The generation of Terahertz (THz) waves via two-color plasma in gas has captured the interest of the research community due to its capability to create intense waves characterized by a wide and adjustable spectrum. Efforts aimed at improving the performance of THz radiation for particular applications entail examining and adjusting several variables. In this study, we introduce a simple method for manipulating THz polarization through the adjustment of chirp and wavelength dispersion. Specifically, we will show that by managing these characteristics, it is possible to produce THz waves with polarizations that can be elliptical, circular, or resemble a "flower" pattern. The implications of these changes on the spatiotemporal path of THz radiation will also be examined.
The generation of Terahertz (THz) radiation using two-color plasma in gases has garnered significant interest within the scientific community for its ability to produce powerful waves with very broad and tunable spectra. Understanding how to modify the characteristics of the generated THz radiation paves the way for optimizing its performance for targeted applications. To achieve this, various approaches have been explored, however, the combined impact of chirp and wavelength dispersion has not yet been thoroughly investigated. Our study aims to understand how the laser chirp and the phase difference caused by air dispersion influence the shape of the THz pulse. The findings highlight that these factors significantly alter the shape of the THz pulse. Specifically, the nature of the chirp (positive or negative) in the pump laser distinctly affects the pulse shape. For instance, a pump laser with positive chirp results in a THz waveform with a negative monopolar configuration, while a laser with negative chirp generates a THz pulse with a positive configuration.
We present findings on High Harmonic Generation (HHG) in solids utilizing a high-energy fiber laser system operating at 1550 nm. The driving laser source comprises an Erbium-Doped Fiber chirped pulse Amplifier (EDFA) combined with a post-compression stage employing a hollow-core photonic crystal fiber (HC-PCF) filled with noble gases. Nonlinear self-compression in the HC-PCF enables the generation of ultrashort pulses with a duration of 50 fs and energy of 0.91 μJ at a repetition rate of 660 kHz. In a first step, harmonics up to H7 were observed when focusing the laser into small bandgap materials such as Zinc Oxide (ZnO). Subsequently, the system was enhanced to measure high harmonics in the extreme ultraviolet (XUV) range, with harmonics up to H25 observed using a large bandgap material, magnesium oxide (MgO). To the best of our knowledge, this represents the first solid-state HHG source driven by a high-energy few-cycle fiber laser in the telecom region.
Countless ultrafast imaging techniques have been developed and even though they brought significant insights, their application out of a laboratory environment is however often quite limited, either by the complexity of operation or by a heavy data processing. We demonstrate flexible single-shot imaging via the combination of sequentially timed all-optical mapping photography with acousto-optics programmable dispersive filtering and digital in-line holography. The frame rate and exposure time can be independently adjusted without complex shaping stages, making the system remarkably agile. In-line holography allows to achieve an even higher simplicity through its lensless operation and the reconstruction on a wide depth of field.
We report on mid-infrared supercontinuum generation from 4 to 9 µm in orientation-patterned gallium-arsenide waveguides pumped by nanojoule-class ultrafast fiber lasers. The QPM waveguide and the laser source are optimized in tandem to pump the waveguides close to the degeneracy by means of sub-picosecond pulses at 2760 nm. The use of a waveguide geometry drastically reduces the required energy to the nanojoule level, thereby opening supercontinuum generation in GaAs platforms to fiber lasers.
We report on the design of OP-GaAs rib waveguides for frequency conversion in the mid-infrared and explore their performances for parametric generation. The samples used are between 10 and 25 mm long and exhibit quasi-phasematched (QPM) periods from 85 to 100 μm. The waveguides are pumped by a femtosecond erbium-doped fluoride fiber laser combined with a soliton self-frequency shift converter delivering sub-300 fs pulses at a wavelength tunable between 2.8 and 3.3 μm. By adjusting the pump wavelength, our OP-GaAs platform can produce ultrashort pulses widely tunable around 4 and 12 μm for the signal and idler, respectively. These results fit quite well our calculations of QPM curves.
Yttrium orthoaluminate (YAlO3) is an attractive laser host crystal for doping with thulium (Tm3+) ions. We report on the absorption and stimulated-emission (SE) cross-sections of this orthorhombic (sp. gr. Pnma) Tm:YAlO3 crystal for the principal light polarizations, E || a, b and c. Polarized absorption data lead to the Judd-Ofelt parameters Ω2 = 1.46, Ω4 = 2.82 and Ω6 = 1.09 [10-20 cm2]. In particular, for the 3H4 → 3H5 transition, it is found a stimulated emission cross section of 0.86×10-20 cm2 at 2278 nm corresponding to an emission bandwidth of ~12 nm (for E || b). Continuous-wave laser operation on this 3H4 → 3H5 transition is achieved with an 1.8 at.% Tm:YAlO3 crystal under laser-pumping at 776 nm. The mid-infrared Tm:YAlO3 laser generated 0.96 W at ~2274 nm with a slope efficiency of 61.8% and a linear laser polarization (E || b). Tm:YAlO3 is promising for mode-locked lasers at ~2.3 μm.
Real-time measurements are now mature enough to cover a wide span of applications from fundamental laser physics and dynamics to applied sciences. In this talk, we focus on the application of time-stretch techniques for the ultrafast imaging of non-repetitive ultrafast events. In particular, we show that amplified time-stretch imaging fills the gap between ultra-high frame rate imaging techniques (burst-mode and/or temporal mapping cameras) and continuous imagers (CCD/CMOS) as it allows MHz frame rates on long - ms - timescales. As an illustration, we demonstrate the real-time tracking - i.e. propagation and reflection - of single laser-induced shockwaves (SWs) with velocities exceeding a few km/s and show that it allows, on the one hand, to monitor its full dynamics, from its deceleration to the observation of the plasma contact wave, and, on the other hand, to easily acquire intensity and velocity statistics on large ensembles of SWs [1]. This study has numerous potential applications in applied physics e.g. in the study of transient phenomena in pulsed laser-material interactions as these dynamics indeed strongly impact many scientific fields such as micromachining, material analysis or high-harmonics generation, to name a few. We also report the use of 1-D amplified time-stretch imaging to capture the rupture of liquid ligaments, which could bring new insights in two-phase flows physics.
[1] Hanzard et al., Appl. Phys. Lett. 112, 161106 (2018)
We present an all-fiber integrated master oscillator power amplifier operating at 1940 nm. The source delivers 422-nJ chirped pulses at a repetition rate of 10.18 MHz corresponding to 4.3 W of average power. The pulses were recompressed down to 900 fs yielding 220 kW of peak power. Stretching the pulse to 200 ps allows further energy scaling beyond the microjoule barrier at low repetition rate (Ep = 4 μJ at 92 kHz, Δτp =1.6 ps).
New wavelengths of laser radiation are of interest for material processing. Results of application of the all-fiber ultrashort pulsed laser emitting in 2 µm range, manufactured by Novae, are presented. Average output power was 4.35 W in a single-spatial-mode beam centered at the 1950 nm wavelength. Pulses duration was 40 ps, and laser operated at 4.2 MHz pulse repetition rate. This performance corresponded to 25 kW of pulse peak power and almost 1 µJ in pulse energy. Material processing was performed using three different focusing lenses (100, 30 and 18 mm) and mechanical stages for the workpiece translation. 2 µm laser radiation is strongly absorbed by some polymers. Swelling of PMMA surface was observed for scanning speed above 5 mm/s using the average power of 3.45 W focused with the 30 mm lens. When scanning speed was reduced below 4 mm/s, ablation of PMMA took place. The swelling of PMMA is a consequence of its melting due to absorbed laser power. Therefore, experiments on butt welding of PMMA and overlapping welding of PMMA with other polymers were performed. Stable joint was achieved for the butt welding of two PMMA blocks with thickness of 5 mm. The laser was used to cut a Kapton film on a paper carrier with the same set-up as previous. The cut width depended on the cutting speed and focusing optics. A perfect cut with a width of 11 µm was achieved at the translation speed of 60 mm/s.
We report on the generation of femtosecond pulses from an all-normal-dispersion fiber laser featuring a large-mode-area
ytterbium-doped photonic-crystal fiber. Nonlinear polarization evolution assisted by passive spectral filtering in
combination with the large-pitch fiber design enables a significant peak power enhancement with the generation of
multi-megawatt pulses. 65 W of average power at 76.5 MHz repetition rate, corresponding to 850 nJ pulses are generated
in a compact oscillator setup. The output pulses are extra-cavity dechirped down to 111fs with 6 MW of peak power. To
the best of our knowledge, these are the highest average and peak powers ever reached by a mode-locked fiber laser.
We report on generation of high-energy pulses in a highly-normal dispersion fiber laser featuring large-mode-area
microstructure fibers. Passive mode-locking is achieved using high modulation depth semiconductor saturable
absorber mirror (SESAM). The total cavity dispersion is varied through insertion of a low-nonlinearity passive
microstructure fiber inside the cavity. We study the effect of the cavity dispersion on the mode-locking performances.
A systematic experimental and numerical description of the laser operation is addressed and the impact
of the spectral filtering on the laser performances is discussed.
We report on the generation of microjoule level picosecond pulses from a mode-locked Yb-doped LMA fiber laser
operating in the purely normal dispersion regime. The self-starting oscillator stabilized with slow relaxation
semiconductor saturable absorber (SAM) emits 11 W of average power at a pulse repetition rate of 10 MHz,
corresponding to a pulse energy of 1.1 μJ. The laser produces a 0.4 nm narrow emission line with 310 ps output pulses.
In the femtosecond operation, the oscillator stabilized with fast relaxation SAM emits 9 W of average power at a pulse
repetition rate of 9.7 MHz, corresponding to a pulse energy of 927 nJ. The laser produces positively chirped output
pulses of 8 ps which are compressed down to 711 fs, corresponding to megawatt peak power. To our knowledge this is
the first time that mode-locked fiber oscillators can generate higher pulse energies of beyond microjoule-level at high
average output power.
We report on an all-normal dispersion passively mode-locked fiber laser based on an ytterbium-doped largemode-
area microstructure fiber and featuring high-energy ultra-short pulses. Mode-locking was achieved with a
high modulation depth semiconductor saturable absorber mirror (SESAM). We investigate the influence of the
modulation depth of the SESAM on the laser performances. We show that mode-locking could be achieved with
a modulation of only 10 %. However, the best performances in term of pulse energy are obtained with the highest
modulation depth (35 %). In this case, the laser delivers 3.3 W average output power with positively-chirped 5.5
ps pulses at a center wavelength of 1033 nm. The pulse repetition rate is 46.4 MHz, which results in an energy
per pulse of 71 nJ. These pulses are extra-cavity de-chirped down to 516 fs using bulk gratings. The average
power of the de-chirped pulses is 2.3 W, which corresponds to a peak power of more than 96 kW.
We report on a all-normal dispersion mode-locked fiber laser based on a large-mode-area ytterbium-doped microstructure fiber and using a high nonlinear modulation depth semiconductor saturable absorber mirror (SESAM). The laser delivers 3.3 W average output power with positively-chirped 5.5 ps pulses at a center wavelength of 1033 nm. The pulse repetition rate is 46.4 MHz, which results in an energy per pulse of 71 nJ. These pulses are extra-cavity de-chirped down to 516 fs using bulk gratings. The average power of the de-chirped pulses is 2.3 W, which corresponds to a peak power of more than 96 kW.
We report on the generation of 265 nJ ultra-short pulses from a mode-locked Ytterbium-doped short-length large-mode-area
fiber laser operating in the dispersion compensation free regime. The self-starting oscillator emits 2.7 W of average
power at a pulse repetition rate of 10.18 MHz. The pulses have been compressed down to 400 fs, corresponding to 500
kW peak power. Numerical simulations confirm the stable solution and reveal the mechanisms for self-consistent intra-cavity
pulse evolution. The pulse energy is one order of magnitude higher than so far reported for fiber oscillators in the
1 μm wavelength region. To our knowledge this is the first time that mode-locked fiber oscillators can compete in terms
of pulse energy and peak power with most advanced bulk solid-state femtosecond lasers.
We report, to the first time to our knowledge, on a passively mode-locked single-polarization single-transverse-mode
large-mode-area photonic crystal fiber laser operating in the dispersion compensation free regime. In the single-pulse
regime, the laser generates 1.6 W of average power with 3.7 ps pulses at a repetition rate of 63 MHz, corresponding to a
pulse energy of 25 nJ. Stable and self-starting operation is obtained by adapting the spot size at the saturable absorber
mirror to the pulse evolution in the low-nonlinearity fiber. The pulses are compressible down to 750 fs. The presented
approach demonstrates the scaling potential of fiber based short pulse oscillators towards high-power ultra-compact allfiber
environmentally-stable configuration.
We report on the generation of self-similar highly-stable femtosecond pulses from a side-pumped ytterbium-doped double-clad fiber laser. Positively-chirped parabolic pulses with 6.4 ps duration and more than 3.2 nJ energy are obtained. These pulses are extra-cavity compressed to 140 fs. The noise measurements using radio-frequency analysis show that this regime of emission ensures low-noise operation with less than 0.05 % amplitude fluctuations.
We report passive harmonic mode locking of a high-power Yb-doped double-clad fiber laser operating in both the normal- and the anomalous-dispersion regimes with a fundamental repetition rate of 20.4 MHz. In the anomalous-dispersion regime (total cavity GVD of -0.1 ps2), 1-ps, 125-pJ pulses are emitted at a repetition rate of 408 MHz. When the total net dispersion is close to zero (about -0.004 ps2), 680 fs, 48 pJ pulses are emitted at a repetition rate higher than 2 GHz. The supermodes suppression is than about 25 dB. In the normal-dispersion regime (total cavity GVD of +0.047 ps2), 116-fs, 1.7-nJ pulses are emitted at a repetition rate of 102 MHz with a supermodes suppression of more than 60 dB. We also report a new regime of multiple pulsing emission observed with this fiber laser : the stable emission of two pairs of bound pulses exhibiting different time separations and uniformly separated in the same cavity round trip.
Keywords: Harmonic mode locking, multiple pulsing, bound states.
We consider an Yb-doped double-clad fiber laser in a unidirectional ring cavity containing a polarizer placed between two half-wave plates. Depending on the orientation of the phase plates, the laser operates in continuous, Q-switch, mode-lock or unstable self-pulsing regime. An experimental study of the stability of the mode locking regime is realized versus the orientation of the half-wave plates. A model for the stability of self-mode locking and cw operation is developed. The model is reduced to a master equation in which the coefficients are explicitly dependent on the orientation angles of the phase plates.
We present an experimental study of the Yb-doped double-clad fiber laser operating in the 1.08 micrometers wavelength. The fiber side-pumped with a high power laser diode using the v- groove technique. Various experimental configurations are performed from the linear cavity to an all-fiber tunable unidirectional ring cavity. Mode-locking of the Yb laser are investigated using the nonlinear polarization rotation.
We theoretically model the polarization properties of an optical fiber by the Jones matrix of an elliptical birefringent plate. The properties of this model are investigated and lead to develop experimental methods to extract the parameters of the model for a real fiber. A magneto-optical method that measures the beat length of the fiber is also presented and gives a more complete description of the fiber. Wavelength dependence of the parameters characterizing the fiber is finally experimentally investigated.
We present an experimental study of the Yb-doped double-clad fiber laser operating in the 1.08 micrometers wavelength. The fiber is side-pumped with a high power laser diode using the v-groove technique. Various experimental configurations are performed from the linear cavity to an all-fiber tunable unidirectional ring cavity.
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