KEYWORDS: Crystals, Picosecond phenomena, Deep ultraviolet, Ultraviolet radiation, Nozzles, Frequency conversion, Thermography, Polarization, Optical surfaces, Laser systems engineering, High power lasers
We report on high average-power, high-energy picosecond fourth-harmonic generation in LBO. The first stages of a Yb:YAG laser chain operating at 1 kHz repetition rate generate few-picosecond 220 mJ chirped pulses at 1030 nm fundamental wavelength. They are frequency-converted in a cascade of three LBO crystals to generate the second-, third-, and fourth harmonics at 515 nm, 343 nm and 257 nm respectively. Crystals thicknesses and angular phase matching detuning were calculated as a function of pulse duration through broadband nonlinear optical numerical simulations. Last crystal is both conduction-cooled on edge and surface-cooled at center through forced-air flow to mitigate heating due to nonlinear absorption in the deep-UV and reduce temperature gradients. Chirped-pulse duration was experimentally adjusted to achieve stable 20% overall conversion efficiency. Near-field beam profiles were continuously recorded at 10 Hz, for all four wavelengths involved, together with corresponding energies, showing no significant beam degradation over 50 hours. Temperatures of the two last crystals were monitored and will help optimize surface cooling for future power ramping-up.
The need for optics that can sustain higher laser fluences and intensities grows as new technological advancements allow laser systems to operate at increased in peak power. This has motivated a substantial effort in recent decades to study laser-induced damage mechanisms and their mitigations. One well known laser-induced damage mechanism is filamentation in fused silica glass, due to Kerr self-focusing of the light [1]. The study of filamentation has been an ongoing effort for the last few decades [2] as it turned out to be a major limitation to laser systems at high peak intensities. Past studies have led to a set of simplified rules that allows for the operation of laser system below the onset point of filamentation to occur, namely what is known as the “IL rule” (intensity times the length before filamenting equals some empirical constant) and the Bespalov-Talanov (BT) perturbation growth theory [3-8]. The necessity to increase the laser beam intensities and optimize the throughput, closer to the point where the optical propagation length in the material is comparable to the predicted filamentation distance, requires revisiting and improving our understanding of the current rule set.
We report on results of fifth harmonic generation in Cesium Lithium Borate (CLBO) using a three-crystal cascaded frequency conversion scheme designed to study the energy balance of the final sum frequency generation stage. The experimental setup independently combines the first and fourth harmonic of a Nd:Glass laser in a 5mm thick CLBO crystal. Energy balance between the incoming and output energy is close to unity when the CLBO is out of phase matching and approximately 80% when the crystal is in phase matching. A detailed analysis of the residual fundamental and fourth harmonic energy indicates 5th harmonic light is being generated but only 26% is unaccounted for. We attribute the missing light to linear transmission loss in the CLBO oven. The ratio of the output to input energy is unity when the missing 5th harmonic is incorporated into the calculations. Two-dimensional plane wave mixing simulations show agreement with the results at lower intensities.
KEYWORDS: Laser induced damage, Crystals, Resistance, High power lasers, Second-harmonic generation, Laser crystals, Frequency converters, Bulk lasers, Diffraction, Frequency conversion
We have laser conditioned a couple of KDP-SHG and DKDP-THG samples thanks to a facility which delivers 6 ns
fundamental (1,053 nm, noted 1ω) pulses, and the harmonics generated by the crystals themselves. The conditioning
ramp has been established according to a model coupling statistics and heat transfer, in order to minimize the generation
of bulk laser damage during the process. Then the efficiency of this procedure has been evaluated for both samples using
two laser damage testing setups, and compared to the best monochromatic conditioning process known to date. For the
KDP-SHG, it appears that this procedure is less efficient than the monochromatic conditioning. But it raises the
resistance to laser damage of the SHG to a level compatible with the use on megajoule-class high power lasers. For the
DKDP-THG, the efficiency of both procedures is quite similar. And even if the conditioned THG still exhibits laser
damage within the range of high power laser working fluences at 351 nm, the density is only a few per mm3.
The temperature-dependent dispersion of the refractive index of optical materials is analyzed by introducing an
appropriate formulation of what we call their "normalized
thermo-optic coefficients" (NTOC). These parameters are
obtained experimentally by performing accurate interferometric measurements of both thermal expansion and changes in
optical thickness of parallelepipedic shaped samples of millimetric size which are submitted to a linear ramp of
temperature. Changes in optical thickness are recorded as function of temperature at a few discrete laser wavelengths;
they are expressed as power series of temperature and the use of a simple vectorial formalism allows then to determine
the thermal behavior of the dispersion law over the whole transparency range of the material. The validity and reliability
of the proposed method has been confirmed by modeling the temperature dependence of the type II second harmonic generation 1.064 μm - 0.532 μm in a KTiOPO4 single crystal, recorded up to 120°C.
An optical bench was developed to measure laser induced damage thresholds in 1:1 and S:1 modes on mirror
and grating samples. The laser based on Ti. Sapphire technology delivers Gaussian pulse of 500fs with a
maximal energy of 3mJ at 1057nm. The experimental setup can deliver on the sample a peak fluence of 9J/cm2
in right section of the beam with a spot size of 200μm (diameter at 1/e2).
Laser induced damage thresholds have been measured on several multidielectric samples produced with
different processes. We present in this paper the damage testing setup in details and give some of the obtained
results.
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