Dispersion management and characterization of ultrashort laser pulses for the optimization of parametric processes

Abstract

This thesis set out to continue the development of a short-wave infrared (SWIR) optical parametric amplifier (OPA). The system is based on a Yb-ber chirped pulse amplification (CPA) laser delivering 400 fs long pulses, centered around 1030 nm, at 200 kHz repetition rate and 40 W average power. The thesis focuses on pulse characterization, through use of frequency-resolved optical gating (FROG), of the output of a previously implemented noncollinear optical parametric amplification (NOPA) stage, amplifying a white-light source with the second harmonic of the fundamental 1030-nm pulses. The results show significant improvements in the pulse structure of the fundamental pulses as well as the amplified white-light pulses through dispersion management optimization. In addition to the pulse characterization, simulations were made for a difference-frequency generation (DFG) and OPA stage, that will be part of the future setup. Most notably, the simulations highlighted two critical aspects: probable bandwidth limitations for the OPA stage and the importance of a smooth spectral pulse shape. The simulations also
demonstrated the feasibility to produce few-cycle pulses.

Popular Abstract

With the invention of the camera, mankind was able to capture moments in time that would otherwise be gone forever. Although the earliest prototypes required an exposure time of twenty minutes, the necessary acquisition time for accurate pictures became shorter and shorter over the years. Eventually shutter speeds became so fast that the capabilities of cameras even surpassed the human eye, making these devices a useful scientific tool. Today, the fastest scientific camera has a frame rate of 10 trillion (10*10^12) frames per seconds, able to capture events on the time scale of 0.0000000000001 s. This nearly incomprehensible speed is so fast, that it can even capture frames of propagating light. Despite this, there are still events so quick that they are impossible to observe with a camera. For example, the movement of electrons inside atoms.

Instead of operating a fast shutter of a camera, a quick motion can be frozen in time by a light flash. The light illuminates the object only for a short time and, hence, captures a snapshot of its evolution in time. The read-out and exposure of the camera then only play a minor role because the scene will be dark without the flash. By using a stroboscopic light and repeatedly illuminating the object, several snapshots taken one after the other reveal the motion of the object. This can be even done on the same photographic plate or sensor if the motion allows to separate the images on the sensor. Otherwise, this technique is limited to a repeatable experiment, where the object is prepared with the same initial conditions and the process is started again for every picture. Then the flash is delayed a bit more for each image so that after a while the entire motion is captured. This is the so-called pump probe technique.

To track the movements of electrons, the flash needs to be super fast, with a duration that is on the same time scale as the motion of the electrons. For this to be possible, the lightwave must be of a very high frequency (short wavelength) as it must be able to complete one full oscillation to propagate. This leads us into the eld of ultrafast laser science and to a process known as high-harmonic generation (HHG). HHG is a nonlinear process occurring when intense femtosecond laser pulses are focused into a gas. Through this process, high-frequency light in the extreme ultraviolet (10 - 124 nm) or even soft x-ray region (0.12 - 5 nm) can be generated. The shortest pulse produced up to date had a duration of only 43 attoseconds (43*10^18, or 0.000000000000000043 s), opening up the possibility to not only observe but also control the realm of electrons.

The key to producing attosecond pulses through HHG is to precisely characterize and optimize the femtosecond pulses driving the process. But how can we measure on the femtosecond time scale? The answer is to use the pulse to measure itself, which has been done in this thesis work with an especially elegant technique known as frequency-resolved optical gating or, more commonly, FROG. The pulse is first split in two and one copy is scanned in delay over the other inside a crystal. Because the pulse has a very high intensity, new photons are created inside the crystal through a nonlinear process known as sum-frequency generation (SFG). A SFG signal is only generated when the pulses overlap in time, hence a temporal measurement of the signal is possible for different pulse overlaps. The spectrum of the SFG signal is measured with a spectrometer, whereafter a computer algorithm is able to reconstruct the pulse, showing its structure.

The FROG technique was used in this thesis to characterize the pulse durations at different stages of an advanced laser that currently is under development in the research group for Attosecond Physics at Lund University. By precisely measuring the shape of the pulses for individual parts of the laser source, special optical components could be used to control it and make these pulses as short as possible. Through this process, the already installed parts were optimized in preparation of stages to the setup that are yet to be implemented. Knowing that the already built parts of the laser are working well, further steps can be taken towards HHG with this new laser source.

• author: Elfving, Axel
• supervisor: Fabian Langer; Ivan Sytcevich; Cord Arnold
• organization: Atomic Physics; Department of Physics
• course: PHYM01 20191
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering; Physics and Astronomy
• keywords: frequency resolved optical gating, optical parametric amplification, difference frequency generation, nonlinear optics, laser pulse compression, lasers, optics
• report number: LRAP 560 (2019)
• language: English
• id: 8981375
• date added to LUP: 2019-06-12 09:17:48
• date last changed: 2019-06-12 09:17:48