Fiber Laser Sensor Research Articles

A contact vibration measurement sensor based on a distributed Bragg reflector fiber laser

A new contact method to measure vibrations with a frequency range of about 30–110 Hz by a distributed Bragg reflector (DBR) fiber laser sensor, based on a beat frequency modulation, has been proposed. In order to demonstrate the plausibility for a DBR fiber sensor to detect vibrations lower than 110 Hz without any complex structures, it is encapsulated in a rectangular slice composed of an epoxy resin glue, with a Young’s modulus of about 2.9 GPa. In experiments, the packaged DBR fiber sensor is placed on a vibration platform to sense the vibration, with a commercial magnet-electrical vibration velocity transducer as a reference. Experimental results indicate that the single DBR fiber laser is able to measure the low-frequency vibration with a few tens of Hertz and several microns of amplitude, offering potential for a low-frequency vibration measurement.

Temperature Self-Compensation High-Resolution Refractive Index Sensor Based on Fiber Ring Laser

In this letter, a temperature self-compensation high-resolution refractive index (RI) sensor based on intracavity intensity-modulated sensing in a fiber ring laser is demonstrated. A fiber-optic multimode interferometer based on single-mode-no-core-single-mode fiber structure is cascaded with a fiber Bragg grating and used as a reflected sensing head to enhance intensity-modulated depth. It is inserted in a fiber ring laser as a wavelength selective filter and the intracavity intensity-modulated RI sensing is induced for the output intensity of the fiber laser. Furthermore, because the lasing wavelength of the sensor system is sensitive to temperature but insensitive to external RI, the temperature self-compensation measurement can be realized. The RI sensitivity is measured to be −4.98 mW/RIU from 1.3349 to 1.3665. Correspondingly, the relative intensity sensitivity achieves −196.1 dB/RIU from 1.3349 to 1.3544 and −744.6 dB/RIU from 1.3544 to 1.3665. The resolution of the fiber laser sensor is obtained as high as $2\times 10^{{{-10}}}$ RIU with the signal-to-noise ratio more than 55 dB.

Performance of dual-frequency ultrasound measurement based on DBR fiber laser hydrophone

The ability of distributed Bragg reflector (DBR) fiber laser sensor in dual-frequency ultrasound (DFUS) measurement has been demonstrated. The influences of DFUS pressure on fiber grating laser sensor were theoretically analyzed, considering the effects of relative distance of the ultrasound field and the amplitude of the DFUS on the degrees of birefringence modulation. In experiment, the birefringence in sensing fiber was modulated by ultrasound signals at 3MHz and 5MHz, and the test distance and driving voltage were set respectively according to the ultrasound frequency. Agreement between theoretical and experimental results was obtained for ultrasound wave propagating from different distances (40mm to 10mm) and different DFUS driving voltage (5V to 20V). The results demonstrate that the DBR fiber grating laser acoustic sensor has a multi-frequency ultrasound recognizable ability, offering a potential for ultrasound medicine and biology

Crack detection in riveted lap joints using fiber laser acoustic emission sensors.

Fiber laser ultrasonic sensors are demonstrated to be capable of measuring acoustic emission generated by cracks in aluminum panels. A single laser sensor is integrated into a riveted lap joint, which is subject to accelerated fatigue. Acoustic emission generated by crack formation in the panel in addition to other acoustic events due to fretting within the joint are clearly resolved by the laser sensor. Localization of an acoustic emission event is demonstrated with a multiplexed array of three laser sensors. This manuscript also calculates the fundamental limit to displacement resolution of the fiber laser sensor and presents measurements of the directional response to Lamb waves. The high measurement resolution and multiplexing capability of the fiber laser ultrasonic sensor makes it an ideal candidate for structural health monitoring applications.

Beat Frequency Characterization and Stabilization for Short-Cavity DBR Fiber Grating Laser Based Sensor

In this paper, we propose our observation, analysis, and alleviation method of the beat frequency fluctuation in short-cavity dual-polarization distributed Bragg reflector (DBR) fiber grating laser sensor in time and frequency domain. According to our observation, beat frequency fluctuation varies from 8 to 800 kHz under different pump power. Research shows frequency domain beat signal linewidth broadening aggravates beat frequency fluctuation due to time domain irregular self-pulsation effect. By introducing self-injection into laser cavity as an auxiliary pump laser with a circulator, self-pulsation effect in DBR fiber laser sensor can be greatly suppressed, resulting in the compression of frequency domain beat signal linewidth as well as the stabilization of beat frequency fluctuation. Experimental results demonstrate that beat frequency fluctuation can be greatly reduced from 800 to 8 kHz within 1 h through self-injection, which is significant for high-accurate sensing.

Sensitivity characteristics of broadband fiber-laser-based ultrasound sensors for photoacoustic microscopy.

High-frequency fiber laser sensor is a new acoustic detector for photoacoustic imaging. However, its performance has not been thoroughly studied. Here, we present a comprehensive characterization of a fiber laser sensor for photoacoustic imaging. Ultrasound waves deform the fiber laser cavity and induce frequency changes in the heterodyning output signal. The sensitivity peaks at 22 MHz, which is associated with an azimuthal mode number l = 2 and a radial mode number n = 1. The broadband acoustic sensitivity in terms of frequency shift is 2.25 MHz/kPa and the noise-equivalent pressure reaches 45 Pa with a sampling rate of 100 MHz. The 3-dB bandwidth is 18 MHz for spherical-wave detection. We characterized the spatial distribution of acoustic sensitivity. The sensitivity along the fiber longitudinal direction varies with the laser spatial mode and is determined by the grating and cavity parameters. The sensitivity at the azimuthal direction presents a |cos(2θ)| dependence as a result of fiber core asymmetry. In the radial direction, the sensitivity is inversely proportional to the square root of the distance between the source and the detector. The acoustic sensitivity can be enhanced by reducing the cavity length. We experimentally show that a short sensor can enhance the contrast and penetration depth of PAM than a long one.

Real-Time Interrogation Technology for a Large-Scale Fiber-Ring Laser Sensor Array

A real-time interrogation technology for a large-scale fiber laser sensor array is proposed and experimentally investigated by employing a hybrid topological structure based on the wavelength- and frequency-division multiplexing techniques. In this system, a 2-D n × m sensor array is established by using a 1 × n coupler and n 1 × m couplers. All sensors are divided into n units by the 1 × n coupler. In each unit, there are m fiber-ring laser sensors with approximately the same wavelength and different beat frequencies due to different laser cavity lengths. Meanwhile, different units are characteristic of different operating wavelengths by splicing different fiber Bragg grating (FBG) as a reflector. Therefore, each sensor has a unique wavelength- and frequency-encoded information. All sensors can be interrogated by the combination of wavelength and frequency multiplexing technology. In the paper, a 4 × 4 sensor array is set up experimentally to verify the interrogation ability, and the sensing information of the strain and temperature has been successfully extracted. The experimental results show that the proposed interrogation system has the ability of real-time monitoring for a large-scale fiber laser sensor array.

Distributed Feedback Fiber Laser Strain Sensor Technology

Distributed feedback fiber laser (DFB FL) sensors have been the subject of considerable research interest over the past decade, due primarily to their remarkable inherent strain sensitivity. Recent research has led to significant advances in both fundamental understanding of DFB FL devices and in practical performance outcomes in demanding real-world applications. Here, we provide an overview of the basic operating principles of wavelength division multiplexed DFB FL sensor systems and review the underlying theory of DFB FL.

Acoustic and Ultrasonic Detection With Radio-Frequency Encoded Fiber Laser Sensors

Fiber lasers have been exploited as sensors for the detection of extremely weak mechanical parameters, by monitoring the radio-frequency beat signal between orthogonal polarized lasing modes. This sensor scheme has presented advantages including advanced measurement capability, low cost and convenience in signal demodulation, resistance to environmental disturbances, and intrinsic multiplexing capability. Here, we demonstrate the capability of the fiber laser sensors for the detection of low-frequency ( 1 MHz (denoted as “high frequency” in this paper) and around 200 kHz (denoted as “medium frequency”), toward different applications. For low-frequency acoustic sensing, the detection limit reaches 74 μPa/Hz1/2 at 1 kHz which is comparable to the zeroth-order sea noise. For high-frequency ultrasound detection, the laser sensor presents a detection capability of 40 Pa over 50 MHz bandwidth, which allows its application as an ultrasound detector in photoacoustic (PA) imaging. Preliminary three dimensional PA imaging is presented, by means of time-to-space reconstruction. Ultrasound detection with a polymer-packaged fiber laser at 100 kHz range is demonstrated, which could find applications in underwater search and targeting. Its imaging capability is also exhibited.

Large-Scale Multilongitudinal Mode Fiber Laser Sensor Array With Wavelength/Frequency Division Multiplexing

A large-scale multilongitudinal mode fiber laser sensor array is proposed and demonstrated experimentally based on wavelength/frequency division multiplexing and beat frequency demodulation technique. In the proposed hybrid multiplexing sensing system, the frequency division multiplexing is realized by designing n fiber lasers with different cavity lengths, while the wavelength division multiplexing is achieved by incorporating a 1 × m coupler and an m -channel wavelength division demultiplexer. By making the operating wavelength of frequency-multiplexed lasers equal to the central wavelength of each channel in the demultiplexer, the light of each laser is coupled independently to the corresponding channel. Therefore, the multiplexing capability of the multilongitudinal mode fiber laser sensors is greatly improved and the maximum number of multiplexed sensors is m × n theoretically. To verify the proposed wavelength/frequency division multiplexing system, a 4 × 4 multilongitudinal mode fiber laser sensor array was set up and investigated. Experimental results show that there is a low crosstalk between the adjacent channels. The proposed wavelength/frequency hybrid multiplexing technology has the ability to monitor simultaneously a large number of fiber laser sensors.

Effects of Intensity Noise in Digital Demodulation for Fiber Laser Sensor

Hydrophone array-based distributed feedback fiber laser (DFB FL) has a small size and large sensitivity; however, it also suffers from intensity noise completely. In this letter, a transfer function for intensity noise propagation through the use of a DFB FL sensor interrogation system is derived, basing on which a symmetric algorithm using $3\times 3$ coupler is improved. Both experimental and simulation data are provided to validate the model. A noise reduction of 20 dB is achieved.

采用3×3耦合器的分布反馈式光纤激光传感器解调技术

采用3×3耦合器的分布反馈式光纤激光传感器解调技术

Hybrid wavelength- and frequency-division multiplexed fiber laser sensor array.

The Letter describes a hybrid multiplexing scheme for a multi-longitudinal-mode fiber laser sensor array by combining wavelength-division multiplexing and frequency-division multiplexing techniques. The proposed multiplexing system consists of many subgroups, each of which has many fiber laser sensors with approximately equal laser-operating wavelengths but different cavity lengths. Each sensor in one subgroup can be interrogated by using frequency-division multiplexing technology because of different sensors with different beat frequencies. For different subgroups, they are characteristic of different laser-operating wavelengths. Each subgroup can be divided into the designed channel by dense wavelength-division multiplexers. At last, a 4×4 sensor multiplexing system was experimentally validated. The strain or temperature applied on each sensor was successfully extracted. The proposed multiplexing system provides a dramatic increase in the number of multiplexed sensors. It greatly reduces the weight, volume, and cost of each sensor and offers a competitive solution for a large-scale monitoring system.

Characteristics of the Fiber Laser Sensor System Based on Etched-Bragg Grating Sensing Probe for Determination of the Low Nitrate Concentration in Water.

The necessity of environmental protection has stimulated the development of many kinds of methods allowing the determination of different pollutants in the natural environment, including methods for determining nitrate in source water. In this paper, the characteristics of an etched fiber Bragg grating (e-FBG) sensing probe—which integrated in fiber laser structure—are studied by numerical simulation and experiment. The proposed sensor is demonstrated for determination of the low nitrate concentration in a water environment. Experimental results show that this sensor could determine nitrate in water samples at a low concentration range of 0–80 ppm with good repeatability, rapid response, and average sensitivity of 3.5 × 10−3 nm/ppm with the detection limit of 3 ppm. The e-FBG sensing probe integrated in fiber laser demonstrates many advantages, such as a high resolution for wavelength shift identification, high optical signal-to-noise ratio (OSNR of 40 dB), narrow bandwidth of 0.02 nm that enhanced accuracy and precision of wavelength peak measurement, and capability for optical remote sensing. The obtained results suggested that the proposed e-FBG sensor has a large potential for the determination of low nitrate concentrations in water in outdoor field work.

Compact two wavelength Brillouin fiber laser sensor with double Brillouin frequency spacing

A compact two wavelength Brillouin fiber laser (BFL) sensor with double Brillouin frequency spacing is proposed and demonstrated. In the experiment, 20 m polarization maintaining fiber is used as the sensing element and Brillouin gain medium. This short cavity configuration not only guarantees single longitudinal mode operation of two Stokes wavelengths, but also can effectively reduce external perturbations, complexity and noise of BFL in the absence of an erbium-doped fiber amplifier in intra-cavity. In experiment, about 2 MHz/°C sensitivity of beat frequency between the pump and the 2nd-order Stokes wavelength keep in good agreement with the theoretical value. Meanwhile, 0.2 °C temperature stability and ±0.1 dB power fluctuation are better than the traditional structure. The system is simple and stable, making it convenient for more applications.

Refractive Index Fiber Laser Sensor by Using Tunable Filter Based on No-Core Fiber

A novel refractive index fiber laser sensor based on a fiber Bragg grating (FBG) integrated with a section of no-core fiber (NCF) is proposed and experimentally demonstrated. The oscillating wavelength of the fiber laser is only decided by the central wavelength of the FBG that is insensitive to the surrounding refractive index (SRI). However, the output power of the fiber laser varies with the SRI because the laser output is filtered by the edge of the bandpass filter (BPF) formed by the multimode interference (MMI) effect in the NCF, which is sensitive to the SRI. By measuring the variation of the output power, a cost-effective power detection RI sensor could be realized. The fiber laser sensor has a linear relationship with SRI and a sensitivity of 113.73 dB/RIU in the RI range of 1.333–1.4076.

Polarimetric Multilongitudinal Mode Fiber Laser for Simultaneous Measurement of Strain and Temperature

A sensing system for simultaneously measuring strain and temperature is proposed by employing a polarimetric multilongitudinal mode fiber laser sensor and beat frequency demodulation device. The proposed fiber laser is built with two fiber Bragg gratings and a piece of Erbium-doped-fiber, which act as cavity reflector and gain medium, respectively. Owing to the intrinsic birefringence of fiber, two orthogonal polarization modes experience slight splitting, thus many longitudinal mode beat signals and polarization mode beat signals are generated in the laser cavity. Strain and temperature can be measured simultaneously by monitoring the frequency shift of longitudinal mode beat signal and polarization mode beat signal due to their different responses to strain and temperature. The selection of beat frequency signal pair for improving the laser sensitivity has also been analyzed and investigated experimentally. In addition, the repeatability and stability performances of the proposed laser were also investigated. The proposed sensing system just uses one polarimetric fiber laser sensor and one beat frequency demodulation equipment, and it shows distinctive advantages of simple structure, portability, high sensitivity, and low cost due to the avoidance of expensive optical detection device for wavelength monitoring.

Implementation of distributed feedback fiber laser sensor for acoustic measurements in hydraulic fracturing

A distributed feedback (DFB) fiber laser strain sensor was implemented to measure acoustic emission induced by the hydraulic fracturing process. A study of practical sensor mounting configurations and their characteristics was carried out to find a practical solution. Combining the suitable mounting configuration and ultrahigh strain sensitivity of the DFB fiber laser, the evolution of the hydraulic fracturing process was well monitored. This study shows that fiber lasers can be useful alternatives to piezoelectric sensors in the field of hydraulic fracturing for gas and oil extraction.

Wide bandwidth dual-frequency ultrasound measurements based on fiber laser sensing technology.

A dual-frequency ultrasound measurement system based on a distributed Bragg reflector (DBR) fiber laser sensor in a liquid medium was presented. To compare the dual-frequency measurement performance of a DBR fiber laser acoustic sensor with that of a piezoelectric (PZT) ultrasound sensor, two experiments were performed. First, we fixed the driving frequencies of two ultrasound signals at 3 and 5 MHz, and decreased the driving voltage from 15 to 3 V. The outputs of the DBR acoustic sensor show flat-balanced response to dual-frequencies, compared with the PZT acoustic sensor whose response to one of the dual-frequency signals (5 MHz in this paper) has been covered by noise at low acoustic pressure. Then we increased the acoustic pressure by fixing the driving voltage at 20 V, and changed the frequency spacing between the two ultrasound signals. By analyzing the frequency response, sensitivity, signal-to-noise ratio, and noise equivalent pressure of two acoustic sensors under different frequencies, we found that the response of the DBR sensor to wideband dual-frequency is stable, while the response of the PZT sensor deteriorates sharply with increasing frequency spacing. The results demonstrate that the DBR fiber laser sensor performs better for wide bandwidth dual-frequency ultrasound measurements.

Study of Rayleigh-backscattering induced coherence collapse in an asymmetric DFB FL sensor

Abstract Rayleigh-back scattering induced coherence collapse of an asymmetric distributed feedback fiber laser (DFB FL) sensor is investigated using a composite cavity model. The coherence collapse threshold condition of the asymmetric DFB FL sensor is measured. The DFB FL sensor shows different dynamic behaviors in different pump configurations. According to the asymmetric behavior to the external optical feedback, a novel method to find the actual phase shift position of the asymmetric DFB FL sensor is presented.