Multi-longitudinal Mode Fiber Laser Research Articles

A real time digital vibration acceleration fiber sensing system based on a multi-carrier modulation/demodulation technique

A multi-longitudinal mode fiber laser sensor (MLMFLS) system based on digital modulation/demodulation technique for vibration acceleration detection is proposed. To the best of the authors’ knowledge, this is the first digital vibration acceleration demodulation system ever reported by a MLMFLS system. A few beat frequency signals (BFS) generated by the MLMFLS are used as signal carriers of applied vibration signals. Based on multi-channel digital down-conversion technique, the weak vibration signals modulated on the BFS are superposed by utilizing the digital universal software radio peripheral (USRP). The signal-to-noise ratio (SNR) of the output vibration signal is greatly enhanced. A spectral subtraction method is also utilized to improve the SNR of the demodulated vibration signal. The acceleration of the vibration signal is obtained after second order differential of the time domain vibration signal. By simply changing the position of window function of short-time Fourier transform, a real-time vibration frequency and acceleration sensing system is achieved. The measurement results demonstrated a minimum detectable acceleration of 0.03 g at vibration frequency of 1.0 Hz by the designed sensing system. It also has a simple structure, high signal-to-noise ratio and stability.

Frequency demodulation of an inhomogeneous medium multi-longitudinal mode fiber laser and its sensing application.

Dispersion characteristic could be a significant factor, which impacts the beat frequency of Multi-longitudinal mode fiber laser (MMFL). In this paper, the mechanism of beat frequency generation in inhomogeneous medium Multi-longitudinal mode fiber laser is discussed. Compared with cavity length-dependent fiber laser sensing system, the proposed model uses a several-millimeter-Fiber-Bragg-Grating (FBG) as the sensing head, which features both high sensitivity and compact size. We designed an experiment to exhibit possible sensing application based on the proposed theory as well.

Magnetic field sensor based on resonant cavity dispersion-frequency mapping of a multi-longitudinal mode fiber laser.

A high-sensitivity and compact-size magnetic field sensor based on a multi-longitudinal mode fiber laser is proposed and experimentally demonstrated in this paper. The resonant cavity is composed of two uniform fiber Bragg gratings (FBGs) and a length of Er-doped fiber. A Terfenol-D rod is used as a transducer to stretch the sensing FBG when applying an external magnetic field. Longitudinal mode beat frequency could be generated in the laser and would shift with the deformation of the sensing FBG caused by the external magnetic field. Experimental results show the sensitivity of the proposed sensor is -47.32k H z/m T.

Multi-longitudinal mode fiber laser sensing system based on resonant cavity dispersion-frequency mapping

In this paper, a multi-longitudinal mode fiber laser (MMFL) sensing system is proposed and experimentally demonstrated. The longitudinal mode beat frequency (LMBF) of the MMFL is related to the effective optical path length of the laser’s resonant cavity, which is determined not only by the physical length of the resonant cavity but also by its dispersion characteristic. Thus, when the total dispersion of the resonant cavity varies with the disturbance caused by external physical factors, the LMBF of the laser would shift and can be used for sensing applications. A chirped FBG was induced in the MMFL to enable such dispersion-frequency mapping. The model of such MMFL system is proposed and verified theoretically and experimentally. Experimental result shows that the proposed sensing system has a more compact size and higher resolution compared to conventional configure.

Fiber-optic temperature sensor based on beat frequency and neural network algorithm

A multi-longitudinal mode fiber laser sensor (MLMFLS) system based on neural network (NN) algorithm is proposed. Gaussian process regression (GPR) is used to denoise the beat frequency signal (BFS). The frequency change of BFS has a linear relationship with the applied external temperature change. Neural network algorithm is used to fit the linear relationship between the frequency of BFS and external temperature change. Processing and analysis of experimental data showed that the method fitted very well with the relationship between the beat frequency signal and external temperature change. The MATLAB App Designer tool is used to display the wide band frequency spectrum, frequency of BFS and corresponding temperature change. The sensitivity of single beat frequency is 5.204 kHz/℃, the maximum absolute error is ± 0.15 ℃ and the average error is 0.072 ℃. The five beat frequencies sensitivity is 5.206 kHz/℃, the maximum absolute error is ± 0.09 ℃ and the average error is 0.055 °C. Through the combination of neural network algorithm and beat frequency sensing system, simple, fast and real time temperature sensing is realized.

Multi-longitudinal mode fiber laser digital vibration-sensing system based on a multi-carrier modulation/demodulation technique.

A multi-longitudinal mode fiber laser sensor (MLMFLS) system based on a digital modulation/demodulation technique is proposed. To the best of the authors' knowledge, this is the first digital vibration-demodulation system ever reported by a MLMFLS system. Multiple beat frequency signals (BFS) generated by the MLMFLS work as signal carriers of applied vibration signals. The vibration signals modulated on several BFS at different frequencies are simultaneously demodulated based on a multi-channel digital down-conversion technique by utilizing the digital universal software radio peripheral (USRP). The demodulated vibration signals from different signal channels are superposed by the USRP. Signal-to-noise ratio (SNR) of output vibration signal is highly improved, which increases sensing stability and accuracy of the system. This is really important for sensing of vibration signals that have an extremely low frequency or weak energy. Measurement results demonstrate that the sensing system has an excellent low frequency vibration-sensing capability, simple structure, high SNR, stability and accuracy.

Multi-longitudinal mode fiber laser sensor system with high signal-to-noise ratio based on laser modes control.

A multi-longitudinal mode fiber laser sensor (MLMFLS) system with high signal-to-noise ratio (SNR) is proposed. To the best of authors' knowledge, this is the highest SNR ever reported by MLMFLS system. The laser sensor consists of a narrow-band fiber Bragg grating (FBG), erbium-doped fiber (EDF) and an optical reflector. The FBG controls the number of laser modes which excited by laser sensor. Beat frequency signals (BFS) modulated by applied measurand are generated by those laser modes and their SNR are controlled by the FBG. Measurement results show the BFS have SNR of 65.0 dB, accuracy of ± 2.0 με, and stability of 0.4 kHz, which demonstrate its high SNR, accuracy and stability.

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.

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.

Multilongitudinal mode fiber laser for highly-sensitive magnetic field measurement

In this paper, a magnetic field sensor based on a multilongitudinal mode fiber laser has been proposed by employing a rotary apparatus modulated by an external magnetic field. Due to the remarkable birefringence change caused by the fiber twist, the polarimetric mode beat frequency (PMBF) of the laser is sensitive to the applied magnetic field intensity. Experimental results show that the PMBF linearly shifts with the variation of the magnetic field intensity and the sensitivity reaches up to 10.4 KHz Oe−1 in the range of 0–270 Oe. Moreover, such a device has the advantages of easy fabrication, low cost and high resolution, offering potentials in the weak magnetic field applications.

Highly Tm(3+) doped germanate glass and its single mode fiber for 2.0 μm laser.

Highly Tm3+ doped optical fibers are urgently desirable for 2.0 μm compact single-frequency fiber laser and high-repetition-rate mode-locked fiber laser. Here, we systematically investigated the optical parameters, energy transfer processes and thermal properties of Tm3+ doped barium gallo-germanate (BGG) glasses. Highly Tm3+ doped BGG glass single mode (SM) fibers were fabricated by the rod-in-tube technique. The Tm3+ doping concentration reaches 7.6 × 1020 ions/cm3, being the reported highest level in Tm3+ doped BGG SM fibers. Using ultra short (1.6 cm) as-drawn highly Tm3+ doped BGG SM fiber, a single-frequency fiber laser at 1.95 μm has been demonstrated with a maximum output power of 35 mW when in-band pumped by a home-made 1568 nm fiber laser. Additionally, a multilongitudinal-mode fiber laser at 1.95 μm has also been achieved in a 10 cm long as-drawn active fiber, yielding a maximum laser output power of 165 mW and a slope efficiency of 17%. The results confirm that the as-drawn highly Tm3+ doped BGG SM fibers are promising in applications that require high gain and high power from a short piece of active optical fiber.

A Low-Cost FDM System for Multi-Longitudinal Mode Fiber Laser Sensor Array

A low-cost and real-time frequency division multiplexing system is proposed based on multi-longitudinal mode fiber laser sensor array and beat signal demodulation technology. The lasers have staggered cavity lengths with corresponding mode spacing. Beat signals of the longitudinal modes of each laser in the array are detected by a common detector. A relationship is worked out between the maximum multiplexing number of the sensors and maximum detectable frequency. A multiplexing array with four laser sensors is investigated experimentally and its potential application for strain measurement is also demonstrated. In the proposed multiplexing system, only one common photodetector and a commercial frequency analyzer are used to monitor all sensors simultaneously, which greatly decreases the cost of the system and makes it more competitive in large-scale monitoring field.

Multiplexed multi-longitudinal mode fiber laser sensor.

A multiplexed multi-longitudinal mode fiber laser sensor system is proposed and demonstrated. By incorporating two matched wavelength division multiplexers (WDMs) and a semiconductor optical amplifier (SOA) into a fiber laser cavity, multiwavelength oscillation is established. Each wavelength corresponding to one channel of WDMs contains multi-longitudinal modes. The multiwavelength output of the laser is directed to another WDM which functions as a demultiplexer. By monitoring the longitudinal mode beat frequency generated at photodetectors following the WDM, the sensing information can be demodulated. Preliminary results for multiplexing two sensors measuring strain and temperature are presented to verify the principle of the system.

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.

High-sensitivity sensor based on mode number-encoded multi-longitudinal mode f iber laser

We propose a high sensitivity sensor based on a mode number-encoded multi-longitudinal mode fiber laser. The fiber laser incorporates a uniform fiber Bragg grating (FBG) and a fiber Fabry-Perot interferometer (FFPI) as sensitive components in the cavity. The sensor counts the number of longitudinal modes (NLM) of fiber laser, which is caused by the mismatch between the reflection band of FBG and the transmission band of FFPI resulting from the application of external perturbation to the FBG. An electrical spectrum analyzer is adopted to analyze the NLM. The strain sensor is experimentally demonstrated to have sensitivity of as high as 0.02 \mu \varepsilon/mode.

Multilongitudinal mode fiber laser for strain measurement

A multilongitudinal mode fiber laser sensor formed by two fiber Bragg gratings and a piece of erbium-doped fiber is first proposed and validated experimentally. When the strain is applied on the sensor, the laser cavity is stretched, which leads to a change of round-trip frequency. Thus the strain can be obtained by measuring the beat frequency of the resonant cavity. The proposed sensor is found to be characterized with simplicity, easy setup, high resolution, low-cost demodulation, and good stability. Experimental results show that the sensor has a sensitivity of -1.1 KHz/microepsilon and the root-mean-square deviation of 3.6 microepsilon.