Optical Fiber Cantilever Research Articles

A dual‐resonance enhanced photoacoustic spectroscopy gas sensor based on a fiber optic cantilever beam microphone and a spherical photoacoustic cell

AbstractWe propose a novel high‐performance dual‐resonance enhanced photoacoustic spectroscopy (DRE‐PAS) gas sensor based on a highly sensitive fiber optic cantilever beam microphone and a high‐Q spherical photoacoustic cell (PAC). The first‐order resonant frequency (FORF) of the spherical PAC is analyzed by finite element analysis to match the FORF of the cantilever microphone for the double resonance enhancement of the photoacoustic signal. The photoacoustic spectroscopy (PAS) system, including the DRE‐PAS sensor, a 1532.8 nm distributed feedback laser, and a high‐speed spectrometer, has been successfully exploited for trace acetylene (C2H2) detection. The experimental results show that the limit of detection (LOD) is 106.8 parts‐per‐billion (ppb) with an integral time of 1 s, and the LOD can be further reduced to 11.03 ppb by Allan‐Werle deviation for 100 s integral time. The normalized noise equivalent absorption coefficient can be obtained as 2.44 × 10−8 cm−1 WHz−1/2. The reported DRE‐PAS gas sensor has the superior characteristics of photoacoustic signal enhancement, high sensitivity, and strong antielectromagnetic interference capability, which can provide a new solution for PAS development.

Fiber optic photoacoustic gas sensor enhanced by multi-pass cell with overlapping phantom spots

A neotype fiber optic photoacoustic gas sensor with overlapping phantom spots based multi-pass is proposed, which greatly improves the reflection times and the length of gas absorption path. Moreover, the detection accuracy of photoacoustic spectroscopy (PAS) system is independent of the optical interference effect caused by overlapping spots, which is different from tunable diode laser absorption spectroscopy (TDLAS) technology. A mathematical model including light beam motion space, spot size and overlap discrimination of multi-pass structure based on overlapping phantom spots is established. The incident light is reflected up to dozens of times in the resonator with a diameter of only 8 mm. The emergent light does not need to converge on a photodetector, which reduces the difficulty of adjusting the optical path and promotes the realization of overlapping phantom spots. The experimental results show that the amplitude of photoacoustic signal is improved by about 40 times compared with the single spot structure. Appling high-sensitivity optical fiber cantilever to detect the sound pressure signal. The detection limit of C2H2 gas reaches 8.2 ppt. The normalized noise equivalent absorption (NNEA) coefficient of the gas sensor is achieved to be 9.3×10−11 cm−1WHz−1/2.

Dense Multibutterfly Spots-Enhanced Miniaturized Optical Fiber Photoacoustic Gas Sensor.

A miniaturized optical fiber photoacoustic gas sensor enhanced by dense multibutterfly spots is reported for the first time. The principle of space light transmission of neglecting paraxial approximation is theoretically analyzed for designing a dense multibutterfly spots-based miniature multipass cell. In a multipass photoacoustic tube with a diameter of 16 mm, the light beam is reflected about a hundred times. The light spots on the mirror surfaces at both ends of the photoacoustic tube form a dense multibutterfly distribution. The volume of the micro multipass gas chamber is only 5.3 mL. An optical fiber cantilever based on F-P interference is utilized as a photoacoustic pressure detector. Compared with that of the single-pass structure, the gas detection ability of the photoacoustic system with dense multibutterfly spots is improved by about 50 times. The proposed miniaturized sensor realizes a detection limit of 3.4 ppb for C2H2 gas with an averaging time of 100 s. The recognized coefficients of minimum detectable absorption (αmin) and normalized noise equivalent absorption are 1.9 × 10-8 cm-1 and 8.4 × 10-10 W cm-1 Hz-1/2, respectively.

An Optical Fiber Cantilever Beam Ultrasound Sensor Based on MI Structure

An Optical Fiber Cantilever Beam Ultrasound Sensor Based on MI Structure

Cantilever enhanced fiber-optic photoacoustic microprobe for diffusion detection of sulfur dioxide

A cantilever enhanced fiber-optic photoacoustic microprobe with the cavity volume of only 119 μL is designed to detect SO2, which has the obvious advantages of passive sensing, anti-electromagnetic interference and low cost. A UV multi-mode fiber and a single-mode fiber are applied to transmit excitation light emitted by UV-LED and probe light, respectively. The lens assembly based on a collimating lens and a focusing lens is added to improve the light energy of UV-LED coupling into UV-fiber. Based on theoretical analysis and finite element method, a photoacoustic cavity with a radius of 1.5 mm and a length of 15 mm is designed, which is conducive to the realization of photoacoustic enhancement and rapid response at high frequencies. SO2 gas diffuses into the PA cavity through a hole with the diameter of 0.5 mm and a gap of the cantilever with the width of 0.5 µm. A fiber-optic cantilever based on F-P interference is adopted to detect sound pressure. The frequency characteristic and response time of the sensor in different backgrounds of SF6 and N2 are analyzed experimentally. Compared with N2, the photoacoustic microprobe operates in SF6 background with lower resonance frequency and longer response time, which is due to the high density and low thermal diffusivity of SF6 gas. With the flow rate of 250 SCCM in the external cavity, the response time of the photoacoustic microprobe to SO2/SF6 and SO2/N2 are 96 s and 40 s, respectively. The detection limits of SO2 in SF6 and N2 background are 35 ppb and 5 ppb, respectively.

Ultrahigh Sensitive Trace Gas Sensing System with Dual Fiber-Optic Cantilever Multiplexing-Based Differential Photoacoustic Detection.

An ultrahigh sensitive trace gas sensing system was presented with dual cantilever-based differential photoacoustic detection. By combining the double enhancement of multipass absorption and optical differential detection, the gas detection sensitivity was significantly improved. The dual-channel synchronous photoacoustic detection was realized by fiber-optic Fabry-Perot interference spectrum multiplexing. The photoacoustic signals detected by two fiber-optic cantilever microphones installed in a differential photoacoustic cell (DPAC) were out of phase, while the detected gas flow noises were in phase. The optical differential detection method achieved both highly sensitive optical interference measurement and differential noise suppression. In the multipass configuration, the interaction path between excitation light and target gas achieved 4.1 m, which improved the photoacoustic signal by an order of magnitude compared with a single reflection. The maximum gas flow allowed by the system based on the DPAC was 250 sccm, which realized the dynamic monitoring of H2S in the SF6 background. The detection limit for H2S in SF6 background was 5.1 ppb, which corresponds to the normalized noise equivalent absorption coefficient of 9 × 10-10 cm-1 W Hz-1/2.

A vector accelerometer based on optical fiber cantilever with hybrid gratings

A vector accelerometer based on optical fiber cantilever with hybrid gratings

All-optical photoacoustic detection of SF6 decomposition component SO2 based on fiber-coupled UV-LED

An all-optical photoacoustic sensor for measuring the concentration of SF6 decomposition derivative SO2 was designed, whose detection limit was in the order of ppb-level. An UV-fiber and a single-mode fiber were utilized to transmit excitation light and probe light, respectively. The technical scheme of all-optical gas sensing has realized the competitive advantages of long-distance detection and anti-electromagnetic interference. As an UV-LED was the excitation light, the challenge of designing all-optical SO2 sensor was to effectively couple UV-LED with large divergence angle into an UV- fiber. Theoretical calculation and experiment have proved that the lens assembly as an effective strategy to realize the efficient coupling and beam collimation, which can achieve high excitation power and low solid photoacoustic background interference. The lens assembly with high focusing property was the key to improve the signal-to-noise ratio (SNR) by 5.2 times. To detect photoacoustic pressure with high sensitivity, a fiber optic cantilever acoustic sensor based on F-P interference was designed. Based on the demodulation algorithm of white light interference, dynamic cavity length of F-P was demodulated with high-resolution. Allan-Werle analysis result indicate that the detection limit of SO2 gas was 48 ppb with an averaging time of 100 s

Micro-machining of in-fibre 45° mirror optical fibre cantilever for dual-axis acceleration measurement

Focused ion beam (FIB) machining has been demonstrated to be capable of fabricating nano- and micro-scale structures. In this paper we demonstrate techniques to design and fabricate 45° micro-mirrors into the end of multi-core fibres using FIB processing. The mirrors are fabricated by a two-step process: a scanning process which is used to make a rough cut followed by a polishing process to create an optical surface finish mirror. The machined 45° mirrors can be accurately aligned with optical fibre cores, which avoids issues associated with the alignment of external turning mirror components. Proof-of-concept demonstration shows that the fabricated structure is capable of measuring two-axis acceleration interferometrically with a linear response from 0.2 to 4 g and an rms. error of 0.03 g. Acceleration measurements of frequency response up to 700 Hz and cross-sensitivity of ∼4.3% are demonstrated.

Detection of SF6 gas decomposition component H2S based on fiber-optic photoacoustic sensing

A high-sensitivity fiber-optic Fabry-Perot (F-P) photoacoustic gas detection system with properties of anti-electromagnetic interference and intrinsically safe was presented for quantitative analysis of SF6 decomposition component H2S. Erbium-doped fiber amplification was combined with acoustic resonance amplification to enhance the photoacoustic signal. A fiber-optic cantilever acoustic sensor (FOCAS) was used to sense weak photoacoustic signals. To enhance the sensitivity and stability of photoacoustic signals detection, the effect of SF6 gas on the responses of FOCAS and photoacoustic cell (PAC) was investigated. When SF6 is the background gas, the resonance frequency of the PAC will be greatly reduced and the sensitivity of the FOCAS will be improved. With the excitation light power of 1 W and an averaging time was 200 s, the minimum detection limit (MDL) of H2S gas in the SF6 background reached 14 ppb, and the normalized noise equivalence absorption (NNEA) was calculated to be 3.14 × 10−9 cm−1·W·Hz−1/2. The combination of excitation enhancement and detection enhancement of the photoacoustic signal solves the problem that the traditional sensor is difficult to improve the detection sensitivity due to the low absorption coefficient of H2S gas in the near-infrared region.

A fiber cantilever beam vibration sensor with air cavity based on intensity modulation

An optical fiber cantilever beam (FCB) vibration sensor is proposed based on an air cavity (AC) composed with two single-mode fiber (SMF) ends. The measuring range is from 50 Hz to 192 kHz with the beam length of 12 mm, which achieves a resolution of 1.64 Hz. The structure of the sensor is simple and flexible, and it can reduce cross-sensitive of temperature. This sensor has potential applications in the fields of machinery, aerospace, and construction.

Low-Cost Fiber Optic Cantilever Accelerometer With a Spherical Tip Based on Gaussian Beam Focusing

We propose and demonstrate a low-cost and simple fiber optic cantilever accelerometer with a spherical tip based on Gaussian beam focusing. The accelerometer consists of ceramic ferrule, ceramic sleeve, receiving fiber and emitting fiber, where both fibers are single mode fiber, the ferrule and sleeve have characteristics of high precision, which reduce the difficulty of optical alignment. The end of the emitting fiber is made into a spherical tip for focusing the Gaussian beam to improve sensitivity. When the accelerometer is in operation, the emitting fiber acts as a cantilever beam, the acceleration can be measured by detecting the transmission power. Further, our experimental results show that the spherical fiber tip can improve the acceleration sensitivity by 67% over 10 Hz-1000 Hz without reducing the working bandwidth. In addition, it is found that the fiber accelerometer has a high signal-to-noise ratio (SNR) up to 60 dB, and a low harmonic distortion of better than -30 dB, rendering a quasi-8-shaped directionality at the working frequency ranging from 10 Hz to 1200 Hz. This clever sensor structure may have potentials for developing high-performance and cost-effective accelerometers and hydrophones.

Optic-fiber vibration sensor based on a reflected 81° tilted fiber grating integrated with a symmetrical flexible hinge.

An optic-fiber vibration sensor based on the reflected 81° tilted fiber grating (81° TFG) integrated with a symmetrical flexible hinge is proposed and experimentally demonstrated in this paper. The vibration sensor is composed of a symmetrical flexible hinge and a reflected 81° TFG, the ends of which are simply fixed on the upper surface of the mass. The theoretical model of the proposed vibration sensor is analyzed, by which the important parameters related to the resonant frequency of the sensor are simulated and discussed; then, the vibration sensing experiments are conducted. Experiment results show that TE/TM mode of the 81° TFG can provide the maximal acceleration sensitivity of 338.28 and 299.94 mV/g at 400 Hz in the flat area of the amplitude-frequency response (50-400 Hz), which is increased by 9.95 and 11.5 times as compared with the optical fiber cantilever beam structure, respectively. Further, the signal-to-noise ratio in the flat area (50-400 Hz) is about ∼66.275dB under the acceleration of 2 g, which is increased by ∼20dB. Furthermore, it can be used for detecting mechanical vibration of medium-high frequency ranging from 50 to 3500 Hz. The proposed 81° TFG vibration sensor has the characteristics of small volume, simple package, high acceleration sensitivity, and wide vibration signal response range, which will ensure it has broad application prospects in the field of mechanical vibration.

Simultaneous measurement of acoustic pressure and temperature using a Fabry-Perot interferometric fiber-optic cantilever sensor.

A Fabry-Perot (F-P) interferometric fiber-optic cantilever sensor is presented for simultaneous measurement of acoustic pressure and temperature, which are demodulated by a single high-speed spectrometer. The acoustic pressure wave pushes the cantilever to produce periodic deflection, while the temperature deforms the sensor and causes the F-P cavity length to change slowly. The absolute length of the F-P cavity of the fiber-optic cantilever sensor is calculated rapidly by using a spectral demodulation method. The acoustic pressure and temperature are obtained by high-pass filtering and averaging the continuously measured absolute cavity length value, respectively. The experimental results show that the acoustic pressure can be detected with an ultra-high sensitivity of 198.3 nm/Pa at 1 kHz. In addition, an increase in temperature reduces the resonant frequency of the acoustic response and increases the static F-P cavity length. The temperature coefficient of the resonance frequency shift and the temperature response of the sensor are -0.49 Hz/°C and 83 nm/°C, respectively. Furthermore, through temperature compensation, the measurement error of acoustic pressure reaches ± 3%. The proposed dual parameter measurement scheme greatly simplifies the system structure and reduces the system cost.

Tube-cantilever double resonance enhanced fiber-optic photoacoustic spectrometer

An ultra-high sensitive trace gas detection method based on tube-cantilever double resonance enhanced fiber-optic photoacoustic spectroscopy (PAS) is proposed. The first-order resonant frequencies of the acoustic resonant tube and the fiber-optic cantilever microphone were both equal to the frequency of the photoacoustic pressure signal. This method combines the amplitude amplification of the photoacoustic pressure wave in an acoustic resonant tube with the response enhancement of the photoacoustic signal by the cantilever, making the gas detection extremely sensitive. An experimental double resonance enhanced photoacoustic spectrometer was built for trace acetylene detection at the wavelength of 1532.83 nm. A noise equivalent detection limit (1σ) was achieved to be 27 ppt with a 200-s averaging time, which is the best value reported so far. In addition, the normalized noise equivalent absorption (NNEA) coefficient reached 4.2 × 10−10 cm−1 W Hz−1/2.

Ultralow-voltage electrothermal MEMS based fiber-optic scanning probe for forward-viewing endoscopic OCT.

We report an ultralow-voltage, electrothermal (ET) micro-electro-mechanical system (MEMS) based probe for forward-viewing endoscopic optical coherence tomography (OCT) imaging. The fully assembled probe has a diameter of 5.5mm and a length of 55mm, including the imaging optics and a 40mm long fiber-optic cantilever attached on a micro-platform of the bimorph ET MEMS actuator. The ET MEMS actuator provides a sufficient mechanical actuation force as well as a large vertical displacement, achieving up to a 3mm optical scanning range with only a 3 VACp-p drive voltage with a 1.5 VDC offset. The imaging probe was integrated with a swept-source OCT system of a 100kHz A-scan rate, and its performance was successfully demonstrated with cross-sectional imaging of biological tissues ex vivo and in vivo at a speed up to 200 frames per second.

An Electro-Thermal Actuation Method for Resonance Vibration of a Miniaturized Optical-Fiber Scanner for Future Scanning Fiber Endoscope Design

Medical professionals increasingly rely on endoscopes to carry out many minimally invasive procedures on patients to safely examine, diagnose, and treat a large variety of conditions. However, their insertion tube diameter dictates which passages of the body they can be inserted into and, consequently, what organs they can access. For inaccessible areas and organs, patients often undergo invasive and risky procedures—diagnostic confirmation of peripheral lung nodules via transthoracic needle biopsy is one example from oncology. Hence, this work sets out to present an optical-fiber scanner for a scanning fiber endoscope design that has an insertion tube diameter of about 0.5 mm, small enough to be inserted into the smallest airways of the lung. To attain this goal, a novel approach based on resonance thermal excitation of a single-mode 0.01-mm-diameter fiber-optic cantilever oscillating at 2–4 kHz is proposed. The small size of the electro-thermal actuator enables miniaturization of the insertion tube. Lateral free-end deflection of the cantilever is used as a benchmark for evaluating performance. Experimental results show that the cantilever can achieve over 0.2 mm of displacement at its free end. The experimental results also support finite element simulation models which can be used for future design iterations of the endoscope.

Parts-per-billion-level detection of hydrogen sulfide based on near-infrared all-optical photoacoustic spectroscopy

A highly sensitive trace hydrogen sulfide (H2S) gas sensing scheme based on all-optical photoacoustic spectroscopy is demonstrated. A high-power erbium-doped fiber amplified near-infrared laser is used as a light source for acoustic excitation. Meanwhile, the second-harmonic photoacoustic signal is measured by a fiber-optic cantilever microphone which is equipped with a white-light interferometric readout. For sensitivity improvement, the demodulated digital photoacoustic signal is processed by a virtual lock-in amplifier. The continuous H2S measurement experiment shows the ability of real-time response. A detection limit is achieved to be 33 ppb with a 10 s measurement time at the wavelength of 1576.29 nm. With both the excitation light and the probe light being transmitted by optical fibers, the designed sensing system has the advantages of remote detection and immunity to electromagnetic interference.

Demonstration of gain multiplication by series assembly of fiber-optic cantilever beam deflection: Measurement of low magnetic field and magnetization of probe samples

We report our experimental realization of fiber-cantilever beam-deflection magnetometer using a series assembly of deflecting fiber-cantilevers for measuring low magnetic field (∼mT) and magnetization of probe nano-particles. Using single-mode fibers coated with optimized cobalt-doped-nickel-ferrite nano-particles, we devise a concatenated twin-fiber-cantilever assembly that identically modulate transmitted fiber-exit beam with varying external magnetic field. We measure surrounding field as low as ∼1 mT and demonstrate gain-multiplication through series-fed signal of fiber-cantilever response. Our cantilever-deflection-transmission analysis is devised to interpret the experimental observation and to determine magnetization of probe-samples. A Sagnac-loop assisted cantilever arrangement is also fabricated and tested that yielded the same sensitivity.

Ultra-high sensitive fiber-optic Fabry-Perot cantilever enhanced resonant photoacoustic spectroscopy

A new scheme of fiber-optic cantilever enhanced resonant photoacoustic spectroscopy (CERPAS), combining high sensitive fiber-optic Fabry-Perot cantilever microphone with resonant photoacoustic spectroscopy, is presented for trace gas detection. A fiber-optic cantilever microphone with high sensitivity at the frequency near 1.4 kHz is designed to match with a first-order longitudinal resonant photoacoustic cell, whose resonant frequency is 1402 Hz. For sensitivity improvement, an erbium-doped fiber amplified near-infrared laser, with the central wavelength of 1532.83 nm and the maximum output power of 1 W, is used as the light source for acoustic excitation. The trace acetylene detection experiment demonstrates that, with the wavelength modulation spectrum and second-harmonic detection methods, the gas detection limit is achieved to be 80 ppt, which is at least one order of magnitude improvement compared with other photoacoustic acetylene sensors reported so far.