Jon Ward

Mid-infrared supercontinuum generation to 12.5μm in large NA chalcogenide step-index fibres pumped at 4.5μm

We present numerical modeling of mid-infrared (MIR) supercontinuum generation (SCG) in dispersion-optimized chalcogenide (CHALC) step-index fibres (SIFs) with exceptionally high numerical aperture (NA) around one, pumped with mode-locked praseodymium-doped (Pr(3+)) chalcogenide fibre lasers. The 4.5um laser is assumed to have a repetition rate of 4MHz with 50ps long pulses having a peak power of 4.7kW. A thorough fibre design optimisation was conducted using measured material dispersion (As-Se/Ge-As-Se) and measured fibre loss obtained in fabricated fibre of the same materials. The loss was below 2.5dB/m in the 3.3-9.4μm region. Fibres with 8 and 10μm core diameters generated an SC out to 12.5 and 10.7μm in less than 2m of fibre when pumped with 0.75 and 1kW, respectively. Larger core fibres with 20μm core diameters for potential higher power handling generated an SC out to 10.6μm for the highest NA considered but required pumping at 4.7kW as well as up to 3m of fibre to compensate for the lower nonlinearities. The amount of power converted into the 8-10μm band was 7.5 and 8.8mW for the 8 and 10μm fibres, respectively. For the 20μm core fibres up to 46mW was converted.

NOMAD spectrometer on the ExoMars trace gas orbiter mission: part 1—design, manufacturing and testing of the infrared channels

NOMAD is a spectrometer suite on board ESAs ExoMars trace gas orbiter due for launch in January 2016. NOMAD consists of two infrared channels and one ultraviolet and visible channel allowing the instrument to perform observations quasi-constantly, by taking nadir measurements at dayside and nightside, and during solar occultations. In this paper, the design, manufacturing, and testing of the two infrared channels are described. We focus upon the optical working principle in these channels, where an echelle grating, used as a diffractive element, is combined with an acousto-optical tunable filter, used as a diffraction order sorter.

Towards supercontinuum-driven hyperspectral microscopy in the mid-infrared

The extension of supercontinuum (SC) sources into the mid-infrared, via the use of uoride and chalcogenide optical fibers, potentially offers the high radiance of a laser combined with spectral coverage far exceeding that of typical tunable lasers and comparable to traditional black-body emitters. Together with advances in mid-IR imaging detectors and novel tunable filter designs, such supercontinua hold considerable potential as sources of illumination for spectrally-resolved microscopy targeting applications such as rapid histological screening. The ability to rapidly and arbitrarily select particular wavelengths of interest from a broad emission spectrum, covering a wide range of biologically relevant targets, lends itself to image acquisition only at key relevant wavelengths leading to more manageable datasets. However, in addition to offering new imaging modalities, SC sources also present a range of challenges to successful integration with typical spectral microscopy instrumentation, including appropriate utilisation of their high spatial coherence. In this paper the application of SC sources to spectrally-resolved microscopy in the mid-IR is discussed and systems-integration considerations specific to these sources highlighted. Preliminary results in the 3-5μm region, obtained within the European FP7 project MINERVA, are also presented here.

Mid infra-red hyper-spectral imaging with bright super continuum source and fast acousto-optic tuneable filter for cytological applications.

Mid-IR imaging spectroscopy has the potential to offer an effective tool for early cancer diagnosis. Current development of bright super-continuum sources, narrow band acousto-optic tunable filters and fast cameras have made feasible a system that can be used for fast diagnosis of cancer in vivo at point of care. The performance of a proto system that has been developed under the Minerva project is described.

Some new developments in acousto-optic and electro-optic devices

Inhomogeneous and anisotropic materials offer a rich variety of possibilities for constructing novel and efficient electro-optic and acousto-optic devices. Here we illustrate this by discussing some aspects of the acousto-optics of TeO2, a material having a high degree of acoustic anisotropy, giving a design example. We also present some recent results concerning the electro-optics and acousto-optics of inhomogeneous structures, in the form of periodically poled LiNbO3.

Towards the mid-infrared optical biopsy

We are establishing a new paradigm in mid-infrared molecular sensing, mapping and imaging to open up the midinfrared spectral region for in vivo (i.e. in person) medical diagnostics and surgery. Thus, we are working towards the mid-infrared optical biopsy (‘opsy’ look at, bio the biology) in situ in the body for real-time diagnosis. This new paradigm will be enabled through focused development of devices and systems which are robust, functionally designed, safe, compact and cost effective and are based on active and passive mid-infrared optical fibers. In particular, this will enable early diagnosis of external cancers, mid-infrared detection of cancer-margins during external surgery for precise removal of diseased tissue, in one go during the surgery, and mid-infrared endoscopy for early diagnosis of internal cancers and their precision removal. The mid-infrared spectral region has previously lacked portable, bright sources. We set a record in demonstrating extreme broad-band supercontinuum generated light 1.4 to 13.3 microns in a specially engineered, high numerical aperture mid-infrared optical fiber. The active mid-infrared fiber broadband supercontinuum for the first time offers the possibility of a bright mid-infrared wideband source in a portable package as a first step for medical fiber-based systems operating in the mid-infrared. Moreover, mid-infrared molecular mapping and imaging is potentially a disruptive technology to give improved monitoring of the environment, energy efficiency, security, agriculture and in manufacturing and chemical processing. This work is in part supported by the European Commission: Framework Seven (FP7) Large-Scale Integrated Project MINERVA: MId-to-NEaR- infrared spectroscopy for improVed medical diAgnostics (317803; www.minerva-project.eu).

A high-performance passband-agile hyperspectral imager using a large aperture acousto-optic tuneable filter

We describe an acousto-optic tunable filter (AOTF) based hyper-spectral imaging microscope system that allows real time unmixing of the combination of cellular morphology staining and multiple biomarker staining on a single microscope slide. We describe several advances in AOTF technology such as novel acoustic apodization schemes in the longitudinal and transverse directions that have greatly improved image quality. In addition we demonstrate construction leading to improved broadband matching which in turn allows in lower power operation allowing up to 16 simultaneous arbitrary-wavelength optical channels to be processed if necessary. We discuss optical schemes and other factors that allow low stray light and rejection of out-of-band light. A hyper-spectral imaging bright field microscope using these advances demonstrates pathology results that have great potential for clinical use.

Mid-infrared fiber-coupled supercontinuum spectroscopic imaging using a tapered chalcogenide photonic crystal fiber

We present the first demonstration of mid-infrared spectroscopic imaging of human tissue using a fiber-coupled supercontinuum source spanning from 2-7.5 μm. The supercontinuum was generated in a tapered large mode area chalcogenide photonic crystal fiber in order to obtain broad bandwidth, high average power, and single-mode output for good imaging properties. Tissue imaging was demonstrated in transmission by raster scanning over a sub-mm region of paraffinized colon tissue on CaF2 substrate, and the signal was measured using a fiber-coupled grating spectrometer. This demonstration has shown that we can distinguish between epithelial and surrounding connective tissues within a paraffinized section of colon tissue by imaging at discrete wavelengths related to distinct chemical absorption features.

Acousto-optic devices for operation in the infrared

The recent MINERVA project set out to develop new supercontinuum sources operating from 2μm up to (and beyond) 10µm together with related enabling technologies, and deploy them in a spectral-imaging system aimed at the early detection of certain cancers. As part of the project a number of Acousto-Optic devices suitable for operation with the new generation of supercontinuum sources were developed. The design and performance characteristics of any AO device are influenced by the operational wavelength. In particular, the acoustic power and hence the RF drive power required to achieve efficient diffraction scales non-linearly with increasing wavelength. As a result, care must be exercised when designing an AO device for operation at wavelengths above about 1µm, and at wavelengths beyond about 2μm the drive power requirement and consequential management of RF/acoustic energy becomes a significant issue. The criteria for selecting the most appropriate AO interaction medium is reviewed, with an emphasis on factors affecting operation at IR wavelengths. We describe some of the devices developed for operation in the 2μm-4·5μm region. These include an AO Q-Switch for operation at 2·9μm and its deployment in a record peak-power Er:ZBLAN fibre laser. Together with a series of narrowband AO Tunable Filters specifically configured for operation with single spatial-mode white-light sources. The devices, based on the quasi-collinear AO interaction utilise the acoustic power with good efficiency, reducing the required drive power. Finally we describe a technique that is particularly suited to large-aperture AO devices such as imaging AO Tunable Filters.

Fast hyper-spectral imaging of cytological samples in the mid-infrared wavelength region

A prototype mid-infrared spectral imaging system for rapid assessment of cells for cytological diagnosis is reported. Based on a fibre optic super-continuum source that has large spectral brightness and is coupled in to an acousto-optic tuneable filter that can rapidly scan over a set of wavelengths that are chosen to give a high level of selectivity for a specific skin disease. The system has the potential to collect an image cube of 100 wavelengths and 300k pixels in 2 seconds so that cells on living people could be analysed. The system has been evaluated with colon cells over 2700- 3100 cm-1.