‘Printing’ lasers as tools for IPAS research – talking papers series, #1
‘Printing’ lasers as tools for IPAS research
Recent work published by IPAS senior researcher David Lancaster brings the team a step closer to being able to ‘print’ (or burn / write) highly specialized lasers required for further research into a wide range of photonics and advanced sensing technologies. With the right kind of industry partner, this work could lead to extremely fast and cheap ways to fabricate lasers for the mass market. Earlier this week I caught up with David to discuss his recently published paper “Fifty percent internal slope efficiency femtosecond direct-written Tm3+:ZBLAN waveguide laser”.
On the way to David’s office at the Institute for Photonics and Advanced Sensing (IPAS) at the University of Adelaide I had one last read through the abstract of this curiously titled paper. Having circled the words ‘doped’, ‘pumped’, ‘resonator’ my attention was grabbed by the final phrases ‘the most efficient laser’ (created in a glass host) via ‘femtosecond waveguide writing’. I figured I would ask about ‘writing a waveguide’ in glass first, then tackle the mysterious ‘fifty percent internal slope efficiency’.
Our conversation began with a brief description of using a precisely focussed laser beam to create tiny ‘bubbles’ of plasma that altered the all important refractive index property of the host glass. (The host glass in this case being a heavy-metal fluoride called ZBLAN). David showed me some microscope images of these tiny bubbles arranged in various patterns and very patiently explained how these patterns created wave-guides similar to those already being fabricated in the IPAS labs. (By a process of extruding molten glass into cylindrical pre-forms which are then re-heated and drawn to the same geometry but less than the thickness of a human hair). I have to admit that I really did not understand how these dots that are ‘written’ (or ‘burned’) into the host glass could do anything useful. Then an object on David’s bookshelf caught my attention – it was a glass cube that had a three dimensional image of a racing car burned (written or 3D-printed) into it.
Then the penny dropped – I knew these glass art works were created by making a design using C.A.D. software then ‘printing’ the three dimensional design into the glass using tightly focussed laser beams to burn (or ‘write’) tiny holes at various x,y,z coordinates. David (and his collaborators at Macquarie University) are doing the same thing – designing three dimensional patterns (that resemble those of the micro-structured fibres fabricated in the IPAS labs) in software and ‘printing’ (or writing) them into the host glass. So there it is – using a laser to ‘write’ (or print / burn) a structure that can guide light (waveguide) in such a way that it can behave as a new laser. It turns out that this technique has been around for a while – the surprise finding described in this paper is just how efficient the resulting laser produced by this inherently fast and cheap process is. We are talking around 12 seconds to write a small batch of lasers at a cost of hundreds of dollars for short runs in lab conditions – plenty of room for dramatic cost reductions in a large-scale commercial process.
So that covers the ‘Femtosecond direct-written Tm3+:ZBLAN waveguide laser’ half of the title, or nearly. Femtosecond (1×10-15 seconds) is unit of the extremely brief duration of each pulse of the tightly focussed writing laser. Each tiny dot is written into the host glass in a few such pulses. Tm3+ is the form of rare-earth ionThulium that emits the photons we desire when excited by another “pump” laser. ZBLAN is our heavy-metal fluoride host glass that has been ‘doped’ with Tm3+ as mentioned earlier.
What, then, is the big deal about the “Fifty percent internal slope efficiency”? Well, it is a good numeric indicator for the efficiency and therefore usefulness of the resulting waveguide laser. At the time this work was being undertaken the best researchers could manage for a similar regime was around 2%. The highest efficiency ever reported for any type of directly written waveguide laser in glass was 21% (for lasers that work in much less elusive wavelengths). The techniques explored by IPAS gave an extremely encouraging result on first try with plenty of room for improvement and customization. The high internal slope efficiency translates to very user-friendly output that is inherently low-loss and thermally efficient. This means that much less engineering is required to manage heat (a problem for most lasers). Furthermore this host/dopant combination is very stable and not particularly prone to ‘self-erasing’ – a problem in many direct written waveguide lasers.
So what for the future of this technique? David tells me he is confident that his team will be able to design waveguide geometries that include the ‘resonator’ function (which was provided by a pair of external mirrors at each end) of the waveguide laser. This removes the need for a whole bunch of engineering and makes for a very useful laser produced entirely with this rapid and cheap technique. Next the team plan to experiment with and characterize combinations of slightly different host glass, dopant and waveguide geometry which will enable IPAS to design and manufacture lasers with very specific properties and to chase those elusive long (mid-infrared) wavelengths that are particularly useful in high end applications.
Speaking of applications, David is optimistic about the potential uses of this technique to develop low cost tools for sensing in general and gave a couple of example scenarios that should be possible. One is in the area of tissue imaging – which requires lasers with very specific characteristics to ‘see through’ various types of internal organs etc. Another is in the area of coherent L.I.D.A.R. which currently requires very expensive specialised lasers. The military are always interested in lasers that work in the mid-infrared beacause that is where they can see heat. Spectroscopy is another area that could benefit from small, cheap, portable lasers that will enable sensing to move out of the labs and into the field – for example to detect contaminants or pathogens in-situ.
In a subsequent chat with Prof. Tanya Monro, Director of IPAS, I learned how the idea of being able to create new & specialised tools for research is entirely consistent with her vision for ‘platform technologies’ coming out of the Institute. The work described in this paper creates a pathway to accessible and scaleable technologies that will produce cheap, robust and useful lasers at wavelengths that currenly require expensive and bulky equipment which can only be operated by experts.
If this story has captured your imagination, take a listen to these brief audio comments from Associate Professor David Lancaster and IPAS Director, Professor Tanya Monro.
Mike Seyfang for IPAS “talking papers” series