Richard Zeleny

Nearly zero dispersion-flattened photonic crystal fiber with fluorine-doped three-fold symmetry core

Abstract. A novel design of a photonic crystal fiber is presented. A nearly zero dispersion regime of operation has been achieved by using a three-fold symmetry core, which is improved by the avoidance of high-index doping in a central region. The core consists of pure silica surrounded by three fluorine-doped regions and three airholes. It can be confirmed by the numerical simulations using the finite difference frequency domain method that flattened dispersion characteristics upon wavelength over the range of 1250 to 1700 nm can be achieved in an optical fiber. Further, the potential fiber geometry imperfections caused some concern. Finally, the presented fiber design is compared to selected fiber designs with a dispersion close to zero.

Broadband submicron flattened dispersion compensating fiber with asymmetrical fluoride doped core

The goal of this paper is to investigate selected fluoride optical materials and to present a photonic crystal fiber designed for specific applications in dispersion compensation by using those materials. The idea how to restrict chromatic dispersion is to increase the index contrast by using calcium fluoride or barium fluoride in the first ring of holes, which lower the effective index. In general, fluoride materials compared to standard silica glass in many aspects offer better mechanical and optical properties. The use of fluorides allows achieving broadband dispersion suppression impossible to achieve in standard fibers with similar geometry. The presented result comprises a numerical model of a photonic crystal fiber in a submicron lattice, specific for its negative dispersion coefficient achieved for broad spectrum of telecommunication wavelengths, i.e. 1300 – 1700 nm. The core consists of pure silica surrounded by three doped regions and three air-holes. Holes doped with fluoride materials enhance negative dispersion coefficient to -438 ps.nm-1.km-1. The diameter of doped regions is about 1 micrometer. Simulations were done by using the full-vector FDFD method. The wavelength evolution of refractive index of materials was introduced by using the Sellmeier approximation. The major advantage of the designed fibers is their material composition, low attenuation and broadband utilization.

Broadband dispersion decreasing photonic crystal fiber for compression of optical pulses

A novel photonic crystal fiber for compression of optical pulses is designed and studied in this paper. The fiber comprises a silica core surrounded by nine rings of air-holes, where air-hole diameter of the innermost ring is gradually reduced along the entire fiber length. In order to obtain the required wavelength dependence of the effective refractive index, finite difference frequency domain method is employed. The calculated chromatic dispersion is flat from 1250 to 1700 nm at the fiber output, and therefore the photonic crystal fiber can be used at a desired wavelength in this range. On the contrary to other studies, chromatic dispersion in this paper is decreasing along the fiber length with the effective mode area. Therefore, during the propagation of solitary waves, the fiber nonlinear parameter increases and consequently the compression ratio is increased. Compression of solitary waves is investigated at the wavelengths of 1250, 1310, 1400, 1550, and 1700 nm. The compression ratio up to 30 for the first-order solitary wave with the length of 1550 nm can be achieved primarily by dispersion varied from 137 to 6 ps·nm-1·km-1 during the wave propagation.

An improved non-linear nearly-zero dispersion flattened photonic crystal fiber with the threefold symmetry core

A new type of dispersion flattened photonic fiber is presented. The fiber has an improved threefold symmetry core that consists of a silica core surrounded by three low-index regions and three air-holes. It can be observed from numerical simulation employing the full-vectorial finite difference frequency domain method that nearly-zero ultra-flattened dispersion can be obtained over the wavelength range of 1250-1700 nm. All fibers parameters are found to be non-immune to imperfections of geometry. An attention should though be paid to the potential fabrication process. The chromatic dispersion behavior with fabrication tolerances of 1 % and 2 % has been numerically demonstrated. Finally, fiber designs with five different hole-to-hole spacing (pitch) have been proposed. Each of the proposed fibers exhibits remarkable chromatic dispersion properties, such as nearly-zero ultra-flattened dispersion over wide wavelength range or zero dispersion at the wavelength of 1550 nm.

Broadband dispersion compensating photonic crystal fiber

Presented work is a study of dispersion properties of photonic crystal fibers. The main objective is to design photonic crystal fibers suitable for potential compensation of group velocity dispersion of optical signals. New fiber structures are proposed, for example, a flattened-dispersion compensating photonic crystal fiber for broadband utilization in high-speed transmission systems is presented. The structure shows flattened negative value of dispersion over the O, C, and L bands. The fiber can eventually get applied in broadband optical signal recovery in systems with wavelength division multiplexing. Last but not least, a dual-core compensating microstructured fiber is optimized in order to achieve low dispersion and low loss at the C-band. Results presented in this work are obtained by using the FDFD method.

Design of As 2 S 3 -based chalcogenide photonic crystal fibre with large mode area and low bending loss for mid-infrared

In this paper, an effectively single mode reasonably bendable leakage channel fibre is designed having the highly nonlinear As2S3 chalcogenide glass as the fibre background. The fibre is designed to have effective mode area as large as possible as well as to have low confinement loss in the bent fibre. The fibre modal properties are calculated through the finite difference frequency domain method to investigate supercontinuum generation inside the designed fibre. Using the light pump near the zero dispersion wavelength at 4.9 μm, the supercontinuum is numerically observed as nearly flat and broad from 3 to 8 μm.

Investigation of optical thin films printed on the surface of facets of photonic crystal fibers

Optical fibres are widely used in various applications as a medium for optical signals or optical transfer. This transport can be realized on long distance, compared to free space optics, which significantly extends reach of applications. Free space optics and fibre optics are combined in practice to yield the maximum performance of individual components forming a particular system. In such cases, light coupling from free space into fibres is required and it is frequently implemented with the use of lenses. An optical signal coupled into a fibre may also need certain modifications of spectral and spatial properties to allow its propagation down the fibre or reduce the amount of power carried in. The above requirement has been fulfilled by modifying surface of facets of photonic crystal fibres. By extrusion of a certain amount of host material from the surface, it is possible to obtain a structure resembling a thin film or an opaque layer for certain wavelengths. Several different structures of photonic crystal fibres and materials are considered to show influence of such thin-film on signal properties. This investigation is carried out in context of abilities of ablation of material from surfaces of photonic crystal fibres. Only certain shapes and geometrical arrangements can be considered. One of the goals is to specify, which of them are key for potential modification of spectral characteristics of photonic crystal fibres. The printed structures could potentially work like a thin-film ablation. Rigorous and versatile finite difference method has been employed to model propagation of light, its incidence onto a surface of the photonic crystal fibre, and subsequent propagation down the fibre. The simulations are carried on small pieces of photonic crystal fibres, with the length of tens of micrometres, due to well-known demands of the simulation technique on computational resources. Nevertheless, such a simplification is valid, since the structure is longitudinally uniform beyond the thin-film layer. However, this is aspect is not covered in the presented paper and it is our ongoing effort. Finally, the goal is to verify if the investigated structures can work as a slot waveguide.

Dispersion limits of the small mode area photonic crystal fibers designed as a broadband compensator

Nonlinear photonic crystal fibers with small effective mode area allow to control chromatic dispersion in the near-infrared region. In this paper the chromatic dispersion is controlled entirely by structural parameters and the influence of each structural parameter is examined and described in detail. Understanding of the influence not only permits fiber design and dispersion tailoring, but also predicts the potential manufacturing tolerances. As a consequence, the fiber structural parameters are modified to found the balance between the operating bandwidth and the high negative dispersion parameter. We found that the limit value for the dispersion parameter is of about −1600 ps•nm-1•km-1 at 1550 nm whereas the desired dispersion slope is achieved over the 120 nm wide band. We predict that the negative dispersion parameter cannot be higher in the small effective mode area photonic crystal fibers operating over the bandwidth larger than the one considered in our paper. The results are calculated by the full-vectorial finite difference frequency domain method.

Precise optical differential thermometer sensor based on interferometric measurement of thermal expansion coefficient

Knowing a thermal expansion coefficient and measured exact thermal expansion, it is possible to design a very sensitive sensor measuring temperature differential. A Michelson interferometer is used to determine temperature changes. It measures linear expansion on a metal object, e.g. a copper rod, as a change in length in response to a change in temperature. Based on the obtained interferograms and knowing the value of thermal expansion coefficient, temperature differential can be calculated. The accuracy of the procedure can be determined by using the exact differential method based on the measurement errors for linear expansion, and initial length. The contribution of this paper is the employment of Michelson interferometer to design a very sensitive differential thermometer measuring with the accuracy of one thousandth degree Celsius. It results from the achieved precision of measuring the optical path length changes in the range of hundreds nanometers. The advantage of this sensor is its precision and noncontact procedure.

Limits of advanced modulation formats for transition in fiber optic telecommunication systems to increase speeds from 10, 40, 100 Gb•s-1 to higher bit rates

In this paper we investigate limits of intensity and phase modulation formats used in optical communications. Non- Return to Zero, Return to Zero, Chirped Return to Zero, Carrier-Suppressed Return to Zero, Binary Phase Shift Keying, and Quadrature Phase Shift Keying including the most actual solutions, such as Polarization Division Multiplexing Quadrature Phase-Shift Keying, are investigated in terms of spectral efficiency, Bit Error Rate to find the limits for selected topologies and spectral grids in Dense Wavelength Division Multiplexing. Differential Phase-Shift Keying and mainly Differential Quadrature Phase-Shift Keying offer improvements in Bit Error Rate and transmission reach, among others. There are practical conclusions about transition from 10 Gb•s-1 to much higher bit rates. We study the potential increase of efficiency of Wavelength Division Multiplexing. We investigate the performance of Polarization Division Multiplexing Quadrature Phase-Shift Keying in very high speed optical systems that are promising even for terabit transmission.