide.
The longitudinal half power beam width, for a grating length of 100 µm, when ß = 0 and s = 80 nm is found to be0.82. It may be further reduced by increasing the grating length. The longitudinal beam width reduces to 0.5° at agrating length of 0.2 mm. Higher values for grating lengths could not be simulated due tocomputational constraints. But the theory and the trend assure a decrease in beam width with higher grating lengths.
The grating coupler is studied for three different etch depths, i.e., 10 nm, 20 nm, and 30 nm to deduct the influence ofetch depths on various beam properties like peak gain, side mode suppression ratio (SMSR) and beam width. SMSRis a measure of the quality of the beam and is defined as the ratio of power in the main lobe to that in the most prominent side lobe. The peak gain and SMSR can be seen to be increasing when the etchdepth is increased. This may be due to the increased radiation efficiency of the grating at higher etch depths. TheHPBW of the beam is also seen to be reducing for shallowetch depths. The lowest etch depth of 10 nm is chosen to obtain lowest HPBW while maintaining SMSR > 10 dB andpeak gain > 30 dB.
The dimension of the serpentine spring was widely varied, and the optimum design is used to present the results. The width of the individual beams in the spring is 2 µm while the width at the turns is 2.5 µm. The turns are connected to the beams by smooth transitions and there are two full periods in the optimised spring structure. The maximumrequired displacement of ~ 140 nm can be obtained by applying a potential difference of ~ 2.4 V.
The 2D beamsteering achieved by the device, for a total of nine different combinations is plotted as shown in FIG.7. The combinations are represented as a function of [ß, s] - a) [-70°, 180nm], b) [0°, 180nm], c) [70°, 180nm], d) [-70°, 80nm], e) [0°, 80nm], f)[70°, 80nm], g) [-70°, 40nm], h) [ 0°, 40nm] and i) [ 70°, 40nm].The left to right steering is achieved using the optical phased array, by changing the phase difference, ß. Whereas the top to bottom steering is achieved using the tunable grating coupler by varying its effective mode index. The collection aperture of the device is ~100 µm × 100 µm. The horizontal (transverse) tuning is achieved by the phased array usingphase shifters, whereas vertical tuning (longitudinal) is achieved by tunable grating coupler using MEMS.

, Claims:WE CLAIM:
1. A device for 1D optical beam steering (100) utilizing Micro-Electro-Mechanical System (MEMS) grating coupler, which feeds signal to a pair of photodiodes, the device comprising:
a vertically actuated split waveguide core (110) having a top segment (111) suspended above a bottom fixed segment (112) using springs (120), the segments being separated by air, fluid,or vacuum gap (108), wherein,
the waveguide core has an internal grating (113) either underneath the top segment or on the bottom segmentor both.

2. The device as claimed in claim 1,wherein the positions of the top and the bottom layers are interchanged.

3. The device as claimed in claim 1, wherein the internal grating (113) is positioned near the center of the waveguide core (110).

4. The device as claimed in claim 1, wherein the split separation (108) between the top segment and bottom segment is changed by MEMS actuation.

5. The device as claimed in claim 1, wherein a layer of cladding (130) is positioned above and below the waveguide core.

6. The device as claimed in claim 1, the waveguide core is composed of a transparent material exhibiting a high refractive index relative to the material utilized for the cladding.

7. The device as claimed in claim 1, wherein a contact material layer is (140) positioned above the top cladding and below the bottom cladding of the waveguide core each, out of which at least one is transparent.

8. The device as claimed in claim 3, wherein the transparent contact material is a Transparent Conducting Oxide.

9. The device as claimed in claim 1, comprises a semiconducting or other dielectricwaveguideplaced between the photo detector, LED or laser and the grating, designed to selectively absorb,or reflect unintentionally coupled wavelengthswhile passing the desired wavelengths.

10. A device for 2D beam steering (200), using a phased array of 1D optical beam steering devices (100), as claimed in claim 1, the device comprising:
an optical phased array of vertically actuated MEMS tunable waveguide grating coupler (201),
wherein the integrated optical phase shifters in the optical phased array induce progressive phase difference for angle tuning in transverse direction; and
wherein the 1D MEMS grating coupler system (100) is utilized to steer the beam in longitudinal direction by varying the effective mode index of the MEMS grating coupler system.

11. The device as claimed in claim 10,wherein the optical phased array of vertically actuated MEMS tunable waveguide grating coupler (201), comprising the split wave guidecore(100), is provided with a cladding(203) on its top and bottom with a material having refractive index lower than that of the waveguide core.

12. The device as claimed in claim 10,wherein MEMS actuation is achieved through one of electrostatic, piezoelectric or electromagnetic means.

Dr V. SHANKAR
IN/PA-1733
For and on behalf of the Applicants