Single-mode Polymer Optical Fiber Research Articles

Bragg Gratings Inscribed in Solid-Core Microstructured Single-Mode Polymer Optical Fiber Drawn From a 3D-Printed Polycarbonate Preform

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.

Optical Strain Measurement with Step-Index Polymer Optical Fiber Based on the Phase Measurement of an Intensity-Modulated Signal.

Polymer optical fibers (POFs) have been proposed for optical strain sensors due to their large elastic strain range compared to glass optical fibers (GOFs). The phase response of a single-mode polymer optical fiber (SM-POF) is well-known in the literature, and depends on the physical deformation of the fiber as well as the impact on the refractive index of the core. In this paper, we investigate the impact of strain on a step-index polymer optical fiber (SI-POF). In particular, we discuss the responsivity of an optical strain sensor which is based on the phase measurement of an intensity-modulated signal. In comparison to the phase response of an SM-POF, we must take additional influences into account. Firstly, the SI-POF is a multi-mode fiber (MMF). Consequently, we not only consider the strain dependence of the refractive index, but also its dependency on the propagation angle . Second, we investigate the phase of an intensity-modulated signal. The development of this modulation phase along the fiber is influenced by modal dispersion, scattering, and attenuation. The modulation phase therefore has no linear dependency on the length of the fiber, even in the unstrained state. For the proper consideration of these effects, we rely on a novel model for step-index multi-mode fibers (SI-MMFs). We expand the model to consider the strain-induced effects, simulate the strain responsivity of the sensor, and compare it to experimental results. This led to the conclusion that the scattering behavior of a SI-POF is strain-dependent, which was further proven by measuring the far field at the end of a SI-POF under different strain conditions.

Thermal effects on the photoelastic coefficient of polymer optical fibers.

We measure the radial profile of the photoelastic coefficient C(r) in single-mode polymer optical fibers (POFs), and we determine the evolution of C(r) after annealing the fibers at temperatures from 40°C to 80°C. We demonstrate that C(r) in the fibers drawn from a preform without specific thermal pre-treatment changes and converges to values between 1.2 and 1.6×10-12 Pa-1 following annealing at 80°C. The annealed fibers display a smoothened radial profile of C(r) and a lowered residual birefringence. In contrast, the mean value of C(r) of the fiber drawn from a preform that has been pre-annealed remains constant after our annealing process and is significantly higher, i.e., 4×10-12 Pa-1. The annealing process decreases the residual birefringence to a lower extent as well. These measurements indicate the impact of annealing on the thermal stability of the photoelastic coefficient of POFs, which is an essential characteristic in view of developing POF-based thermo-mechanical sensors.

Etching Process Related Changes and Effects on Solid-Core Single-Mode Polymer Optical Fiber Grating

Etching process related material and mechanical changes on solid-core single-mode polymer optical fiber (POF) and their influence on the characteristic properties of polymer fiber Bragg gratings (PFBGs) are studied. A poly(methyl methacrylate)-based POF is etched to different diameters, and it is experimentally demonstrated that etching can lead to a change in the Young's modulus of the fiber. It is found that etching process induced material changes of the polymer fiber can enhance the reflectivity and inherent sensitivity of PFBGs. It is demonstrated that gratings based on etched POF exhibit higher reflectivity with a shorter exposure time compared with unetched ones. Highest reflectivity of 98.54% is observed within 7 seconds of exposure for a diameter of 85 $\mu\text{m}$ . The stability of fabricated Bragg gratings is also studied. By tailoring the etching diameter of POF, Bragg gratings with high sensitivity and high reflectivity can be fabricated.

Intrinsic High-Sensitivity Sensors Based on Etched Single-Mode Polymer Optical Fibers

The significance of etched single-mode polymer optical fibers and their potential for the development of high-sensitivity sensors are presented. A polymethyl methacrylate-based single-mode polymer optical fiber is etched to various diameters and it is observed that etching can lead to change in the material properties of the fiber, such as Young’s modulus and thermal expansion coefficient. This can play a vital role in improving the intrinsic sensing capabilities of sensors based on etched polymer optical fiber. To demonstrate that the modified material properties of the etched polymer fiber can enhance its intrinsic sensing capabilities, sensing characteristics of etched polymer fiber Bragg gratings for strain, temperature, and pressure are obtained. From the results, it is confirmed that the sensors based on etched polymer fibers exhibit high intrinsic sensitivity compared with un-etched ones. The potential of developing a sensing system for simultaneous measurement of strain and temperature is also demonstrated.

Inscription of Multiple Bragg Gratings in a Single-Mode Polymer Optical Fiber Using a Single Phase Mask and Its Analysis

Multiple Bragg gratings are inscribed in a photosensitive single-mode polymer fiber using phase mask method and also by applying strain to the fiber. Due to the large strain tuning range of the polymer fiber, multiple Bragg gratings are inscribed with a large peak wavelength separation between adjacent peaks. The effect of hysteresis of the polymer fiber is also considered and its influence on the fabrication of the grating is studied. From this paper, it is found that a fiber elongation up to 0.9% of the total fiber length can be used to inscribe multiple Bragg gratings and give a quality grating array. Fiber elongation ${>}{0.9\%}$ of the total length affects the grating reflectivity and bandwidth due to the creep and hysteresis developed in the polymer fiber from the large strain. Within the 0.9% elongation limit, multiple gratings can be inscribed with a maximum peak wavelength separation of 12 nm between adjacent fiber gratings. The fabricated single-mode polymer fiber Bragg grating array can be used in applications, where temperature and humidity compensations are required, while measuring the strain or as multipoint sensors.

Polymer micro-fiber Bragg grating

Polymer micro-fibers with inscribed Bragg gratings are reported in this Letter. Starting with a single-mode polymer optical fiber and implementing a two-stage tapering process, a 16 μm diameter micro-fiber is fabricated and a Bragg grating is inscribed in it that exhibits a peak reflected wavelength circa 1530 nm. The growth dynamics of the polymer micro-fiber Bragg grating are also observed and analyzed. A maximum reflectivity of 5% is obtained after an exposure time of 3 min to a 50 mW power He-Cd laser of 325 nm wavelength. The temperature and strain characterization results of the micro-fiber Bragg grating with different diameters are also presented. Such polymer micro-fiber Bragg gratings can be used as sensors for high-sensitivity measurements in a number of application areas.

Validation of a single-mode polymer optical fiber sensor and interrogator for large strain measurements

A single-mode polymer optical fiber (POF) in a Mach–Zehnder interferometer configuration is validated for the measurement of tensile nominal elongation of the POF up to 10%. The single-mode POF sensors were unmounted and surface mounted on aluminum tensile coupons for strain measurements. The measured strains from the POF sensors were compared to extensometer measurements for validation. The phase response of the interferometer was measured with a 3 × 3 coupler interrogator. The coupler arrangement was configured to permit the extraction of potential intensity changes in the sensor arm. The phase-shift–strain response of the POF sensors was repeatable for the loading and unloading measurements. The nonlinearity of the phase-shift–strain response was greater than that measured during pure tensile loading of the POF, presumably due to the behavior of the adhesive between the optical fiber and the aluminum coupons.

使用BDK掺杂光敏聚合物光纤布拉格光栅的高灵敏度温度传感器的研究

A single-mode polymer optical fiber (POF) with highly photosensitive core doped with benzil dimethyl ketal (BDK) is fabricated and used for writing Bragg grating through the two-beam interference method. The Bragg wavelength of the grating is about 1570 nm, while the full-width at half-maximum (FWHM) of the reflection peak is 0.3 nm. The temperature response of POF Bragg grating is theoretically analyzed and experimentally measured in contrast to silica optical fiber Bragg grating (FBG). The result shows that the temperature character of POF Bragg grating is negative, which is opposite to the silica optical FBG. The absolute value of the temperature response of POF Bragg grating is one order of magnitude higher than that of the silica optical FBG, making POF Bragg grating appear to be very attractive for constructing temperature sensors with high resolution.

Special structured polymer fibers for sensing applications

We report our recent development on polymer optical fibers with the ultimate goal to realize all polymer-based systems for sensing applications. The progress on the development of dopant polymer fibers and a low-cost, compact polymer optical sensing system is reported. Trans-4-stilbenemethanol doped photosensitive single-mode polymer optical fiber was fabricated, and fiber Bragg gratings were successfully photo-written onto the fiber. Single-mode perfluorinated polymer optical fibers is demonstrated, with the transmission loss of <0.2 dB/m in the wavelength range of 1410–1540 nm and <0.5 dB/m for wavelengths up to 1610 nm is achieved. A novel design of a photosensitive microstructured polymer optical fiber is proposed. The new hole-assisted single-mode polymer optical fiber design eliminates the influence of dopant diffusion and to increase the ultra-violet writing efficiency in fiber Bragg grating inscription.

Writing Parameters for 3D Refractive Index Structures in Polymethyl Methacrylate Using Femtosecond Radiation at 400 nm

Sub-surface modification by a femtosecond laser, producing pulses of 100fs duration at 400nm wavelength for fabrication of optical components is investigated, within sub-millimetre thick sheets of undoped polymethyl methacrylate (PMMA). For effective structuring, the influence of laser writing parameters and focusing conditions were studied to optimise the pulse energy, pulses spacing and focusing depth in the material. Permanent sub-surface refractive index structures with a minimum feature size (similar to 3 mu m) at a depth of similar to 500 mu m demonstrated the feasibility of writing flexible 3-D microstructures within a thin PMMA substrate. This investigation of focusing depth location in PMMA bulk material enabled photonic structures at the desired spatial resolution to be written within the core of single mode polymer optical fibre under controlled laser writing conditions. A long period fibre grating (LPG) consisting of a 2.8 mu m wide refractive index structure with a periodicity of 250 mu m and containing a series of attenuation bands in visible range, demonstrated a photonic structure written into a single mode polymer optical fibre.

High-sensitivity ultrasound interferometric single-mode polymer optical fiber sensors for biomedical applications

This work describes the results of ultrasonic wideband sensors based on single-mode polymer optical fibers that may be used for biomedical applications. We have compared the ultrasonic sensitivities of two Mach-Zehnder interferometric intrinsic optical fiber sensors. One is based on a single-mode polymethylmethacrylate optical fiber and the second on single-mode silica optical fiber, both operating at 632.8 nm. At a frequency of 1 MHz these sensitivities are 13.1 and 0.85 mrad/kPa, respectively. The ultrasonic phase sensitivity of the polymer optical fiber is more than 12 times larger than that from the fused silica fiber in the 1-5 MHz range.

Birefringent azopolymer long period fiber gratings induced by 532 nm polarized laser

Based on the photoinduced birefringence in azo materials, the model of the birefringent long period fiber gratings (BLPFGs) has been proposed and the transmission of such birefringent fiber gratings was further simulated by the coupled mode theory. In order to obtain the optimum condition for making birefringent azopolymer optical fiber (APOF) gratings the photoinduced birefringence of azo dye and methyl methacrylate (MMA) copolymers was systematically studied. After photosensitivity study of azo samples, using Teflon technique, azo dye containing single mode polymer optical fiber (POF) was fabricated. Finally, BLPFGs can be fabricated in azopolymer fiber using polarized 532 nm laser. A 120 μm LPFG with a 50% duty cycle has been successfully written within the core of the fiber with 532 nm laser by an amplitude mask method. Under polarized microphotography, the grating was observed when the optical axis was set at 45° with the direction of the first polarizing lens of the polarization microscope.

Calibration of a single-mode polymer optical fiber large-strain sensor

We calibrate the phase shift as a function of applied displacement in a polymethylmethacrylate (PMMA) single-mode optical fiber interferometer, operating at a wavelength of 632.8 nm. The phase sensitivity is measured up to 15.8% nominal strain in the fiber. The measured phase–displacement response is compared to a previous analytical formulation for the large deformation response of the polymer optical fiber strain sensor. The formulation includes both the finite deformation of the optical fiber and nonlinear strain-optic effects at large deformations. Using previously measured values for the linear and nonlinear mechanical response of the fiber, the nonlinear strain-optic effects are calibrated from the current experimental data. This calibration demonstrates that the nonlinearities in the strain-optic effect are of the same order of magnitude as those in the mechanical response of the PMMA optical fiber sensor.

Cleaving of solid single mode polymer optical fiber for strain sensor applications

Single mode polymer optical fibers (smPOFs) can be applied for measuring large strains in numerous applications, such as civil engineering infrastructure assessment and health monitoring. Because of the large light attenuation of solid smPOFs, small lengths of the fiber would need to be coupled to silica optical fibers (SOFs) for practical applications of the smPOF as a strain sensor. This coupling requires smooth cleaving of the smPOFs. In this work, several cleaving techniques previously demonstrated to provide smooth cross-sections of multimode POFs were applied to the smPOF. From these techniques, hot-knife cutting was determined to be a feasible method for cleaving when the blade was heated to 80°C and the smPOF heated in the range of 30°C to 40°C. In addition, focused ion beam machining which produces high-precision cleaves of the solid smPOF cross-section, was performed to set a bench mark and thus evaluate the quality of cleaving from other methods used in this study.

Behaviour of intrinsic polymer optical fibre sensor for large-strain applications

This paper derives the phase response of a single-mode polymer optical fibre for large-strain applications. The role of the finite deformation of the optical fibre and nonlinear strain optic effects are derived using a second order strain assumption and shown to be important at strain magnitudes as small as 1%. In addition, the role of the core radius change on the propagation constant is derived, but it is shown to be negligible as compared to the previous effects. It is shown that four mechanical and six opto-mechanical parameters must be calibrated to apply the sensor under arbitrary axial and transverse loading. The mechanical nonlinearity of a typical single-mode polymer optical fibre is experimentally measured in axial tension and is shown to be more significant than that of their silica counterpart. The mechanical parameters of the single-mode polymer optical fibre are also measured for a variety of strain rates, from which it is demonstrated that the strain rate has a strong influence on yield stress and strain. The calibrated constants themselves are less affected by strain rate.

Electrically tunable Bragg gratings in single-mode polymer optical fiber

We present what is to our knowledge the first demonstration of a tunable fiber Bragg grating device in polymer optical fiber that utilizes a thin-film resistive heater deposited on the surface of the fiber. The polymer fiber was coated via photochemical deposition of a Pd/Cu metallic layer with a procedure induced by vacuum-ultraviolet radiation at room temperature. The resulting device, when wavelength tuned via joule heating, underwent a wavelength shift of 2 nm for a moderate input power of 160 mW, a wavelength to input power coefficient of -13.4 pm/mW, and a time constant of 1.7 s(-1).

Strain and temperature sensitivity of a single-mode polymer optical fiber

We have measured the optical phase sensitivity of fiber based on poly(methyl methacrylate) under near-single-mode conditions at 632.8 nm wavelength. The elongation sensitivity is 131 +/- 3 x 10(5) rad m(-1) and the temperature sensitivity is -212 +/- 26 rad m(-1) K(-1). These values are somewhat larger than those for silica fiber and are consistent with the values expected on the basis of the bulk polymer properties.

Analysis of Photosensitivity of Copolymer Optical FibrePreform

The photosensitivity of copolymer optical fibre preform is analysed incomparison with the doped one. The effects of write conditions such aspump power and pump time have been studied. Then, the preform is drawninto single mode polymer optical fibre with core refractive index of1.499, and core-cladding refractive-index difference of 0.008.Long-period birefringence gratings with period of 120 um are fabricatedin the fibre. The duty cycle is 50%, and the refractive index changein the exposed area is about 1×10−3.

Stimulated Raman spectrum threshold in poly(methyl methacrylate) optical fibers

In this paper, the Raman differential cross-section of poly(methyl methacrylate) (PMMA) was measured relative to that of toluene and the gain coefficient was calculated. Then the stimulated Raman threshold in the polymer optical fibers (POF) was deduced, and a concise expression of pump threshold was obtained. Through simulation, we found that the threshold power is only several watts in single-mode POF with fiber several meters long, which shows the potential to develop POF Raman amplifiers.