Noise Equivalent Displacement Research Articles

A wavelength-division-multiplex depolarized Sagnac-based interferometer for noise suppression

The Sagnac-based interferometer (SI) has been developed over several decades and is widely recognized for its sensitivity and stability. Most SIs utilize polarization-maintaining (PM) fiber and polarizers to ensure polarization reciprocity, or depolarizers to mitigate the effects of polarization non-reciprocity. The depolarizing method was considered more promising from a marketability standpoint. Additionally, broadband optical sources were typically used in SIs to eliminate the Kerr effect and backscatter. However, broadband light source SIs are hindered by high excess relative intensity noise (excess RIN) generated by the self-interference of different spectral components. To enhance the performance of SIs, it is crucial to suppress the excess RIN. In this study, we demonstrate a wavelength-division-multiplex depolarized Sagnac-based interferometer and suppress the excess RIN by utilizing the overlap of two interfering lights, which shifts the primary frequency of noise outside the detection range. Experimental results show that the noise equivalent displacement at a light intensity of 42 μW achieves 170.18 fm/√Hz, which encompasses both shot noise and non-optical noise, indicating successful suppression of excess RIN.

Optical Feedback FM-to-AM Conversion with integrated Micro-Ring Resonator for Displacement Sensing Applications

In this study, we show the capability of integrated Micro-Ring Resonators (MRRs) to perform frequency-to-amplitude (FM-to-AM) conversion of optical feedback interferometry (OFI) signals with improved signal-to-noise ratio compared to conventional AM OFI signals. Further, contrary to traditional OFI FM-to-AM conversion techniques using gas cell-based edge filters and free-space or fiber Mach-Zehnder Interferometers (MZI), integrated photonic processing offers greater compactness and perturbation resilience, enhancing noise performance through improved temperature control and immunity to parasitic mechanical vibrations. The OFI FM-to-AM conversion was performed with a fabricated silicon nitride MRR of radius 120 μm and a quality factor of 130,000. The FM-to-AM conversion factor achieved was 0.61 GHz−1, with a noise equivalent displacement of only 4.9 nm for a 1 kHz bandwidth. This demonstration highlights the potential of integrated edge filters to replace traditional freespace and fiber architectures, more prone to environmental perturbations, in OFI signal processing for vibrometric applications.

Native signal self-mix interferometer has less than 1 nm noise equivalent displacement.

We demonstrate that the native configuration of a self-mixing interferometer attains a minimum detectable displacement of 0.72 nm or λ/1870 at the laser wavelength of 1310 nm, obtained by the bare laser diode package including a monitor photodiode, observed in the electrical domain by means of an oscilloscope and without any electronic processing of the signal.

3D Profilometry With a Self-Mixing Interferometer: Analysis of the Speckle Error

We show that the speckle phase error incurred when the laser spot is scanned over the diffusing surface can be made as low as 10– $20~\mu \text{m}$ for object sizes in the range 3–10 cm. This result is new, to the best of our knowledge, because it is usually assumed that when the spot moves of its size, the phase becomes uncorrelated and the measurement cannot be done anymore. We carry out numerical simulations of the process and derive an analytical model to evaluate the phase error and the noise equivalent displacement of the profile measurement by a self-mixing interferometer.

Electrostatic Feedback Actuation for Enhancing the Dynamic Range of MOEMS Displacement Sensors

In the past years, researches demonstrated the great potential of novel MOEMS vibration sensors regarding their displacement resolution and sensitivity. The authors show that it is possible to further enhance the dynamic range of these sensors. This is achieved by utilizing feedback mechanisms such as electrostatic actuation without any crosstalk to the optical readout. Furthermore, this allows for canceling out unwanted ringing. The discussed proof of concept prototypes incorporate two electrostatic comb-drive actuators. They enable the seismic mass to reset along the main oscillation axes over a path of several microns and up to the resonance frequency of 400 Hz. It was proven that there is no unwanted crosstalk between the actuation and the position readout. The MOEMS readout features a sensitivity of approximately 200 mV/μm with a noise equivalent displacement of 35 pm/√Hz.

MOEMS Vibration Sensor for Advanced Low-frequency Applications with pm Resolution

Abstract The majority of MEMS vibration sensors requires relatively high resonance frequencies of several kilohertz to avoid mechanical contact of moving parts such as electrode plates. In contrast to that, our hybrid micro-opto-electro- mechanical system (MOEMS) enables sensor applications at low frequencies. This work describes a distinct MOEMS featuring extremely low resonance frequencies of below 200 Hz. It is operated ambient air without closed loop feed- back, extensive electronics and cooling. Due to the soft suspension of the micro-mechanical sub-system the funda- mental limit, the Brownian noise floor, is reached. The resulting noise equivalent displacement for frequencies above the resonance is 1.9 pm/ √Hz, which is equivalent to 0.29 μg/ √Hz below the resonance. We describe the design space for sensors with further enhanced sensitivity and resonance frequencies far below 100 Hz.

An integrated fiber-optic microfluidic device for detection of muscular force generation of microscopic nematodes

This paper reports development of an integrated fiber-optic microfluidic device for measuring muscular force of small nematode worms with high sensitivity, high data reliability, and simple device structure. A moving nematode worm squeezed through multiple detection points (DPs) created between a thinned single mode fiber (SMF) cantilever and a sine-wave channel with open troughs. The SMF cantilever was deflected by the normal force imposed by the worm, reducing optical coupling from the SMF to a receiving multimode fiber (MMF). Thus, multiple force data could be obtained for the worm-SMF contacts to verify with each other, improving data reliability. A noise equivalent displacement of the SMF cantilever was 0.28 μm and a noise equivalent force of the device was 143 nN. We demonstrated the workability of the device to detect muscular normal forces of the parasitic nematodes Oesophagotomum dentatum L3 larvae on the SMF cantilever. Also, we used this technique to measure force responses of levamisole-sensitive (SENS) and resistant (LERV) O. dentatum isolates in response to different doses of the anthelmintic drug, levamisole. The results showed that both of the isolates generated a larger muscular normal force when exposed to a higher concentration of levamisole. We also noticed muscular force phenotype differences between the SENS and LERV worms: the SENS muscles were more sensitive to levamisole than the LERV muscles. The ability to quantify the muscular forces of small nematode worms will provide a new approach for screening mutants at single animal resolution. Also, the ability to resolve small differences in muscular forces in different environmental conditions will facilitate phenotyping different isolates of nematodes. Thus, the present technology can potentially benefit and advance the current whole animal assays.

An optical fibre interferometer for remote detection of laser generated ultrasonics

A 15 MHz bandwidth active homodyne fibre-optical interferometer for remote detection of laser-induced ultrasonic waves in metallic and non-metallic targets is reported. The fibre interferometer design and associated electronics are described in sufficient detail for it to be easily constructed. The interferometer uses a low-power He-Ne laser and the reflected signal from the natural surface of the target at a distance of 200 mm. Despite its use of an open air path in the sensing arm and an all-fibre design, the interferometer achieved a single-pulse noise equivalent displacement of 0.4 nm and a figure of merit of 0.1 pm .

Measurement of a very high displacement sensitivity of the beat frequency in an He-Ne laser

An He-Ne laser with enhanced sensitivity to small displacements is described. The laser is forced to oscillate in two longitudinal modes by an internal mode selector of the Fox-Smith interferometric type. Displacements are induced in one arm of the mode selector and cause a frequency modulation of the beat frequency between the two modes. Experimental data is presented which demonstrate that the frequency modulation varies quadratically with the displacements. This behavior is explained in terms of changes in the dispersive index of refraction of the amplifying medium, induced by the displacements. The laser beat frequency has a displacement sensitivity of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4 \times 10^{11}</tex> Hz/cm and a noise equivalent displacement of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> cm/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> . The system was used to measure displacements on the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> Å.