UV laser pulses the easy way
This ’10 % problem’ that the MINIMODS team set out to overcome in 2013 had been bothering the industry for quite a while. Until now, accessing some laser spectral regions - notably the ultraviolet (UV) band around 300 nm - was far from being a walk in the park: UV pulses had to be generated by means of frequency converters based on light propagation models or simple numerical simulations. A process that allowed for near infrared pulses to become UV pulses by summing up energy of the fundamental pulse photons. The main shortcoming of that approach, however, was that conversion efficiencies stagnated at around 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 maximise it,’ Dr Michal Nejbauer, from the Faculty of Physics of the University of Warsaw, recalls. ’10 % is as good as one can get with this approach.’ The tripler solution MINIMODS’ tripler solution is not only three times as efficient, but it also fits on a finger tip. It uses a ‘sandwich’ of nonlinear and birefringent crystals to convert 190 femtosecond pulses of 1040 nm light from an ytterbium-crystal laser into ultraviolet light. It fits directly in the laser head, can be hermetically sealed, and comes with an open-source simulation package named Hussar. ‘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,’ explains Tomasz Kardas, who developed the software. ‘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. This kind of quantitative agreement between what one gets on the screen and then measures in the lab is rather uncommon in nonlinear optics.’ Further development The tripler component will be integrated within the company’s line of ultrashort-pulsed lasers. In the meantime, the team are working to improve their software so that it can be used in more optical design-related applications. The latter is available on an open-access basis for non-commercial use and can also be purchased by companies willing to use it as a product. ‘I believe the new generation of the 3D pulse propagation software may actually be a big step forward in the design of many devices where nonlinear broadband pulse propagation is involved, for example parametric amplifiers,’ Piotr Wasylczyk, lead author of the paper published in Nature, says. ‘My impression now is that most people use simplified modeling and can only get so far with this approach.’ For more information, please see: project website
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