The intrinsic phase noise of distributed feedback (DFB) fiber laser greatly reduces the signal to noise ratio (SNR) of unbalance interferometric fiber sensor system, which has a negative influence on the demodulation of tiny signal. In order to suppress the phase noise of DFB fiber laser (DFB-FL), a self-injecion locking DFB-FL was presented. Result of experiment demonstrated that the phase noise can be suppressed by the self-injection locking structure and the effect of suppression was improved with the increase of the length of locking ring. When the length of locking ring is two meters, the phase noise above 500Hz decreases by 8 dB / √Hz demodulated by an one meter optical path difference asymmetric Michelson Interference, and the mode is not hoping in eight hours. Contrast with the unlocked DFB-FL, the pump efficiency of self-injection locking DFB-FL is increased by 30%.
A sensor system based on fiber Bragg grating (FBG) is presented which is to estimate the deflection of a lightweight flexible beam, including the tip position and the tip rotation angle. In this paper, the classical problem of the deflection of a lightweight flexible beam of linear elastic material is analysed. We present the differential equation governing the behavior of a physical system and show that this equation although straightforward in appearance, is in fact rather difficult to solve due to the presence of a non-linear term. We used epoxy glue to attach the FBG sensors to specific locations upper and lower surface of the beam in order to measure local strain measurements. A quasi-distributed FBG static strain sensor network is designed and established. The estimation results from FBG sensors are also compared to reference displacements from the ANSYS simulation results and the experimental results obtained in the laboratory in the static case. The errors of the estimation by FBG sensors are analysed for further error-correction and option-design. When the load weight is 20g, the precision is the highest, the position errors ex and ex are 0.19%, 0.14% respectively, the rotation error eθ, is 1.23%.
The large scale fiber grating array sensing system has found lots of applications in fields distributed in underwater, land, sky and space, which is often configured using TDM/WDM multiplexing schemes. In recent years, the integrated fabrication method without any fiber splicing points plays important roles in providing the largest system high reliability and robust detection performance. However, in such fiber grating array, the synthesis of fiber grating array without bringing any damage becomes a difficult problem. The existing reconstruction algorithms are only used for single fiber bragg grating.This paper will start with the fiber grating array which is formed by two low reflectivity FBGs and regarded as a special chirp fiber grating, then a proposal of synthesis the fiber grating array in time domain is presented, which can also suppress the multi-reflections induced signal crosstalk. The research results will provide a solution to the synthesis of the integrated fiber grating array and a new signal processing method to suppress the array signal crosstalk.
A sonobuoy system based on a fiber optic vector hydrophone (FOVH) is demonstrated. Phase Generated Carrier– Arctangent (PGC-ATAN) demodulation algorithm was used to acquire real-time underwater acoustic signals. After the optimal design of the laser configuration, the background noise of the FOVH is -104.3dB re rad √ Hz at 1 kHz, with an acceleration sensitivity of 41.5dB re rad/g which allows the system detecting signals at DSS0. The theoretical derivation of FOVH directivity is proposed and the design criterion is discussed. The ratio of the minimum to the maximum amplitude of the FOVH directivity is -35dB by symmetrical structure design of the FOVH. A lake trial shows that the maximum detection range of the sonobuoy system is more than 15km for an acoustic signal of 210dB re μPa, and the bearing of a moving target can be estimated.
An approach for reflectivity measurement of ultra-weak fiber Bragg gratings is demonstrated. The scheme uses a referenced high-reflectivity FBG to combine with the measured FBG. They have different center wavelengths. By using two referenced FBGs, the reflectivity of one measured FBG is calculated to be 0.298% and 0.287% respectively. For reflectivity measurement of ultra-weak FBG, to eliminate the influence of the reflection spectra side lobe for the referenced FBG, we get the reflection spectrum of the referenced FBG and measured FBG respectively under the same input intensity. The reflectivity of an ultra-weak FBG is measured to be 0.00916% (-40.38dB) and 0.00803%(-40.95dB) based on the two referenced FBGs. The results presents that the method is feasible.
To realize the miniature of fiber laser accelerometers, a metal-shell-packaged single-cylinder mandrel-structured distributed feedback (DFB) fiber laser accelerometer was proposed, whose key sensing component is the DFB fiber laser with a cavity length of 16mm. Simulation results show that when the weight of the mass is 400g, the radius of the thin shell cylinder is 0.5cm, we will find that the resonance frequency of the sensor is 900Hz and its sensitivity reaches 18.1pm/g. It is also shown that its sensitivity achieved 42.8dB.re.rad/g while demodulated by an unbalanced Michelson optical fiber interferometer with 1m path difference. In addition, the effects of its structure and material parameters on the acceleration sensitivity are also studied.
An integrated polarization maintaining fiber polarized Bragg grating is proposed, which achieves reflecting and polarizing functions simultaneously. This integrated polarized grating was designed to replace the traditional fiber polarizer and Bragg grating by a single fiber device, which may induce enormous performance promotion to optical fiber systems incorporating the two devices, such as fiber ring laser, some fiber sensing networks, etc. An integrated fiber polarized grating was fabricated, and the polarization states of both the reflection and transmission were measured. The temperature and strain responses were also tested which indicated sufficient stability. The integrated fiber polarized grating was then applied to the fiber ring laser, and results indicated that a robust single-longitudinal and single-polarization mode laser can be realized by utilizing this design type of integrated polarized grating.
The acoustic pressure sensitivities of hollow-core photonic bandgap fibers (HC-PBFs) with different thicknesses of silica outer-cladding and polymer jacket were experimentally investigated. Experiment with a HC-PBF with 7 μm-thick silica outer cladding and 100 μm-thick Parylene C jacket demonstrated a pressure sensitivity 10 dB higher than the commercial HC-1550-02 fiber and 25 dB higher than a standard single mode fiber. The significant enhancement in sensitivity would simplify the design of fiber hydrophones and increase the number of sensors that could be multiplexed in a single fiber.
Fiber Bragg grating written in polarization-maintaining fiber is strongly polarization-dependent, and its spectral character depends on the polarization state of light that interrogates the grating. We propose a novel and practical method to test the spectra character of the polarization-maintaining fiber Bragg grating (PM-FBG) interrogated by a linearly polarized light with a different azimuth for the first time. We specially designed a splice with a different angle in the system to alter the azimuth of the input linearly polarized light. Using this method, we got the transmission and reflection spectra of the PM-FBG measured at different splice angles and then the experimental results were analyzed originally. The analysis result shows that the relationship between the transmission dip and the splice angle obeys the Gaussian distribution. The reflectivity of the two wavelengths of the PM-FBG depends on the azimuth of the input linearly polarized light.
The random fluctuation in the input polarization to an interferometric optical fiber sensor can result in not only the
variation of the visibility but also the generation of excess phase noise in the output. In this paper, the relationship
between the visibility and polarization-induced phase noise coefficient is described and theoretically confirmed. The use
of polarization diversity receivers (PDR) to reduce phase noise is theoretically and experimentally demonstrated. The
result shows that the PDR that improves the visibility can reduce the polarization-induce phase noise and the signal can
be correctly demodulated.
The influence of coherence collapse due to Rayleigh backscattering on the scale of DFB fiber laser sensor array is
investigated. The largest scales of a two-element sensor array and a three-element sensor array are measured to be 160 m
and 250 m. Results indicate that coherence collapse due to Rayleigh backscattering does not limit the multiplexing
capability of DFB fiber laser sensor array when the multiplexing capacity remains to be several tens, which is significant
for the scale extension of DFB fiber laser sensor array.
The fiber optic hydrophone research in China has a history of over 20 years. This talk will summarize some remarkable
millstones in this history. Recent progress toward the fiber optic hydrophone research and application will be discussed.
Some commercialization explorations will also be introduced.
Polarization fluctuation in polarization maintaining fiber (PMF) resonator is one of the major noise sources in resonant fiber optic gyroscope (R-FOG). 90-deg polarization-axis rotated splicing in R-FOG is an effective way to suppress the polarization-fluctuation induced noise. 90-deg polarization-axis rotated splicing error influences the noise suppression effect. Here, a polarization-coupling testing system based on white-light interferometry is designed to control 90-deg splicing error in double-coupler PMF resonator first time and a result of 0.37-deg splicing error is obtained for the first time. Then the resonant characteristics of the double-coupler PMF resonator are tested using the saw-tooth waveform scanning method. The finesse of this double-coupler PMF resonator is 24.0 and the phase interval of the two eigenstates of polarization (ESOPs) is π.
This paper presents a PGC interrogation scheme when a three-mode DBR fiber laser is used for fiber optic
interferometer. By carefully adjusting the path difference of interferometer, ▵L, a stable interferential signal can be
generated when ▵L is 2g times of DBR resonator length, where g is an integer. When acoustic signal acts on
the interferometer, the PGC demodulation result is in consistent with that obtained with a single mode, high coherent fiber laser.
Kanakidis et al presented several kinds of all-optical chaotic communication systems using two encoding techniques and various dispersion compensation maps [1]. It shows that the permitted transmission distances are different for various dispersion compensation maps and various encoding techniques. In order to explore the upper limits of the transmission distance, the parameters of the all-optical chaotic communication system introduced by D. Kanakidis et al. [1] is optimized using genetic algorithm. Some useful results are presented.
Bragg fibres have many special characteristics. Therefore this kind of fibre attracts more and more attention. In this paper, genetic algorithm is applied to design Bragg fibres to realise desired dipersion and attenuation characteristics.
The random deviation of the periodicity of the gratings will affect the performance of the fiber gratings. The random errors would not accumulate when the gratings were cascaded. But we found a kind of fabricating system errors induced by the method for the side writing of fiber gratings, which would accumulate when cascaded. So laser with the less pulse energy should be used to write the gratings to developing the system's performance.
A novel widely tunable fiber laser which can rapidly select the desired ITU-T wavelength has been proposed. This multiwavelength source, based on vernier caliper component(two sampled fiber Bragg gratings with a slight different in channel separation) appears very promising for wavelength division multiplexing since which has many advantages such as a widely tuning range of selective wavelength, easily lock to the wavelength separation, cheap costly, stable performance and so on.
In this paper, we demonstrate a chirped grating designed to compensate for both second- and third-order fiber dispersion. The fabrication technique of chirped gratings is the phase-mask beam scanning method by using a standard unchirped phase mask and by tapering a fiber in the region of the grating. We have made theoretical analysis on the chirp induced by linear and nonlinear taper profiles. The reflectivity and time delay curve of such fiber Bragg grating have been calculated.
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