[0001] This application claims priority to U.S. Provisional Application No. 62/845,090 filed on May 8, 2019, the entire disclosure and contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] This invention relates generally to tunable optical resonators used for LIDAR applications, namely wind sensing and species detection, that are designed to operate as Fabry-Perot instrumentation.

BACKGROUND OF THE INVENTION

[0003] Tunable optical resonators are used for LIDAR applications, namely wind sensing and species detection. These ultra-compact resonators can be designed to operate as Fabry-Perot instrumentation, thus providing rugged and extremely compact instrumentation for ground and space LIDAR and passive observations. The option to tune the resonator instead of the light source removes the limitations on light source availability and potentially allows for a more compact, rugged measurement system.

Atmospheric Measurements

Direct Detection Technique

[0004] Atmospheric measurements, including wind, aerosol content and species concentration, are critical to the understanding of weather patterns and model verification. While coherent techniques yield good results in the planetary boundary layer, the aerosol concentration is too low for higher altitude (>5 km) wind measurements. Conversely, the Direct Detection LIDAR technique, capable of operating with and without aerosols, relies on molecular scattering for the backscattered signal at higher altitudes and has yielded wind measurements up to the lower mesosphere as disclosed by Abreu, V. A., Barnes, J.E., Hays, P.B“Observations of winds with an incoherent LIDAR detector ,” App. Optics 31, 22 (1992). Ranging was achieved using a circle to line optics (CLIO) and CCD array detector to image fringes generated by a Fabry-Perot interferometer as disclosed by Irgang, T.D. Hays, P.B., Skinner, W.R.,“ Two-channel direct-detection Doppler LIDAR employing a charge-coupled device as a detector ,” App. Optics 41, 6 (2002).

Doppler Shifts Measurements

Fabry -Perot Interferometer

[0005] An illustration of how the atmospheric parameters are extracted from the collected spectrum using direct detection is shown in Figure 1. The recorded interferometric pattern is a composite of the aerosol, molecular and background continuum. An amplitude profile of a single fringe is shown in Figure 1. The phase, or center, of the spectrum is measured relative to a reference to deduce the Doppler shift of the signal; this is used to determine the line-of-sight wind speed and direction. The amplitude of the molecular signal is proportional to the number of molecules that scatter light and therefore is proportional to atmospheric density. Through analysis, these components are separated to measure wind velocity and density.

SUMMARY OF THE INVENTION

[0006] This invention resides in methods and apparatus for a Doppler shift measurement device, comprising: a tunable resonator device operatively configured to receive backscattered light and to scan across the spectral range of the backscattered light so as to identify spectral locations of the morphology dependent resonances therein; a detector operatively configured to receive resonance data from the tunable resonator device; and a processor circuit operatively configured to receive and process the resonance data into output data representing Doppler shifts in the backscattered light in response to resonance patterns in the backscattered light, wherein the resonator device is formed to controllably deform a morphological structure thereof so as to adjustably tune a transmission spectrum of the backscattered light inputted thereinto in response to the output data.

[0007] According to a preferred embodiment, the Doppler shift measurement device further comprises an A/D detector operatively connected to receive the resonance data in analog form from the detector and to digitize the resonance for inputting into the processor circuit. [0008] In another embodiment, the Doppler shift measurement device further comprises a driver circuit operatively connected to output a driver signal to the tunable resonator so as to tune the resonator device.

[0009] In another embodiment of the Doppler shift measurement device, the tunable resonator device includes a resonator and a waveguide both fixedly mounted on an upper surface of a substrate such that an evanescent field of the waveguide couples into the resonator.

[0010] In another embodiment, the Doppler shift measurement device further comprises a light source operatively connected to emit coherent light; a beam splitter operatively configured to receive the emitted coherent light and to generate a first split coherent light portion and a second split coherent light portion, the first split coherent light portion being directed to the tunable resonator device; and a collecting optics device operatively configured to receive the backscattered light resulting from backscattering of the second split coherent light portion emitted to a volume of atmosphere, wherein the backscattered light is transferred from the collecting optics device to the tunable resonator device.

[0011] In another embodiment, the Doppler shift measurement device further comprises a controller circuit operatively connected to generate control signals to control the tunable resonator device; and a driver circuit operatively connected to the controller circuit to receive the control signals to control the tunable resonator device, the driver circuit being configured to generate drive signals for controllably tuning the tunable resonator device.

[0012] In further embodiments of the Doppler shift measurement device, the resonator is a racetrack-shaped resonator with the waveguide is positioned parallel and adjacent to an elongated portion of the racetrack-shaped resonator. Alternatively, the resonator is a racetrack-shaped resonator with the waveguide is positioned perpendicular to an elongated portion of the racetrack shaped resonator and adjacent to a curved portion of the racetrack-shaped resonator.

[0013] In an even further embodiment of the Doppler shift measurement device, the tunable resonator device further includes a piezoelectric layer formed on a lower surface of the substrate, the piezoelectric layer being operatively configured to deform the substrate in response to the processor circuit.

[0014] In further embodiments, the Doppler shift measurement device further comprises a controller circuit operatively connected to generate control signals to control the tunable resonator device; and a driver circuit operatively connected to the controller circuit to receive the control signals to control the tunable resonator device, the driver circuit being configured to generate drive signals for controllably tuning the tunable resonator device, wherein the tunable resonator device includes a resonator and a waveguide both fixedly mounted on an upper surface of a substrate such that an evanescent field of the waveguide couples into the resonator, and a piezoelectric layer formed on a lower surface of the substrate, the piezoelectric layer being operatively configured to deform the substrate in response to the drive signals.

[0015] In a method for measuring Doppler shifts for a LIDAR system, the present invention comprises the steps of: emitting coherent light into a volume of atmosphere of interest so as to generate backscattered light therefrom; receiving the backscattered light in a tunable resonator device operatively configured to scan across the spectral range of the backscattered light so as to generate resonance data indicating morphology dependent resonances therein; processing the resonance data into data indicating Doppler shifts in the backscattered light in response to resonance patterns in the backscattered light; controllably tuning the resonator device by deform a morphological structure thereof so as to adjustably tune a transmission spectrum and spectral location of the resonances of the backscattered light inputted thereinto in response to the Doppler shift data.

[0016] In another embodiment of the Doppler shift method, the present invention further comprises the steps of: providing the tunable resonator device, wherein the tunable resonator device includes a resonator and a waveguide both fixedly mounted on an upper surface of a substrate such that an evanescent field of the waveguide couples into the resonator.

[0017] In further embodiments of the Doppler shift method, the resonator is formed as a racetrack shaped resonator with the waveguide is positioned parallel and adjacent to an elongated portion of the racetrack-shaped resonator. Alternatively, the resonator is formed a racetrack-shaped resonator with the waveguide is positioned perpendicular to an elongated portion of the racetrack-shaped resonator and adjacent to a curved portion of the racetrack-shaped resonator.

[0018] In an even further embodiment of the Doppler shift method, the tunable resonator device further includes a piezoelectric layer formed on a lower surface of the substrate, the piezoelectric layer controllably deforms the substrate. A tunable resonator device operatively configured to receive backscattered light and to scan across the spectral range of the backscattered light so as to identify morphology dependent resonances therein; a detector operatively configured to receive resonance data from the tunable resonator device; and a processor circuit operatively configured to receive and process the resonance data into output data representing Doppler shifts in the backscattered light in response to resonance patterns in the backscattered light.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention is illustrated in the accompanying drawings, wherein:

Figure 1 illustrates how the atmospheric parameters are extracted from the collected spectrum using direct detection;

Figure 2(a) illustrates the structure and operation of a direct detection device using a Fabry-Perot interferometer;

Figure 2(b) illustrate the structure and operation of at least a preferred embodiment of the present invention;

Figures 3(a) and 3(b) show how the TE field is calculated for an all-pass ring resonator and for a disk;

Figures 4(a) and 4(b) are graphical illustrations of a bus waveguide transmission for (a) a fixed length waveguide and (b) a tunable waveguide, respectively;

Figures 5(a) and 5(b) show at least first and second embodiments of the waveguide of the present invention;

Figures 5(c) and 5(e) show a first implementation for deforming the waveguide according to the present invention; Figures 5(f) and 5(h) show a second implementation for deforming the waveguide according to the present invention; and

Figure 6 shows a block diagram of a LIDAR module according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] As discussed above with respect to Figure 1, atmospheric parameters are extracted from the collected spectrum using direct detection. The recorded interferometric pattern 102 shown in Figure 2(a), for example, illustrates a composite of the aerosol, molecular and background sunlight continuum. An amplitude profile of a single fringe representing a Molecular Signal and an Aerosol Signal is shown in Figure 1. The phase PH, or center, of the spectrum (i.e., the Molecular Signal, the Aerosol Signal, and Sunlight) is measured relative to a reference at Zero wind to deduce the Doppler Shift of the signal (shown as a shifted curve of the spectrum); this is used to determine the line-of-sight wind speed and direction. The amplitude of the Molecular Signal is proportional to the number of molecules that scatter light and therefore is proportional to atmospheric density. Similarly, the amplitude of the Aerosol Signal is proportional to the number of aerosol particles that scatter light. Through analysis, these components are separated to measure wind velocity and density.

[0021] Referring to Figure 2(a), the Direct Detection technique uses a Fabry-Perot interferometer 100 to determine the Doppler shifts in the LIDAR return. The detection approaches include either the edge technique which uses narrow band filters to transform the shift into light intensity or an imaging technique which uses a detector 104, such as a CCD array, to image the fringes and their spatial position with respect to the unshifted reference. In both configurations, the interferometer output contains the spectral information over the free spectral range. This technology forms the basis of present LIDAR systems.

[0022] As illustrated in Figure 2(b), according to the present invention, a tunable resonator 200 performs the same operation as that of a Fabry-Perot interferometer by scanning across the free spectral range and identify resonance locations. These tunable resonator 200 offers an alternative to Fabry-Perot interferometers when size, weight and power requirements are stringent. Whispering Gallery Mode Resonators

[0023] The principle of operation of this all-optical sensor is based on the shifts of whispering galley optical modes (or morphology dependent resonances) of a dielectric resonator as disclosed by Benner, R.E., Hill, S.C.,“Morphology-dependent Resonances” in Optical Effects Associated with Small Particles. P.W. Barber and R.K. Chang, Eds., Advanced Series in Applied Physics, World Scientific (1988). Optical whispering gallery modes (WGM), also known as morphology dependent resonances (MDR), observed as sharp dips in the transmission spectrum through the resonator. The observed line width, dn, is related to the quality factor, Q=v/5v. Using silica microspheres, Q values up to 10 ⁸ have been obtained by our collaborators at Southern Methodist University (SMU). A free spectral range greater than 25 GHz is commonly achieved. Because of their inherent high quality -factors, WGM resonances are highly suitable to measure Doppler shifts with excellent accuracy. Optical micro-resonators are microfabricated and afford a Q-factor larger by several orders of magnitude compared to micro Fabry -Perot interferometers.

[0024] Thus, a Doppler shift measurement device according to at least a first embodiment of the present invention, using a micro-resonator 200, the Doppler-shifted backscattered light is passed through the optical micro-resonator 200 and the output thereof is inputted into and measured by using a detector 204, such as a conventional photodiode or photomultiplier tube (PMT).

[0025] The transmission spectrum 206 exhibits narrow resonances that are identified as sharp drops in the signal level as opposed to an interference fringe pattern from a Fabry-Perot interferometer. The micro-fabrication process gives the micro-resonator-based sensor unprecedented compactness and ruggedness. However, unlike the Fabry-Perot interferometer, typically the resonators are passive; they require the light source to be scanned over the free spectral range in order to observe resonances. The present invention embodies a tunable resonator for Doppler shifts measurements. Tuning is achieved by perturbing the resonator morphology, as will be discussed in detail hereinbelow through the various embodiments of the present invention.

[0026] Figures 3(a) - 3(b) show the transverse electric wave (TE) field calculated for an all-pass ring resonator (Figure 3(a)) and for a disk (Figure 3(b)), respectively. The calculations were performed using the Meep package (Meep: MIT Electromagnetic Equation Propagation). As shown in Figures 3(a) - 3(b), a ring resonator or a disk resonator, respectively, is placed within the evanescent field of a Si waveguide. At resonance, the wavelength multiplied by a mode number matches the periphery of the resonator (/ = 18 in the displayed results (empirically, the optimal circumferential mode number lies in the range of / = 1,000 and 8,000); mode confinement will be higher for larger mode numbers).

[0027] At resonance, the energy is absorbed by the resonator 202 and the transmitted energy through the waveguide drops. This is modeled as shown in Figures 4(a)-4(b), wherein the ratio of transmitted light and incident field in the bus waveguide is equal to:

Ipass

Ij _n with f is the single-pass phase shift, and a and r are the loss and self-coupling, respectively, show the resonances for 4p phase shift, for a 480 pm ring diameter (Figure 4(a)), and a for a racetrack resonator stretched between -/+100 nanometers (Figure 4(b)), respectively, for a 532 nm wavelength. The tunable range covers the range required to measure Doppler shifts in a LIDAR return. This is disclosed in Bogaerts, W. et ak,“ Silicon micro-ring resonators," Laser Photonics Rev., 6, No. 1, (2012).

[0028] Figures 5(a) and 5(b) show 3D models of a racetrack resonator according to the present invention. In the resonator element 500 as shown in Figure 5(a), the cross-section of the waveguide 502 and the resonator 504 mounted on a substrate 506 is between 5 and 10 micrometers in size and magnified here for visibility. The waveguide 502 is positioned adjacent the resonator 504 parallel to the straight, long side of the racetrack-shaped resonator 504, such that the evanescent field of the waveguide 502 couples into the resonator 504. The gap between the waveguide 502 and resonator 504 may be -100 nanometers. The racetrack width can range between 0.5 mm and 25 mm. Light is coupled into the waveguide 502 with pigtailed fibers. Alternatively, in the resonator element 500’ as shown in Figure 5(b), the cross-section of the waveguide 502’ and the resonator 504’ mounted on a substrate 506’ is also between 5 and 10 micrometers in size and magnified here for visibility. In this alternative embodiment, the waveguide 502’ is positioned along the curved side of the racetrack-shaped resonator 504’ and perpendicular to the straight, long side of the racetrack-shaped resonator 504 to avoid interference that can arise over long coupling sections, while the evanescent field of the waveguide 502’ couples into the resonator 504’ . Here also, the gap between the waveguide 502’ and resonator 504’ may be -100 nanometers. The racetrack width can also range between 0.5 mm and 25 mm. Light is also coupled into the waveguide 502’ with pigtailed fibers.

[0029] In a further embodiment of the present invention, the resonator element 500, as an example, is formed so as to be tunable, as discussed with respect to Figures 4(a) - 4(b). Specifically, resonator 504 is physically deformed so as to modify its dimensional characteristics and thus its resonance characteristics. In one implementation, the resonator 500, as shown in Figure 5(c), is deformable under a force applied to at least a single point or a tip 506a of the substrate 506. Force applied perpendicularly to, for example, the single point or tip 506a (as illustrated by the arrow F) bends the substrate 506 and stretches the resonator 504. The resonator may be shaped as a ring (not shown) or as a racetrack as shown in Figure 5(c). Alternatively, if a force were applied, for example, laterally parallel to the long side of the resonator 504, the substrate 506 may be deformed in an equivalent manner to the application of force at the single point or tip.

[0030] In at least one implementation for controllably deforming the substrate 506, Figure 5(d) shows the top view and Figure 5(e) shows the bottom view of the resonator 500, wherein the substrate 506 may be formed with a piezoelectric layer 510 at the bottom of the substrate 506. In this implementation, the waveguide 502 is formed parallel to the long side of the racetrack-shaped resonator 504 as in Figure 5(a). The piezoelectric layer 510 may be formed as piezoelectric electrodes 510a, 510b, each electrode having finger portions 511 that are interlaced with each other parallel to the long side of the resonator 504. When electrically charged, the interlaced finger portions 511 will apply a force perpendicular to the long side on the compliant substrate 506, alternatingly pulling in and pushing out on the substrate 506 under voltage. [0031] In a second implementation for controllably deforming the substrate, the resonator 500’, as shown in Figure 5(f), is deformable under a force applied to at least a single point or a tip 506b of the substrate 506’. Force applied perpendicularly to, for example, the single point or tip 506b (as illustrated by the arrow F’) bends the substrate 506’ and stretches the resonator 504’. Again, the resonator may be shaped as a ring (not shown) or as a racetrack as shown in Figure 5(f). Alternatively, if a force were applied, for example, laterally perpendicular to the long side of the resonator 504’, the substrate 506’ may be deformed in an equivalent manner to the application of force at the single point or tip. In this implementation, Figure 5(g) shows the top view and Figure 5(h) shows the bottom view of the resonator 500’, wherein the substrate 506’ may be formed with a piezoelectric layer 510’ at the bottom of the substrate 506’. In this second implementation, the waveguide 502’ is formed perpendicular to the long side of the racetrack-shaped resonator 504’ and adjacent to the curved of the racetrack-shaped resonator 504’ as in Figure 5(b). The piezoelectric layer 510’ may be formed as piezoelectric electrodes 510a’, 510b’, each electrode having finger portions 51 1’ that are interlaced with each other perpendicular to the long side of the resonator 504’. When electrically charged, the interlaced finger portions 51 G will apply a force parallel to the long side on the compliant substrate 506’ alternatingly pulling in and pushing out on the substrate 506’ under voltage.

[0032] Figure 6 shows the block diagram of the LIDAR module 600 according to the present invention. In this embodiment, the LIDAR module 600 incorporates a tunable resonator 602 which receives as an input 603 backscattered light collected from collecting optics 604, such as a telescope. As noted above, in at least one embodiment, the collecting optics 604 may be connected to backscattered light input 603 via pigtailed fibers, for example, into the tunable resonator 602 via the waveguide 602a. The tunable resonator 602 generates an output signal 605 in response to the input 603 and a driver signal 611 from a driver circuit 612. The output signal 605 is directed to a detector 606, such as photomultiplier tube (PMT), that generates a detector signal 607 based on the resonances detected from the output 605. The detector signal 607 is digitized by an A/D converter 608 and outputted to a data buffer 609, and then to a processor 610. The processor 610 receives the digitized output 609 of the A/D converter 608 via the data buffer 611. The processor 610 then generates the appropriate spectral information on Doppler shifts based on the resonance patterns in the backscattered light received from the collecting optics 604. A controller circuit 614 is provided to send control signals 615 to the A/D converter 608, the driver circuit 612 and a coherent light source 616, such as a laser. The coherent light source 616 generates the laser light 617 that is used to produce the backscattered light that becomes the input 603. The laser light 617 when generated by the light source 616 is put through a beam splitter 618 that splits a portion of the laser light 617 as part of the light inputted into the waveguide 602a of the resonator 602. Like the backscattered light input 603, the split portion of the laser light 617 may be inputted via the waveguide 602a using pigtailed fibers, as an example. For purposes of keeping the entire package compact, one implementation of the processor 610 may be the use of a processing circuit such as Raspberry Pi. In this at least one embodiment, the estimated size of the LIDAR module 600 may be less than 15 cm x 15 cm x 12 cm.

[0033] In operation, as the controller 614 controls the light source 616 to emit the laser light 617, the split portion of the laser light 617 and the backscattered light input 603 from the collecting optics 604 are inputted via the waveguide 602a of the tunable resonator 602. The resonator element 602b tuned by the controller 614 via drive signals from the driver circuit 612 generates dips in the transmission spectrum of the light energies propagating through it which indicates the Doppler shifts in the backscattered light input 603. The transmission spectrum generated in the tunable resonator 602 is then inputted into the detector or PMT 606 and then into the A/D converter 608 to then generate the digitized output 609. The digitized output 609 is then inputted into the data buffer 611 and then processed by the processor 610 into Doppler shift data that may be used to, among other parameters, line-of-sight wind speed and direction, atmospheric density, and wind velocity and density. The Doppler shift data from the processor 610 may also be used by the controller 614 to generate and adjust the controller signals 615 used to control the driver circuit (which then tunes the resonator 602), the A/D converter 608 and the light source 616. As a result, the controller 614 can scan across the wavelength range of the Doppler shift data and determine resonance locations via the tuning performed through the driver circuit 612.

[0034] With respect to the embodiment using deformation of, for example, the substrate 506 of the tunable resonator 500 shown in Figure 5(a) to tune the resonator 502, in the implementation of the tuning mechanism using the piezoelectric layer 510, the deformation in terms of the amount the substrate 506 needs to be deformed may be in the range of 100 pm or less for most applications of the LIDAR module of the present invention. It has been found that conventional piezoelectric materials that can be formed as the piezoelectric layer 510 can deform under voltage to stretch the substrate easily over such a range. For instance, for a 1 cm diameter ring-shaped resonator element or a racetrack-shaped resonator element having a curved portion with a 1 cm diameter, n=2.6, and a wavelength of 1.3 x 10-6 m , the free spectral range (FSR) = 20.7 pm. If for example the radius of the resonator is stretched 1 um by the deformation of the substrate, the resonance shift will be 1.17 pm.

[0035] In summary, the sensor according to the invention will:

• Be capable of fulfilling the same functions as a Fabry-Perot interferometer,

• Operate with and without aerosols present in the atmosphere, thus enabling operation in completely clear atmosphere,

• Be free of alignment and thermal stability requirements, and

• Be extremely compact, rugged, and suitable for wind measurements from space.

[0036] Although the present invention has been fully described in connection with the preferred embodiment thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.