This PDF file contains the front matter associated with SPIE Proceedings Volume 9135, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.

We present our measurements of the key spectroscopic properties over the temperature range of 77 K to 450 K for Nd³⁺ ions doped in Y₃Al₅O₁₂ (YAG). From room to liquid nitrogen temperature (LNT), the peak absorption cross section around 808 nm increased by almost 3 times, in conjunction the bandwidth of this absorption line reduced by the same factor. At LNT the peak of the absorption line was blue shifted by 0.25 nm with respect to that at 300 K. The fluorescence spectrum between 850 nm – 1450 nm was measured, from which the emission cross sections for the three main transitions were calculated. One note of particular interest for the dominant emission wavelengths around 1064nm and 1061nm (⁴F_3/2 → 4I11/2) was the switch in their relative strength below 170K, and at LNT the 1061 nm line has almost twice the cross section as at 1064nm.. The fluorescence and lifetime of the upper laser level (⁴F_3/2) was measured and the effective emission cross section determined by the Fuchtbauer-Ladenburg (F-L) method. The effective emission cross section for 946 nm (R₁ → Z₅) increased by more than two times over the 300 K to 77 K range. A numerical fit for the temperature dependent emission cross section at 946 nm and 1064 nm and also calculated absorption coefficient at 808 nm pump diode laser have also obtained from the measured spectroscopic data.

A diode-cladding-pumped mid-infrared passively Q-switched Ho³⁺-doped fluoride fiber laser using a reverse designed broad band semiconductor saturable mirror (SESAM) was demonstrated. Nonlinear reflectivity of the SESAM was measured using an in-house Yb³⁺-doped mode-locked fiber laser at 1062 nm. Stable pulse train was produced at a slope efficient of 12.1% with respect to the launched pump power. Maximum pulse energy of 6.65 μJ with a pulse width of 1.68 μs and signal to noise ratio (SNR) of ~50 dB was achieved at a repetition rate of 47.6 kHz and center wavelength of 2.971 μm. To the best of our knowledge, this is the first 3 μm region SESAM based Q-switched fiber laser with the highest average power and pulse energy, as well as the longest wavelength from mid-infrared passively Q-switched fluoride fiber lasers.

A gain switched pulsed laser based on ytterbium doped rod PCF type fiber is presented. The high performance pump system was based on 976 nm laser diodes incorporating high speed and high current laser diode drivers with active feedback loop based control that enable high pulse to pulse stability. Furthermore the temperature control ensure the adequate output spectrum of the pump laser diodes in order to match maximum of the absorption peak of Yb doped active medium. The pulses duration in range between 48 ns to 75 ns were achieved with peak powers up to 3.6 kW. Further, the change of the laser output spectrum in regard to the pump pulse power is observed.

We demonstrate a new approach to designing a compact RF standard based on the transfer of frequency stability of hyperfine transitions in molecular iodine to the stability of a laser cavity length. We use frequency doubled Nd:YVO₄ laser operating in Kerr lens mode-locked regime and frequency lock it to hyperfine transitions in molecular iodine with further detecting the beat note signal between longitudinal modes on a fast photodiode. A similar system is used for estimating the standard Allan deviation of RF signal which is 2.1 x 10^-14 at the time 100 s.

We extend the existing theory of continuous-wave lasers by systematically considering spontaneous emission. In a simple rate-equation approach, the laser eigenvalue, defined as the ratio of coherent photons coupled out of the resonator divided by the number of photons coupled in via spontaneous emission, emerges as the fundamental parameter describing a continuous-wave laser. We derive a general equation for the laser eigenvalue and confirm the point at which coherence manifests itself in the resonator. The theory describes all types of lasers of three-level, four-level, or any intermediate nature.

Nowadays lasers cover a broad spectrum of applications, like laser material processing, metrology and communications. Therefore a broad variety of different lasers, containing various active media and resonator setups, are used to provide high design flexibility. The optimization of such multi-parameter laser setups requires powerful simulation techniques. In literature mainly three practical resonator modeling techniques can be found: Rigorous techniques, e.g. the finite element method (FEM), approximated solutions based on paraxial Gaussian beam tracing by ABCD matrices and the Fox and Li algorithm are used to analyze transversal resonator modes. All of these existing approaches have in common, that only a single simulation technique is used for the whole resonator. In contrast we reformulate the scalar Fox and Li integral equation for resonator eigenmode calculation into a fully vectorial field tracing operator equation. This allows the flexible combination of different modeling techniques in different subdomains of the resonator. The work introduces the basic concepts of field tracing in resonators to calculate vectorial, transversal eigenmodes of stable and unstable resonators.

We present the world’s first laser at 515 nm with sub-picosecond pulses and an average power of 445 W. To realize this beam source we utilize an Yb:YAG-based infrared laser consisting of a fiber MOPA system as a seed source, a rod-type pre-amplifier and two Innoslab power amplifier stages. The infrared system delivers up to 930 W of average power at repetition rates between 10 and 50 MHz and with pulse durations around 800 fs. The beam quality in the infrared is M² = 1.1 and 1.5 in fast and slow axis. As a frequency doubler we chose a Type-I critically phase-matched Lithium Triborate (LBO) crystal in a single-pass configuration. To preserve the infrared beam quality and pulse duration, the conversion was carefully modeled using numerical calculations. These take dispersion-related and thermal effects into account, thus enabling us to provide precise predictions of the properties of the frequency-doubled beam. To be able to model the influence of thermal dephasing correctly and to choose appropriate crystals accordingly, we performed extensive absorption measurements of all crystals used for conversion experiments. These measurements provide the input data for the thermal FEM analysis and calculation. We used a Photothermal Commonpath Interferometer (PCI) to obtain space-resolved absorption data in the bulk and at the surfaces of the LBO crystals. The absorption was measured at 1030 nm as well as at 515 nm in order to take into account the different absorption behavior at both occurring wavelengths.

This paper presents the results on second harmonic generation in a tuneable Yb-fibre laser with an enhancement cavity partially coupled to the laser resonator. The maximal second harmonic output power was 880 mW at 536 nm when pumped with 6.2 W at 976 nm. The output radiation line width of the second harmonics of the Yb-fibre laser did not exceed 0.5 nm with a tuning range of 521–545 nm and the output power at the ends of this range 220 and 450 mW respectively. Further presented is an analysis of different frequency doubling configurations both with and without an enhancement cavity in a broad range of output powers of the fundamental radiation.

In this work, we present for the first time a method for quasi-smooth tuning of the second harmonic radiation of an Nd:YVO₄/LBO laser within a 60-GHz range. Practicality of this method is demonstrated at the radiation output power of 1.5 W at 532 nm. The proposed method features automatic stitching of 12-GHz continuously tuneable ranges to the precision of the laser output line width (5 MHz). The stitching does not require a precision wavelength meter and is based on a high-finesse scanning confocal interferometer.

The realization of a first-order interferometric autocorrelator in a nearly collinear geometry is reported as an alternative method to measure the minimum pulse duration of femtosecond deep-UV laser pulses. The Fourier limited duration of 257-nm femtosecond pulses is measured with high accuracy, and compared to what obtained by spectrally measuring the bandwidth of the pulses. The agreement between the two methods is excellent, thus indicating the interferometric autocorrelator as a useful tool to extract the chirp of femtosecond UV pulses when used together with a second-order autocorrelator to measure the actual pulse duration.

The mid-IR wavelength range has gained increased interest due to its applications in gas sensing, medicine, defense, and others. Optical parametric devices play an important role in the generation of radiation in the mid-IR. Low thermal load of nonlinear crystals promises high average power outputs if powerful pump laser is available. We have developed 75-W average power pump laser operating at 100 kHz repetition rate. The pulses of Yb-fiber laser oscillator at 1030-nm wavelength are stretched by a chirped volume Bragg grating from 5 ps to 180 ps and inserted into a cavity of regenerative amplifier with an Yb:YAG thin-disk. The amplified pulses are compressed by a chirped volume Bragg grating with an 88% efficiency. We have proposed a wavelength conversion system generating picosecond pulses tunable between 2 and 3 μm. The seed signal radiation is acquired by the optical parametric generation in the first nonlinear crystal. Signal pulse energy is increased in the subsequent optical parametric amplifiers. Each amplification stage consists of a crystal pair in the walkoff compensating arrangement. The wavelength of the signal beam is tunable between 1.6 and 2.1 μm. The 2.1 - 3 μm tunable source will be the idler beam taken from the last amplification stage. Calculations show the output power of ten watt can be achieved for 100 W pump. The results of preliminary experiments with seeded optical parametric generation and subsequent amplification are presented and discussed.

The goal of this work is to prove the feasibility of building a laser system that can generate mid-infrared radiation with the parameters required for the measurement of the hyperfine splitting in the ground state of the mounic hydrogen spectroscopy. The first experimental results of a very straightforward scheme that, to our knowledge, has not been considered in the literature, are presented. We study a laser test bench system emitting nanosecond pulses of infrared tunable radiation in the spectral range 6.78 μm with high energy and narrow line-width, based on direct difference frequency generation (DFG), in non-oxide nonlinear crystals, using as pump lasers a single-mode Nd:YAG laser and tunable narrowbandwidth Cr:forsterite laser. The investigated system is based on lithium thioindate (LiInS₂) and silver thiogallate (AgGaS₂) crystals cut for type II difference frequency generation. The pulses of the Nd:YAG laser (1,064 μm) are combined with the pulses at ~ 1.262 μm of the Cr:forsterite laser through a dichroic mirror and sent to the nonlinear crystals in different optical geometries. The generated radiation reaches an output energy up to 80 μJ in a single pass optical geometry, has 10 ns long pulses at 50 Hz frequency repetition rate and is tunable in the range 6595 – 6895 nm. These first results prove the suitability of such an approach for building the laser system for the muonic-hydrogen experiment.

The fast growing market of organic electronics, including organic photovoltaics (OPV), stimulates the development of versatile technologies for structuring thin-film materials. Ultraviolet lasers have proven their full potential for patterning single organic layers, but in a multilayer organic device the obtained layer selectivity is limited as all organic layers show high UV absorption. In this paper, we introduce mid-infrared (IR) resonant ablation as an alternative approach, in which a short pulse mid-infrared laser can be wavelength tuned to one of the molecular vibrational transitions of the organic material to be ablated. As a result, the technique is selective in respect of processing a diversity of organics, which usually have different infrared absorption bands. Mid-IR resonant ablation is demonstrated for a variety of organic thin films, employing both nanosecond (15 ns) and picosecond (250 ps) laser pulses tunable between 3 and 4 microns. The nanosecond experimental set-up is based on a commercial laser at 1064 nm pumping a singly resonant Optical Parametric Oscillator (OPO) built around a Periodically-Poled Lithium Niobate (PPLN) crystal with several Quasi-Phase Matching (QPM) periods, delivering more than 0.3 W of mid-IR power, corresponding to 15 μJ pulses. The picosecond laser set-up is based on Optical Parametric Amplification (OPA) in a similar crystal, allowing for a comparison between both pulse length regimes. The wavelength of the mid-infrared laser can be tuned to one of the molecular vibrational transitions of the organic material to be ablated. For that reason, the IR absorption spectra of the organic materials used in a typical OPV device were characterized in the wavelength region that can be reached by the laser setups. Focus was on OPV substrate materials, transparent conductive materials, hole transport materials, and absorber materials. The process has been successfully demonstrated for selective thin film patterning, and the influence of the various laser parameters is discussed.

A quantum cascade laser (QCL) tuning mechanism based on an external laser cavity containing a Micro ElectroMechanical System (MEMS) spatial light modulator in the form of a two-dimensional digital micromirror array (DMA) is described. The laser is tuned by modulating the reflectivity of DMA micromirror pixels under computer control. The resulting functionality enables fast (<0.1ms switching time) digitally controlled random-access wavelength tuning, high-bandwidth wavelength modulation (~30kHz modulation rate), and stable wavelength locking of the laser output. With one or more QCL gain elements built into the cavity, it is possible to cover a wide portion of the mid-wave and/or long-wave spectral range with a single device. The fast wideband digitally controlled laser tuning technology described is applicable to other tunable laser including solid-state, diode, gas, and fiber lasers.

We report a high power, single frequency, linearly polarized master oscillator power amplifier emitting 110 ns, 1 kW peak power pulses at 2050 nm. A 20 % slope efficiency and a beam quality of M² = 1.2 are achieved with a three stages double clad Tm³⁺-doped fiber architecture. Various pump schemes are compared leading to the conclusion that 793 nm pump wavelength is the most efficient for amplification at 2050 nm. Based on a numerical simulations, the Brillouin gain coefficient around 2 μm in Tm³⁺ highly doped silica fiber is estimated to 1.2x10^-11 m/W. Output peak power is limited by Stimulated Brillouin scattering to 535 W without mitigation and to 1 kW with application of a strain distribution along the doped fiber.

We report on recent developments in fibre laser component technology for use in 2-micron laser systems. A range of ‘building block’ components has been built to allow novel fibre laser architectures that exploit the advantages of fibre lasers based on Thulium and Holmium active fibres. Fibre lasers operating around 2-microns are becoming widely used in an increasing number of applications, which is driving the need for components that can operate reliably at high powers and also integrate easily with other components. To that end, we have designed and built a range of fused fibre, acousto-optic and magneto-optic devices that can be readily integrated into a range of novel fibre laser systems. Research has been carried out into improving fused fibre technology for components operating at 2um wavelengths. Side-coupled feed through combiners have been developed with signal losses as low as 0.02dB and kilowatt level end-coupled pump couplers. Alongside this a range of taps, splitters and WDMs have been developed which allows for the implementation of a variety of laser architectures. Optical isolators based on new Faraday materials have been developed, providing over 30dB isolation, low insertion loss and 30W power handling in a fibre-in, fibre-out version. New cell designs and materials for Acousto-Optic devices have been researched leading to the development of fibre-coupled Acousto-Optic Modulators (AOM) and allows for the realisation of all fibre Thulium and Holmium Q-switched and pulsed fibre lasers. Novel Acousto-Optic Tunable Filters (AOTF) designs have been realised to produce narrow resolution AOTFs and zero-shift AOTFs.

Thulium-doped fiber lasers are gaining in popularity since they emit at about 2 μm, a wavelength particularly interesting for many industrial, sensing and medical applications, and, moreover, in the so-called “eye-safe” spectral region. Despite the many advantages, however, thulium-doped fiber lasers with power high enough to allow practical applications have still limited deployment mainly due the high cost per emitted watt. The paper investigates alternative paths to high power CW emission at about 2 μm by exploring two complementary approaches: the development of specific pump combiners and the study of new pumping schemes that take advantage of co-doped fibers. The developed pump combiners are based on fused fiber technology and are characterized either by the use of “non-standard” fiber dimensions to allow pumping through an ytterbium-doped fiber laser or by a large number of input ports (up to 39) to provide adequate levels of pump power through the efficient coupling of several fiber pigtailed diodes with emission wavelength suitable for pumping thulium. On the other hand, a co-doped ytterbium-thulium fiber is also studied to analyze the possibility of using ytterbium ions as pump source for thulium ions. The use of ytterbium, either as co-dopant or as laser source, is particularly interesting because it allows taking advantage of the remarkable advancements made in the pump diodes for such a laser system, and specifically of the favorable cost per emitted watt. Preliminary experimental results have demonstrated the feasibility of the proposed approaches and have shown that the joint use of the “ad-hoc” pump combiners and of the ytterbium-thulium co-doping can lead to the development of lasers with power suitable for industrial applications, although the efficiency needs further improvements.

We report Q-switched pulse generation in Tm-doped fiber lasers by introducing piezoelectric-driven microbend into an elliptical coating fiber in a fiber ring resonator. Compared with the untreated circular fiber having a diameter of 240 μm, the elliptical coating fiber was flattened to have a major axis diameter of about 300 μm. We employed a pair of comblike plates attached on the piezoelectric actuators in order to bend the fiber from both sides. The output pulse power is improved by optimizing the tooth-width and spatial period of the comb-like plates, so that the elliptical coating fiber is easily bent and the propagation mode is efficiently coupled to radiation modes around λ = 1.9 μm. The Tm-doped fiber is pumped by a laser diode emitting at 1.63 μm and the pump light is introduced to the fiber ring resonator via the wavelength division multiplexing coupler. The emission spectra showed that the center oscillation wavelength was typically 1.92 μm. When the pump power was increased to 156 mW, the output pulse showed a peak power of 42.5 W with a pulse width of 1.06 μs. We expect that the in-fiber Q-switching technique will provide simple laser systems for environmental sensing and medical applications.

Methods for the machining of metals based on the use of ultra-short pulsed laser radiation continue to gain importance in industrial production technology. Theoretical considerations and experimental studies on laser drilling of steel are discussed. The applicability of geometrical optics to calculate the absorbed energy distribution inside small blind holes is investigated theoretically. A model for melt transport during ultra-short pulsed drilling is proposed and verified experimentally. It confirms that helical drilling is advantageous for machining burr-free holes.

Progressive developments in temporal shaping of short laser pulses offer entirely new approaches at influence and investigate laser-matter-interactions. Commonly used parameters for describing the behavior of short or ultrashort pulses or pulse trains are fluence and intensity. However, fluence does not imply any information about the temporal behavior of energy input during specific pulse duration τ while using the pulse intensity as describing parameter is more meaningful. Nevertheless it still is an averaging over pulse duration and no change in intensity can be determined if the temporal pulse shape changes within a certain combination of pulse duration and pulse energy. Using a flexible programmable MOPA fiber laser experimental studies on the impact of temporal energy distribution within one single laser pulse in micro machining applications were therefore carried out. With this laser source a direct modulation of the temporal pulse shape in the nanosecond regime can easily be controlled. Experiments were carried out with moved as well as with un-moved beam resulting in areas and dimples respectively drilling holes. The presented results clearly show that any averaging over pulse duration results in missing information about time-dependent interactions but can at the same time lead to significant differences in ablation results. Thus, resulting surface roughness Sa can be decreased up to 25 % when changing the pulse shape at constant parameters of fluence and pulse peak power at a pulse duration of 30 ns. It can be observed that the combination of an intensity peak and a lower edge within one pulse can lead to increasing ablation efficiency as well as higher ablation quality compared to the commonly used Gaussian-like temporal pulse shape.

A novel approach for efficient manufacturing of three-dimensional (3D) microstructured scaffolds designed for cell studies and tissue engineering applications is presented. A thermal extrusion (fused filament fabrication) 3D printer is employed as a simple and low-cost tabletop device enabling rapid materialization of CAD models out of biocompatible and biodegradable polylactic acid (PLA). Here it was used to produce cm- scale microporous (pore size varying from 100 to 400 µm) scaffolds. The fabricated objects were further laser processed in a direct laser writing (DLW) subtractive (ablation) and additive (lithography) manners. The first approach enables precise surface modification by creating micro-craters, holes and grooves thus increasing the surface roughness. An alternative way is to immerse the 3D PLA scaffold in a monomer solution and use the same DLW setup to refine its inner structure by fabricating dots, lines or a fine mesh on top as well as inside the pores of previously produced scaffolds. The DLW technique is empowered by ultrafast lasers - it allows 3D structuring with high spatial resolution in a great variety of photosensitive materials. Structure geometry on macro- to micro- scales could be finely tuned by combining these two fabrication techniques. Such artificial 3D substrates could be used for cell growth or as biocompatible-biodegradable implants. This combination of distinct material processing techniques enables rapid fabrication of diverse functional micro- featured and integrated devices. Hopefully, the proposed approach will find numerous applications in the field of ms, microfluidics, microoptics and many others.

The spatial and temporal behavior of ultrashort pulses has drawn more and more attention. Especially in laser material processing, such spatio-temporal behavior has significant influences. In this paper we present a brief analysis on the pulse front tilt (PFT) in simultaneous spatial and temporal focusing (SSTF) mathematically and in simulations. We apply paraxial field tracing based on the Collins integral for modeling the spatio-temporal focusing process. Using the shift theorem of the Fourier transformation, we present an explanation of the PFT in focus for general input pulses. Next, by assuming a Gaussian lateral pulse shape, an analytical solution for the field distribution at any position in the region is obtained. In this work we take the influence of an initial PFT before focusing into considerations as well and find potential way to control the PFT during the focusing process. Finally with the optical modeling software VirtualLab^TM we present rigorous simulations of the SSTF to verify our mathematical conclusions.

A method is described that is promising for application metal conductors on ceramic substrates during printed-circuit boards (PCBs) production without masking plate. The main idea of laser-induced metal deposition from solution (LCLD) consists of implementation of chemical micro reactor by using a focused laser beam. In this reactor the red/ox reaction would be initiated due to heating of a reaction medium. We used a 532 nm DPSS laser (power: 2100 mW) and water solutions of organic alcohols with low molecular weight, ethanol and isopropanol as reductants. The results of deposition were studied using the SEM, EDX methods and impedance spectroscopy. The equivalent resistance-capacitance circuit of copper tracks was constructed. The experiments showed that increasing the rate of deposition of nanostructured copper tracks up to 50 μm/s with electrical resistivity 5 Ohm/cm is possible by replacing the well-known reductants such as formaldehyde and D-sorbitol with iso-propanol.

Single layer thin-film materials such as aluminum, polyethylene, polypropylene, and their multi-layer combinations such as aluminum-paper have been exposed to different laser radiation. A wide number of samples have been processed with 10 - 12.5 ns IR and Green, and 500 - 800 ps IR laser radiation at different translating speeds ranging from 50 mm/s to 1 m/s. High quality incisions have been obtained for all tested materials within the experimental conditions. The presented results provide the necessary parameters for an efficient cut and processing of the tested materials, for the employment of pulsed laser sources in the packaging industry, allowing the laser to prevail in lieu of more costly and energy intensive methods.

Laser induced forward transfer (LIFT) is a freeform, additive patterning technique capable of depositing high resolution metal structures. A laser pulse is used to generate small droplets from the donor material, defined by the spot size and energy of the pulse. Metallic as well as non-metallic materials can be patterned using this method. Being a contactless, additive and high resolution patterning technique, this method enables fabrication of multi-layer circuits, enabling bridge printing, thereby decreasing component spacing. Here we demonstrate copper droplet formation from a thin film donor. The investigation of the LIFT process is done via shadowgraphy and provides detailed insight on the droplet formation. Of particular importance is the interplay of the droplet jetting mechanism and the spacing between donor and receiving substrate on a stable printing process. Parameters such as the influence of laser fluence and donor thickness on the formation of droplets are discussed. An angle deviation analysis of the copper droplets during flight is carried out to estimate the pointing accuracy of the transfer. The possibility of understanding the droplet formation, could allow for stable droplets transferred with large gaps, simplifying the process for patterning continuous high-resolution conductive lines.

Metal coatings have been produced using a Pulsed Laser Deposition (PLD) process based on a dynamic prism system. A nanosecond pulsed Nd:YVO4 laser emitting at a wavelength of 1064 nm is steared through a couple of prisms into a metallic target placed inside a vacuum chamber. This work presents the study of the parameters applied to achieve ablation of two metallic targets, aluminum and brass, in order to obtain their respective metallic coatings on glass substrates by pulsed laser deposition. The results obtained allow to standardize the optimal laser and set-up parameters in order to obtain homogeneous layers, tunable thickness and/or semi-transparent mirror behaviour through the formation of an adequate plasma plume. Microstructure and transmittance spectra of these coatings are also reported.

Compensation of low order aberrations is essential for high power solid state slab laser. With the increase of output power, the peak-to-valley of wavefront distortion increase to dozens of micrometer. It’s difficult to control the wavefront with deformable mirrors which always has limited stroke(<20μm). In this paper, a reflective beam shaping system is designed to shaping the beam spot from rectangular to squarer. The beam shaping system consists of two x-oriented cylindrical mirrors and two y-oriented cylindrical mirrors. Simulations of PID control algorithm for actively compensating of low-order aberrations with reflective beam shaping system are presented. It shows that different combinations of defocus, 0o astigmatism and 45° astigmatism, which is the main contributor of beam aberrations in slab laser, can be well compensated by adjustment of distance and rotation angle of mirrors. And the convergence is fast when the control error signal is set to a suitable combination of low order Zernike coefficients. For beam with wave aberrations (PtV=82.6λ, RMS=18.2λ, Z4=23.6, Z5=7.1, Z6=19.6), the adjustment of distance between mirrors is below 100mm, and the rotation angle about z-axis is below 2 degree. The wavefront aberrations are decreased to a low level (PV=0.16λ, RMS=0.04λ) which can be easily corrected later with DM.

Many commercial laser systems deliver a beam having a Gaussian intensity profile, however, numerous applications require other intensity profiles (top-hat, hollow beam, Bessel beam,…) which are in general obtained by converting the standard Gaussian beam (GB) through transparent diffractive optical elements (DOE). Laser beam shaping (LBS) by use of DOE’s is a topic that has been intensively developed over a long time whether the DOE is programmable or unprogrammable. Although, DOE's are a great help to many LBS problems they have an inherent drawback which is a relatively high cost. Recently, we have experimentally demonstrated the LBS ability resulting from the coaxial superposition of two CW coherent Gaussian beams. This technique is classified under interferometric LBS techniques contrasting with the usual ones based on diffraction. In particular, we demonstrate the reshaping of a Gaussian beam into a bottle beam or flat-top beam in the focal plane of a focusing lens.

The oil and gas industry has attempted for many years to find new ways to analyze and determine the type of rocks drilled on a real time basis. Mud analysis logging is a direct method of detecting oil and gas in formations drilled, it depends on the "feel" of the bit to decide formation type, as well as, geochemical analysis which was introduced 30 years ago, starting with a pulsed-neutron generator (PNG) based wireline tool upon which LWD technology was based. In this paper, we are studying the feasibility of introducing a new technology for real-time geochemical analysis. Laser-induced breakdown spectroscopy (LIBS) is a type of atomic emission spectroscopy, It is a cutting-edge technology that is used for many applications such as determination of alloy composition, origin of manufacture (by monitoring trace components), and molecular analysis (unknown identification). LIBS can analyze any material regardless of its state (solid, liquid or gas), based upon that fact, we can analyze rocks, formation fluids' types and contacts between them. In cooperation with the National Institute of Laser Enhanced Science, Cairo University in Egypt, we've done tests on sandstone, limestone and coal samples acquired from different places using Nd: YAG Laser with in addition to other components that are explained in details through this paper to understand the ability of Laser to analyze rock samples and provide their elemental composition using LIBS technique. We've got promising results from the sample analysis via LIBS and discussed the possibility of deploying this technology in oilfields suggesting many applications and giving a base for achieving a quantitative elemental analysis method in view of its shortcomings and solutions.

We present a theoretical and experimental analysis of a pulsed 1645 nm optical parametric oscillator (OPO) to prove the feasibility of such a device for a spaceborne laser transmitter in an integrated path differential absorption (IPDA) lidar system. The investigation is part of the French-German satellite mission MERLIN (Methane Remote Sensing Lidar Mission). As an effective greenhouse gas, methane plays an important role for the global climate. The architecture of the OPO is based on a conceptual design developed by DLR, consisting of two KTA crystals in a four-mirror-cavity. Using numerical simulations, we studied the performance of such a setup with KTP and investigated means to optimize the optical design by increasing the efficiency of the OPO and decreasing the fluence on the optical components. For the experimental testing of the OPO, we used the INNOSlab-based ESA pre-development model ATLAS as pump laser at 1064 nm. The OPO obtained 9.2 mJ pulse energy at 1645 nm from 31.5 mJ of the pump and a pump pulse duration of 42 ns. This corresponds to an optical/optical efficiency of 29%. After the pump pulse was reduced to 24 ns, a similar OPO performance could be obtained by adapting the pump beam radius. In recent experiments with optimized optical design the OPO obtained 12.5 mJ pulse energy at 1645 nm from 32.0 mJ of the pump, corresponding to an optical/optical efficiency of 39%. Two different methods were applied to study the laser damage thresholds of the optical elements used.

The availability of suitable laser sources is one of the main challenges in future space missions for accurate measurement of atmospheric CO₂. The main objective of the European project BRITESPACE is to demonstrate the feasibility of an all-semiconductor laser source to be used as a space-borne laser transmitter in an Integrated Path Differential Absorption (IPDA) lidar system. We present here the proposed transmitter and system architectures, the initial device design and the results of the simulations performed in order to estimate the source requirements in terms of power, beam quality, and spectral properties to achieve the required measurement accuracy. The laser transmitter is based on two InGaAsP/InP monolithic Master Oscillator Power Amplifiers (MOPAs), providing the ON and OFF wavelengths close to the selected absorption line around 1.57 μm. Each MOPA consists of a frequency stabilized Distributed Feedback (DFB) master oscillator, a modulator section, and a tapered semiconductor amplifier optimized to maximize the optical output power. The design of the space-compliant laser module includes the beam forming optics and the thermoelectric coolers. The proposed system replaces the conventional pulsed source with a modulated continuous wave source using the Random Modulation-Continuous Wave (RM-CW) approach, allowing the designed semiconductor MOPA to be applicable in such applications. The system requirements for obtaining a CO₂ retrieval accuracy of 1 ppmv and a spatial resolution of less than 10 meters have been defined. Envelope estimated of the returns indicate that the average power needed is of a few watts and that the main noise source is the ambient noise.

We present the experimental investigations of different designs of resonant waveguide-grating mirrors (RWG) which are used as intracavity folding mirror in an Yb:YAG thin-disk laser. The studied mirrors combine structured fused silica substrates, a thin-layer waveguide (Ta₂O₅), a buffer layer (SiO²) and partial reflectors. The grating period was chosen to be 510 nm to allow resonances at an angle of incidence of ~10° for TE polarization. The waveguide layer has a thickness of 236 nm. It is followed by the buffer layer with a thickness of 580 nm and the subsequent alternating Ta₂O5/SiO₂ layers. The exact coating sequence depends on the two design approaches which were investigated in this work: either introducing different partial reflectors, i.e. stacks of quarter-wave layers on top of the waveguide while keeping the groove depth of the grating constant, or increasing the grating depth, while keeping an identical partial reflector. The investigation was focused on the rise of the surface temperature due to the coupling of the incident radiation to a waveguide mode, as well as on the laser efficiency, polarization and wavelength selectivity. It is found that, when compared to the simplest RWG design which consists of only a single Ta₂O₅ waveguide layer, damage threshold as well as laser efficiency can be significantly increased, while the laser performances in terms of polarization- and wavelength selectivity are maintained. So far, the presented RWG allow the generation of linear polarization with a narrow spectral linewidth down to 25 pm FWHM in a fundamental mode Yb:YAG thin-disk laser. Damage thresholds of 60kW/cm² have been reached where only 63K of surface temperature increase was observed. This shows that the improved mirrors are suitable for the generation of kW-class narrow linewidth, linearly polarized Yb:YAG thin-disk lasers.

Laser diode wavelength stability is vital for applications such as spectroscopy and data communication, and the emitted wavelength is a function of temperature. In a conventional system, the laser diode temperature is controlled using a Peltier element with a temperature-sensing thermistor, the latter placed at a short distance from the laser diode chip. Despite the use of good thermal design and a case, a change in ambient temperature may cause a change to internal thermal gradients, resulting in a systematic error in the laser diode wavelength. In this paper we describe a novel system to measure the temperature of the laser diode junction via measurement of the junction voltage. The method has been applied to a 1651 nm DFB laser diode for use in tunable diode laser spectroscopy (TDLS) of methane. The wavelength stability of both thermistor- and voltage- control systems are compared over a period of 30 minutes and with different ambient temperatures. Over 30 min at constant ambient temperature, thermistor control provided a precision of ± 0.4 pm (40 MHz) and junction voltage control gave a similar ± 0.6 pm (70 MHz). For an ambient temperature change of 20°C, conventional thermistor control suffered a wavelength change of 76 pm (8.4 GHz), whereas junction voltage control reduced this to 0.6 pm (70 MHz), at or below the level of long-term wavelength precision.

We report on a novel type of laser in which a semiconductor optical amplifier (SOA) receives frequency-selective feedback from a glass-waveguide circuit. The laser we present here is based on InP for operation in the 1.55 μm wavelength range. The Si₃N₄/SiO₂ glass waveguide circuit comprises two sequential high-Q ring resonators. Adiabatic tapering is used for maximizing the feedback. The laser shows single-frequency oscillation with a record-narrow spectral linewidth of 24 kHz at an output power of 5.7 mW. The hybrid laser can be tuned over a broad range of 46.8 nm (1531 nm to 1577.8 nm). Such InP-glass hybrid lasers can be of great interest in dense wavelength division multiplexing (DWDM) and as phase reference in optical beam-forming networks (OBFN). The type of laser demonstrated here is also of general importance because it may be applied over a huge wavelength range including the visible, limited only by the transparency of glass (400 nm to 2.35 μm).

Multiwavelength lasing in the random distributed feedback fiber laser is demonstrated by employing an all fiber Lyot filter. Stable multiwavelength generation is obtained, with each line exhibiting sub-nanometer line-widths. A flat power distribution over multiple lines is also obtained, which indicates the contribution of nonlinear wave mixing towards power redistribution and equalization in the system. The multiwavelength generation is observed simultaneously in first and second Stokes waves.

We report on a Yb:YAG thin-disk multipass amplifier for ultrashort laser pulses delivering an average output power of 1.1 kW which to the best of our knowledge is the highest output power reported from such a system so far. A modified commercial TruMicro5050 laser delivers the seed pulses with an average power of 80 W at a wavelength of 1030 nm, a pulse duration of 6.5 ps and a repetition rate of 800 kHz. These pulses are amplified to 1.38 mJ of pulse energy with a duration of 7.3 ps. To achieve this, we developed a scheme in which an array of 40 plane mirrors is used to geometrically fold the seed beam over the pumped thin-disk crystal. Exploiting the incoming linear polarization, an overall number of 40 double-passes through the disk was realized by using the backpath through the amplifier with the orthogonal linear polarization state. Thermal issues on the disk were mitigated by zero-phonon line pumping at a wavelength of 969 nm directly into the upper laser level and by employing a retroreflective mirror pair. The amplifier exhibits an optical efficiency of 44 % and a slope efficiency of 46 %. The beam quality was measured to be better than M²=1.25 at all power levels. As this system can deliver high pulse energies and high average output powers at the same time without the need of a CPA technique, it can be very suitable for high productivity material processing with ultrashort laser pulses.

Glass drilling realized with the help of femtosecond lasers attract industrial attention, however, desired tasks may require systems employing high numerical aperture (NA) focusing conditions, low repetition rate lasers and complex fast motion translation stages. Due to the sensitivity of such systems, slight instabilities in parameter values can lead to crack formations, severe fabrication rate decrement and poor quality overall results. A microfabrication system lacking the stated disadvantages was constructed and demonstrated in this report. An f-theta lens was used in combination with a galvanometric scanner, in addition, a water pumping system that enables formation of water films of variable thickness in real time on the samples. Water acts as a medium for filament formation, which in turn decreases the focal spot diameter and increases fluence and axial focal length. This article demonstrates the application of a femtosecond (280fs) laser towards rapid cutting of different transparent materials. Filament formation in water gives rise to strong ablation at the surface of the sample, moreover, the water, surrounding the ablated area, adds increased cooling and protection from cracking. The constructed microfabrication system is capable of drilling holes in thick soda-lime, hardened glasses and sapphire. The fabrication time varies depending on the diameter of the hole and spans from a few to several hundred seconds. Moreover, complex-shape fabrication was demonstrated.

Cladding waveguides have been realized in Nd:YAG by direct writing with a femtosecond-laser beam. A classical method that inscribes many tracks around the waveguide circumference with step-by-step translations of the laser medium, and a new technique in which the laser medium is moved on a helical trajectory and that delivers waveguides with well-defined walls were employed. Laser emission on the 1.06 μm ⁴F₃/2→⁴I_11/2 transition and at 1.3 μm on the ⁴F_3/2→4I_13/2 line was obtained under the pump with a fiber-coupled diode laser. Thus, laser pulses at 1.06 μm with energy of 1.3 mJ for the pump at 807 nm with pulses of 12.5-mJ energy were recorded from a circular waveguide of 100-μm diameter that was inscribed in a 5-mm long, 0.7-at.% Nd:YAG single crystal by the classical translation technique. A similar waveguide that was realized in a 5-mm long, 1.1-at.% Nd:YAG ceramic increased the 1.06-μm laser pulse energy to 2.15 mJ for the pump pulses of 13.1-mJ energy. Furthermore, a circular waveguide of 100-μm diameter that was inscribed in the Nd:YAG ceramic by the helical-movement method yielded pulses at 1.06 μm with increased maximum energy of 3.2 mJ; the overall optical-to-optical efficiency was 0.24, and the laser operated with a slope efficiency of 0.29. The same device outputted laser pulses at 1.3 μm with energy of 1.15 mJ.

Strong ion migration in shown to enable the production of high refractive index contrast waveguides by fs-laser writing in a commercial (Er,Yb)-doped phosphate based glass. Waveguide writing was performed using a high repetition rate fslaser fibre amplifier operated at 500 kHz and the slit shaping technique. Based on measurements of the NA of waveguides, the positive refractive index change (Δn) of the guiding region has been estimated to be ∼1-2 x10^-2. The compositional maps of the waveguides cross-sections performed by X-ray microanalysis evidenced a large increase of the La local concentration in the guiding region up to ~25% (relative to the non-irradiated material). This large enrichment in La was accompanied by the cross migration of K to a neighbouring low refractive index zone. The refractive index of the La-phosphate glass increases linearly with the La₂O₃ content (Δn per mole fraction increase of La₂O₃ ≈ 5x10^-3) mainly because of the relative mass of the La³⁺ ions. The density increase without substantial modification of the glass network was confirmed by space-resolved micro-Raman spectroscopy measurements showing minor variations in the (PO₂)_sym vibration Raman band. These results provide evidence for the feasibility of adapting the glass composition for enabling laser-writing of high refractive index contrast structures via spatially selective modification of the glass composition.

The goal of this contribution is to provide a guideline for Kerr-lens mode-locking (KLM) of thin-disk oscillators. This includes cavity design, hard and soft-aperture optimization, handling of thermal effects in intra-cavity optics as well as methods of average power and energy scaling. The main differences and similarities between mode-locking of Ti:sapphire bulk and Yb:YAG thin-disk oscillators are presented.

We present in this study a PM all-fiber laser oscillator passively mode-locked (ML) at 1.03 μm. The laser is based on Nonlinear Polarization Evolution (NPE) in polarization maintaining (PM) fibers. In order to obtain the mode-locking regime, a nonlinear reflective mirror including a fibered polarizer, a long fiber span and a fibered Faraday mirror (FM) is inserted in a Fabry-Perot laser cavity. In this work we explain the principles of operation of this original laser design that permits to generate ultrashort pulses at low repetition (lower that 1MHz) rate with a cavity length of 100 m of fiber. In this experiment, the measured pulse duration is about 6 ps. To our knowledge this is the first all-PM mode-locked laser based on the NPE with a cavity of 100m length fiber and a delivered pulse duration of few picosecondes. Furthermore, the different mode-locked regimes of the laser, i.e. multi-pulse, noise-like mode-locked and single pulse, are presented together with the ways of controlling the apparition of these regimes. When the single pulse mode-locking regime is achieved, the laser delivers linearly polarized pulses in a very stable way. Finally, this study includes numerical results which are obtained with the resolution of the NonLinear Schrodinger Equations (NLSE) with the Split-Step Fourier (SSF) algorithm. This modeling has led to the understanding of the different modes of operation of the laser. In particular, the influence of the peak power on the reflection of the nonlinear mirror and its operation are studied.

This work demonstrates that fibre lasers mode-locked due to non-linear polarisation evolution (NPE) feature a very broad range of their pulse parameter variation in different generation regimes of these lasers. Both numerical modelling and experimental studies confirm that pulse parameters, such as duration and spectrum width may differ by an order of magnitude and more. This ultra-broad variability of pulse parameters in fibre lasers mode-locked due to NPE is unique and has no analogues in other mode-locked lasers. Relatively broad range of key pulse parameters of fibre lasers modelocked due to NPE requires that at least the duration and spectral width of the generated pulses be interactively controlled. For instance, adjustment of polarisation controllers in these lasers may change the pulse duration from 4 to 80 ps and the spectrum width from 0.2 to 7.4 nm.

Birefringence influences the beam quality and output power of high power solid-state lasers. Inhomogeneous distribution of the thermal field inside the laser crystal rod leads to thermal strains and birefringence, due to the photoelastic effect. Analytical models have used the plane stress and plane strain assumption for an axial sym- metric pumped crystal. This leads in case of an [111]-cut to an axially symmetric birefringence pattern. However, numerical calculations of birefringence show a threefold symmetry pattern due to the anisotropic behavior of the photoelastic tensor. This disturbs the ideal use of a radial or azimuthal polarised beam. We analyzed a laser rod pumped at three sides with threefold symmetry, in order to reduce the effect of birefringence. Simulation results show birefringence is affected by rotation of the crystal around its [111]-axis. Smallest birefringence can be obtained by an optimal rotation with respect to the edges of the crystal. Therefore the output beam of this laser device is more suitable for generating radial or azimuthal polarisations.

An all fiber laser with master-oscillator-power-amplifier (MOPA) configuration at 1064nm/1550nm for the high-resolution three-dimensional (3D) imaging light detection and ranging (LIDAR) system was reported. The pulsewidth and the repetition frequency could be arbitrarily tuned 1ns~10ns and 10KHz~1MHz, and the peak power exceeded 100kW could be obtained with the laser. Using this all fiber laser in the high-resolution 3D imaging LIDAR system, the image resolution of 1024x1024 and the distance precision of ±1.5 cm was obtained at the imaging distance of 1km.

Influence of temperature on Pr:YAlO₃ laser behaviour in the near-infrared (747 nm), red (662 nm), orange (622 nm), and green (547 nm) spectral range is reported. To realize Pr:YAlO3 laser systems operating down to the cryogenic temperature, a microchip geometry formed by dielectric films directly deposited on the crystal facets has been proposed and employed. This geometry is particularly attractive due to its compact and rugged design. The best results were demonstrated for the 747 nm wavelength; more than 300 mW of output power with the slope efficiency over 55 % has been extracted from the cryogenically cooled Pr:YAlO₃ active material under 1-W InGaN diode pumping. In addition, the first diode-pumped Pr:YAlO3 orange laser is described, as we believe.

A new method to calculating the wavefront of slap laser is studied in this paper. The method is based on the ray trace theory of geometrical optics. By using the Zemax simulation software and Matlab calculation software, the wavefront of rectangular beam in beam reshaping system is reconstructed. Firstly, with the x- and y-slope measurement of reshaping beam the direction cosine of wavefront can be calculated. Then, the inverse beam path of beam reshaping system is built by using Zemax simulation software and the direction cosine of rectangular beam can be given, too. Finally, Southwell zonal model is used to reconstruct the wavefront of rectangular beam in computer simulation. Once the wavefront is received, the aberration of laser can be eliminated by using the proper configuration of beam reshaping system. It is shown that this method to reconstruct the wavefront of rectangular beam can evidently reduce the negative influence of additional aberration induced by beam reshaping system.

The aim of this study was design and construction of a Nd:YAG laser system allowing laser generation in the nearinfrared region at three separate switchable wavelengths 1.06, 1.32, or 1.44 μm. This integrated multi-wavelength laser system can be useful for application especially in medicine and spectroscopy. We used 1.1 at. % Nd/Y doped Nd:YAG active medium 4 x 102 mm in dimensions with anti-reflection coated faces for 1.06, 1.32, and 1.44 μm. The laser crystal was placed along Xe flashlamp into the diffuse pump cavity (the pump pulse duration was 800 μs FWHM). The laser system was formed by six mirrors including one output coupler, three rear mirrors, and two dichroic mirrors. These dichroic mirrors were specially designed and their characteristics were as follow: HR/T @ 1440/1320 and 1064 nm and HR/T @ 1320/1064 nm. The output coupler reflectivity was 17 %, 80 %, and 82 %, for 1.06, 1.32, and 1.44 μm wavelengths, respectively. Particular laser Nd:YAG line selection was realized by adequate shutters. The output laser characteristics in terms of output energy, spatial beam structure, and temporal profile were recorded. For 62 J pumping energy, the obtained output energies were 0.80 J, 0.45 J, and 0.19 J, for 1.06, 1.32, and 1.44 μm wavelengths, respectively. Specific absorption properties of the designed wavelengths in water and water vapor together with the sufficient reached energy predetermine this compact Nd:YAG laser system for utilization in medical and industry applications.

The goal of our work was preparation and investigation of Er-doped potassium ytterbium lanthanum orthoand metha-phosphate glassy mixtures developed as a solid-state laser active medium. The tested samples were prepared by rapid quenching of molten mixture of starting K₂CO₃, YbPO₄, LaPO₄, YbPO₄ and P₂O₅. Their cations molar concentration was as follows n(K) = 0.7, n(Yb) = 0.135, n(La) = 0.16 and n(Er) = 0.005 and it was the same in all tested samples. The additions of the P2O5 to the individual starting charges were batched so the following compositions of resulting glasses should be obtained: (i) pure meta-phosphate, (ii) mixture of 80 mol% of meta-phosphate and 20 mol% of ortho-phosphate, (iii) mixture of 50 mol% of meta-phosphate and 50 mol% of ortho-phosphate, (iv) mixture of 25 mol% of meta-phosphate and 75 mol% of ortho-phosphate, and (v) pure ortho-phosphate. The glassy samples were prepared in the form of discs about 8mm in diameter and 2mm in height. The absorption spectra were measured in broad range from 200 up to 2500 nm to identify possible impurities, mainly the residual OH-absorption and to calculate absorption cross-section for pumping and laser transition ⁴I_15/2 → ⁴I_11/2 and ⁴I_13/2 → ⁴I_15/2, respectively. For particular transitions fluorescence spectra and fluorescence decay time were recorded simultaneously. It was found that the fluorescence decay time, corresponding to upper laser level ⁴I_13/2 depopulation, progressively increases with the content of orthophosphate in glass composition starting from 2ms for sample (i) up to 8ms for sample (iv). The laser action at 1.54 μm under 975nm pulsed laser diode pumping was successfully demonstrated using the sample with the longest upper laser level lifetime.

In this contribution we present spectroscopic and laser properties of TGT (temperature gradient technique) grown Nd,Y:SrF₂ crystals with neodymium concentration of 0.4, 0.65 and 0.8 at.%. The absorption cross-section, fluorescence spectra and fluorescence decay time were measured. For the laser experiments, the noncoated crystal samples 3.5 or 5 mm thick were pumped by a 796 nm laser diode matching the Nd:SrF2 absorption peak. Several output couplers with reflectivity ranging from 70 to 98 % at the generated wavelength were tested. In the pulsed pumping regime (pulseduration 2 ms, frequency 10 Hz), the maximum average output power of 75 mW was obtained with the slope efficiency as high as 48 % and the optical-to-optical efficiency of 42 % with respect to the absorbed pump power. The output beam spatial profile was nearly Gaussian in both axes, oscillations started at the wavelength of 1057 nm. At higher pumping levels, the second emission line at 1050 nm appears corresponding to our fluorescence measurements. Wavelength tuning using birefringent filter from 1048 to 1070 nm is probably given by crystal-field splitting of the ⁴F_3/2 manifold in Nd³⁺. True-CW laser operation was also successfully obtained at lower pumping level with the maximum output power of 90 mW using output coupler reflectivity of 98 %.

Laser welding is a promising joining process for copper interconnections. A key criterion of quality for these welds is the penetration depth. The penetration depth is subject to intrinsic variation, i.e. by the nature of the welding process. Online detection of penetration depth enables quality assurance and furthermore welding of joint configurations with tighter tolerances via closed-loop control. Weld pool geometry and keyhole optical emission in the wavelength interval of 400-1100 nm are investigated with regard to how suitable they are for the detection of penetration depth in laser welding of copper Cu-ETP. Different penetration depths were induced by stepwise modulation of laser power in bead-on-plate welds. The welds have been monitored with illuminated high-speed videography of the work piece surface and spectrometry. Increase of the weld pool length (in direction of travel) corresponding to increase in penetration depth has been observed while no noticeable change was observed of the weld pool width (transverse to the direction of travel). No significant lines were observed in the spectrum. The radiant power in VIS-spectrum was observed to increase with increasing penetration depth as well. As future work, with increasing understanding and experimental data, online monitoring by indirectly measuring the penetration depth would be possible. The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no 260153 (QCOALA: Quality Control for Aluminium Laser-Welded Assemblies).

In this work we have constructed a range gated imaging LIDAR with the aim to show the potential of this technique as well as to further determine aspects such as the typical energy per pulse needed, the illumination distribution of the laser source and the safety class. For this, we built a custom frequency doubled nanosecond pulsed Nd:YAG laser as illumination source, a CCD coupled to a generation II image intensifier and a simple progressive delays set for the camera gate using a pulse delay generator. At low levels of the illumination pulse and assuming safety perimeter around the system of approx. 1.5 m, the LIDAR could be classified as class 2M. In these conditions, we could resolve objects as far as 690 m.

The simplest model for a description of the random distributed feedback (RDFB) Raman fiber laser is a power balance model describing the evolution of the intensities of the waves over the fiber length. The model predicts well the power performances of the RDFB fiber laser including the generation threshold, the output power and pump and generation wave intensity distributions along the fiber. In the present work, we extend the power balance model and modify equations in such a way that they describe now frequency dependent spectral power density instead of integral over the spectrum intensities. We calculate the generation spectrum by using the depleted pump wave longitudinal distribution derived from the conventional power balance model. We found the spectral balance model to be sufficient to account for the spectral narrowing in the RDFB laser above the threshold of the generation.

This work presents for the first time the results of study of one of the simplest and most reliable configurations of a ring fibre laser passively mode-locked due to nonlinear polarisation evolution. The laser arrangement under consideration comprises a single phase retarding element in contrast to most widely used configurations with several wave plates or two polarisation controllers. By means of numerical simulation based on coupled non-linear Schrödinger equations for orthogonal polarisation components, we investigate mode-lock domain in terms of pump power and phase delay introduced by the single polarisation control element. Changing pump power, we demonstrate the capacity of such a simple cavity layout with only one polarisation element to operate in different lasing regimes including generation of conventional laser pulse trains at the fundamental repetition rate, generation of double-scale partially coherent and noiselike pulses and generation of multiple pulses per round-trip. Besides the results of a detailed numerical study, we also announce experimental results obtained from an Er fibre laser with a single polarisation controlling element based on an electronically driven liquid crystal. Our experimental observations are in good qualitative agreement with simulation results and constitute a platform for creation of new simple, low-cost, and reliable self-starting fibre lasers with ultrashort optical pulses.

The techniques applying laser beams or optical systems are limited by the diffraction limit of the optical heads used. We demonstrate theoretically and experimentally that the use of the photonic jet allows an improvement in the optical resolution to achieve smaller etching without reducing the wavelength of the source. The potential of the photonic jet using a nanosecond pulsed near-infrared laser for micro-fabrication is also demonstrated. These lasers are the most common type of laser used in industrial processes because of their price and the fact that well-packaged sources are available. Their typical spatial resolution in laser etching is limited by the spot size of their focus point at around 25-70 μm. This is the reason why a photonic jet, a high spatial concentration onto a half-wavelength spot of a beam that emerges in the vicinity of a dielectric microsphere, is of great interest. In our experiments, micro-scale glass (ns = 1.5) and BaTiO₃ spheres (n_s = 1.9) have been used to achieve photonic jets. The etching process has been tested on two substrates: silicon wafers, which have a significant absorption at 1064 nm, and glass plates, which have a lower absorption at this wavelength. The smallest marking achieved on silicon has an average diameter of 1.3 μm and despite the low absorption, micrometric etchings have also been achieved on glass using larger microspheres.

Nowadays additive manufacturing becomes more and more popular. It depends on the results of last achievements in developing of the new constructions for modern machine tools. One of the most developed AM technology is SLM or SLS. About twenty years ago the technology of rapid prototyping started to grow up from building prototypes and developed to real functional item production. Especially this becomes more important in producing medical implants in the full accordance with individual digital 3D-model from metallic powder as Ti6Al4V or CoCr. The additive technology gives the possibility to reduce additional steps in implants production process as work preparation process, forwarding a work piece from one shop to another one, post treatment etc. This approach is very topical to production of tooth, knee and coxal implants. This idea is realized in the commercial SLM machines as EOS M280, SLM Solutions 125HL (Germany), Phenix systems PXS/PXM Dental (France) (fig. 1).

We present the determination of the energy transfer upconversion (ETU) coefficient for Nd:YAG via the z-scan technique, achieved by studying the irradiance dependence of the transmission of a pump laser tuned to the absorption peak around 808 nm. A spatially dependent two-level rate equation model has been utilized to predict the transmission dependence as a function of the sample’s position in the z-scan experiment, with the ETU coefficient the only free parameter. Comparing experimental results with the model’s output, the ETU coefficient for 1 at.% Nd:YAG is determined to be 5.1 ± 0.4 x 10^-17 cm³/s.

For coherent pumping of Fe:ZnSe and Fe:ZnMgSe lasers generating giant pulses in mid-infrared part of spectrum the radiation with wavelength ~ 3 μm and pulse-length in the range of hundreds of nanoseconds (for room temperature operation) are needed. For more flexibility in pumping of Fe-based mid-infrared materials also new Er:YSGG laser was designed and characterized and compared with previously designed Er:YAG laser. The giant pulses with the energy 53 mJ and length of pulse ~ 80 ns were obtained for pumping 24 J. The generated wavelength was measured to be 2.79 μm and space structure was close to Gaussian.

In the field of precise measurement of optical frequencies, laser spectroscopy and interferometric distance surveying the optical frequency synthesizers (femtosecond combs) are used as optical frequency references. They generate thousands of narrow-linewidth coherent optical frequencies at the same time. The spacing of generated components equals to the repetition frequency of femtosecond pulses of the laser. The position of the comb spectrum has a frequency offset that is derived from carrier to envelope frequency difference. The repetition frequency and mentioned frequency offset belong to main controlled parameters of the optical frequency comb. If these frequencies are electronically locked an ultrastable frequency standard (i.e. H-maser, Cs- or Rb- clock), its relative stability is transferred to the optical frequency domain. We present a complete digitally controlled signal processing chain for phase-locked loop (PLL) control of the offset frequency. The setup is able to overcome some dropouts caused by the femtosecond laser non-stabilities (temperature drifts, ripple noise and electricity spikes). It is designed as a two-stage control loop, where controlled offset frequency is permanently monitored by digital signal processing. In case of dropouts of PLL, the frequency-locked loop keeps the controlled frequency in the required limits. The presented work gives the possibility of long-time operation of femtosecond combs which is necessary when the optical frequency stability measurement of ultra-stable lasers is required. The detailed description of the modern solution of the PLL with remote management is presented.