A research team from the University of Warsaw’s Faculty of Physics, Poland, has developed a miniature device, called a tripler, capable of producing femtosecond laser pulses in the ultraviolet (UV) range. In addition to having an efficiency that is three times higher when compared to the devices used earlier, the distinctive software package used in the device - which was developed in Warsaw at the design stage - enables it to fit on a finger tip.
Despite the fact that innovative technologies enable lasers to cover more spectral regions, specific wavelengths, such as the UV band close to 300 nm, cannot yet be accessed easily - specifically when high intensities and/or short pulse durations are necessitated.
In many instances, nonlinear processes (e.g. sum frequency generation or second harmonic generation) are employed for generating UV pulses. In such processes new photons of new color and higher energy are formed as a result of summing up of energies of the fundamental pulse photons. However, this process, in which near infrared laser pulses can be converted into UV, has lesser efficiency.
For a long time, frequency converters were designed by using simple numerical simulations or analytical light propagation models, which enabled researchers to adjust various device parameters, conventionally one by one. While converting un-amplified infrared femtosecond lasers to the UV third harmonic, this approach yielded restricted conversion efficiencies of nearly 10%.
It was like coming to the lab, tweaking one knob here, one knob there, while looking at the UV output power and trying to maximize it. And 10% is as good as one can get with this approach.
Michal Nejbauer, Faculty of Physics of the University of Warsaw
The readily accessible computational power in combination with intelligent programming techniques enabled first-time application of global optimization of the infrared to UV frequency conversion process.
Our newly developed, open-source simulation package - called Hussar - allows even an inexperienced user to build a complex, 3-dimensional, accurate simulations of multiple pulse propagation and interaction using simple blocks: input pulse parameters, material properties of the media and the processes involved. Once we define the input pulse parameters, such as energy, duration and spatial beam profile, we essentially start searching for the best design over a large space of parameters: the nonlinear crystal thicknesses, the beam size, the beam waist position, etc. And, to our surprise, once we found these optimum values, built the device and measured its performance, the output UV pulses were exactly as simulated.
Tomasz Kardas, Researcher
Such a quantitative accordance between the value obtained on screen and the one obtained from the lab is quite atypical in nonlinear optics.
However, enhancing the efficiency of the tripling process by a factor of three, that is, by more than 30%, was just the initial move. The original goal of the research team was miniaturization, that is, instead of employing numerous components set up on the laboratory table, the third harmonic generator, or the tripler, developed by them is merely a tiny block of crystals accumulated together.
“In fact, the 1-inch metal holder that keeps all the elements together is the biggest part of the whole setup” explained Pawel Wnuk, who had a major role in the experiments for characterizing the device. Consequently, the overall volume of the tripler prototype is nearly 1000 times less than the conventional designs used earlier.
The miniature frequency tripler was created within the MINIMODS consortium, which includes industry partners Radiant Light (Spain), Laseroptik (Germany), and Time-Bandwidth Products (Switzerland), with coordination from Glasgow-based M Squared Lasers LTD. The University of Warsaw, Poland, and the Fraunhofer Centre for Applied Photonics, UK, are the research partners.
The project was conducted between 2013-2015, and was supported by the EC’s Seventh Framework Programme FP7-SME. The main aim of the project was to eliminate the challenges of innovation and expansion in the photonics industry, with an emphasis on developing compact and cost-effective devices and tools that can be integrated into laser systems.
Working in close collaboration with industrial partners was a new, interesting experience. We have learned a lot about how they approach research and product development. I am not sure if they learned a lot from us, but the feedback we got from them on what we did and how was very positive.
Piotr Wasylczyk, University of Warsaw
The outcomes of the research have been recently published in the journal Scientific Reports.