Frequency-shifted Interferometry Research Articles

Amplified frequency-shifted interferometry fiber loop ringdown glucose sensors using side-polished fiber

An amplified frequency-shifted interferometry fiber loop ringdown (FSI-FLRD) sensor is proposed for the concentration measurement of glucose solutions. It detects glucose concentrations by measuring the ringdown distance of the continuous light in the fiber ringdown loop (RDL) instead of the ringdown time of the pulsed light in the loop as in traditional FLRD glucose sensors. A bidirectional erbium-doped fiber amplifier (Bi-EDFA) and a segment of side-polished fiber (SPF) are respectively inserted into the RDL to compensate the loop attenuation and to act as the sensing probe. The proposed optical sensor was tested with glucose solutions in the concentration range from 0 to 14 % at room temperature, which exhibits a linear response with a sensitivity of 0.0013 km−1/% (4.12x10−4 dB/%) for the 0–8 % glucose solutions. Additionally, it also achieved a system stability of 0.75 %, with a measuring resolution of around 2.63 %. The experimental results show the proposed sensor has simple structure, low cost, robust stability and good resolution, and it is a suitable alternative for the detection of glucose concentrations in biological detection, component analysis and on-line quality control in food industry.

Detection limit improvement for a gas concentration measurement system using space-domain active fiber cavity ring-down sensing technology

Trace gas sensors have received extensive attention and have become particularly attractive for applications in toxic gas detection, medical diagnostics, pollution monitoring, industrial emission measurement, and so on. However, it is still a critical challenge for gas sensors to simultaneously achieve high sensitivity and low detection limits while performing well in stability. Here, a space-domain active fiber cavity ring-down (FCRD) gas sensing technique was proposed to develop a high-performance gas sensor. Taking advantage of the active FCRD sensing technique and frequency-shifted interferometry, the proposed gas sensor exhibited a high sensitivity of 1.122km−1/%. Importantly, introducing a bidirectional EDFA into the fiber cavity further enhances the light–gas interaction, resulting in a low detection limit of 55 ppm, which is much lower than that of most reported gas sensors. Moreover, a good stability of 0.58% can be achieved due to the combined effect of the proposed technology using differential detection to eliminate DC noise, as well as common-path interference to eliminate common-mode noise and the use of continuous light to stabilize the optical power in the fiber cavity for suppressing the gain fluctuations of EDFA.

A novel active continuous-wave FLRD strain sensor based on frequency-shifted interferometry

ABSTRACT An active continuous-wave fiber loop ringdown strain sensor based on frequency-shifted interferometry is presented and experimentally investigated. By measuring the decay rate of the continuous light in the ringdown loop (RDL), the sensor achieves strain measurement without optical pulsation and rapid detection. A bidirectional erbium-doped fiber amplifier is embedded in the loop to form an active fiber RDL, to compensate the inherent loop loss. The strain sensor head was built by splicing a segment of normal graded index multimode fiber (MMF) into the RDL composed of single-mode fiber. In this configuration, the core-cladding mode conservation and insertion loss of the MMF changes with the strain applied, making it possible to detect different levels of strains. The proposed device was tested with the strains in the range from 0 to 2.25 mϵ, which exhibits a linear response with a sensitivity of 0.29 km-1·mϵ-1, with a measurement resolution of 22 mϵ.

Fiber cavity ring-down displacement sensor with picometre resolution using frequency-shifted interferometry

Fiber cavity ring-down displacement sensor with picometre resolution using frequency-shifted interferometry

Displacement measurement of sub-nanometer resolution using space-domain active fiber cavity ring-down technology

A displacement sensor of sub-nanometer resolution using space-domain active fiber cavity ring-down (FCRD) technology was developed. In our scheme, by combining active FCRD with frequency-shifted interferometry technology, a sensitivity of 0.05 km−1 ·nm−1 was achieved and the stability was improved mostly because this scheme is free from thepowerfluctuationofalight source, benefited to the use of low-noise Bi-EDFA and the differential detection method to reduce the direct current noise. Besides the first sensitivity amplification process is involved in this technique due to the multiple times rounds of interaction between the light pulse and the sensor head in the fiber cavity, the secondary sensitivity amplification process is realized by using a Bi-EDFA to compensate the inherent cavity loss of fiber cavity, which results in a significantly improvement in resolution. The experimental results show that our proposed displacement sensor can achieve a resolution of up to 0.3 nm with stability of 1.3 %.

Optical sensor using space-domain active fiber cavity ringdown technique

A novel active fiber cavity ringdown (FCRD) technique using frequency-shifted interferometry (FSI) is proposed for the first time. Using this scheme, external parameters can be monitored in the space domain by measuring the ringdown distance instead of ringdown time. A bidirectional erbium-doped fiber amplifier (Bi-EDFA) is employed to compensate the inherent cavity loss for achieving higher sensitivity. And two band-pass filters are used to reduce the amplified spontaneous emission (ASE) noise of the Bi-EDFA. Compared with the well-known time-domain active FCRD scheme, our proposed method enables us to avoid using pulsed laser needed in time-domain active FCRD, it uses continuous-wave laser to inject into the fiber cavity and stabilize the optical power in the fiber cavity, which can suppress the baseline drift of ringdown signal caused by the gain fluctuations of the EDFA and thus improve the detecting precision. Moreover, this novel method enables us to use differential detection method for further reducing the ASE noise, and thus eliminating the baseline drift of ringdown signal. A magnetic field sensor was developed as a proof-of-concept demonstration. The experimental results demonstrate that the proposed sensor with a sensitivity of 0.01537 (1/km·Gs) was achieved. This is the highest magnetic field sensitivity compared to the time-domain active FLRD method. Due to the reduced ASE noise, the stability of the proposed sensing system was also greatly improved.

Multipoint space-domain active fiber cavity ringdown technique for quasi-distributed magnetic field monitoring

A multipoint space-domain active fiber cavity ringdown (FCRD) sensing technique based on frequency-shifted interferometry (FSI) is proposed and investigated for quasi-distributed magnetic field measurements. Multiple FSI-based fiber ringdown cavities (RDCs) are cascaded together to constitute a sensor array, share one continuous-wave optical source and one slow balanced detector, and thus the system is economical and highly sensitive. In each RDC a bidirectional erbium-doped fiber amplifier (Bi-EDFA) is inserted to compensate the inherent cavity loss for achieving higher sensitivity. Compared to the time-domain active FCRD techniques, the amplifier has lower gain fluctuation because the continuous light (instead of pulse light) coupled into the cavity enables more stable optical power, and has lower amplified spontaneous emission noise due to the use of two narrow bandpass filters and differential detection. Magnetic fields in different locations are synchronously resolved in the space domain by measuring the ringdown distances of continuous light, rather than measuring the ringdown times of pulse light in the time domain as in conventional multipoint FCRD techniques. A dual-point active FSI-FCRD magnetic field sensor was experimentally built, and two side-polished fibers coated with magnetic fluid were incorporated inside the fiber cavities as the sensing probes. By applying the external magnetic field from 0 to 230 Gs, the sensitivities of 6.68 × 10−3 and 4.45 × 10−3/(km Gs) were respectively achieved, which are at least one order of magnitude higher than the conventional passive FCRD techniques. After repeated measurements, the obtained ringdown baseline stabilities of the two units were 0.91% and 2.00%, which were much higher than those of time-domain active FCRD sensing systems. Using power budget analysis, the maximum number of identical sensing units can be expected to be 48 over a 25.89-km distance under the nearly same experimental conditions. The proposed multipoint active FCRD sensing scheme has the merits of low cost, high sensitivity, good stability, and strong multiplexing ability, which would find good application prospects at electric grids, national defense, biology, and medicine for monitoring quasi-distributed magnetic field.

A quasi-distributed fiber magnetic field sensor based on frequency-shifted interferometry fiber cavity ringdown technique

A quasi-distributed fiber magnetic field sensing system based on frequency-shifted interferometry fiber cavity ringdown (FSI-FCRD) technique is proposed and demonstrated. In this scheme, multiple sensing units are connected in series in a frequency-shifted interferometer, and share one continuous-wave light source, one slow detector and one low-frequency data collector to lower the cost. In each unit, a side-polished single-mode fiber coated with the diluted water-based ferrofluid serves as the sensing element for enhancing sensitivity and mechanical strength. The cavity loss of each unit varies with the change of applied magnetic field, due to the tunable refractive index of the ferrofluid and the evanescent wave effect of the polished fiber. Therefore, the measurement of magnetic field can be realized by monitoring the cavity loss change. A dual-unit FSI-FCRD magnetic field sensing system was experimentally investigated. The cavity losses and locations of the two sensing units about 2500 m apart were synchronously achieved from a spatial domain ringdown signal of the system. The measured cavity loss of each sensing unit exhibited a good linear relationship with the magnetic field over a range of 0 to 250 Oe. The sensitivities were separately 6.7894 × 10−4 and 7.4980 × 10−4 dB/Oe, with the corresponding detection limits of 2.56 and 3.63 Oe. Through repeated measurements, the obtained baseline stabilities of the two units were 0.15% and 0.18%, respectively. By theoretical analysis, the maximum multiplexing number of sensing units can reach 30 over a 15.8-km distance under the same experimental settings. It indicates that multi-unit FSI-FCRD provides an alternative technique with low cost, high sensitivity and high stability for quasi-distributed magnetic field detection in many fields such as smart grids, biomedicine and national defense.

Multipoint Magnetic Field Measurement Based on Magnetic Fluid and FSI-FLRD

We present a multipoint fiber magnetic field sensing system based on magnetic fluid (MF) and frequency-shifted interferometry fiber loop ringdown (FSI-FLRD). This system contains a number of cascaded sensing channels, and shares a common continuous-wave light source and slow electronics instead of a pulsed light source and high-speed electron devices as required in traditional multichannel FLRD techniques. Each sensing channel is formed by incorporating an MF-coated etched fiber as the sensing head inside a fiber ringdown loop. Each sensing head is mounted on a temperature control device. We experimentally fabricated a dual-channel FSI-FLRD magnetic field sensing system. First the system's temperature response was investigated, which indicates the MF-based sensing heads had temperature cross sensitivity. Then after keeping the temperature at 20 °C for improving testing accuracy and applying different magnetic fields, the two sensing channels were separately shown to have sensitivities of - 9.38×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> and - 2.28×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> /(km ·Oe) with a linear response in the range of 221-721 Oe. The obtained sensing channel stabilities were respectively 0.02% and 0.04%, which enhance more than one order of magnitude compared with those of most conventional FLRD techniques.

Fiber loop ring-down temperature sensor using frequency-shifted interferometry technology

Fiber loop ring-down temperature sensor using frequency-shifted interferometry technology

Large-dynamic-range fiber loop ringdown strain sensor using Frequency-shifted Interferometry

Large-dynamic-range fiber loop ringdown strain sensor using Frequency-shifted Interferometry

Fiber Loop Ring-Down Magnetic Field Sensor Using Frequency-Shifted Interferometry and D-Shaped Fiber Coated With Magnetic Fluid

In this letter, we demonstrate a low-cost fiber loop ring-down (FLRD) magnetic field sensor using frequency-shifted interferometry.  A D-shaped fiber coated with magnetic fluid was used as the sensor head for enhancing the sensitivity and improving the stability. With this scheme, the magnetic field was sensed in the space domain by measuring the ring-down distance, and its measurement accuracy was improved by introducing a temperature controller. The experimental results showed that a sensitivity of 2.53 × 10−3 (km−1ċG−1) was achieved, and a stability of 0.146% was obtained, which is better than those of the current time-domain FLRD sensors based on evanescent field effect.

A Multichannel Spatial-Domain Fiber Cavity Ringdown Pressure Sensor

We propose and demonstrate a multichannel spatial-domain fiber cavity ringdown (FCRD) pressure sensing scheme based on frequency-shifted interferometry (FSI). In contrast to existing multichannel FCRD techniques, multichannel FSI-FCRD measures intensity decay rates of continuous-wave (CW) light from different fiber ringdown cavities (RDCs) in the spatial domain, rather than those of pulse light in the time domain. It shares one CW light source, one slow detector and one slow data collector, which greatly reduces the system cost. We experimentally investigated a dual-channel FSI-FCRD pressure sensing system. The locations and pressures applied of the two FCRDs were obtained by measuring the corresponding ringdown distances. The measurement sensitivities were 0.030 (km $^{-1}\cdot $ MPa−1) and 0.042 (km $^{-1}\cdot $ MPa−1), with the minimum detectable pressure of 0.126 MPa and 0.403 MPa, respectively. By power budge analysis, the maximum sensor number was predicted to be 37 over a 50-km distance under the same experimental settings. The experimental and simulated results show that the proposed scheme has the advantages of low cost, high sensitivity, good linear response and good stability, which can enhance the multiplexing capacity and meet the requirements for multipoint measurement.

Long-distance vibration measurement based on laser frequency-shifted feedback interferometry

Long-distance vibration measurement based on laser frequency-shifted feedback interferometry

Multipoint Fiber Loop Ringdown Sensors for Large Strain Measurement Using Frequency-Shifted Interferometry.

A novel multipoint fiber loop ringdown (FLRD) strain sensing system using frequency-shifted interferometry (FSI) is proposed and experimentally validated. Compared to conventional multipoint FLRD techniques, this scheme measures the decay rate of the continuous wave (CW) light in the space domain and thus greatly reduces the cost without the requirement of expensive devices. A serial dual-point strain sensing system was experimentally constructed and a biconical tapered multimode fiber (MMF) as the sensor head was used for obtaining the large measuring range. By applying different strains on the sensor heads through translation stages, a linear response between strain and additional loss induced by strain sensor was obtained, and the static strain sensitivities of 0.13676 dB/mε and 0.19665 dB/mε were achieved, corresponding to the detection limit of 0.0123 dB and 0.0360 dB, respectively. Moreover, a large measuring range of approximately 6 mε was achieved for both strain sensors. The experimental results indicate that our proposed method offers a promising multipoint strain sensor which has the advantages of low cost, a simple sensing structure and a large measuring range.

Single-Arm Frequency-Shifted Interferometry Using a Bidirectional Electro-Optic Modulator

Multiplexing fiber-optic sensors is an effective way to perform large-area quasi-distributed sensing. Among many sensor multiplexing and interrogation methods, frequency-shifted interferometry (FSI) emerges as a unique low-cost technique that is capable of resolving sensor locations using a continuous-wave broadband source, a bidirectional optical frequency shifter, and a slow photo detector. An acousto-optic modulator is typically used as the frequency shifter in FSI due to its high modulation efficiency and bidirectional operation. But its relatively low modulation bandwidth results in low spatial resolution, on the order of meters. To improve spatial resolution, single-arm FSI (SA-FSI) that employs an electro-optic modulator (EOM) as the frequency shifter was proposed, but the EOM's broad modulation bandwidth cannot be fully utilized because conventional EOMs cannot support bidirectional modulation at gigahertz frequencies due to the speed mismatch between the optical wave and the applied RF wave, especially when the two are counter propagating. In this paper, we modified a commercial EOM to increase the modulation efficiency of the counter-propagating optical wave and used it to interrogate 15 weak fiber Bragg gratings (FBGs) in an array, achieving a spatial resolution of ∼3 cm and a sensing range of >1 km. A signal-to-noise ratio as high as ∼18 dB was obtained with a 15-mW broadband amplified spontaneous emission source without any optical amplification. This is the first demonstration of both spatially resolved and spectrally resolved SA-FSI, and it is the first time that FBGs are interrogated with the SA-FSI technique.

Simultaneous measurement of gas composition and concentration combined fiber cavtiy ringdown and frequency-shifted interferometry

Abstract A fiber cavity ringdown gas sensing system combined with frequency-shifted interferometry (FSI) for the simultaneous measurement of gas composition and concentration was proposed. With this technique, the loss spectrum of target gas was experimentally achieved by scanning the wavelength of laser source. And its characteristic absorption wavelength agreed well with that of acetylene provided by the Hitran database, which indicates the target gas was exactly the acetylene sample. The gas concentration was also measured simultaneously with a standard deviation of ±0.03%. The corresponding detection sensitivity was 263.48 ppm, which can be further improved by using the gas cell with smaller insertion loss. The experimental results show that the proposed gas sensing system has the advantages of low-cost, good linear responses, high stability and high sensitivity.

Multichannel continuous-wave fiber cavity ringdown gas sensing utilizing frequency-shifted interferometry.

We present a multichannel continuous-wave (CW) fiber cavity ringdown (FCRD) gas sensing method based on frequency-shifted interferometry (FSI). This scheme detects gas concentration by measuring the intensity decay rates of continuous light from different ringdown cavities in the spatial domain, unlike conventional FCRD techniques, which measure the decay rates of pulse light in the time domain. This method shares one CW light source, one slow detector, and one slow data collector. In order to illustrate the theory, acetylene gas concentration measurement in a two-channel FSI-FCRD system was experimentally conducted in the range of 0%-1%. A linear relation was established between concentration and absorption loss, which is proportional to the intensity decay rate, and the measurement resolutions of 3.871%/dB and 3.658%/dB were achieved, respectively. The results reveal that the proposed system has the advantages of low cost, high sensitivity, high precision, and good stability in multichannel gas detection.

Continuous-Wave Fiber Cavity Ringdown Pressure Sensing Based on Frequency-Shifted Interferometry.

We present a continuous-wave fiber cavity ringdown (FCRD) pressure-sensing method based on frequency-shifted interferometry (FSI). Compared with traditional CRD or FCRD techniques, this FSI-FCRD scheme deduces pressure by measuring the decay rate of continuous light exiting the fiber ringdown cavity (RDC) in the spatial domain (i.e., the CRD distance), without the requirement for optical pulsation and fast electronics. By using a section of fiber with the buffer layer stripped in the fiber RDC as the sensor head, pressures were measured within the range from 0 to 10.4 MPa. The sensitivity of 0.02356/(km∙MPa) was obtained with a measurement error of 0.1%, and the corresponding pressure resolution was 0.05 MPa. It was found that the measurement sensitivity can be improved by enlarging the interaction length of the sensor head. The results show the proposed sensor has the advantages of simple structure, low cost, high sensitivity, and high stability in pressure detection.

Synchronous Reflectivity Measurement of a Weak Fiber Bragg Grating Array Using Frequency-Shifted Interferometry

We demonstrate a method of reflectivity measurement for a weak fiber Bragg grating (FBG) sensor array based on frequency shifted interferometry. In the scheme, the measured weak FBG sensor array was connected after a referenced weak FBG, by using frequency shifted interferometry, the reflectivity of the referenced FBG and the reflection amplitude spectrum of each FBG from the sensor array were obtained, respectively, and then each grating reflectivity was iteratively calculated by using the spectral shadowing removal method. A sensor array of 60-weak FBGs with an average peak reflectivity around −25 dB was experimentally investigated. The obtained peak reflectivity ranged from −37.7 to −21.7 dB with an average value of −26.0 dB. Error analysis indicates the average reflectivity measurement stability for all FBGs was ±1.93 dB. The results reveal that this paper presents a feasible, simple, and low-cost method of reflectivity measurement for large-scale weak FBG sensor arrays.