Distributed Feedback Fiber Laser Sensor Research Articles

Improved DFB-FL Sensor Interrogation With Low Harmonic Distortion Based on Extended Kalman Filter

In the distributed feedback fiber laser sensor interrogation based on 3×3 coupler, the asymmetry of the 3×3 coupler and the inconsistency of detectors lead to harmonic distortion in the result. This paper presents an improved method that combines the 3×3 coupler and the extended Kalman filter (3×3-EKF method) to effectively suppress the harmonic distortion in real-time. The coefficients of the three ellipses formed by every two-channel interference fringes were estimated using EKF. The system parameters were corrected based on these coefficients to eliminate the dependence on the idealization of 3×3 coupler and detectors. The result shows that the 3×3-EKF method can update the system parameters with a frequency up to 500 kHz. A total harmonic distortion less than 0.1% can be achieved with signal frequency ranging from 20 Hz to 1 kHz. Besides, the system shows a minimum detectable wavelength shift of 3.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> pm/√Hz @ 1 kHz, and a large dynamic range of 135 dB @ 100 Hz.

Dual-Polarization Distributed Feedback Fiber Laser Sensor Based on Femtosecond Laser-Inscribed In-Fiber Stressors for Simultaneous Strain and Temperature Measurements

We propose and experimentally demonstrate a novel dual-polarization distributed feedback fiber laser (DFB-FL) sensor based on femtosecond laser-inscribed in-fiber stressors. The resonant cavity of the DFB-FL sensor consists of a phase-shifted fiber Bragg grating (PS-FBG) in the fiber core of a section of heavily erbium-doped fiber (EDF), together with two stressors in the fiber cladding around the PS-FBG. The PS-FBG was fabricated by means of a scanning UV laser beam and shielded phase mask technique. The two in-fiber stressors, i.e. a sawtooth stressor and a straight stressor, were directly inscribed at two orthogonal planes in the EDF cladding using a near-infrared femtosecond laser. The two stressors could precisely adjust the fiber birefringence in the laser cavity, and hence the DFB-FL sensor can operate in two polarization modes. Moreover, simultaneous measurement of axial strain and temperature was demonstrated by detecting the polarization beat frequency ν <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> and lasing wavelength λ of the dualpolarization DFB-FL sensor. Experimental results exhibit strain sensitivities of 34.5 kHz/με, 1.25 pm/με and temperature sensitivities of 684.6 kHz/°C, 11.5 pm/°C, respectively. As such, the proposed dual-polarization DFB-FL sensor could be used for simultaneous strain and temperature measurements, and hence is promising for application in many areas, such as smart structures and intelligent robotics.

DFB fiber laser sensor for simultaneous measurement of acoustic and magnetic fields

A novel distributed feedback (DFB) fiber laser sensor, which can measure acoustic and magnetic fields simultaneously, is proposed. The magnetic field can be measured by detecting the change of resonant frequency of the fiber laser, and the acoustic pressure can be measured by detecting the phase shift of the fiber laser. Both of the signals can be simultaneously demodulated in the frequency domain without affecting each other. Experimental studies show that the acoustic pressure sensitivity of this sensor is about ?130 dB (0 dB re 1 pm/μPa) and the sensor has a good linearity with a magnetic field sensitivity of 0.57 Hz/mT.

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

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.

Acoustic emission detection using intensity-modulated DFB fiber laser sensor

The bonded distributed feedback (DFB) fiber laser (FL) acoustic emission sensor and the intensity response of the DFB-FL to external acoustic emissions are investigated. The dynamic sensitivity of the DFB-FL is calibrated by a referenced piezoelectric receiver. In the DFB-FL we used here, the minimum detectable signal is 2×10−6 m/s at 5 kHz. Using wavelet packet technology, the collected signals are analyzed, which confirms that an intensity-modulated DFB-FL sensor can be used to detect acoustic emission signals.

Effects of Rayleigh backscattering on the stability of distributed feedback fiber laser sensors

Rayleigh backscattering induces mode hopping of distributed feedback (DFB) fiber lasers in the sensor array, and the critical length related to Rayleigh backscattering limits the size of DFB fiber laser sensor networking. Based on a three-mirror cavity model, the critical length for a DFB fiber laser is derived. It increases nearly exponentially with the coupling coefficient for the ideal π-shifted DFB fiber lasers. The reflectivity of the sub-fiber Bragg grating at the lasing wavelength is the main factor to resist Rayleigh backscattering for a nonideal DFB fiber laser. The corresponding experiments have been carried out, and the critical length of larger than 150 m was achieved for 42-mm-long DFB fiber lasers.

Distributed Feedback Fiber Laser Strain Sensors

The distributed feedback (DFB) fiber laser strain sensor has demonstrated strain resolution comparable to that obtained from high-performance fiber-optic interferometry. This manuscript describes the characteristics and performance of this fiber laser strain sensor and discusses the technological developments necessary to obtain comparable performance from a multiplexed array of laser sensors. The design of the Bragg grating and doped fiber are discussed, where possible providing simplified equations to quantify the relevant design parameters. Techniques based on fiber-optic interferometry to decode the wavelength shifts of the laser are presented and potential noise sources are described. Measurements conducted on a test laser demonstrate the capability of the DFB fiber laser to resolve effective length changes to less than 0.76 fm/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> at 2 kHz. The accuracy of the strain measurement, calculated by subtracting the output of two lasers subjected to the same strain, is found to be less than 1%. Issues relating to multiplexing lasers, such as pump power depletion and optical feedback, are described along with methods to maximize the number of lasers serially multiplexed on a single fiber. Finally, the strain transduction mechanism and methods to mount the laser sensor are described. It is shown that for certain applications, the DFB fiber laser sensor provides significant performance benefits when compared with remotely interrogated fiber-optic interferometric sensing techniques.

DFB FL Sensor Cross-Coupling Reduction

The effect of weak external reflections on distributed feedback (DFB) fiber laser (FL) sensor frequency noise has been studied. For multiplexed DFB FL arrays, it was demonstrated that out-of-band reflections from DFB FL raise the noise level of adjacent lasers by up to 50 dB. It was further demonstrated that by incorporating a complex phase- and amplitude-apodized FL grating design with high sidelobe suppression, cross coupling due to out-of-band reflections can be reduced to negligible levels, and Rayleigh backscatter limited noise performance can be achieved. Practical limitations to acoustic sensitivity of DFB FL sensors due to increased laser frequency noise are discussed, and ways to reduce DFB FL sensitivity to external reflections are proposed

Distributed feedback fiber laser sensor for hydrostatic pressure

A polarimetric distributed feedback fiber laser sensor for the measurement of fluid pressure up to 50 MPa is investigated between room temperature and 250/spl deg/C. The isotropic pressure acts directly on the fiber, which eliminates the need of a mechanical transducer. A pressure resolution of about 20 kPa (0.2 bar) is demonstrated. Furthermore, the protection of the silica fiber against deterioration by the fluid and the thermomechanical effects of various fiber coatings on the sensor response are studied.

Polarimetric distributed feedback fiber laser sensor for simultaneous strain and temperature measurements

We report the application of a dual polarization distributed feedback (DFB) fiber laser as a strain and temperature sensor. By measurement of the absolute wavelength of one polarization as well as the polarization beat frequency, strain and temperature were determined simultaneously. The sensor has an accuracy of +3 microepsilon and +/-0.04 degrees C. Self-heating of the DFB fiber laser as a function of pump power was measured with this sensor.