A Bragg grating-tuned fiber laser strain sensor system

Design of feedback wavelength demodulation and compensation system for FBG-tuned CW fiber laser

A fiber Bragg grating (FBG) sensor system using a fiber ring laser with a hybrid amplifier is proposed and demonstrated. The hybrid amplifier comprises an erbium doped waveguide amplifier and a semiconductor optical amplifier. The experiment shows that such the hybrid amplifier has a high amplifier spontaneous emission power and gain spectrum. Moreover, this fiber ring laser can provide a stable multiwavelength output with an optical signal-to-noise ratio over 50 dB even if the FBGs are located at a 25 km remote sensing position.

Fiber Bragg grating (FBG) sensors offer a significant advantage for structural health monitoring due to their ability to simultaneously monitor both static and dynamic strain while being durable, lightweight, capable of multiplexing, and immune to electro-magnetic interference. Drawing upon the benefits of FBG sensors, this research explores the use of a series of long-gage fiber optic sensors for damage detection of a structure through dynamic strain measurements and curvature analysis. Typically structural monitoring relies upon detecting structural changes through frequency and acceleration based analysis. However, curvature and strain based analysis may be a more reliable means for structural monitoring as they show more sensitivity to damage compared to modal parameters such as displacement mode shapes and natural frequency. Additionally, long gage FBG strain sensors offer a promising alternative to traditional dynamic measurement methods as the curvature can be computed directly from the FBG strain measurements without the need for numerical differentiation. Small scale experimental testing was performed using an aluminum beam instrumented with a series of FBG optical fiber sensors. Dynamic strain measurements were obtained as the aluminum beam was subjected to various loading and support conditions. From this, a novel normalized parameter based on the curvature from the dynamic strain measurements has been identified as a potential damage sensitive feature. Theoretical predictions and experimental data were compared and conclusions carried out. The results demonstrated the potential of the novel normalized parameter to facilitate dynamic monitoring at both the local and global scale, thus allowing assessment of the structures health.

We have proposed and constructed a system to multiplex fiber Bragg grating (FBG) strain sensors with same reflection wavelength, adopting the synthesis of optical coherence function technique (SOCF), which has realized dynamic strain measurement. The ability to control the coherence peak position of SOCF allows random access in the spatial domain. In this paper, the function of random access strain measurement has been realized by switching the coherence peak position at every FBG, synchronizing with FBG spectrum measurement, to use the sampling rate to the full. In addition, we have proposed and confirmed the novel method for improving FBG interval by simply modulating the LD intensity.

A new multiple fibre Bragg grating (FBG) sensor system for high speed strain signal is proposed. The new system using the CDMA technology is able to separate out the signals of multiple FBG sensors. Linearity of the response for the applying strains was observed. The rapid response of the system for the dynamic strain to FBG sensors was also confirmed. Crosstalk levels between sensor channels appear low. If broadband light source is used and length of PRBS is sufficient, CDMA technology can make it possible to multiplex numerous sensors.

Based on the fiber Bragg gratings (FBGs) and high nonlinear photonic crystal fiber (HN-PCF), a novel dual-wavelength erbium-doped fiber (EDF) laser is proposed and demonstrated. The experimental results show that, owing to the contributions of two degenerate four-wave mixings in the HN-PCF, the proposed fiber laser is great stable and two output signals are uniform at room temperature. With adjustment of the attenuator, our fiber laser can selectively realize one wavelength lasing.

A fiber Bragg grating (FBG) sensor using wide and fast mode-locked wavelength-swept laser is demonstrated. The key device is the optical source, wide and fast wavelength-tunable mode-locked fiber laser based on dispersion tuning. This laser has wide tuning range (over 100nm) and a fast tunable speed (up to 20 kHz). Hence, it is suited for the dynamic FBG sensor system. Our sensor system enables dynamic measurement of strain in real-time at a high measurement rate. In this system,the signals of the sensor are obtained by analyzing temporal waveforms of refleceted light from the FBG without observing the spectrum. A measurement speed as high as 1 kHz is achieved when the wavelength-tunable source is swept at 1 kHz rate. The stability (indicated by the standard deviations of the results) is 30-60 pm in optical spectrum.

Intra-cavity spectroscopic measurements may be obtained with any material that has an absorption signature under the gain bandwidth of a fiber laser. The high sensitivity of the intra- cavity technique combined with the compact size of a fiber laser makes this approach very attractive for a wide range of applications. Experiments have been carried out with acetylene and a tunable erbium-doped fiber laser. Compared to regular absorption measurements, an enhancement in sensitivity limited mainly by laser noise is observed. Since intra-cavity spectroscopy is essentially a single beam technique, dual beam noise reduction is not possible with this arrangement. However, if we consider the fact that a single mode fiber can support two modes of polarization, we can create two lasers using the fiber as a common element for both optical cavities. The proposed fiber laser configuration uses a polarization beam splitter to create two independent cavities (x and y polarization) with the same noise, of which one cavity contains the absorber. This permits the convenient use of Balanced Ratiometric Detection in conjunction with sensitive intra-cavity absorption. Although tested using bulk devices, this fiber laser sensor can easily be implemented in an all- fiber version using off the shelf fiber optic components, which makes it a good candidate for compact field spectroscopic measurements.

We report an experimental study of the effect of fiber length and laser linewidth on self-pulsing dynamics and output stabilization of a single-mode Yb-doped double-clad CW fiber laser. It is found that initiation of self-pulsing under low-level pumping conditions is due to relaxation oscillations and saturable absorption in the weakly pumped region of the doped fiber, irrespective of the fiber length and the laser linewidth. However, with an increase in pump power, depending on fiber length and laser linewidth, the pulses initiated due to relaxation oscillation get amplified, and result in short-duration giant pulses due to either stimulated Brillouin scattering (SBS) or stimulated Raman scattering (SRS). In the case of fiber lasers that employ a broadband mirror and wherein the fiber length is sufficient to reach the SRS threshold, the giant self-pulses are generated by SRS, whereas in the case of fiber lasers using a fiber Bragg grating, characterized by narrowband reflection and with sufficient fiber length to reach the SBS threshold, the giant self-pulses are generated by SBS. Output stabilization and, hence, elimination of self-pulsations can be achieved either by suppressing the relaxation oscillations with the addition of an appropriate length of a passive fiber to sufficiently increase the cavity photon lifetime, or by increasing the pump power to achieve gain uniformity along the doped fiber such that relaxation oscillations and reabsorption effects are suppressed.

On-line measurement of fiber core-temperature is very important for the investigations of high-power fiber lasers. Using thermal infrared imager is currently the most common approach to detect the temperature of fiber, even though it only monitor the surface temperature. So the measurement result is greatly different from the actual temperature of fiber core. Fiber Bragg Gratings (FBGs) have been widely used in detecting many physical quantities, such as strain, temperature, compression strength and curvature due to its intrinsic advantages, including simple manufacture technology, low insertion loss. Here, a new method based on FBG for measuring the temperature of fiber core is presented and experimentally demonstrated. We use a mutiple inscription method that inscribe a low reflectivity FBG on the high reflectivity chirp grating location as a fiber core temperature on-line monitor. Due to the resonant wavelength of chirp grating (1080nm) is far away from FBG's resonance peak (1550nm), it means that the presence of FBG will not impact the performance of the chirp grating. Alongside this composite optical fiber structure is applied to fiber oscillator we inject a amplified spontaneous emission (ASE) light with the wavelength range near 1550 nm from the signal fiber of (2+1)*1 combiner. And then, the reflection spectrum of FBG can be record from the port 2 of circulator. The temperature of chirp grating and low reflectivity FBG can be measure by recording the shift of FBG's resonance peak after the lineal relation between resonance wavelength and temperature are cofirmed. In order to detect the accuracy of this new structure, we also use Optical Frequency Domain Reflectometer (OFDR) to measure the temperature of the chirp grating. Experiment results prove the high accuracy of proposed temperature monitor. It is strongly believed that the novel proposed structure can be used to achieve measurement of fiber core temperature in high-power fiber lasers. This work also provides a novel idea for manufacturing multi-functional composite fiber structure.

Optical fibres (OF) with inscribed arrays of fibre Bragg gratings (AFBG) are interesting for different types of fibre devices such as a fibre sensors and fibre lasers and optical signal management. The most advanced method of manufacturing such AFBG is to write them directly in-line during the fibre drawing process. This gives the possibility of making a huge number of fibre Bragg gratings (FBG) in a single OF. The parameters of these AFBG could be constant or variable along the OF length. We present results of an investigation AFBG produced during fibre drawing process. Typical parameters of such AFBG are: working wavelength into 1550 nm band, spectral width 0.2mm (3dB level), reflective contrast (excess to Rayleigh backscattering level) up to 45 dB. We use polymer or aluminum as AFBG coating. AFBG can be written either in single or multi core preforms. OF with fibre Bragg gratings A(FBG) arrays written over a long length and in-line metal coating have been made for the first time. The optical parameters of the FBG arrays are tested by the optical frequency domain reflectometer method in a wide temperature range, demonstrating no degradation in reflection at heating up to 600C for a fibre with Al coating. Moreover, array of metal coated FBG can not be erased thermally. The mechanical strength of the developed fibre is practically the same as for “ordinary” OF with similar coating, showing the absence of the influence of FBG writing process on fibre strength. Some possibilities of the developed AFBG use in advanced sensor systems with special resolution about 10 mm, Brillouin random fibre laser with linewidth less 40 kHz are considered.

A fiber Bragg grating can be worked as reflector, it is constructed in a short section of optical fiber that reflects light of specific wavelength and allow to pass remaining wavelengths. This is possible by creating a periodic change in refractive index within the fiber core. The change in the modal index of fiber caused due to strain results in a Bragg wavelength. This work describes performance of Fiber Bragg Gratings as a strain sensor at 1550nm wavelength is considered for simulation, corresponding dynamic strain and wavelength shifts are analysed. This performance analysis is useful especially for intrusion detection system where environments are harsh. This sensor is compatible with data communication system and can be used for remote sensing.

We propose a tunable erbium doped fiber laser based on a Fabry-Perot (F-P) cavity tuned by an electrostatic actuator. The device is made of single crystalline silicon. The F-P cavity consists of two Bragg mirrors, one being displaced by a comb-drives actuator. The F-P cavity, grooves for optical fibers and electro-mechanical structure are fabricated by deep reactive ion etching on a 70 μm silicon on insulator wafer and are integrated in a ring fiber laser. The resulting tunable fiber laser has a tuning range of 35 nm in the C-band and a spectral width of less than 0.06 nm. The maximum applied voltage for full tuning of the laser is 37 V. The mechanical resonance frequency of the actuated mirror is 14.4 kHz allowing fast tuning of the laser. The maximum output power is 1.8 mW.

The investigation of fluid-structure interaction during the impact of a flexible plate on a water surface has received much attention. Measurement of highly transient, distributed strain of the plate during the slamming event is of great interest. Multiplexed fiber Bragg grating (FBG) strain sensors provide a promising solution for such measurement since these sensors are inherently waterproof and are immune to electromagnetic interference. However, in order to monitor the highly transient, distributed strain responses (up to 20 kHz), high-speed simultaneous interrogation of multiplexed FBG sensors is required, which is challenging by using commercial optical interrogators. We present a tunable-wavelength-filter-based optical interrogation system for high-speed simultaneous interrogation of multiplexed FBG strain sensors and demonstrate its application for structural monitoring of a flexible plate during the slamming event. The interrogation system employs a piezoelectric-transducer-controlled Fabry–Perot tunable filter. By operating the tunable filter at its resonant frequency and demodulating the sensor signal based on a peak tracing method, we demonstrated an interrogation speed of 100 kHz, an interrogation range of 98 nm, and an interrogation resolution of 5 pm. For proof-of-performance, the interrogation system was used to monitor the vibrational responses of a cantilever plate under impact loading and the measurement of vibration modes up to 6.785 kHz was demonstrated. Finally, the slamming experiments were carried out with six multiplexed FBG strain sensors mounted on a flexible plate. The dynamic strain measurement of the plate during the slamming event was successfully demonstrated by using the high-speed FBG interrogation system.

Mechanical and electrical forces of a three-phase induction motor (TIM), in operation, can cause strain in the stator, which may lead to frustrating vibration problems. These problems may greatly reduce reliability and ultimately lead to increased maintenance cost. This paper shows the method to overcome this problem by carrying out the dynamic strain measurement analysis in a TIM running at no-load condition, using an optical fiber sensor. The dynamic strain induced by mechanical and electrical forces was measured by using fiber Bragg gratings (FBGs), installed inside the motor, in between two stator teeth. The electromagnetic immunity and reduced size of the FBGs render them suitable for this application. The frequency components of the strain in TIM were predicted using a model of the forces present in the air gap, considering static and dynamic rotor eccentricities. Two TIM motors, one of 2-poles and the other of 4-poles, and each equipped with two FBGs strain sensors, were used. The sensors were positioned between two stator teeth, separated by 120° from each other in the 2-pole motor and 90° in the 4-pole motor. The experimental analysis shows that the frequency components of the dynamic force spectrum presented by the two motors are the same as those predicted theoretically. Rotor eccentricity was achieved by attaching an unbalanced load to the rotor shaft. The amplitude of vibration at rotor rotational frequency increased when the unbalanced load on the rotor shaft increased. The measurement presented in this paper can be used to determine some TIM parameters, such as the effect of the motor power supply on its vibration signal. It can also be used as a potential maintenance tool for the continuous monitoring of the TIM with high sensitivity.