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.