We demonstrate a pathway for producing large-core fiber preforms with optimized characteristics for high-power fiber laser applications. Reactive powder sintering (REPUSIL) was used for producing large volumes of doped silica fiber preforms with predictable, homogeneous, and precise compositional profiles, focusing on formulations near the equimolar dopant ratio of P:Al = 1 to suppress Yb-related photodarkening (PD). Spectroscopic and structural properties are related to PD performance for both preform and optical fiber materials. All preform samples exhibit a radial dip-free, step-like refractive index profile with relative average index fluctuations of less than 2%. Reduced excess PD loss of 10 dB /m after 21 h exposure time and lower PD rates are obtained when the P content is adapted at a slight excess over Al, even when the overall Al content is high.

A novel, to the best of our knowledge, sensor based on the controlled collapse of the hollow core of a thin-wall silica capillary tube (SCT) spliced between two single-mode fibers is proposed for the monitoring of external liquid refractive index (RI). The conical shape of the splice excites multimodal interference in the cladding of the SCT. For a range of 1.333-1.341 RIU, when isolating the dominant frequencies, a maximum sensitivity of 4421 nm/RIU is achieved, which is an outstanding result for an all-silica-based device in this RI range. The low-cost one‑step fabrication process, in conjunction with the ultrahigh sensitivity, displays remarkable potential for real‑time detection in chemical processing applications, where precise RI monitoring is required.

An all-silica-based sensor comprising a section of capillary fiber spliced between two singlemode fibers (SMFs) is proposed for the simultaneous measurement of strain and temperature. By intentionally introducing a controlled transversal offset at one of the fusion splice points, core and cladding modes are simultaneously excited in the capillary, enabling the coexistence of two distinct guiding mechanisms within the sensor. The resulting spectral response exhibits two superimposed modulations associated with antiresonance (AR) guidance and a Mach–Zehnder interferometer (MZI). A comprehensive numerical model is developed to describe the interaction between the two mechanisms as a function of the offset. The model is experimentally validated through characterization of the spectral response for increasing offsets, confirming the coexistence and evolution of the AR and MZI components through free spectral range and visibility analysis. The two interference components allow for independent tracking of their wavelength shifts, enabling simultaneous strain and temperature measurements with estimated resolutions of 11.9 με and 0.45 °C, respectively. Owing to the single-element, one-step fabrication process, and the entirely silica-based configuration, the proposed sensor offers a compact and cost-effective solution for localized multiparameter monitoring.

A multiparameter sensor based on a single section of a hollow square core fiber (HSCF) spliced between a single mode fiber and a silica capillary tube is proposed. In a reflection scheme, several Fabry-Perot (FP) cavities are enhanced in different areas of the HSCF. In a 439 mu m long sensing head, three FP cavities are excited. Using the Fourier band-pass filter method, each cavity is individually monitored towards the measurand variation. The distinct responses of the FP cavities allow for a hybrid application for simultaneous measurement of pressure, temperature, and curvature. Additionally, a sinusoidal behavior is found in the curvature response when bending is applied for different relative positions of the fiber. The proposed sensor is robust with simple fabrication and small dimensions, demonstrating promising potential for applications where the simultaneous measurement of several physical parameters is required.

In light of the increasing demand for sensing devices with larger sensitivities and precision, this work presents an optical fiber sensor based on multimode interference, fabricated by manipulating the fusion parameters for a silica capillary tube spliced between two sections of singlemode fiber. The sensor, with a length of similar to 2.5 mm, is studied for a variation of refractive index ranging from 1.3342 RIU to 1.3452 RIU. To circumvent the complex spectral response of the sensor, a frequency filtering of the response strategy is implemented, resulting in sensitivities of 1391.0 nm/RIU and 2807.5 nm/RIU, and a correspondent resolution of 7.6 x 10(-5) RIU and 9.4 x 10(-4) RIU, when considering the first two main frequencies of the spectral response.

Crystals and fibers doped with Ce3+ are highly valued for their wide-ranging fluorescent emission, which covers both visible and near-infrared wavelengths [1], [2]. Among these, their broad fluorescence spectrum makes them suitable for potential use in lighting technologies [3]. Additionally, Ce3+-doped silica fibers hold particular importance in non-invasive biomedical techniques like Optical Coherence Tomography (OCT) [4]. This work focuses on examining the fluorescence behaviour of a Ce3+-doped silica fiber and glass when pumped with 405 nm.

We present examples of recent developments on specialty optical fibers at Leibniz-IPHT. A dual-core dual-clad fiber for multi-modal analyses was fabricated by a stack and draw technique, where MCVD Ge-doped core rods, pure and F-doped fused silica components were combined. For imaging applications fibers with a higher number of cores without cross talk were drawn. For application as feed through thin walled capillaries were drawn. By careful control of the drawing conditions and pressure both wall thickness and hole diameter were prepared with a variation better than 1 mu m.

Bisphenol A (BPA) is an endocrine disruptor found in food-contact materials and, even at very low concentrations, poses a serious risk to human health. Therefore, its presence in food should be monitored. Optical fiber sensors are a highly advantageous option for detecting chemical contaminants; however, due to the low concentrations at which these compounds are found, the surface needs to be modified to promote interaction with the target compound. In this work, we present an optical fiber sensor incorporating a microstructured fiber that is sensitive to external media and, therefore, to refractive index variations. To increase sensitivity, the sensor was coated with a chitosan film, a polysaccharide with high biosorbent ability that has been proposed as an effective adsorbent for several contaminants, including BPA. The sensor was then characterized by its response to variations in BPA concentration from 0 to 0.1 mg/mL. The chitosan-coated sensor exhibited a sensitivity over three times higher than that of the uncoated sensor, with a resolution of 9.98 × 10−4 mg/mL. Washing assays revealed that, although the coating cannot be fully regenerated, the sensor can be reused without requiring a complex or time-consuming procedure.

In this work, we present the development of a novel hybrid optical fiber sensor designed for the simultaneous measurement of pressure and temperature. The sensor consists of a short section of hollow-core microstructured fiber (HCMF) spliced to standard single mode fiber. In the splice region an air bubble is created, forming two Fabry-Perot interferometers, one in air and the other one occurring in silica cavities. The manufacturing process is straightforward and cost- effective, requiring only a fusion splicer, making it suitable for scalable production. The sensor displays distinct sensitivities for each parameter, with the air cavity exhibiting a pressure sensitivity of (4.01 +/- 0.04) nm/MPa and temperature sensitivity of (0.66 +/- 0.06) pm/degrees C, while the silica cavity demonstrates a pressure sensitivity of (0.914 +/- 0.004) nm/MPa and a temperature sensitivity of (8.27 +/- 0.07) pm/degrees C. These results underscore the sensor capability to independently measure pressure and temperature variations with high precision, providing a robust solution for multi-parameter sensing in a range of applications.

During recent years, the optical-fiber-based simultaneous sensing of strain and temperature has attracted increased interest for different applications, e.g., in medicine, architecture, and aerospace. Specialized fiber layouts further enlarge the field of applications at much lower costs and with easier handling. Today, the performance of many sensors fabricated from conventional fibers suffers from cross-sensitivity (temperature and strain) and relatively high interrogation costs. In contrast, customized fiber architectures would make it possible to circumvent such sensor drawbacks. Here, we report on the development of a high-quality coupled-core fiber and its performance for sensors—from the initial fiber layout via elaboration of the preform and fiber up to the sensor evaluation. A compact, high-speed, and cost-effective interrogation unit using such a specialized coupled-core fiber has been designed to monitor reflectivity changes while even being able to distinguish the direction of the force or impact. Several fiber core material techniques and approaches were investigated, which made it possible to obtain a sufficient volume of material for the required fiber core number and a specialized fiber core geometry in terms of core distances and radial refractive index profile, whilst handling the non-symmetrical fiber architectures of such modeled, complex structures and balancing resources and efforts.

Fiber optic bending sensing has potential use in industrial and medical applications. Thus, so far, several configurations have been reported with that end, but the state-of-the-art sensors are either complex, temperature dependent, or cannot be multiplexed easily. To circumvent these important limitations, we have developed a sensing platform based on an asymmetric coupled-core optical fiber that is combined with conventional Bragg gratings. The asymmetric fiber was designed with three cores arranged in an equilateral triangle. The said fiber supports supermodes that suffer drastic changes when it is bent. Consequently, the reflection of a Bragg grating inscribed close to the asymmetric fiber changes drastically, but its wavelength position is not altered. We demonstrate experimentally that our sensing platform allows the development of highly sensitive bending sensors that have important practical assets. The latter include compactness and simple fabrication, capability of distinguishing the direction of bending and simultaneous detection of temperature and bending. Moreover, the sensors can be multiplexed easily and can be interrogated with commercially available fiber optic sensor read out units.

In the food industry, acetic acid is a compound often used as a preservative or additive, but it is also produced during the fermentation of various food products and constitutes a key marker of food safety and quality. Although accurate, sensitive, and selective, the traditional methods for determining acetic acid are also expensive and time-consuming. In this regard, optical fiber sensors have emerged as a possible alternative for the real-time monitoring of small molecules. In this work, we propose an optical fiber sensor for the simultaneous measurement of refractive index (RI) and temperature using a balloon-shaped structure with a single sensing element. The sensor, based on cladding modal interference, was characterized by its response to RI in the range from 1.320 to 1.352 RIU, using acetic acid solutions with concentrations between 0% and 50%(v/v), and by its response to temperature between 23 and 43 °C. A maximum sensitivity of 170.66 nm/RIU was obtained for RI, equivalent to 110.7 pm/%(v/v) in terms of concentration, and a maximum sensitivity of −119.2 pm/°C was obtained for temperature.

We present here the concept of a MEMS-mirror based nonlinear endomicroscopic probe for coherent anti-Stokes Raman scattering (CARS), two-photon excitation fluorescence (TPEF), and second harmonic generation (SHG). The rigid probe head is 5 mm in diameter and 4 cm in length, offering a large field of view (FOV) with a high numerical aperture (NA). It incorporates a double-core fiber delivering two excitation wavelengths of CARS in isolated cores, with a large cladding area to increase the collection efficiency of the nonlinear signal from the tissue. A diffractive grating element is included in the probe to compensate for the spatial offset of two emission cores.

Applying the highly versatile and flexible MCVD technology at Leibniz-IPHT two new designs for optical sensing fibers were realized by co-doping of fused silica. For FBG sensing a Ge/B co-doped fiber with a mode field diameter adapted to standard single mode telecom fibers was prepared. The influence of boron on the attenuation at the inscription wavelength 1550 nm is visible. For distributed Brillouin sensing applications a preform with lateral separated germanium and aluminum doped regions and nearly step-index characteristic was fabricated by the MCVD in combination with the solution doping technique. Theoretical analysis of the acoustic properties and Brillouin spectrum have been shown, that this design is a potential candidate for strain and temperature discrimination. Because of the high temperatures during the preparation processes the radial refractive index and dopant concentration profiles of both fiber designs are influenced by diffusion.

Applying the highly versatile and flexible MCVD technology at Leibniz-IPHT two new designs for optical sensing fibers were realized by co-doping of fused silica. For FBG sensing a Ge/B co-doped fiber with a mode field diameter adapted to standard single mode telecom fibers was prepared. The influence of boron on the attenuation at the inscription wavelength 1550 nm is visible. For distributed Brillouin sensing applications a preform with lateral separated germanium and aluminum doped regions and nearly step-index characteristic was fabricated by the MCVD in combination with the solution doping technique. Theoretical analysis of the acoustic properties and Brillouin spectrum have been shown, that this design is a potential candidate for strain and temperature discrimination. Because of the high temperatures during the preparation processes the radial refractive index and dopant concentration profiles of both fiber designs are influenced by diffusion.