Joseph Muna Karanja

Micro-structured femtosecond laser assisted FBG hydrogen sensor.

We discuss hydrogen sensors based on fiber Bragg gratings (FBGs) micro-machined by femtosecond laser to form microgrooves and sputtered with Pd/Ag composite film. The atomic ratio of the two metals is controlled at Pd:Ag = 3:1. At room temperature, the hydrogen sensitivity of the sensor probe micro-machined by 75 mW laser power and sputtered with 520 nm of Pd/Ag film is 16.5 pm/%H. Comparably, the standard FBG hydrogen sensitivity becomes 2.5 pm/%H towards the same 4% hydrogen concentration. At an ambient temperature of 35°C, the processed sensor head has a dramatic rise in hydrogen sensitivity. Besides, the sensor shows good response and repeatability during hydrogen concentration test.

Femtosecond Laser Ablated FBG Multitrenches for Magnetic Field Sensor Application

Three-dimensional multitrench microstructures, femtosecond laser ablated in fiber Bragg grating (FBG) cladding, TbDyFe sputtered is proposed and demonstrated for magnetic field sensing probe. Parameters such as the number of straight microtrenches, translation speed (feed rate), and laser pulse power of laser beam have been systematically varied and optimized. A 5-μm-thick giant Terfenol-D magnetostrictive film is sputtered on to FBG microtrenches, and acts as magnetic sensing transducer. Eight microtrench samples produced the highest central wavelength shift of 120 pm, nearly fivefold more sensitive compared with nonmicrostructured standard FBG. An increase in laser pulse power to 20 mW generated the magnetic sensitivity of 0.58 pm/mT. Interestingly, reduction in translational speed contributed dramatically to the rise in the magnetic sensitivity of the samples. These sensor samples show magnetic response reversibility and have great potential in the magnetic field sensing domain.

Microstructured FBG hydrogen sensor based on Pt-loaded WO_3

Hydrogen gas sensing properties of Pt-WO3 films on spiral microstructured fiber Bragg grating (FBG) has been demonstrated. Pt-WO3 film was prepared by hydrothermal method. The spiral microsturctured FBG was fabricated using femtosecond laser. Spiral microstructure FBG hydrogen sensor can detect hydrogen concentration from 0.02% H2 to 4% H2 at room temperature, and the response time is shortened from a few minutes to 10~30 s. Double spiral microstructure at pitch 60 μm and sputtered with 2 μm Pt-WO3 film recorded hydrogen sensitivity of 522 pm/%(v/v) H2 responding to hydrogen gas in air. This translated to approximately 2~4 times higher than the unprocessed standard FBG. The humidity has little effect on the sensing property. The sensor has fast response time, good stability, large detection range and has the good prospect of practical application for hydrogen leak detection.

Femtosecond laser inscription of phase-shifted grating by post-processing

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.

Multi-curvities femtosecond laser aided microstructures for magnetic field sensing

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 pm/mT as compared to 0.14 pm/mT on non-microstructured standard FBG sensor. The depth of the deposited magnetostrictive film was measured as ~5 μm.

Research on ultrasonic vibration aided femtosecond laser machining process of transparent materials

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.

Feature regulation and applications of M-FBG by laser ablation

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.

Novel FBG sensors based on cladding surface microstructures

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.