Mateusz Kaskow

250 mJ, self-adaptive, diode-side-pumped Nd:YAG slab laser

A scheme of self-adaptive, closed-loop, diode-side-pumped Nd:YAG slab laser was presented. As a result of four-wave mixing of standing waves intersecting at a small angle in a closed-loop cavity, the self-adaptive process of beam cleaning leading to fundamental mode operation despite severe asymmetry of the inversion profile and thermal-optical distortion was achieved. The output beam was extracted from the cavity as the first- order diffraction beam on the dynamic gain gratings created in an active medium. The near-diffraction-limited (parameter M(2)<1.4, divergence of 1 mrad) output beam of 250 mJ energy and the optical slope efficiency of 30% at a repetition rate of up to 25 Hz in a free-running regime were demonstrated.

Optimization of end-pumped, actively Q-switched quasi-III-level lasers

The new model of end-pumped quasi-III-level laser considering transient pumping processes, ground-state-depletion and up-conversion effects was developed. The model consists of two parts: pumping stage and Q-switched part, which can be separated in a case of active Q-switching regime. For pumping stage the semi-analytical model was developed, enabling the calculations for final occupation of upper laser level for given pump power and duration, spatial profile of pump beam, length and dopant level of gain medium. For quasi-stationary inversion, the optimization procedure of Q-switching regime based on Lagrange multiplier technique was developed. The new approach for optimization of CW regime of quasi-three-level lasers was developed to optimize the Q-switched lasers operating with high repetition rates. Both methods of optimizations enable calculation of optimal absorbance of gain medium and output losses for given pump rate.

Diode-pumped, actively Q-switched Nd:YAG laser with self-adaptive, reciprocal, closed-loop resonator

We demonstrate, for the first time to our knowledge, active Q-switching in self-adaptive, reciprocal, closed-loop, diode-side-pumped Nd:YAG slab laser. Pulse energy of 19 mJ with 9.5 ns pulse duration, corresponding to 2 MW of peak power with near-diffraction-limited beam quality was achieved.

Resonantly pumped, Q-switched Ho:YLF laser with output energy of 5 mJ at 1 kHz

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 highly efficient resonantly pumped Ho:YAG laser

An efficient high-peak-power Ho:YAG hybrid laser resonantly pumped by a 20 W linearly polarized Tm:fiber laser at the wavelength of 1908 nm was developed. At room temperature a maximum continuous output power of 10.7 W with a slope efficiency of over 55% with respect to the incident pump power was achieved. In Q-switching regime an acousto-optic modulators were applied. The research was conducted for normal and Brewsters angle Q-switches respectively. In CW pumping regime the repetition rate was changed from 500 Hz to 5000 Hz. For the best case, for 5 kHz repetition rate, pulses of 1.6 mJ energy and 123 kW peak-power were achieved at the wavelength of 2090.2 nm with an M2 ≈ 1.6.

The new optimization method of Q-switched quasi-three-level lasers

The aim of work is to develop efficient theoretical model enabling analysis and optimization of Q-switched quasi-threelevel lasers. The model consists of two parts: pumping part and Q-switched part, which can be separated in a case of active Q-switching regime. For the pumping of quasi-three-level gain medium the semi-analytical model was developed, enabling the calculations for average occupation of upper laser level for given pump power and pump duration, spatial pump beam profile, length and dopant level of gain medium. Moreover, ground-state-depletion, up-conversion parasitic relaxation and temperature effects were considered in the model. The new approach for optimization of CW regime of quasi-three-level lasers was developed for Q-switched lasers operating with high repetition rates. Moreover, for long pump durations comparable to laser upper level lifetimes, the optimization procedure based on Lagrange multiplier technique was developed. The simple analytical formulae for effective pump duration needed to achieve the quasistationary inversion for given pump power density and up-conversion parameter were derived. The model enables the optimization of gain medium length and absorbance, average pump area and out-coupling losses for wide class of quasithree- level lasers.

Analysis of pumping schemes for high brightness diode-side-pumped lasers

Several schemes of side-pumping by novel, high brightness, 2D laser diode stacks were analyzed. The three most promising schemes were chosen, for which the optical set-ups have been designed, manufactured and preliminary characterized. The special, robust, compact cavity with high tolerances to misalignments, mechanical disturbances and shocks was designed. The analysis of temperature sensitivity of pump unit and laser was performed. In preliminary characterization 180 mJ of output energy with 2-mrad full divergence angle (parameter M2 ~ 5) in free running mode for 0.8-J of incident pump energy was demonstrated in such a compact cavity with 80% transmission of integrated output coupler. The main drawbacks in such design found in preliminary experiments are: low threshold of self-lasing due to high gain density and inhomogeneities in transverse beam profile. The strategy of mitigation of these drawbacks was discussed.

Self-adaptive, passively Q-switched, diode-side-pumped Nd:YAG slab laser

Passively Q-switched, closed-loop, self-adaptive resonator with a Nd:YAG as an active medium is presented. For maximal pump energy of 840 mJ Q-switched generation provided 5 pulse series with total energy of 120 mJ. Single pulse width was 24 ns. The beam quality parameter M2 was 1.6. Four-wave mixing and linear resonators were compared.

Diode pumped Yb-lasers Q-switched by V:YAG saturable absorber

V:YAG saturable absorber, developed mainly for 1.3 μm lasers Q-switching, was used as a passive Q-switch for the 1.03 μm Yb-doped YAG (10% Yb/Y, 3mm long) and LuAG (15% Yb/Lu, 1mm long) lasers. Longitudinally diode pumped gain medium together with the V:YAG crystal were placed inside the 22mm long hemispherical laser cavity. For Yb-doped crystal excitation fibre-coupled (fibre core diameter 100 μm) laser diode (max power amplitude 20W, emission wavelength 968 nm) was used. The laser diode was operating in a pulsed regime (repetition rate 10 Hz, pumping pulse width 2 ms) to reduce parasitic thermal effects inside the gain medium. Stable Q-switching was obtained for laser output coupler reflectivity 70% and V:YAG initial transmission 70% at Yb laser emission wavelength. For the both tested active media the parameters of the generated giant pulses were similar. Pulses with duration of 2.5 ns (FWHM), energy about 0.3 mJ, and peak power up to 120kW were generated. The maximal Q-switched pulses repetition rate inside the single pumping pulse was 6.6 kHz in case of Yb:YAG and 8.6 kHz in case of Yb:LuAG. The beam transversal profile was close to the fundamental Gaussian mode. The output was partially polarized.

Diffraction-limited, grazing-incidence Nd:YVO4 slab laser side pumped by 2D laser diode stack

A single and double-bounce grazing-incidence Nd:YVO4 laser is presented. The output pulse energy of ~20 mJ with slope efficiency reaching up to 24.5% was achieved. The beam quality parameter M2 was 1.25.