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We proposed a hydrogel grating sensor functionalized with phenylboronic acid (PBA) group for glucose concentration detection. A PBA functionalized polyacrylamide hydrogel film was first prepared via ultraviolet polymerization. Then, the diffraction grating was written on the hydrogel film via the femto-second (fs) laser point-by-point direct inscription. Binding between the PBA groups in the hydrogel and glucose molecules would lead to the swelling of hydrogel and the thus grating structure, thus modifying the diffraction properties of the grating. We experimentally characterized the swelling and transmission of the grating with different glucose concentrations. Sensitivity of the sensor was defined as variations in relative diffraction efficiency in response to glucose concentration changes, and was experimentally found to 0.61%/mM. The proposed sensor showed fast response towards the presence of glucose, and its reusability and biocompatibility were also confirmed. The use of fs-laser inscription technique does not require a pre-fabricated template, and would allow to directly modify the fabrication parameters such as scanning speed, pulse energy and frequency. Therefore, one is able to conveniently optimize the grating structure and improve the inscription efficiency. The proposed hydrogel grating could be potentially fabricated into wearable sensors, namely, contact lenses, for continuous monitoring of tear glucose level with rapid response.

An erratum is presented to correct the laser pulse energy applied on the fiber during grating fabrication in Opt. Lett. 47(2), 249 (2022)10.1364/OL.450047.

In this paper, we proposed an in-fiber Mach-Zehnder temperature sensor based on a dual-core fiber (DCF) in which one core, working as the sensing arm, is suspended in an embedded fluidic channel filled with silicone oil, while the other one, working as the reference arm, locates eccentrically in the DCF. Temperature variations would change the refractive index of silicone oil infiltrated as well as the effective index of the guided mode in the suspended core, thus shifting the interference spectra. Both experiments and numerical simulations were carried out to characterize the sensor. The spectrum shifts measured experimentally agreed well with the theoretical results. Experimental sensitivity of the sensor using a DCF infiltrated with ~20 cm-long silicone oil was found to be as high as −1.42 nm/°, comparable to those of the SPR fiber sensors and other interferometric sensors. The measuring range of the sensor was more than 120°. The proposed sensor could be easily fabricated with good robustness and stability, which makes the sensor promising for applications such as environment and architecture monitoring.

We report fiber Bragg grating manufacturing in poly(methyl methacrylate) (PMMA)-based polymer optical fibers (POFs) with a diphenyl disulfide (DPDS)-doped core by means of a 266 nm pulsed laser and the phase mask technique. Gratings were inscribed with different pulse energies ranging from 2.2 mJ to 2.7 mJ. For the latter, the grating reflectivity reached 91% upon 18-pulse illumination. Though the as-fabricated gratings decayed, they were recovered by post-annealing at 80°C for 1 day, after which they showed an even higher reflectivity of up to 98%. This methodology for the fabrication of highly reflective gratings could be applied for the production of high-quality tilted fiber Bragg gratings (TFBGs) in POFs for biochemical applications.

Bragg gratings can now be produced in various types of polymer optical fibers. However, compared to silica optical fibers, it is still difficult to obtain highly reflective gratings in polymer optical fibers because of material swelling resulting from larger UV absorption. Thus, real-time grating evolution monitoring in whole inscription process is highly demanded. In this paper, a convenient method is systematically introduced using an objective imaging system for permanent fiber alignment between a polymer optical fiber and a silica optical fiber before inscription process allowing analyzing spectral evolution for further grating improvement. Finally, the performance of fiber connectorization is evaluated with a core coupling loss of ~ -6.33 dB.

We experimentally report fiber Bragg gratings (FBGs) in a single mode step-index polymer optical fiber (POF) with a core made of TOPAS and cladding made of ZEONEX using 520 nm femtosecond pulses and a point-by-point (PbP) inscription method. With different pulse energies between 9.7 nJ and 11.2 nJ, 12 FBGs are distributed along the cores of two pieces of POFs with negative averaged effective index change up to ~6 × 10−4 in the TOPAS. For POF 1 with FBGs 1–6, the highest reflectivity 45.1% is obtained with a pulse energy of 10.6 nJ. After inscription, good grating stability is reported. Thanks to the post-annealing at 125 °C for 24 h, after cooling the grating reflectivity increases by ~10%. For POF 2 with FBGs 7–12, similar FBG data are obtained showing good reproducibility. Then, the FBGs are annealed at 125 °C for 78 h, and the average reflectivity of the FBGs during the annealing process increases by ~50% compared to that before the annealing, which could be potentially applied to humidity insensitive high temperature measurement.

The detection of circulating tumor cells (CTCs) still faces a huge challenge partially because of low abundance of CTCs (1-10 cells/mL). In this work, a plasmonic titled fiber Bragg grating biosensor is proposed for detection of breast cancer cells. The biosensor is made by an 18° TFBG with a 50 nm-thick gold nanofilm coating over the surface of the fiber, further immobilized with a specific antibody against GPR30, which is a membrane receptor expressed in many breast cancers, serving as bait. In vitro tests have confirmed that the proposed biosensor can detect breast cancer cells in concentration of 5 cells/mL within 20 minutes and has good linearity in the range of 5–1000 cells/mL, which has met the requirement of CTC detection in real conditions. Furthermore, theoretical analysis based on the experimental results shows that the limit of detection can even reach single-cell level. Our proposed biosensor has a simple structure, is easy to manufacture, is of small size, and has a good performance, making it a good choice for real-time, label-free, and milliliter-volume detection of cancer cells in future.

A novel FBG inscription method in cyclic transparent fluoropolymer (CYTOP) polymer optical fiber through the over-clad using phase mask technique and a femtosecond (fs) pulsed laser at 400 nm is reported. It shows that 8 mm-long uniform grating is obtained in less than 20 s with 500 µW average beam power.

In this Letter, we demonstrate 8°-tilted fiber Bragg grating (TFBG) inscription in single-mode step-index TOPAS/ZEONEX polymer optical fibers (POFs) using a 520 nm femtosecond laser and the line-by-line (LbL) writing technique. As a result of the tilt angle and the fiber refractive index, a large spectral range of cladding mode resonances covering 147 nm is obtained. The evolution of the transmitted spectrum is analyzed as a function of the surrounding refractive index (SRI) in a large range from 1.30 to 1.50. The cutoff cladding mode shows a refractive index sensitivity of 507 nm/RIU (refractive index unit). For single-resonance tracking near the cutoff mode, the sensitivity is at least 6 nm/RIU, depending on the exact wavelength position of the cladding modes. The main originality of our work is that it produces, for the first time, to the best of our knowledge, a TFBG in POF that operates in the refractive index range of aqueous solutions. The sensing capability for a large range of refractive index values is also relevant for (bio)chemical sensing in different media.

A simple plastic optical fiber (POF) based surface plasmon resonance (SPR) sensor is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature. The sensor consists of a series of V-grooves along the POF and a side-polish structure at the other side of the fiber. The V-groove structure can alter the SPR excitation angle and act as a mode filter, effectively enhancing the SPR effect and narrowing the SPR wavelength width. After coating a layer of thermosensitive material—polydimethylsiloxane (PDMS) film on half part of the fiber probe, a dual-parameter sensor probe is obtained for RI and temperature measurement. Experimental results show the RI sensitivity of the prepared probe can reach 1546 nm/RIU in the RI range of 1.335–1.37 RIU and the temperature sensitivity is −0.83 nm/°C in the temperature range of 20–80°C. The sensor is simple in structure and low cost, and has potential applications in the biochemical sensing fields.

Soil moisture measurement is very important for soil system monitoring. Compared to the traditional thermo-gravimetric technique, which is time-consuming and can be only performed in labs, the optic-fiber technique has unique advantages, such as small size, remote application in fields, fast response time and immunity to electromagnetic fields. In this paper, the soil moisture is measured by using a polymer optical fiber Bragg grating (POFBG) probe with a packaged dimension of 40 mm × 15 mm × 8 mm. Due to the intrinsic water-absorbing property of poly (methyl methacrylate) (PMMA), optical fiber Bragg gratings based on PMMA have been widely investigated for humidity measurement. Taking advantage of this, a sensor based on the POFBG is investigated to verify the soil condition. The POFBG is protectively integrated inside a stainless-steel package. A window is opened with a thin polypropylene mat as a filter, which allows the air to go through but prevents the soil from going inside to pollute the POFBG. The sensor probe is embedded in soils with different gravimetric soil moisture contents (SMCs) ranging from 0% to 40% and, then, insulated by polyethylene films to minimize the impact from the external environment, showing an average temperature cross sensitivity of −0.080 nm/°C. For a constant temperature, an exponential relationship between the Bragg wavelength and the SMC is obtained. For the SMCs between 8% and 24%, linear relationships are presented showing a temperature-corresponded sensitivity between 0.011 nm/% and 0.018 nm/%. The maximal sensitivity is calculated to be 0.018 nm/% at 20 °C, which is 28 times as high as that in the previous work. For the SMC over 24%, the sensor becomes insensitive because of humidity saturation in the cavity of the sensor probe. Though temperature cross sensitivity is problematic for SMC measurement, the influence could be eliminated by integrating another humidity-insensitive temperature sensor, such as a silica FBG temperature sensor.

[EN] In this work, we obtained chirped Bragg gratings by post-coating uniform poly (methylmethacrylate) (PMMA) microstructured polymer optical fiber Bragg gratings (POFBGs) with an UV cure adhesive. The main advantage of the proposed method consists on being a cost-effective and simple solution, since no special phase mask, addi tional etching or gradient thermal annealing are required. A comparison between the fabricated chirped Bragg grating¿s performance and previous reported methods in the literature is presented. As a compact sensor with two different active areas (adhesive coated and uncoated sections of the POFBG), the developed sensing device can be used for multiparameter sensing, namely for strain, humidity and temperature monitoring. Allying the proposed flexible and low-cost fabrication technique with the intrinsic characteristics of POFs, one may conclude that the developed sensor has high potential for multiparameter sensing in possible future engineering and biomedical applications., National Natural Science Foundation of China (62003046) ; The Spanish Ministerio de Ciencia, Innovacion y Universidades RTI2018-101658-B-I00 FOCAL Project; Guangdong Basic and Applied Basic Research Foundation (2021A1515011997) ; Guangdong Recruitment Program of Foreign Experts (2020A1414010393) ; C. Marques ac-knowledges Fundacao para a Ciencia e a Tecnologia (FCT) through the CEECIND/00034/2018 (iFish project) and this work was developed within the scope of the project i3N, UIDB/50025/2020 &UIDP/50025/2020, financed by national funds through the FCT/MEC. Luis Pereira and Tiago Paixao also acknowledge FCT for the grant with references SFRH/BD/146295/2019 and PD/BD/128265/2016, respectively.

We propose and experimentally demonstrate a fiber refractometer based on a C-shaped fiber and the Vernier effect. The sensor is fabricated by cascading a single mode fiber (SMF) pigtail together with a C-shaped fiber segment and another SMF segment. Thus, the C-shaped fiber would constitute an open cavity (sensing cavity) in which test analytes could be filled, while the SMF segment would constitute another reference cavity. Due to the similar optical path length of these two cavities, the Vernier effect would be activated, thus forming spectral envelops in the reflection spectrum of the sensor. Variations in the refractive index (RI) of analytes would result in the shifts of the spectral envelops. Both theoretical calculations and experiments are carried out in the characterization of the sensor measuring liquid and gaseous analytes. The experimental sensitivity of the sensor is found to be ∼37238 nm/RIU for gas RI measurement. The proposed sensor features the advantages such as ease of fabrication, extremely high sensitivity, capability of sensing of both gaseous and liquid analytes, small footprint, and good mechanical strength. Compared to other existing Vernier effect-based fiber refractometers typically fabricated using PCFs, the proposed sensor would allow analytes to have much easier and quicker access to the sensor probe.

We experimentally report high reflectivity on the poly(methyl methacrylate) (PMMA)-based polymer optical fiber Bragg gratings by means of a 266 nm pulsed laser and phase mask technique. In the first recipe, fiber Bragg gratings (FBGs) were manufactured with a single pulse up to 3.7 mJ. After post-annealing, a stable refractive index change up to 4.2 × 10 − 4 was obtained. In the second recipe, FBGs were inscribed by 22 pulses with a lower pulse energy of 1.4 mJ, showing a stable refractive index change of 6.2 × 10 − 4 . Both behaviors may mainly be attributed to the movement of initiating radicals arising from benzyl dimethyl ketal (BDK) under UV irradiation. The high refractive index change in step-index fibers paves the way to tilted FBG manufacturing with large tilt angles potentially for biomedical applications.

The authors wish to make a change to the published paper [...]

We demonstrate a dual-core fiber-based Mach–Zehnder interferometer that could be used for precise detection of variations in refractive indices of gaseous samples. The fiber used here have a solid germanium-doped silica core and an air core that allows gases to flow through. Coherent laser beams are coupled to the two cores, respectively, and thus excite guiding modes thereby. Interferogram would be produced as the light transmitted from the dual cores interferes. Variations in refractive index of the hollow core lead to variations in phase difference between the modes in the two cores, thus shifting the interference fringes. The fringe shifts can be then interrogated by a photodiode together with a narrow slit in front. The resolution of the sensor was found to be ~1 × 10−8 RIU, that is comparable to the highest resolution obtained by other fiber sensors reported in previous literatures. Other advantages of our sensor include very low cost, high sensitivity, straightforward sensing mechanism, and ease of fabrication.

In this paper, an endlessly single mode microstructured polymer optical fiber (mPOF) in a Mach–Zehnder (M–Z) interferometer configuration is demonstrated for temperature and strain measurement. Because there is no commercial splicer applied for POF-silica optical fiber (SOF) connectorization, prior to the M–Z interferometric sensing, we introduce an imaging projecting method to align a polycarbonate mPOF to a SOF and then the splice is cured permanently using ultraviolet (UV) glue. A He-Ne laser beam at 632.8 nm coupled in a SOF is divided by a 1 × 2 fiber coupler to propagate in two fiber arms. A piece of mPOF is inserted in one arm for sensing implementation and the interference fringes are monitored by a camera. For non-annealed fiber, the temperature sensitivity is found to be 25.5 fringes/◦ C for increasing temperature and 20.6 fringes/◦ C for decreasing temperature. The converted sensitivity per unit length is 135.6 fringes/◦ C/m for increasing temperature, which is twice as much as the silica fiber, or 852.2 rad/◦ C/m (optical phase change versus fiber temperature), which is more than four times as much as that for the PMMA fiber. To solve the sensitivity disagreement, the fiber was annealed at 125◦ C for 36 h. Just after the thermal treatment, the temperature measurement was conducted with sensitivities of 16.8 fringes/◦ C and 21.3 fringes/◦ C for increasing and decreasing process, respectively. One month after annealing, the linear response was improved showing a temperature sensitivity of ~20.7 fringes/◦ C in forward and reverse temperature measurement. For the strain measurement based on non-annealed fiber, the sensitivity was found to be ~1463 fringes/%ε showing repeatable linear response for forward and reverse strain. The fiber axial force sensitivity was calculated to be ~2886 fringes/N, showing a force measurement resolution of ~3.47 × 10−4 N. The sensing methodology adopted in this work shows several advantages, such as very low cost, high sensitivity, a straightforward sensing mechanism, and ease of fabrication.

In this paper, we report a capillary-based Mach&ndash, Zehnder (M&ndash, Z) interferometer that could be used for precise detection of variations in refractive indices of gaseous samples. The sensing mechanism is quite straightforward. Cladding and core modes of a capillary are simultaneously excited by coupling coherent laser beams to the capillary cladding and core, respectively. An interferogram would be generated as the light transmitted from the core interferes with the light transmitted from the cladding. Variations in the refractive index of the air filling the core lead to variations in the phase difference between the core and cladding modes, thus shifting the interference fringes. Using a photodiode together with a narrow slit, we could interrogate the fringe shifts. The resolution of the sensor was found to be ~5.7 &times, 10&minus, 8 RIU (refractive index unit), which is comparable to the highest resolution obtained by other interferometric sensors reported in previous studies. Finally, we also analyze the temperature cross sensitivity of the sensor. The main goal of this paper is to demonstrate that the ultra-sensitive sensing of gas refractive index could be realized by simply using a single capillary fiber rather than some complex fiber-optic devices such as photonic crystal fibers or other fiber-optic devices fabricated via tricky fiber processing techniques. This capillary sensor, while featuring an ultrahigh resolution, has many other advantages such as simple structure, ease of fabrication, straightforward sensing principle, and low cost.

The monitoring of human physiological signs and body functions, such as respiration rate, heart rate, foot pressure, and joint movement, plays an important role in medical advances and life quality improvements. Polymer optical fibers are made of polymer materials with attractive characteristics compared with silica fibers, such as low Young’s modulus, high failure strain levels, high flexibility, and bio-compatibility. These advantages are well-aligned with the instrumentation requirements for monitoring human physiological and body functions. In this perspective, this paper provides a comprehensive review about polymer optical fiber for human physiological and body functions monitoring, namely mechanisms, materials, and applications, which makes it possible to envisage a widespread implementation of such sensors in this research field in the next few years.

This review discusses recent achievements on grating fabrications in polymer optical fibers doped with photosensitive materials. First, different photosensitive dopants in polymer optical fibers (POFs) are summarized, and their refractive index change mechanisms are discussed. Then, several different doping methods to fabricate the photosensitive POFs are presented. Following that, the principles of gratings, including standard fiber Bragg gratings (FBGs), tilted fiber Bragg gratings (TFBGs), chirped fiber Bragg gratings (CFBGs), phase-shifted fiber Bragg gratings (PSFBGs), and long period fiber gratings (LPFGs), are reported. Finally, fabrications of different gratings based on photosensitive POFs in the last 20 years are reported. We present our article clearly and logically, so that it will be helpful for researchers to explore a broad perspective on this proposed topic. Overall, the content provides a comprehensive overview of photosensitive POF fabrications and grating inscriptions in photosensitive POFs, including previous breakthroughs and recent advancements.

[EN] We demonstrate a chirped microstructured polymer fiber Bragg grating based on taper technology for strain sensing application. The effective bandwidth of the grating is dependent on strain and remains practically constant with respect to temperature and humidity changes. We report a sensitivity of 0.90 pm/mu e for the central wavelength under stable temperature and humidity values. The 3-dB bandwidth of the grating has been measured under different temperature and humidity conditions., The authors acknowledge the financial support from FCT through the fellowship SFRH/BPD/109458/2015, program UID/EEA/50008/2013 by the National Funds through the Fundacao para a Ciencia e a Tecnologia/Ministerio da Educacao e Ciencia, and the European Regional Development Fund under the PT2020 Partnership Agreement. This work was also supported by the Research Excellence Award Programme GVA PROMETEO 2017/103 and the Science Foundation of Heilongjiang Province of China (2018026).

[EN] We demonstrate the Bragg gratings inscription in the 850-nm spectral region by using trans-4-stilbenemethanol-doped poly methyl methacrylate step-index optical fiber and a 248-nm krypton fluoride (KrF) excimer laser system. The gratings inscription only takes 0.4 s. A detailed study of the grating fabrication process using different pulse repetition rates is also reported. In addition, temperature, humidity, and strain sensitivities are measured to demonstrate the potentiality of these components for different sensing applications., The research leading to these results has received funding from the Fundacao para a Ciencia e Tecnologia (FCT)/MEC national funds and when applicable co-funded by FEDER-PT2020 Partnership Agreement under Project UID/EEA/50008/2013. This work was also supported in part by the Research Excellence Award Programme GVA Prometeo 2017/103 Future Microwave Photonics Technologies and Applications, and in part by the Fundamental Research Funds for the Heilongjiang Provincial Universities under Grant KJCXZD201703. The work of C. Marques was supported by FCT through the Fellowship SFRH/BPD/109458/2015.

This article discusses recent advances in biocompatible and biodegradable polymer optical fiber (POF) for medical applications. First, the POF material and its optical properties are summarized. Then, several common optical fiber fabrication methods are thoroughly discussed. Following that, clinical applications of biocompatible and biodegradable POFs are discussed, including optogenetics, biosensing, drug delivery, and neural recording. Following that, biomedical applications expanded the specific functionalization of the material or fiber design. Different research or clinical applications necessitate the use of different equipment to achieve the desired results. Finally, the difficulty of implanting flexible fiber varies with its flexibility. We present our article in a clear and logical manner that will be useful to researchers seeking a broad perspective on the proposed topic. Overall, the content provides a comprehensive overview of biocompatible and biodegradable POFs, including previous breakthroughs, as well as recent advancements. Biodegradable optical fibers have numerous applications, opening up new avenues in biomedicine.

In this paper, we proposed an in-fiber Mach-Zehnder temperature sensor based on a dual-core fiber (DCF) in which one core, working as the reference arm, is located in the center of fiber and the other core, working as the sensing arm, is suspended in an embedded fluidic channel partially filled with silicone oil. Temperature variations would mainly change the refractive index of the silicone oil and therefore the effective index of the guided modes in the suspended core, while the guided modes in the center core were barely affected. Phase difference between guided modes in the two cores would be accumulated along the DCF, thus shifting the interference spectrum of the sensor. Both experiments and numerical simulations were carried out to characterize the sensor. The spectrum shifts measured experimentally agreed well with the theoretical results. Experimental sensitivity of the DCF sensor filled with 33.5-cm-long silicone oil was found to be as high as −2.18 nm/°C, comparable to that of other interferometric sensors. The proposed sensor could be easily fabricated with good robustness and stability, which makes the sensor promising for applications such as environment and architecture monitoring.