Noncollinear optical parametric amplification of a supercontinuum

Abstract

During this thesis, work has been done towards realizing a short-wave infrared (SWIR) laser source based on optical parametric amplification. It is planned that this setup should be able to produce few-cycle laser pulses at 2μm with a stable carrier-envelope phase (CEP). This will be done by using various nonlinear processes to convert the output from a 1030-nm, 200 kHz laser source, based on Yb-doped rod-fiber amplifiers.
During my work, the first part of the envisioned SWIR source has been implemented: A super- continuum has been generated, to provide a broadband seed for a non-collinear optical parametric amplifier (NOPA). The NOPA also requires a pump beam, which was produced using second-harmonic generation to obtain light at 515 nm. Finally, the NOPA was implemented using the supercontinuum seed and the pump beam.
The supercontinuum was generated through spectral broadening in a YAG crystal. The focusing conditions and the length of the crystal were adjusted to obtain a flat and stable spectrum in the wavelength range of interest (620 - 750 nm). The NOPA utilized a BBO crystal as a nonlinear medium. Phase-matching simulations of the NOPA process was carried out to design an optimal geometry for the amplification of the white light in the range of 640 - 750 nm. The NOPA was subsequently implemented using the optimal parameters obtained from the simulations. The resulting NOPA produced an output of 440 mW, using an input of approximately 5 - 6 mW of supercontinuum power. The spectrum of the amplified supercontinuum stretched between 640 - 695 nm. The gain of the NOPA is satisfying, however, the bandwidth is narrower than desired. Further improvements can be done to the amplifier such as removing residual chirp from the supercontinuum to further increase the bandwidth.
In future work, a stage performing difference frequency generation will be implemented, which will provide CEP-stable SWIR pulses.

Popular Abstract

Optical amplification of white light as a first step towards ultrashort laser pulses

In this master thesis, an infrared, pulsed laser was used to produce colorful, so called "white light", of which a part of was amplified using an optical amplifier. This is the first part of a planned larger setup, which can be used to, for example, produce ultraviolet light and possibly X-rays. Laser pulses at these wavelengths can be extremely short, which can be used to study extremely fast events, like the motion of electrons. Ultrashort laser pulses can also be used for such different things as the study of dissociation of molecules and eye surgery.

Ultrashort laser pulses are the shortest event in time created by humans. Currently it is possible to generate laser pulses in the timescale of attoseconds, which also is the timescale of electron motion. An attosecond is 10^−18 s, or 0.000000000000000001 seconds. To put this into perspective, a really fast blink of the eye takes 0.1 s. This means that there is literally 100 million billion attoseconds during the blink of an eye. I suggest you blink right now to really get the idea.
In order to generate such short laser pulses, the wavelength of the light must also be very short. For attosecond laser pulses the light must at least be in the extreme ultraviolet (XUV). Unfortunately, there are no lasers today that can produce light with such a short wavelength, so alternative methods are required. A popular method is to take the light from an existing laser (usually an infrared laser) and convert this into light with much shorter wavelength (XUV).
My master thesis is the very of the first step of this process. We have taken infrared light at a wavelength of 1030 nanometers (nm, 10^−9 m) from an existing laser and used it to produce so called white light. The light is in fact not necessarily white, it rather has a green/yellow color, but just like white light it has a very broad spectrum of colors. The white light was produced by sending intense infrared light through a crystal. Part of the white light, with an orange/red color, was then amplified using an optical parametric amplifier. This kind of amplifier works by using strong light to amplify weaker light. This is done by sending both beams of light through a suitable crystal. In this case, we used very intense green light to amplify the white light. The goal was to obtain amplified light between 620 - 750 nm.
The result is that a working amplifier was implemented. The amplifier provided an output power of 440 mW, which is a very good number. Unfortunately, the spectrum of the amplified light only covered 640 - 695 nm. This should however be possible to improve with further work.
The journey towards very short wavelength light and attosecond pulses will be continued by converting the amplified light back to infrared, but this time with a wavelength of 2000 nm. So the purpose of the whole process of creating white light and then amplifying it with green light, is to eventually end up with light that has roughly twice the wavelength of what we started with. This light can then finally be used for new and exciting investigations of generation of XUV light in gases as well as solid materials. The reason to why we want a longer wavelength, is that the process that we use to produce XUV light, called high-harmonic generation, depends on the wavelength. Rather counterintuitively, it produces light with a shorter wavelength, the longer wavelength light you put into it. The generated light can both be used to obtain information of the properties of the material and to produce ultrashort, attosecond laser pulses.

• author: Åkesson, Henrik
• supervisor: Fabian Langer; Ivan Sytcevich
• organization: Atomic Physics; Department of Physics
• course: PHYM01 20191
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy; Technology and Engineering
• keywords: optical parametric amplification, supercontinuum generation, nonlinear optics, laser pulse compression, lasers, optics
• report number: LRAP 557 (2019)
• language: English
• id: 8974177
• date added to LUP: 2019-04-25 19:01:25
• date last changed: 2019-04-25 19:01:48