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Stimulated Raman scattering (SRS) enabled by the emerging gas-filled low-loss anti-resonant hollow-core fiber (ARHCF) technology opens up a competitive way towards the development of novel lasers in the molecular fingerprint region. In this article, the characteristics of noise and long-term stability of near- and mid-infrared (near-IR and mid-IR) gas-filled fiber Raman lasers have been investigated for the first time. The results reveal that an increase in Raman pulse energy is associated with a decrease in noise, and that the relative pulse peak intensity noise (RIN) is always lower than the relative pulse energy noise (REN). We also demonstrate that long-term drift of the pulse energy and peak power are directly linked with the high amount of heat release during the Raman Stokes generation. The demonstrated noise and long-term stability performance provide necessary references for potential spectroscopic applications as well as further improvements of the emerging IR gas-filled ARHCF Raman laser technology. [ABSTRACT FROM AUTHOR]

In this article, two configurations are presented for simultaneous measurement of strain and temperature by reducing the cross-section area in small regions of the fiber where the Bragg gratings were inscribed, to achieve dual sensitivity to strain and handle the cross-sensitivity to temperature of a single grating. Each configuration used a single Bragg grating inscribed in a 2-ring undoped poly (methyl methacrylate) microstructured polymer optical fiber (mPOF) with a pulsed Q-switched Nd:YAG laser system. To reduce the cross-section area, a femtosecond laser system was used to remove portions of the mPOF, creating micromachined slots in the fiber, with different lengths for each configuration. The result was the appearance of a second peak when strain is applied, with a higher strain sensitivity. The thermal, humidity and refractive index response of these gratings were analyzed, revealing a thermal sensitivity almost twice the value of a common Bragg grating inscribed in the same mPOF. The maximum root mean square errors obtained when both strain and temperature are applied in these grating devices were 52 μϵ and 0.6 °C, respectively. These results show that the method used to produce these devices could be a suitable and reliable option to fabricate very compact sensors to simultaneously measure strain and other parameters, such as temperature. Moreover, these devices may be used as phase-shift gratings since the position of the reflective peaks and their relative spectral separation may be modulated by applying strain to the optical fiber. [ABSTRACT FROM AUTHOR]

This paper reports the first microstructured solid-core fiber drawn from a 3D-printed preform and the first fiber Bragg gratings inscribed in a fiber of this type. The presented fiber is made of polycarbonate and displays single-mode behavior. The fiber attenuation was the lowest reported so far for a POF drawn from a 3D-printed preform across a broad range of wavelengths. In addition, extensive fiber characterization results are presented and discussed including: fiber attenuation, mode simulations, dynamic thermomechanical analysis and thermo-optic coefficient. Fiber Bragg gratings are successfully inscribed in the produced fiber using three different lasers: a continuous wave helium-cadmium laser, a pulsed femtosecond frequency doubled ytterbium laser and ultra-violet nanosecond krypton fluoride laser. Mechanical testing of the fiber showed that the 3D printing approach did not introduce any unexpected or undesirable characteristics. [ABSTRACT FROM AUTHOR]

Supercontinuum (SC) lasers provide an excellent opportunity for absorption spectroscopy by virtue of their high brightness and large bandwidth. Here we present the detection of multiple industrial toxic gasses from 1480 nm to 1700 nm, with a simple custom SC laser source. Using readily available optical components, we demonstrate an all-fiber system for the detection of ammonia (NH3) and methane (CH4) in a 2.4 m long hollow-core photonic bandgap fiber with 20- $\mu \text{m}$ core diameter. The responsivity, selectivity, and performance of the system for continuous detection are demonstrated. [ABSTRACT FROM AUTHOR]

We report on demonstration of high energy pulse laser at 2 μm by employing a normal dispersion fiber attained by a waveguide dispersion engineering. The normal dispersion fiber has a W-type refractive index profile, with a Thulium(Tm)-doped core. In contrast to a strong anomalous dispersion at 2 μm in a step-index fiber, the design allows strong waveguide dispersion that can generate normal dispersion at the Tm emission band. Mode area scalability of this fiber is discussed under the normal dispersion requirement. The Tm-doped fiber (TDF) is deployed in a ring cavity to produce mode-locked pulses near 2 μm. Subsequently, the pulse is amplified with more than 27 dB gain through the normal dispersion TDF. The output pulse energy reaches up to ∼525 nJ at 1852 nm without pulse breaking due to the engineered dispersion. The achieved pulse energy is record high for all-fiber configuration near 2 μm, to our knowledge. [ABSTRACT FROM AUTHOR]

Solvents can be used in the fabrication process of Polymer Optical Fiber (POF) sensors, as tapering or etching agents. We present a general approach—the use of Hansen solubility parameters—for identifying usable solvents for etching and present the first results on etching of TOPAS and Zeonex POFs. We also present alternatives to acetone in the form of trichloroethylene and THF, as etching solvents for PMMA, as well as results on the etching rate dependence on fiber orientation and annealing. [ABSTRACT FROM AUTHOR]

The smallest all-polymer optical fiber Bragg grating based transducer element for pH sensing is presented. We show that, considering it's size and robustness, it out-performs similar state-of-the-art fiber Bragg grating based pH sensors regarding both sensitivity and response time. A 5–10 $\mu$ m thick pH sensitive hydrogel coating is placed on a PMMA-based microstructured polymer optical fiber Bragg grating. The hydrogel expands or contracts depending on the pH and thus changes in pH are monitored by following the fiber strain induced changes in the reflected Bragg wavelength  $ \boldsymbol{\lambda _B}$. Prior to applying the hydrogel coating the mPOF is etched from 150 to 80 $\mu$ m to enhance sensitivity and surface crazing is introduced with a 50/50 vol% solution of acetone and methanol to enhance spreading of the hydrogel during the application and adhesion after cure. With this design, we achieved a sensitivity of ${ \Delta \mathbf {\lambda }_B=\text{73 pm/pH} \pm \text{2 pm/pH}}$ and response times below 4.5 mins. for pH 5–7 and 4–7, respectively, and a thermal cross sensitivity of $\text{31.4 pm}/^{\circ}\text{C} \ \pm \text{0.4 pm}/^{\circ}\text{C}$. [ABSTRACT FROM AUTHOR]

Polymer optical fiber Bragg gratings (POFBGs) can be solvent etched to achieve higher sensitivity sensors. To analyze the influence of etch on the mechanical and thermal performance of PMMA-based microstructured POFBGs (mPOFBGs), we etched annealed and unannealed samples in pure acetone, while at the same time monitoring the induced Bragg wavelength shifts. Optical-thermodynamical interpretation of these shifts together with thermal and mechanical testing of etched fibers, reveal that solvent etching can lead to trapping of residual plasticizing solvent in the polymer matrix and consequently to change of Young's modulus and thermal instability of the etched mPOFBG. Removal of these solvent residuals by thermal treatment results in a stable linear response with a thermal sensitivity indistinguishable of that of the unetched mPOFBG. The maximum found change of Young's modulus due to acetone plasticization during etch is ∼5% for both unannealed and annealed mPOFBGs. The mPOFBG force sensitivity increases greatly with reduced diameter and the relatively small changes in strain sensitivity and Young's modulus introduced by the etching process in this case do not have a big effect. [ABSTRACT FROM AUTHOR]

In this paper, a linearly chirped fiber Bragg grating (CFBG) inscribed in a microstructured polymer optical fiber (mPOF) has been demonstrated for detecting temperature profiles during thermal treatments. A CFBG of 10-mm length and 0.98-nm bandwidth has been inscribed in an mPOF fiber by means of a KrF laser and uniform phase mask. The CFBG has a high temperature sensitivity of −191.4 pm/ $^{\circ }$ C. The CFBG has been used as a semidistributed temperature sensor, capable of detecting the temperature profile along the grating length, for scenarios that account minimally invasive biomedical treatments. Two experiments have been designed to validate the CFBG temperature reconstruction, using a linear gradient, and a research-grade radiofrequency ablation setup to apply Gaussian-shaped temperature spatial profiles. The result is that the higher sensitivity of the CFBG supports the detection of spatially nonuniform temperature fields by means of spectral reconstruction. [ABSTRACT FROM AUTHOR]

We demonstrate 20 dB long period grating (LPG) fast inscription in microstructured polymer optical fibers (mPOFs) using a point-by-point technique obtaining an LPG total length of 25 mm. Two 248 nm UV laser pulses of 15 ns duration have been employed for every inscription point, which means a time reduction by over 21 times compared with the fastest inscription time already reported in literature. The device has been fabricated in a single-mode mPOF with a core that has been doped with benzyl dimethyl ketal for photosensitivity enhancement. Moreover, we characterize the strain and temperature responses and the stability of the fabricated gratings response under different conditions in order to assess the viability for different applications. [ABSTRACT FROM PUBLISHER]

In this work, we experimentally validate and characterize the first phase-shifted polymer optical fiber Bragg gratings (PS-POFBGs) produced using a single pulse from a 248-nm krypton fluoride laser. A single-mode poly (methyl methacrylate) optical fiber with a core doped with benzyl dimethyl ketal for photosensitivity improvement was used. A uniform phase mask customized for 850-nm grating inscription was used to inscribe these Bragg structures. The phase shift defect was created directly during the grating inscription process by placing a narrow blocking aperture in the center of the UV beam. The produced high-quality Bragg grating structures, presenting a double dips, reject 16.3 dB (97.6% reflectivity) and 13.2 dB (95.2% reflectivity) of the transmitted power, being therefore appropriate for sensing or other photonic applications. Its transmission spectrum possesses two sharp transmission notches, allowing a significant increase in measurement resolution compared to direct interrogation of a single grating. The reflection and transmission spectra when multiple phase shifts are introduced in the fiber Bragg grating structure are also shown. The PS-POFBG's strain, temperature, pressure, and humidity characteristics have been experimentally analyzed in detail to assess their potential usage as sensors. [ABSTRACT FROM PUBLISHER]

In this letter, we investigate the response of poly(methylmethacrylate) (PMMA) microstructured polymer optical fiber Bragg gratings (POFBGs) after immersion in methanol/water solutions at room temperature. As the glass transition temperature of solution-equilibrated PMMA differs from the one of solvent-free PMMA, different concentrations of methanol and water lead to various degrees of frozen-in stress relaxation in the fiber. After solvent evaporation, we observe a permanent blue-shift in the grating resonance wavelength. The main contribution in the resonance wavelength shift arises from a permanent change in the size of the fiber. The results are compared with conventional annealing. The proposed methodology is cost-effective as it does not require a climate chamber. Furthermore, it enables an easy-to-control tuning of the resonance wavelength of POFBGs. [ABSTRACT FROM PUBLISHER]

We report the fabrication and characterization of a polycarbonate (PC) microstructured polymer optical fiber (mPOF) Bragg grating (FBG) humidity sensor that can operate beyond 100°C. The PC preform, from which the fiber was drawn, was produced using an improved casting approach to reduce the attenuation of the fiber. The fiber loss was found reduced by a factor of two compared to the latest reported PC mPOF [20], holding the low loss record in PC based fibers. PC mPOFBG was characterized to humidity and temperature, and a relative humidity (RH) sensitivity of 7.31± 0.13 pm/% RH in the range 10–90% RH at 100°C and a temperature sensitivity of 25.86±0.63 pm/°C in the range 20–100 °C at 90% RH were measured. [ABSTRACT FROM PUBLISHER]

We present in vitro sensing of glucose using a newly developed efficient optical fiber glucose sensor based on a compound parabolic concentrator (CPC)-tipped polymer optical fiber (POF). A batch of nine CPC-tipped POF sensors with a 35-mm fiber length is shown to have an enhanced fluorescence pickup efficiency with an average increment factor of 1.7 as compared with standard POF sensors with a plane cut fiber tip. in vitro measurements for two glucose concentrations (40 and 400 mg/dL) confirm that the CPC-tipped sensors can efficiently detect both glucose concentrations. [ABSTRACT FROM PUBLISHER]

In this paper, we report on a new method to cleave polymer optical fibre. The most common way to cut a polymer optical fibre is chopping it with a razor blade; however, in this approach both the fibre and the blade must be preheated in order to turn the material ductile, and thus, prevent crazing. In this paper, we make use of the temperature-time equivalence in polymers to replace the use of heating by an increase of the cleaving time and use a sawing motion to reduce fibre end face damage. In this way, the polymer fibre can be cleaved at room temperature in seconds with the resulting end face being of similar quality to those produced by more complex and expensive heated systems. [ABSTRACT FROM PUBLISHER]