Fiber Laser Sensor Research Articles

A fiber laser temperature sensor based on SMF core-offset structure

A simple and low cost fiber laser temperature sensor based on all-fiber single-mode core-offset structure has been demonstrated. When temperature is applied on the core-offset structure filter, the pass-band varies and the wavelength of the output laser shifts correspondingly. A good temperature sensitivity of 0.0449nm/°C and a detection limit of 0.42°C are achieved with measuring range from 30°C to 270°C. Compared with the conventional temperature sensing system, a 980nm laser diode is used as the light source and less detection limit, higher output power and the side-mode suppression ratio (SMSR) are obtained by the proposed fiber laser sensor.

DBR Fiber Laser Sensor With Polarization Mode Suppression

A distributed Bragg reflector fiber laser sensor based on a commercial high-birefringence Er-doped fiber has been proposed. The spectral overlap of two uniform fiber Bragg gratings has been employed as filtering technique to achieve a sensing head that operates in the single longitudinal mode regime. The high-birefringence Er-doped fiber has been employed to remove a polarization mode, only allowing the oscillation of a single polarization mode. In this way, a single frequency operation with a very stable laser output has been achieved. The proposed fiber laser exhibits a linear response for both strain and temperature variations, keeping the SLM regime and the polarization mode suppression.

Effect of Pump Light Polarization and Beat Note Stabilization for Dual-Polarization Fiber Grating Laser Sensors

Dual-polarization fiber grating lasers have been exploited as high-sensitivity photonic sensors for a variety of measurands by monitoring the frequency of the output beat signal. In this paper, we demonstrate that the environmentally induced perturbations on polarization orientation of the pump light can cause considerable changes in output beat frequency, which can significantly affect the resolution of the sensor especially for static or quasi-static measurements. We found that the beat frequency change with the pump orientation is mainly caused by the resonantly enhanced nonlinearity associated with the generation of green fluorescent light, rather than thermal effect or Kerr electro-optic effect. In order to obtain stable beat signal and improve the accuracy of the fiber laser sensor, the fiber lasers are annealed in advance and then connected to polarization-maintaining fibers and devices to avoid the influence of external perturbations. The frequency fluctuation has been reduced from 1.5 to about 0.1 MHz, and the corresponding resolution of the sensor to static transverse load is improved from 1.76 × 10 -4 to 1.17 × 10 -5 N/mm.

A High-Frequency Accelerometer Based on Distributed Bragg Reflector Fiber Laser

We propose an optical accelerometer based on distributed Bragg reflector (DBR) fiber laser sensor. The transducer's vibration reduces the birefringence of the preloaded fiber laser and consequently changes the beat frequency of the two orthogonally polarized lasing modes, thus the frequency can be used to retrieve the acceleration information. The DBR probe is a segment of bare erbium fiber, which has a very high natural frequency of vibration. Experimental results show that sensitivity of 4.810 MHz/g with a dynamic range of 0.01g-5g (where g = 9.8 m/s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is the gravity of the earth) and natural frequency of ~1.3 kHz can be obtained with this sensing system.

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.

Investigation of Bending Sensitivity in Partially Doped Core Fiber for Sensing Applications

A fiber based high sensitive bend sensor is proposed and demonstrated using a uniquely designed partially doped core fiber (PDCF). The fabrication method of PDCF with two core regions, namely an undoped outer region with a diameter of ~ 9.5 μm encompassing a doped, inner core region with a diameter of 4.00 μm is explained. The mechanism of bending effect in proposed PDCF and the experimental setup for amplified spontaneous emission (ASE) based sensor and fiber laser based sensor is illustrated. For ASE sensor, the higher ASE power level loss as the spooling radius is reduced from 20 to 3 cm is measured. The gain peak shift to shorter wavelength with respect to the decrease of the spooling radius from 20 to 3 cm due to higher bending loss at smaller bending radius is observed. The results are in agreement with overlap factor variation of PDCF. As expected from ASE peaks variation, the fiber laser sensor spectral operation is changed from 1539 to 1530 nm range. This phenomenon is due to higher mode field diameter of longer wavelength and result of optical filtering at longer wavelengths. The experimental results showed the output of the ASE is also highly stable, with no observable variation in the power output over a measurement period of 1 h. The PDCF is also temperature insensitive.

Sensitivity enhancement of Faraday effect based heterodyning fiber laser magnetic field sensor by lowering linear birefringence

In this paper, we demonstrate that the sensitivity of Faraday effect based heterodyning fiber laser sensors for magnetic field can be effectively enhanced by lowering the intrinsic linear birefringence inside the fiber laser cavity. Well explained by theoretical analysis and confirmed by birefringence tuning through transversal force, it shows that the sensitivity to magnetic field intensity is inversely proportional to the linear birefringence. A CO(2)-laser treatment is therefore proposed to tune the intra-cavity linear birefringence. With CO(2)-laser treatment, the intra-cavity linear birefringence can be lowered permanently to effectively enhance the sensitivity of a heterodyning fiber laser sensor to magnetic field.

Passively mode-locked fiber laser sensor for acoustic pressure sensing

The development is reported of a multi-longitudinal mode fiber laser sensor based on passive mode locking employing carbon nanotubes in the laser cavity. A polymer membrane is employed beneath the pre-strained erbium-doped fiber (EDF) to convert the sound pressure disturbance into axial strain, alter the cavity length, and induce a shift of the longitudinal modes beat. Hence, acoustic pressure measurement can be carried out by detecting the shift of the beat frequency. Experimental results show comparable strain and sound pressure sensitivity of ~0.5 kHz/με and 147.2 Hz/Pa, respectively. The proposed sensor is an alternative for the measurement of acoustic pressure and possesses the advantages of good stability and ease of interrogation.

Polarimetric heterodyning fiber laser sensor for directional acoustic signal measurement

A DBR fiber grating laser acoustic sensor based on polarization beat signal modulation analysis has been demonstrated for directional acoustic signal measurement. The acoustic sensor was fabricated in birefringent erbium-doped fiber, and the influences of external-acoustic pressure on fiber grating laser sensor were analyzed, considering the effect of relative orientation of the acoustic wave on the degrees of birefringence modulation. In experiment, the birefringence in sensing fiber was modulated by ultrasonic pressure. Agreement between theoretical and experimental results was obtained for ultrasound wave propagating from different directions (0-360 degrees in 15 degrees intervals) corresponding to a nonlinearly change in beat frequency modulation rates. The results demonstrate that the DBR fiber grating laser acoustic sensor has an orientation recognizable ability, offering a potential for acoustic vector signal detection.

Temperature sensing characteristics of tapered Yb-doped fiber amplifiers

We analyze numerically the use of a tapered Yb-doped fiber to explore the temperature response of fiber amplifiers and their feasibility to be used as a sensing element in temperature fiber sensors. The impact of using different pump schemes and different longitudinal shapes of the tapered doped fiber is investigated, and we found that a parabolic shape of the taper and the co-propagating pump scheme are more sensitive to temperature changes at low pump powers. Our results can be used to describe the temperature response of tapered fiber amplifiers doped with other rare-earths, and contribute with new information for the development and optimization of temperature fiber laser sensors.

Regenerated distributed Bragg reflector fiber lasers for high-temperature operation

This Letter presents distributed Bragg reflector (DBR) fiber lasers for high-temperature operation at 750°C. Thermally regenerated fiber gratings were used as the feedback elements to construct an erbium-doped DBR fiber laser. The output power of the fiber laser can reach 1 mW at all operating temperatures. The output power fluctuation tested at 750°C was 1.06% over a period of 7 hours. The thermal regeneration grating fabrication process opens new possibilities to design and to implement fiber laser sensors for extreme environments.

Fiber-Optic Anemometer Based on Distributed Bragg Reflector Fiber Laser Technology

We present a novel fiber-optic anemometer based on a distributed Bragg reflector (DBR) fiber laser. The anemometer consists of a fiber laser pressure sensing element and a Venturi tube designed to convert the wind velocity into differential gas pressure that can be measured by the fiber-laser sensor. A DBR fiber laser that has two polarization modes is designed and fabricated as the major sensing element in this anemometer. The fiber-laser sensor is demodulated by the beat frequency of the two polarization modes, which has higher sensitivity and lower cost over wavelength-demodulated fiber-laser sensors. The experimental results show that the sensor's response to wind velocity is quadratic in the measurement range of 8 ~ 40 m/s, the short term repeatability is better than 0.5%, and its sensitivity is impacted by the power of pump laser. Additionally, this anemometer has the potential capability to be used as a flow meter for both gas and liquid flows.

Simultaneous measurement of strain and temperature with a multi-longitudinal mode erbium-doped fiber laser

A multi-longitudinal mode erbium-doped fiber laser sensor system for simultaneous measurement of strain and temperature is proposed and experimentally demonstrated. The beat frequency signal of the laser shifts with both strain and temperature, while, the output power of the laser only changes with temperature because of the erbium-doped fiber used in the laser cavity. By monitoring both of the beat frequency signal and the output power, the strain and temperature can be achieved at the same time. The maximum experimental errors of simultaneously obtained strain and temperature are within ±17.5με and ±1.8°C over ranges of 0–1100με and 20–140°C.

Theoretical study on Brillouin fiber laser sensor based on white light cavity

We present and theoretically study a superluminal fiber laser based super-sensor employing Brillouin gain. The white light cavity condition is attained by introducing a phase shift component comprising an additional ring or Fabry-Perot cavity into the main cavity. By adjusting the parameters of the laser cavity and those of the phase component it is possible to attain sensitivity enhancement of many orders of magnitude compared to that of conventional laser sensors. The tradeoffs between the attainable sensitivity enhancement, the cavity dimensions and the impact of the cavity roundtrip loss are studied in details, providing a set of design rules for the optimization of the super-sensor.

Multimode fiber laser for simultaneous measurement of strain and temperature based on beat frequency demodulation

A multimode fiber laser sensor system for simultaneous measurement of strain and temperature is proposed and demonstrated. Because of the long cavity and birefringence, longitudinal mode beat frequency and polarization mode beat frequency are achieved in the beat frequency signals of the multimode fiber laser. The strain and temperature can be obtained by monitoring both of them for their different strain and temperature responses. The experimental measurement errors are within ± 16.2 με and ± 1.9 °C. The usage of only one fiber laser and one demodulation system makes the system simple, cost-effective and portable.

Fiber laser sensor based on a phase-shifted chirped grating for acoustic sensing of partial discharges

Acoustic emission monitoring is often used in the diagnosis of electrical and mechanical incipient faults in the high voltage apparatus. Partial discharges are a major source of insulation failure in electric power transformers, and the differentiation from other sources of acoustic emission is of the utmost importance. This paper reports the development of a new sensor concept — a fiber laser sensor based on a phase-shifted chirped fiber grating — for the acoustic emission detection of incipient faults in oil-filled power transformers. These sensors can be placed in the inner surface of the transformer tank wall, not affecting the insulation integrity of the structure and improving fault detection and location. The performance of the sensing head is characterized and compared for different surrounding media: air, water, and oil. The results obtained indicate the feasibility of this sensing approach for the industrial development of practical solutions.

Sensitivity and stability characterization of linear cavity erbium‐doped fiber laser for pressure measurement

AbstractThis article reports a linear cavity Erbium‐doped fiber (EDF) laser for pressure measurement.The fiber laser structure comprises an EDF amplifier, a circulator, an optical coupler, and a fiber Bragg grating (FBG) which is fitted to the shock tube where the pressure is applied. The FBG acts simultaneously as a lasing wavelength selecting component as well as a pressure sensor. The novelty of this sensor lies in its enhanced characteristic with 0.122 nm/bar sensitivity and very good stability with an average power fluctuation of 0.0025 dB over a period of 60 min. Preliminary results have proven to show that the fiber laser sensor is expected to be an attractive choice for long‐distance pressure monitoring. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:2447–2449, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27107

Modeling of temperature sensitivity on tapered Yb-doped fiber lasers

The temperature sensitivity of a tapered Yb-doped fiber laser is numerically investigated. The laser rate equations are modified to analyze the output characteristics of the tapered fiber laser in the continuous wave regime under different temperature conditions. Numerical analysis shows that for different pump schemes, high sensitivity can be achieved when the pump power is reduced to close to the threshold value. Our results are reproducible and contribute new information to the development and optimization of tapered Yb-doped fiber lasers and temperature fiber laser sensors.

Quasi distributed hybrid Brillouin fiber laser sensor system

A hybrid quasi distributed sensing system combining point fiber Bragg gratings and long integral Brillouin scattering transducers is presented. It is able to measure global temperature changes along the sensing line as well as punctual changes at the critical locations of the structure. A 20 km proof-of-concept system has been experimentally demonstrated with a temperature resolution of 0.47 °C.

Simultaneous Measurement of Strain and Load Using a Fiber Laser Sensor

There are longitudinal-mode beat frequency and polarization-mode beat frequency in the beat frequency spectrum of a fiber laser. Thus, a fiber laser sensor for simultaneous measurement of strain and load by monitoring both the longitudinal-mode beat frequency and polarization-mode beat frequency is proposed and demonstrated in this paper. The maximum experimental errors of strain and load obtained are within and g over ranges of 0-960 and 0-600 g. The presented method makes full use of the sensing information of the fiber laser sensor and improves its sensing ability significantly.