Haifeng Xuan

Long period gratings in air-core photonic bandgap fibers

Long period fiber gratings in hollow-core air-silica photonic bandgap fibers were produced by use of high frequency, short duration, CO2 laser pulses to periodically modify the size, shape and distribution of air holes in the microstructured cladding. The resonant wavelength of these gratings is highly sensitivity to strain but insensitive to temperature, bend and external refractive index. These gratings can be used as stable spectral filters and novel sensors.

Fiber-Optic Fabry–Pérot Acoustic Sensor With Multilayer Graphene Diaphragm

A fiber-optic Fabry-Pérot acoustic sensor with a ~100-nm-thick multilayer graphene diaphragm is reported. Acoustic testing demonstrates a pressure-induced deflection of 1100 nm/kPa and a noise equivalent acoustic signal level of ~ 60 μPa/Hz1/2 at the frequency of 10 kHz. The sensor exhibits a flat frequency response from 0.2 to 22 kHz and may be useful for highly sensitive acoustic sensing.

Long-period gratings in wavelength-scale microfibers

We report the fabrication of long-period gratings (LPGs) in wavelength-scale microfibers with diameters from 1.5 to 3 microm. The LPGs were fabricated by use of a femtosecond IR laser to periodically modify the surface of the fibers. These LPGs have grating periods of a few tens of micrometers, much smaller than those in conventional optical fibers. A compact 10-period LPG with a device length of only approximately 150 microm demonstrated a strong resonant dip of >20 dB around 1330 nm. These microfiber LPGs would be useful in-fiber components for microfiber-based devices, circuits, and sensors.

Sensing with hollow-core photonic bandgap fibers

This paper examines the unique properties of hollow-core photonic bandgap fibers and discusses potential sensing applications of such fibers. Guidance of light in air instead of silica reduces the effect of material on light propagation and is advantageous to applications such as fiber gyroscopes. An air–silica microstructure has novel mechanical and thermal properties and can be beneficial to applications such as acoustic pressure sensors. Holey cladding provides extra flexibility for geometry modification through post thermal treatment or selective filling of the air holes and can be exploited for developing novel devices such as in-line polarizers/polarimeters, modal interferometer, wavelength filters and sensors. The confinement of gas and liquid phase materials and optical guided modes simultaneously within the hollow core allows strong light/sample interaction over an extended length and may be useful for the development of highly sensitive gas and liquid sensors.

In-fiber polarimeters based on hollow-core photonic bandgap fibers

In-fiber polarimeters or polarization mode interferometers (PMIs) are fabricated by cascading two CO2-laser-induced in-fiber polarizers along a piece of hollow-core photonic bandgap fiber. Since the two interfering beams are the orthogonal polarizations of the fundamental mode, which are tightly confined to the core and have much lower loss than higher order modes, the PMIs can have either short (e.g., a few millimeters) or long (tens of meters or longer) device length without significantly changing the fringe contrast and hence provide design flexibility for applications required different device lengths. As examples of potential applications, the PMIs have been experimentally demonstrated for wavelength-dependent group birefringence measurement; and for strain, temperature and torsion sensors. The PMI sensors are quite sensitive to strain but relatively insensitive to temperature as compared with fiber Bragg grating sensors. The PMIs function as good directional torsion sensors that can determine the rate and direction of twist at the same time.

Influence of strain and pressure to the effective refractive index of the fundamental mode of hollow-core photonic bandgap fibers

We investigate the phase sensitivity of the fundamental mode of hollow-core photonic bandgap fibers to strain and acoustic pressure. A theoretical model is constructed to analyze the effect of axial strain and acoustic pressure on the effective refractive index of the fundamental mode. Simulation shows that, for the commercial HC-1550-02 fiber, the contribution of mode-index variation to the overall phase sensitivities to axial strain and acoustic pressure are respectively approximately -2% and approximately -17%. The calculated normalized phase-sensitivities of the HC-1550-02 fiber to strain and acoustic pressure are respectively 1 epsilon(-1) and -331.6 dB re microPa(-1) without considering mode-index variation, and 0.9797 epsilon(-1) and -333.1 dB re microPa(-1) when mode-index variation is included in the calculation. The latter matches better with the experimentally measured results.

Tunable comb filters and refractive index sensors based on fiber loop mirror with inline high birefringence microfiber

Highly birefringent (Hi-Bi) microfiber-based fiber loop mirrors (FLMs) were studied for tunable comb filters and refractive index (RI) sensors. The use of two cascaded Hi-Bi microfibers instead of a single microfiber allows more flexibility in controlling the transmission/reflection characteristics of the FLM. The length of Hi-Bi microfibers is of the order of centimeters, one or even more than two orders of magnitude shorter than the conventional Hi-Bi fiber-based FLM devices. The transmission/reflection spectra are sensitive to the RI surrounding the microfibers, and RI sensitivity of 20,745 nm/RIU was experimentally demonstrated.

Robust microfiber photonic microcells for sensor and device applications

We report the fabrication of in-line photonic microcells (PMCs) by encapsulating tapered microfibers (MFs) inside glass tubes. The encapsulation isolates MFs from external environment and makes them more suitable for real-world applications. Based on PMCs with encapsulated highly birefringent (Hi-Bi) MFs, we demonstrated pressure, temperature and refractive index (RI) sensors as well as long period grating devices. A fiber Sagnac loop interferometer incorporating a Hi-Bi microfiber PMC demonstrated RI sensitivity of 2024 nm per RI unit (nm/RIU) in gaseous environment and 21231 nm/RIU in water.

Fiber-optic ferrule-top nanomechanical resonator with multilayer graphene film

Compact ferrule-top nanomechanical resonators with multilayer graphene (MLG) diaphragms as vibrating elements are demonstrated. The resonators comprise a suspended MLG film supported by a ceramic ferrule with a bore diameter of ∼125 μm. The mechanical resonance of the graphene film is excited and detected by an all-fiber optical interrogation system. Based on a beam-shape graphene mechanical resonator, a force sensitivity of ∼3.8 fN/Hz1/2 was theoretically predicted. The integration of nanomechanical graphene film with optical fiber simplifies the excitation and interrogation of the resonator and would allow the development of practical fiber-optic sensors for force, mass, and pressure measurements.

Rocking Long Period Gratings in Single Mode Fibers

We report novel rocking long period gratings (R-LPGs) made by introducing permanent periodic twist along a standard single mode fiber. Very high coupling efficiency of 32.5 dB was achieved with 23 periods and a 2° rocking angle. The responses of R-LPGs to temperature, strain, and torsion were tested. Compared with a normal LPG written under the same conditions, the R-LPGs were found to have a similar sensitivity to temperature, five times smaller sensitivity to strain, and insensitive to twist.