Refractometer based on fiber Bragg grating Fabry-Pérot cavity embedded with a narrow microchannel

Fabrication of Multiple Parallel Microchannels in a Single Microgroove via the Heating Assisted MIMIC Technique

For the first time, multiple parallel microchannels in a single microgroove have been fabricated by the heating-assisted micromolding in capillaries technique (HAMIMIC). Microchannel development, cross-sectional shape, and length were all explored in depth. The factors affecting the cross-sectional shape and length of the double-microchannel were also discussed. Finally, a special-shaped PDMS guiding mold was designed to control the cross-sectional shape and length of multiple parallel microchannels for controlled growth. The HAMIMIC technique provides a low-cost, straightforward, and repeatable way to create multiple parallel microchannels in a single microgroove, and will promote the progress of bifurcated vessels and thrombus vessels preparation technology.

Thermal tuning of a fiber-integrated Fabry-Pérot cavity

Here, we present the thermal tuning capability of an alignment-free, fiber-integrated Fabry-Pérot cavity. The two mirrors are made of fiber Bragg gratings that can be individually temperature stabilized and tuned. We show the temperature tuning of the resonance wavelength of the cavity without any degradation of the finesse and the tuning of the individual stop bands of the fiber Bragg gratings. This not only permits for the cavity's finesse to be optimized post-fabrication but also makes this cavity applicable as a narrowband filter with a FWHM spectral width of 0.07 ± 0.02 pm and a suppression of more than -15 dB that can be wavelength tuned. Further, in the field of quantum optics, where strong light-matter interactions are desirable, quantum emitters can be coupled to such a cavity and the cavity effect can be reversibly omitted and re-established. This is particularly useful when working with solid-state quantum emitters where such a reference measurement is often not possible once an emitter has been permanently deposited inside a cavity.

Optical Fiber Sensors by Direct Laser Processing: A Review

The consolidation of laser micro/nano processing technologies has led to a continuous increase in the complexity of optical fiber sensors. This new avenue offers novel possibilities for advanced sensing in a wide set of application sectors and, especially in the industrial and medical fields. In this review, the most important transducing structures carried out by laser processing in optical fiber are shown. The work covers different types of fiber Bragg gratings with an emphasis in the direct-write technique and their most interesting inscription configurations. Along with gratings, cladding waveguide structures in optical fibers have reached notable importance in the development of new optical fiber transducers. That is why a detailed study is made of the different laser inscription configurations that can be adopted, as well as their current applications. Microcavities manufactured in optical fibers can be used as both optical transducer and hybrid structure to reach advanced soft-matter optical sensing approaches based on optofluidic concepts. These in-fiber cavities manufactured by femtosecond laser irradiation followed by chemical etching are promising tools for biophotonic devices. Finally, the enhanced Rayleigh backscattering fibers by femtosecond laser dots inscription are also discussed, as a consequence of the new sensing possibilities they enable.

Determination of geometry-induced positional distortion of ultrafast laser-inscribed circuits in a cylindrical optical fiber

Positional distortion is a defocusing phenomenon in ultrafast laser inscription of fiber optic circuits induced by the cylindrical geometry of an optical fiber. In this Letter, a study on the positional distortion of ultrafast laser processing assisted by tightly focusing optics has been conducted. Attention has been paid to the effect of numerical aperture (NA) of the focusing optics and location of the laser-writing plane. The occurrence of convex positional distortion that decreased with the NA was observed in an array of laser-inscribed optical tracks when scanning across the fiber. It exhibited a maximum distortion of 28.9 and 23.8 μm in the center plane of the fiber for the 0.42-NA and 0.85-NA dry objective lenses, respectively, but only a negligible positional distortion in the track array written in an off-center plane.

High-Q, low-index-contrast photonic crystal nanofiber cavity for high sensitivity refractive index sensing

We present the design of simultaneous high-quality (Q)-factor and high-sensitivity (S) photonic crystal nanofiber cavities (PCNFCs) made of single silica nanofiber that have a low-index contrast (ratio=1.45). By using the three-dimensional finite-difference time-domain method, two different resonant modes, dielectric mode (DM) and air mode (AM), are designed and optimized to achieve an ultrahigh figure of merit (FOM), respectively. Numerical simulations are performed to study the Q-factors and sensitivities of the proposed PCNFCs. It shows that for both DM- and AM-based PCNFCs, respectively, the Q-factors and sensitivities of Q∼1.1×10⁷, S=563.6  nm/RIU and Q∼2.1×10⁵, S=736.8  nm/RIU can be estimated, resulting in FOMs as high as 4.31×10⁶ and 1.13×10⁵, respectively. To the best of our knowledge, this is the first silica nanofiber cavity geometry that simultaneously features high Q and high S for both DM and AM in PCNFCs. Compared with the state of the art of nanofiber-based cavities, the cavity Q-factor to mode volume (V) ratio (Q/V) in this work has been improved more than two orders of magnitude. The demonstration of a high Q/V cavity in low-index-contrast nanofibers can open up versatile applications using a broad range of functional and flexible fibers. Moreover, due to the extended evanescent field and small mode volumes, the proposed PCNFCs are ideal platforms for remote ultra-sensitive refractive-index-based gas sensing without the need for complicated coupling systems.

Inline fiber optic power sensor featuring a variable tap ratio based on a tightly focused femtosecond laser inscription

We propose and demonstrate an inline fiber optic power sensor (IFPS) resorting to an embedded waveguide tap, which is formed to traverse across the cladding and core of a standard single-mode fiber. The tap was produced via a single-step inscription based on the femtosecond laser direct-writing method. A tightly focused pulsed laser beam has been particularly exploited to suppress the elongation along the laser propagation direction, thereby improving the cross-sectional symmetry of the created tap waveguide. The fabricated fiber optic tap has been stably combined with a photodiode via a compact package. The achieved tap ratio could be tuned from 1.0% to 5.9% at the wavelength of 1550 nm by adjusting the applied laser power, while the induced excess loss was kept below 0.6 dB. The proposed IFPS will be highly suitable for real-time power monitoring in a variety of applications, including optical communication networks and systems.

Refractive index sensor based on a multi-notched plastic optical fiber

We propose a plastic optical fiber (POF) with a multi-notched structure as a long-period grating for refractive index (RI) sensing. A new approach to modify the structure of POFs with enhanced RI sensitivity was carried out. The multi-notched structure was made on the surface of the fiber by pressing a thread rod against the POF. The RI sensing performances for straight and macro-bending (U-shaped) POFs with this structure were studied. It is found that the POF probes with straight multi-notched structures were not sensitive enough for RI measurements. After bending the multi-notched structure into U-shaped probes, the RI sensing performance was improved markedly. By altering the structural parameters, the RI sensing performance of the U-shaped POF probes with multi-notched structures were optimized, and the highest sensitivity of 1130%/RIU with a resolution of 8.44×10^-4RIU in the RI range of 1.333-1.410 was obtained. In addition, the characteristic of the temperature dependence of the sensor was presented. The probe is a low-cost solution for RI sensing purpose, which has the features of simple structure, easy fabrication, compact size, and intensity modulation at visible wavelengths.

Investigation of refractive index sensing based on Fano resonance in fiber Bragg grating ring resonators

In this paper we theoretically investigate a ring resonant cavity obtained by closing on itself a π-shifted fiber Bragg grating, to be used for refractive index sensing applications. Differently from a conventional π-shifted fiber Bragg grating, the spectral structure of this cavity is characterized by an asymmetric splitting doublet composed by a right side resonance having an asymmetric Fano profile and a left side resonance having a symmetric Lorentzian profile. The right side resonance shows a narrower and sharper peak than all the other kinds of resonance achievable with both conventional ring resonators and π-shifted fiber Bragg gratings. A reduction of the resonant linewidth with respect to a conventional π-shifted Fiber Bragg grating and a fiber ring resonator, having the same physical parameters, is theoretically proved, achieving up to five orders of magnitude improvement with respect to the usual ring resonator. Due to these resonance features, the π-shifted Bragg grating ring resonator results suitable for RI sensing applications requiring extremely narrow resonances for high resolution measurements. In particular, by assuming a refractive index sensing to detect the presence of sugar in water, the sensor can show a theoretical resolution better than 10^-9 RIU.

Chemical-assisted femtosecond laser writing of lab-in-fibers

The lab-on-chip (LOC) platform has presented a powerful opportunity to improve functionalization, parallelization, and miniaturization on planar or multilevel geometries that has not been possible with fiber optic technology. A migration of such LOC devices into the optical fiber platform would therefore open the revolutionary prospect of creating novel lab-in-fiber (LIF) systems on the basis of an efficient optical transport highway for multifunctional sensing. For the LIF, the core optical waveguide inherently offers a facile means to interconnect numerous types of sensing elements along the optical fiber, presenting a radical opportunity for optimizing the packaging and densification of diverse components in convenient geometries beyond that available with conventional LOCs. In this paper, three-dimensional patterning inside the optical fiber by femtosecond laser writing, together with selective chemical etching, is presented as a powerful tool to form refractive index structures such as optical waveguides and gratings as well as to open buried microfluidic channels and optical resonators inside the flexible and robust glass fiber. In this approach, optically smooth surfaces (~12 nm rms) are introduced for the first time inside the fiber cladding that precisely conform to planar nanograting structures when formed by aberration-free focusing with an oil-immersion lens across the cylindrical fiber wall. This process has enabled optofluidic components to be precisely embedded within the fiber to be probed by either the single-mode fiber core waveguide or the laser-formed optical circuits. We establish cladding waveguides, X-couplers, fiber Bragg gratings, microholes, mirrors, optofluidic resonators, and microfluidic reservoirs that define the building blocks for facile interconnection of inline core-waveguide devices with cladding optofluidics. With these components, more advanced, integrated, and multiplexed fiber microsystems are presented demonstrating fluorescence detection, Fabry-Perot interferometric refractometry, and simultaneous sensing of refractive index, temperature, and bending strain. The flexible writing technique and multiplexed sensors described here open powerful prospects to migrate the benefits of LOCs into a more flexible and miniature LIF platform for highly functional and distributed sensing capabilities. The waveguide backbone of the LIF inherently provides an efficient exchange of information, combining sensing data that are attractive in telecom networks, smart catheters for medical procedures, compact sensors for security and defense, shape sensors, and low-cost health care products.

Tunable phase-shifted fiber Bragg grating based on femtosecond laser fabricated in-grating bubble

We present a type of phase-shifted fiber Bragg gratings based on an in-grating bubble fabricated by femtosecond (fs) laser ablation together with a fusion-splicing technique. A microchannel vertically crossing the bubble is drilled by fs laser to allow liquid to flow in or out. By filling different refractive index (RI) liquid into the bubble, the phase-shift peak is found to experience a linear red shift with the increase of RI, while little contribution to the change of phase shift comes from the temperature and axial strain. Therefore, such a PS-FBG could be used to develop a promising tunable optical filter and sensor.

Highly sensitive in-fiber interferometric refractometer with temperature and axial strain compensation

A novel fiber-optic refractometer is proposed and demonstrated to achieve temperature- and axial strain-compensated refractive index measurement using highly sensitive outer-cladding modes in a tapered bend-insensitive fiber based Mach-Zehnder interferometer. Peak wavelength shifts associated with different spatial frequency peaks are calibrated to obtain a wavelength-related character matrix (λ)M(RI,T,ε) for simultaneous measurement of multiple environmental variables. A phase-related character matrix (Φ)M(RI,T,ε) is also acquired by direct determination of refractive index, temperature, and axial strain induced phase shifts of the corresponding sensing modes.

Towards the control of highly sensitive Fabry-Pérot strain sensor based on hollow-core ring photonic crystal fiber

A high sensitivity Fabry-Pérot (FP) strain sensor based on hollow-core ring photonic crystal fiber was investigated. A low-finesse FP cavity was fabricated by splicing a section of hollow-core ring photonic crystal fiber between two standard single mode fibers. The geometry presents a low cross section area of silica enabling to achieve high strain sensitivity. Strain measurements were performed by considering the FP cavity length in a range of 1000 μm. The total length of the strain gauge at which strain was applied was also studied for a range of 900 mm. The FP cavity length variation highly influenced the strain sensitivity, and for a length of 13 μm a sensitivity of 15.4 pm/με was attained. Relatively to the strain gauge length, its dependence to strain sensitivity is low. Finally, the FP cavity presented residual temperature sensitivity (~0.81 pm/°C).

A sensitivity-enhanced refractive index sensor using a single-mode thin-core fiber incorporating an abrupt taper

A sensitivity-enhanced fiber-optic refractive index (RI) sensor based on a tapered single-mode thin-core diameter fiber is proposed and experimentally demonstrated. The sensor head is formed by splicing a section of tapered thin-core diameter fiber (TCF) between two sections of single-mode fibers (SMFs). The cladding modes are excited at the first SMF-TCF interface, and then interfere with the core mode at the second interface, thus forming an inter-modal interferometer (IMI). An abrupt taper (tens of micrometers long) made by the electric-arc-heating method is utilized, and plays an important role in improving sensing sensitivity. The whole manufacture process only involves fiber splicing and tapering, and all the fabrication process can be achieved by a commercial fiber fusion splicer. Using glycerol and water mixture solution as an example, the experimental results show that the refractive index sensitivity is measured to be 0.591 nm for 1% change of surrounding RI. The proposed sensor structure features simple structure, low cost, easy fabrication, and high sensitivity.