Oscar G. Ibarra Manzano

Formation of parabolic optical pulses in passive optical fibers

We have investigated a nonlinear pulse reshaping towards parabolic pulses in the passive normal dispersive optical fibers. We have found that pulses with parabolic intensity profile, parabolic spectrum and linear chirp can be obtained due to the passive nonlinear reshaping at the propagation distance exceeding a few dispersion lengths. These pulses preserve parabolic profile during subsequent pulse propagation in a fiber. We have examined the influence of initial pulse parameters and fiber parameters on the resulted pulse shape.

All-normal dispersion photonic crystal fiber for parabolic pulses and supercontinuum generation

We have designed an all-normal dispersion photonic crystal fiber optimized for pumping at 800 nm with initial pulses which are typical for conventional Ti:Sapphire lasers. Parabolic pulse formation and supercontinuum generation in this fiber is analyzed both in time and frequency domains.

Optical power limiter on the basis of 2D photonic crystal

In this work we investigated the possibility of the radiation intensity stabilization using the 2D nonlinear photonic crystal. Numerical experiment approves this assumption. The numerical experiment is carried out for saturable nonlinear material with different saturation threshold. Due to the utilization of photonic crystals, such a devices can be easily integrated to photonic circuits.

All-Normal-Dispersion Photonic Crystal Fibers Under Prism of Supercontinuum Generation and Pulse Compression

We discuss properties of all-normal-dispersion photonic crystal fibers in context of supercontinuum generation and compression of ultrashort pulses. The application of pump pulses typical for the state of the art Ti:Sapphire lasers allows obtaining quite flat and broad spectra extending more than one octave in this fiber. The influence of initial pump pulse parameters such as pulse energy, duration, and pump wavelength on the SC generation was investigated. It was shown that compression of pulses with such SC spectra allows obtaining a few cycle pulses up to 8.1 fs, if a simple quadratic compressor is used and single cycle pulses up to 2.5 fs, if full phase compensation is provided.

Ultrafast Nonlinear Fiber Optics: Single-Pulse Supercontinua and Specialty-Shape Pulses

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

Density of modes maps for design of the photonic crystal devices

Photonic crystal (PhC) waveguides are the prospective structures for high-speed optical micro-devices. Possessing high-effective light localization, such waveguides allow introducing active materials making additional all-optical signal control possible. The most effective from the technological point of view is embedding the PhC filters with characteristics close to the ones of the background PhC (i.e. with the same refractive index, PhC period, etc.). As has been shown in previous papers,1, 2 such filters possess spectral characteristics that allow using them for processing the ultra-short pulses in case of either passive (implemented with linear materials) of active (with nonlinear materials) devices. Moreover, from the technological point of view such PhC waveguides with filters can be easily integrated into the electronic circuit which reduces costs and production time. Unfortunately, optical losses and back reflection from such a filters makes them unsuitable for high-speed integrated optical circuits. In this work we propose analysing both density of modes (DOM) and the transmission spectra to optimize the parameters of the PhC filters made of linear or nonlinear material. Proposed characteristics have been first introduced to optimize the PhC filters used for wavelength division demultiplexing.3 However, since waveguiding properties have not been taken into account, the photonic density of states maps as well as transmittance maps can be applied with high accuracy to a bulk PhC but not to confined PhC filter. Taking into account waveguiding properties provides great enhancement to the precision of characterization of the confined PhC filters.

Comprehensive analysys of the PhC-based filters for optical micro-devices engineering

We propose a new design method for the photonic crystal (PhC) wideband passive optical filters. For this, we join the PhC spectral analysis with investigation of guiding properties of the PhC-waveguides and filters. The method allows improvement of the characteristics of the demultiplexers presented in earlier works, which has been designed by means of photonic band gap maps analysis.