Femtosecond laser-assisted selective infiltration of microstructured optical fibers

A Magnetic Field Sensor Based on Directional Coupling in a Magnetic Fluid-Filled Photonic Crystal Fiber

In this paper, a dual-core photonic crystal fiber (DC-PCF) sensitivity sensor filled with magnetic liquid is introduced and investigated with the finite element method (FEM). To regulate the energy coupling involving the two cores, the magnetic fluid is filled into the pore between the two cores. To adjust the coupling between the supermodes in the DC-PCF, the refractive index (RI) of the air hole filled magnetic fluid may change due to the external magnetic field. This specifically created a magnetic fluid-filled DC-PCF; the magnetic fluid-filled hole is not used as the core for energy transmission, thus avoiding transmission loss. The dip wavelength and the magnetic field displayed an excellent linear connection between 80 and 260 Oe, depending on the numerical data. The detection sensitivity of the magnetic field reached 515.75 pm/Oe at a short fiber length of 482 µm. The designed magnetic fluid-filled DC-PCF has high sensitivity and small volume and has great application prospects in magnetic field detection in the medical and industrial fields.

Designing of Hollow Core Grapefruit Fiber Using Cyclo Olefin Polymer for the Detection of Fuel Adulteration in Terahertz Region

A grapefruit-shape hollow-core liquid infiltrated photonic crystal fiber (LI-PCF) is proposed and evaluated to identify the percentage of kerosene in adulterated petrol. The proposed hollow-fiber sensor is designed with Cyclo Olefin Polymer (Zeonex) and likely to be filled with different samples of petrol which is adulated by the kerosene up to 100%. Considering the electromagnetic radiation in THz band, the sensing properties are thoroughly investigated by adopting finite element method (FEM) based COMSOL Multiphysics software. However, the proposed sensor offers a very high relative sensitivity (RS) of 97.27% and confinement loss (CL) less than 10^-10 dB/m, and total loss under 0.07 dB/cm, at 2 THz operating frequency. Besides that, the sensor also possesses a low effective material loss (EML), high numerical aperture (NA), and large Marcuse spot size (MSS). The sensor structure is fabrication feasible through existing fabrication methodologies consequently making this petrol adulteration sensor a propitious aspirant for real-life applications of petrol adulteration measurements in commercial and industrial sensing.

Two-channel photonic crystal fiber based on surface plasmon resonance for magnetic field and temperature dual-parameter sensing

In this paper, a dual-parameter sensor based on surface plasmon resonance (SPR)-photonic crystal fiber (PCF) is proposed, which can be applied in detecting the magnetic field and temperature. In this sensor, two elliptical channels are designed on both sides of the fiber core. The left channel (Ch 1) is coated with gold film and filled with magnetic fluid (MF) to achieve a response to the magnetic field and temperature using SPR. The right channel (Ch 2) is coated with gold film as well as Ta₂O₅ film to improve the SPR sensing performance. Finally, Ch 2 is filled with polydimethylsiloxane (PDMS) to achieve a response to the temperature. The mode characteristics, structural parameters and sensing performance are investigated by the finite element method. The results show that when the magnetic field is in the range of 50-130 Oe, the magnetic field sensitivities of Ch 1 and Ch 2 are 65 pm Oe^-1 and 0 pm Oe^-1, respectively. When the temperature is in the range of 17.5-27.5 °C, the temperature sensitivities of Ch 1 and Ch 2 are 520 pm °C^-1 and 2360 pm °C^-1, respectively. By establishing and demodulating a sensing matrix, the sensor can not only measure the temperature and magnetic field simultaneously but also solve the temperature cross-sensitivity problem. In addition, when the temperature exceeds a certain value, the proposed sensor is expected to achieve dual-parameter sensing without a matrix. The proposed dual-parameter SPR-PCF sensor has a unique structure and excellent sensing performance, which are important for the simultaneous sensing of multiple basic physical parameters.

Simulation of a Temperature-Compensated Voltage Sensor Based on Photonic Crystal Fiber Infiltrated with Liquid Crystal and Ethanol

A simulated design for a temperature-compensated voltage sensor based on photonic crystal fiber (PCF) infiltrated with liquid crystal and ethanol is presented in this paper. The holes distributed across the transverse section of the PCF provide two channels for mode coupling between the liquid crystal or ethanol and the fiber core. The couplings are both calculated accurately and explored theoretically using the finite element method (FEM). The influence of voltage on the alignment of the liquid crystal molecules and confinement loss of the fiber mode are studied. Liquid crystal molecules rotate which changes its properties as the voltage changes. As the characteristics of the liquid crystal will be affected by temperature, therefore, we further fill using ethanol, which is merely sensitive to temperature, into one hole of the PCF to realize temperature compensation. The simulated results show that the sensitivity is up to 1.29977 nm/V with the temperature of 25 °C when the voltage ranges from 365 to 565 V. The standard deviation of the wavelength difference is less than 2 nm within the temperature adjustment from 25 to 50 °C for temperature compensation. The impacts of the construction parameters of the PCF on sensing performances of this voltage sensor are also analyzed in this paper.

Design of photonic crystal fiber to excite surface plasmon resonance for highly sensitive magnetic field sensing

To improve the sensing performance of optical fiber magnetic field sensor based on magneto-refractive effect, a D-shaped photonic crystal fiber-surface plasmon resonance (PCF-SPR) sensor based on magneto-refractive effect is proposed and its magnetic field sensing characteristics are investigated. The designed D-shaped PCF has a core-analyte-gold structure. Within the D-shaped PCF, the side polishing surface is coated with the gold film and the special hole is sandwiched between the core and the gold film. To realize the high magnetic field sensitivity for the fiber SPR magnetic field sensor, the special hole is filled with magnetic fluid (MF). In this paper, we analyze the mode transmission characteristics and magnetic field sensing characteristics of this fiber sensor by finite element method. We also obtain a general rule for the optimization of PCF-SPR sensors by analyzing the dispersion curves, the energy of the surface plasmon polariton mode and the core mode on the sensing performance of the designed fiber sensor. The maximum refractive index sensitivity and magnetic field sensitivity of the optimized fiber are 59714.3 nm/RIU and 21750 pm/mT (50-130 Oe), respectively. Compared with optical fiber magnetic field sensors based on magneto-refractive effect reported previously, the magnetic field sensitivity in this paper is nearly two orders of magnitude higher and it can initially achieve nT magnitude magnetic field resolution and testing capability. The proposed fiber sensor has the advantages of simple structure, easy production, high sensitivity, and strong environmental adaptability. It not only improves the sensing performance of optical fiber magnetic field sensors, but also provides an ideal alternative platform for biosensors like microfluidics because of its high refractive index sensitivity and the special structure.

A Multi-Parameter Integrated Sensor Based on Selectively Filled D-Shaped Photonic Crystal Fiber

We propose and numerically investigate a multi-parameter integrated sensor based on a selectively filled D-shaped photonic crystal fiber (PCF). The simple structure can be used to comprehensively detect refractive index, magnetic field, temperature, and voltage. According to the surface plasmon resonance and directional coupling effect, the PCF is coated with a gold nano-film to detect the refractive index of the external environment. In addition, magnetic fluid (water-based Fe₃O₄), toluene, and nematic liquid crystal (NLC E7) are selectively filled into different cladding air holes of the D-shaped PCF to realize the different sensing of the magnetic field, temperature, and voltage. The measurement of refractive index, magnetic field, temperature, and voltage are independent of each other, so these four parameters can be measured simultaneously. The sensing characteristics of the proposed structure are investigated systematically by the finite element method. The results show that the sensitivities of refractive index, magnetic field, temperature, and voltage are 4600 nm/RIU, 1.375 nm/Oe, 15.143 nm/°C, and 0.971 nm/V, respectively. The presented design based on materials selectively filled with D-shaped PCF might enable promising application in multi-parameter optical sensing.

Surface Plasmon Resonance Sensor Based on Polymer Liquid-Core Fiber for Refractive Index Detection

In this work, a surface plasmon resonance (SPR) sensor based on a novel liquid-core polymer optical fiber (POF) is proposed and numerically analyzed for refractive index (RI) detection. The polytetrafluoroethylene (PTFE) fiber is selected as the platform for SPR sensing. We combine the PTFE-based POF with the liquid-core structure by introducing a hole filled with analyte into the fiber center. The hole also acts as the fiber core to guide the incident light. This design helps to realize the detection of solutions with low RI values (around 1.33), while keeping the distinguished sensing characteristics of the liquid-core structure. Two side air holes are introduced into the cladding and a thin silver film protected by a titanium dioxide layer is plated on the wall of one air hole, which helps to control the mode coupling. In order to optimize the design of this sensor, the impacts of parameters such as metal layer thicknesses and the central hole radius are investigated using the full-vector finite element method (FEM). After optimization, our design shows a wavelength interrogation sensitivity reaching up to 16,750 nm/RIU and an average full-width at half-maximum (FWHM) of 42.86 nm in the RI range of 1.325–1.35.

Cancer cell detection by a heart-shaped dual-core photonic crystal fiber sensor

This paper contributes a novel design of sensor with a heart-shaped dual-core photonic crystal fiber (PCF) to detect cancerous cells in human cervical, blood, adrenal glands, and breast. Cancer-infected cells and their normal cells are considered in liquid form having their own refractive indices. In the designed PCF, the two heart-shaped cores separated by a large circular air hole serve as two independent waveguides. The large circular air hole is infiltrated by sample cells from different body parts. Detection of cancer-contaminated cells by the proposed PCF is based on the mode-coupling theory. According to the mode-coupling theory, the guided optical light transmits periodically from one core to another, throughout the PCF length. During this transmission, the optical light interacts with the cancerous cell, which is filled in the center air hole of the PCF. Due to this interaction, the dip wavelength of the transmission spectrum is sensitive to the corresponding cancerous cell filled in the center air hole of the PCF. The variation in the PCF transmission spectrum for cancerous cells and their normal cells is observed by using the finite element method. The dip wavelength shift of the cancer cell in reference to its normal cell has been measured from the transmission spectrum to determine the sensing performance of the proposed sensor. The sensitivity achieved of the proposed sensor for cervical cancer cell, blood cancer cell, adrenal gland cancer cell, and breast cancer cells are 7916.67 nm/RIU, 8571.43 nm/RIU, 9285.71 nm/RIU, and 10,000 nm/RIU, respectively, with a maximum detection limit of 0.024. Therefore, the proposed PCF sensor suggests high sensitivity with a rapid cancer detection mechanism.

Fiber-interface directional coupler inscribed by femtosecond laser for refractive index measurements

A novel fiber-interface directional waveguide coupler was inscribed on the surface of a coreless fiber by femtosecond laser, and was successfully applied to highly sensitive refractive index (RI) measurements. The primary arm was first inscribed to couple light from a lead-in single mode fiber to the fiber interface, then back to a lead-out single mode fiber. A side arm was inscribed parallel and in close proximity to the primary arm. Light propagating in the primary arm could then be efficiently coupled into the side arm when a phase-matching condition was met, which produced a dramatic spectral dip at the coupling wavelength. The proposed device achieved a sensitivity as high as ∼8249 nm/RIU over an RI range of 1.44-1.45, due to strong evanescent fields excited in fiber-interface waveguides. The proposed in-fiber directional coupler exhibits high mechanical strength, a compact configuration, and excellent RI sensitivity. As such, it has significant potential for practical applications in biochemical sensing.

Helicity Enhanced Torsion Sensor Based on Liquid Filled Twisted Photonic Crystal Fibers

A highly sensitive torsion sensor can be constructed by combining a twisted photonic crystal fiber with a liquid-filled waveguide in its air-hole cladding. The torsion sensitivity of this type of sensor is determined directly by the phase-matching conditions between the fiber core mode and the liquid waveguide mode, which can be improved by tuning the helicity (denoted by the initial twist rate, α₀) of the twisted photonic crystal fiber. The enhancement mechanism of α₀ on the sensitivity of the proposed torsion sensor is investigated theoretically, followed by experimental verifications, and a torsion sensitivity as high as 446 nm∙mm∙rad⁻¹ can be obtained by tailoring these parameters. Experimental results show that the torsion sensitivity increases with α₀ decreasing from 3.142 to 3.925 rad/mm, which are in consistence with that of the numerical predictions. The demonstrated torsion sensor is expected to contribute to the development of highly sensitive torsion-related photonic crystal fiber devices.

Analysis of a highly sensitive flat fiber plasmonic refractive index sensor

In this paper, we propose an efficient double-layered flat fiber (DLFF) plasmonic refractive index sensor having high resolution and linearity. Thin gold film is used as surface plasmon resonance (SPR) active material protected by a titanium dioxide layer, both deposited on the upper flat surface of DLFF. The sensor consists of an analyte channel in the central core hole as well as on the top of the fiber. Structural parameters of DLFF and thickness of gold and titanium dioxide layer are analyzed based on the finite element method. The optimized structure is studied based on wavelength and amplitude interrogation techniques in the near-infrared region. Numerical results show average wavelength sensitivity of 12172 nm/RIU with a resolution of 8.21×10^-6RIU (refractive index unit) in the highly refractive index (RI) range from 1.445 to 1.490. Further, amplitude sensitivity of this probe is found to be 2910RIU^-1 with a resolution of 3.44×10^-6RIU, which is the highest among all reported PCF SPR sensors, as per the authors' best knowledge. Compared with traditional photonic crystal fiber, the designed DLFF makes the sensor configuration simple to fabricate as well as a potential candidate for developing biochemical sensors and portable devices.

Ultra-short polarization splitter based on a plasmonic dual-core photonic crystal fiber with an ultra-broad bandwidth

A compact polarization beam splitter based on a gold-filled photonic crystal fiber with a square lattice is proposed. The full vector finite element method is used to design and characterize the proposed ultra-compact and ultra-broadband polarization splitter. The plasmonic plays an important role in order to achieve an ultra-short length of 56.33 µm with a high extinction ratio of 132.92 dB at the wavelength of 1.55 µm. It can ensure an ultra-broad bandwidth of 530 nm, from 1225 to 1755 nm, covering all the communication bands with an extinction ratio better than 20 dB. The proposed polarization splitter may be a promising candidate in communication due to its ultra-short length and ultra-broad bandwidth.

Prospects of Photonic Crystal Fiber as Physical Sensor: An Overview

Photonic crystal fiber sensors have potential application in environmental monitoring, industry, biomedicine, food preservation, and many more. These sensors work based on advanced and flexible phototonic crystal fiber (PCF) structures, controlled light propagation for the measurement of amplitude, phase, polarization and wavelength of spectrum, and PCF-incorporated interferometry techniques. In this article various PCF-based physical sensors are summarized with the advancement of time based on reported works. Some physical PCF sensors are discussed based on solid core as well as hollow core structures, dual core fibers, liquid infiltrated structures, metal coated fibers, grating incorporated fibers. With the advancement of sensing technology the possibilities of temperature, pressure, strain, twist, curvature, electromagnetic field, and refractive index sensing are discussed. Also, limitations as well as possible solutions and future hopes are outlined.

Ultrashort polarization beam splitter based on liquid-filled dual-core photonic crystal fiber

An ultrashort polarization beam splitter (PBS) is proposed based on liquid-filled dual-core photonic crystal fiber (DCPCF). The two cores of DCPCF are formed by two side elliptical holes and a central circular hole in the horizontal direction. The properties of the PBS are analyzed first with a non-filled DCPCF by the finite element method. Then, the performances of the PBS are discussed when the DCPCF is filled with liquids in the central hole. As a result, an ultrashort PBS is realized with a length of 78 μm when glycerol solution with a concentration of 37% is filled in the central hole. In this case, an extinction ratio of 87 dB is obtained at 1550 nm wavelength. The significantly short device shows a great advantage when being integrated in ultra-compact optical systems.

Supercontinuum generation at 1.55 μm in As2S3 core photonic crystal fiber

This paper proposes a design and mathematical study of As₂S₃ chalcogenide photonic crystal fiber (PCF) for broadband supercontinuum generation. The proposed design offers a large nonlinearity coefficient and ultra-flattened dispersion. The proposed design was analyzed using the full-vectorial finite element method. Through this method, it is shown that an ultra-broad supercontinuum spectrum of 0.8-4.5 μm is attained using an As₂S₃ core PCF design with 20 fs pump pulse width and a length of 10 mm, having 3 kW power at a -40  dB spectral and temporal intensity. The proposed octagonal PCF has shown a low zero dispersion wavelength at the pump wavelength of 1.55 μm.

Optofluidic gutter oil discrimination based on a hybrid-waveguide coupler in fibre

Discriminating edible oils from gutter oils has significance in food safety, as illegal gutter oils cannot meet a variety of criteria such as the acid value, peroxide value and quality. To discriminate these illegal cooking oils, we propose an ultrasensitive optofluidic detection method based on a hybrid-waveguide coupler. Prior to the straight waveguide inscription in the cladding of the silica tube using a femtosecond laser, a section of coreless fibre is firstly spliced with the ST to supply a platform for the inscription of an S-band waveguide. Then a pair of microfluidic channels are ablated on the ST using the fs laser to enable liquid analytes to flow in and out of the air channel. In the transmission spectrum, a unique resonant loss dip can be observed, which is produced by coupling the light from the laser inscribed waveguide to the liquid core when the phase-matching condition is met. This hybrid-waveguide coupler with a simplified structure realizes dynamic optofluidic refractive index sensing with an ultrahigh sensitivity of -112 743 nm RIU^-1, a detection limit of 2.08 × 10^-5 RIU and a refractive index detection range from 1.4591 to 1.4622. This novel method can be used for food safety detection, specifically, for the discrimination of gutter oils.

Simultaneous measurement of refractive index and temperature based on SPR in D-shaped MOF

A surface plasmon resonance (SPR) sensor based on D-shaped microstructured optical fiber (MOF) is proposed to realize the simultaneous measurement of refractive index (RI) and temperature. The D-shaped flat surface coated with a gold layer is in direct contact with analyte as a sensing channel of RI, and one of the air holes near the fiber core is filled with chloroform to detect temperature. Two separate channels and birefringence caused by the asymmetric structure can distinguish the variations of RI and temperature independently, thus completely solving the cross-sensitivity problem. This is the first time to realize the simultaneous measurement of multiple parameters without matrix equations, to the best of our knowledge. Results show that the y-polarized peak supported by channel I only shifts with RI variation and is unaffected by the temperature floating. Similarly, the x-polarized peak supported by channel II is only influenced by the change of temperature in the external environment. The effect of gold layer thickness is investigated numerically, and the sensor sensitivity is identified both in wavelength and amplitude interrogations. This work is very helpful for the design and implementation of a highly sensitive, real-time, and distributed SPR sensor for multi-parameter measurement applications.

Versatile dispersion characteristics of water solution of glycerine in selective filling of holes in photonic crystal fibers

Using a glycerine-water solution with various concentrations, we investigate the dispersion characteristics of photonic crystal fibers by selective filling of holes. Our analysis is based on a simple but accurate semi-vectorial solution of Helmholtz's equation by the finite difference method devised with a mode-field convergence technique and crosschecked by results with those from a deeply involved multipole method. Significantly, a better ultra-flatness but near-zero group velocity dispersion is revealed with a 20% glycerine-water solution that is superior to pure water of a very recent case when the holes of the first ring of the fiber are filled. This versatile effect in management of holes of identical diameter with liquid is expected to play a guiding role in studies of supercontinuum generation.

In-line optofluidic refractive index sensing in a side-channel photonic crystal fiber

An in-line optofluidic refractive index (RI) sensing platform is constructed by splicing a side-channel photonic crystal fiber (SC-PCF) with side-polished single mode fibers. A long-period grating (LPG) combined with an intermodal interference between LP₀₁ and LP₁₁ core modes is used for sensing the RI of the liquid in the side channel. The resonant dip shows a nonlinear wavelength shift with increasing RI over the measured range from 1.3330 to 1.3961. The RI response of this sensing platform for a low RI range of 1.3330-1.3780 is approximately linear, and exhibits a sensitivity of 1145 nm/RIU. Besides, the detection limit of our sensing scheme is improved by around one order of magnitude by introducing the intermodal interference.

Antiresonant reflecting guidance mechanism in hollow-core fiber for gas pressure sensing

A gas pressure sensor based on an antiresonant reflecting guidance mechanism in a hollow-core fiber (HCF) with an open microchannel is experimentally demonstrated for gas pressure sensing. The microchannel was created on the ring cladding of the HCF by femtosecond laser drilling to provide an air-core pressure equivalent to the external environment. The HCF cladding functions as an antiresonant reflecting waveguide, which induces sharp periodic lossy dips in the transmission spectrum. The proposed sensor exhibits a high pressure sensitivity of 3.592 nm/MPa and a low temperature cross-sensitivity of 7.5 kPa/°C. Theoretical analysis indicates that the observed high gas pressure sensitivity originates from the pressure induced refractive index change of the air in the hollow-core. The good operation durability and fabrication simplicity make the device an attractive candidate for reliable and highly sensitive gas pressure measurement in harsh environments.

High Sensitivity Refractive Index Sensor Based on Dual-Core Photonic Crystal Fiber with Hexagonal Lattice

A refractive index sensor based on dual-core photonic crystal fiber (PCF) with hexagonal lattice is proposed. The effects of geometrical parameters of the PCF on performances of the sensor are investigated by using the finite element method (FEM). Two fiber cores are separated by two air holes filled with the analyte whose refractive index is in the range of 1.33-1.41. Numerical simulation results show that the highest sensitivity can be up to 22,983 nm/RIU(refractive index unit) when the analyte refractive index is 1.41. The lowest sensitivity can reach to 21,679 nm/RIU when the analyte refractive index is 1.33. The sensor we proposed has significant advantages in the field of biomolecule detection as it provides a wide-range of detection with high sensitivity.

Prospective effect in dispersion properties of photonic crystal fibers by selective water-filling of holes

Based on a simple but accurate semivectorial solution of Helmholtz's equation by the finite difference method devised with a mode-field convergence technique, we have shown an interesting and significant effect showing an almost ultraflat zero group velocity dispersion in photonic crystal fiber when the holes of the first ring of the fiber are filled with water. Crosschecking our results with earlier results involving a deeply involved multipole method for the central core of photonic crystal fiber filled with water and fused silica, our observation in the case of filling the first ring holes with water reveals potential information in studies of supercontinuum generation.

Sensitive hydrogen sensor based on selectively infiltrated photonic crystal fiber with Pt-loaded WO₃ coating

A sensitive hydrogen sensing device based on a selectively infiltrated photonic crystal fiber (PCF) coated with Pt-loaded WO₃ is demonstrated. With Pt-loaded WO₃ coating acting as the catalytic layer, hydrogen undergoes an exothermic reaction with oxygen and releases heat when the device is exposed to gas mixtures of air and hydrogen, which induces local temperature change in the PCF and hence leads to the resonant wavelength shift of the proposed device. The maximum wavelength shift of 98.5 nm is obtained with a 10-mm-long infiltrated PCF for 4% (v/v) H₂ concentration, and a hydrogen sensitivity of 32.3 nm/% (v/v) H₂ is achieved within the range of 1%-4% (v/v) H₂ in air.

Laser-assisted lateral optical fiber processing for selective infiltration

We propose a new technique to perform precise selective infiltration of an air hole in the photonic crystal fiber (PCF). To carry out the infiltration process, the end face of the PCF is covered by a mask, which is fabricated by femtosecond laser inscription from the lateral direction. This proposed method overcomes the conventional limitation of maximum mask thickness. An analytical model is further proposed and demonstrated accurate determinations of the fabricated channel diameter in the mask.

Anti-resonant reflecting guidance in alcohol-filled hollow core photonic crystal fiber for sensing applications

Mechanism and sensing applications of antiresonant reflecting guidance in an alcohol-filled simplified hollow-core (SHC) photonic crystal fiber (PCF) are demonstrated. By filling one air hole in the air cladding of the PCF with alcohol, anti-resonant reflecting guidance of light can be achieved and energy leakage of the core modes can be induced at resonant wavelengths of the Fabry-Pérot (F-P) resonator formed by the alcohol-filled layer combined with the silica cladding in the cross-section of the PCF. The proposed structure exhibits periodic lossy dips in the transmission spectrum, of which the visibilities are sensitive to the refractive index of surrounding medium due to the reflectivity variation of the F-P resonator. Water level sensing is experimentally realized with this principle and the lossy dip exhibits a linear decrease against water level with a sensitivity of 1.1 dB/mm. The sensor is also sensitive to environmental temperature and a temperature sensitivity of -0.48 nm/°C is obtained between room temperature and 60 °C.

Suspended core photonic microcells for sensing and device applications

In-line fiber-optic microcells are fabricated by postprocessing NKT LMA10 photonic crystal fibers. The cells are suspended core (SC) elements created by locally inflating some of the air holes while the core is being tapered. Based on a SC microcell with six air holes, a cantilever beam accelerometer is demonstrated. The microcells could also be used as gain and absorption cells for amplifier and spectroscopy applications.

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.

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/με 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/με. The experimentally obtained static strain sensitivity values are ~22 and ~3.8 pm/με 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).

Dispersion properties of liquid-core photonic crystal fibers

Dispersion properties of liquid-core photonic crystal fibers (PCFs) with large air fraction in clads between 300 to 2000 nm have been calculated by a multipole method for various liquids including CS(2), toluene, chloroform, and water for different core diameters. In calculations, air holes are assumed to be arranged in a regular hexagonal array in fused silica, and a central hole is filled with liquid to create a core. The results are compared with those obtained by a fully vectorial effective index method, and fitting parameters for core sizes are found for each liquid except for water, where the latter method does not give correct dispersions at short wavelengths. Also, the power ratios inside liquid cores and effective core areas were calculated at different wavelengths.

Efficient and short-range light coupling to index-matched liquid-filled hole in a solid-core photonic crystal fiber

A photonic crystal fiber (PCF) with a section of one of the holes next to the solid core filled with an index-matched liquid is studied. Liquid filling alters the core geometry, which locally comprises the original silica core, the liquid channel and the silica around it. It is demonstrated that when light reaches the filled section, it periodically and efficiently couples to the liquid, via the excitation of a number of modes of the composite core, with coupling lengths ranging from tens to hundreds of microns. The resulting modal-interference-modulated spectrum shows temperature sensitivity as high as 5.35 nm/°C. The proposed waveguide geometry presents itself as an interesting way to pump and/or to probe liquid media within the fiber, combining advantages usually found separately in liquid-filled hollow-core PCFs (high light-liquid overlap) and in solid-core PCFs (low insertion losses). Therefore, pumping and luminescence guiding with a PCF filled with a Rhodamine solution is also demonstrated.

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.

Ultrafast nonlinear optofluidics in selectively liquid-filled photonic crystal fibers

Selective filling of photonic crystal fibers with different media enables a plethora of possibilities in linear and nonlinear optics. Using two-photon direct-laser writing we demonstrate full flexibility of individual closing of holes and subsequent filling of photonic crystal fibers with highly nonlinear liquids. We experimentally demonstrate solitonic supercontinuum generation over 600 nm bandwidth using a compact femtosecond oscillator as pump source. Encapsulating our fibers at the ends we realize a compact ultrafast nonlinear optofluidic device. Our work is fundamentally important to the field of nonlinear optics as it provides a new platform for investigations of spatio-temporal nonlinear effects and underpins new applications in sensing and communications. Selective filling of different linear and nonlinear liquids, metals, gases, gain media, and liquid crystals into photonic crystal fibers will be the basis of new reconfigurable and versatile optical fiber devices with unprecedented performance. Control over both temporal and spatial dispersion as well as linear and nonlinear coupling will lead to the generation of spatial-temporal solitons, so-called optical bullets.