Grazing incidence pumped Zr x-ray laser for spectroscopy on Li-like ions

X-ray laser fluorescence spectroscopy of the 2s-2p transition in Li-like ions is promising to become a widely applicable tool to provide information on the nuclear charge radii of stable and radioactive isotopes. For performing such experiments at the Experimental Storage Ring ESR, and the future NESR within the FAIR Project, a grazing incidence pumped (GRIP) x-ray laser (XRL) was set up at GSI Darmstadt using PHELIX (Petawatt High Energy Laser for heavy Ions eXperiments). The experiments demonstrated that lasing using the GRIP geometry could be achieved with relatively low pump energy, a prerequisite for higher repetition rate. In the first chapter the need of a plasma XRL is motivated and a short history of the plasma XRL is presented. The distinctive characteristic of the GRIP method is the controlled deposition of the pump laser energy into the desired plasma density region. While up to now the analysis performed were mostly concerned with the plasma density at the turning point of the main pump pulse, in this thesis it is demonstrated that also the energy deposition is significantly modified for the GRIP method, being sensitive in different ways to a large number of parameters. In the second chapter, the theoretical description of the plasma evolution, active medium and XRL emission properties are reviewed. In addition an innovative analysis of the laser absorption in plasma which includes an inverse Bremsstrahlung (IB) correction factor is presented. The third chapter gives an overview of the experimental set-up and diagnostics, providing an analytical formula for the average and instantaneous traveling wave speed generated with a tilted, on-axis spherical mirror, the only focusing system used up to now in GRIP XRL. The fourth chapter describes the experimental optimization and results. The emphasis is on the effect of the incidence angle of the main pump pulse on the absorption in plasma and on output and gain in different lasing lines. This is compared to the theoretical results for two different incidence angles. Significant corrections for the temperature evolution during the main pump pulse due to the incidence angle are demonstrated in comparison to a simple analytical model which does not take into account the pumping geometry. A much better agreement is reached by the model developed in this thesis. An interesting result is also the appearance of a central dip in the spatially resolved keV emission which was observed in the XRL experiments for the first time and correlates well with previous near field imaging and plasma density profile measurements. In the conclusion also an outlook to the generation of shorter wavelength XRL’s is given.

Multi-stage ytterbium fiber-amplifier seeded by a gain-switched laser diode

We demonstrated all-fiber amplification of 11 ps pulses from a gain-switched laser diode at 1064 nm. The diode was driven at a repetition rate of 40 MHz and delivered 13 µW of fiber-coupled average output power. For the low output pulse energy of 325 fJ we have designed a multi-stage core pumped pre-amplifier in order to keep the contribution of undesired amplified spontaneous emission as low as possible. By using a novel time-domain approach for determining the power spectral density ratio (PSD) of signal to noise, we identified the optimal working point for our pre-amplifier. After the pre-amplifier we reduced the 40 MHz repetition rate to 1 MHz using a fiber coupled pulse-picker. The final amplification was done with a cladding pumped Yb-doped large mode area fiber and a subsequent Yb-doped rod-type fiber. With our setup we reached a total gain of 73 dB, resulting in pulse energies of >5.6 µJ and peak powers of >0.5 MW. The average PSD-ratio of signal to noise we determined to be 18/1 at the output of the final amplification stage.

Measurement of plasma electron density generated in an experiment of laser shock processing, utilizing the H­α­-line

In this work we have realized plasma diagnosis produced by Laser (LPP), by means of emission spectroscopy in a Laser Shock Processing (LSP). The LSP has been proposed as an alternative technology, competitive with classical surface treatments. The ionic species present in the plasma together with electron density and its temperature provide significant indicators of the degree of surface effect of the treated material. In order to analyze these indicators, we have realized spectroscopic studies of optical emission in the laser-generated plasmas in different situations. We have worked focusing on an aluminum sample (Al2024) in air and/or in LSP conditions (water flow) a Q-switched laser of Nd:YAG (λ = 1.06 μm, 10 ns of pulse duration, running at 10 Hz repetition rate). The pulse energy was set at 2,5 J per pulse. The electron density has been measured using, in every case, the Stark broadening of H Balmer α line (656.27 nm). In the case of the air, this measure has been contrasted with the value obtained with the line of 281.62 nm of Al II. Special attention has been paid to the self-absorption of the spectral lines used. The measures were realized with different delay times after the pulse of the laser (1–8 μs) and with a time window of 1 μs. In LSP the electron density obtained was between 1017 cm−3 for the shortest delays (4–6 μs), and 1016 cm−3 for the greatest delays (7,8 μs).

Direct femtosecond laser inscription in transparent dielectrics

Since 1996 direct femtosecond inscription in transparent dielectrics has become the subject of intensive research. This enabling technology significantly expands the technological boundaries for direct fabrication of 3D structures in a wide variety of materials. It allows modification of non-photosensitive materials, which opens the door to numerous practical applications. In this work we explored the direct femtosecond inscription of waveguides and demonstrated at least one order of magnitude enhancement in the most critical parameter - the induced contrast of the refractive index in a standard borosilicate optical glass. A record high induced refractive contrast of 2.5×10-2 is demonstrated. The waveguides fabricated possess one of the lowest losses, approaching level of Fresnel reflection losses at the glassair interface. High refractive index contrast allows the fabrication of curvilinear waveguides with low bend losses. We also demonstrated the optimisation of the inscription regimes in BK7 glass over a broad range of experimental parameters and observed a counter-intuitive increase of the induced refractive index contrast with increasing translation speed of a sample. Examples of inscription in a number of transparent dielectrics hosts using high repetition rate fs laser system (both glasses and crystals) are also presented. Sub-wavelength scale periodic inscription inside any material often demands supercritical propagation regimes, when pulse peak power is more than the critical power for selffocusing, sometimes several times higher than the critical power. For a sub-critical regime, when the pulse peak power is less than the critical power for self-focusing, we derive analytic expressions for Gaussian beam focusing in the presence of Kerr non-linearity as well as for a number of other beam shapes commonly used in experiments, including astigmatic and ring-shaped ones. In the part devoted to the fabrication of periodic structures, we report on recent development of our point-by-point method, demonstrating the shortest periodic perturbation created in the bulk of a pure fused silica sample, by using third harmonics (? =267 nm) of fundamental laser frequency (? =800 nm) and 1 kHz femtosecond laser system. To overcome the fundamental limitations of the point-by-point method we suggested and experimentally demonstrated the micro-holographic inscription method, which is based on using the combination of a diffractive optical element and standard micro-objectives. Sub-500 nm periodic structures with a much higher aspect ratio were demonstrated. From the applications point of view, we demonstrate examples of photonics devices by direct femtosecond fabrication method, including various vectorial bend-sensors fabricated in standard optical fibres, as well as a highly birefringent long-period gratings by direct modulation method. To address the intrinsic limitations of femtosecond inscription at very shallow depths we suggested the hybrid mask-less lithography method. The method is based on precision ablation of a thin metal layer deposited on the surface of the sample to create a mask. After that an ion-exchange process in the melt of Ag-containing salts allows quick and low-cost fabrication of shallow waveguides and other components of integrated optics. This approach covers the gap in direct fs inscription of shallow waveguide. Perspectives and future developments of direct femtosecond micro-fabrication are also discussed.

Femtosecond laser writing of buried waveguides in erbium III-Doped oxyfluoride glasses and nano-glass-ceramics

Femtosecond-pulsed laser writing of waveguides, a few mm long, is demonstrated; waveguides were written orthogonally to the writing beam inside the bulk of ErIII-doped oxyfluoride glasses at a depth of 160 mum. The writing beam was 795 nm wavelength, 54 fs pulse duration and 11 MHz repetition rate. Tracks were written at pulse energies of 13.1 nJ to 26.1 nJ and sample translational velocity of 10 mmmiddot.s-1 to 28 mmmiddots-1. The influence of translational velocity and pulse energy on the cross-sectional shape and integrity of the written tracks is reported. Tracks tend to be narrower as the pulse energy is lowered or translational velocity decreased. Above 22.9 nJ, pulse energy, tracks tend to crack. The estimated refractive index profile of one track has a maximum increase of refractive index of 0.003 at the centre. These glasses normally form nano-glass-ceramics on heat treatment just above the glass transformation temperature (Tg). Here, a post-fs-writing heat-treatment just above Tg causes nano-ceramming of the glass sample and removes a light-guiding peripheral region of the fs-written tracks suggesting that this region may have been fs-modified by stress alone. Waveguiding at 651 nm and 973 nm wavelengths, and upconversion, are demonstrated in optimally written tracks.

Mode-locking in 25-km fibre laser

We demonstrate mode-locking and single-pulse generation in fibre laser with record-setting cavity length of 25 km. Substantial increase in the pulse round trip duration leads to ultra-low repetition rate of 8.097 kHz and pulse energy of 3.7 uJ.

Low-loss waveguides fabricated by femtosecond chirped-pulse oscillator

Recent results on direct femtosecond inscription of straight low-loss waveguides in borosilicate glass are presented. We also demonstrate lowest ever losses in curvilinear waveguides, which we use as main building blocks for integrated photonics circuits. Low-loss waveguides are of great importance to a variety of applications of integrated optics. We report on recent results of direct femtosecond fabrication of smooth low-loss waveguides in standard optical glass by means of femtosecond chirped-pulse oscillator only (Scientific XL, Femtolasers), operating at the repetition rate of 11 MHz, at the wavelength of 800 nm, with FWHM pulse duration of about 50 fs, and a spectral widths of 30 nm. The pulse energy on target was up to 70 nJ. In transverse inscription geometry, we inscribed waveguides at the depth from 10 to 300 micrometers beneath the surface in the samples of 50 x 50 x 1 mm dimensions made of pure BK7 borosilicate glass. The translation of the samples accomplished by 2D air-bearing stage (Aerotech) with sub-micrometer precision at a speed of up to 100 mm per second (hardware limit). Third direction of translation (Z-, along the inscribing beam or perpendicular to sample plane) allows truly 3D structures to be fabricated. The waveguides were characterized in terms of induced refractive index contrast, their dimensions and cross-sections, mode-field profiles, total insertion losses at both 633 nm and 1550 nm. There was almost no dependence on polarization for the laser inscription. The experimental conditions – depth, laser polarization, pulse energy, translation speed and others, were optimized for minimum insertion losses when coupled to a standard optical fibre SMF-28. We found coincidence of our optimal inscription conditions with recently published by other groups [1, 3] despite significant difference in practically all experimental parameters. Using optimum regime for straight waveguides fabrication, we inscribed a set of curvilinear tracks, which were arranged in a way to ensure the same propagation length (and thus losses) and coupling conditions, while radii of curvature varied from 3 to 10 mm. This allowed us to measure bend-losses – they less than or about 1 dB/cm at R=10 mm radius of curvature. We also demonstrate a possibility to fabricate periodical perturbations of the refractive index in such waveguides with the periods using the same set-up. We demonstrated periods of about 520 nm, which allowed us to fabricate wavelength-selective devices using the same set-up. This diversity as well as very short time for inscription (the optimum translation speed was found to be 40 mm/sec) makes our approach attractive for industrial applications, for example, in next generation high-speed telecom networks.

2 μm mode-locked thulium doped fiber laser using tilted fiber grating

A nonlinear polarization rotation based all-fiber passively mode-locked Tm3+-doped fiber laser is demonstrated by using a 45° tilted fiber grating (TFG) as an in-line polarizer. Stable soliton pulses centered at 1992.7 nm with 2.02 nm FWHM bandwidth were produced at a repetition rate of 1.902 MHz with pulse duration of 2.2 ps and pulse energy of 74.6 pJ. With the increased pump power, the laser also can operate at noise-like regime with 18.1 nm FWHM bandwidth and pulse energy of up to 250.1 nJ. Using the same 45° TFG, both stable soliton and noise-like mode-locking centered at ∼1970 nm and ∼2050 nm, were also achieved by shortening and extending the length of Tm3+-doped fiber, respectively, exhibiting advantages of broadband and low insertion loss at 2 μm band.

All-fiber polarization locked vector soliton laser using carbon nanotubes

We report an all-fiber mode-locked erbium-doped fiber laser (EDFL) employing carbon nanotube (CNT) polymer composite film. By using only standard telecom grade components, without any complex polarization control elements in the laser cavity, we have demonstrated polarization locked vector solitons generation with duration of ~583fs , average power of ~3 mW (pulse energy of 118pJ ) at the repetition rate of ~25.7 MHz.

Efficient 2.87 μm fiber laser passively switched using a semiconductor saturable absorber mirror

A passively switched Ho3+, Pr3+ codoped fluoride fiber laser using a semiconductor saturable absorber mirror (SESAM) is demonstrated. Q-switching and partial mode-locking were observed with the output power produced at a slope efficiency of 24% with respect to the absorbed pump power. The partially mode-locked 2.87 µm pulses operated at a repetition rate of 27.1 MHz with an average power of 132 mW, pulse energy of 4.9 nJ, and pulse width of 24 ps.

Dual wavelength Q-switched cascade laser

A diode-cladding-pumped dual wavelength Q-switched Ho3+ -doped fluoride cascade fiber laser operating in the mid-infrared is demonstrated. Stable pulse trains from the 5|6 -> 5|7 and 5|7 -> 5|8 laser transitions were produced, and the µs-level time delay between the pulses from each transition was dependent on the pump power. At maximum pump power and at an acousto-optic modulator repetition rate of 25 kHz, the 5|8 -> 5|7 transition pulse operated at 3.005 µm, a pulse energy of 29 µJ, and a pulse width of 380 ns; the 5|7 -> 5|8 transition pulse correspondingly produced 7 µJ pulse energy and 260 ns pulse width at 2.074 µm. To the best of our knowledge, this is the first demonstration of a Q-switched fiber laser operating beyond 3 µm.

Graphene based widely-tunable and singly-polarized pulse generation with random fiber lasers

Pulse generation often requires a stabilized cavity and its corresponding mode structure for initial phase-locking. Contrastingly, modeless cavity-free random lasers provide new possibilities for high quantum efficiency lasing that could potentially be widely tunable spectrally and temporally. Pulse generation in random lasers, however, has remained elusive since the discovery of modeless gain lasing. Here we report coherent pulse generation with modeless random lasers based on the unique polarization selectivity and broadband saturable absorption of monolayer graphene. Simultaneous temporal compression of cavity-free pulses are observed with such a polarization modulation, along with a broadly-tunable pulsewidth across two orders of magnitude down to 900 ps, a broadly-tunable repetition rate across three orders of magnitude up to 3 MHz, and a singly-polarized pulse train at 41 dB extinction ratio, about an order of magnitude larger than conventional pulsed fiber lasers. Moreover, our graphene-based pulse formation also demonstrates robust pulse-to-pulse stability and widewavelength operation due to the cavity-less feature. Such a graphene-based architecture not only provides a tunable pulsed random laser for fiber-optic sensing, speckle-free imaging, and laser-material processing, but also a new way for the non-random CW fiber lasers to generate widely tunable and singly-polarized pulses.

Femtosecond laser-induced microstructures on diamond for microfluidic sensing devices applications

This paper reported a three-dimensional microfluidic channel structure, which was fabricated by Yb:YAG 1026?nm femtosecond laser irradiation on a single-crystalline diamond substrate. The femtosecond laser irradiation energy level was optimized at 100?kHz repetition rate with a sub-500 femtosecond pulse duration. The morphology and topography of the microfluidic channel were characterized by a scanning electron microscope and an atomic force microscope. Raman spectroscopy indicated that the irradiated area was covered by graphitic materials. By comparing the cross-sectional profiles before/after removing the graphitic materials, it could be deduced that the microfluidic channel has an average depth of ~410?nm with periodical ripples perpendicular to the irradiation direction. This work proves the feasibility of using ultra-fast laser inscription technology to fabricate microfluidic channels on biocompatible diamond substrates, which offers a great potential for biomedical sensing applications.

Low-loss waveguides fabricated by femtosecond chirped-pulse oscillator

Recent results on direct femtosecond inscription of straight low-loss waveguides in borosilicate glass are presented. We also demonstrate lowest ever losses in curvilinear waveguides, which we use as main building blocks for integrated photonics circuits. Low-loss waveguides are of great importance to a variety of applications of integrated optics. We report on recent results of direct femtosecond fabrication of smooth low-loss waveguides in standard optical glass by means of femtosecond chirped-pulse oscillator only (Scientific XL, Femtolasers), operating at the repetition rate of 11 MHz, at the wavelength of 800 nm, with FWHM pulse duration of about 50 fs, and a spectral widths of 30 nm. The pulse energy on target was up to 70 nJ. In transverse inscription geometry, we inscribed waveguides at the depth from 10 to 300 micrometers beneath the surface in the samples of 50 x 50 x 1 mm dimensions made of pure BK7 borosilicate glass. The translation of the samples accomplished by 2D air-bearing stage (Aerotech) with sub-micrometer precision at a speed of up to 100 mm per second (hardware limit). Third direction of translation (Z-, along the inscribing beam or perpendicular to sample plane) allows truly 3D structures to be fabricated. The waveguides were characterized in terms of induced refractive index contrast, their dimensions and cross-sections, mode-field profiles, total insertion losses at both 633 nm and 1550 nm. There was almost no dependence on polarization for the laser inscription. The experimental conditions – depth, laser polarization, pulse energy, translation speed and others, were optimized for minimum insertion losses when coupled to a standard optical fibre SMF-28. We found coincidence of our optimal inscription conditions with recently published by other groups [1, 3] despite significant difference in practically all experimental parameters. Using optimum regime for straight waveguides fabrication, we inscribed a set of curvilinear tracks, which were arranged in a way to ensure the same propagation length (and thus losses) and coupling conditions, while radii of curvature varied from 3 to 10 mm. This allowed us to measure bend-losses – they less than or about 1 dB/cm at R=10 mm radius of curvature. We also demonstrate a possibility to fabricate periodical perturbations of the refractive index in such waveguides with the periods using the same set-up. We demonstrated periods of about 520 nm, which allowed us to fabricate wavelength-selective devices using the same set-up. This diversity as well as very short time for inscription (the optimum translation speed was found to be 40 mm/sec) makes our approach attractive for industrial applications, for example, in next generation high-speed telecom networks.

Femtosecond laser-induced microstructures on diamond for microfluidic sensing device applications

This paper reported a three-dimensional microfluidic channel structure, which was fabricated by Yb:YAG 1026?nm femtosecond laser irradiation on a single-crystalline diamond substrate. The femtosecond laser irradiation energy level was optimized at 100?kHz repetition rate with a sub-500 femtosecond pulse duration. The morphology and topography of the microfluidic channel were characterized by a scanning electron microscope and an atomic force microscope. Raman spectroscopy indicated that the irradiated area was covered by graphitic materials. By comparing the cross-sectional profiles before/after removing the graphitic materials, it could be deduced that the microfluidic channel has an average depth of ~410?nm with periodical ripples perpendicular to the irradiation direction. This work proves the feasibility of using ultra-fast laser inscription technology to fabricate microfluidic channels on biocompatible diamond substrates, which offers a great potential for biomedical sensing applications.