Pairs of arrays of point-by-point fiber Bragg Gratings written with a femtosecond laser were anchored with K-type thermocouple within an asymmetrical titanium substrate. Three anchoring methods were used, including silica based, graphite based glue commercially available as well as Yttria Stabilized Zirconia deposited by Atmospheric Plasma Spraying. The optical fibers were first recoated with an inorganic coating, specifically developed to resist to high temperatures above 800°C. The titanium plate was exposed to an intense heat flow delivered by tungsten halogen lamps to reach temperature of around 800°C. High temperature and strain measurements was performed in situ with a relative error of less than 10% at 800°C.
We demonstrate an innovative design dedicated to scalable amplification at 1030 nm, which enables the combination of very high gains at room temperature with output pulse energies of several tens milliJoules (mJ). The optical architecture involves an edge-pumped, thick slab of Yb3+: YAG, which is operated in a fairly compact and scalable, non-resonant multi-pass cavity. The laser performance is experimentally benchmarked by means of a 12- pass multiplexing scheme. Thanks to seeding 3ns-long seed pulses with an energy of 300 μJ at 1 Hz, as permitted by usual fibered sources, we experience straightforward linear amplification up to 20-50 mJ with net gains in the range of 15-20 dB. The optical performance needs to be optimized by controlling the operating conditions in a convenient way, in terms of the pump-beam spatial overlap and of alignment. This takes advantage of ultimately high Gain x Energy products, in excess of 10dB.J, in a single-stage unit.
Over the last decade, frequency comb spectroscopy have led to significant developments in view of the identification of varied species and of the understanding of the structure of matter.
Highly efficient amplification of frequency comb femtosecond oscillators in the high pulse energies regime should allow future applications using this approach to Lidar-type measurements.
We report on the millijoule level design of femtosecond amplifiers near 2 μm wavelength having a great optical efficiency and compactness in order to be carrier in satellites. In addition to space applications, laser systems at 2 μm become more and more popular because they offer elegant solutions to generate ultra-broad band super-continuum in the mid-infrared and for material processing.
Our study helps to compare the optical performance of Tm:YAG, Tm:YAP and Tm:YLF crystals as active media, for designing ultrashort pulse regenerative amplifiers with a high gain and wall-plug efficiencies up to 10%. We will present our approach to ensure the conservation of the initial phase shift between the envelope and the carrier of pulses during amplification.
We primarily discuss an innovative model which proposes a gradual path towards the optimization of any regenerative amplifier using crystalline thulium-based, end-pumped doped rods. This also involves the analysis of sizing criteria based on the assumption of rod-based active media, including the doping content, the length of the rod and the beam size inside.
KEYWORDS: Picosecond phenomena, Thermal effects, Fiber lasers, High power fiber amplifiers, Laser development, High power fiber lasers, Cladding, Photonic crystal fibers, Color centers
Thermally induced transverse modal instabilities (TMI) have attracted these five years an intense research efforts of the entire fiber laser development community, as it represents the current most limiting effect of further power scaling of high power fiber laser. Anyway, since 2014, a few publications point out a new limiting thermal effect: fiber modal degradation (FMD). It is characterized by a power rollover and simultaneous increase of the cladding light at an average power far from the TMI threshold together with a degraded beam which does not exhibit temporal fluctuations, which is one of the main characteristic of TMI.
We report here on the first systemic experimental study of FMD in a high power photonic crystal fiber. We put a particular emphasis on the dependence of its average power threshold on the regime of operation. We experimentally demonstrate that this dependence is intrinsically linked to regime-dependent PD-saturated losses, which are nearly three times higher in CW regime than in short pulse picosecond regime. We make the hypothesis that the existence of these different PD equilibrium states between CW regime and picosecond QCW pulsed regime is due to a partial photo-bleaching of color centers in picosecond regime thanks to a higher probability of multi-photon process induced photobleaching (PB) at high peak power. This hypothesis is corroborated by the demonstration of the reversibility of the FMD induced in CW regime by simply switching the seed CW 1064 nm light by a short pulse, picosecond oscillator.
We propose an innovating model to describe the kinetics of competing photo-darkening and photo-bleaching phenomena in high-power, Ytterbium-Doped-Fibre-Amplifiers. This model makes use of aggregated species of trivalent Ytterbium and divalent ions, which operate as primarily efficient color-centers. This ensures multi-photon excitation, partly from the pump and partly from the signal. The fit of numerical computations with dedicated experiments help to validate our theoretical assumptions, in the definition of the involved physics. Potential applications of this study include further discussions for the selection of processing options with fibre-manufacturers and the optimization of operating conditions.
We present a comprehensive experimental study of the technique of Longitudinal Mode Filling (LMF) applied to the
reduction of Stimulated Brillouin Scattering (SBS), in Ytterbium Doped Fibre Amplifiers (YDFA) at the wavelength of
1064 nm. Pulse durations and Mode Field Diameters (MFD) lie in the ranges of 10 - 100 ns and 10 - 35 μm,
respectively. Input pulse-shaping is implemented by means of direct current modulation in multimode Laser-Diode
seeds. This evidences a number of interests in the development of robust and low cost Master Oscillator Power
Amplifiers (MOPA). Highly energetic, but properly shaped, nanosecond pulses may be produced this way without any
need of additional electro-optical means for in-line phase and amplitude modulation. Seeds consist of Distributed Feed-
Back (DFB) and Fibre Bragg Gratings (FBG) with different fibre lengths. We demonstrate the benefit of LMF with
properly controlled mode spacing, in combination with chirp effects due to fast current transients in the semiconductors,
in order to deal with SBS thresholds in the range of a few to some hundred μJ. The variations of the SBS threshold are
discussed versus the number of longitudinal modes, the operating conditions of the selected seed and pulse-shaping
conditions.
The development of ultra-broadband oscilloscopes is mainly governed by the needs of future telecom networks. But
other applications are requesting the availability of true real-time acquisition oscilloscopes. Systems able to be used in
single-shot operation are of prime interest for Inertial Confinement Fusion (ICF) and for the related R&D for plasma
physics.
We previously demonstrate a single-shot, 100GHz design of an all-optical sampling oscilloscope at 1μm (MULO). This
laboratory system has been improved in stability and compactness to make an all-in-one box prototype. More, by the
addition of an opto-electro-optics (OEO) sub-system at the input, we developed the ability to use this oscilloscope to
analyze an electrical input signal up to 60GHz. This new integrated subset also increases the range of wavelength for
optical input signal, from 300nm up to 2μm. Furthermore, it allows the use of inexpensive opto-electronic components at
telecom wavelength for this system regardless of the signal to be analysed. In parallel with these improvements, by
optimizing the heart of the system, we get a very high sampling rate, up to 500Gs/s and more; this allows considering
much higher bandwidths in the future.
In this talk, we will present latest developments and integration of this system. It will also allow us to give more details
on the innovative OEO sub-system.
An original polarization - maintaining Sagnac switch is proposed for use in optical sampling and short pulse measurement applications, in the range of signal wavelengths of interest for Inertial Confinement Fusion. Our design is implemented using highly-nonlinear
photonic-crystal fibres. It enables the search of huge switching contrasts together with very large sampling bandwidths, in relationship with an elevated temporal resolution. A unique
two-pass Sagnac loop is fed with input signal pulses at 1053nm while triggered with pump pulses at 1550nm. Starting from a
single-pass contrast and a temporal resolution in the ranges of 30dB and of a couple of picoseconds, the two-pass architecture provides optical contrasts in excess of 45dB and sub-picosecond gating durations. Thanks to two-pass operation, we can get nearly free from any environmental perturbation. Furthermore the spectral and the temporal clipping features related to switching are analyzed using comprehensive modeling with higher order dispersion effects. The issue of the optimization of the sampling bandwidth is discussed in details by means of the synchronization of the pump return, which involves a sub-picosecond precision. This way, the output energy from the switch can be kept constant and proportional to the signal power, whatever the input pulse width. The sampling bandwidth then extends up to RF frequencies in the range 300-500GHz.
A new theoretical approach for modeling the saturated single pass gain in a three-level fiber amplifier is presented, relevant to the behavior of rare-earth-doped silica fibers. A basic approximation considering the stimulated emission rate Ws(z,ν) as a dimensionless parameter S, independent of the spatial and frequency variables z and ν, allows to obtain analytic expressions for input and output pump, ASE and signal powers inside the fiber core. We show that these expressions only depend on the S parameter, which is determined by solving the photon balance equation, and S is shown to be fully representative of the saturation in the medium. The main result of the model is that the pump repartition P(z) takes a simple analytical form, which can be separated into two parts, below and above the saturated absorption length L0, which is a function of S. The first part 0〈z〈L0 is the saturated absorption where the pump distribution is linear and the second part L0〈z〈L is the non saturated absorption region with an exponentially decreasing pump distribution. Compared to other analytical models with the assumption of an averaged inversion population 〈N2(z)〉, we obtain a good description of the difference between the co- and counter-propagating ASE behaviors accounting for the fiber length and the pumping level. The model, which is well suited to longitudinal pumping, can also describe a side pumped fiber amplifier by simple adjustment of some of the model parameters.
An original design for electro-optical pulse shaping is presented, enabling the operation of programmable high voltage pulses with fast Pockels cell amplitude modulators. The basic idea is to make use of diode lasers together with photoconductors, inside a simple arrangement comprised of two resistivity coupled microstrip lines. We demonstrate a prototype with better than 200 ps a time resolution of more than 200 ps within 3.5 ns wide pulse, at apeak output voltage of 2 kilovolts and more than 50 dB amplitude dynamics.
We developed a number of active devices for use in the regenerative amplifier, which is one of the specific sub- assemblies of the preamplifying section in the french L.M.J. design. The first one is a modular-square-diode pumped laser head, which provides 0.65 Joules pumping energy at 800 nm into a 4 mm side Nd3+:phosphate glass, in the form of a close coupling-uniform-configuration. Our original side pumping scheme makes use of symmetric diode stacks and optimized rod holders with a thermal conductivity. Some heat is waste in the volume of glass, as a result of the pumping process, and it is efficiently--since very closely--removed by the latter holders. A (pi) /2 rotation of one pumping section with respect to the next one allows uniform pumping and thermal conduction together with birefringence reduction. No water is required. A complete 3D thermal model is developed, in order to describe temperature and stress distributions, inside the laser head and glass rod. Peak stress values in glass at F equals 10 Hz, with flexible rod holders including Indium parts, equal 5.8 Mpa when the mean thermal power is 22 watts. Preliminary laser tests are experienced at 10 Hz with two similar laser heads around a square Nd3+:LHG750 glass rod. With 1.3 Joule pump energy at 400 microsecond(s) pumping duration, the assembly is placed inside a Qswitched stable, multimode plano-concave, resonator. The output energy is 40 mJ, within 80 ns fwhm pulses.
We describe the performances we obtained in a variety of high power diode pumped Nd3+:LNA heads. Two pumping structures are analyzed, which were designed for cylindrical and square cross section rods. The cylindrical rods are tested with 1 joule pumping energy during 200 microsecond(s) . They provide high regenerative gains, in the range 107 to 108 at 20 mJ output energy, with less than 20 passes. The gain transverse distribution looks like a nonuniform cross distribution, at the opposite of that obtained with the square head. The central volume in the rod, whose diameter typically equals 2 mm, only participates to amplification. The purpose of the square structure is both to solve this problem and to optimize the temperature behavior at 10 Hz. Preliminary results are given. In a last step, we also present some spatio-temporal computations in order to understand the growth of the amplified beam as a function of the number of passes in the rod.
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