Minwei Yang

Fiber in-line Mach-Zehnder interferometer fabricated by femtosecond laser micromachining for refractive index measurement with high sensitivity

We report a compact fiber in-line Mach-Zehnder interferometer for refractive index sensing with high sensitivity and precise sensing location. One arm of the interferometer contains a microcavity formed by removing part of the fiber core near the core and cladding interface by femtosecond laser micromachining, and the other arm remains in line with the remaining part of the fiber core. Such a fiber in-line Mach-Zehnder interferometer exhibits an extremely high refractive-index-sensitivity of −9370 nm/RIU (refractive index unit) within the refractive index range between 1.31 and 1.335.

High-Temperature Sensing Using Miniaturized Fiber In-Line Mach–Zehnder Interferometer

A miniaturized single fiber in-line Mach-Zehnder interferometer is proposed for high-temperature sensing. The interferometer has a microcavity in one of its arms, formed by removing part of the fiber core and cladding, while the other arm remains in the fiber core. Because the fiber core exhibits a temperature coefficient of refractive index which is different from that of air, the interferometer is sensitive to temperature variation. The microcavity structured interferometer also has excellent sustainability to high temperatures up to 1100°C . The system is compact, reliable and can detect the temperature at precise location.

Fiber In-Line Mach–Zehnder Interferometer Embedded in FBG for Simultaneous Refractive Index and Temperature Measurement

An ultracompact optical fiber sensor based on a Mach-Zehnder interferometer (MZI) embedded in fiber Bragg grating (FBG) is proposed and experimentally demonstrated for simultaneous refractive index (RI) and temperature measurement. By use of the resonant wavelength of the FBG and the interference dip of the MZI, the RI and temperature of the surrounding medium can be unambiguously determined. The interesting properties of the sensor include good operation linearity, extremely high RI sensitivity up to ~ -9148 nm/RI unit in the RI range between 1.30 and 1.325, and precise sensing location, determined by the MZI cavity created.

Selectively Infiltrated Photonic Crystal Fiber With Ultrahigh Temperature Sensitivity

By selective filling of one of the air holes in the photonic crystal fiber, the fundamental core mode can be effectively coupled to the fundamental mode of the adjacent liquid rod waveguide at the resonant wavelength with extremely high temperature sensitivity. The spectral power of the rod mode can be filtered out by fusion splicing the selectively infiltrated photonic crystal fiber with conventional single-mode fiber, resulting in a sharp dip in the transmission spectrum. Such a device is demonstrated in our experiment by filling standard 1.46 refractive index liquid into one of the air holes of the commercially available photonic crystal fiber by use of femtosecond laser-assisted selective infiltration technique. The average temperature sensitivity achieved is ~54.3 nm/°C.

Refractive index sensor based on a microhole in single-mode fiber created by the use of femtosecond laser micromachining

A compact in-fiber refractive index (RI) sensor is presented that is based on a microhole created in a conventional single-mode fiber by the use of femtosecond laser micromachining. The transmission properties of such a device with a microhole of different diameters have been investigated in the wavelength region of 1500-1600 nm and in the RI range of 1.30-1.45. It is found that the relationship between the transmission and the RI is critically dependent on the size of the microhole in the fiber core region. The highest resolution obtained is 6.70x10(-5), in the RI range of 1.37-1.42, when the microhole diameter is approximately 8 microm, close to the fiber core size. The in-fiber RI sensor developed in this work is easy to fabricate and can be used to implement temperature-independent measurements.

Fiber Bragg gratings with enhanced thermal stability by residual stress relaxation

Fiber Bragg gratings with greatly enhanced thermal stability have been fabricated by the use of femtosecond laser pulse irradiation on optical fibers with relaxed residual stress, through using high temperature annealing treatment. The grating reflectivity and resonant wavelength can be maintained for periods up to 20 hours using isothermal measurements and temperatures up to 1200 degrees C. No hysteresis was observed in the wavelength response when the gratings were annealed and the temperature cycled repeatedly between room temperature and 1200 degrees 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 fabricated micro Mach-Zehnder interferometer with Pd film as sensing materials for hydrogen sensing

In this paper, a femtosecond laser fabricated fiber inline micro Mach-Zehnder interferometer with deposited palladium film for hydrogen sensing is presented. Simulation results show that the transmission spectrum of the interferometer is critically dependent on the microcavity length and the refractive index of Pd film and a short microcavity length corresponds to a high sensitivity. The experimental results obtained in the wavelength region of 1200-1400 nm, and in the hydrogen concentration range of 0-16%, agree well with that of the simulations. The developed system has high potential in hydrogen sensing with high sensitivity.

Long period fiber grating formed by periodically structured microholes in all-solid photonic bandgap fiber

A new type long period fiber grating is fabricated in all-solid photonic bandgap fiber by periodically drilling microholes using femtosecond laser pulse irradiation. Such a structure introduces a strong refractive index modulation in the waveguide structure and hence exhibits a compact grating dimension. Both the simulation and the experimental results confirm the existence of the light coupling from core mode to the LP(11) cladding mode. The refractive index sensing capability of the device has been investigated and the resonant wavelength shift corresponding to the refractive index change from 1.30 to 1.35 is 23.7 nm. The average refractive index sensitivity obtained is 537 nm/RIU (refractive index unit).

Prestressed Fiber Bragg Grating With High Temperature Stability

A prestressed fiber Bragg grating with high temperature stability has been successfully fabricated by use of 800 nm femtosecond laser pulse irradiation through high-temperature annealing, followed by a rapid cooling treatment. The reflectivity and the resonant wavelength of the grating are maintained constants for more than 26 h in an isothermal measurement up to 1200°C, showing significant improvement over conventional femtosecond laser pulse induced fiber gratings. Longitudinal tension tests demonstrate that the prestressed fiber exhibits enhanced mechanical robustness compared with the residual stress relaxed fiber. This paper reveals that residual stress plays an important role and essentially provides a new solution to enhance the thermal stability of fiber gratings at elevated temperatures and offering high reliability.