Semiconductor-based photodetectors have received wide attention. Traditional photodetectors are based on electrical test methods, which inevitably disturbed by dark current noise. In order to overcome this problem, this paper proposes an all-optical photodetection scheme based on a microfiber/SiC-nanowire directional coupling structure, which directly utilizes the refractive index change of SiC nanowire caused by photogenerated carriers instead of the change of photocurrent. The device is fabricated by transferring a single SiC nanowire to a microfiber with diameter of about 1 μm by microscopic operating system. When the device is irradiated by a 266 nm deep ultraviolet laser, its light detection sensitivity reaches up to 103 pm/(W/cm<sup>2</sup>).
A new type of refractive index (RI) sensor based on a side-polished fiber Bragg grating (FBG) has been experimentally demonstrated. At first, a FBG was inscribed in a single mode fiber by means of 267 nm fs laser irradiation through a uniform phase mask, which has an excellent spectral shape with a transmission loss of −9 dB (i.e., a high reflectivity of 87.41%), a 3dB bandwidth of 0.78 nm. Then, the portion of the fiber cladding enclosing the FBG was side-polished into a D-shaped configuration, and the D-shaped FBG was employed as an RI sensor. The removal of the fiber cladding via side-polishing technique ensures that the propagating core-mode of the FBG can interact with the external medium, and hence the Bragg wavelength of the D-shaped FBG will be sensitive to the RI change of the external medium. A conventional unpolished FBG demonstrated a linear relationship between the Bragg wavelength and the surrounding RI with a low sensitivity of ~1.8 pm/RIU at an RI of 1.45. For comparison, the side-polished FBG exhibited a considerably increased RI sensitivity of up to ~10 nm/RIU at an RI of 1.45. Hence, it may be attractive for biochemical sensing applications.
Gas pressure sensor based on an antiresonant reflecting guidance mechanism in a hollow-core fiber (HCF) with an open microchannel is experimentally demonstrated. The microchannel is created on the ring cladding of the HCF by femtosecond laser to provide an aircore pressure equivalent to the external pressure. The HCF cladding functions as an antiresonant reflecting waveguide, which induces sharp periodic losses in its transmission spectrum. The proposed sensor is miniature, robust, and exhibits a high pressure sensitivity of 3.592 nm/MPa, a low temperature cross-sensitivity of 7.5 kPa/°C.
In this paper, an egg-shaped microbubble is proposed and analyzed firstly, which is fabricated by the pressure-assisted arc discharge technique. By tailoring the arc parameters and the position of glass tube during the fabrication process, the thinnest wall of the fabricated microbubble could reach to the level of 873nm. Then, the fiber Fabry–Perot interference technique is used to analyze the deformation of microbubble that under different filling pressures. It is found that the endface of micro-bubble occurs compression when the inner pressure increasing from 4Kpa to 1400KPa. And the pressure sensitivity of such egg-shaped microbubble sample is14.3pm/Kpa. Results of this study could be good reference for developing new pressure sensors, etc.
Negative-index fiber Bragg gratings (FBGs) were fabricated using 800 nm femtosecond laser overexposure and thermal regeneration. A positive-index type I-IR FBG was first inscribed in H<sub>2</sub>-free fiber with a uniform phase mask, and then a highly polarization dependent phase-shifted FBG (PSFBG) was created from the type I-IR FBG by overexposure. Subsequently, the PSFBG was annealed at 800 °C for 12 hours. A negative-index FBG was obtained with a reflectivity of 99.22%, an insertion loss of 0.08 dB, a blue-shift of 0.83 nm, and an operating temperature of up to 1000 °C.
A Graphene Oxide (GO) modified surface plasmon resonance (SPR) sensor based on the silver-coated side-polished fiber was demonstrated. Stable GO aqueous dispersion was prepared through sonication, confirmed by UV-vis absorption spectrum and Tyndall effect. GO nanosheets were decorated on the Octadecanethiol (ODT)-silver sensor surface, where ODT act as a link between the silver and GO nano-films. The GO decoration process was in-situ monitored by SPR wavelength interrogation method. The proposed SPR fiber sensor show a refractive index sensitivity up to 2252.0 nm/RIU in the range of 1.30 ∼ 1.40 RIU and can be used as promising candidate in biosensing.
We reported a Bragg grating inscribed in gold-coated fiber (FBG) by NIR femtosecond laser (fs) for space application. Gold coating can shield the FBG from ultraviolet radiation and oxygen atom erosion. Cryogenic test, high temperature test, and gamma irradiation test were carried out. The reflectivity of the H2-free FBG remained stable at ± 120 °C for 100 h or with 50.4 krad γ irradiation, and the central wavelength shifted within 5 pm and 1.6 pm respectively. Regeneration of the fs-FBG was observed in case the FBG was annealed at 800 °C for 5 h, and the remained 5% in reflectivity after 19 h. Such fs-FBGs inscribed in gold-coated fiber could be employed as high performance fiber sensors for space application.
We investigated experimentally liquid crystal (LC) filled photonic crystal fiber’s temperature responses at different temperature ranging from 30 to 80°C. Experimental evidences presented that the LC’s clearing point temperature was 58°C, which is consistent with the theoretical given value. The bandgap transmission was found to have opposite temperature responses lower and higher than the LC’s clearing point temperature owing to its phase transition property. A high bandgap tuning sensitivity of 105 nm/°C was achieved around LC’s clearing point temperature.
We demonstrated an ultrasensitive temperature sensor based on a unique fiber Fabry-Perot interferometer (FPI). The FPI was created by means of splicing a mercury-filled silica tube with a single-mode fiber (SMF). The FPI had an air cavity, which was formed by the end face of the SMF and that of the mercury column. Experimental results showed that the FPI had an ultrahigh temperature-sensitivity of up to -41 nm/°C, which was about one order of magnitude higher than those of the reported FPI-based fiber tip sensors. Such a FPI temperature sensor is expected to have potential applications for highly-sensitive ambient temperature sensing.
This paper reports a new silica fiber-tip Fabry-Perot interferometer with thin film and large surface area characteristic for high pressure and vacuum degree detection simultaneously, which is fabricated by etching a flat fiber tip into concave surface firstly, with subsequent arc jointing the concave fiber into a inline Fabry-Perot cavity, then drawing one surface of the F-P cavity into several micrometers scale by arc discharge and finally etching the surface into sub-micrometer scale integrally. As the silica fiber-tip Fabry-Perot interferometer film thickness could be tailored very thinly by HF acid solution, plus the surface area of thin film could be expanded during the chemical etching process, the variation of the bubble cavity length is very sensitive to the inner/outer pressure difference of the fiber-tip Fabry-Perot interferometer. Experimental result shows an high sensitivity of 780nm/MPa is feasible. Such configuration has the advantages of lowcost, ease of fabrication and compact size, which make it a promising candidate for pressure and vacuum measurement.
We proposed and experimentally demonstrated four kinds of high-sensitivity gas pressure sensors based on in-fiber devices, including a sub-micron silica diaphragm-based fiber-tip, a polymer-capped Fabry-Perot interferometer, an inflated long period fiber grating and a twin core fiber-based Mach-Zehnder interferometer, which have sensitivities of 1036, 1130, 1680, 9600 pm/MPa, respectively.
Fiber Bragg gratings (FBGs) in gold-coated SMF have been successfully inscribed with NIR femtosecond laser and a phase mask for high temperature sensing application. The spectrums of FBGs inscribed by femtosecond laser are broader and asymmetrical with flat-toped profile which degrades the accuracy of FBGs interrogation with common peak detection techniques. A smart interrogation algorithm based on pattern matching (PMSIA) is reported in this paper. In this algorithm, an adjustable fitting spectrum template was proposed which enables the ability to suit for various spectrum patterns was proposed. The results of simulation and experiment demonstrate the noise immunity and threshold reliability of PMSIA. Less than 7pm interrogation error PMSIA was obtained even if the spectrum changes greatly in the very large sensing temperature range (up to 700°C).
We reported a few high-sensitivity optical strain sensors based on different types of in-fiber FPIs with air bubble cavities those were fabricated by use of a commercial fusion splicer. The cavity length and the shape of air bubbles were investigated to enhance its tensile strain sensitivity. A FPI based on a spherical air bubble was demonstrated by splicing together two sections of standard single-mode fibers, and the spherical air bubble was reshaped into an elliptical air bubble by mean of repeating arc discharge, so the strain sensitivity of the FPI based on an elliptical air bubble was enhanced to 6.0 pm⁄με owe to the decrease of the air cavity length. Moreover, a unique FPI based on a rectangular air bubble was demonstrated by use of an improved technique for splicing two sections of standard single mode fibers together and tapering the splicing joint. The sensitivity of the rectangular-bubble-based strain sensor was enhanced to be up to 43.0 pm/με and is the highest strain sensitivity among the in-fiber FPI-based strain sensors with air bubble cavities reported so far. The reason for this is that the rectangular air bubble has a sharply taper and a thin wall with a thickness of about 1 μm. Moreover, those strain sensors above have a very low temperature sensitivity of about 2.0 pm/oC. Thus, the temperature-induced strain measurement error is less than 0.046 με/oC.
An improved arc discharge technique was demonstrated to inscribe high-quality LPFGs with a resonant attenuation of - 28 dB and an insertion loss of 0.2 dB by use of a commercial fusion splicer. Such a technique avoids the influence of the mass which is prerequisite for traditional technique. Moreover, no physical deformation was observed on the LPFG surface. Compared with more than 86 grating periods required by traditional arc discharge technique, only 27 grating periods were required to inscribe a compact LPFG by our improved arc discharge technique.
We present a new type of phase-shifted FBGs based on an in-grating bubble fabricated by femtosecond laser ablation together with fusion splicing technique. A micro-channel vertically crossing the bubble is drilled by femtosecond laser to allow liquid to flow in or out. By filling different refractive index liquid into the bubble, the phase-shift peak is found to experience a linear red shift with the increase of refractive index. Such a PS-FBG could be used to develop a promising tunable optical filter and sensor.
We demonstrated a high-sensitivity strain sensor based on an in-line Fabry-Perot interferometer with an air cavity whose was created by splicing together two sections of standard single mode fibers. The sensitivity of this strain sensor was enhanced to 6.02 pm/με by improving the cavity length of the Fabry-Perot interferometer by means of repeating arc discharges for reshaping the air cavity. Moreover, such a strain sensor has a very low temperature sensitivity of 1.06 pm/°C, which reduces the cross-sensitivity problem between tensile strain and temperature.
Microstructured optical fibers are usually divided into two different types of fibers: solid-core photonic crystal fibers and air-core photonic bandgaps fibers. This paper presents long period fiber gratings written in both solid-core PCFs and aircore PBGs by use of a CO<sub>2</sub> laser. A sensitive stain sensor was demonstrated by use of a CO<sub>2</sub>-laser-written long period fiber grating in a solid-core photonic crystal fiber. An in-fiber polarizer based on a long period fiber grating was written by use of a focused CO<sub>2</sub> laser beam to notch periodically on a solid-core photonic crystal fiber. Moreover, a novel long period fiber grating was written in air-core photonic bandgap fibers by use of a CO<sub>2</sub> laser periodically collapse air holes in the fiber cladding.
A novel intensity-modulated strain sensor based on a fiber in-line Mach-Zehnder interferometer is proposed and demonstrated, which is constructed by splicing a thin core fiber between two single mode fibers with a core offset. Such an interferometer exhibits a large fringe visibility of more than 15 dB. When used in axial strain sensing from 0 to 400 με, the interferometer operates at intensity mode of detection with a high sensitivity of -0.023 dB/μεwithout the cross sensitivity between temperature and strain. Its ease of fabrication, high strain sensitivity and intensity mode of detection makes it a low-cost alternative to existing sensing applications.
A novel bio-detecting chip configuration based on the fiber surface plasmon enhancement mechanism is proposed and analyzed. Our improvement is proposing to couple the specialized shell-isolated gold nanoparticles into the sensing region of the opened fiber-integrated microfluidic chip, and achieving drastic surface plasmon enhancement by employing the guided optical mode. Simulation shows that the optical intensity distribution near the surface of exposed fiber hole is enhanced drastically, which could be beneficial to the fluorescence or Raman enhancement. Our work could contribute to searching novel microfluidic chip based bio-detecting methods such as for tracing poisonous and harmful substances detection.
A fiber in-line Michelson interferometer based on open micro-cavity is demonstrated, which is fabricated by femtosecond laser micromachining and thin film coating technique. In refractive index sensing, this interferometer operates in a reflection mode of detection, exhibits compact sensor head, good mechanical reliability, wide operation range and high sensitivity of 975nm/RIU (refractive index unit) at the refractive index value of 1.484.
A selective-filling technique was demonstrated to improve the optical properties of photonic crystal fibres (PCFs). Such a technique can be used to fill one or more fluid samples selectively into desired air holes. The technique is based on drilling a hole or carving a groove on the surface of a PCF to expose selected air holes to atmosphere by the use of a micromachining system comprising of a femtosecond infrared laser and a microscope. The exposed section was immersed into a fluid and the air holes are then filled through the well-known capillarity action. Provided two or more grooves are fabricated on different locations and different orientation along the fibre surface, different fluids may be filled into different airholes to form a hybrid fibre. As an example, we filled half of a pure-silica PCF by a fluid with n=1.480 by carving a rectangular groove on the fibre. Consequently, the half-filled PCF became a bandgap-guiding structure (upper half), resulted from a higher refractive index in the fluid rods than in the fibre core, and three bandgaps were observed within the wavelength range from 600 to 1700 nm. Whereas, the lower half (unfilled holes) of the fibre remains an air/silica index-guiding structure. When the hybrid PCF is bent, its bandgaps gradually narrowed, resulted from the shifts of the bandgap edges. The bandgap edges had distinct bend-sensitivities when the hybrid PCF was bent toward different directions. Especially, the bandgaps are hardly affected when the half-filled PCF was bent toward the fluid-filled region. Such unique bend properties could be used to monitor simultaneously the bend directions and the curvature of the engineering structures.
Two promising post-processing techniques, i.e. applying tensile strain and rising temperature, are demonstrated to
enhance mode-coupling efficiency in the CO<sub>2</sub>-laser-carved long period fiber grating with periodic grooves. Such two
post-processing techniques can be used to enhance the resonant attenuation of the grating to achieve a LPFG-based filter
with an extremely large attenuation and to tailor the transmission spectrum of the grating to exactly equalize the gain of
erbium-doped fiber amplifiers.
We filled a refractive index matching liquid into the air holes of a Ge-doped solid-core microstructured optical fiber
(MOF) with a fiber Bragg grating (FBG) to investigate its switching functions. A type of thermo-optic in-fiber switch
based on the tunable bandgap effect was demonstrated in the fluid-filled FBG at the Bragg wavelength of 830nm, and its
extinction ratio depends strongly on the reflectivity of the FBG. Another type of optical switch with an extinction ratio of
30 dB was developed in the fluid-filled MOF at a long wavelength of 1200 or 1400nm, attributing to the absorption of
the filled liquid. Such two types of switches can turn on/off the light transmission via a small temperature adjustment of
±5 or ±10ºC, respectively, and will find useful applications in all-fiber optical communication systems.
The paper reports preparation and applicative aspects of two types of index guiding microstructured fibers (MOFs) with
germanium doped cores. The first fiber type has a solid core with graded germanium profile. It shows a high
photosensitivity compared to pure silica MOFs. We inscribed high-quality Bragg gratings with a reflectivity of 73%
without hydrogen loading. The solid core germanium doped MOF was spliced with standard silica fiber. The minimum
splice loss was about 1 dB at 1550 μm wavelength. A more complex MOF type was prepared with germanium doped
holey core in a silica holey cladding. The germanium doped core area includes seven holes in hexagonal arrangement
with equal diameter and pitch sizes. The holey core propagates a large area annulus mode. We show the suitability of this
MOF for chemical gas sensing by filling the core cavities with hydrocarbon analytes.
Sensor related properties of a small core (4.1μm) Ge-doped photonic crystal fiber (PCF) are being reported. Fiber Bragg
gratings with 35% and almost 100 % reflectivity were written in the Ge-doped PCF before and after hydrogen loading,
respectively, by use of a UV laser. A 5.6pm/°C temperature sensitivity of the FBG was observed. Additionally, a novel
method is demonstrated to splice such PCF by use of a commercial fusion splicer with default splice parameters for
standard single mode fibers (SMF). No parameter adjustments are required to splice the PCF to various SMFs and a low
splice loss of 1.0 ~ 1.4dB can be achieved. No splice interface emerges at the splice joint, which is of advantage for the
sensing applications of such a PCF.
In this paper, we propose a novel compound high order microring resonator all-pass filter by employing an assistant microring between two cascaded microrings. It improves the dispersion compensation ability and provides a wide bandwidth. The extension of group delay range is shown in its group delay response, which allows flexible choice of the microring size for technical convenience and bend loss improvement. The careful design of coupling coefficients is able to optimize the group delay response.
It is proposed and demonstrated that the fiber-optic Mach-Zehnder (MZ) interferometry can measure accurately the elector-optic (EO) coefficients of, not only the polymer thin film, but also the polymer waveguide. Furthermore, the tensor components, both r<sub>13</sub> and r<sub>33</sub>, of the EO coefficient can be measured simultaneously. In contrast with the free space MZ interferometer, the fiber-optic MZ interferometer owns some advantages, such as fewer devices, simple experiment configuration, easy operation, and good stability. The most outstanding advantage is that the second electrode need not be fabricated on the top of the polymer thin film. So, the measurement system is especially
suitable to measure E-O coefficients of the polymer samples on trial. The closed loop control system of the phase bias of the MZ interferometer decreases the requirement of the environment stability and increases the measurement precision.
A novel fast tunable electro-optic (EO) polymer waveguide grating is proposed and designed. Its resonant wavelength can be linearly tuned by first-order EO effect with a high sensitivity of 6.1pm/V. Its spectrum characteristics depend strongly on many grating parameters, such as refractive index modulation, modulation function, grating period and period number. Material selection, fabrication technology, EO tuning ability and polarization dependence of EO polymer waveguide grating are also discussed. This waveguide grating not only overcomes the shortages of optic fiber gratings, such as slow wavelength tuning ability, and large-scale integration inconveniency, but has many advantages, such as high resonant wavelength tune sensitivity, same fabrication technology as semiconductor, and polarization independence.