Periodic Reporting for period 1 - WISDOM (Wavelength conversIon in diSpersion engineereD Optical fibres for Mid-IR applications)
Reporting period: 2015-06-01 to 2017-05-31
The MIR region of the electromagnetic spectrum is attractive for a very broad range of applications, which stem from the overlap with the two widest atmospheric transmission bands (3-5μm, 7-12μm), like thermal imaging and molecular sensing. Absorption based spectroscopy techniques provide a method for detecting such molecules with high sensitivity and specificity and can thus be employed e.g. for biomedical applications, to monitor the composition of gas/liquid mixtures in industrial processes, or to identify trace components down to extremely low concentrations for purity analysis, environmental monitoring and security applications.
ACCESSING THE MIR REGION - A TECHNOLOGICAL BOTTLENECK
The MIR applications mentioned above require optical devices operating at wavelengths currently non accessible. Most common optical laser sources and detectors operating today in the NIR and MIR rely on technologies where the emission wavelengths are inherently set.
A powerful and versatile solution for accessing MIR wavelengths consists in exploiting optical nonlinear interactions in a nonlinear medium to convert one or more input wavelengths into new ones. The concept is well known and has been exploited in bulk nonlinear crystals and silicon waveguide platforms for frequency conversion. As an alternative nonlinear medium, glass optical fibre platform presents several advantages over these two former platforms thanks to its versatility, lightweight and compact format which naturally lends itself to an all-fibre architecture with no moving parts nor alignments needed.
In recent years, the reliability of silica glass fibre technology has allowed significant achievements in the NIR, such as coherent Supercontinuum Generation (SCG), generation of quantum correlated photon pairs, fibre optical parametric oscillators and amplifiers or frequency comb generation. However, because of its poor transparency for wavelengths above 2.5 μm, silica glass is not suitable for MIR applications and alternative glass systems with a suitable MIR transparency and higher nonlinearity must be used.
To develop a novel optical fibre technology that will enable the development of versatile MIR sources through the exploitation of nonlinear effect in optical fibre. It will enable the development of cost effective and efficient MIR sources and detectors with a range of specifications best adapted to applications having important social repercussions such as the development of devices for early diagnosis and point-of-care, remote sensing, or security instruments.
The efficient exploitation of nonlinear effect in optical fibre requires the ability to engineer the overall dispersion profile of the fibre. Such engineering requires the development of high numerical aperture fibre, ie where the refractive indices of the core and the clad glasses are significantly different
1.1 Development of highly nonlinear and MIR transparent glasses
Considering the requirement for both MIR transmission and high optical nonlinearity, we have focused our interest on the development of a core and clad glass pair based on the TeO2 glass system which possesses adequate properties.
We succeeded in developing a core/clad glass pair with enhanced thermo-mechanical properties with respect to standard tellurite glass compositions.
1.3 Preform fabrication and Fibre drawing
A core/clad optical fibre preform was successfully developed through a combination of extrusion and glass cane drawing. A large core fibre (multimode) and a small core fibre (single-mode) shown in Figure 1, were manufactured. Attenuation losses were measured to be 0.26 dB/m in the multimode fibre. The loss of the small core fibre were measured to be 2.6 dB/m at 1.55 um The increase of the losses for the smaller core fibre indicates the interface scattering centre as the prime source of attenuation in this latter fibre. Nonetheless, typical fibre lengths required for SC generation are of the order of few tens of centimetres at most.
RESEARCH LINE II - Development of optical fibre bundle for thermal imaging
We identified this multimaterial fibre as ideal for producing fibre bundle for thermal imaging applications. Actually thanks to the continuous progress of Mid-Infrared (MIR) detectors, related electronics and lenses, thermal imaging has now become a standard inspection technique in numerous industrial sectors, as well as in biomedical science and defence. Fibre endoscopy is a well-developed technology but so far, the development of a MIR version that can be applied to thermal rather than optical imaging has proved challenging.
As illustrated in Figures 2 to 5, we developed a fibre bundle composed of 1200 pixels that operates in the MIR. We performed thermal imaging of objects maintained at temperatures ranging from 250 º C down to human body temperature along bundles 65 cm and 1.15m long.
EXPLOITATION AND DISSEMINATION
The results obtained during the course of this 2 years research program are yet preliminary but are already of interest both for the academic and industrial sector. In particular, the specifications of the MIR fibre bundle for thermal imaging are of direct interest to the industry as there is no equivalent device on the market. Possible commercial exploitation through the EU/H2020 supported schemes is under consideration. In terms of dissemination, the results mentioned above were presented at 5 international conferences whilst two journal articles are under preparation.
The nonlinear glass material developed during the project possess a unique combination of the three crucial properties below.
i) Long mid-IR range transmission
ii) High nonlinearity
iii) High glass transition temperature and thermal stability
The enhanced quality of the glass and fibre fabrication process developed during the project were confirmed by the low fibre loss value (0.26dB/m) achieved. Although achieved in a multimode fibre, this value is the lowest reported in the literature to date. The fabrication of high quality glass and fibre during the project was a significant milestone towards the development of high power density and stable coherent supercontinuum spectrum across the MIR. The range of application for such devices is broad, in particular in the medical field.
RESEARCH LINE II. Development of optical fibre bundle for thermal imaging
The novel approach developed during the project allow to produce thermal imaging bundles which already meet some of the industrial requirements.
With the continuous development of more performing and cheaper IR detector thermal imaging has now become a standard inspection technique in numerous research and industrial sectors. While for some of these applications MIR imaging bundle would prove to be helpful in other cases the lack of such component is actually preventing the use of thermal imaging. For instance, in the case of search and rescue interventions, the use of such bundle would allow the use of thermal imaging technique to inspect inaccessible area and identify the presence of potential survivors.