We demonstrate transient changes in the optical properties, specifically the loss, of antiresonant hollow core fibres (HCFs) due to a combination of the sub-atmospheric gas pressure inside the fibre holes post-fabrication and the subsequent gas induced differential refractive index (GDRI) between the core and cladding elements of the fibre; this is temporarily created while the gas pressures inside the core and cladding elements are evolving after the HCF ends are opened up to surrounding atmospheric pressure. Here we show experimental evidence of this effect in two different HCF designs; for both fibres, the transmitted power initially increases, reaches a maximum, and then reduces to its initial level. We show via gas flow simulations that the timeline of this behaviour is consistent with the gas flow rates into the core and cladding elements of the tubular HCF studied and the subsequent transient differential gas pressure. The experimental results also show (in line with GDRI expectations) that this transmission (loss) change is higher at shorter wavelengths. Our results imply that this transient change in the fibre’s optical properties must be considered for accurate fibre characterisation; this is particularly true for long fibre lengths where the equalisation of the fibre’s internal gas pressure with atmospheric pressure could take many weeks.

Microstructured optical fibres (MOFs), also known as Photonic Crystal Fibres (PCFs) or Holey Fibres (HFs), play an important role in supercontinuum generation because of their high nonlinearity and ability to tailor the dispersion profile. Nowadays, studies have extended to the multimode environment to enhance spectral broadening by taking advantage of various intermodal nonlinear effects. Recently, it has been shown that these types of fibres can also be able to provide spatially cleaned output beams through a novel nonlinear spatial effect called Kerr-induced beam self-cleaning (KBSC), which makes these fibres suitable for a variety of multimode-based applications. In this paper, a full modal analysis of germanium (Ge)-doped graded-index multimode PCFs with different geometrical sizes is provided. The influence of fibre size on modal properties, such as effective refractive index, birefringence and dispersion, was investigated using the finite element method. It was found that the changes in the geometric parameters can significantly affect the modal properties and then the frequency generation and spectral broadening. Experimentally, high-efficiency frequency generation with different frequency detunings was obtained in a short length of each PCF by launching input pulses at 1064 nm pump wavelength. For longer fibre length and high input power, supercontinuum generation with improved broadening from visible to near IR by intermodal four-wave mixing and stimulated Raman scattering was observed in all PCFs. A study of the effect of input peak power, input polarization axis and fibre length on supercontinuum spectrum is presented. Experimental evidence of the KBSC effect as a function of various fibre and laser parameters is also reported. The evolution of spatial output beam pattern from higher-order mode to fundamental mode with low-peak-power self-cleaning threshold was observed for all the fibres by launching a 1064 nm pump laser with 1 ns pulse duration.

Mid-infrared absorption spectroscopy is nowadays considered as a routine analysis and sensing tool providing highly discriminatory information on organic and inorganic molecules. The specific needs of the transport, aerospace or energy industries may involve applications for which the spectroscopic sensor has to withstand high temperatures. This paper reports the design and fabrication of a silica hollow-core anti-resonant fiber with 8 non touching capillaries primarily designed for mid-infrared CO_x sensing inside engines. Numerical and analytical simulations performed to ensure fiber transparency in the 4-5 µm region are reported. Optical transmission between 3.9 µm and 4.7 µm is observed and demonstrate attenuation below 1 dB/m for single capillary ring fibers in this range. A preliminary scheme for CO₂ sensing using a quantum cascade laser operating in one of the fiber’s transmission windows (λ=4.3 µm) is depicted.

Holmium-doped aluminosilicate fibers are frequently used in holmium-doped fiber lasers (HDFL) thanks to their strong emission at 2 μm. Fluorescence lifetime is one of the most important parameters to determine the suitability of holmium doped optical fibers for use in fiber lasers. One of the potential mechanisms for the shortening of fluorescence lifetime is the diffusion of RE ions and Al2O3 at high temperatures during the fiber preparation process. We have prepared a Ho-doped aluminosilicate optical fiber preform using MCVD combined with nanoparticle doping. The prepared preform was subjected to various fabrication processes such as preform elongation, fiber drawing or additional heat treatment, the fluorescence lifetime was measured in all stages of the experiment and its dependency on the fabrication process was discussed. The original preform exhibited a long fluorescence lifetime of 1.433 ms. Gradual application of fiber fabrication processes such as preform elongation or fiber drawing resulted in a decline of fluorescence lifetime down to 1.174 ms in the case of overcladded optical fiber. The decrease of fluorescence lifetime was ascribed to the diffusion of dopants and the changes in the Ho3+ ion environment, which increased the rate of multiphonon relaxation, as well as clustering of holmium ions, which increased concentration quenching.

The advent of 5G mobile communications has amplified the demand for a scalable and sustainable telco networking model. Traditionally, the L-band or long band use to expand the capacity of terrestrial Dense Wavelength Division Multiplexing (DWDM) optical networks. With the acceleration of internet usage and connectivity worldwide, submarine cables carry upwards of 99 percent of global Internet traffic between continental landmasses. On-demand basis, submarine cables reaching close to Shannon’s limit which made the role for the L-band to double the maximum amount of data traffic. This paper deliberates regarding the performance of CzechLight TM (CLA) bi-directional (BiDi) Erbium Doped Fibre Amplifier (EDFA) in L-band specifically in 1595-1610 nm of 100 GHz ITU Grid with a single mode fibre transmission system. The tunable laser in this experiment finds an efficient technique to have an ultra-stable frequency gain over conventional techniques in the optical networks.

We report on the design of a quasi-monolithic fiber chirped pulse amplification (FCPA) system operating at 1040 nm. Chirped-pulse amplification has been achieved using an all-solid step-index PM VLMA Yb-doped fiber, designed and drawn at Photonics Bretagne. The amplifier is integrated to the system through an internally developed mode field adapter (MFA), and bended with a 14 cm diameter, giving a truly single-mode behavior. 540 fs pulses are obtained without any fiber length optimization. An average power of 50 W has been achieved at the fundamental repetition rate of the laser, but also energies up to 41 µJ per pulse at lower repetition rate, pump power limited. The pulses are stretched using a thermally–controlled tunable chirped fibered Bragg grating (CFBG), allowing fine dispersion tuning, and compressed using a chirped volume Bragg grating (CVBG).

An improvement to the Granulated Silica Method (GSM) by using thermally densified “green compacts” is described and implemented in order to produce high alumina content fibres. High alumina fibres are suitable for harsh environments at elevated temperatures due to their mechanical and optical properties. Fibres with up to 70at.% alumina in the core and up to 35at.% in the cladding (to lower the core-cladding index step) have been produced. The cores appear to be transparent for visible light. By EDX analysis diffusion of cladding material (e.g. silica) into the core, but not in the opposite direction from the core to the cladding, have been verified and are confirmed by qualitative index measurements.

e present preliminary characterization of an optical fiber amplifier for wavelengths beyond the L-band based on a core pumped thulium-doped fiber developed and fabricated in-house. We explore adaptation of an existing broadband fiber optic thulium-doped source of amplified spontaneous emission, which generates radiation in a spectral region around 2 micrometer wavelengths, for the purpose of signal amplification. The amplifier is pumped by an erbium-doped fiber laser at 1566 nm in a backward configuration with respect to the pump power. We characterize the amplifier using a narrowband input signal at 1950 nm and 1996 nm. We optimize the amplifier setup by varying the length of the active fiber and pump power. Results show that amplification of <30 dB at 1950 nm with maximum output power ~300 mW is achievable with existing setups.

Optical fibre consisting of a pure silicon core in silica cladding combines the advantageous properties of silicon waveguides with the convenience of optical fibre. However, the optical quality of these fibres is highly dependent on the crystalline structure and the purity of the silicon. The fabrication of these fibres requires engineering of the thermal gradients during the drawing process to ensure optimal crystallisation of the silicon. Here, we investigated the effects of draw speed and analyse the induced stresses at multiple stages in the fabrication process. The thermal exposure of the silicon while in contact with a silica cladding was found to increase the optical losses. This was attributed to the diffusion of impurities from the silica cladding into the silicon core.

In this work, we demonstrate optomechanical measurements of radiation induced alterations of the acoustic velocity in a fluoroacrylate polymer coating of a silica optical fiber. The optomechanical measurement is based on forward Brillouin scattering initiated in the fiber core which stimulates acoustic waves that reach the fiber coating. The measurement may serve as an additional metric to quantify the dose of ionizing radiation to which the fiber was exposed. We have demonstrated that the stiffness of the coating increases following gamma irradiation, as measured by the time of flight of radial acoustic waves through the coating. The measurement was performed on few meters long fiber, but can be extended to a spatially distributed analysis in longer fibers. The tests showed a linear dependence of the acoustic time-of flight on the overall dosage of gamma irradiation. The time of flight decreased by as much as 15% following exposure to 180 Mrad from a 60Co source. In a follow-up study, we found that the stiffness of the previously exposed fiber coatings continued to slowly increase over months, after extraction from the radiation field. These results reveal the vulnerability of the specific coating to ionizing radiation and the potential complexities involved with dosimetry.

Stress is a normal physiological and behavioral response to a stimulus that somehow disturbs the maintenance of homeostasis, leading to changes in cortisol levels. When stress is persistent and uncontrolled, it can severely affect several areas, such as human health and some marine biology sectors, including aquaculture production. Currently, the detection of cortisol is performed in laboratories using conventional techniques with several disadvantages, one of them being the long waiting time for a response. Therefore, it is essential the development of miniaturized analytical devices capable of monitoring in real-time, detecting and quantifying cortisol in point of care (POC). Special optical fiber structure, in this case, D-shape in silica optical fibers (SOF) and polymeric optical fibers (POF) coated with gold (Au) were used in this work for the development of immunosensors based on surface plasmon resonance (SPR) for cortisol (stress hormone) detection. In the laboratory, Au coated SOF (Au-SOF) and Au coated POF (Au-POF) were initially characterized at refractive index (RI) with eight glucose concentrations ranging from 1.333 to 1.386 RI units (RIU). The obtained sensitivities were, respectively, 1646.67 ± 91.66 nm/RIU, being lower than the simulated one with 2138.95 ± 142.65 nm/RIU, and 1566.81 ± 96.87 nm/RIU. Subsequently, the fibers were functionalized with anti-cortisol antibodies using cysteamine as the intermediate linker to allow the immobilization of the antibodies to the Au surface. After this procedure, both immunosensors were tested for cortisol concentrations ranging from 0.1 to 100 ng/ml to compare the performances, in which the Au-SOF and Au-POF immunosensors presented a total resonance wavelength shift of 3.22 and 2.10 nm, and sensitivities of 1.08 ± 0.21 nm/log(ng/mL) and 0.52 ± 0.03 nm/log(ng/mL), respectively. Different limits of detection (LODs) were calculated using different methods for each type of immunosensor. One method consisted in performing a Hill fitting to the results and another considering the response of the control interferents. For Au-SOF and Au-POF, the LODs attained through the first method were 0.12 and 0.13 ng/mL (considering the resonance wavelength shift), respectively, and 0.14 ng/mL considering the intensity variation of Au-POF. The second method enabled a LOD of 1.75 and 1.97 ng/mL for Au-SOF and Au-POF, respectively.

This work presents the development of plastic scintillator (BCF-10) based optical fibre sensors for medical radiotherapy dosimetry. Two different designs of BCF10 joined to PMMA (Polymethyl methacrylate) fibre were considered, based on simple Plug and Play designs for the rapid and effective assembly of radiation sensors. The first design was a simple butt-coupling arrangement sheathed in tubing, with an outer diameter of <2 mm. The second design explored the coupling joint of a cylindrical protrusion and hollow part of BCF10-PMMA that were achieved using femtosecond laser machining; the purpose of which was to maintain the original 1-mm fibre diameter for the sensor probe. The two fibres were pressed together and sealed with UV curing, hence the reference to a Plug and Play architecture. Both sensors exhibit higher output counts at the higher dose rate (due to the higher number of radiation pulses), although a discernible signal is observed at 50 MU/min for 6 MV, 15 MV energies and both sensors. When comparing both sensors with the different joint coupling designs, the flat surface connection of BCF-10 to PMMA demonstrates slightly higher photon counts compared with the micro-machined sensor (Plug n Play). However, the difference is small and the Plug n Play sensor benefits from the smaller sensor diameter (1 mm diameter), which is suitable for inserting into a small applicator or in-vivo monitoring. In the second section, micro-pulses of X-Ray radiation from Siemens Linear Accelerator (linac) were obtained and compared for two different energies and dose rates. Both of the sensors demonstrate the feasibility to be used for characterisation of X-ray pulses from a clinical linac.

We present the design of an optical fibre tip sensor for monitoring dissolved oxygen. It is based on the creation and modification of an all-fibre Fabry-Pérot interferometer, produced in a three-step process. First, fibre cavities are created in two fibres using a femtosecond laser that are then joined using arc fusion splicing, thereby forming a miniature fibre Fabry- Pérot interferometer. Finally, the femtosecond laser is used to create two symmetrical micro holes exactly at the location of the Fabry-Pérot cavity and on opposite sides of the fibre, allowing liquids to access the cavity and perform refractive index sensing. For the dissolved oxygen sensing, the fibre sensor was first immersed in a hemoglobin solution to deposit a thin layer at the walls of the cavity to give oxygen discrimination. The sensitivity to the ratio of dissolved carbon dioxide to dissolved oxygen was demonstrated via the conversion of carboxyhemoglobin to oxyhemoglobin inside the cavity, thus changing the refractive index of the hemoglobin. The complete sensor was tested in two solutions with different oxygen concentrations and having the same refractive index and temperature. The first sample was a PBS solution dissolved in deionized water, which absorbs CO₂ from the air after it is produced. The second sample was a PBS solution with sodium bicarbonate to ensure that all solution-state hemoglobin was converted to oxyhemoglobin, by converting the dissolved carbon dioxide into carbonate groups. We observed the difference between the two solutions that have the same refractive index and different oxygen levels by the means of a wavelength shift and amplitude change in the Fabry-Pérot interferometer spectrum due to the oxygenation of the hemoglobin.

It is known that graded-index fibers (GIF) provide efficient Raman conversion of multimode radiation into a Stokes beam with improved beam quality known as Raman beam cleanup effect, whereas in step-index fibers (SIF) this effect is weak. Here we study propagation of low-coherent (~5 nm wide) highly-multimode (M2~30) pump radiation of high power 940-nm laser diodes (LDs) in GIF and SIF of ~100 µm core diameter. We measured pump beam shapes at the output of fibers with different length. It has been shown that the output beam shape is almost independent of the input pump beam shape approaching to rectangular or parabolic profile after propagation in only ten meters of SIF or GIF, respectively. The output beam shape mimics the refractive-index profile in the multimode fiber core due to random mode coupling. This means that Raman gain is nearly the same for all transverse modes in SIF and has maximum gain for the fundamental mode in GIF that is the main reason of strong Raman beam cleanup effect in GIF. We tested Raman lasing in 1-km GIF/SIF with 940-nm LD pumping of up to ~200 W power with a cavity formed by fiber Bragg gratings (FBGs). The output FBG fs-inscribed in GIF/SIF improves the output beam quality due to spatial filtering property, but in GIF the effect is much stronger.

In this paper, we present the results of numerical simulations of Octagonal Photonic Crystal Fiber (O-PCF). The fiber has a ring core made up of Germanium doped silica and cladding is made up of silica glass with aire-holes arranged in octagonal lattice. The O-PCF is capable of propagating 6 OAM modes with highest mode quality-OAM purity equal to 92% at 1.55µm. Other parameters investigated are OAM purity, Neff, Effective Area, Dispersion, Confinement loss and non-linearity. Finite Element Method (FEM) is used for analysis of O-PCF characteristics using COMSOL Multiphysics. High mode purity, flat dispersion and low confinement loss makes the proposed O-PCF a potential candidate for telecom applications such as high-speed, high-capacity, transmission, Space Division Multiplexing (SDM), 6G applications as well as for non-telecom applications such as Supercontinuum Generation (SCG).

Spatially and spectrally resolved imaging, the so-called S2 imaging technique, is a method to determine the modal content of multimode fibers, the power carried by each mode, and the Multi-Path Interference (MPI) level. However, a drawback of this approach is the need for a highly stable and precisely tunable laser source, since Discrete Fourier Transformation (DFT) is used to extract time delays and reconstruct the powers of mode beatings. This work demonstrates both theoretically and experimentally that a “DFT-free S2 method” can identify and quantify the modal content of a fiber probed under different conditions by simple spatial correlation analysis of images recorded at different wavelengths.

The solid core Photonic Crystal Fiber (PCF) with circular air holes arranged in triangular lattice and in hexagonal pattern is analyzed with variation in number of air hole rings array around the core region. In this fiber, the modified internal reflection mechanism of optical wave guiding has been explored. The core of higher index inhibits the passage of the light in the cladding, thereby resulting in light guidance. To validate the guiding phenomenon of this fiber, Finite Element Method (FEM) computation is applied. This method involves the sub-division of the geometrical area of interest into small composite elements. The refractive index of the cladding region is considered by using Sellmeier Equation to incorporate the dispersion effect of the fiber. The light is launched in the core region and the Electric-field intensity is confined at the center of solid core PCF. The mode confinement loss of this PCF decreases with increase in air hole ring layer. Also, the propagation loss is less for lower order wavelength in conventional optical wavelength band. Therefore this design of solid core PCF is suitable for sensing with lower number of air hole ring array, and suitable for communication with higher number of air hole ring array.

Cyclic transparent optical polymer (CYTOP) based fibre Bragg grating (FBG) sensors are of high interest recently due to their lower optical loss compared with the sensors fabricated in other polymeric materials, such as poly(methyl methacrylate). Numerous scientific reports have shown that polymer based FBGs are usually preferred over their silica counterparts due to their enhanced sensitivity to stress and pressure, and their affinity to humidity. Temperature monitoring with polymer FBGs is also extensively demonstrated, but with inconsistent results and non-linear responses, since most of the polymer optical fibres have a negative thermo-optic coefficient and positive thermal expansion coefficient that work to cancel out each other to some extent, resulting in mixed temperature sensitivities. In this work, an optical fibre with a CYTOP core and a Xylex cladding is used to investigate fibre pre-strain effects on the temperature sensitivity of FBG sensors. The sensors were placed in an environmental chamber with controlled temperature and relative humidity, and their response to temperature was evaluated under various fibre pre-strain values. Without any applied fibre strain, the thermal expansion coefficient slightly prevails over the thermo-optic effect, as a result the Bragg wavelength shifts in longer wavelengths. Under sufficient fibre strain, the thermal expansion coefficient is eliminated, and the temperature sensitivity is greatly enhanced, shifting the Bragg wavelength to shorter wavelengths. This paper demonstrates the possibility to have an array of Bragg grating sensors, some being temperature insensitive and some highly temperature sensitive along the same fibre.

Optical fibres have played an important role in the advancement of real-time dosimetry in clinical applications in recent years. Significant work has been done to increase precision and accuracy in detecting radiation doses during treatment, to avoid the negative effect that can ensue from irradiating healthy tissue around the tumour. The drive to develop distributed measurement in optical fibres has been limited to the slow scanning speed systems from optical time domain reflectometry (OTDR), however for radiotherapy dosimetry, with often short radiation pulse durations, fibre Bragg grating (FBG) interrogation is a better alternative because of the fast-scanning speed. The work presented here includes the preliminary results in the characterisation of CYTOP FBGs on exposure to X-ray radiation emitted from a clinical linear accelerator (linac) machine. A blue shifted linear response of the Bragg wavelength with sensitivity of 6.655 pm/Gy, 6.519 pm/Gy and 7.153 pm/Gy at the three main peaks (1522 nm, 1542 and 1561 nm), was recorded for a 9 Gy of radiation at a dose rate of 1.758 Gy /min with an amplitude fluctuation within the duration of radiation. The response demonstrates the potential for its use in low dose radiation dosimetry, providing for quasi-distributed sensing in radiotherapy.

Heart failure (HF) is a serious and debilitating cardiovascular disease responsible for high mortality and morbidity rates worldwide, as well as expensive healthcare costs. In HF, there is an increase in circulating concentrations of cardiac biomarkers such as N-terminal B-type natriuretic peptide (NT-proBNP), which can be used for its diagnosis and prognosis. Biosensors have emerged as an appealing alternative to conventional methods of detection, being capable of providing a quick response and allowing detection at point of care. Tilted fiber Bragg gratings (TFBGs) present a high sensitivity to refractive index (RI) variations occurring in the surrounding medium and hence were employed in this work to develop a biosensor for NT-proBNP detection. Two immunosensors were exploited based on a bare TFBG and a gold coated TFBG (Au-TFBG), and, in both cases, distinct spectral demodulation methods were studied to evaluate the sensing performance. The NT-proBNP detection was carried out using the immunosensors properly biofunctionalized, in the 0.01-1000 ng/mL concentration range. The best performance results were obtained by computing the lower envelope of the spectra and following its variations for both bare and Au-coated TFBGs, with the latter exhibiting a lower limit of detection of 0.19 ng/mL. Moreover, a control test was done for each immunosensor to test the specificity. The findings reached in the present work highlight the great diversity of spectral demodulation methods and their impact on the sensing performance, having to be considered upon comparison between TFBG-based biosensors.

The number of eigenmodes in an optical fibre depends on the wavelength of the excitation laser beam as well as on the exact geometry and refractive index profile of the fibre. The latter is often proprietary information and, when available, is only specified to within manufacturing tolerances. We here present a method for obtaining the number of fibre modes, as well as their shape, which requires no knowledge about the fibre, save a very approximate core radius. The method is based on the singular value decomposition (SVD) of a set of speckle patterns, measured at the output end of the fiber, which is then expanded onto a set of orthonormal basis functions. We present two possible approaches for the field expansion, where the first approach uses a generic orthonormal basis, such as Laguerre–Gaussian or Zernike functions, and the second one is a basis-free approach where the set of speckled patterns themselves form the basis. Using a set of simulated speckles patterns, we observed that the correct number of fibre modes can be obtained through the SVD decomposition, even at high levels of additive random noise. With a slight extension, using speckle patterns obtained at multiple excitation wavelengths (or equivalently, for different lengths of the same fiber) the method can also retrieve the shape of the actual fibre modes, by forming an appropriate linear combination of SVD modes.