This PDF file contains the front matter associated with SPIE Proceedings Volume 11029, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Publisher’s Note: This conference presentation, originally published on 14 May, 2019, was withdrawn on 19 May, 2019 per author request.

We report on OH diffusion effects at preparation of hollow core fibers with large central hexagonal and square shaped cores. The fibers with up to 30 μm central hole diameter are suitable for chemical sensor applications, especially for gases. We demonstrate the single mode guidance at selective bands in the UV, VIS and NIR region. Key feature of low loss in specific spectral windows of such fiber structures is the control of thickness and homogeneity of the web bridges. The fibers achieve a minimum loss of 2 dB/m and effective single mode propagation in the wavelength range between 270 nm and 1500 nm. The thinness of the bridges beneath one micrometer results in a deep impregnation of OH by diffusion from the cavities during thermal processing, e.g. fiber drawing. Up to 1000 ppm by weight of OH had been measured in the silica web surrounding the hollow core. Obviously, the OH sources are atmospheric humidity and condensed water originating from using a hydrogen-oxygen torch at final preform fabrication. The paper shows the good agreement between OH diffusion simulation and experimental observation of OH impregnation in the hollow core web.

High-speed tracking of single nanoobjects is a gateway to understanding physical, chemical, and biological processes at the nanoscale. Here I will present our recent results on tracking single individual nanoobjects inside optofluidic optical fibers via elastic light scattering. The nanoobjects are located within an aqueous environment inside a well-selected channel of microstructured optical fiber. Light from the core mode which hits the freely diffusing nano-object scatters off and can be detected transversely. Tracking of unlabeled dielectric particles as small as 20 nm as well as individual cowpea chlorotic mottle virus (CCMV) virions at rates of over 2 kHz for durations of tens of seconds has been achieved in nanobore optical fibers, whereas full 3D information about the nano-object’s trajectory are retrieved in modified step index fibers. From the light scattering intensities and the diffusion constants we were able to determine key properties of the particles such as size or hydrodynamic radius.

Publisher’s Note: This conference presentation, originally published on 14 May, 2019, was withdrawn on 19 May, 2019 per author request.

We present the preparation and characterization of holmium-doped silica-based optical fibers for fiber lasers operating around two micrometers. The fibers were prepared by a modified chemical vapor deposition process and co-doped with aluminum oxide. Alumina was doped with the use of two different methods – solution doping and nanoparticle doping. Prepared optical preforms and fibers were characterized according to their optical, spectral and laser properties. It was observed that the doping by alumina nanoparticles improve a fluorescence lifetime and laser characteristics such as laser threshold and slope efficiency. The comparison of both doping methods is presented and results are discussed.

We experimentally investigated loss of multimode optical fibers (MMFs) drawn of thermally shaped optical fiber preforms (OFPs). Such preforms are typically used for fabrication of double clad active fibers. The investigation involved undoped shaped MMFs coated with a low refractive index polymer. The fibers were drawn of silica rod, prepared by collapsing a pure silica tube (Heraeus F300, OH content is 0.2 ppm) in the MCVD lathe. Background losses of undoped MMFs with inner cladding of various geometries shaped by CO₂ laser were measured via cut-back method. Losses of the shaped MMFs were compared to the loss of the circular and of the mechanically shaped MMF. Constraints, drawbacks and advantages of shaping the fiber preform using the CO₂ laser are discussed. Shaping OFPs with CO₂ beam provides advantages of quick polishing, smooth surface, and freeform shape. Results show that mechanical polishing technique leads to significant OH content elimination, which is expressed as reduced absorption peaks at wavelengths of 0.945 μm and 1.24 μm, which correspond to the third and second overtone, respectively. The average perimeter length of fibers cross section governs absorption at polymer-glass interface.

2-5 μm mid-infrared lasers are of great use in many aspects such as the applications of remote monitoring, free space communication, laser surgery and infrared countermeasures. The fiber gas Raman source, combining the advantages of traditional solid core fiber lasers and gas lasers, firstly demonstrated in 2002, has aroused extensive attentions and now is still a hot topic for having better beam quality, higher damage threshold, higher peak power and narrower linewidth. By particularly designing hollow-core fibers (HCFs), selecting proper gases filled in the HCFs, mid-infrared lasers with various wavelengths can be easily obtained. So far, fiber gas Raman laser sources of different bands such as 1.5 μm, 2.0 μm, 3.0 μm, and 4.0 μm has been demonstrated through the sitmulated Raman scattering (SRS) of different gases such as methane, hydrogen and ethane. However, due to the transmission properties of current low-loss mid-infared HCFs, no efficient mid-infared fiber gas Raman laser sources beyond 4 μm have been reported up to now. In this paper, we provides a method of generating mid-infrared laser through the SRS of gases filled in the HCFs under cascaeded sturcture. In our experiments, two kinds of HCFs with different transmission spectra are used respectively in the first stage and second stage of generating Raman laser. In the first stage, a commercial 1064 nm high peak power laser is coupled into a methan-filled HCF through reflectors and planoconvex lens, and a 1.55 μm high peak power Raman laser is achieved by the first-order vibrational SRS of methane. In the seconde stage, the output 1.55 μm Raman laser is coupled into a hydrogen-filled HCF as the pump source, and finally a 4.3 μm Raman laser is generated by the first-order vibrational SRS of hydrogen. The transmission spectra of the HCF are the key factors in determining whether to generate 1.55 μm laser by the SRS of methane in the first stage or generate 1.9 μm laser by the SRS of hydrogen in the first stage. This work provides a vital way to realize a wide wavelength range of mid-infrared, even far-infrared fiber laser sources from available commercial 1 μm lasers with appropriate HCFs and different active gases.

A novel core dopant called diphenyl disulphide is used in PMMA-based fibers, permitting both increase of the refractive index of the fiber, together with good photosensitivity. FBGs can be inscribed in these single mode fibers within 7 ms using 325 nm laser and exhibit significant growth post-UV irradiation. In the present work, we investigate the grating growth behavior upon long UV irradiation time. Furthermore, we demonstrate the fabrication of fiber Bragg gratings with irradiation time ranging from 7 ms up to 10 s using a 325 nm laser and we investigate their growth post UV irradiation over 7 months. It is demonstrated that all the FBGs exhibit over 10 dB growth within the several month post-UV irradiation period. Raman spectroscopy measurements were carried out for several months on thin films post-UV irradiation, and significant changes in the molecular bounds of both diphenyl disulphide and PMMA are recorded, establishing the UV induced photo-chemical reaction responsible for the FBG growth. Finally, the reliability of the novel core dopant and the potential use of the fiber for in-vivo sensing was investigated by using humidity cycling at temperatures near the human body core temperature.

Even though optical fibers have reached great success in a multitude of areas, applications such as wide-field fiber-based endoscopy or random photon collection in quantum optics demand collecting light under comparable large angles. Here reaching satisfactory coupling efficiencies is difficult with fibers that have unstructured end faces due to small acceptance angles of commonly used step-index fibers. For instance the SMF-28 has as an NA of only 0.16 and therefore an acceptance angle of only 7°, thus leading to a diminishing light collection performance for incident angles >20°. Here we report on a plasmonic nano-structure enhanced step-index fiber for the efficient collection of light at extremely large incident angles (> 30 °), reaching a regime that is inaccessible for fibers with plane faces. For the experimental demonstration, we have implement arrays of gold nano-dots (diameter: 480 nm, height: 40 nm) on the end face of a SMF-28 exactly at the location of the core via electron-beam lithography and measured the light collection efficiency at different incident angles under various circumstances including variations in incidence wavelength, array lattice constant and input polarization. The measurements show several orders of magnitude of improvement in light-coupling efficiency when using the nano-structure functionalized fiber for incident angles ranging from 30° to 80°, while fibers with plane end faces only show measurable coupling efficiencies up to angles of 25°. In addition to the mentioned improvement, we observe an additional local enhancement in coupling efficiency, which is located at around 40° to 60° and is associated with lattice constant of nano-dot array, i.e., with the -1st diffraction order. To analyze the impact of the nano-structure on the fiber-coupling efficiency from the theoretical perspective, we developed a toy model which includes the phase evolution of the incident beam across the region of the core mode at the location of the fiber end face and conducted full 3D numerical simulations on the basis of the Finite-Element method. Both the theoretical analysis and the data obtained from toy model indicate a significant enhancement of the coupling efficiency that solely originates from optical response of the gold nano-dot array. Since bare step-index fibers severely suffer from poor incoupling efficiencies at large incident angles, particular bioanalytical applications such as fiber-based endoscopy or in-vivo Raman spectroscopy will greatly benefit from the presented concept of boosting fiber incoupling efficiencies via nano-structure functionalized fiber end faces. Our concept is of generic origin and is not restricted to nano-dot arrays, but rather paves the way towards high-performance fiber-based photonic devices that include sophisticated nano-structures such as dielectric metasurfaces in order to boost incoupling efficiencies even further. Due to its unique performance regarding light-collection efficiency at incident angles being out-of-reach for fibers with plane end faces, we strongly believe that our concept will generate great impact in various fields of research and applications, including biophotonics, green photonics and quantum technologies.

We present the results on fiber Bragg gratings inscription with femtosecond laser pulses in a Fibercore SM- 7C1500(6.1/125) 7-core single-mode optical fiber. By focusing femtosecond pulses into the volume of the fiber and by controlling the transverse spatial position of the pulse absorption region we selectively modify the individual fiber cores of the fiber and at the same time specify geometry of the each grating. We show that different longitudinal profiles of coupling coefficient can be realized for the FBG, including uniform, chirped and apodized ones.

The inscription of fibre Bragg gratings directly into active fibres has great potential compared with typical, nonmonolithic fibre laser configurations, offering more compact and robust lasers since the ‘weak’ sections of the laser, such as the free space components and splice joints, which are usually the main reasons for failures, are removed from the optical cavity. The last few years thulium doped fibres have attracted great interest for fibre laser applications, due to their performance and emission wavelength range, which is in the range of 1.9 to 2.1 μm. Here the laser operates in the ‘eye-safe’ region and offers advantages over the 1 μm lasers both for industrial and military applications. In this paper, we report on the inscription of a high reflectivity fibre Bragg gratings directly into different Thulium-doped silica fibres using the plane-by-plane femtosecond laser inscription method and their demonstration as monolithic fibre lasers. Fibre Bragg gratings were inscribed and characterised as fibre lasers using three fibres with different concentration of thulium ions, which influence the fluorescence properties. The monolithic structures were designed for continuous wave operation in the 1970 nm ‘eye-safe’ region and characterised in fibre laser configurations regarding the power output slope efficiency, stability and effective resonator length.

The turbid nature of refractive index distribution, common for all living tissues, introduces severe aberrations to light propagation thereby severely compromising light-based observations using currently available non-invasive techniques. Numerous approaches of endoscopy, based mainly on fibre bundles or GRIN-lenses, allow imaging in extended depths of turbid tissues. Their footprint, however, causes profound mechanical damage to overlying regions and their imaging performance is very limited. Optical systems have traditionally been understood as assemblies of components acting in a predefined and determinate manner. This notion is currently undergoing a transformation due to rapid advances in the technology and methods for spatial light modulation. Computer-controlled holographic modulators now facilitate the deployment of unusual and complex optical media with properties that bring unique advantages to biomedical applications. Progress in this domain enabled a new generation of minimally invasive, high-resolution endoscopes by substitution of the Fourier-based image relays with a holographic control of light propagating through apparently randomizing multimode optical fibres. The use of multimode fibres (MMFs) as ultranarrow endoscopes is a promising example of this concept, since it allows one to overcome the trade-off between the size of the optical element and the attainable resolution. The nature of light transport through MMFs leads to the transformation (or scrambling) of incident wavefronts into seemingly random speckle patterns. Adaptive optics provide a means for overcoming this signal degradation. A number of recently developed techniques in this domain have enabled the randomized output fields to be tailored into any desired distribution across the distal fibre facet or an arbitrarily remote plane. The most common form of laser scanning microscopy relies on the formation of diffraction-limited foci behind the fibre, combined with image reconstruction from fluorescence signals that are collected and guided backwards. Digital micromirror devices (DMDs) have recently opened up a range of opportunities in this domain by increasing the achievable light modulation refresh rates by several orders of magnitude compared to well-established nematic liquid crystal-based devices. The foci behind a MMF can be scanned at several tens of kHz, thus acquiring images at speeds approaching video rates. Furthermore, it has also been shown that DMDs generate foci of higher quality compared to other modulators. Building on these advances, the focus of researchers is currently shifting towards implementations in biomedically relevant settings, including in vivo applications. I will review our fundamental and technological progression in this domain and introduce several applications of this concept in bio-medically relevant environments. I will present isotropic volumetric imaging modality based on advanced modes of light-sheet microscopy: by taking advantage of the cylindrical symmetry of the fibre, it is possible to facilitate the wavefront engineering methods for generation of both Bessel and structured Bessel beam plane illumination. Further, I will demonstrate the utilization of multimode fibres for imaging in living organism and present a new fibre-based geometry for deep tissue imaging in brain tissue of a living animal model. Lastly I will show the development and exploitation of highly specialised fibre probes for numerous advanced bio-photonics applications including high-resolution imaging and optical manipulation.

Phosphate glasses are attractive materials for the engineering of photonic devices, due to their easy processing, good thermal stability, excellent optical properties and high rare-earth ions solubility. Besides, phosphate glasses with a P2O5 content of 50 mol% have been shown to be suitable for fiber drawing [1]. In this presentation, we will first discuss how to develop new active glasses within the P2O5 – SrO – Na2O composition and how to draw them into fiber. The fabrication process, thermal, structural, and optical properties of the fiber will be presented and we will show that broad luminescence over 70 nm of bandwidth can be obtained from Er/Yb-codoped phosphate fiber. Finally, we will explain that it is possible to combine both biocompatibility and suitable optical properties in fibers. We will explain how to test the bioactivity and optical properties of the fiber in the prospect of developing an innovative biosensor. We will clearly show that the results pave the way towards the development of new bioactive fiber sensors for therapy monitoring. Indeed, such biophotonic fiber would allow one to monitor “in situ” the optical and biological response of an optical glass fiber in aqueous media. Such biodegradable or bioactive optical fiber could be resorbed or lead to new soft tissue once the sensing utility has been accomplished. In this way, the surgical removal of the fiber sensor is not needed. LP acknowledges the Academy of Finland (Academy project-308558). References [1] C. Vitale-Brovarone, et al., Mater. Sci. Eng., C, 31, 434-442, 2011.

Optical fibers are becoming popular in medicine and biomedical research together with optical methods such as endoscopy and sensing, optogenetics or photodynamic therapy. Typically, silica optical fibers are used because they offer very good transparency from visible to near infrared range of the electromagnetic spectrum and they are biocompatible, i.e. they are not toxic and they do not provoke a negative response from the immunity system. However, their use pose a certain risk to the body in case of a breakage of such fiber since the fragments are not easily detectable by X-ray imaging and they can move through venous system. Optical fibers based on water-soluble non-toxic materials such as phosphate glass can solve this issue. In this work, we present a fabrication and both in-vitro and in-vivo characterization of an optical fiber made of undoped sodium phosphate glass.

We demonstrate a biodegradable and biocompatible unclad optical fiber made from poly(D,L-lactic acid) (PDLLA), which is a well-known and commercially available amorphous polyester. We first deal with the chemical and optical characterization of the bulk polymer material and we report on the influence of the processing on the molecular weight and thermal properties of the polymer, during both the preform preparation and the fiber drawing process. We then proceed to the optical characterization based on spectral attenuation measurements using the cutback method and dispersion measurements. We also determine the thermo-optic coefficient. Finally, we confirm the in vitro degradation in phosphate buffered saline (PBS) of our PDLLA fibers. From the results and considering that PDLLA is an FDAregulated material, we anticipate that our optical fibers are valid candidates for medical applications involving in vivo light delivery, such as for example photodynamic therapy.

One of the main obstacles that limited the performances in visible light networking system is the ability to transmit high data communication rate. Wavelength division multiplexing (WDM) is a good solution for increasing data bitrate communication of photonic crystal fiber (PCF) and multicore polymer optical fiber (MC-POF) based visible light communication (VLC) system. In order to overcome this obstacle, we propose two new designs for an RGB demultiplexer, one is based on silicon-nitride (Si3N4) multicore PCF structure and the second is based on polycarbonate (PC) MC-POF structure. The new design is based on replacing several air-holes areas with Si3N4 rods in PCF and PC rods in POF over the fiber length which enables controlling the light propagation direction between the core layers. The locations of the Si3N4 / PC rods and the key geometrical parameters of the device were optimized and analyzed utilizing the beam propagation method (BPM) combined with Matlab codes. Results show that RGB operated wavelengths can be demultiplexed after light propagation of 5.5 mm for PCF and 20 mm for POF with an excellent crosstalk of -19.436 to - 26.474 dB and a large bandwidth of 5.6 to 16.3 nm.

We study a new class of micro-structured fiber known as defective core photonic crystal fiber for ultrashort pulse compression. The underlined fiber can fulfill the self-similar condition for pulse compression in a relatively simple means, without the necessity for a complex approach of changing the size of air holes in the cladding. The fiber obey exponential decrease (increase) in dispersion (nonlinearity) along the fiber length and hence compresses the pulse under self-similar technique. We show that the self-similar technique is superior in terms of quality as well as in efficiency with high degree of compression factor, than the adiabatic route of compressing the pulse.

The evolution of radiation polarization in a non-uniform medium with a gradient refractive index profile using the quantum mechanical method of coherent states is investigated. It is shown that the degree of polarization of both linearly and circularly polarized radiation decreases with distance due to the interaction between polarization (spin) and trajectory (orbital angular momentum). The effect of asymmetry for the right- and left- circularly polarized radiation takes place in relation to the sign of helicity of the trajectories of sagittal rays. The wave nature of depolarization is emphasized. Depolarization disappears when the wavelength tends to zero λ→0. Depolarization increases with an increase in the axial displacement of the beam, the gradient parameter of the waveguide and the wavelength of the radiation. The oscillations of the polarization degree of pure diffraction origin in the propagation of radiation in a singlemode optical waveguide are found. It is shown that the increase in the angle of rotation of the polarization plane with the distance is irregular and depends on the axial displacement or the angle of inclination of the beam to the waveguide axis. The fluctuations of Berry’s phase, which have a wave nature, in the propagation of radiation in an inhomogeneous medium are found. It is shown that the dispersion of the angle of rotation of the polarization plane increases with distance and can be determined from measurements of the degree of polarization of radiation.

Fiber laser sources from visible to near-infrared wavelengths have driven innovative developments, impacting various domains such as telecommunications, biology, and medicine. The development of such fiber laser relies on the accurate knowledge of both optical properties as chromatic dispersion and material properties. On the other hand, quantum metrology is one of the promising field enabled by quantum technologies. It allows to get precise results compare to classical methods when measuring physical properties. A very common approach is to inject non classical states of light in interferometers to increase accuracy as well as sensitivity. Recently, this scheme has been used for detecting gravitational waves for example [1]. During the conference, we show how we take advantage of these capabilities to gather optical fiber photonic engineering with quantum optics. More specifically, we aim at presenting two quantum-based method for (i) high-accuracy (10-5) and dispersion-free measurement of refractive index difference and (ii) chromatic dispersion measurement based on the concept of quantum white-light interferometry that allows absolute measurement of chromatic dispersion with ~2.5 times improved accuracies compared to state-of-the-art realizations at telecom wavelengths. [1] B. P. Abbott et. al., ”Observation of Gravitational Waves from a Binary Black Hole Merger”, Phys. Rev. Lett., 116, 061102 (2016)

Dispersion measurements on a birefringent hollow-core (HC-800-02) and a solid-core (LMA-PM-5) photonic crystal fiber (PCF) are presented using a windowed Fourier-transform (WFT) spectral interferometric method. We investigate the optimal value of the spectral window function of the WFT method to reach the highest accuracy in the dispersion measurement. This requires the knowledge of the precise position of the polarization axes of the fibers. In order to determine the position of the polarization axes we have developed a method based on analyzing the WFT signals, which were obtained from a series of interferograms at different excitation ratios of the polarization modes of the PCFs.

The design and performance evaluation of an innovative, small size, compactly packaged high temperature interferometric optical sensor for harsh environments is reported. The sensor was built with a short segment of strongly coupled multi-core optical fiber (MCF) spliced to a typical single mode fiber (SMF). Matlab MathWorks and PhotonDesign simulation programs were used to design the sensor to monitor the widest temperature range possible with a commercial sensor interrogator. The SMF-MCF structure was protected by two temperature-resistant tubes: an inner ceramic tube with an internal hole that matched the MCF diameter in order keep it tightly in axial direction to avoid the effects of bending and/or vibrations that could be misinterpreted as temperature measurements, and an outer metallic tube to provide protection against impacts. The calibration of our packaged MCF sensor was carried out in a high temperature furnace at the facilities of the Aeronautical Technologies Center (CTA) and a calibrated standard K-type thermocouple was used as a reference. The calibration was performed repeatedly in the range from 200 to 950 °C and the gathered spectra were processed entirely by a custom program made in Matlab MathWorks. Our sensor responded lineally in the tested range with an average temperature sensitivity of 29.1 pm/°C and showed high robustness against vibrations. Results indicate that our MCF sensor is as accurate as the K-type thermocouple with the advantage of its appropriate passiveness for harsh environmental industrial applications.

The focus of the work is on development and implementation of competitive optical elements based on tunable interference wedged structures. Such a structure can be a single interference wedge (two reflecting surfaces separated by a gap with increasing thickness) or a composition of two or more superimposed wedged layers with adjusted parameters. We used these structures to build a new wavelength division multiplexing (WDM) element and realized coupling of these elements with a fiber optical system as an issue of essential interest for optical communications. Under illumination with a multi-wavelengths beam, the composed WDM structure in the fiber system provides precisely controllable wavelength selection (resolution better than 0.01 nm) within the range of more than 15 nm and with controlled continuously variable transitivity from 1-3 to 80 %. The non-transmitted power with the other non-selected and completely reflected light is directed to the next output (theoretical loss of the system ~ 5 %). The WDM-structure works at completely independent spectral selection of each output/input without any influence between the tuning of the channels.

In this study, we developed and fabricate an array of microstructured MIR fibers of silver halide crystals. Firstly, the modal analysis of photonic crystal fiber arrays was carried out by means of program packages, which are based on offsurface integral- and differential-equation formulations, and the optimal parameters for medical thermography application were chosen. Then, the fibers were fabricated using extrusion technique and seven fiber segments were assembled. Each individual fiber has a matrix with the diameter of 525 μm, six hexagonally arranged inserts with lower refractive index than matrix’s one, and a central insert with a larger refractive index than matrix’s one. It was measured, that its cross-talk between adjacent fibers doesn’t exceed 5%. It was experimentally confirmed that the fiber structure allows transmitting infrared radiation with the wavelengths of 9.2-9.4 μm in a single-mode regime, yielding up to 100 μm in the mode field diameter for each individual fiber in the array. These wavelengths correspond to a temperature range of the human body, therefore the arrays may transmit thermal images of human tissues, including internal organs. We expect that such arrays can improve medical thermography techniques in the future.

Radiation-induced absorption (RIA) of light in the visible spectral region is studied in three differently coated puresilica- core, F-doped-silica cladding optical fibers ("pure-silica fibers", PSFs) drawn from the same preform under γ-irradiation up to 1 kGy, the irradiation temperature being varied in the range ±60 °C. The coating types include aluminum, copper and polyimide. It is found out that the extremum of the temperature dependence of RIA due to strainassisted self-trapped holes (STHs) in Al- and polyimide-coated PSFs is shifted towards higher temperatures (≥+60 °C) as compared to acrylate-coated PSFs, of which the extremum temperature was previously found to be around 0 °C. Additional heat treatment in the process of application of the above coatings is argued not to the main factor influencing RIA, which increases primarily to the very presence of a metal layer on the silica surface. The latter, in turn, increases strain in the silica network and the STH population. Photobleaching of the STH-associated bands and UV-tail produced by the probe white-light halogen lamp under γ-irradiation at a dose of ~1 kGy and dose rate of 1.31 Gy/s is assessed quantitatively to be in the range from 20% to 90%.

Radiation-induced attenuation (RIA) of light is studied in a pure-silica-core Panda fiber under γ-irradiation in the dose range 1-10⁶ Gy at the dose rates in the range 0.015-5 Gy/s. A 5-mW erbium fiber superluminescent source or a white-light lamp (integral power of <0.5 μW) are used as the probe light source. Both RIA dose dependences at separate wavelengths in the near-IR range and RIA spectra in the near-IR and visible ranges at fixed doses are investigated. The spectra are Gaussian deconvolved and analyzed. 1.55-μm light (500 μW) is found to produce photobleaching not only of the 0.95- and 1.12-eV RIA bands, the effect observed previously, but also of the 2.6- and 1.63-eV RIA bands associated with inherent and strain-assisted self-trapped holes (STHs), respectively. RIA at λ=1.55 μm is found not to depend on dose rate at those below 0.15 Gy/s, which opens up the possibility to simply estimate the upper bound of RIA gained by the end of space mission. In particular, the RIA upper bound at λ=1.55 μm of ~1.5 dB/km is predicted for a 1-kGy space mission. RIA at λ=1.55 μm in pure-silica-core Pandas is also found to cease to depend on dose rate and probe light power at high doses (~10⁵ Gy). The reason is that all the short-lived RCCs sensitive to the former parameters completely disappear at high doses, whereas RIA at λ=1.55 μm becomes wholly determined by the long-wavelength RIA band peaking at λ<1.7 μm and known to be dose-rate-independent and photobleaching-insensitive.

The creation of self-trapping waveguides within a photopolymer material and the resultant permanent self-written waveguides (SWW) has been investigated for their propagation properties. We look at two light beams as they converge on the photopolymer and interact with the material to produce two separate waveguides fabricated within the material. Here we study these self-written waveguides and investigate their coupling as they propagate within the PVA/AA and combine to form a single SWW within the material. The angle of insertion is interrogated to identify the best angle to produce the SWW coupling effect. The single coupled SWW that is produced, is measured for attenuation so as to characterize the coupled waveguide.

Recently optical capillaries modified by Bragg reflection mirrors applied on the inner walls have been investigated for transmitting radiation of MIR lasers. Such capillaries include Bragg and omniguide fibers, holley fibers, or silica Kagome like fibers. Although OmniGuide fibers are commercially available and have been used for delivery of radiation of CO₂ fibers at 10.6 μm, novel types of hollow-core fibers are still investigated for MIR applications. In this paper a novel approach for the preparation of capillary optical fibers for MIR region is presented. This approach employs the application of thin layers of arsenic sulfide glass and acrylate polymer from their solutions onto the inner wall of silica capillary. Arsenic sulfide forms high-index and polymers the low-index parts of reflection mirrors. By controlling optical thicknesses of such layers, Bragg mirrors can be obtained. In experiments, input solutions of arsenic sulfide in n-propylamine and UV-curable acrylate in acetone were prepared. Such solutions were applied by dip-coating method on glass slides in order to obtain samples of single layers and multilayer coatings for the determination of thicknesses and refractive indices. Acrylate layers were UV cured and arsenic sulfide layer were thermally treated at 80°C. By passing columns of the input solutions through a silica capillary with a hole diameter of 80 μm and a length of 50 cm multilayer coatings on the inner capillary wall were prepared. The column velocity for each solution was controlled as a main factor influencing the layer thickness. Applied layers were UV cured or thermally treated under a nitrogen flow through the capillary. Coatings of three pairs of the high- and low-index layers were fabricated. Single layers and multilayers applied on planar substrates were characterized by transmission spectroscopy and by optical microscopy. Attenuation coefficients of internally coated capillary fibers of 10-20 dB/m were determined at a wavelength of 1940 nm.

Optical fibers and optical fiber bundles are often used for endoscopy and related (minimally invasive) medical methods because they offer good transparency together with flexibility. The ability to perform the operation, monitoring and chemical analysis of tissues with minimal disruption of the skin or internal organs of the patient is very promising in the medical field. Traditionally, silica optical fibers are used. Although silicon oxide is a biocompatible material, its use involves a serious health risk due to its fragility and the fact that potential fiber fragments can freely move inside the body and they are not detectable by conventional methods such as X-ray imaging. A possible solution to this issue can be the development of optical fibers based on biodegradable materials. Important benefit of bioresorbable fibers is that they do not need to be explanted after their use. We report on the optical power transmission tests of recently developed bioresorbable optical fibers based on phosphate glasses. Continuous-wave fiber lasers at 1080 and 1060 nm with output powers up to 7 W and a picosecond laser source at 515 nm with MW pulse peak power were used.