WO2015080662A1

WO2015080662A1 - Opto-mechanical accelerometer

Info

Publication number: WO2015080662A1
Application number: PCT/SG2014/000535
Authority: WO
Inventors: Bin Dong; Hong Cai; Julius Ming-Lin Tsai; Aiqun LIU
Original assignee: Agency For Science, Technology And Research; Nanyang Technological University
Priority date: 2013-11-27
Filing date: 2014-11-14
Publication date: 2015-06-04

Abstract

An accelerometer is provided. The accelerometer comprises a proof mass and at least a displacement sensor comprising a ring resonator. A displacement of the proof mass is configured to cause a shift in a resonance wavelength of the ring resonator.

Description

OPTO MECHANICAL ACCELEROMETER PRIORITY CLAIM

[0001] The present application claims priority to Singapore Patent Application No. 201308803-4, filed on 27 November 2013.

FIELD OF THE INVENTION

[0002J The present invention generally relates to accelerometers, and more particularly relates to an opto-mechanical accelerometer.

BACKGROUND

[0003] An accelerometer is an important device in various applications from consumer electronics to inertial navigation. Traditionally, microelectromechanical systems (MEMS) accelerometers play a dominant role in the accelerometer market due to its low cost and small size, which employ various techniques including piezoresistive, capacitive and tunneling accelerometers. However, further reduction in size is difficult for MEMS accelerometers.

[0004] Optical methods are becoming an alternative approach for accelerometers with smaller size and higher resolution. Generally, optically enabled devices offer superior sensitivity and resolution and resilience to electromagnetic interference. Most optical accelerometers employ free space light modulation, such as grating and diffraction, which require out of plane detection. The out-of plane detection, however, is difficult and makes on-chip integration difficult. [0005] Thus, what is needed is an optical accelerometer having easy on-chip integration. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.

SUMMARY

[0006] According to an aspect, an accelerometer is provided. The accelerometer includes a proof mass and at least a displacement sensor comprising a ring resonator, A displacement of the proof mass is configured to cause a shift in a resonance wavelength of the ling resonator.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to illustrate various embodiments and to explain various principles and advantages in accordance with a present embodiment.

[0008] FIG. 1 depicts a schematic diagram of an accelerometer in accordance with a present embodiment.

[0009] FIG. 2 (a) depicts a schematic diagram of the accelerometer as depicted in FIG.l in operation; FIG. 2 (b) depicts a graph of the resonance wavelength shift of a ring resonator in the accelerometer as depicted in FIG. 2 (a). [0010] FIG. 3 (a) depicts a graph of effective refractive index of the ring resonator as a function of gap in both in- and off-plane direction in accordance with the present embodiment; FIG. 3 (a) insert depicts electrical Field distribution for TE mode at different gaps in accordance with the present embodiment; and FIG. 3 (b) depicts a graph of change of the effective refractive index as a function of gap at in-plane direction in accordance with the present embodiment.

[0011] FIG. 4 depicts a graph of the proof mass displacement and the resonance wavelength shift of the accelerometer as depicted in FIG. 2 (a).

[0012] FIG. 5 depicts a graph of a frequency response of the accelerometer as depicted in FIG. 2 (a).

[0013] Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily be depicted to scale. For example, the dimensions of some of the elements in the schematic diagrams may by exaggerated in respect to other elements to help to improve understanding of the present embodiments.

DETAILED DESCRIPTION

[0014] The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description. It is the intent of the present embodiment to present a novel opto-mechanical accelerometer. This accelerometer employs a Whisper Gallery Mode (WGM) ring resonator, the resonance wavelength shift of which is used for displacement detection of a proof mass. The mechanical displacement of the proof mass is enhanced by the WGM ring resonator and, accordingly, the displacement is precisely detected via a resonance wavelength shift of the ring resonator. This accelerometer also employs a waveguide type displacement sensor which has advantages of low loss, small foot print and easily integrated with other devices. This accelerometer not only allows for chip-scale integration and light proof mass, but also provides high resolution and high bandwidth acceleration sensing. Furthermore, this accelerometer can be formed from silicon chip with CMOS compatible process, thus allowing for further on-chip electronics integration and packaging.

[0015] Referring to FIG. 1, a schematic diagram of an accelerometer 100 in accordance with a present embodiment is depicted. The accelerometer 100 comprises a proof mass 102, two displacement sensors 110 and 120, and two detectors 118 and 128. The proof mass 102 may be suspended from a substrate (not shown) and have holes for suspension purpose. The proof mass 102 may also be supported by two anchors 108. It is understood that a proof mass is a mass of known quantity. Further, the proof mass 102 may have a dimension of 20 μηι x 40 μηι. Various mass and dimension ma be chosen dependent on requirements. In an embodiment, the proof mass has multiple layers. Multiple layers may increase the mass.

[0016] In FIG. 1, the displacement sensors 110 and 120 are symmetrically located on opposite sides of the proof mass 102. The displacement sensors are located along an axis in which the proof mass is accelerated. It is understood that different locations of the displacement sensors can be chosen dependent on requirements so long as the mechanical movement of the proof mass causes optical signal changes in the displacement sensors. It is understood that different numbers of the displacement sensors can be used dependent on requirements, for example, one or four displacement sensors. The displacement sensor 110 comprises an arc 112 attached to the proof mass 102, a waveguide 116 and a ring resonator 114. The ring resonator 114 is located between the arc 112 and the waveguide 116 and coupled to both the arc 112 and the waveguide 116. The ring resonator 114 may have a diameter of substantially 60 μηι which will result in a high Q-factor of up to 13,000. The arc 112 may have a curve following the ring resonator 114 such that a gap therebetween has a substantially constant distance. The ring resonator 114 may be fixed to the substrate from which the proof mass 102 is suspended. To enable on-chip integration, it is preferred that the ring resonator 114 is in a plane where the arc 112 and the proof mass 102 are and that this plane is parallel to the substrate. The displacement sensor 120 comprises similar elements as the displacement sensor 110. In an alternate embodiment, the displacement sensor 120 may comprise a ring resonator having different size from the ring resonator 114. A light source 104 is used to provide broad band light which is coupled into an input port, splitted and transmitted in the waveguides 116 and 126. The waveguides 116 and 126 may be nano- waveguides.

[0017] The present ring resonator is separated from the proof mass. This ensures a better optical performance and mechanical performance, for example, by tolerating the environment disturbance.

[0018] A WGM ring resonator is presented in the present embodiment. It is understood that other types of resonators, such as photonic crystal resonator, can alternatively be used. The WGM resonator may have various designs, such as track-type resonator. A high Q-factor of the resonator is important for performance evaluation. [0019] The accelerometer can be fabricated on silicon on insulator (SOI) wafer with 220 nm silicon structure layer. With the development of silicon micro -fabrication technology, silicon based accelerometers are preferred due to subsequent cost reduction.

[0020] Referring to FIG. 2 (a), a schematic diagram illustrates the working principles of the accelerometer 100 in accordance with the present embodiment. The proof mass 102 experiences an acceleration in the presence of an inertial load which provides damping, The acceleration is along x-axis as shown in FIG. 2 (a) and the proof mass 102 moves along the x-axis with an acceleration rate a. The acceleration rate a is presented as

Mx + Dx + x = Ma (1) wherein M is the mass of the proof mass, D is the damping factor and K is the spring constant. The movement of the proof mass 102 causes a change in the gap between the arc 112 and the ring resonator 114. The gap g is given by g = gO ± Δχ, where go is the initial gap between the ring and arc, Δχ is the displacement induced during acceleration. The change in the gap between the arc 112 and the ring resonator 114 affects the surrounding evanescent wave perturbation and, thereby, affects the coupling therebetween. This gap change, in turn, shifts the resonance wavelength of the ring resonator 114. Consequently, the resonance wavelength shift affects coupling with the waveguide 116 and is detected by detector 118. The coupling between the ring resonator 114 and the waveguide 116 is now described. The light is coupled into the input port C of the waveguide 116 as shown in FIG. 2(a). Due to the coupling effect between the ring resonator 114 and the waveguide 116, the light is transmitted to the ring resonator 114 as shown as arrow A in FIG. 2 (a), circled in the ring resonator 114 (E) and transmitted to the waveguide 116 as shown arrow B in FIG. 2 (a). The light is finally transmitted to the output port D and detected for determining the resonance wavelength shift of the ring resonator 114. The resonance wavelength shift of the ring resonator 114 may be detected in two ways. A direct way is to measure the resonance wavelength shift by connecting the detected light to an optical spectrum analyzer, which can directly sense the resonance wavelength shift. Another way is to measure the resonance wavelength shift by detecting a difference in the transmission power of detected light when the accelerometer 100 is at rest and during acceleration. The transmission power of light with the wavelength of the light set equal to the resonance wavelength of the ring resonator 114 when the accelerometer 100 is at rest is measured. During acceleration, the resonance wavelength shift will result in an increase or decrease in the power of light transmission.

[0021] Similar resonance wavelength shift occurs to the ring resonator 124 and is detected by detector 128. The resonance wavelength of the ring resonator 124 shifts in an opposite direction to that of the ring resonator 114 as the two ling resonators are located in opposite sides of the proof mass 102. As the proof mass 102 moves towards one ring resonator, it moves away from the other ring resonator. As shown in FIG. 2(b), if Δλ] represents the resonance wavelength change of the ring resonator 114, Δλ₂ will represent the resonance wavelength change of the ring resonator 124. Δ ι will have a similar amount with Δλ₂ in case that the ring resonator 114 is in a similar dimension with the ring resonator 124. In an alternative embodiment, Δλ₁ will have a different amount with Δ ₂ in case that the ring resonator 114 is in a different dimension with the ring resonator 124. In a further embodiment, Δλι will have a same direction with Δλ₂ in case that the ring resonator 114 is in the same side of the proof mass 102 with the ring resonator 124. By utilizing two displacement sensors, the resonance wavelength shift (e.g. | λι \ + 1 Δλ₂1 ) is amplified, which improves the sensitivity as compared to a single ring configuration.

[0022] Referring to FIG. 3 (a), a graph 300 illustrates the effective refractive index (η^) of the ring resonator 114 as a function of the gap in both in-plane and off-plane direction. The resonance wavelength shift is detenriined by the effective refractive index. The in- plane direction means a direction within the plane where the proof mass 102 and the displacement sensor are and the off-plane direction means a direction out of the plane. It can be seen that the effective refractive index decreases as the gap increases at in-plane direction. In particular, the effective refractive index decreases sharply while the gap increases from 0 nm to 75 nm and the decrease of the effective refractive index slows down while the gap further increases to 200 nm. Also, it can be seen that the effective refractive index is limited and negligible in off-plane direction. In particular, the effective refractive index in off-plane direction remains around 2.1 which is the effective refractive index of the ring resonator at rest.

[0023] The insert 320 in FIG. 3 (a) illustrates the electrical field distribution for TE mode. The electrical filed distribution is enhanced when the gap narrows down from 150 nm to 50 nm. It shows that the field distribution is sensitive to the gap. In an embodiment utilizing an optimized spring (e.g. for providing better stability for in-plane movement), the off-plane displacement can be well controlled and limited.

[0024] Refening to FIG. 3 (b), a graph 340 illustrates a variation in the effective refractive index as a function of the gap at in-plane direction. It can be seen that larger variation is obtained when the gap is smaller, for example, from 0 nm to 75 nm, and that the variation is close to zero for gaps larger than 100 nm. It is desirable to have larger variation as this will give stronger optical signal. However, smaller gaps require better lithography and fabrication technology. A variation (not shown) in the effective refractive index as a function of the gap at off-plane direction is hundred times smaller than that of the in-plane direction. In other words, at the same displacement of the proof mass, the wavelength shift is hundred times larger for in-plane movement. It is estimated that off- plane sensitivity is only 0.2% or less of the in-plane sensitivity. This cross-sensitivity of 0.2% or less is advantageous as this enables on-chip integration with high sensitivity.

[0025] Referring to FIG. 4, a graph 400 illustrates displacement 410 of the proof mass and the wavelength shift 420 with acceleration. It can be seen that the displacement 410 shows a linear relationship with acceleration and higher acceleration results in higher displacement. As acceleration increases from 0 to 50 g, the displacement of the proof mass increases from 0 to 1.2 nm. A displacement of 22 pm/g is calculated from the graph 410. It also can be seen that the wavelength shift shows a liner relationship with acceleration and higher acceleration results in higher wavelength shift. As acceleration increases from 0 to 50 g, the resonance wavelength shifts from 0 to 165 pm. A wavelength shift of 3.279 nm g is calculated from the graph 420. That is, the wavelength shifts 3.279 nm/g while the proof mass moves at 22 pm/g. The above results are obtained where there is only one displacement sensor. It can be inferred that double displacement sensors will give double sensitivity and thus improve the performance of the accelerometer.

[0026] Referring to FIG. 5, a graph 500 illustrates the frequency response of the accelerometer at various damping conditions. Graph 510 shows the frequency response at under damping condition and graph 520 shows the frequency response at critical damping condition. The frequency response determines the working bandwidth of the accelerometer. Due to the small dimension of the accelerometer (e.g. 20 μηι x 40 μπι), it has a high eigenfrequency (f_m=103 kHz), which corresponds to a high operating bandwidth (e.g. up to 30 kHz) at critical damping. This high operating bandwidth is desirable as the accelerometer is capable to measure fast motion and vibration. As the damping rate can be controlled by optical gradient force excited between the ring resonator and the arc, this provides an easy way to calibrate the accelerometer.

[0027] Thus, in accordance with the present embodiment, an advantageous, on-chip optomechanical accelerometer has been presented which overcomes the drawback of the prior art. With CMOS compatible process, this optical accelerometer can be easily packaged and integrated with other photonic devices. Optical displacement sensor with WGM ring resonator promises a high resolution. The opto-mechanical accelerometer can be potentially used at hash environment including oil industry, or military usage in a complex electromagnetic environment. While exemplary embodiments have been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. For example, those skilled in the ail will realize from the teachings herein that the present technology may also be applied to optical resonator type gyroscope and mechanical force sensor.

[0028] It should further be appreciated that the exemplary embodiments are only examples, and are not intended to limit the scope, applicability, operation, or configuration of the invention in any way. Rather, the foregoing detailed description wilL provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements and method of operation described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.

Claims

CLAIMS What is claimed is:

1. An accelerometer comprising:

a proof mass; and

at least a displacement sensor comprising a ring resonator, wherein a displacement of the proof mass is configured to cause a shift in a resonance wavelength of the ring resonator.

2. The accelerometer of claim 1, wherein the displacement sensor further comprises an arc attached to the proof mass and coupled to the ring resonator such that the displacement of the proof mass causes a change in a gap between the arc and the ring resonator, resulting in the shift in the resonance wavelength of the ring resonator.

3. The accelerometer of claim 2, wherein the arc has a curve following the ring resonator.

4. The accelerometer of claim 1, further comprising a waveguide located adjacent to the ring resonator and coupled thereto, the waveguide being configured to connect with a detector for measuring the shift in the resonance wavelength of the ring resonator.

5. The accelerometer of claim 2, further comprising a waveguide located adjacent to the ring resonator and coupled thereto, the waveguide being configured to connect with a detector for measuring the shift in the resonance wavelength of the ring resonator.

6. The accelerometer of claim 3, further comprising a waveguide located adjacent to the ring resonator and coupled thereto, the waveguide being configured to connect with a detector for measuring the shift in the resonance wavelength of the ring resonator.

7. The accelerometer of claim 1, further comprising a substrate, wherein the proof mass is suspended from the substrate and the ring resonator is fixed on the substrate.

8. The accelerometer of claim 2, further comprising a substrate, wherein the proof mass is suspended from the substrate and the ring resonator is fixed on the substrate.

9. The accelerometer of claim 3, further comprising a substrate, wherein the proof mass is suspended from the substrate and. the ring resonator is fixed on the substrate.

10. The accelerometer of claim 4, further comprising a substrate, wherein the proof mass is suspended from the substrate and the ring resonator is fixed on the substrate.

11. The accelerometer of claim 5, further comprising a substrate, wherein the proof mass is suspended from the substrate and the ring resonator is fixed on the substrate.

12. The accelerometer of claim 6, further comprising a substrate, wherein the proof mass is suspended from the substrate and the ring resonator is fixed on the substrate.

13. The accelerometer of claim 7, wherein the substrate is made of silicon,

14. The accelerometer of claim 4, further comprising a substrate, wherein the proof mass is suspended f om the substrate and the ring resonator is fixed on the substrate, and wherein the proof mass, the displacement sensor and the waveguide are arranged in a plane parallel to the substrate.

15. The accelerometer of claim 1, wherein the diameter of the ring resonator is substantially 60 μηι.

16. The accelerometer of claim 1, comprising two displacement sensors symmetrically located aside the proof mass.

17. The accelerometer of claim 2, comprising two displacement sensors symmetrically located aside the proof mass.

18. The accelerometer of claim 4, comprising two displacement sensors symmetrically located aside the proof mass.

19. The accelerometer of claim 7, comprising two displacement sensors symmetrically located aside the proof mass.

20. The accelerometer of claim comprising two displacement sensors symmetrically located aside the proof mass.