Optical-mechanical integrated microwave clock

Abstract

The invention relates to an opto-mechanical microwave clock (100, 300, 400) integrated on a chip (103), comprising:-an optical waveguide (102) arranged on the chip (103),-a nanocavity (101) arranged substantially parallel to the optical waveguide (102), in which a modulation frequency of an optical mode excited in the nanocavity (101) corresponds to an oscillation frequency of a mechanical mode of the nanocavity (101), the modulated optical mode is evanescently coupled with the optical waveguide (102) in at least one coupling region; wherein the opto-mechanical clock (100, 300, 400) further comprises:-mode locking means configured to control opto-mechanical oscillation stability of the opto-mechanical microwave clock, the mode locking means comprising at least one of a contactless radio frequency mode locking means, an acoustic passive automatic mode locking means, an optical mode locking means comprising an optical ring resonator.

Description

Technical Field

[0001] This invention relates to an optomechanical microwave clock integrated on a chip. In particular, this invention relates to an optomechanical clock for integrating photonic circuits.

[0002] The field of this invention is the field of optomechanical transducers, and more specifically, the field of optomechanical transducers in the microwave field. Background Technology

[0003] Microwave oscillators are used in a variety of technological fields, such as timing protocols, precision positioning systems, sensors, and wireless telecommunications. Microwave photonics (MWP) bridges the gap between microwave electronic circuits and photonic components in the telecommunications band. MWP oscillators (where the clock signal is directly available for distribution in the optical domain) take advantage of immunity to electromagnetic interference and the low propagation loss of optical signals. Furthermore, these technologies require low-power communication devices and benefit from the development of nanoscale integrated and transportable MWP oscillators. The most important requirements for these devices are the spectral purity of the generated microwave signal (characterized by low phase noise) and its long-term stability.

[0004] An example of an MWP oscillator is the optoelectronic oscillator (OEO). These oscillators are highly stable, but they are bulky because they require several kilometers of optical fiber (which also introduces a sufficiently long time delay). Therefore, on-chip integration is not possible.

[0005] On the other hand, small-sized oscillators based on electronic devices (e.g., crystal oscillators and voltage-controlled oscillators (VCOs)) meet on-chip integration standards but are inherently sensitive to electromagnetic interference. Furthermore, their inherent frequencies are in the MHz range, requiring up-conversion for GHz applications. However, this up-conversion comes at the cost of sacrificing spectral purity.

[0006] Therefore, miniaturized, stable on-chip microwave photonic oscillators that operate directly in the GHz band and in harsh electromagnetic environments remain a complex task, especially for applications in satellite communications and 5G networks.

[0007] Against this backdrop, next-generation optomechanical oscillators have attracted attention as candidates to overcome the limitations of current OEOs. Their working principle is as follows: by simultaneously confining optical and mechanical modes at telecommunication wavelengths, optomechanical oscillators (OMOs) directly imprint GHz oscillations onto an optical carrier. Among numerous optomechanical systems, two-dimensional or one-dimensional semiconductor optomechanical crystals, with their inherent compactness, represent one of the best systems for nanoscale integration of such oscillators. Demonstrations of Brownian motion in one-dimensional optomechanical crystals have been reported on numerous platforms in the GHz frequency range, such as silicon-based materials (e.g., silicon (Si), silicon nitride (SiN)) or III-V semiconductors (e.g., gallium arsenide (GaAs), indium gallium phosphide (InGaP)) or lithium niobate (LN).

[0008] Compared to silicon, III-V semiconductors exhibit larger electronic band gaps and piezoelectric properties. These properties result in low two-photon absorption, thus enabling highly efficient devices in the telecommunications wavelength range. Piezoelectricity allows for direct on-chip electromechanical control of OMOs.

[0009] However, even though such OMOs benefit from good integration, they do not exhibit the required spectral purity and long-term stability. Summary of the Invention

[0010] The purpose of this invention is to overcome at least one defect of the known technology.

[0011] The purpose of this invention is to propose an optomechanical clock with stable optical and mechanical frequencies.

[0012] Another object of the present invention is to provide a portable optomechanical clock with a compact size, thereby allowing its integration into mobile systems (e.g., telecommunications systems).

[0013] At least one of these objectives is achieved by an optomechanical microwave clock integrated on a chip, the optomechanical microwave clock comprising:

[0014] - Optical waveguides, which are arranged on the chip.

[0015] - Nanocavities, which are arranged essentially parallel to the optical waveguides.

[0016] The optical mode excited in the nanocavity is modulated by the oscillation frequency of the mechanical mode of the nanocavity, and the modulated optical mode is evanescently coupled to the optical waveguide in at least one coupling region.

[0017] The optomechanical clock also includes:

[0018] - A mode-locking device configured to control the optomechanical oscillation stability of the optomechanical microwave clock.

[0019] According to different aspects of the present invention, the mode-locking device of the optomechanical clock is implemented according to different architectures:

[0020] - Contactless radio frequency mode-locking device,

[0021] - Including acoustic passive automatic or self-locking devices for acoustic cavities, and

[0022] - An optical mode-locking device including an optical ring resonator.

[0023] Different architectures of the mold-locking device will be described in detail below.

[0024] The optomechanical clock according to the present invention represents an optomechanical oscillator integrated on a chip, which can be integrated with photonic circuits.

[0025] The optomechanical clock according to the invention can be excited by injecting an optical signal with an adapted wavelength into an optical waveguide, which then excites optical and mechanical modes in a nanocavity.

[0026] The modulation frequency (i.e., clock frequency) is in the GHz range.

[0027] An optical waveguide can have a grating at its end. Through the grating, an optical signal can be coupled from the optical fiber into the waveguide, and vice versa.

[0028] This invention proposes an optomechanical oscillator heterogeneously integrated on a semiconductor circuit, combining the properties of oscillator materials with the advantages of this platform in terms of cost, manufacturability, and integrability. The optomechanical oscillator with integrated mode-locking allows for improved stability of optical signal modulation by stabilizing one or both of the optical and mechanical modes of the nanocavity. In fact, the phase noise of the nanocavity oscillation is significantly reduced. Subhertz linewidth oscillations can be achieved. The optomechanical clock according to the invention, along with its on-chip integration, represents a strong contender for oscillator applications.

[0029] According to one aspect of the invention, the mode-locking device is a contactless radio frequency (RF) mode-locking device. The RF mode-locking device is achieved through radio frequency electrical excitation without any mechanical components physically connected to the optomechanical clock, and generates an RF field oscillating in air at a frequency corresponding to a mechanical mode of a nanocavity.

[0030] RF electrical excitation devices allow for resonant RF injection within a nanocavity. This can be achieved, for example, by placing two electrodes on either longitudinal side of the nanocavity at a distance from it. The electrodes establish an electric field suitable for inducing displacement of the nanocavity in a completely contactless configuration. The induced displacement will be in a direction perpendicular to the optical mode.

[0031] Contactless RF molding devices allow for enhanced stability of mechanical modes within small package sizes.

[0032] According to another aspect of the invention, the mold-locking device is an acoustic passive automatic mold-locking device.

[0033] The acoustic passive automatic mold-locking device does not require excitation by sound waves.

[0034] A passive automatic mode-locking device may include an acoustic cavity. For mode-locking, this cavity operates at the frequency of the mechanical mode of the nanocavity.

[0035] Then, the acoustic cavity allows for passive automatic mode-locking of the optomechanical clock.

[0036] The acoustic cavity can be realized through a passive acoustic loop.

[0037] Passive acoustic loops may include acoustic waveguides. Preferred acoustic waveguides are on-chip acoustic waveguides.

[0038] In this scenario, phonons leaking from the nanocavity propagate through a passive loop and are guided back to the nanocavity with a time delay, thereby achieving a passive automatic mode-locking architecture.

[0039] According to another aspect of the invention, the mode-locking device is an optical mode-locking device comprising an optical ring resonator or an optical frequency comb generator.

[0040] An optical frequency comb generator allows the optical mode of a nanocavity to be locked to the mode of the frequency comb generator. This allows for enhanced stability of the optical mode.

[0041] The optical frequency comb generator is preferably an on-chip generator.

[0042] Advantageously, the mode-locking devices according to different aspects of the invention can be combined with each other in the same optomechanical clock. The same optomechanical clock can utilize different combinations of the presented mode-locking architectures (contactless RF mode-locking architecture, passive automatic mode-locking architecture, and / or optical mode-locking architecture). Thus, the effects of different mode-locking devices can be combined to stabilize both the optical and mechanical modes of the nanocavity.

[0043] Preferably, the optical waveguide of the optomechanical clock is a silicon waveguide.

[0044] Silicon is particularly suitable for guiding optical signals with wavelengths in the telecommunications range.

[0045] Preferably, the optomechanical clock according to the invention can be implemented on a silicon-on-insulator (SoI) platform, i.e., the chip integrating the optomechanical microwave clock includes a silicon-on-insulator substrate. In particular, the nanocavity can be fabricated on top of a commercially available SoI photonic circuit containing a predefined silicon waveguide.

[0046] Silicon-on-insulator platforms or substrates offer advantages in terms of integrability and ease of fabrication.

[0047] Alternatively, the optical waveguide of an optomechanical clock can also be silicon nitride (SiN) or any other suitable dielectric material.

[0048] According to an embodiment, the nanocavity can be suspended in the air above the optical waveguide by an anchoring device parallel to the optical waveguide.

[0049] Anchoring devices can be located at both ends of the nanocavity to control the optical coupling between the optical waveguide and the nanocavity.

[0050] Nanocavities can also be called nanobeams (because of their appearance) or photonic lines.

[0051] According to a preferred embodiment, the nanocavity comprises a photonic crystal.

[0052] Photonic crystals can be, for example, gallium phosphide (GaP) photonic crystals.

[0053] As a III-V semiconductor, GaP possesses attractive optomechanical properties. GaP is a suitable choice due to its favorable physical properties for both optical and acoustic applications. This material has a large band gap, thus exhibiting weak two-photon absorption. It also possesses a piezoelectric coefficient, allowing for efficient acoustic actuation.

[0054] This material is a promising option for emerging quantum technologies.

[0055] Integrating GaP photonic crystal nanocavities into mature silicon photonics technology presents stringent requirements for on-chip systems in terms of manufacturability, transportability, and integrability. Such integrated systems should also be less sensitive to environmental interference.

[0056] Photonic crystals can also be, for example, gallium arsenide (GaAs) or indium phosphide (InP) crystals, or any other group III-V semiconductor compound.

[0057] Alternatively, photonic crystals can be made of lithium niobate (LiNbO3), aluminum gallium nitride compounds (Al2O3), etc. x Ga1₋ x (N) or indium gallium phosphide compounds (In x Ga1₋ x Made from P.

[0058] According to alternative embodiments, the nanocavity may include a microdisk or any other device suitable for simultaneously limiting optical and acoustic modes.

[0059] Description of the Drawings and Detailed Description of the Embodiments

[0060] Other advantages and features will become apparent from a study of the detailed description of the non-limiting embodiments and the accompanying drawings, in which:

[0061] - Figure 1 A schematic diagram of an embodiment of an integrated optomechanical microwave clock with a contactless RF mode-locking architecture for realizing electrodes according to the present invention is shown;

[0062] - Figure 2 The electrical spectrum of an optomechanical clock according to an embodiment of the present invention is shown;

[0063] - Figure 3 A schematic diagram of another embodiment of the integrated optomechanical microwave clock with a passive automatic mode-locking architecture for realizing a acoustic cavity, according to the present invention, is shown; and

[0064] - Figure 4 A schematic diagram of yet another embodiment of an integrated optomechanical microwave clock with an optical mode-locked architecture for realizing an optical frequency comb generator, according to the present invention, is shown.

[0065] It will be readily understood that the embodiments described below are by no means limiting. In particular, variations of the invention may be envisioned that include only selected portions of the features described below, separate from the other described features, provided that such selection of a feature is sufficient to provide a technical advantage or to distinguish the invention from the prior art. Such selection includes at least one, preferably functional, feature without structural details, or only a portion of structural details, provided that this portion alone is sufficient to provide a technical advantage or to distinguish the invention from the prior art.

[0066] In the accompanying drawings, elements common to multiple drawings may use the same reference numerals.

[0067] Figure 1 This is a schematic diagram of an embodiment of an integrated optomechanical microwave clock according to the present invention. In this embodiment, the microwave clock has a contactless radio frequency (RF) mode-locking architecture for realizing electrodes.

[0068] The optomechanical clock 100 includes an optical waveguide 102 disposed on a chip 103 or a semiconductor platform. The optomechanical clock 100 also includes a nanocavity 101 disposed substantially parallel to the optical waveguide 102.

[0069] In such Figure 1 In the illustrated embodiment, the nanocavity 101 comprises a GaP optomechanical crystal. The nanocavity 101 is referred to as a nanobeam. The nanocavity 101 is fabricated on top of a silicon-on-insulator (SoI) photonic circuit. The optomechanical clock 100 also includes a pair of electrodes 104a and 104b. One of the electrodes 104a and 104b is positioned on either of the longitudinal sides of the nanobeam 101, at a distance from the nanobeam 101.

[0070] The SoI platform includes at least one silicon (Si) waveguide 102. In particular, the platform may include multiple predefined silicon waveguides. At least one waveguide 102 has a grating 105 at each end, allowing coupling from optical signals to optical fibers 106.

[0071] Figure 1 a) A schematic diagram of an optomechanical clock 100 with a contactless RF mode-locked architecture is shown, with an optical fiber 106 located above a waveguide grating 105.

[0072] Figure 1 b) is a cross-sectional view of an optomechanical clock 100 with a contactless RF mode-locked architecture, showing the different layers of the optomechanical clock 100 and the positions of electrodes 104a, 104b near the nanobeam 101.

[0073] The nanobeam 101 is preferably made of a III-V semiconductor material, such as gallium phosphide (GaP), gallium arsenide (GaAs), indium phosphide (InP), or indium gallium phosphide (In). x Ga1₋ x P).

[0074] By arranging them in parallel, repeatable and efficient evanescent coupling can be achieved between the silicon waveguide 102 and the III-V cavity 101.

[0075] The nanobeam 101 is suspended in air by two anchor points 107 located at corresponding ends of the nanobeam 101, parallel to the silicon waveguide 102. (Reference) Figure 1 (b) A dielectric layer 109, such as SiN (Si3N4) or SiO2, beneath the III-V semiconductor layer keeps the III-V semiconductor layer (from which the nanobeam 101 is made) elevated, thus preventing it from contacting the silicon waveguide 102. Once the III-V semiconductor layer is dry-etched, the dielectric layer beneath the nanobeam 101 is removed. Therefore, the nanobeam 101 is suspended, while the anchor 107 remains disposed on the dielectric layer.

[0076] The electric field between electrodes 104a and 104b can be used to drive the nanobeam 101 in a contactless configuration, meaning there is no physical connection between electrodes 104a and 104b and the nanobeam 101. The simulated electric field... Figure 1 In d), arrows are used to indicate the potential, and the potential is shown in grayscale.

[0077] Preferably, the nanocavity 101 is a photonic crystal (PhC) cavity. It is specifically designed to overlap optical and mechanical modes, resulting in a high optomechanical coupling ratio. The geometric design parameters are the height of the elliptical aperture and the distance between adjacent apertures in the crystal. In this example, both increase with a Gaussian distribution at each step from the cavity center to the mirror portions at the ends of the nanobeam 101. These end portions consist of identical and equidistant apertures that act as mirrors for the target optical and mechanical frequencies. This co-design allows for good confinement of the optical and GHz mechanical modes within the nanocavity 101.

[0078] The mechanical motion of the nanobeam 101 modulates the light coupled to the nanocavity 101, and this modulation is imprinted on the light leaving the nanocavity 101 and entering the waveguide 102. The output optical signal can then be read using a photodiode and an electrospectral analyzer.

[0079] Figure 2 Typical electrical spectra in the 0–4 GHz range are shown. On the recorded spectrum, peaks up to 500 MHz correspond to unconfined mechanical modes within a nanocavity, whose displacement fields extend along the entire length of a photonic line or nanobeam. Conversely, a peak at 3.35 GHz corresponds to localized mechanical modes.

[0080] Mechanical modes in the nanocavity can be initially excited by injecting a single-mode laser near the resonant wavelength into the nanobeam via a silicon waveguide. This leads to the excitation of the mechanical intrinsic modes of PhC.

[0081] An important quality factor of the spectral purity of a mechanical mode is phase noise, which provides information about the linewidth and short-term stability of the oscillation. Phase noise depends on the wavelength detuning from the resonance and the injected optical power.

[0082] The short-term stability of the mechanical modes oscillations within the nanocavity can be further optimized and controlled (i.e., locked) by driving on-chip electrodes. This RF actuation of nanobeams can be achieved due to the piezoelectric properties of III-V semiconductors. Therefore, to optimize RF coupling without interfering with the optical field, the nanobeams are placed between two electrodes, aligned relative to the center of the nanocavity. The electrodes can be, for example, 1 μm wide gold electrodes. The distance from the electrodes to the sides of the nanobeams can be 1 μm. When a potential is applied to the electrodes, a transverse electric field is generated between the nanobeams for capacitive actuation. The actuation efficiency can be optimized using a field oriented along the longitudinal direction of the nanobeams; however, this configuration would require complex fabrication processes.

[0083] By applying a periodic signal to the electrodes while coupling a laser close to the resonance of the nanobeam, the RF drive converted to the optical domain can be observed. When the driving frequency approaches the self-sustaining oscillation of the nanobeam, the self-sustaining oscillation locks onto the signal generator, where higher injected electrical power results in a wider lock-in range.

[0084] The distance and shape of the electrodes can be adjusted to improve the mode-locking characteristics of the optomechanical clock.

[0085] Figure 3 This is a schematic diagram of another embodiment of the optomechanical microwave clock according to the present invention. In this embodiment, the microwave clock has a passive automatic mode-locking architecture.

[0086] exist Figure 3 In the embodiment shown, the passive automatic mode-locking device of the optomechanical clock 300 includes a acoustic cavity generated by a passive acoustic loop 312.

[0087] The passive acoustic loop 312 enables passive automode-locking of the optomechanical clock 300. Phonons leaking from the nanobeam 101 propagate through the passive loop 312 and are guided back to the nanobeam 101 for the passive automode-locking architecture. The length and shape of the passive loop 312 can be adjusted by dispersion engineering to produce a suitable time delay for automode-locking improvements in the optomechanical clock 300 with a passive automode-locking acoustic architecture.

[0088] Figure 4 This is a schematic diagram of another embodiment of an optomechanical clock with an optical mode-locking architecture according to the present invention.

[0089] In this embodiment, the mode-locking device of the optomechanical clock 400 includes an optical frequency comb generator 420. This generator 420 is an optical micron-sized resonator made of a material with nonlinear characteristics. To overcome material dispersion, group velocity dispersion must be designed by adjusting the geometric parameters of the micron-sized resonator.

[0090] An optical frequency comb generator 420 is optically coupled to the same optical waveguide 102 as the nanobeam 101. Injected laser light generates the optical frequency comb, one line of which handles the resonance of the nanobeam 101. The spacing and stability of the frequency comb can be adjusted by regulating the power and wavelength of the injected laser. The resonance of the nanobeam 101 is locked to a stable beam, thereby improving the overall phase noise performance of the optomechanical clock 400.

[0091] According to Figure 1 , Figure 3 and Figure 4 In the detailed embodiments described, the mode-locking devices can be combined with each other in an optomechanical clock. The same optomechanical clock can utilize different combinations of the presented mode-locking architectures (contactless RF mode-locking architecture, passive automatic mode-locking architecture, and / or optical mode-locking architecture).

[0092] Of course, the present invention is not limited to the examples described above.

Claims

1. An optomechanical microwave clock (100, 300, 400) integrated on a chip (103), comprising: - An optical waveguide (102) is disposed on the chip (103), - A nanocavity (101) is arranged substantially parallel to the optical waveguide (102). The modulation frequency of the optical mode excited in the nanocavity (101) corresponds to the oscillation frequency of the mechanical mode of the nanocavity (101), and the modulated optical mode is evanescently coupled to the optical waveguide (102) in at least one coupling region. The optomechanical clocks (100, 300, 400) also include: - A mode-locking device configured to control the optomechanical oscillation stability of the optomechanical microwave clock, the mode-locking device comprising at least one of the following: • Non-contact radio frequency mode-locking device • Acoustic passive automatic mold-locking device • Optical mode-locking device, which includes an optical ring resonator.

2. The optomechanical clock (100, 300, 400) according to claim 1, characterized in that, The contactless radio frequency mode-locking device includes a pair of electrodes (104a, 104b) configured to generate an electric field through the nanocavity (101), wherein one of the electrodes (104a, 104b) is disposed on a chip on either longitudinal side of the nanocavity (101).

3. The optomechanical clock (100, 300, 400) according to claim 1 or 2, characterized in that, The contactless radio frequency mode-locking device includes a radio frequency electroexcitation device that oscillates at the frequency of the mechanical mode of the nanocavity.

4. The optomechanical clock (100, 300, 400) according to any one of the preceding claims, characterized in that, The acoustic passive automatic mold-locking device includes an acoustic cavity (312).

5. The optomechanical clock (100, 300, 400) according to any one of the preceding claims, characterized in that, The optical ring resonator includes an optical frequency comb generator (420) optically coupled to the optical waveguide (102), the optical frequency comb generator being configured to generate an optical frequency comb, one of the frequency lines of the optical frequency comb being aligned with the resonant frequency of the nanocavity (101).

6. The optomechanical clock (100, 300, 400) according to any one of the preceding claims, characterized in that, The optical waveguide (102) is a silicon waveguide.

7. The optomechanical clock (100, 300, 400) according to any one of the preceding claims, wherein, The nanocavity (101) is suspended in the air above the optical waveguide (102) by an anchoring device (107) parallel to the optical waveguide (102).

8. The optomechanical clock (100, 300, 400) according to any one of the preceding claims, wherein, The nanocavity (101) includes a photonic crystal.

9. The optomechanical clock (100, 300, 400) according to the preceding claim, characterized in that, The photonic crystal is a GaP photonic crystal.

10. The optomechanical clock (100, 300, 400) according to any one of the preceding claims, characterized in that, The chip (103) includes a silicon-on-insulator substrate.

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