On chip suspended waveguide based FIM-FAM device

Abstract

A method for generating a Brillouin signal resonance response by forward inter-modal Brillouin scattering with fundamental acoustic modes (FIM-FAM), comprising: generating a first optical mode in said first input optical fiber as described herein, wherein the first optical mode is a fundamental optical mode; generating a second optical mode in said second input optical fiber as described herein, wherein the second optical mode is a higher-order optical mode; coupling through said mode combiner as described herein the first optical mode to the second optical mode to become coupled optical modes; driving a fundamental acoustic mode in said multi-mode FIM-FAM active waveguide as described herein, wherein at least one Brillouin signal resonance response is generated within said multi-mode FIM-FAM active waveguide.

Description

DOCKET NO: 1134-203 PCT TITLE ON CHIP SUSPENDED WAVEGUIDE BASED FIM-FAM DEVICE

[0001] This invention was made with government support under ECCS-1943658 awarded by the National Science Foundation and N000142312704 and N000142412012 awarded by the Office of Naval Research. The government has certain rights in the invention.

[0002] This application claims priority from U.S. Provisional Application No. 63 / 586,595, filed September 29, 2023, which is incorporated herein by reference. FIELD

[0003] This application relates to the field of optomechanical interactions and, in particular, the generation of stimulated Brillouin scattering (SBS). BACKGROUND

[0004] Brillouin interactions date back 100 years with essential applications for lasers, delay lines, sensing, and fast, tunable and high resolution microwave photonic filters. In short, optical and acoustic waves couple through electrostrictive forces where a beat wave between two optical waves induces a density variation in the material which propagates at the speed of sound and modulates the refractive index which then reflects and Doppler shifts one optical tone to amplify the other optical tone. It is a lossless parametric interaction for which the interaction frequency is determined by simple energy and momentum conservation laws. In the standard configuration, the acoustic frequency is lower than the optical frequency by the ratio of the light speed to sound speed, giving ~10 GHz for most solids. The linewidth of the response is given by the decay rate of the acoustic waves which for this high frequency is ~40 MHz linewidth. Since the acoustic-wave parameters are fixed by the material, the linewidth response and the performance of devices based on previous interactions is fundamentally limited. SUMMARY

[0005] One aspect of the application relates an integrated platform with an on-chip forward inter-modal Brillouin scattering with fundamental acoustic modes (FIM-FAM) device, comprising: an input mode converter; a first single mode input waveguide, wherein said first single mode input waveguide is connected to said input mode converter; a second single mode input waveguide, wherein said second single mode input waveguide is connected to said input mode converter; a mode combiner, wherein said mode combiner is connected toboth the first single mode input waveguide and the second single mode input waveguide; an input multi-mode waveguide, wherein said input multi-mode waveguide is connected to an output from said mode combiner; an on-chip FIM-FAM device, wherein said on-chip FIM FAM device comprises a multi-mode FIM-FAM active waveguide that is axially homogeneous, and wherein said on-chip FIM-FAM device is connected to the mode combiner via an input from said input multi-mode waveguide to said on-chip FIM-FAM device; an output multi-mode waveguide, wherein said output multi-mode waveguide is connected to an output from said on-chip FIM-FAM device; a mode splitter, wherein said mode splitter is connected to said on-chip FIM-FAM device via said output multi-mode waveguide; an output mode converter, wherein said output mode converter is connected to said mode splitter by a first output single mode waveguide and a second output single mode waveguide; and said first output single mode waveguide comprising a first output adiabatic taper and said second output single mode waveguide comprising a second output adiabatic taper, wherein said first output adiabatic taper and said second output adiabatic taper are adiabatic for different spatial optical modes.

[0006] Another aspect of the application relates to a system for generating a Brillouin signal resonance response by forward inter-modal Brillouin scattering with fundamental acoustic modes (FIM-FAM), comprising: an integrated platform with an on-chip FIM-FAM device as described herein; a first input optical fiber coupled to said first input mode converter; a second input optical fiber coupled to said second input mode converter; a first output optical fiber coupled to said first output mode converter; a second output optical fiber coupled to said second output mode converter.

[0007] Another aspect of the application relates to a method for generating a Brillouin signal resonance response by forward inter-modal Brillouin scattering with fundamental acoustic modes (FIM-FAM), comprising: generating a first optical mode in said first input optical fiber as described herein, wherein the first optical mode is a fundamental optical mode; generating a second optical mode in said second input optical fiber oas described herein, wherein the second optical mode is a higher-order optical mode; coupling through said mode combiner as described herein the first optical mode to the second optical mode to become coupled optical modes; driving a fundamental acoustic mode in said multi-mode FIM-FAM active waveguide as described herein, wherein at least one Brillouin signal resonance response is generated within said multi-mode FIM-FAM active waveguide.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG.1 shows two counter propagating optical fields, pump and Stokes are coupled via a freely propagating longitudinal acoustic wave.

[0009] FIG.2 shows Backward Brillouin interaction. (Upper Panel) acoustic dispersion with the optical driving wavevector overlaid. The intersection point is the allowedinteraction, where the phonon has an approximate wavevector magnitude of |^^^^| ≈ 2^^^^^^^^.(Lower Panel) Measurements from the PI’s group of the standard high frequency 37-MHz response from an optical fiber.

[0010] FIG.3 shows (Panel a) Phase matching and energy conservation diagrams for anti-Stokes scattering process. (Panel b) Frequency response of a backward Brillouin system showing gain and loss bands created in the presence of a strong optical pump.

[0011] FIG.4 shows Forward Intermodal (FIM) Brillouin scattering. (Panel a) Two co-propagating optical fields, pump and stokes, in different spatial optical modes are coupled either through a higher order guided acoustic mode (non-tunable FIM) or fundamental acoustic modes (FIM-FAM). (Panel b) The acoustic dispersion diagram showing phase matching with higher-order acoustic modes for well-known non-tunable FIM processes and the proposed FIM-FAM interactions.

[0012] FIG.5 shows a taper-based FIM-FAM device. (Panel a) An illustration of the FIM-FAM process in a few-mode taper. Regions of the taper are labeled above each section. Pump (^^^^^^^^) and Stoke (^^^^^^^^) beams input respectively into the fundamental and higher-order spatial modes of the input fiber core couple through the transition region into the fundamental mode and higher-order mode family of the taper waist, where the pump light is optomechanically coupled to the Stokes field through the FIM-FAM process before being coupled out of the device in the two modes of the output fiber core through the second transition region. The phase-matching relations for the FIM-FAM interaction indicating the optical wavevector difference between the axial wavevector of the two participating optical modes, Δ^^^^, must equal the acoustic wavevector, ^^^^, is inset below the device. (Panel b) The acoustic dispersion profiles for the modes of the taper waist. While the higher-order acousticmodes (magenta) have a fixed nonzero cutoff frequency at ^^^^ = 0, the fundamental modefrequencies (Ω) and wavevectors (^^^^) extend continuously to zero (cyan). Optomechanical interactions are possible at frequencies where the optical wavevector difference (Δ^^^^) intersects the acoustic dispersion curves. In contrast to interactions with the higher-ordermodes, FIM-FAM interactions with the fundamental modes can be tuned in frequency by varying the optical wavevector difference, Δ^^^^.

[0013] FIG.6 shows the wide new range of acoustic frequencies, lifetimes, and linewidths that can be accessed through FIM-FAM optomechanical interactions.

[0014] FIG.7 shows (Panel a) integrated FIM-FAM waveguided and (Panel b) simulated acoustic frequency as a function of cross-sectional dimensions.

[0015] FIG.8 shows on-chip FIM-FAM device. Light is coupled from standard single mode fibers and the mode profile to enhance the Brillouin gain is engineered on chip.

[0016] FIG.9 shows (Upper Panel): Simulations showing phase matching for the TE0 mode and the TE1 modes for waveguides of 310nm and 1200nm width, respectively. The height is 350nm. (Lower Panel): FEM calculation showing 100% coupling of TE0 mode to TE1 mode.

[0017] FIG.10 shows panels (a) to (f) fabrication process for the Si3N4FIM-FAM platform.

[0018] FIG.11 shows detuning of higher-order acoustic mode frequencies under various conditions, distinguished in different colors. (Panel a) With a fixed waveguide height of 320 nm and zero wavevector, and the waveguide width changes from 1250 nm to 1350 nm. (Panel b) With a fixed waveguide width of 1300 nm and zero wavevector, and the waveguide height changes from 300 nm to 340 nm. (Panel c) The dispersion curves of the acoustic modes, including both fundamental and higher-order modes as a function of the acoustic wavevector q. The waveguide geometry is fixed at 1300 nm × 320 nm.

[0019] FIG.12 shows characterizing an on-chip multimode waveguide. A broadband source is focused into the device under test (DUT), in this case it is an on-chip multimode waveguide. The incident field excites multiple spatial modes in the waveguide. The resultant exiting field is imaged using a couple of onto a sing mode fiber (SMF) probe and analyzed on an optical spectrum analyzer (OSA).

[0020] FIG.13 shows (Panel a) The electric field the ^^^^^^^^1and (Panel b) ^^^^^^^^0modes and (Panel c) the displacement field of the fundamental flexural acoustic mode for FIM-FAM interactions in integrated Si3N4waveguides with 1200nm width and 350nm height. (Panel d)- f) Zoom-in of the top row.

[0021] FIG.14 shows torsional acoustic mode driven by ^^^^^^^^0and ^^^^^^^^0modes

[0022] FIG.15 shows an outline design for the device described herein.

[0023] FIG.16 shows guided optical and acoustic modes designed for the FIM-FAM process in triple-suspended waveguides. (Panel a) The TE1 optical mode. (Panel b) The TM0optical mode. (Panel c) The y-polarized fundamental flexural acoustic mode in the main waveguide. (Panel d) The y-polarized fundamental flexural acoustic mode in one of the side waveguides.

[0024] FIG.17 shows (a) scanning electron microscopic images of the fabricated triple suspended FIM-FAM devices. Left Panel, simple triple suspended waveguides closed spaced horizontally to each other. Right Panel, several horizontal anchors periodically attached to the side of the side waveguides; (b) shows a view of a triple suspended waveguide from a side angle.

[0025] FIG.18 shows FIM-FAM characterizations of a typical triple suspended waveguide without any anchor. Upper Panel, Measured FIM-FAM spectra of the phase- matched sideband. The Fano-like FIM-FAM resonances are emphasized in different colors. Multiple FIM-FAM peaks with sub-MHz level linewidths are measured with FIM-FAM gain as high as ~ 3000 W-1m-1 in the 0-40 MHz region. Lower Panel, simulated FIM-FAM resonances between different pairs of optical modes and different acoustic modes in the system.

[0026] FIG.19 shows FIM-FAM characterizations of a typical triple suspended waveguide with one anchor at the middle of each side waveguide. Upper Panel, Measured FIM-FAM spectra of the phase-matched sideband. The Fano-like FIM-FAM resonances are emphasized in different colors. Multiple FIM-FAM peaks with sub-MHz level linewidths are measured in the 0-50 MHz, as well as the beyond 150 MHz regions. Lower Panel, simulated FIM-FAM resonances between different pairs of optical modes and different acoustic modes in the system. DETAILED DESCRIPTION

[0027] Reference will be made in detail to certain aspects and exemplary embodiments of the application, illustrating examples in the accompanying structures and figures. The aspects of the application will be described in conjunction with the exemplary embodiments, including methods, materials and examples, such description is non-limiting and the scope of the application is intended to encompass all equivalents, alternatives, and modifications, either generally known, or incorporated here. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. One of skill in the art will recognize many techniques and materials similar or equivalent to those described here, which could be used in the practice of the aspects and embodiments of the present application. Thedescribed aspects and embodiments of the application are not limited to the methods and materials described.

[0028] As used in this specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless the content clearly dictates otherwise.

[0029] Ranges may be expressed herein as from "about" one particular value, and / or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as "about" that particular value in addition to the value itself. For example, if the value "10" is disclosed, then "about 10" is also disclosed. It is also understood that when a value is disclosed that "less than or equal to" the value, "greater than or equal to the value" are also disclosed, as appropriately understood by the skilled artisan. For example, if the value "10" is disclosed, the "less than or equal to 10" and “greater or equal to 10” is also disclosed. When two or more value are disclosed, all possible ranges between any two values are disclosed. Brillouin Scattering

[0030] In Brillouin scattering, optical and acoustic waves couple through electrostrictive forces (materials become compressed in the presence of a strong electric field gradient). The beat wave between two counter-propagating optical waves with a small frequency difference induces a density variation in the material (Fig.1). For the appropriate difference frequency, the density variation will propagate at the speed of sound. This periodic density variation also modifies the refractive index with the same spatial period, which reflects and Doppler shifts the pump light to the frequency of the counter-propagating probe light. In other words, the two optical waves generate an acoustic wave, which then amplifies the probe light.

[0031] Brillouin has now been demonstrated in many systems, with useful devices developed in fiber and integrated waveguides. Brillouin has been proven effective for reversing atmospheric distortions in high power lasers, increasing spatial optical coherence, enabling ultranarrow laser sources, and creating optical delay lines. Brillouin is also a very effective sensor for strain and temperature and has more recently seen growing impact for anew form of elastic sensing in tissues. Finally, Brillouin scattering enables fast, tunable and high resolution optical filters that are well-suited for microwave photonic systems. Backward Brillouin Scattering

[0032] Energy and momentum conservation laws determine whether the interaction is allowed and if so, what the frequency will be. By understanding these simple conservation laws, it can be seen why the frequency of traditional backward interactions is fixed, and why FIM-FAM interactions are extremely flexible. Considering an optical pump wave with frequency ωp, and wavevector (or momentum) ^^^^^^^^, an optical probe (from now on called Stokes) wave with frequency ^^^^^^^^, and wavevector ^^^^^^^^, and an acoustic wave with frequency Ω^^^^, and wavevector ^^^^^^^^. The conservation equations can be written simply as: ^^^^^^^^ = ^^^^^^^^ + Ω^^^^ (1a)

[0033] In a waveguide these wavevectors are simplified to scalar quantities for the wavevector component along the waveguide axis. Generally, these two equations can be solved by plugging Eq.1a into 1b and solving for Ω^^^^. The resulting transcendental equation,using the optical wavevector difference, Δ^^^^ = ^�^^⃗^ ^^^^ − ^�^^⃗^ ^^^^, is expressed as Δ^^^^=^^^^(Ω^^^^). Because the acoustic dispersion can become complicated (e.g. in waveguides) it is convenient to solve this equation graphically by plotting the optical wavevector difference, Δ^^^^ , on a plot of the acoustic dispersion, ^^^^(Ω^^^^), and identifying the allowed interactions as the points of intersection (e.g. Fig.2, upper panel).

[0034] In the traditional backward interaction, the optical pump and Stokes fields are counter-propagating, which can be represented by adding a negative sign to the counter-propagating Stokes wavevector. In this case Δ^^^^=≈ 2^^^^^^^^(^^^^^^^^). The Ω^^^^ dependence is ignored because the acousticfrequency (~10 GHz) is orders of magnitude smaller than the optical frequencies(~200 THz). Energy and momentum conservation, Δ^^^^(^^^^^^^^,Ω^^^^) = ^^^^(Ω^^^^), for a fixedoptical pump frequency (^^^^^^^^), now tell us that the acoustic wave accessed by the backwardinteraction simply has to satisfy ^^^^(Ω^^^^) = 2^^^^^^^^. The dispersion for freely propagatingacoustic waves is ^^^^(Ω^^^^) =^^^^^^^^ is the acoustic velocity. Graphically, thisacoustic dispersion (^^^^(Ω^^^^)) line is plotted with slope ^^^^^^^^and the wavevector difference isplotted as a single value of ^^^^, at 2^^^^^^^^. The intersection point determines the allowedBrillouin interaction aswhere ^^^^ is the refractive index of the medium and ^^^^0is the optical wavelength of the pump.

[0035] The acoustic frequency for standard backward Brillouin interactions for most solids is ~10 GHz for near-infrared optical pumping. This backward interaction occurs with the most ubiquitous freely propagating acoustic waves, which also explains why this is the most common and well-studied interaction. The backward interaction also occurs in acoustic waveguides, but because the acoustic wavevector is so large, the waveguide does not significantly alter the response from the bulk case. An example response measured in the PI’s lab of the most common standard single-mode fiber yields a strong peak at 10.8 GHz, with several much weaker peaks from higher order acoustic modes (Fig.2, lower panel). The linewidth of the response given by the decay rate of the acoustic waves is largely determined by the frequency of the interaction, with higher frequencies giving broader linewidths. The 37 MHz linewidth from Fig.3b is representative of the high frequency backward interactions and corresponds to acoustic waves that are short lived, decaying after only 100 microns in the fiber. Since the acoustic-wave parameters are fixed by the material, the linewidth response and the performance of devices based on standard backward interactions is fundamentally limited by intrinsic material constraints. Overcoming The Limitations Of Backward Brillouin Scattering

[0036] The limitations of traditional backward Brillouin scattering can be overcome by engineering the momentum through appropriately designed optical and acoustic waveguides.

[0037] The Brillouin coupling strength is quantified by a gain coefficient, ^^^^0, which depends on system parameters including the optical wavelength, refractive index, speed of sound, density of the medium and its electrostrictive constant. The gain coefficient quantifies how much amplification the Stokes optical field will experience through ^^^^^^^^^^^^^^^^ ^^^^^^^^ ^^^^0^^^^^^^^^^^^^^^^ = ^^^^^^^^ ^^^^ ,

[0038] where ^^^^^^^^^^^^^^^^is the initial Stokes power, ^^^^^^^^^^^^^^^^^^^^is the final Stokes power, ^^^^^^^^is the pump power, and ^^^^ is the length of the Brillouin active medium. ^^^^0is in units ofsuch that the exponential’s power is unitless. The overall amplification is increased by increasing the pump power, ^^^^^^^^, the device length, ^^^^, or the intrinsic Brillouin gain, ^^^^0. Therefore, higher Brillouin coupling strength leads to comparable devices with shorter lengths and lower power requirements.

[0039] Note that energy flow in a Brillouin interaction is one-directional, from the pump to the Stokes field. It does not oscillate between the pump and Stokes optical fields, even if the pump is heavily depleted. The phonons generated through the Brillouin process have a finite lifetime typically much smaller than optical lifetimes. New pump photons can be only created when the Stokes photon absorbs a phonon to create a pump photon, thereby transferring energy into the pump field. Phonons generated through the Brillouin process are lost comparatively quickly and consequently, the pump field continually loses energy, even if it already is depleted.

[0040] In the Stokes processes described so far, energy is transferred from the optical pump to an acoustic wave and a redshifted optical Stokes tone. Alternatively, in the anti- Stokes process energy from an acoustic wave combines with energy from a pump to create a higher energy, blue-shifted (higher frequency) anti-Stokes optical tone. Importantly, the acoustic wave mediating the anti-Stokes process propagates in the opposite direction to that from the Stokes process (Fig.3). In the absence of any external acoustic waves, the anti- Stokes tone experiences exponential loss as it propagates, in contrast to exponential gain experienced by the Stokes tone, and its output power (^^^^^^^^^^^^^^^^^^^^^^^^) can be written as ^^^ ^ −^^^^ ^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^ = ^^^^^^^^^^^^^^^^^^^^ ^^^^ 0 ^^^^ . (3)

[0041] Selecting between Stokes / anti-Stokes process forms the basis of Brillouin based applications including lasers, amplifiers, and filtes; anti-Stokes processes are well- suited to notch filtering applications.

[0042] With a single optical mode, pump and Stokes light propagating in the same direction can couple to guided acoustic waves, leading to so-called forward Intra-modal Brillouin interactions. While these interactions open up new frequencies and have enabled new devices, as described in more detail below, they are still limited in frequency, linewidth and coupling, but now by the device geometry. Forward Inter-modal Brillouin Scattering

[0043] With two optical modes with different effective indices much more flexibility becomes available for Brillouin interactions. For a pump propagating in a mode with refractive index ^^^^1and the Stokes field propagating in another mode with refractive index ^^^^2,the optical wavevector difference becomes Δ^^^^ = ^^^^^^^^ − ^^^^^^^^ ≈ (^^^^1 − ^^^^2)^^^^0 = ^^^^^^^^^^^^0, where ^^^^^^^^is the difference in refractive index of the two optical modes. Let’s consider a two-mode waveguide for light and a solid cylinder for acoustic guidance (Fig.4, Panel a). To determine the available interactions, from our analysis above, we look at the dispersion of all of theacoustic modes (Fig.4, Panel b) and look for all of the points where the single value of Δ^^^^ (Fig.4, Panel b) intersects these curves.

[0044] Although this process uses the same acoustic higher order modes as intra- modal scattering (upper oval region in Fig.4, Panel b), the Stokes and anti-Stokes process in inter-modal scattering uses acoustic waves traveling in opposite directions which is advantageous for applications requiring single-sideband responses. However, like the intramodal interactions, intermodal interactions with the higher order modes are higher frequency with negligible tunability because the higher order modes do not vary in frequency with wavevector.

[0045] Using standard fiber geometries, the interaction strength of forward Brillouin is very weak because the light in the fiber core has a very small overlap with the acoustic mode extending out through the cladding. Forward Inter-modal Brillouin Scattering with Fundamental Acoustic Modes (FIM- FAM)

[0046] The fundamental acoustic modes exist at all frequencies in bulk and waveguide systems and do not have the lower frequency bound that limits the higher-order acoustic modes. Without a strong dependence of the frequency on the geometry, the fundamental acoustic modes offer a unique opportunity to decouple the frequency (and linewidth) from the confinement and therefore strength of the interaction. As can be seen from the vertical graph line in Fig.4, Panel b, the frequency of the interaction can be tuned (dots in Fig.4, Panel b) by the choice of relative effective indices of the participating opticalmodes (Δ^^^^ = ^^^^^^^^^^^^0).

[0047] FIM-FAM is a coherent optomechanical process through which a fundamental acoustic mode with frequency Ω and wavevector ^^^^ mediates parametric coupling between two distinct optical spatial modes (pump with frequency ^^^^^^^^and wavevector ^^^^^^^^, and Stokes with frequency ^^^^^^^^and wavevector ^^^^^^^^). For coupling to occur, the interaction must satisfyphase matching (Δ^^^^ = ^^^^^^^^ − ^^^^^^^^ = ^^^^) and energy conservation (Ω = ^^^^^^^^ − ^^^^^^^^). Theseconditions can be succinctly expressed graphically by examining the acoustic dispersion lines (^^^^(Ω)) and the difference in optical wavevectors between the two optical modes, Δ^^^^ (Fig.5, Panel b). In this picture, interactions are possible at the frequencies where these lines intersect. Intermodal Brillouin interactions of this type allow for stimulated gain, single- sideband amplification, and non-reciprocal processes because of distinct phonon modes mediating Stokes and anti-Stokes processes. In addition, because the interaction frequency isdetermined by Δ^^^^, by engineering the differential effective index of the participating optical modes, Δ^^^^, the frequency of the interaction, Ω, can be tuned. However, as illustrated in Fig. 5b, when changing Δ^^^^ for the higher-order modes (Fig.5, Panel b) that are the subject ofprevious studies of intermodal Brillouin scattering, the frequency remains close to its ^^^^ = 0value, which is fixed by the geometry. In contrast, with the fundamental acoustic modes (Fig.5, Panel b), there is no low-frequency cutoff, and all frequencies become available. FIM-FAM, therefore, offers a wide new window of opportunity for traveling-wave optomechanical interactions (see also Xu et al., Optica, Vol.10, No.2, pp.206-213, Feb. 2023, incorporated herein by reference).

[0048] Novel device design techniques are required to achieve strong confinement of acoustic waves with the long wavelengths needed for large acoustic lifetimes. However, if strong optical coupling can be achieved with the frequency-agile fundamental acoustic modes, this versatile optomechanical interaction can enable unprecedented access to simultaneous strong coupling and the narrow linewidths associated with lower frequency modes (Fig.6). An on-chip forward inter-modal Brillouin scattering with fundamental acoustic modes (FIM-FAM) device

[0049] An aspect of the application is an integrated platform with an on-chip forward inter-modal Brillouin scattering with fundamental acoustic modes (FIM-FAM) device, comprising: an input mode converter; a first single mode input waveguide, wherein said first single mode input waveguide is connected to said input mode converter; a second single mode input waveguide, wherein said second single mode input waveguide is connected to said input mode converter; a mode combiner, wherein said mode combiner is connected to both the first single mode input waveguide and the second single mode input waveguide; an input multi-mode waveguide, wherein said input multi-mode waveguide is connected to an output from said mode combiner; an on-chip FIM-FAM device, wherein said on-chip FIM FAM device comprises a multi-mode FIM-FAM active waveguide that is axially homogeneous, and wherein said on-chip FIM-FAM device is connected to the mode combiner via an input from said input multi-mode waveguide to said on-chip FIM-FAM device; an output multi-mode waveguide, wherein said output multi-mode waveguide is connected to an output from said on-chip FIM-FAM device; a mode splitter, wherein said mode splitter is connected to said on-chip FIM-FAM device via said output multi-mode waveguide; an output mode converter, wherein said output mode converter is connected tosaid mode splitter by a first output single mode waveguide and a second output single mode waveguide; and said first output single mode waveguide comprising a first output adiabatic taper and said second output single mode waveguide comprising a second output adiabatic taper, wherein said first output adiabatic taper and said second output adiabatic taper are adiabatic for different spatial optical modes.

[0050] In certain embodiments, the multi-mode FIM-FAM active waveguide is a two- mode waveguide.

[0051] In certain embodiments, the FIM-FAM active waveguide is suspended; wherein any other waveguides waveguides comprise all on-chip and combiner / coupler waveguides, and the other waveguides may be structurally one or more selected from the group comprising no top cladding, buried and suspended.

[0052] In certain embodiments, the first input single mode waveguide comprising a first input adiabatic taper and said second input single mode waveguide comprising a second input adiabatic taper, wherein said first input adiabatic taper and said second input adiabatic taper are adiabatic for different spatial optical modes.

[0053] In certain embodiments, the multi-mode FIM-FAM active waveguide is suspended.

[0054] In certain embodiments, the multi-mode FIM-FAM device has specific height and width dimensions selected to support driving a flexural fundamental acoustic mode.

[0055] In certain embodiments, the multi-mode FIM-FAM device has specific height and width dimensions selected to support driving a torsional fundamental acoustic mode.

[0056] In certain embodiments, the multi-mode FIM-FAM device has specific height and width dimensions selected to support driving a longitudinal fundamental acoustic mode.

[0057] In certain embodiments, the multi-mode FIM-FAM active waveguide is made of silicon nitride (Si3N4).

[0058] In certain embodiments, the multi-mode FIM-FAM active waveguide is made of silicon dioxide (SiO2).

[0059] In certain embodiments, the multi-mode FIM-FAM active waveguide is made of silicon (Si).

[0060] In certain embodiments, the integrated platform further comprises: a silicon wafer; a silicon oxide layer, wherein said silicon oxide layer is layered over said silicon wafer; a plurality of silicon nitride waveguides.

[0061] In certain embodiments, the integrated platform further comprises: a silicon wafer; a plurality of silicon oxide waveguides, wherein said silicon oxide waveguides are all suspended waveguides.

[0062] In certain embodiments, integrated platform further comprises: a silicon-on- insulator (SOI) wafer; a silicon device layer, wherein said silicon device layer is layered over said SOI wafer on buried silicon dioxide; a plurality of silicon waveguides. A system for generating a Brillouin signal resonance response

[0063] Another aspect of the application is a system for generating a Brillouin signal resonance response by forward inter-modal Brillouin scattering with fundamental acoustic modes (FIM-FAM), comprising: an integrated platform with an on-chip FIM-FAM device as described herein; a first input optical fiber coupled to said first input mode converter; a second input optical fiber coupled to said second input mode converter; a first output optical fiber coupled to said first output mode converter; a second output optical fiber coupled to said second output mode converter. A method for generating a Brillouin signal resonance response

[0064] Another aspect of the application is a method for generating a Brillouin signal resonance response by forward inter-modal Brillouin scattering with fundamental acoustic modes (FIM-FAM), comprising: generating a first optical mode in said first input optical fiber as described herein, wherein the first optical mode is a fundamental optical mode; generating a second optical mode in said second input optical fiber as described herein, wherein the second optical mode is a higher-order optical mode; coupling through said mode combiner as described herein the first optical mode to the second optical mode to become coupled optical modes; driving a fundamental acoustic mode in said multi-mode FIM-FAM active waveguide as described herein, wherein at least one Brillouin signal resonance response is generated within said multi-mode FIM-FAM active waveguide.

[0065] In certain embodiments, the fundamental acoustic mode is a flexural mode.

[0066] In certain embodiments, the fundamental acoustic mode is a torsional mode.

[0067] In certain embodiments, the fundamental acoustic mode is a longitudinal mode.

[0068] The present application is further illustrated by the following examples that should not be construed as limiting. The contents of all references, patents, and published patent applications cited throughout this application, as well as the Figures and Tables, are incorporated herein by reference.EXAMPLES Example 1: Integrated FIM-FAM development in Si3N4, SiO2, and Si material platforms

[0069] Integrated devices combine exquisite control over device parameters with a robust, compact and widely producible platform lead to new physics and more desirable devices for applications. With this thrust, we will develop FIM-FAM devices in integrated platforms enabling complete exploration of fundamental performance limits and versatile application-customizable devices. The integrated devices will feature suspended waveguides which support both optical and acoustic modes in a simple rectangular cross-section with a solid-air boundary (Fig.7). Full FEM simulations are run modeling FIM-FAM devices in three suspended material candidates, silicon nitride (Si3N4), silicon dioxide (SiO2), and silicon (Si). Cross-sections designed to support the first three modes with a realistic 20nm fabrication tolerance for FIM-FAM at 1MHz all yield incredible gains of a million per Watt per meter, which is 3-4 orders higher than previous Brillouin platforms. These higher gains also enable much lower powers and shorter effective waveguides for applications.

[0070] The on-chip design includes mode converters, combiners, in addition to the FIM-FAM active waveguides. The ratio between spatial optical modes is controlled by coupling light into single mode waveguides at the edge of the chip and converting the light through a directional coupler that is phase matched to the desired mode. The on-chip FIM- FAM devices will be fabricated on standard photonic platforms that are compatible with CMOS processing. Advanced but established undercutting techniques can be used for each platform. These can be designed with kHz acoustic frequencies where the gain is anticipated to grow by more than another order of magnitude and the linewidth is anticipated to drop below the kHz level, where new acoustic leakage mechanisms are expected to play a role.

[0071] An initial design for FIM-FAM operation is a suspended waveguide structure in which both optical and acoustic modes are supported by a simple rectangular cross-section with a solid-air boundary (Fig.7). Starting from a square geometry small enough that only TE0and TM0are well guided, increasing the width enables additional TE1guidance as the effective refractive indices of both TE0and TE1modes increase. Allowing this additional mode allows for the intermodal interactions required by FIM-FAM. Interestingly, if the contrast between width and height is large enough, the neffof the TE1mode will approach that of the TM0mode and eventually even surpass it. This means that for intermodal interactions with the flexural mode, Δk can be tuned arbitrarily through zero with simplegeometry variations, which allows for complete tunability of the FIM-FAM frequency (Fig. 7, Panel b). Exampe 2: Manufacture

[0072] The on-chip design includes mode converters, combiners, in addition to the FIM-FAM active waveguides (Fig.8). The ratio between spatial optical modes is controlled by coupling light into single mode waveguides at the edge of the chip and converting the light through a directional coupler that is phase matched to the desired mode. For example, for a silicon nitride waveguide with a thickness of 350nm and width of 1200nm that supports the TM0 and TE1 optical modes, we will couple light onto the chip from two optical fibers to two waveguides with an oxide mode converter / inverse nanotaper combination for optimum coupling efficiency. One waveguide will adiabatically taper to a width of 1200nm. The second waveguide will adiabatically taper to a width of 310nm. The simulations show that the TE0 of the second waveguide is phase matched to the TE1 mode of the original waveguide at this width (Fig.9). The study will evanescently couple the TE0 mode into the TE1 mode using a directional coupler. This coupler is based on previous work manipulating higher order modes in photonic devices. By controlling the coupling between the modes we can engineer any mode combination and power ratios needed. To couple light in and out of the chip with high efficiency, we can use oxide mode converters to attach an array of optical fibers to the photonic chip. The oxide mode converters improve the optical mode matching between an optical fiber and the photonic waveguide. They also provide an ideal point to fuse the fiber directly to the chip avoiding the use of optical adhesives.

[0073] For Silicon Nitride the study will grow 4µm of thermal oxide on a silicon wafer (Fig.10, Panel a) and then deposit 300 to 500nm (depending on the design) of silicon nitride using low pressure chemical vapor deposition (LPCVD) at 800°C (Fig.10, Panel b). The study patterns the silicon nitride waveguides using electron beam lithography on MaN 2405 ebeam resist, which is negative tone. The study etches the nitride in an inductively coupled reactive ion etcher (ICP-RIE) using a tetrafluoromethane and oxygen (CHF3 / O2) chemistry to achieve vertical sidewalls (Fig.10, Panel c). The study deposits two microns of silicon dioxide using plasma enhanced chemical vapor deposition (PECV) to clad the device and add oxide mode converters (Fig.10, Panel d). Next, the study uses photolithography to open a region to suspend the silicon nitride waveguides and enable the acoustic modes (Fig. 10, Panel e). After patterning, the study suspends the silicon nitride waveguides by immersing the sample in dilute hydrofluoric acid to isotropically etch the silicon dioxide followed by drying the sample in a critical point drying to prevent stiction of the suspendedsilicon nitride waveguides (Fig.10, Panel f). (If the suspended region is long, during the water drying period, the droplet sticks to the bottom and the suspended waveguide van der Waals forces, which is not desired. Critical point drying is not needed for short, suspended regions as we are currently investigating.)

[0074] For Silicon Dioxide the study thermally grow the desired thickness of silicon dioxide on a silicon wafer (330nm or 650nm depending on the design). Since the oxide has a lower refractive index than the silicon substrate, the entire waveguide structure must be suspended to enable optical and acoustic mode guiding. Thus, in this case we will use anchor pads to hold the waveguides. The anchors are placed at multiple locations with a random spacing to minimize the generation of standing waveguides. The study uses inverse tapers to maximize the optical coupling from the fibers onto the photonic chip. The waveguides are patterned using electron beam lithography and etched in an ICP-RIE. Finally, the study suspends all the waveguides using Xenon Difluoride (XeF2) etching to undercut the silicon. Since the XeF2 is a dry process, there’s no need for critical point drying (CPD). An alternative undercut method is immersion in hot Phosphoric acid followed by CPD.

[0075] For the Silicon device, the study starts from a silicon on insulator wafer (SOI) with a device layer (i.e., silicon) thickness of 240nm on 3µm of buried silicon dioxide. The thickness of the silicon device layer can be adjusted through thermal oxidation to reduce to, for example, 210nm. The waveguide devices are patterned through electron beam lithography and etched in Hydrogen Bromide (HBr) in an ICP-RIE. After cladding and patterning the oxide mode converter, the study uses photolithography to open a window to undercut the silicon waveguide and enable the acoustic mode propagation. After patterning, the study undercuts the silicon using vapor HF or immersion in HF in a process similar to the one used for the silicon nitride platform.

[0076] Because each of the three material platforms is anticipated to have similar performance, after the first round of fabrication, the study will down-select for the most promising platform based on fabrication challenges as well as characterization of the FIM- FAM responses. Using the down-selected platform the study will redesign as described above to push the accessible acoustic frequency to its lower limit to investigate FIM-FAM performance in the extreme limit of high gain and ultranarrow linewidth. Example 3 Geometry and optical and acoustic modal analysis

[0077] Brillouin measurements enable characterization of the waveguide geometry. The suspended waveguide yields intra-modal or inter-modal optomechanical interactions with higher-order acoustic modes at GHz. Any fabrication imperfectness causing unwantedmode crosstalk or non-complete mode coupling before the light reaches the suspended region will lead to multiple modes propagating in the suspended waveguide with multi-GHz-peak optomechanical responses.

[0078] The two types of forward scattering have different geometry sensitivity: for the intra-modal response the acoustic frequency is solely dependent on the waveguide geometry (Fig.11, Panel a-b). For the inter-modal response, the waveguide height and width determine the cutoff frequencies of these higher-order acoustic modes, in the same way as the intra-modal response. On the other hand, they also influence the guided mode indices, then indirectly alter the acoustic wavevector q which is proportional to Δ^^^^^^^^^^^^^^^^(the mode index differences between two interacting optical modes). Different acoustic modes have distinct dispersion behaviors (Fig.11, Panel c), with the same wavevector changes, resulting in various frequency shifts. In the intra-modal and inter-modal forward scattering experiments, the number and the exact central frequencies of the acoustic responses serve as probing tools for the guided acoustic modes, revealing accurate information on the actual waveguide dimensions. In addition, with enough samples to collect sufficient data points on the acoustic resonances with different dimensions, the mechanical property of silicon nitride, like Young's modulus, material density, and Poisson's ratio determining the acoustic dispersion curves, can also be calibrated to compensate for the possible deviation from their typical values. These calibrations are essential for future FIM-FAM characterizations for a more accurate theoretical model anticipating and explaining the FIM-FAM responses.

[0079] In silicon nitride, the photoelastic constants, p11 and p44, have not yet been experimentally determined. These can be extracted for the first time with Brillouin measurements. With forward intra-modal and inter-modal scattering, different input mode combinations generating optical forces excite separate groups of higher-order acoustic modes; these forces containing the photoelastic constants contribute to the coupling strength. Based on experimentally measured optomechanical gains, we can accumulate data points on how three photoelastic constants influence the gain value in each interaction and estimate p11, p12, and p44. These measurements contribute to the FIM-FAM modeling and would also be the first measurements of the complete photoelastic constants matrix for silicon nitride, an essential material for integrated photonics.

[0080] The optical modes can be characterized directly with spatially and spectrally resolved imaging known as S2imaging of multi-mode waveguides. The S2imaging technique is based on gathering spatially resolved mode beating data, as a function of spaceand frequency. One possible implementation of this technique (Fig.12) features a broadband source coupled into the on chip multimode waveguide of interest. The output beam exiting the waveguide passes through a polarizer and is imaged onto a single mode fiber. The light entering the single mode fiber is analyzed on a optical spectrum analyzer (OSA). The SMF probe is raster-scanned along two directions (x, y) perpendicular to the beam propagation. At any given location (x,y) the optical spectrum is measured. Owing to the group delay difference between different spatial modes, the intensity at any given point (x,y) will vary as a function of wavelength. To measure modal group delays, the spectral dependence of intensity at any one point (x,y) is sufficient. Reconstructing the mode profiles of the various modes is achieved by measuring the spectral variation at every point. Example 4: Si3N4 FIM-FAM analysis and design

[0081] Full FEM simulations are run to design suitable FIM-FAM devices. The waveguide supports only the first three modes to minimize unwanted cross-coupling (Fig. 13). The dimensions are designed such that in the unsuspended regions, the same optical modes still exist. The calculated gains and frequencies are listed in Table 1 assuming a modest quality factor of 100. In addition to the flexural acoustic mode, the torsional mode (Fig.14) can be driven for the first time by with only the first two modes, ^^^^^^^^0and ^^^^^^^^0modes, by introducing a slight asymmetry in the dimensions of a square waveguide. This mode has unique zero-dispersion and has not yet been driven optically and could enable further design flexibilities for applications. Waveguide designs (see, e.g., Fig.15) for this new interaction are also listed in Table 1 below. Table 1cases acoustic modes (flexural and torsional) and frequency.*Note that p11and p44have not be reported for Si3N4; calculations assume the values for Si. This research will also measure this for the first time.

[0082] The initially predicted gain for inter-modal FIM-FAM interactions is more than 3 orders of magnitude higher than current state-of-the-art Brillouin systems due to thestrong acousto-optic overlap and the low acoustic frequencies. As described above, higher gains enable lower powers and shorter effective waveguides for applications. Example 5: On-chip waveguide design enabling large Brillouin gain

[0083] To demonstrate narrow linewidth FIM-FAM on-chip in Si3N4, the cross- section of the suspended structure is engineered to achieve large FIM-FAM gain at low acoustic frequency. One approach is to combine multiple suspended waveguides closely together with different dimensions, so that the optical modes are guided by the combinations of waveguides, while the acoustic waves are confined in individual waveguides instead, isolated from each other by the air in between. If the three waveguides shared an acoustic mode the gain would be lower due to cancelling force contributions, but if they are separated the gain can be high for the targeted central waveguide. A triple suspended waveguide structure therefore has been designed, fabricated and experimentally measured. The design freedom of the geometrical dimensions of the triple suspended waveguide structure is large, which includes the width and height of the main waveguide in the middle and the two waveguides on either side (Fig.16). The low-frequency FIM-FAM is designed for the optical TM0and TE¬1modes, and the fundamental flexural acoustic mode polarized in the vertical direction. To increase the fabrication feasibility, the heights of the three waveguides are designed to be the same, with two side waveguides sharing the same width, which differs from that of the main waveguide. A 2D array of FIM-FAM on-chip devices were fabricated with different widths of the main and side waveguides, as well as different air gaps among the three suspended waveguides, targeting different FIM-FAM frequencies by varying the effective refractive indices of the TE1 and TM0 modes in different ways. In addition, to see how the acoustic modes are influenced by the waveguide structures, various horizontal anchors perpendicular to the light propagation directions are also added on the side waveguides on certain FIM-FAM devices (Fig.17).

[0084] The FIM-FAM devices are designed with FIM-FAM frequencies around 10 MHz. Practically, due to the fabrication uncertainties, the measured FIM-FAM resonances will differ from the designed values. FIM-FAM spectra of several samples have been extensively measured. For example, on a triple suspended device without anchor, multiple FIM-FAM resonances are observed at different frequencies from near zero all the way up until ~150 MHz. (Fig 18) The acoustic linewidths are found to be around 100 kHz-400 kHz, with high FIM-FAM gain over 1000 W-1m-1 for the prominent peaks, several orders of magnitudes stronger than that of previously measured single suspended waveguides. Thelargest gain is measured in this case is 1542 W-1m-1 at ~12.2 MHz with a linewidth of 203 kHz.

[0085] Due to the design of the suspended structure, multiple optical modes are guided including the TE0, TE1, TE2, TM0 and TM1 modes, and FIM-FAM can happen between any two of them with different phase-matched acoustic wavevectors. In addition, interactions can occur with either one of the four fundamental acoustic modes with various velocities, including the x-polarized and y-polarized flexural modes, the torsional mode and the longitudinal mode. With mode cross-coupling at the two ends of the suspended region, multiple FIM-FAM responses will exist. Three suspended waveguides also have different FIM-FAM responses at the same wavevectors, due to their geometrical dissimilarity. Also, the phase-matched and non-phase-matched FIM-FAM sidebands have identical resonances, indicating that even though the acoustic wave is only optically driven in one direction, the acoustic wave still exists in both direction along the suspended waveguide. Low-frequency FIM-FAM leads to long phonon lifetime as long as ~2us, corresponding to acoustic decay lengths at several mm, much longer than the length of the suspended region. With small reflection loss at the buried-suspended boundaries, FIM-FAM may excite acoustic cavities in the suspended regions. As a result, even with a fixed phase-matched wavevector, multiple cavity modes will also be excited by the FIM-FAM process in the same waveguide. To reduce these additional responses a variation of the triple suspended waveguide is made which attenuates acoustic waves in the side waveguides through anchors on the side waveguides. Characterized (Fig.19) reveals completely different spectral features compared with the previous no-anchor sample, as expected. Targeting the single-resonance FIM-FAM, multiple improvements can be implemented on top of the triple suspended FIM-FAM waveguides: • Re-design the waveguide dimensions based on the fabrication uncertainty to reduce the number of guided optical modes, while still maintaining a small acoustic wavevector. • Dissipate the acoustic wave on the side waveguides to suppress FIM-FAM responses from the side waveguides. • Optimize the interface structure between the buried and the suspended regions to reduce mode cross-coupling. • Elongate the suspended region or introduce high acoustic loss at the buried-suspended interfaces to avoid the formation of acoustic cavities.

[0086] Meanwhile, other waveguide cross-section designs can also enhance the FIM- FAM response at low frequencies, engineering the effective indices of the optical modes and the distribution of the guided modes on the cross-sections. One example is to design triple suspended photonic crystal waveguides. Here the vertical optical guidance is offered by the index difference between the waveguide material and air, and horizontal optical confinement is offered by 2-D photonic crystal waveguides with air in the holes, which also helps with the fabrication process. The triple suspended photonic crystal waveguides differ in a way that there is also a main waveguide horizontally separated from the remaining photonic crystal structure along the propagation direction, so that acoustic modes can be tightly confined in the main waveguide, and the optical guidance is still defined by the photonic crystal structures. Different materials will also have different optimum cross-sectional designs. For example, Silicon and Silica are two other promising materials which will have a different force cancellation effect and fabrication procedure. In all useful cases, the overlap between light and sound must be large and the light and the sound must both be well confined.

[0087] While various embodiments have been described above, it should be understood that such disclosures have been presented by way of example only and are not limiting. Thus, the breadth and scope of the subject compositions and methods should not be limited by any of the above-described exemplary embodiments but should be defined only in accordance with the following claims and their equivalents.

[0088] The above description is for the purpose of teaching the person of ordinary skill in the art how to practice the present invention, and it is not intended to detail all those obvious modifications and variations of it which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such obvious modifications and variations be included within the scope of the present invention, which is defined by the following claims. The claims are intended to cover the components and steps in any sequence which is effective to meet the objectives there intended, unless the context specifically indicates the contrary.

Claims

WHAT IS CLAIMED IS:

1. An integrated platform with an on-chip forward inter-modal Brillouin scattering with fundamental acoustic modes (FIM-FAM) device, comprising: an input mode converter; a first single mode input waveguide, wherein said first single mode input waveguide is connected to said input mode converter; a second single mode input waveguide, wherein said second single mode input waveguide is connected to said input mode converter; a mode combiner, wherein said mode combiner is connected to both the first single mode input waveguide and the second single mode input waveguide; an input multi-mode waveguide, wherein said input multi-mode waveguide is connected to an output from said mode combiner; an on-chip FIM-FAM device, wherein said on-chip FIM FAM device comprises a multi- mode FIM-FAM active waveguide that is axially homogeneous, and wherein said on-chip FIM-FAM device is connected to the mode combiner via an input from said input multi-mode waveguide to said on-chip FIM-FAM device; an output multi-mode waveguide, wherein said output multi-mode waveguide is connected to an output from said on-chip FIM-FAM device; a mode splitter, wherein said mode splitter is connected to said on-chip FIM-FAM device via said output multi-mode waveguide; and an output mode converter, wherein said output mode converter is connected to said mode splitter by a first output single mode waveguide and a second output single mode waveguide; and said first output single mode waveguide comprising a first output adiabatic taper and said second output single mode waveguide comprising a second output adiabatic taper, wherein said first output adiabatic taper and said second output adiabatic taper are adiabatic for different spatial optical modes.

2. The integrated platform of Claim 1, wherein said multi-mode FIM-FAM active waveguide is a two-mode waveguide.

3. The integrated platform of Claim 1 or 2, wherein the FIM-FAM active waveguide is suspended; wherein any other waveguides waveguides comprise all on-chip and combiner / coupler waveguides, and the other waveguides may be structurally one or more selected from the group comprising no top cladding, buried and suspended.

4. The integrated platform of Claim 1 or 2, wherein said first input single mode waveguide comprising a first input adiabatic taper and said second input single mode waveguide comprising a second input adiabatic taper, wherein said first input adiabatic taper and said second input adiabatic taper are adiabatic for different spatial optical modes.

5. The integrated platform of Claim 1 or 2, wherein said multi-mode FIM-FAM active waveguide is suspended.

6. The integrated platform of any one of Claims 1-5, wherein said multi-mode FIM- FAM device has specific height and width dimensions selected to support driving a flexural fundamental acoustic mode.

7. The integrated platform of any one of Claims 1-5, wherein said multi-mode FIM- FAM device has specific height and width dimensions selected to support driving a torsional fundamental acoustic mode.

8. The integrated platform of any one of Claims 1-5, wherein said multi-mode FIM- FAM device has specific height and width dimensions selected to support driving a longitudinal fundamental acoustic mode.

9. The integrated platform of any one of Claims 1-8, wherein said multi-mode FIM- FAM active waveguide is made of silicon nitride (Si3N4).

10. The integrated platform of any one of Claims 1-8, wherein said multi-mode FIM- FAM active waveguide is made of silicon dioxide (SiO2).

11. The integrated platform of any one of Claims 1-8, wherein said multi-mode FIM- FAM active waveguide is made of silicon (Si).

12. The integrated platform of Claim 9, wherein said integrated platform further comprises: a silicon wafer; a silicon oxide layer, wherein said silicon oxide layer is layered over said silicon wafer; and a plurality of silicon nitride waveguides.

13. The integrated platform of Claim 10, wherein said integrated platform further comprises: a silicon wafer; and a plurality of silicon oxide waveguides, wherein said silicon oxide waveguides are all suspended waveguides.

14. The integrated platform of Claim 11, wherein said integrated platform further comprises:a silicon-on-insulator (SOI) wafer; a silicon device layer, wherein said silicon device layer is layered over said SOI wafer on buried silicon dioxide; and a plurality of silicon waveguides.

15. A system for generating a Brillouin signal resonance response by forward inter- modal Brillouin scattering with fundamental acoustic modes (FIM-FAM), comprising: the integrated platform with an on-chip FIM-FAM device of any one of Claims 1-14; a first input optical fiber coupled to said first input mode converter; a second input optical fiber coupled to said second input mode converter; a first output optical fiber coupled to said first output mode converter; and a second output optical fiber coupled to said second output mode converter.

16. A method for generating a Brillouin signal resonance response by forward inter- modal Brillouin scattering with fundamental acoustic modes (FIM-FAM), comprising: generating a first optical mode in said first input optical fiber of Claim 15, wherein the first optical mode is a fundamental optical mode; generating a second optical mode in said second input optical fiber of Claim 15, wherein the second optical mode is a higher-order optical mode; coupling through said mode combiner of any one of Claims 1-14 the first optical mode to the second optical mode to become coupled optical modes; and driving a fundamental acoustic mode in said multi-mode FIM-FAM active waveguide any one of Claims 1-14, wherein at least one Brillouin signal resonance response is generated within said multi-mode FIM-FAM active waveguide.

17. The method of Claim 16, wherein said fundamental acoustic mode is a flexural mode.

18. The method of Claim 16, wherein said fundamental acoustic mode is a torsional mode.

19. The method of Claim 16, wherein said fundamental acoustic mode is a longitudinal mode.

20. The integrated platform of any one of Claims 1-19, wherein the multi-mode FIM- FAM active waveguide is a triple suspended waveguide structure.

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