Coherently parallel fiber-optic distributed acoustic sensing using dual Kerr soliton microcombs.

Fiber-optic distributed acoustic sensing (DAS) has proven to be a revolutionary technology for the detection of seismic and acoustic waves with ultralarge scale and ultrahigh sensitivity, and is widely used in oil/gas industry and intrusion monitoring. Nowadays, the single-frequency laser source in DAS becomes one of the bottlenecks limiting its advance. Here, we report a dual-comb-based coherently parallel DAS concept, enabling linear superposition of sensing signals scaling with the comb-line number to result in unprecedented sensitivity enhancement, straightforward fading suppression, and high-power Brillouin-free transmission that can extend the detection distance considerably. Leveraging 10-line comb pairs, a world-class detection limit of 560 fε/√Hz@1 kHz with 5 m spatial resolution is achieved. Such a combination of dual-comb metrology and DAS technology may open an era of extremely sensitive DAS at the fε/√Hz level, leading to the creation of next-generation distributed geophones and sonars.

High-performance towing cable hydrophone array with an improved ultra-sensitive fiber-optic distributed acoustic sensing system.

A high-performance towing cable hydrophone array based on an improved ultra-sensitive fiber-optic distributed acoustic sensing system (uDAS) with picostrain sensitivity is demonstrated and tested in sea trial, for the first time. A new composite transducer is designed and optimized to enhance the acoustic pressure sensitivity significantly. A sea trial is carried out to test the performances of such a hydrophone array, including flow noise, underwater acoustic signal capture capacity, beamforming processing and localization of artificial source targets. The array exhibits high sensitivity and low noise floor. An average sensitivity of -129.23 dB re rad/µPa at frequencies from 10 Hz to 1500 Hz has been achieved. The localization at distances of 5 km and 10 km is realized, respectively, validating the excellent remote detection and positioning capability of the hydrophone system. The proposed towing cable system, with high sensitivity, simple structure and remote target localization ability, may pave a way for development of the next generation of high-performance light-weighting hydrophone arrays for towing applications.

Kilometers Long Graphene-Coated Optical Fibers for Fast Thermal Sensing.

The combination of optical fiber with graphene has greatly expanded the application regimes of fiber optics, from dynamic optical control and ultrafast pulse generation to high precision sensing. However, limited by fabrication, previous graphene-fiber samples are typically limited in the micrometer to centimeter scale, which cannot take the inherent advantage of optical fibers-long-distance optical transmission. Here, we demonstrate kilometers long graphene-coated optical fiber (GCF) based on industrial graphene nanosheets and coating technique. The GCF shows unusually high thermal diffusivity of 24.99 mm2 s-1 in the axial direction, measured by a thermal imager directly. This enables rapid thermooptical response both in optical fiber Bragg grating sensors at one point (18-fold faster than conventional fiber) and in long-distance distributed fiber sensing systems based on backward Rayleigh scattering in optical fiber (15-fold faster than conventional fiber). This work realizes the industrial-level graphene-fiber production and provides a novel platform for two-dimensional material-based optical fiber sensing applications.

Cladding softened fiber for sensitivity enhancement of distributed acoustic sensing.

Fiber-optic distributed acoustic sensing (DAS) technology with high spatial and strain resolutions has been widely used in many practical applications. New methods to enhance the phase sensitivity of sensing fiber are worth exploring to further improve DAS performances, although the standard single-mode fiber (SSMF) has been widely used for DAS technology. In this work, we propose and demonstrate the concept of enhancing the phase sensitivity of DAS by softening the cladding of the sensing fiber, for the first time. The theoretical analysis indicates that softening sensing fiber cladding is an effective way to improve phase sensitivity. Thus, we fabricated cladding softened fibers (CSFs) and tested their phase sensitivities experimentally. According to the results, it is found that the phase sensitivity of the CSF with 0.48 WT% phosphorus-doping concentration and 80 µm cladding diameter is 22% and 54% higher than that of the non-phosphorus-doping fiber with 80 µm cladding diameter and SSMF, respectively. The results show that by reducing fiber cladding Young's modulus with higher phosphorus-doping concentration, the DAS phase sensitivity can be enhanced effectively, verifying the theoretical analysis. Also, we found that the phase sensitivity enhancement of the sensing fiber has a linear relationship with the cladding phosphorus-doping concentration, i.e. Young's modulus. In conclusion, the reported CSF paves a way for improving the DAS phase sensitivity and would be applied to other major optical fiber sensing systems as a better sensing element over SSMF due to the enhancement in the elasto-optical effect of the sensing fiber.

Ultrasensitive all-fiber inline Fabry-Perot strain sensors for aerodynamic measurements in hypersonic flows.

This work proposes an ultrasensitive, temperature-insensitive, all-fiber inline Fabry-Perot (FP) strain sensor for aerodynamic coefficients measurements of a hypervelocity ballistic correlation model 2 in a Φ1 hypersonic wind tunnel. The FP sensors fabricated using 157 nm laser micromachining system are structurally simple, small-sized, and high-temperature resistance. 16 FP sensors are installed on a six-force balance, which is mounted inside the model, to sense the aerodynamic forces and moments of the model, and then the model's aerodynamic coefficients are calculated based on aerodynamic theory according to the test data. A new temperature-compensated method is proposed to improve measurement accuracy of aerodynamic coefficients via eliminating temperature-induced measurement errors. Experimental results show, at high temperatures, the FP sensors based on the balance (FP balance) exhibits a high-repeatability precision of the aerodynamic coefficients measurement of less than 1%, and match well with the results of the traditional method using foil-resistive strain sensors. This enhanced-sensitivity FP sensor is currently the most promising alternative to foil-resistive strain sensors for aerodynamic tests among kinds of fiber-optic strain sensors to the best of our knowledge. The FP balance satisfies the requirements of practical application of aerodynamic characteristic tests, and opens up another test system of the field.

Temperature-insensitive fiber-optic tip sensors array based on OCMR for multipoint refractive index measurement.

A temperature-insensitive fiber-optic tip sensors array is proposed for multipoint refractive index measurement using optical carrier based on microwave reflection (OCMR). The tip sensors array is made of a series of cleaved fiber end-faces and is spatially multiplexed by physically connecting with a fiber-optic splitter with different lengths of short delay fiber. A sensors array with eight sensing tips is demonstrated for multipoint refractive index measurement. Experimental results show that it can offer a high refractive-index resolution of 3.60 × 10-6 RIU and a low temperature-refractive index cross sensitivity of 3.74 × 10-7 RIU/°C. Such a sensors array not only possesses excellent sensing performances, but also can be integrated into a chip for biochemical sensing applications.

Hypersonic Aerodynamic Force Balance Using Micromachined All-Fiber Fabry⁻Pérot Interferometric Strain Gauges.

This paper presents high-sensitivity, micromachined all-fiber Fabry-Pérot interferometric (FFPI) strain gauges and their integration in a force balance for hypersonic aerodynamic measurements. The FFPI strain gauge has a short Fabry-Pérot cavity fabricated using an excimer laser etching process, and the deformation of the cavity is detected by a white-light optical phase demodulator. A three-component force balance, using the proposed FFPI gauges as sensing elements, was fabricated, calibrated, and experimentally evaluated. To reduce thermal output of the balance, a simple and effective self-temperature compensation solution, without external temperature sensors, is proposed and examined through both oven heating and wind tunnel runs. As a result of this approach, researchers are able to use the balance continuously throughout a wide range of temperatures. During preliminary testing in a hypersonic wind tunnel with a free stream Mach number of 12, the measurement accuracies of the balance were clearly improved after applying the temperature self-compensation.

Highly Integrated All-Fiber FP/FBG Sensor for Accurate Measurement of Strain under High Temperature.

Accurate measurement of strain is one of the most important issues for high temperature environments. We present a highly integrated all-fiber sensor to achieve precise measurements of strain/high-pressure, which consists of a fiber Bragg grating (FBG) inscribed by an 800 nm femtosecond laser cascaded with a micro extrinsic Fabry⁻Perot (FP) cavity fabricated by the 157 nm laser micromachining technique. FBG is sensitive to temperature, but insensitive to strain/pressure, whereas the FP is sensitive to strain/pressure, but has a small dependence on temperature. Therefore, such a cascaded sensor could be used for dual-parameter measurement and can work well at high temperatures. Experimental results indicate that this device exhibits a good strain characteristic at high temperatures and excellent high-pressure performance at room temperature. Due to its highly sensitive wavelength response, the proposed sensor will have remarkable potential applications in dual parameter sensing in harsh environments.

Integrated FP/RFBG sensor with a micro-channel for dual-parameter measurement under high temperature.

An integrated sensor via overlapping a micro Fabry-Perot (MFP) cavity with a micro-channel on a regenerated fiber Bragg grating (RFBG) is constructed for dual-parameter sensing of temperature, strain, and gas pressure under a high temperature (600°C). The MFP is fabricated by using a 157 nm micro-machining on H2-loaded bendinsensitive fiber. A fiber Bragg grating (FBG) is inscribed at the same position of the MFP using 248 nm laser exposure, and then successfully regenerated after a required annealing process which enhances the strain sensitivity of MFP more than three times. The micro-channel created on the MFP is used to improve gas pressure sensitivity of the MFP nearly 100 times. Since the MFP and RFBG have different sensitivities to gas pressure, strain, and temperature, the sensor head could be used to perform dual-parameter measurement by simultaneous measurement of high temperature and strain, and high temperature and gas pressure.

Laser-machined microcavities for simultaneous measurement of high-temperature and high-pressure.

Laser-machined microcavities for simultaneous measurement of high-temperature and high-pressure are demonstrated. These two cascaded microcavities are an air cavity and a composite cavity including a section of fiber and an air cavity. They are both placed into a pressure chamber inside a furnace to perform simultaneous pressure and high-temperature tests. The thermal and pressure coefficients of the short air cavity are ~0.0779 nm/°C and ~1.14 nm/MPa, respectively. The thermal and pressure coefficients of the composite cavity are ~32.3 nm/°C and ~24.4 nm/MPa, respectively. The sensor could be used to separate temperature and pressure due to their different thermal and pressure coefficients. The excellent feature of such a sensor head is that it can withstand high temperatures of up to 400 °C and achieve precise measurement of high-pressure under high temperature conditions.

Hybrid LPFG/MEFPI sensor for simultaneous measurement of high-temperature and strain.

A hybrid fiber-optic sensor consisting of a long-period fiber grating (LPFG) and a micro extrinsic Fabry-Perot (F-P) interferometric (MEFPI) sensor is proposed and demonstrated for simultaneous measurement of high-temperature and strain. The LPFG written by using high-frequency CO(3+) laser pulses is used for high-temperature measurement while the MEFPI sensor fabricated by using 157nm F(2) laser pulses is used for strain measurement under high temperature. The distinguishing feature of such a hybrid fiber-optic sensor is that it can stand for high temperature of up to 650 masculineC and achieve precise measurement of strain under high temperature conditions simultaneously.

Long-distance fiber Bragg grating sensor system with a high optical signal-to-noise ratio based on a tunable fiber ring laser configuration.

A novel tunable fiber ring laser configuration with a combination of bidirectional Raman amplification and dual erbium-doped fiber (EDF) amplification is proposed for realizing high optical signal-to-noise ratio (SNR), long-distance, quasi-distributed fiber Bragg grating (FBG) sensing systems with large capacities and low cost. The hybrid Raman-EDF amplification configuration arranged in the ring laser can enhance the optical SNR of FBG sensor signals significantly owing to the good combination of the high gain of the erbium-doped fiber amplifier (EDFA) and the low noise of the Raman amplification. Such a sensing system can support a large number of FBG sensors because of the use of a tunable fiber Fabry-Perot filter located within the ring laser and spatial division multiplexing for expansion of sensor channels. Experimental results show that an excellent optical SNR of approximately 60 dB has been achieved for a 50 km transmission distance with a low Raman pump power of approximately 170 mW at a wavelength of 1455 nm and a low EDFA pump power of approximately 40 mW at a wavelength of 980 nm, which is the highest optical SNR achieved so far for a 50 km long FBG sensor system, to our knowledge.