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A novel concept of acoustic fiber Bragg grating (AFBG) was conceived and experimentally validated as an effective technique for high-temperature sensing. Similar to the well-known optical fiber Bragg grating, the AFBG employs interactions between waves and periodic structures on an elongated waveguide. Acoustic waves reflected from periodic structures interfere with each other and result in the emergence of resonance frequency when acoustic wavelength matches with pitch length of the periodic structures. When the single-mode operation condition is satisfied, the resonance frequency location can be accurately resolved and thus be used as an effective indicator for temperature sensing. To demonstrate this concept, an AFBG sample was fabricated on a telecommunication optical fiber (125- $\mu \text{m}$ diameter) with femtosecond laser micromachining setup, and it demonstrated sensing capability up to 700 °C, which was limited by the fiber material (fused silica) and could be over 1400 °C if other material, such as sapphire, was used as the waveguide. With the features of various applicable material, low system cost, and potential multiplexing capability, this AFBG technology is a promising candidate for high-temperature sensing, even in a distributed manner.

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.

A fiber-optic Fabry-Perot 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.

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.

In this paper we propose a new and cost-effective fabrication scheme for porous polymer optical fibers. Different porous polymer fibers made from polycarbonate (PC) and poly(methyl methacrylate) (PMMA) using this method have been thermally drawn and characterized. Porosity in the fiber cladding is introduced by the absorbed water in one layer of the polymer fiber preforms under heat treatment and/or thermal drawing, and can be controlled by adjusting the water concentration. In addition, we have shown that the fabricated porous polymer fibers have the potential application in localized drug delivery for cancer treatment.

A novel class of polarization converters (PCs) in highly birefringent (Hi-Bi) microfibers is proposed and investi- gated numerically by use of the coded full vectorial finite difference beam propagation method. The novel PCs may be implemented by periodically deforming the surface along “one-side” of a Hi-Bi microfiber. Simulation shows that complete polarization may be achieved with a device length of less than 300 μm.

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.

A novel class of polarization converters (PCs) in highly birefringent (Hi-Bi) microfibers is proposed and investigated numerically by use of the coded full vectorial finite difference beam propagation method. The PCs could be implemented by inducing periodic deformations along the surface of a Hi-Bi microfiber. Simulation shows that complete conversion between two linearly polarized eigen modes may be achieved with a device as short as 150 μm.

We present a large-core single-mode “windmill” single crystal sapphire optical fiber (SCSF) design, which exhibits single-mode operation by stripping off the higher-order modes (HOMs) while maintaining the fundamental mode. The “windmill” SCSF design was analyzed using the finite element analysis method, in which all the HOMs are leaky. The numerical simulation results show single-mode operation in the spectral range from 0.4 to 2 μm in the windmill SCSF, with an effective core diameter as large as 14 μm. Such fiber is expected to improve the performance of many of the current sapphire fiber optic sensor structures.

A new type of single crystal sapphire optical fiber (SCSF) design is proposed to reduce the number of guided modes via a highly dispersive cladding with a periodic array of high and low index regions in the azimuthal direction. The structure retains a “core” region of pure single crystal (SC) sapphire in the center of the fiber and a “cladding” region of alternating layers of air and SC sapphire in the azimuthal direction that is uniform in the radial direction. The modal characteristics and confinement losses of the fundamental mode were analyzed via the finite element method by varying the effective core diameter and the dimensions of the “windmill” shaped cladding. The simulation results showed that the number of guided modes were significantly reduced in the “windmill” fiber design, as the radial dimension of the air and SC sapphire cladding regions increase with corresponding decrease in the azimuthal dimension. It is anticipated that the “windmill” SCSF will readily improve the performance of current fiber optic sensors in the harsh environment and potentially enable those that were limited by the extremely large modal volume of unclad SCSF.

We introduce a femtosecond-laser-based technique for selective opening of airholes in photonic crystal fibers (PCFs). With this technique, selective filling and inflation of the airholes in the PCF cladding are demonstrated. The technique may find important applications in tailoring or altering PCF characteristics and make it possible to seamlessly integrate various components/functions into PCFs.

Highly birefringent (Hi-Bi) air-clad silica microfibers (MFs) with wavelength and sub-wavelength scale transverse dimensions are studied theoretically and experimentally. Hi-Bi MFs are taper-drawn from the standard SMF-28 single mode fibers that are "pre-processed" by "cutting away" parts of the silica cladding on opposite sides of the fiber with a femtosecond infrared laser. Such Hi-Bi MFs have approximately elliptical cross-sections and are approximated by a three-layer model comprising a small central Ge-doped region surrounded by an elliptical silica region and an air-cladding. Theoretical modeling shows that phase and group birefringence of the order 10(-2) can be achieved with such air-clad Hi-Bi MFs. Experiments with an air-clad elliptical fiber with a major diameter of 0.9 microm and a minor/major diameter ratio of 0.9 demonstrated a group birefringence of approximately 0.015, agreeing well with the theoretical predictions. The Hi-Bi MFs are useful for micron/nanoscale polarization maintaining transmission and phase-sensitive interferometric sensors.

Air-cladding micro/nano fibers can be taper-drawn from standard SMF-28 fibers. Firstly, highly birefringent (Hi-Bi) noncircular air-clad silica microfibers (MFs) with wavelength and sub-wavelength scale transverse dimensions are studied. These Hi-Bi MFs are also taper-drawn from the SMF-28 fibers that are "pre-processed" by "cutting away" parts of the silica cladding on opposite sides of the fiber with a femtosecond infrared laser. Such Hi-Bi MFs have approximately elliptical cross-sections. Experiments with an air-clad elliptical fiber with a major diameter of 0.9 μm and a minor/major ratio of 0.9 demonstrated a group birefringence of ~ 0.015. Secondly, long period gratings (LPGs) are fabricated in the circular air-cladding silica microfiber by use of a femtosecond infrared 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. Such a LPG has small temperature sensitivity and is truly a compact device with a grating length of only -150 μm.

A series of low-contract photonic band-gap (PBG) fibers were fabricated by filling the holes of a commercial air-silica hollow-core PBG fiber with different refractive index liquids. The PBGs and the transmission characteristics of these fibers were investigated theoretically and experimentally. An increase in the refractive index of liquid filling the holes causes blue-shift of the PBG and a narrow down of the PBG width, which may be exploited for sensitive refractive index measurement.

In this paper, an in-line optical fiber spectral analysis system is presented and analyzed, which will be used in the field for measuring the mount of components of oil, water and gas in down-well. Instead of full spectrum analysis of crude oil with a spectroscopy on the ground, this system uses a disperse-spectrum-selecting algorithm, which makes the system in small and compact size and easy to use in down-well. The correlation of the model of disperse wavelengths is equivalent to the model of full range wavelengths and the system measurement accuracy reaches 2 percent by experiments. In addition, the math's model of the system is established, and several regression methods such as Multivariate linear regression (MLR) and Principle components regression (PCR) are discussed and compared in detail. The performance of the system is analyzed with the math's model also. The result indicates that the PCR has a higher accuracy then MLR and 6 channels are enough for practical system used in the field. Besides, an accuracy of less than 1 percent may be got by the results of performance analysis.

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.

In this paper, an on-line down-well crude oil analysis system is presented, which is applied to measurement of the mount of components in down-well oil, water and gas. Instead of full spectrum of oil, the disperse-spectrum-selecting algorithm was presented to simplify the design of conventional spectrum analysis system. By the disperse-spectrum-selecting algorithm, the system can be designed in small size and compact structure, and the experiment results show that the correlation of the model using disperse wavelength is equivalent to the model using the all wavelengths and system measurement accurate reach to 2 percent.

A novel optical fiber null coupler (OFNC) sensor based on acousto-optic interaction is developed, which can be used in the structure health monitoring of the medical materials. The OFNC sensors can be response to 10MHz supersonic wave, and their signal-to noise ratio are higher then Piezo Ceramic Transducers(PZT). A kind of Perspex with a 1mm hole is employed as the sample, where the OFNC sensor is glued on, and the reflected signal of ultrasonic wave by the hole is detected .

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.

Long period gratings (LPGs) are fabricated by use of focused high frequency CO(2) laser pulses to periodically modify the transverse dimension of silica microfibers. A 20-period LPG with a 27 dB attenuation dip is realized in a microfiber with a diameter of approximately 6.3 microm. The resonant wavelength has a negative temperature coefficient and a high sensitivity to external refractive index. The microfiber LPGs may be useful in micron scale in-fiber devices and sensors.

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.

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.