AFRL-AFOSR-UK-TR-2017-0017 Near-field imaging of optical fibres in the mid-infrared for new Mid-Wave Infrared Fiber Science

This Report is a collation of the information supplied to EOARD 2014-2017 comprising the original Proposal, intermediate Reports consisting of PowerPoint slides (Feb. 2015) and interim Progress Report (June 2015) and the Final Report. The EOARD funding award ($50k) was used in its entirety to purchase a bespoke midinfrared quantum cascade laser (MIR-QCL) for use in far-field and near-field assessment of selenidechalcogenide optical fibers fabricated in-house at the University of Nottingham from glasses melted inhouse. In addition, a cost-effective pixelated pyrometer detector has been bought and commissioned and a far-field optical rig has been constructed and commissioned for use in the near-infrared (NIR) for selenidechalcogenide fiber assessment of numerical aperture. Initial results are reported here together with a full scanning electron microscope imaging and elemental analysis of in-house made selenide-chalcogenide step-index fiber (SIF). However, these initial results reveal a problem in that the NA of the laser source used for far-field must be greater than the NA of the fiber to be tested which can be > 1. Successful near-field NIR assessment is reported here on in-house made selenide-chalcogenide fiber. Future work will hook up the new MIR-QCL with the nearand far-field rigs. We also report novel resonant pumping of rare earth ion doped selenide chalcogenide fiber towards achieving MIR fiber lasing above 4 m wavelength. We report here on two world records achieved during same time period as the EOARD funding: (i) widest MIR supercontinuum generation in a chalcogenide fiber (published in Nature Photonics 2014) and (ii) lowest optical loss Ge-As-Se chalcogenide fiber, with 80 dB/km minimum loss and MIR transmission through 82 m of fiber. DISTRIBUTION A. Approved for public release: distribution unlimited. Professor AB Seddon, MIR Photonics Group, University of Nottingham, UK. E: [email protected] T: +44(0)115 8466755 Page 3 of 53 1. Technical Proposal to EOARD, 10 July 2013 1.1. Long-term Aim and Aim of this Proposal 1.2. Relevant Background 1.3. Proposed research tasks and milestones 1.4. Justification of costing 1,5. Suitability of University of Nottingham References 1.1 Long-term Aim and Aim of this Project The long-term aim is to create a new paradigm in mid-infrared (MIR) fiberoptic power-output, sensing and imaging. New, MIR fiber-based bright-sources, as well as passive conduit fibers, are key to this aim. The aim of this Project, is to construct a novel experimental rig for assessing the geometrical arrangement of glass, and MIR modality, of our in-house fabricated MIR fiber, to give us important iterative feedback on MIR fiber design and glass processing. 1.2. Relevant Background 1.2.1 Usefulness of MIR spectral region There is currently great interest in the MIR spectral region. Soref, a leading contemporary US visionary in photonics and electronics declared in his Jan. 2013 SPIE keynote lecture [1.1]: ‘the mid-infrared is alive!’. The MIR spectral region is defined as 3-50 m and covers the important atmospheric windows of 3-5 and 8-12 m wavelength regions enabling, for example, MIR ship-to-ship free space communications and MIR aircraft free-space counter-measures and collision avoidance. The MIR spectral region also encompasses the molecular fingerprints, that is the characteristic MIR fundamental vibrational absorptions, of numerous gases, liquids and solids as diverse as: combustion gases (giving potential for controlling energy efficiency); ground, water and air pollutants and greenhouse gases (for controlling the environment); pharmaceuticals; toxic agents (for security); soft materials such as plastics and biological tissue (for laser machining and laser surgery and real-time tissue imaging). These matters are discussed further in our papers [1.2, 1.3]. Exploiting the MIR effectively requires the development of a raft of new fiber and waveguide based, narrow and broadband sources, sensors, imaging systems, power delivery and components. An ultimate aim is to achieve analogous flexibility and capability (although not long-haul communications) at the MIR wavelengths as is presently delivered at telecom wavelengths using predominantly silica-glass based fiber [1.2]. For instance to date there are no fiber lasers available for general use beyond 3 m wavelength. 1.2.2 Suitability and potential of chalcogenide glass fiber-optics for MIR Chalcogenide glasses provide transparency across the MIR wavelength range and are sufficiently chemically and physically robust materials for development. Chalcogenide glass based optical fibers not only exhibit MIR passive transmission, but also have the potential to be very bright narrow and broadband, MIR fiber-optic sources. The latter would facilitate: free-space communications; new laser machining; new wavelengths for laser medicalsurgery; real-time molecular sensing and spectral imaging; MIR coherent imaging and MIR power delivery [2.2]. These are the nuts and bolts of MIR communication systems which could provide real-time molecular information to inform decision-taking in many walks of life and enable greater control to be had in real-time over a diversity of processes. Achieving MIR fiber lasing is an important step in developing MIR photonics, and this is briefly covered in the following section. 1.2.3 Chalcogenide glass MIR fiber lasers Commercially available fiber lasers can cover the wavelength range: 400 nm-3 μm. The problems with obtaining longer wavelengths are connected with the host glass material. Lanthanide ions have many photoluminescent transitions in the MIR, but these are quenched by the host material of many currently used fibers [1.3, 1.4]. For example, the most popular material to date for the realization of fiber lasers is silica glass. However, the phonon DISTRIBUTION A. Approved for public release: distribution unlimited. Professor AB Seddon, MIR Photonics Group, University of Nottingham, UK. E: [email protected] T: +44(0)115 8466755 Page 4 of 53 energy of silica is 1100 cm which is the reason why wavelengths above 2 μm are strongly quenched in this material. In order to construct a MIR fiber laser, a host material with a low phonon energy must be manufactured. A candidate is the chalcogenide glasses, which can have very low phonon energies compared to other glasses used to produce fiber lasers, such as ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) and silica. We have modelled [1.5] fiber lasers based on Pr , Dy and Tb rare earth ions doped in selenide-chalcogenide glass fibers. Fig. 1, from [1.5], shows the dependence of the output fiber-laser power on the fiber-laser length for selected values of the fiber optical loss. In all cases the doping concentration is 1500 ppmw and the pump power: 5 W. This study shows that significant output power can be achieved even with losses as high as 3 dB/m. Figure 1.1 Calculated dependence of fiber-laser output power on fiber length for different levels of fiber optical loss for: a) Dy; b) Pr and c) Tb doped into a selenide-chalcogenide glass fiber [1.5]. 1.2.4 Making chalcogenide glass MIR fiber-optics We make multimode core/clad. passive transmission fiber [1.6] lowest optical loss ~ 1 dB m, cf. the champion optical loss achieved for sulfide-chalcogenide glass optical fibers is 12 dB/km [1.7]. We make fiber monomode in the near-infrared (NIR) (paper (i) in preparation). We have measured MIR photoluminescence in a multimode rare earth ion doped optically-clad fiber (paper (ii) in preparation). 1.2.5 Importance of iterative feedback when making chalcogenide fiber-optics It is imperative that we have iterative feedback on the MIR fibers we make, in order to optimize our glass processing and fiber-making. The first step in making chalcogenide optical fiber for MIR active or passive operation is fiber design. Our fiber design capability is based on electromagnetic wave propagation modeling and fiber laser modeling. To realize the fiber design, the minimum requirements are: (i) low optical loss fiber and (ii) control over the geometry of the finished fiber. For instance, a simple fiber geometry could be a tightly-controlled core-diameter, and core-location, along the fiber length, of a circular core that exhibits guiding in the core. We measure fiber optical loss in the MIR using the standard cut-back method and FTIR (Fourier transform infrared) spectroscopy which gives us reassuring feedback on our precursor sourcing, glass chemistry, glass processing and fiber fabrication processing routes. Iterative feedback of the geometrical arrangement and modality of the finished fiber is presently done by imaging fibers in the near-field using SOTS (standard-of-the-shelf) near-infrared (NIR) laser diodes/cameras. This capability is in common with thousands of labs. across the world, for this is the frequency realm of silica glass fiber. DISTRIBUTION A. Approved for public release: distribution unlimited. Professor AB Seddon, MIR Photonics Group, University of Nottingham, UK. E: [email protected] T: +44(0)115 8466755 Page 5 of 53 However, SOTS NIR components are not appropriate for fibers destined for MIR use. There are several reasons why not, as follows. Firstly, chalcogenide glasses exhibit anomalous (steeply changing) refractive index dispersion in the NIR, yet normal refractive index dispersion in the MIR. Secondly, the shorter wavelengths in the NIR that fiber designed and fabricated to be monomode in the MIR ((all other things being equal) would be multimode in the NIR. In short, the feedback got from measuring NIR near-field fiber propagation and modality is not helpful for successful MIR fiber development and operation. Therefore we wish to measure near-field fiber propagation and modality in the MIR. This is the initial research aim of this Project. 1.3. Proposed research tasks and milestones To achieve our Project aim of measurement of near-field fiber propagation and modality in the MIR, we shall purchase a suitable MIR laser, detector, and nano-positioner. A focal plane array detector would be ideal but beyond the scope of costing here. Instead we shall buy an entry-level non-monolithic, and as multi-facet as possible, detector and one idea is to develop a 2-D razor-blade-edge technique (to block the light) together with motorized, computerized nano-positioning stage. The stage position will be synchronized with the light detected in order to map the MIR light emitted from a fiber and thereby understand the fiber near-field and fiber geometry. The MIR necessarily means longer wavelengths of operation which means the diffraction limit is larger (unhelpful) but core sizes etc. are potentially bigger than for NIR use. Aiming for measurement in the 3-5 m wavelength region will be helpful because detection is more sensitive there rather than at 5-12 m. Task 1 Buying equipment for MIR near-field rig Source and purchase suitable MIR source(s), detector(s) and nano-positioner(s). Milestone 1: delivery of source, detector and nano-positioner. Task 2 Commissioning MIR near-field rig using multimode core/clad. MIR fiber The source, detector and nano-positioner will be assembled as an optical rig and first tested with a simple multimode core/clad. chalcogenide fiber, whose optical loss spectrum has been measured. The aim is to make good the MIR optical circuit and characterize it for repeatability. A typical fiber geometry, for a particular fiber draw batch, will be first characterized using analytical scanning electron microscopy (SEM) mapping, as a calibration of the new MIR rig. We will work out: (i) how to rastor across the near-field of the exit face of the known multimode fiber; (ii) how to capture repeatably, small spatial parts of the exiting light; (iii) how to synchronize in time both (i) and (ii), and (iv) how to perform this measurement for one fiber reproducibly and so that geometrically it matches what is seen by the SEM mapping calibration. Computer control of the rastoring and light acquisition will be carried out using MatLab software. Milestone 2: repeatable near-field MIR imaging of a multimode core/clad fiber of measured optical-loss and whose geometry has been calibrated by SEM mapping, to  5 m 2-dimensional (2D) spatial resolution. Task 3 Pushing equipment to its highest resolution to measure monomode core/clad. fiber and other structured fiber types Task 3 involves pushing what has been achieved during Task 2 to greater limits of spatial resolution using well characterized monomode core/clad fiber and other structured fibers. Milestone 3: repeatable near-field MIR imaging of a monomode core/clad fiber whose geometry has been calibrated by SEM mapping, to  1 m 2D spatial resolution and if possible to 0.5 m 2D repeatability. The main aim of this Project will be fundamental scientific results concerning the properties of MIR optical fibers, which we would wish to publish in high impact journals and at conferences. If a successful prototype MIR near-field rig is developed, we would seek to protect the intellectual property associated with a patent. DISTRIBUTION A. Approved for public release: distribution unlimited. Professor AB Seddon, MIR Photonics Group, University of Nottingham, UK. E: [email protected] T: +44(0)115 8466755 Page 6 of 53 1.4. Justification of costing The whole of this budget will be required to purchase the equipment. 1.5. Suitability of University of Nottingham, UK The University of Nottingham is research-led and ranked as in world’s 100 best universities. The University of Nottingham is an elite UK Russell Group university. updated: In the last UK Government Research Excellence Framework (REF, December 2014) the University of Nottingham was ranked 8 in the UK. The University of Nottingham is a truly global university with campi in Kuala Lumpur, Malaysia, and Ningbo, China. The University of Nottingham, UK, is internationally renowned for world-changing, award-winning research and its reputation for excellence attracts distinguished researchers to the staff – many of whom are internationally recognized in their field. The strength of their work lies in their commitment to go beyond traditional boundaries, carrying out research across a vast range of disciplines. Notably, the Nobel Prize was awarded to Sir Peter Mansfield of the University of Nottingham for MRI imaging and medical imaging remains a powerful strand of research at the University. The MIR Photonics Group lies within the Wolfson Centre for Materials Research with access to a raft of local equipment and broad range of on-campus, state-of-the-art equipment.