We experimentally demonstrate laser-driven deposition of single-walled carbon nanotubes (CNTs) in suspended core fibers (SCFs) for the first time. Three samples for each SCF type with three (SCF1) and four (SCF2) air holes are individually inserted in a syringe loaded with a 0.5 mL solution of CNTs dispersed in methanol, and a high-power laser at 980 nm is injected into the fibers during distinct periods: SCF1 (20, 30 and 45 minutes), SCF2 (5, 10 and 15 minutes). The CNT solution thermally expands, accelerating and depositing nanotubes on the fiber tip. The material and dimensions of the CNTs deposited on the SCFs' cross-section are characterized, revealing nanotube bundles increasing in thickness from the fiber core to the bridges and inside the holes. The results indicate that shorter laser periods increase nanotube distribution within the SCFs’ core and overall cross-section. In contrast, longer periods focus high-pressure particles, enhancing ablation and depositing thinner layers in the fiber core. These achievements are significant for nanoparticle deposition and for the fabrication of microscale devices on tips and within specialty and hollow fibers, enabling innovative fiber sensors, mode-locked fiber lasers, in-fiber acousto-optic modulators, and broadband photoacoustic transmitters for imaging and neurostimulation.

We demonstrate a new all-fiber electrically tunable modulation method which significantly reduces the response time of a Bragg grating acousto-optic modulator. A 4 cm long device is fabricated with a 1 cm grating inscribed in a suspended core fiber. An acoustic pulse train is switched out of phase along the fiber, damping unwanted natural resonant vibrations inside the grating. The device rise time is decreased from 56 to 9 mu s by tuning the duty cycle of the driven electrical signal, contributing to achieve the shortest switching time of 15.6 mu s. This tunable temporal response reveals unique features to change the profile of optical pulses. High pulse modulation depths are achieved employing a compact acousto-optic modulator, pointing to fast switching of all-fiber photonic devices.

For high power fiber lasers, codoping with Al, P or both is necessary to prevent rare earth (RE) clustering in the silica network of the laser active core material. Here, we present a complementary infrared (IR) based multispectral method combined with elemental analysis data on core/cladding to describe the structure of the doped core material as fabricated by chemical vapor deposition with gas phase doping of Al and P. The resulting 2D image and its corresponding 3D visualization of the data enable an alternative and convenient way to characterize the main species of the dopants aside from NMR measurements.

A Mach–Zehnder interferometer (MZI) sensor based on a coupled four-core sapphire-derived fiber (FSDF) for temperature sensing is proposed and demonstrated. The coupled FSDF with a high refractive index (RI) difference between core and cladding can support four LP01-like supermodes (LP01s modes) and eight LP11-like supermodes (LP11s modes). We have fabricated and investigated two sensor samples with a sensing length of 3.1 cm (Sensor I) based on LP01s-LP11s mode interference and with a sensing length of 9.0 cm (Sensor II) based on LP01s-LP01s mode and LP11s-LP11s mode interference. The high-temperature behaviors of both sensors were studied after annealing at temperatures up to 900 °C. The experimental results show that the temperature sensitivity of Sensor I is around 70 pm/°C. For Sensor II, multiple supermode interferences are involved providing different sensitivities. The sensitivity based on the first-order LP01s and second-order LP01s interference is about 136 pm/°C, and the sensitivity based on the first-order LP01s and fourth-order LP01s mode interference is about 36 pm/°C. The sensitivity based on two LP11s mode interference achieves about 64 pm/°C. The experimental results are compared to theoretical simulation. The theoretical simulations give additional information on how the geometry of the coupled multicore fiber affects its temperature properties. Adjusting the core radius and the core pitch is shown to have a considerable influence on the achievable sensitivity and may even change the sign of the sensor sensitivity to temperature. Therefore, designing a specific geometric structure of the coupled fiber is beneficial to optimize the sensing characteristic of this type of sensor.

AbstractUltrafast and low-noise supercontinuum (SC) sources based on all-normal dispersion (ANDi) fibers are emerging as key-enabling technology for new applications in spectroscopy, microscopy, and ultrafast photonics. In this chapter we review the fundamental physics, fiber designs, and applications of this unique white light source.

Applications associated with complex, structured optical fibers having no circular symmetry often require information about their specific angular orientation. Such cases include, for example, splicing, sophisticated mode coupling, selective fiber Bragg gratings inscription, or direction‐sensitive sensing. Besides the external angular orientation of the fiber, additionally an intrinsic twist of the internal fiber structure has been identified as a critical parameter for applications. Such an intrinsic fiber twist might occur as a side effect of the fiber drawing process, or it may have been introduced intentionally as a special fiber property. In all such cases, a measurement of the local orientation along the fiber is highly desirable. A method for such measurements based on the analysis of the refractive–diffractive scattering patterns is presented. By comparing such measured patterns with reference patterns from modeling or from reference measurements, the orientation of the fiber can be identified with high accuracy and without measuring or reconstructing the detailed cross section of a fiber. The method, therefore, provides an approach for quick identification of a local angular fiber orientation status.

A novel multicore optical waveguide component based on a fiber design optimized towards selective grating inscription for multiplexed sensing applications is presented. Such a fiber design enables the increase in the optical sensor capacity as well as extending the sensing length with a single optical fiber while preserving the spatial sensing resolution. The method uses a multicore fiber with differently doped fiber cores and, therefore, enables a selective grating inscription. The concept can be applied in a draw tower inscription process for an efficient production of sensing networks. Along with the general concept, the paper discusses the specific preparation of the fiber-based sensing component and provides experimental results showing the feasibility of such a sensing system.

In order to provide high accuracy in shape measurement with multicore optical fibres, characterization and calibration procedures are an important part of sensor preparation. Some procedures can be considered mandatory for adequate shape reconstruction, while others can help to enhance the measurement accuracy. Several of such procedures are discussed and experimentally applied for demonstrating the possible performance enhancement of curvature sensing, a fundamental phase of the shape reconstruction process. The maximum error observed in curvature calculation for a test object has been proved to be almost halved, decreasing from 2.48% to 1.36%, by applying such calibration corrections. The overall average relative accuracy of curvature measurement was improved from 0.89% to 0.5% (an improvement of 44%).

The optical analog of the Vernier effect applied to fiber interferometers is a recent tool to enhance the sensitivity and resolution of optical fiber sensors. This effect relies on the overlap between the signals of two interferometers with slightly detuned interference frequencies. The Vernier envelope modulation generated at the output spectrum presents magnified sensing capabilities (i.e., magnified wavelength shift) compared to that of the individual sensing interferometers that constitute the system, leading to a new generation of highly sensitive fiber sensing devices. This review analyses the recent advances and developments of the optical Vernier effect from a fiber sensing point-of-view. Initially, the fundamentals of the effect are introduced, followed by an extensive review on the state-of-the-art, presenting all the different configurations and types of fiber sensing interferometers used to introduce the optical Vernier effect. This paper also includes an overview of the complex case of enhanced Vernier effect and the introduction of harmonics to the effect.

We demonstrate acousto-optic modulation of a fiber Bragg grating in a birefringent suspended core fiber up to frequencies of 5.2 MHz for the first time. At acoustic resonances lower than 820 kHz, the reflectivity of the orthogonal polarization modes is electrically switched to a superposed reflection band with a maximum modulation depth of 51% at 10 V. The wavelength peak of the polarization modes is dynamically shifted by tuning the driven electrical signal from 1.92 to 5.2 MHz. A novel side-coupled based acousto-optic device is demonstrated, indicating new possibilities for ultracompact, fast and efficient all-fiber integrated devices.

Highly efficient and compact acousto-optic modulator of a fibre Bragg grating is reported for the first time. The device is composed of millimetre scaled components and a 1 cm grating inscribed in a four air holes birefringent suspended core fibre. The reflection of the orthogonal polarization modes is tuned by a sinusoidal electrical signal at the resonance frequency of 469 kHz. A significant modulation depth of 45% is achieved at a maximum drive voltage of 10 V. The demonstrated 4 cm long all-fibre modulator is 60% shorter compared to previous similar setups, indicating new possibilities for stable and fast switching of fibre-integrated photonic devices.

An innovative approach using the vapor axial deposition (VAD), for the preparation of silica-based high-power fiber laser preforms, is described in this study. The VAD uses a plasma deposition system operating at atmospheric pressure, fed by a single, chemically adapted solution containing precursors of laser-active dopants (e.g., Yb2O3), glass-modifier species (e.g., Al2O3), and the silica matrix. The approach enables simultaneous doping with multiple optically active species and overcomes some of the current technological limitations encountered with well-established fiber preform technologies in terms of dopant distribution, doping levels, and achievable active core diameter. The deposition of co-doped silica with outstanding homogeneity is proven by Raman spectroscopy and electron probe microanalysis. Yb(2)O(3 )concentrations are realized up to 0.3 mol% in SiO2, with simultaneous doping of 3 mol% of Al2O3.

Achieving a new generation of enhanced sensors requires the development of structures that result from the fusion of different concepts and effects. In this paper, we combine a special strain sensing structure with an optical sensitivity magnification, through harmonics of the Vernier effect. The recently demonstrated harmonics of the Vernier effect result from increasing the optical path length (OPL) of one of two interferometers by multiple times the OPL of the other interferometer. The effect generates higher magnification factors, proportional to the order of the harmonics. The sensing structure is demonstrated for strain and temperature discrimination, allowing compensation for temperature fluctuations. We explore the complex case of the optical Vernier effect in series, where both interferometers are used as sensing interferometers (no reference interferometer is used). Our results also suggest that the magnification enhancement provided by harmonics of the Vernier effect for a configuration in series is the same as for a configuration in parallel: the magnification factor increases proportionally to the order of the harmonics.

The incorporation of phosphorus into silica soot material strongly changes during the multistep preparation process of the MCVD technology in combination with solution doping for Al and rare earths. We report on the influence of various process parameters on the phosphorus concentration, the bond types of phosphorus atoms and the relative density of the soot material. By optimization of the process the phosphorus concentration of the presintered soot could be increased by around 10% in comparison to the conventional treatment. The understanding of the interdependencies allows an improvement of the preparation process of phosphorus co-doped RE doped silica laser fibers with MCVD technology.

The optical Vernier effect consists of overlapping responses of a sensing and a reference interferometer with slightly shifted interferometric frequencies. The beating modulation thus generated presents high magnified sensitivity and resolution compared to the sensing interferometer, if the two interferometers are slightly out of tune with each other. However, the outcome of such a condition is a large beating modulation, immeasurable by conventional detection systems due to practical limitations of the usable spectral range. We propose a method to surpass this limitation by using a few-mode sensing interferometer instead of a single-mode one. The overlap response of the different modes produces a measurable envelope, whilst preserving an extremely high magnification factor, an order of magnification higher than current state-of-the-art performances. Furthermore, we demonstrate the application of that method in the development of a giant sensitivity fibre refractometer with a sensitivity of around 500 µm/RIU (refractive index unit) and with a magnification factor over 850.

Based on favorable properties with respect to beam quality, efficiency and applicability, fiber-integrated lasers replace costly bulk lasers in various application fields, such as in academia, industry, medical technology and life sciences. Additionally, rare-earth doped fibers exhibit broad gain regions, providing huge potential to develop broadly tunable fiber lasers for spectroscopy. Recently a fiber-integrated tuning concept based on an FBG array as discrete spectral filter was demonstrated, exhibiting a unique flexibility to tailor spectral and temporal emission properties. In this work, we present the prospects of this concept to address Differential Absorption LIDAR (DIAL) for environmental sensing. With tunable multi-wavelength operation for enhanced measurement speeds, and customized spectral emission lines to probe specific molecules, it may provide a fast and cost-efficient solution with excellent usability.

The optical Vernier effect magnifies the sensing capabilities of an interferometer, allowing for unprecedented sensitivities and resolutions to be achieved. Just like a caliper uses two different scales to achieve higher resolution measurements, the optical Vernier effect is based on the overlap in the responses of two interferometers with slightly detuned interference signals. Here, we present a novel approach in detail, which introduces optical harmonics to the Vernier effect through Fabry-Perot interferometers, where the two interferometers can have very different frequencies in the interferometric pattern. We demonstrate not only a considerable enhancement compared to current methods, but also better control of the sensitivity magnification factor, which scales up with the order of the harmonics, allowing us to surpass the limits of the conventional Vernier effect as used today. In addition, this novel concept opens also new ways of dimensioning the sensing structures, together with improved fabrication tolerances.

This paper presents an innovative one-step doping approach for the preparation of Al-Yb co-doped silica glasses for fiber preforms. Today, fiber-lasers are of great interest in industry due to highest precision and flexibility in system design combined with high power output and excellent beam quality. Industrially established processes such as modified chemical vapor deposition (MCVD), outside vapor deposition (OVD) and reactive powder sintering technology (REPUSIL) are used to fabricate co-doped silica glasses for laser fibers. However, none of these processes is able to simultaneously incorporate laser active dopants increasing the refractive index (rare earth elements, RE), glass matrix modifiers (e.g. aluminum, Al2O3) and dopants reducing the refractive index (e.g. fluorine, F). Instead, the incorporation of the individual refractive index changing dopants, into a silica glass matrix, has to be carried out in subsequent and separate steps. The novel approach pursues to overcome this limit by application of atmospheric-pressure microwave plasma with oxygen used as reactive gas in combination with a powder sintering process, targeting the preparation of tailored rareearth doped preforms for high power fiber-laser applications. As a proof of principle, silica powders doped with Al3+ and Yb3+ have been synthesized successfully. These have been proven to perfectly suit the subsequent processing via the powder sintering process. The plasma generated Al2O3 doped SiO2 particles have an averaged particle size of 30 nm a specific surface area of about 55 m2/g, at an Al2O3 concentration of up to 3 mol%. In a second set of experiments, microwave atmospheric pressure plasma-based co-doping of SiO2 with Al and Yb species has been successfully demonstrated for the first time.

New miniaturized sensors for biological and medical applications must be adapted to the measuring environments and they should provide a high measurement resolution to sense small changes. The Vernier effect is an effective way of magnifying the sensitivity of a device, allowing for higher resolution sensing. We applied this concept to the development of a small-size optical fiber Fabry–Perot interferometer probe that presents more than 60-fold higher sensitivity to temperature than the normal Fabry–Perot interferometer without the Vernier effect. This enables the sensor to reach higher temperature resolutions. The silica Fabry–Perot interferometer is created by focused ion beam milling of the end of a tapered multimode fiber. Multiple Fabry–Perot interferometers with shifted frequencies are generated in the cavity due to the presence of multiple modes. The reflection spectrum shows two main components in the Fast Fourier transform that give rise to the Vernier effect. The superposition of these components presents an enhancement of sensitivity to temperature. The same effect is also obtained by monitoring the reflection spectrum node without any filtering. A temperature sensitivity of −654 pm/°C was obtained between 30 °C and 120 °C, with an experimental resolution of 0.14 °C. Stability measurements are also reported.