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Terahertz (THz) technology has witnessed a significant growth in a wide range of applications, including spectroscopy, bio-medical sensing, astronomical and space detection, THz tomography, and non-invasive imaging. Current THz microstructured fibers show a complex fabrication process and their flexibility is severely restricted by the relatively large cross-sections, which turn them into rigid rods. In this paper, we demonstrate a simple and novel method to fabricate low-cost THz microstructured fibers. A cyclic olefin copolymer (TOPAS) suspended-core fiber guiding in the THz is extruded from a structured 3D printer nozzle and directly drawn in a single step process. Spectrograms of broadband THz pulses propagated through different lengths of fiber clearly indicate guidance in the fiber core. Cladding mode stripping allow for the identification of the single mode in the spectrograms and the determination of the average propagation loss (~ 0.11 dB/mm) in the 0.5–1 THz frequency range. This work points towards single step manufacturing of microstructured fibers using a wide variety of materials and geometries using a 3D printer platform.

An ultrasensitive optical nanofiber coupler (NFC) for thermal and refractive index sensing with nanoscale dimension is proposed and experimentally demonstrated. The NFC operating near the cut-off region of the higher order modes is fabricated by fusing and pulling two standard telecom fibers using the modified flame brushing technique. Due to the sub-micron diameter, the NFC exhibits an anomalous behavior in close proximity to the odd supermode cut-off wavelength, where the high extinction ratio spectral oscillations exhibit very slow oscillations. The sensitivity of this region is exploited for thermal sensing over a broad range of temperatures (84 °C–661 °C). An average temperature sensitivity of 55 pm/°C and a maximum sensitivity of 60 pm/°C at the highest temperature were demonstrated. Owing to its small size, the thermal sensor exhibits a fast response time of ~7.2 ms. The sensor capability for detecting refractive index changes is also examined with different ethanol concentrations. A record sensitivity of $4.80\times 10^{5}$ nm/RIU was achieved, which is the highest reported value in all optical fiber refractometers.

While recent advances in femtosecond writing have allowed rapid and flexible realization of lab-on-chip and integrated optics systems in various media, miniaturisation of such fs-written waveguide architectures remains hampered by high bend loss α b . Despite efforts to reduce bend loss, e.g. by annealing the waveguides [1] or writing stress structures [2], the minimum bend radius at telecommunications wavelengths remains large (> 16mm) in silica [1]. To overcome this, we demonstrate a novel bend loss reduction method by inducing stress to fabricate a microcrack on the outer bend edge of the waveguide. The large index difference at the core-microcrack interface (1.45 vs 1.0) enhances modal confinement and inhibits radiation loss. Laser induced stress cracking is often used for dicing glass sheets with excellent smoothness (roughness < 30nm) [3]; hence such cracks will not exacerbate scattering losses when integrated alongside a waveguide.

Here we discuss several different pathways of exploiting optical fibers for sensing applications. We demonstrate that fused optical fiber couplers can operate as a sensitive refractometers and thermometers. Their potential for bio-sensing applications is also discussed. We also discuss application of plastic fibers for accessing mid-IR spectral region for optical sensing.

The removal of polarization and modal degeneracy in the excitation of higher order modes is realised in all-fiber mode-selective weakly-fused-couplers by controlling the coupler geometry. Experiments carried out with short couplers show strong polarization dependency, while long couplers with a slow varying cross-section have shown to remove polarization and spatial degeneracy in the first high order mode.

Microstructured polymer optical fibers (MPOFs) have long attracted great interest due to their wide range of applications in biological and chemical sensing. In this manuscript, we demonstrate a novel technique of manufacturing MPOF via a single-step procedure by means of a 3D printer. A suspended-core polymer optical fiber has been extruded and directly drawn from a micro-structured 3D printer nozzle by using an acrylonitrile butadiene styrene (ABS) polymer. Near-field imaging at the fiber facet performed at the wavelength λ~1550 nm clearly indicates guidance in the fiber core. The propagation loss has been experimentally demonstrated to be better than α = 1.1 dB/cm. This work points toward direct MPOFs manufacturing of varieties of materials and structures of optical fibers from 3D printers using a single manufacturing step.

Miniature lasers producing ultrashort phase-coherent pulses at high repetition rates by stable mode-locking in ambient conditions can offer unique capabilities in various applications, spanning from microwave photonics to telecom and biological imaging techniques. Here, the operation of graphene mode-locked lasers based on channel waveguides written by femtosecond and picosecond laser pulses in Ti:sapphire crystals is demonstrated. Trains of pulses of 41.4-fs duration at a 21.25-GHz repetition rate were generated by capitalizing on the formation of solitons in their monolithic resonators through Gires–Tournois interferometers. The latter, allow for effective pulse shaping via tuning of the intracavity group delay dispersion while simultaneously enabling ultralow laser operating thresholds. A number of features of these sources, including their high-brightness and broad bandwidth, are essential ingredients for achieving high longitudinal resolution and sensitivity, which are the primary performance metrics of the Fourier domain/spectral domain variant of optical coherence tomography systems. A further doubling of the laser repetition rate to 42.5 GHz was achieved by coherent pulse interleaving in optical fiber technology, thereby underlining the potential of the Ti:sapphire waveguide lasers to produce highly stable, wide-spaced combs of phase-coherent optical frequency lines.

Mid-infrared (mid-IR) optical fibers have long attracted great interest due to their wide range of applications in security, biology and chemical sensing. Traditionally, research was directed towards materials with low absorption in the mid-IR region, such as chalcogenides, which are difficult to manipulate and often contain highly toxic elements. In this paper, we demonstrate a Polyethylene Terephthalate Glycol (PETG) hollow-core fiber (HCF) with guiding properties in the mid-IR. Guiding is provided by the fiber geometry, as PETG exhibits a material attenuation 2 orders of magnitude larger than the HCF propagation loss. The structured plastic fiber preforms were fabricated using commercial 3D printing technology and then drawn using a conventional fiber drawing tower. The final PETG fiber outer diameter was 466 µm with a hollow-core diameter of 225 µm. Thermal imaging at the fiber facet performed within the wavelength range 3.5–5 µm clearly indicates air guidance in the fiber hollow-core.

Indoor optical wireless communication with optical beamsteering capability is currently attracting a lot of attention. One major two-dimensional (2D) optical beamsteering scheme is realized by 2D grating or its active counterpart, which is usually based on a spatial light modulator (SLM). However, there is a fundamental trade-off between the field of view (FoV) and power efficiency due to the inherent feature of gratings. In this Letter, we propose a new class of 2D beamsteering, named cyclically arranged optical beamsteering (CAO-BS), which can break that trade-off. Traditional 2D gratings extend the optical beam in the Cartesian coordinates (1D grating in horizontal + 1D grating in vertical), while CAO-BS extends the optical beam in the polar coordinates (1D grating + angular rotation). Since only 1D grating is engaged, the power efficiency increases with the number of grating lobes reduced. In the polar coordinates, the angle rotation tuning in a SLM is quasi-continuous in a full 2π range. The CAO-BS is demonstrated at the receiving end in an indoor experimental system. The FoV is 18° by 360° in polar coordinates without any additional mechanical parts. Based on the CAO-BS, 40 Gbit/s on-off keying data is also successfully transmitted over 1 km single-mode fiber and 0.5 m free space.

Silica fibres doped with small (< 1 mol%) concentrations of gadolinium ions have been fabricated and tested in a 1T magnetic field showing a Faraday rotation two orders of magnitude larger than that reported in conventional telecom fibres. This is the largest Faraday rotation ever reported in low-loss optical fibers and could pave the way for a new class of optical isolators.

Silica waveguides with polarization-selective transmission were written by femtosecond laser inscription using linear and circular polarization. By tailoring writing parameters, polarization-dependent losses from 0.5 to 24dB at 1310nm were achieved.

We demonstrate the single-step manufacturing of a microstructured polymer optical fiber (MPOF) by extruding and directly drawing the polymer from a structured 3D printer nozzle.

Optical fiber couplers with ∼300nm waist diameter are used as compact temperature sensors from room temperature to 661oC, showing an average temperature sensitivity of 55 pm/oC and a maximum sensitivity of 60pm/ oC at the highest temperature.

Weakly-fused optical fibre couplers can be simplified as a system of two optical fibre tapers, which are often called nanofibers when having sub-micron diameters. While conventional optical fibre couplers exhibit a wide spectral region of modal interference where two modes provide a continuous oscillatory behaviour associated to the large difference between phase constants of interfering supermodes, nanofiber couplers (NFC) exhibit an anomalous behaviour (Fig. 1) in close proximity to the supermodes cut-off wavelengths, where the high extinction ratio oscillations can exhibit a very slow oscillation. This region of interest exhibits flat transmission over nearly 20nm and results from the variation of the supermodes dispersion near their cut-off. This hybrid behaviour, where a flat transmission region is proximal to rapid spectral oscillations, allows the NFC to operate at high sensitivity as well as large dynamic range for application related to optical detection. &more...

A multi-port microcoil resonator magnetic field sensor based on a microfiber coupler coil resonator (MMCR) is presented. The microfiber coupler coil is fabricated by coiling a four-port microfiber coupler with a uniform waist region around a low index support rod. The MMCR is embedded in a low refractive index polymer to increase the robustness and operation stability. The enhanced sensor response to the magnetic field is ascribed to the diverse MMCR response to the light polarization state. The MMCR magnetic field sensor is compact and low cost, and exhibits a magnetic field sensitivity of 37.09 dB/T with an estimated minimum detection limit (DL) of ~ 27 µT.

UV light has been generated in optical fibers using nonlinear optics (harmonic generation) and rare earth doping with Gd3+ Asia Communications and Photonics Conferene (ACP)

Orbital angular momentum (OAM) beam generation has been investigated using all-fibre and free space configurations. In the first approach, the composite fused coupler is based on a single mode fibre (SMF) and an air-core fibre. The second approach exploits geometrical phase introduced by nanostructuring of silica glass. Both approaches are demonstrated to achieve power coupling efficiencies in excess of 80% at telecom wavelengths.

The generation of UV light in solid core silica fibres has been achieved using four wave mixing in optical fibre tapers or rare earth doping with Gd3+.

A polarization selective weakly-fused fiber coupler composed of a few mode fiber and a single mode fiber is demonstrated with high efficiency higher than 90%. The coupler profile optimization has allowed to remove polarization and spatial degeneracy in the first and second high order mode.

Optical microfiber waveguides with diameters close to the wavelength of light possess an intriguing combination of properties, such a tight modal confinement, tailorable dispersion, and high nonlinearity, which have been utilized in many passive applications. Here, the key fabrication techniques and optical properties of microfibers are introduced, followed by a discussion of the various passive microfiber devices and sensors. Applications exploiting their strong confinement are reviewed, including harmonic generation, supercontinuum sources, gratings, tips for optical trapping and intracellular sensing and subwavelength light sources, as well as devices based on large evanescent fields such as couplers, interferometers, optical manipulators, sensors, and resonators. Furthermore, the properties and practical intricacies of manufacturing various microfiber resonators are evaluated, with a focus on their applications in sensing ranging from temperature monitoring to current, pressure, refractive index and chemicals detection.

Optical microfiber‐ and nanofiber‐based resonators are extremely flexible devices with the ability to tailor dispersion, increase nonlinearity and phase‐match different modes propagating in them. These properties have been used for a variety of nonlinear effects, including supercontinuum generation, second‐, third‐, and one‐third‐harmonic generation, pulse shaping, and nonlinear switching. In this chapter, we review our work in this area and also briefly discuss future plans and prospects.

Optical microfiber-based temperature sensors have been proposed for many applications in a variety of industrial uses, including biomedical, geological, automotive, and defense applications. This increasing demand for these micrometric devices is attributed to their large dynamic range, high sensitivity, fast-response, compactness and robustness. Additionally, they can perform in-situ measurements remotely and in harsh environments. This paper presents an overview of optical microfibers, with a focus on their applications in temperature sensing. This review broadly divides microfiber-based temperature sensors into two categories: resonant and non-resonant microfiber sensors. While the former includes microfiber loop, knot and coil resonators, the latter comprises sensors based on functionally coated/doped microfibers, microfiber couplers, optical gratings and interferometers. In the conclusions, a summary of reported performances is presented.

We model and demonstrate a simple mode selective all-fiber coupler capable of exciting specific higher order modes in two- and few-mode fibres with high efficiency and purity. The coupler is based on inter-modally phase-matching the propagation constants in each arm of the asymmetric fused coupler, formed by dissimilar fibres. At a specific coupler diameter, the launched fundamental LP(01) mode is coupled into the higher order mode (LP(11), LP(21), LP(02)) in the other arm, over a broadband wave-length range around 1550 nm. Unlike other techniques, the demonstrated coupler is composed of a multimode fiber that is weakly fused with a phase matched conventional single mode telecom fiber (SMF-28). The beating between the supermodes at the coupler waist produces a periodic power transfer between the two arms, and therefore, by monitoring the beating while tapering, it is possible to obtain optimum selection for the desired mode. High coupling efficiencies in excess of 90% for all the higher order modes were recorded over 100 nm spectral range, while insertion losses remain as low as 0.5 dB. Coupling efficiency can be further enhanced by performing slow tapering at high temperature, in order to precisely control the coupler cross-section geometry.

Because of the strong confinement and large refractive index contrast, silica nanowires can efficiently generate light in the UV through intermodal phase matching. Similarly, the fraction of power in the evanescent field allows for many applications in sensing.

We demonstrate an efficient method for DNA Sensing using simple devices, the microfiber coupler, and the Multi port Microcoil Resonator. These devices promise, high sensitivities, up to 200nm/RIU, and low detection limit of 10-6 RIU.

We experimentally demonstrate 2 × 2 and 3 × 3 fused fiber couplers made from dual mode fiber. A unique mode dependent power transfer characteristics as a function of pulling length is obtained that support various optical functionalities. Exploiting this we demonstrate several devices of interest for mode division multiplexed data transmission including LP11 mode filter, LP11 mode tap coupler, and 50:50 power splitter for both LP01 and LP11 modes.

Due to their strong modal confinement, optical microfibres (OMs) provide an excellent opportunity for studying nonlinear effects and in particular intermodally phase matched second (SHG) and third (THG) harmonic generation with predicted efficiencies up to 10-1 over several cm in an ideal uniform silica OM for both cases. Experimentally however, fabricating the required phase matching diameter uniformly over such lengths remains challenging. A simple solution is to reconfigure the OM as a resonator, such that pump recirculation near resonance increases conversion without the need for higher power sources or OM lengths greater than a few mm. Here, we theoretically study surface-SHG and THG in OM loop resonators by numerically solving the underlying coupled mode equations to determine the dependence of the enhancement on the resonance properties.

In this work, it is experimentally demonstrated the enhanced surface SHG using a nonlinear microfiber loop resonator, in which the recirculation of the resonant pump wavelengths enhances the second harmonic conversion and reduces the required pump power.

We design and optimize a simple, low-cost and compact biosensor based upon a microfiber coupler, with a predicted sensitivity and detection limit up to S~104 nm/RIU and DL~10-6 RIU respectively.

Intermodally phase matched up- and down-conversion processes based on the third order nonlinearity can be used to efficiently generate UV and mid-IR wavelength regions in solid core silica optical fibers and optical fiber tapers. We theoretically study the waveguide parameters and practical considerations required for optimum conversion.

We model and demonstrate resonantly enhanced third harmonic generation in microfiber loop resonators, in which the large pump field intensity is exploited to improve the conversion on resonance. Silica microfibers were fabricated with waist diameters near 0.76 μm to ensure intermodal phase matching. When pumped with λ=1.55 μm 4 ns pulses at 100 W peak power, the conversion efficiency is 3×10(-6) over an estimated interaction length of ∼1 mm near the waist. The resonator is then formed by manually translating and twisting the microfiber ends to produce loop diameters down to 6 mm and resonant enhancements up to 7.7 dB for the same pump parameters.

A new type of multi-port optical microfiber resonator is presented. The coupler-based coil resonator was fabricated by coiling a four port microfiber coupler around a low index support rod and then embedding the resulting device in a low refractive index polymer to improve its robustness. In this structure, light propagates along the coil whilst the beating between the supermodes of the coupler still present, giving an increased extinction ratio and an output spectrum strongly dependent on the microfiber coupler diameter.

A new type of self-coupling multi-port microcoil resonator using a microfiber coupler is presented. The microresonators, a simple combination of a microfiber coupler and microcoil resonator, were fabricated by coiling a four port microfiber coupler around a low index support rod to induce optical resonance via coupling between adjacent turns. Light propagates along the coil whilst the beating between the supermodes of the coupler is still present, giving an increased extinction ratio and an output spectrum strongly dependent on the microfiber coupler diameter. The multiport microcoil resonator was embedded in a low refractive index polymer to improve its robustness and the polarization dependence was further analyzed.

A proof-of-principle experimental demonstration with theoretical modeling is presented for resonantly enhanced Faraday rotation in a microcoil current sensor. The recirculation of resonant light within the coil gives rise to an accumulated polarization rotation and thus improved responsivity. According to simulations, microcoil resonators with sharper resonances could offer significantly larger enhancements. This new type of current sensor has the potential to be ultrafast, compact, and low-cost. Refereed/Peer-reviewed

Issues and benefits of using optical microfibers for current sensing are discussed.1. IntroductionCurrent sensors exploiting the Faraday Effect in optical fibers [1] have attracted considerable interest because of their robustness and configurability, thus compatibility with remote sensing. Yet, in conventional optical fibers the minimization of bending losses results in a large sensor size (typically in excess of ~10cm). Moreover, as optical fibers exhibit small responsivities to currents, long lengths of fiber have to be deployed in a single sensor. Since the sensor response time is related to the light time of flight into the sensing device, long lengths of fiber are associated with response times longer than ms. Since 2003, when Tong and co-workers demonstrated that optical microfibers (OMs) can have low propagation losses [2] and therefore can be used to manufactured devices, OMs have been proposed for a wealth of sensing applications [3]. Because of their reduced size, OM coils [4] and coil resonators [5] have found application in current sensing, with mm size sensors being demonstrated. In this paper the use of OMs for current sensing is reviewed, with an analysis of its benefits and potential issues.

We model and experimentally investigate second harmonic generation in silica microfibers and loop resonators, in which the second order nonlinearity arises from the glass-air surface dipole and bulk multipole contributions. In the loop resonator, the recirculation of the pump light on resonance is used to increase the conversion. The effect of the loop parameters, such as coupling and loss, is theoretically studied to determine their influence on the resonance enhancement. Experimentally, microfibers were fabricated with diameters around 0.7 μm to generate the intermodally phase matched second harmonic with an efficiency up to 4.2 × 10−8 when pumped with 5 ns 1.55 μm pulses with a peak power of 90 W. After reconfiguring the microfiber into a 1 mm diameter loop, the efficiency was resonantly enhanced by 5.7 times.

Mid-infrared Raman sources based on spontaneous Raman scattering have been demonstrated in a 15-lm diameter glass-clad crystalline germanium optical fiber and in a germanium-wire. A quantum cascade laser was used as pump at a wavelength of 5.62 lm. Because of their ultra-high optical nonlinearities and extremely broad transparency window, germanium core fibers offer the possibility of fabricating compact and efficient mid-infrared sources.

Optical fibre sensors can be classified in point, quasi-distributed and distributed sensors. This tutorial will review the different types of optical fibre sensors, with a focus on recent developments in the distributed fibre sensors.

Selective modal multiplexing and demultiplexing is demonstrated in few-mode fibers using weakly-fused microfiber couplers.