This PDF file contains the front matter associated with SPIE Proceedings Volume 11773, including the Title Page, Copyright information, and Table of Contents.

A short piece of specialty optical fiber acts as the backbone of various fiber-based devices such as high-power laser, amplifier, sensor, etc. suitable for various applications like communication, medical diagnosis, industrial as well as advance basic research. The performance of this specialty optical fiber will depend on the selection of materials, fabrication process technology used, and suitable optimization of various process steps. Accordingly the fabrication of a good quality silica-based specialty fiber doped with suitable dopants in a reliable and repeatable manner is a key challenge from a technological viewpoint and will be briefly discussed.

Current trends in optical fibers will be reviewed such as fibrs with suppressed stimulated Brillouin scattering, silica hollow core optical fibers with extended spectral transmission range, transmission of giant pulses over hollow core fiber lasers, and hollow core fiber gas lasers.

Given their EM immunity, intrinsic safety, small size and weight, autoclave compatibility and capability to perform multi-point and multi-parameter sensing remotely, optical fibers and fiberoptic-based sensors are seeing increased acceptance and new uses for a variety of bio-medical applications—from laser delivery systems, to disposable blood gas sensors, to intra-aortic pressure probes, to digital X-rays to name a few. This tutorial will provide a broad overview on how optical fibers and fiber-based sensors are being utilized in the biomedical arena, highlighting their intrinsic characteristics, advantages and requirements. Key industry applications, challenges and trends will be discussed, along with their future prospect and overall commercial outlook.

We will present our recent work using noble and Raman-active gas-filled anti-resonant hollow-core fiber (ARHCF) technology. First, we will present the generation of supercontinuum spanning from 200 nm to 4000 nm based on nonlinear effects of soliton self-compression and phase-matched deep‑ultraviolet (DUV) dispersive wave (DW) emission in Argon (Ar)-filled ARHCFs pumped at 2.46 μm wavelength with 100 fs pulses and ~8μJ pulse energy. Then we will discuss our recent work on stimulated Raman scattering (SRS) effect in a hydrogen (H2)-filled ARHCF, to achieve near- and MIR Raman lasers. By employing the single-stage vibrational SRS effect, a 4.22 μm Raman laser line is directly converted from a linearly polarized 1.53 μm pump laser. A quantum efficiency as high as 74% was achieved, to yield 17.6 µJ pulse energy. The designed 4.22 μm wavelength is well overlapped with the strongest CO2 absorption, therefore constituting a promising way for CO2 detection. In addition, we report a multi-wavelength Raman laser based on the cascaded rotational SRS effect. Four Raman lines at 1683 nm, 1868 nm, 2100 nm, and 2400 nm are generated, with pulse energies as high as 18.25 µJ, 14.4 µJ, 14.1 µJ, and 8.2 µJ, respectively. The energy of these Raman lines can be controlled by tuning the H2 pressure from 1 bar to 20 bar.

High-speed tracking of nano-objects is a gateway to understanding biological processes at the nanoscale. Here we will present our results on tracking single or ensembles of nano-objects inside optofluidic fibers via elastic light scattering. The nano-objects diffuse inside a channel of a microstructured fiber and the light scattered by the nano-object is detected transversely via a microscope. We will present the fundamentals of this approach and focus on selected results including retrieval of the full 3D trajectory of a diffusing nano-sphere, the simultaneous detection of hundreds of nano-objects in hollow core anti-resonant fibers and first results on inactivated SARS-CoV-2.

One of the main challenges of laser-based gas sensingis the development of sensors delivering simultaneously high sensitivity, selectivity, fast-response time and non-complex design. Mostly, the detection capability of such sensors depends on the interaction path length between the laser light and the measured gas. Hence, long optical paths are highly desired for e.g. low-concentration gas sensing. Our proposal is to use Antiresonant Hollow-Core Fibers (ARHCFs), which filled with the target gas mixture form absorption cells with potentially any length, delivering low-volume, long and versatile optical paths within the sensor configuration. Currently, the ARHCF core is filled with the target gas via specially designed bulk-optics-based cells placed at the fiber’s ends. This solution provides relatively fast fiber core filling time, however being only efficient while an overpressure is used to force the gas flow through the core, not the diffusion. Therefore, searching for alternative ways of fiber filling with the target gas is necessary. We propose a method of processing the fiber structure using a femtosecond laser allowing for non-invasive accessing the fiber core for more efficient and faster gas diffusion into it through the fabricated microchannels. The fiber structure modification was optimized in a way that does not introduce any unwanted damage of the fiber e.g. cracks on the glass parts or cladding capillaries. The performed experiments have indicated that the laser-processing of the ARHCF structure introduces negligible transmission loss regardless of the number of fabricated microchannels and their length (0.2dB loss for 25 microchannels), confirming the proposed method suitability.

Coupling of light into optical fibers is important for many applications, while for commonly used step-index optical fibers it massively drops for oblique incident angles <15 degrees, limiting their operational range to a narrow angle interval. In this work, we address this issue via inclusion of dielectric concentric ring-type nanostructures located in the core region of commercially available step-index fibers. Modification of fiber facet with the optimized ring-like nanostructure leads to polarization- and azimuthally-independent enhancement of in-coupling efficiency across the entire angle interval from 15 to 85 degrees. We develop the analytical model and show the percent-level of light in-coupling efficiency even at angles as large as 70 degrees, addressing a domain that is out-of-reach for fibers with unstructured end faces. The main result of this work is the enhancement of the in-coupling efficiency at large incident angles (<30 degrees) by several orders of magnitude with respect to a bare fiber. The results obtained are promising for any application that demands to remotely collect light under large angles, such as in-vivo spectroscopy, biosensing or quantum technology.

The polymer-salt method was applied to synthesize nanoscale Gd₂O₃:Nd³⁺ phosphors in the form of thin films on the inner surfaces of capillaries which organize the structure of a silica hollow-core anti-resonant optical fiber. To obtain luminescing centers, the preform of a hollow-core anti-resonant optical fiber was impregnated with a homogeneous mixture of Gd(NO₃)₃ and NdCl₃ dissolved in water and organic solvent (polyvinylpyrrolidone). This procedure was followed by a few post-processing steps, including drying of the impregnated preform in normal conditions and its thermal treatment at temperature 1000 °C. As a result, Gd₂O₃:Nd³⁺-based thin films were produced inside the capillaries. Finally, the modified preform was drawn into the hollow-core anti-resonant optical fiber of 120 μm in diameter at temperature 1850 °C. The analysis of crystallographic structure of the initial Gd₂O₃:Nd³⁺ nanopowder and the same nanophosphor inside the fabricated fiber revealed the absence of structural and phase transformations of synthesized nanocrystals with an average size 35 nm after drawing. The data on spectral-luminescent properties of the fabricated fiber confirmed the presence of Gd2O3:Nd3+ nanophosphors in silica glass with the main emission peak at wavelength 1064 nm. Presented method of modifying the structure of a hollow-core anti-resonant optical fiber allows formation of active silica layers without using technologically complicated and expensive CVD processes.

Hollow core optical fibres have many unique properties, especially compared to traditional glass-core optical fibres [1]. Firstly, the light path is accessible and light can thus interact with the gas inside over long lengths, making them interesting for applications in gas sensing or for nonlinear processes in gasses. Hollow core fibres can also operate at wavelengths, where silica glass has poor transmission and their chromatic dispersion is not compromised by the chromatic dispersion of bulk glass. Yet another unique feature is weak interaction of light with the guiding medium (typically air), significantly increasing the damage threshold and thus making them a good candidate for high-power (average or peak power) light delivery. Another group of unique features is related to how their properties (little) change with temperature. In the presentation, we will firstly show where the common fibre optics wisdom (gained from work with standard optical fibres) tends to fail. In the second part, we will discuss how differently hollow core fibre change with temperature as compared to standard optical fibres and how it can be used for various applications, including fibre interferometry and time-stable signal transmission.

Anti-resonant hollow-core silica fiber is used to demonstrate near- and mid-infrared laser absorption spectroscopy of methane. Molecular transitions near 6057 cm^-1 (~1651 nm) and at 3057.7 cm^-1 (~3270 nm) are targeted. Distributed feedback laser diode and interband cascade lasers are used as tunable laser sources. Detection of methane in ambient air is demonstrated using this mid-infrared system.

In the paper we discuss current state of the art in the field of tapered fiber development. The best results in term of high peak and high average power achieved with this type of fibers are presented together with requirements to the tapered fiber amplifier design. The report is mainly focused on tapered fiber amplifiers operated near 1 μm (Yb-doped tapered fibers), but also extension of this technique to 1.55 μm spectral range is discussed.

In this paper we systematically study a limitation for maximum of Yb dopant concentration in silica based fibers. Two the most popular glass matrixes (F-Al₂O₃-SiO₂ and Al₂O₃-P₂O₅-SiO₂) were thoroughly investigated in this respect. A possibility to introduce ultra-high doping level of Yb₂O₃ (in excess of 2.5 mol%) with a relatively low optical losses in fibers was demonstrated. At the same time it was investigated that at ultra-high Yb concentration in the fiber core even with maintaining very low background losses (both initial and induced by photodarkening) such a fiber can nearly completely lose its active properties. Optimal glass matrixes and optimal concentration of Yb dopant in the glass core, which allow keep up lasing properties of the developed fibers high, were studied. An ultra-short length fiber amplifier (~3.5 cm) based on the developed Al2O3-P2O5-SiO2 glass core fiber doped with 1.2 mol% of Yb₂O₃ (Yb ions absorption was about 1000 dB/m at 920 nm) was created.

We report on the demonstration and characterization of Raman laser generating at the wavelength of ~1090 nm with total output power of up to 5 W based on the 7-core passive fiber with coupled cores. The Raman gain in all cores is provided by the pump laser connected to the FBG-free central core, whereas the laser cavity is formed by two sets of highly-reflective fiber Bragg gratings (FBGs) inscribed by fs pulses in all peripherical cores at the both ends of the 7-core fiber. The output FBG set has got a random shift along the axis between individual FBGs thus forming a random array of FBGs. Along with the Stokes line narrowing reasoned by the reduction of spectral broadening via nonlinear effects due to the enlargement of effective mode area in the multicore fiber with coupled cores in comparison with a standard singlemode fiber Raman laser, the additional line narrowing effect induced by the multicore random FBG array has been also revealed. It results in the generation of single peak of <30 pm linewidth near the threshold, whereas the linewidth broadens to ~250 pm at maximum power. At that, the single peak generation at low powers is not stable in time converting at some moments to multiple 20- pm peaks with random spacing and amplitudes defined by the interference of beams reflected from individual output FBGs with random longitudinal shifts. The ways to stabilize the generated spectrum are discussed.

Thulium-doped fiber lasers (TDFL) are currently in focus of intense research worldwide with a great application potential in a spectral region around 2 μm. Their broad utilization includes among others medicine, defense or material processing. TDFL are in foreground of the interest especially thanks to a thulium energy level structure which enables a so-called two-for-one cross-relaxation (CR) process. This CR process presents a way to generate two photons at 2 μm from one pump photon at around 790 nm, and thus it allows to efficiently generate emission at 2 μm from available high brightness laser diodes emitting around 790 nm. Although the CR process is very promising and high-power fiber lasers based on it have already been presented, there are still reserves in its practical exploitation. In order to push the practical limits, reliable theoretical models are necessary. Among all parameters needed for the modelling, those describing energy transfers (ET) between thulium levels pose the main uncertainty. In this contribution, we present a method of energy transfer coefficients evaluation using rate equation modelling. This approach was based on a set of rate equations relating populations of energy levels with spectroscopic data. The coefficients were derived from fluorescence measurements by fitting fluorescence decay curves with theoretical equations. Studied fibers were pumped at two wavelengths – 793 nm and 1620 nm. Fluorescence curves were collected at 800 nm and 2 μm. All combinations of pumping and fluorescence measurements were examined for various pump power in a range up to 70 mW. Calculated energy transfer coefficients will be used in theoretical investigations and optimization of thulium-doped silica-based fiber lasers.

Active gain fiber is a critical component to realize fiber laser sources for various applications such as cutting, welding and other material processing. To meet the requirement for various laser applications, the power scaling of fiber lasers is still concern, where active gain fiber is very important. There are several fabrication technologies for active fibers with low NA, large core diameter and high absorption, which characteristics for high power lasers are typically required. Here, novel fabrication technology based on VAD (Vapor Axial Deposition) for active fibers is introduced.

We have investigated how the physical dimensions of a fiber Bragg grating (FBG) may affect the performance of a monolithic fiber laser system. In particular, we perform a series of direct laser inscriptions in a Holmium-doped single mode fiber using the plane-by-plane femtosecond laser inscription method, using exactly the same inscription conditions but having different grating widths. Specifically, the gratings were divided in to three groups; group 1, gratings inscribed with planes having widths smaller of the core size but in the center of the fiber; group 2, gratings inscribed with plane widths similar to the core diameter and group 3, gratings with plane widths larger the core diameter that were extended uniformly in to cladding region. All the gratings were characterized in a fiber laser configuration and their performance were analyzed using as metrics the threshold power, the effective length and the power slope efficiency. We note that all the gratings were designed to have a resonance Bragg wavelength at 2.1 μm, having the same length and inscribed directly through the fiber coating. The monolithic fiber laser strands were pumped using Thulium-doped fiber laser operating at 1.95 μm. The results clearly show that the spatial dimensions of the FBGs are certainly an important parameter that is required for consideration during the development of the monolithic fiber lasers, especially for medium and high-power fiber laser systems.

The spectral region between 1600 and 1800 nm is still a band where optical amplifiers struggle to achieve satisfactory gain and output power levels. The output power typically does not exceed few tens of milliwatts, what severely limits some applications. In this paper, the bismuth-doped fiber amplifier (BDFA) operating beyond 1600 nm is presented. We demonstrate the in-house developed BDFA capable of providing output powers that exceed 200 mW for wavelengths near 1700 nm. The performance of the amplifier is discussed and various properties of the device are presented such as gain characteristics and noise figure.

In recent years, there has been a growing interest towards compact high peak-power pulsed laser sources for applications such as LIDAR, range findings, remote sensing, communications and material processing. A common laser architecture used to realize these sources is the Master Oscillator Power Amplifier (MOPA), in which a master oscillator produces a highly coherent beam and a fiber amplifier boosts the output power, while preserving its main spectral properties. Phosphate glasses are recognized to be an ideal host material for engineering the amplification stage of a pulsed MOPA since they enable extremely high doping levels of rare-earth ions to be incorporated in the glass matrix without clustering, thus allowing the fabrication of compact active devices with high gain per unit length. With the aim of realizing compact optical fiber amplifiers operating at 1 and 1.5 µm, a series of highly Yb3+- and Yb3+/Er3+-doped custom phosphate glass compositions were designed and fabricated to be used as active materials for the core of the amplifiers. Suitable cladding glass compositions were explored and final core/cladding glass pairs were selected to realize single-mode and multi-mode optical fibers. Core and cladding glasses were synthesized by melt-quenching technique. The core glass was then cast into a cylindrical mold to form a rod, while the cladding glass was shaped into a tube by rotational casting method or extrusion technique. The latter has been extensively employed for the manufacturing of tellurite and germanate glass preforms, but only recently the first example of active phosphate fiber preform fabricated by this method has been reported by our research team. Phosphate fibers were then manufactured by preform drawing, with the preform being obtained by the rod-in-tube technique. Preliminary results of pulsed optical amplification at 1 and 1.5 µm are presented for a single-stage MOPA.

In this contribution experimental investigation of acousto-optically Q-switched erbium-doped fluorozirconate fiber laser is presented. Under the repetition rate of 1 kHz laser produces pulses with the shortest duration of 20 ns and the maximum pulse energy of 180 μJ, corresponding to a maximum peak power of 9 kW.

We consider a comparative study of radiation effects (γ and electron) on fibre Bragg gratings (FBGs) that were inscribed using a femtosecond laser in single mode silica optical fibre. The FBGs were inscribed using the point-by-point and the plane-by-plane inscription methods. The FBGs were exposed to a total accumulated radiation dose of 15 kGy in both γ and electron cases. The gratings’ spectra were measured and analysed before and after the exposure to the radiation, while complementary characterisation was undertaken using Raman and Fourier transform infrared spectroscopy. In addition, the changes of the temperature coefficient of the FBGs were analysed comparatively prior to the irradiation to explain how material changes responded to the particular types of radiation. Finally, we consider which of the two inscription methods proves more robust in such harsh environments.

We present simple and robust designs for optical fiber radiation sensors for dosimetry applications, focusing on improved light gathering efficiency. More specifically, we examined the implementation of compound parabolic concentrators (CPC) using scintillator-based optical fibers. The fabrication of the parabolic concentrator is achieved by femtosecond micromachining at the end face of the polymer fiber. Furthermore, we consider the luminous properties of Gadolinium Oxysulfide (GADOX), an inorganic compound usually used in ceramic scintillators, as an alternative and combine it with laser-shaped polymer optical fibers (POF). The simplicity and ease of implementation of the sensor designs offers the prospect of distributed sensors; adding a wavelength shifting element is discussed to make the sensor more adaptable depending on the selected interrogation system.

We report on a novel bend sensor with high flexibility and elasticity based on Bragg grating structures in polymer optical fibres. The concept is very simple and relies on the inscription of eccentrical Bragg gratings into multimode graded-index polymer optical fibres via contact exposure with a krypton fluoride excimer laser in the ultraviolet region and an optimized phase mask. Depending on the polymer optical fibre deformation, the lattice constant of the inscribed Bragg grating is strained or compressed due to its position relative to the fibre core. This in turn results in a shift of the Bragg wavelength of up to 1.3 nm to the red or blue wavelength region, respectively. Therefore, deformation along one axis can be observed with a single Bragg grating with a sensitivity of 50 pm/m⁻¹. Moreover, multiple Bragg gratings inscribed into the same polymer optical fibre at different positions would allow to determine the shape deformation of the fibre relative to a reference frame. Consequently, this technology could form the basis for new applications in the areas of medical diagnostics, robotics or augmented reality in the future.

Two slightly shifted wavelength gratings are inscribed one over the other in a single mode fiber by shifting the phasemask between two positions. The inscription setup includes a NIR femtosecond laser, a phase-mask, a defocusing spherical lens and a cylindrical focusing lens. A first fiber Bragg grating (FBG) is inscribed, while the second overlapping FBG is inscribed only after a slight shift of the phase-mask, enabling a slight wavelength shift. The transmission spectrum of the complex structure is like that of a phase shifted grating, while the inscription process is very fast and simple compared to other standard methods. A high-quality phase shifted grating with two −20 dB transmission dips, a 15 dB transmission peak with a 30 pm transmission bandwidth at 3 dB is achieved. It is also observed that this phase shifted grating structure is birefringent.

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.

In order to test and compare the suitability of various radiation hardened optical fibers, we have established a test site in the 5MW research grade open pool reactor located at Soreq Nuclear Research Center, Israel. Several commercial fiber samples were coiled inside a specially designed apparatus which was lowered into the reactor core. The input and output legs of each fiber coil were prepared sufficiently long to extend outside the reactor pool, and were attached to a source (input) and detector (output). The transmission/attenuation could then be measured during, and in between, operations of the reactor. Since the reactor does not operate continuously, we were able to monitor in real time both transmission changes under very high radiation rates (approx. 0.5 MGy/hr) and doses (about 20 MGy) of gamma rays and neutrons, as well as recovery processes after each reactor shut down. Results are presented comparing the RIA and recovery kinetics of different commercial radiation hardened fibers under identical exposure/relaxation cycles. This study also examined effects on fibers with different coating materials and a fiber with inscribed Bragg gratings.

In this work, we demonstrated the possibility realize a continuous measurement of liquid level based on light diffusing fibers (LDFs). The sensor consists of two parallel LDFs coupled together. The illuminating fiber is connected to a laser diode. The light scattered into the liquid is then coupled, always by the scattering, to the detection fiber and delivered to a detector. By setting a working wavelength that is strongly absorbed by the liquid, the power coupled between the fibers depends on liquid level. The sensor is made by a polymeric beam of Polyvinyl chloride (PVC), on which two LDF (3M Fibrance) with a diffusion length of 1m have been glued side by side at a distance of 1.5mm. The fiber has a core diameter of 170 μm, a low-index polymeric cladding with a diameter of 230 μm, and loose tube PVC jacket with an outer diameter of 900 μm. As light source a 1550nm fiber coupled laser diode is used. At this wavelength, water, employed as the test liquid, exhibits a strong absorption (=1210 m-1). A high sensitivity photodetector connected to a data acquisition module (DAQ) is used for measuring the detection fiber output power at different liquid levels The optical coupling phenomena between the fibers could be modelled by coupled power equations. Co-propagation and counter-propagation coupling configurations have been analyzed and experimentally validated. The measurements results are in good agreement with the theory, and demonstrate that both configurations could be used for liquid level sensing. The counter-propagation configuration exhibits a nonlinear response as function of the liquid level, while the co-propagation coupling configuration response is linear simplifying the calibration procedure. In the co-propagation configuration, the resolution ranges from ±8mm at low liquid level up to ±2mm at high liquid level over a 1m length measurement range.

Fibre based transmission of ultra-precise time and ultra-stable optical frequency is quickly becoming common reality, not only in the fibres used for research but also in other operational fibre networks. At the moment, a fibre represents the best precision of such transmissions. In order to achieve highest possible transmission stability up to 10-18for 1000 s averaging, the bidirectional transmission within single fibre is required including exclusive optical amplification. We present here the use and results of Optical Time Domain Reflectometry (OTDR) technique for detection of disturbances as connector losses, reflections, bending etc. on live fibres with present Amplified Spontaneous Emission (ASE) from bidirectional Erbium Doped Fibre Amplifiers (EDFAs).

In this work, we present the design and fabrication of a fiber device that performs digital droplet microfluidics for molecular diagnostics. A variety of fibers and capillaries were used to build three connected modules dedicated to droplet generation, incubation, and fluorescence detection which enables a uniaxial arrangement. This is in contrast to the traditional 2-dimensional lab-on-a-chip architecture. We characterize our fiber device using a fluorescein dilution series. Our observed detection limit is on the order of 10 nM fluorescein. We demonstrate our all-fiber device for the fluorescence readout after loop-mediated isothermal amplification (LAMP) of synthetic SARS-CoV-2. Our results suggest that this fiber device can successfully distinguish between positive and negative samples in molecular diagnostics. We propose that our fiber device offers benefits over microfluidic chip techniques such as easier optical integration, much simpler sample loading, and faster diagnosis with high specificity and sensitivity.

Here for the first time the technique of the resonant modes coupling is used for spectrally-selective fundamental mode suppression. In our work we considered the fiber design consisting of the low-index core surrounding by the appropriately chosen high-index absorbing rods. Mode suppression in this case happens due to the resonant core mode deformation owing to mode-anticrossing effect and its partial absorption into the rods. According to our calculations it was established that stop-band of the core fundamental mode can be easily adjusted for different practical aims by fiber bending. Furthermore, in the present work we implemented and studied passive fiber with three high-index absorbing rods incorporated into fiber cladding. The Sm was chosen as an absorbing element of the high-index rods.

We report a white-light interferometric method for broad-band measurements of chromatic dispersion of higher-order-modes (HOMs) selectively excited in an optical fiber using a spatial light modulator (SLM). To excite a specific mode we appropriately modulated a phase distribution across a supercontinuum input beam with the SLM used in reflective configuration. For this purpose, the SLM surface was divided into azimuthally and radially distributed sectors which introduce the phase shifts equal alternately to 0 or π radians, similarly as in the targeted mode. The voltage applied to respective sectors of the SLM was corrected versus wavelength to ensure broad-band dispersion measurements for the required mode. For a given voltage setting, the dispersion measurements were possible without any correction over 250 nm in the visible and over even greater range in the infrared. We demonstrate feasibility of the proposed approach in the measurements of chromatic dispersion for all modes supported by Corning SMF-28e, i.e., LP01, LP11, LP21, LP02, and LP31. The measurements were conducted in the spectral range from 450 nm up to the cut-off wavelengths of respective higher order modes and up to 1600 nm for the fundamental mode.

One of the common methods to increase data bitrate using visible light communication (VLC) fibers system is to use wavelength division multiplexing (WDM). However, the implementation of WDM requires additional components and more energy consumption which can limit the system performances. To overcome these issues, we propose a new method for designing an RGB multiplexer based on multicore polymer optical fiber (MC-POF). The new design is based on replacing seven air-holes areas with polycarbonate (PC) layers along the fiber length. The PC layer length sizes are suitable to the light coupling of the operating wavelengths which allows us to control the light switching between closer PC layers and to obtain an RGB multiplexer device without adding more devices. Results show that after a 20 mm light propagation the PC MC-POF RGB multiplexer can be obtained with a low power loss of 0.6 to 1.02 dB, large bandwidth of 7.3 to 28.4 nm and good isolation between the transmission of the input RGB channels.

A highly birefringent polymer photonic crystal fiber (PCF) for polarization maintaining is investigated in this work. A triangular structure of circular air holes included in polytherimide polymer (PEI) with a defected core for high birefringence is modeled. The properties of this structure are simulated with a full vector finite element method (FVFEM) using PML (Perfect Matched Layer) as boundary condition. The optimized design ensures a very high birefringence of 4.9 10⁻² at a wavelength of 1550nm. Furthermore, we have achieved an extremely low confinement loss around 10⁻⁶dB/km and negative chromatic dispersion of 180ps/(nm.km) along the y polarization at the operating wavelength. Owing to the excellent polarization maintaining properties, the proposed fiber design could be easily suitable for optical sensor applications. The proposed structure could also enhance the dispersion compensating devices in high bite rate transmission network.

The structural properties of PMMA, which has been melt-spun and treated using a specific cooling profile, is investigated in order to evoke desired optical and mechanical properties. Several PMMA fibres, which had been melt spun and subsequently processed with different temperature profiles, were analysed by small-angle X-ray scattering (SAXS) measurements. These results will be compared to a combination of numerical models, which consider the quenching of a filamentary PMMA polymer melt in water. This multi-scale simulation considers macroscopically the cooling process in the water and within the fiber. The spatially resolved cooling rates, which have been simulated at different locations serve as input for a 3D-Monte-Carlo polymer simulation model, which takes, among others, the Lennard-Jones, the bending and bond potentials into account in order to predict the resulting PMMA structure of the fabricated fiber These simulated structures are then evaluated in order to analyse their macroscopic properties. These comprise for instance the polymer entanglement, which describes the interaction of neighboring polymer chains leading to stronger, but stiffer fibers. Entanglement will also affect the glass-transition temperature, which determines the maximum operation temperature. But this can also lead to increased optical scattering, which will be subject to investigations, as well.

With the development of LLL night vision technology from the second and third generation to 4G, the specifications of imaging have been significantly improved. In particular, the contrast of imaging, which determines the clarity, even resolution of the image. The contrast of imaging also refers to the Stray Light Crosstalk (SLC) among optical fibers. How to characterize the contrast of Optical Fiber Imaging Elements (OFIE) by detecting the SLC has become an important problem that must be solved. At present, the contrast performance is often characterized by the Knife-edge Response Value (KERV), which is the transmittance value of light passing through the knife edge through optical fiber imaging element. However, KRV has some disadvantages, such as inaccurate measurement value, harsh test conditions, complex sample preparation and great influence on the measurement result. The most important disadvantage is that KRV is an indirect detection, which needs to slice and grind the tested sample, and the slice position often cannot represent the overall contrast performance of the tested OFIE. In this paper, the digital imaging equipment (high-precision CMOS camera + high-resolution microscope) is used to take photo of the end face of the OPIE placed on the black-and-white boundary of the USAF resolution target. The process of light passing through the black-and-white edge provides accurate information for the contrast change. Through the computer analysis and processing of the digital image, the SLC in different positions of the OPIEs is obtained. The SLC can be used to analyze the degree of crosstalk, or contrast. The digital imaging equipment mainly includes light source system, precision transmission system, CCD camera system, software analysis system and control system. The equipment has the advantages of direct detection, simple operation, high precision, good repeatability and reliability, convenient maintenance, and can be used to test and analyze the imaging contrast of all optical fiber imaging elements. It has been proved that the device is effective in detecting the SLC, and completely replaces the KRV method.

Silica anti-resonant hollow-core fiber (ARHCF) is a promising platform for optofluidic applications because it can act as fluid-cell, permitting intense fluid-light interaction over extended length with low optical loss from ultra-violet to midinfrared region. For this kind of applications, an all-fiberized and compact structure is necessary. However, a prerequisite for this purpose is that micro-channels must be created on the side of the fiber, to provide access for the diffusion of fluids (i.e. liquid or gas) into the core. Several attempts based on femtosecond laser micro-machining technology have been made to create micro-channels through the silica cladding, but significant loss could be induced due to the damage of the cladding capillaries of ARHCF. Here, we report a high-precision and repeatable micro-machining technique using focused ion beam (FIB) milling on a nodeless ARHCF. Ga+ ion beam is employed to bombard a 43 μm thick outer cladding of ARHCF for 30 minutes, to create a 50 μm deep fluidic channel. The micro-channel in the silica cladding is precisely drilled at the middle position of two adjacent capillaries with a 2.8 μm gap, providing direct access for liquid/gas to diffuse into the hollow-core region, while avoiding the damage of the capillaries. Corroborating results from simulation of such a structure are presented to demonstrate that no additional loss is induced by the milled structure.

The paper deals with the determination of group and phase birefringence of an experimental highly birefringent (Hi-Bi) optical fiber across a wide wavelength span ranging from 1200 nm to 1700 nm. Several approaches for birefringence determination are used and the results are discussed. For the investigation, a set of two plane polarizers, a broadband light source, and an optical spectrum analyzer are used. The front face of the Hi-Bi fiber is illuminated by a linearly polarized light from a broadband light source and the end face of the fiber is connected to a fiber plane polarizer. At the output of the fiber plane polarizer, a typical spectrum – an interference pattern consisting of a quasi-periodic distribution of maxima and minima is measured by an optical spectrum analyzer. From the positions of subsequent minima of the interference pattern measured by the optical spectrum analyzer, the group birefringence dispersion is determined. The mean value obtained is in good agreement with that calculated from a differential group delay. The phase birefringence dispersion is determined by finding the appropriate dispersion functions representing the phase birefringence by fitting the measured spectrum to a calculated one, as the character of the spectrum depends on the phase birefringence.

In this work, phase-separated fibers of the system SiO₂-ZrO₂ doped by thulium and holmium ions were prepared by modified solution-doping method combined with MCVD. The ZrO₂ concentration in both fibers was approx. 3 mol. %. The rare-earth ion concentrations were 270 and 740 ppm, respectively. The presence of ZrO₂-based nanoparticles in the optical fiber preforms was confirmed by scanning electron microscope (SEM). The background losses of the fibers were in the range of 0.1 – 0.6 dB/m. The fibers exhibited strong emission in the near-infrared region thanks to 4f-4f transitions of rare-earth ions. The photoluminescence decay of both fibers exhibited double exponential character, most likely due to the incorporation of rare-earth ions in different optically active sites, i.e. ZrO₂-based nanoparticles and grain boundaries or amorphous silica matrix. The fibers were tested as active mediums in fiber laser setup. The thulium-doped fiber exhibited threshold for laser operation of 233 mW and slope efficiency of 72.7 %. The holmium doped fiber failed to manifest lasing properties. An improved laser performance may be achieved by higher proportion of rare-earth ions incorporating in the favorable environment of the nanoparticles.

Photonic crystal fibers (PCFs) are microstructure optical fibers which demonstrate unique optical properties by exploiting the guiding mechanism of electromagnetic waves through the periodic formation of refractive indices. PCF with high negative dispersion and enhanced nonlinearity is often desirable for improving the signal quality in long-haul light-wave communication and different nonlinear optical applications. Investigations have been carried out previously on dispersion and nonlinearity for several numerical designs of PCF through the variation of structural parameters. However, the designs of photonic fibers, which are comprised of noncircular air holes, are difficult and challenging to fabricate with existing technologies. In this work, an analytical design of hexagonal photonic crystal fiber (H-PCF), which consists of all circular air holes is proposed. The primary aim of the proposed numerical design is to attain desired optical characteristics by using circular air holes only to make the fiber simple and feasible for standard fabrication process. The proposed H-PCF consists of a regular hexagonal lattice structure, where the size and location of the few air holes are changed in order to obtain high optical dispersion and enhanced nonlinearity. The corresponding modal properties resulting from geometrical modification and the optimal values of the geometrical parameters are investigated using the numerical electromagnetic solver based on finite element method (FEM). The numerical results show that our proposed H-PCF achieves a large dispersion of −2304 ps/(nm. km) and nonlinearity of 110.8 W⁻¹km⁻¹ at the operating wavelength of 1.55 µm. The proposed structure offers design flexibility since only circular air holes are involved in the design. Our proposed H-PCF structure can be considered a prospective candidate for dispersion compensation in long-haul optical communication and several other applications such as optical modulation and amplification.

Thulium-doped fiber lasers have been extensively investigated as the most promising source of efficient laser emission at wavelengths around 2 μm, i. e., in the eye-safer spectral region and in the atmospheric window as well. It allows for wide range of applications including medicine, defense, distance measurement or materials processing. To enhance pump absorption efficiency along the active double-clad fiber, good overlap of the pump light and doped fiber core should be achieved along the fiber length. The overlap can be increased by breaking the circular symmetry of the inner cladding by shaping its cross-section. Further mode-mixing and better pump absorption can be achieved by coiling and twisting of double-clad fibers. In this work we present experimental measurement of 792 nm pump cladding absorption of a series of double-clad thuliumdoped fibers with respect to their bend radius, the inner cladding cross-sectional shape and twist rate. With these fibers, we assembled a set of fiber lasers with different resonator setups and tested their performance. Twisting was introduced to fiber during drawing from an octagonal, CO2 laser-shaped or mechanically grinded preform so that the twist remained frozen in the drawn fiber. We have shown that the fiber twist significantly improves the pump absorption even in the case of straight or coiled fibers with large coil radii. We provide a preliminary comparison of two fiber laser resonators.

In this paper, we present results of long-term stability tests of a low-loss (<0.55 dB) hollow core fiber (HCF) to standard optical fiber interconnection prepared by modified gluing-based fiber-array technology. We measured insertion loss of three interconnected HCF samples over a period of 100 days at room temperature, observing a variation in insertion loss of less than 0.02 dB. Subsequently, we placed the HCF samples in a climatic chamber and heated to +85°C in four cycles. Maximum insertion loss variation of 0.10 dB was observed for HCF samples with angled 8° interconnections and only 0.02 dB for a HCF sample with a flat interconnection.

The high power optical fibers with a doped active core are subject to heating due to non-radiative absorption of pumping power in the core area. The heat generated in the central part of a fiber is conducted to its outer parts resulting in temperature distribution on the fiber cross section. Efficient cooling is needed in order to avoid thermal damage of the fiber, in particular on the boundary between silica cladding and fiber coating. Temperature distribution in the fiber and its surrounding can be estimated by solving a steady-state heat conduction problem in a domain of fiber cross-section characterized by different values of thermal conductivity of different materials present. In this work, the problem is solved numerically by a finite element method for several kinds of fiber cross sections including a fiber with an octagonal cladding structure. It is shown that the temperature on the critical boundary cladding-coating is increasing linearly with the heat load with the slope determined by the boundary radius. The effect of different shapes of metal slot guiding the fiber is demonstrated.

We report on experimental setup and characterization of a broadband fiber-optic thulium-doped source of amplified spontaneous emission, which generates radiation in a spectral region around 2 micrometer wavelengths. We present a broadband source based on core-pumped thulium-doped fiber fabricated in house using the modified chemical vapor deposition method and solution doping method, pumped by erbium-doped fiber laser at 1566 nm. The source in a backward configuration with respect to the pump operates in a single-ended configuration achieved using a simple all-fiber geometry and produces radiation with an output power of up to 350 mW. The output spectrum is combined from two local emission peaks of the Tm-doped fiber, at around 1850 nm and at around 1950 nm, with total 3-dB width of more than 140 nm and output power of 130 mW.

In recent years, photoacoustic imaging, as a new type of biomedical imaging method, combines the advantages of high selectivity in pure optical tissue imaging and deep penetration in pure ultrasound tissue imaging to obtain high-resolution and high-contrast tissue images. The use of photoacoustic imaging technology to deal with complex medical tissue problems is still a new research direction. How to compress large amounts of data and quickly transmit and store important value information has become a problem waiting for optimization. This paper uses the StagewiseOMP and tracking algorithm to combine it with the photoacoustic imaging of the k-wave simulation toolbox to rebuild a virtual simulation platform for blood vessel imaging. On the one hand, compressed sensing can reduce the sampling rate and speed up imaging. On the other hand, it can modify the demand for hardware equipment to facilitate data transmission and storage. A simulation model of photoacoustic field propagation, photoacoustic signal recording and image reconstruction was established using the k-wave simulation toolbox. We have used the excellent performance of the simulation platform through imaging technology to complete the imaging restoration of part of the blood area tube tissue.