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Taming optical parametric amplification. stable few cycle pulses at 210 to 10000 nm from Ti:Sapphire and Yb-based lasers
Taming optical parametric amplification. stable few cycle pulses at 210 to 10000 nm from Ti:Sapphire and Yb-based lasers
Due to a limited number of known laser media technically relevant femtosecond laser systems are restricted to a few laser frequencies only. Tunability based on these sources is enabled by the process of optical parametric amplification (OPA). A small portion of the laser output is used to generate a seed pulse. To enable tunability over a large spectral range a bulk continuum is used. With a strong pump pulse a selected spectral part can be amplified in a suitable nonlinear medium. In order to obtain highly stable and ultrashort pulses on the 10 fs scale a deep understanding of each individual step in the process is necessary. In this work Ti:sapphire and Yb-based laser systems are used to pump OPAs. In the used topology it is possible to utilize repetition rates from 1 kHz to 1 MHz and input energies from 10 to 300 µJ without major changes of the design. We obtain ultrashort pulses from the UV to the MIR. New aspects on bulk continuum generation are presented. For generation with 1030 nm light various crystals are compared regarding the visible and near infrared continuum side. The hitherto unconsidered GSO is found to be superior. Self-compression due to continuum generation is shown without the need for external compression. It bases on a 1 mm sapphire or YAG plate and an astigmatism-free, achromatic telescope . a Schiefspiegler. The self-compressed, unchirped continuum is used to seed a NOPA (noncollinear OPA) for ultra-broadband pulse generation. With this system quantum efficiencies of up to 45% in the amplification process and 18 µJ output are demonstrated and compression down to 6.7 fs. The transmitted seed light after optical parametric amplification and the amplified output can propagate in significantly different directions. This is explained as a consequence of Kerr lensing at the needed pump intensities. A pump induced Kerr lens is acting on the signal/seed pulse. This induces a deflection in the amplifier medium at imperfect input coupling. Locating the amplifier crystal behind the focal plane of the pump minimizes the self-lensing effect due to nonlinear balancing of the beam divergence. This also allows reducing the pump intensity simply by moving the crystal further away from the focal plane. For MHz repetition rates serious thermal issues in a NOPA arise due to the high average power that is needed for the nonlinear processes. For third harmonic generation (THG) of an Yb-fiber laser, second harmonic generation (SHG) with subsequent sum-frequency mixing is demonstrated. Two-photon absorption of the UV leads to measureable heat in the THG crystal. The phase-matching conditions change and the UV power decreases over time. With a time delay compensation plate (aBBO) between the SHG and THG crystal this effect can be minimized. By temporally pre-compensating the fundamental with respect to the second harmonic, the generation locus of the main UV power is shifted to the end of the THG crystal and the volume for absorption is minimized. Stable UV pulses result. This enables a long term stable UV pumped NOPA output even at 1 MHz repetition rate. An OPA for the generation of an octave spanning middle infrared pulse centered around 8 µm is presented. A 515 nm pumped NOPA with a subsequent collinear, 1030 nm pumped amplifier based on LGS is utilized. The chirp management is entirely by bulk material and selected optical filters. The MIR field is temporally characterized by electro-optical sampling. As gate pulse the self-compressed fundamental is used in an extremely simple setup. Electro-optical sampling reveals a compression down to 1.4 cycles of the MIR field and an intrinsically phase-locked CEP stability of better than 94 mrad over one hour.
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Wittmann, Emanuel
2019
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Wittmann, Emanuel (2019): Taming optical parametric amplification: stable few cycle pulses at 210 to 10000 nm from Ti:Sapphire and Yb-based lasers. Dissertation, LMU München: Faculty of Physics
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Abstract

Due to a limited number of known laser media technically relevant femtosecond laser systems are restricted to a few laser frequencies only. Tunability based on these sources is enabled by the process of optical parametric amplification (OPA). A small portion of the laser output is used to generate a seed pulse. To enable tunability over a large spectral range a bulk continuum is used. With a strong pump pulse a selected spectral part can be amplified in a suitable nonlinear medium. In order to obtain highly stable and ultrashort pulses on the 10 fs scale a deep understanding of each individual step in the process is necessary. In this work Ti:sapphire and Yb-based laser systems are used to pump OPAs. In the used topology it is possible to utilize repetition rates from 1 kHz to 1 MHz and input energies from 10 to 300 µJ without major changes of the design. We obtain ultrashort pulses from the UV to the MIR. New aspects on bulk continuum generation are presented. For generation with 1030 nm light various crystals are compared regarding the visible and near infrared continuum side. The hitherto unconsidered GSO is found to be superior. Self-compression due to continuum generation is shown without the need for external compression. It bases on a 1 mm sapphire or YAG plate and an astigmatism-free, achromatic telescope . a Schiefspiegler. The self-compressed, unchirped continuum is used to seed a NOPA (noncollinear OPA) for ultra-broadband pulse generation. With this system quantum efficiencies of up to 45% in the amplification process and 18 µJ output are demonstrated and compression down to 6.7 fs. The transmitted seed light after optical parametric amplification and the amplified output can propagate in significantly different directions. This is explained as a consequence of Kerr lensing at the needed pump intensities. A pump induced Kerr lens is acting on the signal/seed pulse. This induces a deflection in the amplifier medium at imperfect input coupling. Locating the amplifier crystal behind the focal plane of the pump minimizes the self-lensing effect due to nonlinear balancing of the beam divergence. This also allows reducing the pump intensity simply by moving the crystal further away from the focal plane. For MHz repetition rates serious thermal issues in a NOPA arise due to the high average power that is needed for the nonlinear processes. For third harmonic generation (THG) of an Yb-fiber laser, second harmonic generation (SHG) with subsequent sum-frequency mixing is demonstrated. Two-photon absorption of the UV leads to measureable heat in the THG crystal. The phase-matching conditions change and the UV power decreases over time. With a time delay compensation plate (aBBO) between the SHG and THG crystal this effect can be minimized. By temporally pre-compensating the fundamental with respect to the second harmonic, the generation locus of the main UV power is shifted to the end of the THG crystal and the volume for absorption is minimized. Stable UV pulses result. This enables a long term stable UV pumped NOPA output even at 1 MHz repetition rate. An OPA for the generation of an octave spanning middle infrared pulse centered around 8 µm is presented. A 515 nm pumped NOPA with a subsequent collinear, 1030 nm pumped amplifier based on LGS is utilized. The chirp management is entirely by bulk material and selected optical filters. The MIR field is temporally characterized by electro-optical sampling. As gate pulse the self-compressed fundamental is used in an extremely simple setup. Electro-optical sampling reveals a compression down to 1.4 cycles of the MIR field and an intrinsically phase-locked CEP stability of better than 94 mrad over one hour.