We are making new types of optical fibre that transmit, and can emit, long-wavelength mid-infrared light. Why? Mid-infrared light-waves oscillate at frequencies within a range that matches the frequency-range of characteristic vibrations of molecular bonds. The molecular bond vibration increases in amplitude on (resonantly) absorbing mid-infrared light of the same frequency. For the first time, we made a record optical fibre that, on being laser pumped, emitted a record broad range of frequencies of mid-infrared light: 'a mid-infrared rainbow' called a mid-infrared supercontinuum (SC) of light [1]. Shining this broad SC of mid-infrared light onto a molecular sample, and collecting the light again after its interaction with the sample, reveals some mid-infrared frequencies are diminished in brightness, gone to stimulate particular molecular vibrations in the sample. This is called mid-infrared spectroscopy and it allows us to sense and image the molecular makeup of a molecular sample, including numerous molecular gases, liquids and solids as diverse as: greenhouse-gases, explosives, food and biological tissue.
To date, because mid-infrared light sources have been characteristically weak the source/sample/ detector have all had to be in close proximity. The new bright [1,2] mid-infrared fibre SC sources are a disruptive technology which will help establish a new paradigm in PORTABLE, REAL-TIME mid-infrared molecular sensing and imaging, opening up the mid-infrared spectral region for more general use. We are developing this new paradigm through focused development of portable fibre devices and systems which are robust, functionally designed, safe, compact and cost effective, and which are based on mid-infrared optical fibers. Hyper-pure optical fibres are required to increase the efficiency of the SC sources and to realise long-wavelength mid-infrared fibre lasers, and also for passive routeing of mid-infrared light to where it is needed.
Currently, making and purifying the long-wavelength mid-infrared optical fibres is rather intricate and takes about 8 man-weeks. This long-winded processing could be hugely cut, and hyper-purity improved, by applying the innovative processing methods to be developed in this Project.
Currently, resistive heating is used for glass-melting and purification but production times are exceedingly long. We demonstrated for the first time [3] that microwave-assisted heating can achieve high-speed glass-melting. In this Project, we will develop the microwave approach, optimise microwave-cavities and carry out rapid glass melting and follow-on rapid glass hyper-purification via microwave-heating.
Why is the microwave heating so fast? -because the microwaves directly couple to the mid-infrared glass melt and not to the container, which remains cold and uncompromised. Far higher glass-melting temperatures can be attained than normal which: (i) gives faster melt-homogenisation (0.5 h instead of 36 h) and (ii) facilitates high vapour pressures for fast multi-distillations of the glass-melt.
This Project aims to achieve novel cold-container processing to enable the unprecedented rapid manufacture of long-wavelength mid-infrared selenide and telluride chalcogenide glasses of new levels of hyper-purity needed to make new long-wavelength mid-infrared glass fibre-optic devices for disruptive portable, real-time molecular sensing and imaging.
In this Project we will demonstrate, by means of new rapid processing:
i. new hyper-pure fibres for conduiting long-wavelength mid-infrared light;
ii. new hyper-pure long-wavelength mid-infrared SC fibre sources for efficient portable molecular sensing and
iii. first time long-wavelength mid-infrared fibre lasers for pumping the fibre SC.
REFERENCES
1. Petersen, Tang, Benson, Seddon et al., NAT. PHOTON. 8 830(2014).
2. Yu et al., Opt. Lett., 40(6)1081(2015).
3. Prasad, Seddon et al., J. Non-Cryst. Solids, 356(41-42) 2134(2010).
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