Intensity Demodulation Research Articles

Intensity Demodulation Technology of FBG Time Division Multiplexing Based on Sagnac Ring

Intensity Demodulation Technology of FBG Time Division Multiplexing Based on Sagnac Ring

TWDM-Assisted Active Quadrature Demodulation of Fiber-Optic Fabry–Perot Acoustic Sensor Network

In this article, we propose a hybrid time and wavelength division multiplexed (TWDM) active quadrature demodulation technique for fiber-optic Fabry–Perot acoustic sensor network. A modulated grating Y-branch (MG-Y) laser is utilized to achieve fast and programmable wavelength tuning. For each sensing channel, two operating wavelengths with quadrature phase difference are determined by the initial cavity length of the sensor. According to the DC voltages of the detected signal, the operating wavelength is feedback-controlled to locate on the spectral slope, maintaining the high sensitivity of the sensing system. Compared with traditional passive quadrature demodulation and quadrature point intensity demodulation methods, the feedback control logic is simple and flexible. Moreover, the proposed active quadrature demodulation scheme is easily applicable to sensors with different cavity lengths. Combined with a hybrid TWDM technology, simultaneous multi-point acoustic detection can be achieved, requiring only one tunable laser and one photodetector. A two-point sound velocity sensing system and a four-point sound source localization system are constructed. Experimental results show that different sensing channels can work effectively without crosstalk.

Twice-FFT demodulation for signal distortion in optical fiber FP acoustic sensor

A Twice-FFT demodulation method for signal distortion state is proposed and experimentally demonstrated in an optical fiber Fabry-Perot (FP) acoustic sensor. Here the fiber FP acoustic sensor element is build with fiber end face and combination of diaphragms which is composed of a small round aluminum foil and a polymer PET film. The hypotenuse intensity demodulation method is applied to acquire the acoustic signal in time domain assisted with a Photodetector (PD) and an oscilloscope. The first Fast Fourier transform (FFT) processing results show harmonic distortion on the frequency domain spectrum. To demodulate the acoustic frequency and amplitude information, twice-FFT processing is preformed. Experimental results reveal an accuracy up to 95.6% of acoustic signal in the frequency range 2–100 Hz. Our scheme provides a new option for signal demodulation of optical fiber acoustic sensors (OFAS).

Dual Enhanced Fiber-Optic Microphone Using Sensitive Diaphragm-Free Transducer and Ultrahigh-Resolution Spectral Demodulation

Interferometric fiber-optic microphones (FOMs) with outstanding sensitivity are ideal method to detect weak acoustic signals. In this work, we demonstrated an ultrasensitive fiber-optic Michelson microphone with dual enhancement by using a diaphragm-free acoustic transducer and a high-resolution optical path difference (OPD) demodulation device. A cylindrical polyvinyl chloride (PVC) tube wrapped around by the optical fiber was employed as a transducer to enhance the responsivity to weak acoustic wave. A high-resolution spectrum demodulation-based white-light interferometer (WLI) was utilized to measure the variations of OPD. In theory, the FOM owns features to overcome defects of the conventional diaphragm-based or intensity demodulation method-based FOMs, such as quadrated point (Q-point) drifting and being sensitive to environmental interference. Experimental results showed that this FOM had an ultrahigh sensitivity of ~6800 nm/Pa at the resonance frequency of 1455 Hz. The sensitivity and the noise-limited minimum detectable sound pressure level (MDSPL) at 1000 Hz were calculated to be 315.3 nm/Pa and - 20.4 dB/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> (1.9μ Pa/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> ), respectively.

Design and analysis of high-frequency fiber Bragg grating vibration sensor

The fiber Bragg grating vibration sensor has received a lot of attention due to its unique performance. However, the natural frequency of the sensor limits its development and application. This paper presents an acceleration sensor with an isosceles triangular cantilever and mass block structure, which has high natural frequency and high sensitivity. The optical fiber sensing system consists of two fiber gratings. One section of the fiber grating is pasted on the cantilever and serves as an accelerometer; the other is used as a reference grating. The first-order natural frequency of the accelerometer is 8356 Hz, and its sensitivity is 0.46 pm g−1. A vibration test confirms the reliability of the sensor, and the resonant frequency of the sensor is 8193 Hz. The principle of light intensity demodulation is adopted in the sensor system and its sensitivity is 45.82 mV g−1 with a linearity of 0.997. The sensor has the advantages of high frequency and sensitivity, and thus can be applied to the rail transit bearing systems

All-fiber humidity sensor based on Michelson interferometer with hyaluronic acid and polyvinyl alcohol composite film

Abstract All-fiber humidity sensor based on Michelson interferometer with hyaluronic acid (HA) and polyvinyl alcohol (PVA) composite film is investigated. The interferometer is fabricated by fusing 691 μm thin-core fiber (TCF) with single mode fiber (SMF) together and melting the other end of the TCF into a circular shape. The outer of the interferometer is coated with HA and PVA composite film. As far as we know, hyaluronic acid is used in relative humidity (RH) measurement for the first time. Through wavelength and intensity demodulation, the proposed sensor’s maximum sensitivity is −0.1679 nm/%RH and −0.148 dB/%RH, respectively. Besides, the sensor’s cross-sensitivity is low (0.0727%RH/°C). Therefore, the proposed sensor with hyaluronic acid has great potential in humidity measurement.

Eccentric reflective optical fiber MEMS micro-pressure sensor

An eccentric reflective optical fiber micro-electro-mechanical system (MEMS) micro-pressure sensor is proposed in this paper. The core part of the sensor consists of a dual fiber collimator (a fiber collimator with two pigtails) in an eccentric position and a sensitive silicon diaphragm. The sensitive silicon diaphragm adopts the BM (beam-membrane) structure with small structural parameters manufactured by MEMS manufacturing technology. Simulation results show that the BM structure has good sensitivity and high natural frequency. Overall structure of the sensor with the measurement range of 0 ∼ 10 kPa is designed. The way of intensity demodulation ensures the performance and stability of the sensor and makes the sensing system easier to process signals. By building a static test platform and conducting experiments, we can conclude that the sensitivity of the sensor is −0.32 dB kPa−1. Furthermore, the repeatability of the sensor is 1.26%FS (full-scale), the hysteresis of the sensor is 0.95%FS and the zero drift of the sensor is 0.615%FS h–1. By building a dynamic experimental platform and conducting experiments, it can be seen that the response time of the sensor is 0.47 ms (<1 ms), the sensor has good dynamic response.

A MEMS based Fabry–Pérot accelerometer with high resolution

Optical MEMS has become exceedingly popular because of its high performance and resistance to electromagnetic interference. A MEMS based Fabry–Perot accelerometer consisting of a G-shaped mass-spring structure sensing chip, laser diode, cube beam splitter and photo translating system integrated by 3D printed sensor package is investigated. The sensitivity and resolution calibrated by intensity demodulation method are respectively, 183.793 V/g and 300 ng. The results show that the adopted G-shaped cantilever-mass structure sensing chip combined with the Fabry–Perot interfere technology can obtain good performance, and the 3D printed sensor package makes the interference optical path and accelerometer more robust and portable.

Composite Modal Interferometer for Simultaneous Triple-Parameter Measurement Based on Cascaded No-Cladding Fiber and Thin-Core Fiber Structure

In this paper, an in-fiber composite modal interferometer (CMI) is proposed and experimentally demonstrated based on cascaded no-cladding fiber (NCF) and thin-core fiber (TCF) structure, which respectively corresponds to multimode interferometer (MMI) and Mach-Zehnder interferometer (MZI). The self-image effect of MMI is deeply analyzed and the interference fringes distribution of CMI is accurately designed by the selective the length of NCF and TCF. The comprehensive tests are then conducted and the experimental results show that interference fringes present similar temperature response but the apparent differences in strain and refractive index (RI) tests owing to the asymmetrical structure. MMI has a near-zero intensity sensitivity of strain and red-shifts with the increased RI. But MZI has the sensitivity of −19.3nm/RIU and a 4.94 dB increment in the range of 300– $1100~\mu \varepsilon $ . Thus high-discrimination simultaneous triple-parameter measurement is achieved by means of joint wavelength and intensity demodulation. The proposed CMI has the merits of high sensitivity, low cost and ease of fabrication, and is very promising and potential for the engineering monitoring and sensing applications.

Measurement of CO2 and Ultrasonic Vibration Based on Tunable Fiber Ring Laser

In this paper, we propose a method to simultaneously measure carbon dioxide (CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) concentrations and ultrasonic vibration using fiber sensors with a tunable fiber ring laser based on a loop design. The in-line interferometric vibration sensor consists of a single-mode-multimode-single-mode fiber structure and detects continuous dynamic vibrations through intensity demodulation. The evanescent wave absorption based CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sensor is composed of a no-core fiber coated with silica derived from tetraethylorthosilicate. The fiber ring laser output is tuned to the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> absorption wavelength of 1574.6 nm, which also best matches the quadrature bias point of the vibration sensor. The vibration frequency signals from 10 Hz to 50 kHz, and fast detection of CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentrations from 10% to 100% are experimentally demonstrated. The proposed hybrid sensor system is a promising method for monitoring the gas pipelines operating state, gas turbines, nuclear power plants, and automotive engines, where both vibration and CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentration measurements are essential.

Up-down Taper Based In-Fiber Mach-Zehnder Interferometer for Liquid Refractive Index Sensing.

A novel in-fiber Mach-Zehnder interferometer based on cascaded up-down-taper (UDT) structure is proposed by sandwiching a piece of polarization maintaining fiber between two single-mode fibers (SMF) and by utilizing over-fusion splicing method. The dual up tapers respectively act as fiber splitter/combiner, the down taper acts as an optical attenuator. The structure parameters are analyzed and optimized. A larger interference fringe extinction ratio ~15 dB is obtained to achieve refractive index (RI) sensing based on intensity demodulation. The experimental results show that the RI sensitivity is −310.40 dB/RIU with the linearity is improved to 0.99 in the range of 1.3164–1.3444. The corresponding resolution can reach 3.22 × 10−5 RIU, which is 6.8 times higher than wavelength demodulation. The cross sensitivity which caused by temperature fluctuation is less than 1.4 × 10−4.

A novel method for simultaneous measurement of temperature and strain based on EFPI/FBG

A modified intensity demodulation method for fiber optic Fabry–Perot cavity and fiber Bragg grating combined sensing system is proposed, through which the cross-sensing error can be reduced effectively and high sensing speed can be realized at the same time. The system is based on an Extrinsic Fabry–Perot Interferometer for strain sensing, and a fiber Bragg grating for temperature sensing. Three-wavelength digital phase demodulation method and passive linear filtering demodulation method are adopted for the demodulation of F–P cavity length and fiber Bragg grating central wavelength, respectively. Meanwhile temperature varied from 25 °C to 105 °C under constant pressure, the demodulation accuracy of strain after correction is ±0.6με, which is improved by 2 times than the direct demodulation result. The demodulation accuracy of temperature is 1.4 °C after correction, with pressure ranging from 0 to 10 MPa at constant temperature. Temperature demodulation error is decreased to 1/3 of its original value. Strain sensing accuracy after correction was ±1με. Experimental studies show that the proposed sensing system has a more compact size and higher measurement accuracy than the previous ones.

High-sensitivity refractive index and temperature sensor based on cascading FBGs and droplet-like fiber interferometer

In this paper, a highly sensitive refractive index (RI) and temperature sensor based on two fiber Bragg gratings (FBGs) cascaded with a droplet-like fiber interferometer (DLFI) is proposed and experimentally demonstrated. The DLFI was fabricated by mechanically bending a section of coating-stripped single-mode fiber. As the surrounding RI changes, the transmission spectrum of the DLFI shifts, which causes the intensity difference of the two FBGs change. Whereas, the temperature change induces a Bragg wavelength shift of the FBGs. Thus, RI and temperature can be simultaneously measured by the senor through monitoring the intensity-difference change and the wavelength change. In our experiment, the proposed sensor exhibits a RI sensitivity of 385.2 dB/RIU (refractive index unit) in the range of 1.3388–1.3998, which is the highest in RI sensors based on intensity demodulation so far. Meanwhile, the temperature sensitivity is 11 pm/℃ in the range of 25–70 ℃.

Influence of light source on the sensitivity of a Polarization Maintaining Fiber Loop Mirror vibration sensor

Wavelength or light intensity demodulation method for a polarization maintaining fiber loop mirror (PM-FLM) sensor is not only expensive but also unsuitable for vibration monitoring. This paper studies the voltage demodulation principle and explores theoretically the influence of light source on the sensitivity. Then a PM-FLM sensor is applied to vibration monitoring. A low-power narrow-band light source reflected from FBG and a high-power laser light source are used respectively as input. The time domain voltage signals of vibration from a photo-detector are collected by a data acquisition card and converted via a fast Fourier transform algorithm. The experimental result shows that a PM-FLM sensor using a high-power narrow-band light source as input has a higher sensitivity to vibration than the one using a low-power narrow-band light source. In practical application, we can choose a high-power narrow-band light source as input to improve the sensitivity to vibration.

108-km Distributed Acoustic Sensor With 220-p$\varepsilon /\surd$Hz Strain Resolution and 5-m Spatial Resolution

The measurement distance is one of the most important parameters for distributed acoustic sensor (DAS). In this paper, we report a long-distance and high-sensitivity DAS system based on time-gated digital optical frequency domain reflectometry. The bi-directional distributed Raman amplification is adopted to realize long measurement distance. The shape of optical pulse is pre-distorted to be a standard Hanning window, which provides a theoretical peak-side lobe ratio of 46 dB to suppress the crosstalk. The interference fading and polarization fading are well suppressed, and hence phase-demodulation method is adopted, instead of intensity demodulation method. As a result, the sensitivity is enhanced and the full information (amplitude, phase, and frequency) of the vibration can be obtained. In experiments, the fiber length is about 108 km, whereas the spatial resolution is 5 m. A weak vibration with peak-peak amplitude of 14.7 nε is correctly located at the distance of 98 km with a high SNR of 30 dB. It is the first time that 220-pε/√Hz strain sensitivity is realized over 100-km-level fiber and the vibration waveform is retrieved linearly without harmonics.

Intensity modulation-based spectral polarization measurement method of coded aperture

This paper presents a spectral polarization measurement method for single-frame imaging. The method is based on intensity-modulated, coded aperture measurement, comprising a spectral polarization intensity modulation and a compressive sensing spectral imaging system. The incident light is modulated by two multistage phase retarders and a polarizer, and the spectrum’s Stokes parameters are shifted to different frequency channels. The spectrum of the modulated light is obtained by a spectral imaging system composed of a coded aperture and a dispersive prism. The modulated spectral information is reconstructed using the TwIST algorithm, and the Stokes elements can be demodulated from it. Taking a pixel as an example, numerical simulations of the modulation of incident light intensity modulation, spectral reconstruction, and demodulation of Stokes parameters show that the method can acquire spectral polarization information of sparse images. This process needs only one measurement of the object and is therefore capable of high-speed acquisition.

Compact gas refractometer based on a tapered four-core fiber.

A compact in-line interferometer is proposed and experimentally demonstrated for gas refractive index (GRI) measurement. The sensor comprises a tapered four-core fiber (TFCF) sandwiched between two single-mode fibers (SMFs), forming an in-line SMF-TFCF-SMF structure. The fiber taper acts as a bridge between the external GRI variation and the multimode interference within the TFCF segment. A high sensitivity of 1280.94dB/refractive index unit is obtained in GRI measurement around 1.0. Temperature change only shifts the interference wavelength, and the cross-sensitivity of temperature can be ignored by intensity demodulation. The proposed gas refractometer, with its improved performance, can be a good candidate for chemical sensing or bio-sensing.

Thin-Core Fiber Taper-Based Multi-Mode Interferometer for Refractive Index Sensing

We demonstrate a thin-core fiber (TCF)-based hybrid modal interferometer for high-sensitivity refractive index (RI) sensing. It mainly consists of a tapered TCF with an air bubble fabricated by the arc discharge technique using a fusion splicer machine. Due to mode mismatch and the tapering of the TCF, high-order cladding modes are excited, which can be used for high-sensitivity index sensing. The selected two interference dips of the sensor can obtain the index sensitivities of 370.37 and −72.22 dB/RIU in the RI range from 1.333 to 1.3799. Using the differential intensity demodulation method, a total index sensitivity as high as 442.59 dB/RIU can be achieved. Featured with high sensitivity, linear response, small size, ease of fabrication, and low cost, the proposed sensor may have various applications in biochemical sensing field.

A Linear Optical Current Transducer Based on Newton’s Ring Sub-Wavelength Grating

This paper presents a novel optical current transducer based on Newton’s ring sub-wavelength grating. It converts the Faraday rotation angle into synchronized rotation of the spot image and locates the spot with a four-quadrant detector to achieve real-time linear measurement of the current. The Newton’s ring grating is used instead of a radial grating as a polarization detection device so the extinction ratio is distributed uniformly in the radial directions and the polarization characteristics have no change with the radial direction, thereby a higher measurement accuracy has been achieved. The angle measurement range can reach ±40 degrees which breaks through the limitations of the intensity demodulation mode. The performance of the transducer has been verified by experiments and a 0.2 S class accuracy achieved with a linear measurement, no interference of optical power fluctuation, and minimized the effect of birefringence.

Liquid-level sensing based on a hollow core Bragg fiber.

A novel optical fiber liquid level sensor based on a hollow core Bragg fiber (HCBF) was proposed and demonstrated. The HCBF was first designed and successfully fabricated with periodic transmission band in the spectrum and a transmission loss of ~3.48 dB/cm. An inline optical fiber liquid-level sensor was fabricated by simply sandwiching a piece of HCBF between two single mode fibers. The sensing performance was experimentally tested. A linear liquid-level sensitivity of ~1.1 dB/mm, and fast response time less than 3s was obtained by the intensity demodulation measurement. The temperature and refractive index cross-sensitivities were also investigated. The experimental results indicate that our proposed structure has tiny temperature and RI dependence, which makes it a promising liquid level sensing platform for different liquids.