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Next Generation Optical Fibres - Towards Development of Novel Spectroscopic Laser Tool in Mid-IR

Internationaliseringsstipendium | 03/06/2016

The mid-IR wavelength range of 2–20 µm is a spectral region of tremendous scientific and technological interest. First of all, it contains the strong characteristic absorption lines of many important molecules, making mid-IR laser sources crucial for applications in skin-cancer detection, materials processing and biomolecular sensing[1]. Therefore, the question for us to answer is whether it is possible to make a new kind of optical fibres that could be used to develop novel lasers for all the aforementioned applications. Such achievement would have a big impact on Danish research society through the generation of new knowledge and innovation directly related to industry.

The current project constitutes an efficient way of developing novel optical fibres that are able to generate and transmit light in the mid-IR range. More importantly, the proposed fibres will be functional and able to respond to external changes such as voltage and temperature in ways never reported before. To achieve such functionalities, different materials (i.e glasses, polymers, metals) have to be combined forming a “multi-material fibre” with distinct thermo-mechanical and optical properties[2]. These fibres will then be used to generate very bright and broad light (known as supercontinuum generation) that can be used for spectroscopy in the mid-IR or “molecular fingerprint” region.

Fig. 1. Schematic representation of an optical fibre constituted by several materials for the mid-IR region.

Mid-Infrared Materials – Chalcogenide Glass Fibres 

The most suitable candidates as emerging materials for the mid-IR region are perhaps chalcogenide glasses. This glass category contains one or more chalcogenide elements sulphur (S), selenium (Se) and tellurium (Te) having unique optical properties in the mid-IR region such as high transparency, extremely high optical nonlinearity, high refractive indices, photo- tuneable properties, etc.[3] Several research groups are working with these glasses in order to develop tuneable hybrid fibres and devices[3-6]. However, one of the main goal is to move towards the realisation of supercontinuum (SCG) laser sources covering the mid-IR molecular fingerprint region [7] up to 20 µm based on chalcogenide glasses. Fibre-based SCG is very important for spatially coherent laser sources. A recent development in mid-IR sources was achieved by our group (Fibre Sensors and Supercontinuum Group headed by Prof. Ole Bang) of DTU Fotonik, where we demonstrated one of the first SCG up to 13.3 µm using a step-index chalcogenide fibre [7].

Controlling the Fibre Properties

One way to control the optical properties of a fibre is by controlling the material’s refractive index to cause light to travel faster or slower through it. This is a particularly good option for materials that naturally alter their refractive index according to the intensity of light to which they are exposed. Such materials thus behave differently depending on whether the light passing through comes from a low-power source or a high-power laser. These materials such as chalcogenide glasses are known as optically nonlinear and high nonlinearity is most of the time considered an attractive trait.

Fig. 2 Example of spectrum evolution in a chalcogenide fibre pumped with intense laser pulses (the dashed and dotted lines represent the ZDW and pump wavelength). respectively.

One of the most crucial parameters controlling the efficiency of nonlinear effects in a fibre is the location of the zero-dispersion wavelength (ZDW) relative to the wavelength, which is used to “excite” the fibre (known as pump wavelength). One way to tune the pump wavelength close to the ZDW of the fibre is to use an ultrafast laser (duration of a few femtoseconds) with a tuneable optical parametric amplifier (TOPA). However, this approach is often impractical and expensive, because the pump system is expensive, complicated to operate and requires very large area of bulk optics and thus constructing a real, compact, and portable mid-IR laser is basically unachievable. Therefore, there is a huge need for new nonlinear fibres whose properties - in particular the ZDW- can be externally tuned using for example electrical or thermal signals. In such case, there is no need to tune the pump wavelength of the laser. In this project, we aim to make a fibre from different materials enabling the ability to control and tune the fibre properties in the mid-IR range. Furthermore, these fibres could be made in a scalable and low-cost way and with the desired performance. The fabricated fibres will then be used to develop a mid-IR supercontinuum source.

Relevance to Society and Industry

The mid-IR spectral region encompasses the molecular fingerprints, i.e. the characteristic set of mid-IR fundamental molecular vibrational absorptions of numerous gases, liquids and solids as diverse as greenhouse gases; ground, water and air pollutants; pharmaceuticals; toxic agents; food and drink; oil, oil products and plastics and, as addressed here, biological tissues and cells, and detection of early-stage skin-cancer[8]. Therefore, the development of a novel, compact, portable, and relatively simple broadband laser source (such as a supercontinuum source) would have a tremendous impact on society as new systems will be able to fulfil the requirements for all the aforementioned applications.  On the other hand, the proposed project might be a novel solution for the laser industry since new directions and possibilities will open for the development of compact supercontinuum sources. From a scientific point of view, even today,  there  is  no  research  group  in  Denmark  nor in  Europe, which is able  to  fabricate the proposed fibres.  The proposed project is fitting ideally with the strategic long-term plan of DTU Fotonik, which in September 2016 will get a new building with brand new laboratories for a state-of-the-art fibre draw tower, chalcogenide glass chemistry laboratory and other advanced equipment for glass manufacturing such as a glass extruder. Therefore, the proposed project will create a completely new research activity in Denmark and in Europe. 

What Motivates Me?

For the past five years I have had the opportunity to work in multidisciplinary projects aimed at a variety of applications. Even during my undergraduate studies I was always excited about materials science and how new materials could open up new avenues in several fields. My first experiment with a laser during my doctoral degree was enough for me to realise that working with light is beyond exciting. That is when I decided that this was the final direction I would like to follow. However, my passion and motivation was always to combine the two areas: Materials science and Photonics. I was very enthusiastic about investigating how light behaves when it propagates in new and functional materials. The best way to achieve this interaction in my opinion is by “trapping” the light in a fibre made from exotic materials and investigate how the two act together.  With the Carlsberg Foundation’s Internationalisation Fellowship I was given the opportunity to visit the Multi-material Optical Fibre Devices Group of Professor Abouraddy’s group at CREOL, University of Central Florida, USA[9]. My visit in Professor Abouraddy’s group significantly enhanced my knowledge on how to make optical fibres based on different materials. More importantly, I also had the opportunity to visit the Mid-Infrared Photonics Group headed by Professor Angela Seddon at the University of Nottingham, UK.  Professor Seddon’s group is one of the best groups in the world at making ultra-high purity chalcogenide glasses and at the moment they hold the world record for ultra-low loss chalcogenide fibres[10].

Therefore, this project was a great opportunity for me to visit different research groups and to establish new collaborations as well as teaming up with complementary groups in order for us all to explore new pathways in the fibre-optics field.  Last, but not least, this project was a great opportunity for me to further enhance my independent research profile and pursue a career in Denmark.

References

[1]                 A. Schliesser  et al.  Nat. Phot. 6, 440 (2012);

[2]                 A. F. Abouraddy, et al.   Nat. Materials  6, 336 (2007)

[3]                 Eggleton, B.J et al. Nat. Phot. 5, 141 (2011)

[4]                 C. Markos et al.   Opt. Express 20, 14814 (2012)

[5]                 C. Markos et al. Proc. SPIE 9634 (2015)

[6]                 C. Markos et al. Sci. Rep. 4, 6057 (2014)

[7]                 C.R. Petersen, Nat. Phot. 830 (2014)

[8]                 Angela B. Seddon Proc. SPIE 9703 (2016)

[9]                 G. Tao, C.Markos, A. F. Abouraddy, Invited review article, to be submitted Optical Materials Express (2016)

[10]               Z. Tang, Opt. Mater. Express 5, 1722-1737 (2015).