Fiber in-line Mach-Zehnder interferometer based on an inner air-cavity for high-pressure sensing.

We demonstrate a fiber in-line Mach-Zehnder interferometer based on an inner air-cavity with open micro-channel for high-pressure sensing applications. The inner air-cavity is fabricated by combining femtosecond laser micromachining and the fusion splicing technique. The micro-channel is drilled on the top of the inner air-cavity to allow the high-pressure gas to flow in. The fiber in-line device is miniature, robust, and stable in operation and exhibits a high pressure sensitivity of ∼8,239 pm/MPa.

Microfiber in-line Mach-Zehnder interferometer for strain sensing.

An elegant way of achieving an ultracompact optical fiber in-line Mach-Zehnder interferometer is to create an inner air cavity in a section of microfiber. The sandwich structure splits the light propagating in the fiber into two beams: one passes through the inner air cavity and the other travels along the silica wall of the cavity before recombining at the cavity end, resulting in an interference fringe pattern. Such a device is applied for strain measurement with a high sensitivity of 6.8 pm/µÎµ.

Temperature-insensitive refractive index sensing by use of micro Fabry-Pérot cavity based on simplified hollow-core photonic crystal fiber.

A temperature-insensitive micro Fabry-Pérot (FP) cavity based on simplified hollow-core (SHC) photonic crystal fiber (PCF) is demonstrated. Such a device is fabricated by splicing a section of SHC PCF with single mode fibers at both cleaved ends. An extremely low temperature sensitivity of ~0.273 pm/°C is obtained between room temperature and 900°C. By drilling vertical micro-channels using a femtosecond laser, the micro FP cavity can be filled with liquids and functions as a sensitive refractometer and the refractive index sensitivity obtained is ~851.3 nm/RIU (refractive index unit), which indicates an ultra low temperature cross-sensitivity of ~3.2×10(-7) RIU/°C.

Miniaturized fiber in-line Mach-Zehnder interferometer based on inner air cavity for high-temperature sensing.

We demonstrate a miniaturized fiber in-line Mach-Zehnder interferometer based on an inner air cavity adjacent to the fiber core for high-temperature sensing. The inner air cavity is fabricated by femtosecond laser micromachining and the fusion splicing technique. Such a device is robust and insensitive to ambient refractive index change, and has high temperature sensitivity of ∼43.2 pm/°C, up to 1000°C, and low cross sensitivity to strain.

Embedded coupler based on selectively infiltrated photonic crystal fiber for strain measurement.

A photonic crystal fiber (PCF) with embedded coupler is demonstrated for strain measurement. The embedded coupler is constructed by the selective filling of one of the air holes in the PCF. Light propagated in the fiber core can be efficiently coupled to the liquid-filled rod waveguide under phase-matching conditions, resulting in sharp decreasing of resonant wavelength intensity. The highest strain sensitivity is calculated to be ~23.8 pm/µÎµ due to the coupling between core mode and fundamental mode of the liquid rod, when the refractive index (RI) of the liquid is 1.46. With the increase of the RI, the resonance can also be observed between the core mode and the higher-order modes of the liquid rod, whereas the strain sensitivity drops to ~6.4 pm/µÎµ. The experimentally obtained static strain sensitivity values are ~22 and ~3.8 pm/µÎµ for the coupling between the core mode and the fundamental mode or linearly polarized LP(11) modes of the liquid rod, respectively, which are in good agreement with the simulations. The dynamic strain measurement resolution obtained is ~1.2 nε/(Hz)(1/2).

Optical fiber Fabry-Perot interferometer cavity fabricated by femtosecond laser micromachining and fusion splicing for refractive index sensing.

We demonstrate a fiber in-line Fabry-Perot interferometer cavity sensor for refractive index measurement. The interferometer cavity is formed by drilling a micro-hole at the cleaved fiber end facet, followed by fusion splicing. A micro-channel is inscribed by femtosecond laser micromachining to vertically cross the cavity to allow liquid to flow in. The refractive index sensitivity obtained is ~994 nm/RIU (refractive index unit). Such a device is simple in configuration, easy for fabrication and reliable in operation due to extremely low temperature cross sensitivity of ~4.8 × 10(-6) RIU/°C.

Fiber in-line Mach-Zehnder interferometer constructed by selective infiltration of two air holes in photonic crystal fiber.

A fiber in-line Mach-Zehnder interferometer is fabricated through selective infiltrating of two adjacent air holes of the innermost layer in the solid core photonic crystal fiber, assisted by femtosecond laser micromachining. The liquid infiltrated has higher refractive index than that of the background silica, and, hence, the two rods created can support a guide mode with lower effective refractive index than that of silica. The interference is produced by the fiber fundamental mode and the guide mode. The free spectral range (FSR) of the interferometer is found to be dependent on the photonic crystal fiber length, and a large FSR corresponds to a short photonic crystal fiber length. Such an interferometer device is robust and exhibits extremely high temperature sensitivity (∼7.3 nm/°C for the photonic crystal fiber length of 3.4 cm) and flexible operation capability.

Femtosecond laser-assisted selective infiltration of microstructured optical fibers.

A new method of selectively infiltrating microstructured optical fibers with the assistance of femtosecond laser micromachining is presented. With this technique, any type of air-holes in the cross-section of the microstructured optical fibers can be selectively infiltrated with liquids, which opens up a highly efficient, precise, flexible and reliable way of selective infiltrating and has high potential in the fabrication of novel hybrid-structured optical fibers and the devices based on them.

Femtosecond laser fabricated fiber Bragg grating in microfiber for refractive index sensing.

Fiber Bragg grating (FBG) is fabricated in the microfiber by the use of femtosecond laser pulse irradiation. Such a grating can be directly exposed to the surrounding medium without etching or thinning treatment of the fiber, thus possessing high refractive index (RI) sensitivity while maintaining superior reliability. The grating in the microfiber may have a number of propagation modes in its transmission spectrum, depending on the fiber diameter, and the higher order of mode has larger RI sensitivity. The RI sensitivity also depends on the fiber diameter and a smaller diameter corresponds to a large sensitivity. The maximum sensitivity obtained is approximately 231.4 nm per refractive index unit at the refractive index value of approximately 1.44 when the fiber diameter is approximately 2 microm. The FBG fabricated in the microfiber has high potential in various types of optical fiber sensor applications.

A new method for sampled fiber Bragg grating fabrication by use of both femtosecond laser and CO2 laser.

A new method for sample fiber Bragg grating fabrication by use of both femtosecond laser and CO(2) laser has been proposed and demonstrated. Such a method exhibits the advantages of high fabrication flexibility, and good thermal stability. The sampling period and duty cycle can be easily varied by changing the CO(2) laser beam scanning pattern during operation. The gratings produced have potential applications in optical communications, fiber lasers, and optical fiber sensors.

Study of spectral and annealing properties of fiber Bragg gratings written in H2-free and H2- loaded fibers by use of femtosecond laser pulses.

The spectral and annealing properties of a series of fiber Bragg gratings (FBGs) written in both H(2)-loaded and H(2)-free fibers by use of 800nm femtosecond laser pulse irradiation and created through a phase mask, have been investigated. It is found that type II FBGs inscribed in H(2)- loaded fibers exhibit superior spectral quality when compared with those written in H(2)-free fibers. Isochronal annealing tests shows that type II FBGs written in H(2)-free fibers have the highest thermal stability, followed (in order of stability) by H(2)-loaded type II, H(2)-free type I and then H(2)-loaded type I FBGs. The thermal stability of the H(2)-loaded type II FBGs can effectively be increased by using a high temperature pre-annealing treatment. After the treatment, type II FBGs written into both H(2)-free and H(2)-loaded fibers can sustain long-term annealing (for more than 12 hours) at temperatures of more than 1000 masculineC while their high reflectivities can still be maintained. This demonstrates the real potential of the FBGs developed and investigated in this work to be used as the ideal sensing elements for a series of high temperature measurement applications.