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Self-compression in a multipass cell

Abstract : We demonstrate self-compression of short-wavelength in-frared pulses in a multipass cell (MPC) containing a plate of silica. Nonlinear propagation in the cell in the anomalous dispersion regime results in the generation of 14 μJ 22 fs pulses at 125 kHz repetition rate and 1550 nm wavelength. Periodic focusing inside the cell allows us to circumvent catastrophic self-focusing, despite an output peak power of 440 MW well beyond the critical power in silica of 10 MW. This technique allows straightforward energy scaling of self-compression setups and control over the spatial manifestation of Kerr nonlinearity. More generally, MPCs can be used to perform, at higher energy levels, temporal manipulations of pulses that have been previously demonstrated in waveguides. Ultrafast lasers have allowed a plethora of applications, ranging from industrial processing to a large number of scientific advances such as multiphoton imaging, ultrafast dynamics studies, and strong field physics. In the short-wavelength infra-red (SWIR) and mid-infrared (MIR) wavelength ranges, applications such as high-harmonic generation in gases [1], solids [2], and multidimensional molecular spectroscopy [3] are pushing the development of high-power microjoule-millijoule laser systems with pulse durations in the few-cycle regime. In this wavelength range, the most widespread sources of ultrashort pulses are optical parametric chirped pulse amplifiers (OPCPAs) that readily deliver pulses with sub-100 fs durations at high repetition rates [4-8]. Access to shorter pulse durations is often allowed by implementing a nonlinear compression setup at the output of the laser system. Propagation of the pulse in a solid or gas material induces spectral broadening through self-phase modulation (SPM), corresponding to a shorter Fourier-transform limited pulse. For sources in the near-infrared, where material dispersion is normal, additional negative dispersion must be added after the SPM stage [9]. For sources in the SWIR and MIR, most optical materials exhibit anomalous dispersion, and soliton dynamics results in compression in the initial stage of propagation. Although self-compression can be performed in gaseous [10] and bulk solid [11] media, these experiments often show low energy transfer to the compressed pulse and more difficult experimental control because of the complex spatio-temporal dynamics. Alternatively, the use of a waveguide allows large interaction lengths and negates the spatial Kerr effect, resulting in efficient and well-controlled setups. Energy scaling in waveguides is challenging, requiring, for instance, the use of gas-filled anti-resonant photonic crystal fiber designs to increase the critical power for self-focusing, ensure low losses, and induce anomalous dispersion [12]. Recently, a concept allowing the use of a bulk nonlinear medium while retaining the advantages of waveguides has emerged [13]. It consists of placing the nonlinear medium inside a multipass cell (MPC) and accumulating the nonlinearity over a large number of passes, so that nonlinear propagation in the medium can be seen as a small perturbation to free-space propagation as far as spatial effects are concerned. This can be compared to waveguides where spatial nonlinear effects change slightly the mode structure [14] without significant physical impact. It can also be considered as an extension of multi-plate setups [15-17] with a distribution of the nonlinearity over tens of passes in the material instead of a few. Although it was proposed as early as 2000 [18], it is only recently that this idea has been demonstrated [13]. The concept was first implemented with a solid nonlinear medium [13,19,20], where it was shown that the peak power can exceed the critical power in the medium without affecting the spatial profile at the output. The MPC idea was then used with a gas medium [21,22], at higher energy levels of 140 μJ and 2 mJ, respectively. These demonstrations have all been done in setups where the net dispersion in the MPC was either negligible or slightly normal, resulting in an essentially stationary temporal profile during propagation and necessitating negative dispersion at the output to compress the pulse. Here we report the use of an MPC in net anomalous dispersion regime to take advantage of soliton dynamics and self-compress 19 μJ 63 fs pulses generated by an OPCPA at 1.55 μm down to a 22 fs pulse duration with a transmission of 73%. The experimental data are compared to a 3D numerical model, and the output beam is characterized in terms of spatio-spectral couplings. This experiment establishes the use of MPCs as a simple way to scale the energy of nonlinear Letter Vol. 43, No. 22 /
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Submitted on : Tuesday, November 5, 2019 - 2:23:35 PM
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Gaëtan Jargot, Nour Daher, Loic Lavenu, Xavier Delen, Nicolas Forget, et al.. Self-compression in a multipass cell. Optics Letters, Optical Society of America, 2018, 43 (22), pp.5643. ⟨10.1364/OL.43.005643⟩. ⟨hal-02348758⟩

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