The method of fabricating phase-shifted fiber Bragg grating (PSFBG) that does not need a phase mask by femtosecond laser (fs) post-processing is demonstrated. A central region of standard single mode fiber Bragg grating (FBG) is irradiated by fs laser. A rotating jig is designed to hold and rotate the fiber when the fiber is irradiated by laser. The bandwidth of transmission peak of PSFBG and the transmission loss is growing with increasing of laser energy. PSFBG produced by this method is simple, fast and reproducible.
Fabricating of phase-shifted fiber Bragg grating (PSFBG) by a femtosecond (fs) laser postprocessing of standard single mode fiber Bragg grating (FBG) without phase mask is demonstrated. A central region of grating is irradiated by an fs laser assisted with a rotating jig, which produces a π phase shift at the central region of the grating and forms a π phase-shifted FBG. The procedure is simple, fast, and has good reproductivity. The bandwidth of transmission peak of PSFBG grows with an increasing amount of laser energy or length of irradiation and decreasing of translation speed; the transmission loss decreases with increasing irradiation length. Additionally, the repeatability of fs postprocessing and temperature stability of PSFBG were investigated.
A novel optic fiber hydrogen sensor is proposed in this paper. Two Bragg gratings with different reflectivity were written in single mode fiber with phase mask method by 248 nm excimer laser. The end-face of singe mode fiber was deposited with WO<sub>3</sub>-Pd-Pt multilayer films as sensing element. The peak intensity of low reflectivity FBG is employed for hydrogen characterization, while that of high reflectivity FBG is used as reference. The experimental results show the hydrogen sensor still has good repeatability when the optic intensity in the fiber is only 1/3 of its initial value. The hydrogen sensor has great potential in measurement of hydrogen concentration.
Magnetic sensors utilizing direct magneto-optic field coupling in an optical fiber Bragg grating (FBG) is proposed and demonstrated. The FBG’s cladding is micro-machined into micro-curvities aided by femtosecond laser, and coated with TbDyFe, magnetic sensing element. Number of micro-curvities and laser energy under a laser beam were optimized during FBG micromachining and dramatically improved sensor performance. Six-micro-groove sensor is four times more sensitive as compared to non-micro-grooved standard FBG sample. The effect of 18 mW laser pulse power impacted magnetic sensitivity of magnitude 0.6 <i>pm/mT </i>as compared to 0.14 <i>pm/mT</i> on non-microstructured standard FBG sensor. The depth of the deposited magnetostrictive film was measured as ~5 μm.
A new process of femtosecond laser micromachining with ultrasonic vibration aided is proposed. An ultrasonic aided device has been designed, and the laser micromachining experiments of transparent materials have been carried out. The effects of the ultrasonic vibration with different power on surface quality and the drilling depth have been investigated, and the mechanism of the ultrasonic vibration aided laser machining has been analyzed. After introducing the ultrasonic vibration device, the residue debris on surface of the ablated trench is significantly reduced, and the drilling depth is increased. These results show that, ultrasonic vibration can effectively improve the surface quality of material processing, increase the depth of the drilling hole and promote the processing efficiency of the femtosecond laser.
Fabricating microstructures into the cladding of fiber Bragg grating, the FBG sensors will have wider applications in magnetic field measurement or gas sensing. In present paper, we regulate the physical feature of FBG by ablating single or cross spiral micro-trench with femtosecond laser. The influences of different processing parameters on M-FBG (microstructured FBG) have been investigated. The waveform variations and its controlling method have been discussed. It is shown that, the central wavelength shift enlarged with increasing of the laser energy, or decreasing of scanning speed. Finally, a cross spiral type M-FBG magnetic field probe and a temperature probe are also demonstrated.
Different novel kinds of 3D microstructure manufactured in the cladding of FBG are proposed. Femtosecond laser and special rotary are used to fabricate microstructure such as single thread, double thread and annulus. The thread pitch is 60μm, 80μm controlled by feed and rotary speed. In addition, a thickness of 300nm copper film is coated in the microgrooves by magnetron sputtering technology. Temperature experiment has also been set up and done. Experiment result shows that the FBG sensor with double thread of 80μm is the most sensitive one responding to temperature change. It is approximately five times higher than the standard FBG sensor .This new type of FBG sensor shows great prospect in temperature sensing.
Novel FBG sensors based on cladding microstructures are proposed, which include micro-holes array, single or double spiral and straight groove types. Through depositing different sensitive film, the microstructured FBG sensors can be applied to fiber optic magnetic field sensor, hydrogen sensor and humidity sensor. As a developing example, a spiral type microstructured FBG magnetic field sensor is demonstrated. The testing results show that, the sensitivity of the spiral types probe can be promoted 4-6 times more than that of the non-microstructured standard FBG probe. The double spiral probe is better than the single spiral type.
Real-time, on-line measurement of key physical parameters as well as their variations is the most crucial problem for
safety running of reciprocating compressors in petrochemical plants. In this paper, a method for condition monitoring
and fault diagnosis of reciprocating compressor based on FBG sensors is proposed. A fiber optic accelerometer is
demonstrated, which is used for detecting the vibration of crankcase, crosshead and cylinder of the compressor. FBGbased
temperature sensors are used to monitor the variation of the suction/vent valve temperature. A monitoring system
included hardware and software for the reciprocating compressors is developed and applied in a petrochemical plant.
Currently, the vibration measurement and monitoring of the rotating machines has become the primary means of fault
diagnosis. This paper proposed to develop a magnetic coupling non-contact machinery vibration detection sensor based
on fiber Bragg grating (FBG). The magnetic coupling technology and elastic diaphragm are adopted to achieve the
vibration detection by changing the axial strain of optical fiber. Theoretical analysis and static experiment show that the
function of output wavelength and the measured spacing is exponential decrease. Dynamic vibration measurement shows
that the proposed FBG diaphragm accelerometer provides a wide frequency response range (0-90Hz), identifying it as a
good candidate for machinery health monitoring.
Currently, sapphire is widely used in the field of optoelectronic devices and micro-mechanical components. One of the
problems in using sapphire is the difficulty in cutting and micro-structuring due to the hardness of sapphire itself. In this
paper, laser micromachining characteristics of sapphire are investigated using 157nm DUV laser micro-ablation system.
Under laser fluence of 3-4 J/cm<sup>2</sup>, the maximum ablation rate could reach to 400nm/s. For 3D laser ablation, it is
necessary to select a proper combination of process parameters. Several 3D micro-structures are produced in sapphire
wafers and sapphire fibers. As a whole, the ablation equality is good for use.
157nm deep-ultraviolet laser is considered as one of good 3D micro-structuring tools. In this study, a micro-fabrication
system based on the 157 nm laser is conducted to micro-structuring experiments of photonic materials. For laser ablation
of fused silica, the ablation rate is about 80nm/pulse under laser fluence of 5 J/cm2. A micro-hole array is produced in
silica glass chip, and several 3D microstructures are ablated into SMF-28 optical fibers. Experiments and analysis show
that, material removal is dominantly photon-chemical process in case of 157nm laser machining.
Silica is a good material of MOEMS, however, it is difficult to be micro-processed to form a complicated appearance.
Introduced in this paper is process analysis of fluorine (F<sub>2</sub>) 157-nm laser micro-ablating silica. The interaction and
mechanism of ablative photo decomposition between the photons of 157-nm laser and silica are discussed in detail. To
test 157-nm silica ablating effect, the profile of micro holes on the end cross section of silica photonic crystal fiber (PCF)
was employed under the exposure of 157-nm beam, to quantitatively analyze the ablating depth and degree. The results
show that under 7.9eV photon energy of 157-nm laser, the defect formations in silica are accumulated to produce a large
quantity of free electrons. Meanwhile, the dopants in silica fiber or glass considerably reduce the breakdown threshold.
Because the rate of single-photon absorption outclasses the rate of multi-photon absorption, it can be inferred that the
mechanism of interactions between 157-nm laser and silica is a process of single-photon absorption of electron-avalanche.
The actual ablating velocity 210nm/pulse showed that the 157-nm laser could be absorbed strongly by silica
material. It also illustrated that there did be thermal process during the ablation, however, because the velocity of damage
caused by ablating was very fast, the heat was therefore limited. A good quality of ablating result could be ensured.