This PDF file contains the front matter associated with SPIE Proceedings Volume 6998, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

We demonstrate that resonant high-modulation-depth saturable absorbers allow efficient pulse shortening in Q-switched lasers. Using a 70% modulation depth resonant saturable absorber mirror we achieved 8 ns pulses that are close to the limit set by the cavity length and are, to our knowledge, the shortest pulses demonstrated to date from passively Qswitched fiber lasers.

The multi-core fiber laser is a promising fiber laser concept since it combines good beam quality and a high mode field diameter to reduce nonlinear effects especially for pulsed laser operation. Therefore this concept is a good candidate for high power fiber laser operation with a good beam quality. In the present paper we report on the characterization and the laser operation of a fiber laser with a hexagonal array of 19-cores. The near and the far field intensity distribution of the emitted beam as well as the bending-induced transversal mode selection have been investigated. The obtained experimental and simulation results and show a good agreement.

We report on the observation of bound state of some hundreds of solitons in a passively mode-locked Er:Yb-doped fiber laser. A double-clad fiber is used in a unidirectional ring cavity where the mode-locking is obtained thanks to the nonlinear polarization rotation. The phenomenon is described theoretically using a multiscale approach to the gain dynamics.

I review our recent work in the area of high power 2 μm silica fibre laser development particularly in the area of highly efficient Tm³⁺-doped silica and Ho³⁺-doped silica fibre lasers that are excited with diode lasers operating at 1150 nm.

We report on an all-normal dispersion passively mode-locked fiber laser based on an ytterbium-doped largemode- area microstructure fiber and featuring high-energy ultra-short pulses. Mode-locking was achieved with a high modulation depth semiconductor saturable absorber mirror (SESAM). We investigate the influence of the modulation depth of the SESAM on the laser performances. We show that mode-locking could be achieved with a modulation of only 10 %. However, the best performances in term of pulse energy are obtained with the highest modulation depth (35 %). In this case, the laser delivers 3.3 W average output power with positively-chirped 5.5 ps pulses at a center wavelength of 1033 nm. The pulse repetition rate is 46.4 MHz, which results in an energy per pulse of 71 nJ. These pulses are extra-cavity de-chirped down to 516 fs using bulk gratings. The average power of the de-chirped pulses is 2.3 W, which corresponds to a peak power of more than 96 kW.

This communication describes the fabrication of Yb doped silica preforms by modified chemical vapor deposition (MCVD) method using Yb(C₁₁H₁₉O₂)₃ and AlCl₃ in vapor phase. In order to investigate the optical quality of the preforms a systematic spectroscopic study was carefully carried out on various samples with different Yb and Al doping levels. In this way, the influence of the preforms composition on the Yb spectroscopic properties was studied in details, allowing the prediction of the composition favoring the laser emission with the highest efficiency. The predictions have then been validated after pulling the preforms in large mode area fibers (LMA).

The development of femtosecond lasers has continued rapidly over the past decade from laboratory systems to an impressive range of commercial devices. Novel materials, notably quantum-dot semiconductor structures, have enhanced the characteristics of such lasers and opened up new possibilities in ultrafast science and technology. In our most recent work, it has been demonstrated that quantum-dot structures can be designed to provide an efficient means for the generation and amplification of ultrashort optical pulses at high repetition rates. The work also confirms that quantum dot based semiconductor saturable absorber mirrors exhibit a degree of flexibility which allows control and tuning of the ultrashort pulse laser systems. Further developments in ultrashort-pulse solid-state, fibre and semiconductor external cavity lasers, by means of both active and passive semiconductor quantum dot components are also presented.

The Tm:YLF laser end-pumped by a fiber coupled laser diode bar, actively Q-switched with tunability option was worked out and examined for low duty factor and true cw pumping regimes. The numerical model of such a laser including thermo-optic effects was developed to examine properties of Q-switching regimes. Above 7-W and near 30% slope efficiency was demonstrated in free-running mode for low duty factor pumping for the best case. Above 2-W output power was obtained in cw pumping regime for 220-mm long cavity and 15% transmission of output mirror. The cw output power was limited here by reabsorption losses and thermal lensing. The Lyot's filter tuning in 1845-1935 nm range with 3-nm linewidth was achieved for free-running regime. The divergence angle was about 4.3 mrad and estimated parameter M² < 1.3. For Q-switching regime, the acousto-optic modulator made of fused silica was deployed. In the best case of low duty factor pumping (10-ms pump duration, 10-Hz repetition rate) pulse energy was 10.5 mJ, pulse duration was about 20 ns corresponding to near 0.5 MW of peak power. In case of cw pumping for maximum incident pump power of 19 W we have demonstrated 10-mJ pulses of 220 kW peak power for 133 Hz of repetition rate. The output energy and peak power was limited here by damages of applied laser elements. The peak power of 12 kW and 1.7 W of average output power were achieved for the highest rep. rate of 1 kHz.

We present a new optical device for pulse picking and Q-switching based on a LiTaO₃-crystal together with polarizers. LiTaO₃ is piezoelectric, hence when a harmonic voltage course with a proper frequency is applied to the crystal it will start to oscillate resonantly in a mechanical eigenmode. Due to photo-elasticity an artificial modulated birefringent is induced by this oscillation such that the polarization of trough-going light is modulated. Together with polarizers the transmission of the whole setup can oscillate between 0 and 100%. The applied voltage amplitude is usually in the order of below 10 V. With a special choice of the crystal dimensions it is possible that the first shear eigenmode has exactly three times the frequency of the first longitudinal eigenmode. Both modes have qualitatively the same influence on polarization, such that with a proper superposition of these two modes a short opening time of the setup can be achieved, which can be used to enforce pulsed laser operation. The latter was realized with a small end pumped fibre laser. A pulse sequence with 127 kHz and a ratio of peak power to average power of ~30 was achieved.

A diode-pumped chirped-pulse amplifier (CPA) system based on Yb:glass and Yb:YAG as a picosecond pump source for a future ultra-high peak-power optical parametric chirped-pulse amplifier (OPCPA) is currently under development. Pulses as short as 300 fs generated in an Yb:glass mode-locked oscillator have been stretched to the nanosecond level. The seed pulses are then amplified up to an output pulse energy of 100mJ in a diode-pumped Yb:glass regenerative amplifier and a subsequent Yb:YAG booster. At a small-signal gain of 10³ in Yb:YAG the initial pulse bandwidth of 4 nm (FWHM) has been gain-narrowed to 1.5 nm, which allows the re-compression to 1.1 ps. With a multi-pass Yb:YAG amplifier which has been seeded by a Q-switched sub-10-nanosecond laser an output pulse energy of 2.9 J has been achieved. In quasi-cw-mode a peak output power of 7.6kW and a tuning range of 5nm have been obtained. The foot-print-size of the multi-pass amplifier is 0.8m×1.1m which illustrates the degree of system compactness.

We experimentally compared the co- and counter-propagative pumping scheme for the amplification of ultra-short optical pulses. According to pumping direction we show that optical pulses with a duration of 75 fs and 100mW of average output power can be obtained for co-propagative pumping, while pulse duration is never shorter than 400 fs for the counter-propagative case. We show that the impact of non-linear effects on pulse propagation is different for the two pumping configurations. We assume that Self Phase Modulation (SPM) is the main effect in the copropagative case, whereas the impact of Stimulated Raman Scattering is bigger for the counter-propagative case.

The Q- switched laser system for short pulse laser beam generation are presented. Through the creation of required configuration of photon field at short cavity using external cavity short and intensive laser pulse is generated. The investigations of spatial characteristics of laser field in the center of the converging beams are carried out.

Beams with radial polarization have attracted an increasing interest during the past few years because of their attractive properties. An overview of the different intra- or extra-cavity techniques to generate such beams is given in the present paper. The design, fabrication and characterization of a multilayer polarizing grating mirror developed for an Yb:YAG thin-disc laser resonator are reported. The potential of the proposed solution is discussed together with the first demonstration of a radially polarized Yb:YAG thin-disc laser.

In order to improve the resonator design of solid state lasers, we provide a new simulation tool to calculate the output power of the outgoing laser beam and its beam quality in terms of the M²-factor. Furthermore, the application of our method to actively Q-switched lasers provides detailed information about the pulse shape. Using a time-dependent 3D simulation with Gaussian modes and Finite Elements, we take into account different pumping types, thermal lensing effects, and the gain process in the crystal as well as optical apertures in the resonator. Therefore, our model, which we call 'Incoherent-Multimode-Analysis' (IMMA), combines the simulation of resonator optics with the computation of structural mechanics and a modified set of rate equations in the crystal to describe complex effects like mode competition.

To qualify passive fibers for (high power) laser beam delivery, different experimental approaches (interferometric, heterodyn, M², ...for beam characterization at fiber output are under test in the community. Measurement of the individual strength of different components (eigenmodes) contained in the superposition at the fiber output in dependence for example on bending radius seems to be very promising. This can be done by means of optical correlation filters based on DOEs. For a standard telecommunication fiber SMF-28, operated at 633 nm, this could be demonstrated earlier¹ by us. Here we present experimental results for quantitative proof of LP modes in LMA fibers as well as in SMF-28 fibers by means of such correlation filters, and demonstrate potential and limitations of this approach.

With the help of a newly implemented circular perfectly matched layer for complex coordinate stretching a fast and accurate calculation of radiation losses of optical waveguides is reported in the present contribution. We will show the results of a fully vectorial finite-element calculation used for the design of special fibers for highpower high-brilliance beam delivery. In particular, we have investigated the propagation losses in the so-called hollow-core and solid-core Bragg-type fibers. These optical fibers have claddings consisting of alternating high and low index layers and offer an asymptotically single-mode behavior even for large core sizes. In the case of the hollow-core Bragg fibers, the preferred mode is a non-degenerated azimuthally polarized doughnut mode (TE01) because it experiences the lowest losses, whereas for the solid-core Bragg fiber a two fold degenerated linearly polarized mode (LP01) experiences the lowest losses. We will describe how to design Bragg fibers for minimal propagation losses and how to reduce the bending sensitivity of these structures. Combining high mode-field diameter with low losses and a low bending sensitivity makes these fibers suitable for high-power single-mode beam delivery systems.

We describe design and performance of efficient linear-to-radial polarization and mode converter. The converter is a spatially variable retarder (SVR), comprised of eight appropriately cut half- wave plate segments. The SVRs were applied to perform two different tasks. First, a linearly polarized Nd:YAG TEM₀₀ beam was converted into a radiallypolarized LG_01* beam with polarization-purity of 98% and 96% measured in the near-field and far-field respectively. The total power-loss in the transformation was 18%. Second, a 70W Nd:YAG radially-polarized beam with beamquality M²=2.6 was converted into a linearly polarized nearly-Gaussian beam with beam-quality M²=1.4. Taking into account power-losses, the experimental beam-brightness was increased by a factor of 2.6. The SVR manufacture was optimized for application to high power lasers, where minimum phase-front distortion and maximum cylindrical- polarization purity is required. SVRs have so far been successfully tested to the kW level. The proposed converging methods can be of high practical importance due to unique properties of radially-polarized beams.

The objective of the present work is to develop a fiber suitable for high-power fundamental-mode beam delivery over a useful length for material processing (~100 m). The investigated design is based on evanescent-field coupled waveguides, also called multi-core fibers (MCFs) [6]-[8]. The investigated MCF consists of a hexagonal array of 7 cores in which the fundamental mode, the so-called in-phase supermode, has an effective mode area (A_eff) of 348 μm² and a numerical aperture (N.A.) of 0.035. We show how the bending-induced losses and the mode-mixing depend on the bending radius and on the structure of the waveguide. The experimental results on the behavior of the near-field and the far-field for different bending radii will also be reported. Additionally, we will show another method to reproduce experimental results with multi-mode fibers supporting few modes where the conventional approach leads to irreproducible results due to the mode-mixing effect.

Quality evaluation of a laser beam is a subject of interest to both designers and users of lasers. A well known method is to measure the M² second-moment from the longitudinal evolution of beam width determined from intensity profile monitored by a CCD camera. This standard procedure is time consuming, and costly. It is therefore difficult to be implemented for checking, for instance, each VCSEL diode moving out of the assembly line of a high volume production factory. In this paper, we propose an alternative fast method allowing to separate single transverse mode from multiple transverse modes oscillation. This method is based on an electronic analysis (locking amplifier) of the local slope of the output laser characteristic, i.e. laser output power versus pumping intensity. The only optoelectronics component used is a cheap photodiode with a large sensitive area (active diameter 5mm) for measuring the laser output power. Correlation between M² and local slope variations, as the pumping intensity is increased, has been experimentally demonstrated with different lasers : Nd:YVO₄ and VCSEL.

A novel coupling method for injecting a high peak power laser into a multimode optical fiber through cone-channel condenser is introduced. The novel coupler is investigated by experiments and theories. The design minimizes the irradiance on the fiber input face and reduces its dependence on the system alignment. A simple lens and a special designed cone-channel condenser operate together to transform a laser beam with 5 mm diameter into a smaller one that fits on the 400 μm or 600 μm diameter fiber face. The method resolves the problem that laser induce damage to fiber input end faces. The design principle and method of cone-channel condenser are described by the light transmission theory. The prototype was fabricated without anti-reflection coatings on the end faces. The experimental results show that the transmission efficiency of cone-channel condenser is up to 90%. Though there was 1 mm gap between the cone-channel condenser and a fiber, the coupling efficiency of cone-channel condenser to fiber reach 73%. The maximum transmitted energy before front-face of cone-channel condenser breakdown is 84.5mJ. The transmission capacity of fiber increases by 2-3 times comparing with the traditional method. The interest in this new coupling method is related to the development of transmitting high peak powers through multimode fibers applied to laser-based firing systems for initiating explosives and driving flyer, et al.

Stimulated Raman scattering (SRS) in BaWO4 crystal under 1.56 μm pumping was investigated. Several (up to fourth) Stokes components were observed. Mid IR spectral range 2.75 µm and 3.7 μm radiation was obtained using BaWO4 crystalline Raman shifter. Under 1.318 μm pumping 4.3 μm and 4.7 μm oscillations in low phonon PbGa₂S₄:Dy³⁺ laser were obtained.

We report tunable thulium/holmium-doped single mode fiber laser passively Q-switched by an antimonide-based semiconductor saturable absorber mirror (SESAM) and boosted in Tm/Ho amplifier up to 2 W of average power. Pulse operation tunable from 1960 nm to 1990 nm with pulse energies up to 30 μJ has been achieved. The study presents the first demonstration of 2 μm Q-switched fiber laser using antimonide semiconductor technology.

Cr:ZnSe laser active material is one of the favourite possibility how to generate broadly tunable mid-infrared laser radiation at room-temperature. The aim of this study was to demonstrate and analyze pulsed as well as continuous-wave laser action in bulk Cr:ZnSe crystals grown by the floating-zone method or by the Bridgman method. The absorption spectra of Cr:ZnSe were measured to be from 1500 to 2000 nm, therefore various lasers were utilized for coherent longitudinal pumping of Cr:ZnSe laser, namely flashlamp-pumped Er:YAP laser (generated wavelength 1658 nm), diode-pumped Tm:YLF laser (generated wavelength 1912 nm), and diodepumped Tm:YAP laser (generated wavelength 1980 nm). In the first case, the Cr:ZnSe crystal grown by the Bridgman method was investigated. In the second case, the Cr:ZnSe crystal grown by the floating zone method was studied. In both cases, the homogeneity of the active Cr:ZnSe crystals was found reasonable good. The emission spectrum was from 2000 up to 2800 nm. The Cr:ZnSe laser generated radiation was broadly continuously tunable in the range from 2050 nm up to 2750 nm. The generated radiation beam spatial structure was close to TEM₀₀.

Diode-pumping with appropriately modulated pulses is used to expose subtle input / output pulse-timing characteristics of Er³⁺:Yb³⁺-codoped systems, particularly Er³⁺:Yb³⁺:glass and Er³⁺:Yb³⁺:YAG, and study their relation to laser efficiency. In free-running operation, these systems may persist to lase for uncommonly long time periods (up to a ms) after the pump stops. In Q-switched operation, maximum laser output is obtained only if Q-switching is purposely delayed for some significant additional time (typically 50-500μs) following the end of the pump pulse. Furthermore, pump-pulse duration affects profoundly the Er³⁺:Yb³⁺-laser output performance, especially in Q-switched mode. Numerical modeling simulations are presented, accounting for the observed effects and extrapolating them to novel regimes of Er³⁺:Yb³⁺-laser operation.

Lasing and fluorescence behavior of thulium doped YVO₄, GdVO₄, and LuVO₄ single crystals were investigated under pulsed pumping with variable duty cycle up to CW. This allowed us to study properties of these crystals in dependence on thermal load in a broad range. Following crystals were investigated: Tm:YVO₄ (5 at.% Tm/Y, grown by the Czochralski technique), Tm:GdVO₄ (2, 4, and 6 at.% Tm/Gd, grown by the floating-zone technique), and Tm:LuVO₄ (3 at.% Tm/Y, grown by the floating-zone technique). For pumping a fibre-coupled (core diameter 400 μm) laser diode operating in range from 800 up to 803nm was used (available CW power 20 W). All tested crystals were investigated under CW and pulsed pumping (pulse length 4 ms). Under pulsed pumping (4% duty cycle), the lasing was demonstrated with all samples. Under CW pumping only Tm:GdVO₄ crystal was lasing. For Tm:YVO₄ and Tm:LuVO₄ crystals, a lasing was not reached for pumping with duty cycle higher than 60 %, and the strong blue emission was observed. Detailed measurement of visible emission for broad range of pumping duty cycles (from 4 up to 60%) showed the exponential increase of Tm³⁺ integral emission intensity in bands around 480 and 700 nm. Comparison with the results obtained for fixed duty cycle and variable crystal holder temperature (290 - 310 K) allowed us to find a relation between the duty cycle and temperature of pumped part of the crystal. Measurement of infrared fluorescence temporal behavior in dependence on duty cycle gives us possibility to study a relative population of lasing level in dependence on temperature.

Various lengths of Yb:Er:YVO4 were end pumped by a quasi-continuous wave 967 nm diode laser. The best slope efficiency with respect to absorbed pump power for gain switched operation was 8 % for a 5 mm long crystal. Co:MgAl₂O₄ saturable absorbers of 98 % and 93 % initial transmission were used to passively Q-switch the cavity. For the 98 % initial transmission absorber, average pulses energies of 44 µJ were measured. The average pulse width and repetition rate were ~256 ns and 36 kHz, respectively. For the 93% initial transmission absorber, a single output pulse of 37 µJ energy and 22 ns duration per pump pulse was measured when the crystal was pumped for a pumping duration of 1.7 ms.

In this paper we demonstrate an efficient dual frequency operation of a pulsed diode pumped Nd:GdVO₄ laser at 1.06 μm spectral region using polarization control of the generated radiation. Generation at the commonly reported 1063 nm line with π polarization (parallel to crystal c-axis) can be changed to the 1066 nm line with σ polarization by tuning of the wave plate in the laser resonator. For pump energy of 15 mJ from laser diode the laser output energy is 5.2 mJ at 1063 nm, or 4.23 mJ at 1066 nm or 4 mJ in simultaneous dual wavelength regime. The investigation of the spatial and temporal structure of generated radiation was also performed. A source allowing wavelength selection can be attractive for applications in spectroscopy etc.

We studied diode pumping of the locally disordered Yb-doped NaY(WO₄)₂ crystal. An a-cut 1.514-mm thick sample (6.9 at % Yb doping or 4.52×10²⁰ cm^-3) was placed under Brewster angle in a z-shaped astigmatically compensated cavity, without any active cooling. It was oriented for pumping and emission in π-polarization (E//c). Laser experiments were performed applying a high brightness laser diode as a pump source. The laser diode delivered up to 2.1 W of input power and the emission wavelength was selected in the broad absorption peak of Yb:NaY(WO₄)₂ around 961nm. Continuous-wave operation in the 1-μm range was obtained for output coupler transmission between 1% and 10%. A maximum output power of ≈180 mW and slope efficiencies up to 30%, related to the absorbed power, were achieved. For femtosecond mode-locked operation, a semiconductor saturable absorber mirror as well as two SF10 prisms were included in the cavity. Aligning the cavity for shortest pulses, we obtained stable passive mode-locking with pulse durations as short as 97 fs directly from the oscillator. Applying external compression, the pulse duration could be further reduced to 90 fs with a corresponding time-bandwidth-product of 0.321. The average output power amounted to 59 mW at a repetition rate of 90 MHz and the corresponding output spectrum was centered at 1044 nm.

Yb:CaGdAlO₄ is a near infrared material which recently demonstrated very interesting features for the development of the next generation of diode-pumped femtosecond lasers. This material presents two main assets as far as diode pumped high power lasers are concerned. First, it has to our best knowledge, the broadest and the flattest emission band of the Yb:doped crystals. This allowed the generation of 68 fs pulses at an average power of 520 mW. The measured thermal conductivity for 2 at. % Yb:CaGdAlO4 is 6.9 and 6.3 W K^-1 m^-1 along the a and c axis, respectively. These values are very similar to those of the doped Yb:YAG and allow to reach high output power with limited thermal effects. In the present work, we investigate the performance of Yb³⁺:CaGdAlO₄ for ytterbium concentration ranging between 2% and 5%, under low power diode pumping, in order to optimize the absorption and wavelength tunability. Then the 2% Yb:CaGdAlO₄ under high power diode pumping (100 W @ 980 nm) has been evaluated in a standard laser cavity and 15 W of output laser power have been obtained for 42 W absorbed.

Nonlinear optical conversion of 1.064 μm pulses from a Q-switched Nd:YAG laser to the mid-infrared is demonstrated experimentally. The setup is based on a two-stage master-oscillator/power-amplifier (MOPA) design with a KTiOPO₄ based MOPA in the first stage and a KTiOAsO₄/ZnGeP₂ based MOPA in the second stage. We obtain more than 8 mJ at 8 μm with a beam quality factor M² ≈ 3.6.

A new family of nonlinear optical (NLO) crystals is presented for the generation of ultraviolet (UV) light by frequency conversion. Included in this family are REAl₃(BO₃)₄ with RE = Y, Lu and other rare earths. Phase matching properties of YAl₃(BO₃)₄ (i.e., YAB) are described in detail.

High power or miniaturized laser systems are limited by the laser damage resistance of optical components, particularly of nonlinear crystals. The laser damage of optical components depends on many factors such as wavelength, frequency, pulse duration, spot-size,... Moreover, in nonlinear crystals, the anisotropy of physical parameters may cause anisotropy of the Laser Induced Damage Threshold (LIDT). Thus, the LIDT may depend on polarization or propagation direction of the laser beam. The aim of this paper is to discuss the laser damage results of two nonlinear crystals: KTiOPO4 (KTP) and RbTiOPO4 (RTP). In general, due to its higher effective electro-optic coefficients, RTP is more used for electrooptic applications, whereas KTP is popular for second harmonic generation. Laser damage tests in KTP and RTP reveal that for both crystals the LIDT depends on the polarization. The laser damage tests were carried out at 1064nm with a nanosecond Nd:YAG laser. The tests were performed with the polarization and the propagation direction of the light along a principal crystal axis, and all configurations were tested with a parallel beam (waist diameter 75μm). As they belong to the same crystal family, RTP and KTP crystals have similar nonlinear optical properties. This work also reveals that the laser resistance of KTP and RTP is very close. Functional laser damage tests of RTP for Pockels cell applications and SHG-cut KTP were performed too. We also discuss the influence of second harmonic generation on the LIDT.

Ultraviolet single mode laser light is proposed by frequency quadrupling the output of a Nd:YLF laser with two successive frequency doubling stages. For a simple linear cavity laser configuration investigated at 1312 nm, we have obtained an intracavity power of 310 W for 16 W of absorbed pump power (λ_p ~ 806 nm). Up to 10 mW tunable single-frequency laser (λ_2ω=656-658 nm) is observed. This has been achieved by intracavity second-harmonic generation of a diode-pumped Nd:YLiF₄ linear laser oscillating on the σ-polarized ⁴F_3/2-⁴I_13/2 transition (λ_ω ~1314 nm) with a β-Barium Borate (BBO) crystal. We also report the experimental measurement of the red-uv conversion efficiency according to the waist size in the BBO crystal. Obtained value is compared to those given by the Boyd-Kleinman theory.

Characterization of diode pumped Er:YVO4 microchip laser working in an "eye-safe" spectral region is done. Two active materials for microchip laser based on Er:YVO₄ and Er:YVO₄ + CaO crystals were investigated. The dimension of the microchips was in both cases the same: an aperture 8.3 x 10.4 mm and thickness 2.9 mm. The concentration of active ions Er³⁺ was 0.5 at% in both samples and 0.6at% CaO was added to Er:YVO₄ + CaO crystal. The resonator mirrors were deposited directly on the crystal faces: a rear mirror was HR for the 1.6 μm wavelength and HT for 0.97 μm pumping radiation and as the output coupler a dielectric coatings with the 0.5% transmission at 1.6 μm wavelength was prepared. As a pumping source a fiber coupled (core diameter-200 μm) laser diode emitting radiation at wavelength 0.976 μm was used. Laser diode was operating in pulsed regime (pulse width 3 ms, repetition rate 20 Hz, maximum mean pumping power 1.13 W). The diode radiation was focused into the uncooled microchip sample by two achromatic doublet lenses with the focal length of 75 mm. As the result 175 mW and 152 mW output peak powers were obtained for the Er:YVO4 and Er:YVO₄ + CaO, respectively. The laser emission was observed in detail in range 1.529 μm up to 1.604 μm for Er:YVO₄ microchip in dependence on pumping conditions. For Er:YVO4 + CaO crystal 1.6041 μm was generated only. Up-conversion radiation for both materials in dependence on pumping was also studied.

This article was originally published online on April 16, 2008. The following errors were discovered by the authors after publication: incorrect author order, equations 10 and 11 were written twice.

The goal of the work was the investigation of hollow waveguide utilization for near infrared laser radiation delivery. As basic delivery unit, a new thin cyclic olefin polymer coated silver hollow glass waveguide with diameters 100/190 μm or 250/360 μm and length up to 20 cm was used. Four near infrared laser sources were based on the Nd:YAG crystals. The first one - Nd:YAG laser passively Q-switched by LiF:F^2- saturable absorber - was coherently pumped by Alexandrite radiation. The system generated 1.06 μm wavelength radiation with 6 ns length of pulse and 0.7 mJ maximum output energy. The second and third laser systems were compact longitudinally diode pumped Nd:YAG lasers generating radiation at wavelength 1.06 μm and 1.44 μm. These lasers were operating in a free-running regime under pulsed pumping (pulse repetition rate 50 Hz). Mean output power 160 mW (90 mW) with pulse length 0.5 ms (1 ms) was generated at wavelength 1.06 μm (1.44 μm). The last radiation source was the Nd:YAG/V:YAG microchip laser pumped by laser diode and generating the radiation at 1.34 μm wavelength. The output power, pulse length, and repetition rate were 25 mW, 6 ns, and 250 Hz, respectively. All lasers were generating beam with gaussian TEM₀₀ profile. These radiations were focused into thin a waveguide and delivery radiation characteristics were investigated. It was recognized that the output spatial structure is significantly modified in all cases. However a compact delivery system can be useful for near infrared powerful radiation delivery in some special technological and medical applications.

We report on the first, promising production of Nd:YAG ceramics made in Poland. The Nd:YAG ceramics was produced by a solid-state reaction of high-purity (4N) nanometric oxides powders i.e. Al₂O₃, Y₂O₃ and Nd₂O₃. After sintering process mean grain sizes of 2%Nd:YAG samples were about 20 μm and transparency of its were comparable to 0.9% Nd:YAG single crystal. Two types of active elements: rods and slabs were fabricated and characterized in several diode pumping schemes. In end pumping configuration as a pump source 20-W fiber coupled laser diode operating in low duty cycle regime (1 ms pump duration /20 Hz) was deployed. In the best case, 3.7 W of output power for 18 W of absorbed pump power, M² <1.4 were demonstrated for uncoated ceramics Nd:YAG rod of φ4x3mm size in preliminary experiments. For the ceramics of two times lower Nd dopant level above 30% slope efficiency was achieved. In case of slab ceramic side pumped by 600-W laser diode stack above 12 W was demonstrated with slope efficiency of 3.5%.

The results of spectroscopy of disordered Er-doped calcium-niobium- gallium garnets (CNGG: Er) are displayed. Based on the Judd-Ofelt theory, three intensity parameters Ω_t (t=2, 4, 6) for Er ions concentration from 6 at. % in these crystals were obtained. The oscillator strength for hypersensitivity transition ⁴I_15/2→²H_11/2 and intensity parameter Ω₂ of Er3+ ions in CNGG: Er crystals are higher than corresponding values for other Er-doped garnets. These results we explain by the presence in these crystals of optical centers Er with the environment symmetry lower than D₂. Gain crosssection for laser transition ⁴I_13/2 → ⁴I_15/2 of Er³⁺ ions was obtained using absorption and emission cross-data for transition ⁴I_15/2 → ⁴I_13/2 and ⁴I_13/2 → ⁴I_15/2 respectively.

For laser detonator application, high-peak power pulsed Nd:YAG laser is transmitted through all-silica optical fiber. The transmission properties of step-index fibers are investigated, using a high-peak power pulsed Nd: YAG rod laser with beyond 1MW power and Q-switch mode. The fibers are step-index multimode fibers with 400 or 600 μm core diameters, 440 or 660 μm cladding diameters. The power delivery characteristics were studied by theory and experiments. The results show that the fiber core diameter, NA, length and so on affect the transmission efficiency for high power laser. When the laser power is beyond a certain threshold, the SRS and SBS will be serious; the quantity of fiber end-face limits to the raising of laser power passing through fibers; the zero-probability damage threshold is calculated according to ISO/DIS standard 11254-1.2, which is 58.6J/cm². Energy distribution of output beam from fibers will be uniform. Even the fiber end-face was partly damaged, laser power is still deliverable, and the transmission efficiency is related to the fiber damage grade.

Single amplified ultra-short laser pulse is very important in basic scientific researches as well as in ultra-high power laser applications. The high intensity ultra-short laser pulse usually comes from the mode-locked pulses train. It is important to develop a laser amplifier configuration that can optimally amplify single ultra-short laser pulse. We present in this research, first by controlling the synchronization of the amplifiers chain, and then optimally adjust the shape of amplified spontaneous emission of each amplifier, we can select the single amplified ultra-short laser pulse out of the seed ultra-short pulses train and boost the laser pulse to the maximum power amplification. An adjustable repetitive single pulse high intensity ultra-short pulse UV laser system was developed and the laser outputs were extracted at various ends and characterized in detail.

Potassium dihydrogen phosphate (KH2PO4 or short KDP) is one of the major nonlinear optical crystals for frequency conversion and electro-optic switching in high power lasers. In particular, this material has been chosen for the frequency converters of the Laser Mega Joule in France and the National Ignition Facility in the US. These laser work close to the damage threshold of the crystals and large efforts have been provided to improve the laser induced damage threshold for KDP at different wavelength. We present in this paper first results of a new setup dedicated to the correlation of non destructive luminescence spectroscopy and destructive laser damage tests. We concentrate on the differences between conventionally grown KDP and KDP-crystals that have been produced by the rapid growth method that has been developed in the last years especially for the large laser installations LMJ and NIF. Different photoluminescence spectra are obtained from conventionally and rapidly grown KDP for both pump configurations: (i) pulsed pumping by the forth harmonic of a Nd:YAG laser at 266nm, and (ii) continuous pumping using a frequency doubled Argon ion laser at 244nm.

We report on a passively Q-switched end pumped Nd:YLF laser including a noncritically phase-matched KTP singly resonant intracavity optical parametric oscillator (IOPO-KTP). For the Q-switching operation we have used Cr:YAG saturable absorber. The optimized passively Q-switched Nd:YLF laser without IOPO generated linearly polarized pulses of 11.5 ns and 1.07 mJ at 1047 nm. The conversion efficiency of the optimized Q-switched pulse energy at 1047 nm to 1547 nm of signal approached about 47%. For optimizing both Nd:YLF laser and IOPO we have numerically solved theoretical model. We have achieved 1.6-ns duration pulses at 1547 nm with energy of 0.5 mJ and peak power above of 300 kW. The beam quality was excellent (M²≈1).

The performance of a flashlamp pumped miniature 2.94 μm Er:YAG laser is optimized by using a mathematical model. The model is based on the rate equations with spectroscopic data including energy transfer processes. Using this model, we can explain the experimental phenomenon that the laser pulse slightly extends beyond the pumping pulse in some cases. A method of optimizing the reflectivity of the output coupler was also presented.

The first demonstration of a pulsed Nd:YCOB laser at 1060 nm is reported, with results for both gain switching and Qswitching presented. Active Q-switching is achieved using the spinning disc technique pulses of 50 ns duration with pulse energies up to 0.6 mJ are obtained. Optimisation is performed for both pulse energy and slope efficiency of the laser. A Q-switched slope efficiency of 56% is achieved.

Correlation between Yb³⁺ optical properties and structural disorder of glass network has been investigated by using good quality Yb-doped silica glasses fabricated by MCVD method. Absorption spectra strongly depend on the fictive temperature, which is a good indicator of structural disorder in silica glass. Their dependences are affected by the codopants (Al and F). Fictive temperature dependence of refractive index has been also elucidated.

Raman spectral bandwidths of tellurite glasses are widened by using Raman active components of suitable concentration in appropriate base glasses. The MoO₆ octahedra were found to have high octahedral distortion; therefore, have high Raman polarizability compared to WO₆, NbO₆, and TaO₆ octahedra and PO₄ tetrahedra. This high Raman polarizability enabled broadening of the spectral width up to ~350 cm^-1 while maintaining high Raman scattering intensities. Although similar bandwidths could be achieved using combined generation of WO₆ octahedra and PO₄ tetrahedra, the resultant Raman scattering intensity of such glasses is only half of that could be achievable using MoO₆. It is shown that the simplest tellurite glass showing wide spectral broadening is a quaternary system comprising a network modifier (BaO or Bi₂O₃) and two Raman oscillators (NbO6 and MoO₆ octahedra). Using the newly developed gain medium gain flattened S+ C+ L ultrabroadband fiber Raman amplifier are designed by solving the inverse amplifier design problem. The relative gain flatness and the effective bandwidth of new gain medium are better and larger than those of conventional tellurite fibers.

The characterization of the coefficient of the nonlinear optical Kerr effect, the nonlinear refractive index (n₂), of several femtosecond laser crystals with compositions derived by total or partial replacement of D²⁺ in DXO_4, X = Mo or W, is presented. Tetragonal (space group I4) Na-based double tungstates NaT(WO₄)₂ (T = Y, La, Gd, Lu and Bi) and double molybdate NaY(MoO₄)₂, as well as the monoclinic (space group C2/c) Li₃Gd₃Ba₂(MoO₄)₈ crystals, have been measured by the z-scan technique. All these crystals present structural local disorder, and among them the tetragonal ones exhibit significant n₂ values, which should allow their efficient laser pulsed operation by Kerr-lens mode locking, especially NaBi(WO₄)_2, 68x10^-16 cm²/W (for σ light), which is about twice than for the others. This feature is attributed to the high polarizability associated to the lone electron pair of Bi³⁺.

We report the active Q-switching of a Yb:Er:YVO4 laser for the first time. A Yb:Er:YVO4 crystal was end pumped by a quasi-continuous wave laser diode emitting at 967 nm with a peak power of up to 48 W. The laser cavity was actively Qswitched using the spinning disc technique. At a repetition rate of 19.2 kHz, the Q-switched slope efficiency and threshold were 4 % and 62 mJ respectively. In comparison, the same system had a slope efficiency of 5% and a threshold of 75 mJ without mechanical Q-switching. Single pulse of energy up to 90 μJ and duration as short as 110 ns were obtained for the single output pulse per pump pulse operation.

In this paper the gain dynamics of an erbium-doped fiber laser (EDFL) with an inhomogeneous active medium in the presence of ion pairs is modeled. A two-level model for single ions and a three-level model for ion pairs are employed to write the propagation and rate equations of inhomogeneous laser medium. The governing equations are an uncountable system of partial differential equations (PDEs). By employing the moment method, the system of PDEs is converted to a finite system of ordinary differential equations (ODEs). The Solution of the system of ODEs is used to analyze the output power of a high concentration EDFL. As it expected, theoretical results show that the threshold pumping power increases and the output power decreases by increasing the ion pair concentration.

In this paper, we propose a novel passively mode-locked waveguide laser using carbon nanotubes saturable absorber integrated with gain medium, in which the carbon nanotubes saturable absorber is directly sprayed on the Er-Yb doped phosphate glass waveguide, a ring cavity is chosen. The mode-locking mechanism is the interaction between the evanescent field of guiding mode and the carbon nanotubes. An elementary research on the proposed passively mode-locked waveguide laser is presented.

Near- and mid-infrared fibre lasers find many applications in areas such as remote and chemical sensing, lidar and medicine, and tellurite fibres offer advantages over other common fibre glasses such a lower phonon energy and higher rare-earth ion solubility than silicate glasses, and greater chemical and environmental stability than fluoride glasses. Rate equation modelling is a very useful tool for the characterisation and performance prediction of new rare earth transitions in these novel fibre materials. We present the numerical rate equation modelling results for a ~2 μm Tm³⁺-doped tellurite fibre laser when pumped with a 1.6 μm Er³⁺/Yb³⁺-doped double-clad silica fibre laser. A maximum slope efficiency of 76% with respect to launched pump power was achieved in the experimental fibre laser set up with a 32 cm long fibre. The high slope efficiency is very close to the Stokes efficiency limit of ~82% which is due to the in-band pumping scheme employed and the lack of pump excited state absorption. The two-level rate equations involving absorption and emission between the Tm³⁺: ³H₆ and ³F₄ levels have been solved iteratively using a fourth-order Runge-Kutta algorithm and the results compared with the experimental results. For the 32 cm fibre with output coupler reflectivities of 12%, 50%, 70% and 90%, the respective theoretical slope efficiencies of 73%, 64%, 53% and 29% are in very good agreement with the experimentally measured values of 76%, 60%, 48% and 33%.

TeO₂-ZnF₂-PbO-Nb_2O5 based fluorotellurite glasses were synthesized and studied for the first time for laser applications. The property characterizations including XRD and thermal analysis as well as optical properties measurement were performed. It is demonstrated that this fluorotellurite system has good glass formation ability; and increasing the ZnF₂ concentration to 30 mol% can significantly increase the thermal stability of the glass. Adding ZnF₂ also reduced remarkably the hydroxyl (OH) concentration of the glass resulting in lower optical absorption in the infrared region, which is crucial for infrared laser applications. In addition, the glass absorption cut-off edge near 400 nm blue-shifted with increasing ZnF₂ addition.