An increasing number of scientific applications require highly energetic ultrafast laser system like OPA pumping, THz generation or filamentation. To address their requirements, a compact diode-pumped Terawatt-class laser was developed, this laser can deliver 220 mJ pulses at 50 Hz repetition rate and 415 fs pulse duration. The system is based on a CPA scheme starting with a synchronizable oscillator, a spectral shaper, then a regenerative amplifier and two multipass amplifiers. The laser’s modular structure allows for an upgrade with additional amplification stage. In order to maintain a large spectrum throughout the whole amplification, a spectrum modulator is used to shape the spectral amplitude of the seeded pulses. It allows the versatile shaping of the spectral bandwidth to efficiently compensate the spectral narrowing induced by the amplifiers. This spectrally shaped pulse is then seeded to a regenerative amplifier based on a Yb:CaF2 crystal. Thanks to its wide emission cross section and excellent storage efficiency, the pulse energy reached 30 mJ with a spectral bandwidth of 6nm centered at 1030 nm. This spectral shape is adapted to the two amplification stage which are based on Yb:YAG crystal. The first amplification multipass stage allows to increase the pulse energy to 90 mJ. The main amplification stage then increases the energy to 250 mJ. The energy is compressed by a pair of highly efficient gratings. The pulses are successfully compressed down to 415 fs with an output energy of 220 mJ. The beam after compression has an excellent spatial Gaussian shape, a remarkable pointing stability and a high Strehl ratio.
In recent past years, solid state laser have been attracted attentions in industry and scientific researches to achieve both high average power and energies with good beam quality. As an example, high energy laser with average powers over the kW have been demonstrated [1]. In these systems, predicting and managing heat generated during optical pumping is critical, as it can result in unwanted thermo-optical and thermo-mechanical effects such as thermal lensing or thermal stress fracture.
The determination of temperature distribution inside the laser gain medium is essential for optimizing the laser operation. The first step in thermal analysis is to estimate the thermal volume flow deposited within the gain medium over optical pumping, but this very first point is extremely difficult to accurately measure. In this work, we propose a novel method, based on a both an experimental thermal imaging set-up and a new theoretical model, that we use to accurately measure the deposited thermal flux in a laser crystal during pumping.
We use a high frequency thermal camera and a pulsed laser diode ( Pump LD 969 nm or 940 nm) to submit a photo-thermal excitation to the front surface of a non-contacted Yb:YAG laser crystal. Based on singular value decomposition and inverse methods, thermal diffusivity and the heat losses were estimated using the thermal evolution of the crystal over the relaxation step. These results and the thermal evolution of the crystal over the solicitation allowed estimating the thermal volume flow deposited within the crystal over optical pumping.
A high intensity Gamma source is required for Nuclear Spectroscopy, it will be delivered by the interaction between accelerated electron and intense laser beams. Those two interactions lasers are based on a multi-stage amplification scheme that ended with a second harmonics generation to deliver 200 mJ, 5 ps pulses at 515 nm and 100 Hz.
A t-Pulse oscillator with slow and fast feedback loop implemented inside the oscillator cavity allows the possibility of synchronization to an optical reference. A temporal jitter of 120 fs rms is achieved, integrated from 10 Hz to 10 MHz.
Then a regenerative amplifier, based on Yb:YAG technology, pumped by fiber-coupled QCW laser diodes, delivers pulses up to 30 mJ. The 1 nm bandwidth was compressed to 1.5 ps with a good spatial quality: M2 of 1.1. This amplifier is integrated in a compact sealed housing (750 x 500 x 150 mm), which allows a pulse-pulse stability of 0.1 % rms, and a long-term stability of 1,9 % over 100 hours (with +/-1°C environment).
The main amplification stage uses a cryocooled Yb:YAG crystal in an active mirror configuration. The crystal is cooled at 130 K via a compact and low-vibration cryocooler, avoiding any additional phase noise contribution, 340 mJ in a six pass scheme was achieved, with 0.9 of Strehl ratio. The trade off to the gain of a cryogenic amplifier is the bandwidth reduction, however the 1030 nm pulse was compressed to 4.4 ps. As for the regenerative amplifier a long-term stability of 1.9 % over 30 hours was achieved in an environment with +/-1°C temperature fluctuations
The compression and Second Harmonics Generation Stages have allowed the conversion of 150 mJ of uncompressed infrared beam into 60 mJ at 515 nm.
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