Qianbo Lu

Minimizing cross-axis sensitivity in grating-based optomechanical accelerometers.

Cross-axis sensitivity of single-axis optomechanical accelerometers, mainly caused by the asymmetric structural design, is an essential issue primarily for high performance applications, which has not been systematically researched. This paper investigates the generating mechanism and detrimental effects of the cross-axis sensitivity of a high resoluion single-axis optomechanical accelerometer, which is composed of a grating-based cavity and an acceleration sensing chip consisting of four crab-shaped cantilevers and a proof mass. The modified design has been proposed and a prototype setup has been built based on the model of cross-axis sensitivity in optomechanical accelerometers. The characterization of the cross-axis sensitivity of a specific optomechanical accelerometer is quantitatively discussed for both mechanical and optical components by numerical simulation and theoretical analysis in this work. The analysis indicates that the cross-axis sensitivity decreases the contrast ratio of the interference signal and the acceleration sensitivity, as well as giving rise to an additional optical path difference, which would impact the accuracy of the accelerometer. The improved mechanical design is achieved by double side etching on a specific double-substrate-layer silicon-on-insulator (SOI) wafer to move the center of the proof mass to the support plane. The experimental results demonstrate that the modified design with highly symmetrical structure can suppress the cross-axis sensitivity significantly without compromising the sensitivity and resolution. The cross-axis sensitivity defined by the contrast ratio of the output signal drops to 2.19% /0.1g from 28.28%/0.1g under the premise that the acceleration sensitivity of this single-axis optomechanical accelerometer remains 1162.45V/g and the resolution remains 1.325μg.

A MOEMS Accelerometer Based on Diffraction Grating with Improved Mechanical Structure

(250 Words) In this paper, an improved MOEMS accelerometer is described, which is based on integrated grating with phase modulation. This device is composed of a laser diode, an optoelectronic processing circuit and a sensing chip consisted of a piezoelectric translator, an integrated grating as a reflective mirror on a transparent substrate and a mechanical part of a bulk silicon proof mass suspended by four cantilevers whose upper surface acted as another mirror. This device generates a series of interference fringes by two diffracted beams when illuminated with coherent light, whose intensities are modulated by the relative distance between the grating and the proof mass. The intensities of the interference fringes varied with the distance alteration caused by external accelerations, which is proportional to the acceleration. It is realizable to get the magnitude of acceleration by using a differential circuit to detect the distance. A modified structure is introduced in this paper to obtain high sensitivity and reduce cross-sensitivity between different sensitive axes. Compared to experimental results before the simulation and theory analysis demonstrate that this modified MOEMS accelerometer has a good performance with higher static acceleration sensitivity of 3×10V/g and very low crosstalk.

Single Chip-Based Nano-Optomechanical Accelerometer Based on Subwavelength Grating Pair and Rotated Serpentine Springs

Optical coupling between subwavelength grating pairs allows for the precise measurement of lateral or vertical displacement of grating elements and gives rise to different types of displacement and inertial sensors. In this paper, we demonstrate a design for a nano-optomechanical accelerometer based on a subwavelength grating pair that can be easily fabricated by a single Silicon-on-insulator (SOI) chip. The parameters of the subwavelength grating pair-based optical readout, including period, duty cycle, thickness of grating and metal film, and the distance of the air gap, were optimized by combining a genetic algorithm and rigorous coupled wavelength analysis (RCWA) to obtain the optimal sensitivity to the displacement of suspended grating element and the acceleration. A corresponding mechanical design was also completed to meet the highly sensitive acceleration measurement requirement while considering the mechanical cross-axis sensitivity, dynamic range, bandwidth, and fabrication feasibility. This device was verified by both RCWA and finite-different-time-domain methods, and a tolerance analysis was also completed to confirm that it is able to achieve the extremely high optical displacement sensitivity of 1.8%/nm, acceleration-displacement sensitivity of 1.56 nm/mg, and acceleration measurement sensitivity of more than 2.5%/mg, which is almost one order of magnitude higher than any reported counterparts. This work enables a single SOI-based high performance accelerometer, and provides a theoretical basis and fabrication guides for the design.

Optical Acceleration Measurement Method with Large Non-ambiguity Range and High Resolution via Synthetic Wavelength and Single Wavelength Superheterodyne Interferometry

Interferometric optomechanical accelerometers provide superior resolution, but the application is limited due to the non-ambiguity range that is always less than half of the wavelength, which corresponds to the order of mg. This paper proposes a novel acceleration measurement method based on synthetic wavelength and single wavelength superheterodyne interferometry to address this issue. Two acousto-optical modulators and several polarizers are introduced to the two-wavelength interferometry to create four beams with different frequencies and polarization states, and two ultra-narrow bandwidth filters are used to realize the single wavelength measurement simultaneously. This technique offers the possibility to expand the non-ambiguity range without compromising the high resolution. Also, the superheterodyne phase measurement and the corresponding processing algorithm are given to enable real-time measurement. A prototype is built and the preliminary experimental results are compared with the simulation results, showing good agreement. The results prove an estimated acceleration measurement resolution of around 10 μg and a non-ambiguity range of larger than 200 mg, which is more than 100 times that of the single wavelength-based optical accelerometer.

The analysis of temperature effect and temperature compensation of MOEMS accelerometer based on a grating interferometric cavity

There is a temperature drift of an accelerometer attributed to the temperature variation, which would adversely influence the output performance. In this paper, a quantitative analysis of the temperature effect and the temperature compensation of a MOEMS accelerometer, which is composed of a grating interferometric cavity and a micromachined sensing chip, are proposed. A finite-element-method (FEM) approach is applied in this work to simulate the deformation of the sensing chip of the MOEMS accelerometer at different temperature from -20°C to 70°C. The deformation results in the variation of the distance between the grating and the sensing chip of the MOEMS accelerometer, modulating the output intensities finally. A static temperature model is set up to describe the temperature characteristics of the accelerometer through the simulation results and the temperature compensation is put forward based on the temperature model, which can improve the output performance of the accelerometer. This model is permitted to estimate the temperature effect of this type accelerometer, which contains a micromachined sensing chip. Comparison of the output intensities with and without temperature compensation indicates that the temperature compensation can improve the stability of the output intensities of the MOEMS accelerometer based on a grating interferometric cavity.

Mechanical design optimization of a single-axis MOEMS accelerometer based on a grating interferometry cavity for ultrahigh sensitivity

The ultrahigh static displacement-acceleration sensitivity of a mechanical sensing chip is essential primarily for an ultrasensitive accelerometer. In this paper, an optimal design to implement to a single-axis MOEMS accelerometer consisting of a grating interferometry cavity and a micromachined sensing chip is presented. The micromachined sensing chip is composed of a proof mass along with its mechanical cantilever suspension and substrate. The dimensional parameters of the sensing chip, including the length, width, thickness and position of the cantilevers are evaluated and optimized both analytically and by finite-element-method (FEM) simulation to yield an unprecedented acceleration-displacement sensitivity. Compared with one of the most sensitive single-axis MOEMS accelerometers reported in the literature, the optimal mechanical design can yield a profound sensitivity improvement with an equal footprint area, specifically, 200% improvement in displacement-acceleration sensitivity with moderate resonant frequency and dynamic range. The modified design was microfabricated, packaged with the grating interferometry cavity and tested. The experimental results demonstrate that the MOEMS accelerometer with modified design can achieve the acceleration-displacement sensitivity of about 150μm/g and acceleration sensitivity of greater than 1500V/g, which validates the effectiveness of the optimal design.

The modulation and demodulation module of a high resolution MOEMS accelerometer

A MOEMS accelerometer with high precision based on grating interferometer is demonstrated in this paper. In order to increase the signal-to-noise ratio (SNR) and accuracy, a specific modulator and an orthogonal phase-lock demodulator are proposed. Phase modulation is introduced to this accelerometer by applying a sinusoidal signal to a piezoelectric translator (PZT) amounted to the accelerometer. Phase demodulation module consists of a circuit design and a digital design. In the circuit design, the modulated light intensity signal is converted to a voltage signal and processed. In the digital part, the demodulator is mainly composed of a Band Pass Filter, two Phase-Sensitive Detectors, a phase shifter, and two Low Pass Filters based on virtual instrument. Simulation results indicate that this approach can decrease the noise greatly, and the SNR of this demodulator is 50dB and the relative error is less than 4%.

Analysis and amelioration about the cross-sensitivity of a high resolution MOEMS accelerometer based on diffraction grating

Cross-sensitivity is a crucial parameter since it detrimentally affect the performance of an accelerometer, especially for a high resolution accelerometer. In this paper, a suite of analytical and finite-elements-method (FEM) models for characterizing the mechanism and features of the cross-sensitivity of a single-axis MOEMS accelerometer composed of a diffraction grating and a micromachined mechanical sensing chip are presented, which have not been systematically investigated yet. The mechanism and phenomena of the cross-sensitivity of this type MOEMS accelerometer based on diffraction grating differ quite a lot from the traditional ones owing to the identical sensing principle. By analyzing the models, some ameliorations and the modified design are put forward to suppress the cross-sensitivity. The modified design, achieved by double sides etching on a specific double-substrate-layer silicon-on-insulator (SOI) wafer, is validated to have a far smaller cross-sensitivity compared with the design previously reported in the literature. Moreover, this design can suppress the cross-sensitivity dramatically without compromising the acceleration sensitivity and resolution.

A novel scheme design of a high-g optical NEMS accelerometer based on a single chip grating with proper sensitivity and large bandwidth

In this paper, an innovative scheme design of an optical NEMS accelerometer based on a grating with proper sensitivity and dynamic range is proposed, which can be fabricated by a single SOI chip and IC integration technique. An optical micro cavity composed of a grating and the bottom bulk silicon coated with argentum serves as a high sensitivity displacement sensor. The optical displacement detecting principle is specifically discussed in this paper. The parameter of grating, which acts as both the inertial mass and optical element, has been designed through the optimization of FDTD. In addition, a quad serpentine beam structure is introduced to this accelerometer to transfer the acceleration to the displacement of the inertial mass. The performance of the mechanical structure is optimized by the ANSYS, which can acquire high mechanical sensitivity and extremely low crosstalk between different sensitive axes.

Detection of defects in optics based on scanning

In this paper, a method to detect internal pocks and bubbles of optical elements based on laser line source scanning is proposed. In dark field environment, a laser line source is used to illuminate from one side of the glass under test, a high-resolution CCD camera is used to take pictures in front of the glass sample. Images which contain information of defects are acquired through rough scanning and accurate scanning. Accurate three-dimensional coordinates of the internal defects are acquired after image processing, which descript the characteristic information of internal defects quantificationally. Compared with the microscope imaging measurement, this proposed detection of defects in optics based on laser line source scanning has a relative aberration smaller than 2%. In addition, the detection time is approximately reduced to 20 minutes from 1 hour dramatically. The analysis indicates that the error of the position of defects is much smaller than the size of them, which means the position of the defects can be acquired accurately by this approach.