Cavity optomechanics offers significant reduction in noise and drift for inertial sensing devices by providing high signal-to-noise displacement sensing. Strong coupling between mechanical motion and optical resonances such as whispering gallery mode resonances has been shown to detect displacements at the femtometer scale. By using a photonic integrated circuit (PIC) ring resonator to detect the motion of a micro-electro-mechanical systems (MEMS) structure as it moves within the optical evanescent field, sensitivities 100-10,000x better than existing low-cost inertial sensors can be achieved. To design optimised accelerometers and vibratory gyroscopes, accurate finite element simulations of the PIC resonators are required to match the sophistication of MEMS modelling. We describe comparisons between frequency domain methods and 2D finite-difference time-domain (FDTD) methods for silicon ring resonators. As a demonstration of our working principle, we show experimental data where we measure the motion of a mechanical cantilever. We also characterise the thermo-optic induced shifting of the ring resonator resonance by measuring this effect, showing good agreement with FDTD simulations.
Silicon photonics can be used to create compact high performance optical inertial sensors by combining photonic integrated circuit structures with micro-electro-mechanical systems engineering. Optically transduced mechanical test-masses benefit from the low noise and long-term stability of stabilised coherent light sources, enabling lower noise floors and improved bias stability compared with capacitive devices. By using optical resonances in the form of whispering gallery modes (WGM) to perform the measurement, we further boost the signal-to-noise ratio of our readout. The dispersive optomechanical coupling between the WGM within a ring resonator and the motion of the test-mass causes a measurable shift to the resonance. We report on progress towards creating an optomechanical accelerometer from silicon-on-insulator wafers, targeting a noise floor of < 1×10-6 ms-2/Hz1/2.
The coupling between mechanical motion and optical fields can be exploited for exquisite force sensing. This optomechanical interaction is further amplified with optical resonances, leading to unprecedented displacement sensitivities beyond 10-18 m/Hz 1/2, as exemplified by the Laser Interferometer Gravitational Wave Observatory (LIGO). In this talk I will introduce a cavity optomechanics platform for motion sensing based on optical whispering gallery mode (WGM) resonances. I will describe the evolution of a WGM accelerometer from the laboratory to a hand-fabricated proof-of-concept prototype, and now, towards chip-scale fabrication. Our goal is to reach acceleration sensitivities below 100 ng/Hz1/2 (where g=9.81 ms-2) whilst ensuring low power operation, high linearity, and low drift. Through the lens of commercial feasibility, we use finite element modelling to simulate the optical, mechanical, and thermal behaviour of a range of MOEMS designs. Preliminary results from chip fabrication and chip-testing will also be presented.
Cavity optomechanical systems show great promise as force and displacement sensors, with scope to operate across the classical to quantum regimes. I will discuss the commercial development of an optical whispering gallery mode (WGM) accelerometer, which relies on a dispersive and dissipative coupling between the cavity resonance and the motion of the cavity. The accelerometer operates at a sensitivity of micro-g Hz-1/2 (g=9.81 ms-2) with plans to approach nano-g Hz-1/2 through tailoring the mechanical and optical properties. I also describe the first prototype assembly, results from outdoor field-trials, and recent work using micro-electro-mechanical systems engineering to produce a chip-scale device.
Light coupled from a tapered optical fiber is used to excite the morphology dependent whispering gallery mode (WGM) resonances of a silica microsphere-cantilever. Using the optomechanical transduction from the WGM supported by the microsphere1, we can simultaneously detect the thermal Brownian motion of both the microsphere-cantilever and the tapered fiber used for coupling. This allows for active feedback cooling of multiple mechanical modes of the tapered fiber and the microsphere-cantilever using the optical dipole force and a piezo-stack2. Stabilisation of the coupling junction by employing simultaneous cooling of both oscillators is also presented2, useful for many hybrid WGM systems coupled with a tapered waveguide.
We describe cooling of the center-of-mass (c.o.m.) motion of silica microspheres using the morphology dependent whispering gallery mode (WGM) resonances excited by light coupled from a tapered optical fibre. This scheme uses passive cooling via the velocity dependent scattering force from the excitation of WGM resonances in one direction1 and active feedback cooling via cavity enhanced optical dipole forces (CEODF)2 along a perpendicular axis. Initial experiments have shown successful laser frequency locking to a WGM using relatively high coupled powers despite thermal bistability and thermally induced frequency shifts in the WGM. We also demonstrate the optomechanical transduction required for feedback by monitoring the transmission through the tapered fibre, demonstrating the ability to resolve displacements of less than a nanometer and velocities less than 40X10-6 ms-1.
Cavity and Doppler cooling of trapped silica nanospheres and microspheres to their motional ground state is described. Characterisation of the levitation of a range of silica spheres from radius 25 nm to 5 µm in both optical and ion traps in vacuum is reported and prospects for realizing cooling in these systems is discussed.
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