Resonator comprising a remote peripheral light guide

Abstract

Resonator (30) comprising: an actuator (38) a resonant structure (31), configured to oscillate, by periodically deforming according to a resonant frequency (), under the effect of the actuator; a peripheral light guide (32), extending around the resonant structure, and configured to oscillate, by periodically deforming, while being driven by the resonant structure; the resonator being characterized in that: the resonant structure (31) is thicker than the peripheral light guide; the peripheral light guide is kept at a distance from the resonant structure by at least one anchor (33).

Description

Title of the invention: Resonator comprising a remote peripheral light guide Technical field

[0001] The technical field of the invention relates to mechanical resonators, which can in particular be used to form an opto-mechanical oscillator. PREVIOUS ART

[0002] The use of an opto-mechanical oscillator allows the formation of a periodic electronic signal whose amplitude and period are controlled. One possible application is the formation of a clock signal.

[0003] In the devices of the prior art, the optomechanical oscillators comprise a light source and a resonator. Under the effect of optical coupling, a portion of the light emitted by the light source is extracted and propagates in the resonator. When the resonant structure oscillates, at a resonant frequency, the efficiency of the optical coupling at a given wavelength varies at the resonant frequency. This results in a periodic variation in the quantity of light extracted in the resonator.

[0004] Generally, in optomechanical devices, the thickness of the structures is limited to a few hundred nanometers so as to maintain a single optical propagation mode. For simplicity, the resonator is usually extracted from a single layer of fixed thickness. The reason is to limit the risks of intermodal coupling, which destroys practical use.

[0005] When these resonators are inserted into oscillators, these thickness restrictions prevent the oscillator from performing well. These performances are quantified using a figure of merit, called the oscillator's phase noise, which must be minimized. The phase noise depends in particular on the "filtering power" of the resonator, i.e. its mechanical quality factor (to be maximized), which can be increased by thickening the resonator.

[0006] The publication Beyazoglu et al “A multimaterial Q-boosted low phase noise opto-mechanical oscillator”, describes an optomechanical oscillator with a low noise level. The oscillator comprises a polycrystalline silicon ring surrounded by a silicon nitride ring, forming a peripheral light guide. The two rings are concentric. The polycrystalline silicon ring forms a resonant structure, vibrated by electrodes, by capacitive effect. The vibration of the resonant structure causes a displacement of the peripheral light guide. The ring forming the light guide is thinner than the ring forming the resonant structure. Two different materials are used to form respectively the resonant structure, vibrated by electrodes, and the peripheral light guide. The peripheral light guide extends in contact with the resonant structure, all around the latter. In order to allow light confinement, the light guide must have a certain width.

[0007] The inventors propose an opto-mechanical resonator having a high mechanical quality factor while limiting the number of optical modes. It also has reduced sensitivity to temperature fluctuations, making it an ideal candidate for use in oscillators. The objective may be to generate an amplitude modulated signal whose power and period are stable. Statement of the invention

[0008] A first object of the invention is a resonator comprising: an actuator; a resonant structure, configured to oscillate, by periodically deforming according to a resonant frequency, under the effect of the actuator; a peripheral light guide, extending around the resonant structure, and configured to oscillate, by periodically deforming, while being driven by the resonant structure;

[0009] the resonator being characterized in that: the resonant structure is thicker than the peripheral light guide; the peripheral light guide is held at a distance from the resonant structure by at least one anchor.

[0010] An intermediate space may in particular extend between the peripheral light guide and the resonant structure, the intermediate space being filled with a gas or a liquid or a vacuum.

[0011] According to one possibility: the thickness of the peripheral light guide is between 100 nm and 600 nm; the thickness of the resonant structure is at least twice or at least three times greater than the thickness of the peripheral light guide.

[0012] The thickness of the resonant structure may be between 600 nm and 1 mm, and preferably between 1 pm and 50 pm.

[0013] The resonant structure may be arranged opposite at least one actuating electrode, the actuating electrode being spaced from the resonant structure by an air gap, the actuating electrode forming the actuator, being configured to generate an oscillation of the resonant structure for example by capacitive effect.

[0014] The actuating electrode may extend into the intermediate space.

[0015] According to one possibility: - the resonant structure comprises a piezoelectric material; - the resonator comprises two actuating electrodes, on either side of the resonant structure, the actuating electrodes forming the actuator.

[0016] The resonant structure and the peripheral waveguide may be formed from the same material.

[0017] The resonant structure may have a cylindrical, circular or polygonal base, or annular shape.

[0018] The resonant frequency may be greater than 1 MHz.

[0019] A second object of the invention is an opto-mechanical oscillator, comprising: - a light source, configured to emit a light beam and propagate the latter along a resonator; - the resonator being configured so that the periodic oscillation of the resonator causes a periodic modulation of a light power of the light beam. - the opto-mechanical oscillator being characterized in that the resonator is a resonator according to the first object of the invention.

[0020] The oscillator may comprise a photodetector, configured to detect the light beam having propagated along the resonator. The photodetector may be configured to supply the resonator with a feedback signal.

[0021] The invention will be better understood upon reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below. FIGURES

[0022] [Fig.lA] and [Fig.lB] diagram the main components of an opto-mechanical oscillator.

[0023] [Fig. 2] illustrates an oscillation of the optical power delivered by an optomechanical oscillator as described in connection with Figures 1A and 1B

[0024] [Fig.3A] is a three-dimensional view schematically illustrating a resonant structure driving a peripheral light guide in an oscillating motion.

[0025] [Fig.3B] shows a schematic sectional view of the resonant structure and the peripheral light guide described in connection with [Fig.3A]. The sectional plane corresponds to a median plane of the peripheral light guide.

[0026] [Fig.3C] shows a sectional view of the resonant structure described in connection with [Fig.3B], in a transverse sectional plane, perpendicular to the median plane of the peripheral light guide.

[0027] [Fig.3D] represents the configuration described in connection with Figures 3A to 3C with a capacitive effect actuator.

[0028] [Fig.4] illustrates the distribution of the electromagnetic field in a waveguide cylindrical light with a thickness of 1 pm, multimode along the vertical axis, and with cylindrical symmetry.

[0029] [Fig.5A] shows a diagram of a second embodiment of the invention, in a transverse section plane as previously defined.

[0030] [Fig.5B] shows schematically the second embodiment of the invention, in a section plane corresponding to the median plane of the peripheral light guide.

[0031] Figures 6A to 61 diagram a method of producing the resonator as described in connection with [Fig.3D].

[0032] [Fig.7] shows a third embodiment, in which the actuation of the resonant structure is carried out by piezoelectric effect. PRESENTATION OF SPECIAL METHODS OF IMPLEMENTATION

[0033] Figures 1A and 1B diagram components of an optomechanical oscillator. The oscillator comprises a light source 10, emitting a light beam F. In this example, the light beam is transported by a light guide 20 to a photodetector 25.

[0034] The oscillator comprises a resonator 30 extending along the light guide 20. The resonator 30 is configured to oscillate, by deforming, at a resonant frequency f . The resonant frequency is preferably greater than 1 MHz, and is preferably in the range 1 MHz - 100 GHz. The resonator is, in this example, configured to be animated by a vibration movement in the plane, according to the resonance frequency, under the effect of a capacitive actuation exerted by an actuator 38. The actuator 38, shown in [Fig. 1B], is fixed relative to the resonator 30. The actuator 38 is located on either side of the resonator 30. The electrostatic actuation is carried out through an air gap 37, extending between the fixed actuator 38 and the resonator 30. The capacitive actuation is controlled by a control unit 40.

[0035] The photodetector is configured to form an electrical signal modulated according to a modulation frequency, at the resonant frequency of the resonator. The frequency modulated signal can be used to form a clock signal. The control unit 40 is powered by a feedback loop resulting from the photodetector 25.

[0036] In this example, the resonator 30 is a cylinder with a circular base. The resonator may extend according to a different geometry, for example a cylinder with a polygonal base or a ring.

[0037] The light guide 20 can be produced on the surface Si layer of a SOI (Silicon on Insulator) type substrate. The cross-section can for example be a few hundred nm by a few hundred nm, by example of the order of 600 nm x 200 nm. By cross section, we mean a section in a plane perpendicular to the axis of propagation of the light. Preferably, the light guide 20 is configured to carry out propagation according to a single mode, at a wavelength which can be 1550 nm, a usual wavelength in the field of telecommunications.

[0038] The resonator 30 is optically coupled to the light guide 20, for example by evanescent coupling. The distance between the resonator and the light guide 20 is for example 100 nm. Thus, when a light beam propagates along the light guide 20, a portion of the light beam is extracted and propagates in the resonator 30.

[0039] [Fig. 2] illustrates an optical coupling bandwidth between the light guide 20 and the resonator 30. The abscissa axis corresponds to the optical wavelength. The ordinate axis corresponds to the light power extracted from the optical light guide 20 by the resonator 30. The solid line curve represents the fixed configuration, in which the resonator is fixed. Under the effect of the oscillation of the resonator, the optical coupling between the waveguide and the resonator varies. In [Fig. 2], this results in a periodic variation of the coupling bandwidth, between the two dashed curves, reflecting a spectral shift of the light guide 20 / resonator 30 coupling.

[0040] In Figure 2, the dotted vertical line corresponds to the emission wavelength Xe of the beam F propagating in the light guide 20. Under the effect of the periodic variation of the coupling bandwidth, the power of the optical beam F propagating in the light guide 20 is modulated according to a pulsation (A, according to the expression

[0041] P=P0 + acos(ùV)

[0042] where: - Pq corresponds to the power propagating in the light guide “at equilibrium”, that is to say in the absence of oscillation of the peripheral light guide 32; - a corresponds to the modulation amplitude, that is to say the variation in power extracted in the peripheral light guide 32 under the effect of the oscillation of the resonant structure 31. = 17Tfr (2)

[0043] Figures 3A to 3D represent an example of a resonator 30 according to the invention, which can be integrated into an oscillator as described in connection with Figures 1A and 1B. [Fig.3A] corresponds to a perspective view. Figures 3B and 3D are sectional views.

[0044] A special feature of the resonator is that the mechanical quality factor is high, while the sensitivity to temperature fluctuations is reduced.

[0045] When it is part of an oscillator, the resonator 30 acts as a bandpass filter, attenuating noise sources whose frequency components are located outside the passband of the resonator. The width of this passband is inversely proportional to the mechanical quality factor, thus the oscillating movement of the resonator will have better spectral purity if it benefits from high quality factors. The modulation of the optical beam, carried out by the optomechanical transduction system, also benefits from this improved spectral purity. It is therefore important to have a high mechanical quality factor Q. The more the thickness of a mechanical resonator increases, the more the quality factor increases.

[0046] The mechanical quality factor of the resonator corresponds to the sharpness of the resonance peak of the resonator oscillation. When the resonator is inserted into an oscillator, the figure of merit of the latter, also called the phase noise, must be minimized. It is possible to show that the phase noise depends on the ratio 1 / Q2, Q being the mechanical quality factor. The interest here is therefore to work with mechanical quality factors as high as possible.

[0047] It is also considered that the spectral density of the noise includes a thermal component which can be considered as inversely proportional to the mass of the resonator, therefore to its thickness.

[0048] Thus, in order to increase the mechanical quality factor and reduce the sensitivity to noise, it is preferable to increase the thickness of the resonator 30. However, it is preferable for the light to propagate through a structure whose thickness is sufficiently small so as to allow only single-mode light propagation in the cross-section of the guide.

[0049] In [Fig.4], a cross-sectional view of a light guide, having a thickness of 1 μm, is shown. A spatial distribution of the intensity of an electric field in the cross-section of the light guide is shown. The gray levels represent the intensity of the electric field. Two maxima are observed in each cross-section: this reflects the fact that the light propagates according to several modes, which is not desirable, due to the risk of intermodal coupling.

[0050] The resonator 30, shown in Figures 3A to 3D, comprises a resonant structure 31, configured to be vibrated by an actuator. The resonator also comprises a peripheral light guide 32, extending around the resonant structure, at a non-zero distance from the latter. Anchors 33 connect the peripheral light guide 32 to the resonant structure 31. As shown in [Fig.3B], the anchors are arranged so as to transmit the oscillations of the resonant structure 31 to the peripheral light guide 32, while maintaining the peripheral light guide 32 remote from the resonant structure 31.

[0051] In [Fig.3A], the peripheral light guide is shown: - in shade, when it is animated by an oscillating movement transmitted by the anchors 33; - in wired form, in a motionless state: it then describes a ring around the resonant structure 32.

[0052] An intermediate space 34 extends between the peripheral light guide 32 and the resonant structure 31. The intermediate space 34 extends around the resonant structure 31, between each anchor 33. Thus, along at least 80%, or even 90% of the contour of the peripheral light guide 32, the latter is separated from the resonant structure 31 by the intermediate space 34. The intermediate space 34 is filled by the ambient medium: it may be a gas, such as air, or a vacuum or a liquid whose refractive index is lower than that of the material forming the peripheral light guide.

[0053] [Fig.3B] shows a sectional view of the elements shown diagrammatically in [Fig.3A], in a sectional plane corresponding to a median plane of the peripheral light guide 32.

[0054] The distance of the peripheral light guide 32 from the resonant structure 31 makes it possible to confine the light propagating in the peripheral light guide. Thus, the resonant structure 31 and the peripheral light guide 32 can be formed from the same material, for example Si. The anchors 33 can be formed from the same material, the effect of the anchors on the confinement of the light being negligible due to their small contact surface with the contour of the peripheral light guide: typically less than 20%, or even less than 10%, or even less than 5% of the contour of the peripheral light guide, opposite the resonant structure 31, is occupied by an anchor.

[0055] Figure 3C shows a sectional plane along the thickness of a portion of the resonator, along a line shown in dashes in Figure 3B. The resonant structure 31, set in motion by an actuator, extends along a thickness 8 preferably between 600 nm and 1 mm, and more preferably between 1 μm and 50 μm. The thickness e of the peripheral light guide 32 is preferably between 50 nm and 600 nm, or between 100 nm and 600 nm, for example 200 nm. Generally speaking, the thickness of the resonant structure 31 is at least twice or at least three times greater than the thickness of the peripheral light guide 32.

[0056] The resonant structure 31 is connected to a base 35 by a pillar 36. The resonant structure 31, as well as the peripheral light guide 32, extend around a central axis A. During the oscillations of the resonant structure 31, the central axis A remains fixed. The central axis A preferably forms an axis of symmetry of the assembly formed by the resonant structure 31 and the peripheral light guide 32. In [Fig.3C], the anchors 33 were materialized by dashes.

[0057] In Figure 3D, an actuation electrode 38 is shown, allowing actuation of the resonant structure by capacitive effect. The actuation electrode acts as an actuator. An insulating air gap 37 extends between the actuation electrode 38 and the resonant structure 31. The actuator is formed by an electrode polarized by an alternating current generating an electric field £ in the air gap. The thickness of the air gap is of the order of a hundred nm. Preferably, the air gap 37 is filled by the ambient medium: gas, vacuum or liquid. The actuating electrode is held by a support 39, the latter being connected to the base 35. In [Fig.3D], the anchors 33 are represented by dashes, the latter being distributed, preferably symmetrically with respect to the axis A, between the peripheral light guide 32 and the resonant structure 31.

[0058] Figures 5A and 5B show an embodiment in which the electrode actuating electrode 38 extends between the resonant structure 31 and the peripheral light guide 32. Figures 5A and 5B are respectively sections in the transverse plane and in the median plane as previously defined. Such an embodiment increases the surface area over which the capacitive coupling is carried out between the electrode 38 and the resonant structure 31. This results in better coupling between the actuating electrode 38 and the resonant structure 31. The air gap 37 then corresponds to a part of the intermediate space 34 extending between the resonant structure 31 and the peripheral light guide 32.

[0059] Figures 6A to 61 diagram the main steps of a method for manufacturing a resonator 30 as described in connection with [Fig.3D]. There is a substrate 40 of SOI type, comprising a superposition of a bulk layer 41, an intermediate layer 42 of SiO2 and an upper layer 43 of thinned Si, the thickness of the latter being for example 220 nm. Cf. [Fig.6A].

[0060] The upper layer 43 is etched, so as to define several elements: cf. [Fig.6B]. A peripheral, annular element 43a forms a base of a support for the actuating electrode. A central, cylindrical element 43c corresponds to a base of the resonant structure. An annular element 43b forms the peripheral light guide and, locally, the anchors, the latter extending between the central element 43c and the annular peripheral element 43a.

[0061] The central element 43c and the element 43a are grown by epitaxy. Cf. [Fig.6C],

[0062] A layer 44 of SiO2 is deposited: cf. [Fig.6D], then is subjected to deep etching, so as to delimit the central element 43c: cf. [Fig.6E]. A superficial etching is applied, so as to free the upper surface of the stack, around the central element. Cf. [Fig.6F].

[0063] A conductive layer 45, for example metallic, is applied to the upper surface of the stack: see [Fig.6G]. The conductive layer 45 is then thinned: see [Fig.6H]. Wet etching then removes the SiO2 44, leaving residues intended to form a pillar 36 as well as a part of the support 39. See [Fig.61]. In [Fig.61], the main elements forming the resonator 30 can be seen as described in connection with [Fig.3D].

[0064] Figure 7 represents a third embodiment, in which the actuation of the resonant structure 31 is piezoelectric. The elements 32, 35 and 36 are as described in connection with Figures 4D or 5A. The resonant structure is formed of a piezoelectric material, for example aluminum nitride (AIN), lithium niobate (LNO). The base 35 comprises a lower electrode 38;. The resonant structure comprises an upper electrode 38s. Under the effect of an amplitude-modulated electric field E between the upper electrode 38s and the lower electrode 38;, the resonant structure deforms, according to a vector * as represented in [Fig.7]. The electrodes can be formed of metal, metal alloy, or doped semiconductor.

[0065] The invention makes it possible to form a resonator configured to modulate a light beam in amplitude in a stable manner, and according to a stabilized frequency. When such a resonator is integrated into an oscillator, this makes it possible to form a clock signal whose amplitude and frequency are controlled.

Claims

Claims

1. Resonator (30) comprising: - an actuator (38) - a resonant structure (31), configured to oscillate, by periodically deforming according to a resonant frequency ( / ), under the effect of the actuator; - a peripheral light guide (32), extending around the resonant structure, and configured to oscillate, by periodically deforming, while being driven by the resonant structure; the resonator being characterized in that: - the resonant structure (31) is thicker than the peripheral light guide; - the peripheral light guide is kept at a distance from the resonant structure by at least one anchor (33).

2. A resonator according to claim 1, wherein an intermediate space (34) extends between the peripheral light guide (32) and the resonant structure (31), the intermediate space being filled with a gas or liquid or vacuum.

3. Resonator according to any one of the preceding claims, wherein: - the thickness of the peripheral light guide is between 100 nm and 600 nm; - the thickness of the resonant structure is at least twice or at least three times greater than the thickness of the peripheral light guide.

4. A resonator according to claim 3, wherein the thickness of the resonant structure is between 600 nm and 1 mm.

5. A resonator according to any preceding claim, wherein the thickness of the resonant structure is between 1 pm and 50 pm.

6. A resonator according to any preceding claim, wherein the resonant structure is arranged facing at least one actuating electrode (38), the actuating electrode being spaced from the resonant structure by an air gap (37), the actuating electrode forming the actuator, being configured to generate an oscillation of the resonant structure by capacitive effect.

7. A resonator according to claim 6 and claim 2, wherein the electrode extends into the intermediate space (34).

8. Resonator according to any one of claims 1 to 5, in which - the resonant structure comprises a piezoelectric material; - the resonator comprises two actuating electrodes (38;, 38s), on either side of the resonant structure, the actuating electrodes forming the actuator.

9. A resonator according to any preceding claim, wherein the resonant structure and the peripheral waveguide are formed from the same material.

10. A resonator according to any preceding claim, wherein the resonant structure has a cylindrical, circular or polygonal base, or annular shape.

11. A resonator according to any preceding claim, wherein the resonant frequency is greater than 1 MHz.

12. Opto-mechanical oscillator, comprising: - a light source (10), configured to emit a light beam and propagate the latter along a resonator (30); - the resonator being configured so that the periodic oscillation of the resonator causes a periodic modulation of a light power of the light beam; - the opto-mechanical oscillator being characterized in that the resonator is a resonator according to any one of the preceding claims.

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