Optomechanical systems have been exploited in ultrasensitive measurements of force, acceleration and magnetic fields. The fundamental limits for optomechanical sensing have been extensively studied and now well understood—the intrinsic uncertainties of the bosonic optical and mechanical modes, together with backaction noise arising from interactions between the two, dictate the standard quantum limit. Advanced techniques based on non-classical probes, in situ ponderomotive squeezed light and backaction-evading measurements have been developed to overcome the standard quantum limit for individual optomechanical sensors. An alternative, conceptually simpler approach to enhance optomechanical sensing rests on joint measurements taken by multiple sensors. In this configuration, a pathway to overcome the fundamental limits in joint measurements has not been explored. Here we demonstrate that joint force measurements taken with entangled probes on multiple optomechanical sensors can improve the bandwidth in the thermal-noise-dominant regime or the sensitivity in the shot-noise-dominant regime. Moreover, we quantify the overall performance of entangled probes with the sensitivity–bandwidth product and observe a 25% increase compared with that of classical probes. The demonstrated entanglement-enhanced optomechanical sensors would enable new capabilities for inertial navigation, acoustic imaging and searches for new physics. Joint force measurements with entangled optical probes on two optomechanical sensors are demonstrated. The force sensitivity is improved by 40% in the shot-noise-dominant regime. The sensing bandwidth is improved by 20% in the thermal noise limit. 

/pdf/entanglement-enhanced-optomechanical-sensing-3h3kflgr.pdf

Entanglement-enhanced optomechanical sensing

In this work, we have synthesized the nickel oxide (NiO) nanopowders via twofold facile solvothermal synthesis. Later, the products are calcined at 300 °C, 400 °C, and 500 °C in an air atmosphere to form NiO nanopowders, which are deposited onto the SiO2/Si substrate via the spin coating technique. The morphological studies depict the high porosity at 400 °C calcinated NiO thin film than other conditions. Further, the Brunauer-Emmett-Teller (BET) surface analysis reveals that at 400 °C calcinated NiO sample exhibits a higher surface area of 52.3 m2g-1 due to the existence of porous grain-like surface morphology. To examine the gas sensing performance, Ag/NiO/Ag hydrogen gas sensor device is fabricated by depositing planar-type silver (Ag) electrodes onto the NiO thin film via thermal evaporation technique. The gas sensing behaviour of the fabricated gas sensor test-devices has been examined at various operating temperatures (100 °C–300 °C) and concentrations (100 ppm–300 ppm). The hydrogen gas sensor test-device fabricated using NiO nanopowder calcined at 400 °C shows good crystalline nature, a highly porous surface with efficient oxygen vacancies, and narrowed bandgap, which results in the maximum responsivity of 97.2% at the operating temperature of 250 °C with response and recovery time of 56 s and 21 s, respectively. Therefore, the Ag/NiO/Ag gas sensor test-device can be a potential route toward the development of hydrogen gas sensor devices. 

Effect of calcination temperature on NiO for hydrogen gas sensor performance

Optical sensing technologies for hydrogen monitoring are of increasing importance in connection with the development and expanded use of hydrogen and for transition to the hydrogen economy. The past decades have witnessed a rapid development of optical sensors for hydrogen monitoring due to their excellent features of being immune to electromagnetic interference, highly sensitive, and widely applicable to a broad range of applications including gas sensing at the sub-ppm range. However, the selection of hydrogen selective metal and metal alloy plays an important role. Considering the major advancements in the field of optical sensing technologies, this review aims to provide an overview of the recent progress in hydrogen monitoring. Additionally, this review highlights the sensing principles, advantages, limitations, and future development.

/pdf/recent-advances-in-optical-hydrogen-sensor-including-use-of-cz4eabg4.pdf

Recent Advances in Optical Hydrogen Sensor including Use of Metal and Metal Alloys: A Review

We demonstrated a hybrid sensor of fiber Bragg grating (FBG) and Fabry-Perot interferometer (FPI) based on fiber-tip microcantilever for simultaneous measurement of temperature and humidity. The FPI was developed using femtosecond (fs) laser-induced two-photon polymerization to print the polymer microcantilever at the end of a single-mode fiber, achieving a humidity sensitivity of 0.348 nm/%RH (40% to 90%, when temperature = 25 °C ± 0.1 °C), and a temperature sensitivity of -0.356 nm/°C (25 to 70 °C, when RH% = 40% ± 1%). The FBG was line-by-line inscribed in the fiber core by fs laser micromachining, with a temperature sensitivity of 0.012 nm/ °C (25 to 70 °C, when RH% = 40% ± 1%). As the shift of FBG-peak on the reflection spectra is only sensitive to temperature rather than humidity, the ambient temperature can be directly measured by the FBG. The output of FBG can also be utilized as temperature compensation for FPI-based humidity measurement. Thus, the measured result of relative humidity can be decoupled from the total shift of FPI-dip, achieving the simultaneous measurement of humidity and temperature. Gaining the advantages of high sensitivity, compact size, easy packaging, and dual parameter measurement, this all-fiber sensing probe is anticipated to be applied as the key component for various applications involving the simultaneous measurement of temperature and humidity.

Simultaneous measurement of humidity and temperature based on fiber-tip microcantilever cascaded with fiber Bragg grating.

Nanomechanical sensors made from suspended graphene are sensitive to pressure changes. However, these devices typically function by obtaining an electrical signal based on the static displacement of a suspended graphene membrane and so, in practice, have limited sensitivity and operational range. The present work demonstrates an optomechanical Au/graphene membrane-based gas pressure sensor with ultrahigh sensitivity. This sensor comprises a suspended Au/graphene membrane appended to a section of hollow-core fiber to form a sealed Fabry-Pérot cavity. In contrast to conventional nanomechanical pressure sensors, pressure changes are monitored via resonant sensing with an optical readout. A miniature pressure sensor based on this principle was able to detect an ultrasmall pressure difference of 1 × 10-7 mbar in the ultrahigh-vacuum region with a pressure range of 4.1 × 10-5 to 8.3 × 10-6 mbar. Furthermore, this pressure sensor can work over an extended pressure range of 7 × 10-6 mbar to 1000 mbar at room temperature, outperforming commercial pressure sensors. Similar results were obtained using both the fundamental and higher-order resonant frequencies but with the latter providing improved sensitivity. This sensor has a wide range of potential applications, including indoor navigation, altitude monitoring, and motion detection.

Nano-optomechanical Resonators for Sensitive Pressure Sensing.

Hydrogen is widely used in industrial production and clinical medicine, and as fuel. Hydrogen becomes explosive when the hydrogen-air mixture ranges from 4 to 76 vol%; thus, a rapid hydrogen concentration measurement is particularly important in practical applications. We present a novel fiber optic hydrogen sensor with fast response fabricated from a graphene-Au-Pd sandwich nanofilm and an ultrashort fiber Bragg grating. The response time is only 4.3 s at a 3.5 vol% hydrogen concentration. When the measured hydrogen concentration was increased from 0 to 4.5 vol%, the optical resonance dip in the sensor near 1550 nm shifted by 290 pm. In addition, the sensor has an insertion loss of only -2.22 dB, a spectral contrast of 10.8 dB, and a spectral finesse of 5. Such a flexible, fast-response sensor is expected to be used in the development of hydrogen sensors with low power consumption.

/pdf/fiber-optic-hydrogen-sensor-based-on-a-fabry-perot-4vb8ji41sq.pdf

Fiber optic hydrogen sensor based on a Fabry-Perot interferometer with a fiber Bragg grating and a nanofilm.

Nanomechanical bolometers have proven to be well suited to the analysis of light. However, conventional wafer-based devices have limited practical applications because they require special vacuum chambers, cryogenic temperatures, bulky space optical components, and/or complex circuitry. The present work developed a nanoscale optomechanical bolometer intended for photothermal sensing using an all-optical actuation and measurement approach. The proposed bolometer is compact and has an integrated all-optical-fiber structure based on fabricating a Fabry–Perot interferometer incorporating multilayer graphene at the fiber tip and packaged in a small vacuum-sealed tube. This microscale vacuum packaging doubled the signal-to-noise ratio compared with that in air. This miniature all-fiber nanoscale optomechanical bolometer also exhibited a high resolution with photothermal sensitivities of approximately 6.23 and 6.44 kHz/μW when using the second-order mode at room temperature and 0 °C, respectively. This design could be beneficial for applications outside specialized laboratories with uses in the fields of medicine, industrial manufacturing, nanoscience, and astronomy. 

Room-Temperature Fiber Tip Nanoscale Optomechanical Bolometer

Nanofilm resonators combine ultracompact and highly mechanically sensitive properties, making them intriguing devices for sensing applications. For trace hydrogen detection, we demonstrate an optomechanical nanofilm resonator by employing a Pd- and Au-decorated graphene onto a fiber end facet. The Pd layer is a sensitive layer for selective absorption of hydrogen. Hydrogen sensing is achieved by all-optical measuring of the resonant frequencies shift of the optomechanical nanofilm resonator induced by hydrogen-related mechanical stress change. Using the approach, we realize highly sensitive hydrogen sensing at room temperature with a low detection limit, challenging the state-of-the-art. When the measured hydrogen concentration increases from 0 to 1000 ppm (v/v), the mechanical resonance frequencies of the sensor at 511.7 kHz, 1253.4 kHz, and 2231.7 kHz blue-shift by 100.4 kHz, 257.5 kHz, and 400.6 kHz, respectively. The response and recovery time are 120.3 and 91.3 s at a 1000 ppm hydrogen concentration. Such a sensor exhibits a low detection limit of 741 ppb and good repeatability in the measurement process, which makes the practical application of the sensor possible.

Highly Sensitive Hydrogen Sensor Based on an Optical Driven Nanofilm Resonator.

The widespread utilization of hydrogen energy has increased the demand for trace hydrogen detection. In this work, we propose a fiber-optic hydrogen sensor based on a Fabry-Pérot Interferometer (FPI) consisting of a fiber-tip graphene-Au-Pd submicron film cantilever. The palladium (Pd) film on the cantilever surface is used as hydrogen-sensitive material to obtain high sensing sensitivity. Hydrogen sensing is realized by monitoring the resonant frequency shift of the FPI introduced by the interaction between Pd film and hydrogen molecules. The hydrogen sensor is proven for low-hydrogen-concentration detection with hydrogen concentrations in the range of 0-1000 ppm, and experimentally characterized by a highest sensitivity of 30.3 pm ppm-1 in a low hydrogen concentration of 0-100 ppm, which is more than two orders higher than for previously reported FPI-based sensors. In real-time hydrogen monitoring, a rapid reaction time of 31.5 s was achieved. This work provides a compact all-optical solution for the safe detection of low hydrogen concentrations, which is an interesting alternative for trace hydrogen detection in the aerospace industry, energy production, and medical applications.

Fiber-tip Fabry-Pérot interferometer with a graphene-Au-Pd cantilever for trace hydrogen sensing.

The paper presents a novel fiber-optic vector magnetic field sensor using a Fabry-Perot interferometer, which consists of an optical fiber end face and a graphene/Au membrane suspended on the ceramic ferrule end face. A pair of gold electrodes are fabricated on the ceramic ferrule by femtosecond laser to transmit electrical current to the membrane. Ampere force is generated when an electrical current flows through the membrane in a perpendicular magnetic field. The change in Ampere force causes a shift in the resonance wavelength in the spectrum. In the magnetic field intensity range of 0 ∼ 180 mT and 0 ∼ -180 mT, the as-fabricated sensor exhibits magnetic field sensitivity of 5.71 pm/mT and 8.07 pm/mT. The proposed sensor has great potential application in weak magnetic field measurements due to its compact structure, cost-effectiveness, ease to manufacture, and good sensing performance.

Ampere force fiber optic magnetic field sensor using a Fabry-Perot interferometer.

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