Zbyszek Holdynski

Experimental study of dispersion characteristics for a series of microstructured fibers for customized supercontinuum generation

We demonstrate an experimental study of the chromatic dispersion properties for a series of microstructured fibers (MSFs) dedicated for a supercontinuum generation. With white-light interferometry application we analyze experimentally how the small variations of structural parameters, i.e. an air-hole diameter and a lattice constant, influence dispersion characteristics in different groups of MSFs. Our study provides useful information on how to design the fiber which is less sensitive to the fabrication imperfections. Moreover those investigations are the initial step to the development of the customized or tunable supercontinuum light sources based on MSFs with slightly changed structural parameters which can generate light with a different spectrum range, adapted to a proper application.

Experimental Investigation of Supercontinuum Generation in Photonic Crystal Fibers Pumped With Sub-ns Pulses

We experimentally investigate a supercontinuum generation in a series of photonic crystal fibers pumped with sub-ns pulses. The fibers are designed to have zero dispersion wavelengths close to 1064 nm, but they differ with respect to the lattice constant and air hole size. We focus on the supercontinuum generation mechanism and on the shape of spectra generated with the use of these photonic crystal fibers. A comprehensive study on the influence of the fiber structural parameters on the supercontinuum outcome indicates how to tailor these parameters for specific application where particular spectral characteristics are desired.

Low loss coupling and splicing of standard single mode fibers with all-solid soft-glass microstructured fibers for supercontinuum generation

In this work we would like to present the results of low loss coupling of a novel soft glass fiber for super continuum generation with standard single mode fiber by a filament splicing method. For our experiment we used an all solid soft glass microstructured fiber (MSF) made from a composition of F2 lead-silicate glass and NC21 borosilicate glass. The structure and material properties of the fiber were optimized to achieve all normal dispersion (ND) flattened around 1560 nm, which offers two general advantages for supercontinuum generation. The ND supercontinuum avoids soliton dynamics, hence it is less sensitive to pump laser shot noise and has larger degree of coherence than supercontinuum in the anomalous dispersion range. Furthermore flattening around 1560 nm indicates optimal supercontinuum pump wavelength, which is readily available from erbium doped femtosecond fiber lasers. Using Vytran splicing station (GPX3400) we were able to achieve repeatable splice loss between a standard fused-silica single mode fiber (SMF28) and the low-melting-temperature soft glass MSF as low as 2.12 dB @1310 nm and 1.94 dB @ 1550 nm. The developed very low loss splicing technology together with the above mentioned all solid soft glass MSF advantages give very promising perspectives for commercial applications.

Low power and inexpensive microstructured fiber Mach Zehnder interferometer as temperature insensitive mechanical sensor

Microstructured fibres (MSFs) reveal unique properties including endlessly single-mode operation from ultraviolet to infrared wavelengths, very high birefringence or nonlinearity, very large or very small effective mode field area, and many others. The size, shape and the location of the air holes allow for tailoring MSF parameters in a very wide range, way beyond the classical fibres, what opens up the possibilities for various applications. Due to their advantages MSFs obtain growing attention for their perspectives in sensing applications. Different MSF sensors have already been investigated, including interferometric transducers for diverse physical parameters. Until now, there have not been any publications reporting on the sensing applications of MSF Mach-Zehnder interferometers, targeting the mechanical measurements of vibrations, dynamic or static pressure, strain, bending and lateral force. Moreover, a critical feature opening the prospective of optical fibre transducer to successfully accomplish a particular sensing task remains its cross-sensitivity to temperature. Studied MSF is made of pure silica glass in the entire cross-section with a hexagonal structure of the holes. Consequently, there is no thermal stress induced by the difference in thermal expansion coefficients between the doped core region and the pure silica glass cladding, in contrast to standard fibres. In this paper we present the experimental comparison of mechanical and temperature sensitivities of Mach- Zehnder interferometer with replaceable FC connectorized sensing fibre arm, such as: off-the-shelf endlessly single mode MSF or standard telecom single mode fibre. Experimental results clearly show very low cross-sensitivity to temperature of studied MSF compared with standard fibre. Additionally, microstructured fibre Mach-Zehnder interferometer with standard FC receptacles allows using different fibres as sensors with the same device. Moreover, investigated interferometer consumes in total extremely low electric power (< 20 mW) due to the implementation of exceptionally effective data analysis electronics and VCSEL as the light source.

Investigation of dispersion characteristics of highly nonlinear microstructured fibre series for customized supercontinuum generation

Dispersion characteristics are the fundamental properties of microstructured fibres (MSFs) with respect to the nonlinear applications. The changes of fibre dispersion may strongly influence the whole chain of diverse nonlinear effects resulting in supercontinuum generation (SG). Transferring the experience from the topics related to tailoring different properties of MSFs to investigate the potential design freedom of dispersion opens novel possibilities of building the customized, all-fibre broadband and bright light sources. The silica nonlinear microstructured fibres, as presented in this paper, become compatible with standard fibre components and technologies (e.g. splicing, connectorization etc). Supercontinuum generated in a small-core MSF is a very interesting nonlinear phenomenon from application-oriented point of view. A development of tailored dispersion of highly non-linear silica MSF offers us the possibility of constructing a customized broadband light source. Therefore, in the paper we present a theoretical and experimental investigation of dispersion characteristics of several different MSFs. Our studies are leading to the development of adapted dispersion properties, allowing construction of customized supercontinuum sources. All fibre, white light interferometry set-up, resulting in extremely high precision measurement of chromatic dispersion, is demonstrated, together with fully computer controlled fringe pattern analysis. Constructed set-up permitted comparison of chromatic dispersion measurements of microstructured fibres with modified fibre cross-section dimensions during the production process. High correlation between modelling and measured data gives possibility to control dispersion level in manufacturing process. Additionally, precisely designed and measured chromatic dispersion, especially around the zero dispersion wavelength, enables superior estimation of MSF nonlinear effects.

Supercontinuum generation in three-fold symmetry microstructured fibers in visible and infrared spectral regions

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP

Highly birefringent dual-mode nonlinear fibers for customised supercontinuum generation

In this paper we present the possibility of supercontinuum generation in the dual mode and single mode regime of highly birefringent microstructured fibers. Unique birefringent fiber geometry enables precise dispersion control in both fundamental and second order modes. Our results are related to create new functionality in the generated supercontinua: polarization based spectrum tunability and flatness control. We analyse experimentally the stability of the generation of nonlinear effects in our series of fibers, related to polarization coupling and dispersion changes. The domination of nonlinear effects can be changed in the fundamental and second order modes according to the fiber geometry in order to generate a spectrally flat broadband source. Here we demonstrate the ability to generate a customized supercontinuum in three regimes of mode operation - dual mode, fundamental mode only and second order mode only - where spectral flatness and range are controlled.

Experimental investigation of temperature influence on nonlinear effects in microstructured fibers

In this paper we present possibilities of tuning spectrum of supercontinuum with the use of temperature change. Our study is based on the information about the role of dispersion characteristics in the process of nonlinear effects generation in nanosecond pulse regime. We obtain tunable spectrum effects in microstructured fiber and we show how to optimize its properties. Our experimental results showing nonlinear effects generation in fiber pumped in normal and anomalous dispersion regime enables to determine how the nonlinear effects depend on temperature changes. We show that even small changes of dispersion characteristic of microstructured fibers enable to obtain significant modification of generated spectra when four wave mixing is dominant effect. Controllable generation of tunable supercontinuum can be used in numbers of potential applications such as diagnostics and measurement systems.

Influence of the mode field diameter on the strain sensitivity of different fibers

Phase sensitivities of temperature, longitudinal strain or pressure, are very important fiber features in sensing and telecommunication applications. The most common ways to modify such sensitivities are to change the material properties (by adjusting the core doping level) or employ microstructured fibers (which properties strongly depend on the cross-section geometry). We decided to investigate strain sensitivity influenced by effective mode field area and mode field diameter as clear consequence of fiber cross-section geometry. In this paper we present the results of a three dimensional numerical analysis of the correlation between the fiber mode field diameter and its longitudinal strain sensitivity. Both conventional and microstructured (commercially available and custom designed) fibers are investigated. Furthermore we compare the theoretical results with experimental data. To measure fiber sensitivity we developed a dedicated all-fiber Mach-Zehnder interferometer which enables the measurement of strain induced phase changes in various fiber types (including conventional and microstructured fibers). As a conclusion of our work we present relationship between strain sensitivity and MFD .

Influence of zero dispersion wavelength on supercontinuum generation in near infrared, visible, and UV range for a series of microstructured fibres

Nonlinear phenomena in microstructured fibers (MSFs) is defined by dispersive properties of a fiber. Zero dispersion wavelength (ZDW) and pump source wavelength play an important role in estimating the nonlinear effects and thus are subject of wide investigations. Multiple nonlinear processes like: four wave mixing (FWM), cross phase modulation (XPM), cannot be very efficient without phase matching which is achieved when a fiber is pumped in anomalous dispersion region. On the other hand, other nonlinear processes, such as self-phase modulation (SPM) and Raman scattering (RS), profit from pumping fiber in normal dispersion region. Thus the efficiency of supercontinuum (SC) generation in a fiber is dependent on its chromatic dispersion properties, which can be tailored by the proper fiber geometry design, and by the pump source wavelength. In our paper we present experimental analysis of SC generation obtained for a series of nonlinear MSFs. Our fibers have different ZDW and therefore when pumped by the same pump source, different nonlinear effects contribute to the SC generation. We analyze and explain the influence of ZDW on nonlinear effects. Comparisons of nonlinear interactions for fibers pumped in anomalous and normal dispersion regimes are provided. In our silica MSFs an ultra-short UV radiation was obtained by nonlinear processes estimation. We provide experimental analysis of MSFs geometrical parameters influence on UV conversion efficiency. Our studies present effective SC generation in near infrared, visible and UV ranges. Unique information about the influence of MSFs geometry on UV generation efficiency gives possibility to increase its application potential.