Michał Nejbauer

Ground and Excited State Double Hydrogen Transfer in Symmetric and Asymmetric Potentials: Comparison of 2,7,12,17-Tetra-n-propylporphycene with 9-Acetoxy-2,7,12,17-tetra-n-propylporphycene

Analysis of time-resolved anisotropy of transient absorption enabled determination of room temperature ground and excited state rate constants for intramolecular double hydrogen transfer in two similar porphycenes, one of them with symmetric and the other, with asymmetric character of a double minimum potential for hydrogen motion. The perturbation preserves a quasi-symmetric minimum in S(0), but the rate decreases approximately two times. In S(1), the perturbed potential becomes strongly asymmetric, and the downhill hydrogen transfer occurs with a rate higher than that observed for a symmetrical compound.

Efficient spectral shift and compression of femtosecond pulses by parametric amplification of chirped light

We present a method for an efficient spectral shift and compression of pulses from a femtosecond laser system. The method enables generation of broadly tunable (615-985 nm) narrow bandwidth (≈10 cm(-1)) pulses from the femtosecond pulses at 1030 nm. It employs a direct parametric amplification--without spectral filtering--of highly chirped white light by a narrow bandwidth (<5 cm(-1)) 515 nm pump pulse. The system, when pumped with just 200 μJ of the fundamental femtosecond pulse energy, generates pulses with energies of 3-9 μJ and an excellent beam quality in the entire tuning range.

Spectral compression of femtosecond pulses using chirped volume Bragg gratings

In this Letter, we demonstrate a 360 fold spectral bandwidth reduction of femtosecond laser pulses using the method of sum frequency generation of pulses with opposite chirps. The reduction has been achieved in a compact setup in which a single chirped volume Bragg grating replaces conventional stretcher and compressor units. Starting with 180 fs pulses, we have obtained, with a 30% overall efficiency, pulses longer than 100 ps with the spectral bandwidth of 0.23  cm-1 (7 GHz). We also discuss our method on theoretical grounds.

Full 3D modelling of pulse propagation enables efficient nonlinear frequency conversion with low energy laser pulses in a single-element tripler

Although new optical materials continue to open up access to more and more wavelength bands where femtosecond laser pulses can be generated, light frequency conversion techniques are still indispensable in filling the gaps on the ultrafast spectral scale. With high repetition rate, low pulse energy laser sources (oscillators) tight focusing is necessary for a robust wave mixing and the efficiency of broadband nonlinear conversion is limited by diffraction as well as spatial and temporal walk-off. Here we demonstrate a miniature third harmonic generator (tripler) with conversion efficiency exceeding 30%, producing 246 fs UV pulses via cascaded second order processes within a single laser beam focus. Designing this highly efficient and ultra compact frequency converter was made possible by full 3-dimentional modelling of propagation of tightly focused, broadband light fields in nonlinear and birefringent media.

Simple all-PM-fiber laser system seeded by an all-normal-dispersion oscillator mode-locked with a nonlinear optical loop mirror

In this paper we report an all-PM-fiber laser amplifier system seeded by an all-normal-dispersion oscillator mode-locked with a Nonlinear Optical Loop Mirror (NOLM). The presented all-normal-dispersion cavity works in a dissipative soliton regime and delivers highly-chirped, high energy pulses above 2.5 nJ with full width at half maximum below 200 fs. The ultrafast oscillator followed by the all-PM-fiber amplifying stage delivered pulses with the energy of 42.5 nJ and time duration below 190 fs. The electrical field of optical pulses from the system was reconstructed using the SPIDER technique. The influence of nonlinear processes on the pulse temporal envelope was investigated.

Convenient scheme for efficient generation of mid-infrared CEP-stabilized femtosecond pulses using 1030-nm source (Conference Presentation)

Here we propose a simple and effective system for conversion of 200-fs, 1030 nm pulses into few-optical-cycles pulses in the 3-6 um spectral range. The scheme works as follows. Small part of pump radiation is converted into supercontinuum and the radiation from the 700-900 nm spectral range is used as a seed in the first OPA. The remaining pump beam passes through a BBO crystal which converts less than 3% of the pulse energy into the second harmonic (515 nm). The green beam is used as a pump in first OPA based on 1 mm-think BBO crystal. Collinear geometry is used to generate idler pulses in the 1.2-1.6 um range without angular chirp. The idler beam is then used as a seed in the second collinear OPA, which is pumped by the unconverted 1030 nm beam. The resulting idler pulses in 3-6 um range are slightly chirped depending on the crystal used (LGS, GS, AGS, LiIO3) and are compressed in the bulk material (silicon or germanium). Such scheme provides a passive stabilization of the carrier-to-envelope phase of the resulting pulses. Because of the lower group velocity mismatch (GVM) between interacting pulses in nonlinear infrared crystals, as compared to Ti:sapphire based pump lasers, bandwidths supporting few-optical-cycles pulses in the 3-6 um spectral range are easily obtained. In the experiment we have used 200 uJ pulses (1030 nm) at repetition rate 1-10 kHz and the total quantum conversion efficiency 25-35% was reached. The resulting mid-infrared pulses were fully characterized.

Study on parameters of fiber loop mirrors as artificial saturable absorbers

This work focuses on practical analysis of the Nonlinear Optical Loop Mirror (NOLM). This all-fiber device serves as a saturable absorbers for passive mode-locking in ultrashort pulse generation. It has a great potential for further development as it’s made in all fiber technology, and can be easily adapted to custom system. We investigate and provide experimental results as well as characteristics of NOLM’s in several different setups. The all-fiber laser system used for testing produced femtosecond pulses with a positive chirp. We present reflection as a function of input power and as a function of the coupler’s coupling. Also, relevant spectral characteristics are shown. These results may be useful in further development of fiber laser systems and other applications.

Femtosecond fiber CPA system in a single pass configuration

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

A Prototype Femtosecond Laser System for Precise Micromachining

In this article we present a concept of a prototype design of the femtosecond laser micromachining system and a femtosecond solid-state Yb:KYW laser developed in our laboratory. Ultra-short laser pulses have many advantages over the long pulses laser micromachining, due to their unique ability to interact with different materials without transferring heat to them. This allows very precise and pure laser-processing, clean cuts and sharp edges. A femtosecond laser micromachining device presented here is equipped with a synchronized motion control system. Application of this advanced motion system results in the maximum use of the femtosecond laser potential for high-precision micromachining.

Efficiency optimization of the square pulse pumped terawatt level optical parametric chirped pulse amplifier

Numerical simulations and analysis of a very efficient and stable non-collinear Optical Parametric Chirped Pulse Amplification (OPCPA) femtosecond system are presented. The system is optimized for a long (nanosecond), rectangular temporal profile and a flat-top spatial profile of the pump laser pulse. We show that a two stage system consisting of a multipass preamplifier and a time-sheared power amplifier operating around 850 nm and pumped by a 532 nm pulse can amplify pulses directly from a femtosecond oscillator up to multi-terawat levels with quantum efficiencies as high as 0.9. We also discuss practical schemes of the few-cycle multi-terawatt OPCPA systems employing different nonlinear crystals. The results of the Monte-Carlo simulations are used to balance the stability and efficiency of the parametric amplifier system.