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.
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.
Acousto-Optic Tunable Filters with large acceptance angle (parallel tangent configuration) are the component of choice for imaging application in visible and NIR region wavelength. AOTF in the wavelength range above 2<i>μm</i> could be impractical due to the λ<sup>2</sup> and interaction length dependencies on acoustic field intensity to achieve peak diffraction efficiency. A potential solution to reduce the RF power requirement for full diffraction efficiency is to realize a resonant acoustic cavity, and "recycle" the phonons. This configuration could give a theoretical advantage factor between 4 and 10. A prototype device with an operational wavelength range between <i>1μm</i> and <i>2μm</i> has been designed and tested and an optimized design to operate between 2<i>μm</i> - 4<i>μm</i> has been prepared and under construction. Due to the presence of standing wave, when the device is not in resonance a feedback signal from the device is affecting the electrical matching and the power delivered to the device is mostly reflected back (VSWR > 25), therefore a special RF driver is required in order to maintain in resonance the device. The resonance frequencies are also affected by the temperature of the device, thus a temperature control mechanism with high accuracy is required. We present the preliminary results of the first prototype, which are in good agreement with the mathematical model and an advantage factor of about 4 has been measured. Further investigation are planned in order to improve the device performance and develop the RF driver for the resonant configuration.