Lukasz Gorajek

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.

Resonantly pumped, high peak power Er:YAG laser

The high peak power hybrid Er:YAG laser, resonantly pumped at the wavelength of 1532 nm was developed. The fused silica acousto-optic modulator driven by 10 W of RF power was deployed as the Q-switch for such a laser. For the best case of Q-switching regime the pulses of 110 kW peak power (4 mJ, 37 ns) were reached at the pump power of 7.8 W and the repetition rate of 500 Hz.

Q-switched Er:YAG lasers resonantly pumped by Erbium fiber laser

The study describes efficient, acousto-optic Q-switching of Er:YAG laser at the 1645 nm eye-safe wavelength. For longitudinal pumping at wavelength 1532 nm, linear-polarized 10 W Erbium fiber laser radiation was used. The investigated Er:YAG crystals were 25 and 40 mm long and their Erbium concentration was 0.20 and 0.25%, respectively. For giant pulse generation, a fused silica acousto-optic modulator was inserted inside the Er:YAG laser oscillator. For a maximum incident pump power of 7.95 W, pulse energy up to 4.1 mJ was generated with pulse duration 34 ns at 500-Hz repetition rate; the corresponding peak power was 119 kW.

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.

Passively Q-switched Nd:YAG laser with diffractive output resonator

Passive Q-switching is presented in a novel, closed-loop, diode side-pumped Nd:YAG slab laser. Q-switched generation provided five pulse series with total energy of 120 mJ, 1 MW peak power and 24 ns single pulse width for maximal pump energy of 840 mJ. Beam quality parameter M 2 was less than 2. Generation parameters in both four-wave mixing and classic linear resonator with similar length were compared.

Laser and thermo-optical investigations of Nd:YAG ceramics

The ‘wet’ technology was implemented for processing of Nd:YAG ceramics. The samples of disk, rod, slab shapes with 1–2% Nd dopant were fabricated and tested. Several method of optical characterization were applied. Near 80% transmission and scattering losses <0.2 cm−1 were demonstrated. The laser action with 34% slope efficiency was obtained for the best case. To characterize the spatial inhomogeneities of output parameters “half-microchip configuration” with output coupler formed by uncoated output facet of plane parallel ceramic sample was implemented. Thermally induced aberrations and birefringence observed in Nd:YAG ceramic of disk samples under high heat load were examined and compared with numerical models.

Actively Q-switched Thulium Lasers

The near and mid infrared range of optical spectrum (1500 – 5000 nm) has attracted great attention over the last decades, mainly because of a numerous set of absorption / emission lines being “finger prints” of specific chemical and biological compounds (see e.g. (Sorokina & Vodopyanov, 2003)). One of the most reliable techniques for stand alone detection and identification of such agents is laser spectroscopy, for which the coherent, highly monochromatic, tuneable laser sources are required. Research on mid infrared laser sources destined for long range detection is one of the most active areas of solid state photonics nowadays (see e.g. (Godard, 2007) (Mirov et al., 2007) etc.) (Schellhorn, et al., 2007) (Eichhorn, 2008)). One of the necessary requirements for such application is high spatial coherence of laser output beam (parameter M2 near 1). Among the most perspective types of lasers operating in a mid infrared range (above 2.2 μm-wavelength) are quantum cascade lasers, Cr2+:AIIBVI lasers and optical parametric generators. For shorter wavelengths laser diodes and erbium (1600 – 1700 nm), thulium (1800 2000 nm), holmium (2050-2200 nm) solid state lasers realised in bulk/crystalline/ or fibre technology are available. The main properties differentiating such groups of lasers are peak power and pulse energy. It has to be pointed out, that for such a wavelength range, damage thresholds of optical elements are severely decreased, comparing to 1-μm wavelength, because of the presence of water vapour and OH groups, having a wide absorption peak just in this wavelength range. The physics of laser action in semiconductor lasers prohibits the high peak power and high energy operation. Moreover, the high CW power 1D or 2D laser diodes stacks characterise low spatial coherence and high beam volume, thus they can be used mainly as optical pumps for other lasers, e.g. holmium or erbium ones. For short infrared range (< 2 μm wavelength) the Tm or Ho–doped fibre lasers sources operating in a CW regime seem to be the most perspective choice (Gapontsev et al., 2007) (McComb et al., 2009). The pulsed operation of fibre lasers, with energies of tens to hundreds of microJoules, can be realized applying Q-switching technique (Eichhorn & Jackson, 2008) (Eichhorn & Jackson, 2009) or gain-switched method. The best result of 2-μm Q-switched Tm:fibre lasers from the point of view of pulse energy (0.6 mJ for 10 Hz of rep. rate) were obtained (Barnes & De Young, 2009). Mid infrared fibre lasers suffer from lack of good quality active glasses and problems with high peak power / high energy operation for

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

Short-pulsed gain-switched Cr2+:ZnSe laser

We report the first demonstration of gain-switched, ultra-low-threshold Cr2+:ZnSe laser generating pulses as short as 1.75 ns. A diode pumped Tm3+:YLF laser delivering up to 5 mJ energy in 11 ns pulses was utilized as a pump source. The laser operated at 20 Hz repetition rate with 0.1 duty factor allowing us to reduce thermal effects in an active crystal. In a short resonator (length, 70 mm) we obtained more than 0.5 mJ of output energy and 300 kW of corresponding peak power. The Cr2+:ZnSe laser was characterized by very low losses manifesting themselves by an extremely low generation threshold of less than 7 μJ and very high slope efficiency (reaching the quantum efficiency) determined with respect to absorbed pump power.

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.