A High Stability Optical Shadow Sensor with Applications for Precision Accelerometers

Gravimeters are devices which measure changes in the value of the gravitational acceleration, \textit{g}. This information is used to infer changes in density under the ground allowing the detection of subsurface voids; mineral, oil and gas reserves; and even the detection of the precursors of volcanic eruptions. A micro-electro mechanical system (MEMS) gravimeter has been fabricated completely in silicon allowing the possibility of cost e-effective, lightweight and small gravimeters. To obtain a measurement of gravity, a highly stable displacement measurement of the MEMS is required. This requires the development of a portable electronics system that has a displacement sensitivity of $\leq 2.5$ nm over a period of a day or more. The portable electronics system presented here has a displacement sensitivity $\leq 10$ nm$/\sqrt{\textrm{Hz}}$ ($\leq 0.6$ nm at $1000$ s). The battery power system used a modulated LED for measurements and required temperature control of the system to $\pm$ 2 mK, monitoring of the tilt to $\pm$ 2 $\mu$radians, the storage of measured data and the transmission of the data to an external server.Comment: 8 Pages, 12 figures, 5 equations, currently submitted and under review at IEEE Sensors SIE

Sub-shot-noise shadow sensing with quantum correlations

The quantised nature of the electromagnetic field sets the classical limit to the sensitivity of position measurements. However, techniques based on the properties of quantum states can be exploited to accurately measure the relative displacement of a physical object beyond this classical limit. In this work, we use a simple scheme based on the split-detection of quantum correlations to measure the position of a shadow at the single-photon light level, with a precision that exceeds the shot-noise limit. This result is obtained by analysing the correlated signals of bi-photon pairs, created in parametric downconversion and detected by an electron multiplying CCD (EMCCD) camera employed as a split-detector. By comparing the measured statistics of spatially anticorrelated and uncorrelated photons we were able to observe a significant noise reduction corresponding to an improvement in position sensitivity of up to 17% (0.8dB). Our straightforward approach to sub-shot-noise position measurement is compatible with conventional shadow-sensing techniques based on the split-detection of light-fields, and yields an improvement that scales favourably with the detector’s quantum efficiency

MEMS Gravity Sensors for Imaging Density Anomalies

Gravimeters measure small changes in the local gravitational acceleration. They are applied for environmental monitoring, oil and gas prospecting and defence and security. Gravimeters used in these applications have a remarkable sensitivity but at a cost of being bulky and very expensive. Recently, a micro-electrical mechanical system (MEMS) gravimeter has been developed, which was cheap, had a comparable sensitivity to commercial gravimeters and maintained its stability over long timescales (10−6 Hz). In this paper we discuss to replace the current shadow sensor readout with an on-chip interferometer. This new readout has a higher sensitivity so that the device can be more robust and reduces the system size. The design of this readout is discussed and the first experimental results are presented. The new readout improves the imaging capabilities of density anomalies of the device

Field tests of a portable MEMS gravimeter

Gravimeters are used to measure density anomalies under the ground. They are applied in many different fields from volcanology to oil and gas exploration, but present commercial systems are costly and massive. A new type of gravity sensor has been developed that utilises the same fabrication methods as those used to make mobile phone accelerometers. In this study, we describe the first results of a field-portable microelectromechanical system (MEMS) gravimeter. The stability of the gravimeter is demonstrated through undertaking a multi-day measurement with a standard deviation of 5.58 × 10−6 ms−2 . It is then demonstrated that a change in gravitational acceleration of 4.5 × 10−5 ms−2 can be measured as the device is moved between the top and the bottom of a 20.7 m lift shaft with a signal-to-noise ratio (SNR) of 14.25. Finally, the device is demonstrated to be stable in a more harsh environment: a 4.5 × 10−4 ms−2 gravity variation is measured between the top and bottom of a 275-m hill with an SNR of 15.88. These initial field-tests are an important step towards a chip-sized gravity senso

Exploring the multiple policy objectives for Primary Care Networks: a qualitative interview study with national policy stakeholders

Objectives: English general practice is suffering a workforce crisis, with GPs retiring early and trainees reluctant to enter the profession. To address this, additional funding has been offered, but only through participation in collaborations known as Primary Care Networks (PCNs). This study explored national policy objectives underpinning PCNs, and mechanisms expected to help achieve these, from the perspective of those driving the policy. Design: Qualitative semi-structured interviews and policy document analysis. Setting and participants: National level policy maker and stakeholder interviewees (n=16). Policy document analysis of Network Contract Direct Enhanced Service draft service specifications. Analysis: Interviews were transcribed, coded, and organised thematically according to policy objectives and mechanisms. Thematic data was organised into a matrix so prominent elements to be identified and emphasised accordingly. Themes were considered alongside objectives embedded in PCN draft service delivery requirements. Results: Three themes of policy objectives and associated mechanisms were identified:(1) Supporting general practice;(2) Place-based inter-organisational collaboration;(3) Primary care ‘voice’. Interviewees emphasised and sequenced themes differently suggesting meeting objectives for one was necessary to realise another. Interviewees most closely linked to primary care emphasised the importance of(1). The objectives embedded in draft service delivery requirements primarily emphasised(2). Conclusions: These policy objectives are not mutually exclusive but may imply different approaches to prioritising investment or necessitate more explicit temporal sequencing, with the stabilisation of a struggling primary care sector probably needing to occur before meaningful engagement with other community service providers can be achieved or a ‘collective voice’ is agreed. Multiple objectives create space for stakeholders to feel dissatisfied when implementation details do not match expectations, as the negative reaction to draft service delivery requirements illustrates. Our study offers policy makers suggestions about how confidence in the policy might be restored by crafting delivery requirements so all groups see opportunities to meet favoured objectives

Normalising Flows for Bayesian Gravity Inversion

Gravity inversion is a commonly applied data analysis technique in the field of geophysics. While machine learning methods have previously been explored for the problem of gravity inversion, these are deterministic approaches returning a single solution deemed most appropriate by the algorithm. The method presented here takes a different approach, where gravity inversion is reformulated as a Bayesian parameter inference problem. Samples from the posterior probability distribution of source model parameters are obtained via the implementation of a generative neural network architecture known as Normalising Flows. Due to its probabilistic nature, this framework provides the user with a range of source parameters and uncertainties instead of a single solution, and is inherently robust against instrumental noise. The performance of the Normalising Flow is compared to that of an established Bayesian method called Nested Sampling. It is shown that the new method returns results with comparable accuracy 200 times faster than standard sampling methods, which makes Normalising Flows a suitable method for real-time inversion in the field. When applied to data sets with high dimensionality, standard sampling methods can become impractical due to long computation times. It is shown that inversion using Normalising Flows remains tractable even at 512 dimensions and once the network is trained, the results can be obtained in $O(10)$ seconds.Comment: 14 pages, 6 figures, submitted for publication in Computers & Geosciences Journa

Microelectromechanical system gravimeters as a new tool for gravity imaging

A microelectromechanical system (MEMS) gravimeter has been manufactured with a sensitivity of 40 ppb in an integration time of 1 s. This sensor has been used to measure the Earth tides: the elastic deformation of the globe due to tidal forces. No such measurement has been demonstrated before now with a MEMS gravimeter. Since this measurement, the gravimeter has been miniaturized and tested in the field. Measurements of the free-air and Bouguer effects have been demonstrated by monitoring the change in gravitational acceleration measured while going up and down a lift shaft of 20.7 m, and up and down a local hill of 275 m. These tests demonstrate that the device has the potential to be a useful field-portable instrument. The development of an even smaller device is underway, with a total package size similar to that of a smartphone

Dual-band single-pixel telescope

Single-pixel imaging systems can obtain images from a wide range of wavelengths at low-cost compared to those using conventional multi-pixel, focal-plane array sensors, especially at wavelengths outside the visible spectrum. The ability to sense short-wave infrared radiation with single-pixel techniques extends imaging capability to adverse weather conditions and environments, such as fog, haze, or night time. In this work, we demonstrate a dual-band single-pixel telescope for imaging at both visible (VIS) and short-wave infrared (SWIR) spectral regions simultaneously under some of these outdoor weather conditions. At 64 × 64 pixel-resolution, our system has achieved continuous VIS and SWIR imaging of various objects at a frame rate up to 2.4 Hz. Visual and contrast comparison between the reconstructed VIS and SWIR images emphasizes the significant contribution of infrared observation using the single-pixel technique. The single-pixel telescope provides an alternative cost-effective imaging solution for synchronized dual-waveband optical applications

Improved fused silica fibres for the advanced LIGO monolithic suspensions

To further increase the sensitivity of the advanced LIGO (aLIGO) gravitational wave detectors, two major upgrades of the monolithic fused silica suspension are considered: a higher stress in the suspension fibres and a longer final suspension stage. One of the challenges for this upgrade will be producing thinner and longer fibres that can hold the test mass safely. We demonstrate that laser power fluctuations during the fibre fabrication process can produce diameter variations and potentially weak fibres. We present a laser intensity stabilisation technology for fused silica fibre fabrication using a camera system to monitor heating. Fibres fabricated with this new technique showed a 34% decrease in the interquartile range of measured breaking stress, which indicates that the application of intensity stabilisation technology can improve the statistical strength of fused silica fibres by tightening the spread of values. As the aLIGO detectors upgrade plan (A+) proposes to use thinner fibres, it is essential to enhance the performance of fused silica fibres

Optical Readout Design for a MEMS Semi-Absolute Pendulum Gravimeter

Gravimetry has many useful applications from volcanology to oil exploration; being a method able to infer density variations beneath the ground. Therefore, it can be used to provide insight into subsurface processes such as those related to the hydrothermal and magmatic systems of volcanoes. Existing gravimeters are costly and heavy, but this is changing with the utilisation of a technology most notably used in mobile phone accelerometers: MEMS –(Microelectromechanical-systems). Glasgow University has already developed a relative MEMS gravimeter and is currently collaborating with multiple European institutions to make a gravity sensor network around Mt Etna - NEWTON-g. A second generation of the MEMS sensor is now being designed and fabricated in the form of a semi-absolute pendulum gravimeter. Gravity data for geodetic and geophysical use were provided by pendulum measurements from the 18th to the 20th century. However, scientists and engineers reached the limit of fabrication tolerances and readout accuracy approximately 100 years ago. With nanofabrication and modern electronics techniques, it is now possible to create a competitive pendulum gravimeter again. The pendulum method is used to determine gravity values from the oscillation period of a pendulum with known length. The current design couples two pendulums together. Here, an optical shadow-sensor pendulum readout technique is presented. This employs an LED and split photodiode set-up. This optical readout can provide measurements to sub-nanometre precision, which could enable gravitational sensitivities for useful geophysical surveying. If semi-absolute values of gravity can be measured, then instrumental drift concerns are reduced. Additionally, the need for calibration against commercial absolute gravimeters may not be necessary. This promotes improved accessibility of gravity measurements at an affordable cost