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1. A surface-coupled optical trap with 1-bp precision via active stabilization

   https://doi.org/10.1007/978-1-4939-6421-5  

   S.R. Okoniewski, A.R. Carter (January 2017, Methods in molecular biology)

   Optical traps can measure bead motions with Å-scale precision. However, using this level of precision to infer 1-bp motion of molecular motors along DNA is difficult, since a variety of noise sources degrade instrumental stability. In this chapter, we detail how to improve instrumental stability by (1) minimizing laser pointing, mode, polarization, and intensity noise using an acousto-optical-modulator mediated feedback loop and (2) minimizing sample motion relative to the optical trap using a three-axis piezo-electric-stage mediated feedback loop. These active techniques play a critical role in achieving a surface stability of 1 Å in 3D over tens of seconds and a 1-bp stability and precision in a surface-coupled optical trap over a broad bandwidth (Δf = 0.03–2 Hz) at low force (6 pN). These active stabilization techniques can also aid other biophysical assays that would benefit from improved laser stability and/or Å-scale sample stability, such as atomic force microscopy and super-resolution imaging. 

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2. Investigation and Mitigation of Noise Contributions in a Compact Heterodyne Interferometer

   https://doi.org/10.3390/s21175788  

   Zhang, Yanqi; Hines, Adam S.; Valdes, Guillermo; Guzman, Felipe (September 2021, Sensors)

   We present a noise estimation and subtraction algorithm capable of increasing the sensitivity of heterodyne laser interferometers by one order of magnitude. The heterodyne interferometer is specially designed for dynamic measurements of a test mass in the application of sub-Hz inertial sensing. A noise floor of 3.31×10−11m/Hz at 100 mHz is achieved after applying our noise subtraction algorithm to a benchtop prototype interferometer that showed a noise level of 2.76×10−10m/Hz at 100 mHz when tested in vacuum at levels of 3×10−5 Torr. Based on the previous results, we investigated noise estimation and subtraction techniques of non-linear optical pathlength noise, laser frequency noise, and temperature fluctuations in heterodyne laser interferometers. For each noise source, we identified its contribution and removed it from the measurement by linear fitting or a spectral analysis algorithm. The noise correction algorithm we present in this article can be generally applied to heterodyne laser interferometers. 

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3. Compact optomechanical accelerometers for use in gravitational wave detectors

   https://doi.org/10.1063/5.0142108  

   Hines, A.; Nelson, A.; Zhang, Y.; Valdes, G.; Sanjuan, J.; Guzman, F. (February 2023, Applied Physics Letters)

   We present measurements of an optomechanical accelerometer for monitoring low-frequency noise in gravitational wave detectors, such as ground motion. Our device measures accelerations by tracking the test-mass motion of a 4.7 Hz mechanical resonator using a heterodyne interferometer. This resonator is etched from monolithic fused silica, an under-explored design in low-frequency sensors, allowing a device with a noise floor competitive with existing technologies but with a lighter and more compact form. In addition, our heterodyne interferometer is a compact optical assembly that can be integrated directly into the mechanical resonator wafer to further reduce the overall size of our accelerometer. We anticipate this accelerometer to perform competitively with commercial seismometers, and benchtop measurements show a noise floor reaching 82 pico-g Hz−1/2 sensitivities at 0.4 Hz. Furthermore, we present the effects of air pressure, laser fluctuations, and temperature to determine the stability requirements needed to achieve thermally limited measurements. 

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4. Optomechanical Accelerometers for Geodesy

   https://doi.org/10.3390/rs14174389  

   Hines, Adam; Nelson, Andrea; Zhang, Yanqi; Valdes, Guillermo; Sanjuan, Jose; Stoddart, Jeremiah; Guzmán, Felipe (September 2022, Remote Sensing)

   We present a novel optomechanical inertial sensor for low-frequency applications and corresponding acceleration measurements. This sensor has a resonant frequency of 4.715 (1) Hz, a mechanical quality factor of 4.76(3) × 105, a test mass of 2.6 g, and a projected noise floor of approximately 5 × 10−11 ms−2/Hz at 1 Hz. Such performance, together with its small size, low weight, reduced power consumption, and low susceptibility to environmental variables such as magnetic field or drag conditions makes it an attractive technology for future space geodesy missions. In this paper, we present an experimental demonstration of low-frequency ground seismic noise detection by direct comparison with a commercial seismometer, and data analysis algorithms for the identification, characterization, and correction of several noise sources. 

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5. Air-Coupled Whispering Gallery Mode On-Chip Microspherical Shell Resonator for High-Frequency Ultrasound Detection

   https://doi.org/10.1109/LSENS.2024.3433472  

   Huang, Chichen; Zhang, Jiayuan; Tadigadapa, Srinivas (September 2024, IEEE Sensors Letters)

   Langfelder, Giacomo (Ed.)

   In this letter, we report on a high-sensitivity whispering gallery mode (WGM) resonator-based air-coupled ultrasound sensor capable of detecting minute pressure variations across an ultrasound frequency spectrum of 0.6–3.5 MHz. The sensor comprises a microspherical glass shell of approximately 450 μm in radius and nonuniform shell thickness of 7–15 μm, which is optically coupled to a tunable laser for resonance excitation. The setup allows for the precise measurement of acoustic signals, benefiting from the high optical Q-factor of ∼2 million of the blown glass microspherical shells. A noise equivalent pressure as low as 40 μPa/ √Hz was obtained at 1.72-MHz ultrasound frequency. A very good correspondence between the simulated axisymmetric resonance frequencies measured using the WGM resonator and a 3D finite-element analysis model in COMSOL was established. The sensor showed an expected linear dependence on the drive voltage of the ultrasound transducer. The distortion of the microspherical shell under acoustic pressure was also independently confirmed using a laser Doppler vibrometer. The sensor’s capability to handle high-frequency ultrasonic waves with significantly better signal-to-noise ratio than conventional piezoelectric- or microphone-based systems is demonstrated, highlighting its suitability for advanced photoacoustic applications. 

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